The Florida State University Hollow Gold Nanosphere Optical Transducers Studied Using Femtosecond...

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4-8-2013

Hollow Gold Nanosphere Optical TransducersStudied Using Femtosecond Time-Resolved LaserSpectroscopyAnne-Marie DowgialloThe Florida State University

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Recommended CitationDowgiallo, Anne-Marie, "Hollow Gold Nanosphere Optical Transducers Studied Using Femtosecond Time-Resolved LaserSpectroscopy" (2013). Electronic Theses, Treatises and Dissertations. Paper 7357.

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THE FLORIDA STATE UNIVERSITY

COLLEGE OF ARTS AND SCIENCES

HOLLOW GOLD NANOSPHERE OPTICAL TRANSDUCERS STUDIED USING

FEMTOSECOND TIME-RESOLVED LASER SPECTROSCOPY

By

ANNE-MARIE DOWGIALLO

A Dissertation submitted to the

Department of Chemistry and Biochemistry

in partial fulfillment of the

requirements for the degree of

Doctor of Philosophy

Degree Awarded:

Spring Semester, 2013

ii

Anne-Marie Dowgiallo defended this dissertation on March 15, 2013.

The members of the supervisory committee were:

Kenneth L. Knappenberger, Jr.

Professor Directing Dissertation

James Brooks

University Representative

Naresh Dalal

Committee Member

Geoffrey F. Strouse

Committee Member

The Graduate School has verified and approved the above-named committee members, and

certifies that the dissertation has been approved in accordance with university requirements.

iii

I dedicated this to my Uncle Billy

iv

ACKNOWLEDGEMENTS

As I conclude my graduate career in chemistry with the completion of this dissertation, I

would like to thank everyone that contributed to my growth and success these past five years. In

particular, I would like to thank my younger sister Jackie for being one of my biggest support

systems. Even though you were living far away in D.C., I knew you were always only a phone

call away and we talked all the time about everything. When you moved to Tallahassee, I was so

happy that I got to live with you, even if it was only for a short time. I seriously could not have

done this without you. Thank you for always encouraging me to achieve my goals no matter

what. Special thanks to my parents for providing me with love and support throughout my years

in college and graduate school. Dad, I would not have an interest in science if it wasn’t for you.

Thank you for instilling a passion in me for learning about science and nature. Your advice about

everything has allowed me to be where I am today and I am eternally grateful for that. Thank

you Mom for always being supportive of me no matter what I wanted to do in life, and

encouraging me to just be myself. You told me when I was younger “dare to be different” and I

have always lived by that. Thank you to my sisters Karin and Carolyn, my grandparents, Nancy

and Bill Piechota, my Aunt Laurie, Uncle Kurt, Aunt Jackie, Uncle Jeff, and Aunt Louise for all

of your encouragement and support throughout my college and graduate school years. Finally I

would like to thank the rest of my extended family.

To my major advisor, Dr. Kenneth L. Knappenberger, Jr., thank you for all of your

support and guidance during these past five years. You have been extremely supportive of my

goals and aspirations since the first day I walked into your office, and I couldn’t be more grateful

for that. Your passion for science and research is inspiring, and I thank you for taking the time to

work with me one-on-one in the lab so that I could develop my own passion for ultrafast lasers

v

and spectroscopy. Thank you for being patient with me and extremely helpful when I would

practice my presentations over and over with you. Also, thank you for giving me the opportunity

to travel to many different conferences. The ACS meeting in San Francisco was remarkable in

terms of the latest research being done in chemistry, and I am thankful I had you as an expert

tour guide to explore that beautiful city!

I would also like to acknowledge my committee members Dr. Naresh Dalal, Dr. Geoffrey

Strouse, and Dr. James Brooks. Thank you Dr. Dalal for your insightful lessons in quantum

mechanics during my second year at FSU. Thank you Dr. Strouse and Dr. Brooks for all of your

invaluable help and guidance during my time as a graduate student. Special thanks to Dr. Rafael

Bruschweiler for being another valuable mentor and teacher while I was at FSU. Your course on

thermodynamics was the first class I had as a graduate student, and that experience made me

realize I was in the right place to further my studies in chemistry. Also thank you to Dr. David

Gormin for being very patient with me when I taught physical chemistry laboratory for the first

time. You made the teaching assistant experience very worthwhile, and I looked forward to

coming to lab every day.

Last but not least, I would like to thank my group members Manabendra, Lenzi, Tom,

Jeremy, Casey, Daniel, Andrew, Chongyue, Domllermut, and Pat. Thank you for coming to

every practice talk I gave and giving me invaluable advice and feedback. Thank you Manabendra

for teaching me how to do second harmonic generation experiments and dark field microscopy.

Lenzi, best of luck to you with continuing my iron porphyrin project, I hope the things I taught

you about hollow gold nanospheres and Raman spectroscopy help you in your future research.

Thank you all for being supportive group members and friends and I wish the best of luck to all

of you in the future.

vi

TABLE OF CONTENTS

List of Tables ................................................................................................................................. ix

List of Figures ..................................................................................................................................x

Abstract ....................................................................................................................................... xvii

1. INTRODUCTION ...................................................................................................................1

1.1 Characteristics and Significance of Hollow Gold Nanospheres ....................................1

1.2 Analysis of Hollow Gold Nanospheres..........................................................................4

1.3 Overview of the Dissertation Study ...............................................................................5

1.4 Description of the Dissertation Chapters .......................................................................7

2. EXPERIMENTAL METHODS ..............................................................................................9

2.1 Nanoparticle Synthesis And Characterization ...............................................................9

2.1.1 Hollow Gold Nanoparticle Synthesis.................................................................9

2.1.2 Solid Gold Nanoparticle Synthesis ..................................................................12

2.1.3 Characterization Tools .....................................................................................13

2.1.4 Aggregation Techniques ..................................................................................14

2.2 Femtosecond Transient Extinction Spectroscopy ........................................................15

2.2.1 Data Fitting Routine .........................................................................................16

3. STRUCTURE-DEPENDENT COHERENT ACOUSTIC VIBRATIONS OF HOLLOW

GOLD NANOSPHERES ...............................................................................................................18

3.1 Introduction ..................................................................................................................18

3.2 Materials and Methods .................................................................................................19

3.2.1 Coherent Data ..................................................................................................19

3.3 Results and Discussion ................................................................................................20

3.3.1 Transient Extinction of HGNs .........................................................................20

3.3.2 Residuals and Fourier Transformations ...........................................................23

3.3.3 Vibrational Frequencies ...................................................................................25

3.3.4 Oscillation Phase ..............................................................................................26

3.3.5 Electron-Phonon Coupling...............................................................................28

3.4 Conclusions ..................................................................................................................30

4. ULTRAFAST ELECTRON-PHONON COUPLING IN HOLLOW GOLD

NANOSPHERES ...........................................................................................................................31

4.1 Introduction ..................................................................................................................31


4.2.1 HGN and SGN Samples...................................................................................34

4.2.2 Two-Temperature Model .................................................................................37


4.3.1 Electron-Phonon Coupling of HGNs ...............................................................38

4.3.2 Size Dependence ..............................................................................................41

4.3.3 Solvent Dependence ........................................................................................46

4.4 Conclusions ..................................................................................................................48

vii

5. INFLUENCE OF CONFINED FLUIDS ON NANOPARTICLE-TO-SURROUNDINGS

ENERGY TRANSFER ..................................................................................................................49

5.1 Introduction ..................................................................................................................49



5.3.1 Phonon-Phonon Coupling in HGNs.................................................................54

5.3.2 Energy Transfer Mechanisms ..........................................................................59

5.3.3 Interfacial Thermal Conductance of HGNs .....................................................61

5.3.4 Solvent Dependence.........................................................................................64

5.4 Conclusions ..................................................................................................................67

6. INTERPARTICLE ELECTROMAGNETIC COUPLING ENHANCEMENT IN HOLLOW

GOLD NANOSPHERE AGGREGATES .....................................................................................68

6.1 Introduction ..................................................................................................................68


6.2.1 Aggregation Techniques ..................................................................................70

6.2.2 Characterization Tools .....................................................................................70

6.2.3 Computational Methods ...................................................................................74

6.3 Electronic Relaxation Dynamics in HGN Aggregates .................................................75

6.3.1 Linear Absorption Spectral Changes ...............................................................75

6.3.2 Transient Extinction of HGN Aggregates........................................................77

6.4 Controlled SPR Properties of HGN Aggregates ..........................................................80

6.4.1 Thiol-Induced Aggregation of HGNs ..............................................................81

6.4.2 SPR Spectra and Electric Field Simulations ....................................................87

6.4.3 Cysteine-Induced Aggregation ........................................................................90

6.4.4 Ethanedithiol-Induced Aggregation .................................................................92

6.5 Conclusions ..................................................................................................................94

7. GOLD NANOPARTICLE AND IRON PORPHRYIN INTERACTION ............................96

7.1 Introduction ..................................................................................................................96


7.2. Aggregation Using FeTMPyP..........................................................................97

7.2.2 Raman Spectroscopy ........................................................................................97

7.3 FeTMPyP-Induced Aggregation of HGNs and SGNs .................................................99

7.3.1 Linear Extinction Measurements .....................................................................99

7.3.2 SERS of FeTMPyP ........................................................................................101

7.4 Conclusions ................................................................................................................104

8. CONCLUSIONS AND FUTURE WORK ..........................................................................105

APPENDICES .............................................................................................................................107

A. HGN AND SGN SIZE DISTRIBUTIONS .............................................................................107

B. HGN AGGREGATE SIZE DISTRIBUTIONS ......................................................................112

C. COPYRIGHT PERMISSION INFORMATION ....................................................................114

viii

REFERENCES ............................................................................................................................122

BIOGRAPHICAL SKETCH .......................................................................................................135

ix

LIST OF TABLES

2.1 Reagent amounts for HGN synthesis ....................................................................................11

2.2 Size distributions for HGN samples ......................................................................................12

2.3 Size distributions for SGN samples .......................................................................................13

4.1 Aspect ratio dependence of the electron-phonon coupling in HGNs ....................................40

4.2 Electron-phonon coupling data for solid gold nanoparticles .................................................42

5.1 Energy transfer half times and structural parameters for citrate-stabilized HGNs ................57

5.2 Energy transfer half times and structural parameters for citrate-stabilized SGNs ................57

6.1 Outer diameters, shell thicknesses, aspect ratios (outer diameter/shell thickness) and SPR

peak positions of isolated HGNs...........................................................................................82

6.2 Average interparticle gap sizes between HGNs in the cysteine-induced aggregates ............85

x

LIST OF FIGURES

2.1 UV-Visible extinction spectra for various HGN samples. The SPR spectral position shifted

to longer wavelengths as the aspect ratio increased ..............................................................13

2.2 TEM images of (a) low aspect ratio HGNs (HGN-2), and (b) high aspect ratio HGNs

(HGN-5). The scale bar applies to both images ....................................................................14

2.3 Laser table layout for femtosecond transient extinction experiments ...................................16

3.1 Two-dimensional transient extinction image plot of the 40 nm OR, R1/R2 = 0.75 HGN

following 400 nm excitation .................................................................................................21

3.2 (a) Transient extinction spectrum acquired for the HGN with R1/R2 aspect ratio of 0.75. The

spectrum was recorded at a pump-probe delay of 1.5 ps following excitation by 400-nm

light. (b) The oscillatory data obtained from temporal integration at the wavelengths

denoted by the vertical lines, 585 nm (blue) and 695 nm (red) from (a) ..............................21

3.3 SPR peak position (black) and spectral width (red) of the R1/R2 = 0.75 HGN plotted as a

function of pump-probe time delay. The period of the SPR fluctuations is about 65 ps,

which is consistent with the oscillations observed in Figure 3.2b ........................................22

3.4 Coherent portion of the transient extinction signal for samples (a) HGN-3, R1/R2=0.38, (b)

HGN-2, R1/R2=0.46 and (c) HGN-13, R1/R2=0.75 ...............................................................23

3.5 Fourier transformation of transient extinction time-domain data for a series of HGNs. The

outer radii were (a) 10, (b) 15, (c) 25, (d) 28, and (e) 40 nm ................................................24

3.6 Summary of coherent acoustic vibration frequencies. (a) The high-frequency mode for a

series of low-aspect ratio HGNs is plotted as a function of the inverse particle outer radius

(open circles). Results from SGNs are included for comparison (filled circles). (b) The low-

frequency vibration measured for high-aspect ratio HGNs is plotted as a function of the

inverse particle outer radius ..................................................................................................26

3.7 Modulated portion of the HGN transient extinction signal for an HGN with R1/R2 = 0.75

(outer radius = 40 nm) along with the fit obtained using the phenomenological response

function given above .............................................................................................................27

4.1 (a) UV-Visible extinction spectra for HGN samples studied here. The SPR spectral position

shifted to longer wavelengths as the aspect ratio increased from 3.5 (HGN-2) to 9.5 (HGN-

15). TEM images of (b) low-aspect ratio HGNs (HGN-5) and (c) high-aspect ratio HGNs

(HGN-15). The scale bar applies to both images ..................................................................35

4.2 UV-visible extinction for solid gold nanoparticle samples SGN-2, SGN-4, and SGN-7 .....36

xi

4.3 TEM images of the solid gold nanoparticle samples used in this study: (a) SGN-2, (b) SGN-

4, and (c) SGN-7, having diameters of approximately 20, 40, and 80 nm, respectively ......36

4.4 (a) Differential extinction for sample J (diameter = 53 ± 11 nm, shell thickness = 5.7 ± 1.0

nm, and aspect ratio = 9.5 ± 2.1) after excitation with 400-nm laser pulses of 90-fs duration

at zero time delay. The dashed line is located at zero differential amplitude as a guide for

the eye. (b) Bleach recovery kinetics observed at the maximum of the SPR band (630 nm

for sample J) for a series of different laser pulse intensities. The raw data (●) were fit to a

bi-exponential decay (—). Higher laser powers gave rise to longer lifetimes ......................39

4.5 Relaxation times determined for the electron–phonon coupling step when different laser

pulse energies were used to excite the sample. The two-temperature model was used to

obtain the zero-point electron–phonon coupling time from the y-intercept of the linear fit

for HGN-2 (squares), HGN-8 (circles), and HGN-12 (triangles). Higher aspect ratio HGNs

exhibited more rapid electron–phonon relaxation ................................................................40

4.6 (a) The room-temperature zero-point electron–phonon coupling times for HGNs (open

circles) and SGNs (closed circles) as a function of aspect ratio and inverse radius,

respectively. (b) The corresponding electron–phonon coupling constants (G) for HGNs and

SGNs, also as a function of aspect ratio and inverse radius, respectively. The coupling

constant was calculated from the room-temperature zero-point electron–phonon coupling

time t0 using t0 = γT0/G (g = 66 J m-3

K-2

for gold and T0=298 K). In both figures, the

dotted line is located at the average value for t0 and G of HGN-2, HGN-4, and HGN-6 and

SGN-2, SGN-4, and SGN-7, at 1.08 ps and 1.84 x 1016

W m-3

K-1

, respectively. In addition,

a linear fit was applied to the t0 and G values for HGN-7, -8, and 11-15 .............................42

4.7 Electron–phonon coupling constants, G, as a function of the total surface area (a) and total

volume (b) of HGNs (samples 2, 5-8, and 11-15) ................................................................45

4.8 Electron–phonon coupling constants, G, as a function of the HGN surface to volume ratio

(total surface area/total volume). The solid line is a linear fit to the data .............................46

4.9 Electron-phonon coupling times as a function of the laser pulse energy for HGN-12 in

water (circles) and methanol (triangles). The room-temperature zero-point electron–phonon

coupling time (or the y-intercept) was 690 and 770 fs in water and methanol, respectively.47

4.10 Electron–phonon relaxation times as a function of the laser pulse energy for HGN-14

dispersed in water (solid circles) and in methanol (solid triangles). The room-temperature

zero-point electron–phonon coupling time (or the y-intercept) was 650 and 990 fs in water

and methanol, respectively....................................................................................................47

5.1 Normalized extinction spectra for select HGN samples used in this chapter. The SPR

maximum wavelength ranges from 550 to 710 nm with increasing outer-diameter-to-shell-

thickness aspect ratio ............................................................................................................52

xii

5.2 Representative TEM images of sample HGN-5 (a) and corresponding EDS data (b). The

scale bar in part a is 20 nm. The images and EDS data indicated that the structures were

composed of a gold shell and a hollow cavity. Cu peaks in panel b arose from the sample

grid and were not indicative of sample contamination .........................................................53

5.3 (a) Spectrally resolved transient extinction spectra of HGN-5. The data were recorded at a

pump−probe time delay of 5 ps following excitation by a 400-nm laser pulse (500

nJ/pulse). (b) Temporally resolved extinction data obtained by monitoring the spectrum

shown in panel a at a probe wavelength of 610 nm (center wavelength of bleach). The

experimental data are plotted along with the best fit to the data, obtained using equation 1.

The dashed vertical line in panel b provides a guide to the point at which the data reflect

nanoparticle-to-surroundings energy transfer kinetics ..........................................................55

5.4 Nanoparticle-to-surroundings energy transfer half times (τET) of HGNs plotted as a function

of their total surface area. These HGNs have cavity radii ranging from 3.3 to 27.5 nm, shell

thicknesses from 5 to 11 nm, and aspect ratios from 3 to 9. The data exhibited behaviors

that were categorized in two classes: HGNs with cavity radii <15 nm and those with cavity

radii ≥15 nm. The data point corresponding to a 15-nm HGN cavity radius is denoted by an

arrow. In both cases, the τET half time was linearly dependent on the total surface area. A

linear fit to the data collected for HGNs with small cavities yielded γ = 20 ± 4 fs/nm2; γ =

65 ± 5 fs/nm2 was obtained for large cavities. x-Axis error bars were determined based on

the outer and inner diameters from TEM images of several particles, and assume uniform

HGN shells. (b) Nanoparticle-to-surroundings energy transfer half times of SGNs as a

function of their total surface area. The τET relaxation time is linearly dependent on the

surface area, with a γ value of 62 ± 3 fs/nm2 ........................................................................58

5.5 HGN (○) and SGN (●) energy transfer half times (τET) plotted as a function of HGN shell

thickness, or SGN radius. The experimental half times are plotted along with calculated

size-dependent interfacial thermal conductivities, G. The values for G were obtained using

equation 8, and bulk values obtained from reference 109 ....................................................61

5.6 Nanoparticle-to-surroundings energy transfer relaxation kinetics obtained for HGN-10. The

raw differential extinction data (black) is plotted along with the result from a two-

component exponential decay (red). The bleach data was inverted for clarity ....................64

5.7 Comparison of the time-resolved extinctions obtained for SGN-5 dispersed in water (black

trace) and methanol (red trace). The raw data reflected a slower transient bleach recovery

for gold nanospheres dispersed in methanol than for those dispersed in water ....................65

5.8 (a) Summary of the energy transfer half times obtained for HGN-5 dispersed in water or

methanol at several excitation pulse energies. The data reflected an increase of τET by a

factor of ∼2.5. (b) Summary of τET for SGN-1 dispersed in methanol or water at several

excitation pulse energies. The data reflected a 3-fold increase in the energy transfer half

time .......................................................................................................................................66

xiii

6.1 Extinction spectra recorded for HGN-5 as an isolated (black) and aggregated (blue) species.

Aggregation was achieved by adding KCl to the HGN-5 aqueous suspension ....................71

6.2 UV-Visible absorption spectra of isolated (black) and aggregated (red) solid gold

nanospheres that an outer diameter of 50 nm. Upon aggregation using a KCl solution, the

SPR band of the isolated SGNs broadens and a new longitudinal band appears at redder

wavelengths, consistent with previous findings.104,105

..........................................................71

6.3 HRTEM image of different regions of HGN-5 aggregates. The images show surface

necking occurring to various degrees, and also that the particles remain hollow .................72

6.4 Dynamic Light Scattering measurements of isolated HGN-5 sample (black) and their

aggregates (red) .....................................................................................................................73

6.5 EDS data for two (a) and several (b) hollow gold nanospheres in the HGN-5 aggregate.

Copper is from the sample grid. The EDS data indicate the aggregates are free of oxides ..74

6.6 Electron diffraction pattern from several hollow gold nanospheres in the HGN-5 aggregate75

6.7 Normalized extinction spectra for isolated and aggregated HGN-10. A noticeable shift of

the SPR λmax to bluer wavelengths is observed upon aggregation ........................................76

6.8 (a) Femtosecond transient extinction spectra of isolated (red) and aggregated (blue) hollow

gold nanoparticles. The nanospheres were excited with a 400 nm pump (500 nJ/pulse) and

probed at 500 fs time delay with a white-light continuum probe. The aggregate spectrum is

clearly blue-shifted with respect to that of the isolated HGN sample. (b) Kinetic traces

resulting from temporal integration of transient extinction bleach from (a) for HGN (red)

and HGN aggregates (blue). The HGN bleach recovery is much slower for the aggregate

system ...................................................................................................................................77

6.9 Electron-phonon coupling relaxation times as a function of relative pump pulse energy for

both isolated (●) and aggregated (○) HGN samples. A linear fit was applied to each set of

data to determine the zero-point electron-phonon coupling time, τel-ph, which is the y-

intercept. The HGN aggregates exhibit a longer electron-phonon coupling lifetime

compared to isolated HGNs and begins to approach bulk values .........................................78

6.10 (a) Femtosecond transient extinction kinetic traces of 50 nm solid gold nanospheres probed

at 520 nm following 405 nm excitation. The excitation pulse energies are: 0.1 μJ/pulse

(blue), 0.2 μJ/pulse (black) and 1 μJ/pulse (red). (b) Electron-phonon relaxation times

plotted as a function of relative excitation pulse energy. The y-intercept from the linear fit

corresponds to the zero-point electron-phonon coupling lifetime for 50 nm SGNs, 770 ±

150 fs .....................................................................................................................................79

6.11 Experimental extinction spectra of isolated HGN and cysteine and ethanedithiol-induced

HGN aggregates. The dashed lines are located at the center of the extinction peaks. A

distinct blue shift of the SPR for HGNs having shell thickness <7 nm (a,b) occurred upon

xiv

ethanedithiol addition. However for HGNs with shell thickness >7 nm (c,d), only a small

red shift or no peak shift occurred after ethanedithiol addition. The insets show the same

spectra normalized at the respective SPR maxima to show the peak shift more clearly ......82

6.12 Cysteine-induced (A) and ethanedithiol-induced (B) aggregates of HGN-6. The scale bars

correspond to 50 nm. In the cysteine-induced aggregates, distinct gaps can be seen and

were usually about 1 nm wide. On the other hand, ethanedithiol-induced aggregates usually

formed small dimers or trimers and showed little to no interparticle gap ............................83

6.13 Absorption spectra of HGN-13 (a), 15 (b), 16 (c), and 17 (d) formed by low concentration

addition of cysteine (2 μL of 5 mM cysteine) .......................................................................84

6.14 Probable hydrogen-bonding scheme of the cysteine-induced HGN aggregates (not to scale) 85

6.15 FTIR spectra of cysteine and cysteine induced aggregates of HGN-15. Absence of SH

stretching vibrational mode at 2564 cm-1

in the spectrum of aggregates suggest that

cysteine is attached to the gold surface by Au−S linkage .....................................................86

6.16 Simulated absorption spectra using FDTD calculations of an HGN having an outer diameter

of 53 nm and shell thickness of (a) 5 nm and (b) 8 nm. These results are similar to the

experimental results presented in Figure 6.11 b (HGN-17) and d (HGN-8). .......................87

6.17 The normalized peak shift (Δλ/λ0) as a function of HGN shell thickness. Panel (a) shows the

experimentally determined values for ethanedithiol-induced HGN aggregates. Panel (b)

shows the FDTD simulated results for HGN dimers (outer diameter of 53 nm) in contact. 88

6.18 Simulated electric field maps using FDTD calculations for various shell thicknesses: from

left to right in each panel, 5, 7, and 10 nm. The outer diameter of the HGNs was fixed at 53

nm for each panel. The top panels are for an isolated HGN, the middle panels are for HGNs

dimers in contact, and the bottom panels are for HGN dimers that are separated by 5 nm ..89

6.19 Simulated SPR peak shift (red shift Δλ for spatially separated dimers divided by the SPR

maximum of the isolated particle, λ0) as a function of interparticle gap to outer diameter

ratio (D/d) for HGN-6. The decay constant (t) value is consistent with that of similarly

sized solid gold nanospheres .................................................................................................91

6.20 Schematic of a stable charge-transfer plasmon where the incident electromagnetic field is

polarized parallel to the interparticle axis of the hollow gold nanospheres. The distance

between the two cavities is given by D .................................................................................93

