relative stability of thiol and selenol based sams on au ...users.uj.edu.pl/~cyganik/pccp10.pdf ·...
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Relative stability of thiol and selenol based SAMs on
Au(111) — exchange experiments
Katarzyna Szelagowska-Kunstman,a Piotr Cyganik,*a Bjorn Schupbachb and
Andreas Terfortb
Received 5th November 2009, Accepted 2nd February 2010
First published as an Advance Article on the web 24th February 2010
DOI: 10.1039/b923274p
Two fully analogue homologue series of thiol and selenol based aromatic self-assembled
monolayers (SAMs) on Au(111) in the form of CH3–(C6H4)2–(CH2)n–S–Au(111) (BPnS/Au(111),
n = 2–6) and CH3–(C6H4)2–(CH2)n–Se–Au(111) (BPnSe/Au(111), n = 2–6), respectively, have
been used to elucidate the relative stability of the S–Au(111) and Se–Au(111) bonding by
monitoring their exchange by alkanethiol and alkaneselenol molecules from their respective
solutions. The exchange process was monitored using infrared reflection absorption spectroscopy
(IRRAS). Two main results obtained by these study are: (1) the selenium-based BPnSe/Au(111)
series is significantly more stable than their sulfur analogues; (2) a clear odd–even effect exists
for the stability of both BPnS/Au(111) and BPnSe/Au(111) SAMs towards exchange processes
with the even-numbered systems being less stable. The results obtained are discussed in view of
previously reported microscopic and spectroscopic data of the same SAMs addressing the issue of
the relative stability of S–Au(111) and Se–Au(111) bonding, which is an important factor for the
rational design of SAMs.
Introduction
The broad range of applications for self-assembled
monolayers (SAMs) in nanotechnology defines this as a
distinguished part of that field.1 Molecular electronics is one
of the most exciting directions in nanotechnology where a
significant part of research is dedicated to electronic transport
mechanisms,2–5 metal electrode work-function modification,6,7
molecular switches,8–12 and memory devices,13,14 for which
SAMs of aromatic thiols on Au(111) substrates are often used.
One important obstacle in using SAMs for molecular
electronics applications is the presence of defects, which have
a profound influence on their electronic properties.15–17 In
particular for purely aromatic thiolate SAMs on Au(111) the
defect density is high18–23 due to the relaxation of stress which
results from the misfit between the lattice preferred by the
aromatic moieties and that of the Au(111) substrate.24 In the
past, we have proposed three different strategies to solve this
problem including: (1) formation of hybrid aromatic–aliphatic
molecules where insertion of a flexible aliphatic chain gives
additional degrees of freedom providing other pathways
to reduce stress without breaking the structure preferred
by the aromatic moieties;24 (2) using systems where the
competition between different structural forces leads to phase
transitions into new lower density structures characterized by
unprecedented structural quality;25–27and (3) changing the
thiol binding group of the aromatic molecule to selenol, as
was demonstrated for biphenyl selenolate based SAMs.28
While the first two strategies require the introduction of an
alkyl chain between the aromatic part and the binding group,
which significantly reduces the conductance of the molecule,
the third method, the simple substitution of the binding atom,
should leave the conductance of the SAM either unchanged
or even increased according to the existing theoretical
calculations29–31 and experimental data32,33 obtained for
aromatic selenolate SAMs. Therefore, the S - Se substitution
seems to be the most attractive method to improve molecular
order in aromatic SAMs on Au(111) substrates, and the recent
reproduction34 of this effect for the purely aromatic SAMs of
anthracene confirms its generality.
