Surface Stress, Kinetics and Structure of
Alkanethiol Self-Assembled Monolayer
Michel Godin,*† P. J. Williams,‡ Olivier Laroche,† Vincent Tabard-Cossa,† L. Y.
Beaulieu,† R. B. Lennox§ and Peter Grütter†
Department of Physics, McGill University, Montreal, Quebec, Canada H3A 2T8,
Department of Physics, Acadia University, Wolfville, Nova Scotia, B0P 1X0, and
Department of Chemistry, McGill University, Montreal, Quebec, Canada H3A 2K6
Abstract
The surface stress induced during the formation of alkanethiol self-assembled monolayers
(SAM) on gold from the vapor phase was measured using a micromechanical cantileverbased chemical sensor. Simultaneous in-situ thickness measurements were carried out
using ellipsometry. Ex-situ scanning tunneling microscopy was performed in air to
ascertain final monolayer structure. The evolution of the surface stress induced during
SAM formation exhibits features associated with the structural phase transitions of the
monolayer. These results show that both the kinetics of SAM formation and the resulting
SAM structure are strongly influenced by the surface structure of the underlying gold
substrate and by the diffusion rate of the vapor alkanethiol towards the gold surface. The
1
adsorption onto gold surfaces having large, flat grains produced high quality selfassembled monolayers, as opposed to those formed on gold with smaller grains. An
induced surface stress of 15.89 ± 0.01 N/m was measured for the formation of a highquality c(4×2) dodecanethiol SAM on gold.
Introduction
Chemical self-assembly is increasingly being used for a multitude of applications
including surface modification and functionalization. The idea of building structures
from the bottom-up is one of the core concepts in nanoscience and promises to
revolutionize many industries.
For example, researchers in the field of molecular
electronics are investigating the use of molecular self-assembly to build new types of
transistors and switches that could replace today’s dependence on silicon electronics. 1,2,3
The drug industry is also interested in using the self-assembly principles to increase the
efficiency of the drug delivery process.4 However, if self-assembly is to become a useful
technology, it is important to understand the fundamental mechanisms that drive this
process, both from a kinetics and structural point of view. This paper will look into the
kinetics of alkanethiol, HS(CH2)nCH3, self-assembled monolayer (SAM) formation as a
model for understanding the self-assembly process.
In a recent review paper, Schwartz5 lists several factors that influence the growth
kinetics during the formation of alkanethiol SAMs. In particular, the issue of poor
2
reproducibility between experiments, especially between laboratories, is discussed.
Schwartz notes that complementary in-situ experimental techniques are needed to fully
investigate the formation process. It is recognized that the formation of an alkanethiol
SAM undergoes several structural phase transitions.6
During this series of phase
transitions, the molecular structure of the SAM is defined by the alkanethiols having their
alkyl chains parallel to the Au surface in a series of lying-down phases, before achieving
a final standing-up phase. Many groups have observed that SAM formation can be
defined in terms of two processes with different time constants.
Schreiber et al.7
observed a rapid process associated with the formation of the lying-down phases,
followed by a second, much slower process governed by the transition into an ordered
standing-up phase. Others5,8 suggest that the transition into the standing-up phase is
accompanied by a reorganization or crystallization of the alkyl chains that accounts for
the slower process.
It is suggested that this slow process involves molecular
organization9,10 (i.e. straightening of alkyl chains, removal of gauche defects) and
reduction of defects.
The time constants associated with these processes of formation
are of particular relevance to the present study. As shown in Figure 8A of Schwartz’s
paper5, a discrepancy of up to two orders of magnitude in the time constants associated
with SAM formation is observed for similar experiments between laboratories.
In the present study, we will show that we are able to track different structural phase
transitions that occur during alkanethiol SAM formation by measuring the induced
surface stress in real-time.
By combining in-situ surface stress and thickness
measurements, as well as ex-situ STM imaging, we obtained complementary information
3
about the different processes occurring during SAM formation. In particular, we have
found that the surface structure of the underlying gold substrate strongly influences both
the kinetics of SAM formation, and the resultant equilibrium SAM structure.
Furthermore, in the case of gold surfaces with small grain sizes, the impingement rate of
the alkanethiols on the gold surface strongly influences the final SAM structure. It is
imperative that these two aspects be controlled if the kinetics of alkanethiol SAM
formation and the equilibrium structure are to be analyzed and compared in a
reproducible manner.
