Steric Effects on the Formation of Self-Assembled Monolayers of Alicyclic Thiol Derivatives on Au(111)

Article abstract of DOI:10.1002/bkcs.12364

To understand the steric effects on the formation of alicyclic thiolate self-assembled monolayers on Au(111), the surface structures and reductive desorption behaviors of self-assembled monolayers on Au(111) derived from cyclohexanethiol (CHT) and 1-methylcyclohexanethiols (MCHT) were characterized using scanning tunneling microscopy (STM) and cyclic voltammetry. STM observations showed that the adsorption of CHT on Au(111) resulted in the formation of well-organized monolayers with an unit cell of a?=?14.5 ± 0.3?? and b?=?10.2 ± 0.3??, while the adsorption of MCHT with a methyl substituent bonded to the 1-position of the cyclohexyl ring resulted in the formation of a disordered phase. The reductive desorption potentials for CHT and MCHT monolayers on Au electrodes were observed at ?0.984 and ?0.974 V, respectively. This difference in potentials can be attributed to the difference in lateral interactions, which depend on the structural order of monolayers.

Full text of DOI:10.1002/bkcs.12364

Article
DOI: 10.1002/bkcs.12364
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S. Seong et al.
KOREAN CHEMICAL SOCIETY
Steric Effects on the Formation of Self-Assembled Monolayers of
Alicyclic Thiol Derivatives on Au(111)
Sicheon Seong,† Seungwook Kwon,† Seulki Han,† Young Ji Son,† Gaeun Lee,† Taeho Yang,†
†,§,
Nam-Suk Lee,‡, and Jaegeun Noh
*
*
†Department of Chemistry, Hanyang University, Seongdong-gu, Seoul 04763, South Korea
‡National Institute for Nanomaterials Technology, Pohang University of Science and Technology, Pohang
37673, South Korea. *E-mail: nslee@postech.ac.kr
§Institute of Nano Science and Technology, Hanyang University, Seongdong-gu, Seoul 04763,
South Korea. *E-mail: jgnoh@hanyang.ac.kr
Received June 3, 2021, Accepted July 7, 2021
To understand the steric effects on the formation of alicyclic thiolate self-assembled monolayers on
Au(111), the surface structures and reductive desorption behaviors of self-assembled monolayers on
Au(111) derived from cyclohexanethiol (CHT) and 1-methylcyclohexanethiols (MCHT) were character-
ized using scanning tunneling microscopy (STM) and cyclic voltammetry. STM observations showed that
the adsorption of CHT on Au(111) resulted in the formation of well-organized monolayers with an unit
cell of a = 14.5 Æ 0.3 Å and b = 10.2 Æ 0.3 Å, while the adsorption of MCHT with a methyl substituent
bonded to the 1-position of the cyclohexyl ring resulted in the formation of a disordered phase. The reduc-
tive desorption potentials for CHT and MCHT monolayers on Au electrodes were observed at À0.984
and À0.974 V, respectively. This difference in potentials can be attributed to the difference in lateral inter-
actions, which depend on the structural order of monolayers.
