4. Physisorption and Chemisorption of
Alkanethiols and Alkylsulfides on Au(111)
4.1. Introduction
The spontaneous ordering of adsorbates to form self-assembled monolayers (SAMs) is a phenomenon with great technological potential. Through the deposition of molecules with specific symmetry or functionality, surfaces can be custom-tailored for particular applications such as electrodes, chemical sensors, or for template patterning1-6. Although several adsorbate/substrate systems have been observed to self-assemble, the deposition of alkanethiols on single-crystal gold surfaces has been studied most frequently2. Due to the relative inertness of the gold surface to most potential contaminants, high-quality alkanethiol monolayers can be deposited from solution. Once prepared, SAMs composed of longchain alkanethiols are air stable and do not desorb at room temperature7.
The principal ingredient for self-assembly is a relatively strong interfacial binding asymmetry of the molecular constituents. In the alkanethiol SAM case, this is provided by the affinity of the sulfur atom for gold and a comparably strong lateral interaction (4-8 kJ/mol per CH28) arising from the van der Waals forces between the chains. The magnitude of this lateral interaction can be controlled by changing the length of the hydrocarbon. While the lateral interactions are relatively well understood, the sulfur-gold interaction in the alkanethiol-gold SAMs has remained the subject of frequent debate.
Until recently, SAM formation was thought to proceed via simple Langmuir-type kinetics9. However, the recent discovery of the existence of a lower density phase (the socalled "striped" phase10) and measurements of the rate of thiol adsorption on gold surfaces (determined by LEED11, STM12, X-ray and atomic beam scattering13,14) have indicated that SAM growth from the vapor occurs in at least two steps. Atomic force microscopy (AFM)15 studies have recently shown that this is also true for SAM growth from solutions of alkanethiols in ethanol. Furthermore, during gas phase deposition, the growth rate shows a complex dependence on the impingement rate (pressure) with evidence of three regimes of linear, quadratic, and saturated growth16. To understand this complex behavior, an improved knowledge of the kinetics and energetics of adsorption is essential. However, only limited data on adsorption energies is available in the literature at present. Based on the adsorption of hexane, Nuzzo et al.17 estimated a methylene-surface interaction of 8 kJ/mol. Nuzzo has also reported an chemisorption energy of 117 kJ/mol for dimethyldisulfide adsorption.
To better understand the head group/substrate interactions responsible for the structure and growth kinetics of SAMs, a program was initiated to produce accurate measurements of the energetics of the interaction of alkanethiols, dialkylsulfides, and dialkyldisulfides with Au(111). The ability of alkanethiols to both physisorb through van der Waals interactions and to chemisorb through the sulfur atom provided an excellent opportunity to also study the role of the physisorbed precursor state in the chemisorption kinetics. By systematic variation of the chain length of the alkanethiol, the dependence of the physisorption and chemisorption energy on the identity of the alkanethiol was explored. By quantifying the rate of chemisorption from a physisorbed precursor state, the energy of the activation barrier to chemisorption was also determined. With this information a more unified picture of the adsorption process will be provided for this important class of molecules with an extremely widely used, but only partially understood, substrate.
4.2. Results and Discussion: Energetics
4.2.1. Energetics of Alkanethiol Adsorption
Temperature programmed desorption measurements were performed on various coverages of several linear 1-alkanethiols (ethanethiol, butanethiol, hexanethiol, octanethiol, and nonanethiol). All thiols were initially deposited on a Au(111) crystal at a surface temperature of 200 K. For each thiol, two thermal desorption peaks are observed (Figure 4-1). Although the peak desorption temperature of the lower temperature peak varied as a function of the chain length of the alkanethiol adsorbate, the higher temperature peak position remained relatively constant.
Figure 4-1. a) Temperature programmed desorption performed with a clean Au(111) crystal (- - -) and a hexanethiol-covered Au(111) surface (---). b) Differentiation of hexanethiol desorption curve shown with (- - -) and without (---) correction for Debye-Waller attenuation of the specular signal during the temperature ramp.
From the hydrocarbon desorption experiments presented in the previous chapter, it was found that the peak desorption temperature of physisorbed species increased linearly with increasing chain length. For comparison, the activation energies for desorption of the linear alkanes and alkanethiols are plotted in Figure 4-2 as a function of the bulk heat of vaporization of the adsorbate. Both series of data display a similar linear relationship between the adsorption energy and the heat of vaporization with a slope of 1.15. This relationship implies that the lowenergy interaction with the surface is due to van der Waals forces which are responsible for the cohesive energy in the bulk phases.
Figure 4-2. Activation energy for desorption of both alkanethiols () and alkanes () from a Au(111) surface plotted as a function of bulk heat of vaporization. The alkanethiols shown are ethanethiol, butanethiol, hexanethiol, and nonanethiol. The alkanes are hexane, heptane, octane, nonane, decane, and dodecane. Linear fits to the alkanethiol (---) and the alkane (- - -) energy data are shown.