Experimental
The adsorption kinetics of dodecanethiol11 (HS(CH2)11CH3) from the vapor phase on
gold was investigated using three complementary techniques. The surface stress induced
during SAM formation was measured in real-time using a custom-made differential
cantilever-based chemical sensor.12
Briefly, two AFM cantilevers are positioned
approximately 2 mm apart in a sealed aluminum cell. The cantilevers have been prepared
by thermally evaporating gold onto one side of both cantilevers as described below. One
cantilever is used as is while the other is used as a reference cantilever.
The reference
cantilever is rendered inert to the presence of alkanethiols by pre-depositing a
dodecanethiol SAM onto its surface. This SAM is formed by incubating the reference
cantilever in a 1 mM dodecanethiol/ethanol solution for 24 hours. The addition of the
SAM to the reference cantilever did not significantly alter its spring constant. Thus, the
reference cantilever is used to subtract unwanted signals resulting from variations in
4
temperature and environmental noise. The deflection of both cantilevers is monitored
using an optical beam deflection technique similar to that used in atomic force
microscopes (AFM), as shown in Figure 1. The deflection of the gold-coated cantilever
is directly proportional to the surface stress induced during monolayer formation.13 The
cantilever deflection can be converted to a surface stress by additionally measuring the
cantilever’s spring constant and geometry.14,15
Figure 1: The optical beam deflection technique is used to monitor the induced deflection
of the cantilever. The laser beam is reflected off the apex of the cantilever. The
displacement of the reflected spot, which is proportional to the cantilever deflection, is
monitored using a position-sensitive photodetector.
In a second system, also reported elsewhere12, a cantilever-based chemical sensor and
an ellipsometer16,17 were combined to yield simultaneous stress and monolayer thickness
measurements. In this case, gold-coated samples of mica18 (0.5 mm × 1.5 mm) were used
for the ellipsometric measurements. Both cantilever and mica samples were placed in the
same cell less than 1 mm apart to insure identical adsorption conditions.
5
Dodecanethiol was introduced into both of these systems by injection through a Teflonlined septum in pure liquid form. 10 to 50 µl of liquid dodecanethiol was injected at a
designated position in the closed cell. The dodecanethiol then evaporated into the vapor
phase under ambient pressure at room temperature before reaching the gold-coated
samples.
Additionally, scanning tunneling microscopy19 (STM) was used ex-situ to image the
resulting SAMs to ascertain their final structure. Alkanethiol-covered samples were
rinsed with anhydrous ethanol and blown dry with UHP nitrogen prior to imaging. All
images were acquired in air at constant current using hand-cut Pt80Ir20 tips.20
The gold films were prepared by thermal evaporation21 at a base pressure of 1.0x10-5
Torr or lower. In this study, two types of gold surfaces were prepared: surfaces having a
relatively small average grain size and surfaces having a large average grain size. The
gold surfaces with small grains were prepared by evaporating 10 nm of titanium22 at a
rate of 0.04 nm/sec followed by 100 nm of gold23 at a rate of 0.14 nm/sec. The samples
were not heated prior to the start of the evaporation, but radiative heating from the
evaporation boats increased the sample temperature to 130 ± 20 oC. The gold with larger
grains was prepared by initially heating the silicon nitride, SiNx, (cantilever) and freshlycleaved mica substrates to 200 ± 1 oC for 30 minutes. 100 nm of gold was then
evaporated at a rate of 0.14 nm/sec. The heater was turned off during evaporation, but the
substrate temperature reached 260 ± 10 oC by the end of the evaporation. Sample
6
temperature was kept under 300oC in order to prevent excessive permanent bending of
the cantilevers. Although Ti was originally used as an adhesion layer, it was found that
adequate adhesion was possible without it.
All experiments were performed using
freshly evaporated gold in order to minimize gold surface contamination due to exposure
to air, which affects SAM formation.
Results
Figure 2 shows STM images acquired in air of the two gold surfaces used in our
studies. Figure 2a shows gold with large grains, whereas Figure 2b reveals gold having
much smaller grains, as described above. The large grains had an average size of 600 ±
400 nm, while the gold with small grains typically had an average size of 90 ± 50 nm.
The RMS roughness of the gold in Figure 2a and 1b are 0.3 ± 0.1 nm and 0.9 ± 0.2 nm,
respectively, on a 200 nm length scale. X-ray diffraction revealed that both the largegrained and small-grained gold were highly ordered Au(111). It should be noted that the
gold on both the mica (for ellipsometry) and on the SiNx cantilever surface (for
cantilever-based stress sensor) were similar, in terms of grain size and roughness, as
verified by STM.
