Self-assembled monolayers of alkaneselenolates on (111) gold and silver

Article abstract of DOI:10.1021/jp045052f

Self-assembled monolayers (SAMs) formed from didodecyl diselenide (C12SeSeC12) and didodecyl selenide (C12SeC12) on (111) Au and Ag substrates were extensively characterized by several complementary techniques. C12SeSeC12 was found to form contamination-free, densely packed, and well-ordered C12Se SAMs on both substrates, whereas the adsorption of C12SeC12 occurred only on Au and resulted in the formation of a SAM-like C12SeC12 film with a low packing density and a conformational disorder. The properties of the C12Se SAMs were compared with those of dodecanethiolate (C12S) SAMs. The packing density, orientational order, and molecular inclination in C12Se/Au and C12S/Au were found to be very similar. In contrast, C12Se/Ag exhibited significantly lower packing density, a lower degree of the conformational and orientational order, and a larger molecular inclination than C12S/Ag. The results suggest a sp ³ bonding configuration for the selenium atom on Au and Ag and indicate a larger corrugation of the headgroup-substrate binding energy surface in C12Se/Ag than in C12S/Ag.

Full text of DOI:10.1021/jp045052f

3898
J. Phys. Chem. B 2005, 109, 3898-3906
Self-Assembled Monolayers of Alkaneselenolates on (111) Gold and Silver
A. Shaporenko,† A. Ulman,‡,| A. Terfort,§ and M. Zharnikov*,†
Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, Im Neuenheimer Feld 253,
69120 Heidelberg, Germany, Department of Chemical and Biological Sciences and Engineering,
Polytechnic UniVersity, Six Metrotech Center, Brooklyn, New York, and Anorganische und Angewandte
Chemie, UniVersita¨t Hamburg, 20146 Hamburg, Germany
ReceiVed: October 29, 2004; In Final Form: December 27, 2004
Self-assembled monolayers (SAMs) formed from didodecyl diselenide (C12SeSeC12) and didodecyl selenide
(C12SeC12) on (111) Au and Ag substrates were extensively characterized by several complementary
techniques. C12SeSeC12 was found to form contamination-free, densely packed, and well-ordered C12Se
SAMs on both substrates, whereas the adsorption of C12SeC12 occurred only on Au and resulted in the
formation of a SAM-like C12SeC12 film with a low packing density and a conformational disorder. The
properties of the C12Se SAMs were compared with those of dodecanethiolate (C12S) SAMs. The packing
density, orientational order, and molecular inclination in C12Se/Au and C12S/Au were found to be very
similar. In contrast, C12Se/Ag exhibited significantly lower packing density, a lower degree of the
conformational and orientational order, and a larger molecular inclination than C12S/Ag. The results suggest
a sp3 bonding configuration for the selenium atom on Au and Ag and indicate a larger corrugation of the
headgroup-substrate binding energy surface in C12Se/Ag than in C12S/Ag.
1. Introduction
binding energy surface, the bonding configuration of the
chemisorbed sulfur headgroup is of importance, being sp3 and
sp on Au(111) and Ag(111), respectively.14-20 In general, the
headgroup-substrate bond is the deciding factor in the balance
of structural forces in aliphatic SAMs on metal substrates. By
varying the character of this bond one can probably change the
properties of these films to a noticeable extent and create novel
systems, which can be of interest for applications. A straight-
forward way to do so is to use another headgroup than thiol,
e.g. selenol.
Frontier areas of modern technology rely on the possibility
to tailor surface properties such as wetting, adhesion, lubrication,
corrosion, and biocompatibility both on microscopic and
macroscopic scales. To a definite extent, this possibility is
provided by self-assembled monolayers (SAMs), which are 2D
polycrystalline films of semirigid molecules that are chemically
anchored to a suitable substrate.1-5 So far, the focus SAM
systems were films of n- alkanethiolates (AT) on (111) gold
and silver substrates.3,4 These films have been used and are still
being used both as an archetypal system to investigate the
properties and behavior of SAMs as well as an efficient platform
for several applications.3-5 It is well-known that the properties
of AT/Au and AT/Ag are rather different, what is mainly related
to the different corrugations of the sulfur-gold and sulfur-
silver binding energy surfaces.1-3,6 In case of Ag, this corruga-
tion is relatevely weak, which allows a dense packing of AT
chains in a (x7×x7)R19.1° incommensurate structure with a
lattice constant of ≈4.67-4.77 Å, close to the next neighbor
distance in bulk paraffin or polyethylene crystals (≈4.4 Å).2,4,7-9
In the case of Au, the corrugation of the sulfur-metal potential
is assumed to be more pronounced,6 which results in the
formation of a nearly commensurate c(4 × 2)-modulated
(x3×x3)R30° lattice, with a larger intermolecular spacing of
≈5.0 Å.2,4,7,10,11 In this lattice, minimization of energy is
achieved by tilting of the AT molecules by ≈30° (the measured
tilt angle depends on the experimental technique used12),10,13,14
which is noticeably larger than the tilt in the AT/Ag systems
(10-12°).8,14 Along with the corrugation of the sulfur-substrate
The chemical properties of sulfur and selenium are quite
similar. Both elements have the same valence electron configu-
rations and are neighbors in the VIB column of the periodic
table. A noticeable difference between them is, however, the
van der Waals radius, which is 1.85 Å in the case of S and 2.0
Å in the case of Se. Despite the similarities, there has been,
however, relatively little work done on selenol-derived SAMs,21-30
including only few publications on alkaneselenols, and only on
gold substrate.21,26,27,29 Samant et al.21 studied a SAM formed
from docosaneselenolate (C22Se) SAM on Au(111) by surface
X-ray diffraction. They found an incommensurate structure with
an oblique unit cell, stated the average tilt angle of the alkyl
chains as 15°, and postulated that the selenium-gold bond is
weaker than the sulfur-gold one. Nakano et al.26 used infrared
spectroscopy, X-ray photoelectron spectroscopy (XPS), elec-
trochemistry measurements, and a quartz-crystal microbalance
to characterize decaneselenolate (C10Se) SAM on Au(111).
