Photoinduced work function changes by isomerization of a densely packed azobenzene-based SAM on Au:...

14302 Phys. Chem. Chem. Phys., 2011, 13, 14302–14310 This journal is c the Owner Societies 2011

Cite this: Phys. Chem. Chem. Phys., 2011, 13, 14302–14310

Photoinduced work function changes by isomerization of a densely

packed azobenzene-based SAM on Au: a joint experimental and

theoretical study

N. Crivillers,a A. Liscio,b F. Di Stasio,c C. Van Dyck,d S. Osella,d D. Cornil,d

S. Mian,cG. M. Lazzerini,

cO. Fenwick,

cE. Orgiu,

aF. Reinders,

eS. Braun,

f

M. Fahlman,fM. Mayor,*

egJ. Cornil,*

dV. Palermo,*

bF. Cacialli*

cand

P. Samorı*a

Received 21st March 2011, Accepted 13th May 2011

DOI: 10.1039/c1cp20851a

Responsive monolayers are key building blocks for future applications in organic and molecular

electronics in particular because they hold potential for tuning the physico-chemical properties of

interfaces, including their energetics. Here we study a photochromic SAM based on a conjugated

azobenzene derivative and its influence on the gold work function (FAu) when chemisorbed on its

surface. In particular we show that the FAu can be modulated with external stimuli by controlling

the azobenzene trans/cis isomerization process. This phenomenon is characterized experimentally

by four different techniques, kelvin probe, kelvin probe force microscopy, electroabsorption

spectroscopy and ultraviolet photoelectron spectroscopy. The use of different techniques implies

exposing the SAM to different measurement conditions and different preparation methods, which,

remarkably, do not alter the observed work function change (Ftrans–Fcis). Theoretical calculations

provided a complementary insight crucial to attain a deeper knowledge on the origin of the work

function photo-modulation.

1. Introduction

Self-assembled monolayers (SAMs) are highly ordered supra-

molecular arrays of functional molecules that can be easily

processed on specific surfaces via chemisorption.1 SAMs are

technologically important as they can be incorporated in devices

for a wide range of applications such as sensors,2 memory

devices3 and organic thin-film transistors.4 From a fundamental

viewpoint, SAMs offer a unique and tunable combination of

physical properties that render them prototypical systems for

various studies in the realm of interfacial chemistry and

more generally self-organization.5 An appropriate design of

the molecules that will self-organize on a substrate is crucial to

confer a specific functionality to the resulting modified surface.

Among various tunable functions of a SAM, the electronic

work function (F) of metallic contacts is very important for

optimizing charge injection and extraction in electronic devices

and hence their performance.6 In general, the presence of an

interfacial dipole associated with the molecular monolayer

at the metal/semiconductor interfaces has been identified as

the main contribution to the work function tuning of metallic

contacts.7

Since the surface of an electrode can be decorated with arrays

of responsive molecules, the design of switchable SAMs, i.e.

monolayers able to undergo significant changes in their physical

or chemical properties as a result of external stimuli, is of

particular interest for the fabrication of smart surfaces and

interfaces.8 Among photochromic systems, azobenzenes are

known to undergo a well-defined isomerization from the trans

to the cis conformation around the NQN double bond, in

response to an external stimulus such as UV light and vice versa

if exposed to visible light or heating.9 These two forms feature

different physico-chemical properties10 that when self-assembled

into monolayers emerge as different wettability,11 conductance12

or dipole moment.13 Accordingly, azobenzenes have been used

a ISIS—CNRS 7006, Universite de Strasbourg,8 allee Gaspard Monge, 67000 Strasbourg, France.E-mail: [email protected]

b ISOF-CNR, via Gobetti 101, 40129 Bologna, Italy.E-mail: [email protected]

c Department of Physics and Astronomy and London Centre forNanotechnology, University College London, Gower Street,London WC1E 6BT, England. E-mail: [email protected]

d Laboratory for Chemistry of Novel Materials, University of Mons,20 Place du Parc, B-7000 Mons, Belgium.E-mail: [email protected]

eDepartment of Chemistry, University of Basel, St. Johannsring 19,4056 Basel, Switzerland. E-mail: [email protected]

f Department of Physics, Chemistry and Biology,Linkoping University, 581 83 Linkoping, Sweden

gKarlsruhe Institute of Technology KIT, Institute for Nanotechnology,P.O. Box 3640, 76021 Karlsruhe, Germany

