Silicon ring strain creates high-conductance pathways in single-molecule circuits

Article abstract of DOI:10.1021/ja410656a

Here we demonstrate for the first time that strained silanes couple directly to gold electrodes in break-junction conductance measurements. We find that strained silicon molecular wires terminated by alkyl sulfide aurophiles behave effectively as single-molecule parallel circuits with competing sulfur-to-sulfur (low G) and sulfur-to-silacycle (high G) pathways. We can switch off the high conducting sulfur-to-silacycle pathway by altering the environment of the electrode surface to disable the Au-silacycle coupling. Additionally, we can switch between conductive pathways in a single molecular junction by modulating the tip-substrate electrode distance. This study provides a new molecular design to control electronics in silicon-based single molecule wires.

Full text of DOI:10.1021/ja410656a

Communication
pubs.acs.org/JACS
Silicon Ring Strain Creates High-Conductance Pathways in Single-
Molecule Circuits
Timothy A. Su,† Jonathan R. Widawsky,‡ Haixing Li,‡ Rebekka S. Klausen,†,§ James L. Leighton,*,†
Michael L. Steigerwald,*,† Latha Venkataraman,*,‡ and Colin Nuckolls*,†
†Department of Chemistry, Columbia University, New York, New York 10027, United States
‡Department of Applied Physics, Columbia University, New York, New York 10027, United States
S
* Supporting Information
junction (STM-BJ) technique.7 Point contacts between the Au
ABSTRACT: Here we demonstrate for the first time that
strained silanes couple directly to gold electrodes in break-
junction conductance measurements. We find that strained
silicon molecular wires terminated by alkyl sulfide
aurophiles behave effectively as single-molecule parallel
circuits with competing sulfur-to-sulfur (low G) and sulfur-
to-silacycle (high G) pathways. We can switch off the high
conducting sulfur-to-silacycle pathway by altering the
environment of the electrode surface to disable the Au−
silacycle coupling. Additionally, we can switch between
conductive pathways in a single molecular junction by
modulating the tip−substrate electrode distance. This
study provides a new molecular design to control
electronics in silicon-based single molecule wires.
tip and substrate electrodes are repeatedly broken and formed
in a solution of the target molecule at room temperature and
low voltage (225 mV). After the Au−Au point contact is
broken, aurophilic thiomethyl groups8 on the molecule bind to
undercoordinated gold atoms to form Au−molecule−Au
junctions. Conductance (current/voltage) is measured across
the gap as a function of tip−substrate displacement (Δx), and
the resulting traces reveal molecule-dependent plateaus
signifying junction formation with conductance values below
G0 (2e2/h), the quantum of conductance describing a single
Au−Au contact.9 Junctions form and break thousands of times,
and we analyze the measured traces using one- and two-
dimensional histograms10 as detailed below.
The results from STM-BJ measurement of 1 in 1,2,4-
trichlorobenzene demonstrate four unique characteristics that
are unlike any molecule we have ever measured. First, as we see
from a two-dimensional conductance-displacement histogram
shown in Figure 1a, we have two distinct conductance states
that differ by an order of magnitude. This difference is
significantly greater than we previously observed for 4,4′-
bipyridine.11 We define the region around 4.2 × 10−4 G0 as the
high G state and the region around 4.0 × 10−5 G0 as the low G
state. Sample conductance traces have plateaus at these
conductance values, as shown in the inset. Second, we note
that both high and low G states occur directly after the Au−Au
contact rupture (Δx = 0); yet, a high G junction can only
sustain a 0.4 nm elongation while a low G junction can sustain a
0.6 nm elongation. Since junction elongation length correlates
with the conduction path length,12 we hypothesize the
conduction path in the high G state is shorter than in the
low G state.
his manuscript unveils the characteristics of strained
T
silicon in single molecule electrical devices. Current state-
of-the-art integrated circuits incorporate strain in the silicon
crystal lattice to improve charge mobility and channel
conductance. As electronic devices continue their ultra-
miniaturization,1−5 understanding the effects of strain in
molecular versions of silicon will be crucial. In this study, we
incorporate the strained silacyclobutane ring into a molecular
wire (1) to investigate how silicon ring strain manifests in
molecular conductance. Previously, we found that permethy-
lated oligosilanes with alkyl sulfide contacts at both ends of the
molecule conduct through a single pathway from sulfur to
sulfur.6 Here, we demonstrate that silane 1 conducts not only
from sulfur to sulfur (red, low G pathway) but also from sulfur
to silacycle (blue, high G pathway). The latter pathway arises as
a consequence of the significant strain of the silacycle.