They have concluded that decaneselenol adsorbs on Au surfaces
to form stable monolayers as alkanethiols, with a similar packing
density. Yee et al.27 studied the functionalization of gold
nanoparticles by didodecyl diselelenide and found that AT
chains pack more orderly on gold than their selenolate ana-
logues. The binding energy of the Au-Se bond was found to
be slightly higher than that of the Au-S bond, which is in a
contradiction with the above statement of Samant et al.21 Very
* Corresponding
author.
E-mail:
Michael.Zharnikov@
urz.uni-heidelberg.de.
† Universita¨t Heidelberg.
‡ Polytechnic University.
§ Universita¨t Hamburg.
| Present address: Department of Chemistry, Bar-Ilan University,
Ramat Gan 52900, Israel 2.
10.1021/jp045052f CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/09/2005

Self-Assembled Monolayers of Alkaneselenolates
J. Phys. Chem. B, Vol. 109, No. 9, 2005 3899
recently, Monnell et al.29 characterized SAMs formed from
didecyl and didodecyl diselelenides on Au(111) by scanning
tunneling microscopy and found that the densely packed phase
of the alkyl selenides is distorted hexagonal and incommensurate
to the underlying gold substrates. The respective lattice constants
were found to be very close to the previously reported X-ray
diffraction values.21
ethanol at room temperature for 24 h. The same cleaning and
sample storage procedure as for the selenium-derived films was
used.
The fabricated films were characterized by synchrotron-based
HRXPS, XPS (data not shown, since HRXPS is superior), angle-
resolved NEXAFS spectroscopy, Fourier transform IRRAS,
ellipsometry, and contact angle measurements. All experiments
were performed at room temperature. The HRXPS, XPS, and
NEXAFS measurements were carried out under UHV conditions
at a base pressure better than 1.5 × 10-9 mbar. The spectra
acquisition time was selected in such a way that no noticeable
damage by the primary X-rays occurred during the measure-
ments.35-38
HRXPS experiments were performed at the bending magnet
beamline D1011 at the MAX II storage ring of the MAX-lab
synchrotron radiation facility in Lund, Sweden. For these
measurements, only the films on mica were used. The HRXPS
spectra were collected in normal emission geometry at photon
energies of 150 eV for the Se 3d range, 350 and 580 eV for the
C 1s range, and 350 eV for the S 2p region. In addition, Au 4f
and Ag 3d spectra were acquired and the O 1s range was
monitored. The binding energy (BE) scale of every spectrum
was individually calibrated using the Au 4f7/2 emission line of
AT-covered Au substrate at 83.95 eV. The latter value is given
by the latest ISO standard.39 It is very close to a value of 83.93
eV, which has been obtained by us for Au 4f7/2 using a separate
calibration to the Fermi edge of a clean Pt-foil.34 The energy
resolution was better than 100 meV, which is noticeably smaller
than the full widths at half-maximum (fwhm) of the photo-
emission peaks addressed in this study.
HRXPS spectra were fitted by symmetric Voigt functions and
either a Shirley-type or linear background. To fit the Se 3d5/2,3/2
and S 2p3/2,1/2 doublets we used a pair of such peaks with the
same fwhms, the branching ratios of 3:2 (3d5/2/3d3/2) and 2:1
(2p3/2/2p1/2), and spin-orbit splittings (verified by fit) of ≈0.86
eV (3d5/2/3d3/2) and ≈1.18 eV (2p3/2/2p1/2).40 The fits were
carried out self-consistently: the same peak parameters were
used for identical spectral regions. The accuracy of the resulting
BE/fwhm values is 0.02-0.03 eV.
NEXAFS spectroscopy measurements were performed at the
HE-SGM beamline of the synchrotron storage ring BESSY II
in Berlin, Germany. The spectra acquisition was carried out at
the C K-edge in the partial electron yield mode with a retarding
voltage of -150 V. Linear polarized synchrotron light with a
polarization factor of ≈82% was used. The energy resolution
was ≈0.40 eV. The incidence angle of the light was varied from
90° (E-vector in surface plane) to 20° (E-vector near surface
normal) in steps of 10-20° to monitor the orientational order
in the SAMs.
In the present work we make a first attempt to compare the
properties of alkaneselenolate SAMs on (111) Au and Ag
substrates and perform an extended characterization of SAMs
formed from didodecyl diselelenide (C12SeSeC12), didodecyl
selelenide (C12SeC12), and dihexadecyl diselenide (C16SeSeC16)
on these substrates. We used several complementary experi-
mental techniques, including high-resolution XPS, near-edge
X-ray absorption fine structure (NEXAFS) spectroscopy, in-
frared reflection absorption spectroscopy (IRRAS), ellipsometry,
and contact angle measurements. The results for the C12SeSeC12
SAMs are presented to the full extent, whereas only some data
and only a general description are given for the C12SeC12 and
C16SeSeC16 films, respectively, which is related to their poor
quality. As a direct reference for the alkaneselenolate films we
used dodecanethiolate (C12S) SAMs on (111) Au and Ag.
In the following section we describe the experimental
procedure and techniques. The results are presented and briefly
discussed in section 3. An extended analysis of the data is given
in section 4 followed by a summary in section 5.
2. Experimental Section
C12SeSeC12 and C12SeC12 were synthesized by refluxing
Na2Se2 (C12SeSeC12) or Na2Se (C12SeC12) with C12H25Br in
ethanol. The product was purified by chromatography on SiO2
using hexane as the eluent. The purified substance was crystal-
lized from a mixture of 2-propanol and heptane. The synthesis
of C16SeSeC16 was performed using the method described in
ref 31, followed by a chromatographic purification using hexanes
on silica. The resulting yellowish solid was obtained in
quantitative yield. Its melting point of 46-48 °C was close to
a literature value of 50-51 °C.32 The substance turned out to
be sensitive to light. C12H25SH (C12SH) was purchased from
Sigma-Aldrich.
