Structures and displacement of 1-adamantanethiol self-assembled monolayers on Au{111}

Article abstract of DOI:10.1021/ja042621o

We have designed monolayers with weak intermolecular interactions for use as placeholders in intelligent self- and directed-assembly. We have shown that these 1-adamantanethiolate monolayers are labile with respect to displacement by exposing them to dilute solutions of alkanethiols. These self-assembled monolayers (SAMs) of 1-adamantanethiol on Au{111} were probed using ambient scanning tunneling microscopy (STM), and their assembled order was determined. Solution deposition of the molecules results in a highly ordered hexagonally close-packed molecular lattice with a measured nearest neighbor distance of 6.9 ± 0.4 A. The SAMs exhibit several rotational domains, but lack the protruding domain boundaries typical of alkanethiolate SAMs, and are similarly stable at room temperature. Co-deposition of alkanethiol and 1-adamantanethiol from solution results in alkanethiolate SAMs, except when using extremely low alkanethiol to 1-adamantanethiol concentration ratios. Facile displacement of low interaction strength SAMs can be exploited to enhance patterning using soft nanolithography.

Full text of DOI:10.1021/ja042621o

Published on Web 05/24/2005
Structures and Displacement of 1-Adamantanethiol
Self-Assembled Monolayers on Au{111}
Arrelaine A. Dameron, Lyndon F. Charles, and Paul S. Weiss*
Contribution from the Departments of Chemistry and Physics, The PennsylVania State
UniVersity, UniVersity Park, PennsylVania 16802-6300
Received December 8, 2004; E-mail: stm@psu.edu
Abstract: We have designed monolayers with weak intermolecular interactions for use as placeholders in
intelligent self- and directed-assembly. We have shown that these 1-adamantanethiolate monolayers are
labile with respect to displacement by exposing them to dilute solutions of alkanethiols. These self-assembled
monolayers (SAMs) of 1-adamantanethiol on Au{111} were probed using ambient scanning tunneling
microscopy (STM), and their assembled order was determined. Solution deposition of the molecules results
in a highly ordered hexagonally close-packed molecular lattice with a measured nearest neighbor distance
of 6.9 ( 0.4 Å. The SAMs exhibit several rotational domains, but lack the protruding domain boundaries
typical of alkanethiolate SAMs, and are similarly stable at room temperature. Co-deposition of alkanethiol
and 1-adamantanethiol from solution results in alkanethiolate SAMs, except when using extremely low
alkanethiol to 1-adamantanethiol concentration ratios. Facile displacement of low interaction strength SAMs
can be exploited to enhance patterning using soft nanolithography.
1
. Introduction
chemical and physical properties of the assembled molecules
can result in control of the chemical and physical properties of
the entire system, an attribute that is sought for molecular
In the field of nanoscale fabrication, self-assembly techniques
are being rapidly developed as device structures are getting
smaller and more intricate. Self-assembly is especially appealing
because of the ease of manufacture and the variety of applicable
systems. As such, self-assembled monolayers (SAMs) have been
extensively studied for use as matrices for molecular compon-
1
1,22,23,
devices.
It is useful to have a library of molecules that
form SAMs with distinctive properties so that in the future it is
possible to create monolayers that are uniquely designed for
their application. This can be achieved by tuning the molecular
topology and the intermolecular interactions within the SAMs.
The most rigorously characterized SAMs to date are al-
1
-8
5,9-119-11
ents,
media for patterning,
and simple models for
organic thin films.9
,12-21
For self-assembled systems, tuning the
1
6,24-29
kanethiolate SAMs on Au{111}.
Alkanethiolate SAMs
are fabricated easily from thiols or disulfides via a variety of
methods because they form spontaneously on Au{111} by
chemisorption of the sulfur headgroup to the gold surface. These
SAMs are well ordered and highly stable because of two
(
(
(
1) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.;
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M.; Weiss, P. S. Science 2001, 292, 2303.
properties: the strong sulfur-gold bond, and the attractive van
(
(
(
(
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13,20,28,30,31
der Waals forces between adjacent alkyl chains.
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3
10.1021/ja042621o CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 8697-8704
9
8697

A R T I C L E S
Dameron et al.
