Self-assembled monolayers of a novel diacetylene on gold

O. Cavalleri a, M. Prato a,*, A. Chincarini b, R. Rolandi a,M. Canepa a, A. Gliozzi a, M. Alloisio c, L. Lavagnino c,

C. Cuniberti c, C. Dell’Erba c, G. Dellepiane c

a INFM and Dipartimento di Fisica, Universita di Genova, Via Dodecaneso 33, 16146 Genova, Italyb INFN, Unita di Genova, Via Dodecaneso 33, 16146 Genova, Italy

c INSTM, INFM and Dipartimento di Chimica e Chimica Industriale, Universita di Genova,

Via Dodecaneso 31, 16146 Genova, Italy

Available online 3 May 2005

www.elsevier.com/locate/apsusc

Applied Surface Science 246 (2005) 403–408

Abstract

We report on the preparation and characterization of self-assembled monolayers (SAMs) of a novel diacetylene monomer,

the 14-N-carbazolyltetradeca-10,12-diyndisulfide. The SAM/gold interface was investigated by means of spectroscopic

ellipsometry (SE), X-ray photoelectron spectroscopy (XPS) and scanning tunnelling microscopy (STM). SE data indicate a

layer thickness compatible with the formation of a monomolecular layer. The occurrence of molecular chemisorption is

confirmed by XPS measurements which indicate the formation of a thiolate species. This result is confirmed by STM imaging

which shows the formation of small pits, one gold layer deep, a typical feature of self-assembled organosulfur monolayers on

gold.

# 2005 Elsevier B.V. All rights reserved.

PACS: 81.16.Dn; 33.60.Fy; 68.37.Ef; 78.6.Qn

Keywords: Self-assembled monolayers (SAMs); Diacetylene; Gold; Spectroscopic ellipsometry (SE); X-ray photoelectron spectroscopy

(XPS); Scanning tunnelling microscopy (STM)

1. Introduction

In the last years, nanoscale design and fabrication

of monolayer assemblies have achieved increasing

* Corresponding author. Tel.: +39 010 3536287;

fax: +39 010 311066.

E-mail address: prato@fisica.unige.it (M. Prato).

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.apsusc.2004.11.045

importance in wide-ranging applications including

optoelectronics and sensors. In particular, sponta-

neously organized monolayers formed from thiol or

disulfide compounds on gold surfaces have been

extensively studied [1]. Recently, the incorporation of

photopolymerizable diacetylene groups within the

monolayer alkyl chains has led to new polymeric

platforms which present relevant durability and

.

O. Cavalleri et al. / Applied Surface Science 246 (2005) 403–408404

robustness as well as unique optical and electronic

properties, connected to the highly conjugated

polydiacetylene backbone [2–4].

A class of particularly attractive polydiacetylenes

is represented by polycarbazolyldiacetylenes, owing

to their outstanding optoelectronic properties related

to the electron-donor character of the carbazolyl

substituents [5], but so far these polymers have not

been utilized in monolayers fabrication. In order to

investigate the influence of the carbazole rings both on

the monolayer properties and on the polymerization

process we have synthetized a novel carbazolyldia-

cetylene, the 14-N-carbazolyltetradeca-10,12-diyndi-

sulfide, hereafter named CDS9, properly designed to

ensure a stable anchoring to the metal substrate

through a S–Au bond. The CDS9/gold interface has

been investigated by means of spectroscopic ellipso-

metry (SE), X-ray photoelectron spectroscopy (XPS)

and scanning tunnelling microscopy (STM).

2. Experimental

CDS9 was synthesized by oxidative coupling

between the acetylenic moiety carrying the carbazolyl

group and the acetylenic moiety carrying the

nonylthiol substituent, through a reactions sequence

set up in our laboratory [6]. As formed, the thiol

derivative spontaneously oxidizes giving the disulfide

of Fig. 1.

