CHEN ET AL. VOL. 7
’ NO. 4
March 29, 2013
C2013 American Chemical Society
Tunable Nanowire Patterning Using
Standing Surface Acoustic Waves
Yuchao Chen,†Xiaoyun Ding,†Sz-Chin Steven Lin,†Shikuan Yang,†Po-Hsun Huang,†Nitesh Nama,†
Yanhui Zhao,†Ahmad Ahsan Nawaz,†Feng Guo,†Wei Wang,‡Yeyi Gu,§Thomas E. Mallouk,‡and
Tony Jun Huang†,*
†Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, Pennsylvania 16802, United States,
‡Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States, and§Department of Food Science,
The Pennsylvania State University, University Park, Pennsylvania 16802, United States
nanoscale electronic circuits,7?10
optoelectronics.11?13Over the past two
decades, researchers have made significant
progress in the synthesis of different kinds
of nanowires.14?16However, postsynthesis
manipulation (alignment, patterning, as-
sembly, and interconnection) of the nano-
wires is usually required for device
applications.17To address this need, a vari-
ety of techniques have been developed for
manipulating nanowires, including Lang-
muir?Blodgett assembly and transfer,18?20
strategies.32,33Each ofthese methods has its
advantages and limitations. For example, in
ing of individual nanowires is determined
by the arrangement of alignment elec-
trodes and/or wells that are first patterned
lithographically.34Thus, the development
anowires are critical components
in diverse applications, including
of more flexible and tunable dielectro-
phoretic methods would be valuable in
several applications. For example, highly
aligned nanowire bundles can dramatically
increase the sensitivity of surface-enhanced
Raman spectroscopy (SERS) for chemical
rent assembly methods provide inadequate
control over the positioning of closely
spaced nanowires, which strongly modu-
lates the SERS signal intensity.36Therefore,
a new approach that can generate control-
lable, tunable patterns is desirable. Herein,
wave (SSAW)-based nanopatterning tech-
nique, which enables the generation of
tunable nanowirepatterns eitherinsuspen-
sion or on the surface of the substrate.
Previously, we have used SSAW-based
methods to manipulate (i.e., pattern, focus,
spheres or cells.37?44In these earlier studies,
acoustic radiation forces caused by pressure
fluctuations in the liquid played a major role
in the manipulation of microscale objects,
*Address correspondence to
Received for review January 1, 2013
and accepted March 29, 2013.
ABSTRACT Patterning of nanowires in a controllable, tunable manner is
important for the fabrication of functional nanodevices. Here we present a simple
approach for tunable nanowire patterning using standing surface acoustic waves
(SSAW). This technique allows for the construction of large-scale nanowire arrays
with well-controlled patterning geometry and spacing within 5 s. In this approach,
SSAWs were generated by interdigital transducers, which induced a periodic
alternating current (ac) electric field on the piezoelectric substrate and consequently patterned metallic nanowires in suspension. The patterns could be
deposited onto the substrate after the liquid evaporated. By controlling the distribution of the SSAW field, metallic nanowires were assembled into
acoustic waves. Additionally, we observed 3D spark-shaped nanowire patterns in the SSAW field. The SSAW-based nanowire-patterning technique
presented here possesses several advantages over alternative patterning approaches, including high versatility, tunability, and efficiency, making it
promising for device applications.
KEYWORDS: surface patterning.nanowire.standing surface acoustic waves (SSAW).virtual electrodes
CHEN ET AL. VOL. 7
’ NO. 4
driving them to pressure nodes or antinodes. In con-
trast, we take advantage here of the SSAW-induced
silver) nanowiresby dielectrophoresis. SSAWs inducea
nonuniform charge distribution and create virtual
electrodes47on the piezoelectric substrate. By control-
ling the distribution of the SSAW field, virtual electrode
arrays are easily repatternable, allowing for the manip-
ulation of nanowires in real time. This flexibility is
unattainable for most conventional approaches,18?34,48
which ordinarily generate static patterns.
In this article, we demonstrate versatile, tunable
patterning of nanowires into parallel and perpendicu-
lar arrays, as illustrated in Figure 1. In a SSAW-induced
ac electric field, disordered nanowires (Figure 1A) sus-
pended in liquid are aligned and assembled along the
electric field lines (Figure 1D). With the application of
one-dimensional (1D) SSAW, the nanowires are as-
sembled into parallel rows with the same orientation
(Figure 1B). In two-dimensional (2D) SSAW fields, the
nanowires form a 2D lattice (Figure 1C) with 3D spark
patterns at the nodes of the network (Figure 1E).