Structures similar to 1-adamantanethiol, such as bis(bicyclo2,2,2-
3
,3,1,1
octylmethyl)disulfide) and bis(tricyclo
decylmethyl)disulfide,
have been assembled on Au{111} and studied by STM.4 The
two possible unit cells assigned to the SAMs of the latter, which
differs from 1-adamantanethiol by an additional methylene unit
4,45
between the adamantane cage and the sulfur atom, are (4
x
3 ×
x
4 3)R30° and (7 × 7). Here, we utilize STM in ambient
conditions to demonstrate that 1-adamantanethiol forms flat,
highly ordered hexagonal close-packed, one-molecule-thick
films on Au{111}. We show that 1-adamantanethiolate SAMs
are labile with respect to displacement, which may make them
useful as temporary protective layers in device fabrication or
as placeholders, creating a means to hold useful molecules or
molecular devices in place without the risk of disrupting their
functionality.
2. Experimental Methods
2
.1. 1-Adamantanethiol Synthesis. 1-Adamantanethiol was syn-
Figure 1. Scanning tunneling microscopy images showing the well-ordered
hexagonal close-packed lattice of 1-adamantanethiolate self-assembled
monolayers on Au{111}: (A) 1500 Å × 1500 Å, sample bias 0.75 V,
current 6.0 pA; (B) 200 Å × 200 Å, sample bias 1.0 V, current 5.0 pA; (C)
33,42,43,46,
thesized based on previously reported methods.
In a three-neck
flask, 10.7 g of 1-bromoadamantane (Aldrich), 50 mL of glacial acetic
acid, 7.6 g of thiourea (Aldrich), and 25 mL of 48% HBr were refluxed
for more than 3 h under argon. Formation of the 1-adamantyl
isothiuronium salt was monitored with thin-layer chromatography, and
after refluxing, the solution was allowed to cool to room temperature
overnight. The salt was filtered from the acidic solution and hydrolyzed
by stirring for 15 h under argon in a solution of 10 g of NaOH in 200
mL of water and 75 mL of ethanol. The resulting solution was acidified
using 1 mM HCl, and the 1-adamantanethiol was separated via liquid-
liquid extraction using chloroform and the acid solution, followed by
rinsing with methylene chloride and hexanes. The resulting extracts
were dried over anhydrous sodium sulfate and filtered, and the
250 Å × 250 Å, sample bias 750 mV, current 3.0 pA; (D) Fourier transform
of C showing first-order and a few second-order reciprocal lattice points.
The structure of 1-adamantanethiolate is given in the lower right corner
for reference.
SAMs pack into a (
x
3 ×
x
3)R30° geometry (with a nearest
neighbor distance of 5.0 Å) with respect to the underlying
Au{111} lattice and related superstructures, and they form large
domains comprised of molecules with identical rotations and
tilts. The boundaries separating the domains are discrete and
1
7,32
1
often contain molecules protruding out of the SAM.
remaining solvent was removed by evaporation under Ar. H NMR
three singlets): δ 2.03, 1.94, 1.67. 13C NMR: δ 47.5, 43.1, 35.8, 30.1.
Mp 100-102 °C. The proton and carbon NMRs were taken in CDCl
using a Bruker DPX-300 NMR.
(
However, because of the prominent defects of alkanethiolate
SAMs, such as tilt boundaries, rotational boundaries, and
3
17,19
stacking faults,
they are not always completely reproducible
2.2. Self-Assembled Monolayer Fabrication. All of the SAMs were
matrices for device fabrication or patterned interfaces. Although
some defects are inherent to the formation of thiolated SAMs
on gold, with the correct choice of molecule, it should be
possible to create a SAM with few domains and indistinct
domain boundaries lacking significant defects.3
assembled on Au{111} via solution deposition in sparged ethanol.
Commercially available Au{111} on mica substrates (Molecular
Imaging, Tempe, AZ) were flame annealed using a hydrogen flame
immediately prior to deposition. Alkanethiolate SAMs were prepared
by immersing the annealed substrates in a 1 mM ethanolic solution for
24 h, while 1-adamantanethiolate SAMs were made similarly from 10
mM solutions. Following deposition, each sample was rinsed in neat
ethanol and blown dry with nitrogen twice.
Co-deposited SAMs were made from solutions containing both
1-adamantanethiol and alkanethiol in specified molar proportion to make
1 mM total thiolate solutions, and were then treated in the same manner
as single-component SAMs. Alkanethiol-inserted SAMs were made by
first fabricating a 1-adamantanethiolate SAM, confirming the presence
of a well-ordered lattice using STM, and then immersing the sample
into a 1 mM solution of alkanethiol for the specified time period.
3-35
As a member of the diamondoid family, with the space group
F 4h 3m, adamantane forms highly crystalline three-dimensional
structures and is the subject of many crystallography and theory
3
6-38
papers.
The molecule is comprised of a 10-carbon cage
39
made of four fused cyclohexane rings in chair conformations.
