Covalent attachment of a transition metal coordination complex to functionalized oligo(phenylene-ethynylene) self-assembled monolayers

Article abstract of DOI:10.1021/ja054378e

We have investigated the reaction of tetrakis(dimethylamido)titanium, Ti[N(CH₃)₂]₄, with N-isopropyl-N-[4-(thien-3- ylethynyl) phenyl] amine and N-isopropyl-N-(4-{[4-(thien-3-ylethynyl) phenyl]ethynyl}-phenyl) amine self-a

Full text of DOI:10.1021/ja054378e

Published on Web 09/23/2005
Covalent Attachment of a Transition Metal Coordination
Complex to Functionalized Oligo(phenylene-ethynylene)
Self-Assembled Monolayers
Abhishek Dube,† Andrew R. Chadeayne,‡ Manish Sharma,†
Peter T. Wolczanski,‡ and James R. Engstrom*,†
Contributions from the School of Chemical and Biomolecular Engineering, and Department of
Chemistry and Chemical Biology, Cornell UniVersity, Ithaca, New York 14853
Received July 1, 2005; E-mail: jre7@cornell.edu
Abstract: We have investigated the reaction of tetrakis(dimethylamido)titanium, Ti[N(CH3)2]4, with N-iso-
propyl-N-[4-(thien-3-ylethynyl) phenyl] amine and N-isopropyl-N-(4-{[4-(thien-3-ylethynyl) phenyl]ethynyl}-
phenyl) amine self-assembled monolayers (SAMs), on polycrystalline Au substrates. The structure of the
SAMs themselves has also been investigated. Both molecules form SAMs on polycrystalline Au bound by
the thiophene group. The longer-molecular-backbone molecule forms a denser SAM, with molecules
characterized by a smaller tilt angle. X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS
have been employed to examine the kinetics of adsorption, the spatial extent of reaction, and the
stoichiometry of reaction. For both the SAMs, adsorption is described well by first-order Langmuirian kinetics,
and adsorption is self-limiting from Ts ) -50 to 30 °C. The use of angle-resolved XPS clearly demonstrates
that the Ti[N(CH3)2]4 reacts exclusively with the isopropylamine end group via ligand exchange, and there
is no penetration of the SAM, followed by reaction at the SAM-Au interface. Moreover, the SAM molecules
remain bound to the Au surface via their thiopene functionalites. From XPS, we have found that, in both
cases, approximately one Ti[N(CH3)2]4 is adsorbed per two SAM molecules.
Introduction
most well-known examples. In all these cases, the so-called
bottom contact is formed using chemically specific adsorption,
Most modern electronic devices are solid-state devices, the
active components of which are constructed essentially entirely
of inorganic materialsssemiconductors, metals, various oxides,
nitrides, and silicides. To date, excepting important applications
such as photoresists in lithography, organic materials have
played a rather secondary role in this technology. This situation
is changing, and considerable interest has developed in the past
5-10 years concerning the use of small molecules in active
components of electronic circuitrysthe field is known as
molecular-scale electronics or molecular electronics.1 A major
challenge in this area is devising chemistries and processes that
can bring together these two diverse materials sets, inorganic
and organic, without doing damage to the delicate organic
structures and functionalities.
alternatively referred to as self-assembly. The solution-phase
deposition of organothiols on gold6 is a well-developed chem-
istry for forming well-ordered self-assembled monolayers
(SAMs). These SAMs can have rigid backbones comprised of
aromatic fragments7 or “floppy” backbones comprised of
aliphatic fragments.8 The former, including oligo(phenylene-
ethynylene) SAMs, have attracted interest due to their structure,
which should promote facile electrical conduction along their
molecular backbone. Indeed, conjugated SAMs have been used
for making sensors,9 rectifiers,10,11 and molecular switches.12
A common motif in these devices is an electrode/molecule/
electrode microstructure, formed by sequential deposition and
patterning steps.4,5,11,12
It is important to note, however, that with all these approaches
a second or top contact with the SAM (an inorganic-on-organic
interface) is required to fabricate functional molecular electronic
Attempts to construct devices incorporating molecules have
taken many designs: break junctions, formed mechanically2 and
electrically,3 nanopores,4 and cross-bar arrays5 are perhaps the
(6) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481-4483.
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Soc. 1995, 117, 9529-9534.
† School of Chemical and Biomolecular Engineering, Cornell University.
‡ Department of Chemistry and Chemical Biology, Cornell University.
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A R T I C L E S
Dube et al.
devices. The tip of either a conducting atomic force microscope
(c-AFM) or a scanning tunneling microscope (STM) has been
used to make this contact.13-15 This approach can work well
for fundamental studies of single to several hundred molecules,
but it obviously does not make a permanent contact and is, in
general, unsuitable for fabricating arrays of devices. Formation
of inorganic or metal thin films on SAMs, whether explicitly
for top contacts or not, has mostly involved evaporative
deposition in a vacuum or liquid-phase deposition. Vapor
deposition of elemental metals (e.g., Ag, Cu, Ti, Al, Fe, Cr,
and Au) on SAMs possessing different terminal organic
functional groups (OFGs) such as -CH3, -OH, -COOH,
-COOCH3, -CN, and -SH has been studied extensively.16-24
In many cases, due to the rather unspecific reactions of many
of these elemental metals, mixed adlayers were formed because
reactions occurred not only with the OFG tail groups but also
apparently with the SAM backbone and headgroups. Such
penetration of the organic monolayer by the metal species, the
extent of which depended on the terminal OFG, as well as the
metal studied, is unacceptable concerning most devices envis-
aged for molecular electronics. Formation of inorganic-on-
organic interfaces via liquid-phase thin-film deposition has also
been problematic. TiO2 thin films have been deposited on
alkyltrichlorosilane SAMs possessing different terminal
OFGs.25-28 The films, in most instances, were rough and
exhibited poor adhesion, while molecular-level details concern-
ing the interfaces were absent from these studies. Finally, X-ray
photoelectron spectroscopy (XPS) revealed that the films
suffered from carbon and chlorine contamination.28
of metal thin films on SAMs (thiols), including Au growth via
[(CH3)3P]AuCH3,29 Pd growth via Cp(allyl)Pd,29c,30 and Al
growth via [(CH3)3N]AlH3.31,32 In the case of Au and Pd
deposition, the selectivity of growth and film morphology were
examined. In the case of Al deposition,31 interfacial chemistry
was examined using XPS, but an explicit examination of the
kinetics of adsorption was not attempted. Recently, we have
completed what is perhaps the most rigorous study of the
reaction of a transition metal coordination complex with
SAMs,33 in this case, the reaction of tetrakis(dimethylamido)-
titanium, Ti[N(CH3)2]4, a TiN precursor,34-41 with alkyltrichlo-
rosilane SAMs possessing -OH, -NH2, and -CH3 terminal
OFGs. We found by XPS that the reaction was self-limiting in
all these cases. Using angle-resolved XPS (ARXPS) to probe
the spatial extent of the reaction, we found that penetration of
the SAM followed by reaction at the SAM/substrate occurred
in the case of the -CH3 SAM. In the case of the -NH2 SAM,
however, no evidence of penetration was found, and reaction
was confined to the terminal -NH2 group.