Keywords: Cyclohexanethiol, 1-Methylcyclohexanethiol, Self-assembled monolayers, Steric effect,
Scanning tunneling microscopy, Cyclic voltammogram
Introduction
standing-up phase via several intermediate phases were
observed as the thiolate surface coverage increases.11,30
Thermal desorption spectroscopy (TDS) measurements
showed that the desorption behaviors and thermal stability
and of alkanethiolate SAMs were influenced by the nature
and surface topography of the metal.31 X-ray photoelectron
spectroscopy (XPS) measurements showed a strong S 2p3/2
peak at 162 eV, implying that the SH headgroup in alka-
nethiols are chemically bound to the gold surfaces during
self-assembly.1,26
Interestingly, branched alkanethiolate SAMs on Au(111)
were also examined to understand the steric effects on the
formation and surface structure of SAMs.32–34 High-
resolution STM imaging revealed that the SAMs on
Au(111) prepared by n-propanethiol (PT) with a straight
The functionalization of solid surfaces with nanoscale
precision can be achieved using self-assembled monolayers
(SAMs) formed by adsorption of various adsorbates.1–16
Therefore, SAMs have been extensively used in nano-
science and nanotechnology such as molecular sensors,17
bio-interface,1,18,19 nanopatterning,1,20 and molecular elec-
tronics.2,21–23 The binding conditions and surface structures
of SAMs on metals markedly affect the physical properties
of SAM-modified devices.1,18,22,23 Therefore, control and
understanding the surface and interface characteristics are
crucial factors for practical application of SAMs. The
adsorption geometry and surface structure of SAMs can be
tuned by deposition time and temperature,1,3,4,11,13 deposi-
tion methods (solution or vapor deposition),3,24 the thermal
annealing process,7,25 and preparation solvent.15,26,27 The
interface characteristics of metals can be easily modulated
via the formation of SAMs with different headgroups on
the metal surfaces.1,2,5,14,28 Alternatively, n-alkanethiols are
the most popular molecular system for studying their self-
assembly phenomena and their surface structures depending
on the alkyl chain length.1,11,13,27,29 Scanning tunneling
microscopy (STM) observations elucidated that the phase
transitions of n-alkanethiolate SAMs on the gold surface
from a loosely packed striped phase to closely packed
alkyl chain have a well-known (√3 Â √3)R30ꢀ packing
structure, whereas those formed by 2-methyl-2-propanethiol
(MPT) with three terminal methyl groups have a different
packing structure that can be described as a (√7 Â √7)R19ꢀ
structure.32 The adsorption geometry and energy for
branched alkanethiolate SAMs on Au(111) were proposed
using a molecular dynamic simulation study combined with
STM.33 The TDS study showed that MPT molecules in the
SAMs on Au(111) were desorbed at a lower temperature
of 390 K compared with PT molecules in the SAMs
at 450 K. The lower desorption temperature of MPT SAMs
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is because of weaker van der Waals interactions
between bulky alkyl chains.34 Alternatively, the SAMs of
cyclohexanethiol (CHT) with an aliphatic cyclic ring (alicy-
clic ring) on Au(111) at saturation have highly ordered
domains with a lower packing density than an analog
alkanethiolate SAM.35–39 Such low packing density for
CHT SAMs on Au(111) is attributed to the two structural
conformers with axial and equatorial forms, which are
required for large molecular space in the SAMs.35,40 As a
result, the loosely packed CHT SAMs are more easily
desorbed from the Au(111) surface than n-hexanethiolate
SAMs with a hexyl chain in ultrahigh vacuum condition.41
CHT SAMs on Au(111) can be utilized for a molecular
template for the growth of another SAM with a high struc-
tural order using a displacement technique.42 Similar alicy-
clic SAMs have also been used for the fabrication of
nanopatterning via electron irradiation.43 In this study, we
synthesized 1-methylcyclohexanethiol (MCHT) to examine
the steric effects on the formation and structure of alicyclic
thiolate SAMs on Au(111). The MCHT molecule is consid-
ered a best model system because it has the simplest methyl
substituent bonded to the 1-position of the cyclohexyl ring
that likely interferes with the formation and growth of
MCHT SAMs on Au(111). To date, there have been no
reports describing the steric effects on the formation of ali-
cyclic thiolate SAMs on Au(111). Therefore, the surface
structures and reductive desorption behaviors of non-
substituted CHT and MCHT SAMs on Au(111) were
investigated and compared using STM and cyclic
voltammetry (CV).