Thus, each carbon is tetrahedrally sp -hybridized, and there is
3
limited strain because each carbon-carbon bond is stag-
gered.3
8,40,41
The addition of a sulfur group, creating 1-adaman-
tanethiol (see Figure 1), allows the carbon cage to be tethered
3
3,42,43
to a gold surface while still keeping its round topology.
1
-Adamantanethiol-inserted alkanethiolate SAMs were made similarly,
but instead used 10 mM 1-adamantanethiol solution. Subsequently, each
sample was submitted to a rinse and dry cycle before returning them
to the STM. All STM measurements were performed under ambient
conditions using a custom beetle-style STM.9
To determine the lattice spacing of the 1-adamantanethiolate SAMs,
the same tip was used to scan a SAM of known spacing (i.e., an
alkanethiolate SAM) before and/or after scanning an 1-adamantanethi-
(
32) Noh, J.; Hara, M. Langmuir 2002, 18, 1953.
(
33) Charles, L. F. M.S. Thesis, The Pennsylvania State University, University
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970, 92, 2377.
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41) Shen, M.; Schaefer, H. F., III; Liang, C.; Lii, J.-H.; Allinger, N. L.; Schleyer,
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(
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959, 31, 2082.
8698 J. AM. CHEM. SOC.
9
VOL. 127, NO. 24, 2005

1
-Adamantanethiol SAMs on Au
{
111}
A R T I C L E S
olate SAM. Alternatively, the lattice spacing was determined from a
multicomponent SAM in which there were distinct ordered areas of
1
-adamantanethiolate and alkanethiolate. In this way, the same tip was
used to determine the lattice spacing of both simultaneously. In both
cases, the measured spacing of the alkanethiolate SAM was used to
calibrate the gain of the piezoelectric scanners and, thus, the measured
lattice parameters. The 1-adamantanethiolate lattice spacing was
measured from Fourier transforms of the images of the 1-adaman-
tanethiolate lattice and was calibrated using the Fourier transform of
the alkanethiolate lattice from the same or subsequent images. SAM
corrugation was calibrated using the known height of the step edges
of the Au{111} substrate in the same or subsequent images.
3. Results and Discussion
3
.1. 1-Adamantanethiolate SAM Structure. Figure 1A
shows two Au{111} terraces covered with a 1-adamantanethi-
olate SAM. A typical SAM of this type consists of very flat
topography with depressions on the order of Au{111} substrate
vacancy islands. With this resolution, it can be seen that
1
-adamantanethiolate SAMs lack the protruding domain bound-
12,21,48,49
aries that are typical in alkanethiolate SAMs.
Closer inspection at higher resolution (Figure 1B and C)
shows individual molecules arranged in a hexagonal close-
packed formation, and in addition to the substrate vacancy
islands, there are apparent depressions of the molecular lattice
of less than 0.5 Å. A Fourier transform of Figure 1C (shown in
Figure 1D) shows a hexagonal lattice with the first- and second-
order reciprocal lattice spots visible. However, these spots are
not single points, but rather a collection of points the same
distance from the center but slightly rotated. This distance in
reciprocal space corresponds to a measured nearest neighbor
distance of 6.9 ( 0.4 Å and a next nearest neighbor distance of
Figure 2. A scanning tunneling microscopy image of the domains of a
1
-adamantanethiolate self-assembled monolayer on Au{111}. The same
1
1.8 ( 0.4 Å. Measuring the angle spanned by the first-order
image is shown underneath with the domains highlighted and the contrast
increased for clarity. The measured angles between the unit cells of the
domains are given in Table 1 using the alphanumeric labels, indicated on
the figure. A slight tip jump, marked by the red arrowheads, approximately
two-thirds of the way up the image, gives the appearance of a change in
rotation of the lattice, but instead the molecules are all uniformly shifted to
the left across the image. The imaged area is 200 Å × 200 Å, sample bias
spots (the angle formed by drawing a line from one end of the
spot to center and back to the other end of the spot) in the
Fourier transforms also shows that there are rotational domains
of the hexagonal lattice with orientations as above.
With careful inspection, one should also observe that the rows
of molecules shift direction slightly throughout the molecularly
resolved images. It can be seen in Figure 2 that the apparent
depressions mentioned above are domain boundaries separating
different rotational domains. Five distinct domains (color coded
and labeled in the bottom image of Figure 2) are observed, and
their rotations with respect to one another are given in Table 1.
