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John William Boley, Edward L. White, and Rebecca K. Kramer*
Metallic nanoparticles (MNPs) have been used since ancient
times.[1] Recent advancements in science and engineering havedemonstrated the utilization of MNPs for applications in a
broad array of fields, including optics,[2] medicine,[3] memory,[4–6]
and semiconductors.[7] Many current applications require a sintering step in order to coalesce the deposited MNPs to enable
functionality; whether it be thermal,[8,9] photonic,[10–12] plasmatic,[13] or chemical in nature.[14–16] Although ambient andlocal temperatures of current sintering methods are significantly lower than the melting point of the bulk materials, difficulties can still arise when integrating MNPs into functional
devices with other materials such as chemically or thermallysensitive polymers. Here, we introduce a new class of MNPs
that are “mechanically sintered” at and below room temperature. Using eutectic gallium-indium (EGaIn) MNPs coatedwith Ga2O3 or thiol, we demonstrate particle coalescence via
the application of low pressures, which ruptures the particle
coatings and releases the low-viscosity liquid-metal. This phenomenon is observed morphologically through scanning electron imaging and electrically through two-probe measurements
during compression. Fusing the results of this study with theunique properties of EGaIn and the semiconductive nature of
Ga2O3[17] will facilitate new applications across a broad array offields such as soft robotics, conformable electronics, wirelesscommunications, micro/nanofluidics, wearable/implantabledevices, and energy storage and transport systems. We also
demonstrate that EGaIn MNPs are compatible with scalable
additive manufacturing technologies such as inkjet printing
and can be used to fabricate flexible/stretchable integrated
devices across multiple length scales.
Recent scientific progress in creating small particles of
Ga–In alloy has seen potential for applications in surfaceenhanced Raman spectroscopy substrates,[18] heavy metal ion
sensors,[19,20] and photo catalysts.[19] We focus this study on
EGaIn nanoparticles (EGaInNPs) formed via sonication inethanol, a system which has been studied,[18] although we haveachieved liquid-metal dispersions via sonication in different
solvents (e.g., toluene and acetone). Two separate modalities ofmechanical sintering are investigated: local/selective sintering,where local areas within a deposit of EGaIn nanoparticles(EGaInNPs) are sintered (sub-mm scale); and global sintering,where the entire deposit is sintered (at and above mm scale).
Figure 1 displays local stress-induced coalescence of
EGaInNPs. The main image in Figure 1a features a deliberately

Dr. J. W. Boley, E. L. White, Prof. R. K. Kramer
School of Mechanical Engineering
Purdue University
585 Purdue Mall, West, Lafayette, IN 47907, USA
E-mail: rebeccakramer@purdue.edu

DOI: 10.1002/adma.201404790

Adv. Mater. 2015, 27, 2355–2360

plowed pile of chips formed from a dried film of EGaInNPs(see Experimental Section). Note the occurrence of microcracks
in the deposits in Figure 1, which is common in dried colloidal
deposits.[21,22] Furthermore, the stability of the chips indicates“solid-like” behavior of the deposit and high interparticle adhesion, both characteristic of the oxide coating each particle.[23]During the plowing process, some local areas of the film experienced stresses large enough to coalesce the EGaInNPs into
a continuous liquid phase (see top right detail in Figure 1a),while others experienced smaller stresses so that the film of
EGaInNPs remained intact (see bottom right detail in Figure 1a).
The detail views in Figure 1a highlight the drastic morphological difference between the noncoalesced and coalesced regionsof the film. To confirm that particle coalescence induces electrical change, two-probe electrical measurements were taken
before and after coalescence. Before coalescence, a high electrical loss through the EGaInNP network is observed, most
likely from electron scattering across the interfaces and microcracks within the deposit (formed during the drying process),resulting in a high breakdown field of ≈50 kV m−1 (Figure S1,
Supporting Information). Note that the characteristics of thisbreakdown field will change to some extent depending on the

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Mechanically Sintered Gallium–Indium Nanoparticles

Figure 1. Observation of local mechanical sintering phenomenon.
a) SEM image of plowed pile of EGaInNP chips (main) with detail viewsof liquid EGaIn formed during the plowing process (right-top) and intact
nanoparticles (right-bottom). Scale bar in larger image is 20 µm in length.
Scale bars in detail views are 5 µm in length. b) Resistance measurement(≈49 Ω) of liquid EGaIn line (left). Detail view of drawn line (middle).
Representative SEM image of drawn line revealing morphological change
in area of contact from nanoparticle network to a coalesced line of liquid(right). Scale bar is 20 µm in length.

