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Reconfigurable Positioning of Vertically-Oriented Nanowires Around Topographical Features in an AC Electric Field Sarah Boehm, Lan Lin, Nermina Brljak, Nicole Famularo, Theresa S. Mayer, and Christine D. Keating Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02163 • Publication Date (Web): 15 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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Reconfigurable Positioning of VerticallyOriented Nanowires Around Topographical Features in an AC Electric Field

Sarah J. Boehm,§ Lan Lin,†ɣ Nermina Brljak,§ Nicole Famularo,§ Theresa S. Mayer,†‡ and Christine D. Keating§* Departments of §Chemistry and †Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States. ‡Virginia Tech University, Blacksburg, Virginia 24060, United States. *Address correspondence to [email protected] KEYWORDS directed assembly, surface patterning, AC field, gold

ABSTRACT

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We report the effect of topographical features on gold nanowire assemblies in a vertically-applied AC electric field. Nanowires 300 nm in diameter × 2.5 µm long, and coated with ~30 nm silica shell, were assembled in aqueous solution between top and bottom electrodes, where the bottom electrode was patterned with cylindrical dielectric posts.

Assemblies

were

monitored

in

real

time

using

optical

microscopy.

Dielectrophoretic and electrohydrodynamic forces were manipulated through frequency and voltage variation, organizing nanowires parallel to the field lines, i.e., standing perpendicular to the substrate surface. Field gradients around the posts were simulated and assembly behavior was experimentally evaluated as a function of patterned feature diameter and spacing. The electric field gradient was highest around these topographic features, which resulted in accumulation of vertically-oriented nanowires around the post perimeters when dielectrophoresis dominated (high AC frequency) or between the posts when electrohydrodynamics dominated (low AC frequency). This general type of reconfigurable assembly, coupled with judicious choice of nanowire and post materials/dimensions, could ultimately enable new types of optical materials capable of switching between two functional states by changing the applied field conditions.

INTRODUCTION Control over nanowire position and orientation is critical for many envisioned optical and electrical applications based on these particles.1-4 Responsive device architectures that can be reconfigured between different particle organizations are appealing for a wide range of switchable devices such as bioelectrocatalysts and optical filters.5-7 New approaches are needed that can provide dynamic positional and orientational control for populations of nanowires. Here, we report an assembly strategy that combines a vertical 2 ACS Paragon Plus Environment

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applied alternating current (AC) electric field to orient gold nanowires with dielectric (photoresist) features that shape the in-plane field to drive their positioning relative to these features. Assemblies of vertically-oriented nanowires adopt different organizations in response to changes in applied frequency. The ability to switch between two assembled states could enable on-demand switching of optical properties, for example, wrapped metasurfaces that control light scattering profiles around an object8,9 to cloak and reveal it. Control over the orientation of anisotropic particles such as nanowires provides a means of controlling the optical response of these particles5,7,10-14 and is additionally important in applications such as solar cells,2,15-20 batteries,21-23 and nanoelectronics1,3,24-26. Orientation of nanowire populations has been achieved a wide range of methods ranging from templated substrates, directional flow, controlled drying, or simply high particle number density to applied magnetic or electric fields.1,3,27-29 These examples have focused primarily on orientation of nanowires that are laying with their long axis parallel to the substrate, for example to span electrodes or form horizontal arrays of parallel wires. Vertically-oriented nanowires, with long axes perpendicular to a substrate, are straightforward to produce during synthesis by growth or etching from a substrate,1,2,30 but such structures have been less-studied for self-assembly of pre-existing nanowires from solution. Approaches to vertical nanowire assembly have included controlled drying, particle designs that take advantage of gravity and interparticle forces, substrate patterning with microwells smaller than the nanowire length, and applied magnetic or electric fields.6,31-43 Of these, strategies based on applied electric field are particularly versatile in terms of particle size and composition while also offering the possibility of 3 ACS Paragon Plus Environment

