Understanding the stabilities of alkanethiolate SAMs at coinage metal surfaces upon
heating and exposure to externally applied forces is of particular importance since these
properties are directly related to performance and efficiency of the systems in technological
applications. To date, there is no theoretical study addressing the force-induced mechanical
response of thiolate-metal interfaces. In the present work, this gap is filled by employing DFT calculations to study the mechanochemical behavior of several thiolate-metal
systems, including propanethiolate SAMs at Au(111), Ag(111), and Au(110) as well as
heptanethiolate SAMs at Au(110). These systems have been chosen to cover the effects of
thiolate chain length, surface plane, and metal substrate.
First of all, the stable structures of the thiolate-metal interfaces of interest in the absence of
external forces are determined since they are needed as initial structures for the investigation
of the nanomechanical properties. Surprisingly, it has been extremely challenging to
unravel the atomic structures of these thiolate-metal interfaces even for the most common
prototype systems, i.e., thiolate SAMs at Au(111), which have been studied for several
decades, but the structures have been discussed controversially for many years, and even in
recent years improved structural models have been found. In this study, the first step was
therefore to identify the most stable structures of alkanethiolate molecules with various
chain lengths adsorbed at different metal surfaces, i.e., Cu(111), Ag(111), Au(111), and
Au(110). Besides, the adsorption of sulfur atoms at these metal surfaces was also studied
for comparison. A variety of surface models covering most of the structures suggested in
previous experimental and theoretical studies was considered. Moreover, vdW interactions
were taken into account for alkanethiolate molecules that contain more than one carbon
atom in the alkyl chain.
The present DFT results confirm that the adsorption of alkanethiolate molecules results in
reconstructions of the Cu(111), Ag(111) and Au(111) surfaces. The most energetically sta√
√
ble phase for atomic sulfur at Cu(111) and Ag(111) is the ( 7 × 7)R19◦ reconstruction,
√
√
but at Au(111) the unreconstructed ( 3 × 3)R30◦ structure yields the lowest surface
energy. In general, the order of the binding strength for sulfur adsorption is Cu(111)
> Ag(111) > Au(111). For methanethiolate and propanethiolate adsorption, the most
preferable structures are the pseudo-(100) reconstruction represented by the [ 51 03 ] supercell
√
√
√
for Cu(111), the ( 7 × 7)R19◦ phase for Ag(111), and the c(4×2), (3×4), and (3×4 3)
phases for Au(111). At Cu(111) and Ag(111), the reconstructed surfaces involve the
presence of surface vacancies. On the other hand, at Au(111) the key constituent for the
three most stable phases is a complex formed by two thiolate radicals linked to a shared
gold adatom.
Moreover, we find that for these thiolate-metal interfaces vdW interactions between thiolate
chains play a very important role for molecular ordering and surface stability. The vdW
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interactions are more important for longer-chain thiolate SAMs, yielding decreased thiolate
tilt angles, binding energies, and surface stabilities with an increase in the thiolate chain
length.
The binding strength is much higher for sulfur adsorption than for alkanethiolate adsorption
at a given metal surface. This is a reason why sulfur atoms are poisons in many catalytic
reactions. Besides, the adsorption of sulfur atoms and alkanethiolate molecules gives rise
to different surface reconstructions.
However, we note that other surface models are still required in some cases, in which the
calculated models show a significant deviation from the experimentally observed structures.
For example, in case of alkanethiolate SAMs at Cu(111) it seems that the [ 51 03 ] cell may
not be the optimum choice for the experimentally observed pseudo-(100) phase because
of the strong distortions in the fourfold coordination of the thiolate molecules and the
existence of several local minima differing in the position and number of the copper atoms
in the reconstructed layer. Therefore, other models with larger unit cells are needed to
allow for a realistic relaxation of the reconstructed layer. Besides, in case of sulfur atoms
√
√
adsorbed at Cu(111) and Ag(111) the ( 7 × 7)R19◦ cell yields the most stable structure,
but several models differing in the number of metal atoms in the reconstructed layer and
the metal-sulfur coordination have very similar surface stabilities. Taking into account
the intrinsic error of the PBE functional, it is not possible to make a conclusive statement
on the most stable structure. Moreover, there may be other structures with a comparable
stability, but they are not considered in this work.
For the adsorption of propanethiolate and heptanethiolate SAMs at the less stable Au(110)
surface, sulfur-headgroup atoms are adsorbed in short bridge positions and form a commensurate c(2×2) structure with respect to the underlying Au(110) surface. Alkanethiolate
molecules bind to the Au(110) surface more strongly than to the Au(111) surface. This is a
result of weaker bonds between gold atoms at the Au(110) surface due to larger interatomic
distance and thus these gold atoms are likely to form stronger bonds with adsorbed atoms
or molecules to lower the surface energy.
The most energetically stable structures for all studied systems together with the corresponding surface energies are summarized in Figures 8.26– 8.28. It is noted that in some
cases not only the most stable structure, but also other comparably stable structures with a
surface stability difference of only a few meV/Å2 are presented.
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S7 (−315)
S @ Cu(111)
S’7 (−207)
S @ Ag(111)
S9 (−313)
S @ Cu(111)
S’9 (−207)
S @ Ag(111)
S8 (−309)
S @ Cu(111)
S’8 (−204)
S @ Ag(111)
S’’4 (−153)
S @ Au(111)
Figure 8.26: The stable structures for sulfur atoms adsorbed at Cu(111), Ag(111), and Au(111).