The robustness of this simple relationship of the physisorption enthalpy to the bulk heat of vaporization is displayed in Figure 4-3. Here, a wide variety of adsorbates ranging from simple alkanes to sulfur-containing molecules of variable structural complexity, as well as some aromatic compounds, show the same linear relationship between the bulk heat of vaporization of the molecule and its enthalpy of physisorption on the Au(111) surface.
Figure 4-3. Enthalpy of desorption of alkanes and various sulfur containing species versus their respective bulk heats of vaporization. The compounds displayed are (A) hexane, (B) heptane, (C) octane, (D) nonane, (E) decane, (F) dodecane, (G) benzene, (H) ethanethiol, (I) butanethiol, (J) hexanethiol, (K) octanethiol, (L) nonanethiol, (M) diethylsulfide, (N) dibutylsulfide, (O) 1,4-butanedithiol, (P) t-butanethiol, (Q) isopropylthiol, and (R) thiophene. A linear fit to the alkane data exclusively (- - -) and to all data points (---) are shown.
Table 4-1 presents the activation energies for desorption corresponding to the observed peak desorption temperatures of all the adsorbates studied. For most adsorbates, more than one TPD peak was observed. These multiple features have been classified by proposed origin and will be discussed individually in more detail in the following sections.
The column labeled "Phys" indicates energies
corresponding to peaks assigned to desorption from the physisorbed
layer, "Hindered" indicates a "lower than normal"
chemisorbed energy state which is not observed for the simple
alkanethiols, "Chem 1" refers to the first peak arising
from chemisorption which has been identified as the "normal"
chemisorption state of the thiols, and "Chem 2" are
peaks from a higher energy chemisorption feature that can only
be obtained at high exposures and that can be annealed away.
The column "Phys(calc)" represents the physisorption
energy calculated by an additive model outlined in Section 4.2.2.
"Deviation" is the percent error between the prediction
of the model and the observed experimental results.
4.2.2. Physisorption Enthalpies
Figure 4-4 displays the activation energies for desorption corresponding to all the desorption features found in the TPD spectra of the linear 1-alkanethiols plotted as a function of the chain length.
Figure 4-4. Plot of desorption
enthalpy as a function of number of carbons for alkanes and alkanethiols.
Desorption enthalpy of alkanethiols from the physisorbed state ()
and the chemisorbed state () are shown. Transient chemisorption
features at higher energies are shown as open diamonds. The dotted
and solid lines are the result of linear fits to the physisorbed
and chemisorbed data, respectively. The dashed line is the linear
fit to the alkane data of Figure 4-2.
The parameters of the linear fits to the alkane
and alkanethiol data are shown in Equations 4-1 and 4-2, respectively.
The incremental adsorption energy per carbon atom for the two
series are similar in accordance with the linear relationship
between the adsorption energy and the bulk heat of vaporization.
HtAu = (6.08 ± 0.74 kJ/mol) × C + (43.5 ± 5 kJ/mol) (4-1)
HaAu
= (6.16 ± 0.16 kJ/mol) × C + (19.4 ± 1.4
kJ/mol) (4-2)
where C represents the number of carbon atoms, HtAu is the activation energy for desorption of the alkanethiols, and HaAu is the activation energy for desorption of the alkanes. The difference between the y-intercepts of the fits of the two series is 24.1 kJ/mol, due primarily to the presence of a highly-polarizable sulfur atom in the alkanethiols. The linear slopes of both energy relationships are indicative of the additive nature of the contribution of the methylene subunits to the desorption enthalpy, implying that the plane of the molecule (as defined by the carbon backbone) lies parallel to the metal surface; with each methylene unit contributing relatively equally via its polarizability.
The methylene contribution to the physisorption energy of the alkanethiols had previously been estimated as 7.9 kJ/mol6. In addition, Dubois et al.18 reported adsorption energies of 48.5 kJ/mol and 58 kJ/mol for methanethiol physisorbed on Au(111). (The presence of the lower energy peak was attributed to multilayering.) For methanethiol (C=1), Equation 4-1 predicts that the physisorption energy will be 49.6 kJ/mol, in good agreement with literature values. For comparison, a study by Sexton and Hughes19 which measured the adsorption of alcohols, ethers, and alkanes on Cu(100) and Pt(111) showed that the contribution of a methylene group to adsorption is 5.06.5 kJ/mol on both metals, and that the oxygen atom contributes 42 kJ/mol to the observed adsorption energy on platinum and 35 kJ/mol on copper. An alkane adsorption study by Teplyakov et al.20 has recently measured the methylene contribution to the activation energy for desorption from Cu(100), 6.3 kJ/mol.
