J. Phys. Chem. C 2007, 111, 16909-16919
16909
Influence of Molecular Structure on Phase Transitions: A Study of Self-Assembled
Monolayers of 2-(Aryl)-ethane Thiols
Piotr Cyganik,*,†,‡ Manfred Buck,*,† Thomas Strunskus,‡ Andrey Shaporenko,§ Gregor Witte,‡
Michael Zharnikov,§ and Christof Wo1 ll‡
EaStChem School of Chemistry, St. Andrews UniVersity, North Haugh, St. Andrews, KY16 9ST,
United Kingdom, Lehrstuhl für Physikalische Chemie I, UniVersitätsstrasse 150, 44801 Bochum,
Germany, and Angewandte Physikalische Chemie, UniVersität Heidelberg, Im Neuenheimer Feld 253,
69120 Heidelberg, Germany
ReceiVed: May 23, 2007; In Final Form: August 11, 2007
Self-assembled monolayers (SAMs) prepared on Au(111) substrates from solutions of ω-(4′-methylbiphenyl4-yl)ethane thiol (CH3(C6H4)2(CH2)nSH, n ) 2, BP2), at room temperature and subsequently annealed at
temperatures of up to 423 K were studied using scanning tunneling microscopy, low-energy electron diffraction,
high-resolution X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy.
Upon annealing a phase transition occurs from the low-temperature (5x3 × 3) structure common to all
SAMs prepared from the series of BPn homologues with n ) even studied so far, to a new structure which
is markedly different from the high-temperature phases of the higher BPn homologues. Although its basic
structure can be approximated by a (2x3 × 2) unit cell, the regular occurrence of line defects running
exclusively along the 〈112h〉 direction is the most characteristic feature of this new phase. Irrespective of these
defects the phase transition dramatically improves the stability of the BP2 monolayer as demonstrated by
exchange experiments. In contrast to BP2, SAMs made from the closely related 2-phenylethane thiol
(C6H5(CH2)2SH, P2) do not show any phase transition. The differences between BP2, its higher homologues,
and P2 highlight the subtleties of the interplay of different factors determining the structure of a SAM.
I. Introduction
A key challenge for molecular electronics is the controlled
arrangement of molecular entities exhibiting electronic functionality on a length scale ranging from externally addressable
down to molecular dimensions. This requires materials which
combine the desired electronic properties with the possibility
of patterning them at very different length scales. Aromatic selfassembled monolayers (SAMs) is one class of systems which
are currently explored along this route.1,2 Experiments performed
in recent years have demonstrated that aromatic SAMs allow
control of charge transfer and tailoring of electronic
functionality3-12 but also offer new opportunities in
lithography.13-19 However, to address the nanometer scale in
aromatic SAMs, a high level of control over the structure and
properties of these systems is of key importance. At present,
unfortunately, it is not possible to precisely control the structure
of a SAM in a rational fashion (i.e., by deliberately adding
functional groups to the organothiol) since the properties of a
SAM are determined by a complex interplay of several factors
including intermolecular interactions, molecule-substrate bonding, and lattice mismatch between the molecular lattice and the
substrate, to just name a few. The latter factor is presumably a
major limiting factor for the structural quality of thiol-derived
SAMs with a purely aromatic backbone, in which the mismatch
between the packing preferred by the rigid aromatic layer20 and
the substrate lattice results in significant stress. This stress is
released predominantly by the formation of defects (i.e., domain
* Corresponding authors. E-mail: [email protected] (P.C.); mb45@
st-and.ac.uk (M.B.)
† St. Andrews University.
‡ Lehrstuhl für Physikalische Chemie I.
§ Universität Heidelberg.
boundaries, dislocation faults, point defects)21 which results in
the observed lack of structural quality in purely aromatic thiolderived SAMs.21-26 As demonstrated in our previous studies
on biphenyl-substituted alkanethiols (CH3-(C6H4)2-(CH2)nSH, BPn, n > 1)27 this problem can be partly overcome by
introducing an alkane spacer chain between the thiol head group
and the aromatic moiety. The individual thiolates forming these
SAMs have additional degrees of freedom (e.g., conformational
ones) through which stress is reduced without breaking up the
structure.27 However, beyond the improvement of the quality
of the film the insertion of the “flexible” alkane spacer also
exerts a crucial influence on the film structure, i.e., the molecular
orientation and packing. Our previous studies28-31 have revealed
that BPn SAMs exhibit a pronounced alternation in molecular
orientation and packing density with the change between n )
odd and n ) even. This odd-even variation is also reflected in
a number of properties of the BPn system such as stability
against exchange by other thiols,32 electrochemical stability,19,33
and electron-induced modification.34 However, the most striking
manifestation of the odd-even behavior is probably, as
demonstrated recently, the molecule-dependent occurrence of
polymorphism35-37 and irreversible phase transitions35,36 which
occurs only for n ) even. Such a transition is accompanied by
a dramatic increase in the level of perfection, resulting in
domains with the area exceeding 105 nm2. Furthermore, this
phase transition causes a surprising switch in stability against
exchange by other thiols when immersed into the corresponding
solutions. The phase transition seen in these biphenyl-based
organothiolate adlayers is fundamentally different from temperature-induced structural transitions of alkanethiolate adlayers
into low-density “flat lying” phases38-43 which are not stable
when immersed in solutions under conditions where a high-
10.1021/jp073979k CCC: $37.00 © 2007 American Chemical Society
Published on Web 10/19/2007
16910 J. Phys. Chem. C, Vol. 111, No. 45, 2007
density phase forms. Detailed spectroscopic and microscopic
investigations of the structures of BPn SAMs28-31 and the phase
transitions observed in SAMs of BP4 and BP6 provided
substantial insight into the basic factors behind these phenomena.36 The model which emerged from these studies explains
the occurrence of phase transitions and their dependence on the
molecular structure by the either cooperative or competitive way
the different factors determining the energetics of a SAM enter
into the energy balance. This is illustrated by a simple qualitative
model (see Figure 11 in ref 36) where the Au-S-C bending
potential and the density of S-Au bonds along with intermolecular interactions are key factors determining the energy of
the system. Whereas for n ) odd all factors act cooperatively,
the Au-S-C bending potential opposes the other factors in
even-numbered BPn SAMS. As a consequence, several structures exist for BPn SAMs with n ) even which are similar in
energy and thus can undergo thermally induced structural
transitions.