Additionally, we find that we can control the pathway through
which conductance occurs and thereby switch between two
distinct conductance states.
We synthesize 1 from 1,1-dichlorosilacyclobutane and 4-
(methylthio)phenylmagnesium bromide (synthetic details and
characterization are described in the Supporting Information
(SI)). We grew single crystals suitable for X-ray diffraction from
methanol vapor diffusion into a solution of 1 in 1,2,4-
trichlorobenzene (TCB). The bond angle about the aryl
carbons and silicon atom describes a standard tetrahedron
(109°) while the silacyclobutane ring is significantly distorted
(CH2−Si−CH2, 79°). We measure single-molecule conduc-
tance using a scanning tunneling microscope-based break-
Received: October 17, 2013
© XXXX American Chemical Society
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Journal of the American Chemical Society
Communication
nearly identical to that of dimethylsilane 2 (Figure S4).6 We
hypothesize that the high G state represents a junction where
current flows between one of the sulfides and the silacycle
(Figure 2a). This junction geometry is consistent with the
shorter elongation length of the high G state. Furthermore, it
explains the lack of the high G peak in 1-bromonapthalene. We
have previously shown that 1-bromonaphthalene binds much
more strongly to undercoordinated Au sites relative to
chlorinated and ethereal solvents.14 Accordingly, the absence
of the high G state suggests that bromonapthalene molecules
are preventing the formation of the weak Au−silacycle contact
in the high G geometry. This Au−silacycle interaction must also
be weaker than the Au-SMe linkage because the low G state is
unaffected in the 1-BN experiment. Indeed, we find that the
Au−silacycle conduit is too fragile to form on its own, as
monosulfide 3 has no measurable conductance (Figure S5).
The secondary Au−SMe linkage is therefore necessary to
organize the silacycle onto a nearby Au atom. Because the Au−
silacycle interaction is easily disrupted, the high G state only
occurs if the Au atoms that span the auxiliary sulfide and
silacyclobutane linkages are uncoordinated, thus explaining the
concentration dependence in trichlorobenzene: at high
concentrations, there are significantly more Au atoms
coordinated by stronger sulfide linkages and, therefore, a
lower probability that the high G junction geometry can be
accessed (Figure 2b).
Figure 1. (a) 2D histogram of the first 5000 traces of 1 (1 mM TCB)
with selected sample traces (inset). (b) Single-junction elongation
(left) and compression traces (right) showing switching events. The
black dotted line denotes the piezoelectric motion while the red solid
line shows the conductive response. (c) 1D histogram tracking
sequential sets of 5000 traces (20 000 total) to show peak evolution
with increasing concentration. (d) 1D histogram of 1 in 1-BN solvent
(1 mM), compared with TCB (10 mM).
Third, we find that we can induce switching from the high to
low G state reliably; however, the reverse occurs infrequently
(see SI for more detailed analysis). To switch from a high to
low G state (elongation), we first form a high G junction and
then pull the junction an additional 0.2 nm as shown in Figure
1b. We find that, with this modified ramp, 65% of the traces
switch from high to low G, 12% stay in high G, and 23% break.
To switch from low to high G, we form a low G junction, then
compress the electrodes by 0.2 nm. The fraction of traces that
switch from low to high G is small (13%). Under compression,
the majority of measured traces remain in the low G state,
indicating that the high G state is not easily accessible once the
low G state is formed.
The fourth and most surprising distinction between 1 and
other molecular wires demonstrating multiple conductance
states11−13 is that the ratio between the high and low G states is
concentration-dependent. The one-dimensional conductance
histogram in Figure 1c shows that as solvent evaporates over
time, the high G peak diminishes as the low G peak grows in.
This effect is reversible: replenishing the solvent subsequent to
evaporation restores the initial peak distribution (Figure S1).
We find the high G peak is more intense in dilute solutions (1
mM) whereas the low G peak is more intense in concentrated
solutions (10 mM) in both trichlorobenzene and trimethox-
ybenzene solvents (Figures S2 and S3). At 1 mM in 1-
bromonapthalene (1-BN) however, the high G peak is
conspicuously absent (Figure 1d, red).