SAMs of CDS9 for XPS and SE measurements

were deposited onto commercial gold films evapo-

rated onto chromium-primed glass substrates (Arran-

deeTM, Germany). SAMs for STM measurements

Fig. 1. Chemical formula of 14-N-carbazolyl

were deposited onto gold films epitaxially grown on

mica as reported in details elsewhere [7]. Arrandee

samples with more homogeneous optical properties

were used for SE measurements, while gold films on

mica characterized by larger (1 1 1) terraces were used

for STM measurements. In all cases, before use, the

gold films were flame annealed in a butane flame to

glowing red and quenched in ethanol. For XPS and

STM measurements, the gold substrates were trans-

ferred into a 1 mM CDS9 solution in chloroform

immediately after flame annealing. Instead, for SE

analysis, the substrates were immersed in the

monomer solution immediately after a SE quality

check of the substrate. This check involved a limited

exposure to atmospheric contamination. The values of

the parameter D at the incidence angle of 708 and at l

�632 nm obtained on about 20 bare substrates showed

a narrow distribution around �109.58. We measured a

typical decrease of D of �0.038 after 300 s. The

dielectric function derived from SE on flame annealed

gold gave results in excellent agreement with

literature [8,9].

The gold substrates were kept in the CDS9 solution

for 40–48 h at room temperature. After extraction they

were thoroughly rinsed with chloroform and dried

under a nitrogen stream.

XPS analysis was carried out with a PHI ESCA

5600 MultiTechnique electron spectrometer. The

system consists of an X-ray Al-monocromatised

source (hn = 1486.6 eV) and a spherical capacitor

electron energy analyser (SCA), used in the fixed

analyser transmission (FAT) mode at a pass energy of

5.85 eV. In the standard configuration the analyser

axis formed an angle (take-off angle) of 258 with the

-tetradeca-10,12-dyin disulfide (CDS9).

O. Cavalleri et al. / Applied Surface Science 246 (2005) 403–408 405

sample surface. The binding energy scale was

referenced by setting the Au 4f7/2 to 84.0 eV.

SE measurements were performed on a rotating

compensator spectroscopic ellipsometer (M-2000, J.A.

Woollam Co. Inc.). The instrument, fully tested in a

recent experiment on LB cadmium stearate thin films

[10], allows simultaneous measurements at 225

different wavelengths in the range 245–725 nm.

Spectra have been collected at several angles of

incidence, in the 55–708 range. Samples were

characterized by SE after flame annealing, immediately

prior to self-assembly, and after the layer deposition.

STM measurements were performed in air with a

Nanoscope II (Digital Instruments) equipped with a

STM A head. Tips were mechanically cut from

0.25 mm in diameter Pt/Ir wire. Typical tunneling

parameters were a few tenths of nA as tunneling

current and a few hundreds of mV as bias.

3. Results and discussion

Representative SE spectra obtained at 658 of

incidence on CDS9-covered samples are shown in

Fig. 2 together with spectra collected on correspond-

ing bare substrates. The data taken on a single sample

were characterized by an extremely high repeatability.

A limited variability was observed over different

samples. The data are therefore presented after

averaging over 15 samples.

The SE results are conventionally presented report-

ing the value of the D and C ellipsometric angles as a

Fig. 2. Representative SE spectra for both bare and CDS9-covered

gold. We report also simulated spectra for CDS9-covered samples,

obtained with the Lorentz model. Note that, for CDS9-covered

samples, experimental data and generated curves are almost super-

imposed.

function of the wavelength [11]. D and C are defined

through the relation r = Rp/Rs = tan C exp(iD), where

Rp and Rs are the complex reflection coefficients for s-

and p-polarized light [11], respectively. The parameter

D carries most of the information on the overlayer

thickness, while C is mainly related to the near-surface

gold electronic structure [12] and is affected by the

organic overlayer to a lesser extent. Fig. 2 indicates an

evident decrease of D after the CDS9 layer deposition.