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amount of Ga2O3 present in the sample due to its semiconductive nature.[17,20] However, as illustrated by Figure 1b, byselectively tapping the deposit (see Experimental Section), thecoalesced area displays a low electrical resistance (≈49 Ω) andappears as a continuous liquid film. This series of observations
confirms that mechanical coalescence of EGaInNPs changes
their electrical properties from nonconductive to conductive.
Moreover, this process is irreversible. Once the protective layers
have been ruptured and the liquid metal coalesces into a single
body, its cohesion allows it to remain intact, as illustrated in
Figure 1a and b. Although plowing is useful for illustrating thesintering process (as shown in Figure 1a), it is difficult to trackwith regard to quantifying mechanical requirements for activating the material.
Compression tests were conducted on drop-casted networksof EGaInNPs in the absence of any stabilizing agents (e.g.,
thiols) in order to demonstrate global sintering and to quantify the required forces to sinter particles of different sizes. The
results from these experiments are summarized in Figure 2.
Figure 2a shows the process for obtaining desired particle sizes(see Experimental Section). Briefly, the raw scanning electron microscope (SEM) images (Figure 2a(i)) were converted
from grayscale to black and white (Figure 2a(ii)), followed bya segmentation step. The output of the segmentation step
(Figure 2a(iii)) was then used to obtain the size distributions foreach sample. Figure 2b shows the average particle diameter andcorresponding standard deviations for different particle formation conditions. Since all samples were prepared using the same
sonication conditions except for sonication time, Figure 2b

Figure 2. Characterization of EGaInNPs without stabilizing agents.
a) Process map for determining particle sizes including i) the raw SEM
image, ii) the black and white converted image, and iii) the resulting
segmented image. Scale bars are 2 µm in length. b) Resulting average
particle diameters as a function of sonication time. Uncertainty bars
are two standard deviations in length. c) Representative voltage output
versus applied force results from compression experiments on samples
of different particle sizes. d) Estimated activation force per particle versus
particle diameter. Vertical and horizontal error bars are two standard deviations in length. Line represents linear fit through the data.

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indicates that the particle size is tunable and that an increase inultrasonic irradiation dosage results in a smaller particle size,
which is in agreement with other sonication studies conductedto produce small metallic particles.[19,24] The results from thecompression tests are featured in Figure 2c (see Experimental
Section for details of the experiment). In general, all samples
followed the same trend: (1) an initial nonconductive state until
a critical activation force is reached, followed by (2) a jumpin voltage corresponding to a metal-like resistance, then (3) adwell period, where the voltage remains relatively constant until
a critical deactivation force is reached, and finally (4) a sudden
drop in voltage, indicating the squeezing of the liquid metal
film from beneath the punch to the surrounding environment.
The first stage (a nonconductive state up to activation force) is
expected considering the electrical results of the unperturbed
network (Figure S1, Supporting Information). In order to gaina better understanding of the dependence of activation force onparticle size, the results were recast in terms of estimated activation force per particle, as shown in Figure 2d. The activation
force per particle f is the force that each particle in contact withthe punch experiences at activation and was calculated by
f =

d2
F
pD 2

(1)