The corresponding surface energies are given in the parentheses in the unit of meV/Å2 .
M7a (−48)
C1S @ Cu(111)
M’6 (−28)
C1S @ Ag(111)
M8 (−48)
C1S @ Cu(111)
M’’9 (−22)
C1S @ Au(111)
M12b (−47)
C1S @ Cu(111)
M’’10 (−22)
C1S @ Au(111)
M’’7 (−21)
C1S @ Au(111)
Figure 8.27: The stable structures for methanethiolate C1 S SAMs at Cu(111), Ag(111), and
Au(111). The corresponding surface energies are given in the parentheses in the unit of meV/Å2 .
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P2 (−61)
C3S @ Cu(111)
P’’3 (−29)
C3S @ Au(111)
P5d (−61)
C3S @ Cu(111)
P’’6 (−28)
C3S @ Au(111)
P’4a (−37)
C3S @ Ag(111)
P’’’2 (−38)
C3S @ Au(110)
(−46)
C7S @ Au(110)
Figure 8.28: The stable structures for propanethiolate C3 S SAMs at Cu(111), Ag(111), Au(111),
and Au(110), as well as heptanethiolate C7 S SAMs at Au(110). The corresponding surface energies
calculated with the DFT-D3 method are given in the parentheses in the unit of meV/Å2 .
Then, we now turn to the mechanochemical studies of propanethiolate SAMs at Au(111),
Ag(111), and Au(110) as well as heptanethiolate SAMs at Au(110). The force-induced
nanomechanical response scenarios of these thiolate-metal interfaces are summarized in
Figure 8.29.
It was found that the mechanical response scenarios strongly depend upon the combination
of directionality and magnitude of external forces and they differ from system to system
due to differences in the nature of chemical bonds at the interfaces, the initial structural
configuration, as well as the surface stiffness and mobility. There are many interesting
surface phenomena observed in the present work, i.e., “surface peeling” for the thiolateAu(110) systems, as well as “surface sliding” and “surface detachment” for the thiolateAu(111) and thiolate-Ag(111) systems. These observations are vastly different from what
has been found by pulling single thiolate molecules normal to the surface, where nanowire
formation is typically observed.
At Au(110), surface peeling of one and a half gold layers was found when tilted forces
consisting of lateral and normal components with a magnitude larger than about 2.0 nN
were applied to propanethiolate SAMs. On the other hand, for heptanethiolate SAMs the
surface peeling was induced by forces larger than about 2.5 nN normal to the Au(110)
surface. Lifting of only the topmost Au(110) layer was not observed probably due to
instability of a Au(110) layer with large interatomic distance between gold surface atoms,
but it is more preferable to lift all four gold atoms from the topmost Au(110) layer together
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C7S@Au(110)
C3S@Au(110)
| FN | >= 2.5 nN
F max
| FT | >= 2.0 nN
0−
3 nN
Surface peeling
Sliding of 2 silver layers
Mechanochemical
response scenarios of
thiolate−metal interfaces
F min
| FL | >= 0.4 nN
Sliding of 1 silver layer
| FN | >= 1.6 nN
Surface sliding
| FL | >= 0.6 nN
Surface detachment
| FN | >= 1.4 nN
C3S@Ag(111)
C3S@Ag(111)
Sliding of 1 gold layer
C3S@Au(111)
C3S@Au(111)
Figure 8.29: Overview of the mechanochemical response scenarios of propanethiolate SAMs at
Au(111), Ag(111), and Au(110) as well as heptanethiolate SAMs at Au(110). The most interesting
surface phenomena include surface sliding, surface detachment and surface peeling, which strongly
depend upon the magnitude and direction of the applied forces. Note that FL , FN , and FT are
abbreviated forms of a lateral, a normal, a tilted force, respectively.
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with two gold atoms from the second one. Besides, we find that different chain orientations
between propanethiolate and heptanethiolate SAMs in the initial structures may cause
variations in the effectively barrier heights for different force directions.
At the Au(111) and Ag(111) surfaces, surface sliding was induced by pulling with forces
parallel to the (111) surface plane and the number of the displaced metal layers was found
to be related to the direction and magnitude of the applied forces. On the other hand,
normal and tilted forces induced a detachment of some metal atoms from the outermost
reconstructed layer, i.e., the detachment of propanethiolate gold adatom complexes from
the Au(111) surface and the detachment of propanethiolate molecules together with at
most four out of five silver atoms from the reconstructed Ag(111) layer.
In general, the order of the force magnitude required for these surface modifications is
as follows: surface peeling and surface detachment > surface sliding, and Au(110) >
Au(111) > Ag(111).
Furthermore, it was found that the response of these thiolate-metal interfaces upon exposure
to external forces is thoroughly different from that upon heating. The interesting mechanical
response scenarios such as surface peeling, surface detachment, and surface sliding are
corresponding to the breaking of metal-metal bonds. In contrast, it is known that the
Au-S and S-C bonds are responsible for the breakdown in thermal dissociation processes,
resulting in the desorption of sulfur-containing species from gold surfaces and nonsulfurcontaining species from silver surfaces.
Overall, a rich spectrum of nanomechanical response scenarios of alkanethiolate SAMs
at the coinage metal surfaces is revealed by these explorative calculations as pictorially
summarized in Figure 8.29. The observations are very important not only for the progress of
mechanochemistry itself, but also for the development of many technological applications,
especially surface modifications using STM and AFM tips.
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