In this work, we consider the reaction of Ti[N(CH3)2]4 with
adsorbed molecules that possess specially chosen head and tail
groups and backbone. In particular, a thiophene headgroup has
been chosen for its affinity for Au surfaces, an isopropylamine
terminal OFG to react with Ti[N(CH3)2]4, and a phenylene-
ethynylene backbone for electrical conduction. Although a
specific device function is not assumed here, this scheme leads
to the aforementioned electrode/molecule/electrode structural
motif used in molecular switches5,12 and rectifiers.11 Thiophenes
offer potential advantages over the related thiols. For example,
thiols may be reduced to thiolates42 or oxidized to disulfides,43
whereas the likelihood of a thiophene group participating in
reactions other than simple molecular adsorption, under our
reaction conditions, is quite small due to its stable ring structure.
Comparative studies of thiophene SAMs and thiol SAMs on
Au are relatively scarce. STM has been used to demonstrate
that thiophene can form well-ordered monolayers on Au(111).44
In another study, the structural evolution of a thiophene SAM
Formation of inorganic-organic interfaces via the use of
transition metal coordination complexes holds tremendous
promise. A key is to tailor both the terminal OFG and the
transition metal complex such that they react with each other
in a specific fashion. Growth that is self-limiting may also be
desirable and feasible with this approach, as uncontrolled
continuous deposition might lead to degradation of the interface.
There are a handful of studies that have examined the deposition
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A R T I C L E S
Figure 1. Schematic for the synthesis of N-isopropyl-N-[4-(thien-3-ylethynyl) phenyl] amine and N-isopropyl-N-(4-{[4-(thien-3-ylethynyl) phenyl] ethynyl}
phenyl) amine.
phenyl]ethynyl}phenyl) amine were synthesized using the scheme that
is shown in Figure 1. Details of the synthetic procedure, as well as all
assignments of the chemical shifts from NMR, are in the Supporting
Information. One ligand (7) has one phenyl ring in its backbone,
whereas the other has two (6). From now on, we will use the shorthand
notation 1P for the former and 2P for the latter, in an obvious reference
to the number of phenyl groups in the molecule. SAMs of these
thiophene ligands and 4-aminothiophenol were prepared on evaporated
gold substrates via a liquid-phase deposition process. Contact angle
measurements, ellipsometry, and X-ray photoelectron spectroscopy
(XPS) were employed to characterize the order, thickness, and
composition of these monolayers. Characterization of the SAMs via
XPS and their reactions with Ti[N(CH3)2]4 were carried out in a custom
built ultrahigh vacuum (UHV) chamber that is described in detail
elsewhere.47
on Au(111) was examined by Fourier transform infrared
reflection absorption spectroscopy.45 The formation of the SAM
was found to occur in two stages: in the first stage, thiophene
orients parallel to the Au surface; in the second stage, the
molecular orientation changes to upright. In other work, XPS46
revealed that the sulfur in thiophene chemically interacts with
Au, indicated by a shift in the binding energy of the S(2p3/2
)
by 2-3 eV.
Herein, we describe our studies of the following: the
synthesis and formation of SAMs possessing a molecular
backbone that should facilitate facile electrical conduction and
an endgroup that should bind in a specific fashion to a Au
substrate; the reactions between the organic functional tailgroups
of these SAMs [-NH(i-C3H7)] and a transition metal coordina-
tion complex, Ti[N(CH3)2]4. We use XPS to probe the nature
of SAMs that are formed, quantify the kinetics of adsorption
of the Ti complex on these layers, and determine the specificity
and/or spatial extent of reaction between the Ti complex and
the SAMs. Ultimately, this strategy could be used to make
“sandwich” structures comprising electrode/molecule/electrode,
or any of a variety of structures where precisely fabricated
inorganic-organic interfaces are key structural elements.
III. Results and Discussion
A. Characterization of the SAMs. We have used contact-
angle measurements, ellipsometry, and XPS to characterize the
layers formed from the adsorption of the two molecules of
interest on polycrystalline Au. In Table 1, we present data for
the advancing and receding contact angles of water, as well as
the hysteresis for both of the SAMs. As may be seen, we observe
a smaller contact angle for the 1P SAM in comparison to the
2P SAM. These results are in qualitative agreement with results
reported for thiol SAMs possessing conjugated backbones of
varying lengths.48 In particular, in this work, it was reported
that increasing the chain length of the conjugated thiols reduced
the tilt angle, which in turn, resulted in a more hydrophobic
II. Experimental Section
Complete details concerning the experimental procedures employed
here are in the Supporting Information, and we only give a brief
summary here. The thiophene ligands, N-isopropyl-N-[4-(thien-3-
ylethynyl) phenyl] amine and N-isopropyl-N-(4-{[4-(thien-3-ylethynyl)
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A R T I C L E S
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Table 1. Properties of Self-Assembled Monolayers
contact angle
SAM
advancing
receding
hysteresis
thickness (ellipsometry)
molecular length
density (cm-2, XPS)
1P
2P
56° ( 3°
66° ( 1°
27° ( 3°
41° ( 2°
56° ( 2°
15°
10°
6.6 ( 0.4 Å
16.6 ( 0.8 Å
10.7 ( 0.8 Å
12.6 Å
19.6 Å
5.6 Å
2.11 ( 0.87 × 1014
3.42 ( 0.79 × 1014
3.24 ( 0.91 × 1014
4-aminothiophenol
character of the longer ring system. Indeed, the ellipsometric
data, also shown in Table 1, confirms this picture. The measured
ellipsometric thicknesses for the 1P and 2P SAMs are 6.6 (
0.4 and 16.6 ( 0.8 Å, respectively, vs estimated lengths (from
molecular models) of 12.6 and 19.6 Å for the molecules
themselves. These results are in good qualitative agreement with
data for other conjugated thiol SAMs,48 and the larger tilt angle
implicated for the shorter 1P SAM can be attributed to weaker
intermolecular forces due to fewer aromatic rings in the
backbone.49
Survey XP spectra have been collected from both SAMs, and
the following peaks have been observed: Au(4f), N(1s), C(1s),
and S(2p). The survey scan was followed by detailed scans for
all of these. The C(1s) feature can be used to estimate the
absolute coverage of the SAMs. To accomplish this, one needs
to account for the photoelectron cross-sections, σ, for the C(1s)
and the Au(4f7/2) peaks, the analyzer transmission, T(E), which
is inversely proportional to the kinetic energy (E ) 968.6 and
1169.6 eV, respectively), the atomic density of the two elements,
N, and the inelastic mean free path, λ, for the photoelectrons.