Building on the studies of BP4 and BP6, the present work
explores the idea of competitive design further. Choosing
molecules which are closely related to the ones investigated
previously but which exhibit systematic differences this work
aims at an improved understanding of the relative importance
of structure-determining factors.
II. Experimental Section
Sample Preparation. The synthesis of the ω-(4′-methylbiphenyl-4-yl)ethane thiol (CH3(C6H4)2(CH2)nSH, BPn, n ) 2)
molecule has been described elsewhere.28 2-Phenylethane thiol
(C6H5(CH2)2SH, P2) has been obtained from Aldrich. Commercially available Au/mica substrates from Georg Albert PVD,
consisting of 150 nm Au evaporated onto mica (rate 2 nm/s,
temperature 340 °C), were flame-annealed in a butane/oxygen
flame and subsequently immersed into a 0.1 mM solution of
BP2 or P2 in ethanol at room temperature for 24 h. After
immersion, samples were rinsed with pure ethanol and blown
dry with nitrogen. Annealing of the SAMs was done in a sealed
container which was purged with nitrogen prior to temperature
treatment.
Scanning Tunneling Microscopy (STM) Measurements.
All STM measurements were carried out in air at room
temperature using a Molecular Imaging Picoscan STM instrument. In all cases tips were prepared mechanically by cutting a
0.25 mm Pt/Ir alloy (8:2, Goodfellow) wire. The data were
collected in constant current mode using tunneling currents
between 400 and 700 pA and a sample bias between 600 mV
and 1.0 V (tip positive). No tip-induced changes were observed.
Low-Energy Diffraction (LEED) Measurements. LEED
data were obtained in a multichamber ultrahigh vacuum (UHV)
system using a microchannel plate LEED system (OCI) and a
charge-coupled device (CCD) camera connected to a frame
grabber card for image acquisition. This system is capable of
recording LEED data with very low electron fluxes (5-15 nA/
cm2)44 and thus allows us to reduce total electron dose during
the entire LEED experiment (up to 5 min) to 1.5-4.5 µC/cm2,
which is far below the doses leading to electron-induced damage
of BP2 and P2 SAMs.34,45,46 Even though a diffraction pattern
of the SAM was observed at room temperature, the Au single
crystal was heated to 340 K for a few minutes to improve the
quality of the data. For the LEED measurements the sample
was cooled down to 110-120 K to reduce the inelastic
background and to increase the intensity of the diffraction spots
which are determined by the Debye-Waller factor.
High-ResolutionX-rayPhotoelectronSpectroscopy(HRXPS)
Measurements. The HRXPS experiments were performed at
Cyganik et al.
the bending magnet beamline D1011 at the MAX II storage
ring of the MAX-lab synchrotron radiation facility in Lund,
Sweden. The HRXPS spectra were acquired in normal emission
geometry at photon energies of 350 and 580 eV for the C 1s
range and 350 eV for the S 2p region, respectively. In parallel,
the Au 4f 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 the
latest ISO standard.47 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.29 The energy
resolution was better than 100 meV (typically ∼60 meV), which
is noticeably smaller than the full width at half-maximum
(fwhm) of the photoemission peaks addressed in this study.
Thus, these fwhms are representative for the natural widths of
the respective lines. HRXPS spectra were fitted by symmetric
Voigt functions and a Shirley-type background. To fit the S
2p3/2,1/2 doublet we used two peaks with the same fwhm, the
standard48 spin-orbit splitting of ≈1.18 eV (verified by a fit),
and a branching ratio of 2 (S 2p3/2/S 2p1/2). The fits were
performed self-consistently: the same fit parameters were used
for identical spectral regions.
Near-Edge X-ray Absorption Fine Structure (NEXAFS)
Spectroscopy Measurements. The 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 P 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 30° (E-vector near surface normal). The raw NEXAFS
spectra were normalized to the incident photon flux by division
through a spectrum of a clean, freshly sputtered gold sample.
The energy scale was referenced to the pronounced π* resonance
of highly oriented pyrolytic graphite at 285.38 eV.
Contact Angle Measurements. Advancing contact angles
of distilled water were measured with a Krüss goniometer,
model G10. The experiments were performed under ambient
conditions with the needle tip in contact with the drop. Averaged
values of at least 10 measurements at different locations on each
sample are reported here. Deviations from the average were less
than (1°.
III. Results
ω-(4′-Methylbiphenyl-4-yl)ethane Thiol (CH3(C6H4)2(CH2)2SH, BP2). STM Measurements. Figures 1 and 2 summarize STM data obtained for the BP2 system prepared at room
temperature from the respective solution and then annealed at
different temperatures. Images collected at large scale (Figure
1a-d) reveal two important features. First, in comparison to
BP2 samples prepared at room temperature (see Figure 5 in ref
31 for direct comparison) the density of substrate depressions49-51
(black areas in the STM images) is strongly reduced due to
Ostwald ripening. Although this is commonly observed upon
annealing of SAMs,52,53 the extent to which this occurs for BP2
is more pronounced compared to other types of thiol SAMs
and is comparable to its higher homologues BP4 and BP6.36
Second, proper annealing conditions yield a perfectly uniform
SAM (Figure 1c), whereas lower (Figure 1, parts a and b) or
higher (Figure 1d) temperatures result in heterogeneous SAMs.