(11.8 mL) in a round-bottomed flask fitted with a stirrer
and reflux condenser. The mixture was refluxed for 48 h
with stirring. Then, a solution containing 4.73 g (3 eq) of
sodium hydroxide in 50 mL of water was added and further
refluxed without stirring for 5 h. After that, the acidified
aqueous layer was separated, and the organic layer was
extracted with 50 mL of dry ether three times. The resulting
mixture was dried with calcium chꢀloride, and the solvent
was removed by evaporation at 30 C under vacuum. The
colorless MCHT liquid was separated from the solution by
distillation at 90 ꢀC. The product was further purified
by column chromatography before use. 1H NMR
(400 MHz, CDCl3): δ 1.50–1.72 (m, 10H), 1.44 (s, 3H).
Single crystal Au(111) substrates were prepared by ther-
mal evaporation of gold atoms onto freshly cleaved mica
sheets at 623 K under ultrahigh vacuum (10À5 to 10À6 Pa).
CHT and MCHT SAMs were prepared by immersing the
Au(111) substrate into a V-vial containing a 1 mM
ethanolic solution of each CHT or MCHT at room tempera-
ture (RT) for 24 h. The prepared SAM samples were care-
fully rinsed with pure ethanol and dried by N2 blowing
before characterization.
STM measurements were conducted using a NanoScope
E (Veeco, Instruments Inc., CA, USA) with a mechanically
cut Pt/Ir tip (80:20). All STM images were obtained in con-
stant current mode under an ambient condition. The bias
voltage was controlled between 300 and 500 mV and the
tunneling current was controlled between 0.30 and 0.60 nA
during scanning. CV measurements were performed with a
BAS-100 W equipped with a typical three-electrode cell.
The CHT or MCHT SAM-modified Au electrode was uti-
lized as the working electrode. A Pt wire and Ag/AgCl
glass electrode were utilized as the auxiliary electrode
and the reference electrode, respectively. An aqueous solu-
tion of 0.1 M KOH was used as the electrolyte. The
solution was degassed with N2 for 20 min before use. The
applied potential was cycled between 0 and 1200 mV at a
rate of 400 mV/s with a voltage interval of 1 mV.
Experimental
CHT (purity ≥98%) was purchased from Tokyo Chemical
Industry Co., Ltd. (Tokyo, Japan) and MCHT was synthe-
sized according to a previously reported method.44 Figure 1
shows the molecular structures of two alicyclic compounds:
(a) CHT and (b) MCHT. The synthetic procedures for
MCHT are described as follows. 1-Methylcyclohexanol
(9.8 mL, 2 eq) was added to a mixture of thiourea (3 g,
1 eq), hydrochloric acid (9.45 mL, 3 eq), and distilled water
Results and Discussion
To probe the steric effects on the formation of alicyclic
thiolate SAMs on Au(111), it is essential to compare the
surface structures of SAMs formed by MCHT with a
methyl substituent bonded at the 1-position of the
cyclohexyl ring with those of non-substituted CHT SAMs.
Therefore, the surface features of CHT and MCHT SAMs
on Au(111) were investigated using STM, the most power-
ful technique to elucidate molecular-scale adsorption
behaviors and surface structures of adsorbates on metal sur-
faces.1–4 STM observations showed that the adsorption of
CHT molecules on Au(111) resulted in the formation
of well-organized SAMs, as shown in the STM images of
Figure 2. Tri-directional ordered domains (Regions I, II,
and III) with a domain angle of 60 Æ 2ꢀ or 120 Æ 2ꢀ were
observed in Figure 2(a), implying that the growth of CHT
Figure 1. Molecular structures of alicyclic compounds: (a) CHT
and (b) MCHT.
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Steric Effects on the Formation of SAMs of Alicyclic Thiolate
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Figure 2. STM images of CHT SAMs on Au(111). (a) Large-scale STM image (80 Â 80 nm2) showing a highly ordered phase with tri-
directional domain orientations (I, II, and III), (b) small-scale STM image (20 Â 20 nm2) showing individually visualized adsorbed mole-
cules in the ordered domains (I and II), and (c) molecularly resolved STM image (8 Â 8 nm2) showing highly ordered molecular
arrangements.