7
A
))
B
))
Figure 2: STM images (3µm × 3µm) of (a) large-grained gold and (b) small-grained
gold. Images were acquired in air with a tip bias of 600 mV and tunneling current of 35
pA. Height contrast scale is 14 nm for both images.
Simultaneous in-situ surface stress and thickness measurements were performed as a
function of time on gold samples having either large grains or small grains. Figure 3
shows that the real-time thickness profiles of the dodecanethiol SAMs as they grow on
the two types of gold are fairly similar, and reach a thickness of 1.5 ± 0.1 nm within
approximately 120 minutes. However, simultaneous surface stress measurements reveal
large differences for the two types of gold, as shown in Figure 4a. Figure 4b shows the
first 25 minutes of the data in Figure 4a.
The deflection signals of the reference
cantilevers are not shown because the reference cantilevers showed negligible deflections
when exposed to dodecanethiol vapor, as compared to the measured deflections of the
active cantilevers. Thus, the surface stress curves shown are purely due to the surface
stress induced during SAM formation. From Figure 4b, we see that for both the small-
8
and large-grained gold, there is an initial rapid development of the surface stress,
reaching a value of approximately 0.4-0.6 N/m after 2.5 minutes. This initial increase in
surface stress is more rapid for the gold with large grains, and a slightly larger surface
stress is measured at this time. Figure 4a shows, however, that the long term evolution of
the surface stress is markedly different for the two systems. The surface stress on the
gold with large grains continues to increase for approximately 10 hours, reaching a final
value of approximately 16 N/m, while the gold with small grains only exhibits a slight
increase in surface stress, with a final value of approximately 0.5 N/m.
Although ellipsometry fails to reveal information about the long term process, the
complementary use of the cantilever-based sensor provides insight into the kinetics of
SAM formation from the vapor phase.
9
Figure 3: The real-time thickness profiles of the dodecanethiol SAMs grown on smallgrained gold and large-grained gold are similar. Dodecanethiol was introduced at time t
= 0 min.
10
(A)
11
(B)
Figure 4: The surface stress induced during the formation of dodecanethiol SAM on goldcoated cantilevers. (a) The SAM grown on large-grained gold exhibits a long-term
increase in surface stress, which is not observed for the SAM grown on small-grained
gold. Dodecanethiol was introduced at time t = 0 hours. The first 25 minutes of SAM
formation are shown in (b).
Ex-situ molecular resolution STM imaging was performed to survey the structure of the
resulting SAM. These images were then analyzed to determine whether the entire SAM
was in an ordered standing-up phase, or whether defects and/or other phases were
present. Although a broad survey of the entire gold surface was not possible, it was
12
found that the SAM resulting from adsorption onto the gold with large grains was fully in
a standing-up phase, as shown in the STM image (44.7 nm × 44.7 nm) of Figure 5a,
where the (√3 × √3)R30o structure and its c(4×2) superlattice are observed in several
adjacent domains. The dark surface features found in Figure 5a are etch pits, which
commonly result from alkanethiol adsorption on gold.24,25,26 Figure 5b is a close-up look
(7.9 nm × 7.9 nm) at the boxed region of Figure 5a, showing the c(4×2) superlattice of
the (√3 × √3)R30o lattice.27 The superlattice’s primitive unit cell, p(3×2√3), is also
shown.
Figure 6 shows a STM image of the SAM grown on gold with small grains. In this
case, we observe alkanethiol domains of the lying-down phase alongside domains of the
standing-up phase. Although we do see some instances of the standing-up phase, a
broader survey of the surface revealed that the lying-down phase is predominantly
present. The width of these stripes is approximately 1.5 nm. From this, we conclude that
this is a striped phase where the alkyl chains form an interdigitated bilayer on the gold
surface, forming a stacked lying-down phase.6 The ellipsometer measures a thickness of
approximately 1.5 nm for this partial monolayer, which is approximately the same
thickness as a fully standing-up SAM.28
13
A
))
B
Figure 5: STM image (44.7 nm × 44.7 nm) of a dodecanethiol SAM on gold. All
alkanethiol domains present in (a) show the standing-up phase. The middle domains
exhibit the c(4×2) superlattice of the (√3 × √3)R30o lattice. The domains in the upper left
and on the right of the image are in the (√3 × √3)R30o phase. A zoom (7.9 nm × 7.9 nm)
of part of the middle domain (boxed region) is shown in (b). Here we see the c(4×2)
superlattice of the (√3×√3) R30o lattice. The p(3×2√3) primitive unit cell (0.85 nm ×
14
1.01 nm) of the superlattice is also depicted. The STM tip bias was 600 mV and the
current was 25 pA.