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dynamic reconfigurability by changing the applied field conditions. We therefore chose to take advantage of applied electric fields in the work described here. In order to also achieve control over the lateral positioning of nanowires, we coupled this approach by patterning the planar electrode surfaces with photoresist microfeatures. Top-down fabrication techniques are often employed when spatial precision is required, but are typically limited to producing static structures with singular functionality. Combinations of top-down and bottom-up methods are increasingly prevalent, particularly when surface patterning is employed to assemble solutionsynthesized particles.27,44,45 Chemical patterning, in the form of charged or hydrophobic/hydrophilic coatings has been used to localize particles.46-52 Topographical features, such as microwells and microposts, have been employed alone33,53-56 or coupled with capillary forces and controlled drying57-67 to assemble particles in or around features with excellent control. An example from Wolf and coworkers demonstrated high precision registry of nanorods in patterned wells with up to a 97% yield over a large area array.66 In an applied electric field, the presence of topographical features alters the field gradient, which can provide additional control over particle assembly. For example, we have used microwells to provide submicrometer lateral positioning of single nanowires assembled across parallel coplanar electrodes.68,69 Dielectric topographical features are commonly used in dielectrophoretic manipulation of biological samples, and have been used for on-chip separations of biomolecules, cells, and particles (e.g., “insulating” DEP).70-74 Here, we report vertically-oriented gold nanowire assemblies formed around lithographically patterned cylindrical posts in AC electric fields. Dielectrophoresis (DEP) 4 ACS Paragon Plus Environment

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aligned the nanowires parallel to the field lines, standing perpendicular to the substrate surface. The patterned features acted as electric field gradient hot spots, which caused particles to assemble around the post perimeters. Electric field profile simulations confirmed high field gradients along the circumference of the posts and low field gradients in the regions between posts. These nanowire assemblies could be reconfigured in two distinct ways: (1) turning the field on/off switched the nanowires between standing and laying orientations and (2) switching between high and low applied frequency conditions organized vertical wires around the patterned posts via DEP or in between posts via electrohydrodynamic (EHD) flow, where field gradients were weakest. RESULTS AND DISCUSSION Our experimental setup, shown in Figure 1, enabled simultaneous particle assembly and observation. The gold-coated glass cover slip assembly surface was lithographically patterned with cylindrical photoresist posts (Figure 1A). Gold nanowires (300 nm diameter, 2.5 µm long, Figure 1B) were synthesized by electrodeposition into porous alumina membranes,75-78 and coated with a ~30 nm silica shell to prevent irreversible aggregation.79 Aqueous suspensions of nanowires (ca. 5×108 NW/mL) were assembled in a 120 µm thick sample chamber between a top indium tin oxide (ITO) electrode and a bottom gold electrode patterned with photoresist posts (Figure 1C). AC voltage was applied across the two electrodes, perpendicular to the substrate surface. For a given experiment, wires were allowed to sediment to the surface for two minutes once the assembly chamber was sealed, ensuring all wires were at the bottom of the sample chamber before electric field application.


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Figure 1. Scheme depicting experimental setup. (A) Transmitted light optical microscope image of photoresist posts patterned on a gold-coated glass cover slip. Note, 30 nm gold coating was partially transparent, thus enabling imaging from below. (B) Transmission electron microscope image of 2.5 µm gold nanowires. (C) Side view schematic of experimental set up (not to scale). Photoresist posts patterned onto a gold-coated glass cover slip, which served as the bottom electrode. An indium tin oxide (ITO) coated glass cover slip served as the top electrode. The double-sided tape spacer contained the nanowire suspension. The AC electric field was applied across the two electrodes, perpendicular to the assembly surface.