SAMs was significantly influenced by strong chemical
interactions between the sulfur headgroups and Au(111)
substrate, as in the case of alkanethiolate SAMs.1,7,18 The
structural ordering of CHT SAMs could also be mainly
driven by energy gain through the S Au bond formation.
Interestingly, most of the vacancy islands (round shaped
dark areas) existed around the domain boundary regions,
not in the ordered domains. This could be attributed to the
facile movement of vacancy islands to the domain bound-
ary regions through Ostwald ripening, which should be
driven by the maximization of lateral interactions between
alicyclic rings through the formation of the two-
dimensional ordered phase for CHT SAMs on Au(111).
Note that the origin and formation mechanism of such
vacancy islands were thoroughly discussed in literature.45
The STM image in Figure 2(b) clearly shows highly
ordered structures of Domains I and II for CHT SAMs on
Au(111), and these domains were separated by the domain
boundary as indicated by the white line. The STM image in
Figure 2(c) clearly shows the well-ordered molecular pack-
ing arrangements of CHT SAMs on Au(111). Based on this
STM observation, the lattice parameters of an unit cell were
extracted: a = 14.5 Æ 0.3 Å and b = 10.2 Æ 0.3 Å. This
Interestingly, CHT SAMs at saturation coverage had a typi-
cal surface feature with zigzag ordered rows with an inter-
molecular distance showing a repeating unit of 5.8 Æ 0.1 Å
and 6.1 Æ 0.1 Å.37,38
Alternatively, the ordered phase observed for CHT
SAMs with a flexible cyclohexyl backbone was different
from that observed for alkanethiol SAMs with an alkyl
chain backbone,1,7,26,39 which can be attributed to the pres-
ence of two energetically stable conformers of the
cyclohexyl ring in CHT SAMs on Au(111), such as axial
and equatorial chair conformers, as demonstrated by SERS
measurements.40,46 The equatorial conformer protruded
from the surface 1.57 Å more than the axial conformer,35,40
and was energetically more stable by 5 kJ/mol.47 Consider-
ing on STM imaging mechanism and structural stability of
two conformers on the Au(111), the bright (indicated by
the filled red circles) and dark (indicated by the filled blue
circles) molecular features in the unit cell of Figure 2(c)
are assigned to the equatorial and axial conformers,
respectively.
The STM images in Figure 3 show that the adsorption of
MCHT SAMs on Au(111) resulted in the formation of a
disordered phase, which is completely different from the
CHT SAMs with a well-organized structural phase, as
shown in Figure 2. The large-scale STM image in Figure 3
(a) showed that the vacancy islands considered good evi-
dence for the formation of chemisorbed monolayers existed
in the disordered phase. Close inspection of the MCHT
SAMs in Figure 3(b) revealed that the disordered phase
was not uniform with bright and dark surface regions as
expected by the different adsorption geometries of MCHT
molecules on Au(111). The STM image in Figure 3(c)
clearly showed randomly adsorbed molecules (bright
molecular spots with round shapes) in the disordered phase.
The STM observation clearly showed that a methyl substit-
uent bonded to the 1-position of the cyclohexyl ring in
MCHT remarkably affected the growth of two-dimensional
packing structure can be described as a (5 Â 2√3)R35ꢀ
structure, which is an identical packing structure for CHT
ꢀ
SAMs on Au(111) formed at À20 C. On the other hand,
different packing structures such as a (5 Â 2√10)R48ꢀ
structure35,36 or (2√7 Â √13)R19.1ꢀ structure38 were also
observed for CHT SAMs on Au(111) formed in a 1 mM
ethanolic solution. CHT SAMs on Au(111) formed in a
10À4 mM ethanolic solution had a (2 Â 4√2)R20ꢀ structure,
while those formed in a 10À2 mM ethanolic solution at RT
had a (5 Â √15)R25ꢀ structure, which means that the pack-
ing structure of CHT SAMs was significantly influenced by
solution temperature. Solvent-dependent structural change
of CHT SAMs on gold was also demonstrated by surface-
enhanced Raman scattering (SERS) measurements.40
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Steric Effects on the Formation of SAMs of Alicyclic Thiolate
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Figure 3. STM images of MCHT SAMs on Au(111). (a) Lage-scale STM image (100 Â 100 nm2) showing an disordered phase with
many vacancy islands, (b) small-scale STM image (20 Â 20 nm2) showing a disordered phase, and (c) high-resolution STM image
(8 Â 8 nm2) showing randomly adsorbed individual molecules in the disordered phase.