Figure 6: STM image (24.5 nm × 24.5 nm) of mixed phases of dodecanethiol SAM on
small-grained gold. The domain on the left side of the image exhibits a periodicity of 0.5
nm, indicating that the SAM is in the (√3 × √3)R30o standing-up phase. The larger
stripes on the lower right side of the image are spaced by 1.5 nm, typical of a stacked
lying-down phase. The STM tip bias was 600 mV and the current was 25 pA.
For the data shown in Figure 4a, the final surface stress was measured to be 0.51 ± 0.03
N/m and 15.89 ± 0.01 N/m for gold with small grains and large grains, respectively. The
15
STM results strongly suggest that the surface stress measured on gold with a large
average grain size is due to the formation of highly-ordered dodecanethiol SAMs
exhibiting the c(4×2) structure.
Previously reported surface stress results13,29,30 for
dodecanethiol SAM formation from the vapor phase are more consistent with the results
obtained in this study for the gold with small grains, which we conclude leads to a highly
defective, incomplete SAM.
Figure 7 shows the initial stages of the induced surface stress for an experiment similar
to the one depicted in Figure 4. The only difference in this case is that the growth was
slowed down by slightly increasing the separation distance between the liquid
dodecanethiol droplet and the gold coated samples.12 As a result, three distinct regions
are now clearly resolvable in the surface stress curves associated with different stages of
SAM formation. These features are also present in the data shown in Figure 4, but are
less pronounced. Based on STM and ellipsometric results, we infer that region A in
Figure 7 is associated with a disordered phase present at the onset of SAM formation.
The process in region B is associated with the transitions into the striped phases. In
region C, the surface stress induced on the cantilever coated with gold with large grains
starts to increase over a long time frame. This long-term increase in stress is related to
the transition into the standing-up phase and its subsequent ordering. For adsorption onto
gold with small grains, the surface stress ceases to increase since the SAM remains in a
mixed phase, where the stacked striped phase is predominantly found.
The time
constants associated with these different regions are a function of various factors,
including cell geometry (i.e. vapor concentration), alkanethiol length, and gold
16
cleanliness. Nevertheless, the time constant associated with the final process is always at
least an order of magnitude larger the first two processes. Finally, although the thickness
profiles of acquired by ellipsometry (e.g. Figure 3) did not always show clear signs of a
multistep process, they did not follow simple Langmuir kinetics.
Figure 7: Initial stages of the surface stress curves induced during dodecanethiol SAM
formation on large- and small-grained gold. The region label A corresponds to the initial
stages of SAM formation. Region B is associated with the transition into the lying-down
phases. In region C, the transition into the standing-up phase begins for the adsorption
onto the large-grained gold, whereas the SAM remains in a stacked lying-down phase for
the small-grained gold.
17
The dodecanethiol vapor concentration in the early stages of SAM formation also has a
strong influence on the evolution of the surface stress induced during SAM growth. In
the following series of experiments, the vapor deposition of dodecanethiol on gold-coated
cantilevers was performed by injecting 150 µl of pure liquid dodecanethiol into a closed
cell. Part of the liquid droplet then evaporated into the cell, and the alkanethiol vapor
concentration increased to saturation in the vicinity of the gold surface. It should be
noted that varying the volume of liquid alkanethiol injected into the cell did not affect the
observed surface stress curves, as long as enough liquid was injected such that some
remained in the cell when the vapor reached saturation. The time constant associated
with this increase in concentration up to saturation increased with the separation distance
between the alkanethiol droplet and the cantilevers. As the SAM formed on the goldcoated cantilevers, the induced surface stress was measured as a function of time for
different distances and cell volumes. This set of experiments was conducted on gold with
small grains.
Figure 8 shows the induced surface stress as a function of time for alkanethiol
droplet/cantilever separations of 3 mm, 23 mm, 96 mm and 246 mm, corresponding to
cell volumes of 3 ml, 3 ml, 12 ml and 32 ml respectively. Note that the shape of the
kinetics curves is quite different for distances of 3 mm and 23 mm, although the volumes
are the same.