Closer inspection of Figure 1, parts a and b, reveals the
coexistence of two ordered phases one of which is the R-phase
SAMs of 2-(Aryl)-ethane Thiols
J. Phys. Chem. C, Vol. 111, No. 45, 2007 16911
Figure 1. BP2 SAMs on Au(111). STM images with different resolutions showing samples prepared at room temperature and subsequently annealed
in N2 atmosphere for 15 h at 373 (a), 393 (b and e), 417 (c and f), and 423 K (d). R and δ indicate the areas covered by different phases as described
in the text. White dotted loops in (d) indicate the destroyed areas of the δ structure. White dashed lines in (e) indicate the translational domains with
the unit cell shifts marked by white solid lines.
known from our previous studies of BPn SAMs. The R-phase
is a rectangular (5x3 × 3) structure with eight molecules per
unit cell and an area per molecule of 27.05 Å2. This structure
is characteristic for BPn systems with an even number of CH2
units prepared from solution at room (or slightly elevated)
temperature.27,30,31 Similar to BP4 and BP6,36 annealing of BP2
SAMs results in a modification of the R-phase. As shown by
the high-resolution image reproduced in Figure 1e narrowly
spaced translational domain boundaries are formed which, as
indicated by the white dashed lines, are about 5-7 nm apart.
We interpret the formation of translational domain boundaries
as relief of stress. As discussed previously27 stress in SAMs
originates from the misfit between the molecular lattice and the
substrate lattice. Upon increase of temperature this stress is then
released by forming domain boundaries. Whereas for purely
aromatic thiol SAMs, e.g., anthracene thiol, a high density of
translational domains is already observed at room temperature,21
for BPn films prepared under corresponding conditions the
density of such defects is rather low. This can be rationalized
by taking into account that the conformational degrees of
freedom of the alkane spacer (in combination with a possible
restructuring of the Au-S interface) make the BPn systems more
tolerant to lattice mismatch.27 However, there seems to be a
limit to this “soft” way of stress reduction and continuous growth
of domains at higher annealing temperatures demands more
effective ways to relieve stress resulting in the break up of the
regular molecular pattern as seen in Figure 1e and observed
previously for BP4 and BP6.36
The other phase labeled δ seen in Figure 1, parts a and b, is
not present in the native BP2 SAM but emerges during
annealing. In contrast to the annealed R-phase with its high
density of translational domain boundaries, the new δ-phase is
of pronounced uniformity as illustrated by Figure 1f. Increasing
the annealing temperature (keeping the same annealing time of
15 h) from 373 to 393 K (Figure 1, parts a and b) results in a
continuous increase of the area occupied by the δ-phase and
yields a complete transition at 408 K (Figure 1c). Even higher
temperatures cause a gradual damage of the SAM resulting in
loss of molecular order in the dark areas marked by dotted loops
in Figure 1d. This process starts mainly at steps for which we
hold responsible possible defects in the SAMs related to (1)
the distortion of the local SAM structure by the substrate step
and/or (2) possible accumulation of contaminations at substrate
steps prior to SAM formation.
For a detailed analysis of the structure of the δ-phase we
turn to the high-resolution STM images presented in Figure 2.
The first characteristic feature of this structure is the appearance
of densely packed rows of molecules which alternate in the STM
contrast. These rows running along the 〈11h0〉 directions are
common for the structures reported previously for the odd and
even BPn systems30,31 and can be assigned to the herringbone
arrangement of the biphenyl moieties. In the case of the δ-phase,
however, the orientation of the alternating rows is along the
〈11h0〉 instead of the 〈112h〉 directions observed for the R-phase.
This is also different from the β-phase of SAMs of BP4/BP6
where the rows also run along the 〈112h〉 direction. This indicates
a significant rearrangement of the molecules during the R f δ
phase transition. Another characteristic feature of the δ-phase
are dark lines running perpendicular to the alternating molecular
rows. From the images taken on a larger scale (Figure 1f) it
follows that these dark lines define a large unit cell of the δ
structure. However, a closer inspection of the structure (see
Figure 2a) reveals that there is not just a single value for the
distance between these lines but discrete values are measured
for the long axis of the unit cell which jump between about 48
and 62 Å. This suggests that these lines correspond to highly
ordered line defects. The high-resolution data presented in Figure
2b together with the corresponding cross sections (Figure 2c)
reveal the arrangement of molecules outside and inside the
region of the line defects. Although the intermolecular distances
appear regular along the 〈112h〉 direction, one can clearly discern
two types of variation of the intermolecular distances along the
〈11h0〉 directions. In the line scan labeled A one molecule is
slightly displaced toward its neighbor, whereas line B reveals
a missing molecule.
The result of the STM analysis is summarized in a schematic
model of the δ-phase presented in Figure 3. Outside the regions
of the line defects, the average distance between molecules along
the 〈11h0〉 and 〈112h〉 directions amounts to 5.9 ( 0.4 and 10.0
( 0.6 Å, respectively, where the averaged values and errors
have been obtained from a set of five different images. The
basis of the δ-phase can, thus, be described as a rectangular
(2x3 × 2) lattice with an area per molecule of 28.7 Å2. The
area of the line defect is characterized by (i) a variation of
intermolecular distances (a ) 5 Å, b ) 6.6 Å in Figure 3) in
every second row but with an average identical to the 5.8 Å of
the unit cell and (ii) by a missing molecule in the adjacent row
resulting in increased intermolecular distances of c ) 7.9 Å
and d ) 9.4 Å. Two things are important to note at this point.
First, even though the proposed commensurate structure describes the dimensions rather accurately one should bear in mind
that it is the mismatch between molecular lattice and substrate
16912 J. Phys. Chem. C, Vol. 111, No. 45, 2007
Cyganik et al.
Figure 3. Proposed model for the molecular arrangement in the δ-phase
of BP2 SAMs on Au(111). Au atoms are marked by open circles.
Adsorption sites of the BP2 molecules (threefold hollow sites are chosen
for the modelssee text) are marked by gray circles. The unit cell of
the rectangular (2x3 × 2) lattice corresponding to an area per molecule
of 28.7 Å2 is marked by the black rectangle. The orientation of the
phenyl rings in the herringbone pattern is also indicated (for clarity
the tilt angle of molecules is omitted). The commensurate rectangular
(2x3 × 2) lattice on the right and left sides is divided by the line
defect arising due to the missing molecules. The positions of the
molecules in the defect area have been obtained from STM images
(see the corresponding distances marked in Figure 2b) and assuming
that the molecules adjacent to the vacancy have changed their adsorption
sites from face-centered cubic (fcc) to hexagonal close-packed (hcp)
(a ) 5.00 Å, b ) 6.52 Å, c ) 7.88 Å, d ) 9.40 Å).