Figure 2. (a) In dilute conditions, an auxiliary sulfur group organizes
the Au−silacycle conduit. (b) At higher concentrations, more
molecules assemble on the electrode surface and the weak Au−
silacycle conduit is more easily displaced. (c) A schematic explaining
the nonadiabatic nature of single-junction switching.
The switching statistics from the elongation and compression
experiments are also consistent with this explanation. Switching
from high to low G is frequent (65%) because the molecule is
easily pulled out of the Au−silacycle interaction to the more
stable low G conformer at Δx = 0.6 nm. Switching from low to
high G is less frequent (13%) because there is poor incentive to
form the Au−silacycle complex, as there are other geometries
the low G state can adopt at Δx = 0.4 nm (Figure 2c). This
model for our high G state is also supported by the findings of
others15,16 who have shown that ethynyltrimethylsilanes form
self-assembled monolayers on gold surfaces through Au−silane
interactions.
We attribute the low G state in 1 to sulfur-to-sulfur
conductance because its conductance-displacement profile is
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The Au−silane conduit observed here is specific to highly
strained silicon rings. The STM-BJ measurement of less
strained silacyclopentane 4 (CH2−Si−CH2, 96° [DFT-
calculated; see SI]) gives a single conductance peak at 5.1 ×
10−5 G0 corresponding to the low G state and analogous to
dimethylsilane 2 (Figure S6). We also find this high G state is
not a generality of strained four-membered rings. All-carbon
cyclobutane analog 5 gives a single conductance peak (1.7 ×
10−4 G0) close in magnitude to that for bis((4-methylthio)-
phenyl)methane (1.2 × 10−4 G0 (Figure S7)). Cyclobutane 5 is
significantly strained, with a ring angle of 87° in the crystal
structure. The fact that cyclobutane 5 does not yield a
secondary conductance state highlights a fundamental differ-
ence between carbon and silicon: though 1 and 5 are
structurally analogous, the electronics of strained silicon enables
coupling between the gold electrode and silacycle.
Conductance in these molecular systems typically occurs
through tunneling, which is strongly distance dependent.5 The
Au−silacycle interaction provides a shorter conduit relative to
the distal methyl sulfide, thereby enabling high conductance
even in structures that do not typically conduct. Previously we
demonstrated that tetramethyldisiloxane 6 does not form
conductive junctions because the oxygen atom disrupts
delocalization across the molecule in the orbital most
commonly ascribed to S-to-S conductance (HOMO).6 DFT
calculations show that the HOMO of bis(cyclobutyl)disiloxane
7 is similarly localized on one end of the molecule (SI). We find
that disiloxane 7 gives a high G peak (2.5 × 10−4 G0) that
strongly diminishes with increasing concentration (Figure S8).
These results can be rationalized if the silacycle is an electronic
contact that opens an alternative conductance pathway.
We find that steric encumbrance near the silacyclobutane
disables the Au−silacycle interaction and suppresses the high G
state. We install ortho-methyl groups in silacyclobutane 8 to
simultaneously frustrate arene rotation17,18 and restrict the Au−
silane interaction. Accordingly, the STM-BJ measurement of 8
yields a single, sharp, low G peak (7.6 × 10−5 G0, Figure S9).
We can also tune the high G state’s sensitivity to concentration
by modifying the structure of the gold-molecule chelate. Butyl-
linked 9 gives both high (3.1 × 10−4 G0) and low G (1.8 × 10−5
G0) peaks at 1 mM (Figure 3a). As solvent evaporates, the high
G peak in 9 disappears entirely (darkest blue line) and
resembles the analogous dimethylsilane 10 (2.6 × 10−5 G0).
This is in contrast with 1, where the high G state is still
accessible at higher concentrations. Like 1, we find the high G
peak in 9 re-emerges when solvent is replenished. These
molecules demonstrate that the behavior of the high G state
whether it diminishes completely or does not appear at all, can
be tuned by changing the linker structure.