7.1 Normalized extinction spectra for isolated SGNs (black) and FeTMPyP-induced SGN

aggregates (red). The changes occurring following aggregation include a slight red-shift of

the transverse SPR band to 535 nm and a new red-shifted band at 820 nm .........................99

xv

7.2 Normalized extinction spectra for isolated HGNs (black) and FeTMPyP-induced HGN

aggregates (red). The SPR band of the isolated HGNs red-shifts slightly to 557 nm and

another red-shifted peak occurs at 685 nm .........................................................................100

7.3 Surface-enhanced Raman spectra of FeTMPyP in the presence of SGN-5 aggregates (black)

and 2.5 x 10-7

M FeTMPyP (red). The solutions were excited using a 785 nm laser. The

peaks are labeled according to reported Raman bands for FeTMPyP ................................101

7.4 Surface-enhanced Raman spectra of FeTMPyP in the presence of HGN aggregates. The

solutions were excited using a 633 nm laser. The peaks are labeled according to reported

Raman bands for FeTMPyP ................................................................................................102

A.1 Size distributions for HGN-1 having d = 16.6 ± 2.9 nm, t = 5.0 ± 1.2 nm, AR = 3.4 ± 0.6,

and R1/R2 = 0.40 ± 0.08 ......................................................................................................107


and R1/R2 = 0.46 ± 0.1 ........................................................................................................107


and R1/R2 = 0.56 ± 0.07 ......................................................................................................107


and R1/R2 = 0.61 ± 0.05 ......................................................................................................108


and R1/R2 = 0.60 ± 0.10 ......................................................................................................108


and R1/R2 = 0.68 ± 0.15 ......................................................................................................108

A.7 Size distributions for HGN-8 having d = 54.6 ± 12.5 nm, t =8.6 ± 2.9 nm, AR = 6.7 ± 1.8,

and R1/R2 = 0.67 ± 0.08 ......................................................................................................108

A.8 Size distributions for HGN-10 having d = 77.9 ± 5.5nm, t = 11.3 ± 2.2nm, AR = 6.9 ± 1.2,

and R1/R2 = 0.75 ± 0.05 ......................................................................................................109


and R1/R2 = 0.70 ± 0.16 ......................................................................................................109


and R1/R2 = 0.73 ± 0.12 ......................................................................................................109


and R1/R2 = 0.74 ± 0.07 ......................................................................................................109

xvi

A.12 Size distributions for HGN-14 having d = 52.2 ± 8.0 nm, t = 5.9 ± 1.0 nm, AR = 9.0 ± 1.6

and R1/R2 = 0.77 ± 0.17 ......................................................................................................110

A.13 Size distributions for HGN-15 having : d = 53.3 ± 10.5 nm, t = 5.7 ± 1.0 nm, AR = 9.5 ±

2.1, and R1/R2 = 0.78 ± 0.05 ...............................................................................................110

A.14 Size distributions for SGN samples: (a) SGN-2, d = 19.8 3.7 nm, (b) SGN-4, d = 37.7

3.3 nm, and (c) SGN-7, d = 83.3 7.5 nm .........................................................................110

A.15 Size distributions for SGN samples: (a) SGN-1, d = 18.3 2.0 nm, (b) SGN-3, d = 25.4

4.2 nm, (c) SGN-5, d = 38.4 4.2 nm ................................................................................110

A.16 Size distribution for SGN-6, d = 59.8 7.8 nm ..................................................................111

B.1 Ethanedithiol-induced HGN aggregate distributions for HGN-2, HGN-6, HGN-13, HGN-

15, HGN-16, and HGN-17. The occurrence of each type of aggregate (i.e. monomer, dimer,

trimer, etc.) was plotted as a histogram up until aggregates comprising 8 HGNs ..............112

B.2 Ethanedithiol-induced HGN aggregate distributions for HGN-8. The occurrence of each

type of aggregate (i.e. monomer, dimer, trimer, tetramer, etc.) was plotted as a histogram

up until aggregates comprising 8 HGNs .............................................................................113

xvii

ABSTRACT

This dissertation presents and evaluates the unique interplay between nanoparticle

structure, environment, and electronic energy relaxation. This knowledge will provide useful

information for tailoring nanoparticle properties so that they can be applied to the development

of more efficient transducers, such as a light-harvesting antenna. In particular, plasmonic gold

nanoparticles have been synthesized, both hollow (HGNs) and solid (SGNs), and their structural

properties have been characterized using transmission electron microscopy (TEM), energy

dispersive spectroscopy (EDS), dynamic light scattering (DLS), and UV/Vis absorption

spectrophotometry. Femtosecond pump-probe transient extinction experiments have been

conducted on both isolated and aggregated HGNs and SGNs in order to elucidate their electronic

energy relaxation properties. While studying how aggregated nanostructures influence optical

and electronic properties, an unexpected spectral blue-shift of the surface plasmon resonance

(SPR) was observed upon aggregation of HGNs using a salt solution, which led to longer

electronic energy relaxation times compared to isolated HGNs. These findings were significant

because previous studies have found that SGNs red-shift upon aggregation and have faster

electronic energy relaxation times. In order to understand further the nature of the blue shift in

HGN aggregates, alkane-thiols were used to induce the aggregation, where it was found that at a

critical thickness of the HGN shell, the SPR blue-shifts due to the interaction of the electric

fields within the hollow cavities of the nanoparticles. These alkane-thiol ligands provide for

more controlled aggregation over the interparticle gap than other aggregating agents such as

potassium chloride salt. Transient extinction experiments at high pulse energies were conducted

to learn about the modulation in the SPR frequency of HGNs following excitation by a

femtosecond laser pulse. The oscillation frequency and phase were determined for a wide range

xviii

of HGN sizes, revealing a size-dependent excitation mechanism of the vibrational modes. In

addition, transient extinction experiments were carried out at low pulse energies in order to

determine the electron-phonon coupling times for a wide range of sizes of HGN and SGN

samples. As the aspect ratio of the HGN increases, the electron-phonon coupling time decreases

(or the electron-phonon coupling increases), whereas for SGNs, the electron-phonon coupling

remains constant with increasing diameter. The electron-phonon coupling enhancement exhibited

by high aspect ratio HGNs was attributed to the large surface to volume ratio of these structures,

which results in non-negligible contributions from their environment. Finally, the phonon-

phonon coupling properties of HGNs were investigated, which is the last step in electronic

energy relaxation in metal nanoparticles. This study revealed that fluids confined to the hollow

core of HGNs have different properties compared to their bulk counterparts, thereby influencing

the particle-to-surroundings energy transfer rates. Hence, the cavity influences the electronic and

mechanical properties of the HGNs. The structural, optical, and electronic studies on the

aforementioned types of metal nanoparticles provide the basis to understand how the surface

plasmons influence light absorption in a nearby molecule. Specifically, how the surface

plasmons of HGNs and their aggregates interact with the discrete electric-dipole transitions of

iron porphyrin molecules. Surface-enhanced Raman spectroscopy (SERS) of iron porphyrin

molecules near SGN and HGN aggregate surfaces was employed to understand the interaction

between the strong electric fields of HGNs and molecular electronic transitions.

1

CHAPTER ONE

INTRODUCTION

1.1 Characteristics and Significance of Hollow Gold Nanospheres

Hollow gold nanospheres (HGNs) have the potential to be used as more efficient optical

transducers, such as light-harvesting antennas. The dimensions of these high surface-to-volume

ratio structures can be used to tailor the efficiency of the conversion from the initial electronic

energy to thermal energy. The nanoparticle structure and surrounding environment of HGNs

significantly influence electronic energy relaxation in these particles, where HGNs exhibit more

efficient electronic energy relaxation than solid gold nanospheres (SGNs). In addition, as

broadband optical antennas, the strong electric field near the surface of the HGNs could lead to

coupling between energy levels in a nearby molecule. The surface plasmon resonance (SPR)

phenomenon exhibited by noble metal nanoparticles can provide selective enhancement of

molecular spectroscopies such as surface-enhanced Raman spectroscopy (SERS)6 and metal-

enhanced fluorescence (MEF).7-9

The SPR is an extremely short-lived species on the order of a

few femtoseconds; hence, the SPR has been utilized as a probe to study longer events on the

picosecond time scale, such as electronic energy relaxation. The electronic energy relaxation

steps that will be described for metal nanostructures are the coherent vibrational resonances,

electron gas-to-metal lattice energy transfer, and lattice-to-solvent energy transfer. These energy

relaxation steps have been well characterized for solid gold nanoparticles, but making the

nanoparticle hollow leads to significant differences. The hollow gold nanosphere is a unique

nanostructure that contains a gold shell and a fluid dielectric core.10-14

The frequency of the SPR

observed for HGNs can be tuned from the visible to the near-IR by altering the structure’s aspect

2

ratio, which can be defined as the ratio of the outer diameter-to-shell thickness. This wide range

of tunability is not observed for solid gold nanospheres. The tunability of the SPR and the high

electron temperatures obtained upon excitation of metals using ultrafast laser pulses15-19

make

HGNs potential materials for use as efficient photothermal therapy agents. In addition, they

possess structure dependent vibrations which could allow them to function as nanomechanical

resonators for ultrasensitive chemical sensing.20,21

HGNs exhibit additional properties that differ from those of solid gold nanospheres. For

example, HGNs exhibit first hyperpolarizabilities that are three times larger than the values

determined for comparably sized SGNs, suggesting that HGNs could be highly useful for

nonlinear optical applications.13

Similarly, increased surface-enhanced Raman scattering by

HGNs over SGNs has been reported by Zhang and co-workers.18

The mechanical properties of

HGNs also differ from those of SGNs; the coherent acoustic oscillations of HGNs exhibit a

longer period compared to SGNs.23–25

Examination of a comprehensive range of particle sizes

demonstrated that the isotropic coherent acoustic vibrations of HGNs are structure dependent.25

In addition, interparticle HGN plasmon resonances are heavily affected by the interior (cavity)

surface, resulting in a blue shift of the SPR resonance rather than the red shift common to SGN

aggregates.11,12

However, the properties of the fluids confined to the nanoscale dimensions of the

HGN interior cavity, and their influence on HGN optical, mechanical, and electronic relaxation

properties, remain unclear. Finally, electron–phonon coupling rates observed for isolated HGNs

are faster (fastest time being 0.59 ± 0.08 ps) to than those seen for HGN aggregates or SGNs

(average value, 1.08 ± 0.08 ps).11

The high surface-to-volume ratio of HGNs leads to more

efficient electron-phonon coupling in these structures. The latter property may significantly

3

impact technologies depending on metal-interface energy transfer, such as nanoelectronics and

photothermal therapy.

The process of nanoparticle-to-surroundings energy transfer in HGNs can enhance the

efficiency of several applications such as micro/nanoelectronics,26

material processing,27

photodynamic therapy,28

and electromagnetic energy transport through patterned nanoparticle

networks.29

The nanostructure synthesis and fabrication techniques currently available allows for

the production of particles over a vast range of sizes and morphologies, which can be exploited

to tune particle-to-environment energy transfer rates.1−5,30

Structure-dependent energy transfer

rates of HGNs were quantified using femtosecond time-resolved transient extinction

spectroscopy, which is a reliable experimental diagnostic for studying the rapid electronic energy

relaxation mechanisms of metal nanostructures.31-34

When HGNs are aggregated, they have the potential to contribute to three-dimensional

photon delivery in nanoscale devices.35

The near-field coupling that takes place in plasmonic

nanoparticle aggregates provides a nanostructured waveguide to promote the coherent

propagation of electromagnetic radiation.11

The combination of facile laboratory

synthesis1,10,15,16,36

and elegant “superstructure” assembly techniques37-42

has stimulated

expectations of superior products in fields such as photovoltaics,43,44

biological labels,45,46

nonlinear optics,47,48

three-dimensional waveguides,49,50

and negative refractive index

materials.51-53

However, the influence of the precise arrangement of plasmonic nanoparticles

within a designed array on the collective properties of composite materials must be understood

before many of these applications can be realized. Hollow gold nanospheres have potential to

contribute to several of these applications due to their wide range of tunability in terms of their

structure, optical, and electronic properties.

4

1.2 Analysis of Hollow Gold Nanospheres

Electronic excitation of metals using short-pulse lasers generates a non-equilibrium electron gas

with high electron temperatures, which relaxes by three successive steps: (i) electron−electron

scattering, (ii) electron−phonon coupling, and (iii) energy transfer to surroundings.32

Following

ultrafast (∼100 fs) electron−electron scattering, the hot electron distribution equilibrates with the

metal lattice on a ∼1-ps time scale via electron−phonon coupling. The final step in this electronic

energy relaxation sequence is energy transfer from the hot electron and phonon subsystems to the

environment. The time scale for the electron–phonon relaxation process (a few picoseconds) is

shorter than the phonon period (typically tens of picoseconds).31,54

Thus, the process of electron-

phonon coupling leads to lattice expansion. For metals, this expansion reduces the material’s

charge density and modifies the metal’s dielectric properties, yielding a modulation of the SPR

frequency.31

As a result, femtosecond time-resolved transient extinction has become a reliable

experimental diagnostic for studying the electronic, optical, and mechanical properties of

plasmon-supporting nanostructures.31,54-66

This time-resolved pump–probe technique monitors

the surface plasmon resonance in the ‘‘probe’’ step to report on the cooling rates of these three

processes.31,32-34,67

In particular, the use of the two-temperature model and measurements of the

electron–phonon coupling rate (step 2) as a function of excitation pulse energy allow for

determination of the metal’s electron–phonon coupling constant, G.68

When HGNs are aggregated, the SPR is shifted to shorter wavelengths than the transverse

SPR of an isolated HGN.11

Finite difference time domain simulations (FDTD) suggest the blue-

shifted SPR can be assigned to newly formed longitudinal SPR of HGN dimers. Two possible

explanations for this surprising SPR blue shift are (i) antibonding modes of hybridized plasmons

of HGN dimers69

or (ii) a charge-transfer plasmon resonance.70-72

In the first case, a SPR blue

5

shift would be observed for an asymmetric nanosphere aggregate when spectral weight is

transferred from the lower-frequency bonding mode to the higher-frequency antibonding hybrid

mode, or other higher-order modes.73

In the case of the charge-transfer model,71,72

a blue shift

would be expected when the particles are either in contact or separated by distances short enough

to permit a conductive overlap; such an overlap would lead to a collective time-dependent charge

oscillation over the two particles comprising the dimer. Thiol linkages were used to form small

aggregates such as nanosphere dimers, trimers, and tetramers, as well as larger extended

structures that can be used to elucidate key parameters that influence collective charge

oscillations in plasmonic assemblies. Our findings indicated that HGNs can exhibit both

hybridized plasmon modes and collective charge transfer resonances when the particles are

assembled into small or large extended aggregate structures. The interparticle gap and shell

thickness were identified as key factors that influenced aggregate optical properties. When

aggregates containing significant interparticle spatial separation were created, a dielectric gap

was formed that screened the two particles. In this case, the optical response was fully described

by the hybridization of surface plasmon modes. In contrast, negligible interparticle gaps led to a

domination of the visible absorption spectra by charge-transfer resonances.

1.3 Overview of the Dissertation Study

The influence of particle size and morphology on the nanostructure’s acoustic vibrations can be

studied directly using ultrafast laser-based techniques.20,21,31

Experimental femtosecond time-

resolved transient extinction data is presented in this work to demonstrate that the excitation

mechanism and the frequency of the isotropic vibrational mode of HGNs are both size-

dependent. This work represents the first study of electronic energy relaxation and coherent

acoustic vibrations of hollow gold nanospheres spanning a comprehensive range of particles

6

sizes. Conclusions are supported by quantitative analysis of the frequency, amplitude and phase

components of the coherent transient data.

In addition, electron–phonon relaxation lifetimes (and the corresponding electron–

phonon coupling constants) have been determined for hollow gold nanospheres having outer

diameter-to-shell thickness ratios (aspect ratios) ranging from 3.5 to 9.5. The electron–phonon

relaxation times were obtained using pump–probe transient extinction spectroscopy. As the HGN

aspect ratios increased, the electron–phonon coupling times decreased, leading to larger

electron–phonon coupling constants. The size-dependence likely arose from non-negligible

environmental contributions to the cooling process that occurred as the HGN total surface-area-

to volume ratio increased. Descriptions of the dependence of the electronic energy relaxation on

the particle composition, size, shape, and surroundings are necessary to develop novel devices

that utilize their unique thermal and electrical transport properties.1-3,5

The last step in the relaxation process involving particle-to-surroundings energy transfer

is analyzed to determine the half times for a series of HGNs having outer diameter-to-shell

thickness aspect ratios ranging from 3 to 9 and total surface areas ranging from 1.0 × 103 to 2.8 ×

104 nm

2. The apparent energy transfer half times were obtained using femtosecond time-resolved

pump−probe transient extinction spectroscopy. As the HGN surface area increased, the energy

transfer half times also increased, but the data showed a discontinuity at a particle cavity radius

of 15 nm. Analysis of HGN interfacial energy transfer indicated small HGNs (cavity radius <15

nm) had interfacial thermal conductivities that were ∼1.9−2.4 times less than those of SGNs and

larger HGNs. This effect was attributed to the difference between the thermal conductivity of

water confined to small HGN cavities and that for bulk water. The apparent energy transfer half

7

times were also sensitive to the surrounding environment, becoming larger when the HGNs were

dispersed in methanol, which has a lower thermal conductivity than water.

Next, the aggregation of HGNs demonstrates control over the SPR spectral position and

electric field spatial distribution of these nanostructures.12

Thiol-induced aggregation forms

structures that contain only a few HGNs, such as dimers and trimers. It was found that the SPR

spectral position could be tuned to either higher or lower frequencies, depending upon the size of

the interparticle gap and the thickness of the HGN shell. Both numerical and experimental results

indicated that the electric field amplitude and spatial distribution of HGN aggregates could be

tailored by changing the dimensions of the individual nanospheres in the aggregate.

1.4 Description of the Dissertation Chapters

The chapters that follow introduce experimental methods and describe the powerful instrumental

technique, femtosecond transient extinction spectroscopy, used to study electronic energy

redistribution in both hollow and solid gold nanospheres. Chapter 2 provides a detailed

description of the synthetic protocol, instrumentation, and data analysis. Specifically, this chapter

presents the synthesis of both hollow and solid gold nanospheres, and their characterization using

UV-Visible absorption spectrophotometry, electron microscopy, dynamic light scattering (DLS),

and energy dispersive spectroscopy (EDS). In addition, the experimental set-up of femtosecond

transient extinction spectroscopy and data fitting routine is discussed.

Chapter 3 presents the size-dependent coherent acoustic vibrations exhibited by HGNs.

The frequency and phase analysis of this data is described and presented to reveal that HGNs

exhibit two different categories for the excitation of acoustic vibrations: (1) direct isotropic

expansion in low aspect ratio particles, and (2) indirectly launched low-frequency modes in high

8

aspect ratio particles. The indirectly-launched modes result from efficient electron-phonon

coupling in high aspect ratio (and hence, higher surface-to-volume ratio) HGNs.

In Chapter 4, the electron-phonon coupling properties of HGNs and SGNs are presented.

It discusses how the higher surface-to-volume ratio of high aspect ratio HGNs leads to shorter

electron-phonon coupling times, or increased electron-phonon coupling constants. In contrast,

SGN samples did not exhibit size-dependent electron-phonon coupling times. In addition, this

chapter describes solvent-dependent measurements.

Chapter 5 describes nanoparticle-to-surroundings energy transfer for both hollow and

solid gold nanospheres. It discusses how HGNs exhibit two different behaviors during this

process depending on the size of their cavities.

The aggregation properties of HGNs is presented in Chapter 6. HGNs were aggregated

using KCl salt solution, cysteine, and ethanedithiol. Cavity plasmon resonances appear to

contribute significantly to interparticle modes that are formed when neighboring particles

undergo near-field coupling.11-13

Aggregation of HGNs by surface necking results in decreased

electron−phonon coupling rates owing to the formation of a continuous nanoparticle network that

has a decreased effective surface-to-volume ratio.11

In Chapter 7, a cationic porphyrin, iron(III) tetrakis(1-methyl-4-pyridyl)porphine

(FeTMPyP), was used to aggregate SGNs and HGNs. This aggregation was verified using UV-

visible absorption spectrophotometry and surface-enhanced Raman scattering measurements.

Future research on this topic will focus on describing the influence of the plasmon on molecular

electric-dipole transitions in FeTMPyP.

Finally, Chapter 8 summarizes the main conclusions from each work.

9

CHAPTER TWO

EXPERIMENTAL METHODS

In this chapter, the synthesis and characterization of both hollow and solid gold nanospheres will

be described. In addition, the method of nanoparticle aggregation will be presented. The

technique used to study the electronic properties of hollow and solid gold nanospheres is

femtosecond transient extinction spectroscopy and will be described here.

2.1 Nanoparticle Synthesis And Characterization

2.1.1 Hollow Gold Nanosphere Synthesis

Hollow gold nanospheres were synthesized by a sacrificial galvanic replacement technique

involving the oxidation of cobalt nanoparticles and the subsequent reduction of gold ions.10

The

materials used for the HGN synthesis included: cobalt chloride hexahydrate (CoCl2·6H2O,

Puratronic, 99.998%, Alfa Aesar), chloroauric acid trihydrate (HAuCl4·3H2O) ACS reagent

grade, 99.99%, Alfa Aesar), sodium citrate tribasic dihydrate (C6H9Na3O9, ACS reagent grade,

>99%, Sigma-Aldrich), and sodium borohydride (NaBH4, 99.99%, Sigma-Aldrich). All water

used in the syntheses was 18.2 MΩ milli-Q filtered. Under deoxygenated conditions and constant

argon flow, cobalt nanoparticles were first synthesized by the sodium borohydride-mediated

reduction of Co2+

ions in the presence of citrate ions. Once hydrogen gas formation had ceased,

the desired amount of gold salt was added to the cobalt nanoparticle suspension where the Co0

oxidized to Co2+

ions and Au3+

ions reduced to Au0 onto the cobalt nanoparticle template.

Exposure to ambient conditions ensured the complete oxidation of the cobalt nanoparticle and

formation of a thin gold shell encapsulating water and dissolved salts. For example, a cobalt

10

nanosphere suspension was first prepared using 100 L of 0.4 M cobalt chloride solution (this

amount is the same for all syntheses) and 400 L of 0.1 M sodium citrate solution in 100 mL of

water. The amounts of each reagent used for each HGN sample are listed in Table 2.1. Cobalt

nanospheres are highly sensitive to dissolved oxygen. Hence, the cobalt nanosphere solution

was vacuumed for 30 minutes and then bubbled with argon gas for 10 minutes. The Co2+

in

solution was reduced to cobalt nanospheres by adding 100 L of a freshly-made 1 M sodium

borohydride solution with vigorous stirring. The cobalt nanoparticles serve as sacrificial

templates to leave a hollow spherical shell of gold remaining. The addition of NaBH4 generated

H2 gas bubbles that need to dissipate before adding any gold to the solution. Hence, the cobalt

nanosphere solution was allowed to stir under constant argon flow for about 45 minutes, or until

there were no visible H2 gas bubbles. The gold solution was prepared by adding 30-80 L of 0.1

M chloroauric acid solution to 10 mL of water in a beaker. Then, 30 mL of the cobalt

nanoparticle suspension was quickly added to this beaker containing the gold solution. Exposing

the solution to air allows the cobalt nanoparticles to be completely oxidized, leaving only a thin

shell of gold remaining with water and dissolved salts on the inside. Using this method, it is

possible to produce different aspect ratios of HGNs samples by using the same cobalt

nanoparticle suspension and different amount of gold solution. HGN-13, HGN-15, and HGN-16

were obtained from the same cobalt nanoparticle solution. HGN-2 and HGN-6 were also

obtained from the same cobalt solution, and finally HGN-12 and HGN-14 were synthesized from

the same batch of cobalt nanoparticles. In addition, HGNs can be produced by instead adding a

high concentration of gold solution.

HGN samples 5, 7, and 8 were prepared in this slightly different manner. First, cobalt

nanoparticles were prepared as described above. Then, 50 L of 0.1 M chloroauric acid was

11

added to the stirring cobalt nanoparticle solution (under constant Ar flow) at 60 seconds intervals

until a total volume of 200 L of gold solution added was reached. The solution was then

exposed to air, allowing the hollow gold nanospheres to form through the oxidation of cobalt and

reduction of gold. This alternative procedure produced 100 mL of HGN sample solution as

opposed to the previous procedure that produced only 30 mL of each HGN sample solution.