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for a large variety of applications14 including modulation of the

interactions between azo-protected nanoparticles, photo-

mechanical responsive systems, liquid-crystal alignment for

displays, optical switching and data storage devices.15 In

particular, azobenzene derivatives hold potential to be incorpo-

rated in electronic switchable devices. Azobenzene-based

SAMs on gold have been exploited to tune reversibly the

work function by photoisomerization.13,16 In contrast to the

conjugated azobenzene used in the present work, many previous

studies were addressed to asymmetric azobenzene dithiols

exhibiting an alkyl chain on the second sulfur binding group.

Such alkyl group led to a 50% surface coverage of azobenzene

units within the SAM, thereby providing the necessary free

volume for the photoisomerization process to occur.17 Never-

theless, in our specific case, we previously demonstrated by

means of scanning tunneling microscopy (STM) that the

terminally thiol functionalized azo-biphenyl (AZO) used for

this study forms highly ordered and tightly packed SAMs

on Au (111) which undergoes photoisomerization in large

domains at surfaces, with an unexpected isomerization yield

exceeding 96%.18 By exploiting such a high yield, bistable

electrical nano-junctions,19 cargo lifter20 as well as optically

modulable field-effect transistors21 could be fabricated.

Here, we report on the change in the Au work function (F)and its modulation caused by the trans- to cis-azobenzene

photoisomerization of AZO-SAMs chemisorbed on Au

(Scheme 1). To this end, we have measured the change in work

function using four complementary experimental techniques:

macroscopic Kelvin Probe (KP), Kelvin Probe Force Micro-

scopy (KPFM), electroabsorption (EA) spectroscopy and

ultraviolet photoelectron spectroscopy (UPS). Theoretical

calculations on the photoisomerization in AZO-SAMs have

been also carried out to complement and explain the experi-

mental results. KP and KPFM have been employed to investi-

gate the surface potential and work function modification due

to the presence of the AZO-SAM chemisorbed Au in a similar

air environment, measurement done under the effect of an

electric field. While KP maps a macroscopic area, the KPFM

features tens of nm spatial resolution. In the case of the EA, an

electric field is also used to modulate the measurable absorp-

tion, the experiments are performed under rough vacuum

(10�2 mbar). This latest technique offers the possibility to

study the effect of the AZO-SAM under similar operational

conditions but when it is embedded in a real device. On the

contrary, it is interesting to observe if when the AZO-SAM is

exposed to completely different measurement condition the

result is altered, such in the case of the UPS.

2. Experimental details

AZO-SAMs were prepared by immersing the gold substrates

in a 0.1 mM solution of the azobenzene derivative in chloroform

for 48 h. The samples were vigorously rinsed with chloroform

and dried under nitrogen. To obtain comparable results

between different techniques, polycrystalline evaporated gold

on SiOx or indium tin oxide (ITO) were used, thus providing

comparable surface roughness, i.e., homogeneous Au grain

size. Au (111) substrates were prepared by evaporating 60 nm

of gold onto freshly cleaved mica after preheating the mica at

450 1C overnight. Thermal deposition was performed at a rate

of 0.01 nm s�1 and a pressure of 1–1.5 � 10�6 mbar keeping

the temperature at 450 1C during the evaporation. Commercial

micro-electrodes were purchased from IPMS Fraunhofer and

consisted of SiO2 substrates exposing pre-patterned gold

electrodes (30 nm Au with 10 nm ITO as adhesion layer).

AFM images on the gold showed similar roughness as the

above mentioned polycrystalline Au evaporated gold on SiOx.

Macroscopic Au surfaces evaporated on mica and SiOx

expose hundreds of nanometer sized atomically flat terraces

having a root mean square roughness (Rrms) of 0.3 nm. In the

case of commercial Au micro-electrodes similar roughness

values are obtained on images recorded on a surface area of

ca. 200 mm2. Similarly, the KPFM images of all the three Au

surfaces show a similar Rrms close to 10 mV.

In the KP and KPFM experiments the same UV-light

source (365 nm) and same sample-light distance were used.