The directional polarizability model
of the previous chapter established that the contributions to
the physisorption energy of hydrocarbons on Au(111) could be broken
down into additive contributions of atoms or groups of atoms.
In particular, a methylene group contributes 6.2 ± 0.1 kJ/mol
in a long linear chain (but 8.1 kJ/mol in cyclic compounds) while
a methyl group contributes 15.5 kJ/mol to the physisorption energy.
In addition, the contribution of a double bond was found to amount
to 6.1 kJ/mol. Because of the additive nature of the adsorption
energies, these energy values were used with the linear alkanethiol
adsorption energies reported in Table 4-1 to extract the contribution
of the SH group. Simple algebra assigns the contribution of the
SH group to the observed adsorption energy as:
HSH
= HS
+ HCH3
- HCH2
(4-3)
The adsorption energies of molecules
with the sulfur atom at the center of the molecule such as dialkylsulfides
and thiophene can also be predicted with this model. Since HS
represents the contribution of a sulfur atom (when added to the
chain without altering the number of methyl groups), its value
(24.1kJ/mol) can be used with the appropriate hydrocarbon contribution
to calculate H for these molecules. As shown in Table 41,
the calculated adsorption energies ("Phys(calc)") agree
with the experimental results.
A challenge for the model was the
rationalization of the experimental adsorption energies of the
three highly-branched thiols: t-butanethiol, 2-propanethiol,
and neopentanethiol. If the contribution of all the atoms in
tbutanethiol are considered, the additive model significantly
overpredicts the value of the adsorption energy by the contribution
of a methyl group. However, excellent agreement is obtained if
only the contributions of the groups that are in proximity to
the surface in the most favorable configuration (as shown in Figure
4-5) are included. For tbutyl thiol, it can
be seen that only three of the four groups attached to the central
carbon will be equidistant from the surface and able to fully
interact.
Lastly, it should be pointed out that
the physisorption energies for the alkyldithiols cannot be understood
in the simple terms reported above. While 1,6-hexanedithiol appears
to rapidly chemisorb so that no value for the physisorption energy
could be determined, the two butanedithiols physisorb with energies
that are smaller than expected. Although the butanedithiols also
chemisorb less strongly than expected, the origin of the reduction
in physisorption energy remains unexplained for lack of sufficient
data.
Figure 4-5. Sketch of
possible orientation of neopentanethiol, 2-propanethiol, and tbutanethiol
physisorbed on Au(111) surface. Only those groups closest to
the surface contribute to binding. The dark spheres represent
carbon atoms, the lighter spheres represent hydrogen atoms, and
the spheres with "S" inscribed are sulfur atoms.
4.2.3. Chemisorption Enthalpy
Figure 4-4 also displays the activation energy for
desorption derived from the second feature in the TPD spectra
of alkanethiols (plotted as , also listed under the heading "Chem
1" in Table 4-1). For this feature, the adsorption energy
does not change with chain length and has an average value of
126 ± 2 kJ/mol. Since chemisorption to the surface probably
occurs through the formation of a chemical bond with the sulfur
atom21, the length of the chain would not be expected
to play a large role in the desorption enthalpy of the chemisorbed
molecules.
At high coverage, the adsorption of alkanethiols
on Au(111) leads to the formation of a well-ordered SAM where
the activation energy for desorption was found previously to be
between 126 -146 kJ/mol22. This adsorption
energy is approximately the same for both alkanethiols and dialkyldisulfides.
Dimethyldisulfide has been reported to desorb from Au(111) as
a disulfide with an energy of approximately 117 kJ/mol17.
The average adsorption energy for the chain-length independent
feature in Figure 4-4 is consistent with the literature.
In the present TPD measurements, it was observed
that alkanethiols composed of eight carbon atoms or less exhibit
both a physisorption and a chemisorption feature. For octanethiol
and longer chain thiols, the physisorption feature becomes difficult
to detect with TPD and a third, higher energy peak becomes apparent
at 148 ± 3 kJ/mol, which is 21 kJ/mol greater than the desorption
energy of the first chemisorption peak (this will be discussed
further in Section 4.2.5.). The disappearance of the physisorption
feature between 100 and 126 kJ/mol is due to the kinetics of chemisorption.
As the surface temperature is increased, the rate of chemisorption
from the physisorbed population also increases. For the short-chain
thiols, the temperature of desorption of the physisorbed molecule
is low, so these molecules will desorb from the surface before
the activation barrier to chemisorption can be easily crossed.
For longer-chain thiols, however, the chemisorption channel will
be able to compete successfully with the desorption channel and
will deplete the observed physisorbed population.
4.2.4. Physisorption vs. Chemisorption
As the chain length of the linear alkanethiols is
increased, the physisorption energy is also observed to increase.