Concerning these, σAu/σC ) 9.8,50 NAu ) 5.88 × 1022
atoms‚cm-3 51 and λAu ) 15.5 Å.52 The atomic density of C in
,
the SAM depends on the coverage or density of the SAM, nSAM
(molecules‚cm-2), and the mean spacing between C in the
backbone, dC. The integrated intensity of the Au (4f7/2) peak
for a clean Au substrate is proportional to σAuNAu
For the C(1s) peak, we must account for the finite thickness of
the layer, and the integrated intensity is proportional to σC (nSAM
λAuT(EAu).
/
dC) λCT(EC)[exp (-ndC/λC cos θ)], where n is the number of C
in the SAM backbone and θ is the takeoff angle. For the inelastic
mean free path of the C(1s) photoelectrons, we use λC ) 24.5
Å.53 Making use of these expressions, we have computed the
density, nSAM, for both the SAMs, and these values are also
given in Table 1. Given the assumptions made here to calculate
these values, we estimate that their absolute accuracy is
approximately (30%. A higher density (3.4 × 1014 vs 2.1 ×
1014 molecules‚cm-2) is observed for the 2P SAM. This can
again be attributed to the fact that more aromatic rings lead to
higher intermolecular forces which in turn account for the
smaller tilt and higher packing density.49 A density of 4.5 ×
1014 molecules‚cm-2 has been reported for a SAM of 4-[4′-
(phenylethynyl)-phenylethynyl]-benzenethiol on Au,54 which has
a similar molecular structure to the 2P SAM, but possesses a
different headgroup and lacks the terminal isopropylamine
group, which in our case may hinder packing of the molecules.
Figure 2. Integrated peak areas as a function of takeoff angle for the 1P
(filled symbols) and 2P SAMs (open symbols) on polycrystalline Au for
the (a) Au(4f), (b) S(2p), and (c) C(1s) features from XPS at Ts ) 30 °C.
The smooth curves represent fits of the data to models that are described
in the text. The parameters found from these fits are given in the text and/
or in Table S1 in the Supporting Information.
Angle-resolved XPS has been used to probe the SAMs, which
is particularly valuable concerning noninvasive depth profiling
of the layers. ARXPS of the Au(4f), S(2p), and C(1s) peaks
have been obtained. The takeoff angle was varied from 0° to
64°. Plotted in Figure 2a are the Au(4f) integrated intensities
as a function of the takeoff angle for both the SAMs we consider
here. The data can be modeled as a semi-infinite (Au) substrate
that is covered by a uniformly thick two-dimensional (SAM)
film: I(θ) ) I0 exp (-dSAM/λ cos θ), where, I0 is the
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Attachment of a Coordination Complex to SAMs
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unattenuated emission achieved at normal takeoff angle. The
smaller integrated intensities for the 2P SAM at all takeoff
angles are consistent with the fact that more attenuation of the
Au (4f) photoelectrons is occurring due to the presence of a
thicker (and denser) overlayer. In terms of the parameters, from
these data, we find dSAM/λ ) 0.47 ( 0.01 and 0.49 ( 0.02 for
the 1P and 2P SAM, respectively. These values for the
parameters, as well as all values obtained in this work from
ARXPS, are also given in Table S1 in the Supporting Informa-
tion. In Figure 2b, we display the S(2p) integrated intensity as
a function of the takeoff angle for both SAMs. In this case, it
is assumed that sulfur atoms are arranged in a 2-D plane at a
distance, d, from the SAM-vacuum interface. The expression
is given by I(θ) ) (I0/cos θ) exp(-d/λcos θ), where again I0 is
the unattenuated emission achieved at normal takeoff angle A
fit to the data shown in Figure 2b gives d/λ ) 1.69 ( 0.57 and
2.05 ( 0.65 for the 1P and 2P SAM, respectively. These values
indicate that the sulfur is a significant distance beneath the
surface, presumably at the SAM-Au interface. Also, the values
found for d/λ increase with increasing length of the SAM. We
will return to a discussion of the physical meaning of the
absolute values below. Results from ARXPS of the C(1s) peak
for both the SAMs are presented in Figure 2c. Here we use the
following expression to model the data: I(θ) ) I0 [1 - exp
(-dSAM/λ cos θ)], which assumes the C is present at some
constant density in a thin film at the surface and where I0 is the
C(1s) emission from a semi-infinite SAM film. Here we find
that dSAM/λ ) 0.32 ( 0.14 and 0.68 ( 0.12 for the 1P and 2P
SAM, respectively. One final set of experiments was conducted
to further characterize the SAMs. In these experiments, first a
scan for Au(4f) was completed on a clean Au substrate, and
this was followed directly by another scan on a Au substrate
covered by the SAM. Again, assuming the SAM covers the
surface uniformly this gives directly the quantity I(θ)/I0 ) exp-
(-dSAM/λ cos θ). We find that dSAM/λ ) 0.21 ( 0.01 for the
1P SAM and 0.29 ( 0.01 for the 2P SAM. These values are in
qualitative agreement with those found in Figure 2a, b, and c.
One explanation for the disparity in dSAM/λ values could be a
difference in attenuation between the Au photoelectrons and
those arising from C and S. For example, as C and S are both
part of the SAM molecules, attenuation by the remaining portion
of the molecule is assured. Such is not the case for the Au
substrate, particularly for a less-than-well-packed SAM over-
layer. In any event, these results are consistent with a SAM
where the molecules are bound exclusively by the thiophene
end of the molecule.
