Arrays Of Highly Complex Noble Metal Nanostructures Using .
NanoscaleView Article OnlinePublished on 19 September 2018. Downloaded by University of Notre Dame on 10/9/2019 5:17:58 PM.PAPERCite this: Nanoscale, 2018, 10, 18186View Journal View IssueArrays of highly complex noble metalnanostructures using nanoimprint lithographyin combination with liquid-phase epitaxy†Eredzhep Menumerov,‡a Spencer D. Golze,‡a Robert A. HughesSvetlana Neretina *a,baandCurrent best-practice lithographic techniques are unable to meet the functional requirements needed toenable on-chip plasmonic devices capable of fully exploiting nanostructure properties reliant on a tailorednanostructure size, composition, architecture, crystallinity, and placement. As a consequence, numerousnanofabrication methods have emerged that address various weaknesses, but none have, as of yet,demonstrated a large-area processing route capable of defining organized surfaces of nanostructureswith the architectural diversity and complexity that is routinely displayed in colloidal syntheses. Here, ahybrid fabrication strategy is demonstrated in which nanoimprint lithography is combined with templateddewetting and liquid-phase syntheses that is able to realize periodic arrays of complex noble metal nanostructures over square centimeter areas. The process is inexpensive, can be carried out on a benchtop,and requires modest levels of instrumentation. Demonstrated are three fabrication schemes yieldingReceived 24th August 2018,Accepted 19th September 2018DOI: 10.1039/c8nr06874grsc.li/nanoscale1.arrays of core–shell, core–void–shell, and core–void–nanoframe structures using liquid-phase synthesesinvolving heteroepitaxial deposition, galvanic replacement, and dealloying. With the field of nanotechnology being increasingly reliant on the engineering of desirable physicochemical responses througharchitectural control, the fabrication strategy provides a platform for advancing devices reliant onaddressable arrays or the collective response from an ensemble of identical nanostructures.IntroductionThe integration of nanomaterials with wafer-based fabricationtechniques remains one of the fundamental challenges inadvancing devices reliant on the remarkable properties accessible when materials are reduced to the nanoscale.1 Substrateimmobilized noble metal nanostructures are particularly compelling in this regard since functionality is derived from arange of properties that include plasmonic resonances,intense near-fields, hot electron generation, plasmonicheating, refractive index sensitivity, and catalytic activity. Theirpotential impact is further amplified when (i) formed inaddressable arrays that allow for the interrogation of individual structures with optical, electrical, or scanning microscopyaCollege of Engineering, University of Notre Dame, Notre Dame, Indiana, 46556,USA. E-mail: sneretina@nd.edubDepartment of Chemistry and Biochemistry, University of Notre Dame, Notre Dame,Indiana, 46556, USA† Electronic supplementary information (ESI) available: Description of thenanoimprint lithography system and additional characterization of the nanostructures. See DOI: 10.1039/c8nr06874g‡ Equal contribution.18186 Nanoscale, 2018, 10, 18186–18194probes, (ii) arranged in patterns that promote a coupledresponse between adjacent nano- and bulk-scale materials, (iii)integrated with biopatterns, (iv) functionalized with analytereceptors, (v) incorporated into circuitry, or (v) integrated intomicrofluidic or lab-on-a-chip platforms. Collectively, suchcapabilities have the potential to revolutionize a diverse rangeof applications with a scope that includes biological andchemical detection,2 photovoltaics,3 metasurfaces,4 and evennanofabrication itself.5 Capitalizing on these opportunities is,however, reliant on the establishment of a processing sciencethat is currently challenged by the often stringent demandsthat these applications place on nanostructure size, composition, architecture, crystallinity, and placement. Advancingmethodologies that address these demands are, hence, ofcritical importance.Electron-beam lithography is the most widely used technique for defining organized patterns of substrate-based noblemetal structures with nanometer-scale dimensions. The advantages of this top-down technique include ultrahigh resolution,the accurate positioning of structures, high fidelity, and easeof integration into solid-state device fabrication. At the sametime, it suffers from numerous deficiencies that include (i) atechnically demanding serial writing process that is cost-prohi-This journal is The Royal Society of Chemistry 2018