Figure 2. BP2 SAMs on Au(111). (a and b) High-resolution STM
images recorded for samples prepared at room temperature and
subsequently annealed in N2 atmosphere for 15 h at 417 K. (c) Height
profiles along the lines depicted in (b).
lattice which gives rise to the line defects, and therefore, the
suggested dimension along the 〈11h0〉 directions should be
considered an approximation. Second, the thiols have been
arbitrarily positioned in threefold hollow sites for referencing
purposes in order to facilitate illustration of the various
intermolecular distances. The true position is not known, and
various recent experiments suggest other adsorption sites of the
sulfur.54,55 Furthermore, as discussed in our previous publication
on phase transitions in BP4 and BP636 and evidenced by studies
of short-chain alkanethiols56,57 a precise model has to include a
restructured S-Au interface.
LEED Measurements. Independent information on the structure of the δ-phase of the BP2 system has been obtained from
the LEED patterns. In order to suppress the inelastic background
signal and to minimize the Debye-Waller attenuation of the
diffraction peaks all LEED data were recorded at a sample
Figure 4. (a and c) LEED data recorded for the δ-phase of BP2 SAMs
on Au(111). The diffraction patterns have been obtained for electron
energies of 80 (a) and 30 (c) eV at 120 K. (b and d) Schematic
presentation of the diffraction patterns for 80 (b) and 30 (d) eV. The
dashed hexagon in (b) indicates the position of the first-order diffraction
spots of the bare Au(111) substrate recorded at the electron energy of
80 eV. (e) A comparison of the observed diffraction spots with the
calculated diffraction pattern of a rectangular (2x3 × 2) structure
including (rotational and mirror) domains. The color of the diffraction
spots marked in (e) directly corresponds to the color of the spots marked
in (b) and (d).
temperature of 120 K. In Figure 4, parts a and c, two LEED
patterns are displayed which were acquired at electron energies
of 80 and 30 eV, respectively, together with a schematic
SAMs of 2-(Aryl)-ethane Thiols
Figure 5. Normalized C 1s (left panels) and S 2p (right panels) HRXPS
data recorded for the R (upper panels) and δ (bottom panels) phases of
BP2 SAMs on Au(111). The spectra were acquired at a photon energy
of 350 eV. The spectra are fitted assuming the presence of two different
components in the case of the C 1s data and a doublet in the case of
the S 2p data (see text for details).
representation of these diffraction patterns (Figure 4, parts b
and d) showing the most intense diffraction spots as colored
balls. The dashed hexagon and gray balls in Figure 4b indicate
the position of the first-order diffraction spots of the bare Au(111) substrate recorded at the same electron energy of 80 eV.
A comparison of the observed diffraction spots with the
calculated diffraction pattern of a rectangular (2x3 × 2)
structure including (rotational and mirror) domains which is
displayed in Figure 4e reveals a close correspondence and thus
confirms the commensurability of the δ-phase. We note that,
due to a complex phase dependence, only few diffraction spots
become visible for a given incident electron energy.
Despite several attempts, no ordered diffraction pattern could
be resolved for the R-phase of the BP2 system. As in our
previous LEED study30 we attribute this effect to the large
rectangular (5x3 × 3) unit cell of the R-phase which leads to
a large number of rather closely spaced diffraction spots which
apparently cannot be resolved (already for one domain of the
rectangular (5x3 × 3) structure about 100 superlattice diffraction spots are expected to appear within the hexagon of the gold
substrate first-order spots). Moreover, due to the large number
of domain boundaries and lateral stacking faults, this phase
reveals only a small coherence length which results in an
additional broadening of the diffraction peaks.
HRXPS Measurements. The S 2p and C 1s HRXPS data of
the R- and δ-phases of BP2/Au are presented in Figure 5. The
spectra are normalized to the intensity of the incident X-ray
beam and the number of scans so that a direct comparison
between the individual spectra in the same spectral range is
possible.
The S 2p HRXPS data of both R- and δ-phases exhibit a
single S 2p3/2,1/2 doublet at a BE of ≈162.05 eV commonly
assigned to the thiolate species,58,59 with no evidence for
disulfides, alkylsulfides, or oxidative products. There is, however, a weak signal at 161.0 eV (S 2p3/2) for the δ-phase, whose
origin is unclear. Molecular species in SAMs not attached by a
thiolate bond have been proposed in previous work,60-63 but
we cannot exclude an assignment to atomic sulfur.64 Since slight
changes in the annealing conditions substantially affect the
quality of the layers (see Figure 1) an onset of thermal
decomposition in a small fraction of the substrate area could
J. Phys. Chem. C, Vol. 111, No. 45, 2007 16913
escape the rather local STM imaging but would be seen in the
data recorded with a spatially averaging spectroscopy like XPS.
The presence of such a heterogeneity is supported by the fact
that the intensity of the 161.0 eV doublet exhibits slight
variations from sample to sample due to inevitably small
variations in the preparation procedure such as slight differences
in annealing temperature.33
The intensities of the thiolate-related doublet in the R- and
δ-phases are essentially identical. This suggests very similar
packing densities of both phases in accordance with the STM
data. The fwhm of the S 2p3/2,1/2 components is quite small for
both R- and δ-phases: it amounts to about 0.50 eV for both
films. Such a small value suggests a small heterogeneity of the
adsorption sites of the BP2 molecules in these films.33 Note
that, since the instrumental broadening is below 100 meV, the
value of the fwhm is essentially determined by the natural line
width of the S 2p3/2,1/2 lines and the inhomogeneity of bonding
configurations of the thiolate headgroups (e.g., occurrence of
several different adsorption sites).