where F is the measured activation force applied by the punch
onto the sample, D = 4.5 ± 0.3 mm is the diameter of the circular polystyrene punch, d is the average particle diameter, andp is the areal packing factor of the particles (assumed to beconstant at 0.82, corresponding to a random packing arrangement).[25] An adjusted R2 value of 0.98 indicates that the activation force per particle increases linearly with average particle size, which agrees with a previous study on the bursting
strength of microscale oil-filled particles with formaldehyde
walls.[26] The physical mechanism behind this effect comesfrom the naturally formed Ga2O3 layer surrounding each of
the particles in the network. When a given particle undergoes
enough strain, this viscoelastic shell will fracture, inducing an
outward flow of encased EGaIn. Since this happens over all particles in contact with the punch, the liquid EGaIn released thencoalesces to form pathways for metal-like electron transport.
The linear trend displayed in Figure 2d can be modeledusing the theory of thin, elastic-perfectly plastic, fluid-filled
membranes. Previous studies have reported that the naturally
forming Ga2O3 outer layer of Ga–In alloys in air is inherently
conformal and fast-forming.[23,27,28] More specifically, X-ray
reflectivity measurements show that the thickness of the Ga2O3
layer is t ≈ 0.5 nm, which is much smaller than the particle
radii in this study.[28] Additionally, from previous experiments,
the stress–strain relationship of the Ga2O3 layer was found tobehave elastically up to the fracture strain, after which there
is no significant change in stress.[23] Therefore, the theory ofthin, elastic-perfectly plastic, fluid-filled shells is applicable.
Using data from Dickey et al.’s previous report, the modulus ofelasticity (E) and the yield stress (σy) of Ga2O3 are estimated tobe ≈39 and ≈1 GPa, respectively.[23] Interpreting the activation
force as the yielding force, the results depicted in Figure 2d forthe compressed network can be effectively modeled as a single
incompressible fluid-filled spherical shell of the same diameter© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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kε =

2m
≈ 3.11tσ y(2)

Using this value for kε and interpolating from Table 1 of Mercadé-Prieto’s finite element results[29] corresponds to an effective compression of 0.44d, which indicates that the effective
particle will rupture when compressed by 44% of its original
diameter. This amount of compression may be larger than thatof a single particle in compression due to the effects of compaction, interparticle interactions, and the occurrence of microcracks within the deposit. The difference in the dwell period(stage three of the sintering process) stems from the amount of
Ga2O3 present with each sample. Samples with smaller particle
sizes inherently contain larger amounts of Ga2O3 (i.e., smaller
particles result in a larger amount of surface area exposed tooxygen, resulting in increased amounts of Ga2O3). As the concentration of Ga2O3 increases, so does the mechanical rigidity,
since Ga2O3 is known to be a viscoelastic material.[23,28] As
mechanical rigidity increases, so does its resistance to flow
or deformations for a given force (i.e., effective surface tension), which explains the observed increased dwell periodwith decreased particle size. Figure 2c also shows an apparent
gradual voltage drop throughout the dwell period. This observed
drop is attributed to the gradual removal of material between
the electrodes during the compression process. Moreover, thedecrease in slope of this drop with decreased particle size canbe explained by the increased structural stability coming fromthe higher amounts of oxide. The maximum output voltage
achieved after reaching the activation force remained constantthroughout all trials (95% confidence interval of 1.23 ± 0.19 V),
indicating indistinguishable electrical resistance effects fromincreased amounts of Ga2O3 with decreased particle size over
the range of particle sizes tested. With the role of the Ga2O3
layer established in the mechanical sintering process, we now
turn our focus to the effects of replacing this layer with a different material.
Steric stabilization and size reduction of MNPs with thiols iswell documented.[18,30] However, it is also important to understand the effects of thiols on the morphology and sintering performance of the deposited material. Therefore, we developed
a process to arrive at an appropriate thiol concentration for agiven sonication condition that leverages the steric stabilizationof the thiol while preserving the mechanical sintering property.
Figure 3 summarizes the results obtained by experiments conducted to characterize these effects for a fixed sonication condition (see Experimental Section). Beginning with deposit morphology (see Figure 3a), for thiol concentrations ≤1 × 10−3 M
it is evident that the particles are polydispersed. However, as
we increase the thiol concentration between 0 and 0.5 × 10−3 M,
we observe a more bimodal-like distribution with increased
clustering of similarly sized particles. We attribute this effectto insufficient thiol levels, where non-thiol-capped particles arelikely to aggregate since there is no thiol providing steric stabilization. This trend levels off at a concentration of 1 × 10−3 M,