As a result, the physisorption energy will become larger than
the chemisorption energy at chain lengths greater than C14H30
as shown in Figure 4-4. To confirm this trend, the activation
energy for desorption of gas-phase deposited hexadecanethiol was
measured to be 150 ± 15 kJ/mol. For comparison to the literature,
Nuzzo et al.21 determined that solution-deposited
hexadecanethiol desorbs from gold with a desorption enthalpy of
approximately 167 kJ/mol. This energy was significantly
higher than the previously reported "normal thiolate"
adsorption energy of 117 kJ/mol17 and was explained
by Nuzzo as being caused by a stabilization through interchain
interactions.
In the present context, interchain attraction does
not need to be invoked to justify the increased value of the desorption
energy since the continuation of the physisorption trend observed
for the shorter chains predicts that physisorbed hexadecanethiol
can still be bound to the surface at temperatures where the "chemical"
S-Au bond is already broken. Furthermore, at the temperature
where hexadecanethiol desorbs (>500K) it is highly unlikely
that the molecules will continue to form islands. At these temperatures,
the molecules are expected to desorb from an uncondensed two-dimensional
gas state since the energy needed for the molecules to detach
from the island borders is lower than that needed to desorb from
the gold.
To extend this trend, additional experiments were
performed with longer chain lengths. However, because of the
low vapor pressure of octadecanethiol, and docosanethiol, the
experiments were carried out with a modified procedure. The gold
crystal was cleaned as usual. The UHV sample chamber was vented
to nitrogen and a solution of one of the thiols in ethanol was
placed on the gold surface. The machine was pumped down and TPD
was carried out as usual. The TPD peaks were not as sharply defined
but clearly showed an activation energy for desorption that continued
to increase with chain length, shown in Figure 4-4.
4.2.5. Higher Energy Chemisorption
The appearance of an additional high energy TPD
peak at 148 ± 3 kJ/mol (shown as in Figure 4-4, listed in
the column "Chem 2" in Table 4-1) was unexpected. This
peak appears under non-equilibrium growth conditions when the
surface has been saturated with adsorbates at high fluxes and
low surface temperatures. To clarify its origin, several experiments
were conducted by dosing the gold surface with decanethiol at
298 K and then waiting for several minutes before performing the
TPD. Figure 4-6 shows the results of three experiments with anneal
times ranging from 0 to 3600 sec. In this series of spectra,
the area of the higher energy peak is largest with no anneal and
consistently becomes smaller with longer anneal time. However,
the temperature locations of the two peaks do not shift significantly.
Figure 4-6. Temperature
programmed desorption spectra of decanethiol annealing experiments.
A layer of decanethiol on Au(111) is annealed at 343 K for a) 0
seconds, b) 900 seconds, and c) 3600 seconds. The higher energy
peak (143 kJ/mol) anneals into the lower energy peak (124 kJ/mol).
After possibly going through an intermediate state,
the normal chemisorption feature becomes larger at the expense
of the higher energy peak, implying that the higher energy population
anneals into the lower energy state. This is surprising since
it is the reverse of what is expected and commonly observed.
A possible explanation that would account for this
apparent contradiction is that the higher energy peak is indicative
of adsorbates desorbing as thiols while the lower energy peak
corresponds to desorption of disulfides. Both thiol and disulfide
desorption from Au(111) surfaces have been observed by Nishida
et al.23. A comparison of Seller's24
calculation of methanethiol adsorption on Au(111) with a Born-Haber
calculation of desorption as a dimethyldisulfide indicates that
desorption as a disulfide is energetically favorable. Under a
highflux dosing condition, there may be enough hydrogen
present on the surface to facilitate the desorption of thiol species.
After annealing (or under slower growth conditions) the hydrogen
would desorb, leaving only the disulfide desorption channel available.
A second, and perhaps more likely explanation, can
be derived on the basis of a recent STM study of methanethiol
adsorption from the gas phase onto reconstructed Au(111) by Feher et al.25
With STM images, the authors show that "the dosing rate
affects the final structure of the monolayer." At high dosing
rates, a very large number of regularly spaced islands of gold
atom vacancies are formed which only upon annealing merge by Ostwald
ripening into large depressions while the monolayer (both in and
out of the depressions) assumes the normal c(4x2) structure.
If one makes the reasonable assumption that before and after the
annealing (which involves motion of the gold atoms on the surface)
the binding energy of the sulfur atoms to the gold is different,
the present results can then be explained.
The fact that the higher energy desorption peak
is observed only for chains with more than eight carbons can also
be explained in the context outlined above. As the annealing
rate of the chemisorbed phase must be a function of the chain
length, both dimerization and gold reorganization rates for chains
shorter than eight carbons may be fast enough to allow the annealing
to be completed during the temperature ramping of the TPD procedure
and before the desorption temperature is reached.
