Self-assembled monolayers of nitrile-functionalized alkanethiols on gold and silver substrates

Article abstract of DOI:10.1021/jp0221690

Self-assembled monolayers (SAMs) formed from nitrile-functionalized alkanethiols (AT), NC(CH₂)₁₆SH (NC-Cl6), on (111) gold and silver substrates were characterized by X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and contact angle measurements. The average chain tilt angles in NC-C16/Ag and NC-C16/Au were estimated to be 29.5° ± 5° and 42.5° ± 5° from the surface normal, respectively, while the data suggest lower ordering for NC-C16/Au. The -C≡N bonds were found to be predominantly oriented in the surface plane with a tilt angle of 65° ± 7° for both NC-C16/Au and NC-C16/ Ag. Comparison with previous data on CH₃-terminated SAMs reveals that substitution of weakly interacting CH₃ groups by the CN entities results in an increase in the average tilt angles of the alkyl chains by ～7.5° and ～17.5° in AT/Au and AT/Ag, respectively. A strong electrostatic interaction between the polar nitrile groups is assumed to underlie the structural behavior by controlling a balance between the headgroupsubstrate and interchain interactions. The near-parallel orientation of the nitrile groups to the surface in both of these SAMs can be explained on the basis of minimization of the unfavorable CN - CN dipole - dipole interactions.

Full text of DOI:10.1021/jp0221690

7716
J. Phys. Chem. B 2003, 107, 7716-7725
Self-Assembled Monolayers of Nitrile-Functionalized Alkanethiols on Gold and Silver
Substrates
S. Frey, A. Shaporenko, and M. Zharnikov*
Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, Im Neuenheimer Feld 253,
69120 Heidelberg, Germany
P. Harder and D. L. Allara
Departments of Chemistry and Materials Science, PennsylVania State UniVersity,
UniVersity Park, PennsylVania 16802
ReceiVed: September 30, 2002; In Final Form: February 26, 2003
Self-assembled monolayers (SAMs) formed from nitrile-functionalized alkanethiols (AT), NC(CH2)16SH (NC-
C16), on (111) gold and silver substrates were characterized by X-ray photoelectron spectroscopy, near-edge
X-ray absorption fine structure spectroscopy, and contact angle measurements. The average chain tilt angles
in NC-C16/Ag and NC-C16/Au were estimated to be 29.5° ( 5° and 42.5° ( 5° from the surface normal,
respectively, while the data suggest lower ordering for NC-C16/Au. The -CtN bonds were found to be
predominantly oriented in the surface plane with a tilt angle of 65° ( 7° for both NC-C16/Au and NC-C16/
Ag. Comparison with previous data on CH3-terminated SAMs reveals that substitution of weakly interacting
CH3 groups by the CN entities results in an increase in the average tilt angles of the alkyl chains by ∼7.5°
and ∼17.5° in AT/Au and AT/Ag, respectively. A strong electrostatic interaction between the polar nitrile
groups is assumed to underlie the structural behavior by controlling a balance between the headgroup-
substrate and interchain interactions. The near-parallel orientation of the nitrile groups to the surface in both
of these SAMs can be explained on the basis of minimization of the unfavorable CN-CN dipole-dipole
interactions.
1. Introduction
studies have concentrated on the wetting properties of SAMs
formed from differently functionalized ATs and paid minor
attention to the effect of the ω-substitution on the structure and
packing density in these systems. Such an influence can be,
however, fairly large, especially in the case in which some
strongly interacting (e.g., polar groups or groups capable of
hydrogen bonding) functional groups are used.13,16 Under these
conditions, the interaction between the tailgroups becomes a
third participant in the structure-building interplay between the
headgroup-substrate and the intermolecular forces, which can
cause a significant change of the structure, orientational order,
and packing density in a SAM.
During the last two decades self-assembled monolayers
(SAMs) attracted considerable attention because they enable one
to fabricate an organic surface of a definite chemical identity
and provide a means to tailor the surfaces properties such as
wetting, adhesion, lubrication, corrosion, and biocompatibility.1-5
SAMs are polycrystalline films of chainlike or rodlike molecules
that are chemically anchored to a suitable substrate. The SAM
constituents consist generally of three parts: a headgroup that
binds strongly to the substrate, a tailgroup that constitutes the
outer surface of the film, and a spacer that connects head and
tail groups. Whereas the balance between the headgroup-
substrate and the intermolecular interactions is of crucial
importance for the molecular structure and packing density of
SAMs,1,2,4,6-9 the identity of the tailgroup is deciding for the
most of the SAM properties, providing a high orientational order
and packing density in the film. A large variety of different
functionalities along with a flexible molecular architecture are
exactly the properties that make SAMs attractive for scientific
and practical applications.
In this paper, we studied SAMs formed from nitrile-
functionalized alkanethiols (AT), NC(CH2)16SH (NC-C16, 17-
mercaptoheptadecanenitrile), on (111) gold and silver substrates.
These systems are interesting from both scientific and practical
standpoints. From the practical point of view, the nitrile-
functionalized SAMs are perspective candidates for SAM-based
lithography17 and the fabrication of non-hydrogen-bond donor
surfaces with low protein affinity.18 From the scientific stand-
point, the polar nitrile group with a relatively large dipole
moment is well-suited to explore to what extent the structure
of a nonsubstituted AT SAM can be affected by strongly
interacting tailgroups. A theory predicts a strong influence of
the electrostatic interactions between the nitrile tailgroups on
the entire system.13 A few published experimental papers
concentrated on some specific issues and have not come to a
clear conclusion concerning the structure of the nitrile-
substituted AT SAMs,10,19-21 even though some of the authors
The properties of differently functionalized SAMs have been
the subject of many studies. In particular, much work has been
published on ω-substituted alkanethiolate (AT) SAMs on noble
metal substrates, which are probably the best studied SAM
systems at present because of their stability and the easiness of
fabrication (e.g., see refs 10-16). However, many of these
* To whom correspondence should be addressed. E-mail:
Michael.Zharnikov@urz.uni-heidelberg.de.
10.1021/jp0221690 CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/10/2003

SAMs of Nitrile-Functionalized Alkanethiols
J. Phys. Chem. B, Vol. 107, No. 31, 2003 7717
reported a low structural order in these systems.21 In addition,
only the SAMs on Au were investigated.
were washed once with 75 mL of brine, dried with MgSO4 and
filtered through a MgSO4 plug, and the ether was evaporated.