It should be noted, however, that apart from the improve-
ment in the molecular order of the SAM, due to the S - Se
substitution, another key factor which decides the real
application of this approach is the stability of the chemical
bonding between the molecule and the substrate. Despite
several studies addressing the relative stability of thiolate
and selenolate SAMs bonding to the Au(111) substrate, this
issue remains unclear. Both higher35–38 and lower32,39 stability
of the Se–Au(111) bonding have been concluded. The only two
reports32,39 concluding lower stability of Se–Au(111) bond as
compared to S–Au(111) exclusively compare phenylthiolate
(PT) and phenylselenolate (PSe) SAMs on Au(111). The same
SAMs were compared in other two reports35,37 which
concluded, in contrast, higher stability of the Se–Au(111)
bonding. To make judgement more difficult, it should be noted
that in each of these four publication different experimental
methods have been used to support the conclusions, namely:
thermal desorption spectroscopy,39 X-ray photoelectron
spectroscopy,32 competitive adsorption of PT and PSe
aDepartment of Physics of Nanostructures and Nanotechnology,Smoluchowski Institute of Physics, Jagiellonian University,30-059 Krakow, Poland. E-mail: [email protected]
b Institut fur Anorganische und Analytische Chemie,Goethe-Universitat Frankfurt, 60438 Frankfurt, Germany
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PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics
monitored by surface enhanced Raman spectroscopy,35 and
electrochemical desorption.37 It is important to note that PT
molecules in contrast to PSe do not form well ordered SAMs
as documented by the STM data presented in one of these
publications.39
For us it seems rather crucial that for a meaningful
comparison of the stabilities, the respective thiol and selenol
molecules need not only have the same carbon backbones, but
also should form ordered structures with the same or very
similar molecular packing. Only under such conditions the
contribution of the intermolecular interactions to the different
stability of such analogue films can be minimized to elucidate
the role of the molecule–substrate bonding on the film
stability. To firmly justify such results, it would be of course
desirable to perform such an analysis not only for a single
analogue pair but for a whole family of molecules.
Following this general idea, which so far has not been
realized, we have decided to compare two analogue
homologue series of SAMs formed on Au(111) substrate by
hybrid aromatic–aliphatic molecules in the form of
CH3–(C6H4)2–(CH2)n–S–Au(111) (BPnS/Au(111), n = 2–6)
and CH3–(C6H4)2–(CH2)n–Se–Au(111) (BPnSe/Au(111),
n = 2–6). Importantly, previous detailed spectroscopic40–42
and microscopic43–45 experiments for BPnS/Au(111) and
BPnSe/Au(111) documented the formation of well-ordered
SAMs exhibiting the same or very similar structures and
molecular packing. In the following contribution we are
reporting exchange experiments of both series of SAMs by
an alkanethiol (CH3–(CH2)15–SH, HDT) and its alkaneselenol
analogue (CH3–(CH2)15–SeH, HDSe) monitored by infrared
absorption reflection spectroscopy (IRRAS).
Experimental
CH3–(C6H4)2–(CH2)n–Se–Se–(CH2)n–(C6H4)2–CH3 and CH3–
(C6H4)2–(CH2)n–SH with n = 2–6 were synthesized according
to the procedures reported earlier.40,46 HDSe was obtained by
the reduction of a hexadecyldiselenide/-triselenide mixture
with lithium aluminium hydride.
Substrates were prepared by thermal evaporation of 150 nm
of Au (99.99%) onto polished single crystal silicon (100)
wafers (ITE, Warsaw) primed with a 7 nm chromium adhesion
layer. The polycrystalline Au films consist of grains (20–50 nm
in diameter) exhibiting predominantly (111) orientation
verified by scanning tunnelling microscopy (STM) measure-
ments.47 The BPnS, BPnSe, HDT and HDSe SAMs have
been prepared by immersion of freshly prepared poly-
crystalline Au(111) substrates into 0.1 mM (BPnS, BPnSe)
or 1 mM (HDT, HDSe) solutions of the respective molecules
in ethanol at room temperature for 24 h. After immersion,
samples were rinsed with pure ethanol and blown dry with
nitrogen and then immediately analysed by IRRAS or used for
exchange experiments. Exchange experiments were performed
by incubation of BPnS/Au(111) and BPnSe/Au(111) samples
in 1 mM ethanolic solution of either HDT or HDSe for
fixed time of 24 h. All exchange experiments were performed
under nitrogen atmosphere to avoid influence of oxidation.
The IRRAS analysis directly followed the exchange
experiments.
IRRAS measurements were performed with a dry-
air-purged Thermo Scientific FTIR spectrometer model
Nicolet 6700 equipped with a liquid nitrogen-cooled MCT
detector. All spectra were taken using p-polarized light
incident at a fixed angle of 801 with respect to the surface
normal. Spectra were measured at a resolution of 2 cm�1 and
are reported in absorbance units A = �log R/R0, where
R is the reflectivity of the substrate with the monolayer and
R0 is the reflectivity of the reference. Substrates covered with
a perdeuterated hexadecanethiolate SAM were used as a
reference.