Because the pitch of both the 1D and 2D structures is
sensitive to the frequency of the SSAW field, this
technique allows for the patterning of nanowires with
tunable spacing and density.
RESULTS AND DISCUSSION
arrays were assembled using a 1D SSAW field as
illustrated in Figure 2A (see Supporting Information,
Figure S1 A and B for device details). Two parallel
interdigital transducers (IDTs) were deposited on a
piezoelectric substrate (Y-128? LiNbO3) to generate
traveling surface acoustic waves (SAWs) along the x
of nanowire patterning, we used two parallel IDTs that
had 20 pairs of electrodes with electrode widths and
spacing gaps of 75 μm. Both IDTs could generate
identical SAWs with a wavelength of 300 μm at a
resonance frequency of 12.6 MHz. The input power
intensity of dielectric forces and the fluidic streaming
effect (Supporting Information, Figure S2).49We used
silver nanowires as a model in this study because of
their wide applications in photonics, electronics, and
biosensors.50?58Ag nanowire suspensions were in-
jected into a 10 ? 10 mm2open microchamber with
a depth of 50 μm between the LiNbO3substrate and a
glass slide. The patterning process was observed in
dark field using an optical microscope.
Figure 2B presents a cross-sectional view of the
piezoelectric substrate along the x axis to illustrate the
mechanism of nanowire patterning in a 1D SSAW field.
The standing mechanical vibrations along the z axis
inducea nonuniform charge distribution ontheLiNbO3
substrate. The periodic displacement nodes (minimum
displacement) correspondtoSSAWpressure antinodes.
When placed in the SSAW field, polystyrene micro-
spheres are driven to the displacement antinodes
(pressure nodes) with a consistent spacing of half the
wavelength, as shown in previous studies.39The peri-
odic distribution of electric charges generates an ac
electric field with electric field lines from positive
charges to negative charges. The electric field was
simulated using COMSOL Multiphysics 3.5a software
(Figure 2C). Polarized by the electric field, nanowires
are aligned to the field lines along the x axis (SAW
propagation direction) due to resultant torques. Then,
dielectrophoretic forces transport the nanowires to the
displacement nodes, where the nanowires experience
counter-balancing forces from both positive and nega-
tive electric charges. To test this hypothesis, we placed
both silver nanowires and polystyrene microspheres in
in Figure 2D. Unlike microspheres, which were trapped
at the pressure nodes, nanowires were patterned into
parallel arrays along the displacement nodes (pressure
antinodes). This observation was consistent with our
hypothesis that nanowires are patterned by dielectro-
phoretic forces rather than by acoustic radiation forces.
To further study the forces exerted on nanowires, we
carried out another control experiment by inserting a
layer of coupling liquid (water) and a Au layer between
the LiNbO3substrate and the nanowire suspension (see
Supporting Information, Figure S1C for device details).
The Au layer was designed to shield the nanowires from
the electric field. As a result, the dielectrophoretic forces
were significantly screened, whereas the acoustic wave
the coupling liquid. Dominated by acoustic radiation
Figure 1. Schematic of the SSAW-assisted nanowire pat-
are assembled into bundles due to the electric fields. (E) In
nodes of the network.
CHEN ET AL. VOL. 7
’ NO. 4
forces, the nanowire bundles had similar behaviors to
microspheres, which were patterned at pressure nodes
without alignment by the electric field (Figure 2E). This
wires experience both dielectrophoretic and acoustic
radiation forces inside a SSAW field, the dielectro-
In our calculation (see Supporting Information), the
acoustic radiationforce anddielectric forceacting ona
nanowire bundle (length ∼50 μm, radius ∼250 nm)
The acoustic radiation force is smaller than the di-
electric force by approximately 3 orders of magnitude.
