Fully Tunable Silicon Nanowire Arrays Fabricated by SoftNanoparticle TemplatingBy Marcel Rey,† Roey Elnathan,‡ Ran Ditcovski,§ Karen Geisel,∥ Michele Zanini,†

Miguel-Angel Fernandez-Rodriguez,†,⊥ Vikrant V. Naik,# Andreas Frutiger,∇ Walter Richtering,∥

Tal Ellenbogen,§ Nicolas. H. Voelcker,‡ and Lucio Isa*,†

†Laboratory for Interfaces, Soft Matter and Assembly, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, CH-8093Zurich, Switzerland‡ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, Future Industries Institute, University of SouthAustralia, Mawson Lakes, 5095, Australia§Department of Physical Electronics, Fleischman Faculty of Engineering, Tel-Aviv University, Tel-Aviv 69978, Israel∥Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, 52056, Aachen, Germany⊥Biocolloid and Fluid Physics Group, Applied Physics, University of Granada, 18071 Granada, Spain#Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, CH-8093 Zurich,Switzerland∇Laboratory of Biosensors and Bioelectronics, Institute for Biomedical Engineering, ETH Zurich, Gloriastrasse 35, ETZ F76,CH-8092 Zurich, Switzerland

*S Supporting Information

ABSTRACT: We demonstrate a fabrication breakthrough toproduce large-area arrays of vertically aligned silicon nanowires(VA-SiNWs) with full tunability of the geometry of the singlenanowires and of the whole array, paving the way towardadvanced programmable designs of nanowire platforms. At thecore of our fabrication route, termed “Soft NanoparticleTemplating”, is the conversion of gradually compressed self-assembled monolayers of soft nanoparticles (microgels) at awater−oil interface into customized lithographical masks tocreate VA-SiNW arrays by means of metal-assisted chemicaletching (MACE). This combination of bottom-up and top-down techniques affords excellent control of nanowire etchingsite locations, enabling independent control of nanowirespacing, diameter and height in a single fabrication route. Wedemonstrate the fabrication of centimeter-scale two-dimen-sional gradient photonic crystals exhibiting continuouslyvarying structural colors across the entire visible spectrumon a single silicon substrate, and the formation of tunable optical cavities supported by the VA-SiNWs, as unambiguouslydemonstrated through numerical simulations. Finally, Soft Nanoparticle Templating is combined with optical lithography tocreate hierarchical and programmable VA-SiNW patterns.

KEYWORDS: Vertically aligned nanowires, nanoscale patterning, microgels, fluid interfaces, 2D photonic crystals, optical cavities

Nanoscale patterning is widely used across disciplines toimpart new functions and properties to materials. Arrays

of vertically aligned silicon nanowires (VA-SiNWs) are aprominent example where control of both their crystallographicstructure (i.e., growth direction) and of their geometricalparameters (diameter, height, and spacing) enables theemergence of different photonic,1−7 electronic,8 mechanical,9

wetting,10 and biointerface properties11,12 compared to the bulkmaterial. These include increased optical absorption due tolight trapping,13 generation of structural colors due to wave-

guiding and resonance phenomena,2,3,7,14 high density of

nanowire field-effect transistors (FETs),8 and the ability to

transfect cells in vitro and in vivo.6,11,15 Future nanowire-based

device technology requires an economically viable path toward

high-throughput, robust, and scalable processing, and ideally

Received: August 25, 2015Revised: December 4, 2015

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the ability to fabricate VA-SiNW arrays with precise control oftheir geometrical parameters.Current nanofabrication of VA-SiNW arrays is either based