Table 2.1: Reagent amounts for HGN synthesis.

Sample Amount of 0.1M

citrate (μL)

Amount of 1M

NaBH4 (μL)

Amount of 0.1M

HAuCl4 (μL) HGN-1 400 300 40

HGN-2 400 100 30

HGN-3 400 300 40

HGN-4 300 200 40

HGN-5 400 200 200

HGN-6 400 100 80

HGN-7 400 200 100

HGN-8 400 200 200

HGN-9 200 300 80

HGN-10 120 160 100

HGN-11 190 180 70

HGN-12 190 180 70

HGN-13 190 180 70

HGN-14 190 180 50

HGN-15 190 180 50

HGN-16 190 180 30

HGN-17 190 180 30

Desired HGN aspect ratios were achieved by altering the relative amounts of citrate, NaBH4, and

HAuCl4 injected into the reaction. In Table 2.2, the dimensions of the HGNs that were

synthesized are listed, and range from 16.6 nm to 77.9 nm in outer diameter, 4.5 to 11.3 nm in

shell thickness, and 3.4 to 11.7 in aspect ratio.

For solvent-dependent studies, both HGNs and SGNs were transferred from water to

methanol solutions. HGN and SGN solutions were subjected to centrifugation at 5000 rpm for 30

min. The aqueous supernatant was discarded, and the pellet was redispersed in methanol via

sonication.

12

Table 2.2: Size distributions for HGN samples.

Sample Outer Diameter

(nm)

Shell Thickness

(nm)

Aspect Ratio

HGN-1 16.6 (± 2.9) 5.0 (± 1.2) 3.4 (± 0.6)

HGN-2 29.9 (± 6.2) 8.5 (± 2.2) 3.5 (± 0.6)

HGN-3 18.0 (± 3.0) 5.0 (± 1.0) 3.6 (± 0.7)

HGN-4 27.9 (± 3.2) 6.3 (± 1.3) 4.4 (± 2.1)

HGN-5 51.1 (± 5.1) 10.0 (± 1.0) 5.1 (± 0.6)

HGN-6 31.2 (± 4.6) 6.3 (± 2.1) 5.4 (± 1.5)

HGN-7 50.7 (± 8.9) 8.2 (± 2.2) 6.4 (± 1.3)

HGN-8 54.6 (± 12.5) 8.6 (± 2.9) 6.7 (± 1.8)

HGN-9 48.0 (± 5.0) 7.0 (± 1.0) 6.9 (± 2.7)

HGN-10 77.9 (± 5.5) 11.3(± 2.2) 6.9 (± 1.2)

HGN-11 46.7 (± 8.5) 7.0 (± 2.1) 7.2 (± 2.1)

HGN-12 53.2 (± 7.2) 7.1 (± 1.7) 7.8 (± 1.6)

HGN-13 54.8 (± 12.2) 6.9 (± 1.6) 8.3 (± 2.2)

HGN-14 52.2 (± 8.0) 5.9 (± 1.0) 9.0 (± 1.6)

HGN-15 53.3 (± 10.5) 5.7 (± 1.0) 9.5 (± 2.1)

HGN-16 49.3 (± 9.7) 5.1 (± 0.8) 9.9 (± 2.0)

HGN-17 51.5 (± 7.8) 4.5 (± 0.8) 11.7 (± 2.5)

2.1.2 Solid Gold Nanosphere Synthesis

Solid gold nanospheres were synthesized by following one of two published protocols: the citrate

reduction of gold method, reported by Ghosh et al.,74

and the seed-mediated growth approach

reported by Jana et al.75

The resulting sizes of SGNs synthesized are listed in Table 2.3. In the

citrate method, 0.25 mM HAuCl4 is heated to a rolling boil, and 0.5 mL of 1% trisodium citrate

solution is added. SGN-1, 2, 3, 5, and 6 were synthesized using the citrate reduction method. In

the seed-mediated growth approach, the seed solution was first prepared by adding 0.6 mL of

ice-cold, freshly prepared 0.1M NaBH4 to 20 mL aqueous solution of 2.5 x 10-4

M HAuCl4 and

2.5 x 10-4

M trisodium citrate. The growth solution was prepared by adding 6 g of solid

cetyltrimethylammonium bromide to 200 mL aqueous solution of 2.5 x 10-4

M HAuCl4, and

heated until it turned a clear orange color. Next, 9 mL of growth solution, 0.05 mL of 0.1 M

ascorbic acid solution, and 1.0 mL of the seed solution were mixed, resulting in nanospheres

having an average outer diameter of about 8.0 nm. To form larger particles, these 8.0 nm

13

particles were used as the seed solution for the next preparation. Again, 9 mL of the growth

solution was mixed with 0.05 mL of 0.1 M ascorbic acid, followed by 1.0 mL of the 8.0 nm

SGNs. SGN-4 and SGN-7 were synthesized by the seed-mediated growth method.

Table 2.3: Size distributions for SGN samples.


(nm) SGN-1 18.3 (± 2.0)

SGN-2 19.8 (± 3.7)

SGN-3 25.4 (± 4.2)

SGN-4 37.7 (± 3.3)

SGN-5 38.4 (± 4.2)

SGN-6 59.8 (± 7.8)

SGN-7 83.3 (± 7.5)

2.1.3 Characterization Tools

The resulting HGNs and SGNs were characterized by UV-Vis absorption spectrophotometry

(Perkin Elmer Lambda 950) and transmission electron microscopy (FEI CM-120 TEM, 120-kV

acceleration voltage).

Figure 2.1: UV-Visible extinction spectra for various HGN samples. The SPR spectral position

shifted to longer wavelengths as the aspect ratio increased.

14

The normalized extinction spectra for several of the HGN samples used in this work are given in

Figure 2.1. Samples were applied to a formvar-coated copper grid and air-dried for 24 hr prior to

TEM image acquisition. TEM images were analyzed using ImageJ software to determine the

HGN size distributions. Particle dimensions (outer diameter, shell thickness, and aspect ratio)

and size distributions were confirmed by analyzing TEM images of more than 100 particles per

sample. Representative images for HGN-5 and HGN-15 (Figure 2.2a and b) show HGNs

containing only a thin gold shell; by contrast, solution-phase HGNs have a fluid dielectric core.

Figure 2.2: TEM images of (a) low aspect ratio HGNs (HGN-2), and (b) high aspect ratio HGNs

(HGN-5). The scale bar applies to both images.

2.1.4 Aggregation Techniques

HGN aggregates were formed by drop-wise addition of concentrated KCl to the colloidal HGN

solution during stirring.10

Absorption measurements were carried out at regular intervals of KCl

addition to confirm the absorption blue shift for HGN aggregates. It is important to note that

continued addition of KCl solution will eventually lead to precipitation. In addition, HGN

aggregates were formed using two different thiols: ethanedithiol and cysteine.12

For

(a) (b)

15

ethanedithiol-induced aggregation, 2 μL of a 5 mM ethanedithiol solution in ethanol were added

to 1 mL of an HGN solution. For cysteine-induced aggregates, 10 μL of a 1 M aqueous solution

of cysteine were added to 1 mL of an HGN solution. Then, the HGN aggregate solutions were

agitated by shaking and allowed to sit at room temperature for one hour before their use.

2.2 Femtosecond Transient Extinction Spectroscopy

Femtosecond pump-probe transient extinction experiments were performed on a 1-kHz

regeneratively amplified Ti:Sapphire laser system that delivered 800-μJ pulse energies centered

at 800 nm (Figure 2.3). The duration of the amplified pulse was typically ~90 fs, and the pulse

was characterized by frequency-resolved optical gating (FROG) pulse diagnostics. The amplified

laser output was frequency doubled to generate 400-nm light (200 μJ/pulse), which was

attenuated and used as the excitation pump pulse. Excitation pulse energies ranged from 50

nJ/pulse to 1.5 μJ/pulse. A small portion (4%) of the fundamental laser output was passed

through a sapphire plate to generate the continuum probe pulse that typically extended from 450

nm to 850 nm. The pump-probe time delay was controlled using a retroreflecting mirror mounted

on a motorized linear translation stage (Newport). Time-resolved differential transient extinction

spectra were collected with a commercial Helios™ transient extinction spectrometer (Ultrafast

Systems LLC). Pump-probe dynamics were monitored by temporally delaying the probe beam

with a linear translation stage capable of step sizes as small as 7 fs with a dynamic range

extending to 3.2 ns. Both pulses were spatially overlapped in the sample-laser interaction region.

Differential extinction of the probe was measured as a function of the time delay between the

pump and probe by mechanically chopping the pump pulse at 500 Hz. Here, the probe was

spectrally dispersed on a silicon diode array to generate a wavelength-resolved differential

extinction spectrum that spanned from 450 nm to 800 nm. Data were acquired for two seconds

16

at each pump-probe delay. The instrument response time (~150 fs) was determined from the non-

resonant response of the pump and probe pulses in water. The full dynamic range of the

measurements extended from 10 ps before to 3.2 ns after time zero. The transient data was fit

using an in-house program that uses an iterative least-squares approach.

Figure 2.3: Laser table layout for femtosecond transient extinction experiments.

2.2.1 Data Fitting Routine

Temporal integration of the SPR bleach measured in the transient extinction spectrum provided

electronic relaxation kinetic traces. The best fits to the incoherent processes (electron-phonon

coupling and phonon-phonon coupling) were obtained by fitting the data to bi-exponential

functions of the form:

(1)

Ael-ph and AET are amplitude coefficients that described the contributions from electron-phonon

relaxation and nanoparticles-to-surroundings energy transfer, respectively, and τel-ph and τET are

the half times for electron-phonon relaxation and nanoparticle-to-surroundings energy transfer

17

(or phonon-phonon coupling), respectively. The pump-probe delay time was given by t. The

transient data was fit using a program written in house, which relies on an iterative least-squares

approach.

18

CHAPTER THREE

STRUCTURE-DEPENDENT COHERENT ACOUSTIC

VIBRATIONS OF HOLLOW GOLD NANOSPHERES

Reproduced with permission from Dowgiallo, A.M., Schwartzberg, A.M. and Knappenberger,

K.L., Jr., Nano Letters, 11 (2011) 3258–3262. DOI: 10.1021/nl201557s.

Copyright 2011 American Chemical Society.

3.1 Introduction

This chapter describes the coherent vibrational response of hollow gold nanospheres following

electronic excitation using femtosecond time-resolved transient extinction. The results from

these experiments indicated that HGNs support an isotropic mode, resulting in periodic

modulation of the surface plasmon differential extinction. Two different categories of coherent

acoustic vibrations, which depend on particle dimensions, were observed for HGNs. Further, the

vibration launching mechanism was dependent upon the dimensions of the HGN. Coherent

vibrations in HGNs characterized by small outer radii (<10 nm) and low cavity-radius-to-outer-

shell radius (R1/R2) aspect ratios (<0.5) were excited by a direct mechanism, whereas the

vibrations observed for the larger particles (>25 nm outer radius) with higher aspect ratios (>0.5)

resulted from an indirect mechanism. Coupling of electrons with the radial expansion mode can

be either direct or indirect. When the pump excites the phonons in an indirect way, the electron

gas is heated by the pump and its relaxation through electron-phonon coupling dumps energy

into the lattice where the nanoparticles expand in response to this rapid heating. Impulsive lattice

heating occurs because the time scale for electron-phonon coupling is faster than the vibrational

http://pubs.acs.org/doi/full/10.1021/nl201557s?prevSearch=%255BContrib%253A%2Bdowgiallo%255D&searchHistoryKey=

19

period. However, impulsive lattice heating is not the complete picture because at high electron

temperatures, the electrons exert a significant force on the nuclei, known as the hot electron

pressure, and this triggers in-phase dilation of the nanoparticles around its equilibrium size. The

dynamics of these two processes are different and they can thus be separated by analyzing the

phase of the observed oscillations.56

These findings may be significant for developing a

predictive understanding of nanostructure optical and mechanical properties.

3.2 Materials and Methods

All HGNs featured here were synthesized using a sacrificial galvanic replacement method.10

The

synthetic protocol yielded HGNs having outer diameters ranging from 20 to 80 nm and shell

thicknesses of 3 to 11 nm (resulting ratios of cavity radius to outer-shell radius were 0.38 to 0.82;

specifically, these are HGN-2, 3, 4, 5, 6, 8, 10, 13, 15, 16, and 17 (Table 2.2)). Exact HGN

dimensions were determined from high-resolution TEM data. Detailed synthetic protocols,

particle size data and extinction spectra for each HGN system are provided in Chapter 2. For all

HGN systems, the interband transition of gold was excited using the 400 nm second harmonic of

an amplified Ti:sapphire laser, and the time-domain relaxation dynamics were recorded using a

white-light continuum probe that monitored the resultant SPR bleach.

3.2.1 Coherent data

To understand more fully the acoustic vibrations of HGNs, the intensity recurrences observed in

the transient bleach were analyzed quantitatively. First, the incoherent electron cooling processes

were fit to the two-component exponential decay function given in 2.2.1, accounting for the

electron-phonon and phonon-phonon coupling processes described previously. Next, the

residuals from the fit, which represented the coherent vibrations, were Fourier transformed to

yield frequency data. This data is also used for the phase analysis of the coherent vibrations. The

20

pronounced oscillatory component of the time-dependent extinction was modeled using the

phenomenological response function

(2)

where A is an amplitude coefficient, τ is the dephasing time and ϕ is the phase of the oscillation.

3.3 Results and Discussion

3.3.1 Transient Extinction of HGNs

These results represent the first study of electronic energy relaxation and coherent acoustic

vibrations of hollow gold nanospheres spanning a comprehensive range of particles sizes.

Conclusions are supported by quantitative analysis of the frequency, amplitude and phase

components of the coherent transient data. For all HGN systems, the interband transition of gold

was excited using the 400 nm second harmonic of an amplified Ti:sapphire laser, and the time-

domain relaxation dynamics were recorded using a white-light continuum probe that monitored

the resultant SPR bleach. Figure 3.1 portrays the SPR bleach wavelength for a 40 nm outer

radius (OR) HGN following 400 nm excitation. This two-dimensional (2D) map captures all of

the features of the transient data; the time-dependent change in SPR wavelength and spectral

width are both evident. The steady-state wavelength-resolved differential extinction spectrum

and transient bleach recovery kinetics for an HGN with an outer radius of 40 nm and an aspect

ratio of 0.75 is shown in Figure 3.2 a and b, respectively. The maximum of the SPR bleach was

located at 640 nm; the time-domain data shown in Figure 3.2b were obtained by monitoring the

bleach at either 585 nm(blue) or 695 nm(red). The kinetic traces exhibited regular periodic

intensity modulations that were exactly out of phase with each other.

21

Figure 3.1: Two-dimensional transient extinction image plot of the 40 nm OR, R1/R2 = 0.75

HGN following 400 nm excitation.

This oscillation stemmed from laser induced lattice expansion that caused a time-dependent

alteration of the SPR spectral position. This behavior was also observed in Figure 3.3 in which

the HGN SPR center frequency is plotted as a function of pump-probe delay time. In fact, the

wavelength modulation observed in Figure 3.3 was characterized by the same period (∼65 ps) as

the intensity recurrences seen in Figure 3.2b.

Figure 3.2: (a) Transient extinction spectrum acquired for the HGN with R1/R2 aspect ratio of

0.75. The spectrum was recorded at a pump-probe delay of 1.5 ps following excitation by 400-

nm light. (b) The oscillatory data obtained from temporal integration at the wavelengths denoted

by the vertical lines, 585 nm (blue) and 695 nm (red) from (a).

22

The time-dependent peak width of the SPR is also shown in Figure 3.3. These data also exhibited

time-dependent modulations but with an approximate π/2 phase shift with respect to the SPR

wavelength.

Figure 3.3: SPR peak position (black) and spectral width (red) of the R1/R2 = 0.75 HGN plotted

as a function of pump-probe time delay. The period of the SPR fluctuations is about 65 ps, which

is consistent with the oscillations observed in Figure 3.2b.

Previous work by Apkarian and co-workers indicates that SPR peak position depends on lattice

strain; further, peak width is sensitive to the strain rate and surrounding dielectric, which is

temperature dependent.63,65

Continuing research will be necessary to fully describe all of the

transient spectral features of HGNs.

In solid gold nanospheres, excitation of the isotropic breathing mode yields similar

oscillations with frequencies that are inversely proportional to nanosphere radius.31

By

comparison, the oscillation frequencies observed for HGNs with an OR of 40 nm (0.5 cm-1

,

Figure 3.2) were 3-fold lower than those predicted for similarly sized SGNs (1.7 cm-1

). These

findings agree well with previous work on core-shell and hollow nanospheres.23-24,76

23

3.3.2 Residuals and Fourier Transformations

Figure 3.4 shows the residuals from the bi-exponential decay fit to the raw data. The residuals for

the HGN sample having an aspect ratio of R1/R2 = 0.75 is shown in Panel (c) of Figure 3.4. In

panel (a), a high-frequency oscillation was clear. The data shown in Figure 3.4(b) included both

high- and low-frequency components, while only one low-frequency component was observed in

the time-domain data shown in Figure 3.4(c).

Figure 3.4: Coherent portion of the transient extinction signal for samples (a) HGN-3,

R1/R2=0.38, (b) HGN-2, R1/R2=0.46 and (c) HGN-13, R1/R2=0.75.

The residuals depicted in Figure 3.4 and for the remaining HGN samples were Fourier

transformed to yield frequency data. Figure 3.5 depicts frequencies for several particles. The

inner-to-outer radius aspect ratio ranged from 0.38 (Figure 3.5a) to 0.75 (Figure 3.5e).

24

Figure 3.5: Fourier transformation of transient extinction time-domain data for a series of

HGNs. The outer radii were (a) 10, (b) 15, (c) 25, (d) 28, and (e) 40 nm.

The Fourier transformation shown in Figure 3.5a was obtained from the vibrations of a 10 nm

OR sample (R1/R2 = 0.38, where R1 is the particle inner radius and R2 is the outer radius). This

particle exhibited a 3.2 cm-1

oscillation, which agreed well with predictions for a SGN with a 10

nm radius. Figure 3.5b corresponds to a 15 nm OR sample (R1/R2 = 0.46) and contains peaks at

two frequencies: 2.5 and 1.1 cm-1

. The value of the higher-frequency component, which

represented 75% of the amplitude, agreed well with SGN predictions. As noted in Figure 3.5c, a

bimodal Fourier transform was also observed for the 25 nm OR sample (R1/R2 = 0.60). Again,

the high-frequency component matched SGN expectations, but the amplitude partitioning had

shifted with the high-frequency component accounting for 60% of the total signal. When the

aspect ratio of the HGNs was increased to 0.67 and 0.75 (Figure 3.5 d and e, respectively), only a

25

single low-frequency component that did not match SGN values was observed. The data shown

in Figure 3.5 clearly indicated that the nature of the coherent oscillations observed for HGNs was

size dependent; high aspect ratio HGNs appeared to vibrate more slowly than low aspect ratio

HGNs. The two frequencies observed for HGNs with intermediate aspect ratios likely arose from

different expansion processes exhibited by the sample rather than from the excitation of multiple

modes within a single particle; the ensemble contained particles that exhibited either the low- or

high-frequency acoustic vibration, but not both.

3.3.3 Vibrational Frequencies

Vibrational frequencies for all HGNs examined here are shown in Figure 3.6 as a function of

particle dimensions, with the high-frequency component depicted in Figure 3.6a and the low-

frequency vibration in Figure 3.6b. For comparison, the frequencies we observed for SGNs are

also included in Figure 3.6a. These values agreed well with the previous works of Hartland and

illustrated the similarity between SGNs and the low aspect ratio HGNs.31

The vibrational

frequencies for these two classes of particles were inversely proportional to the outer radius, as

indicated by the linear fit to SGNs. As demonstrated in Figure 3.6b, the vibrational frequencies

of high aspect ratio HGNs were inversely proportional to the HGN outer radius; the frequencies

increased linearly as a function of 1/outer radius. This trend, which holds over a large range of

radii, was consistent with theoretical predictions.23

Polarization-dependent pump-probe

measurements on these samples revealed that all vibrational modes were isotropic. Low-

frequency modes that were observed previously for high aspect ratio samples have been

attributed to softening of the isotropic breathing mode due to efficient cooling and the inherent

polycrystallinity of the HGN lattice.24

The data in Figures 3.5 and 3.6 represent the first

experimental investigation of HGN vibrational modes over a comprehensive range of particle

26

dimensions, and the results clearly demonstrate the size-dependent nature of the vibrational

frequencies.

Figure 3.6: Summary of coherent acoustic vibration frequencies. (a) The high-frequency mode

for a series of low-aspect ratio HGNs is plotted as a function of the inverse particle outer radius

(open circles). Results from SGNs are included for comparison (filled circles). (b) The low-

frequency vibration measured for high-aspect ratio HGNs is plotted as a function of the inverse

particle outer radius.

3.3.4 Oscillation Phase

The pronounced oscillatory component of the time-dependent extinction was modeled using the

phenomenological response function given in 3.2.1. The coherent portion of the experimental

data was fit well by this equation. The data and fit shown in Figure 3.7 correspond to the 40 nm

(a)

(b)

27

OR (R1/R2 = 0.75) sample; the determined oscillation frequency was 0.5 cm-1

(Tosc = 65 ps),

which is in excellent agreement with the Fourier transform shown in Figure 3.5e.

Figure 3.7: Modulated portion of the HGN transient extinction signal for an HGN with R1/R2 =

0.75 (outer radius = 40 nm) along with the fit obtained using the phenomenological response

function given above.

The damping time of the oscillations was 95 ± 5 ps, and the phase was 1.4. By

comparison, modeling of the lowest aspect ratio HGN (R1/R2 = 0.38) resulted in an oscillation

frequency of 3.1 cm-1

(Tosc ≈ 10.7 ps, data not shown). Coherence damping times did not yield

any meaningful trend, possibly due to sample polydispersity. Comparison of the oscillation

phases observed for both the high and low aspect ratio HGNs provided insight into the

mechanisms responsible for launching the coherent vibrations in these nanostructures. The

apparent phase value of ∼0.35 determined for the high-frequency oscillations of the 10 nm OR,

0.38 aspect ratio HGNs indicated that the periodic signal was sinusoidal in nature. Previous

studies demonstrated that the oscillations of solid metal nanospheres (R < 10 nm), which are best

described by a sine function, could be attributed to a direct launching mechanism via

deformation by the incident laser pulse.59,77

In contrast, the ϕ = 1.4 value obtained for the high

28

aspect ratio HGNs indicated that modeling of the time-dependent response required a cosine

function. Like the large aspect ratio HGNs studied here, large-diameter solid metal nanospheres

also exhibit cosine-like behavior.56,59

This observed π phase shift suggested excitation of the

acoustic vibrations via an indirect mechanism involving efficient electron gas-to-metal-lattice

energy transfer.56,59,78

These results indicated that the larger aspect ratio HGNs exhibited more

efficient electron-phonon coupling than the low aspect ratio structures.

3.3.5 Electron-Phonon Coupling

The electron-phonon coupling strength is described by the electron-phonon coupling constant, G

(W m-3

K-1

). Previously, we reported a G value of 6.6 x 1016

W m-3

K-1

for an HGN similar in

size to the high aspect ratio samples considered here (R1/R2 ≈ 0.6).11

By comparison, the G

values of comparably sized SGNs are typically 2 x 1016

W m-3

K-1

. In the current study, G values

of ∼3 x 1016

and ∼7 x 1016

W m-3

K-1

were determined for HGNs with R1/R2 values of 0.38

(Figure 3.5a) and 0.75 (Figure 3.5e), respectively. Taken together, these findings indicated that

high aspect ratio samples exhibited larger electron phonon coupling strengths than similarly

sized SGNs and low aspect ratio HGNs. Therefore, we attribute the difference in the observed

excitation mechanism of the acoustic vibrations to more efficient electron-phonon coupling for

high aspect ratio HGNs. Electron-phonon coupling strength also influences the amplitude (A) of

the intensity-modulated signal since A is inversely proportional to the electron-phonon coupling

lifetime.56

Therefore, based on our current and previous data,11

HGNs with larger aspect ratios,

which allow for more efficient electron-phonon coupling, should be characterized by high

amplitude oscillations. Indeed, the amplitude of the oscillations observed for the low aspect ratio

sample (R1/R2 = 0.38) was only 65% of the amplitude observed for the R1/R2 = 0.75 sample.

Taken together, the oscillation phases, the degree of electron-phonon coupling, and the

29

amplitude comparison were all consistent with the conclusion that the different vibrational

excitation mechanisms observed for high and low aspect ratio nanospheres were, at least in part,

the result of electron-phonon coupling efficiency.