Kelvin probe techniques

Macroscopic kelvin probe measurements were performed under

ambient conditions using 2 mm diameter gold tip amplifier

(Ambient Kelvin Probe Package from KP Technology Ltd.)

KPFM measurements were performed by employing a

commercial microscope Multimode (Veeco) with Extender

Electronics module. In order to obtain a sufficiently large and

detectable mechanical deflection, we used (k= 2.8 Nm�1) Pt/Ir

coated Si ultra levers (SCM, Veeco) with oscillating frequencies

in the range 60 o o o 90 KHz. AFM and KPFM images are

acquired in the same measurement; a topographic line scan is

first obtained by AFM operating in Tapping Mode and then

that same line is rescanned in Lift Mode with the tip raised to a

lift height of 10 nm.

Both techniques provide a voltage resolution of about 5 mV,

while the lateral resolution lies between a few millimeters and a

few tens of nanometers for KP and KPFM, respectively.

Calibration of the probe was performed against a freshly

cleaved High Oriented Pyrolytic Graphite (HOPG) surface

for both techniques.22 A comprehensive description of the two

techniques can be found in23 and references therein.

Ultraviolet photoelectron spectroscopy (UPS)

Ultraviolet photoelectron spectroscopy (UPS) measurements

of as-received (trans state) and UV-illuminated (60 min in air)

(cis state) AZO-SAM samples were carried out at a base

pressure lower than 1 � 10�9 mbar. The work function was

estimated using the secondary electron cut-off method24 with

an experimental error below 0.05 eV, and with the absoluteScheme 1 Schematic representation of the AZO-SAM.

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work function values calibrated against sputter-cleaned

polycrystalline gold of F = 5.2 eV.

Electroabsorption spectroscopy

The linear dependence of the electroabsorption (EA) signal on

the applied bias was used to determine the nulling voltage (Vnull)

across the light-emitting diodes (LEDs), a detailed description of

the setup and technique can be found in reference.25

The LEDs used for this study were prepared using O2

plasma treated ITO substrates,26 with a thin Au layer (5 nm)

and the AZO-SAM deposited on top. Active layers were made

of poly(9,90-dioctylfluorene-alt-benzothiadiazole), F8BT, and

a 50 nm thick Al cathode was deposited on top via thermal

evaporation (inset Fig. 5) using a shadow mask to obtain eight

pixels for each ITO substrate. AZO-SAMs are exposed to air

prior to spin-coating of the polymer and cathodes evaporation.

Quantum-chemical calculations

Before estimating the work function, the geometrical para-

meters for the corresponding cis- and trans-SAMs on gold

were optimized. In one of our previous studies, it was experi-

mentally established by STM measurements that the trans

isomer exhibits a herringbone packing on the Au (111)

surface18,27 (cell parameters: a = 6.5 � 0.5 A; b = 8.9 � 0.5 A;

a = 841 � 51, Fig. 1a). The unit cell contains two independent

molecules, leading to an area per molecule of 28.8 � 4.1 A2.

The experimental data also indicates that the position of the

anchoring group is not modified during the isomerization.18

Accordingly, we have built a two-dimensional periodic slab

made of five layers of gold with a lattice parameter of 4.1 A

(Au–Au distance of 2.9 A);28 the slabs are separated in the

third direction by a vacuum region of 40 A. We have then

added on top a layer of chemisorbed AZO molecules in

the two forms mimicking the pattern observed in the STM

measurements (in particular, an inversion in the orientation of

the cis form between adjacent layers, see Fig. 1a); we also

chose the same initial position for the sulfur atoms on the gold

surface for the two isomers. Considering the limited size of the

unit cell that we can treat and the gold surface periodicity

constraints, the best matching unit cell that we have adopted

has the parameters: a = 8.7 A; b = 7.7 A, and a = 791; this

represents an area per molecule of 32.9 A2 entering into the

experimental error bars. The geometry of the cells has been

optimized at the density functional theory (DFT) level using

the SIESTA3 package,29 with a full relaxation of the molecule

and top two gold layers. The local density approximation has

been used in conjunction with the Ceperley–Alder exchange–

correlation functional.30 The valence electrons are described

within the LCAO approximation using a DZP basis set29

whereas Trouillier–Martin pseudopotentials are used for the

description of the core electrons. We use a mesh cut-off of

Fig. 1 (a) STM image showing the packing of the cis (left) and trans (right) isomers on the gold surface; Reprinted with permission from ref. 18;

Copyright 2007 by the National Academy of Sciences, USA (b) optimized geometries of the unit cell for the cis (left) and trans (right) isomers.