on bottom-up processes, such as vapor−liquid−solid(VLS)16−18 and metal−organic vapor-phase epitaxy(MOVPE),19,20 or top-down methods, such as reactive ionetching (RIE) and metal-assisted chemical etching(MACE).21−23 Despite significant research efforts,24 achievinghigh-resolution, cheap, and easy-to-implement fabrication viachemical vapor deposition methods currently remains elusive.In contrast, better control of the spatial organization of VA-SiNW arrays can be reached using top-down methodscombined with lithographic steps. Nearly all existing litho-graphical technologies, for example, photolithography, electronbeam lithography,25 dip-pen nanolithography,26 and nano-imprint lithography27 have been used to generate etching masksfor VA-SiNW.25,27−29 Despite the added flexibility, theseprocesses are either sequential, and thus slow and limited tosmall areas, or require expensive equipment of limitedaccessibility.A promising alternative to these methods is colloidal

lithography, namely, the controlled deposition of nano- ormicroparticles onto a solid substrate, which act either as a directmask30 or are used to produce metal masks complementary tothe particle arrangement.31 The advantages are manifold: thelarge availability of highly monodisperse particles allows precisecontrol over the feature size, and the process is inherentlyparallel and scalable to large samples. Yet, the main limitation

lies in the accurate control of the spacing between the features.A possible method to create nonclose-packed (ncp) patterns ofspherical colloids is shrinking close-packed arrays of particlesusing reactive plasma etching.32 However, the size of theoriginal spheres sets the relation between the feature size andspacing, that is, the two cannot be changed independently andcontinuously on the same substrate. Alternatively, ncp maskscan be obtained by exploiting self-assembly and deposition ofcharged colloids at fluid−fluid interfaces,33−35 by depositiononto nanostructured templates36 or by deposition onstretchable elastomers.37 Yet, these methods exhibit drawbacksand limitations. Colloidal lithography at fluid interfaces comeswith limitations over the size range and separation of thecolloids. Templated deposition allows exquisite control over thedeposited pattern but requires the fabrication of templates viaundesired lithography steps. The use of stretchable substrates isan extremely simple and flexible alternative but requires somehigh-temperature steps, which limit its applicability to inorganicparticles (e.g., silica) that cannot be used as MACE masks.In this work, we show that these obstacles can be completely

overcome by using microgel particles. Microgels are soft,compressible nanoparticles consisting of cross-linked, water-soluble polymers, typically showing a more densely cross-linkedcore surrounded by a looser corona38,39 (see Figure 1a). Thisconformation is emphasized when the microgels are confined atan oil−water (o/w) interface, where the corona adsorbs andstretches out to form an extended soft shell around the cores40

(Figure 1b). Upon spreading and compression at an o/w

Figure 1. Schematics of the Soft Nanoparticle Templating process. The central element shows the workings of the experimental setup. The o/wLangmuir trough is schematically drawn, showing the two barriers used to compress the interface. Simultaneous to interface compression, a siliconwafer is lifted through the interface at an angle of 30° and then emerges through the oil−air interface (red arrows). The process produces a gradientin spacing of the deposited microgels (colored in green), as schematically shown on the substrate after lift-off. (a,b) Schematic representation ofmicrogel particles in bulk water and at an o/w interface, respectively. The core−shell nature of the particles is displayed, together with theirdeformation and flattening at the o/w interface. (c,d) Top view of the microstructure of the microgel monolayer at different degrees of compression.The steric contacts between the coronas at the interface are compressed giving rise to hexagonal particle lattices with gradients in the interparticlespacing. Different positions on the silicon substrate correspond to different degrees of compression, with decreasing particle separation from right toleft, corresponding to increasing surface pressure. Bottom row: main steps of the VA-SiNW array fabrication via MACE. The deposited microgelarray (e) is swollen in photoresist, leading to an increased particle height from the substrate (f). A 10 nm layer of Au is deposited (g) and themicrogels provide masking for the contact between the Au layer and the Si substrate. Upon exposure to the etching solution, the MACE reactionproceeds around the swollen microgels, causing the formation of VA-SiNWs that replicate the lateral geometry of the customized microgel shadowmask with high fidelity (h).

