Spectroscopic tracking of molecular transport junctions generated by using click chemistry

Article abstract of DOI:10.1002/anie.200806028

Click to fill the gap: The in situ modular fabrication of molecular transport junctions in nanogaps generated by on-wire lithography is achieved by using click chemistry (see picture). The formation of molecular junctions proceeds in high yields and can b

Full text of DOI:10.1002/anie.200806028

Communications
DOI: 10.1002/anie.200806028
Molecular Electronics
Spectroscopic Tracking of Molecular Transport Junctions Generated by
Using Click Chemistry**
Xiaodong Chen, Adam B. Braunschweig, Michael J. Wiester, Sina Yeganeh, Mark A. Ratner,*
and Chad A. Mirkin*
The development of efficient methods for the construction of
molecular transport junctions (MTJs) and the ability to
spectroscopically identify molecules assembled within the
junctions continues to challenge the field of molecular
electronics.[1,2] Most of the current work in MTJ fabrication
relies primarily on ex situ syntheses of molecular wires (e.g.,
dithiolated molecules) followed by subsequent insertion of
the molecules into the gap devices.[3] The problems associated
with this approach are: 1) the difficulty involved in synthesiz-
ing long molecular wires with thiol groups on both ends
because of the low solubility and reaction yields of these
molecules and 2) complications in bridging the electrodes
because of a strong tendency of such molecular wires to
aggregate.[4] In addition, the small junction sizes (normally
only several nanometers wide) often prohibit the use of
routine spectroscopic tools to identify the contents of MTJs.
Therefore, a modular method for the in situ synthesis of
molecular wires to bridge nanogaps,[4,5] which allows spectro-
scopic tracking of the assembly process, merits development.
Herein, we report a new method for the fabrication of MTJs
by using the alkyne–azide “click reaction” within nanogaps
fabricated by on-wire lithography (OWL), while using sur-
face-enhanced Raman scattering (SERS) to characterize the
assembly processes within the gaps. This strategy for forming
MTJs proceeds in high yields, and, as a result of the accessible
functional-group requirements of click chemistry, is a mod-
ular approach that can be used to form MTJs comprising
different molecular components. Additionally, this approach
is well-suited for studying the transport properties of various
molecular architectures because the resulting triazole formed
by the reaction of the alkyne and azide groups retains the
conjugation required for electronic transport.
structures (e.g., nanogaps and disk arrays) with control over
composition and morphology.[6] The obtained structures have
been used for prototyping nanostructured materials with
advanced functions in the context of molecular electronics[6–8]
and SERS.[9–11] OWL allows the preparation of gaps with
feature-size control down to 2 nm, which makes them
promising testbeds for the fabrication of MTJs.[7] The
characteristics of OWL-fabricated nanogaps include high-
throughputs and tunable, molecular-sized features. Herein,
we use click chemistry for the in situ modular synthesis of
molecular wires within the OWL-fabricated nanogaps. Click
chemistry is a synthetic approach popularized by Sharpless
and co-workers that involves reactions that proceed quickly,
with high yields and specificity, under mild conditions.[12] An
advantage of forming molecular wires by using the click
methodology within the OWL-fabricated nanogaps is that
in situ fabrication within a confined space (nanogap) is
challenging for other existing testbeds, such as scanning
probes[13–15] and wire crossing,[16] because these techniques are
not easily solution-processable. On the other hand, mechan-
ical break-junction techniques[17,18] provide only limited
control over gap size in comparison with nanogaps formed
by OWL. In this study, the copper(I)-catalyzed 1,3-dipolar
Huisgen cycloaddition reaction (click reaction) between azide
and alkyne groups (Figure 1c) is utilized as a model reaction
for preparing molecular wires within OWL-fabricated nano-
gaps to form MTJs.