4.2.6. Steric Hindrance Effects
In an attempt to gain further information on the
bonding of thiols to the gold surface, other related sulfur-containing
molecules were studied. To address the question of disulfide
formation, sterically-hindered thiol groups were chosen to test
the hypothesis that the measured adsorption energy would be greater
if dimer formation was not possible due to steric constraints.
The TPD spectrum of t-butanethiol (with a thiol group
attached to a tertiary carbon) showed a low energy physisorption
peak which was consistent with the additive model (see section
4.2.2.) and a second feature which, contrary to the stated hypothesis,
corresponded to an adsorption energy of only 107 kJ/mol.
No desorption feature around 124 kJ/mol was observed even after
annealing.
Neopentanethiol (a thiol group attached to a primary
carbon with a t-butyl end) showed instead a "normal"
chemisorption feature at 128 kJ/mol. Although S-H bond strengths
differ slightly between the two molecules (368 kJ/mol for ethanethiol
and 364 kJ/mol for tbutanethiol26), it
appears that the sterically hindered thiols are not able to bind
as closely to the surface. Steric hindrance quickly ceases to
be a factor when an additional methylene unit (as in neopentanethiol)
allows the sulfur atom to interact more strongly with the metal
surface without pulling the rest of the molecule into the repulsive
wall of the physisorption potential well. This extra repulsive
energy due to the presence of sterically hindering groups is 20
kJ/mol (or approximately 16% of the desorption energy) and will
produce a relatively small change in the distance of the sulfur
atom from the gold surface.
4.2.7. Dialkyldisulfides and Dialkylsulfides
Another set of experiments directly explored the
adsorption behavior of diethyldisulfide on Au(111). TPD experiments
indicated the presence of only one desorption peak for diethyldisulfide
at 124 kJ/mol, comparable in energy to the chemisorption
peak for the alkanethiols. Since the physisorption energy for
diethyldisulfide was expected to be comparable to either ethanethiol
(if it cleaved upon adsorption) or butanethiol (if it remained
intact), it was surprising that no physisorption peak was observed.
However, the kinetics of chemisorption of the disulfide species
are different than that of the thiols. Experiments by Dubois
et. al.6 found that the sticking probability
for dimethyldisulfide was larger than for methanethiol by several
orders of magnitude. They suggested that for the shorter chain
species, the disulfide bond was more efficiently cleaved and that
disulfides did not exhibit the same activation barrier for adsorption
as the thiols. This is not surprising since the overall thermodynamics
of chemisorption of thiols on Au(111) involves the breakage of
the SH bond as well as the formation of H-Au bonds, H-H
bonds, and other reaction pathways. Instead, disulfide chemisorption
as thiolates could occur by a process which would only cleave
the disulfide bond and form two sulfur-gold bonds. However, according
to Fenter et al.27, disulfides could potentially
chemisorb intact without dissociation.
In a final set of experiments, the dialkylsulfides
(diethylsulfide and dibutylsulfide) were studied to investigate
the probability of cleavage of the C-S bond. In TPD experiments,
both compounds showed physisorption enthalpies consistent with
the correlation with the bulk heat of vaporization. However,
neither molecule showed a chemisorption feature. These findings
are in apparent contradiction to those of Porter28
who demonstrated C-S bond cleavage in several organosulfides under
electro-chemical conditions. Other studies, however, support
the present findings. In an earlier study utilizing dialkylsulfides,
Troughton et al.29 found that dialkylsulfides
formed a poorly organized layer from solution. They found that
the poor quality of the layer was not due to the decomposition
of the dialkylsulfide, but rather to the fact that the adsorbates
were only weakly bonded through the sulfur group. A recent study
by Beulen et al.30 has also shown that a variety
of dialkylsulfides remain intact on the surface and experience
no C-S bond cleavage.
Thiophene, a heterocyclic molecule containing
four carbon atoms and a sulfur atom, also showed only a physisorption
feature for adsorption on gold as in the case of the dialkylsulfides.
The single desorption peak occurs at 60 kJ/mol which is consistent
with desorption from a physisorbed layer. No other desorption
peaks indicative of chemisorption were found, even for samples
prepared under high dosing and/or long annealing procedures.
The thiophene measurements were
stimulated by a recent paper of Dishner et al.31
in which the formation of wellordered thiophene monolayers
on Au(111) was observed. The present work confirms the theoretical
results of Elfeninat et al.32 who concluded
that thiophene could not chemisorb on Au(111) surfaces.
4.3. Results and Discussion: Kinetics
4.3.1. Precursor mediated chemisorption: a brief
introduction
Physisorbed precursor states have been investigated
with respect to their effect on chemisorption kinetics33,34,
and the dynamics of adsorption and desorption35. Typically,
for a direct chemisorption process, sticking coefficients increase
with increased surface temperature. Systems that show a decrease
in sticking coefficient with increased surface temperature are
thought to involve a precursor state36-39. The
precursor is typically a more weakly bound state such as a physisorbed
state. The decrease of sticking coefficient into the chemisorbed
state arises from a kinetic competition between desorption
from the precursor state and the crossing of an activation barrier
to chemisorption. Precursor chemisorption has been found, for
example, for such systems as O2 on Pt(111)40,
CO and CO2 on Ni(100)41,42, and N2
on W(100)43. These systems have been explored using
molecular beam deposition, studying the dependence of the sticking
on the angle of incidence of the molecular beam with the surface,
the kinetic energy of the beam, and the surface temperature.