The gold and silver substrates were prepared by thermal
evaporation of 200 nm of gold or 100 nm of silver (99.99%
purity) onto mica or polished single-crystal silicon (100) wafers
(Silicon Sense) primed with a 5 nm titanium adhesion layer.
Such evaporated films are standard substrates for thiol-derived
SAMs. They are polycrystalline, with a grain size of 20-50
nm as observed by atomic force microscopy. The grains
predominantly exhibit a (111) orientation.33,34 The SAMs were
formed by immersion of freshly prepared substrates into a 5
µmol C12SeSeC12, C12SeC12, or C16SeSeC16 solution in
absolute ethanol at room temperature for 24 h. After immersion,
the samples were carefully rinsed with pure ethanol, blown dry
with argon, and kept for several days in argon-filled glass
containers until the characterization. No evidence for impurities
or oxidative degradation products was found. Note that a µmol
concentration and ethanol as the solvent were found to be crucial
for the film quality. A millimolar concentration, used frequently
for AT SAMs, resulted in poor-quality films. We have also tried
some other solvents instead of ethanol (tetrahydrofuran, etc.),
but this led to poor-quality films, as well.
Raw NEXAFS spectra were normalized to the incident photon
flux by division through a spectrum of a clean, freshly sputtered
gold sample. In the case of Ag substrate, a spectrum of clean
silver was subtracted from the raw spectrum of a SAM sample
before the normalization.17,41 The photon energy (PE) scale was
referenced to the pronounced π1* resonance of highly oriented
pyrolytic graphite at 285.38 eV.42
IRRAS measurements were performed with a dry-air-purged
Bio-Rad FTIR spectrometer Model FTS 175C equipped with a
liquid nitrogen-cooled MCT detector. All spectra were taken
using p-polarized light incident at a fixed angle of 80° with
respect to the surface normal. The spectra were measured at a
resolution of 2 cm-1 and are reported in absorbance units A )
-log R/R0, where R is the reflectivity of the substrate with the
monolayer and R0 is the reflectivity of the reference. Substrates
For most of the experiments, we used SAMs formed from
C12SH, as a direct reference to the selenium-derived films. The
C12S SAMs were prepared by immersion of freshly prepared
Au and Ag substrates into a 1 mmol C12SH solution in absolute

3900 J. Phys. Chem. B, Vol. 109, No. 9, 2005
Shaporenko et al.
Figure 2. Normalized S 2p and C 1s HRXPS spectra of C12S/Au
and C12S/Ag (open circles) acquired at a photon energy of 350 eV,
along with the corresponding fits (solid lines). The BE positions of the
S 2p3/2 component and C 1s emission for C12S/Au are marked by the
dotted lines. A background is shown.
Figure 1. Normalized Se 3d and C 1s HRXPS spectra of C12Se/Au
and C12Se/Ag (open circles) acquired at photon energies of 150 and
350 eV, respectively, along with the corresponding fits (solid lines).
The BE positions of the Se 3d5/2 component and C 1s emission for
C12Se/Au are marked by the dotted lines. A background is shown.
covered with a perdeuterated hexadecanethiolate SAM were
used as a reference.
Ellipsometry measurements were carried out usingthe M-44
ellipsometer of J. A. Woollam Co. Inc. The data were analyzed
with the respective software. The incident angle was set to 75°
and calibrated with a silicon wafer as a reference. A refractive
index of 1.45 was assumed for all films in the calculation of
their thickness.43,44
Advancing contact angles of Millipore water were measured
on freshly prepared samples with a Kru¨ss goniometer Model
G1. The measurements were performed under ambient condi-
tions with the needle tip in contact with the drop. At least three
measurements at different locations on each sample were made.
The averaged values are reported. Deviations from the average
were less than (1°.
Figure 3. Normalized Au 4f7/2 HRXPS spectra of clean Au acquired
at photon energies of 150, 350, and 580 eV. The decomposition of the
spectra into the bulk and surface components is shown.
3. Results
3.1. HRXPS. Normalized Se 3d and C 1s HRXPS spectra of
C12Se/Au and C12Se/Ag are presented in Figure 1, along with
the corresponding fits (we will use the abbreviation C12Se for
the films formed from C12SeSeC12). The results of fitting and
a quantitative analysis of these spectra are shown in Table 1,
along with the analogous data for the C12SeC12 and C12S films
(see below). The Se 3d spectra of both C12Se/Au and C12Se/
Ag exhibit a single Se 3d5/2,3/2 doublet, accompanied by a weak
Au 5p3/2 emission. The BE position of this doublet (54.15 eV)
is distinctly different from that for the bulk C12SeSeC12 (55.3
eV),27 suggesting that upon the adsorption of C12SeSeC12 on
Au and Ag, the covalent Se-Se bonds are cleaved, through an
oxidative addition mechanism, and selenolate-metal bonds are
formed. Note, that there is a similar difference between the BE
positions of the S 2p doublet in nonbound ATs and AT SAMs
on noble metal substrates. As follows from Figure 2, where S
2p and C 1s spectra of C12S/Au and C12S/Ag are depicted,
the BE position of the thiol-related doublet is close to 162.00
eV (see Table 1 for the exact values), whereas this energy for
nonbound ATs is around 163.5 eV.45-48
In addition to the spectra of the headgroup (Se 3d), the
building of the selenolate-metal bond can be monitored using
a substrate emission, e.g., Au 4f7/2 in the case of Au. The Au
4f7/2 spectra of the clean Au substrate in Figure 3 exhibit a
splitting of the emission in two components at ≈83.95 and
≈83.65 eV, which can be assigned to the gold atoms in the
bulk and topmost surface layer, respectively.34 This assignment
is supported by the intensity decrease of the surface component
with increasing kinetic energy of the photoelectrons (see Figure
3) and by the good agreement of the observed surface core level
shift of -0.31 eV with literature values.34,49-51 Upon the
adsorption of C12SeSeC12 on Au substrates, the surface
components shifts to higher binding energy, merging with the
bulk component, as illustrated by Figure 4, where the Au 4f7/2
HRXPS spectra for clean gold and C12Se/Au are presented.