The C 1s HRXPS data of the R- and δ-phases of BP2/Au
are quite similar. The spectra exhibit a main emission peak at
a BE of 284.23 and 284.30 eV, respectively, which is assigned
to the aromatic backbone and a shoulder at ≈0.7 eV higher
BE. Similar shoulders have been observed previously for
different aromatic SAMs and have been assigned to the carbon
atom bonded to the sulfur headgroup or to shakeup processes.63,65,66 Since the probing depth of HRXPS is rather small
at the photon energy used in the present experiments, the former
assignment seems to be rather questionable and an assignment
to a shakeup process is more likely. The fwhm of the main C
1s emission peak for the δ-phase is slightly larger than that for
the R-phase (0.79 vs 0.73 eV) which could be indicative of a
somewhat larger inhomogeneous broadening in the δ-phase due
to, e.g., changes in the crystalline packing.
Like the sulfur signals, the total C 1s intensities of the Rand δ-phases do not differ within the experimental error
supporting that the average packing densities of the BP2
molecules are very similar in both phases, see below. The
thickness of these films was evaluated on the basis of the Au
4f spectra and C 1s relative intensities seen for a photon energy
of 580 eV (data not shown). A standard expression for an
exponential attenuation of the photoemission signal was assumed. The attenuation lengths of the C 1s and Au 4f
photoelectrons were taken in accordance with the data of Lamont
and Wilkes.67 The derived film thickness amounts to ≈14.2 Å
for both R- and δ-phases of BP2/Au.
At first sight the same average thickness as determined by
XPS seems to be at variance with the structural models proposed
for the two phases. Assuming an average spacing between the
line defects in the δ-phase of 51.84 Å (i.e., equivalent to nine
unit cells of the δ-phase), then, from the model presented in
Figure 3, one can obtain an average area per molecule of 30.5
Å2, which is about 12.5% less dense compared to the R-phase
(27.05 Å2). However, considering that the rotational domains
in the R-phase prepared at room temperature are much smaller
compared to the δ-phase, i.e., about 10 and 100-400 nm in
diameter, respectively, the effective difference in density will
be certainly reduced to only a few percent due to the larger
concentration of defects in the R-phase.
NEXAFS Measurements. NEXAFS spectroscopy is a synchrotron-based spectroscopic technique to probe electric dipole
transitions from core levels to unoccupied molecular orbitals
close to the continuum.68 Absorption resonances in NEXAFS
spectra give a signature for a characteristic bond, a chemical
16914 J. Phys. Chem. C, Vol. 111, No. 45, 2007
Cyganik et al.
Figure 6. C 1s NEXAFS spectra for R (a) and δ (b) phases in the
BP2 SAMs on Au(111) samples acquired at X-ray incident angles of
90° and 30° (shadowed).
group, or a molecule. In addition, from the linear dichroism in
X-ray absorption data68 information on the molecular orientation
of adsorbed species can be obtained.
In Figure 6, parts a and b, we present the C 1s edge NEXAFS
spectra obtained for the R- and δ-phase of the BP2 SAMs,
respectively. The spectra exhibit characteristic absorption
resonances (marked in the top part of Figure 6a) related to the
phenyl rings.69-72 The most prominent is the π1* resonance at
≈285.0 eV, which is accompanied by a weaker π2* resonance
at ≈288.9 eV, and several broad σ* resonances at higher photon
energies. The difference in the NEXAFS spectra taken at X-ray
incidence angles of 90° and 30° as seen in Figure 6 for both
phases of the BP2 system reveals a pronounced linear dichroism
and thus directly demonstrates the presence of a high degree of
orientational order. To determine the average tilt angles of the
biphenyl backbones in the R- and δ-phases, the entire sets of
spectra acquired at different incidence angles θ were used. For
simplicity, only the intensity I of the most pronounced π1*
absorption resonance was monitored. The angular dependence
was evaluated according to the following theoretical expression
(for vector-type orbital and substrate with threefold symmetry)68
1
1
I(F, θ) ∝ P 1 + (3 cos2 θ - 1)(3 cos2 F - 1) +
3
2
1
(1 - P) sin2 F (1)
2
[
]
where P denotes the polarization factor (P ≈ 0.82), F corresponds to the angle of the transition dipole moment (TDM) for
the transition in question relative to the surface normal, and
the incidence angle θ of the X-ray beam is defined with respect
to the surface plane. To avoid normalization problems, not the
absolute intensity but intensity ratios I(F, θ)/I(F, 30°) were
analyzed. The experimental results together with best fits based
on eq 1 are presented in Figure 7. The average values of the
angle F determined for the R- and δ-phases are very close, i.e.,
64° and 65°, respectively. Since the TDM of the π1* resonance
is oriented perpendicular to the plane of the phenyl rings (see
Figure 1), the value of the angle φ corresponding to the tilt
angle of the biphenyl moiety is given by the following
equation:28
sin φ )
cos F
cos ϑ
(2)
where ϑ is the twist angle of the coplanar biphenyl moiety
around the 4,4′ axis. Assuming a herringbone arrangement of
the biphenyl moieties, which is typical for aromatic systems,
and a twist angle ϑ as in bulk biphenyl20 (32°),36,69,70,73,74 φ
values of about 31° and 30° were obtained for the R- and
δ-phases, respectively. However, by a combination of NEXAFS
and infrared reflection absorption spectroscopy (IRRAS) data
in our previous study of the BPn (n ) 1-6) homologues,28 a
value of ϑ ≈ 60° was estimated which yields φ values of about
61° and 58° for the R- and δ-phases, respectively. The NEXAFS
data, thus, suggest very similar orientation of the BP2 molecules
in both phases with an indication of a smaller tilt angle φ for
the δ-phase. This is quite unexpected, since the lower density
of the δ-phase would indicate, if any, an appositive change in
the tilt angle, i.e., a higher value for the lower density δ-phase.