The difference in orientation angles between domains A and E
is small, and it is possible that these molecules belong to the
same rotational domain. It should be noted that the exact rotation
with respect to the substrate cannot be determined by imaging
only the adlayer, although it is possible to estimate this rotation
from straight step edges of the Au{111} substrate, which follow
close-packed substrate directions.
1.0 V, current 5.0 pA.
Table 1. Measured Angles between Each of the Observed
Rotational Domains (Shown in Figure 2) of a
1
-Adamantanethiolate Self-Assembled Monolayera
A
B
C
D
E
A
15° (CW) 7° (CW)
10° (CCW) 2° (CCW)
B
C
D
E
15° (CCW)
7° (CCW)
9° (CW)
9° (CCW) 24° (CCW) 15° (CCW)
15° (CCW) 8° (CCW)
9° (CW)
24° (CW) 15° (CW)
15° (CW) 8° (CW)
8° (CW)
2° (CW)
8° (CCW)
a
The direction of each angle is specified as clockwise (CW) or
counterclockwise (CCW). All measured angles have an associated error of
(4°.
Given the above observations, the most likely unit cell
assignments are (7 × 7) with two rotationally equivalent unit
cells of (7 × 7)R21.8° with respect to the Au{111}substrate,
additional possibilities with nearest neighbor distances near 6.9
Å are not ruled out. Using a gold lattice spacing of 2.89 Å, a (7
×
7) unit cell has a lattice constant of 20.23 Å and has nine
molecules per unit cell. Therefore, a (7 × 7) unit cell gives a
or the four rotationally equivalent unit cells (
x
91 ×
x
91)R27°
and (
x
91 ×
x
91)R5.2° with respect to the Au{111} substrate,
nearest neighbor distance of 6.74 Å and a next nearest distance
as shown schematically in Figure 3. It is also possible that the
preferred orientation is some combination of each unit cell;
of 11.68 Å. A (
x
91 ×
x
91) unit cell has a lattice constant of
2
7.57 Å with 16 molecules per unit cell, resulting in a nearest
(
48) Delamarche, E.; Michel, B.; Gerber, C.; Anselmetti, D.; Guntherodt, H. J.;
Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869.
neighbor distance of 6.89 Å and a next nearest neighbor distance
of 11.94 Å. All the calculated distances and angles for these
unit cells are within the error of those measured. Note that these
(49) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993,
1
15, 9389.
J. AM. CHEM. SOC.
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VOL. 127, NO. 24, 2005 8699

A R T I C L E S
Dameron et al.
in alkanethiolate matrices were mimicked.1,3,50,51 Further in-
vestigations of this phenomenon were performed inserting 1 mM
decanethiol, octanethiol, or hexanethiol into preformed 1-ada-
mantanethiolate SAMs for a variety of time scales. Table 2
shows the fraction of inserted decanethiolate or octanethiolate
in images of inserted 1-adamantanethiolate SAMs for three
insertion times. The fractional coverage was measured from
representative images of each sample. The images were scaled
to have high contrast between the two molecules on the surface
based on apparent height differences. The pixel fraction of the
image covered by each apparent height type (and therefore
domains of each molecule) was calculated and then divided by
the total lattice coverage. Defects in the substrates were not
included. All of the coverages were calculated from images
5
00 Å × 500 Å in size that include only one substrate terrace.
The associated error for all the percentages shown is (2%.
For insertion times of less than 1 min in a 1 mM decanethiol
solution, we found spotty coverage of which only a few percent
of the resulting SAM was composed of decanethiolate. After 5
min, patches of decanethiolate covered ∼40% of the surface,
and with longer insertion times, the coverage increase radially
outward from the patches until the entire 1-adamantanethiolate
SAM was replaced by decanethiolate (more than 30 min). These
patches were ordered domains of decanethiolate (see Figure 4A
and B), and it was possible to resolve the lattice of both the
Figure 3. The possible unit cells for 1-adamantanethiolate SAMs on
Au{111} with equivalent rotational unit cells shown for each. The (1 × 1)
unit cell of the Au{111} substrate is shown in black, and the (7 × 7) and
x
( 91 ×
x
91) unit cells of the adlayer are drawn in red and blue,
respectively.
Table 2. Measured Height Differences and Fractional Coverage
1-adamantanethiolate and decanethiolate portions simultaneously
for Solutions of Decanethiol and Octanethiol Inserted into
a
(although this is significantly easier when the apparent heights
of the two components are similar).