These two experiments were performed in the same cell, but the
dodecanethiol droplet was injected at different places in this cell. These results clearly
indicate that the SAM formation is strongly influenced by the vapor concentration in the
early stages of exposure. The development of the surface stress occurs over longer time
18
scales, indicating that the growth is diffusion limited. We also observe that the surface
stress curves exhibit a stress release for large droplet-cantilever distances. In these cases,
ellipsometry reveals that the resultant SAM does not attain full monolayer thickness.
Figure 9 shows that the final thickness reaches 0.7 ± 0.1 nm, instead of the 1.5 ± 0.1 nm
expected for a fully standing-up or stacked lying-down SAM.
From this thickness
measurement, we conclude that the final phase in these experiments is a lying-down
phase where the alkyl chains are not stacked.6,31
Figure 8: Surface stress profiles for various alkanethiol droplet/cantilever distances. For
distances of 23 mm or greater, a release in stress is observed and the final SAM thickness
only reaches 0.7 ± 0.1nm.
19
Figure 9: Simultaneous stress and thickness measurements of a SAM grown with an
initially low vapor concentration. The stress curve exhibits a stress release, while the
ellipsometric thickness monotonically increases, attaining a value of only 0.7 ± 0.1 nm.
The structure of the resultant SAM grown on gold with small grains depends strongly
upon the growth rate. In both cases, the final phase appears to be in a lying-down phase.
For rapid growth rates, STM and ellipsometry indicate that the final phase is a stacked
lying-down phase. For slow growth rates, ellipsometry suggests that the final phase is an
unstacked lying-down phase.
The surface stress results are consistent with this
interpretation, as the final stress measured in the slow growth regime is typically smaller
than the final stress measured in the rapid growth regime. One would expect that the
20
induced surface stress would be larger for the systems where the molecular density in
higher.
Discussion
We have investigated the growth kinetics of alkanethiol SAMs on gold from the vapor
phase. The resultant SAM grown on gold having large grains was in a fully standing-up
phase. STM imaging of these SAMs revealed the presence of the c(4×2) superstructure
of the (√3 × √3)R30o lattice, which is consistent with a crystallized SAM in the standingup phase. The ellipsometer measured a final monolayer thickness of 1.5 nm, consistent
with the thickness of a SAM in the standing-up phase. Adsorption onto gold having a
small average grain size resulted in an incomplete SAM. In this case, STM imaging
revealed that most domains where in a stacked lying-down phase, with some domains in
the standing phase. Thickness measurements also leveled off at 1.5 ± 0.1 nm. This
implies that the thickness of the stacked lying-down phase, as measured by ellipsometry,
is approximately that same as the thickness of a dodecanethiol SAM in the standing-up
phase. Others have measured the thickness of alkanethiol SAMs in the lying-down
phase. Xu et al.31 measured the thickness of an unstacked lying-down domain to be 0.50
± 0.05 nm using an atomic force microscope. This value is smaller than the 0.7 ± 0.1 nm
thickness we measured using ellipsometry. Figure 10 shows a schematic of the lyingdown, stacked lying-down and standing-up phases.
The similarity in the measured
thicknesses of the stacked lying-down and of the standing-up phases remains
questionable. Possible sources of this discrepancy include the fact that a constant index of
refraction was used for all SAM phases. Variations in film index of refraction away from
21
the bulk value during the self-assembly process could account for a systematic error in
thickness results, especially for the lying-down phase. Also, thickness measurements
obtained by AFM could underestimate the actual thickness due to SAM compression by
the AFM tip during scanning. In any case, the similarity in thicknesses of the standingup and the stacked lying-down phases is questionable and makes the analysis of
ellipsometric measurements highly non-trivial, especially at low coverage.
Figure 10: SAM formation is characterized by the transition from the lying-down phase
to the stacked-lying phase before reaching the standing-up phase. The alkanethiol film
thickness increases with coverage. Other intermediate phases occur but are not depicted.
Three distinct processes are clearly resolved in our surface stress measurements, as
shown in Figure 7. The first two processes are associated with the formation of an initial
disordered phase and a transition into a striped phase. The third is a long term process
22
where the large increase in surface stress is due to the full conversion of the striped
phases into a crystallized standing-up phase. This conversion into the standing-up phase
does not occur on the small-grained gold. We note that the average size of atomically flat
areas available for SAM formation on the small-grained gold is of the same order of
magnitude as typical domain sizes that are observed for well-ordered SAMs.32,33 On
small-grained gold, alkanethiol domains of the lying-down phase form, but their growth
is hindered by the limited area available on the gold grain. The result is that the SAM
remains in this state, unable to undergo the transition into the standing-up phase. This is
not an equilibrium process since the coherence length for this growth is substrate-limited.
