On-wire lithography-generated molecule-based transport junctions: A new testbed for molecular electronics

Article abstract of DOI:10.1021/ja800338w

On-wire lithography (OWL) fabricated nanogaps are used as a new testbed to construct molecular transport junctions (MTJs) through the assembly of thiolated molecular wires across a nanogap formed between two Au electrodes. In addition, we show that one can use OWL to rapidly characterize a MTJ and optimize gap size for two molecular wires of different dimensions. Finally, we have used this new testbed to identify unusual temperature-dependent transport mechanisms for α,ω-dithiol terminated oligo(phenylene ethynylene). Copyright

Full text of DOI:10.1021/ja800338w

Published on Web 06/04/2008
On-Wire Lithography-Generated Molecule-Based Transport Junctions: A New
Testbed for Molecular Electronics
Xiaodong Chen, You-Moon Jeon, Jae-Won Jang, Lidong Qin, Fengwei Huo, Wei Wei, and
Chad A. Mirkin*
Department of Chemistry and International Institute for Nanotechnology, Northwestern UniVersity,
2145 Sheridan Road, EVanston, Illinois 60208
Received January 15, 2008; E-mail: chadnano@northwestern.edu
Molecular transport junctions (MTJs) are essential structures for
developing the field of molecular electronics.1–3 Several techniques
have been developed to form such junctions, including ones based
upon nanopores,4 scanning probes,5–7 Hg drop top contacts,8 wire
crossings,9 and template-synthesized materials.10 All of these
techniques can be classified as sequential approaches: the molecules
are first covalently attached to one electrode and then a second
electrode is brought into contact with the molecules or molecular
layers by either mechanical control or metal evaporation. Alterna-
tively, one-step methods for directly assembling molecules into
predefined nanogap electrodes have been developed for constructing
MTJs. With these one-step approaches, nanogaps have been
fabricated by shadow mask evaporation,11 mechanical break
junction techniques,12,13 electroplating,14 local oxidative cutting of
carbon nanotubes,15 and electromigration.16 Although researchers
have used these techniques to make impressive advances in
measuring the conductance of MTJs,5–15 the question of how to
reliably fabricate nanogaps between electrodes with the appropriate
Figure 1. (a) SEM image and (b) TEM image of wires with 3.2 ( 0.4 nm
gaps fabricated by OWL; (c) SEM image of a device prepared with an
OWL-fabricated wire with a ∼3 nm gap; (d) molecular structure of OPE-1
and OPE-2; (e) a diagram of OPE-1 molecules spanning the ∼3 nm gap.
molecular dimensions in a controlled, high-throughput, and scalable
manner remains open.
Recently, we developed an on-wire lithography (OWL)17 tech-
nique to fabricate free-standing nanogaps (Figure 1a and b). The
gaps in these systems can be scaled down to ∼2 nm in a high-
throughput and highly reproducible fashion.18 OWL relies on the
template-directed synthesis of nanorods in an anodized aluminum
oxide membrane (pore diameter ∼360 nm) by the electrochemical
deposition of desired materials.19,20 By selectively introducing a
thin sacrificial layer of Ni, one can subsequently create a gap of
well-defined thickness by selective wet-chemical etching. The gap
size is controlled by the number of coulombs passed in the
electrochemical synthesis of the Ni layer. For instance, 3 nm Ni
segments are obtained when 30 mC of charge is applied during
electrochemical deposition, while 2 nm thick segments result from
20 mC of passed charge. The structure is held together by a thin
coat of silica on one face of the wire, which is introduced prior to
the etching step. Herein, we demonstrate how OWL-fabricated
nanogaps can serve as a new testbed to construct MTJs through
the assembly of thiolated molecular wires across a gap formed
between two Au electrodes. We show how one can use OWL to
easily characterize a MTJ system and optimize gap size for two
molecular wires of different dimensions.