Results
The IRRAS spectra obtained for the BPnS/Au(111) and
BPnSe/Au(111) monolayers before and after their exchange
by HDT and HDSe molecules are summarized in Fig. 1 and 2,
respectively. The first columns in these Figures show spectra
obtained for the native monolayers. Several absorption bands
characteristic for the structure of BPnS(Se)/Au(111) SAMs
can be identified. To simplify such an identification, an
enlarged spectrum for BP3Se/Au(111) is shown in Fig. 3. As
it was discussed in detail in previous spectroscopic studies,40,42
bands observed at B3028 cm�1, B1500 cm�1 and B1005 cm�1
are associated with the biphenyl part of the molecules and
correspond to the C–H stretching, C–C stretching, and C–H
bending, respectively. The aliphatic parts of the BPnS(Se)/
Au(111) SAMs become visible through asymmetric and
symmetric C–H stretching vibrations corresponding to bands
atB2920 cm�1 andB2865 cm�1, respectively. The weak band
at B1381 cm�1 is associated with the C–CH3 symmetric
deformation at the terminal group. The second and third
columns in Fig. 1 show IRRAS spectra obtained after 24 h
incubation of BPnS/Au(111) monolayer in HDT and HDSe
ethanolic solution (1 mM), respectively. For comparison,
spectra of native HDT/Au(111) and HDSe/Au(111) SAMs
are shown at the bottom of the second and third column
in Fig. 1 (see grey box). The exchange data obtained for
BPnSe/Au(111) SAMs are shown in Fig. 2 in a fully analogous
way. The analysis of these spectra is presented in Fig. 4 and 5.
Data shown in Fig. 4 analyse native samples of BPnS/Au(111)
(left panel) and BPnSe/Au(111) (right panel) and display rather
systematic absorbance variation for bands at B1500 cm�1,
B1381 cm�1, and B1005 cm�1 as a function of the parameter
n. This effect in IRRAS has been reported previously for these
SAMs40,42 and is a fingerprint of the characteristic odd–even
changes in the inclination of BPnS(Se) molecules towards the
Au(111) substrate. The combination of complementary
spectroscopic (NEXAFS, XPS, ellipsometry), contact angle,
and microscopic (STM) measurements, applied in previous
studies, demonstrated that these changes in the inclination of
the BPnS(Se) molecules are directly associated with different
packing densities and types of crystallographic lattice adopted
by the odd and even members of BPnS(Se)/Au(111)
SAMs.40,42,44,45
To quantify the exchange of BPnS(Se)/Au(111) SAMs by
aliphatic HDT and HDSe molecules, the intensity of bands
associated with the biphenyl part of the investigated SAMs has
been used. Assuming that the exchange process does not
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change the orientation of the BPnS(Se) molecules with respect
to the Au(111) substrate significantly, but only the fraction of
the sample area covered by them, we can monitor relative
changes in the surface coverage of BPnS(Se) molecules on
Au(111) surface by taking the total intensity of bands related
exclusively to the biphenyl moiety (i.e.B3028 cm�1,B1500 cm�1
and B1005 cm�1) and normalizing it by the respective value
obtained for native BPnS(Se)/Au(111) SAMs. Following this
assumption, we show in Fig. 5 the changes in the relative
surface coverage (calculated in percents) of BPnS and BPnSe
molecules on the Au(111) surface after 24 h of exchange
with HDT and HDSe solutions. Results obtained for
BPnS/Au(111) SAMs show that the exchange by HDT results
in a clear odd–even variations of the efficiency of this process
with the odd members of series being more resistant against
exchange. Interestingly, the exchange of these SAMs with the
selenol analogue (HDSe) results in complete exchange of
BPnS molecules irrespective of the parity of the parameter n
(compare the respective spectra with the spectra obtained for
native HDSe/Au(111) monolayer shown in Fig. 1). The very
opposite results are obtained for BPnSe/Au(111) SAMs, where
the incubation in HDT results in no visible exchange, whereas
incubation in the HDSe shows again a clear odd–even effect,
with the odd members of the series being more resistive
towards exchange.