Figure 3. XP spectra of the Ti(2p) feature for clean Au and 2P SAM
surfaces, both exposed to Ti[N(CH3)2]4 at Ts ) 30 °C. Exposures were
1.01 and 0.50 × 1017 molecules‚cm-2, respectively. Spectra have been fit
to two peaks using Gaussian-Lorentzian product functions.
on the clean Au substrate in comparison to the surface with the
2P SAM. The area of the peaks can be used to estimate the
surface density of Ti. First, we obtained a Ti(2p) spectrum from
a reference single-crystal TiO2 surface. The area under this peak
is proportional to σTiNTiλTiT(ETi), where λTi ) 20.67 Å,55 and
NTi ) 3.2 × 1022 atoms‚cm-3. The Ti atoms in the Ti[N(CH3)2]4
adlayer are modeled as a thin film of thickness dTi and atomic
density N′Ti. The area under the peak from such an adlayer is
proportional to σTiN′TidTiT(ETi)/cos θ, assuming dTi , λTi. Thus,
the quantity N′TidTi, which represents the Ti surface density
(atoms‚cm-2), can be calculated directly. We find that the
(temperature-averaged) saturation Ti densities on the three
substrates are 0.073 ( 0.03 × 1014 (clean Au), 1.2 ( 0.2 ×
1014 (1P SAM), and 2.1 ( 0.2 × 1014 atoms‚cm-2 (2P SAM).
A higher Ti saturation coverage for the 2P SAM is in agreement
with the results given above where a higher coverage of SAM
molecules, and hence reactive endgroups, was implicated.
To examine the kinetics of adsorption, we have acquired
coverage-exposure relationships for both the SAMs at two
different values of Ts (30 and -50 °C). These data have been
fit to first-order Langmuirian kinetics, ns(dθ/dt) ) SR,0F(1 -
θ), where, ns is the density of reactive sites (molecules‚cm-2),
θ is the fractional surface coverage of Ti[N(CH3)2]4, SR,0 is the
initial reaction probability, and F is the incident flux of Ti-
[N(CH3)2]4 (molecules‚cm-2‚s-1). The data and the fits thereof
are presented in Figure 4 for the 1P and 2P SAM. Using our
estimates for the absolute densities of Ti and the estimated
incident flux of Ti[N(CH3)2]4, we can calculate SR,0. At Ts )
30 °C, we find that SR,0 ) 0.024 and 0.028 for the 1P and 2P
SAM, respectively. At Ts ) -50 °C, we find values of 0.010
and 0.020, respectively. Whereas the uncertainties in the absolute
values are on the order of (50%, the uncertainties in the relative
values are much less, perhaps on the order of (10%. Given
these estimates, we conclude that the initial reaction probability
in three of the four cases is within the experimental uncertainties.
For the case that appears to deviate, the 1P SAM at Ts ) -50
°C (SR,0 ) 0.010), we note that the fit to the model at low
B. Reaction of Ti[N(CH3)2]4 with the SAMs. The reaction
of Ti[N(CH3)2]4 with the clean Au substrate and the 1P and 2P
SAMs has been examined using XPS. In our previous work on
trichlorosilane-based SAMs,33 the starting substrate was found
to be the most reactive. In this work, the substrate was
“chemical” silicon oxide, which has a high density of silanol
groups at the surface. To determine if a similar phenomenon
was possible in the systems examined here, we have studied
the reaction of Ti[N(CH3)2]4 with a clean Au substrate at Ts )
30 °C. After a 1 h exposure to Ti[N(CH3)2]4 (dose of ∼1.01 ×
1017 molecules‚cm-2), a Ti(2p) spectra was acquired, which is
displayed in Figure 3. In Figure 3, we also display the results
of a 30 min exposure of the 2P SAM to Ti[N(CH3)2]4. It can
be clearly seen that there are only trace amounts of Ti present
(55) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2-11.
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A R T I C L E S
Dube et al.
Figure 5. Integrated peak areas for the Ti(2p) region as a function of takeoff
angle for a saturation exposure of the 1P (filled symbols) and 2P SAMs
(open symbols) to Ti[N(CH3)2]4 at Ts ) 30 °C. The smooth curves represent
fits of the data to a model that is described in the text. The parameters
found from these fits are given in the text and/or in Table S1 in the
Supporting Information.
adlayer other than reaction with the endgroup, possibly including
displacement/desorption of the adsorbed molecules. These data
are shown in Figure 6a, b, and c, which are identical in layout
to those shown in Figure 2, and have been fit to the corre-
sponding functional forms indicated above. A number of things
are apparent upon comparison of Figures 2 and 6. First, the
qualitative trends for all three peak areas for both SAMs are
unchanged after exposure to Ti[N(CH3)2]4sthe Au(4f) and S(2p)
peaks are attenuated, whereas the C(1s) increases in intensity
at more glancing takeoff angles. These results are consistent
with a situation in which significant structural rearrangement
and/or ligand displacement has not occurred upon Ti[N(CH3)2]4
chemisorption, leaving the molecules still bound to the Au
surface via the thiophene linkage. Indeed, some changes in
features are minimal. For example, a fit of the data for the Au-
(4f) peaks shown in Figure 6a gives dSAM/λ ) 0.46 ( 0.01 and
0.43 ( 0.04 for the 1P and 2P SAM, respectively, values
essentially unchanged from those for the bare SAMs (cf. Table
S1, Supporting Information).
Closer inspection of the data, however, does reveal some
important differences. In particular, for the 2P SAM, exposure
to Ti[N(CH3)2]4 resulted in a rather small increase of ∼6% in
the Au(4f) intensity at all takeoff angles, whereas a more
substantial increase of ∼21% occurred on the 1P SAM.
Although Ti[N(CH3)2]4 chemisorption might produce SAM
desorption, this seems very unlikely because the Ti species does
not compete with the SAM for adsorption sites, but rather
exhibits a propensity to react with the endgroup on the SAM
itself. A more plausible explanation is that reaction between
the Ti species and the SAM has changed the orientation of these
species, making them more upright and less able to cover the
underlying Au substrate. This scenario is consistent with the
larger change seen for the 1P SAM, which, from ellipsometry,
consists of a monolayer that is less dense and less upright.
For the S(2p) peak, cf. Figure 6b, we find that there is a small
change in the absolute intensities for the 1P SAM upon reaction
with Ti[N(CH3)2]4, whereas there appears to be a much more
Figure 4. Coverage-exposure relationships, deduced from XPS, for the
adsorption of Ti[N(CH3)2]4 on (a) the 1P SAM and (b) the 2P SAM, both
for Ts ) -50 and 30 °C. The fits to the data, shown as smooth curves, are
for first-order Langmuirian kinetics.
coverage is not particularly good, and a simple linear fit of the
first three to four data points would have revealed a larger SR,0
,
closer to that implicated in the other three cases.