Preformed 1-Adamantanethiolate SAMs
Coverage for Insertion Times
Apparent
When inserting 1 mM octanethiolate into the 1-adaman-
tanethiol SAMs, the same process occured. Spotty coverage
primarily around the defects was observed for low insertion
times, with increased coverage extending out from the insertion
points for longer time scales. Slightly lower fractions (although
not dramatically different) were observed on the same time
scales as those for decanethiolate insertions. This may correlate
to the difference in hydrocarbon tail lengths and to the difference
in the speed of ordering due to larger intermolecular van der
Waals forces between the decanethiol relative to octanethiol
Inserted Molecule
Height Difference
5 min
10 min
30 min
decanethiol
octanethiol
2.7 ( 0.5 Å
0.4 ( 0.2 Å
40%
32%
55%
43%
g90%b
82%
a
The fractional coverage was measured by counting the number of pixels
with the same apparent height and dividing by the total available coverage.
Substrate defects were not counted toward this total. The fractions were
determined from images of the same size and resolution that cover only a
b
single substrate terrace. Due to the scarcity of 1-adamantanethiol mol-
ecules, the contrast of the SAM was not high enough to determine an exact
fraction, but it was apparent from the images that the SAMs were
predominately composed of decanethiolate.
1
2,14,21,30
molecules.
lattice spacings are significantly larger than those found in
alkanethiolate SAMs.
The measured apparent height differences between 1-ada-
mantanethiol and two alkanethiols with different chain lengths
are given in Table 2. 1-Adamantanethiolate is estimated to stand
.9 Å off the surface, including the 2.36 Å sulfur-gold bond.
Insertion of 1 mM hexanethiol into 1-adamantanethiolate
SAMs also resulted in a mixed SAM with both lattices apparent.
However, instead of the SAM being composed of mostly
1
-adamantanethiolate lattice with patches of hexanethiolate (as
seen for the other alkanethiol insertions), in addition to small
hexanethiolate patches, there were also striped regions and
regions of disorder (see Figure 5 top). The regions of disorder
were likely areas of mixed, unordered 1-adamantanethiolate and
hexanethiolate molecules. The stripes were observed in three
orientations, rotated 120° from each other. As shown in Figure
7
7
Using 1.1 Å per methylene unit length for the alkanethiolate
matrices, the measured apparent height in STM images of the
1
-adamantanethiolate SAM is 8.3 ( 0.5 Å. As discussed below,
because of steric hindrance, there is likely little tilting of the
-adamantanethiolate molecules with respect to the gold surface,
further supporting the idea that the different domains are due
to changes in rotation of the unit cell.
1
6
5
, each stripe set consists of a row of smaller molecules spaced
.1 ( 0.5 Å apart (row A in Figures 5 and 6), then two rows of
larger molecules spaced 11 ( 1.0 Å apart (rows B and C),
followed by another row of molecules that appear less protruding
in the STM images (row D). The molecular spacings of row A
and rows B and C correlate well to the nearest neighbor distance
of an alkanethiolate lattice and the previously discussed
3
.2. Displacement Studies. The regularity and lack of large
defect sites as well as the minimal internal conformational
relaxation make 1-adamantanethiolate SAMs appealing matrices
for molecular electronics. Insertion of molecules with electronic
function into an existing 1-adamantanethiolate SAM by previ-
3
ously described methods was attempted for several molecules,
(
50) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.;
Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am.
Chem. Soc. 1998, 120, 2721.
but resulted in coverages of the inserted species that were too
high to allow single inserted molecules to be probed when
conditions that had been used previously for inserting molecules
(51) Dameron, A. A.; Ciszek, J. W.; Tour, J. M.; Weiss, P. S. J. Phys. Chem.
B 2004, 108, 16761.
8700 J. AM. CHEM. SOC.
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1
-Adamantanethiol SAMs on Au
{
111}
A R T I C L E S
Figure 5. (Top) A scanning tunneling microscopy image of an 1-adaman-
tanethiolate self-assembled monolayer on Au{111} that was immersed for
30 min in a 1 mM hexanethiol solution. The red, blue, and yellow arrows
indicate patches of ordered hexanethiolate, regions of ordered 1-adaman-
tanethiolate, and regions of disorder, respectively; 300 Å × 300 Å, sample
bias 1 V, current 4.0 pA. (Bottom) A model of the 1-adamantanethiolate
and hexanethiolate stripes on Au{111} using a (7 × 7) unit cell for
Figure 4. (A and B) Scanning tunneling microscopy images of 1-adaman-
tanethiolate SAMs with decanethiol inserted for 10 min. The decanethiolate
protrudes beyond the 1-adamantanethiolate in ordered patches, indicated
by the arrows. (A) 1500 Å × 1500 Å, sample bias 1.0 V, current 1.0 pA;
1
-adamantanethiolate SAM and a ( 3 × 3)R30° unit cell for hex-
x x
anethiolate SAM. The white boxes indicate the (4 × 4
and (6 × 3) (right) subunit cells of the striped 1-adamantanethiolate
lattice and hexanethiolate lattice, respectively. The black box indicates the
13 × 2 3) unit cell of the hexanethiolate and adamantanethiolate stripes
together.