Discussion
Data obtained for native BPnS/Au(111) and BPnSe/Au(111)
monolayers are not only the reference for the exchange
experiments but, as shown in Fig. 4, demonstrate that the
monolayers used in our exchange studies exhibit the same
odd–even behaviour as reported in previous experiments.
It should be noted that BP1Se/Au(111) and BP1S/Au(111)
have not been included in our studies, since previous
spectroscopic42 and microscopic45 studies clearly show
ill-defined monolayers for the BP1Se/Au(111) system and
thus lack the possibility of a meaningful comparison with
the BP1S/Au(111) system.
Exchange experiments summarized in Fig. 5 clearly
demonstrate that this odd–even variation in the film structure
is reflected in the odd–even stability of both types of
SAMs towards the exchange processes. The exchange of
BPnS/Au(111) molecules in HDT solution brings two important
information: (1) odd-numbered BPnS/Au(111) films are more
stable towards exchange by HDT; and (2) with increasing
length of the aliphatic spacer the stability of the even-
numbered BPnS/Au(111) films increases, approaching the
stability of odd-numbered systems. It should be noted at this
point that similar effects were observed in previous studies
analysing the exchange of BPnS/Au(111) films by HDT
Fig. 1 IRRAS spectra for BPnS/Au(111) SAMs (n = 2–6). Left column: native BPnS/Au(111) monolayers. Middle column: BPnS/Au(111)
monolayers after 24 h incubation in 1 mMHDT ethanolic solution at RT. Right column: BPnS/Au(111) monolayers after 24 h incubation in 1 mM
HDSe ethanolic solution at RT. For comparison, the grey box at the bottom shows IRRAS spectra for native HDT and HDSe SAMs.
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molecules by monitoring the capacity of the film.48 The
exchange of the BPnSe/Au(111) SAMs by the HDSe molecules
represents the fully analogous experiment with respect to the
S - Se substitution regarding both the monolayer structure
and exchanged molecules. As in the BPnS/Au(111) system,
also in this case odd-numbered SAMs exhibit higher stability.
Moreover, also the stability of even-numbered SAMs increases
with longer aliphatic spacers. For BPnSe/Au(111) SAMs,
however, this last effect is more pronounced, and already
for BP5Se/Au(111) and BP6Se/Au(111) there is very little
difference in stability. It should be also noted that within
our experimental conditions, odd-numbered BPnSe/Au(111)
SAMs are significantly more stable against the exchange by
HDSe molecules than BPnS/Au(111) SAMs against HDT
molecules.
As previously proposed by us,26 the higher stability of the
odd-numbered members of the BPnS/Au(111) homologue
series can be explained by a simple qualitative model. In this
model, the film stability is determined by either cooperative or
competitive action of three different factors determining the
energetics of a SAM, i.e. the molecular density of the films
(that is, the number of Au–S bonds formed per surface unit),
the intermolecular interactions, and the Au–S–C bending
potential contribute to the energy balance. While for
odd-numbered BPnS/Au(111) SAMs all these factors act
cooperatively, the Au–S–C bending potential opposes the
other two factors in the even-numbered BPnS/Au(111) films.
As a consequence, odd-numbered BPnS/Au(111) SAMs are
more stable. Apart from the present exchange experiments, the
higher stability of odd-numbered BPnS/Au(111) SAMs is
also supported by their higher electrochemical desorption
potential,49 and the fact that only even-numbered BPnS/Au(111)
SAMs undergo phase transitions (upon annealing) into new,
more stable structures.25–27 The key element of this pheno-
menological model is the significant contribution of the exact
bonding configuration of the thiolate on the Au(111) surface
(i.e. the Au–S–C bending potential) to the overall energetics of
Fig. 2 IRRAS spectra for BPnSe/Au(111) SAMs (n = 2–6). Left column: native BPnSe/Au(111) monolayers. Middle column: BPnSe/Au(111)
monolayers after 24 h incubation in 1 mM HDT ethanolic solution at RT. Right column: BPnSe/Au(111) monolayers after 24 h incubation in
1 mM HDSe ethanolic solution at RT. For comparison, the grey box at the bottom shows IRRAS spectra for native HDT and HDSe SAMs.
Fig. 3 IRRAS spectrum for BP3Se/Au(111) with indicated charac-
teristic absorption bands. See text for a detailed description.