Yield: 1.93 g, yellowish solid (77%).
We used several complementary experimental techniques,
such as X-ray photoelectron spectroscopy (XPS), near-edge
X-ray absorption fine structure (NEXAFS) spectroscopy, and
contact angle measurements to get information on the chemical
identity, packing density, and orientational and conformational
order in the NC-C16 SAMs on Au and Ag. Of particular interest
was the determination of the orientation of the terminal nitrile
groups from the NEXAFS data. To date, the only reported
example of tail group orientation by NEXAFS has involved the
rather bulky phenyl tailgroups in phenyltricosanthiolate SAMs
on Au.22 As compared to the pronounced π1* resonance of
phenyl, the absorption resonances of the diatomic nitrile group
have much lower intensity. Thus the nitrile SAM provides a
useful test of the limits of NEXAFS spectroscopy in character-
izing functionalized SAMs.
Thioacetic Acid R,S-(16-Cyanohexadecyl) Ester (5). Triphen-
ylphosphine (2.748 g, 10.5 mmol) was dissolved in 28 mL of
dry THF in a 250 mL round-bottom flask. The solution was
cooled to 0 °C. Diisopropyl azodicarboxylate (2.118 g, 10.5
mmol) was added, and the solution was stirred for 40 min when
a solution of 1.4 g of nitrile 4 (5.25 mmol) with 0.8 g of
thiolacetic acid (5.25 mmol) in 28 mL of THF was slowly added
through a transfer needle.
The temperature was kept at 0 °C for 1 h, and the solution
was stirred for another 24 h at room temperature. The THF was
rotovaped, and 10 mL of 15% ethyl acetate/85% hexane was
added to the residue. The solution was filtered, and the filtrate
was separated with column chromatography. The thioacetate 5
came second after a yellow impurity and before the triphen-
ylphosphine oxide. The crude product (main impurity was
triphenyl phosphine oxide) was recrystallized in methanol.
Yield: 669 mg recrystallized product (39.4%). 1H NMR
(CDCl3): δ 2.86 (t, 2H, J ) 7.3 Hz), 2.34 (t, 2H, J ) 7.1 Hz),
2.32 (s, 3H), 1.66 (m, 2H), 1.56 (m, 2H), 1.44 (m, 2H), 1.38-
1.22 (b, 22H).
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
17-Mercaptoheptadecanenitrile (1). Thioacetate 5 (300 mg)
was added to 40 mL of outgassed ethanol. The solution was
warmed until it was clear, and 8 mL of 5 N NaOH was added
with a syringe. The solution was stirred for 2 h at room
temperature. Sixteen milliliters of 5 N HCl, 100 mL of freshly
distilled ether, and 100 mL of water were added successively,
and the ether phase was separated, dried with MgSO4, and
filtered through a MgSO4 plug. NMR shows the methylene
group next to the nitrile group at 2.34 ppm, complete hydrolysis
(no more thioacetate CH3 group at 2.32 ppm). The product was
90% thiol (multiplet at 2.52 ppm) and 10% disulfide (triplet at
2.1. Synthesis of NC-C16. 17-Mercaptoheptadecanenitrile
(1). This compound was prepared from 16-bromohexadecan-
1-ol through a nucleophilic substitution of the bromide with
cyanide followed by the esterification with thiolacetic acid in
the presence of diisopropyl azodicarboxylate and triphenylphos-
phine (Mitsunobu reaction) and finally the cleavage of the
thioacetate to the thiol. All reactions were performed in a
nitrogen atmosphere with A.C.S reagent grade organic solvents
and deionized water. THF was freshly distilled from sodium.
16-Bromohexadecanoic Acid (2). Hexadecanolide (20 g, 78.6
mmol, Aldrich H0893) was dissolved in a mixture of 100 mL
of 48% HBr and 100 mL of glacial acetic acid. The mixture
was refluxed for 24 h and allowed to cool for 15 min, and the
warm solution was poured into a beaker with 200 mL of cold
water. The acid precipitated immediately and was filtered and
washed with water until the filtrate was no longer acidic. The
product on the filter was washed once with hexane, vacuum-
dried, and recrystallized from 200 mL of hexane. Yield: 23.2
g (88%) 16-bromohexadecanoic acid (MW 335.3), white
powder. 1H NMR (CDCl3): δ 3.41 (t, 2H, J ) 6.9 Hz), 2.35 (t,
2H, J ) 7.6 Hz), 1.85 (m, 2H), 1.63 (m, 2H), 1.42 (m, 2H),
1.37-1.22 (b, 20H).
16-Bromohexadecan-1-ol (3). The acid 2 (4.3 g, 12.8 mmol)
was dissolved in 30 mL of dry THF. Thirteen milliliters of 1.5
M borane-tetrahydrofuran complex in THF (19.5 mmol,
Aldrich 43,687-9) was added dropwise at 0 °C. The solution
was refluxed for 24 h, cooled to room temperature, quenched
with 10 mL of water, and added to a separatory funnel with
150 mL of water and 150 mL of ether. The ether phase was
separated, and the aqueous phase was extracted twice with 50
mL of ether. The combined ether phases were dried with
MgSO4, and the solvent was evaporated under vacuum. 1H NMR
(CDCl3): δ 3.64 (t, 2H, J ) 6.6 Hz), 3.41 (t, 2H, J ) 6.9 Hz),
1.86 (m, 2H), 1.57 (m, 2H), 1.44 (m, 2H), 1.37-1.22 (b, 20H).
17-Hydroxyheptadecanenitrile (4). Three grams (9.4 mmol)
of the bromide 3 was dissolved in 60 mL of DMSO. Sodium
cyanide (504 mg, 10.3 mmol) was added, and the mixture was
stirred at 70 °C for 1 h. The mixture was cooled to room
temperature and added to a separatory funnel containing 300
mL of water and 225 mL of ether. The water layer was separated
and extracted three times with ether. The combined ether phases
1
2.68 ppm). H NMR (CDCl3): δ 2.52 (m, 2H), 2.34 (t, 2H, J
) 7.2 Hz), 1.69-1.54 (m, 4H), 1.48-1.20 (b, 24H), 1.33 (t,
1H, J ) 7.7 Hz).