The arrows depict the opening direction of the cis form to compare with the STM image; (c) optimized geometry of the cis form in the gas phase

(left) and of the two independent molecules in the unit cell on the gold surface (right).

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250 Ry and a k-sampling of (5,5,1) in the Monkhorst–Pack

scheme.31 The parameters of the DZP basis have been adapted

in the same way for all atoms so that the work function of the

clean Au (111) surface matches the experimental value of

5.26 eV.28 This theoretical approach has been validated in

the case of model systems such as alkanethiol derivatives by

showing a good quantitative agreement for the vacuum

level shift and its components when compared to another

theoretical approach often used in literature (based on DFT

calculations using plane waves within the VASP code) and

corresponding experimental data.32

Fig. 1b shows the optimized structure of the cell for the two

isomeric forms. The trans isomers pack into a herringbone

structure with an average tilt angle of 7.21 between the surface

normal and the vector connecting the sulfur atom and the top

hydrogen atom. This small angle is consistent with experi-

mental estimates around 201.27 The cis isomer exhibits a

significant opening of the structure (1171) which is larger than

for the molecule in the gas phase (821) due to steric effects in

the SAM. It also appears in our simulations that the phenyl

ring next to the surface is more tilted in average for the

cis isomer (16.11 and 7.11 for the two independent molecules)

than for the trans (13.11 and 6.81). The anchoring position of

the sulfur atoms on the surface is conserved upon isomeri-

zation, in agreement with the experimental observations. The

average SAM thickness is about 18.4 A for the cis isomer and

22.2 A for the trans isomer; the difference of about 3.8 A is a

little bit smaller than experimental studies of azobenzene

SAMs which give typical values around 5–7 A.19,20,33 We

attribute this difference to the larger thickness of the cis form

in our simulations compared to the experimental thickness

of about 16 A;19 on the other hand, the value obtained for

the trans form agrees with corresponding experimental data

(20.4 A in27 and 23.6 � 1.5 A in19).

The electronic density associated to the equilibrium geometry

can then be exploited to plot the plane-averaged electrostatic

potential along the axis normal to the interface; this provides a

direct estimate of the work function shift by comparing the

converged potential on the bare side of the surface and on the

SAM-covered side of the surface. The total dipole moment

perpendicular to the surface at the origin of the work function

shift34 can be obtained by integrating the electronic density.

The total work function shift (DF) may also be casted down

into a molecular contribution (DVSAM) arising from the dipole

supported by the molecular backbone and a bond dipole (BD)

contribution describing the interfacial electronic reorganization

upon formation of the Au–S bond35:

DF = DVSAM + BD (1)

The magnitude of these two contributions depends whether we

are considering that radicals approach the gold surface to

make a covalent Au–S bond or that chemisorption occurs by

replacing in the neutral molecule the hydrogen atom of the

thiol group by a gold atom.36 DVSAM has been estimated here

in the radical scenario by computing at the LDA level with

spin polarization (LSDA) the electrostatic potential profile

across a sheet of radicals obtained by removing the gold atoms

from the entire system while keeping the same geometry for

the molecular part. The value of BD is then obtained by

subtracting DVSAM from DF.

3. Results and discussion

The work function (F) measured by means of KP is an average

of long acquisition time measurements (15 to 60 min) with a

standard deviation lower than 0.02 eV. The work function for

the bare polycrystalline gold on SiO2 substrate was found

to amount to 5.12 eV. Fig. 2 shows the graph of the absolute

F (FAZO�SAM) vs. time while monitoring the trans-cis photo-

isomerization cycles. The UV illumination was carried out

ex-situ, recording F before (trans) and after irradiation (cis).

The thermal recovery in dark could be monitored in situ over

time. The largest DFcis�trans is observed for the first trans to cis

conversion, with an increase of 70 meV (Ftrans = 5.15 eV and

Fcis = 5.22 eV), and an exponential decay during the back

reaction reaching almost saturation, thus indicating that most

of the cis molecules have been converted to the trans form.