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interface, a range of different structures can be produced.41 Inparticular, as the coronas enter into steric contact, the particlecores readily self-assemble into large-area, high-quality 2D ncphexagonal crystals (Figure 1c), which can be compressed untilthe particles experience core−core contacts (Figure 1d).However, after deposition and drying, the height of themicrogels is only of a few nanometers, resulting in insufficientmaterial contrast for use as lithography masks. Here we showthat a simple modification step solves this problem and enablesthe novel use of the microgels as lithography masks for thefabrication of fully tunable, large-area optically active VA-SiNWarrays.The process, that we name “Soft Nanoparticle Templating”,

is based on the following fabrication steps as describedschematically in Figure 1 (full details are in Methods -Experimental Details in Supporting Information). A monolayerof microgel particles (synthesized as in Methods - ExperimentalDetails in Supporting Information) is prepared by spreading acontrolled volume of a microgel suspension at a water−hexaneinterface in a customized Langmuir trough by means of a

precision microsyringe. The trough has a custom-made bath toallow simultaneously compression experiments and monolayerdeposition from an o/w interface. A number of microgels arespread so that before starting the compression, the surfacepressure is approximately 2−3 mN/m. This corresponds toparticles in corona−corona contact that form a hexagonal ncplayer of the cores. After equilibration, the barriers of theLangmuir trough are closed at a speed of 2.3 mm/min and,simultaneously, a silanized 2 × 1 cm2 silicon substrate is raisedthrough the interface at a 30° angle and a speed of 0.3 mm/minso that it has fully crossed the interface when the compressionstops. In this way, we can achieve the complete deposition of aninterfacial monolayer at a continuously varying surfacepressure/area per particle, where different positions on thesubstrate correspond to different compression states of themonolayer. Upon raising the surface pressure up to 18 mN/m,the corona−corona contacts in the ncp microgel crystal areuniformly compressed and we achieve gradients of distancesbetween the particle cores continuously varying from 550 to350 nm on the same substrate (from the lowest compression to

Figure 2. Characterization of gradient samples. (a) 10 × 10 μm2 AFM images of deposited microgels and corresponding top-view SEM micrographsof the VA-SiNW arrays taken at different positions along a gradient sample, corresponding to the surface pressures reported above each image. Scalebar 2 μm for all images. (b) Center-to-center nanowire separation Λ and corresponding surface pressure Π as a function of position x on thesubstrate. Compression proceeds from left to right, causing a progressive and controlled reduction of nanowire separation. (c) SEM cross-section ofthe VA-SiNW array with a height of 1.8 μm and average radius of 90 nm (aspect ratio 20). Scale bar 1 μm. (d) Optical photograph of the gradientsample showing continuously varying structural colors across the whole visible spectrum on a single substrate.

Figure 3. Control over nanowire diameter. (a,b) Top-view (scale bar 1 μm) and cross-section (scale bar 2 μm) SEM micrographs of VA-SiNWsfrom microgel masks obtained after 15 and 40 s O2 plasma etching, respectively. (c) Nanowire radius R and center-to-center nanowire spacing Λ as afunction of exposure time to oxygen plasma. The nanowire radius can be tuned independently of the nanowire separation. (d) Bright-field opticalphotographs at normal incidence corresponding to data points I, II, and III on the graph in which the etched VA-SiNW arrays show vivid colors.

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the highest compression, respectively), while maintaining ahighly 2D hexagonal crystalline structure (see Figure 2). Onlyat the highest surface pressure we observe the formation ofsmall microgel clusters and a weaker dependence ofinterparticle spacing on compression. The procedure is highlyreproducible and the gradient is only defined by the initialamount of spread particles and by the deposition protocol.Different samples prepared in similar conditions give analogousgradients robustly showing the same properties (see SupportingInformation Figure S1). The same method can also be used toproduce cm2 microgel arrays with uniform interparticleseparation, by keeping a constant surface pressure duringdeposition, that is, by adjusting the trough area, so that allpositions on the substrate correspond to the same compressionstate, as typically done in Langmuir−Blodgett depositions (seeMethods - Experimental Details in Supporting Information andFigure 3).After deposition, the microgels are swollen in situ for 30 min