When SAMs are slowly grown on the small-grained gold by reducing the alkanethiol
vapor concentration in the early stages of formation, ellipsometry reveals a final SAM
thickness of 0.7 nm. For SAMs grown more rapidly on small-grained gold, STM reveals
that the monolayer stabilizes in a stacked lying-down configuration, while ellipsometry
yields a thickness value of 1.5 nm. Since the stacked lying-down phase is found to have
approximately twice the thickness of the SAM grown slowly, we assume that the
measured thickness of 0.7 nm is associated with a SAM in an unstacked lying-down
phase. It is possible that the molecular adsorption is slowed down to such a point that the
unstacked lying-down domains grow and stabilize in an energetically stable state. This
phenomenon has been referred to as a kinetics trap by Schreiber,34 and is more
predominant in vapor phase deposition than for SAM formation from solution. For rapid
growth, this intermediate state is not as stable and the transition into a denser, stacked
lying-down phase is possible.
23
We have identified two factors that have a strong influence of the growth of alkanethiol
SAMs on gold: the underlying gold substrate structure and the alkanethiol vapor
introduction conditions. Careful control and characterization of these factors is essential
in order to obtain reproducible results. Nevertheless, there are several other factors that
affect SAM growth, such as temperature, alkanethiol purity, and others outlined in
Schwartz’s review.5 Despite careful control over many of these factors, we find that the
surface stress measurement proved to be extremely sensitive to SAM growth. Although
the qualitative nature of the surface stress curves were quite reproducible, the quantitative
final stress values still tend to show a noticeable degree of variability in the case of SAM
formation on large-grained gold. Sample-to-sample variations in gold substrate structure
might account for some degree of variability. Nevertheless, an understanding of the
origins of the surface stress13,35 induced during the formation of alkanethiol SAMs will
lead to a greater understanding of the mechanisms that drive the self-assembly process.
Ongoing research is aimed at modeling the surface stress associated with SAM formation
for comparison with experimental measurements.
Summary
We have investigated the adsorption kinetics and the final structure of dodecanethiol
SAMs on gold surfaces using a micromechanical cantilever-based surface stress sensor,
an ellipsometer and a scanning tunneling microscope. We have demonstrated that the
grain size of the gold substrate strongly influences the ordering of the SAM. Our data
24
suggests that there is a critical gold grain size that is needed to initiate long-range
ordering in the SAM. We have also shown that the final structure of alkanethiol SAMs
on small-grained gold is strongly influenced by the alkanethiol vapor concentration in the
early stages of exposure. A careful control of the gold substrate quality and of the
alkanethiol introduction conditions is crucial in order to attain well-ordered, defect-free
self-assembled monolayers.
Acknowledgements
We would like to acknowledge the support of the Natural Sciences and Engineering
Research Council of Canada (NSERC) and Le Fonds Québécois de la Recherche sur la
Nature et les Technologies (FQRNT). We would also like to thank Eddie Del Campo for
skillfully machining some of our cells.
Footnotes:
*
Corresponding Author: E-mail: mgodin@physics.mcgill.ca
†
Department of Physics, McGill University
‡
Department of Physics, Acadia University
§
Department of Chemistry, McGill University
1
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The V-shaped cantilevers used were purchased from Thermomicroscopes, California.
Spring constants varied from 0.04 N/m to 0.01 N/m for the 220 µm to 320 µm long
cantilevers, respectively. Leg width and thickness for both cantilevers were 22 µm and
600 nm, respectively.
16
Laroche, O. M.Sc. Thesis, McGill University, Montreal, Quebec, Canada, 2002.
17
Gaertner Scientific, model L116C. The ellipsometer laser used had a wavelength of
632.5 nm and an angle of incidence of 70o. A constant index of refraction of 1.459 was
used for dodecanethiol SAM thickness measurements.
18
V-4 grade mica was purchased from SPI Supplies, PA. Mica substrates were freshly-
cleaved prior to evaporation.
19
Digital Intruments, USA.
20
Goodfellow Cambridge Limited, UK.
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Thermionics Laboratory Inc., USA, model VE-90.
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Alfa Aesar, CAN, 99.995% purity.
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Angstrom Sciences, USA, 99.99% purity.
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