The general scheme for bridging the nanogaps by using
click chemistry for MTJ fabrication is shown in Figure 1a. In a
typical experiment, 4-ethynyl-1-thioacetylbenzene (1)[19] is
first assembled into a monolayer on the surfaces of the
OWL is an electrochemistry-based nanofabrication tech-
nique used to prepare a wide variety of nanowire-based
[*] Dr. X. Chen,[+] Dr. A. B. Braunschweig,[+] M. J. Wiester,[+] S. Yeganeh,
Prof. M. A. Ratner, Prof. C. A. Mirkin
Department of Chemistry and
International Institute for Nanotechnology, Northwestern University
2145 Sheridan Road, Evanston, IL 60208 (USA)
E-mail: ratner@northwestern.edu
chadnano@northwestern.edu
[+] These authors contributed equally to this work.
[**] C.A.M. acknowledges support from the NUNSF-NSEC. and for an
NSSEF Fellowship from the Department of Defense. A.B.B. is
grateful for an NIH Postdoctoral Fellowship. S.Y. thanks the ONR
for an NDSEG fellowship. M.A.R. acknowledges funding from the
MRSEC at NU.
Figure 1. a) Schematic illustration of click chemistry within the nano-
gaps. b) Molecules used in this study. c) The alkyne–azide click
reaction.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200806028.
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electrodes, which are located at opposite ends of an OWL-
fabricated nanogap, by immersing the entire device in a
solution of 1 (1 mm) in dichloromethane/methanol (2:1, v/v)
for 12 h. Concentrated sulfuric acid (50 mL) is added to the
solution to deprotect the thiol groups.[20] The device is rinsed
with dichloromethane/methanol, chloroform, and ethanol,
and then immersed in a solution of 2,7-di-azido-fluorene[21] (2;
1 mm) in DMF (10 mL) containing a solution (200 mL) of
copper sulfate (0.074m) and ascorbic acid (0.148m). One of
the azide groups in compound 2 reacts with the alkyne group
on electrode-immobilized 1 to form a 1,2,3-triazole unit at one
end, whilst the azide group at the other end is left unchanged.
This structure can be further extended by reaction with 2,7-
diethynyl-fluorene (3),[21] which, in turn, can be reacted again
with 2. Following the appropriate number of reaction cycles,
the molecular wires that grow from the opposing electrodes
combine and bridge the nanogap. The point at which the
bridge is formed can be determined from the I–V character-
istics of the device, and the number of reaction steps required
to form a bridge depends on the size of the gap.
As a proof of concept, we first attempted a one-step click
reaction with 1 and 2 to bridge a 2 nm OWL-fabricated gap
and form MTJs. The calculated S–S distance of the target
bridging molecule is 2.6 nm, which is long enough to span the
gap. In a typical experiment, 360 nm diameter wire structures
with 2 nm nanogaps (Figure 2a) were cast onto a substrate
bearing gold microelectrodes and then connected to the
electrodes by electron-beam lithography and subsequent
chromium and gold thermal deposition (Figure 2b). The
two-terminal I–V characteristics of the gap devices were
measured at room temperature before and after click
reactions (Figure 2c). The empty nanogaps, or nanogaps
modified with a monolayer consisting only of 1, exhibit no
conductance within the noise limit of the measurement (<
2 pA; Figure 2c, & and ~). However, following the click
reaction of 1 and 2 within the gap, the I–V characteristics
show a clear molecular response in the mA range (Figure 2c,
!), which indicates the realization of a conjugated molecular
bridge within the nanogap formed as a result of the click
reaction. The yield for working devices is 41% (12 out of 29
devices with I > 0.1 nA at 1 V bias). It should be noted that
the magnitude of the current measured in different MTJ
devices varies from 0.1 nA to 600 mA at 1 V bias, which
presumably results from the different numbers of molecules
that bridge the nanogap in different experiments (see
Figure S5 in the Supporting Information). It is also likely
that the roughness of the electrode surface contributes to the
observed variation. As a control experiment, we synthesized
the dithiol 4 (see the Supporting Information for structure)
ex situ from 1 and 2, and the current amplitude and yield of
working devices is lower (ca. 10%, 3 out of 31) than that of
MTJ devices assembled in situ. This observation is likely to
result from the slow diffusion of the large molecule into the
nanogap.[11]
X-ray photoelectron spectroscopy (XPS) of the proof-of-
concept system on a bulk surface confirmed that the click
reactions proceed on Au substrates. For example, the
assembly of 1 and the click reaction between 1 and 2 were
carried out on a planar Au substrate surface (model system)
and followed by XPS in the S 2p region and the N 1s region.