Since both components in C12Se/Au are difficult to disentangle,
a suitable fingerprint to describe the adsorbate-induced shift of
the surface component is the fwhm of the joint Au 4f7/2 emission.
The values of this parameter for clean Au, C12Se/Au, and C12S/
TABLE 1: Binding Energy Positions (eV) and fwhm’s (eV; in brackets) of the Photoemission Peaks for C12Se, C12S, and
C12SeC12 SAMs on Au and Ag
C12Se
C12S
C12SeC12
Au
Ag
Au
Ag
Au
Ag
C 1s, hν)350 eV
Se 3d or S 2p, hν)150, 350 eV
284.99 (0.82)
54.15 (0.74)
284.58 (0.89)
54.17 (0.51)
284.96 (0.83)
162.02 (0.52)
285.32 (0.77)
161.93 (0.60)
284.35 (1.1)
55.02 (0.61)
284.10 (1.2)
-

Self-Assembled Monolayers of Alkaneselenolates
J. Phys. Chem. B, Vol. 109, No. 9, 2005 3901
Figure 5. Normalized Se 3d and C 1s HRXPS spectra of C12SeC12/
Au and C12SeC12/Ag (open circles) acquired at photon energies of
150 and 350 eV, respectively, along with the corresponding fits (solid
lines). The same intensity scale as in Figure 1 is used. The BE positions
of the Se 3d5/2 component and C 1s emission for C12Se/Au (see Figure
1) are marked by the dotted lines as references. A background is shown.
Figure 4. Normalized Au 4f7/2 HRXPS spectra of clean Au and C12Se/
Au acquired at a photon energy of 350 eV. The decomposition of the
former spectrum into the bulk and surface components is shown.
TABLE 2: fwhm’s (eV) of the Au 4f7/2,5/2 Components for
Clean Au, C12Se/Au, and C12SAu
system\photon energy
350 eV
580 eV
TABLE 3: Effective Thickness (Å) of the C12Se and C12S
SAMs on Au and Ag
clean Au
C12Se/Au
C12S/Au
0.405
0.432
0.46
0.424
0.440
0.47
C12Se
C12S
C12Se
C12S
HRXPS
HRXPS
ellipsometry
ellipsometry
Au
Ag
14.5
13.9
14.9
15.6
14.6
-
14.7
-
Au, where a similar shift of the Au 4f7/2 surface component is
observed,34 are compiled in Table 2. The data for two different
photon energies are presented; the higher fwhm value for the
higher photon energy is related to increasing energy spreading
of the primary photon beam. As seen in Table 2, the Au 4f7/2
fwhm for C12Se/Au is somewhat smaller (by 0.03 eV) than
that for C12S/Au, which suggests that the surface component
shifts closer to the bulk component in the former case. Assuming
the shift of 0.25 eV upon the adsorption of AT on Au,34 one
gets an adsorbate-induced shift of 0.28 eV for the Au 4f7/2
surface component in the case of C12SeSeC12.
substrate (see Figure 2 and Table 1), in accordance with the
incommensurate (x7×x7)R19.1° structure.2,4,7-9
The C 1s HRXPS spectra of C12Se/Au and C12Se/Ag in
Figure 1 exhibit a single emission. The BE position of this
emission for C12Se/Au (284.99 eV) is noticeably higher than
that for C12Se/Ag (284.58 eV). This is distinctly different from
the case of C12S: the BE position of the C 1s emission for
C12S/Au (284.96 eV) is smaller than that for C12S/Ag (285.32
eV). The comparison of the values for the C12Se and C12S
films shows that the BE position of the C 1s emission does not
change significantly upon exchange of thiol for selenol on Au,
but decreases by 0.74 eV on Ag.
A similar analysis is more difficult in the case of the Ag
substrate, since the surface core level shift for the clean Ag
surface is much smaller than that for gold, so that the bulk and
surface components are merged together.34 However, some
tentative conclusions can be derived from the Se 3d spectra. In
the case of the C12Se films, the Se 3d doublet has almost
identical BE positions on Au and Ag, whereas the BE position
of the S 2p doublet in C12S/Ag is lower (by ≈0.1 eV) than
that in C12S/Au (Table 1). A lower BE is indirect indication
of a higher extent of the charge transfer, i.e., a stronger bond.
Therefore, we may assume that the selenium-silver bond is
weaker than the sulfur-silver one. Note, however, that this
statement is only a tentative one, since the charge transfer is
not the only parameter affecting the BE positions of the Se 3d
and S 2p doublets. Another parameter, namely the final state
effects due to conduction electrons of the substrates (see e.g.
ref 52), should be considered in detail, which is rather difficult
for the systems under considerations.
Apart from the above consideration, the intensities of the C
1s emission could be evaluated. They were rather similar for
C12Se/Au and C12Se/Ag, but quite different for C12S/Au and
C12S/Ag, where a larger value is observed for Ag. Based on
the exact values of C 1s intensity, and taking into account the
intensities of the Au 4f and Ag 3d emissions, the thicknesses
of the C12Se and C12S SAMs were determined from the IC1s
/
IAu4f and IC1s/IAg3d ratios. The derived values are presented in
Table 3, along with the ellipsometry data (see below). For the
evaluation, the attenuation lengths reported in ref 53 were used.
