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: piotr.cyganik@.uj.edu.pl

b Institut fur Anorganische und Analytische Chemie,Goethe-Universitat Frankfurt, 60438 Frankfurt, Germany

4400 | Phys. Chem. Chem. Phys., 2010, 12, 4400–4406 This journal is �c the Owner Societies 2010

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.

4402 | Phys. Chem. Chem. Phys., 2010, 12, 4400–4406 This journal is �c the Owner Societies 2010

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.

4404 | Phys. Chem. Chem. Phys., 2010, 12, 4400–4406 This journal is �c the Owner Societies 2010

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

discussion in the text.

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