using a photoresist. After fixation, the resist is washed first withacetone and then ethanol, and the substrate is dried under asteep angle. Ethanol swells the microgels and the recedingdrying front leaves behind swollen microgels that incorporatephotoresist residues still dissolved in the solvent. Uponcomplete evaporation of the ethanol, the photoresist remainsinside the microgels, leading to an increase in the height profileof the dry particles from 30 to 100 nm (more details are in theSupporting Information Figures S2, S3, and S4). Finally, theswollen microgels are exposed to oxygen plasma for up to 1min; after 15 s of plasma time, the particle corona is removed,and further exposure leads to a controlled reduction of theparticle diameter (see Supporting Information Figure S4).

The microgel arrays are then sputter-coated by a 10 nm thickAu layer to produce masks for MACE (details for the etchingconditions in Methods - Experimental Details in SupportingInformation). Specifically, the swollen microgels act as spacersbetween the Au layer and the silicon substrate and, as aconsequence, the wafer is etched preferentially around themicrogels where the catalytic Au layer is in direct contact withthe silicon substrate, causing the formation of VA-SiNWs. Theduration of the etching controls the height of the nanowires,while the diameter of the microgel core, tunable by O2 plasma,defines the nanowire diameter.Because of the smooth, controlled compression of the

interface, these customized masks enable the generation of agradient of spacing for nanowires with the same diameter, onthe same substrate, and in a single deposition. This emphasizesthe potential and the flexibility of Soft Nanoparticle Templatingto produce novel VA-SiNWs platforms, where the geometry ofthe individual nanowires and the properties of the array aredecoupled and can be robustly, accurately, and independentlytuned, over cm2 scales. This feature holds the key to futuretechnological applications and has so far not been demon-strated by any other method.Figure 2 provides an example of such gradient samples. In

Figure 2a, AFM images of the deposited microgels and SEMtop-views of the nanowire array after etching at differentpositions on the substrate are summarized. The differentlocations correspond to different surface pressures (see Figure2b for the relation between position and surface pressure). Aside view of the array, demonstrating that these nanowires havean aspect ratio of 20, is shown in Figure 2c. The continuousgradient in nanowire spacing causes smoothly changingstructural colors of the substrate, covering the entire visible

Figure 4. Large-scale formation of nanowire-supported optical cavities with multiple resonances. (a) Bright-field reflectance spectra for VA-SiNWtaken at a position with average nanowire separation Λ = 395 nm. (b) Reflectance spectra taken at different positions along the sample vs themeasured average local nanowire spacing Λ. (c) Simulated reflectance spectra for Λ = 400 nm. Nanowire radius is 100 nm and nanowire length is 2.4μm. (d,e) Simulated reflectance spectra vs nanowire spacing (radius 100 nm) (d) and nanowire radius (spacing 400 nm) (e). The vertical dashedlines correspond to the spectra in (c). (f−k) Simulations of normalized magnitude of the electric fields in a single unit cell for spectral peaks at 656nm (f), 606 nm (g), and 567 nm (h) and for spectral dips at 635 nm (i), 588 nm (j), and 551 nm (k). The electric field shows confinement of lightdue to formation of optical cavities in the dip positions with consecutive number of nodes (8 in panel i, 9 in panel j and 10 in panel k).