It can clearly be seen that the layers on Au must have a similar
orientation, since the thicknesses are almost the same, while
on Ag the thickness of the C12Se layer is considerably smaller
than the C12S film, suggesting either a larger tilt and/or a looser
packing of the former.
Along with the BE position of the Se 3d doublet, an important
parameter is the fwhm of the Se 3d5/2 and Se 3d3/2 components.
Since the instrumental spreading is negligible in the present case,
the value of fwhm is characteristic of the inhomogeneity of the
bonding configurations (e.g. the distribution of the adsorption
sites) for the selenol headgroups. A noticeable larger Se 3d5/2,3/2
fwhm for C12Se/Au as compared to C12Se/Ag suggests a larger
inhomogeneity (e.g. a coexistence of several different adsorption
sites) in the former case. This fwhm relation is just opposite to
the case of C12S, where a larger fwhm is observed for Ag
The results for the C12Se SAMs can be compared with those
for the C12SeC12 films. Normalized Se 3d and C 1s HRXPS
spectra of C12SeC12/Au and C12SeC12/Ag are presented in
Figure 5, along with the corresponding fits. The results of fitting
and a quantitative analysis of these spectra are compiled in Table
1. The Se 3d spectra of C12SeC12/Au are dominated by a Se
3d5/2,3/2 doublet at a BE of 55.02 eV, which is characteristic of
weakly bound Se and noticeably higher than the respective value
for C12Se/Au (54.15 eV). There is only a weak intensity at the
latter BE position in the spectra of C12SeC12/Au. This suggests

3902 J. Phys. Chem. B, Vol. 109, No. 9, 2005
Shaporenko et al.
Figure 6. IRRAS spectra of the C12Se and C12S SAMs on Au and
Ag. The characteristic absorption bands are indicated. The positions
of these bands for C12S/Au are marked by the dotted lines as a guide
for eye.
Figure 7. IRRAS spectra of C12SeC12/Au and C12SeC12/Ag. The
characteristic absorption bands are indicated. The positions of these
bands for C12Se/Au (see Figure 6) are marked by the dotted lines as
references.
TABLE 4: Positions (cm-1) of the C-H Stretching Modes in
the IRRAS Spectra of C12Se, C12S, and C12SeC12 SAMs
on Au and Ag (ip means in-plane)
This suggests all-trans conformation of these chains with a very
low concentration of gauche defects.13,43 The positions of the
asymmetric and symmetric methylene stretching bands in
νa(CH3) νs(CH3)FR νa(CH2)
νs(CH3)
νs(CH2)
C12Se/Ag are somewhat higher, namely 2924 and 2853.5 cm-1
,
TDM direction
C-CH3 || C-CH3
C-C-C || C-CH3 ip C-C-C
respectively (the methyl bands are shifted as well). In addition,
these bands are broader than the analogous modes in the spectra
of C12Se/Au, C12S/Au, and C12S/Ag. These differences
indicate some gauche conformations around the C-C bonds in
C12Se/Ag. Each gauche bond shifts the vibrational frequencies
of the adjoining methylene groups by 10-20 wavenumbers. A
liquid alkane at room temperature has ∼40% C-C gauche
ip C-C-C
ip H-C-H
C12S/Au
C12S/Ag
C12Se/Au
C12Se/Ag
2965.5
2965.5
2965.5
2967
2937.5
2937.5
2937.5
2939
2920.3
2919.7
2919.7
2924
2878.5
2878.5
2878.5
2880.2
2880.5
2852
2852
2852
2853.5
2855.5
C12SeC12/Au 2966
2937.5
2926.5
that the vast majority of the C12SeC12 species experiences no
cleavage of one of the two C-Se bonds in C12SeC12 (leading
to the formation of a gold selenolate) upon the adsorption on
Au. Under these circumstances, a densely packed SAM cannot
be formed, as evidenced by the C 1s HRXPS spectra of
C12SeC12/Au, where a much lower intensity than for C12Se/
Au (Figure 1) is observed. Also, the BE position of the C 1s
emission in the spectra of C12SeC12/Au is noticeably lower
than that for C12Se/Au. This can be related to the different
structure of the C12SeC12 film and a higher extent of the
substrate-mediated screening of the C 1s photoemission hole
in the thinner C12SeC12/Au.
The situation for C12SeC12/Ag is even more abnormal.
Almost no intensity is observed in the Se 3d spectrum, and there
is only a weak emission in the C 1s spectra. The observed BE
position (284.1 eV) is characteristic of carbon contamination.
Presumably, the C12SeC12 molecules are not able to oust
contamination from the Ag surface. Note, that the surface of
the even freshly prepared Au and Ag substrates is contaminated
to some extent because of their exposure to ambient. This
contamination is, however, displaced upon SAM formation.2
3.2. Ellipsometry. The effective thicknesses of the C12Se
and C12S films on Au derived from the ellipsometry measure-
ments are presented in Table 3. These values agree fairly well
with the thicknesses calculated on the basis of the HRXPS data.
The ellipsometry measurements for the films on Ag were not
performed.
conformations and peak maxima at 2928 and 2856 cm-1 43
.
The relative intensities of the C-H stretching bands are very
similar for C12Se/Au and C12S/Au, but somewhat different for
C12Se/Ag and C12S/Ag (see e.g. the symmetric methyl and
methylene stretching bands). Note that the ratio of the band
intensities is a fingerprint of the molecular orientation in the
SAMs and can even be used for extracting quantitative
information on the film structure.54-56 According to the selection
rules for surface IR spectroscopy, only vibrational modes with
the transition dipole moment (TDM) perpendicular to the surface
can be excited. At an inclined orientation of the TDM, only its
perpendicular component contributes to the absorbance, so that
the orientation of the functional group can be determined if at
least two orthogonal modes are considered. Such an analysis
is, however, not necessary in our case, since we can get the
structural information from the NEXAFS data. We only want
to point out that, in view of the above consideration, the IRRAS
results suggest similar orientations for C12Se/Au and C12S/
Au, but different orientations for C12Se/Ag and C12S/Ag.