2.2. Monolayer Preparation. The gold and silver substrates
for the XPS and NEXAFS experiments were prepared by
thermal evaporation of 200 nm of gold or 100 nm of silver
(99.99% purity) onto polished 3 in. single-crystal silicon (100)
wafers (Silicon Sense) that had been precoated with a 5 nm
titanium adhesion layer. The coated wafers were cut with a
diamond tip into 1 cm2 sample pieces. The substrates for the
wetting experiments were 2 in. silicon (100) wafers with
thermally evaporated 5 nm chromium and 200 nm gold or silver
films. 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, which is, in particular,
supported by the observation of the corresponding forward-
scattering maxima in the angular distributions of the Au 4f
photoelectrons23 and by the characteristic binding energy (BE)
shift of the Au 4f surface component.24 The SAMs were formed
by immersion of freshly prepared substrates into a 1 mM NC-
C16 solution in absolute ethanol at room temperature for 24 h.
After immersion, the samples were carefully rinsed with pure
ethanol and blown dry with argon. No evidence for impurities
or oxidative degradation products was found.
2.3. XPS and NEXAFS Spectroscopy. The NC-C16 SAMs
were characterized by XPS and NEXAFS spectroscopy using
a modified multitechnique ultrahigh vacuum (UHV) chamber25
attached to the HE-TGM 2 beamline26 at the German synchro-
tron radiation facility BESSY I in Berlin. The experiments were

7718 J. Phys. Chem. B, Vol. 107, No. 31, 2003
Frey et al.
performed at room temperature under UHV conditions (base
pressure better than 1.5 × 10-9 Torr). The time for the
NEXAFS/XPS characterization was selected as a compromise
between the spectra quality and the damage induced by
X-rays.17,27-29
electrons originated from the Au 4f photoelectrons (BE below
the S 2p and C 1s ionization thresholds). The relatively poor
signal-to-noise ratio of the S 2p spectra is explained by the
attenuation of the corresponding signal by the overlayer and a
relatively short acquisition time chosen to reduce possible X-ray-
induced damage during the measurements.17,27-29 The com-
paratively poor signal-to-noise ratio of the N 1s spectra stems
from a large inelastic background.
The XPS measurements were performed with a Mg KR X-ray
source and a VG CLAM 2 spectrometer in normal emission
geometry with an energy resolution of ∼0.9 eV. The X-ray
source was operated at 260 W power and positioned ∼1 cm
away from the samples. The energy scale was referenced to
the Au 4f7/2 peak at 84.0 eV,30 which resulted in a binding
energy (BE) of 368.15 eV for the Ag 3d5/2 peak in agreement
with handbook values of 368.3 eV30 and 367.9 eV.31 For each
sample, a wide scan spectrum and the C 1s, O 1s, S 2p, and Au
4f or Ag 3d narrow scan spectra were measured. The narrow
scan spectra were normalized to the integral intensity of the
wide scan spectra to correct for small differences in sample
positions and X-ray source intensities6 and fitted by using a
Shirley-type background32 and symmetric Voigt functions33 with
a Gauss/Lorenz ratio of 4:1. To fit the S 2p3/2,1/2 doublet, we
used a pair of such peaks with the same full width at
half-maximum (fwhm), the standard30 spin-orbit splitting of
In the C 1s spectra in Figure 1a,b, a pronounced C 1s
photoemission maximum with a higher BE shoulder is observed.
The BE positions of the main maxima (284.95 eV for NC-C16/
Au and 285.15 eV for NC-C16/Ag) are very close to the
respective values for nonsubstituted (CH3-terminated) AT SAMs
on Au and Ag (284.85 and 285.25 eV, respectively).24,35
Whereas the main C 1s emission can be unequivocally attributed
to the carbon atoms in the aliphatic matrix, the shoulder at a
higher BE can only be related to the occurrence of the -CN
group: such a shoulder is not observed in the XPS spectra of
nonsubstituted ATs. Exclusively assigning the shoulder to the
nitrile carbon, we fitted it by a single peak and got a ratio
between the intensities of the main and nitrile-related peaks of
4.5-5. After a correction for the attenuation of the C 1s signal,
this intensity ratio gave us a nonrealistic value of ∼8.1 for the
ratio between the numbers of the nitrile and methylene groups.
At the same time, a fit of the shoulder by two peaks10 results in
stoichiometric intensity ratio of ∼16 after the attenuation
correction. These two peaks can only be assigned to the carbon
atoms in the nitrile functionality and nitrile-adjacent methylene
group, which means that the BE of the carbon atom in the latter
group experiences a chemical shift (∼1.3 eV) due to the bonding
to the nitrile group. The nitrile carbon exhibits a larger chemical
shift of ∼2.2 eV with respect to the carbon atoms in the alkyl
matrix.
∼1.2 eV (verified by fit), and the branching ratio of 2 (S2p3/2
/
S2p1/2). The fits were performed self-consistently: the same peak
parameters were used for identical spectral regions.
The NEXAFS measurements were carried out at the C 1s
and N 1s absorption edges in the partial electron yield mode
with a retarding voltage of -150 and -350 V, respectively.
Linear polarized synchrotron light with a polarization factor of
∼92% was used. The energy resolution was ∼0.65 eV. The
incidence angle of the synchrotron 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 within
the films. This approach is based on so-called linear dichroism
in X-ray absorption, that is, the strong dependence of the cross
section of the resonant photoexcitation process on the orientation
of the electric field vector of the linearly polarized light with
respect to the molecular orbital of interest.34
The 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 NC-C16/Ag, a spectrum
of clean silver was subtracted from the raw spectrum of a SAM
sample before the normalization.6,35 The energy scale was
referenced to the pronounced π1* resonance of highly oriented
pyrolytic graphite at 285.35 eV.36
In the N 1s spectra in Figure 1c,d, a single emission related
to the nitrile nitrogen is observed. The BEs of this emission for
Au and Ag are very close to each other: 399.95 eV for NC-
C16/Au and 400.0 eV for NC-C16/Ag.