With the exception of alkanes on transition metals44-49,
larger molecules have not been as extensively studied. In these
systems, a decrease in initial sticking coefficient has been observed
with increasing incident energy and increasing surface temperature.
The gold/alkanethiol system with its relatively
low chemisorption energy and physisorption dependence on chain
length presents the opportunity to probe the phenomenon of precursor-mediated
chemisorption in a way not possible with the diatomic systems.
Since only the sulfur head-group chemisorbs to the gold surface
with an energy that does not depend on chain length, the chemisorption
well remains constant for different chain lengths. Increasing
the alkane chain length, however, increases the physisorption
(precursor) well depth. In this way, the activation barrier to
chemisorption from the physisorbed state can be systematically
varied and the process of precursor mediated chemisorption explored.
Whether the barrier between the two wells is independent of the
chain length or is decreased by the deepening of the physisorption
well had to be determined experimentally. Since the physisorption
interaction is distributed throughout the molecule while the chemisorption
well is localized, the barrier was predicted to be independent
of the chain length.
4.3.2 Chemisorption from a physisorbed layer
Helium atom reflectivity is unable to readily distinguish
between chemisorbed and physisorbed adsorbates. As a result,
the rate of chemisorption during dosing cannot be easily determined
at surface temperatures significantly below the physisorption
desorption temperature. However, by performing TPD after deposition,
it is possible to distinguish between the two adsorbed species
to determine if there is chemisorption of molecules from a physisorbed
precursor state.
To perform these experiments, the Au(111) crystal
was held at a surface temperature low enough to accumulate a physisorbed
monolayer. By increasing the amount of wait time between the
deposition and the TPD, the rate of chemisorption from the physisorbed
population was determined. As the chemisorbed population increases
by depletion of the physisorbed population, the relative populations
of adsorbates will change. In this series of experiments, it
is assumed that the contribution of each molecule to the total
area of the TPD peaks is the same irrespective of the physisorbed
or chemisorbed status of the molecules that produce the specularity
drop. This assumption is probably not strictly correct, but for
the purpose of comparison of similar conditions this should not
represent a serious problem.
Figure 4-7 shows a series of TPD experiments for
butanethiol deposited onto a Au(111) crystal at 208 K. Curve
a) shows the presence of mostly physisorbed species when the waiting
time is zero. The series of curves b), c), and d) represent progressively
longer waiting times showing conversion of the physisorbed species
into chemisorbed species. The final curve d), the result of annealing
the sample for 3600 seconds, exhibits mostly chemisorbed species.
Figure 4-8 displays the ratio of chemisorbed peak area to the
total peak area derived from each of the TPD profiles in Figure
4-7. The data clearly shows that conversion into chemisorbed
species increases with waiting time. The slope of the first three
points at shorter waiting times indicates that, at the surface
temperature of 208 K, the rate of chemisorption of physisorbed
species is on the order of 4 × 10-4 sec-1.
The TPD profiles in Figure 4-7 show also that the
physisorption peak location does not change with decreasing coverage
but that the chemisorption peak location shifts to higher temperatures
with increasing coverage. Annealing at higher temperatures would
be needed to ensure that the gold surface has also equilibrated
after chemisorption has occurred. Therefore no conclusions can
be drawn from the presence of these small shifts but they are
noted for the sake of completeness.
Figure 4-7. TPD spectra
of a butanethiol layer on Au(111) deposited and "annealed"
at 208 K for various times. The higher energy peak (chemisorbed)
becomes more intense over time at the expense of the lower energy
peak (physisorbed).
The final point in Figure 4-8 at 3600 sec indicates
the limit of full conversion into a chemisorbed layer. The y-intercept
of 0.35 corresponding to zero wait time indicates the presence
of partial conversion of the physisorbed population as the surface
temperature is ramped during the TPD cycle. As the surface temperature
increases there is an increase in the rate of chemisorption from
the physisorbed population, therefore some fraction of physisorbed
molecules will be converted before desorption.
Hexanethiol was also deposited on Au(111) at 208
K in a similar set of experiments. Once again, the TPD curve
obtained after zero waiting time (the solid curve of Figure 4-9)
shows the presence of both physisorbed and chemisorbed species.
Unlike butanethiol, there appears to be no further conversion
to the chemisorbed state even after waiting for 2460 sec.