Photoresist posts ~4 µm in height were chosen for ease of fabrication and because this height exceeds the length of our gold nanowires (~2.5 µm long). Due to the size and density of the gold wires, these particles sediment onto the bottom of their sample cell and even when orienting vertically, will maintain one end on the substrate. A variety of post diameters (2.8-19 µm) and spacings (7.7-52 µm between posts) were investigated as a means of altering the local electric field distribution. We first simulated the field gradients and then performed assemblies using these patterned substrates. Electric Field Profile Simulations COMSOL Multiphysics software was used to simulate the electric field gradient for three different post diameter and spacing conditions that correspond to experimental


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samples. Areas of highest field gradient were observed around the post perimeters, while lowest field gradient regions were centered between and atop posts (Figure 2). Posts with the same diameter but different spacing (Figure 2A,B) had noticeably different field gradient profiles, particularly in the regions between posts. When the features were further apart (Figure 2A), much of the interstitial areas between posts had comparatively low field strength. Larger diameter features with smaller spacing (Figure 2C) influenced the electric field gradient over a larger area, and the lowest relative field gradient was only found atop the post. Despite these differences, the general field gradient profiles are similar across the range of feature sizes and spacings evaluated, which in all cases showed a region of highest field gradient around the posts, suggesting that particle assembly could be directed to this region by positive DEP.

Figure 2. Electric field gradient simulations produced using COMSOL Multiphysics software. Patterned photoresist posts as seen from 50 nm above the post tops appear as circles with high field gradient around the perimeter (red) and significantly lower field gradient in the center of the post (dark blue). Field gradient in V2/m3 is shown on a log scale. Post diameter and edge to edge spacing are as follows: (A) 8.4, 21.1 µm; (B) 8.4, 11.2 µm; and (C) 19.3, 7.7 µm. All scale bars are 10 µm.


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Nanowire Assembly Around Patterned Features Prior to electric field application, nanowires diffused freely with their long axes parallel to the substrate surface, as shown in Figure 3A,B. AC voltage and frequency were gradually increased until the wires aligned with their long axes parallel to the field lines, standing perpendicular to the substrate surface. Assemblies were imaged in situ, from below using transmitted light optical microscopy, where standing nanowires appeared as black dots against the bright background. Figure 3A shows the assembly of nanowires between two un-patterned ITO-coated cover slips in the absence (left) and presence (right) of the applied AC field. Similar assemblies in sample chambers containing a patterned bottom electrode showed preferential particle organization around the photoresist post perimeter when the field was on (Figure 3B). Illustrations in Figure 3C,D depict bottom and side views of these nanowire assemblies. While nanowires formed vertical assemblies both with and without patterned features, the posts facilitated spatial localization and organization of the particles.

Figure 3. Effect of photoresist posts on vertical nanowire assembly in an AC electric field. Top panels show optical micrographs of 2.5 µm gold nanowire assemblies between 8 ACS Paragon Plus Environment

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top and bottom electrodes, imaged from below as illustrated in Figure 1C, both in the absence (A), and in the presence of (B) patterned photoresist posts. When the field was off (left images), nanowires laid parallel to the substrate. When the field was turned on (right; 330 V/cm, 750 kHz), the nanowires stood perpendicular to the substrate and appeared as black dots (C, D). Side view illustrations of nanowire assemblies formed on unpatterned substrates (C) and around posts (D) when the field was applied. Electric field directions denoted on right of illustrations.

The nanowire assembly behavior was examined as the field was switched on/off/on at different field conditions. Experimental videos were collected and analyzed to determine the effect of field strength, feature size, and feature spacing on the resulting assemblies. Figure 4A-E shows still frames taken from Movie S1 of 9.5 µm diameter posts, spaced 11.0 µm apart. A corresponding plot details the number of nanowires standing/laying in each frame over time (Figure 4F). The electric field was on for at least two minutes at the desired field conditions (500 V/cm, 750 kHz for Movie S1), prior to movie collection, to allow the nanowires sufficient time to assemble around the patterned features. We set time equal to 0 s as the start time of the movie; the field was turned off at 5 s and turned on again at 16 s. When the field was on for upwards of two minutes, 99.4% of visible wires were standing and appear as dots in the images. When the field was turned off (Figure 4B), nanowires fell down, laying parallel to the substrate and appearing as rods in the image. At 500 V/cm and 750 kHz, the field strength was sufficiently strong enough to cause 95.8% of the wires to stand within 1 s of turning the field back on (Figure 4C). Over time, an increased number of particles assembled around the posts, often clustering in groups that appeared to contain two or more wires.