Adv. Mater. 2015, 27, 2355–2360

where the particles seem to be mostly monodispersed. The
average particle diameter of the EGaInNPs for the 1 × 10−3 M
samples (180 ± 32 nm) is considerably smaller than that ofsamples without thiol (465 ± 65 nm); this effect of decreased
particle size with increased thiol concentration is consistent
with the observations of Hohman et al.[18] Increasing thiol concentration beyond 1 × 10−3 M results in agglomerations of the
EGaInNPs, which appears to be a result of excess thiols. The
excess thiols appear on the samples as a continuous clear film
surrounding the EGaInNP aggregates (see the detail image ofthe 5 × 10−3 M sample in Figure 3a). Rinsing the higher thiol
concentration samples (≥3 × 10−3 M) prior to deposition verifies the presence of this film (see the Supporting Information).
Therefore, the optimized ratio of EGaIn to thiol concentration
for this system is ≈400:1 by weight.
Compression tests were also conducted to see how thiols
affect the mechanical sintering phenomenon. As evident in
Figure 3, the required activation force (F) applied by the punchremains consistent for thiol concentrations ≤1 × 10−3 M. However, the significant size reduction associated with the 1 × 10−3 M
concentration samples results in a much lower activation force
per particle (f ≈ 25 nN). This value is significantly less than thatpredicted by the linear model for thiol-free EGaInNPs of thesame size shown in Figure 2d (168 nN). This observed reduction in activation force per particle indicates a combined effectof softening, weakening, or thinning of the EGaInNP surface inthe presence of the thiols. The rupture mechanism facilitating
the coalescence of the thiol-capped EGaInNPs is considered tobe the same as that of the oxide-encased particles. These results
suggest that the thiol coating ruptures more easily than the
Ga2O3 coating, and therefore the effective surface tension of
the thiol-capped particles must be lower than that of the Ga2O3
coated particles. This decrease in effective surface tension facilitates the production of smaller particles in the presence of thiol,
which also agrees with explanations given by Hohman et al.[18]In this regard, we expect size reduction in the presence of thiol
to occur across multiple sonication conditions. Furthermore, as
indicated by Figure 3b,c, the presence of excess thiol inhibits
the formation of percolation pathways for the EGaIn during
compression, resulting in increased activation forces and film
resistances, as well as their corresponding uncertainties.
Additional experiments were conducted to produce smallerfeatures by selective mechanical sintering. As shown in Figure 4,
arrays of smaller lines can be formed simply by scaling down
the sintering tool. In this study, 1 µm coalesced EGaIn lines
were formed, which are half the size of previously reported
features.[31] Reducing the sintering tool size even further may
achieve submicrometer features.
Mechanically sinterable EGaInNP dispersions also provide
a means for inkjet printing of EGaIn devices, a process notapplicable to the bulk material in an oxygen-containing environment.[32] However, by dispersing EGaInNPs, the resulting
material displays properties more closely resembling the inkjettable carrier solvent. As shown in Figure 5, this material
can be inkjet printed directly onto an elastomer glove surface
to form arrays of strain gauges with intricate wiring and contact pads. This result represents an opportunity to manufacture
liquid-metal devices using high-yield, high-throughput, andscalable processes.

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squeezed between two flat plates, yielding at high deformations.[29] Next, using the slope of the linear fit from Figure 2d(m = 0.78 ± 0.27 nN nm−1) enables us to calculate the MercadéPrieto fitting parameter (kε):[29]2357www.advmat.de

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Figure 3. Effects of thiol concentration. a) Representative SEM images of deposited samples prepared with different concentrations of thiol. Scalebars of main images are 10 µm in length. Scale bars of corresponding detail images are 2 µm in length. b) Thiol dependence of activation force for allconcentrations considered, with accompanying detail at lower concentrations. Error bars are two standard deviations in length. c) Thiol dependence of
resulting resistance across coalesced sample after activation. Inset shows details of dependence at lower concentrations. Error bars are two standard
deviations in length.