In addition to the size-dependent transition from direct to indirect vibrational excitation,

the HGNs examined here also displayed a reduction in oscillation frequency for particles with

large R1/R2 values. Previous experimental and theoretical data published by Guillion et al.

reported similar low-frequency isotropic vibrations for core-shell nanostructures.23

The authors

propose a theoretical model that predicts a linear decrease in the breathing mode frequency as a

function of increasing aspect ratio, and they attribute this behavior to lattice polycrystallinity.

Later, Newhouse et al. adopted the same model to account for low frequency oscillations of

R1/R2 ≈ 0.75 HGNs.24

This predictive model is in relative agreement with our experimental data

on high aspect ratio HGNs, (Figure 3.6b). However, the experimentally observed frequencies

reported here, as well as all previous reports of HGNs and core-shell structures, were still lower

than theory by a factor of at least two.

The HGNs studied here and in previous works contained polycrystalline shells. This

physical property likely contributed to a “softening” of the isotropic vibration. However, the

present data indicated that electron-phonon coupling strength was crucial for determining the

nature of the coherent oscillations. Moreover, the data clearly demonstrated that launching of

acoustic vibrations was structure specific and that femtosecond time-resolved pump surface

plasmon-probe spectroscopy was a sensitive tool for analyzing the structure-specific relaxation

mechanisms of electronically excited HGNs. Previous work on metal nanocubes also revealed

the presence of multiple vibrational frequencies for large structures.77

In that case, the low-

frequency vibration was attributed to inhomogeneous heating of the structure by the laser. This

30

effect should not be relevant here because all HGNs examined in this study contained shells that

were within the skin depth of the metal, leading to homogeneous sample heating.

3.4 Conclusions

In conclusion, size-dependent coherent acoustic vibrations in hollow gold nanospheres were

reported. Detailed frequency and phase analysis of the periodic transient SPR extinction revealed

two different categories for the excitation of acoustic vibrations in HGNs: (1) direct isotropic

expansion in small, low aspect ratio particles and (2) indirectly launched low-frequency modes in

isotropic oscillations in large, high aspect ratio particles. The indirect excitation mechanism

observed for larger particles was attributed to efficient heating of the lattice via electron gas-to-

lattice energy transfer exhibited by high aspect ratio samples. These results are significant for

providing a fundamental understanding of the interplay between nanomaterial structure and

mechanical properties and also for applications where a predictive understanding of interfacial

energy transfer is critical. Rapid cooling, which appears to be a feature of HGNs, may benefit

applications requiring efficient local heating (e.g., photothermal therapy).

31

CHAPTER FOUR

ULTRAFAST ELECTRON-PHONON COUPLING IN HOLLOW

GOLD NANOSPHERES

Phys. Chem. Chem. Phys., 2011, 13, 21585 – 21592. DOI:10.1039/C1CP22743B

Reproduced by permission of the PCCP Owner Societies.

4.1 Introduction

This chapter describes how electronic energy relaxation in HGNs was studied using femtosecond

time-resolved transient extinction spectroscopy. A range of HGNs having outer diameter-to-shell

thickness aspect ratios of 3.5 to 9.5 were synthesized by the galvanic replacement method. The

HGNs exhibited electron–phonon relaxation times that decreased from 1.18 ± 0.16 to 0.59 ± 0.08

ps as the aspect ratio increased over this range. The corresponding electron–phonon coupling

constants, G, ranged from (1.67 ± 0.22) to (3.33 ± 0.45) x 1016

W m-3

K-1

. Electron–phonon

coupling was also determined for SGNs with diameters spanning 20 nm to 83 nm; no size

dependence was observed for these structures. The HGNs with high aspect ratios exhibited larger

electron–phonon coupling constants than the SGNs, whose average G value was (1.9 ± 0.2) x

1016

W m-3

K-1

. By comparison, low-aspect ratio HGNs exhibited values comparable to SGNs.

The electron–phonon coupling of high-aspect ratio HGNs was also influenced by the

surrounding fluid dielectric; slightly smaller G values were obtained when methanol was the

solvent as opposed to water. This coupling enhancement observed for high-aspect ratio HGNs

was attributed to the large surface-to-volume ratio of these structures, which results in non-

negligible contributions from the environment.

http://dx.doi.org/10.1039/C1CP22743B

32

Metal nanostructures exhibit a range of unique optical and mechanical properties.

Continued advances in colloidal synthesis allow the morphology, size, and composition of the

nanoparticles to be tailored so that they possess the desired properties.1–5,10,11,16,28,79,80

In the case

of hollow gold nanospheres (HGNs), which consist of a metal shell and a fluid interior dielectric,

variation of the outer diameter-to-shell thickness aspect ratio provides tunability of the localized

surface plasmon resonance (SPR).10,11,16

In Chapter 3, the difference between the mechanical

properties of HGNs compared to those of SGNs was discussed; the coherent acoustic oscillations

of HGNs exhibit a longer period compared to SGNs.23–25

Examination of a comprehensive range

of particle sizes demonstrated that the isotropic coherent acoustic vibrations of HGNs are

structure dependent and depend on the electron-phonon coupling lifetime.25

In addition, apparent

electron–phonon coupling rates observed for isolated HGNs are faster than those seen for HGN

aggregates or SGNs.11

The latter property may significantly impact technologies depending on

metal-interface energy transfer (e.g. nanoelectronics, photothermal therapy, etc.). This issue is

addressed by systematically studying the size-dependent electron–phonon coupling of a series of

isolated HGNs and SGNs using femtosecond time resolved transient extinction spectroscopy.

Electron–phonon coupling can be studied quantitatively using the pump–probe technique.

Electronic excitation of metals by femtosecond laser pulses results in a non-equilibrium electron

gas with high electron temperatures. These hot electrons subsequently cool in three successive

steps: (1) electron–electron scattering, (2) equilibration with the phonon modes of the metal

lattice (electron–phonon coupling), and (3) lattice-to-solvent energy transfer (phonon-phonon

coupling).31

Time-resolved pump–probe techniques that monitor the surface plasmon resonance

in the ‘‘probe’’ step can report on the cooling rates of these three processes.31–34,67

In particular,

the use of the two-temperature model and measurements of the electron–phonon coupling rate

33

(step 2) as a function of excitation pulse energy allow for determination of the metal’s electron–

phonon coupling constant, G.68

As a result, femtosecond time-resolved transient extinction has

become a widely used experimental tool for quantifying electron–phonon coupling strength in

both bulk films and metal nanostructures.31–34,67,81–92

Electron–phonon coupling times and constants for metal nanoparticles exhibit both

particle size-independent83–89

as well as size-dependent90–92

values, depending on the identity of

the metal and the domain size being examined. Hodak et al. reported electron–phonon coupling

times of 630 ± 100 fs and 500 ± 200 fs for 10 ± 3 nm and 50 ± 10 nm diameter silver particles,

respectively.93

The authors attributed the apparent size independence of the electron–phonon

time to the fact that both the electronic heat capacity and the coupling constant, which determine

the lifetime, are directly proportional to particle volume.93

Link et al. made similar observations;

electron–phonon relaxation times of 1.6, 1.6, and 1.7 ps were determined for gold nanoparticles

with diameters of 9, 22, and 48 nm, respectively.87

These authors proposed that the dominant

electron scattering process occurred at twin boundaries or other particle defects revealed in the

HRTEM images, resulting in size-independent lifetimes.87

In contrast, Del Fatti et al. observed

size-dependent dynamics for silver nanoparticles embedded in a glass matrix; the lifetimes

determined for these structures ranged from ~0.5 ps for particles with a 3-nm radius to 0.9 ps for

particles with a 15-nm radius.34

They proposed that the combination of the spatial confinement

of the electron-lattice interactions in the smaller particles and the increased electron-surface

scattering created by the ultrafast heating of the electrons enhanced the electron–phonon

coupling in small metal nanoparticles.34

In examining Ag and Au nanoparticles of various sizes

that were either supported on a glass slide, suspended in aqueous solutions, or embedded in

polymer matrices, Arbouet et al. found that the electron– phonon time was independent of the

34

environment but dependent upon the particle size.90

In this study, increasing the Ag nanosphere

outer diameter from 3.2 to 30 nm led to a concomitant increase in electron–phonon times from

~0.5 to 0.9 ps; similar results were obtained for Au nanospheres (diameters: 2.2 to 20 nm;

electron–phonon coupling times: ~0.6 to 1.1 ps). These authors again attributed the reduction in

relaxation time for smaller particles to confinement effects that led to increased electron-lattice

interaction.90

In this chapter, electron–phonon relaxation lifetimes (and the corresponding electron–

phonon coupling constants) are presented for hollow gold nanospheres having outer diameter-to-

shell thickness ratios (aspect ratios) ranging from 3.5 to 9.5. The electron–phonon relaxation

times were obtained using pump–probe transient extinction spectroscopy. As the HGN aspect

ratios increased, the electron–phonon coupling times decreased, leading to larger electron–

phonon coupling constants. The size-dependence likely arose from non-negligible environmental

contributions to the cooling process that occurred as the HGN total surface-area-to volume ratio

increased. Descriptions of the dependence of the electronic energy relaxation on the particle

composition, size, shape, and surroundings are necessary to develop novel devices that utilize

their unique thermal and electrical transport properties.94–97


4.2.1 HGN and SGN Samples

The electron-phonon coupling properties of HGN samples 2, 5-8, and 11-15 were examined. In

order to determine the optical properties of these nanostructures, extinction spectra were

collected for each of the HGNs (Figure 4.1a). Each sample was monitored between 300 nm and

1000 nm, revealing a higher-energy (~400 nm), size-independent interband extinction (data not

shown) and a single lower-energy SPR mode. The λmax of the plasmon band for HGN-2, 5-8, and

35

11-15 ranged from 550 nm to 660 nm; as expected, particles with larger aspect ratios (outer

diameter/shell thickness) gave rise to SPR frequencies at longer wavelengths.

Figure 4.1: (a) UV-Visible extinction spectra for HGN samples studied here. The SPR spectral

position shifted to longer wavelengths as the aspect ratio increased from 3.5 (HGN-2) to 9.5

(HGN-15). TEM images of (b) low-aspect ratio HGNs (HGN-5) and (c) high-aspect ratio HGNs

(HGN-15). The scale bar applies to both images.

Particle dimensions (outer diameter, shell thickness, and aspect ratio) and size distributions were

confirmed by analyzing TEM images of more than 100 particles per sample (Table 2.2).

Representative images for HGN-5 and HGN-15 (Figure 4.1b and c) show HGNs containing only

a thin gold shell; by contrast, solution-phase HGNs have a fluid dielectric core. HGNs with ten

different outer diameter/shell thickness dimensions were examined: (HGN-2) 29.9 ± 6.2 nm/8.5

± 2.2 nm, (HGN-5) 51.1 ± 5.1 nm/ 10.0 ± 1.0 nm, (HGN-6) 31.2 ± 4.6 nm/6.3 ± 2.1 nm, (HGN-

36

7) 50.7 ± 8.9 nm/ 8.2 ± 2.2 nm, (HGN-8) 54.6 ± 12.5 nm/8.6 ± 2.9 nm, (HGN-11) 46.7 ± 8.5

nm/7.0 ± 2.1 nm, (HGN-12) 53.2 ± 7.2 nm/7.1 ± 1.7 nm, (HGN-13) 54.8 ± 12.2 nm/6.9 ± 1.6

nm, (HGN-14) 52.2 ± 8.0 nm/5.9 ± 1.0 nm, and (HGN-15) 53.3 ± 10.5 nm/5.7 ± 1.0 nm; their

aspect ratios varied from 3.5 ± 0.6 to 9.5 ± 2.1. Complete particle sizing statistics for the HGN

samples are provided in Appendix A.

Figure 4.2: UV-visible extinction for solid gold nanoparticle samples SGN-2, SGN-4, and SGN-

7.

Figure 4.3: TEM images of the solid gold nanoparticle samples used in this study: (a) SGN-2,

(b) SGN-4, and (c) SGN-7, having diameters of approximately 20, 40, and 80 nm, respectively.

(c)

(a) (b)

37

For comparison purposes, SGNs were also examined; these particles had diameters of 19.8 ± 3.7

nm, 37.7 ± 3.3 nm, and 83.3 ± 7.5 nm. Extinction spectra and representative TEM images for

these SGN samples are shown in Figures 4.2 and 4.3, respectively, and complete particle sizing

statistics are provided in Appendix A.

4.2.2 Two-Temperature Model

The experimental kinetic data were fit to the bi-exponential decay function (equation 1) given in

Section 2.2.1. In order to examine more carefully the power dependence of the bleach recovery,

relaxation times for each sample were plotted as a function of laser pulse energy, and a linear fit

was applied. The zero-point (room temperature) electron–phonon coupling time was determined

using the two-temperature model99

and extrapolating the linear fit to zero laser pulse energy. In

this model, the electron gas and the lattice are treated as two coupled subsystems at different

temperatures because they have non-identical heat capacities. The electron–phonon coupling

time constant depends on particle temperature, which in turn depends upon the excitation laser

pulse energy, and the extent of electron–phonon coupling determines the rate of energy flow

from one subsystem to the other. The two-temperature model can be described using equations

(3)−(5):68

(3)

(4)

(5)

where Te and Tl are the electron and lattice temperatures, and Ce and Cl are the electron and

lattice heat capacities, respectively. The electron–phonon coupling constant, G, quantifies the

coupling of Te and Tl. The dependence of the electronic heat capacity, Ce, on the electronic

38

temperature, Te, gives rise to the temperature dependence of the electron relaxation times

(equation (5)) where γ = 66 J m-3

K-2

for Au.93

Hence, a series of experiments were performed at

different pump laser powers (and thus, different Te) to determine the slope and intercept of the

linear function describing the dependence of the relaxation times on the excitation power for

these gold nanoparticles. The electron–phonon cooling lifetimes (t0) acquired in this manner

were then used to calculate the electron–phonon coupling constant, G, using the relationship,

(6)

where T0 is the ambient temperature (298 K).31


4.3.1 Electron-Phonon Coupling of HGNs

Time-resolved transient extinction experiments were performed to determine electron-phonon

relaxation rates of electronically excited HGNs. A 400-nm laser pulse was used to excite both

HGNs and SGNs. Subsequently, a transient bleach was observed at the λmax of each sample’s

SPR band. Electron cooling kinetics were probed by monitoring the time dependence of the

recovery of the transient bleach. Figure 4.4 portrays the data collected for each sample; for

clarity, data for only one sample (HGN-15) are shown in the Figure. The transient differential

extinction signal for HGN-15 (Figure 4.4a) was centered at ~630 nm.

Kinetic data obtained after excitation of the sample with a range of laser pulse energies

are shown in Figure 4.4b. These time-dependent traces represent the recovery of the bleach and

were generated using the magnitude of the 630-nm signal as a function of time after sample

39

excitation. For this sample, the pump laser power was varied from 200 nJ to 800 nJ per pulse,

with higher laser powers leading to longer relaxation times.

Figure 4.4: (a) Differential extinction for sample J (diameter = 53 ± 11 nm, shell thickness = 5.7

± 1.0 nm, and aspect ratio = 9.5 ± 2.1) after excitation with 400-nm laser pulses of 90-fs duration

at zero time delay. The dashed line is located at zero differential amplitude as a guide for the eye.

(b) Bleach recovery kinetics observed at the maximum of the SPR band (630 nm for sample J)

for a series of different laser pulse intensities. The raw data (●) were fit to a bi-exponential decay

(—). Higher laser powers gave rise to longer lifetimes.

The linear relationship between the experimentally derived electron–phonon relaxation times and

the excitation pulse energy is shown in Figure 4.5 (for ease of viewing, only HGN-2, 8, and 12

are shown, but all ten HGNs exhibited linear trends; extrapolated zero-point values for other

samples are given in Table 4.1).

40

Figure 4.5: Relaxation times determined for the electron–phonon coupling step when different

laser pulse energies were used to excite the sample. The two-temperature model was used to

obtain the zero-point electron–phonon coupling time from the y-intercept of the linear fit for

HGN-2 (squares), HGN-8 (circles), and HGN-12 (triangles). Higher aspect ratio HGNs exhibited

more rapid electron–phonon relaxation.

Table 4.1: Aspect ratio dependence of the electron-phonon coupling in HGNs.

Sample Aspect Ratio 0 (ps) G [1016

W m-3

K-1

]

A 3.5 (± 0.6) 1.12 ( 0.08) 1.76 ( 0.13)

B 5.1 (± 0.6) 1.18 ( 0.16) 1.67 ( 0.22)

C 5.4 (± 1.5) 0.97 ( 0.08) 2.03 ( 0.17)

D 6.5 (± 1.3) 0.90 ( 0.08) 2.19 ( 0.19)

E 6.7 (± 1.8) 0.91 ( 0.08) 2.16 ( 0.19)

F 7.2 (± 2.1) 0.80 ( 0.08) 2.45 ( 0.24)

G 7.8 (± 1.6) 0.70 ( 0.10) 2.82 ( 0.41)

H 8.3 (± 2.3) 0.65 ( 0.23) 3.01 ( 1.05)

I 9.0 (± 1.6) 0.65 ( 0.08) 3.04 ( 0.37)

J 9.5 (± 2.1) 0.59 ( 0.08) 3.33 ( 0.45)

Low fluence laser pulses ranging from 100 nJ to 1.5 mJ were used to excite the samples; the

apparent relaxation time increased with the power of the excitation pulse. The slope of the

dependence of the recovery time upon the pulse power, as well as the extrapolated zero-point

value, was dependent upon the identity of the sample.

As mentioned above, these samples were HGNs whose diameters and shell thicknesses

had been tailored to achieve specific aspect ratios. HGN-2 (aspect ratio: 3.5 ± 0.6) displayed an

41

apparent electron–phonon relaxation time of 1.12 ± 0.08 ps, similar to that observed for a solid

gold nanoparticle. When the HGN aspect ratio was increased to 6.7 ± 1.8 (HGN-8), the electron–

phonon relaxation occurred more quickly (0.91 ± 0.08 ps). The observed relaxation was even

more rapid (0.70 ± 0.10 ps) for HGN-12, which contained HGNs with an aspect ratio of 7.8 ±

1.6. The larger error bars in HGN-12 likely arose from the smaller absorption cross section of

thinner-shelled HGNs, which decreases the optical density, making low power measurements

more difficult to acquire (shell thicknesses: 2—8.5 nm, 8—8.6 nm, 12—7.1 nm). The electron–

phonon coupling times as well as the corresponding coupling constants (G) for all samples are

presented in Table 4.1.

4.3.2 Size Dependence

Coupling times ranged from 0.59 ± 0.08 ps (HGN-15) to 1.18 ± 0.16 ps (HGN-5); G values

spanned (1.67 ± 0.22) x 1016

W m-3

K-1

for HGN-5 to (3.33 ± 0.45) x 1016

W m-3

K-1

for HGN-

15. Figure 4.6a portrays the zero-point electron–phonon coupling times as a function of the

particle aspect ratio; increases in the HGN aspect ratio led to decreases in the electron–phonon

coupling times. Figure 4.6b shows the linear relationship between G and the particle aspect ratio,

suggesting a size-dependence for the HGN electron–phonon coupling. However, the G values

obtained for samples HGN-2 and HGN-5 (aspect ratios 3.5 and 5.1, respectively) were within

error of each other. Further, their electron–phonon cooling times were similar to those observed

for SGNs, suggesting that size dependence becomes more pronounced for higher-aspect ratio

hollow particles.

To determine whether a similar size dependence could be observed in the relaxation

kinetics of solid particles, SGNs with diameters of 20 nm (HGN-2), 38 nm (HGN-4), and 83 nm

(HGN-7) were examined. The apparent electron–phonon coupling times were 1.00 ± 0.10, 1.20 ±

42

0.08, and 1.00 ± 0.08 ps, respectively. Calculation of G values using equation (6) yielded

electron–phonon coupling constants of (1.97 ± 0.07) x 1016

W m-3

K-1

for SGN-2, (1.64 ± 0.08) x

1016

W m-3

K-1

for SGN-4, and (1.95 ± 0.07) x 1016

W m-3

K-1

for SGN-7 (Table 4.2).

Figure 4.6: (a) The room-temperature zero-point electron–phonon coupling times for HGNs

(open circles) and SGNs (closed circles) as a function of aspect ratio and inverse radius,

respectively. (b) The corresponding electron–phonon coupling constants (G) for HGNs and

SGNs, also as a function of aspect ratio and inverse radius, respectively. The coupling constant

was calculated from the room-temperature zero-point electron–phonon coupling time t0 using t0

= γT0/G (g = 66 J m-3

K-2

for gold and T0=298 K). In both figures, the dotted line is located at the

average value for t0 and G of HGN-2, HGN-4, and HGN-6 and SGN-2, SGN-4, and SGN-7, at

1.08 ps and 1.84 x 1016

W m-3

K-1

, respectively. In addition, a linear fit was applied to the t0 and

G values for HGN-7, -8, and 11-15.

Table 4.2: Electron-phonon coupling data for solid gold nanoparticles.

Sample Diameter

(nm) 0 (ps) G [10

16 W m

-3

K-1

] 2 19.8 3.7 1.00 0.08 1.97 0.16

4 37.7 3.3 1.19 0.08 1.64 0.11

7 83.3 7.5 1.01 0.08 1.95 0.15

This observed size independence was consistent with the independent works of Hartland31–

33,83,84,88,89,93 and El-Sayed.

67,81,87 Figure 4.6b contains a comparison of the HGN and SGN

43

relationships between the apparent electron–phonon coupling constants and the dimensions of

the particle. Aspect ratios were used for HGNs, and 1/R (where R is the particle radius) were

used for SGNs. As noted above, low-aspect ratio HGNs yielded electron–phonon coupling

constants that were similar to those noted for solid gold nanoparticles. As the aspect ratio of the

HGNs increased, the larger size and thinner shells of these particles allowed for more extensive

electron–phonon coupling than that seen for SGNs or smaller, thicker-shelled low-aspect ratio

HGNs. The electron–phonon coupling enhancement observed for high-aspect ratio HGNs was

consistent with previous findings.81–85,91–93,100,101

Initially, we studied the electron–phonon coupling properties of HGNs and their

aggregates. We found that HGNs exhibited stronger electron–phonon coupling compared to

SGNs, and when aggregated showed electron–phonon coupling that begins to approach bulk

values.11,102,103

We attributed the enhanced electron–phonon coupling strength in HGNs to spatial

confinement of the electrons. The decreased electron– phonon coupling strength for the

aggregates was the result of electron delocalization over multiple particles. As described above,

Del Fatti et al. proposed that size-dependent electron–phonon coupling in small Ag nanoparticles

arose from increased electron surface scattering and spatial confinement of the electron-lattice

interactions in smaller particles.34

Arbouet et al. also attributed the stronger coupling in smaller

Au clusters to confinement effects leading to electron spill-out.90

These effects may become

more significant for hollow particles with thin gold shells (particles examined here had shell

thicknesses between 5.7 and 8.5 nm) than they are for solid gold nanoparticles. The apparent size

dependence of the electron–phonon coupling times was also consistent with a model developed

by Scherer et al.104,105

These authors reported hot-electron lifetimes that increased from 1 to 3 ps

when 12-nm diameter colloidal Au nanoparticles were arranged in thin films with depths varying

44

from 47.1 to 5.8 nm.104,105

Increased colloidal aggregation (domain size) in thicker films led to

greater nanoparticle coupling and reduced electron– phonon relaxation times that began to

approach bulk values. Their data were analyzed by considering two competing size-dependent

effects: inelastic surface scattering (ISS) and electron oscillation phonon resonance detuning

(EOPRD). ISS results in increased electron–phonon coupling for smaller particles; in contrast,

EOPRD decreases coupling in small particles. EOPRD determines the electron–electron

scattering lifetimes when the electron oscillation frequency is greater than the Debye

frequency.106

Efficient electron-to-lattice energy transfer requires resonance between the electron

oscillation frequency (resulting from reflection of the electron wave by the particle or domain

boundary) and lattice vibrational modes. These frequencies are further apart in small domains or

particles, leading to inefficient electron–phonon coupling. As a result, inelastic electron

scattering from the domain boundaries becomes the dominant contribution to electron–phonon

coupling in small domains. This scattering, or ISS, increases electron–phonon coupling in

smaller domains or particles because they possess larger surface areas or domain boundaries per

unit volume for inelastic scattering. Scherer et al.’s data were consistent with this size

dependence of the ISS and EOPRD phenomena.104,105

The inner and outer surfaces created by the

hollow structure of HGNs results in increased surface area relative to that of an SGN of identical

diameter, which possesses only an outer surface. A solid gold nanosphere with a 30-nm outer

diameter has a surface area of 2.83 x 103 nm

2. By comparison, the HGNs in this study that have

an outer diameter of ~30 nm have total surface areas of 3.34 x 103 nm

2 HGN-2 and 4.13 x 10

3

nm2 HGN-6. Similarly, an SGN with an outer diameter of 50 nm has a surface area of 7.85 x 10

3

nm2, whereas HGNs with ~50 nm outer diameters have total surface areas of 1.37, 1.47, and 1.45

x 104 nm

2 (HGN-12,13, and 15, respectively). In these higher-aspect ratio HGNs, the surface

45

area was almost twice that of an SGN having a similar outer diameter. The inner surface may

serve as an additional electron scattering site, leading to enhanced electron–phonon coupling and

more rapid relaxation in high aspect ratio HGNs compared to solid gold nanoparticles.

