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: [email protected] (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.
Top Related