Unfortunately, a switch fatigue is appreciable in the last cycle.

For this reason we consider that the first isomerization quantifies

the WF change.

The work function shift induced by the photoisomerization

of the SAM, and the following reversible switching when

keeping the sample in dark, was also monitored by KPFM

as shown in Fig. 3a. The measured work function for the trans

initial state of the SAM on Au (111) was 5.17� 0.03 eV, which

is really close to the value obtained by macroscopic KP. The

trans AZO-SAM was irradiated until a very small variation of

the SP was measured in time, i.e. very close to the steady state.

KPFM yields a slightly higher increase for the cis-SAM

(DFcis�trans = 116 � 16 meV) compared to the one acquired

with KP. As a control experiment, a bare gold surface was

irradiated with the same UV lamp and at the same distance

than the functionalized gold, and the surface potential did not

reveal any variation (as shown in the bottom side of Fig. 3a).

Similar value of the work function difference between cis and

trans configurations (DFcis�trans = 125 � 15 meV) was

Fig. 2 Plot of the F AZO�SAM on gold over time monitored by Kelvin

Probe (KP) experiments. UV irradiation was performed ex situ,

measuring the F before (trans) and after irradiation (cis). The thermal

back reaction (cis to trans) was monitored in situ in dark.

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measured on samples of AZO-SAM chemisorbed on commercial

gold micro-electrodes (polycrystalline gold on ITO). The DFdeviation observed for the two different types of gold surfaces

made it possible to get a better insight into the switching

mechanism. To this end, we fitted a stretched exponential

function (SEF) to the experimental data.37

F tð Þ ¼ Ftrans þ DFcis�trans � exp � t� t0

t

� �b� �: cis! trans

ð2aÞ

F tð Þ ¼ Fcis � DFcis�trans � exp � t� t0

t

� �b� �: trans! cis

ð2bÞ

where t is the characteristic time and b o 1 represents the

stretch parameter of the two transitions. When b = 1, the

expression (2a,b) become a simple asymptotic exponential and

b is called decay constant. In the case of exponential decay, the

observable quantity decreases at a rate proportional to b.Thus, kinetics described as an exponential law indicate that

they governed by only one mechanism or when all the sites of

the sample are activated at same time. Differently, when more

mechanisms compete or are activated in different times, the

time constant is no longer constant but shows a time-dependence.

Thus, a kinetics well described by a stretched exponential

function remarks that the variation of the observable (SP in

our case) often shows multiple time constants (implying a

‘‘serial activation’’ of the decay process of different sites of the

sample). In general, the ‘‘stretched behaviours’’ are observed

in amorphous systems as glasses, percolations, excitation

transfer from donor to static defects (see37 and references

therein).

The dynamics of trans - cis conversion measured when the

AZO-SAM is chemisorbed on Au evaporated on mica and on

micro-patterned Au electrodes are reported in Fig. 3b. All

t and b values obtained by fitting the measured time-evolution

work function with the expressions (2a,b) are summarized in

Table 1.

A strong asymmetry in the time-evolution trend is observed

for both substrates, both in shape and characteristic time. The

trans - cis conversion is faster than the inverse one and

presents a work function variation roughly described by a

simple exponential trend (b E 1). By STM, it was observed

that cis isomer formed herringbone domains stabilized by

p–p intermolecular interactions. This stability of the cis

AZO-SAM was corroborated by the slow thermal back isomeri-

zation observed by UV–vis absorption measurements.18 The

fact that the trans to cis is a photoexcited process and the

stability of the cis state in the SAM form, support the larger

t found for the back reaction compared to the t of the trans tocis isomerisation. In addition, the difference in the two values

observed when the SAM is supported on Au with different

degree of crystallinity can be explained by the direct influence

of the nature of the Au on the molecule-molecule interactions

and thus, affecting the packing and order of the molecules

forming the SAM. This will lead to more disordered and

smaller domains for polycrystalline gold. This different ordering

will directly influence the directionality and yield of the

isomerisation, i.e. time scale of the process.

The smaller b values achieved for cis - trans conver-

sions can be expressed in terms of a hierarchical model

introduced by Palmer38 for strongly interacting glassy materials.