nylene) (OPE-1, ∼0.2 mg/mL) for 24 h, rinsed with THF,
dichloromethane, and ethanol, and then blown dry with N2. OPE-1
was selected as a molecular wire because its length is appropriate
to span the 3 nm gap defined by the OWL process, and the
oligo(phenylene ethynylene) moiety is a well-known, conductive
π-conjugated organic molecular wire from which a high quality
monolayer can be prepared.21
The two terminal I-V characteristics of the ∼3 nm gap devices
were measured at room temperature before and after molecular
assembly (Figure 2a). The empty nanogap (black trace, Figure 2a)
exhibits no conductance within the noise limit of the measurement
(<2 pA). However, the nanogap devices loaded with OPE-1 show
a significant I-V response (red curve, Figure 2a), suggesting the
assembly of the molecules across the gap with chemical connectivity
to each of the gold electrodes on opposite sides of the gap.
Consistent with this conclusion, ∼3 nm gap devices loaded with a
monothiol terminated OPE (OPE-2, Figure 1d) did not exhibit
significant current transport across this sized nanogap (red curve,
Figure 2b). In addition, 1-octadecanethiol (ODT), 1,16-hexade-
canedithiol (HDT), and biphenyl-4,4′-dithiol (BPDT) assembled in
such ∼3 nm gap devices all lead to I-V characteristics similar to
open gap structures (Figure 2b). Taken together, these data strongly
suggest OPE-1 assembles on the gold nanowires and spans the gap
in such a way that it can chemically bond to each of the gold
In a typical experiment, 360 nm diameter wire structures with
∼3 nm nanogaps were cast onto a substrate with gold microelec-
trodes and then connected to the electrodes by e-beam lithography
and subsequent chromium and gold thermal deposition (Figure 1c).
The unmodified nanogap device was immersed in a tetrahydrofuran
(THF) solution of R,ω-dithiol terminated oligo(phenylene ethy-
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2008 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. (a) Representative I-V response for ∼3 nm OWL-fabricated
gaps before (black curve) and after (red curve) being modified with OPE-
1. (b) I-V response of a ∼3 nm pure nanogap before (black curve) and
after OPE-2, HDT, ODT, and BPDT modification.
Figure 4. (a) Temperature dependent I-V response of OPE-1 bridging
the 3 nm nanogap fabricated by OWL. (b) Plots of ln(I/V) as a function of
1/T for different biases. (c) Plots of ln(I) vs V0.5 with biases from 0.1 to 1.0
V at different temperature. (d) Plot of the slope of ln(I) - V0.5 vs 1/T. The
inset shows the magnification of the high temperature part with linear fitting.
All of the straight lines are ꢀ2 fits for respective data sets.
assembly, where the initial adsorption event directs sub-
sequent events to create structures with a predominance of one
orientation.
These new MTJs not only provide a rapid way of matching gap
size and electrode composition with molecular electronic compo-
nents of interest (note, we have made electrodes out of Au, Pt, Pd
and Ag thus far),17 but also allow one to rapidly define the intrinsic
transport properties of the molecules that span the gaps. For
example, we studied the temperature dependent I-V characteristics
of the OPE-1-modified junction (Figure 4a) to elucidate the
conduction mechanism of the device. The monotonic decrease of
current with temperature suggests a thermally activated transport
mechanism, such as thermionic emission or hopping conduction.25
However, hopping conduction can be excluded on the basis of the
temperature-dependent voltage dependence.26 When ln(I/V) is
plotted as a function of 1/T the curves should be identical for
hopping conduction; but instead one sees a clear difference as a
Figure 3. Representative I-V response for 2 nm OWL-fabricated gaps
before (black curve) and after (red curve) being modified with OPE-2. (Inset)
A diagram of OPE-2 spanning the 2 nm gap in a nanowire fabricated by
OWL.