Figure 4. Small scale line coalescence. Setup used to write arrays of≈1 µm wide coalesced lines (top-left); detail view of an array of writtenlines (top-right), scale bar is 500 µm in length; detail view featuring asingle coalesced line (bottom-right), scale bar is 20 µm in length; anddetail view of single coalesced line revealing liquid-phase morphology(bottom-left), scale bar is 500 nm in length.

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To summarize, we have employed sonication and thiol selfassembly to create EGaInNPs with tunable average particle
sizes from ≈600 nm down to ≈180 nm. We have characterized
this material as a new class of mechanically sintered MNPs.
The EGaInNPs encased with Ga2O3 requires an activation force
per particle that scales with average particle size (sensitivity of≈0.78 nN nm−1), while capping EGaInNPs with thiol displays asoftening effect (≈15% smaller activation force per particle). In
this study, we also demonstrate two modes of mechanical sintering: global sintering, with the ability to sinter entire deposits
at or above the mm scale; and local/selective sintering, withthe ability to sinter areas within deposits as small as 1 µm andpotentially smaller. Finally, we also show applicability to inkjet
printed devices, a process not applicable to bulk EGaIn in anoxygen-containing environment. Future work may include
dynamic compression/rupture studies evaluating both individual particles and the complex interactions in particle films.
Other future investigations will include the application of thisprocess to create stretchable conductors, sensors, mechanicalswitches and integrated liquid circuits, with applications insensory skins, stretchable electronics, and active conformabledevices.

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Figure 5. Inkjet printing of EGaInNPs. a) Photograph of inkjet system printing EGaInNP dispersion. Scale bar is 5 mm in length. b) Human hand
wearing inkjet functionalized nitrile glove with arrays of strain gauges, intricate wiring, and contact pads comprised of EGaInNPs. c) Same hand holding
a tennis ball, demonstrating stretchability of the electronics.

Experimental Section
Preparation of EGaInNPs: All EGaInNP samples were made using
a QSonica Q700 microtip sonicator (part number 4417) at 30%
amplitude (96 µm) inserted a fixed distance (≈1 mm) above the bottomof a Kimble Chase 3 dram glass vial (See Figure S2a, Supporting
Information). All samples including thiol were sonicated for 60 min.
Prior to sonication, all samples were filled with a fixed mass (362 mg)of EGaIn (product number 495425 as purchased from Sigma Aldrich)using a syringe and a mass balance (OHAUS PioneerTM). Following this,
4 mL ethanol/thiol solutions were added to the vial by micropipetting(BioPette Plus BPP1000) a prescribed volume combination ofhighly concentrated ethanol/thiol solution and neat ethanol (Koptec
part number V1001) to achieve the desired concentration. Highconcentration thiol solutions were made by adding a measured amountof thiol (3-Mercapto-N-nonylpropionamid; part number 686492 aspurchased from Sigma Aldrich) mass to a glass vial, followed by theaddition of the appropriate volume of ethanol. The resulting mixture
was shaken vigorously followed by a 1 min bath sonication (Branson
1800) at room temperature to achieve uniformity. Each EGaInNP
dispersion sample was allowed to settle for 24 h. Just prior to creating
samples for particle characterization and compression testing, eachsample was shaken vigorously and magnetically stirred for 5 min
to ensure uniform dispersion of EGaInNP. Uniformity of EGaInNP
concentration was confirmed by measuring the post-evaporation massof 1 mL samples. Multiple dispersions (N = 5) were made for each setof conditions (thiol concentration and sonication time) for confidence.
High speed centrifugation was employed to rinse the 3 and 5 × 10−3 M
samples to confirm the presence of excess thiols. Sonicated samples
were subjected to 6238 relative centrifugal force in a centrifuge (Cole
Parmer WU-39065-05) for 20 min. After centrifugation, the aliquot was
discarded, fresh ethanol was added, and the pellet was resuspended ina light sonication bath (Branson 1800) for ≈3 min. This process was
repeated twice for each sample.
Plowing and Line Drawing: The plowed pile of EGaInNP chips shownin Figure 1a was formed by drop casting 50 µL of EGaInNP dispersion
onto a Si wafer (0.0035–0.0038 Ωcm Si wafers (Silicon Sense, Inc.)),followed by a line-carving step with a diamond scribe. The coalesced lineof liquid EGaIn was formed by gently tapping a similarly drop-casted filmof EGaInNPs with an X-Acto knife.
I–V Characterization: Samples for I–V characterization were made byspin-coating EGaInNPs/ethanol dispersions (0 × 10−3 M) onto Kapton
substrates. Two-probe measurements were conducted on the nonsintered samples by positioning electrical probes (Signatone SE-TZ)
via micromanipositioners (Signatone S-926) onto the sample a spacingof ≈1 mm apart tracking the supplied voltage and measured current
through a Keithley 2410-C SourceMeter. Voltage sweeps were performed
manually (See Figure S1, Supporting Information), waiting≈30 s for
signal stabilization after each step prior to recording.