Fig. 3 (a) Work function variation (J) corresponding to trans and cis SAMs due to the UV illumination recorded in situ by KPFM on

evaporated gold films on mica substrate. Acquired in the same conditions, the work function variation (’) of the bare Au substrate is reported in

the bottom side of the image. (b) Work function variations after UV illumination (sample in dark) acquired for different gold substrates (red J)

evaporated gold on mica (Au (111)) and (&) polycrystalline gold (commercial micro-electrodes). DSP ¼ SPUV � SPdark ! Fcis�Ftrans

�e

Table 1 t and b extracted from fitting a stretched exponential law (eqn (2a)–(b)) to the experimental data (Fig. 3b)

t/min b

cis - trans trans - cis cis - trans trans - cis

Au (111) on mica 40 � 15 20 � 5 0.5 � 0.1 0.8 � 0.1Polycrystalline Au (commercial micro-electrodes) 35 � 10 15 � 5 0.7 � 0.1 0.9 � 0.1

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Using such model, we suggest that the molecular relaxation

occurs in stages, and the constraint imposes by a fast relaxa-

tion process must relax before a subsequent slower relaxation

process sets in. This implies that the time scale of relaxation of

one molecule (or molecular cluster) is subordinated to the

relaxation of the close ones. KPFM images do not show

anisotropies on the surface, this experimental evidence implies

that the cluster’s lateral dimension is smaller than the lateral

resolution of the technique, amounting to 100 nm.

UPS measurements reveal a work function shift upon

isomerisation amounting to 80 � 50 meV (Ftrans = 4.32 eV

and Fcis = 4.40 eV) when the functionalized AZOtrans-Au was

irradiated with UV-light (365 nm) under ambient conditions

for 60 min (Fig. 4). The work function of the bare polycrystalline

gold on SiO2 was measured to be 4.50 eV without being

sputter cleaned prior to measurement. The difference with

the polycrystalline gold surface prepared in UHV through

sputtering (WF = B5.2 eV) used for the calibration is due to

the hydrocarbon absorption at the surface which reduces the

work function down to B4.5 eV for a Bmonolayer coverage

through the well known ‘‘pillow’’ effect.24,39 The function-

alized gold gave as expected for UPS measurements, lower

work function values (few hundreds of meV) than KP, which

is attributed to the fact that the KP method measures an

average of all work function values found in potentially

different (and/or non-homogenous) domains existing under

the probe head. Instead, UPS is most sensitive to the lowest

work function in the probed area, as the secondary electrons

with the lowest kinetic energy are used for the determination.40

The observed FSAM-Au shift caused by the isomerization of the

photoresponse SAM upon irradiation is in agreement with

that observed by KP method. Importantly, in the KP and

KPFM also the water thin film contribution is measured,

which highly contributes to the difference in absolute value

respect to UPS performed in UHV. Indeed, while the absolute

values are different, the relative change in WF (cis-trans

difference) is consistent, and are in good agreement for the

three experimental techniques, suggesting uniform films and

that although in air, when the difference between the two

states is made, the water contribution can be removed.

As a further proof of the work function measured by the

above described techniques when the SAM is embedded in a

device, we performed EA measurements on LEDs incorporating

the transAZO-SAM, with the following structure: ITO/Au(5 nm)/

AZO-SAM/F8BT(100 nm)/Al(50 nm). All measurements were

carried out at room temperature at the EA signal peak, by

irradiating with a monochromated beam (from a Xe-lamp) at

l = 505 nm, while applying a sinusoidal voltage VAC = 0.5 V

at a frequency f = 2 kHz, and with the optical probe entering

the LED through the semitransparent ITO/Au electrode. The

wavelength at which the beam operates will not induce

the trans to cis isomerisation since it only takes place when

the sample is irradiated with UV light. The zero-crossing

voltages were obtained from an average of 10 measurements

over 2 pixels. Fig. 5 shows the EA signal as a function of the

applied DC bias. In the ideal condition of negligible charge

being present within the semiconductor (F8BT in our case) the

EA signal is null when the applied DC voltage compensates

the built-in voltage VBI, which is generated by the equalisation

of the Fermi energy throughout the whole device and, thus, is

equal to the work function difference between the electrodes.41

For the trans AZO-SAM the zero crossing voltage (Vnull) was

determined to be 1.39 � 0.03 V.