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Figure 4. Progression of nanowire assembly around posts after the field was turned off and then reapplied. (A-E) Optical micrographs of 2.5 µm gold nanowire assemblies between top and bottom electrodes, imaged from below. “Field on” conditions were 500 V/cm, 750 kHz. (F) Plot of visible standing (blue) and laying (red) nanowires in selected still frames taken from Movie S1. The 0 s data point on the plot corresponds to the image in panel (A), and the grey box highlights the time frame during which field was turned off. Images in (A-E) are cropped from still frames from Movie S1 and correspond to each point on the plot.

Reconfigurable Assembly Behavior The effect of applied frequency on the nanowire assembly behavior was studied for the largest diameter posts (19 µm, 7.7 µm spacing). At frequencies ≥100 kHz particles organized around the posts, in areas of highest field gradient, which was indicative of positive DEP driven assembly.80 As the frequency was decreased from 100 to 50 kHz, the majority of nanowires moved from post edges to standing throughout the regions void of features (Figure 5B). From 50-25 kHz, only a few wires remained at the post perimeters, while the majority stood in the center of the areas between posts (Figure 5C). When frequency was decreased from 25-15 kHz (Figure 5D), the standing wires were concentrated closer together. At 5 kHz, the fluid force was strong enough to cause the majority of wires laid down (Figure 5E). Our simulations indicate low field gradients in the sites between each set of four posts and atop each post; for this post diameter, gradients are smallest atop the posts (Figure 2). Negative dielectrophoresis can concentrate particles in low field gradient regions, however the absence of nanowires atop the posts argues against this explanation. Indeed, we have not observed negative 10 ACS Paragon Plus Environment

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DEP for metallic particles of these dimensions and frequencies in other electrode geometries. Figure S1 and Movie S2 show movement of fluorescent tracer particles towards, and some accumulation of tracers at, these sites at 5 kHz but not at 100 kHz. Therefore we attribute the observed assembly behavior to EHD flow concentrating particles in stagnation points where the field gradient was weak.80 The observed particle accumulation between features rather than atop posts at low frequency is consistent with EHD because fluid stagnation points occur at the lowest topographical regions. The cartoon in Figure 5F illustrates the side view of these assemblies under high and low frequency conditions. Switching between DEP and EHD enabled the reversible assembly of nanowires around and in between posts. The reversible and reconfigurable control of assemblies, as we show here, could be beneficial for future optical and electronic applications.

Figure 5. Nanowire assemblies could be reconfigured by changing the applied frequency. (A-E) Transmitted light, inverted optical micrographs of a 2.5 µm gold nanowire assembly as frequency was decreased, with constant voltage (330 V/cm). (A) At frequencies ≥100 kHz, dielectrophoretic (DEP) forces dominated, causing the nanowires to assemble in the regions of highest field strength, around the posts. (B-D) As frequency was decreased, electrohydrodynamic (EHD) flow dominated, causing fluid flow to move the wires to stagnation points, where the field was weakest, between the posts. (E) The decreased frequency concentrated standing wires closer together, and eventually forced


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the majority of particles to lay down. (F) Side view illustrations of nanowire assembly at high and low frequency.

Analysis of Clustering Behavior Upon closer inspection of assembly videos where the field was switched on/off/on, we saw an increased number of wires after the field was turned off. Indeed, Figure 4F shows roughly 20% more total particles could be counted with the field off as compared to on. Wires that appeared to be single particles when the field was on were sometimes found to be two or more wires when the field was turned off; this scenario can be seen in group i of Figure 6A. It was also common to see dimers when the field was on, and then 3 or 4 wires in the dimer’s place upon turning the field off, highlighted as group ii in Figure 6A. These observations suggest that the nanowires chain vertically in the electric field, taking on a staggered arrangement to satisfy dipolar interparticle interactions. Only those wires closest to the coverslip are visible when the field is on; any chaining particles that are offset in the z direction are not in the focal plane and therefore cannot be seen until the field is switched off and they fall to the surface. This staggered chaining of nanowires in an applied ac field is reminiscent of the horizontal 2D lattices that we have reported previously for gold nanowires in an AC field that was applied horizontally between coplanar electrodes.79 There, nanowires formed a staggered, running-bond brickwork structure to minimize repulsive dipolar interactions from neighboring wires. The staggered chains formed here are considerably less particle-dense than in references 79

and

5

due to the different electrode geometries but appear to otherwise show similar

interparticle interactions.