A similar, but even more pronounced, effect was observed in
our previous study of the R f β phase transition in the BP4
and BP6 system.36 Calculating the above value of the tilt angle
φ (using eq 2), it was assumed that the twist angle ϑ remains
unchanged during the phase transition, and therefore, we take
the discrepancy described above as an indication that the
simplifying assumption of a phase-independent constant value
of ϑ is not valid. As discussed in more detail previously36 one
can reasonably assume that the change in packing density by
about 6% upon the R f δ phase transition is accompanied by
a change in the twist angle ϑ of the phenyl rings. In addition
(and in contrast to the previously reported R f β transition),
the R f δ phase transition reported here leads to a reorientation
of the herringbone structure relative to the Au(111) substrate
(see the model in Figure 3), and thus, apart from change in the
density this is another factor which may be associated with the
change in the twist angle ϑ upon the phase transition.
Contact Angle Measurements. To compare the relative
stability of the R- and δ-phases of the BP2/Au(111) system
against exchange by other thiols, a series of contact angle
measurements was performed. For this purpose respective
samples were incubated at room temperature in 1 mM ethanolic
solution of ω-mercaptohexadecanoic acid (HS-(CH2)15COOH) for a given time. The drop in the contact angle value
(toward the value of about 50° obtained for ω-mercaptohexadecanoic acid SAMs formed by adsorption on clean Au(111)
substrate) was used to monitor the exchange process. The results
obtained by this procedure are summarized in Figure 8.
Comparison of the exchange process for the R- and δ-phases
shows that the R f δ phase transition results in a pronounced
improvement of the film stability against exchange by other
thiols.
2-Phenylethane Thiol (C6H5(CH2)2SH, P2). Figures 9 and
10 summarize STM data obtained for the P2 SAMs prepared
at room temperature from the respective solution and then
investigated either as prepared or annealed at different temperatures. Images collected at larger scale are presented in Figure
9. For the samples prepared at room temperature (Figure 9ac), formation of an ordered structure has been observed with,
however, some disordered regions located at the borders between
the rotational domains (see Figure 9c). As documented by Figure
9, parts d and e, additional annealing at 373 K for 15 h leads to
a significant increase in the size of the well-ordered domains,
i.e., from 5-10 to 20-50 nm. The high-resolution STM data
did not indicate any change in the packing of the molecules.
At the same time, the density of the substrate depressions (black
islands in the STM images) decreased due to Ostwald ripening
as already mentioned above. The STM data shown in Figure 9,
parts f and g, reveal that annealing at an even higher temperature
SAMs of 2-(Aryl)-ethane Thiols
J. Phys. Chem. C, Vol. 111, No. 45, 2007 16915
Figure 7. Angular dependence of the π1* resonance intensity ratio I(θ)/I(30°) for the R (a) and δ (b) phase of BP2 SAMs on Au(111) samples (θ
denotes the X-ray incidence angle). The experimental data are presented together with the best fits (solid line) according to eq 1. The values of the
derived average TDM tilt angle (F) are given together with the respective fits.
Figure 8. Static water contact angles recorded for R (filled circles)
and δ (open circles) phases of BP2 SAMs on Au(111) as a function of
the incubation time in 1 mM ethanolic solution of ω-mercaptohexadecanoic acid at room temperature.
of 393 K (keeping the same annealing time of 15 h) leads to a
partial destruction (starting from defects located at the boundaries of the rotational domains) of the film with, however, no
structural changes in the remaining ordered areas. The highresolution image presented in Figure 10a reveals details of the
P2 film structure. Similar to BP2 and other BPn systems, also
in this case one can observe the appearance of densely packed
rows of molecules (along the 〈112h〉 directions) exhibiting an
alternating contrast in STM. As discussed above, this effect can
be attributed to a herringbone-type arrangement of the phenyl
rings. The orientation of the rows along the 〈112h〉 directions is
the same as for BP2 and other BPn SAMs prepared at room
temperature. The cross sections taken from this image (Figure
10b) reveal periodicity of about 11.7 ( 0.6 and 4.8 ( 0.4 Å in
the 〈11h0〉 and 〈112h〉 directions, respectively (the averaged values
and errors have been obtained from a set of five different
images). These distances are very close to the corresponding
values of 11.52 and 4.98 Å for the commensurate rectangular
(4 × x3) lattice with an area per molecule of 28.7 Å2. A
schematic molecular arrangement for the P2 SAM on Au(111)
is shown in Figure 10c, with, as previously commented for the
BP2 system, arbitrarily assigned adsorption sites.
IV. Discussion
The annealing behavior of BP2 and P2 SAMs exhibits
remarkable differences. On one hand in BP2 SAMs a phase
transition is observed which results in a structure distinctly
different from the ones observed in SAMs made from the higher
homologues BP4 and BP6. In P2 SAMs, on the other hand, no
temperature-induced phase transition is observed. The substantial
difference in the behavior of these closely related molecules
which differ only in the number of methylene spacer units or
phenyl rings, respectively, illustrates the subtle balance of the
factors which determine the energy and the structure of thiol
SAMs.
Microscopic and diffraction data show that thermal annealing
of the BP2 system leads to an irreversible phase transition from
the R-phase described by a rectangular (5x3 × 3) unit cell into
the δ-phase which is based on a rectangular (2x3 × 2) lattice
but which has very characteristic regular line defects. On the
one hand, BP2 represents another manifestation of the pronounced odd-even difference in the annealing behavior of BPn
SAMs on Au(111) with only n ) even exhibiting polymorphism
as a consequence of the competitive design mentioned in the
Introduction and discussed in detail before.35,36 On the other
hand, the R-phase which is common to all three even-numbered
BPn SAMs (n ) 2, 4, 6) studied so far evolves into a structure
very different from those seen for BP4/BP6 (see the summary
of even BPn/Au(111) structures shown in Figure 11). The latter
two form very large domains with a perfectly homogeneous
arrangement of molecules described by an oblique (6x3 × 2x3)
unit cell,35,36 whereas BP2 realizes a defect-rich structure. It is,
however, quite remarkable that the defects are very regular, that
is, they occur as parallel line defects which run exclusively along
the 〈112h〉 directions at a rather close distance of 8-11 unit cells
(4.8-6.2 nm).