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range, as visualized in Figure 2d and later measured in Figure5b.Additionally, as previously described, the diameter of the

nanowires can also be independently tuned by shrinking theswollen microgels using an O2 plasma. Figure 3 shows examplesof nanowire arrays with the same center-to-center spacing butwith continuously varying radii in the range between 90 and 50nm. In line with previous findings,3 these substrates also showdifferent colors at normal incidence in reflection modedepending on the nanowire diameter (Figure 3d). The imagesand data points in Figure 3 have been collected on separatesamples prepared independently by depositions at the sameconstant surface pressure (8 mN/m), demonstrating both thecapability of producing cm2 arrays with controlled uniformpitch and the robustness and reproducibility of the process.We then proceeded to carefully characterize the remarkable

optical properties of the tunable VA-SiNW gradient arrays.Previous reports have described some of the intriguing opticalproperties of VA-SiNWs,2,3,7,13,14 which depend on the localgeometrical parameters of the nanowires, for example, their sizeand shape, and on their nonlocal arrangement, for example,their period and lattice type. Here, we describe the opticalproperties of the gradient VA-SiNW samples and show thatthey display a rich combination of interesting opticalphenomena that have not been comprehensively studied beforeand that are directly relevant for applications, for example, insensing.Figure 4a shows a reflectance spectrum (see Methods -

Experimental Details in Supporting Information for furtherdetails) taken at a position (x = 9 mm) along the gradient,where the average center-to-center distance between thenanowires was Λ = 395 nm (corresponding to the centralAFM/SEM image in Figure 2a). Unlike in the case ofpreviously examined 1 μm-long VA-SINWs in arrays with a 1μm spacing, which produced single broad dips in reflectance3

(∼50−100 nm FWHM), we observed multiple sharp dips witha FWHM of ∼20 nm. Figure 4b shows that these spectralfeatures persist throughout the entire gradient VA-SiNWsample. In particular, the spectral dips appear at relativelyconstant spectral positions, become sharper for smallernanowire separations, and start to fade when the separationsget close to ∼350 nm. Moreover, for Λ approaching ∼350 nmthe sample no longer forms well-defined periodic arrangementsof nanowires because the microgels enter into core−corecontacts, leading to the formation of nanowire clusters (Figure

2a for position x ≥ 16 mm) and fading of the reflection dips(Figure 4b).The observed features in the reflectance cannot be simply

attributed to conventional interferences in thin films but arerelated to the potential of the VA-SiNW to confine and guidelight. To identify univocally the origin of these spectral features,we performed finite-difference-time-domain simulations of thebright-field reflectance configuration (see Methods - Exper-imental Details in Supporting Information). Figure 4c,d showsthe simulated reflectance spectrum for Λ = 400 nm, andadditionally for varying separations between 350 and 500 nm,respectively. The simulated results agree with the features thatappear in the experimental spectra of Figure 4a,b. Mostimportantly, they reproduce the multiple dips and theirsharpening for smaller nanowire separations. The smalldifferences observed between the simulations and experimentalresults stem from the experimental polydispersity of nanowirediameter and spacing as, for instance, described by the errorbars in Figures 2b or 3c, which lead to homogeneousbroadening of the resonances. To support this claim further,in Figure 4e we show a simulation of the reflectance versusnanowire radius R for fixed nanowire separation Λ = 400 nm. Itcan be seen that small changes in the nanowire radius lead tolarge shifts in the spectral position of the multiple dips.Additionally, surface roughness of the nanowires (see Methods- Experimental Details in Supporting Information) may explainthe lower experimental values of reflectance compared to thenumerical simulations.42 Roughness and the slight polydisper-sity in the nanowire diameter may also be responsible for thefact that the fine features of the spectra in the short wavelengthregion (400−500 nm) visible in the simulations are somewhatblurred in the experiments (Figure 4b,d).There are numerous reasons and optical phenomena that can

lead to reflection dips. In the case of the examined VA-SiNWsamples with L ≈ 2.4 μm, R ≈ 100 nm, 350 nm ≤ Λ ≤ 500 nm,the observed dips in normal incidence/reflection mode arisemainly from the coupling of incident light to Fabry−Perotresonances of the VA-SiNWs, which at small nanowireseparations also lead to confinement of the light in the gapsbetween the nanowires. To corroborate this conclusion, weshow in Figure 4f−k plots of the simulated electric field aroundthe nanowires (see Methods - Experimental Details inSupporting Information) for the corresponding reflectancepeaks and dips shown in Figure 4c. It can be seen that for thedips the field is confined in the gap between the nanowires and