To discriminate between the two size-dependent models described above, we analyzed

the value of G as a function of both surface area (ISS model) and volume (confinement). The

electron–phonon coupling constants were plotted as a function of total surface area (Figure 4.7a)

and total volume (Figure 4.7b) for all HGN samples. No clear trend was seen in either case.

However, when the electron–phonon coupling constants were plotted as a function of the ratio of

these two quantities (surface-to-volume ratio), a linear dependence was observed (Figure 4.8);

the electron–phonon coupling constant increased linearly as the surface-to-volume ratio

increased.

Figure 4.7: Electron–phonon coupling constants, G, as a function of the total surface area (a)

and total volume (b) of HGNs (samples 2, 5-8, and 11-15).

This same trend has been reported for the matrix-assisted phonon-phonon cooling process of

dielectric core-shell nanospheres.23

Electron–phonon coupling in thin films is also sensitive to

electron-boundary conditions and can be influenced by the thermal conductivity of the film’s

46

substrate.103

Taking into consideration the previous results on core-shell particles and films, our

current data suggested that the increased surface-to-volume ratio of HGNs resulted in sensitivity

to the thermal conductivity of the embedding media (here, the solvent used to disperse the

HGNs).

Figure 4.8: Electron–phonon coupling constants, G, as a function of the HGN surface to volume

ratio (total surface area/total volume). The solid line is a linear fit to the data.

4.3.3 Solvent Dependence

To determine whether environmental parameters influenced the observed electron–phonon

coupling, high-aspect ratio HGNs (HGN-12 and HGN-14) were subjected to centrifugation and

redispersed in methanol. The time-resolved experiments were then repeated on these samples.

The thermal conductivity of methanol is one third that of water107,108

and, therefore, an increase

in the electron–phonon cooling time (decrease in G) was expected. The experimentally measured

electron relaxation dynamics changed as expected (the results for HGN-12 are in Figure 4.9 and

those for HGN-14 are in Figure 4.10); a slightly longer electron–phonon relaxation time was

observed when methanol was used as the dispersion solvent (HGN-12 in MeOH: 800 ± 80 fs,

H2O: 690 ± 100 fs; HGN-14 in MeOH: 990 ± 80 fs, H2O: 650 ± 80 fs).

47

Figure 4.9: Electron-phonon coupling times as a function of the laser pulse energy for HGN-12

in water (circles) and methanol (triangles). The room-temperature zero-point electron–phonon

coupling time (or the y-intercept) was 690 and 770 fs in water and methanol, respectively.

Figure 4.10: Electron–phonon relaxation times as a function of the laser pulse energy for HGN-

14 dispersed in water (solid circles) and in methanol (solid triangles). The room-temperature

zero-point electron–phonon coupling time (or the y-intercept) was 650 and 990 fs in water and

methanol, respectively.

These data suggested that the enhanced electronic energy relaxation of high-aspect ratio

HGNs resulted from increased contributions from the surroundings, a conclusion that is

consistent with a solvent dependence of the electron–phonon coupling.103

This finding highlights

48

the importance of environmental conditions in determining the properties of plasmon-tunable

nanostructures. This effect is not observed for solution-phase solid gold nanospheres of

comparable diameter.90

We do note that El-Sayed and co-workers reported environmental-

dependent electronic relaxation of SGNs dispersed in both liquid and solid media.109

These

previous results are consistent with our current solvent dependent studies.

4.4 Conclusions

Femtosecond time- and wavelength-resolved transient extinction pump–probe spectroscopy was

used to investigate electron– phonon coupling in hollow gold nanospheres with aspect ratios

(outer diameter/shell thickness) ranging from 3.5 to 9.5. Extrapolated room-temperature

electron–phonon relaxation times ranged from 590 ± 80 fs to 1180 ± 160 fs. Calculation of

electron– phonon coupling constants yielded G values of (1.67 ± 0.22) x 1016

W m-3

K-1

to (3.33

± 0.45) x 1016

W m-3

K-1

, demonstrating concomitant increase in aspect ratio and G. By

comparison, solid gold nanospheres with outer diameters ranging from 20 to 83 nm displayed

size-independent electron–phonon coupling. The aspect ratio-dependence of the electron–phonon

coupling observed for hollow particles was attributed to the large surface-to-volume ratio of

these nanostructures and their sensitivity to the local environment. This was also consistent with

experiments carried out in solvents with different thermal conductivities (water and methanol).

In these experiments, a slightly larger electron–phonon coupling constant was observed when the

HGNs were dispersed in water, which has a larger thermal conductivity than methanol. The

current data demonstrated that HGN particle dimensions and environments can be tuned to yield

desired electron–phonon coupling properties. Detailed structure-specific descriptions of these

nanostructure properties will be necessary for achieving predictive functionalities.

49

CHAPTER FIVE

INFLUENCE OF CONFINED FLUIDS ON NANOPARTICLE-

TO-SURROUNDINGS ENERGY TRANSFER

Reproduced with permission from A.M. Dowgiallo and K.L. Knappenberger, Jr., J. Am. Chem.

Soc., 2012, 134, 19393-19400. DOI: 10.1021/ja306644p.


5.1 Introduction

The influence of confined fluids on the rate of nanoparticles-to-surroundings energy transfer will

be described in this chapter for both hollow and solid gold nanospheres. The HGNs exhibited

energy transfer half times that ranged from 105 ± 10 ps to 1010 ± 80 ps as the total particle

surface area increased from 1,005 to 28,115 nm2. These data showed behaviors that were

categorized into two classes: energy transfer from HGNs to interior fluids that were confined to

cavities with radii <15 nm and ≥15 nm. Energy transfer times were also determined for solid

gold nanospheres having radii spanning 9−30 nm, with a similar size dependence where the

relaxation times increased from 140 ± 10 to 310 ± 15 ps with increasing nanoparticle size.

Analysis of the size-dependent energy transfer half times revealed that the distinct relaxation rate

constants observed for particle-to-surroundings energy transfer for HGNs with small cavities

were the result of reduced thermal conductivity of confined fluids. These data indicate that the

thermal conductivity of HGN cavity-confined fluids is approximately one-half as great as it is for

bulk liquid water. For all HGNs and SGNs studied, energy dissipation through the solvent and

transfer across the particle/surroundings interface both contributed to the energy relaxation

http://pubs.acs.org/doi/full/10.1021/ja306644p?prevSearch=%255BContrib%253A%2Bdowgiallo%255D&searchHistoryKey=

50

process. The current data illustrated the potential of fluid-filled hollow nanostructures to gain

insight into the properties of confined fluids.

Light-driven activation of metal nanostructures results in the formation of a non-

equilibrium electron gas, which relaxes by three successive steps: (i) electron−electron

scattering, (ii) electron−phonon coupling, and (iii) energy transfer to surroundings.32

Ultrafast

(∼100 fs) electron−electron (e−e) scattering forms a hot electron distribution that subsequently

equilibrates with the metal lattice on a ∼1-ps time scale via electron−phonon (e−ph) coupling.

The final step in this electronic energy relaxation sequence is energy transfer from the hot

electron and phonon subsystems to the environment. This final particle-to-surroundings energy

transfer process plays a critical role in determining the efficiency of many applications that

feature metal nanostructures as functional hosts including micro/nanoelectronics,26

material

processing,27

photodynamic therapy,28

and electromagnetic energy transport through patterned

nanoparticle networks.29

The repertoire of nanostructure synthesis and fabrication techniques

currently available allows for the production of particles over a vast range of sizes and

morphologies, which can be exploited to tune particle-to-environment energy transfer rates.1-5,30

Structure-dependent energy transfer rates can be quantified using femtosecond time resolved

transient extinction spectroscopy, which is a reliable experimental diagnostic for studying the

rapid electronic energy relaxation mechanisms of metal nanostructures.31−34

The high surface areas of HGNs may provide a useful route for tailoring particle-to-

environment relaxation rates of electronically excited gold nanostructures. However, the

properties of the fluids confined to the nanoscale dimensions of the HGN interior cavity, and

their influence on HGN optical, mechanical, and electronic relaxation properties, remain unclear.

For example, cavity plasmon resonances appear to contribute significantly to interparticle modes

51

that are formed when neighboring particles undergo near-field coupling.11−13

HGNs also exhibit

size dependent electron−phonon equilibration rates; the electron−phonon coupling constant

increases linearly with increasing particle surface-to-volume ratio.110

This phenomenon is not

observed for similarly sized solid gold nanospheres (SGNs). By comparison, electron−phonon

coupling sensitivity to the surface-to-volume ratio does not occur for low-aspect-ratio HGNs,

which exhibit electron−phonon coupling values comparable to SGNs. Aggregation of HGNs by

surface necking results in decreased electron−phonon coupling rates owing to the formation of a

continuous nanoparticle network that has a decreased effective surface-to-volume ratio.11

In

addition, HGNs exhibit oscillations at frequencies lower than those observed for SGNs.25

Possible contributing factors include the increased lattice polycrystallinity of HGNs compared to

SGNs as well as structure-dependent energy dissipation for HGNs, which may be modified by

the fluid-filled cavity.

Here, particle-to-surroundings energy transfer half times are reported for a series of

HGNs having outer diameter-to-shell thickness aspect ratios ranging from 3 to 9 and total surface

areas ranging from 1.0 × 103 to 2.8 × 10

4 nm

2. The apparent energy transfer half times were

obtained using femtosecond time-resolved pump−probe transient extinction spectroscopy. As the

HGN surface area increased, the energy transfer half times also increased, but the data showed a

discontinuity at a particle cavity radius of 15 nm. Analysis of HGN interfacial energy transfer

indicated small HGNs (cavity radius <15 nm) had interfacial thermal conductivities that were

∼1.9−2.4 times less than those of SGNs and larger HGNs. This effect was attributed to the

difference between the thermal conductivity of water confined to small HGN cavities and that for

bulk water. The apparent energy transfer half times were also sensitive to the surrounding

52

environment, becoming larger when the HGNs were dispersed in methanol, which has a lower

thermal conductivity than water.


Hollow gold nanospheres were synthesized following the method described in Section 2.1.1. In

order to characterize the HGN optical properties and structure, extinction spectra (Figure 5.1)

and TEM data (Figure 5.2) were collected.

Figure 5.1: Normalized extinction spectra for select HGN samples used in this chapter. The SPR

maximum wavelength ranges from 550 to 710 nm with increasing outer-diameter-to-shell-

thickness aspect ratio.

On the basis of the statistics obtained from TEM images of at least 200 particles for each HGN

sample, the syntheses yielded particles with nine different outer diameter/shell thickness

dimensions: (HGN-1) 16.6 ± 2.9 nm/5.0 ± 1.2 nm, (HGN-2) 29.9 ± 6.2 nm/8.5 ± 2.2 nm, (HGN-

4) 27.9 ± 3.2 nm/6.3 ± 1.3 nm, (HGN-5) 51.1 ± 5.1 nm/10.0 ± 1.0 nm, (HGN-6) 31.2 ± 4.6

nm/6.3 ± 2.1 nm, (HGN-9) 48.0 ± 5.0 nm/7.0 ± 1.0 nm, (HGN-10) 77.9 ± 5.5 nm/11.3 ± 2.2 nm,

(HGN-12) 53.2 ± 7.2 nm/7.1 ± 1.6 nm, and (HGN-14) 52.2 ± 8.0 nm/5.9 ± 1.0 nm. Their outer-

53

diameter-to-shell-thickness aspect ratios were the following: 3.4 ± 0.6, 3.7 ± 0.6, 4.5 ± 0.7, 5.1 ±

0.6, 5.4 ± 1.5, 6.9 ± 1.7, 6.9 ± 1.2, 7.8 ± 1.6, and 9.0 ± 1.6, respectively. In addition, SGNs were

examined that had outer diameters of (SGN-1) 18.3 ± 2.0 nm, (SGN-3) 25.4 ± 4.2 nm, (SGN-5)

38.4 ± 4.2 nm, and (SGN-6) 59.8 ± 7.8 nm. The maximum wavelengths of the SPR responses for

the HGNs ranged from 550 to 710 nm, with longer SPR wavelengths being observed for HGNs

with larger outer-diameter-to-shell-thickness aspect ratios.

Figure 5.2: Representative TEM images of sample HGN-5 (a) and corresponding EDS data (b).

The scale bar in part a is 20 nm. The images and EDS data indicated that the structures were

composed of a gold shell and a hollow cavity. Cu peaks in panel b arose from the sample grid

and were not indicative of sample contamination.

54

Representative transmission electron micrographs of HGN-5 are given in Figure 5.2. TEM

images, along with energy dispersive analysis (Figure 5.2b), indicated that the HGNs consisted

of a thin gold shell and a hollow cavity. Taken together, the optical and TEM data provided

evidence that the solution-phase samples used for transient extinction spectroscopy

measurements were gold shells with fluid-filled cavities; cavity radii ranged from 3.3 to 27.5 nm.

SGNs were prepared by citrate reduction of gold, following the method reported by Ghosh et

al.74

and described in Section 2.1.2. For solvent-dependent studies, both HGNs and SGNs were

transferred from water to methanol solutions using the method reported in Section 2.1.1.

Femtosecond pump−probe transient extinction experiments were performed on a 1-kHz

regeneratively amplified Ti:Sapphire laser system that delivered 800-μJ pulse energies centered

at 800 nm as described in Section 2.2. Temporal integration of the SPR bleach measured in the

transient extinction spectrum provided electronic relaxation kinetic traces. The transient data was

fit with an in-house program that uses an iterative least-squares approach.112,113

The best fits to

the data were obtained using Equation 1, which accounted for both electron−phonon and

phonon−phonon relaxation rates, τel‑ph and τET.


5.3.1 Phonon-Phonon Coupling in HGNs

After the initial structural and optical characterization, time resolved transient extinction

experiments were performed to examine the relaxation dynamics of electronically excited HGNs

and SGNs. Both HGNs and SGNs were excited using a 400-nm laser pulse, and the relaxation

dynamics of the electron and phonon systems were subsequently probed using a continuum laser

pulse. The transient extinction spectrum obtained from one sample (HGN-5) is shown in Figure

55

5.3a; the spectrum was recorded at a pump−probe delay of 5 ps, following excitation by a 500-

nJ, 400-nm pump pulse.

Figure 5.3: (a) Spectrally resolved transient extinction spectra of HGN-5. The data were

recorded at a pump−probe time delay of 5 ps following excitation by a 400-nm laser pulse (500

nJ/pulse). (b) Temporally resolved extinction data obtained by monitoring the spectrum shown in

panel a at a probe wavelength of 610 nm (center wavelength of bleach). The experimental data

are plotted along with the best fit to the data, obtained using equation 1. The dashed vertical line

in panel b provides a guide to the point at which the data reflect nanoparticle-to-surroundings

energy transfer kinetics.

A broad transient bleach centered at 610 nm was observed, which was consistent with the λmax of

the sample’s SPR band. Coincidence of the center wavelength of the transient bleach and the

SPR maximum obtained from linear extinction measurements was observed for all samples

studied. The kinetics of electronic relaxation were determined by measuring the transient bleach

56

recovery in the time domain. The cooling of sample HGN-5 is shown in Figure 5.3b. These time-

resolved transient extinction traces depict the magnitude of the 610-nm signal as a function of

the pump−probe time delay; data for all samples correspond to the center wavelength of the

transient bleach in the time domain.

Each HGN and SGN sample examined here yielded time-resolved transient data that

exhibited two distinct components: (1) an initial, fast decay that was completed within ∼1 ps and

(2) a slower decay that persisted for hundreds of picoseconds. The fast component 1 of this HGN

relaxation process has been discussed previously.110

These two distinct relaxation processes are

also observed for large SGNs (>15-nm diameter).114,115

Hartland and co-workers attribute the

first component to coupling between the photoinduced hot electron system and lattice phonons of

the particle.114,115

They assign the second component to energy transfer as heat from the particle

to the surroundings. The observation of a distinct transition from the fast to the slow component

was important, because it indicated that the hot electrons equilibrated with the particle’s phonon

bath prior to energy transfer to the surroundings. Energy transfer did not compete with

electron−phonon coupling for any of the HGNs studied here. Although competitive energy

transfer and electron-phonon coupling was observed for some smaller HGNs, those samples

were not included in the current analysis. The experimental data shown in Figure 5.3b are plotted

along with the fit results obtained using equation 1, which allowed for quantitative analysis of

the structure dependent energy dissipation half times. The dimensions of each HGN sample and

their respective energy transfer half times (τET) are summarized in Table 5.1. Similar information

is provided for the SGN samples in Table 5.2. Time-resolved transient extinction measurements

were carried out in triplicate at several excitation pulse energies. The energy transfer half times

were independent of laser power (unlike the electron−phonon coupling times); hence, the

57

relaxation times obtained for different powers were averaged to determine the energy transfer

half times for each sample.

Table 5.1: Energy transfer half times and structural parameters for citrate-stabilized HGNs.

Sample Outer diameter, shell

thickness (nm)

Surface Area

(nm2)

S:V

(nm-1

)

τET (ps)

HGN-1 16.6 ± 2.9, 5.0 ± 1.2 (1.00 ± 0.65) x 103 0.45 105 ± 10

HGN-2 29.9 ± 6.2, 8.5 ± 2.2 (3.34 ± 2.62) x 103 0.26 165 ± 30

HGN-4 27.9 ± 3.2, 6.3 ± 1.3 (3.17 ± 1.24) x 103 0.33 155 ± 40

HGN-5 51.1 ± 5.1, 10.0 ± 1.0 (1.13 ± 0.45) x 104 0.21 245 ± 25

HGN-6 31.2 ± 4.6, 6.3 ± 2.1 (4.13 ± 1.90) x 103 0.33 195 ± 40

HGN-9 48.0 ± 5.0 , 7.0 ± 1.0 (1.09 ± 0.43) x 104 0.29 160 ± 15

HGN-10 77.9 ± 5.5, 11.3 ± 2.2 (2.86 ± 0.61) x 104 0.18 1010 ± 80

HGN-12 53.2 ± 7.2, 7.1 ± 1.6 (1.36 ± 0.68) x 104 0.29 325 ± 70

HGN-14 52.2 ± 8.0, 5.9 ± 1.0 (1.37 ± 0.83) x 104 0.34 330 ± 30

Table 5.2: Energy transfer half times and structural parameters for citrate-stabilized SGNs.

Sample Outer diameter

(nm)

Surface Area

(nm2)

S:V

(nm-1

)

τph-ph (ps)

SGN-1 18.3 ± 2.0 (1.05 ± 0.22) x 103 0.33 140 ± 10

SGN-3 25.4 ± 4.2 (2.03 ± 0.67) x 103 0.24 170 ± 10

SGN-5 38.4 ± 4.2 (4.64 ± 1.02) x 103 0.16 210 ± 20

SGN-6 59.8 ± 7.8 (1.12 ± 2.92) x 104 0.10 310 ± 15

In order to summarize size-dependent nanoparticle energy transfer, the time constants

obtained for energy transfer from the photoexcited nanoparticles to the surroundings were plotted

with respect to the total surface area for each of the nanoparticles. Figure 5.4a shows the energy-

transfer-to-solvent half times for all of the HGN samples as a function of total surface areas. Two

distinct regions of the plot were observed, both of which exhibited linear surface area

dependencies for the energy transfer half times. HGNs with small surface areas resulted in a

shallower slope. In addition, the shallow-sloped portion of Figure 5.4a corresponded to HGNs

with cavity radii <15 nm, whereas the data for the HGNs with cavity radii ≥15nm fell on the

portion of Figure 5.4a characterized by the steeper slope.

58

Figure 5.4: Nanoparticle-to-surroundings energy transfer half times (τET) of HGNs plotted as a

function of their total surface area. These HGNs have cavity radii ranging from 3.3 to 27.5 nm,

shell thicknesses from 5 to 11 nm, and aspect ratios from 3 to 9. The data exhibited behaviors

that were categorized in two classes: HGNs with cavity radii <15 nm and those with cavity radii

≥15 nm. The data point corresponding to a 15-nm HGN cavity radius is denoted by an arrow. In

both cases, the τET half time was linearly dependent on the total surface area. A linear fit to the

data collected for HGNs with small cavities yielded γ = 20 ± 4 fs/nm2; γ = 65 ± 5 fs/nm

2 was

obtained for large cavities. x-Axis error bars were determined based on the outer and inner

diameters from TEM images of several particles, and assume uniform HGN shells. (b)

Nanoparticle-to-surroundings energy transfer half times of SGNs as a function of their total

surface area. The τET relaxation time is linearly dependent on the surface area, with a γ value of

62 ± 3 fs/nm2.

A linear dependence of the energy transfer time constants upon the surface area was also

observed for the SGN samples (Figure 5.4b). As established by Hartland, the slope of the linear

59

relationship between τET and the particle’s total surface area is γ (sec/nm2), or the time constant

of energy transfer per unit surface area.116

A linear fit to the data in Figure 5.4a yielded γ = 20 ±

4 fs/nm2 for HGNs with cavity radii <15 nm, and γ = 65 ± 5 fs/nm

2 for HGNs with cavity radii

≥15 nm. The same analysis resulted in γ = 62 ± 3 fs/nm2 for SGNs, in agreement with previous

studies.104

These data showed that HGNs with large cavities (radii ≥15 nm) transferred energy at

a rate comparable to that observed for SGNs. In contrast, HGNs in which the interior fluid was

confined to small (<15-nm radii) cavities exhibited energy transfer rates that differed from SGNs

and larger HGNs by a factor of ∼3.1−3.3.

5.3.2 Energy Transfer Mechanisms

In order to understand the origin of the discontinuity observed at r = 15 nm in the energy transfer

time constants of HGNs, it is necessary to consider all possible contributing mechanisms: (1)

energy transfer across the nanoparticle/ surroundings interface and (2) heat dissipation through

the surroundings. If energy transfer across the interface were the rate-limiting step, the energy

transfer time constants would be expected to scale linearly with the particle’s surface-to-volume

ratio.116

On the other hand, if heat dissipation through the solvent were limiting, the relaxation

time constants would be expected to scale linearly with the particle’s surface area.116

For systems

in which the particle-to-surroundings energy transfer is limited by diffusion through the

surroundings, the heat dissipation half times (τd) depend on the surface area (SA) of the particle

and the thermal conductivity (Λs), density (ρs), and heat capacity (Cs) of the surroundings as

shown in Equation 7. The data shown in Figure 5.4, which showed a linear dependence of the

energy transfer time constant on both HGN and SGN total surface areas, identify heat diffusion

within the surroundings of the nanoparticle as an important component in the relaxation process.

However, the fact that none of the data in Figure 5.4 included a value of zero for the y-axis

60

intercept indicated that equation 7 did not fully account for the data. Therefore, energy transfer

across the metal/surroundings interface was included in the data analysis.

(7)

The time required for energy transfer across the nanoparticle interface (τi) increases as a

linear function of SGN radius (HGN shell thickness; R − r) and the particle’s volumetric heat

capacity (Cp). The interfacial energy transfer time is inversely dependent upon interfacial thermal

conductivity, G:

(8)

Equation 8 describes interfacial energy transfer for HGNs; for SGNs (R − r) is replaced by r.

Interface effects become significant when τd and τi are comparable. As such, a critical value for

G, which reflects the onset of interfacial contributions to the relaxation dynamics, can be

obtained by equating equations 7 and 8.116,117

(9)

When G greatly exceeds Gcritical, energy diffusion through the solvent dominates heat dissipation

by excited nanoparticles. Equation 9 was used to calculate the critical interface thermal

conductance for the HGNs studied here. The resultant values of Gcritical spanned from ∼265 to

∼600 MW m-2

K-1

. Cf is the heat capacity of the fluid and Λf is the thermal conductivity of the

fluid. Previous studies on SGNs in water yielded G = 100−110 MW m-2

K-1

.32,118,119

Taken

together, our calculations and previous experimental results indicated interfacial energy transfer

61

must be included in the analysis of the relaxation dynamics for all HGN samples. Importantly,

since Gcritical > G for all HGNs, the discontinuity observed in Figure 5.4a did not result from a

size-dependent crossover from interface- to diffusion-controlled nanoparticle-to-surroundings

energy transfer.