For a quantitative data analysis we simulated the spectra

by using a model based on a stack of laminar structures

with sharp interfaces. Since photoabsorption measure-

ments on CDS9 in solution (Fig. 3a) exhibit distinct

absorption bands at about 260 and 290 nm, due to

carbazole and diacetylene groups, and at about 320 and

350 nm, due to the carbazolylic substituents [13], we

attempted a model including absorption features

through the introduction of Lorentz oscillators [11].

The wavelength dependences of the optical functions,

refractive index n and extinction coefficient k, obtained

with the same model are shown in Fig. 3b. The best fit

(mean squared error [14] = 4.24) was obtained with five

oscillators at energies E1 = 3.67 � 0.05 eV, E2 = 3.83

� 0.05 eV, E3 = 4.25 � 0.05 eV, E4 = 4.75 � 0.05 eV,

Fig. 3. (a) Absorbance spectrum obtained for 1 mM CDS9 solution

in chloroform. (b) Optical functions n and k obtained for CDS9 SAM

on gold through SE data analysis with Lorentz model.

O. Cavalleri et al. / Applied Surface Science 246 (2005) 403–408406

E5 = 5.36 � 0.05 eV, corresponding respectively to

335, 325, 290, 260 and 230 nm, in good agreement

with photoabsorption data. The presence of a broad

absorption band at E6 = 1.90 � 0.10 eV (645 nm) was

also found. The interpretation of this absorption feature

is less straightforward; work is in progress to understand

whether it could be related to a partial CDS9

photopolymerization. With this model, we obtain a

layer thickness d = 1.5 � 0.2 nm that, by comparison

withtheestimatedlengthofthemolecule(about2.2 nm),

can be interpreted as the formation of a quite dense

molecular layer of possibly tilted molecules, with a

coverage in the 0.6–0.8 range.

The occurrence of CDS9 deposition was confirmed

by XPS measurements. Fig. 4a and b show the S 2p

and C 1s core level regions of a CDS9 layer on gold.

The S 2p spectrum is quite noisy due to the relatively

low signal intensity. Notwithstanding this we tried to

Fig. 4. S 2p (a) and C 1s (b) core level regions of a CDS9 sample.

The thin continuous lines are the experimental data. The thick

continuous lines represent the best fits to the experimental data.

The dotted and dashed lines are the resolved components contribut-

ing to the signals.