Angle-resolved XPS has been used to probe the spatial extent
of the reaction between Ti[N(CH3)2]4 and the SAMs, similar to
what was conducted for the SAMs themselves (Figure 2). To
probe for the spatial location of Ti in the adlayer, XP spectra
for the Ti(2p) features have been acquired at four different
takeoff angles from 0° to 64°. In Figure 5, we display the Ti-
(2p) integrated intensity, plotted as a function of takeoff angle,
for both SAMs and saturation exposures to Ti[N(CH3)2]4.
Clearly, for both adlayers, we see that the intensity increases
as a function of takeoff angle, which points to the presence of
Ti at the SAM-vacuum interface, as opposed to being buried
at the SAM-Au interface. The Ti (2p) has been fit to the
expression used for the S(2p) peak above (Figure 2b), namely,
I(θ) ) (I0/cos θ) exp(-d/λ cos θ). Here we force the quantity
d/λ to be a positive definite. A fit of the data (shown by the
smooth curves) gives d/λ ) 0.0003 ( 0.3 for the 1P SAM and
d/λ ) 0.0003 ( 0.2 for the 2P SAM. From these values, we
can safely conclude that all the Ti is present at the SAM-
vacuum interface in both cases. Penetration, followed by reaction
at the SAM-Au interface, can be ruled out on the basis of these
data and is unlikely due to the negligible reactivity observed
for Ti[N(CH3)2]4 on the clean Au substrate. In summary, these
results indicate that the Ti is at the SAM-vacuum interface,
bound to the -NH(i-C3H7) SAM endgroup in both cases.
Angle-resolved XP spectra of the Au(4f), S(2p), and C(1s)
peaks have also been acquired from adlayers representing
saturation exposures of both SAMs to Ti[N(CH3)2]4. These
experiments were conducted to determine if exposure of the
SAMs to Ti[N(CH3)2]4 resulted in significant changes in the
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Attachment of a Coordination Complex to SAMs
A R T I C L E S
Figure 7. Relationship between the concentration of Ti in the saturated
adlayers and the concentration of molecules (and functional endgroups) in
the SAMs. The datum for the -NH2-terminated alkylsilane is from ref 33.
the (bare) SAMs, we have been able to compute the SAM
density. Likewise, following a saturation exposure to Ti-
[N(CH3)2]4, we have been able to compute the density of Ti
present. Combining these results allows us to calculate the Ti/
SAM ratio for both SAMs. In Figure 7, we plot the Ti density
vs the SAM density at saturation. Also plotted are our data for
the reaction of Ti[N(CH3)2]4 with a tricholorosilane-based
alkylamine SAM on SiO2.33 As may be seen the ratio is ∼1:2
in both cases considered here and for the alkylamine (-NH2
endgroup), a result that is consistent with different scenarios.
In one scenario, it can be argued that each Ti[N(CH3)2]4
molecule is reacting with two SAMs via the -NH(i-C3H7)
group, effectively occupying all possible reactive endgroups.
In another scenario, one could argue that on an average only
one-half of the -NH(i-C3H7) groups are reacting with Ti-
[N(CH3)2]4. This situation is also plausible because the SAM
ligands considered here are bulky and possess a relatively
inflexible backbone, where steric hindrance might limit Ti-
[N(CH3)2]4 to react only with every alternate reactive endgroup.
Additional insight into the stoichiometry of the adlayer can
be gained by examining the N/Ti ratio. Concerning the scenarios
considered above, for Ti[N(CH3)2]4 reacting with only one-half
of the SAMs present, and simple ligand exchange for chemi-
sorption [loss of one N(CH3)2 ligand via formation of HN-
(CH3)2], we would expect a ratio of 5:1 in the absence of
attenuation effects. Alternatively, for the case where each Ti-
[N(CH3)2]4 reacts with two SAMs, again by simple ligand
exchange, we would expect a ratio of 4:1. In Table 2, we give
the N/Ti ratios for the saturated adlayers for both SAMs and
for our previous results on the alkylamine. Also indicated in
Table 2 are the substrate temperatures at which the experiments
were conducted. The N/Ti ratios have been calculated after
making suitable corrections for the photoelectron cross-sections.
Clearly, a N/Ti ratio of approximately 3:1 is observed for both
the SAMs considered here, independent of substrate temperature.
At Ts ) -50 °C, a ratio of ∼4:1 is observed for the alkylamine.
The ratio of 3:1 is unexpected, but nonetheless in poor
agreement with the scenario where only one-half of the SAMs
react with Ti[N(CH3)2]4. Alternatively, exclusively attenuation
Figure 6. Integrated peak areas as a function of takeoff angle for the 1P
(filled symbols) and 2P SAMs (open symbols) after saturation exposures
to Ti[N(CH3)2]4 at Ts ) 30 °C for the (a) Au(4f), (b) S(2p), and (c) C(1s)
features from XPS. The smooth curves represent fits of the data to models
that are described in the text. The parameters found from these fits are
given in the text and/or in Table S1 in the Supporting Information.
substantial (∼50% at 0° takeoff angle) attenuation of this peak
for the 2P SAM. Given the results for the Au(4f) peak discussed
above, differing behavior of the two layers is not unexpected.
For example, restructuring of the 1P adlayer upon reaction into
a more sparse adlayer, which is also thicker, might produce
minimal changes in the intensity. On the other hand, reaction
and deposition leading to minimal restructuring of the 2P SAMs
could further attenuate the S(2p) signal coming from the Au-
SAM interface. Finally, for the C(1s) peak, cf. Figure 6c, we
find that there is minimal change in the absolute intensities (less
than 5%), and fits to the data give dSAM/λ ) 0.42 ( 0.04 and
0.98 ( 0.05 for the 1P and 2P SAM, respectively. These
somewhat larger values for dSAM/λ might reflect a thicker C
layer due to the presence of the N(CH3)2 ligands from the Ti-
[N(CH3)2]4.
To obtain more insight into the structure and composition of
the adlayer formed at a saturation exposure, we further consider
the results from XPS. In particular, from the C(1s) feature of
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A R T I C L E S
Dube et al.
Table 2. Stoichiometry of the Saturated Adlayers, Ti[N(CH3)2]4 on
SAMs
substrate
substrate
SAM
temperature
N/Ti
polycrystalline Au
1P
1P
2P
2P
-50 °C
30 °C
-50 °C
30 °C
30 °C
-50 °C
2.9 ( 0.2
3.0 ( 0.2
3.3 ( 0.4
2.9 ( 0.2
4.0 ( 0.3
4.0 ( 0.3
4-aminothiophenol
-NH2-terminated
alkylsilane
SiO2
-NH2-terminated
alkylsilane
30 °C
3.0 ( 0.2
effects would also seem to be insufficient to explain a reduction
in the theoretical ratio of 4:1 for Ti[N(CH3)2]4 bound to two
SAMs. One remaining possibility is the additional loss of ligand
via pathways that do not involve ligand exchange.