Patterning Nanowires in Suspension. We demonstrate
nanowires in a real-time fashion. As the propagation
direction of SAWs changes, the alignment direction of
the nanowires can be switched simultaneously, allowing
for dynamic patterning. As soon as the input power is
turned off, the nanowire arrays are disturbed by the
fluidic flow, but they can be repatterned into various
geometries by applying different SSAW fields. It was also
possible to switch between patterns without turning off
the power during the process. The dynamic patterning
concept is illustrated in Figure 3. After turning off the
radio frequency (RF) signal, patterned nanowire bundles
When polarized again by the SSAW-induced 1D electric
field, small nanowire bundles simultaneously connected
end-to-end to form a longer chain of wires, which took
less than 1 s (Figure 3B). Due to the dielectrophoretic
forces along the x and y directions,24these bundles
tended to move close to each other while reorienting
this process, some bundles would merge together as
shown in Figure 3D and E. The red arrows in Figure 3D
and E indicate a typical combination process of two
nanowire bundles located in close proximity. Finally, all
the nanowire bundles were aligned and patterned into
one row anchored in the SSAW displacement node
(pressure antinode) with almost the same spacing along
Figure 2. (A) Schematic of 1D nanowire patterning. Two parallel IDTs generate SSAW to align and pattern nanowires in the
suspension. (B) Cross-section view of IDT-deposited piezoelectric substrate along the x axis. The mechanical vibration of the
substrate is induced by SSAW, and the electric field is generated by nonuniform charge distribution. Nanowires and
microspheres are patterned in the microchamber due to dielectrophoretic forces and acoustic radiation forces, respectively.
(C) Simulation result shows the periodic electric field distribution induced by the 1D SSAW. The values of the color bar are
nanowire bundles are trapped at pressure nodes (displacement antinodes) by acoustic radiation forces and do not align.
CHEN ET AL.VOL. 7
’ NO. 4
5 s, which was comparable to the conventional dielec-
trophoretic approach.21A stable pattern of nanowires
was achieved when the electrostatic repulsion forces
tic forces along the y direction were properly balanced,
resulting in a very regular spacing of nanowire bundles
(Supporting Information, Figure S3).34
Patterning Nanowires on Substrates. Patterning nano-
wires onto the surface of a substrate is important in
many applications, particularly in the fabrication of
optical and electrical devices. In the SAW-assisted
patterning technique, we have shown that Ag nano-
wires can be patterned in suspension (Figure 3). We
also found that the nanowire patterns could settle
down from the suspension onto the piezoelectric
substrate without destroying the patterns (Figure 4).
dispersed in the microchamber (Figure 4A). A few
nanowire clusters were observed as bright dots by a
Figure 3. Dynamic process of nanowire patterning in a 1D
The red arrows in (D) and (E) indicate a typical combination
process of two adjacent nanowire bundles.
Figure 4. Surface patterning of nanowires. (A) Before applying the SSAW, nanowires uniformly dispersed in the suspension.
(B) Nanowires aligned and patterned in the suspension when applying a 1D SSAW field. SEM images of (C) aligned nanowire
bundles on thesubstrateand (D)a typicalnanowire bundlewithin the surfacepatterns. (E) Dry transferof nanowirepatterns
with polydimethylsiloxane (PDMS) stamp.
CHEN ET AL. VOL. 7
’ NO. 4
dark-field microscope. When a 1D SSAW field was
applied, the nanowires were polarized and subse-
quently assembled into bundles with the long axis
along the propagation direction of the SAWs
(x direction), as shown in Figure 4B. Ultimately, aligned
nanowires were patterned into parallel rows located at
the SSAW displacement nodes (pressure antinodes),
x direction). The average length of the nanowire
bundles and their average spacing along the y direc-
tion were dependent on the concentration and, there-
fore, could be controlled by adjusting the nanowire
concentration in the suspension (Supporting Informa-
tion, Figure S4). The spacing of nanowire arrays along
the x direction (distance between the centers of two
adjacent rows) was around 150 μm, which was equiva-
lent to the half-wavelength of the SSAWs. After the
medium in the open chamber gradually evaporated
within 2?3 min. The SSAWs were retained during the
evaporation process to prevent the patterns from
being destroyed by convection or capillary forces.