fit S 2p doublets to the data. Two doublets were

necessary to reasonably reproduce the experimental

data. The spin-orbit energy splitting and the corre-

sponding branching ratio were set to the values of

1.18 eV and 2:1, respectively, as predicted by spectro-

scopy basic theory. Conveniently identifying the

doublets with the energy of their 2p3/2 component,

the lower BE doublet (SA species) occurs at 162 eV

and the higher BE doublet (SB species) is found at

163.2 eV. Each doublet has been fitted by two Pseudo-

Voigt functions (GL), the sum of a gaussian and a

lorenzian function with adjustable weights, with the

same full width at half maximum (fwhm) which

resulted to be 1.2 eV. A sulfur signal at 162 eV has

been reported in a numerous series of papers on

organosulfur SAMs on gold and has been assigned to a

thiolate species due to the molecule chemisorption

(i.e. the formation of S–Au bond) [7,15–19]. The

assignment of the SB species is less straightforward

and needs a more detailed discussion. S 2p signals in

the 163.2–163.6 BE range have been observed in

pristine samples and assigned to unbound molecules

on poorly rinsed long chain alkanethiol SAMs [17] or

to second layer molecules hydrogen-bonded to the first

layer molecules in short chain SAMs [20,21]. A

second possible assignment of the SB species takes

into account the occurrence of X-ray-induced mole-

cular damage. In fact, a S 2p component around

163.2–163.4 eV was found on long chain alkanethiol

SAMs damaged by X-ray radiation. This sulfur

species was attributed either to disulfide formation

[22,23] or, more recently, to the incorporation of

sulfide entities into the alkylic chains [24]. In the

present study, due to the low signal intensity, relatively

long acquisition times (about 6 h) were necessary in

order to collect the data. The long beam exposure can

reasonably account for the presence of a beam-

induced S 2p signal around 163 eV. From the S 2p data

we can therefore infer that CDS9 chemisorbs on gold

through –S–S– bond cleavage and subsequent thiolate

formation, but we cannot make definite statements on

the percentage of chemisorbed molecules in pristine

CDS9 SAMs. To obtain an indirect evaluation of the

CDS9 layer coverage we performed comparative

measurements on octadecanethiol SAMs, a well

characterized system known to form compact mono-

layers on gold [25]. From the S 2p/Au 4f intensity ratio

calculated for CDS9 and octadecanethiol SAMs,

O. Cavalleri et al. / Applied Surface Science 246 (2005) 403–408 407

Fig. 5. STM image of the CDS9/gold interface. The small pits

indicate the occurrence of a layer deposition. Image size

(320 � 145) nm2. Tunneling current 0.3 nA, bias 400 mV.

under the assumption of a similar sulfur signal

screening due to the organic layer in the two systems,

we could infer the CDS9 coverage to be 60–70% of that

from the octadecanethiol. This result correlates well

with the above reported SE data analysis. CDS9

deposition is confirmed by the analysis of the C 1s

spectral region (Fig. 4b). Two GL functions have been

fitted to the C 1s lineshape. The signal analysis indicates

the presence of a main CA state at 284.5 eVand a lower

intensity CB state at 285.3 eV. Based on literature, the

CA state can be assigned to the methylene carbons of the

hydrocarbon chain while the CB state can be attributed

to –C C– and –CBBC– carbons [26]. It must be noted,

however, that, as shown by check measurements on bare

gold substrates, adventitious carbon both present in the

gold substrates and due to atmospheric contamination,

contributes to the CA signal intensity, thus explaining its

high intensity.

Fig. 5 shows an STM image of a CDS9-covered

Au(1 1 1) sample. A monoatomically flat terrace

decorated with pits, a few nanometers in size and one

gold monoatomic layer in depth, can be observed.

These depressions, absent on bare gold, are due to the

reorganization of the gold topmost layer occurring

during the monomer chemisorption. They are covered

by the organic layer as well as the surrounding terrace

[27,28]. The presence of these pits, which is well

known for alkanethiol SAMs on gold, confirms the

formation of the CDS9 SAM [27,29].

4. Conclusions

Altogether, our data indicate the formation of an at

least partially chemisorbed CDS9 SAMs on gold with

a molecular coverage in the range of 0.6–0.7

compared to a full coverage alkanethiol SAM with

unsubstituted alkylic chains. This result can be

reasonably explained by taking into account that the

steric hindrance of the carbazole groups can induce

some tilt/disorder in the molecular packing, which in

turn, could affect the photopolymerization process of

the monolayer. Work is in progress to investigate the

polymerization of the CDS9 layers upon UV exposure

and its dependence on the substrate properties and film

homogeneity. Preliminary data obtained by Raman

spectroscopy do not give evidence of polymer

formation on these Au(1 1 1) terraces, presumably

because no resonance condition is present at the

available wavelengths. However, surface enhanced

Raman spectroscopy (SERS) experiments carried out

on CDS9 monolayers on rough surfaces gave a

spectrum whose peak positions and intensities are

consistent with a polymeric structure formed by

monomeric chains nearly normal arranged to the

substrate.

Acknowledgments

The authors thank G. Gonella for assistance and F.

Gatti for the evaporation chamber use. This project has

been partly financed by Genoa University and INFM.

Funding from the FIRB project ‘‘Molecular Nano-

Devices’’ and from the FIRB 2001-2003 ‘‘Molecules

and organic/inorganic hybrid structures for photo-

nics’’ are also acknowledged.

References

[1] A. Ulman, An Introduction to Ultrathin Organic Films, Aca-

demic Press, New York, 1991, and references therein.

[2] D.N. Batchelder, S.D. Evans, T.L. Freeman, L. Haussling, H.

Ringsdorf, H. Wolf, J. Am. Chem. Soc. 116 (1994) 1050.

[3] T. Kim, R.M. Crooks, Tetrahedron Lett. 35 (1994) 9501.