electrodes that are on opposite sides of the gap. It should be noted
that the magnitude of the current measured with different devices
is different (from 0.1 nA to 300 µA with 1 V bias), which is
presumably due to the number of molecules that actually span the
nanogap and make contact with the gold segments (Supporting
Information). The roughness of the electrode surface likely
contributes to the observed variation.
function of voltage (Figure 4b). However, if one plots ln(I) vs V0.5
,
Significantly, the nanogap size can be rapidly tailored to match
the size of a given molecular wire. For instance, when OPE-2 with
the length of ∼2.6 nm is assembled within ∼2 nm gaps fabricated
by OWL, an MTJ is established and rectifying behavior is observed
(Figure 3). The rectifying behavior is likely a result of the different
modes of contact between the different ends of the molecules and
the gold electrodes that span the gap. One end can chemisorb to
the electrode while the other is limited to physical contact (Figure
3, inset). These two different metal-molecule contacts induce
different electronic coupling of the interface, different injection
barriers and unequal voltage drops.22–24 For different devices, the
current magnitude varies (from 0.07 to 400 nA with 1 V bias), but
the rectifying behavior is always observed with this size gap with
OPE-2 (Supporting Information). It also should be noted that the
rectification ratio, which is defined as the forward current divided
by the reverse current, is different in each gap device. This
observation may be related to the different possible orientations of
OPE-2 within the gap due to its dissymmetric molecular structure
(Figure 3, inset). Finally, the rectifying response may imply that
the molecules do not create structures with a 1:1 ratio of the two
different possible general orientations (determined by the electrode
to which thiol adsorption occurs). This could be due to templated
a linear dependence is observed (Figure 4c), which is characteristic
of thermionic emission.25,27 This dependence comes from the
following relationship28
qV
qΦ - q
4πꢀ ꢀd
ꢀ
0
2
(
)
I ) AT exp -
(1)
kT
where A is the effective Richardson constant multiplied by the
current injection area, Φ is the thermal emission barrier height, k
is the Boltzmann’s constant, q is the electron charge, ꢀ0 is the
vacuum dielectric constant, ꢀ is the relative dielectric constant of
the OPE-1 layer, and d is the thickness of the OPE-1 monolayer.
It is important to note that the disordered film model, which
describes charge injection from a metal into a disordered organic
film, predicts a current-voltage dependence similar to the thermi-
onic emission model.29–31 This model is often used to describe
charge injection in light emitting diodes (LEDs), which have
disordered multilayers of organic molecules between the metal
electrodes in the device. We use the thermionic emission model to
describe our system because the monolayer of OPE-1 molecules
are chemisorbed to the electrodes on opposite sides of the gap and
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C O M M U N I C A T I O N S
significantly more oriented and ordered than the types of molecules
found in conventional LEDs.
also thank Drs. Gengfeng Zheng, Ling Huang, Jacob W. Ciszek,
Dwight S. Seferos, Mr. David Giljohann, and Miss Louis Giam
for experimental assistance and valuable discussion.
At high temperatures (>120 K) (red line, Figure 4d), the response
of the MTJ fits well with a thermionic emission mechanism, which
involves thermally activated charge injection from the electrode to
the OPE-1. From the slope of the line formed from plotting ln(I)
- V0.5 as a function of temperature, one can determine the relative
dielectric constant of the OPE-1 layer to be 16.4, the thermal barrier
height to be 0.19 eV, and the Richardson constant to be 21 A/cm2.