In the S 2p spectra in Figure 1e,f, a single S 2p3/2/S 2p1/2
doublet is observed for both NC-C16/Au and NC-C16/Ag. This
doublet is usually associated with a thiolate species bonded to
the metal surface and is typical for the intact thiol-derived
SAMs.10,12,35,37,38 Similar to nonsubstituted AT SAMs,24,35 the
BEs of this doublet for Au and Ag are very close to each other:
162.1 eV for NC-C16/Au and 161.85 eV for NC-C16/Ag.
The integral intensities of the C 1s, N 1s, and S 2p emissions
for NC-C16/Ag are larger by 15-35% than those for NC-C16/
Au, which implies a higher packing density and a smaller
inclination of the alkyl chains on Ag. The same conclusion
results from the analysis of the Au 4f and Ag 3d spectra (not
shown) for NC-C16/Au and NC-C16/Ag, respectively. Consid-
ering the attenuation of the Au 4f and Ag 3d photoelectrons by
the hydrocarbon overlayer, we got an effective thickness of 19.7
and 23.5 Å for NC-C16/Au and NC-C16/Ag, respectively. Note
that the attenuation lengths of the Au 4f and Ag 3d photoelec-
trons were taken as 31 and 26 Å, respectively (Mg KR X-ray
source).5,39 Note also that the comparison of the derived effective
thicknesses to the analogous values for nonsubstituted AT SAMs
on Au and HO-substituted AT films on Ag suggested lower
packing densities of NC-C16/Au and NC-C16/Ag.
2.4. Wetting Properties. The uncut 2 in. wafer samples were
put on a Rame´-Hart contact angle goniometer equipped with
a motor to move the sample stage horizontally under the droplet.
The latter was attached to a needle at the end of a syringe filled
with ultrapure water (Millipore system, 18 MΩ cm). Ten
microliter water droplets were put on the surface, and images
of the drop silhouette were taken with a video camera and stored
for analysis on a computer while the stage was moved at a speed
of 0.05 mm per second.
3. Results
3.1. XPS Measurements. XPS spectra provide quantitative
information on the composition, chemical identity, and effective
thickness of the investigated films. The C 1s, N 1s, and S 2p
spectra of the NC-C16 SAMs on Au and Ag are presented in
Figure 1. The vertical scale is the same for the spectra in
identical spectral regions. The higher background intensity and
lower signal-to-noise ratio of the S 2p and C 1s spectra for NC-
C16/Au as compared to NC-C16/Ag are related to inelastic
3.2. NEXAFS Measurements. NEXAFS experiments pro-
vide both chemical and structural information about the occur-
rence and average orientation of unoccupied molecular orbitals
within organic films of interest. The C 1s NEXAFS spectra of
NC-C16/Au and NC-C16/Ag are depicted in Figure 2, panels a

SAMs of Nitrile-Functionalized Alkanethiols
J. Phys. Chem. B, Vol. 107, No. 31, 2003 7719
Figure 1. The C 1s (a, b), N 1s (c, d), and S 2p (e, f) XPS spectra collected from the NC-C16 films on Au (a, c, and e) and Ag (b, d, and f)
substrates. The C 1s emission structures are fitted by three Voigt functions (solid lines). Only one Voigt function was used to fit the N 1s spectra.
The S 2p3/2/S 2p1/2 doublet was decomposed using two Voigt functions with the same fwhm and Gauss/Lorentz proportion. See text for details.
and b, respectively. The corresponding N 1s spectra are
presented in Figure 3a,b.
tational order for both NC-C16/Au and NC-C16/Ag, while the
signs of the resonance-related peaks in the difference spectra
imply an upright orientation of the alkyl chains. Also, a distinct
π*(CtN*) resonance in the N 1s NEXAFS spectra of these
films exhibits a clear linear dichroism, which suggests that the
-CN moieties have a predominant orientation. The photon
energy (PE) position of this resonance is 400.83 and 400.82
eV for NC-C16/Au and NC-C16/Ag, respectively. These values
are somewhat lower than the theoretical estimate for long-chain
n-alkylnitriles (401.87 eV)48 but slightly exceed the respective
experimental PEs for HCN49 and CH3CN.50 Considering the
errors of the theoretical calculations48 and an increase in the
All spectra in Figure 2 exhibit a C 1s absorption edge related
to C 1s f continuum excitations, several pronounced π* and
σ* resonances, and two overlapping R* resonances (excitations
into orbitals of dominantly Rydberg character).40-42 The as-
signments of these resonances are given in Table 1; they were
performed in accordance with refs 34, 40, and 43-47. The
intensities of the resonances vary strongly when the angle of
incidence of the X-rays is changed, which is additionally
emphasized by the difference spectra (bottom curves). The
pronounced linear dichroism suggests a relatively high orien-

7720 J. Phys. Chem. B, Vol. 107, No. 31, 2003
Frey et al.
TABLE 1: Positions and Assignments of the C 1s NEXAFS Resonances for NC-C16/Au and NC-C16/Ag
spectral
feature
photon energy for
NC-C16/Au (eV)
photon energy for
NC-C16/Ag (eV)
fwhm
(eV)
reference photon
energy (eV)
assignment
-C*H2-
step 1
step 2
peak 1
287.8
289.6
285.3
287.8
289.6
285.3
1.7
1.7
1.2-3.05
287.8 [44]
from XPS
285.5 [46]
285.1 [47]
C*N-C*H2-
contamination,
excitation into
alkane-metal orbitals
π*(C-N)
peak 2
peak 3
peak 4
peak 5
peak 6
peak 7
286.8
287.4
288.0
289.4
292.8
300.0
286.9
287.5
288.1
289.4
293.2
300.0
0.6
0.6
0.6
1.1
3.4
286.4 [34]
287.4 [40]
288.1 [40]
Rσ*
Rπ*
σ-like or C*-S
σ*(C-C)
293.4 [44]
301.6 [44]
11.5-12.0
σ*(C-C′)
Figure 3. The N 1s NEXAFS spectra collected from NC-C16/Au (a)
and NC-C16/Ag (b). The bottom curves in panels a and b represent
the difference of the spectra acquired at incidence angles of 90° and
20°.
Figure 2. The C 1s NEXAFS spectra collected from NC-C16/Au (a)
and NC-C16/Ag (b). The bottom curves in panels a and b represent
the difference of the spectra acquired at incidence angles of 90° and
20°.
typical fit of C 1s NEXAFS spectra of NC-C16/Au and NC-
C16/Ag is depicted in Figure 4; the 55° spectra are taken as an
example.