Adv. Mater. 2015, 27, 2355–2360

Particle Characterization: Samples for particle characterization wereprepared by depositing 50 µL via micropipette (BioPette Plus BPP200)
onto Si wafers (Silicon Sense, Inc., 0.0035–0.0038 Ωcm) (See bottommiddle image of Figure S2b, Supporting Information for process).
Scanning electron microscope (SEM) images were then obtained (Philips
XL-40 FEI, 15 kV, 3 µm beam spot size). All images were analyzed
using the following procedure through ImageJ: (1) each raw grayscale
SEM image was converted to a binary image (Image → Adjust →Threshold, with a lower cutoff of ≈15 and an upper cutoff of ≈230
and Dark background option); (2) each resulting binary image wassegmented using the built-in ImageJ watershed routine (Process →Binary → Watershed); (3) the sizes for all segmented particles wereobtained using ImageJ’s particle analysis package (Analyze → Analyze
Particles, with size range going from 10 square pixel units to infinity,
circularity from 0 to 1, showing bare outline of particles); (4) the
diameter for each particle was computed from the area output by
ImageJ under a spherical approximation. This process was repeatedover multiple experimental trials (N = 5) for each formulation condition.
Energy-dispersive X-ray spectroscopy (EDS) was employed (FEI Quanta
3D FEG Dual-beam SEM with an Oxford INCA Xstream-2 silicon drift
detector with Xmax 80 window) to measure elemental composition insamples (results shown in Figures S3–S7, Supporting Information).
The SEM system used to inspect the morphology of the rinsed samples(results shown in Figures S8 and S9, Supporting Information) is thesame used for particle characterization on the thiol free and nonrinsed
samples.
Compression Experiments: Polydimethylsiloxane (PDMS) devices
containing copper wires were made to serve as EGaInNP reservoirs forthe compression tests (see Figure S2b, Supporting Information, far rightdetail image for photograph of device). Specifically, PDMS (Sylgard 184)
was mixed as received from Dow Corning with a 10:1 weight ratio ofbase to crosslinking agent. The resulting mixture was spincoated onto
glass slides, yielding a thickness of ≈350 µm. After fixing copper wires
to the cured PDMS films, a second coat of ≈200 µm was applied via
spincoating. Finally, samples were cut and circular reservoirs ≈200 µm
deep with a diameter of ≈2.5 mm were made in a single step using alaser ablation technique. The compression tests were conducted using
a materials tester. Each device was fixed to a platen loaded into a single
column materials testing unit (Instron model 3345) fitted with a 1 kN
load cell. The electrodes of the device were attached to a Wheatstonebridge, where the three other legs of the bridge were 20 Ω resistors
and the voltage supply was 3 V. The output voltage of the bridge wasconnected to the data acquisition of the materials tester and tracked
with applied force during the experiments. All samples were compressed
at a rate of 1 mm min−1. This process was repeated over multiple
experimental trials (N = 5) for each formulation condition.
Small Scale Line Drawing Experiments: Arrays of thin lines were
produced (See Figure 4 for photograph of setup) via mechanicalsintering by moving an EGaInNP-deposited sample (1 × 10−3 M) in© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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straight lines relative to a stationary mechanically cleaved Si needle(radius of needle ≈1 µm). The substrate motion relative to the needle
was controlled through a three-axis motion system (PI: 2 X M-531.DD,
M-501.PD, C-843.41). Prior to motion, contact between the Si needle
and the sample was observed through a 3D optical profiler (Zeta-20 witha customized tiltable turret).
Inkjet Printing Experiments: The ink used in the inkjet experiments were
0.5 × 10−3 M thiol samples sonicated at 60 min and filtered through a 1 µm
polytetrafluoroethylene syringe filter (Cole-Parmer 32816-34). Substrates
employed for these experiments were purple nitrile gloves (KimberlyClark) precleaned with ethanol. All inkjet printing experiments wereconducted using a single ≈65 µm orifice nozzle from an HP thermal inkjet
picojet system. Samples were manipulated via two-axis stage (AnoradXKY-C-150-150-AAA0 stage controlled by Anorad-CM-2 controller).
Printing commands were employed through in-house software.