View Article OnlinePublished on 19 September 2018. Downloaded by University of Notre Dame on 10/9/2019 5:17:58 PM.Nanoscalebitive, time-consuming, and often impractical when formingnanostructures over square centimeter areas, (ii) a resolutionthat is challenged to define nanogaps between plasmonic nanostructures that maximize the intensity of hot spots, (iii) the useof adhesion layers that severely damp plasmonic resonances,6and (iv) an inability to define three-dimensional multi-elementstructures with the crystallinity and architectural diversity thatis now routinely demonstrated using colloidal syntheses.Attempts to mitigate these deficiencies have relied on increasingly complex e-beam instrumentation and procedures7 as wellas post-processing of the lithographically-defined structures soas to promote crystallinity8 and alloying.1a,9 The so-formedstructures are, however, still comparatively simple whencompared to their colloidal counterparts.With e-beam lithography having its shortcomings, numerous methods and innovative concepts have emerged that overcome many of its deficiencies. Nanoimprint lithography (NIL),which is of specific relevance to the current study, hasemerged as an attractive process for forming periodic arrayswith nanoscale features over large areas with a high throughput.10 In this process, a lithographically-defined stamp is usedto imprint a pattern into a moldable polymeric resist. Eventhough the stamp itself is made using e-beam lithography, itscommercial availability and reusability make NIL a relativelylow-cost and technologically straightforward benchtop process.The metal patterns produced can also undergo post-processingprocedures such as templated dewetting1a to obtain crystallinestructures with even smaller dimensions.11 Other low-costmethods for forming periodic arrays of metal nanostructuresinvolve the use of lithographically-defined shadow masks12 orself-assembled surfaces formed using colloids,13 anodizedaluminum oxide (AAO),14 or diblock copolymers.15 Shadowmasks with nanoscale openings, however, have limited reusability while self-assembled surfaces are typically limited bythe number of patterns accessible and defects that disruptlong-range order. The Mirkin group has forwarded an alternative approach for forming metal nanoparticle arrays that usesarrays of scanning probes to site-selectively deposit attoliterdroplets containing chemical precursors that when heatedeach form a single metal nanostructure.16 While all of theaforementioned techniques have advanced array fabrication,the structures formed remain relatively simple in terms ofarchitecture and composition. While such structures unquestionably have numerous functionalities, they are somewhat atodds with the field of nanotechnology that is, to a largedegree, driven by engineering desirable physicochemicalresponses through architectural control.With solution-based chemistry providing the only means toengineer a library of complex nanostructure geometries, anumber of hybrid strategies have emerged where lithographically-defined features are used to regulate the placement of colloidal nanostructures. These techniques, which fall underheadings such as capillary assembly,1,17 template guidedassembly,18 chemical contrast patterning,19 and DNA-mediatedassembly,20 have all proven effective in the site-selective placement of colloidal nanostructures. While unquestionablyThis journal is The Royal Society of Chemistry 2018Paperimpressive, the downside of these techniques include a yieldthat is limited by the homogeneity of the colloid, target sitesthat are fabricated using e-beam lithography, and challengesassociated with the alignment of asymmetric structures. Analternate approach, which has been advanced by our group,uses solid-state dewetting to form architecturally simple nanostructures that are then transformed into complex nanostructures using liquid-phase chemistry.21 This work, alongwith the work of other groups22 who have carried out variations to this approach, have collectively fabricated the mostsophisticated substrate-based nanostructures produced todate. These demonstrations have, however, yielded either randomly distributed structures or periodic arrays over smallareas. Here, we demonstrate a hybrid nanofabrication schemefor forming nanostructured arrays of highly complex nanostructures over a square centimeter area through the integration of NIL, directed-assembly, and solution-based growthmodes. The method advances the goal of fabricating organizedsurfaces of engineered nanostructures in a manner that isresponsive to