Figure 5. Colored backscattering due to formation of tunable gradient spacing, large scale, hexagonal photonic crystal. (a) Illustration ofbackscattering measurement scheme. (b) Color photograph of position-dependent backscattering from the gradient spacing sample. The graysemicircle at the top marks the position of the sample holder during microgel deposition. The colors range corresponds to surface pressures between2 and 18 mN/m going from the largest spacing (red) to the smallest one (blue). (c) Normalized, position-dependent, backscattering spectra fromthe sample in (b). The top axis is the average NW separation calculated from eq 1, which corresponds to the measured wavelength axis (bottom).The white circles are local center-to-center distances between nanowires (Λ) measured with SEM.

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that for consecutive dips there are consecutive number ofoptical nodes in the gaps, which demonstrates the formation ofFabry−Perot cavity modes. The fact that these modes aresupported by the nanowire waveguides is demonstrated nicelyin Figure 4e where for thin nanowires there are no waveguidemodes due to transverse cutoff of the modes, while fornanowires with larger radii the lowest order transversewaveguide modes start to appear and for even larger nanowireradii the signature of higher order transverse modes emerges.Finally, because the dips are Fabry−Perot resonances, a dip isobserved whenever the roundtrip length, 2L, is equal to aninteger multiple of the effective wavelength of the nanowireguided modes, mλq = 2L, where m is an integer number andλq = λ0/neff, where λ0 is the light wavelength in vacuum and neffis the effective refractive index experienced by the guided mode.The nanowires in our sample are long enough in order to meetthis criterion several times for the spectrum of a xenon lamp.This effect can be seen also in the simulations data reported inFigure 4f−h.In addition to complex reflectance, ordered nanostructured

materials can also lead to strong scattering of light and creationof vivid structural colors due to photonic crystals that areformed in the materials. This phenomenon can also beobserved in nature as a coloring mechanism, for example, forcertain types of insects, butterflies and birds,43 and it is evidentin Figure 2d. To characterize this effect in our samples, wemeasured the backscattering of white light from a tilted gradientsample. Illustration of the experimental configuration is shownin Figure 5a (see Methods - Experimental Details in SupportingInformation). Because of the gradually varying spacing betweenthe nanowires, different regions in the sample backscatter lightat different wavelengths and Figure 5b shows a colorphotograph of the backscattering for θ = 54°. It can be seenthat the backscattering changes with position along the gradientand that it shifts from blue to red (see also SupportingInformation Figure S5 for additional details). Figure 5c showsthe corresponding normalized spectral measurements as afunction of position on the sample in Figure 5b using animaging spectrometer. In this case, contrary to the bright fieldnormal reflection case, we observed only one reflection peakthat shifts to shorter wavelengths with decreasing center-to-center spacing between nanowires. The results nicely fit theprediction for light diffraction of 2D hexagonal lattice photoniccrystals44 with a gradient in spacing

λθ

Λ =xx

( )( )

3 sin( ) (1)

where λ is the position-dependent peak diffracted wavelength infree space (eq 1). The calculated center-to-center distance is

shown as the top axis in Figure 5c and can be directly comparedto the one measured with SEM (white circles), demonstratingthat indeed the samples behaves as a tunable 2D photoniccrystal.In addition to the features reported above, a unique aspect of