PE for the π*(CtN*) resonance with increasing length of the
aliphatic chain,48 the obtained PE positions of this resonance
in NC-C16/Au and NC-C16/Ag agree well with the previous
results.
The derived intensities of the most pronounced π*(C*tN),
σ*(C-C), and R* resonances are presented in Figure 5 as
functions of the incidence angle of the X-rays, along with the
intensities of the π*(CtN*) resonance taken from the N 1s
NEXAFS spectra. For comparison, the theoretical dependencies
for different tilt angles of the alkyl chains and -CN moiety are
added as thin solid lines. These dependences were calculated
on the basis of the standard expressions describing angular
dependence of the transition matrix elements for the cases of
so-called “vector” and “plane” orbitals.34 The π*(C*tN) and
R* (Rσ* + Rπ*) orbitals were considered as plane orbitals, while
the σ*(C-C) was regarded as a vector orbital following the
assignments of ref 45. Accordingly, γ is the angle between the
sample normal and the normal to the π*(CtN) or R* plane,
while æ is the angle between the sample normal and the vector
along the σ*(C-C) orbital. The derived values of γ and æ are
indicated at the respective data.
Except for the qualitative conclusion on the orientational order
in the NC-C16 SAMs, values of the average tilt angle of the
alkyl chains and -CN functionalities in these films can be
obtained by a quantitative analysis of the angular dependence
of the NEXAFS resonance intensities.34 Whereas the respective
intensities can be derived directly from the N 1s spectra, a
decomposition of the C 1s spectra is necessary. This decomposi-
tion was performed self-consistently for the entire spectra series;
several Gauss-like peaks and two adsorption edges were used
to fit the spectra following the recipes of ref 34. The second,
low-intensity absorption edge was introduced to represent the
carbon atoms in the nitrile group and the nitrile-adjacent
methylene moiety. This edge was shifted upward by ∼1.8 eV
(an average of 1.3 and 2.2 eV) with respect to the major edge
following the difference between the BE positions of the
respective maxima in the C 1s XPS spectra in Figure 1a,b. A
Because the R* orbital is perpendicular to the alkyl chain
axis and the σ*(C-C) orbital is directed along this axis,40,44,45

SAMs of Nitrile-Functionalized Alkanethiols
J. Phys. Chem. B, Vol. 107, No. 31, 2003 7721
Figure 5. The intensity of the π*(C*tN) (a, b), σ*(C-C) (c, d), R*
(a, b), and π*(CtN*) (a, b) resonances for NC-C16/Au (a, c) and NC-
C16/Ag (b, d) as a function of the incidence angle of the X-rays. For
comparison, the theoretical dependencies for different tilt angles of the
alkyl chains and CtN moiety are added as thin solid lines. The π*-
(CtN) and R* orbitals were considered as plane orbitals, while the
σ*(C-C) was regarded as a vector orbital. Accordingly, γ is the angle
between the sample normal and the normal to the π*(CtN) or R*
plane, while æ is the angle between the sample normal and the vector
along the σ*(C-C) orbital. The derived values of γ and æ are indicated
at the respective data. Note that the superposition of the Rσ* and Rπ*
orbitals can be considered as a plane orbital because both vector
constituents are perpendicular to each other and have similar oscillator
strengths.34,40
Figure 4. A fit of the C 1s NEXAFS spectra of NC-C16/Au (a) and
NC-C16/Ag (b). The 55° spectra are taken as an example. The entire
spectra sets in Figure 2a,b were fitted self-consistently.
both γ and æ are average tilt angles of the alkyl chains. The
derived values of γ are 40° (Au) and 38° (Ag) for the R* orbital,
while the values of æ are 45° (Au) and 36° (Ag) for the σ*-
(C-C) orbital. Taking the mean of these values, we got an
average tilt angle of the alkyl chain of 42.5° and 37° for NC-
C16/Au and NC-C16/Ag, respectively.
As to the -CN moiety, the angle γ describes the orientation
of the CtN axis with respect to the sample normal. Considering
the data for π*(CtN*) resonance, one sees that the respective
dependences do not follow the theoretical curves, which makes
impossible the estimate of γ. We believe that this problem is
related to the difficulties in the normalization of the N 1s
NEXAFS spectra (Figure 3), which do not reveal a pronounced
absorption edge. At the same time, the data for the π*(C*tN)
resonance can be roughly described by the theoretical depend-
ences, especially in the case of NC-C16/Ag. The derived value
of γ is ∼65° for both NC-C16/Au and NC-C16/Ag.
where Ip(θ) and Ip(θ1) are the resonance intensities and C is a
normalization constant that depends on the excitation probability
from the C 1s core level into a given molecular orbital. Even
though the latter parameter is unknown, the expression 1 can
be used for the evaluation of the NEXAFS spectra, if one has
a reference sample with the same molecular orbitals and a
known molecular structure.
As such reference samples, we used SAMs formed from
CH3(CH2)15SH (C16), CH3(CH2)17SH (C18), and HO(CH2)16-
SH (HO-C16) on Au and films fabricated from HO-C16 on Ag.
In accordance with previous NEXAFS investigations,16,44 we
assumed that the average tilt angles of the alkyl chains in these
systems are equal to 35° for C16/Au, C18/Au, and HO-C16/
Au. Considering that no significant changes in the orientation
of the alkyl chains in nonsubstituted AT SAMs on Au occur
upon replacing the terminal -CH3 group by the -OH moiety,16
we assumed that the average tilt angle of the alkyl chains in
HO-C16/Ag does not differ significantly from that in the
nonsubstituted AT SAMs on Ag12,35,38,52 and is approximately
equal to 12°.
It should be mentioned that the fits presented in Figure 4 are
the result of the optimization of all relevant parameters. In doing
such an optimization, a large importance has a position and a
width of the major absorption edge, which have to be preset
before the fitting procedure. Even though these parameters are
optimized in the course of the fitting procedure and the resulting
values agree well with literature data,16,44 the initial settings
influence the results. To avoid this ambiguity and to perform
an independent proof of the derived tilt angles, we processed
the C 1s NEXAFS data using the difference spectra, which are
presented in Figure 6. As described in refs 34, 43, and 51, upon
subtracting two NEXAFS spectra recorded at different X-ray
incidence angles θ and θ1, one gets for a plane orbital
We used R* and π*(C*tN) resonances to derive qualitative
and quantitative information on the orientation of the alkyl
chains and -CN moiety in the NC-C16 SAMs. The intensities
Ip(θ) - Ip(θ1) ) C(1 - cos2 γ)(cos2 θ - cos2 θ1) (1)

7722 J. Phys. Chem. B, Vol. 107, No. 31, 2003
Frey et al.