ordering of MCHT SAMs on Au(111). Based on this STM
study, it is clear that the methyl substituent interferes sig-
nificantly with the adsorption of the SH group bonded to
the same position of the cyclohexyl ring during self-assem-
bly, and furthermore prevents the maximization of lateral
interactions between alicyclic rings, resulting in the forma-
tion of disordered phase. The schematic view in Figure 4
shows that MCHT molecules on Au(111) generated disor-
dered SAMs because of the steric hindrance of the methyl
substituent during SAM formation. This result is supported
by previous reports showing that the SAMs of thiophene
on Au(111) formed in the ethanolic solution had ordered
phase,48,49 while the SAMs of 2,5-dimethylthiophene
(DMT) with two methyl substituents have only an
disordered.
phase. Because of the steric hindrance of these two
methyl groups bonded to the thiophene ring of DMT, direct
adsorption of thiophene sulfur on Au(111) can be hampered
considerably, resulting in the formation of a disordered
phase.48 Additionally, branched alkanethiols with bulky
alkyl substituents can form loosely packed monolayers and
an inhomogeneous interface compared with alkanethiol
SAMs with a straight alkyl chain.32–34 These results mean
that branched alkyl chains could significantly affect the for-
mation and growth of monolayers. Therefore, the surface
and interface structures of the branched alkanethiol SAMs
on Au(111) were different from those of n-alkanethiol
SAMs.32–34
CV measurements for reductive desorption of organic
monolayers adsorbed on Au electrodes provide vital infor-
mation regarding the binding strength of the headgroup–Au
bond and lateral interactions between molecular back-
bones.50–54 It was reported that reductive desorption peaks
appeared at more negative potentials when the headgroup–
Au bond strength strengthened.50 For example, the reduc-
tive desorption peak for benzeneselenolate SAMs on Au
were approximately 200 mV more negative potential than
that of benzenethiolate SAMs because the binding strength
of Se Au bond is stronger than that of S Au bond.50 The
chemical structures of the molecular backbones of adsor-
bates on Au is another factor that affects the reductive
desorption behavior because they could significantly modu-
late the headgroup–Au bond strength via different electron
transfer processes.51,52 In addition, reductive desorption
peaks of alkanethiol SAMs on Au shifted to more negative
potentials with increasing alkyl chain lengths because of an
Figure 4. A schematic illustration showing the formation of disordered MCHT SAMs on Au(111) resulting from the steric hindrance of
the methyl substituent bonded to the 1-position of the cyclohexyl ring during molecular self-assembly.