Consequently, the nanowires patterned in the suspen-
sion were deposited onto the substrate. The integrity
of the surface patterns is comparable to those in the
freestanding suspension. A scanning electron micro-
scope (SEM) image of nanowire surface patterns on a
bundles were well aligned in the same direction after
the liquid evaporated, with only a small number of
nanowires changing their orientations. Figure 4D
shows the structure of a typical nanowire bundle, in
bundle. After the nanowire patterns were deposited
onto the LiNbO3substrate, they could be successfully
transferred onto polydimethylsiloxane (PDMS) stamps
with high fidelity (Figure 4E). The stamp was lightly
attached onto the substrate to make a conformal
contact. The nanowire patterns were then picked up
with the PDMS stamp by peeling off the stamp with a
high speed. It should be possible to redeposit the
nanowires on these PDMS stamps to rigid or flexible
are already well developed.59?62
Tunability of Nanowire Patterning. As demonstrated
to prepare highly ordered nanowire bundles, which
explored the ability of this SSAW-assisted patterning
technique in tuning the structure of nanowire patterns
(particularly, the spacing of nanowire bundles). The
method allowed control of the spacing along the x
direction by simply tuning the frequency of SSAWs. In
the 1D SSAW field, nanowire bundles are patterned
along the nodal positions with the spacing equal to
the half-wavelength; therefore an SSAW with tunable
frequency should be able to control the spacing of
chirped IDTs,41which generated two traveling SAWs
and formed a 1D SSAW field, were used to control the
spacing ofnanowires along the x direction.The chirped
IDTs had a gradient width of the electrode (25?50 μm),
which could generate SAWs with different wavelengths
(100?200 μm) at a resonance frequency between 18.5
and 37 MHz. A nanowire suspension with a concentra-
are shown in Figure 5. As the frequency of SSAW
gradually increased,nanowire bundles indifferentrows
were moved closer to each other along the x direction
average spacing measured at different frequencies
produced values similar to each frequency's corre-
sponding half-wavelength. For example, at the fre-
quency of 19 MHz and a half-wavelength of 100 μm,
the average spacing was 97.6 μm. When the frequency
increased to 37 MHz, resulting in a corresponding half-
48.4 μm. The insets of Figure 5 show that the density of
nanowire bundles in the pattern increases with the
frequency of the SSAW. By tuning the frequency, the
shift of the nodal positions induced a rearrangement of
the nanowire patterns.
The ability to control the structure of nanowire
patterns renders the SSAW-based method a versatile
tool for nanowire alignment and surface patterning.
The 1D parallel nanowire array is one of the most
commonly used nanowire patterns with applica-
tions that include molecular sensors,3nanowire
transistors,63and nanowire batteries.64Compared to
the conventional dielectrophoretic approach, this tech-
nique does not require prefabricated electrodes and is
tuned in situ.
Figure 5. Spacing of nanowires along the x direction
(horizontal axis in the inset images) as a function of SSAW
frequency. Black squares represent the spacing of nano-
wires patterned by chirped IDTs at five frequencies from 19
to 37 MHz. Red open circles represent calculated values of
nanowire spacing, which equals the acoustic half-wave-
length. Insets show representative patterns of nanowires
at frequencies of 19, 28, and 37 MHz. White dashed lines in
the inset images indicate the centers of each row.
CHEN ET AL. VOL. 7
’ NO. 4
2D Patterning of Nanowires. 2D SSAW fields could also
be generated, resulting in nanowires patterned into
2D electric field generated by two orthogonal pairs of
IDTs. An array of virtual dot electrodes was created on
the piezoelectric substrate due to the interaction be-
tween the two orthogonal SSAWs, a result that was
quite different from the 1D simulation results (see
Figure 2C). Figure 6B illustrates the mechanism of the
formation of a 2D electric field. When 2D SSAW was
generated, the interaction of positive displacement
antinodes (red lines) formed antinode points with
positive charges (red filled circles), and the interaction
of negative antinodes (blue lines) formed antinode
in the 2D displacement field (green open circles),
which had no displacement, were located at the
positions that were equidistant from the positive and
in perpendicular directions, as shown in Figure 6C. 2D
networks were formed with angles of 90? between the
nanowire bundles. Nanowires patterned into 3D spark
structures were observed at the displacement anti-
nodes due to the high intensity of the electric field. A
control experiment similar to 1D patterning was car-
ried out by screening the electric field (Figure 6D).