As a final point of comparison, we have examined the
adsorption of 4-aminothiophenol (4-ATP) on polycrystalline Au
and its reaction with Ti[N(CH3)2]4. This molecule, which has
been examined by a number of groups concerning its use as an
adhesion promoter,56-58 possesses a much simpler structure,
-SH and -NH2 groups separated (para) by a phenyl group. In
one study,58 from electrochemical deposition, a saturation
coverage of 5.5 ( 0.4 × 1014 molecules‚cm-2 was reported.
As with the molecules of primary interest here (the 1P and 2P
SAMs), we first prepared an adlayer of the SAM itself and
characterized it using contact angle measurements, ellipsometry,
and ARXPS. Subsequently, this layer was exposed to Ti-
[N(CH3)2]4 and the layer was again characterized using ARXPS.
For this adlayer, we did not attempt a characterization of the
kinetics of adsorption. The results of the contact angle and
ellipsometry studies are displayed in Table 1. The contact angle
(sessile drop method) of 26.9 ( 3.1 is smaller than that observed
for the 1P and 2P SAMs. The ellipsometric thickness of 10.7
( 0.8 Å, unlike the results for the 1P and 2P SAMs, is larger
than expected, even for a molecule bound to the surface upright
(calculated length of 5.6 Å). One expects this molecule to be
bound upright, and if it is bound with a higher density than the
1P and 2P SAMs, then it could produce a larger apparent
thickness from ellipsometry due to the assumptions concerning
the dielectric constant.
Figure 8. Integrated peak areas as a function of takeoff angle for the
4-aminothiophenol SAM on polycrystalline Au, prior (open symbols) and
subsequent (closed symbols) to a saturation exposure to Ti[N(CH3)2]4 at Ts
) 30 °C for the (a) Au(4f), (b) S(2p), and (c) C(1s) features from XPS.
The smooth curves represent fits of the data to models that are described
in the text. The parameters found from these fits are given in the text and/
or in Table S1 in the Supporting Information.
0.11 ( 0.10 for Ti[N(CH3)2]4 on aminothiophenol, which
indicates that all of the Ti is present at the SAM-vacuum
interface.
Concerning ARXPS of 4-aminothiophenol, the results are
presented in Figure 8a, b, and c for the Au(4f), S(2p), and C(1s)
peaks (pre- and postexposure) and in Figure 9 for the Ti(2p)
peak (exposure was ∼1.01 × 1017 molecules‚cm-2). As shown
Figure 8, the results for ARXPS on the unreacted SAM is
qualitatively similar to that observed for the 1P and 2P SAMs:
increasing attenuation of the Au(4f) and S(2p) features with
increasing takeoff angle, higher intensity for the C(1s). Again
these features have been fit to the models used to fit the data in
Figures 2 and 6. Shown in Figures 8 and 9 are the results for
ARXPS on the SAM after reaction with Ti[N(CH3)2]4. In Figure
9, we fit the data for the Ti(2p) peak, using the form used above
in connection with Figure 5, and arrive at a value of d/λ )
Examining the results shown in Figure 8, we observe the
following: for the Au(4f) feature, a fit of the data shown in
Figure 8a gives dSAM/λ ) 0.49 ( 0.02 (bare SAM) and 0.45 (
0.01 (reacted SAM), a minimal change. More interesting is the
fact that we observe an increase in the Au(4f) emission at all
takeoff angles after exposure to Ti[N(CH3)2]4, where the
percentage increase (21.5%) is essentially that seen for the 1P
SAM (21.2%) in the same situation. As indicated above, if
chemisorption results in local densification of the adlayer about
the Ti center, then this could occur in the absence of SAM
desorption. For the S(2p) peak displayed in Figure 8b, we see
that exposure to Ti[N(CH3)2]4 has changed the photoemission
intensity only perhaps a small amount. These results would seem
to confirm that the S-Au bond is intact, even in the presence
of reaction with the -NH2 tailgroup. Finally, for the C(1s)
(56) Schlieben, O.; Hormes, J. J. Synchrotron Rad. 1999, 6, 793-795.
(57) Hooper, A. E.; Werho, D.; Hopson, T.; Palmer, O. Surf. Interface Anal.
2001, 31, 809-814.
(58) Batz, V.; Schneeweiss, M. A.; Kramer, D.; Hagenstro¨m, H.; Kolb, D. M.;
Mandler, D. J. Electroanal. Chem. 2000, 491, 55-68.
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Attachment of a Coordination Complex to SAMs
A R T I C L E S
(50%, a relative comparison can be made with more confi-
dence, as the method of calculating SR,0 was identical in both
cases. Thus, a difference in reactivity in the two cases is a factor
of 3.6-4.2, with the primary alkylamine being more reactive.
One expects some difference between the -R-NH2 group
compared to the [-Ar-NH(i-C3H7)] group. First, the N-H
bond dissociation energy of the former is expected to be higher
by about 6-7 kcal‚mol-1 59
, suggesting lower reactivity for the
-R-NH2 group. Additionally, a monoalkylamine species is less
acidic than an N-alkyl aniline. Hence, if the transamination
reaction were mediated by deprotonation of the incoming ligand,
then the long-alkyl-chain -R-NH2 species would be moder-
ately less reactive toward substitution. On the other hand, steric
effects will certainly be in play for the 1P and 2P SAMs (e.g.,
the bulkier i-C3H7 and phenyl groups), in comparison to the
long-alkyl-chain -R-NH2. On the basis of our results, it
appears that these steric effects dominate the substitution
chemistry in our system, probably influencing both the pre-
exponential factor and the barrier to reaction.
Figure 9. Integrated peak area for the Ti(2p) region as a function of takeoff
angle for a saturation exposure of the 4-aminothiophenol SAM to Ti-
[N(CH3)2]4 at Ts ) 30 °C. The smooth curves represent fits of the data to
a model that is described in the text. The parameters found from these fits
are given in the text and/or in Table S1 in the Supporting Information.