Supporting Information
Supporting Information is available from the Wiley Online Library orfrom the author.

Acknowledgements
We would like to thank Professor Dimitrios Peroulis and Dr. Nithin
Raghunathan for access to and training on electrical characterizationequipment. Many thanks to Professor George T.-C. Chiu for access tothe inkjet system employed in this study and to the HP Corvallis teamfor providing the inkjet print-head system. We would also like to thankMichelle Yuen for help with ImageJ and photography and Jennifer
Case for finding literature. Special thanks to Alex Bottiglio for cover art
work. Additional thanks to Chia-Ping Huang of the Purdue Life Science
Microscopy Facility and to Dr. Seok-Hee Hyun for technical support on
the high resolution EDS system. E.L.W. is supported by the National
Science Foundation Graduate Research Fellowship Program underGrant No. DGE-1333468. Any opinions, findings, and conclusions or
recommendations expressed in this material are those of the authorsand do not necessarily reflect the views of the National Science
Foundation.
Received: October 16, 2014
Revised: December 22, 2014
Published online: February 27, 2015

[1] M.-C. Daniel, D. Astruc, Chem. Rev. 2004, 104, 293.
[2] C. J. Murphy, T. K. Sau, A. M. Gole, C. J. Orendorff, J. Gao, L. Gou,
S. E. Hunyadi, T. Li, J. Phys. Chem. B 2005, 109, 13857.
[3] L. Zhang, F. X. Gu, J. M. Chan, A. Z. Wang, R. S. Langer,
O. C. Farokhzad, Clin. Pharmacol. Ther. 2008, 83, 761.
[4] S. Paul, C. Pearson, A. Molloy, M. A. Cousins, M. Green,
S. Kolliopoulou, P. Dimitrakis, P. Normand, D. Tsoukalas,
M. C. Petty, Nano Lett. 2003, 3, 533.