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Figure 6. Nanowire clustering behavior. (A) Still frames showing examples of wire clusters that appear to contain less wires when the field is on (left) than when the field is off (right). Illustrations of nanowire organization within these clusters, viewed from the side to emphasize the vertical offset between particles that allows the particles to satisfy dipolar interactions. Box i highlights an apparently single wire (A, left) that was shown to be two wires when the field was turned off (A, right). Box ii highlights an apparent dimer (A, left) that was actually three wires when the field was turned off (A, right).

Experimental videos were used to quantify the nanowire clusters in two ways: (1) counting the number of wires visible in still frames when the field was on, turned off, and turned back on (Figure 7A, Movies S1, S3-S5), and (2) characterizing individual clusters by counting the number of wires visible in a given cluster when the field was on and when the field was subsequently turned off (Figure 7B, Movies S4-S6). We can see from Figure 7A that approximately one third of the particles were not visible when the field was initially on at t = 4 s, as compared to when the field was off at t = 11 s. This observation is consistent with some of the particles assembling above the focal plane. Once the field was turned on again, the number of nanowires decreased and approached the number of wires visible prior to turning the field off. This behavior was consistent for both post diameters (9.5 and 19 µm) at 170 and 500 V/cm. For both post sizes, the higher voltage condition (500 V/cm) facilitated a faster recovery to vertical chaining than when 13 ACS Paragon Plus Environment

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170 V/cm was used. Videos of the larger posts at constant frequency (750 kHz) and three voltages (170, 330, 500 V/cm) were used to identify trends in the cluster behavior. We grouped observations by number of nanowires visible in a cluster when the field was on, with the number of nanowires in that same cluster when the field was turned off. For example, group i in Figure 6 appeared to be a single nanowire when the field was on, but was in fact two nanowires. This observation was classified as 1→2 as shown in the histogram in Figure 7B. Similarly, group ii in Figure 6 was classified as 2→3. Figure 7B shows the distribution of the most common occurrences across the three voltages studied. At the two lower field strengths, the majority of the nanowires fell into one of two categories: 1→1 and 2→3. At 500 V/cm, there were few nanowires in the 1→1 category, with the majority in the 2→3 category. It was also observed that free clusters formed more readily in areas where features were spaced farther apart.


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Figure 7. Quantification of visible nanowire counts, as viewed from below, with field on and off. (A) Analysis of still frames from Movies S1-S4 of nanowires visible when the field was on (t = 4 s), turned off (t = 11 s), and turned back on again (t = 17, 30, 40 s) for 9.5 and 19 µm diameter posts at 170 and 500 V/cm, 750 kHz. Grey box highlights region where field was off. (B) Distribution of nanowire cluster classifications for 19 µm diameter posts at three different field strengths, but constant frequency (750 kHz). Data collected from Movies S3-S5. Classifications contain two numbers, where the first is how many wires a cluster appears to be when the field was on, and the second is how many wires fell down from that same cluster when the field was turned off (i.e. 2 to 3 indicates a dimer that consisted of three wires when the field was turned off).

As a comparison, an analogous sample chamber consisting of two ITO coated cover slips without photoresist features (electric field was still applied vertically) was used to examine the clustering behavior. At 330 V/cm, 750 kHz, 33.5% of occurrences were 1→1, 27.7% were 1→2, and 23.6% were 2→3. These results, shown in Figure S2, were similar to the patterned sample at the same field conditions (Figure 7A), where the same categories had 40.1%, 16.1%, and 30.4%, respectively. In the same experiment, the field was left on at 830 V/cm and 10 MHz for >2 hours. When the field was turned off, a small population of nanowire clusters were found to be irreversibly stuck together, but moved freely in solution; an example is shown in Figure S3. The observed chain-link structure further validates the hypothesis that the nanowires formed staggered, vertical chains in the electric field to satisfy interparticle dipolar interactions. Effect of Feature Size and Field Strength Posts with sizes ranging from 2.8 to 19 µm in diameter were studied. The most uniform assemblies were formed with 9.5 and 19 µm diameter posts with spacing of 11.0 and 7.7 µm, respectively, and are used for the remainder of the discussion. To quantify the nanowire assembly behavior, we identified and categorized five different types of