Concerning the structure of the SAMs, both before and after
exposure to Ti[N(CH3)2]4, work on related systems is very
limited. Most relevant are structural studies of thiophene,46,60
and oligo(phenylene-ethynylene)61 monolayers. STM studies60
suggest that thiophene forms an ordered paired-row structure
feature, we see from Figure 8c that the intensity increases with
increasing exposure to Ti[N(CH3)2]4. This situation is consistent
with an increased amount of C in the layer due to the N(CH3)2
ligands. As discussed above in connection with Figure 6c, we
found a modest (∼20%) decrease in the C(1s) intensity for both
the 1P and 2P SAMs upon exposure to Ti[N(CH3)2]4. It is
important to note that there is considerably less C in the 4-ATP
(6 atoms vs 16 and 23), and a larger relative increase is expected.
In terms of the parameters used to fit Figure 8c, the increase in
the C fraction is about 0.88 ( 0.41 or 5.3 ( 2.5 C atoms if no
SAM is lost to desorption.
on Au(111), with a density of ∼5.2 × 1014 molecules‚cm-2
.
However, adding methyl groups to the molecule in the case of
2,5-dimethylthiophene destroys the ability to form this ordered
structure, and no ordered monolayers are observed. In the case
of oligo(phenylene-ethynylene) monolayers (thiol headgroup,
unfunctionalized phenyl tail group), on the basis of results from
contact mode AFM,61 a (x3 × x3)R30° structure was formed,
which would correspond to a density of ∼4.6 × 1014
molecules‚cm-2. Addition of a -NO2 group at the central phenyl
group did not seem to affect ordering of this monolayer. Finally,
in this study, on the basis of results from infrared spectroscopy,61
an average tilt angle of ∼32-39° was reported.
As with the results for the 1P and 2P SAMs, XPS can also
be used to quantify the stoichiometry of the adlayer: the ratios
Ti/SAM and Ti/N at saturation. Following a procedure identical
to that used for the 1P and 2P SAMs, we calculate that the
absolute coverage of 4-ATP for our conditions was 3.24 ( 0.91
× 1014 molecules‚cm-2. For Ti, we estimate a saturation
coverage of 1.58 ( 0.36 × 1014 molecules‚cm-2. These results
are plotted in Figure 7, and we see that they are in good
agreement with the other systems we have studied with -NHR
termination: a Ti/SAM ratio of ∼1:2. For the N/Ti ratio, we
find a value of N/Ti ) 4.03 ( 0.25 for 4-ATP and Ti[N(CH3)2]4
adsorption at Ts ) 30 °C. This is essentially the result we
achieved with an -NH2-terminated alkylsilane SAM on SiO2
but seems to differ from the results we report here for the 1P
and 2P SAMs and the alkylsilane at Ts ) 30 °C, where the
apparent stoichiometry was ∼3:1.
In the work we report here, we use XPS to deduce structural
properties of the monolayers. For the 2P SAM, we estimate
that the saturation density is 3.4 × 1014 molecules‚cm-2, which
would represent a coverage of 0.245 on Au(111), consistent
with a (2 × 2) overlayer. The ellipsometric data implies a tilt
angle of θ ) cos-1(16.6/19.6) ) 32.1°, remarkably close to
that reported elsewhere for the oligo(phenylene-ethynylene).
How do the results for the 1P SAM fit into this picture? Let us
assume that the 1P SAM forms a (2 × m) overlayer, where a
larger tilt angle results in m > 2. That is, as the tilt angle
increases, the spacing between adjacent chains perpendicular
to their molecular axis does not. Again making use of the
ellipsometric data, a tilt angle of 58.4° is implicated for the 1P
SAM. If the spacing between the molecular axes has not
changed from that for the 2P SAM, then the increase in tilt
angle would make m ≈ 2 cos(32.1°)/cos(58.4°) ) 3.23. The
density of such a layer of the 1P SAM would be 2/3.23 that of
the 2P SAM layer, or (2/3.23)(3.4 × 1014) ) 2.1 × 1014
molecules‚cm-2, exactly the result we estimate from XPS.
Although this result fits perfectly into the structural model we
Reviewing our results as a whole, we will focus on the
following three important issues: (i) the kinetics of adsorption;
(ii) the structure of the reacted and unreacted SAMs; and (iii)
the stoichiometry of the saturated adlayer. Concerning the first
of these, in previous work,33 we examined the reactions of Ti-
[N(CH3)2]4 on alkylsilane SAMs on SiO2. For a -(CH2)12-
NH2 SAM, we found initial reaction probabilities of ∼0.13, 0.10,
and 0.14 for Ts ) -50, 30, and 110 °C, respectively. Focusing
on the results at Ts ) 30 °C for the 1P and 2P SAMs, we found
values of SR,0 ) 0.024 and 0.028, respectively. Although the
absolute accuracy of both sets of results may be no better than
(59) Gomes, Jose’ R. B.; Ribeiro da Silva, Maria D. M. C.; Ribeiro da Silva,
Manuel A. V. J. Phys. Chem. A 2004, 108, 2119-2130.
(60) Noh, J.; Eisuke, I.; Araki, T.; Hara, M. Surf. Sci. 2003, 532-535, 1116-
1120.
(61) Stapleton, J. J.; Harder, P.; Daniel, T. A.; Reinard, M. D.; Yao, Y.; Price,
D. W.; Tour, J. M.; Allara, D. L. Langmuir 2003, 19, 8245-8255.
9
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A R T I C L E S
Dube et al.
Figure 10. Possible reaction pathways for Ti[N(CH3)2]4 reacting with the 1P and 2P SAMs.
have just described, we hesitate to assign too much quantitative
importance to it due to the use of XPS as a quantitative measure.
In a relative sense, however, XPS is quite reproducible and
reliable, and these results certainly highlight the important
differences in the 1P and 2P SAMs. In particular, the lower
density and larger tilt angle implicated for the 1P SAM,
whatever the exact structure may be.
forming metallacycles and/or imines64 is likely not kinetically
feasible at Ts e 30 °C. However, for the Ti[N(CH3)2]2(a)
fragment bound by siloxane bridges examined by Beaudoin and
Scott,63 just such a mechanism was proposed for prolonged (∼10
h) “exposure” to vacuum at room temperature. Thus, including
this mechanism, two pathways appear reasonable on the basis
of precedent. First, a dimethylamide could abstract a hydrogen
from the adjacent Ti-N(CH3)2 functionality to generate free
HN(CH3)2 and an azametalacyclopropane (alternately described
as a bound imine), (L-N(i-C3H7))2Ti(η2-CH2NCH3)) (pathway
A, Figure 10). â-Abstractions from dialkylamide groups are well
established in early transition metal chemistry.63,65-70 A second
possible route involves abstraction of an isopropyl group
hydrogen to afford HN(CH3)2 and the azametalacyclobutane,
(L-N(i-C3H7))(L-κ2-N,C-NCH(CH3)CH2)Ti(N(CH3)2) (path-
way B, Figure 10). While not as common, in complexes that
are sterically crowded, abstractions of peripheral hydrogens can
occur concomitant with cyclometalation of the ligand.71-73
Subsequent loss of propene from the azametalacyclobutane
might occur to give (L-N(i-C3H7))(L-Nd)Ti(N(CH3)2), and
this 3-coordinate species would be expected to add any number
of CH bondsseither intra- or intermolecularlysacross the imido
A final point of discussion concerns the stoichiometry and
structure of the reacted adlayer, and we consider possible
scenarios in Figure 10. Chemisorption most likely involves
transamination ligand exchange reactions with the -Ar-NH-
(i-C3H7) and -Ar-NH2 groups at the SAM-vacuum interface
[forming HN(CH3)2(g)], and these may be facile at Ts e 30 °C.