This assumes a monolayer thickness of 3 nm, the average size of
the nanogap. These values are comparable to ones reported for other
organic molecular wires.26,27 However, at low temperatures (<120
K), charge transport is dominated by tunneling.32 Consistent with
this conclusion, the I-V response is minimally dependent upon
temperature (green line, Figure 4d). Therefore, there are two
different charge transport mechanisms depending on temperature
for the OPE-1 MTJs based on the OWL-fabricated nanogaps: at
low temperature (>120 K), electrons tunnel across the barrier
through the intervening states; as the temperature increases, thermal
emission of electrons over a barrier of 0.19 eV dominates electron
injection from Au to the OPE-1 layer, thus reducing the contribution
from the tunneling process. The transition from tunneling to
thermionic emission at high temperature is likely the result of
thermal fluctuations and the onset of torsional fluctuations of the
phenyl rings in OPE-1. Others have invoked this type of torsional
fluctuation to explain transitions between different transport mech-
anisms.33 It is worth noting that this is the first experimental
observation of a transition from tunneling to thermionic emission
in a MTJ, although there are two examples of other kinds of
transport transitions: tunneling to hopping (depending on temper-
ature)34 and tunneling to field emission (depending on bias).35 The
different transition behaviors suggest the importance of local
environment and the means by which molecules are con-
tacted.36
In summary, this work is important for the following reasons.
First, it shows how one can use OWL to rapidly construct electrodes
with gaps small enough to accommodate “molecular wires”, and
that the gap size can be tailored for a given molecule. Second, the
process is high-throughput, with respect to gap fabrication, and
extremely flexible with respect to the type of materials one can
use as an electrode. Anything that can be plated in an electrochemi-
cal experiment is an electrode candidate material. Third, the
approach allows one to easily study the properties of molecular
wires, and we have created an initial testbed to identify unusual
transport mechanism differences for one type of molecular wire
(OPE-1) as a function of temperature. Indeed, from a molecular
electronics point of view, OWL-fabricated structures offer a
transport testbed of remarkable simplicity, stability, and scalability.
In addition, OWL-fabricated MTJs may be used for a variety of
applications that extend beyond electronics, including chemical and
biological sensing when more sophisticated molecules are used to
make them.
Supporting Information Available: Complete ref 15; experimental
procedures including molecule synthesis, device fabrication, and
characterization data. This material is available free of charge via the
Internet at http://pubs.acs.org.
References
(1) Lindsay, S. M.; Ratner, M. A. AdV. Mater. 2007, 19, 23–31.
(2) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384–1389.
(3) Tao, N. J. Nat. Nano. 2006, 1, 173–181.
(4) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286,
1550–1552.
(5) Xu, B. Q.; Tao, N. J. Science 2003, 301, 1221–1223.
(6) Cui, X. D.; Primak, A.; Zarate, X.; Tomfohr, J.; Sankey, O. F.; Moore,
A. L.; Moore, T. A.; Gust, D.; Harris, G.; Lindsay, S. M. Science 2001,
294, 571–574.
(7) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.;
Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705–
1707.
(8) Holmlin, R. E.; Haag, R.; Chabinyc, M. L.; Ismagilov, R. F.; Cohen, A. E.;
Terfort, A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2001,
123, 5075–5085.
(9) Kushmerick, J. G.; Holt, D. B.; Pollack, S. K.; Ratner, M. A.; Yang, J. C.;
Schull, T. L.; Naciri, J.; Moore, M. H.; Shashidhar, R. J. Am. Chem. Soc.
2002, 124, 10654–10655.
(10) Mbindyo, J. K. N.; Mallouk, T. E.; Mattzela, J. B.; Kratochvilova, I.; Razavi,
B.; Jackson, T. N.; Mayer, T. S. J. Am. Chem. Soc. 2002, 124, 4020–4026.
(11) Tang, J. Y.; Wang, Y. L.; Klare, J. E.; Tulevski, G. S.; Wind, S. J.; Nuckolls,
C. Angew. Chem., Int. Ed. 2007, 46, 3892–3895.
(12) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science
1997, 278, 252–254.
(13) Reichert, J.; Ochs, R.; Beckmann, D.; Weber, H. B.; Mayor, M.; von
Lohneysen, H. Phys. ReV. Lett. 2002, 88, 176804.