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particle organization, as shown in Figure 8A. The red boxes labeled 1 and 2 indicate single nanowires standing freely or around the post perimeter, respectively. The red boxes labeled 3 and 4 show clusters of nanowires standing freely or around the post perimeter, respectively. The arrow labeled 5 points to a shadow on the top of the post, which corresponds to a portion of a nanowire that is above the focal plane, laying along the top of the post. Figure S4 shows images taken at different focal points confirming that these shadows correspond to portions of nanowires that are above our normal focal plane. Experimental videos were used to quantify the average number of single standing nanowires around a post and the average number of clusters assembled around a post. Figure 8B,C shows the analysis of Movies S1-S4, where the field was switched on/off/on in assembly regions containing 9.5 and 19 µm diameter posts. Applied frequency was held constant at 750 kHz, and two different voltages, 170 V/cm, and 500 V/cm, were examined. For the 9.5 µm posts (blue and red traces in Figure 8B,C), all nanowires were standing before the field was turned off and the majority of nanowires around the posts were in clusters (78% for 170 V/cm, 90% for 500 V/cm), as opposed to single wires. Wires found in the areas between posts, however, were populated mainly by single wires (88% for 170 V/cm, and 81% for 500 V/cm). Plots containing data for the number of free single nanowires and the number of free clusters can be found in Figure S5. When the field was turned off, >98% of particles fell down, laying parallel to the substrate, for both field conditions. Upon reapplication of the field at the lower voltage, 170 V/cm, the majority of wires remained laying, with ca. one quarter of the particle population standing as single wires. For the higher field strength (500 V/cm), >95% of the wires were standing within 1 s of turning the field back on and single nanowires were observed


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assembling around the posts. Over time, both field conditions produced assemblies where the number of single nanowires decreased as wires preferentially formed clusters, both around the post perimeters and in the regions between posts.

Figure 8. Analysis of nanowire assembly behavior from Movies S1, S3-S5. (A) Optical image of a single post with labels showing five distinct nanowire behaviors: single nanowire standing freely (1) or around the post (2), cluster of nanowires standing freely (3) or around the post (4), and arrow points to shadow of nanowire on top of the post (5). Electric field was on for >2 minutes prior to movie collection. t = 0 s corresponds to the start of the movie. The region of the plot where the field was off is highlighted by the grey box. Still frames at five time points were examined by counting the number of standing nanowires categorized in two ways: (B) average number of single nanowires per post, and (C) average number of clusters per post. Red and blue traces correspond to 9.5 µm diameter posts at 170 and 500 V/cm, respectively. Green and black traces correspond to 19 µm diameter posts at 170 and 500 V/cm, respectively. Note, black trace stops at 30 s because fewer frames were collected for Movie S4.

The green and black traces in Figure 8 show the assemblies around the larger, 19 µm diameter features spaced 7.7 µm apart, which followed similar trends as those around the smaller posts. Nanowires laid down when the field was off and stood as single wires around and between posts once the field was reapplied. The average number of single nanowires and clusters organized around the larger posts was higher than for the 9.5 µm