Figure 7. Plots of the intensities of the R* (a) and π*(C*tN) (b)
difference peaks (see Figure 6) for NC-C16/Au (2) and NC-C16/Ag
(1) versus cos2 θ - cos2 θd (θd ) 20°) with the respective linear fits
(the solid lines) using least-squares analysis. The analogous fits for
the reference samples, C16/Au (the dotted line) and HO-C16/Ag (the
dashed line) are presented in panel a for comparison. A fit for NC-
C16/Au (dashed line) in panel b was performed both with and without
the consideration of the data point derived from the “90°-20°”
spectrum.
Figure 6. The differences of the NEXAFS spectra recorded at X-ray
incidence angles 40°-90° and 20° for NC-C16/Au (a) and NC-C16/
Ag (b).
of the respective difference peaks were derived by numerical
integration of the difference spectra (Figure 6) in the energy
intervals 286.1-287.0 and 287.0-288.6 eV for the π*(C*tN)
and R* resonances, respectively. The plots of the intensities of
the R* and π*(C*tN) difference peaks for NC-C16/Au and
NC-C16/Ag versus cos2 θ - cos2 20° are depicted in Figure 7,
along with the lines that are best fits to the data using least-
squares analysis based on eq 1. The analogous fits for two
reference samples, C16/Au (dotted line) and HO-C16/Ag
(dashed line), are presented in Figure 7a for comparison. It is
clearly seen that the slopes of the linear fits for NC-C16/Au
and NC-C16/Ag are noticeably smaller than the slopes of the
lines for the respective reference samples: C16/Au for NC-
C16/Au and HO-C16/Ag for NC-C16/Ag, which means a larger
tilt of the alkyl chain in the case of the -CN substitution.
Quantitative evaluation based on the C16/Au, C18/Au, and HO-
C16/Au standards gives values of 44°, 44°, and 41°, respectively,
for the average tilt angle of alkyl chains in NC-C16/Au. For
NC-C16/Ag, the same procedure results in average tilt angles
of 30° and 29° using the C16/Au and HO-C16/Ag reference
samples, respectively. Thus, the values derived on the basis of
different standards are very close to each other, which empha-
sizes their reliability. In addition, the values for NC-C16/Au
practically coincide with the average tilt angle derived via the
fitting of the resonance intensities. Only in the case of NC-
C16/Ag, there is a difference of ∼7° between the average tilt
angles obtained from the spectra difference approach (29°, 30°)
and from the fitting of the resonance intensities (37°).
a conclusion on relative orientation of these functionalities in
NC-C16/Au and NC-C16/Ag can be made. Considering the data
presented in Figure 7b, one finds that the linear fits for NC-
C16/Au and NC-C16/Ag have almost the same slopes, as soon
as the data point derived from the “90°-20°” spectrum for NC-
C16/Au is not taken into account. The inclusion of this point
will result in only a minor increase in the slope of the respective
linear fit, which means only a slight increase in the tilt angle of
the -CN functionality. Thus, average tilt angle of the -CN
functionality in HO-C16/Au is very close to or slightly exceeds
that for NC-C16/Ag, which agrees well with the results of the
direct fitting (∼65° for both NC-C16/Au and NC-C16/Ag).
3.3. Water Contact Angle Measurements. NC-C16/Au
showed advancing (θa) and receding contact angles (θr) of 75°
and 54°, respectively (see Figure 8). On Ag, both angles are
slightly sharper with θa ) 69° and θr ) 52°. The hysteresis θa
- θr is relatively large in comparison with the previously53
reported <10° differences for similarly wettable methyl ester
surfaces, especially if we consider that (cos θa - cos θr) is the
real thermodynamic measure for the difference between the
surfaces immediately before and after wetting. The larger
hysteresis of the nitrile-terminated surfaces indicates that the
small -CN end group has some space for rearragements. The
nitrile end groups are buried inside the nonwetted film and
rearrange and become more exposed as soon as the droplet
contacts the film surface.
In the case of the π*(C*tN) resonance, a quantitative
evaluation of the data presented in Figure 7b is not possible
because of the absence of a suitable reference sample containing
the -CN functionality in a known orientation. Therefore, only
The average (θa + θr)/2 contact angle is 64.5° on Au and
60.5° on Ag. That is far less hydrophobic than methyl- or
methylene-terminated surfaces (100°-115°) and even below the
85°-78° for methoxy-terminated monolayers. The dipole mo-

SAMs of Nitrile-Functionalized Alkanethiols
J. Phys. Chem. B, Vol. 107, No. 31, 2003 7723
the N 1s NEXAFS spectra. Because of the same reason, the
difference spectra approach does not work for the N 1s data as
well.
In the case of the C 1s NEXAFS spectra, both fitting of the
resonance intensities and the difference spectra approach lead
to reasonable results. The former procedure gives values of 42.5°
and 37° for the average tilt angle of the alkyl chains in NC-
C16/Au and NC-C16/Ag, respectively, and a value of 65° for
the average tilt angle of the -CN moieties in both of these
systems. The derived tilt angles vary somewhat depending on
the preset of the absorption edges within the spectra fitting
procedure. The difference spectra approach results in values of
41°-44° and 29°-30° for the average tilt angle of the alkyl
chains in NC-C16/Au and NC-C16/Ag, respectively. For the
-CN moiety, only a qualitative estimate was possible. The data
suggest a similar orientation of the -CN moieties in NC-C16/
Au and NC-C16/Ag with a slightly larger tilt of these moieties
in NC-C16/Au than in NC-C16/Ag.
The contact angle measurements indicate that the SAM/
ambient interface consists, as expected, of the terminal -CN
groups. The large hysteresis of the contact angle implies that
these groups have some space for rearrangements, which agrees
with their orientation derived from the NEXAFS data. The in-
surface-oriented nitrile groups can easily rearrange and become
more exposed as soon as the droplet contacts the film sur-
face.