(14) Hu, W. P.; Nakashima, H.; Furukawa, K.; Kashimura, Y.; Ajito, K.; Liu,
Y. Q.; Zhu, D. B.; Torimitsu, K. J. Am. Chem. Soc. 2005, 127, 2804–
2805.
(15) Guo, X. F.; et al. Science 2006, 311, 356–359.
(16) Park, H.; Lim, A. K. L.; Alivisatos, A. P.; Park, J.; McEuen, P. L. Appl.
Phys. Lett. 1999, 75, 301–303.
(17) Qin, L. D.; Park, S.; Huang, L.; Mirkin, C. A. Science 2005, 309, 113–
115.
(18) Qin, L. D.; Jang, J. W.; Huang, L.; Mirkin, C. A. Small 2007, 3, 86–90.
(19) Hurst, S. J.; Payne, E. K.; Qin, L. D.; Mirkin, C. A. Angew. Chem., Int.
Ed. 2006, 45, 2672–2692.
(20) Martin, C. R. Science 1994, 266, 1961–1966.
(21) Tour, J. M. Acc. Chem. Res. 2000, 33, 791–804.
(22) Dhirani, A.; Lin, P. H.; Guyot-Sionnest, P.; Zehner, R. W.; Sita, L. R.
J. Chem. Phys. 1997, 106, 5249–5253.
(23) Taylor, J.; Brandbyge, M.; Stokbro, K. Phys. ReV. Lett. 2002, 89, 138301.
(24) Kushmerick, J. G.; Holt, D. B.; Yang, J. C.; Naciri, J.; Moore, M. H.;
Shashidhar, R. Phys. ReV. Lett. 2002, 89, 086802.
(25) Wang, W. Y.; Lee, T.; Reed, M. A. Phys. ReV. B 2003, 68, 035416.
(26) Zhou, C.; Deshpande, M. R.; Reed, M. A.; Jones, L.; Tour, J. M. Appl.
Phys. Lett. 1997, 71, 611–613.
(27) Chen, J.; Calvet, L. C.; Reed, M. A.; Carr, D. W.; Grubisha, D. S.; Bennett,
D. W. Chem. Phys. Lett. 1999, 313, 741–748.
(28) Sze, S. M. Physics of Semiconductor DeVices; Wiley: New York, 1981; p
403.
(29) Burin, A. L.; Ratner, M. A. J. Polym. Sci. B: Polym. Phys. 2003, 41, 2601–
2621.
(30) Barth, S.; Wolf, U.; Bassler, H.; Muller, P.; Riel, H.; Vestweber, H.; Seidler,
P. F.; Riess, W. Phys. ReV. B 1999, 60, 8791–8797.
(31) Wolf, U.; Arkhipov, V. I.; Bassler, H. Phys. ReV. B 1999, 59, 7507–7513.
(32) Segal, D.; Nitzan, A.; Davis, W. B.; Wasielewski, M. R.; Ratner, M. A. J.
Phys. Chem. B 2000, 104, 3817–3829.
(33) Selzer, Y.; Cai, L. T.; Cabassi, M. A.; Yao, Y. X.; Tour, J. M.; Mayer,
T. S.; Allara, D. L. Nano Lett. 2005, 5, 61–65.
(34) Selzer, Y.; Cabassi, M. A.; Mayer, T. S.; Allara, D. L. J. Am. Chem. Soc.
2004, 126, 4052–4053.
Acknowledgment. CAM acknowledges support from AFOSR,
NSF-NSEC, and DARPA. CAM is also grateful for an NIH
Director’s Pioneer Award. We thank Professor Mark Ratner for
his insight and preliminary review of this manuscript. The authors
(35) Beebe, J. M.; Kim, B.; Gadzuk, J. W.; Frisbie, C. D.; Kushmerick, J. G.
Phys. ReV. Lett. 2006, 97, 026801.
(36) Nitzan, A. Annu. ReV. Phys. Chem. 2001, 52, 681–750.
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