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the BPnS film. The existence of such a bending potential was
demonstrated by previous experiments40 showing that the
odd–even changes in the packing and orientation of the
BPnS molecules are reversed once the substrate is changed
from Au(111) to Ag(111), and, thus, the preferred value
of the substrate–S–C angle changes from B1041 to B1801,
respectively.
Considering that the orientation of BPnSe/Au(111) SAMs
on Au(111) and Ag(111) substrates exhibits the same effect,42
and that the molecular packing of these SAMs is also very
similar,45 the same qualitative model could be envisaged for
the BPnSe/Au(111) system. However, the stability of these
SAMs has not been addressed so far, so that the present
exchange data showing an odd–even effect for the BPnSe/
Au(111) exchange are the first experiments which may confirm
the applicability of the above model of stability to BPnSe/
Au(111) SAMs. Following this model, we can also intuitively
explain the influence of the spacer length on the exchange
experiments. Since the odd–even effect in the structure is
not observed for purely aliphatic SAMs it is clear that the
competition effect observed in hybrid aliphatic–aromatic
BPnS(Se)/Au(111) SAMs is caused by introduction of the
rigid and relatively long biphenyl group. In even-numbered
systems this group becomes, due to sterical reasons, more
tilted and thus opposes high density packing. By increasing the
length of the aliphatic chain, which links the rigid biphenyl
part with the anchoring S(Se) atom, we introduce more
flexibility in the system permitting to reduce the competition
between the optimal configuration of the S(Se)–Au(111)
bonding and the two other factors i.e. packing density and
intermolecular interactions mainly governed by aromatic part
of the system. Thus, the stability of even-numbered systems
with longer spacers may approach that of odd-numbered ones
which do not suffer such competition.
So far we have discussed similarities in the structure of
BPnS/Au(111) and BPnSe/Au(111) SAMs which result in the
observation of the odd–even effect in their structure and, as
show our experiments, also in their stability towards exchange
by alkanethiol (HDT) and alkaneselenol (HDSe) molecules,
respectively. Such exchange experiments are completely
symmetric with respect to the S 2 Se substitution regarding
both the monolayer structure and exchanged molecules. In the
next step, we need to discuss the exchange experiments in
which a S - Se or a Se - S substitution should occur, i.e. the
results obtained for BPnS/Au(111) and BPnSe/Au(111) SAMs
exchange by HDSe and HDT, respectively. These experiments
demonstrate a complete exchange of the BPnS/Au(111)
films by HDSe molecules and complete lack of exchange for
BPnSe/Au(111) by HDT. Before concluding on these results,
several experimental facts should be noted. The STM data44,45
obtained for odd-numbered BPnS/Au(111) and BPnSe/Au(111)
show for both types of monolayers exactly the same structure
which is close to the commensurate (2O3 � O3)R301 with
an area per molecule of 0.216 nm2. The analogous STM
measurements44,45 for even-numbered BPnS/Au(111) and
BPnSe/Au(111) monolayers show somewhat different
structures which can be transformed into each other by
uniaxial expansion of the (2O3 � O3)R301 lattice along
the h11�2i substrate directions. While for even-numbered
BPnS/Au(111) SAMs such expansion is homogeneous, leading
to the periodic rectangular (5O3 � 3) lattice with an area per
molecule of 0.27 nm2, an undefined periodicity was observed
for even-numbered BPnSe/Au(111) SAMs.45 It should
be noted, however, that the calculated45 average area per
molecule in even-numbered BPnSe/Au(111) SAMs remains
essentially the same as for their thiol analogues i.e.
0.260–0.275 nm2. The uniaxial expansion of the molecular
lattice in even-numbered BPnS(Se)/Au(111) SAMs as
Fig. 4 Intensities of the characteristic absorption bands at 1005, 1381
and 1500 cm�1, for BPnS/Au(111) SAMs (left column) and BPnSe/
Au(111) (right column) as a function of the parameter n. Data
obtained from the left columns in Fig. 1 and 2.
Fig. 5 Changes in the relative surface coverage for BPnS/Au(111)
(left column) and BPnSe/Au(111) (right column) after 24 h incubation
in 1 mM HDT or HDSe ethanolic solution at RT as a function of the
parameter n. Data obtained from Fig. 1 and 2. See text for the
description of the relative surface coverage estimation.