In general, the S 2p spectra are composed of 2p3/2 and 2p1/2
peaks with an intensity ratio of 2:1, as theoretically deter-
mined from the spin-orbit splitting effect. Binding peaks at
162.4 (S 2p3/2) and 163.9 eV (S 2p1/2), which are assigned to
the bound sulfur atoms, are shown in Figure S3 in the
Supporting Information.[22] Furthermore, when a surface-
bound monolayer of 1 reacts with 2 in the presence of CuI, a
peak at 400.2 eV is observed, which arises from the presence
of both triazole and azide groups (N 1s).[23] These spectral
signatures confirm that the 1,3-dipolar cycloaddition reaction
between azide and alkyne groups proceeds successfully on
monolayer-modified Au surfaces.
SERS measurements carried out directly on the nanogaps
confirm that the click reaction proceeds within these confined
spaces. OWL-fabricated nanogaps less than 100 nm in width
have been shown to act as Raman “hot spots” with enhance-
ment factors as large as 108,[9,11] therefore molecules assem-
bled within nanogaps can be efficiently identified by
SERS.[24,25] To evaluate the potential of OWL-fabricated
nanogaps for simultaneous assembly and spectroscopic iden-
tification, we fabricated sub-100 nm nanogap structures, with
Au segments on the opposite sides of the nanogaps. In a
typical experiment, the molecules were assembled by click
chemistry in a nanogap ((98 Æ 11) nm; Figure 3a) as de-
scribed above for the MTJ fabrication, and the Raman spectra
and image of the gap area were measured by confocal
scanning Raman microscopy (WiTec Alpha300). For the gap
structures modified with 1, the Raman spectrum (Figure 3c,
Figure 2. a) SEM image of a 2 nm OWL-fabricated nanogap. b) An
SEM image of 2 nm nanogap–MTJ device. c) Representative I–V
response for 2 nm OWL-fabricated gaps before (&), after (~)
modification with 1, and the bridging click reaction of 2 with 1 (!).
The plain gray line shows the theoretical fitting of the I–V curve.
ꢀ
Spectrum 1) clearly shows the presence of alkyne groups (C
C symmetric stretch at 2108 cmÀ1) and benzene rings (aro-
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Information). In addition, a single-level model[28] was used to
fit the experimental I–V curve (see the Supporting Informa-
tion for details). When transport in the Landauer regime of
coherent tunneling was assumed,[29–31] a transport equation
dominated by one channel (single-level picture) was formu-
lated to obtain the electrode–molecule coupling (0.037 eV on
both sides) and the energy gap between the Fermi level and
the molecular level (0.75 eV). By taking an Au Fermi level at
around 5 eV, our results indicate that hole transport (that is,
through the HOMO) dominates. The experimental results
(Figure 2c, !) and theoretical fit (Figure 2c) are in good
agreement.
Multistep reactions can also be carried out within the
nanogaps in a stepwise approach that is analogous to solid-
phase synthesis. The ability to extend and vary the chemical
structure within MTJs in situ allows the determination, in a
high throughput, combinatorial fashion, of how the changes in
molecular structure within the gaps affect transport proper-
ties. To this end, we have synthesized 3, a fluorene derivative
with alkyne substituents. When 3 is reacted with the available
azide from 2 on the Au electrode under click reaction
conditions, the oligomeric fluorene chain is extended, which
either bridges the gap or leaves an azide group available for
further reaction (Figure 4). The number of fluorene mono-
mers in the oligomeric chain is controlled precisely by the
number of reaction steps and the gap size. We have produced
Figure 3. a) SEM image of an OWL-fabricated nanowire with a 100 nm
nanogap. b) 3D confocal scanning Raman images of an OWL-fabri-
cated gap structure modified with 1. The inset shows a simultaneously
obtained optical image that shows the position of the nanogap. Scale
bar: 2 mm. c) Representative Raman spectrum taken for the different
steps: 1) 1 within the gap, 2) following addition of 2, and 3) following
addition of 3.
matic C C stretch of 1 at 1585 cmÀ1). In addition, when
À
compared with the spectrum of the neat solid of 1 (Figure S6
in the Supporting Information), the absence of thioacetyl
(634 cmÀ1) and SH (bending, 915 cmÀ1) vibration modes
[26]
À
indicates Au S bonding.