Figure 8. Dynamic advancing and receding water contact angles of
NC-C16/Au (a) and NC-C16/Ag (b). The tip with the attached water
droplet was kept still while a motor moved the underlying sample stage
horizontally with a constant speed of 0.05 mm per second.
ment of the -CN group and the free electron pair at the nitrogen
atom produce a surface that is as hydrophilic as a surface with
densely packed methyl ester groups53 or oligo(ethylene glycol)4
derivatives such as -(OCH2CH2)-OCH3. The nitrile group is
the only two-atom end group so far that both is hydrophilic
and fulfills all four molecular characteristics18 that usually
correlate with low protein adsorption (large polarity, a hydrogen
bond accepting group, no hydrogen bond donating groups, and
no net charge).
Considering all experimental data together, we can conclude
that the results of the different experimental techniques and
different data processing approaches agree quite well. A larger
packing density, a smaller inclination of the alkyl chains, and
a higher degree of the orientational and conformational order
is observed for NC-C16/Ag as compared to NC-C16/Au (the
latter conclusion is also supported by the preliminary infrared
reflection spectroscopy (IRS) results).54 Also, similar orientations
of the -CN functionalities, predominantly in the surface plane,
occur in both systems. The average tilt angle of the alkyl chains
in NC-C16/Au was estimated to be 42.5°. In the case of NC-
C16/Ag, we have to choose between the somewhat different
values obtained from the difference spectra approach (29.5°)
and from the fitting of the NEXAFS resonance intensities (37°).
Taking into account the noticeable difference in the XPS-derived
effective thicknesses for NC-C16/Au and NC-C16/Ag, the
above-mentioned ambiguity of the spectra fitting, and prelimi-
nary results of infrared spectroscopy experiments,55 we rather
trust the results of the difference spectra approach, which infers
that an average tilt angle of the alkyl chains in NC-C16/Ag is
equal to 29.5°. The average tilt angle of the -CN moieties in
NC-C16/Au and NC-C16/Ag was found to be 65°.56 The error
of the derived angles is estimated to be (5° and (7° for the
tilt angles of the alkyl chains and -CN moieties, respectively.
A value of (5° is a standard accuracy for the NEXAFS-derived
tilt angles, whereas the second value ( (7°) takes into account
large deviations of the respective experimental data from the
theoretical curves (Figure 5). Note that the obtained tilt angles
for the -CN moieties agree quite well with the theoretical
estimate from ref 13. The latter value is equal to 71.5°. It could
be derived on the basis of the calculated values for out-of-plane
and in-plane components of the average dipole moment of the
-CN functionality in NC-C16/Au.13
4. Discussion
The XPS, NEXAFS, and contact angle data imply that NC-
C16 forms ordered, densely packed SAMs on polycrystalline
gold and silver substrates with predominantly (111) orientation.
The NC-C16 molecules are bonded to the substrate in the same
manner as those in nonsubstituted AT SAMs, as concluded from
the observation of the characteristic S 2p doublet in the XPS
spectra.
The XPS data suggest a higher packing density or a larger
effective thickness for NC-C16/Ag as compared to NC-C16/
Au. The latter parameter was estimated to be 23.5 and 19.7 Å
for NC-C16/Ag and NC-C16/Au, respectively. A chemical shift
of ∼2.2 eV for nitrile carbon and ∼1.3 eV for the carbon atom
in the nitrile-adjacent methylene group were found in good
agreement with previous results.10
The C 1s and N 1s NEXAFS spectra reveal absorption
resonances that are characteristic for the intact NC-C16 SAMs.
A pronounced linear dichroism of these resonances suggests a
good orientational order in the NC-C16 films. The signs of the
resonance peaks in the difference NEXAFS spectra implies an
upright orientation of the alkyl chains and a predominant
orientation of the -CN moieties within the surface plane for
both NC-C16/Au and NC-C16/Ag. A quantitative evaluation
of the NEXAFS data was performed in two different wayss
via the fitting of the resonance intensities and via the difference
spectra approach.
The resonance intensities derived from the N 1s NEXAFS
spectra behave in a similar way for NC-C16/Au and NC-C16/
Ag but do not follow a theoretical expression for the resonance
intensity vs the incidence angle of the X-rays, which is
presumably related to the difficulties in the normalization of
These results have to be compared with the analogous
parameters for the nonsubstituted AT SAMs on Au and Ag. In
this context, it is worth mentioning that even for these “simple”
systems the measured tilt angles depend on the technique
applied:39 slightly different values are derived from IR specros-
copy,12 X-ray diffraction,4,57,58 and NEXAFS43-46 data. The

7724 J. Phys. Chem. B, Vol. 107, No. 31, 2003
Frey et al.
difference is presumably related to the intrinsic features of the
above-mentioned methods. In particular, X-ray-diffraction probes
only the ordered (crystalline) regions,4 whereas NEXAFS
spectroscopy gives an average value of the tilt angle over the
X-ray spot region, including both ordered and disordered
(domain boundaries, defect sites, etc.) areas. This is probably
the reason that the latter tilt angles exceed the values derived
from X-ray diffraction by several degrees. Keeping this differ-
ence in mind, we still prefer to use the tilt angles determined
by the same method as that used in the present study (NEXAFS
spectroscopy) for the comparison.
5. Summary and Future Work
Chemical identity, packing density, and orientational and
conformational order in SAMs formed from nitrile-functional-
ized alkanethiols on (111) gold and silver substrates were studied
by using XPS, NEXAFS spectroscopy, and contact angle
measurements. The NEXAFS spectra were acquired at the C
1s and N 1s absorption edges with both direct fitting and the
difference spectra approach applied to process the data. The
results of the different experimental techniques and different
procedures of the data processing agree quite well with one
another and suggest that NC-C16 form well-ordered, densely
packed SAMs on (111) gold and silver substrates. A larger
packing density, a smaller inclination of the alkyl chains, and
a higher degree of the orientational and conformational order
were found in NC-C16/Ag as compared to NC-C16/Au. The
average tilt angles of these chains in NC-C16/Ag and NC-C16/
Au were estimated to be 29.5° ( 5° and 42.5° ( 5°,
respectively. The -CN functionalities were found to be
predominantly oriented in the surface plane with same tilt angle
of 65° ( 7° in both NC-C16/Au and NC-C16/Ag. This
orientation is beneficial to minimize the electrostatic interaction
between the -CN moieties but different from the standard
orientation of the functional groups in nonsubstituted AT SAMs
of the same chain length.