Figure 3. 1D histograms of 9 and 10; (b) 11 and 12. (a) For 9, the
lightest line compiles the first 2000 traces while the darkest line
compiles the last 2000 traces over 15 000 traces.
synthesized 11 and 12 with meta-thioanisole linkers. Previous
studies have shown that molecular wires with meta-substituted
linkers are poor π-conductors due to quantum interference
effects,19−22 in which the nodes at the meta-position prevent
the distal sulfur pπ orbitals from coupling through the
molecule. Our own investigations of oligoenes,21 inorganic
clusters,23 fluorenes,24 and longer silanes (Figure S10) have
shown that while molecules with para-substituted thioanisole
linkers strongly conduct, meta-substituted linkers yield little to
no conductance. In the monosilanes studied here, we find that
the magnitude of conductance is not significantly affected by
whether the thioanisole linkers are para- or meta-substituted.
Meta-linked 11 gives two sharp conductance peaks (2.7 × 10−4
G0, 1.4 × 10−5 G0) similar to the high and low G states for 1
(Figures 3b, S11). Dimethylsilane analog 12 furnishes a single
peak (1.7 × 10−5 G0) similar to the low G state in 11.
Conductance in 11 and 12 implies the σ-framework is the
dominant conduit for diarylmonosilanes. The presence of the
high G state in 11 further suggests the strained silane ‘shorts’
end-to-end conductance.
The silacyclobutane family is useful in silicon-directed
organic synthesis. The Lewis acidity at the silicon center
activates an array of organic transformations that are otherwise
difficult to achieve with unstrained silanes.25−27 This Lewis
acidity implies that the Si−C bond is strongly polarized toward
the carbon. We speculate that it is these vicinal carbons or
polarized Si−C bonds that contact the gold electrode and
propose that the Lewis acidic properties of silacyclobutanes in
organic reaction chemistry manifest here as a new conductance
state. Drawing such an analogy between reaction chemistry and
molecular electronics opens the possibility that the vast store of
synthetic methodology can be applied to create new types of
molecular devices.
We demonstrate here for the first time that strained silanes
couple directly to gold electrodes. This coupling enables
strained silicon molecular wires with alkyl sulfide leads to
conduct through two distinct pathways: sulfur-to-sulfur and
sulfur-to-silacycle. We find that the molecular wire ‘chooses’ its
conductive pathway based on the surface environment of the
Au electrodes. Furthermore, we can controllably switch from
the high to low G state in a single junction by modulating the
tip−substrate distance. Our studies here uncover the electronic
effects of straining silicon at the molecular level and provide a
foundation for the realization of strained silicon molecular
components.
To elucidate the conductance mechanism in these
monosilane systems (and particularly the strained silanes), we
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dx.doi.org/10.1021/ja410656a | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(16) Marques
́
-Gonzal
́
ez, S.; Yufit, D. S.; Howard, J. K.; Martín, S.;
ASSOCIATED CONTENT
■
́
Osorio, H. M.; García-Suarez, V. M.; Nichols, R. J.; Higgins, S. J.; Cea,
P.; Low, P. J. Dalton Trans. 2013, 42, 338.
S
* Supporting Information
Synthetic procedures, STM-BJ procedures, table of conductan-
ces, additional figures, characterization data, computational
details, and crystallographic data (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(17) Vonlanthen, D.; Rotzler, J.; Neuburger, M.; Mayor, M. Eur. J.
Org. Chem. 2010, 2010, 120.
(18) Venkataraman, L.; Klare, J. E.; Nuckolls, C.; Hybertsen, M. S.;
Steigerwald, M. L. Nature 2006, 442, 904.
(19) Ke, S.; Yang, W.; Baranger, H. U. Nano Lett. 2008, 8, 3257.
(20) Solomon, G. C.; Andrews, D. Q.; Goldsmith, R. H.; Hansen, T.;
Wasielewski, M. R.; Van Duyne, R. P.; Ratner, M. A. J. Am. Chem. Soc.
2008, 130, 17301.
(21) Aradhya, S. V; Meisner, J. S.; Krikorian, M.; Ahn, S.;
Parameswaran, R.; Steigerwald, M. L.; Nuckolls, C.; Venkataraman,
L. Nano Lett. 2012, 12, 1643.
AUTHOR INFORMATION
■
Corresponding Authors
leighton@chem.columbia.edu
mls2064@columbia.edu
lv2117@columbia.edu
cn37@columbia.edu
́
(22) Guedon, C. M.; Valkenier, H.; Markussen, T.; Thygesen, K. S.;
Hummelen, J. C.; van der Molen, S. J. Nat. Nanotechnol. 2012, 7, 305.