Since the only difference between the molecules is the length
of the alkane spacer this must account for the different structures
of the δ- and β-phases. Thus, the thermally induced transformation to different structures for BP2 and BP4/BP6 highlights the
vital importance of the spacer for the structure of SAMs beyond
the odd-even effect. As revealed by studies on the homologue
series of BPn SAMs with n ranging from 0 to 6 the spacer is
crucial for coping with stress arising from the mismatch between
the molecular lattice and the underlying substrate lattice.27
Whereas BPn SAMs with n < 2 yield poorly ordered SAMs,
i.e., the stress is released in a rather random way by forming
small, irregular domains, one obtains well-ordered layers for n
g 2. We have suggested that the conformational degrees of
freedom (in combination with relaxation processes at the S-Au
interface27) are responsible for accommodating the mismatch
of molecular and substrate lattices and, thus, reducing stress.
For the phases generated by annealing this still holds for BP4
and BP6, whereas for BP2 it seems that there are limits to the
extent to which the alkane spacer can cope with stress.
Therefore, BP2 seems to represent a system located at the
boundary between SAMs which cannot cope with stress other
16916 J. Phys. Chem. C, Vol. 111, No. 45, 2007
Cyganik et al.
Figure 9. P2 SAMs on Au(111): STM images with different resolutions recorded for samples prepared at room temperature (a-c) and subsequently
annealed in N2 atmosphere for 15 h at 373 (d and e) and 393 K (f and g). In (b) the white rectangle marks the area shown in (c). In (c) black lines
mark the areas of defects in arrangement of molecules in the rectangular (4 × x3) lattice (marked by black rectangles) identified outside these
defect regions.
than by forming defects/domain boundaries and SAMs which
have “soft” ways of stress relief without breaking the molecular
order. A most interesting aspect is that defect formation is highly
ordered, i.e., the stress is highly anisotropic. This does not only
result in well-oriented defect lines but induces a long-range order
as illustrated by Figure 1, parts c and f. Unfortunately, since
numerous factors determine the structure of a SAMs a reliable
modeling of these systems (which at present does not exist)
would be required to elucidate the relative importance of the
different factors involved such as intermolecular interactions,
conformational states, and the structure and energetics of the
S-Au interface. At this state we can only say that the stress
anisotropy might not be too surprising keeping in mind that
the C-S-Au plane has preferred direction/orientation and that
the packing of the biphenyl moieties in SAMs on Au(111)
substrate requires an anisotropic deformation with respect to
the preferred packing of biphenyl moieties in biphenyl crystal.
We note that a related anisotropy effect has been observed in
BP6 SAMs where annealing at temperatures low enough to
avoid transition to the β-phase results in highly elongated
domains.31
Concluding the comparison of BP2 SAM with its higher
homologues, the R f δ phase transition in the BP2 system leads
to a phase with a lower density similar to what has been
observed for the R f β phase transition in the BP4 and BP6
systems. Despite the decreased density, which is about 30%
lower compared to a densely packed (x3 × x3)R30°-type
structure of alkanethiols, both the β- and the δ-phase are
surprisingly stable against exchange by other thiols, in contrast
to the R-phases of BP2 (Figure 8) and BP4/BP6.36 However,
even though BP2 and BP4/BP6 SAMs are of similar stability,
there is a remarkable difference in their molecular packing. The
β-phase of the latter forms very large domains of a uniform
structure, whereas BP2 is characterized by missing molecules
along lines parallel to the 〈112h〉 direction. It is interesting that
despite these defects the BP2 SAMs in the δ-phase are perfectly
stable against exchange by other thiols or back-conversion to
the R-phase upon extended immersion in BP2 solution. Also
thermal destruction of δ-phase at higher temperatures is not
initiated from these defects but only from the step edges
indicating high stability of the δ-phase. Although for BP4/BP6
kinetic stabilization cannot be completely ruled out it fits well
to the interpretation that the phase transition is thermodynamically driven by lowering the energy related to stress and possibly
restructuring of the interface.36 In this context it is noteworthy
that, like the β-phase of BP4/BP6 SAMs, the δ-phase of BP2
SAM lacks any Moiré-type contrast variation in the STM
images, in contrast to the R-phase.
We now turn to an STM analysis of the temperature-induced
changes in morphology as seen for the P2 SAMs. In this case,
preparation at room temperature results in the formation of a
rectangular (4 × x3) structure with an area per molecule of
28.7 Å2. This structure is not consistent with the (7 × x3) lattice
reported in an earlier STM study of this system26 which
corresponds to a structure with a much lower molecular density
(50.3 Å2/molecule). The reason for this difference is not clear
at present. It should be noted, however, that the preparation
conditions were different and that the resolution of the previously reported micrographs was somewhat inferior to the data
presented here. We would like to note that the long side of the
rectangular (7 × x3) unit cell (20.16 Å) proposed in the
previous study26 is very close to the distance between the black
lines in Figure 9c, which corresponds to defect lines embedded
in the rectangular (4 × x3) lattice. Such defects have been
commonly observed by us for P2 SAMs prepared at room
temperature (but not for the film annealed at 373 K, see Figure
9, parts d and e). Their presence is evidenced by a change in
the STM contrast (dark lines in Figure 9a) and by a change in
the arrangement of molecules (Figure 9c) visible in images taken
at lower and higher magnification, respectively. Interestingly
and independent of these differences in the observed structures
[(7 × x3) vs (4 × x3)], the difference between P1/P3 SAMs26
exhibiting a (2x3 × x3)R30° structure and P2 SAMs is another
manifestation of the odd-even behavior28 and, thus, the crucial
influence of the SAM/substrate interface. Although the structures
of Pn SAMs parallel the odd-even effect of BPn SAMs, the
SAMs of 2-(Aryl)-ethane Thiols
J. Phys. Chem. C, Vol. 111, No. 45, 2007 16917
Figure 11. Summary of structures observed for even BPn SAMs on
Au(111) substrate: (a) room-temperature R-phase observed for BP2,
BP4, and BP6 characterized by a rectangular (5x3 × 3) and an area
per molecule of 27.05 Å2; (b) β-phase observed for BP4 and BP6 after
annealing (15 h at 423 K for BP4 and 24 h at 423 K for BP6)
characterized by an oblique (6x3 × 2x3) unit cell and an area per
molecule of 32.4 Å2; (c) δ-phase observed for BP2 after annealing (15
h at 417 K) characterized by a rectangular (2x3 × 2) unit cell and an
area per molecule of 28.7 Å2; (d) schematic presentation of the relative
size and orientation of R-, β-, and δ-phase.