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posts at both field conditions. At t >30 s, there were significantly more clusters per post for the large posts (13 for 170 V/cm, and 10.5 for 500 V/cm) than the smaller ones (3.1 for 170 V/cm, and 3.4 for 500 V/cm). These observations correspond well with the trends shown in Figure 7, as high field strengths typically produced more clusters than single nanowires. Additionally, the higher field strength (500 V/cm) greatly increased the rate at which the particles stood up, as well as assembled around the patterned features, regardless of post diameter. The observed experimental trends showed larger diameter posts with smaller spacing between posts assembled the most nanowires, with very few free wires in the regions between posts. The larger post assemblies also recovered faster upon reapplication of the field. Effect of Post Diameter on Number of Particles per Post and Their Spacing We quantified the distribution of the number of nanowires and their spacing around patterned posts. Although we anticipate that more uniform spacings will be observed for high nanowire concentrations (see Figure S4), such conditions can also result in greater numbers of nanowires in the regions between posts. Therefore in these experiments a much lower particle concentration (10% of that used in all experiments above) was used to ensure all wires present were assembled around posts. Wires assembled alone, in clusters, and as single nanowires on top of the post. Since it was not always apparent how many wires made up a group, each of the aforementioned assembly types were deemed “groups” to aid quantification. In this way, the number of groups around each post were counted for 9.5 and 19 µm diameter posts at 500 V/cm, 750 kHz (Figure 9A). The median number of groups around the two post sizes was found to be 7 and 8, respectively. The edge-to-edge distance between groups for the 19 µm posts surrounded 18 ACS Paragon Plus Environment

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by 7, 8, and 9 groups (>60% of all posts) was measured using the segmented line tool in NIH ImageJ to follow the post curvature. Figure 9B-D shows the spacing measurement distributions. Posts with 7 groups had the broadest spacing distribution of the three, and a median spacing of 7.9 µm. Posts with 8 and 9 groups had a narrower spacing distribution and medians of 6.7 and 5.6 µm, respectively. Particle spacing became more uniform with increased particle concentration. For a small sample size of 19 µm diameter posts with 22 groups around the perimeter, the average spacing was measured to be 1.4 ± 0.5 µm. Increased control of particle spacing could be useful for optical applications such as metamaterials.7,81,82

Figure 9. Distribution of number of particle groups per post and their spacing. Groups were defined as clusters and single nanowires around the post perimeters as well as wires laying on top of posts. (A) Number of groups around a given post at 500 V/cm, 750 kHz, for 9.5 (red) and 19 µm (blue) diameter posts. (B-D) Distribution of spacing between groups around a 19 µm post for (B) 7 groups per post, (C) 8 groups per post, and (D) 9 groups per post, at the same field conditions described for (A).

CONCLUSIONS Reconfigurable vertical assemblies of gold nanowires were achieved using AC electric fields and patterned surface topography. We found that large features with close spacing assembled the most particles around the post perimeters, with wires preferentially


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organized in clusters, particularly at higher voltages. Uniform spacing of particles around features was achieved at high nanowire concentrations. This general assembly approach could be useful for future applications such as wrapped metasurfaces83,84 in which metallic elements are assembled around a central cylinder to alter its scattering profile. In such

a

case,

the

central

feature

to

be

cloaked/revealed

by

metasurface

wrapping/unwrapping could in principle direct metasurface assembly around itself due to its impact on the electric field gradient. Particle orientation, position, and clustering would all impact optical response; understanding these aspects of nanowire assembly behavior is a first step towards rational design of such structures. More generally, incorporation of micropatterned dielectric features provides an additional level of control over local electric field gradients, which can find use in a wide variety of assemblyfocused efforts in which particle placement, orientation and/or local concentration are important. METHODS Nanowire Synthesis Nanowires were synthesized using previously reported methods of templated electrodeposition of metal salts (Orotemp 24 and Silver Cyless R plating solutions, Technic Inc.) into an anodic alumina membrane with a nominal pore diameter of 0.2 µm (Whatman).76-78 Nanowire length was dependent on deposition time and diameter was fixed by pore size. After release from the membrane using a 3M NaOH solution, wires were subsequently coated with a thin layer of amorphous silica (~30 nm) in a modified sol-gel method (tetraethoxy silane, Gelest).75 All wires were characterized by