Figure 4-9. Temperature
programmed desorption spectra of hexanethiol layer deposited at
208 K after a waiting time of a) 0 seconds and b) 2460 seconds.
The relative intensity of lower energy peak (physisorbed) and
higher energy peak (chemisorbed) remain relatively unchanged in
this range of waiting time.
A close examination of the increase in peak area
of the chemisorbed peak from no anneal to 2460 sec indicates a
much slower growth rate of about 110-5 sec-1.
This reduction in chemisorption rate compared to butanethiol
could be due either to a change in the activation barrier from
the physisorbed to the chemisorbed state or a change in the pre-exponential
factor. This question is considered in the next section.
4.3.3. Chemisorption from a steady-state physisorbed
population
A more convenient experiment is to study the growth
of the chemisorbed layer in realtime at a constant surface
temperature and with a small, constant physisorbed population
on the surface. By conducting growth experiments in a temperature
regime high enough that the desorption rate of the physisorbed
molecules is significant (approximately 20 K below the peak physisorption
desorption temperature), a steady state of physisorbed molecules
can be achieved. Additional decreases of specular intensity after
this physisorbed population has been established can then be assigned
to an increasing population of chemisorbed species on the surface.
However, since under these conditions there is a chance that
direct chemisorption may contribute to the chemisorption rate,
this effect must be taken into account.
A steady-state approximation can be applied to the
simple Langmuir growth equation (Equation 2-1) to determine the
coverage expected when the rates of adsorption and desorption
are balanced:
ss = ka / ( ka
+ kd ) (4-4)
where ss is the steady state coverage
of adsorbates, ka is the adsorption rate (controlled
in part by the partial pressure of the adsorbing molecules), and
kd is the desorption rate (controlled by the surface
temperature).
Figure 4-10. Specular
decay of Au(111) caused by adsorption of ethanethiol from the
gas phase at a surface temperature of 223 K. The solid curve
is the raw helium specular signal. The dotted line depicts
the partial pressure of ethanethiol in the UHV sample chamber.
As shown in Figure 4-10, the specularity initially
drops to a finite value (0.75) when exposed to a constant flux
of molecules which corresponds to a steady state coverage of physisorbed
molecules. However, even though the partial pressure of thiols
remains constant for 1600 sec, a slower decrease in specularity
is observed. This is attributed to conversion of the physisorbed
molecules into chemisorbed species, thereby "permanently"
occupying a fraction of the surface sites and decreasing the fraction
of surface available to the physisorption equilibrium. At the
end of the growth experiment, the thiols are evacuated from the
chamber and a rapid partial recovery of specularity is found.
The rate of recovery is consistent with desorption of the physisorbed
species. The chemisorption rate is identified as the rate of
the slower specular decay and is determined by differentiating
this portion of the decay curve and normalizing to the coverage.
By conducting this same type of experiment over a range of temperatures,
an Arrhenius plot for the physisorption to chemisorption conversion
process can be constructed.
Figure
4-11 shows these Arrhenius plots for ethanethiol, butanethiol,
hexanethiol, and decanethiol. For all four species, the same
slope is found with different y-intercepts,
as shown in Table 4-2. Since the rates found from the TPD "annealing"
experiment in the previous section fall within the data on this
Arrhenius plot, any direct process contribution to the rate of
chemisorption must be small compared to the rate of precursor
mediated chemisorption.
Figure 4-11. Arrhenius
plot of the chemisorption rate from a steady-state physisorbed
population for ethanethiol (), butanethiol (), hexanethiol (),
and decanethiol ().
While the slopes of the Arrhenius
plots have sizable errors, they all correspond to 29 kJ/mol
within 5%, independent of chain length. Nuzzo6 had
estimated the barrier to chemisorption of methanethiol from the
gas phase as 25 kJ/mol. However, his model predicts that the
activation barrier would decrease with chain length. This is
due to the fact the chemisorption well for all alkanethiol chain
lengths remains fixed and the physisorbed well becomes deeper
with increased chain length. In his model, the transition state
is viewed as being stabilized by attractive dispersion forces
to a degree comparable to that reflected by the increased heats
of adsorption of the molecular precursors6. Therefore,
increased chain length should decrease the activation barrier.
Within the limits imposed by the errors, the present data suggest
that while the physisorbed well depth increases the location of
the curve crossing with the chemisorbed state remains the same,
and therefore the activation barrier to chemisorption remains
constant with increasing chain length.
In this model of fixed activation barrier, the rate
of chemisorption is controlled in part by the lifetime of the
precursor state. Therefore, the overall rate of chemisorption
is a branching ratio between chemisorption of the physisorbed
molecules and their desorption. With a fixed activation barrier,
the rate for chemisorption increases overall with increased surface
temperature. The fact that physisorbed TPD features become difficult
to detect above chain lengths greater than octanethiol is consistent
with this model, since the longer chains remain on the surface
at higher temperatures resulting in faster chemisorption.