This difference is, however, not as significant as the difference
between the SAMs formed from the dialkyl diselenides and
dialkyl selenides. The IRRAS spectra of C12SeC12/Au and
C12SeC12/Ag are presented in Figure 7. The spectra of
C12SeC12/Au exhibit all characteristic C-H stretching modes
of the methyl and methylene groups, but intensity relations
between these modes are noticeably different from those for
C12Se/Au. Also, the positions of the asymmetric and symmetric
methylene stretching bands in C12SeC12/Au (2926.5 and 2855.5
cm-1, respectively) are somewhat higher than those in C12Se/
Au (2919 and 2851 cm-1). Along with a significant broadening
of the absorption peaks for C12SeC12/Au, this suggests a
disordered structure with a high percentage of the gauche defects
in the C12SeC12 film on Au. In the case of Ag, no characteristic
modes of the methyl and methylene groups are observed, in
full agreement with the HRXPS results. There is only carbon
contamination on the Ag surface.
3.3. IRRAS. IRRAS spectra of C12Se and C12S SAMs on
Au and Ag are presented in Figure 6. All spectra exhibit
characteristic asymmetric (νa) and symmetric (νs) C-H stretch-
ing bands of methyl and methylene (marked in the figure) and
a Fermi-resonance band of methyl (νs FR); the positions of these
bands are compiled in Table 4. The positions of the asymmetric
and symmetric methylene stretching bands in C12Se/Au, C12S/
Au, and C12S/Ag are almost identical. The corresponding values
are ≈2920 and ≈2852 cm-1, respectively. Such band positions
are characteristic for crystalline-like packing of the alkyl chains.

Self-Assembled Monolayers of Alkaneselenolates
J. Phys. Chem. B, Vol. 109, No. 9, 2005 3903
absorption edge related to C 1s f continuum excitations and
all characteristic absorption resonances of extended alkyl chains,
namely a sharp resonance at ≈287.7 eV, and two broader
resonances at ≈293.4 eV and ≈301.6 eV. The two latter
resonances are commonly related to valence, antibonding C-C
σ* and C-C′ σ* orbitals,57,58 while the resonance at 287.7 eV
is alternatively attributed to the excitations into pure valence
orbitals,57,59 predominantly Rydberg states60,61 and mixed
valence/Rydberg states.62 This resonance consists of several
individual resonances, which are merged together.59-61 We will
denote it as a R* resonance but take into account a possible
admixture of antibonding C-H* orbitals. The molecular orbitals
related to the R* resonance are supposed to be orientated
perpendicular to the alkyl chains,58,63,64 whereas the transition
dipole moments of the orbitals corresponding to the C-C σ*
and C-C′ σ* resonances are believed to be directed along the
chain axis.63 Thus, the orientations of these orbitals unequivo-
cally determine the orientation of the alkyl chains in the
investigated films.
The NEXAFS spectra of C12Se/Au and C12Se/Ag in Figures
8 and 9 exhibit a pronounced linear dichroism, i.e., a dependence
of the absorption resonance intensity on the incidence angle of
X-rays, which suggests a high orientational order in these SAMs.
Considering the TDM directions of the R*, C-C σ*, and C-C′
σ* orbitals and the character of the observed intensity changes,
an upright orientation of the alkyl chains in C12Se/Au and
C12Se/Ag can be assumed. A fingerprint of the orientational
order and average molecular inclination is a difference spec-
trum.19,57 Such spectra of C12Se/Au and C12S/Au in Figure 8
are almost identical, which suggest a comparable orientational
order and similar molecular inclinations in both films. In
contrast, the peak amplitudes in the difference spectrum of
C12Se/Ag in Figure 9 are noticeably smaller than those in the
spectrum of C12S/Ag, which imply a lower orientational order
and/or a larger molecular inclination in C12Se/Ag.
Figure 8. Carbon K-edge NEXAFS spectra of C12Se/Au acquired at
X-ray incident angles of 90°, 55°, and 20°, along with the difference
between the 90° and 20° spectra. For comparison, the analogous
difference spectrum of C12S/Au is also given by the dotted line. It
overlaps completely with the spectrum of C12Se/Au. The dashed line
corresponds to zero. The characteristic absorption resonances are
indicated.
Apart from these qualitative considerations, a quantitative
analysis can be performed, and the average tilt angles of alkyl
chains can be determined. For this purpose, the intensity of an
absorption resonance I should be monitored as a function of
the X-ray incidence angle θ and the resulting dependence
evaluated according to theoretical expression (for a plane-type
orbital)57
Figure 9. Carbon K-edge NEXAFS spectra of C12Se/Ag acquired at
X-ray incident angles of 90°, 55°, and 20°, along with the difference
between the 90° and 20° spectra. The difference spectrum is normalized
to the R* intensity ratio between the 55° spectra of C12Se/Ag and
C12Se/Au, for direct comparison to the respective spectrum of C12Se/
Au. For comparison, the analogous difference spectrum of C12S/Ag
is also given by the dotted line. The dashed line corresponds to zero.
The characteristic absorption resonances are indicated.
I(R,θ) ) A{P × (2/3)[1 - (1/4)‚(3‚cos2θ - 1)‚(3‚cos2γ -
1)] + (1 - P) × (1/2)‚(1 + cos2γ)} (1)
where A is a constant, P is a polarization factor of the X-rays,
and γ is the angle between the sample normal and the normal
of the molecular orbital plane.