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compared to odd-numbered analogues is also reflected in
corresponding spectroscopic data40,42 showing a significantly
larger tilting of the even-numbered molecules towards the
substrate. This effect can be explained by the directional
character of the Au–S–C and Au–Se–C bonds in BPnS/Au(111)
and BPnSe/Au(111).
Essentially, all existing microscopic and spectroscopic data
clearly show that BPnS/Au(111) and BPnSe/Au(111)
monolayers have the same or very similar structures for
odd- and even-numbered monolayers, respectively. Thus,
considering that intermolecular interactions in BPnS/Au(111)
and BPnSe/Au(111) are essentially the same, we conclude that
the observed higher stability of BPnSe/Au(111) monolayers as
compared to BPnS/Au(111) towards exchange is related to the
higher stability of the Se–Au(111) bond as compared to
S–Au(111). Such conclusion is also in line with our previous
microscopic results which demonstrated that adsorption of
BPnSe molecules on Au(111) surface leads to the reorientation
of Au(111) substrate step edges upon adsorption of BPnSe
molecules. This indicates a higher adsorbate-induced mobility
of the Au atoms in the top layer, and thus, stronger adsorbate
bonding to the substrate in comparison to their thiol
analogues (BPnS) where such reorientation is only observed
at elevated temperatures.45 At the same time also the size of
the domains observed for BPnSe/Au(111) SAMs is about
5 times larger than for their thiol analogues prepared under
the same conditions. We relate this feature to a smaller
corrugation of the binding energy hypersurface for
Se–Au(111) as compared to S–Au(111).45 Taking together
conclusions reached in the present exchange study and
previous microscopic experiments, we suppose that the
Se–Au(111) binding energy is higher than the S–Au(111)
binding energy, while the corresponding binding energy
hypersurface is less corrugated at the same time, as schemati-
cally shown in Fig. 6. Whereas higher binding energy would
account for higher stability of selenol based SAMs on
Au(111), the lower corrugation explains the higher structural
order observed for these systems28,34,45 as a result of easier
relaxation of the stress resulting from the misfit between
structures preferred by the adsorbed molecules and the
substrate, respectively.24
Conclusion
We investigated the relative stabilities of a homologous series
of BPnS/Au(111) and BPnSe/Au(111) SAMs against their
exchange by alkanethiol and alkaneselenol molecules. IRRAS
was used to quantify the extent of the exchange. Our
experiments show higher stabilities of the BPnSe/Au(111)
monolayers in this process. Since previous microscopic and
spectroscopic studies demonstrated very similar or exactly the
same structure for both types of monolayers, the contribution
of the intermolecular interactions to the different stabilities of
such analogue films can be neglected. Therefore, in contrast to
the previous studies which compared SAMs with different
structures, the higher stability of the Se–Au(111) bonding in
comparison to the analogue S–Au(111) bonding can be clearly
demonstrated in this case. Considering this information to-
gether with our previous microscopic studies, we propose a
higher binding energy of Se–Au(111) as compared to the
S–Au(111) bond, simultaneous with a lesser corrugated
binding energy hypersurface. In our opinion these two factors
are responsible for higher stability and better structural order,
respectively, of selenium-based SAMs on Au(111) surface
as compared to their thiol analogues. In the light of
these conclusions we consider selenium-based SAMs on
Au(111) as a superior alternative for commonly used sulfur
analogues.
Acknowledgements
This work was supported by the Polish Ministry of Science
and Higher Education (0061/B/H03/2008/34). PC greatly
acknowledges a Homing fellowship by the Foundation for
Polish Science. AT and BS appreciate the financial support by
the DFG through the graduate school 611 (‘‘Functional
materials’’).
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Fig. 6 Schematic drawing showing the anticipated differences in the
S–Au(111) and Se–Au(111) bonding: higher binding energy for
Se–Au(111) case with at the same time lower corrugation of the
binding energy hypersurface. The arrows mark higher (right) and
lower (left) surface mobility of Se and S atoms, respectively. See
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4406 | Phys. Chem. Chem. Phys., 2010, 12, 4400–4406 This journal is �c the Owner Societies 2010