[4] M.D. Mowery, A.C. Smith, C.E. Evans, Langmuir 16 (2000)

5998, and references therein.

[5] M. Alloisio, S. Sottini, P. Riello, E. Giorgetti, G. Margheri, C.

Cuniberti, G. Dellepiane, Surf. Sci. 554 (2004) 68, and

references therein.

[6] C. Dell’Erba, private communication.

[7] O. Cavalleri, G. Gonella, S. Terreni, M. Vignolo, L. Floreano,

A. Morgante, M. Canepa, R. Rolandi, Phys. Chem. Chem.

Phys. 6 (2004) 4042.

O. Cavalleri et al. / Applied Surface Science 246 (2005) 403–408408

[8] D.E. Aspnes, E. Kinsbron, D.D. Bacon, Phys. Rev. B 21 (1980)

3290.

[9] E.S. Kooij, H. Wormeester, E.A.M. Brouwer, E. van Vroon-

hoven, A. van Silfhout, B. Poelsema, Langmuir 18 (2002)

4401.

[10] G. Gonella, O. Cavalleri, I. Emilianov, L. Mattera, M. Canepa,

R. Rolandi, Mater. Sci. Eng. C 22 (2002) 359.

[11] R.M.A. Azzam, N.M. Bashara, Ellipsometry and Polarized

Light, North Holland, Amsterdam, 1977.

[12] J. Shi, B. Hong, A.N. Parikh, R.W. Collins, D.L. Allara, Chem.

Phys. Lett. 246 (1995) 90.

[13] J.W. Robertson, Handbook of Spectroscopy, vol. 2, CRC

Press, Cleveland, 1974.

[14] J.A. Woollam, B. Johs, C.M. Herzinger, J. Hilfiker, R. Syno-

wicki, C.L. Bungay, Crit. Rev. Opt. Sc. Tech. CR72 (1999) 1.

[15] K. Heister, M. Zharnikov, M. Grunze, L.S.O. Johansson, J.

Phys. Chem. B 105 (2001) 4058.

[16] T. Ishida, N. Choi, W. Mizutani, H. Tokumoto, I. Kojima, H.

Azehara, H. Hokari, U. Akiba, M. Fujihira, Langmuir 15

(1999) 6799.

[17] D.G. Castner, K. Hinds, D.W. Grainger, Langmuir 12 (1996)

5083.

[18] Y.W. Yang, L.J. Fan, Langmuir 18 (2002) 1157.

[19] M. Zharnikov, M. Grunze, J. Phys.: Condens. Matter 13 (2001)

11333.

[20] A. Abdureyim, S. Kera, H. Setoyama, K.K. Okudaira, R.

Suzuki, S. Masuda, N. Ueno, Y. Harada, Appl. Surf. Sci.

144–145 (1999) 430.

[21] G. Gonella, O. Cavalleri, S. Terreni, D. Cvetko, L. Floreano, A.

Morgante, M. Canepa, R. Rolandi, Surf. Sci. 566–568 (2004)

638.

[22] M. Wirde, U. Gelius, T. Dunbar, D.L. Allara, Nucl. Instr.

Methods Phys. Res. B 31 (1997) 245.

[23] D. Zerulla, T. Chasse, Langmuir 15 (1999) 5285.

[24] K. Heister, M. Zharnikov, M. Grunze, L.S.O. Johansson, A.

Ulman, Langmuir 17 (2001) 8.

[25] F. Schreiber, Prog. Surf. Sci. 65 (2000) 151.

[26] NIST X-ray Photoelectron Spectroscopy Database

[27] J.A.M. Sondag-Huethorst, C. Schonenberger, L.G.J. Fokkink,

J. Phys. Chem. 98 (1994) 6826.

[28] O. Cavalleri, A. Hirstein, K. Kern, Surf. Sci. 340 (1995)

L960.

[29] K. Edinger, A. Golzhauer, K. Demota, Ch. Woll, M. Grunze,

Langmuir 9 (1993) 4.