3.4. NEXAFS. In NEXAFS experiment, core level electrons
(e.g. C1s for a C K-edge spectrum) are excited into nonoccupied
molecular orbitals, which are characteristic for specific bonds,
functional groups, or molecules. The PE positions of the
respective absorption resonances give then a clear signature of
these entities. In addition, information on molecular orientation
can be derived from the experimental data, since the cross-
section of the resonant photoexcitation process depends on the
orientation of the electric field vector of the linearly polarized
synchrotron light with respect to the molecular orbital of interest
(so-called linear dichroism in X-ray absorption).57
Carbon K-edge NEXAFS spectra of C12Se/Au and C12Se/
Ag acquired at X-ray incident angles of 90°, 55°, and 20° are
presented in Figures 8 and 9, respectively, along with the
difference between the 90° and 20° spectra for the C12Se and
C12S SAMs. All spectra in Figures 8 and 9 exhibit a C1s
For the evaluation, the R* resonance as the most intense one
in the spectra has been selected. The respective orbital was
considered as a plane-type one. To avoid normalization prob-
lems, not the absolute intensities but the intensity ratios I(θ)/
I(90°) were analyzed,57 where I(θ)and I(90°) are the intensities
of the R* resonance at X-ray incidence angles of θ and 90°.
The angular dependences of the R* resonance intensity ratio
I(θ)/I(90°) for the C12Se films are presented in Figure 10, along
with the best theoretical fits according to the eq 1. The derived
values of the average tilt angles of the alkyl chains in C12Se/
Au and C12Se/Ag are given at the respective fit curves; they
are 28° and 37°, respectively. The accuracy of these values is
(3-5°, which is just a general accuracy of the NEXAFS
experiment and data evaluation procedure. The value for C12Se/
Au is very close to that for C12S/Au (≈30°), whereas the

3904 J. Phys. Chem. B, Vol. 109, No. 9, 2005
Shaporenko et al.
gold one, even if insignificantly. This is in contrast with the
intuitive suggestion by Samant et al.21 but in accordance with
the STM data by Dishner et al.,22 the substitution experiments
by Huang et al.,24 and electrochemistry measurements by Sato
and Mizutani.30 In particular, in the experiments by Huang et
al., it was observed that diphenyl diselenide (DPDSe) displaces
benzenthiolate from gold, but diphenyl disulfide does not
displace benzeneselenolate.24 The adsorption of DPDSe was
found to be more favorable by 0.7 kcal/mol,24 which is rather
small as compared to the sulfur-gold interaction energy in AT/
Au (44 kcal/mol).66
Figure 10. The angular dependence of the R* resonance intensity ratio
I(θ)/I(90°) for C12Se/Au (up filled triangles) and C12Se/Ag (down
open triangles) along with the best theoretical fits according to eq 1
(dotted and solid lines, respectively). The derived tilt angles are given
at the respective fit curves.
The analysis of the Se 3d spectra suggests that the selenium-
silver bond is weaker than the sulfur-silver one, even if
insignificantly. This result is in accordance with the substitution
experiments by Hun et al.,24 in which the adsorption of
benzenethiolates was favored over the benzeneselenolate ad-
sorption. The respective difference in the free energy of
adsorption was calculated to be 0.3 kcal/mol, which is, similar
to the case of Au, small as compared to the energies of Ag-S
and Ag-Se bonds in diatomic systems (51.9 and 48.4 kcal/
mol, respectively).67
TABLE 5: Advancing (θadv) and Receding (θrec) Water
Contact Angles of the C12Se, C12S, and C12SeC12 SAMs on
Au and Ag
C12Se
C12S
C12SeC12
θadv
θrec
θadv
θrec
θadv
θrec
Au
Ag
110°
111°
101°
102°
111°
112°
102°
101°
100°
98°
89°
75°
The differences in the binding energy and exact bonding
configurations of selenium on Au and Ag cause some differences
in the structure of the C12Se and C12S films on these substrates.
The packing density, orientational order, and molecular inclina-
tion in C12Se/Au were found to be very similar to those in
C12S/Au. This suggests the same bonding configurations, i.e.,
sp3, for both sulfur and selenium on Au. The only noticeable
difference between C12Se/Au and C12S/Au is a superposition
of different adsorption sites in C12Se/Au, which follows from
the relatevely large value of the Se 3d5/2,3/2 fwhm. This
conclusion is in excellent agreement with the recent STM data
for the C10Se and C12Se SAMs on Au, which suggest the
multiple binding sites for alkylselenolates on Au(111).29
average tilt angles in C12Se/Ag is noticeably larger than that
in C12S/Ag (12-15°).
3.5. Contact Angle Measurements. The values of the
advancing (θadv) and receding (θrec) water contact angles of the
C12Se, C12S, and C12SeC12 SAMs on Au and Ag are given
in Table 5. The θadv and θrec values in the C12Se and C12S
films are practically identical and characteristic of well-ordered
AT SAMs with the CH3 termination.44,65 The difference between
the advancing and receding contact angles, i.e., the contact angle
hysteresis, is only 9-11° in these films, which suggest that their
film-ambient interface is rather smooth.
The θadv and θrec values on the C12SeC12 films are noticeably
smaller than those on the C12Se and C12S SAMs, and the
contact angle hysteresis on Ag is very large (23°). This suggests
a partial exposure of the CH2 groups in C12SeC12/Au to the
surface and a rather rough “film”-ambient interface in C12SeC12/
Ag.
3.6. C16SeSeC16. Despite all our efforts and an extended
variation of the parameters of the preparation procedure (solvent,
the substance concentration, sonification, etc.), we have not
succeeded to prepare high-quality C16Se SAMs on Au and Ag.
Even though the IRRAS spectra were characteristic of crystalline
alkylselenol SAMs, there were additional emissions except for
the selenolate doublet in the Se 3d HRXPS spectra. At the
moment, we cannot conclude whether there is a physical reason
behind these difficulties or just a broader variation of the layer
preparation parameters is required. We think that further work
is necessary to clarify this issue.