our Soft Nanoparticle Templating approach is the easyintegration with other existing parallel patterning techniques.In particular, the combination with standard optical lithographyenables the fabrication of large-scale microstructured patternsof VA-SiNWs using the same process flow (see Methods -Experimental Details in Supporting Information for moredetails). This combined lithography relies on the strongadhesion of the soft nanoparticles on the silanized siliconsubstrates, fixing the microgel array during the opticallithography steps, in which a photoresist pattern is created ontop of the microgels. The exposed particles are entirelyremoved by oxygen plasma, whereas the protected ones stayintact. Subsequent rinsing with acetone and ethanol, aspreviously described, exposes and swells the initially protectedmicrogels, enabling the fabrication of the hierarchicallithography masks that define micropatterned arrays of VA-SiNWs, as demonstrated in Figure 6 (see also SupportingInformation Figure S6 for additional details). Three examplesof such structures with circular, linear, and triangular micro-patterns are shown.We have demonstrated that harnessing the self-assembly of

microgels at fluid (o/w) interfaces enables a novel nanoscalepatterning paradigm that addresses and overcomes currentbottlenecks in the fabrication of fully tunable VA-SiNW arraysover large areas. In particular, the continuous and simultaneouscompression and deposition of interfacial monolayers ofmicrogels enable us to produce a customized lithographicalmask to grow VA-SiNW arrays with adaptable and program-mable designs by means of MACE. Importantly, this approachenables independent control over spacing and diameter of thenanowire elements, defined by the deposition protocol and themicrogel size. This feature was exploited for the fabrication ofVA-SiNW arrays with gradients in spacing and constantdiameter. Future work with differently sized microgels willextend the range of available geometrical parameters. Our fullytunable samples show extremely rich optical behavior in thevisible range in particular via the formation of controlled opticalcavities that confine the optical fields to the gaps between theNWs and thus can potentially be used as for sensing, stimulatedemission, and nonlinear optics applications. Finally, thepossibility to produce hierarchical patterns of VA-SiNWspaves the way for new applications in optical circuits, complexplatforms for cell studies, and surfaces with programmableadhesion and wettability.

Figure 6. Hierarchical structures from combined lithography. (a−c) Examples of hierarchical, micropatterned VA-SiNW arrays from combined SNTand photolithography. (a) Scale bar 50 μm; (b) scale bar 20 μm; (c) scale bar 50 μm. The scale bars in the insets correspond to 1 μm.

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■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.nano-lett.5b03414.

All the experimental details in the Methods Section plusadditional figures and data. (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: lucio.isa@mat.ethz.ch.Author ContributionsM.R. and R.E. contributed equally to this work.L.I. designed the study. M.R., R.E., N.H.V., M.A.F.R., M.Z.,

and L.I. performed the substrate fabrication, the microscopymeasurements and the corresponding data analysis. R.D. andT.E. performed the optical characterization and the numericalsimulations. K.G. and W.R. synthesized the microgel particles.M.R. and A.F. did the optical lithography and swelling of theparticles. V.V.N. and M.R. did the IR measurements andanalysis. All authors contributed to the discussion and wrote thepaper.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSL.I. would like to thank Professor N. D. Spencer, Professor A.Studart, and Professor J. Voros for access to instrumentation.L.I. and M.Z. acknowledge financial support from the SwissNational Science Foundation Grant PP00P2_144646/1.N.H.V. kindly acknowledges fellowship support from theAlexander von Humboldt Foundation. M.A.F.R. acknowledgesfinancial support from the COST action: Smart and GreenInterfaces (STSM MP1106-14761) and would like to thankProfessor R. Hidalgo-Alvarez, Professor M. A. Rodriguez-Valverde, and Professor M. A. Cabrerizo-Vilchez for supportinghis COST fellowship. K.G. and W.R. acknowledge financialsupport from the German Research Foundation within theCollaborative Research Center SFB985 “Functional Microgelsand Microgel Systems”. T.E. acknowledges financial supportfrom the European Commission Marie Curie CareerIntegration Grant (Grant 333821). Marc Cirera is acknowl-edged for the illustrations in Figure 1a−h.

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Nano Letters Letter

DOI: 10.1021/acs.nanolett.5b03414Nano Lett. XXXX, XXX, XXX−XXX

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