5.3.3 Interfacial Thermal Conductance of HGNs

Interfacial conductivity was estimated by plotting the experimentally determined energy transfer

half times with respect to HGN shell thickness and SGN radius (Figure 5.5).

Figure 5.5: HGN (○) and SGN (●) energy transfer half times (τET) plotted as a function of HGN

shell thickness, or SGN radius. The experimental half times are plotted along with calculated

size-dependent interfacial thermal conductivities, G. The values for G were obtained using

equation 8, and bulk values obtained from reference 109.

These data were fit to equation 8 to obtain G. In the case of SGNs, analysis of our experimental

data resulted in G = 85 MW m-2

K-1

, which provided good agreement with previous

research.32,118,119

However, the interfacial thermal conductance obtained for HGNs ranged from

G = 35 to G = 45 MW m-2

K-1

, which was significantly reduced compared to SGNs. These data

represented a reduction in interfacial thermal conductance by a factor of ∼1.9−2.4, which

62

indicated that the thermal conductivity (Λs) of confined water is less than that of bulk water. We

do note that the experimental data provides an estimate of Λs, on the basis of the assumption that

interfacial conductance was the rate-limiting step (i.e., Gcritical > G). The data indicated that heat

diffusion through the fluid also contributed to the relaxation dynamics.

The fluid thermal conductivity is related to the experimentally determined interfacial

conductance as G = Λ/h, where h is the thickness of the solvent layer required to dissipate the

energy transferred across the nanoparticle/fluid interface.117

The thermal conductivity of liquid

water is 0.6 W m-1

K-1

.120

Our experimental value for G from SGNs (85 MW m-2

K-1

) implies h

∼7 nm. Assuming energy transfer through 7 nm of cavity-confined water, the thermal

conductivities obtained for the HGNs (G = 35−45 MW m-2

K-1) indicate Λ = 0.25−0.31 W m

-1 K

-

1. These data imply the thermal conductivity of cavity-confined fluids are ∼1.9−2.4 times less

than that of bulk water.

The nature of the cavity interface is not well understood. Citrate ions were used to

passivate the HGN surface. These ions have a molecular diameter of 0.6 nm,30

which may limit,

but not prohibit, their diffusion to the cavity during the galvanic replacement process.

Nonetheless, we assume the outer HGN surface is more completely passivated than the cavity.

As a result cavity-confined fluids can more readily access the metal surface. Previous studies

focused on the influence of capping agent concentration on interface thermal conductivity.118

These results show that lower capping agent concentration increases, not decreases, the value of

G. This effect occurs because incomplete surface passivation allows water molecules access to

the nanoparticle surface, resulting in increased thermal conductivity of the interface. Therefore,

the reduced G that we observed for HGNs with cavity radii <15 nm reflected differences

between the thermal conductivity of bulk and cavity-confined fluids, rather than incomplete

63

passivation of the nanoparticle cavity surface by the capping agent. As such, the G values should

have reflected the properties of the cavity fluids, although some energy transfer to water at the

outer surface could also have contributed to the relaxation dynamics. Previous computational

studies also show that the thermal conductivity of water that is restricted to nanoscale dimensions

can be distinct from that of bulk water.121

In addition, the low frequency vibrational modes of

water shift to higher energies when water is confined to nanometer-sized pools, which range

from 1.5 to 9.0 nm in radius.122

These changes result in a decrease in the effective heat capacity

upon going from bulk to confined water. Although more research is necessary to understand the

properties of the water confined to small HGN cavities, the current time-domain data clearly

indicated that the thermal conductivities of these fluids were ∼1.9−2.4 times less than that

observed for bulk water. This apparent step function in the thermal properties of water must have

its break around 15 nm because HGNs with cavity radii ≥15 nm were characterized by similar

properties as those observed for bulk water.

The kinetic traces obtained for nanoparticle-to-surroundings energy transfer were fit

using a single time constant for eight of the nine HGNs studied. The ability of a single-

exponential function to describe the energy relaxation data for most of the HGNs indicated that

the cavity-confined water was the reservoir for nanoparticle energy transfer; largely unrestricted

access of water to the cavity surface favored energy transfer to the interior fluid. By comparison,

the data obtained for the largest HGN (HGN-10) required a second exponential, yielding time

constants of 110 ± 10 and 1010 ± 80 ps (Figure 5.6). The fast time constant was comparable to

the value obtained for an HGN with a 3.5-nm cavity radius. The HGN lattice can be porous,

having pinholes of 1−2 nm in diameter.123

These pinholes likely accommodate some water

molecules, which serve as a low-temperature sink for energy transfer from hot HGNs.

64

Figure 5.6: Nanoparticle-to-surroundings energy transfer relaxation kinetics obtained for HGN-

10. The raw differential extinction data (black) is plotted along with the result from a two-

component exponential decay (red). The bleach data was inverted for clarity.

5.3.4 Solvent Dependence

In order to test further the nature of the nanoparticle energy relaxation process, the influence of

the dispersing medium on the energy transfer from the particle to the surroundings was also

determined. The time-resolved transient bleach data obtained for one SGN sample (20-nm

radius) dispersed in both water and methanol are compared in Figure 5.7. It was clearly evident

that the energy transfer rate slowed down when the dispersion medium of the nanosphere was

changed from water to methanol. This effect was also observed for HGN samples. For example,

HGN-12 displayed an energy transfer time constant of 325 ± 70 ps in water; this value changed

to 590 ± 60 ps in methanol. Similar results were obtained for HGN-14 (330 ± 30 ps in water; 600

± 50 ps in methanol). By comparison, the energy transfer time constants of HGN-5 were 245 ±

25 ps in water and 600 ± 35 ps in methanol. In these three samples, the energy transfer time

constants increased, on average, by a factor of ∼2.1 when the dispersion medium was altered

from water to methanol.

65

Figure 5.7: Comparison of the time-resolved extinctions obtained for SGN-5 dispersed in water

(black trace) and methanol (red trace). The raw data reflected a slower transient bleach recovery

for gold nanospheres dispersed in methanol than for those dispersed in water.

The energy transfer time constants obtained for HGN-5 in both water and methanol,

using several excitation pulse energies, are depicted in Figure 5.8a. The ratio of the bulk thermal

conductivities (ΛH2O/ ΛCH3OH) is 3.120

Therefore, the energy transfer time constants of HGNs and

SGNs were expected to increase by a factor of 3 when methanol was used as the dispersing

medium instead of water. The same solvent-dependent analysis was carried out for SGN samples

(Figure 5.8b). In all cases the energy transfer time constant increased by a factor of 3 when the

samples were dispersed in methanol instead of water, as expected on the basis of bulk thermal

conductivities. These results were consistent with another study on SGNs:124

for 15-nm SGNs,

changing the surrounding matrix from an aqueous solution to an organic gel leads to a large

increase in the phonon−phonon coupling time constant. This increase is also attributed to the

lower thermal conductivity of the gel compared to that of water, which results in less efficient

heat transfer from the particles to the surrounding matrix.124

The size- and solvent-dependent

kinetics of nanoparticle-to-surroundings energy dissipation observed for HGNs, which were

66

distinct from those noted for SGNs, indicated that the properties of both cavity-confined water

and methanol are different from those of bulk fluids.

Figure 5.8: (a) Summary of the energy transfer half times obtained for HGN-5 dispersed in

water or methanol at several excitation pulse energies. The data reflected an increase of τET by a

factor of ∼2.5. (b) Summary of τET for SGN-1 dispersed in methanol or water at several

excitation pulse energies. The data reflected a 3-fold increase in the energy transfer half time.

Therefore, HGNs provide a novel platform for investigating the properties of confined fluids.

Taken together, the size- and solvent-dependent energy relaxation time constants obtained here

for HGNs indicated that heat diffusion within the surroundings and energy transfer across the

nanoparticle/surroundings interface were both important processes mediating particle-to-

surroundings energy transfer in both HGNs and SGNs.

67

5.4 Conclusions

Nanoparticle-to-surroundings energy transfer was studied for citrate-stabilized hollow and solid

gold nanospheres with outer diameters ranging from 17 to 78 nm using femtosecond time

resolved transient extinction spectroscopy. The HGNs had fluid-filled cavities with radii ranging

from 3 to 27.5 nm. In all cases, energy transfer across the nanoparticle/fluid interface and heat

diffusion through the surroundings were both contributing energy relaxation mechanisms. The

energy transfer half times ranged from 105 ± 10 ps to 1010 ± 80 ps for the HGNs and 140 ± 10

ps to 310 ± 15 ps for the SGNs. The data obtained for the preferential energy transfer from hot

HGNs to cavity confined fluids indicated that the HGNs could be split into two classes: those

with cavity radii <15 nm and those with cavity radii ≥15 nm. In the former case, the kinetic data

reflected an ∼3-fold reduction in the thermal conductivities of confined water with respect to

bulk values. In the latter case, HGN and SGN kinetics were similar, indicating that the thermal

properties of water confined to cavities with radii ≥15 nm approached bulk values. Experiments

on HGNs and SGNs dispersed in methanol also supported the idea that fluids confined to

nanoscale dimensions (radius <15 nm) had different thermal properties than those observed for

bulk fluids. In contrast, solvent-dependent data obtained for SGNs were consistent with

predications based on bulk thermal conductivities. These data indicated that hollow

nanostructures are useful for understanding the properties of fluids confined to nanoscale

dimensions.

68

CHAPTER SIX

INTERPARTICLE ELECTROMAGNETIC COUPLING

ENHANCEMENT IN HOLLOW GOLD NANOSPHERE

AGGREGATES

Reproduced with permission from Knappenberger, K. L., Jr., Schwartzberg, A. M., Dowgiallo, A.

M. and Lowman, C. A., Journal of the American Chemical Society, 131 (2009) 13892–13893.

DOI: 10.1021/ja903086g; Chandra, M., Dowgiallo, A. M. and Knappenberger, K. L., Jr.,

Journal of the American Chemical Society, 132 (2010) 15782–15789. DOI: 10.1021/ja106910x.


6.1 Introduction

Plasmon-supporting nanoparticles have potential to be used in devices capable of

functioning throughout the visible and near-infrared electromagnetic regions. A thorough

understanding of the interplay between the precise arrangement of these particles within a

designed array and the collective properties of composite materials is required. The

electromagnetic surface fields formed by the interaction of nanospheres and nanorods have been

studied by several research groups.11,130-1134,64,69-73

It has been previously observed that a large

spectral red-shift of the plasmon resonance occurs when nanospheres aggregate in such a manner

that a large (>1 nm) interparticle gap exists between the individual nanoparticles.73,131

This type

of behavior results from the formation of a bonding hybridized plasmon resonance mode.73

However, a detailed description of the aggregate properties when nanoparticles are in contact

with little to no gap is still needed.

http://pubs.acs.org/doi/full/10.1021/ja903086g?prevSearch=%255BContrib%253A%2Bdowgiallo%255D&searchHistoryKey=

http://pubs.acs.org/doi/full/10.1021/ja106910x?prevSearch=%255BContrib%253A%2Bdowgiallo%255D&searchHistoryKey=

69

Femtosecond transient extinction measurements of electronic relaxation and interparticle

electromagnetic coupling in hollow gold nanospheres (HGNs) and HGN aggregates will be

described. HGNs exhibit a tunable SPR, and upon aggregation, a systematic blue shift of this

resonance occurs. The blue-shifted SPR narrows significantly as compared to isolated HGN,

indicative of preserved plasmon coherence. Finite-difference time-domain (FDTD) calculations

confirm that this blue shift is due to the delocalization of the Fermi-gas over multiple particles.

The relaxation of the excited electrons proceeds by: (1) ultrafast electron scattering, (2) electron-

phonon coupling and (3) energy dissipation to the solvent. A 48-nm HGN with a shell thickness

of 7 nm has an electron−phonon coupling lifetime of 300 ± 100 fs, and upon aggregation, this

lifetime increases to 730 ± 140 fs, indicating Fermi-gas delocalization over multiple particles.

In addition, HGNs having dimensions ranging from 29.9 nm/8.5 nm (outer diameter/shell

thickness) to 51.5 nm/4.5 nm and having aspect ratios spanning 3.5-11.7 were employed to

investigate the SPR and electric field enhancement of HGN aggregates by variation of the aspect

ratio, interparticle gap, and cavity spatial separation. HGN aggregation was achieved using either

ethanedithiol or cysteine, resulting in dimeric structures in which monomer subunits were

spatially separated by <3 Å and 1.2 ± 0.7 nm, respectively. Particle dimensions and separation

distances were confirmed by TEM. Experimental extinction spectra obtained for high-aspect

ratio HGNs aggregated by ethanedithiol exhibited an obvious blue shift of the SPR relative to

that observed for isolated HGNs. One explanation for the blue-shifting is due to a charge-transfer

plasmon resonance at the dimer interface. In addition, the blue shifting depended on the

thickness of the hollow shell. Thin-shelled HGNs that were in contact due to aggregation from

the dithiol exhibited the largest blue-shift, whereas thick-shelled (≥7 nm) HGNs did not display a

significant SPR shift when the individual particles were in contact. On the other hand, cysteine

70

resulted in large interparticle gaps (>1 nm) and a red-shift of the SPR for all of the HGN

samples. This effect results from the coupling of the plasmons of each nanoparticle surface,

resulting in a lower energy hybridized plasmon mode. Electric field maps were simulated by

FDTD calculations and showed that the inner HGN surface plays a significant role in the

interparticle coupling mechanism. These findings, which describe structure-dependent SPR

properties, may be significant for applications where energy transport is nanoscale devices is

required.


6.2.1 Aggregation Techniques

Hollow gold nanospheres were synthesized according to the sacrificial galvanic replacement

method described in Chapter 2. HGN aggregates were formed by two different methods. The

first method involved adding KCl to the colloidal HGN suspension while stirring. The extinction

was monitored during drop-wise addition of the KCl to confirm aggregation of the nanoparticles.

The second method involved using two different thiol ligands, ethanedithiol and cysteine. For

ethanedithiol syntheses, 2 μL portions of a 5 mM solution of ethanedithiol in ethanol were

injected into 1 mL of each HGN solution. The solution was agitated by shaking and then allowed

to equilibrate at room temperature for 1 h before performing experiments. For preparing

cysteine-based HGN aggregates, 10 μL portions of 1 M aqueous solution of cysteine were added

to 1 mL of each HGN solution.

6.2.2 Characterization Tools

UV-Visible absorption, high-resolution transmission electron microscopy (HRTEM), energy

dispersive spectroscopy (EDS) and electron diffraction analyses confirmed HGN integrity

71

following aggregation. Absorption measurements were carried out with a Cary 300 UV-Vis

spectrophotometer at regular intervals of KCl addition to confirm the absorption blue shift for

HGN aggregates (Figure 6.1). The absorption intensity decreased for the aggregate sample due to

dilution of the colloidal HGN suspension by the KCl solution.

Figure 6.1: Extinction spectra recorded for HGN-5 as an isolated (black) and aggregated (blue)

species. Aggregation was achieved by adding KCl to the HGN-5 aqueous suspension.

Figure 6.2: UV-Visible absorption spectra of isolated (black) and aggregated (red) solid gold

nanospheres that an outer diameter of 50 nm. Upon aggregation using a KCl solution, the SPR

band of the isolated SGNs broadens and a new longitudinal band appears at redder wavelengths,

consistent with previous findings.104,105

72

Unlike the HGNs, aggregates formed from 50-nm solid gold nanospheres exhibit an absorption

red shift. This is consistent with the previous reports in references 54 and 95b. The absorption

properties of solid 50-nm gold nanospheres and aggregates are compared in Figure 6.2. TEM

measurements were conducted on a JEOL-2011 electron microscope that has a LaB6 filament,

and was operated at 200 kV. This instrument has lattice and theoretical point resolutions of 0.14

nm and 0.23 nm, respectively. The TEM facility is in part supported by the National High

Magnetic Field Laboratory (NHMFL) under cooperative agreement NSF DMR-0654118, and the

State of Florida. The measurements were conducted on carbon-coated copper grids. Particle

diameters were calculated using ImageJ software [Rasband, W.S., ImageJ, U. S. National

Institutes of Health, Bethesda, Maryland, USA, http://rsb.info.nih.gov/ij/, 1997-2005].

Representative TEM images of HGNs and HGN aggregates are provided in Figure 6.3 a, and b

and c, respectively.

Figure 6.3: (a) HRTEM image of different regions of HGN-5 aggregates. The images show surface

necking occurring to various degrees, and also that the particles remain hollow.

These measurements indicated the particles (HGN-5) had an outer diameter of 51.1 ± 5.1 nm and

a shell thickness of 10.0 ± 1.0 nm (Table 2.2). The HRTEM in Figure 6.3 b and c shows two

73

different regions of HGN-5 aggregates, and both clearly show the formation of a bi-continuous

surface due to particle necking.

Dynamic light scattering (DLS) measurements indicate the HGN-5 aggregate diameter is

800 ± 300 nm. DLS measurements were acquired using a Dynapro TITAN Serial Number 99-

172, Model 99D (Wyatt Technologies) with a laser wavelength of 824.6nm at the FSU Physical

Biochemistry Facility. Measurements were collected at 20°C with a 10 second acquisition time

and 1 second read interval at 100% laser power for the colloidal HGNs and 50% laser power for

the aggregates. Prior to analysis, solutions were filtered through a 0.2 μm Supor Membrane

Acrodisc Syringe Filter and centrifuged to remove dust particles and then added to a clean quartz

cuvette. Scattered light was collected at a fixed angle of 90°. The correlator was operated with

248 channels at 0.48 μs. The calculations of the particle size distributions and distribution

averages were performed with the software DYNAMICS®. Several typical DLS results for both

HGN and HGN aggregates are shown in Figure 6.4. Peaks that are assigned to water, HGN-5

(black), HGN-5 aggregates (red) and dust are clearly distinguished. Aggregate samples show

little or no sign of isolated HGN.

Figure 6.4: Dynamic Light Scattering measurements of isolated HGN-5 sample (black) and their

aggregates (red).

74

Analysis of DLS measurements indicate the hollow gold nanospheres had a radius of 22 ± 2 nm

and the aggregate radius is 400 ± 150 nm. For energy dispersive X-ray spectroscopy (EDS), the

TEM instrument described above is equipped with a Si(Li) PGT detector. The EDS spectra in

Figure 6.5 show that the HGN aggregate contains only Au and very little oxygen. The presence

of Cu is attributed to the copper grids and no other elements were detected within the detection

limit.

Figure 6.5: EDS data for two (a) and several (b) hollow gold nanospheres in the HGN-5 aggregate.

Copper is from the sample grid. The EDS data indicate the aggregates are free of oxides.

In addition, electron diffraction patterns were collected for multiple HGNs in the aggregate and

is shown in Figure 6.6. The selected area diffraction patterns were taken from a group of HGNs

within the aggregated sample. Both patterns are indexed to be fcc gold, with a lattice parameter

of a = 4.118 Å.

6.2.3 Computational Methods

The absorption spectra of all experimentally studied HGNs and their dimers (at distinct

interparticle distances) were numerically simulated using the finite difference time domain

75

(FDTD) technique.135

Simulations were performed using FDTD Solutions software from

Lumerical. Absorption spectra of HGNs and HGN dimers were simulated using 2 nm mesh and

an environmental dielectric constant of unity. The material dielectric constants were taken from

Johnson and Christy.136

In the case of HGN dimers, the incident electromagnetic field was

polarized parallel to the interparticle axis.

Figure 6.6: Electron diffraction pattern from several hollow gold nanospheres in the HGN-5 aggregate.

6.3 Electronic Relaxation Dynamics in HGN Aggregates

6.3.1 Linear Absorption Spectral Changes

Ultrafast transient extinction measurements revealed interparticle electromagnetic coupling in

aggregates of hollow gold nanospheres based on comparison of the electron-phonon scattering

76

rates in isolated and aggregated HGNs. The normalized extinction spectra of colloidal and

aggregated HGN-10 are shown in Figure 6.7. The λmax of the SPR for colloidal HGN-10 solution

occurred at 605 nm, which is consistent with previous findings.10

The HGN extinction was

monitored from 350 nm to 2 μm, and no other additional peaks were observed.

Figure 6.7: Normalized extinction spectra for isolated and aggregated HGN-10. A noticeable

shift of the SPR λmax to bluer wavelengths is observed upon aggregation.

Upon KCl-induced aggregation, there was an unexpected blue-shift of the SPR band. This effect

was observed for several other HGN samples. The fact that the shift is small indicates that the

plasmonic nature of the nanospheres was retained upon aggregation. This blue-shifted absorption

has not been previously reported for other metal nanoparticle aggregates. For solid gold

nanoparticles, aggregation leads to the formation of red-shifted and broader absorption bands

(Figure 6.2).104,105

Blue-shifting of the SPR with respect to Mie theory-based approximations has

been previously observed for small gas-phase Au and Ag nanoclusters, and was attributed to

electron confinement effects.125-127

The blue shift observed for HGN aggregates was attributed to

77

delocalization over multiple particles, leading to the formation of a new SPR. This conclusion

was supported by time-resolved transient extinction measurements and FDTD calculations.

6.3.2 Transient Extinction of HGN Aggregates

The transient extinction spectra recorded at a 500-fs probe temporal delay for both isolated and

aggregated HGNs are shown in Figure 6.8a.

Figure 6.8: (a) Femtosecond transient extinction spectra of isolated (red) and aggregated (blue)

hollow gold nanoparticles. The nanospheres were excited with a 400 nm pump (500 nJ/pulse)

and probed at 500 fs time delay with a white-light continuum probe. The aggregate spectrum is

clearly blue-shifted with respect to that of the isolated HGN sample. (b) Kinetic traces resulting

from temporal integration of transient extinction bleach from (a) for HGN (red) and HGN

aggregates (blue). The HGN bleach recovery is much slower for the aggregate system.

The transient bleach for the aggregated sample is also blue-shifted with respect to the isolated

HGN. The kinetic traces for both the isolated and aggregated HGNs (Figure 6.8b) were obtained

78

by temporal integration of the spectrum at the center wavelength of the transient bleach. These

kinetic traces show that both the electron-electron and electron-phonon coupling lifetimes are

shorter for isolated HGNs compared to aggregated HGNs or similarly sized SGNs.60-62,83,84

The

electron-electron scattering lifetimes were determined by convoluting the fit function (Equation

1) with the instrument response and were found to be 150 ± 70 fs for the isolated HGN and 300 ±

50 fs for the aggregate. The well established two-temperature model was then used to determine

the electron-phonon coupling times (τel-ph) and constant (G) for these systems.128

The electron-

phonon coupling time as a function of relative excitation pulse energy is shown in Figure 6.9 and

is the y-intercept from the linear fits.

Figure 6.9: Electron-phonon coupling relaxation times as a function of relative pump pulse

energy for both isolated (●) and aggregated (○) HGN samples. A linear fit was applied to each

set of data to determine the zero-point electron-phonon coupling time, τel-ph, which is the y-

intercept. The HGN aggregates exhibit a longer electron-phonon coupling lifetime compared to

isolated HGNs and begins to approach bulk values.

Transient extinction measurements were taken at low-powers (50 nJ to 1μJ), in the linear

excitation regime. The resulting electron-phonon coupling times for isolated and aggregated

HGNs were 300 ± 100 fs and 730 ± 140 fs, respectively. These lifetimes were converted to

79

electron-phonon coupling constants (G) using Equation 6 and had values of 6.6 × 1016

W m-3

K-1

and 2.7 × 1016

W m-3

K-1

for isolated and aggregated HGNs, respectively. The electron-phonon

coupling lifetime of colloidal HGNs was remarkably fast, whereas the aggregate system had a

lifetime that approached values previously observed for SGNs and bulk gold systems.31,129

For

comparison, our results from examination of solid 50-nm particles are provided in Figure 6.10.

Figure 6.10: (a) Femtosecond transient extinction kinetic traces of 50 nm solid gold nanospheres

probed at 520 nm following 405 nm excitation. The excitation pulse energies are: 0.1 μJ/pulse

(blue), 0.2 μJ/pulse (black) and 1 μJ/pulse (red). (b) Electron-phonon relaxation times plotted as

a function of relative excitation pulse energy. The y-intercept from the linear fit corresponds to

the zero-point electron-phonon coupling lifetime for 50 nm SGNs, 770 ± 150 fs.