2360

wileyonlinelibrary.com[5] K. Szot, W. Speier, G. Bihlmayer, R. Waser, Nat. Mater. 2006, 5, 312.
[6] W. Guan, S. Long, M. Liu, Z. Li, Y. Hu, Q. Liu, J. Phys. D: Appl. Phys.
2007, 40, 2754.
[7] J. Park, J. Joo, S. Kwon, Y. Jang, T. Hyeon, Angew. Chem. Int. Ed.
2007, 46, 4630.
[8] K.-S. Moon, H. Dong, R. Maric, S. Pothukuchi, A. Hunt, Y. Li,
C. P. Wong, J. Electron. Mater. 2005, 34, 168.
[9] T. Bhuvana, W. Boley, B. Radha, B. D. Dolash, G. Chiu,
D. Bergstrom, R. Reifenberger, T. S. Fisher, G. U. Kulkarni, Micro
Nano Lett. 2010, 5, 296.
[10] J. Perelaer, M. Klokkenburg, C. E. Hendriks, U. S. Schubert, Adv.
Mater. 2009, 21, 4830.
[11] D. Angmo, T. T. Larsen-Olsen, M. Jørgensen, R. R. Søndergaard,
F. C. Krebs, Adv. Energy Mater. 2013, 3, 172.
[12] J. Chung, S. Ko, N. R. Bieri, C. P. Grigoropoulos, D. Poulikakos,
Appl. Phys. Lett. 2004, 84, 801.
[13] I. Reinhold, C. E. Hendriks, R. Eckardt, J. M. Kranenburg, J. Perelaer,
R. R. Baumann, U. S. Schubert, J. Mater. Chem. 2009, 19, 3384.
[14] S. Magdassi, M. Grouchko, O. Berezin, A. Kamyshny, ACS Nano
2010, 4, 1943.
[15] M. Grouchko, A. Kamyshny, C. F. Mihailescu, D. F. Anghel,
S. Magdassi, ACS Nano 2011, 5, 3354.
[16] S. B. Walker, J. A. Lewis, J. Am. Chem. Soc. 2012, 134, 1419.
[17] M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui, S. Yamakoshi,
Appl. Phys. Lett. 2012, 100, 013504.
[18] J. N. Hohman, M. Kim, G. A. Wadsworth, H. R. Bednar, J. Jiang,
M. A. LeThai, P. S. Weiss, Nano Lett. 2011, 11, 5104.
[19] W. Zhang, J. Z. Ou, S.-Y. Tang, V. Sivan, D. D. Yao, K. Latham,
K. Khoshmanesh, A. Mitchell, A. P. O’Mullane, K. Kalantar-zadeh,Adv. Funct. Mater. 2014, 24, 3799.
[20] V. Sivan, S.-Y. Tang, A. P. O’Mullane, P. Petersen, N. Eshtiaghi,
K. Kalantar-zadeh, A. Mitchell, Adv. Funct. Mater. 2013, 23, 137.
[21] J. Ma, G. Jing, Phys. Rev. E 2012, 86, 061406.
[22] A. G. Mailer, P. S. Clegg, J. Colloid Interface Sci. 2014, 417, 317.
[23] M. D. Dickey, R. C. Chiechi, R. J. Larsen, E. A. Weiss, D. A. Weitz,
G. M. Whitesides, Adv. Funct. Mater. 2008, 18, 1097.
[24] K. S. Suslick, Science 1990, 247, 1439.
[25] H. H. Kausch, D. G. Fesko, N. W. Tschoegl, J. Colloid Interface Sci.
1971, 37, 603.
[26] Z. Zhang, R. Saunders, C. R. Thomas, J. Microencapsulation 1999,
16, 117.
[27] R. C. Chiechi, E. A. Weiss, M. D. Dickey, G. M. Whitesides, Angew.
Chem. Int. Ed. 2008, 47, 142.
[28] M. J. Regan, H. Tostmann, P. S. Pershan, O. M. Magnussen,
E. DiMasi, B. M. Ocko, M. Deutsch, Phys. Rev. B. 1997, 55,
10786.
[29] R. Mercadé-Prieto, R. Allen, D. York, J. A. Preece, T. E. Goodwin,
Z. Zhang, Chem. Eng. Sci. 2011, 66, 1835.
[30] R. A. Caruso, M. Ashokkumar, F. Grieser, Langmuir 2002, 18,7831.
[31] B. A. Gozen, A. Tabatabai, O. B. Ozdoganlar, C. Majidi, Adv. Mater.
2014, 26, 5211.
[32] J. W. Boley, E. L. White, G. T.-C. Chiu, R. K. Kramer, Adv. Funct.
Mater. 2014, 24, 3474.

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