Perhaps the most striking result we have is the implied
stoichiometry of the adlayer, Ti/SAM ≈ 1:2, for four rather
different amine-terminated monolayers (cf. Figure 7). The best
explanation for these results is successive ligand exchange
reactions with adjacent molecules in the SAM (-L-NHR),
forming a (-L-NR)2-Ti[N(CH3)2]2(a) species. This model is
a perfect fit for the results for 4-aminothiophenol and the -NH2-
terminated alkylsilane at Ts ) -50 °C, based on the implicated
N:Ti ratios (cf. Table 2). It is also consistent with results reported
by Scott and co-workers for Ti[N(CH3)2]4 reacting on silica
surfaces,62,63 where (≡Si-O-)2-Ti[N(CH3)2]2(a) species were
implicated. However, for both the 1P and 2P SAMs, and the
alkylsilane at Ts ) 30 °C, a stoichiometry of 3:1 is indicated.
A further loss of dimethylamine from (L-N(i-C3H7))2Ti-
[N(CH3)2]2 is a plausible way in which the predicted 4:1 ratio
of N/Ti can be lowered. As discussed elsewhere,33 if the kinetics
of gas-phase unimolecular decomposition of Ti[N(CH3)2]4 is
used as a guide, then decomposition of Ti[N(CH3)2]x(a) species
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Attachment of a Coordination Complex to SAMs
A R T I C L E S
linkage.74,75 In these scenarios, the ultimate N/Ti ratio would
be 3:1. The feasibility of these two mechanisms is currently
being investigated using quantum chemistry calculations.76
Independent of this proposition, however, is the unambiguous
preference of Ti[N(CH3)2]4 to react with these layers via two
successive ligand-exchange reactions. As discussed above, our
results for the 1P SAM seem to indicate that the bare SAM is
a low-density layer characterized by molecules closer to parallel
to the surface, rather than perpendicular. Reaction with Ti-
[N(CH3)2]4 would likely change that, particularly if two
molecules are bound to the same Ti center. Indeed, a more
upright (-L-NR)2-TiN(CH3)2(a) species might be expect to
occupy a smaller area of the Au surface and lead to less
attenuation of the Au signal. This is, of course, exactly what
we see from XPS. In contrast, the more-upright 2P layer may
lead to species covering similar amounts of the Au surface,
leading to little change in attenuation of the Au signalsagain,
this is what is observed in XPS. More definitive conclusions
concerning the structure of the reacted adlayers await direct
structural studies using techniques such as STM.
these SAMs with Ti[N(CH3)2]4 in both cases is self-limiting,
and the kinetics of adsorption are in good agreement with first-
order Langmuirian kinetics. The initial probability of reaction
on these SAMs is about a factor of 4 smaller than that observed
on a -NH2 terminated alkylsilane, which we attribute to steric
effects caused by the i-C3H7 group on these SAMs. Angle-
resolved XPS conducted after the reaction of the SAMs with
Ti[N(CH3)2]4 shows clearly that the reaction occurs cleanly with
the terminal isopropylamine group, and there is no reaction at
the SAM-Au interface. From XPS, we find that the stoichi-
ometry in the saturated adlayers for both SAMs is ∼1:2 for
Ti/SAM and ∼3:1 for N/Ti. These results are best explained
by a model where chemisorption involves ligand-exchange
reactions resulting in one Ti[N(CH3)2]x fragment bound to two
SAMs via the isopropylamine group. Taken as a whole, these
results indicate that transition metal complexes can bind very
specifically and noninvasively to the endgroups on SAMs that
possess a backbone that may be useful in fields such as
molecular electronics.
Acknowledgment. This research was supported by the
Nanoscale Interdisciplinary Research Team on Inorganic-
Organic Interfaces (NSF-ECS-0210693). Additional support was
provided by the Cornell Center for Materials Research (CCMR),
a Materials Research Science and Engineering Center of the
National Science Foundation (DMR-0079992), and the Semi-
conductor Research Corporation via the Center for Advanced
Interconnect Science (SRC task 995.011). A.D. wishes to thank
Applied Materials for a Graduate Fellowship.
IV. Conclusions
We have investigated the synthesis and characterization of
conjugated thiophene SAMs with an isopropylamine endgroup,
as well as their reaction with tetrakis(dimethylamido)titanium.
Using contact angle measurements and ellipsometry, we have
found that the shorter 1P SAM produces an adlayer that is less
dense than the longer 2P SAM. This may be due to the nature
of the intermolecular interactions in the 2P SAM, which lead
to an adlayer that involves molecules that are more upright than
the 1P SAM. These results are confirmed by results from XPS,
which also indicate a higher density for the 2P SAM. Angle-
resolved XPS indicates that the molecules for both SAMs are
bound to the Au surface via the thiophene headgroup, with the
amine termination at the SAM-vacuum interface. Reaction of
Supporting Information Available: Details of the experi-
mental procedures including materials used, synthesis, as well
as all assignments of the chemical shifts from NMR for the
thiophene ligands, substrate preparation, characterization of
SAMs, and the adsorption kinetics experiments; and values for
all the parameters obtained from ARXPS curve fits both before
and after exposure to TiN[(CH3)2]4. This material is available
free of charge via the Internet at http://pubs.acs.org.
(74) Bennett, J. L.; Wolczanski, P. T. J. Am. Chem. Soc. 1997, 119, 10696-
10719.
(75) Slaughter, L. M.; Wolczanski, P. T.; Klinckman, T. R.; Cundari, T. R. J.
Am. Chem. Soc. 2000, 122, 7953-7975.
(76) Haran, M. K.; Clancy, P. Personal communication.
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