Nanowire bundles were trapped at the SSAW pressure
nodes, which showed a different pattern from the one
generated by the electric field. After careful examina-
tion of the 3Dspark structure in Figure 6E, it was found
that the nanowire bundles were aligned along the
electric field lines with one end pointing toward the
displacement antinodes. When the power was off, the
Figure 6. (A) Simulation of the electric field distribution in a 2D SSAW field. The values of the color bar are normalized. (B)
Mechanism of 2D displacement field distribution. Red lines represent maximum positive displacements that carry positive
charges. Blue lines represent maximum negative displacements that carry negative charges. (C) Image of 2D nanowire
patterning. 3D nanowire spark patterns are positioned at the displacement antinodes (pressure nodes). (D) Control
experiment in which the nanowire suspension is screened from the electric field. Nanowires are trapped at the pressure
off the power, nanowires in the spark pattern settle in the suspension.
CHEN ET AL.VOL. 7
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nanowire bundles stopped spinning and settled in the
suspension, as shown in Figure 6F. In addition, the
nanowire bundles in the spark structure appeared to
become longer after turning off the power, indicating
that they had been aligned along different tilt direc-
tions in a 3D configuration in the electric field.
Nanowires can be controllably patterned into dif-
ferent geometries in a rapid, simple process by using
electric fields generated by SSAW. The spacing of
nanowire bundles can be adjusted by tuning the
frequency of the SSAW. With 1D SSAW patterns, the
direction of nanowire alignment can be reoriented in
real time, and 1D and 2D patterns are interconvertible
by switching the distribution of the SSAW field. The
of versatility and tunability to dielectrophoretic pat-
terning of metal nanowires. As a contactless and
template-free patterning technique, SSAW patterning
include electronics, biotechnology, and microfluidics.
MATERIALS AND METHODS
Device Fabrication and Experimental Setup. In this study, we used
substrates. To deposit IDTs on the substrate, the LiNbO3wafer
was spin-coated with a layer of photoresist (SPR3012, Micro-
Chem, Newton, MA, USA). The IDT design on the mask was
developer (MF CD-26, Microposit). After depositing a metal
double layer (Cr/Au, 50 Å/500 Å) with an e-beam evaporator
(Semicore Corp), IDTs were formed by a lift-off process.
Two spacers with a height of 50 μm were created by
attachingpolyimide Kaptontapesonthesubstrate. Aftercover-
ing with a glass slide (10 ? 10 mm2) on the top, an open
microchamber between the glass slide and the substrate was
formed to contain the nanowire suspension. In the control
experiments, two open microchambers were created to form
a coupling liquid (water) layer at the bottom and a suspension
layer at the top (Supporting Information, Figure S1).
The device was immobilized on the stage of a microscope
(Nikon, Eclipse Ti, LV100DA-U). A camera (Nikon, DS-Fi1) was
connected to the microscope to capture the dynamics of
nanowire patterning in dark field. An RF signal generator
to 250 mW to generate SSAWs on the piezoelectric substrate
(Supporting Information, Figure S2). When chirped IDTs were
used, the frequency of the acoustic waves could be tuned by
controlling the input frequency of the ac signals.
Preparation of Silver Nanowire Suspension. Silver nanowires were
purchased from Blue Nano (NC, USA). The nanowires had
average diameters and lengths of 60 nm and 40 μm, respec-
tively. The nanowires were dispersed in ethanol at an original
concentration of 7 mg/mL, and the suspensions were then
diluted by adding deionized water to a final concentration of
5?45 μg/mL (Supporting Information, Figure S5). The diluted
nanowire suspensions were then sonicated for 15 min to
disperse aggregates and resuspend the nanowires.
SEM Image of Nanowires. SEM images were taken on dried
nanowires deposited on the surface of the piezoelectric sub-
strate. After the nanowires were patterned by the SSAW fields,
we kept the acoustic power on until the liquid evaporated to
deposit the nanowire patterns on the surface.
Conflict of Interest: The authors declare no competing
Acknowledgment. We gratefully acknowledge financial
support from National Institutes of Health (Director's New
Innovator Award, 1DP2OD007209-01), the National Science
Foundation, and the Penn State Center for Nanoscale Science
(MRSEC) under grant DMR-0820404. Components of this work
were conducted at the Penn State node of the NSF-funded
National Nanotechnology Infrastructure Network. We thank
Joey Rufo for his helpful discussion.
Supporting Information Available: (1) Device configuration;
(2) patterning time; (3) force analysis; (4) dependence of
nanowire patterns on sample concentration; (5) estimation of
acoustic radiation force and dielectric force. This material is
available free of charge via the Internet at http://pubs.acs.org.
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