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Steric Effects on the Formation of SAMs of Alicyclic Thiolate
KOREAN CHEMICAL SOCIETY
increase in lateral interactions between alkyl chains.53,54 To
understand the reductive desorption behaviors of CHT and
MCHT SAMs on Au(111), the reductive desorption poten-
tials of CHT and MCHT SAMs modified on the Au work-
ing electrodes were examined, as shown in Figure 5. Peak
potentials for CHT and MCHT SAMs were observed at
À0.984 and À0.974 V, respectively. Even though the
observed peak potentials were nearly the same, the MCHT
SAMs with a disordered phase have less negative potential
by ~10 mV than CHT SAMs with an ordered phase. Note
that reductive desorption measurements were performed for
three times and it was conformed that the RD peaks
for MCHT SAMs were reproducibly appeared at less nega-
tive potentials than those for CHT SAMs within an error
range. Therefore, the small difference in potentials is
mainly because of the difference in van der Waals interac-
tions. It is well known that these interactions can be
increased when the ordered phase is formed. It was
reported that the peak potentials for closely packed
hexanethiolate SAMs with an aliphatic hexyl chain were
observed near À1.07 V, which is a more negative potential
for CHT and MCHT SAMs because hexanethiol with a
straight alkyl chain can increase van der Waals interactions,
forming more closely packed monolayers than CHT or
MCHT with an alicyclic ring (relatively bulky backbone),
as demonstrated by STM.35,37,41 Alternatively, the reduc-
tive potentials of benzenethiolate SAMs with a benzene
ring were observed near À0.6 V,51,52 which is much less
negative than those of CHT or MCHT SAMs. Considering
the above results, the relatively high reductive potentials of
CHT and MCHT SAMs were mainly because of the strong
binding strength of S Au in CHT or MCHT SAMs and
not because of the lateral interactions.
It also should be valuable to discuss the surface cover-
ages of both SAMs to understand the steric effects of the
methyl substituent of MCHT on the formation of SAMs.
Based on the reductive desorption peaks of CHT and
MCHT SAMs on Au(111), the surface coverages of both
SAMs were calculated according to the previously reported
equations.53,55–57 It was found that the surface coverages of
CHT and MCT SAMs on Au(111) are 4.49 Â 10À10 and
3.53 Â 10À10 mol/cm2, respectively, suggesting that MCHT
SAMs had 21.4% more loosely packed monolayers com-
pared with that of CHT SAMs. This result strongly implies
that the methyl substituent of MCHT significantly reduces
the adsorption amount of MCHT molecules, resulting
in the formation of loosely packed and disordered
monolayers.
Conclusions
The surface structures and electrochemical reductive
desorption behaviors of CHT and MCHT SAMs on Au(111)
were characterized by STM. STM observations revealed that
the adsorption of CHT on Au(111) resulted in the formation
of highly ordered SAMs with an unit cell of a =
14.5 Æ 0.3 Å and b = 10.2 Æ 0.3 Å, while the adsorption of
MCHT with a methyl group bonded to the 1-position of the
cyclohexyl ring generated disordered monolayers showing a
randomly adsorbed molecules. Therefore, the methyl substit-
uent significantly prevents the two-dimensional growth and
ordering of MCHT SAMs on Au(111). As a result, the disor-
dered phase was dominant. The reductive desorption poten-
tials for CHT and MCHT SAMs on Au electrodes were
observed at À0.984 and À0.974 V, respectively. The lateral
interactions should be a reason for this small difference in
potentials because the reductive desorption peak of CHT
SAMs with an ordered phase was observed at more negative
potential than the MCHT SAMs with a disordered phase.
Molecular-scale STM observations clearly revealed that the
methyl substituent of MCHT significantly affected the for-
mation and two-dimensional growth of alicyclic MCHT
SAMs on Au(111).
Acknowledgments. This research was supported by the
National Research Foundation of Korea (NRF) funded by
the Ministry of Education (NRF-2012R1A6A1029029,
NRF-2018R1D1A1B07048063,
NRF-
2020R1A6A1A06046728, and NRF-2021R1A2C2010917).
This research was also supported by the Commercialization
Promotion Agency for R&D Outcomes funded by the Min-
istry of Science and ICT (2021-RE-G02, Development of
In-situ IV measurement TEM holder).
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Figure 5. Cyclic voltammograms of the reductive desorption of
(a) CHT and (b) MCHT SAMs on Au working electrodes in a
0.1 M KOH solution. Ag/AgCl glass electrode was used as the
reference electrode.
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