(23) Roy, X.; Schenck, C. L.; Ahn, S.; Lalancette, R. A.;
Venkataraman, L.; Nuckolls, C.; Steigerwald, M. L. Angew. Chem.,
Int. Ed. 2012, 51, 12473.
Present Address
§Department of Chemistry, The Johns Hopkins University,
Baltimore, MD 21218, USA.
(24) Klausen, R. S.; Widawsky, J. R.; Su, T. A.; Li, H.; Chen, Q.;
Steigerwald, M. L.; Venkataraman, L.; Nuckolls, C., unpublished
results.
(25) Myers, A. G.; Kephart, S. E.; Chen, H. J. Am. Chem. Soc. 1992,
114, 7922.
(26) Denmark, S. E.; Griedel, B. D.; Coe, D. M. J. Org. Chem. 1993,
58, 988.
(27) Matsumoto, K.; Oshima, K.; Utimoto, K. J. Org. Chem. 1994, 59,
7152.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The STM-BJ experiments were supported as part of the Center
for Re-Defining Photovoltaic Efficiency Through Molecular-
Scale Control, as Energy Frontier Research Center funded by
the U.S. Department of Energy (DOE), Office of Science,
Office of Basic Energy Science under Award Number DE-
SC0001085. T.A.S. is supported by the NSF Graduate Research
Fellowship under Grant No. 11-44155. H.L. is supported by the
Semiconductor Research Corporation and New York CAIST
program. J.L.L. acknowledges support from NSF Grant CHE-
11-52949. We thank Arunabh Batra for helpful discussions and
Dr. Yasuhiro Itagaki for mass spectrometry assistance. We
thank the NSF (CHE-0619638) for acquisition of an X-ray
diffractometer and Dr. Wesley Sattler and Serge Ruccolo for
crystallographic analysis of 1 and 5.
REFERENCES
■
(1) Aviram, A. J. Am. Chem. Soc. 1988, 110, 5687.
(2) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541.
(3) Heath, J. R.; Ratner, M. A. Phys. Today 2003, 56, 43.
(4) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384.
(5) Tao, N. J. Nat. Nanotechnol. 2006, 1, 173.
(6) Klausen, R. S.; Widawsky, J. R.; Steigerwald, M. L.;
Venkataraman, L.; Nuckolls, C. J. Am. Chem. Soc. 2012, 134, 4541.
(7) Xu, B.; Tao, N. J. Science 2003, 301, 1221.
(8) Park, Y. S.; Whalley, A. C.; Kamenetska, M.; Steigerwald, M. L.;
Hybertsen, M. S.; Nuckolls, C.; Venkataraman, L. J. Am. Chem. Soc.
2007, 129, 15768.
(9) Agraït, N.; Rodrigo, J.; Vieira, S. Phys. Rev. B 1993, 47, 12345.
(10) Kamenetska, M.; Koentopp, M.; Whalley, A.; Park, Y.;
Steigerwald, M.; Nuckolls, C.; Hybertsen, M.; Venkataraman, L.
Phys. Rev. Lett. 2009, 102, 126803.
(11) Quek, S. Y.; Kamenetska, M.; Steigerwald, M. L.; Choi, H. J.;
Louie, S. G.; Hybertsen, M. S.; Neaton, J. B.; Venkataraman, L. Nat.
Nanotechnol. 2009, 4, 230.
(12) Meisner, J. S.; Kamenetska, M.; Krikorian, M.; Steigerwald, M.
L.; Venkataraman, L.; Nuckolls, C. Nano Lett. 2011, 11, 1575.
(13) Li, C.; Pobelov, I.; Wandlowski, T.; Bagrets, A.; Arnold, A.;
Evers, F. J. Am. Chem. Soc. 2008, 130, 318.
(14) Fatemi, V.; Kamenetska, M.; Neaton, J. B.; Venkataraman, L.
Nano Lett. 2011, 11, 1988.
(15) Marchenko, A.; Katsonis, N.; Fichou, D.; Aubert, C.; Malacria,
M. J. Am. Chem. Soc. 2002, 124, 9998.
D
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