Figure 10. P2 SAMs on Au(111). In (a) a high-resolution STM image
of a sample prepared at room temperature and subsequently annealed
in N2 atmosphere for 15 h at 373 K is shown. Unit cell and topographic
cross sections A and B are marked in red. In (b) topographic cross
sections A and B taken in (a) are presented. In (c) an illustration of the
molecular arrangements of P2 SAMs on Au(111) is shown. Au atoms
are marked by open circles. Adsorption sites of the P2 molecules
(threefold hollow sites were arbitrary chosen) are marked by gray
circles. The unit cell of the rectangular (4 × x3) lattice with an area
per molecule of 28.7 Å2 is marked by a black rectangle. The schematic
arrangement of the phenyl rings in the herringbone pattern is also
marked (for clarity the tilt angle of molecules is not included).
most important finding in the present STM study of P2 SAMs
is that, in contrast to the even-numbered BPn systems, no
thermally induced phase transitions could be observed. As for
odd-numbered BPn SAMs, only an increase in the size of
domains was seen before a gradual destruction of the film
structure upon further increasing the annealing temperature.
The comparison of BP2 SAMs with P2 SAMs clearly reveals
the crucial importance of an additional phenyl ring for the
properties of the SAMs, in particular with respect to its annealing
properties. This conclusion is consistent with a recent study of
terphenyl-substituted alkanethiols [C6H5(C6H4)2(CH2)nSH, TPn,
n ) 1-6] on Au(111) substrates. Preparation of a TP6 SAM at
elevated temperature75 shows the formation of the β-phase
already at 333 K, whereas in the corresponding experiment with
BP6 the onset of the transition is seen only at a significantly
higher temperature of 393 K.
Besides the odd-even effect, which is seen not only for Pn
and BPn SAMs but also persists in TPn SAMs,73,75 the phenyl
ring dependent polymorphism is another manifestation of how
sensitively the structure of a SAM depends on the various factors
entering into its energy balance. As outlined in the Introduction,
this balance is determined by partially competing factors31,35,36
and is affected significantly by adding another aromatic moiety.
As a consequence of the increased contribution of phenylphenyl intermolecular interactions the energy landscape is
substantially altered, and new pathways for energy optimization
reflected by phase transitions open up.
In general, (small) changes of the molecular structure in the
organothiol used to fabricate a SAM can result in structural
transitions either by lowering the activation barrier for the
transition from the initial to the final structure or by changing
the energy difference between two different structures. The
former implies that the transition is kinetically controlled. If
the activation energy for the phase transition becomes too high,
the competing channel of thermal desorption will open upon
raising the temperature and the phase transition may cease to
be a viable route. This may be the case for the P2 SAMs. The
latter is a thermodynamic argument, and the detailed contributions to the change in total energy, which result from a complex
interplay between intermolecular interactions, molecular degrees
of freedom, and structural effects at the SAM-substrate
interface, are particularly difficult to predict. Most likely both
types of mechanism play a role, and the difference in the
transition temperature between BP6 and TP6 suggests a change
in the transition state. However, as we have pointed out
previously,35,36 for a full understanding of the polymorphism
in BPn SAMs it is necessary to understand the extent to which
the energetics of the interface plays a role and how it is affected
by the molecular structure. The need to understand these issues
16918 J. Phys. Chem. C, Vol. 111, No. 45, 2007
in detail is, first, highlighted by the crucial influence of the
molecular structure on the energetics of the interface as revealed
by ion- and electron-induced desorption experiments.46 In this
study a higher stability of the Au-S and S-C bonds was found
for P2 SAMs compared to BP2 SAMs. Second, the change of
step directions and the disappearance of stress upon phase
transitions is another sign that restructuring of the interface is
one of the key factors toward the understanding of the phase
transitions.35,36 Third, two recent studies of alkanethiols which
found a significant deviation of the thiol-Au interface from
the bulk-terminated Au(111) structure is another piece of
evidence illustrating the importance to understand the SAMAu interface.56,57
V. Conclusions
The structure of BP2 SAMs obtained after annealing is
remarkable since, to the best of our knowledge, it is the first
example of a thiol SAM where defects form in such a welldefined way. In the series of homologue BPn SAMs, BP2
appears as a boundary case between n < 2 which do not form
well-ordered layers and n ) 4, 6 where defect-free structures
are formed. A comparison of the new high-temperature phase
of the BP2 system with the corresponding ones of BP4 and
BP635,36 demonstrates that the details of such phase transitions
with respect to transition temperature, time, and resulting
structure are the result of a subtle balance of factors. Nevertheless, despite the differences between the structures of BP2, BP4,
and BP6, the occurrence of phase transitions across the range
of CH2 units and results for TP675 show that this is a general
phenomenon which occurs for a large number of related
compounds.
However, the lack of such a transition in P2 SAMs demonstrates that the relative magnitude of the factors entering the
energy balance is crucial. This can be expected to be of
particular importance for the competitive design. A too unbalanced contribution of the various factors causes the SAM
structure to be dominated by one (or more cooperatively acting)
factor(s) with the consequence that one structure is energetically
strongly favored and phase transition impeded. It will be
interesting to see how far the concept of competitive design
can be carried with respect to structural variations and controlled
introduction of defects.
Acknowledgment. This work was supported by The Leverhulme Trust, German Science Foundation, EPSRC, and
SHEFC. P.C. was a Postdoctoral fellow of the Alexander von
Humboldt Foundation. This work has been supported by the
German BMBF (05KS4VHA/4) and European Community
(Access to Research Infrastructure action of the Improving
Human Potential Program).
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