x
7/3)R30° (left)
(
B) 200 Å × 200 Å, sample bias 1.0 V, current 2.0 pA. (C) A decanethiolate
x
SAM with 1-adamantanethiol inserted for 30 min. No regions of remaining
1
-adamantanethiolate are observed; 700 Å × 700 Å, sample bias 1.0 V,
(
x
current 2.0 pA.
measured next nearest neighbor distance of an 1-adamantanethi-
olate lattice, respectively. On the basis of these spacings and
the relative sizes of the two molecules, the molecules of rows
A and D are assigned as hexanethiolate and the molecules in
the middle rows B and C as 1-adamantanethiolate. Seen most
clearly in the inset of Figure 6, the alkanethiolate molecules in
row A are positioned directly across from those in row D, while
the 1-adamantanethiolate molecules in row B are staggered with
the molecules in row C (i.e., the molecules in row B are nearest
neighbors of the molecules in row C). Furthermore the 1-ada-
mantanethiolate rows in a stripe set are staggered with respect
to the equivalent rows in a neighboring stripe set. Upon closer
inspection of the hexanethiolate rows, additional hexanethiolate
molecules are observed. The arrow in the inset of Figure 6 shows
the apparent pairing of two hexanethiolate molecules in row D
with a third molecule in the nearest neighbor position between
rows D and C. Other hexanethiolate molecules were observed
along row D as well as above row A. The extra molecules did
not occur all the way along the row and did not appear uniformly
J. AM. CHEM. SOC.
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A R T I C L E S
Dameron et al.
between two 1-adamantanethiolate rows (B-C) is approximately
4
Å; this is possible because of the staggered arrangement of
the molecules.
As shown in Figure 5, it is possible to model the striped
x
regions using a ( 3 ×
x
3)R30° unit cell for hexanethiolate
and a (7 × 7) unit cell for 1-adamantanethiolate with respect to
the Au{111} surface. Using the orientation of the hexanethiolate
lattice with respect to a substrate step edge (not shown), it is
possible to determine the orientation of the Au{111} lattice and,
therefore, the associated orientation to the 1-adamantanethiolate
lattice. Given the above observation that the rows of hexanethi-
olate molecules are aligned with one another and the 1-ada-
mantanethiolate molecules are not, it is determined that two
hexanethiolate rows must be an even integer apart (i.e., every
second, fourth, sixth, etc. row), while the 1-adamantanethiolate
rows must be an odd integer apart (i.e., first (nearest neighbor),
third, etc. row). From the measured row spacings, the most likely
orientation is the one shown in the model, with the hexanethi-
olate rows repeating every four rows and the 1-adamantanethi-
olate rows paired and repeating every three rows. In this
orientation, the hexanethiolate molecules are arranged in a (6
×
x
3) unit subcell, and the adamantanethiolate molecules
form a (4 × 4 7/3)R30° unit subcell with respect to the
3) unit
x
Au{111}. Both molecules together form a (13 × 2
x
cell (shown in black in Figure 5). The model also demonstrates
that there is enough room for an extra hexanethiolate molecule
near the hexanethiolate nearest neighbor positions, although not
always at the preferred nearest neighbor alkanethiolate spacing,
explaining the strained appearance.
When 1-adamantanethiol is inserted from a 10 mM solution
into alkanethiolate SAMs, very little displacement is observed.
As shown in Figure 4C, the adamantanethiol molecules insert
sparsely around the defects of the decanethiolate SAMs, but
they do not displace the existing molecular lattice. Insertion
times of 1-adamantanethiol up to 24 h still result in nearly pure
alkanethiolate SAMs.
Co-adsorption studies were also performed in which deposi-
tion times of 24 h were used to make SAMs from two-
component solutions in which the relative molar ratio of
decanethiol to 1-adamantanethiol was varied. The type of SAM
and the resulting order were monitored, and examples are shown
in Figure 7. An ordered SAM of decanethiolate results when
the solution consisted of molar ratios of less than 90:10
1
-adamantanethiol:decanethiol (Figure 7A). However, the de-
Figure 6. At the top is a scanning tunneling microscopy image of a
canethiolate SAMs have more and smaller domains than a
typical decanethiolate SAM made with a 24 h deposition time
and are often disordered at domain boundaries. For molar
percentages of 10% decanethiol in solution, a disordered SAM
of both decanethiolate and 1-adamantanethiolate results (see
Figure 7B). For molar percentages of less than 5% decanethiol,
an ordered SAM of 1-adamantanethiolate results (Figure 7C)
with small clusters of decanethiolate molecules randomly
dispersed throughout the SAM, similar to the samples seen for
the insertion studies with low insertion times.