The confocal Raman images
(Figure 3b) obtained by integrating the spectral intensity
from 1520 to 1620 cmÀ1 and the bright-field optical image
show that the hot spots are localized in the gap. When 2 reacts
with the monolayer of 1 under click reaction conditions, new
peaks at 967 and 1010 cmÀ1 (the triazole ring stretch),[27]
1606 cmÀ1 (aromatic C C stretch of 2), and 2198 cm
À1
Figure 4. Representative I–V response curves for the multistep click
reactions within OWL-fabricated gaps before and after the final click
reaction step. a) 5 nm OWL-fabricated gap, two-step click reaction and
b) 7 nm OWL-fabricated gap, three-step click reaction.
À
(asymmetric stretching of azide groups) appear, and the
peak at 2108 cmÀ1 disappears (Figure 3c, Spectrum 2), which
indicates that the click reaction of 1 and 2 proceeded
successfully. Furthermore, when 3 reacts with 2 under click
reaction conditions, the relative intensity of the peak at
1606 cmÀ1 increases (Figure 3c, Spectrum 3), which confirms
the occurrence of the reaction between 3 and 2. These SERS
experiments demonstrate the direct observation of the
chemical reactions within nanogaps and, as a result, confirm
the chemical composition of the molecules confined within
the MTJs.
Theoretical calculations were performed to characterize
the transport behavior observed across the MTJs. Density
functional theory calculations (B3LYP, 6-31G*) were carried
out on the gas-phase (geometry optimized) molecular wire,
the HOMO–LUMO gap of which was determined to be
3.8 eV (HOMO (highest occupied molecular orbital):
À5.6 eV vs. vacuum, LUMO (lowest unoccupied molecular
orbital): À1.8 eV vs. vacuum; see Figure S8 in the Supporting
MTJs with three and five fluorene monomers from two and
three click reaction steps in 5 and 7 nm gaps, respectively.
Transport is observed only after the appropriate number of
reaction steps to bridge the gap have been carried out. In the
case of the 5 nm gap, no current is seen until two reactions
have been carried out, and, in the case of the 7 nm gap, no
current is seen until three reactions have been carried out.
These experiments demonstrate the ability to carry out
multiple reaction steps within the gap.
In summary, we have demonstrated a new method for the
in situ, modular fabrication of MTJs through click chemistry
in OWL-fabricated nanogaps, whereby the Raman enhance-
ment inherent to these nanostructures is used to spectroscopi-
cally characterize the molecular assembly processes within
the gaps. The use of click chemistry to form MTJs proceeds in
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[11] G. F. Zheng, L. D. Qin, C. A. Mirkin, Angew. Chem. 2008, 120,
high yields and can be used to test different molecules; the
resulting triazole form maintains conjugation in the molecular
wires. In addition, this method overcomes a major challenge
in the field of molecular electronics, that is, the ability to
spectroscopically track the assembly processes of MTJs within
such tiny gaps. By using the azide–alkyne click reaction to
affix molecules within the gap, the transport properties of
different functional building blocks can be explored. We have
demonstrated this concept by synthesizing fluorene oligomers
of different lengths within the gaps and studying their
transport properties. We anticipate that this new method of
forming and characterizing MTJs will be used to create
nanoelectronic devices with diverse functions and applica-
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Received: December 10, 2008
Published online: February 19, 2009
Keywords: click chemistry · fluorene · molecular electronics ·
.
nanowires · on-wire lithography
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