Comparison with the SAMs formed from the nonsubstituted
ATs suggests that the strongly interacting nitrile entities have a
strong influence on the molecular orientation, packing, and
structure of AT films on both Au and Ag substrates. The
substitution of the weakly interacting methyl groups by the
nitrile entities results in increase in the average tilt angles of
the alkyl chains by ∼7.5° and ∼17.5° in AT/Au and AT/Ag,
respectively. This increase is presumably related to the disorder-
ing of the alkyl chains (via the formation of gauche defects) in
NC-C16/Au and their rearrangement (as compared to the
respective nonsubstituted AT SAM) in NC-C16/Ag due to a
strong electrostatic interaction between the polar -CN moieties,
which distorts the underlying 2D-lattice formed by AT chains.
These types of effects are very important because they offer a
means to control and understand the detailed structure of a SAM
and its ambient surface for applications such as tailored wetting
and biologically active surfaces. Accordingly, to better define
the conformational order, molecular orientation, and translational
order correlations in these SAMs, work is underway on the
application of quantitative infrared reflection spectroscopy (IRS)
and molecular resolution atomic force microscopy. The pre-
liminary IRS results support both the conclusion about a
significant decrease in chain conformational order and molecular
orientation changes in NC-C16/Au compared to SAMs on Ag.54
The combination of these results with the XPS and NEXAFS
data will provide more accurate structural characterizations.
NEXAFS spectroscopy was found to be capable of providing
information on the orientation of small tail groups in self-
assembled monolayers, even though some problems related to
a correct normalization of the absorption spectra have to be
overcome in the future.
The NEXAFS-derived (average) tilt angles of the alkyl chains
in the nonsubstituted AT SAMs on Au and Ag are ∼35° and
∼12°, respectively.35,38,43-46 It is then obvious that the substitu-
tion of weakly interacting -CH3 tailgroups by strongly interact-
ing polar groups results in a noticeable disordering or reorien-
tation or both of the AT SAMs on both Au and Ag. The
disordering presumably occurs through the formation of gauche
defects in the alkyl chains, which have a zigzag planar
conformation in nonsubstituted, long-chain AT SAMs. For NC-
C16/Au, the degree of this disordering is comparable with that
for AT SAMs with -COOH tailgroup,16 in which the average
tilt angle of the alkyl chains is larger by 7.5°-8° than this value
in nonsubstituted AT/Au. The reason for the disordering in the
carboxylic-acid-substituted SAMs is presumably strong hydro-
gen bonds between the -COOH groups, which lead to their
dimerization and subsequently prohibit the formation of well-
ordered films. In the case of the -CN groups, it is a strong
electrostatic interaction between these moieties that distorts the
orientational order. Theoretical estimates showed that such an
interaction can even result in the formation of an ordered,
ferroelectric structure at the SAM-ambience interface.13 The
respective 2D lattice formed by the polar -CN groups should
be rotated and distorted with respect to the underlying arrange-
ment formed by AT chains.13 The formation of this hypothetical
ordered lattice cannot of course be proven by either XPS or
NEXAFS spectroscopy but rather by surface-sensitive scattering
experiments such as He beam diffraction (e.g., see ref 59).
What can, however, be done, it is to consider the orientation
of the -CN groups in the NC-C16 SAMs and compare it with
the expected orientation in the case of all-trans conformation
and the standard12 tilt and twist of the AT chains. Assuming
the substrate-S-C bonding angles around 104° for AT/Au and
180° for AT/Ag,7,8,12,52 one gets “theoretical” tilt angles of the
-CN moiety of 59.3° and 25.7° for NC-C16/Au and NC-C16/
Ag, respectively. Another reference is the set of experimental
values for the tilt angle of the methyl entity in the AT SAMs
with an even number of methylene units in the alkyl chain.
These angles were estimated to equal 53°52 and 58°60 for AT/
Au and 27°52 or 38.1°60 for AT/Ag, close to the calculated
values. Comparing these values with the derived average tilt
angles of the -CN moieties in NC-C16/Au and NC-C16/Ag
(65°), one finds that there is only a slight reorientation of the
-CN moiety in NC-C16/Au due to the electrostatic interaction,
whereas the reorientation of these groups in NC-C16/Ag is quite
large (27°-39°). This higher extent of the reorientation agrees
well with a large increase in the average tilt angle of alkyl chains
in NC-C16/Ag as compared to the nonsubstituted AT SAMs
on Ag. This increase can be related to both a disordering and a
completely new arrangement of the AT chains in NC-C16/Ag
as compared to the nonsubstituted SAMs. Taking into account
the IR data for NC-C16/Ag, we rather believe that a rearrange-
ment is the case.
Another interesting issue is the orientation of the -CN
functionalized AT SAMs with an odd number of methylene units
in the alkyl chain. Preliminary experiments on SAMs formed
from NC(CH2)15SH (NC-C15) on Au exhibit noticeable differ-
ences as compared to NC-C16/Au, giving rise to so-called odd-
even effects.54 This work will be reported in future publications.
Acknowledgment. The authors are very grateful to M.
Grunze for his support. We thank the BESSY staff, especially

SAMs of Nitrile-Functionalized Alkanethiols
J. Phys. Chem. B, Vol. 107, No. 31, 2003 7725
(29) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.; Ulman,
A. Langmuir 2001, 17, 8.
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M. Mast for technical help, K. Heister for assistance at BESSY,
G. Albert for preparation of the substrates, and Ch. Wo¨ll
(Universita¨t Bochum) for providing us with experimental
equipment. This work has been supported by the German BMBF
( Grants 05 SF8VHA 1 and GRE1HD), Deutsche Forschungs-
gemeinschaft (Grant JA 883/4-1), the Fonds der Chemischen
Industrie, the U.S. Defense Advanced Research Program Agency
and Office of Naval Research (Grant DARPA/ONR; for D.L.A.
and P.H.), and the U.S. National Science Foundation (D.L.A.).
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