1
3
-adamantanethiolate self-assembled monolayer on Au{111} immersed for
0 min in a 1 mM hexanethiol solution. The inset shows a detailed single
stripe set with the 1-adamantanethiolate molecules zig-zagging on the
interior, displayed in yellow (rows B and C), and the hexanethiolate
molecules lined up on the exterior, displayed in red (rows A and D). The
black arrows indicate example additional hexanethiolate molecules close
to the nearest neighbor position of the hexanethiolate molecules in row D;
1
50 Å × 150 Å, sample bias 1 V, current 4.0 pA (top). Profiles of the
adamantanethiolate and hexanethiolate rows (lines on the STM image in
blue and red, respectively) are shown below.
spaced, nor were they uniformly the same apparent height,
possibly indicating strained conformations.
Figure 8 schematically shows possible explanations for the
susceptibility of 1-adamantanethiolate SAMs to displacement.
SAMs of n-alkanethiolates are stabilized considerably by the
presence of intermolecular van der Waals forces and tilt so as
to maximize these chain-chain attractive interactions.1
The measured distance between the hexanethiolate rows (A-
D) is 22 ( 2 Å. The measured distance between a hexanethiolate
and the nearest 1-adamantanethiolate (A-B) row is 8 ( 2 Å
and between a hexanethiolate and the farthest 1-adamantanethi-
olate row (A-C) is 12 ( 2 Å. Therefore, the measured distance
2,21,30,48,49
The 1-adamantanethiolate SAMs have substantially lower van
8702 J. AM. CHEM. SOC.
9
VOL. 127, NO. 24, 2005

1
-Adamantanethiol SAMs on Au
{
111}
A R T I C L E S
Figure 8. Proposed schematic of the propensity for 1-adamantanethiolate
displacement. (1) Compared to alkanethiol SAMs, there are much reduced
van der Waals interactions between the neighboring molecules because of
the distance between them (blue). (2) The sulfur-gold bond is not in its
most favorable conformation (green). (3) The steric repulsion from the lower
hydrogens cause the bonds tethering the molecules to the gold to extend
(red).
cage experience steric repulsion from the gold surface, which
may further weaken the bond to the surface. The substrate access
between molecules, the lack of substantial intermolecular
stabilization, and the weakened Au-molecule bonds all make
adamantanethiolate susceptible to displacement.
The exact mechanism for displacement by alkanethiol mol-
ecules is unknown. One possibility is that the slimmer al-
kanethiol molecules first insert in and around the defects in the
1
-adamantanethiolate SAM. The presence of the first alkanethi-
olate molecules then causes instability in the surrounding
-adamantanethiolate molecules, causing those to be displaced
1
and allowing further insertion of alkanethiolate molecules. In
this way, the alkanethiolate islands grow outward from defects
in the 1-adamantanethiolate SAMs, as observed.
The reason for the striping of inserted hexanethiolate mol-
ecules rather than the patches observed for octanethiolate and
decanethiolate is most likely a result of lowered van der Waals
forces between hexanethiol molecules because of the shorter
alkyl tails. With decreased tail length, the direction of tilt of
the alkanethiol molecules shifts toward the nearest neighbor
direction because of a balance between the amount of molecular
strain and the degree of molecular packing order, both of which
5
2
are influenced by the hydrocarbon tail length. Alkanethiolate
molecules with alkyl tail lengths below eight carbons demon-
strate an affinity for the striped phase in pure SAMs, especially
1
8,53-57
for shorter deposition times.
conditions, it has been demonstrated that alkanethiolate mol-
3) rectangular unit mesh with three
Under low surface coverage
ecules form a (p ×
x
19
Figure 7. Scanning tunneling microscopy images of co-deposited 1-ada-
mantanethiolate and decanethiolate SAMs: (A) A 50:50 (1-adamantanethiol:
decanethiol) solution molar ratio results in a decanethiolate SAM; 300 Å
equivalent orientations. Furthermore, Noh et al. have shown
that recrystallization of a disordered hexanethiolate SAM results
in a (6 ×
x
3) structure with alternating missing rows in the
nearest neighbor direction.