Figure 6.10a displays the laser pulse energy dependence of the electronic energy relaxation

times. In Figure 6.10b, the relaxation times extracted from the kinetic traces in Figure 6.10a are

plotted with respect to relative pulse energy, and the y-intercept from the linear fit represent the

electron-phonon coupling lifetime at room temperature and is 770 ± 150 fs. The electron-phonon

coupling time exhibited by isolated HGNs was about 40% shorter compared to the value

obtained for HGN aggregates and similarly sized SGNs. Small (2-5 nm) nanoclusters also

exhibited short electron-phonon coupling times that were 35%-50% of bulk values.128

These

80

previous findings for small nanoclusters suggest that the faster electron-phonon coupling times

of HGNs result from the higher surface-to-volume ratio of these structures, where the electrons

are spatially confined in the thin shell. The blue shift of the SPR and relaxation lifetimes

approaching values of large SGNs and bulk gold suggest that the HGN aggregates exhibit

interparticle coupling. This can be most simply rationalized as a volume effect where the

electronic energy seems to delocalize through the aggregate. More importantly, this occurs

because the electron is spatially confined within the thin shell of the HGN. These initial findings

show that HGN aggregates have potential to be used to control energy transport over nanometer-

length scales.

6.4 Controlled SPR Properties of HGN Aggregates

In addition to SPR shifts, the aggregation of hollow gold nanospheres leads to changes in the

spatial distribution of the strong electric fields generated at the surface of these nanoparticles.

This section will demonstrate that these properties can be tailored using thiol-ligands, instead of

KCl solution, to form aggregates that contain only a few HGNs, such as dimers and trimers.

Furthermore, the SPR spectral position and electric field distribution depends heavily upon the

size of the interparticle separation and the HGN shell thickness. Since the shell thickness is

typically less than 10 nm, HGNs can be used to study the interaction between the interior

(cavity) surfaces that are separated by very small distances.10,16

As described in Section 6.3, the

SPR of aggregated surface-necked HGNs shifts to shorter wavelengths compared to the

transverse SPR of an isolated HGN.11

FDTD simulations were performed and suggest that the

blue-shifted SPR can be assigned to a newly formed longitudinal SPR of HGN dimers. Two

possible explanations for this blue shift are (1) antibonding modes of hybridized plasmons of two

HGNs69

or (2) a charge-transfer plasmon resonance.70-72

In the first case, a SPR blue shift would

81

occur in an asymmetric nanosphere aggregate. The individual plasmon modes of each HGN

hybridize to form a lower energy bonding mode and a higher energy antibonding mode. For an

asymmetric dimer, the spectral weight would be shifted from the bonding to the antibonding

mode, or other higher-order modes.74

In the case of the charge-transfer model,71,72

conductive

overlap may occur when the particles are either in contact leading to the blue shift; such an

overlap would lead to a collective time-dependent charge oscillation over the dimer. The results

presented here indicate that HGNs can exhibit both hybridized plasmon modes and collective

charge transfer resonances when the particles are assembled into small or large extended

aggregate structures. Dielectric screening occurs for aggregates formed using cysteine molecules

where a significant interparticle gap between adjacent HGNs exists. For these structures, the

optical properties can be explained using the hybridization model of surface plasmon modes. In

contrast, the spectral shifts exhibited by contact HGN dimers results from charge-transfer

resonances.

6.4.1 Thiol-Induced Aggregation of HGNs

In order to study the structure dependence of interparticle coupling, the aggregates of HGN-2,

HGN-6, HGN-8, HGN-13, HGN-15, HGN-16, and HGN-17 were examined. Dimensions, along

with their standard deviations, for all HGNs investigated are provided in Table 6.1. The different

thiols used as aggregating agents formed two different types of aggregate structures.

Ethanedithiol-induced aggregates were characterized by HGNs in close contact with one another,

usually as dimers. Cysteine-induced aggregates were characterized by HGNs separated by >1 nm

gap. The extinction spectra of both isolated and aggregated HGN-2, HGN-6, HGN-8, and HGN-

17 are shown in Figure 6.11. This spectral position depends directly on the outer diameter-to-

shell thickness aspect ratio. For all seven HGNs examined in this study, cysteine addition yielded

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an extinction spectrum that was broader and red-shifted relative to that observed for isolated

HGNs (Figure 6.11).

Table 6.1: Outer diameters, shell thicknesses, aspect ratios (outer diameter/shell thickness) and

SPR peak positions of isolated HGNs.


(nm)

Shell Thickness

(nm)

Aspect

Ratio

SPR

(nm)

Δλ = (λdimer – λHGN)

(nm) HGN-2 29.9 (± 6.2) 8.5 (± 2.2) 3.5 (± 0.6) 548 +4

HGN-6 31.2 (± 4.6) 6.3 (± 2.1) 5.4 (± 1.5) 577 -9

HGN-8 54.6 (± 12.5) 8.6 (± 2.9) 6.7 (± 1.8) 0.67 0

HGN-13 54.8 (± 12.2) 6.9 (± 1.6) 8.3 (± 2.2) 647 -10

HGN-15 53.3 (± 10.5) 5.7 (± 1.0) 9.5 (± 2.1) 658 -9

HGN-16 49.3 (± 9.7) 5.1 (± 0.8) 9.9 (± 2.0) 695 -47

HGN-17 51.5 (± 7.8) 4.5 (± 0.8) 11.7 (± 2.5) 713 -26

Figure 6.11: Experimental extinction spectra of isolated HGN and cysteine and ethanedithiol-

induced HGN aggregates. The dashed lines are located at the center of the extinction peaks. A

distinct blue shift of the SPR for HGNs having shell thickness <7 nm (a,b) occurred upon

ethanedithiol addition. However for HGNs with shell thickness >7 nm (c,d), only a small red

shift or no peak shift occurred after ethanedithiol addition. The insets show the same spectra

normalized at the respective SPR maxima to show the peak shift more clearly.

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This observation was consistent with previous work on cysteine-mediated aggregation of solid

gold nanoparticles.137

By comparison, addition of ethanedithiol to the HGN dispersion resulted in

distinct blue shifts of the SPR for all particles having a shell thickness <7 nm (Figure 6.11 a,b).

For HGNs with thicker shells, ethanedithiol addition led to either no peak shift or a small red

shift with almost no change in the width of the peak (Figure 6.11 c,d). Absorption was measured

out to 1200 nm, and no additional absorption peaks were observed. The insets in Figure 6.11

show the effect of the aggregation agent on optical properties more clearly. TEM images (Figure

6.12) clearly demonstrated that addition of cysteine and ethanedithiol to the colloidal HGN

solutions resulted in two different structures.

Figure 6.12: Cysteine-induced (A) and ethanedithiol-induced (B) aggregates of HGN-6. The

scale bars correspond to 50 nm. In the cysteine-induced aggregates, distinct gaps can be seen and

were usually about 1 nm wide. On the other hand, ethanedithiol-induced aggregates usually

formed small dimers or trimers and showed little to no interparticle gap.

Cysteine addition led to the formation of large, extended aggregates, whereas the addition of

ethanedithiol yielded small aggregates comprised primarily of dimers, trimers, and tetramers.

84

The statistical distribution of aggregate sizes (e.g., dimers, trimers, etc.) formed after

ethanedithiol addition to the HGNs was also determined and is displayed in Appendix B.

Approximately 40-50% of the HGNs in solution aggregated upon ethanedithiol addition. Of

these, most (30% of [HGN]total) were found in dimeric structures. Cysteine-induced aggregation

could also be controlled to restrict the aggregate size to a few nanospheres by keeping the

cysteine concentration low (2 μL, 5 mM), as was done for addition of ethanedithiol. These

results are shown in Figure 6.13 where a small red shift of the SPR occurs.

Figure 6.13: Absorption spectra of HGN-13 (a), 15 (b), 16 (c), and 17 (d) formed by low

concentration addition of cysteine (2 μL of 5 mM cysteine).

A striking difference between the particles produced using the two types of thiol-mediated

aggregation was the resulting spatial separation of the monomers forming the aggregate. The

85

average interparticle distance of cysteine-induced aggregates was approximately 1.2 ± 0.7 nm.

The average interparticle spacings for all of the cysteine-induced HGN aggregates are listed in

Table 6.2.

Table 6.2: Average interparticle gap sizes between HGNs in the cysteine-induced aggregates.

Sample Interparticle Spacing

(nm) HGN-2 1.17 ± 0.33

HGN-6 1.00 ± 0.35

HGN-8 1.80 ± 0.60

HGN-13 1.91 ± 0.67

HGN-15 1.14 ± 0.65

HGN-16 1.37 ± 0.44

HGN-17 1.58 ± 0.44

In contrast, HGN aggregates formed using ethanedithiol did not have significant interparticle

gaps where the nanoparticles are in or nearly in contact. As a result, the HGNs are within the

conductive limit.132,133

Previous studies have found that cysteine forms extended aggregate

structures for solid nanoparticles through a hydrogen bond network, as shown in Figure 6.14.138

Figure 6.14: Probable hydrogen-bonding scheme of the cysteine-induced HGN aggregates (not

to scale).

The measured average interparticle distance of 1.2 ± 0.7 nm approximates the length of two

cysteine molecules arranged through hydrogen bonding. FTIR measurements of the cysteine-

86

induced HGN aggregates supported this type of hydrogen-bonding scheme and are shown in

Figure 6.15.

Figure 6.15: FTIR spectra of cysteine and cysteine induced aggregates of HGN-15. Absence of

SH stretching vibrational mode at 2564 cm-1

in the spectrum of aggregates suggest that cysteine

is attached to the gold surface by Au−S linkage.

The disappearance of the S-H stretching vibrational mode at 2564 cm-1

indicated the formation

of a Au-S bond.139

In addition, broadening of the N-H stretching band at 3274 cm-1

suggests

hydrogen bonding of the amide proton.139

Hence, the TEM and FTIR results supported the

hydrogen bonding scheme presented in Figure 6.14 for cysteine-induced aggregates. Conversely,

the ethanedithiol-induced aggregates had different structures. It is possible that the terminal

sulfur atoms of ethanedithiol may coordinate to the HGN surface atoms, or the molecule may

form a bridge between two HGNs.140

Either of these arrangements would result in HGNs in

contact or separated by a very subtle gap (≤3 Å).

87

6.4.2 SPR Spectra and Electric Field Simulations

FDTD calculations were used to simulate the absorption spectra of HGN aggregate structures

with interparticle gaps and those in contact. HGN dimers were chosen as representative

aggregate structures. The simulated spectra for HGNs having an outer diameter of 53 nm with

shell thicknesses of 5 and 8 nm, and their two different dimers (in contact and separated by 5

nm) are shown in Figure 6.16 and qualitatively reproduced our experimental data.

Figure 6.16: Simulated absorption spectra using FDTD calculations of an HGN having an outer

diameter of 53 nm and shell thickness of (a) 5 nm and (b) 8 nm. These results are similar to the

experimental results presented in Figure 6.11 b (HGN-17) and d (HGN-8).

The SPR red-shifted for HGN dimers separated by 5 nm regardless of the shell thickness, which

was consistent with our experimental absorption measurements on cysteine-induced aggregates.

These findings indicated that the outer surface plasmon modes contributed the most to

interparticle coupling for particles separated by a distinct gap. Dimers formed by placing HGNs

in contact exhibited a spectral response that was strongly dependent upon the shell thickness of

the HGN. The SPR red-shifted slightly for aggregates of HGNs with thick (8 nm) shells; whereas

it blue-shifted for thinner shells (5 nm). The SPR shifts more to the blue as the shell thickness

88

decreases (<4 nm), which agrees with our experimental results. In Figure 6.17, the experimental

and calculated relative spectral shifts (Δλ/λ0) are plotted as a function of shell thickness to show

the transition from red-shifted to blue-shifted SPR in HGNs dimers in contact.

Figure 6.17: The normalized peak shift (Δλ/λ0) as a function of HGN shell thickness. Panel (a)

shows the experimentally determined values for ethanedithiol-induced HGN aggregates. Panel

(b) shows the FDTD simulated results for HGN dimers (outer diameter of 53 nm) in contact.

Hence, there is significant interaction between the cavity modes of two HGNs with shell

thicknesses below a critical value. In Figure 6.18, electric field simulations show how dimer

plasmon properties depend on the HGN shell thickness. The electric fields were simulated for

HGNs having an outer diameter of 53 nm and shell thicknesses of 5, 7, and 10 nm. These electric

field maps correspond to isolated HGNs, dimers in contact, and dimers separated by a 5 nm gap.

A significant electric field was observed along the cavity walls for contact HGNs with 5 nm

shells (Figure 6.18, left column, middle row). However, the electric field intensity predominated

at the conical region of the particle interface for HGNs with 10 nm shells (Figure 6.18, right

column, middle row). Hence, the experimental and computational data indicate that cavity modes

greatly influenced the plasmon properties of HGNs in contact.

89

Figure 6.18: Simulated electric field maps using FDTD calculations for various shell

thicknesses: from left to right in each panel, 5, 7, and 10 nm. The outer diameter of the HGNs

was fixed at 53 nm for each panel. The top panels are for an isolated HGN, the middle panels are

for HGNs dimers in contact, and the bottom panels are for HGN dimers that are separated by 5

nm.

HGN-16 yielded a very large SPR blue-shift following ethanedithiol aggregation (Table

6.1). However, TEM images indicate that this structure included many defects in the surface.

Therefore, the structural and optical properties are reported in Table 6.1 for reference but are not

included in the overall analysis. The influence of the HGN shell thickness or aspect ratio on the

direction (red or blue) of the SPR shift was determined (data in Table 6.1). The absorption

90

spectrum of HGN-6 (outer diameter: 31.2 nm, thickness: 6.3 nm) displayed a distinct SPR blue

shift upon ethanedithiol addition; by comparison, the SPR did not shift for HGN-8 (outer

diameter: 54.57 nm, thickness: 8.57 nm). The aspect ratios for HGN-6 and HGN-8 and were 5.37

and 6.71, respectively. If the HGN aspect ratio controlled the peak shift in contact dimers, then

the lower aspect ratio species should behave more like a solid particle. However, this dependence

is not observed for the HGN aspect ratio and SPR shift. The shell thicknesses for HGN-6 and

HGN-8 were different, at 6.30 and 8.57 nm, respectively. The blue shift observed for HGN-6

indicated that its thinner shell allowed cavity modes to interact strongly with each other, even

thought HGN-6 has a lower aspect ratio. This type of interaction would be weaker in a contact

dimer containing HGNs with thick shells, such as HGN-8. As a result, the shell thickness rather

than the aspect ratio dictated the direction of the SPR spectral shift for HGNs in or near contact.

6.4.3 Cysteine-Induced Aggregation

The absorption spectra of cysteine-induced HGN aggregates have been explained by a

hybridization model originally developed for both isolated and aggregated metal dielectric core-

shell particles.69

The outer surface plasmon modes interact with the inner surface (cavity)

plasmon modes, forming a symmetric low-energy bonding mode and an antisymmetric high-

energy antibonding mode. The splitting between these states depends on the spatial separation of

the inner and outer surfaces. The large dipole moment of the bonding plasmon results in a larger

absorption cross section for this mode than the antibonding plasmon. As a result, the absorption

spectra of HGNs are red-shifted compared to similarly-sized SGNs. The coupling of these

hybridized plasmon modes in HGN aggregates leads to the formation of even more combinations

of distance-dependent hybridized modes. In symmetric HGN homodimers, these modes may

include: surface plasmon(1)-surface plasmon(2), cavity plasmon(1)-cavity plasmon(2), and

91

other, higher-order modes. Similarly, the symmetric lower energy bonding mode dominates

aggregate absorption.131,69-73,132-134

In asymmetric heterodimers, there is reduced symmetry,

yielding multiple SPR peaks.134

For cysteine-induced aggregates, the large red shift and

broadening of the SPR was consistent with the symmetric surface plasmon(1)-surface

plasmon(2) hybridization model described above, which is also observed for SGN aggregates.131

In Figure 6.19, the simulated SPR spectral position as a function of HGN outer diameter (d) to

interparticle gap (D) ratio for several HGN dimers is plotted.

Figure 6.19: Simulated SPR peak shift (red shift Δλ for spatially separated dimers divided by the

SPR maximum of the isolated particle, λ0) as a function of interparticle gap to outer diameter

ratio (D/d) for HGN-6. The decay constant (t) value is consistent with that of similarly sized

solid gold nanospheres.

These simulations show that HGN dimers exhibit the “plasmon ruler” behavior described for

SGNs, where a distance-dependent spectral shift of the dimer plasmon results.141-143

This is

consistent with the symmetric surface-surface coupling model used to describe cysteine-linked

HGN dimers where the red shift is due to dielectric screening effects. The electric field is

localized to the space between particles at a gap of ∼1.2 nm where there is large dipolar coupling

among surface plasmons. The FDTD electric field simulations in the bottom row of Figure 6.18

92

verify this behavior. Spectral broadening may have occurred due to the formation of large and

extended aggregate structures, as observed in the electron microscope images (Figure 6.12).

6.4.4 Ethanedithiol-Induced Aggregation

By comparison, ethanedithiol-induced aggregates resulted in HGNs that were in or near contact

and complex plasmonic behavior. In order to describe the plasmon properties of touching HGNs,

the following contributions were considered: (1) reduced dielectric screening which led to

decreased red shifting of the SPR, (2) increased contributions from antibonding or higher-order

modes, and (3) formation of a charge-transfer plasmon resonance. If dielectric screening effects

contributed, then a lower-frequency SPR (spectral red shift) would have been observed

(refractive index of ethanedithiol is 1.5589 and that of water is 1.33 at 20°C).144

However, this is

not consistent with either the experimental and theoretical data on ethanedithiol-induced

aggregates. Therefore, changes in dielectric screening alone cannot be used to account for the

experimentally observed SPR blue shift of HGN dimers in or near contact.

Thinner HGN shells allows cavity and surface plasmons to interact more, resulting in a

more dominant antibonding mode that gains spectral weight from the bonding mode.69-73,132-134

In

the case of an HGN dimer, the plasmon modes of one particle can interact with those of the other

particle. The hybridized symmetric bonding mode and antisymmetric antibonding mode of each

HGN should couple to their counterparts to give rise to the hybridized states of the dimer.73

A

red shift of the SPR would occur when the coupling involves only the bonding modes of two

HGNs.145

In contrast, a blue shift of the SPR would result when the coupling involves the

antibonding HGN modes. Higher order modes may also be present when two HGNs interact, and

would appear as several absorption peaks at SPR frequencies that depend on the aspect ratio and

the interparticle spatial separation. However, only a single SPR band upon surface contact was

93

observed and is not consistent with a system exhibiting higher-order modes. In heterodimers,

these interactions become increasingly significant due to the reduced symmetry.134

Although the

structures reported here were homodimers, it is possible that sample polydispersity led to

reduced symmetry.

A charge transfer plasmon resonance occurs when nanospheres are close enough to allow

a collective charge oscillation between the two particles.70,73

This charge oscillation results in a

time-varying total charge for each nanosphere, which is highly sensitive to the interfacial

structure of the dimer.73

The large interfacial volume of HGNs makes them ideal materials to

study collective charge oscillations. Figure 6.20 illustrates the charge cloud oscillation following

interaction with an EM wave having the electric field polarized parallel to the interparticle

longitudinal axis of the dimer.

Figure 6.20: Schematic of a stable charge-transfer plasmon where the incident electromagnetic

field is polarized parallel to the interparticle axis of the hollow gold nanospheres. The distance

between the two cavities is given by D.

Since the HGNs in the dimer are coupled within the near-field limit, the two surfaces in contact

will be instantaneously polarized with opposite signs for the charge, leading to a stable charge-

transfer configuration. This is consistent with the experimentally observed (Figure 6.17a) and

theoretically confirmed (Figure 6.17b) thickness dependence of the SPR shift. The coupling

strength increased as the spatial separation between the cavities decreased, or the shell became

94

thinner. Asymmetric hybridized plasmon modes are still considered as a possible contribution to

the observed blue shifting of the SPR of contact HGNs. However, the charge transfer plasmon

resonance may contribute to HGN contact dimer properties due to the strong shell thickness

dependence and the absence of higher-order plasmon frequencies in the aggregate absorption

spectra. Further studies using advanced spectroscopic techniques and rigorous quantum

mechanical calculations may provide more insight into the unique SPR properties of HGNs.

6.5 Conclusions

Both experimental and computational studies of SPR properties for small and extended HGN

aggregates have been described. The SPR spectral position of HGN aggregates could be tuned to

either higher or lower energies compared to those of the isolated HGNs, which is not observed

for solid nanospheres. A charge-transfer plasmon model involving interparticle coupling was

used to account for the SPR blue-shift that depended strongly on HGN shell thickness and aspect

ratio. These results are supported by earlier findings where the HGN aggregates formed through

KCl addition led to longer electron-phonon coupling times. Additionally, the direction of the

SPR shift (red or blue) was dictated by the interparticle gap separating the HGN dimers. All

examined aggregates that possessed a large gap (1.2 ± 0.7 nm) showed a broad and red-shifted

SPR peak, which was fully described by symmetric bonding plasmon modes. However,

aggregation of thin-shell, high-aspect-ratio HGNs using short-chain dithiols exhibited a

pronounced, newly formed longitudinal SPR that was blue-shifted with respect to the transverse

SPR of isolated HGNs. Aggregates of low-aspect ratio particles formed by the same manner did

not show any significant spectral change. FDTD simulations reproduced the experimental

measurements and also indicated that the resultant nanoscale electric field amplitudes and spatial

profiles were extremely sensitive to nanosphere shell thickness. These findings highlight the

95

critical role of the inner HGN cavity surface in composite nanostructures. Moreover, HGNs

provide a versatile platform not only for plasmonic engineering but also for developing coupled

composite plasmonic nanostructures.

96

CHAPTER SEVEN

GOLD NANOPARTICLE AND IRON PORPHYRIN

INTERACTION

7.1 Introduction

The addition of a cationic porphyrin, such as iron(III) tetrakis(1-methyl-4-pyridyl)porphine

(FeTMPyP), to a solid gold nanoparticle (SGN) or hollow gold nanoparticle (HGN) solution led

to the formation of SGN and HGN aggregates. The aggregation was indicated through the

observed spectral changes in the linear absorption spectrum of the nanoparticle solution. It is

important to note that FeTMPyP in aqueous solution undergoes several equilibria with changing

pH.132

At pH values less than 4, the porphyrin exists solely as the monoaquo ferric porphyrin

[FeTMPyP(H2O)] with spectral features at 400, 515, and 640 nm.147

At pH values greater than 4,

FeTMPyP forms a μ-oxo dimer species in aqueous solution.148

Our absorption spectrum showed

a strong peak corresponding to the Soret band at 425, a Q0 band at 600, and a Qb band at 630 nm.

These spectral positions are consistent with a system that contained both the monomer

(FeTMPyP(OH)2) and dimer ([FeTMPyP(OH)]2O) forms, where the Soret, Q0, and Qb bands

occurred at 424, 603, and 632 nm, respectively.147

The dimerization reaction

2[FeTMPyP(OH)2]3+

⇌ [[FeTMPyP(OH)]2O]6+

+ H2O

is believed to result in a low-spin, six-coordinate complex with the iron atoms bonded by an

Fe(III)-O-Fe(III) μ-oxo bridge.148

Hence, Raman experiments were carried out using porphyrin

solutions at pH 10 to ensure that there were predominantly FeTMPyP dimers. In addition,

evidence of strong antiferromagnetic coupling between the two Fe(III) ions results in much

lower magnetic susceptibilties for the μ-oxo bridged dimer than for the monomer.149

97


7.2.1 Aggregation Using FeTMPyP

HGNs were prepared according to Section 2.1.2. In order to form HGN aggregates, 10 μL of

6.25 x 10-5

M FeTMPyP in water (pH adjusted to 10 using sodium hydroxide) was added to 1

mL of the gold nanoparticle solution and vortexed. The aggregated sample was then used

immediately for the Raman experiments. Gold colloids were prepared by citrate reduction of

gold, following the method reported by Ghosh et al. and described in Section 2.1.2.74

Specifically, 50 mL of water containing 0.25 mM HAuCl4 was heated to boiling. Then, 0.50 mL

of 1% trisodium citrate was added to the boiling solution with vigorous stirring, reducing the

Au3+

and forming the gold nanoparticles. The solution was boiled for at least 10 minutes before

allowing the solution to cool to room temperature. This procedure resulted in solid gold

nanospheres having an average outer diameter of 38.4 ± 1.2 nm. In order to form the gold

nanoparticle aggregates, 4 μL of 6.25 x 10-5

M FeTMPyP in water (pH adjusted to 10 using

sodium hydroxide) was added to 1 mL of the gold nanoparticle solution and vortexed. The

aggregated sample was then used immediately for the Raman experiments.