Estimation of the work function of the AZO-SAM function-

alized electrode on the basis of this Vnull requires knowledge of

the work function of the Al cathode and of the lowest

unoccupied molecular orbital (LUMO) energy of F8BT, in

case of cathodic pinning of the Fermi energy to the LUMO.

We consider the possibility of pinning unlikely since the

LUMO of F8BT is accepted to lie in the range 3.2–3.5 eV

and the Al work function only decreases below 3.5 eV upon

exposure to significant amount of oxygen during or after

Fig. 4 Ultraviolet Photoelectron Spectroscopy (UPS) secondary electron

cut-off spectra for the AZO-SAM before irradiation (trans) and after UV

irradiation (cis).

Fig. 5 Electroabsorption signal at l = 505 nm and VAC = 0.5 V as

a function of applied DC voltage for the light emitting diode: ITO/Au/

AZO-SAM/F8BT/Al. The zero crossing voltage is 1.39 V.

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evaporation (B300 L or more; the work function of Al in

UHV is B4.1–4.2 eV).42 Previous KP measurements of our

evaporated Al electrodes gave an average Al work function in

our diodes of 3.70–3.80 eV. On that basis, we can calculate the

work function of the trans AZO-SAM functionalized anode

using eqn (3):

Vnull = (Fanode–Fcathode)/e (3)

Where Fanode and Fcathode are the work function of the anode

(AZO-SAM) and the cathode (Al) respectively and e is the

electronic charge. Using this equation, we extracted a value for

the SAM-functionalized anode of 5.06–5.22 eV, in agreement

with the other determinations.43

The above experimental results prompted us to perform

theoretical calculations to help in elucidating the origin of the

small F shift encountered between the two isomeric configu-

rations. Fig. 6 displays the plane averaged potential profiles

associated to the optimized structures of the unit cell for the

cis and trans isomers. The work function of gold modified with

the cis and trans isomers are 4.14 eV and 4.04 eV, respectively,

which represents a decrease by �1.12 eV and �1.22 eV with

respect to the clean metal surface. The higher work function

observed for the cis form and the difference between the two

forms (100 meV) are entirely consistent with the experimental

data. The large work function shifts are comparable to other

theoretical values reported in the literature35,44 as well as with

experimental UPS data in UHV conditions.45 However, these

shifts are much higher than those measured in the present

study. The comparison is most likely hampered by the actual

nature of the gold sample and by the presence of contaminants

in the KP, KPFM, and EA measurements. In order to

investigate the bond dipole versus molecular contributions to

this shift, we have also computed the potential profile of the

free SAM in Fig. 6. For the cis form, the molecular contri-

bution is �1.79 eV; leading to a bond dipole of +0.67eV. A

very similar value of �1.72 eV is obtained for the trans form,

which translates into a bond dipole about +0.50 eV. According

to our calculations, the shift of the work function is dominated

by the electronic reorganization upon formation of the Au–S

bond between the gold surface and the radical rather than by

the difference of dipole moment between the two isomeric

forms. In order to further shed light on the difference between

the bond dipole of the two forms, we have plotted in Fig. 7 the

evolution of the generated bond dipole when moving from

the left side to right side of the interface for both isomers.

Fig. 6 Plane averaged potentials of the cis (a) and trans (b) SAMs on gold surface. The zero energy is set at the Fermi level so that the work

function is directly readable. We display curves for the bare Au (111) gold surface, the gold surface covered by the SAM, and the free SAM system.

In the left part, we can identify the five gold layers and the SAM contribution moving away from the surface. The two arrows on the right side

points to the work function shift of the entire system (light grey) and of the contribution arising from the free SAM (black). The difference between

these shifts corresponds to the bond dipole.

Fig. 7 Graph illustrating the build up of the bond dipoles as an

electron crosses the metal/SAM interface from the left to the right side.

The dashed lines correspond to the cis and trans isomers. Reference

vertical lines corresponding to the gold surface and different positions

along the molecular backbone have been added. These positions are

shown in the superimposed trans geometry. The bond dipole first

decreases in the vicinity of the Au-S bond. This is attributed to the

repulsion of the gold electronic cloud at the surface by the atoms of the

deposited molecule (i.e., pillow effect). Then, a delocalized polariza-

tion of the electronic density extending from the sulfur to the attached

biphenyl unit induces a growing barrier leading to the final bond

dipole. The continuous line corresponds to the difference between the

bond dipoles of the cis and trans forms. This shows that the origin of

the bond dipole difference is not only concentrated around the Au–S

bond but also takes place over a spatial region encompassing the sulfur

and attached biphenyl unit.