×
300 Å, sample bias 1.0 V, current 5.0 pA. (B) A 90:10 solution molar
ratio results in a disorder SAM containing both 1-adamantanethiolate and
5
4
decanethiolate; 700 Å × 700 Å, sample bias 1.0 V, current 2.0 pA. (C) A
It is postulated that the inserted hexanethiolate molecules
assemble in linear stripes rather than growing radially in patches
because of the combination of lowered van der Waals molecular
interactions (related to the decreased alkyl tail length) and
anisotropy created by the insertion of a tilted molecule. Because
of the lowered van der Waals interactions, there is no driving
force for the hexanethiol molecules to insert near existing
9
9:1 solution molar ratio results in an ordered 1-adamantanethiolate SAM
with decanethiolate clusters (the small protrusions apparent in the image);
200 Å × 1200 Å, sample bias 1.0 V, current 7.0 pA.
1
der Waals forces than those that strengthen and order n-
alkanethiolate SAMs because of the larger distance between the
molecules, the limited intermolecular contact, and the insig-
nificant conformational relaxation.1
7,20,28
Also, due to the length
(
52) Fenter, P.; Eberhardt, A.; Liang, K. S.; Eisenberger, P. J. Chem. Phys.
of the thiolate tether to the gold and the bulky shape of the
molecule, it is unlikely that the adamantanethiolate molecules
could be significantly tilted from normal to the gold surface
without straining the thiolate-gold bond. We expect, based on
molecular models, that the lower hydrogens on the adamantane
1
997, 106, 1600.
(
(
53) Noh, J.; Hara, M. Mol. Cryst. Liq. Cryst. 2000, 349, 223.
54) Noh, J.; Hara, M. Langmuir 2001, 17, 7280.
(55) Noh, J.; Nakajima, K.; Hara, M.; Sasabe, H.; Knoll, W.; Lee, H. Korea
Polym. J. 1998, 6, 307.
(
56) Poirier, G. E. Langmuir 1999, 15, 1167.
(57) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966.
J. AM. CHEM. SOC.
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A R T I C L E S
Dameron et al.
hexanethiolate molecules; however, should one insert near an
existing hexanethiolate molecule, the alkyl tails tilt so as to
maximize interactions, creating linear anisotropy. This aniso-
tropy causes strain of the adamantanethiolate molecules in that
direction, thereby enabling the next hexanethiol molecules to
displace the neighboring strained adamantanethiolate molecules.
It is unclear if there is a preferred insertion direction with
respect to gold and/or if one 1-adamantanethiolate molecular
site is more likely to be displaced than another. Further
displacement studies altering the tether length of the adaman-
tanethiol molecules in addition to changing the structure of the
molecules inserted into the 1-adamantanethiol SAMs are ongo-
ing.
molecules upon insertion of alkanethiol molecules into the
matrix and the co-deposition of both species from solution. It
was found that 1-adamantanethiolate molecules are easily
displaced by alkanethiolate molecules, and this eventually leads
to complete replacement of the 1-adamantanethiolate SAM. We
propose that displacement of 1-adamantanethiolate SAMs can
be exploited for use in patterned SAMs without risk of disturbing
the patterned features, and that these transient SAMs can be
used as temporary layers in soft lithography for protection of
volatile or reactive surfaces, and can then be easily removed
prior to initiation of the next fabrication step. We are currently
attempting to exploit displacement in this manner.
Acknowledgment. The authors thank Dr. P. A. Lewis, N.
Santagata, and Prof. K. F. Kelly for their analytical assistance,
R. K. Smith for synthetic aid, B. A. Mantooth for hardware
and software support, and Dr. E. C. H. Sykes and Dr. L. C.
Fern a´ ndez-Torres for helpful discussions. The Air Force Office
of Scientific Research, Army Research Office, Defense Ad-
vanced Research Projects Agency, National Science Foundation,
Office of Naval Research, and Semiconductor Research Cor-
poration are gratefully acknowledged for their support.
4. Conclusions and Prospects
Novel 1-adamantanethiolate SAMs with fewer defects than
alkanethiolate SAMs have been fabricated and studied with
STM. A measured nearest neighbor distance of 6.9 ( 0.4 Å
and five rotational domains of the lattice are observed, implying
some combination of (7 × 7), (7 × 7)R21.8°, (
x
91 ×
x
91)R27°, or ( 91)R5.2° unit cells. The bonding of
91 ×
x x
the adamantanethiolate SAMs to the Au{111} was explored by
monitoring both the displacement of the 1-adamantanethiolate
JA042621O
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VOL. 127, NO. 24, 2005