The identification of the "bottleneck"
to chemisorption is difficult. Typically, for diffusion controlled
processes on surfaces, the activation barrier increases with chain
length while the pre-exponential remains relatively constant50-52.
The present data show that the preexponential factor is
greatest for ethanethiol and smallest for the longer chains.
Indeed, hexanethiol, octanethiol, and decanethiol have approximately
the same pre-exponential factor. While the pre-exponential factor
should not be too greatly emphasized since it is notoriously difficult
to measure, these data lead to the tentative conclusion that diffusion
of the molecules across the surface is not likely to be responsible
for this "bottleneck" to chemisorption. It seems more
likely that orientation of the sulfur group with respect to the
surface is important, and that shorter chain lengths such as ethanethiol
have an advantage over the longer ones. However, this advantage
is bound to saturate with increased chain length because of the
increased chain flexibility. In this way the effect would become,
for the longer chains, independent of chain length.
4.4. Conclusions53
Alkanethiols physisorb to the surface of gold through
van der Waals interactions that generate adsorption energies on
the order of their bulk heats of vaporization. The molecules
tend to bind more strongly to the gold surface than to each other
in the bulk by a factor of about 1.15. This holds true for a
wide variety of sulfur containing alkanes of differing degrees
of branching as well as thiophene. The physisorption enthalpy
per methylene group for alkanethiols is on the order of 6.1 kJ/mol,
a value very similar to that observed for n-alkanes and
1-alkenes. The physisorption energy contributed by the thiol
group (SH) is on the order of 33 kJ/mol while the sulfur atom
alone contributes ~24 kJ/mol.
Alkanethiols show a chemisorption enthalpy of 126
kJ/mol which is independent of chain length. This implies that
for chain lengths greater than 14 carbons, the physisorption enthalpy
should be higher than the chemisorption energy. Indeed, longer
thiols exhibit a single TPD feature at temperatures which are
consistent with the projection of the physisorption energy with
chain length. The TPD spectra for thiols shorter than octanethiol
show both physisorption and chemisorption desorption features.
Octanethiol and longer chain thiols exhibit both the normal chemisorption
feature and a higher energy peak at 148 kJ/mol. This higher energy
peak can be annealed into the lower energy chemisorption peak.
This is likely due to a rearrangement of the gold surface atoms
which takes some time to occur and has an influence on the value
of the sulfurgold surface bond. The lack of physisorption
features for chain lengths longer than octanethiol is attributed
to the fact that the chemisorption rate is large enough around
350 K that as the TPD temperature ramps through this region, the
molecules will chemisorb instead of desorbing from the physisorbed
state.
Sterically hindered thiol groups show less binding
energy with the surface. Both tbutanethiol and 2-propanethiol
show physisorption enthalpies consistent with predictions based
on an additive contribution to binding per methylene unit, and
a lower-than-normal chemisorption enthalpy of 107 kJ/mol. Adding
one methylene group between the thiol and the t-butyl group
forms neopentanethiol. This species behaves "normally"
in that it has a predicted physisorption enthalpy and a chemisorption
enthalpy at 124 kJ/mol. The extra methylene group removes the
steric hindrance of the t-butyl group.
Diethylsulfide and dibutylsulfide both show only
physisorption peaks. This is consistent with earlier studies
of dialkylsulfides on Au(111) that showed them to be poorly organized,
physisorbed monolayers. Diethyldisulfide, however, showed only
a chemisorption feature. This is consistent with Nuzzo's finding
that the disulfide species tend to chemisorb more readily than
the comparable thiols. Disulfides probably have a lower barrier
to chemisorption due to the fact that they can chemisorb without
the need to eliminate hydrogen.
By probing the rate of conversion from physisorbed
layer to chemisorbed layer at 208 K for butanethiol and hexanethiol,
it is found that the rate decreases from 4 × 10-4
sec-1 for butanethiol to 3.3 × 10-5
sec-1 for hexanethiol. These rates are similar to
those found in constant exposure experiments that create a steady
state coverage of physisorbed molecules from which chemisorption
can occur. Arrhenius plots generated with the chemisorption rate
for ethanethiol, butanethiol, hexanethiol, and decanethiol indicate
that the activation barrier to chemisorption for these molecules
is about 29 kJ/mol irrespective of the chain length. This is
consistent with a model involving a constant chemisorption well
depth as the physisorption well depth increases with increased
chain length. In this case,
the increased physisorption energy does not affect the height
of the barrier but does increase the residence time of the molecules
on the surface at temperatures where the chemisorption occurs
more easily. The systematic study described in this paper has
clarified several issues related to the adsorption of alkylsulfides
and other sulfur-containing molecules on Au(111) and has provided
new clear challenges for those interested in the theoretical simulation
of the behavior of this very popular system
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