7.2.2 Raman Spectroscopy

In order to observe the surface-enhanced Raman scattering (SERS) signal, the appropriate

excitation wavelength for the samples was determined based on resonance with the longitudinal

plasmon band. Hence, an excitation wavelength of 785 nm was used for the SGN aggregates,

which is close to the longitudinal plasmon band that occurred at 760 nm. For the HGN

aggregates, a laser excitation source at 633 nm was used because the longitudinal plasmon band

occurred at 685 nm.

98

Raman scattering measurements were carried out using a Horiba Jobin Yvon HR800 UV

microRaman spectrograph. The laser excitation source had a wavelength of either 632.8 nm or

784.65 nm. The 633 nm laser was a linearly polarized, 17 mW HeNe laser (Melles-Griot 25-

LHP-925-249) which gave about 4.5 mW of power at the sample. The 785 nm laser was also

linearly polarized, 80 mW grating stabilized diode laser (Toptica DL 100) and had about 5 mW

of power at the sample. The confocal microscope had a focal length of 800 nm and used two

Semrock filters: a bandpass filter to clean up the laser (LL01-633-12.5 for 633 nm, LL01-785-

12.5 for 785 nm) and an long-wave-pass edge filter (LP02-633RE-25 for 633 nm, LP02-785RU-

25 for 785 nm) to couple the laser into the microscope and reject the Rayleigh scattering while

allowing longer wavelengths (>50 cm-1

to the red of 633 nm, >110 cm-1

to the red of 785 nm) to

pass through to the spectrograph. The grating used had 600 lines/mm. An Olympus BX30M

microscope was used that had a macrosample mirror attachment to couple the microscope to a

cuvette holder (Quantum Northwest TLC 50-F, thermoelectric temperature control). The

temperature control feature of this cuvette holder was not used in the experiments described here.

The CCD detector was from Wright Instruments (ATECCD-1024X56-0), which was an 1024 x

256 pixel, open electrode EEV CCD, with 26 µm x 26 µm pixels, and four stage

thermoelectrically cooled to -70 deg °C. The computer software was LabSpec V4.08 from

Horiba JY. The Raman scattering signal was collected from 100 to 1700 cm-1 with an overlap of

244 pixels and 467 pixels for the experiments conducted using the 785 and 633 nm lasers,

respectively. The Raman scattering signal was collected for 180 and 240 seconds, three times

each and then averaged, for the experiments using the 785 and 633 nm lasers, respectively. These

experiments were carried out on both solid and hollow gold nanosphere solutions containing

99

FeTMPyP, as well as on blank solutions that contained only millipure water. Then, the data from

the blank experiments were subtracted from that of the SGN and HGN samples.

7.3 FeTMPyP-Induced Aggregation of HGNs and SGNs

7.3.1 Linear Extinction Measurements

The isolated nanoparticles strongly absorb at their characteristic surface plasmon resonance

wavelength, in this case occurring at 530 nm for SGNs and 550 nm for HGNs. Upon addition of

the iron porphyrin, an immediate color change from red to purplish-gray for the SGNs and purple

to gray for the HGNs was observed. Figure 7.1 shows the extinction spectra of isolated SGNs

and FeTMPyP-induced SGN aggregates. Following addition of FeTMPyP to a SGN solution, the

plasmon band red shifts to 535 nm and a new broad band appears at 820 nm, indicating the

formation of large, polydisperse SGN aggregates.

Figure 7.1: Normalized extinction spectra for isolated SGNs (black) and FeTMPyP-induced

SGN aggregates (red). The changes occurring following aggregation include a slight red-shift of

the transverse SPR band to 535 nm and a new red-shifted band at 820 nm.

100

Figure 7.2 shows the SPR for isolated and FeTMPyP-induced HGN aggregates. Following

addition of FeTMPyP to an HGN solution, the plasmon band red shifts to 560 nm and a new

broad band appears at 685 nm, indicating the formation of large, polydisperse HGN aggregates.

Figure 7.2: Normalized extinction spectra for isolated HGNs (black) and FeTMPyP-induced

HGN aggregates (red). The SPR band of the isolated HGNs red-shifts slightly to 557 nm and

another red-shifted peak occurs at 685 nm.

The SGNs and HGNs used here were prepared using sodium citrate where the citrate ion serves

as the stabilizing agent (synthetic method described in Section 2.1.1 and 2.1.2). The citrate anion

contains three negatively charged oxygen atoms that bind to the surface of the particle, forming a

negatively charged surface. A porphyrin cation can then be electrostatically adsorbed onto the

negatively charged nanoparticle surface. Isolated SGNs and HGNs do not aggregate due to the

electrostatic repulsive forces between the colloidal particles. However, the presence of a cationic

species such as FeTMPyP reduces the electrostatic repulsion barrier, inducing SGN or HGN

aggregate formation.

101

7.3.2 SERS of FeTMPyP

In conjunction with the linear absorption spectra, surface-enhanced Raman spectroscopy (SERS)

has been used to determine the oxidation state, spin state, and orientation of metalloporphyrins

on metal surfaces.146,148

The frequency and intensity of the vibrational bands in the Raman

spectra of FeTMPyP are also pH-dependent. This pH-dependence has been used to determine the

bands that differentiate the dimer (at high pH) from the monomer (at low pH) form. In the

measured Raman spectrum of FeTMPyP in the presence of SGN-5 aggregates shown in Figure

7.3, the vibrational bands occurring at 661, 790, 850, and 1184 cm-1

are assigned to the N-CH3+

groups.

Figure 7.3: Surface-enhanced Raman spectra of FeTMPyP in the presence of SGN-5 aggregates

(black) and 2.5 x 10-7

M FeTMPyP (red). The solutions were excited using a 785 nm laser. The

peaks are labeled according to reported Raman bands for FeTMPyP.

These peaks suggest a close interaction between the N-CH3+ groups and the SGN surface. The

shoulder at 386 cm-1

and the peak at 1080 cm-1

indicate that dimers are present in solution.

102

Raman bands occurring at 365 cm-1

and 370 cm-1

in porphyrin solutions at pH 10 were assigned

to the symmetric Fe-O-Fe stretching mode. Both of these reported peaks are blue-shifted from

the band at 388 cm-1

reported for the resonance Raman of FeTMPyP at pH 10. In addition, a

peak at 1098 cm-1

is only present in FeTMPyP solutions at high pH (around pH 10) where a

dimer species is prevalent. This band has been assigned to the symmetric bending mode of Cβ–

H.132

This band occurred in our spectra at 1080 cm-1

, indicating the presence of a dimeric

porphyrin in our SGN aggregates. In addition, the band reported at 905 cm-1

is close to our band

at 901 cm-1

which only appears at high pH where the dimer species is present.148

The Raman

spectrum of FeTMPyP in the presence of HGN aggregates is displayed in Figure 7.4, where the

vibrational bands occurring at 662, 777, 864, and 1178 cm-1

are assigned to the N-CH3+ groups.

Figure 7.4: Surface-enhanced Raman spectra of FeTMPyP in the presence of HGN aggregates.

The solutions were excited using a 633 nm laser. The peaks are labeled according to reported

Raman bands for FeTMPyP.

These peaks suggest a close interaction between the N-CH3+ groups and the HGN surface. The

peaks at 375 cm-1

and 1090 cm-1

indicate that dimers are present in solution. In addition, the

103

band reported at 905 cm-1

is close to the band we measure at 896 cm-1

which only appears at high

pH where the dimer species is present.148

The possible orientations of the FeTMPyP molecule on the SGN and HGN surfaces can

be described by using a nanoparticle dimer with a finite gap as a representative aggregate. When

FeTMPyP is present as a μ-oxo dimer species, both porphyrin molecular planes (the plane that

the porphyrin ring lies in) may be oriented parallel to the interparticle axis of the SGN dimer,

where four N-CH3+ groups (two from each porphyrin) would be electrostatically adsorbed on the

surface of each nanoparticle. In this “edge-on” configuration, the Fe-O-Fe bond would be

aligned perpendicular to the interparticle axis. In the Raman spectra of FeTMPyP in the

presence of SGN and HGN aggregates, a band appeared at 386 cm-1

and 375 cm-1

for SGNs and

HGNs, respectively, and was assigned to the stretching mode of the Fe-O-Fe bond. In a pH 10

solution, this band appeared at 365 cm-1

when the porphyrin was oriented “face-on” to a Ag

electrode surface.148

A “face-on” configuration would have the Fe-O-Fe bond parallel and the

porphyrin molecular plane perpendicular to the interparticle axis of an SGN or HGN dimer. In

addition, a “face-on” orientation would lead to a decrease in this vibrational frequency due to

surface interaction of the Fe atom. Hence, the Fe(III)TMPyP μ-oxo dimer is oriented “edge-on”

since our band occurred at a higher wavenumber (386 cm-1

for SGNs and 375 cm-1

for HGNs),

indicating that there was less interaction between the Fe atom and the gold surface.

Another piece of evidence for the “edge-on” configuration comes from the linear

absorption spectra of the HGN aggregates. Based on our earlier studies on HGN aggregate

formation, a red shift of the plasmon band accompanied by the appearance of a longer

wavelength band occurred when the HGNs were separated by a gap that was couple of

nanometers wide. However, when the HGNs are in contact with one another and separated by a

104

gap less than 5 Å, the plasmon band is instead blue-shifted and there is not a second band present

at longer wavelengths. When the iron porphyrin dimer is in an “edge-on” configuration, it would

create a gap of at least 1.5 nm, which is the length of one FeTMPyP molecule. The length of the

Fe-O-Fe bond is less than 0.5 nm, which would lead to a blue shift in the absorption spectrum if

the Fe-O-Fe bond was parallel to the interparticle axis; however, a blue shift was not observed.

Hence, the red-shifting of the plasmon band and appearance of a longitudinal band at longer

wavelengths in the HGN aggregates is consistent with a gap larger than 0.5 nm, which would be

created when the iron porphyrin dimer is oriented in an “edge-on” configuration.

7.4 Conclusions

Cationic FeTMPyP molecules can be used as aggregating agents for both solid and

hollow gold nanospheres. Aggregation was confirmed using linear absorption measurements as

well as surface-enhanced Raman scattering (SERS) measurements. It is believed that the

FeTMPyP dimer is oriented in an “edge-on” configuration with respect to the nanoparticle

surface for both solid and hollow gold nanospheres. This nanoparticle assembly may be useful

for understanding the influence of intense surface plasmon fields on molecular dynamics.

105

CHAPTER EIGHT

CONCLUSIONS AND FUTURE WORK

The goal of this dissertation is to describe energy relaxation mechanisms in nanoscale materials

in order to tailor their properties so that they can function as efficient transducers, such as a light-

harvesting antenna. In particular, plasmonic gold nanoparticles, both hollow and solid, have been

synthesized and characterized using transmission electron microscopy, energy dispersive

spectroscopy, dynamic light scattering, and UV/Vis absorption spectrophotometry. Femtosecond

pump-probe transient extinction experiments on both isolated and aggregated HGNs and SGNs

have been carried out in order to elucidate their electronic energy relaxation properties.

Transient extinction experiments were carried out at high pulse energies to analyze the

SPR to learn about the electronic and mechanical properties of HGNs following excitation by a

femtosecond laser pulse. The oscillation frequency and phase were determined for a wide range

of HGN sizes, revealing size-dependent vibrational modes. In addition, transient extinction

experiments were conducted at low pulse energies in order to determine the electron-phonon

coupling times for a wide range of sizes of HGN and SGN samples. As the aspect ratio of the

HGN increases, the electron-phonon coupling time decreases (or the electron-phonon coupling

increases), whereas for SGNs, the electron-phonon coupling remains constant with increasing

diameter. The electron-phonon coupling enhancement exhibited by high aspect ratio HGNs was

attributed to the large surface to volume ratio of these structures, which results in non-negligible

contributions from their environment. The electronic energy relaxation properties of HGNs were

investigated further by determining their phonon-phonon coupling times, which is the last step in

electronic energy relaxation in metal nanoparticles. The fluids confined to the hollow core of

106

HGNs have different properties compared to their bulk counterparts, thereby influencing the

particle-to-surroundings energy transfer rates.

While studying how aggregated nanostructures influence optical and electronic

properties, an unexpected spectral blue-shift of the surface plasmon resonance (SPR) occurred

upon aggregation of HGNs using a salt solution, which led to longer electronic energy relaxation

times compared to isolated HGNs. These findings were significant because previous studies have

found that SGNs red-shift upon aggregation and have faster electronic energy relaxation times.

In order to understand further the nature of the blue shift in HGN aggregates, alkane-thiols were

used to induce the aggregation, and at a critical thickness of the HGN shell, the SPR blue-shifts

due to the interaction of the electric fields within the hollow cavities of the nanoparticles. These

findings may indicate that confined fluids dramatically impact the HGN properties, and future

work in this area will be to understand the properties of confined fluids. The potential impact of

this further research may benefit applications beyond nanoscience, because water confined to

small volumes is thought to mediate many important chemical and biochemical processes.

The structural, optical, and electronic studies on the aforementioned types of metal

nanoparticles provide the basis for the research on molecule-plasmon interactions, where the

surface plasmons may influence light absorption in a nearby molecule. Specifically, future

studies will focus on the interaction between the surface plasmons of HGNs and their aggregates

with the discrete electric-dipole transitions of iron porphyrin molecules. These early studies on

the surface-enhanced Raman spectroscopy (SERS) of iron porphyrin molecules near SGN and

HGN aggregate surfaces will facilitate the understanding of the interaction between the strong

electric fields of HGNs and molecular electronic transitions.

107

APPENDIX A

HGN AND SGN SIZE DISTRIBUTIONS

Key: d: outer diameter; t: shell thickness; AR: aspect ratio (outer diameter/shell thickness);

R1/R2: inner radius/outer radius

Figure A.1: Size distributions for HGN-1 having d = 16.6 ± 2.9 nm, t = 5.0 ± 1.2 nm, AR = 3.4

± 0.6, and R1/R2 = 0.40 ± 0.08.


± 0.6, and R1/R2 = 0.46 ± 0.1.


± 2.1, and R1/R2 = 0.56 ± 0.07.

108


± 0.6, and R1/R2 = 0.61 ± 0.05.


± 1.5, and R1/R2 = 0.60 ± 0.10.


± 1.3, and R1/R2 = 0.68 ± 0.15.

Figure A.7: Size distributions for HGN-8 having d = 54.6 ± 12.5 nm, t =8.6 ± 2.9 nm, AR = 6.7

± 1.8, and R1/R2 = 0.67 ± 0.08.

109

Figure A.8: Size distributions for HGN-10 having d = 77.9 ± 5.5nm, t = 11.3 ± 2.2nm, AR = 6.9

± 1.2, and R1/R2 = 0.75 ± 0.05.


± 2.1, and R1/R2 = 0.70 ± 0.16.

Figure A.10: Size distributions for HGN-12 having d = 53.2 ± 7.2 nm, t = 7.1 ± 1.7 nm, AR =

7.8 ± 1.6, and R1/R2 = 0.73 ± 0.12.


8.3 ± 2.3, and R1/R2 = 0.74 ± 0.07.

110


9.0 ± 1.6 and R1/R2 = 0.77 ± 0.17.

Figure A.13: Size distributions for HGN-15 having : d = 53.3 ± 10.5 nm, t = 5.7 ± 1.0 nm, AR =

9.5 ± 2.1, and R1/R2 = 0.78 ± 0.05.

Figure A.14: Size distributions for SGN samples: (a) SGN-2, d = 19.8 3.7 nm, (b) SGN-4, d =

37.7 3.3 nm, and (c) SGN-7, d = 83.3 7.5 nm.

Figure A.15: Size distributions for SGN samples: (a) SGN-1, d = 18.3 2.0 nm, (b) SGN-3, d =

25.4 4.2 nm, (c) SGN-5, d = 38.4 4.2 nm.

111

Figure A.16: Size distribution for SGN-6, d = 59.8 7.8 nm.

112

APPENDIX B

HGN AGGREGATE SIZE DISTRIBUTIONS

Figure B.1: Ethanedithiol-induced HGN aggregate distributions for HGN-2, HGN-6, HGN-13,

HGN-15, HGN-16, and HGN-17. The occurrence of each type of aggregate (i.e. monomer,

dimer, trimer, etc.) was plotted as a histogram up until aggregates comprising 8 HGNs.

113

Figure B.2: Ethanedithiol-induced HGN aggregate distributions for HGN-8. The occurrence of

each type of aggregate (i.e. monomer, dimer, trimer, tetramer, etc.) was plotted as a histogram up

until aggregates comprising 8 HGNs.

114

APPENDIX C

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135

BIOGRAPHICAL SKETCH

EDUCATION

Florida State University; Tallahassee, FL

Physical Chemistry Graduate Student

Ph.D. program, degree expected May 2013

Advisor: Dr. Kenneth L. Knappenberger, Jr. 2008–present

Towson University; Towson, MD

B.S., Chemistry, May 2008

Magna cum laude 2004–2008

AWARDS

American Chemical Society (ACS) Physical Chemistry Division

Outstanding Student Poster Award 2011

Hoffman Teaching Merit Fellowship, Florida State University,

Department of Chemistry and Biochemistry 2008 –2009

Analytical Division (ACS) Award in Analytical Chemistry, Towson

University, Department of Chemistry 2008

Towson University Provost Scholarship 2004–2008

Maryland State Senatorial Scholarship 2004–2008

Towson University Department of Music Scholarship 2004–2005

EXPERIENCE

Research Assistant – to Dr. Kenneth L. Knappenberger, Jr., Assistant Professor, Florida

State University May 2009–Fall 2011, Summer 2012–Fall 2012

Understand electron and energy transfer processes in nanoscale systems using femtosecond

transient extinction spectroscopy

Characterize electric fields amplification by nanoparticle aggregates using single particle

second harmonic generation (SHG) microscopy, bright and dark field scattering

microscopy

Operation of pulsed femtosecond laser systems

Synthesis of plasmonic nanostructures

Nanostructure characterization by optical spectroscopy, transmission electron microscopy

(TEM), and dynamic light scattering (DLS)

Teaching Assistant – to Dr. David Gormin, Florida State University

Spring 2012, Spring 2013

Undergraduate physical chemistry II laboratory

Responsible for educating and ensuring the safety of one lab section, and grading lab

reports

136

Teaching Assistant – to Dr. Rafael Bruschweiler, Dr. Heidi Mattoussi, and Dr. Oliver

Steinbock, Florida State University

Spring 2012, Spring 2013

Undergraduate physical chemistry II course

Responsible for grading home-works, quizzes, and exams

Teaching Assistant – to Dr. Stephanie Dillon, Florida State University

August 2008-May 2009

Undergraduate general chemistry laboratory

Responsible for educating and ensuring safety of two lab sections and assigning/grading

lab reports

Biological Lab Technician, United States Department of Agriculture (USDA) College

Student Internship Program in Beltsville, MD

June 2006–August 2008

Research Aid for Microbiologist, Dr. Zafar A. Handoo, USDA-ARS Nematology

Laboratory

Processed soil and plant samples to recover nematode specimens

Maintained and established nematode cultures on plants in greenhouse and growth chamber

Prepared and performed routine maintenance for nematode slides for the USDA Nematode

Collection

Kept detailed records in USDA Nematode Collection of experimental data obtained from

host test experiments on various crops

PUBLICATIONS

K.L. Knappenberger, Jr., A.M. Dowgiallo, M. Chandra, and J.W. Jarrett “Optical Plasmonic

Nanoparticle Transducers Studied Using Femtosecond Laser-based Spectroscopy” J. Phys.

Chem. 2013, In Press.

A.M. Dowgiallo and K.L. Knappenberger, Jr. “The Influence of Confined Fluids on

Nanoparticle-to-Surroundings Energy Transfer” J. Am. Chem. Soc., 2012, 134, 19393-

19400.

M. Chandra, A.M. Dowgiallo, K. L. Knappenberger, Jr. “Magnetic Dipolar Interactions in

Solid Gold Nanosphere Dimers” J. Am. Chem. Soc., 2012, 134, 4477 – 4480.

S.M. Stagg, K. L. Knappenberger, Jr., A.M. Dowgiallo, M. Chandra “Three-Dimensional

Interfacial Structure Determination of Hollow Gold Nanosphere Aggregates” J. Phys.

Chem. Lett., 2011, 2, 2946 – 2950.

A.M. Dowgiallo and K. L. Knappenberger, Jr.”Ultrafast Electron-Phonon Coupling in Hollow

Gold Nanospheres” Phys. Chem. Chem. Phys., 2011, 13, 21585 – 21592.

A.M. Dowgiallo, A.M. Schwartzberg, K. L. Knappenberger, Jr.”Structure-Dependent Coherent

Acoustic ibrations of Hollow Gold Nanospheres” Nano Lett., 2011, 11 (8), 3258 – 3262.

137

M. Chandra, A.M. Dowgiallo, K. L. Knappenberger, Jr. “Controlled Plasmon Resonance

Properties of Hollow Gold Nanosphere Aggregates” J. Am. Chem. Soc., 2010, 132 (44),

15782–15789.

M. Chandra, A.M. Dowgiallo, K. L. Knappenberger, Jr. “Two-photon Rayleigh Scattering

from Isolated and Aggregated Hollow Gold Nanospheres” J. Phys. Chem. C, 2010, 114

(47), 19971–19978.

K. L. Knappenberger, Jr., A. M. Schwartzberg, A.M. Dowgiallo, C.A. Lowman “Electronic

Relaxation Dynamics in Isolated and Aggregated Hollow Gold Nanoparticles” J. Am.

Chem. Soc., 2009, 131 (39), 13892–13893.

PRESENTATIONS

A.M. Dowgiallo, K.L. Knappenberger, Jr. “Ultrafast Electronic Energy Relaxation in Hollow

Gold Nanospheres” Oral Presentation at Florida State University Department of Chemistry

and Biochemistry, Physical Chemistry Seminar, December 6, 2012, Tallahassee, FL.

A.M. Dowgiallo, K.L. Knappenberger, Jr. “Ultrafast Electron-Phonon Coupling in Hollow

Gold Nanospheres” Poster Presentation at the 63rd

Southeastern Regional Meeting

American Chemical Society (SERMACS), October 26 - 29, 2011, Richmond, VA.

A.M. Dowgiallo, K.L. Knappenberger, Jr. “Ultrafast Electron-Phonon Coupling in Hollow

Gold Nanospheres” Poster Presentation at the 242nd

American Chemical Society National

Meeting and Exposition, August 28 - September 1, 2011, Denver, CO, Section F: Excited

State Dynamics: Theory and Experiment.

A.M. Dowgiallo, M. Chandra, K.L. Knappenberger, Jr. “Spectroscopy of Tailored

Nanostructures” Oral Presentation at Towson University Department of Chemistry,

November 4, 2010, Towson, MD.

A.M. Dowgiallo, K.L. Knappenberger, Jr. “Spectroscopy of Tunable Plasmonic Nanoparticles”

Oral Presentation at Florida State University Department of Chemistry and Biochemistry,

Physical Chemistry Seminar, April 19, 2010, Tallahassee, FL.

A.M. Dowgiallo, C.A. Lowman, K.L. Knappenberger, Jr. “Interparticle Electromagnetic

Coupling Enhancement with Tunable Plasmon-Supporting Hollow Gold Nanospheres”

Poster Presentation at the 239th

American Chemical Society National Meeting and

Exposition, March 21-25, 2010, San Francisco, CA, Section N: Multiscale Nanomaterials,

Polymer, and Biomolecular Dynamics.

A.M. Dowgiallo, C.A. Lowman, K.L. Knappenberger, Jr. “Interparticle Electromagnetic

Coupling Enhancement with Tunable Plasmon-Supporting Hollow Gold Nanospheres”

Poster Presentation at the Inter-American Photochemical Society 20th

Winter Conference,

January 2-5, 2010, St. Petersburg Beach, FL.

138

A.M. Dowgiallo and K.L. Knappenberger, Jr. “Electronic Coupling Enhancement with

Tunable Plasmon-Supporting Hollow Gold Nanoparticles” Oral Presentation at the Florida

Annual Meeting and Exposition (FAME) of the American Chemical Society, May 14-16,

2009, Orlando, FL, Physical Chemistry II Symposium.

The Florida State University Hollow Gold Nanosphere Optical Transducers Studied Using Femtosecond...

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