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measurement of TEM images using NIH ImageJ, collected with a JEOL JEM 1200 EXII at 80 keV using an ORIUS® 830 SC200 CCD camera. The nanowire concentration was determined using a Hausser Scientific Neubauer Hemocytometer. Device Fabrication Circular glass cover slips (35 mm diameter, number 2 thickness, GlycoTech) were rinsed with DI water, ethanol, and dried with nitrogen gas. A Kurt Lesker Lab-18 electron beam evaporator was used to deposit 15 nm Ti, then 30 nm gold onto clean glass cover slips. The gold-coated substrates were then patterned using photolithographic processes. Hexamethyldisiloxane (HMDS, MicroChem) was spun onto the substrate and baked at 105°C for 2 min. SPR 955 (MicroChem) was then spun using the following program (ramp rates in parentheses): resist was deposited during a 10 s dynamic period at 500 RPM (1,000 RPM/s), spread for 1 s at 100 RPM (1,000 RPM/s), thinned for 35 s at 1,200 RPM (10,000 RPM/s), and finished for 1 s at 100 RPM (1,000 RPM/s). The coated samples were then baked at 105°C for 2 min, then exposed for 0.6 s using a GCA8000 iline Stepper tool. Samples were developed in MF-CD-26 (2.4% tetramethylammonium hydroxide solution, MicroChem) for 90 s and rinsed in a water bath for 1 min. The substrates were then exposed for 10 min using an OAI Deep UV Flood Exposure tool to crosslink the remaining resist, then baked at 105°C for 10 min. Patterned post dimensions (diameter and spacing) were measured from optical images using NIH ImageJ. Nanowire Assembly Ethanol was used to open a small window on the patterned coverslip in order to make electrical contact to the underlying gold electrode. The circular spacer (4.5 mm diameter, 21 ACS Paragon Plus Environment

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120 µm thick) was cut from Secure Seal spacers (Life Technologies) to make a sealable assembly chamber surrounding the patterned posts. Nanowire solution (2.75 µL) was pipetted into the center of the spacer and sealed with an indium tin oxide-coated cover slip (8-12 Ω, SPI) which served as the top electrode. Top and bottom electrodes were attached to an Agilent 33210A 10 MHz Function/Arbitrary Waveform Generator via gold wires (Ametek Electronic Components and Packaging, Coining) and silver conductive adhesive (Electron Microscopy Sciences) that led to conductive plates where leads of the function generator (not terminated in 50 Ω) could be secured. Assemblies were imaged in real time using a Nikon TE300 inverted optical microscope with a halogen lamp for transmission imaging. The assembly concentration for each nanowire solution was set to 5 × 108 NW/mL, ca. 0.6% standing nanowire coverage of the assembly area. During nanowire assembly, voltage and frequency were slowly increased until all visible nanowires were standing. We aimed to use relatively low field strengths to study the behavior of the standing nanowires, as to avoid irreversible aggregation of particles that can be caused by high field strengths. Electric field conditions of 500 V/cm, 750 kHz produced standing nanowire assemblies for all spacing and sizes of patterned features, enabling us to compare effects of surface topography on assemblies. Experimentally applied voltages supplied by the function generator and applied across the electrodes (e.g., 2, 4, and 6V) were chosen to demonstrate the effect of field strength on the assembly behavior as well as for ease of experimental procedure. To aid comparison to related literature, the voltages are reported in V/cm; our sample chamber (i.e. the distance between top and bottom electrodes) was 120 µm high, therefore the converted voltage values were 170, 330, 500 V/cm for applied voltages of 2, 4, and 6V.


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Multiphysics Simulations Finite Element Method (COMSOL MULTIPHYSICS Version 4.4) with the AC/DC Electric Current module was used to simulate electric-field gradient distribution. Voltage was applied between top and bottom electrodes with frequency of 750 kHz and an electric field of 660 V/cm. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *Address correspondence to [email protected] Present Addresses ɣ

Intel Corporation, 2501 NW 229th Avenue, OR, UNITED STATES, 97124

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This research was supported by the Penn State Materials Research Science and Engineering Center (MRSEC, NSF DMR-1420620). TEM images were acquired at the Penn State Microscopy and Cytometry Facility and electrodes were fabricated at the


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Pennsylvania State University NSF NNIN Site. Undergraduate N. B. was funded by NSF NNIN REU (ECCS-0335765).


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