Note that the headgroup hybridization cannot be exclusively
associated with a definite adsorption site (e.g. a sp3 hybridization
with a 3-fold hollow site) since there is a second important
contribution to the hybridization. It is the electronic structure
of the substrate, governing a specific overlap of the headgroup’s
orbitals with the frontier orbitals of the substrate. Thus, an sp3
hybridization can be characteristic of both C12Se/Au and C12S/
Au, even though the adsorption sites or their superposition might
be different. Note, that recent studies favor a top adsorption
site for AT/Au,68,69 although previous results suggested either
3-fold hollow2,6,70 or bridge71,72 positions.
The obtained value (28°) of the average tilt angle of alkyl
chains in C12Se/Au is markedly different from the value
reported by Samant et al. (15°).21 Note, however, that these
authors determined the average tilt angle of alkyl chains in
docosanethiol on Au as 12°, which is obviously wrong in view
of a vast majority of all other studies on AT/Au (≈30°).10,13,14
Thus the angle value reported for C12Se/Au by Samant et al.
can be strongly underestimated, just in the same way as it
occurred for the docosanethiol SAM.
4. Discussion
All experimental data suggest that C12SeSeC12 form con-
tamination-free, densely packed, and well-ordered C12Se SAMs
on (111) Au and Ag substrates. The molecules are bound to
the substrate via a selenolate-gold linkage, while the alkyl
backbones have an upright orientation. The selenolate-gold
linkage is formed after the cleavage of the Se-Se bond, which
occur by an oxidative addition mechanism, upon the adsorption,
similar to the formation of the thiolate bonds from the disulfide
precursors.
In contrast to the case of Au, C12Se/Ag exhibits significantly
lower packing density, a lower degree of the conformational
and orientational order, and a larger molecular inclination than
C12S/Ag. A possible explanation for this difference can be the
difference in the bonding configurations of selenium and sulfur
on silver, i.e., sp3 and sp, respectively. The sp3 hybridization
will favor a larger inclination of the alkyl chains in C12Se/Ag,
similar to C12Se/Au. An alternative explanation is the larger
corrugation of the selenium-silver binding energy surface as
compared to the sulfur-silver case. This assumption is in accord
The slightly larger shift of the Au 4f7/2 surface component
upon the adsorption of selenium as compared to sulfur suggests
that the selenolate-gold bond is stronger than the thiolate-

Self-Assembled Monolayers of Alkaneselenolates
J. Phys. Chem. B, Vol. 109, No. 9, 2005 3905
with the narrow Se 3d5/2,3/2 emissions for C12Se/Ag (Figure
1), which can be associated with a single adsorption site for
the C12Se molecules on Ag. Note, that in the case of a strong
corrugation of the headgroup-substrate binding energy surface,
the headgroups will be pinned at the energetically favorable
positions, so that the energy minimization in a SAM can only
be achieved by a molecular tilt.2
An interesting finding is the difference in the BE position of
the C1s emission in C12Se/Ag and C12S/Ag (284.58 and 285.32
eV, respectively; see Table 1). The high value for C12S/Ag is
untypical for saturated hydrocarbons and noticeably larger than
that for C12S/Au. It was related to the detailed molecular
arrangement in the alkyl matrix of AT/Ag, resulting in a specific
screening of the C 1s photoemission hole by intermolecular
chain transfer.73 It is then understandable that a change in the
molecular arrangement, as it for example happens at going from
C12S/Ag to C12Se/Ag, will be accompanied by a drastic
decrease in the BE position of the C 1s emission.
In contrast, C12Se/Ag exhibits significantly lower packing
density, a lower degree of the conformational and orientational
order, and the larger molecular inclination than C12S/Ag. In
particular, the average tilt angle of aliphatic chains in C12Se/
Ag was found to be 37°. The difference between C12Se/Ag
and C12S/Ag was tentatively explained by the difference in the
bonding configurations of selenium and sulfur on silver, which
are assumed to be sp3 and sp, respectively. Alternatively, a larger
corrugation of the selenium-silver binding energy surface as
compared to the sulfur-silver case can be assumed. The analysis
of the experimental data suggests that the selenium-gold bond
is stronger than the sulfur-gold one, whereas the inverse relation
takes place for silver.
In addition to the C12Se and C12SeC12 films, SAMs formed
from dihexadecyl diselenide have been studied. So far, the
results are not completely convincing, and further work is
necessary.
Acknowledgment. We thank M. Grunze for the support of
this work, Thimo Bastuck for the help with the ellipsometry
measurements, Ch. Wo¨ll (Universita¨t Bochum) for providing
us with the experimental equipment for the NEXAFS measure-
ments, L. S. O. Johansson (Karlstad University) for the
cooperation at MAX-lab, and the BESSY II and MAX-lab staff
for the assistance during the synchrotron-based experi-
ments. This work has been supported by the German BMBF
(05KS4VHA/4), European Community (Access to Research
Infrastructure action of the Improving Human Potential Pro-
gramme), and the Fonds der Chemischen Industrie. A.U. thanks
the NSF for partial support through the MRSEC for Polymers
at Engineered Interfaces.
Along with the results for dialkyl diselenides, we have found
that dialkyl selenides do not form high-quality SAMs on Au
and Ag substrates. The vast majority of the C12SeC12 molecules
do not undergo cleavage of the C-Se bond, leading to the
formation of a gold selenolate, upon the adsorption. As a result,
steric constrains provided by both chain parts do not allow a
dense molecular packing and a high orientational and confor-
mational order. This occurs in the case of C12SeC12/Au, where
a SAM-like film is formed, but with a low packing density and
a conformational disorder. In the case of Ag, the C12SeC12
species are not capable to remove contamination from the Ag
surface and no SAM-like film, even if a bad quality, is formed.
The observed behavior of dialkyl selenides agrees quite well
with that of dialkyl sulfides. Even though there was a contro-
versy concerning the properties of the latter systems,74-79 recent
studies state that the maximum coverage achieved with a dialkyl
sulfide is considerably lower than that with ATs, and substantial
amounts of contamination remain on the substrate.80
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