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The difference between cis and trans isomers is also plotted.

This difference is zero on the left side and amounts to a total

value of 177 meV on the right side. This graph nicely illustrates

that the bond dipole does not build up exclusively in the

vicinity of the Au–S bond since the dipole is significantly

growing from the sulfur atom to the end of the molecule.

The bond dipole difference between cis and trans is only about

12 meV near the sulfur atom and keeps growing until the

nitrogen atoms are reached (up to 193 meV). The difference in

electronic reorganization thus takes place over a spatial region

encompassing the sulfur and attached biphenyl unit.

The absence of contribution from the dipole moment of the

molecular backbone is intuitively unexpected in view of the

large difference between the dipole moment of the isolated

radical molecule (2.81 D for the cis and 5.30 D for the trans, as

calculated with SIESTA in a large unit cell to avoid inter-

molecular interactions). This behavior is mostly driven by

steric hindrance that increases the bending of the cis isomers

by about 351 in the SAM in contrast to the gas-phase structure

(see Fig. 1c). This yields a dipole moment evolving from

2.81 D in the gas phase to 5.80 D and 6.31D for isolated

radical molecules adopting the structure of the inequivalent

units in the SAM; in contrast, the dipole slightly changes from

5.30 D to 5.44 D and 5.18 D for trans, respectively. Depolari-

zation effects next come into play to modify these individual

dipoles when accounting for the intermolecular interactions

within the SAM.46 The total dipole of the unit cell based on

the two independent radical molecules is 11.28 D for cis and

9.62 D for trans, thus showing a depolarization of about 8%.

The use of periodic conditions further reduce the dipole of the

cell by up to 70%, leading finally to a similar dipole value of

about 3–3.1D (3.1 D for cis and 2.95 D for trans) per cell that

rationalizes the lack of contribution of the molecular part to

the work function shift between the two forms.

4. Conclusions

The work function variation (DF) between the two isomeric

states, trans and cis, of a tightly packed AZO-based SAM on

gold has been studied. The work function shift upon photo-

isomerization of the SAM has been found to be rather small

(70–125 meV). This result is fully supported by a comparative

study based on measurements performed using different

techniques. In spite of the small work function tuning by this

SAM, the comparison between the techniques allows us to get

a deeper knowledge on our system. KP and KPFM confirmed

that the switching capacity of the work function is extended

from the nano to the macroscale. Moreover, interestingly, a

very similar work function has been obtained by EA, showing

that the tuning of the work function caused by the chemisorp-

tion of the SAM is not altered in a device structure such as a

LED. Very similar values of the DF between cis and trans were

confirmed by UPS. The quantum-chemical calculations

further provide work function shifts in quantitative agreement

with the experimental data. We attribute the deviations in the

absolute work function values obtained by KP, KPFM, EA

and UPS with respect to the calculations to the different

environmental conditions. Interestingly, the large difference

between the calculated dipoles of the isolated trans and

cis forms in the gas phase are not recovered in the SAM,

due to the interplay of steric and depolarization effects in the

solid state. The modulation of the work function is found to be

dominated by the electronic reorganization of the radical when

the Au-S bond is formed.

Acknowledgements

This work was supported by the EC Marie-Curie

IEF-OPTSUFET (PIEF-GA-2009-235967), ITN-SUPERIOR

(PITN-GA-2009-238177) and RTN-THREADMILL (MRTN-

CT-2006-036040), the EC FP7 ONE-P large-scale project no.

212311, the NanoSci-E+ project SENSORS and the Inter-

national Center for Frontier Research in Chemistry (FRC,

Strasbourg). The work in Mons is further supported by the

Belgian National Fund for Scientific Research (FNRS). C.V.D.

and J.C. are FNRS research fellows. Financial supports by the

Swiss Nationals Science Foundation (SNF) and the Swiss

Nanoscience Institute (SNI) are also gratefully acknowledged.

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Photoinduced work function changes by isomerization of a densely packed azobenzene-based SAM on Au:...

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