scalability, throughput, and cost-effectiveness.2. Results2.1.Fabrication strategyThe strategy used to fabricate periodic arrays of complex metalnanostructures relies upon a three-stage processing route. Thefirst stage utilizes NIL as a means to impose a periodicity overthe substrate surface by creating an array of openings througha deposited resist that act as target sites for the formation ofmetal nanoparticles. The second stage sees the deposition ofthe metal through the openings, the removal of the resist, anda heat treatment that results in the agglomeration and crystallization of the metal at the center of each target site. In thefinal stage, the substrate-immobilized nanoparticles are subjected to wet-chemistry protocols in which each particle seedsan identical reaction whose product is a nanostructure ofincreased sophistication. In the ensuing sections, we demonstrate this nanofabrication route by forming periodic arrays ofAu seeds using NIL and then transform them into progressively more intricate nanostructures using three different solution-based protocols.2.2.Seed fabricationThe processing route used to fabricate periodic arrays of Auseeds is shown schematically in Fig. 1a. A moldable polymericresist is spin-coated onto a planar (0001)-oriented sapphire substrate and baked. The resist is then embossed with a 1 cm2silicon stamp consisting of a periodic array of pillars (diameter 275 nm, height 350 nm) arranged in a square pattern (pitch 580 nm), a procedure carried out at elevated temperatures andpressures using a commercially available stamp in combinationwith a home-built pneumatic nanoimprinting press (see ESI,Fig. S1†). At this stage, the bottom of each cylindrical hole doesnot reach the substrate surface as denoted by the red arrow inFig. 1b. Openings to the surface are made by exposing theNanoscale, 2018, 10, 18186–18194 18187
View Article OnlinePublished on 19 September 2018. Downloaded by University of Notre Dame on 10/9/2019 5:17:58 PM.PaperNanoscaleFig. 1 (a) Schematic of the six step procedure used to form periodic arrays of Au nanostructures using NIL followed by templated dewetting. Tiltview SEM images of the (b) imprinted resist, (c) the imprinted resist after being exposed to an RIE treatment, (d) polycrystalline Au discs formedthrough deposition and lift-off, and (e) crystalline Au seeds. Note that a scratch was made in the resist to expose the edges seen in (b) and (c). (f )Top-view SEM images of the same process over larger areas.resist to a reactive ion etch (RIE) (Fig. 1c) that gradually thinsthe resist while leaving any exposed substrate intact (i.e., thesubstrate remains planar). Room temperature physical vapordeposition is then used to deposit an 18 nm thick layer of Auinto the openings and onto the remaining resist. Films produced in this manner are polycrystalline with nanometer-scalegrains.23 A lift-off procedure is then carried out to dissolve theresist and remove the Au deposited on its surface, a processthat leaves behind an array of circular Au discs on the substrate (Fig. 1d). When heated, the discs undergo a dewettingprocess that sees them transformed into weakly faceted structures with a near-hemispherical geometry (Fig. 1e) of smallerdiameter than the initial Au disc (yellow arrows in Fig. 1d ande). It is these structures that act as seeds for solution-basedgrowth modes. Fig. 1f shows top-view SEM images of the sameprocess over larger areas.Forming periodic arrays of nanostructures using NIL incombination with templated dewetting, like any nanofabrication technique, has both advantages and limitations. Its foremost advantage is that it provides a large-area capability (seeESI, Fig. S2†). The NIL stamp used to produce the array inFig. 1e, for example, results in the formation of 2 108 Aunanostructures over a centimeter-square substrate. In addition,it is a relatively straightforward process, requires only modestlevels of instrumentation, can be carried out on a benchtop infew hours, and does not require clean-room facilities. The NILstamps do, however, ultimately limit the density at which18188 Nanoscale, 2018, 10, 18186–18194structures can be produced and show damage over time thatleads to arrays with missing structures (see ESI, Fig. S3†). Theuse of templated dewetting not only results in the formation ofcrystalline structures but also presents the opportunity to crystallographically align the structures if a heteroepitaxialrelationship is formed with the substrate.21b Substrates thatare likely amenable to Au heteroepitaxy are [0001]-oriented surfaces with a hexagonal crystal structure or [111]-oriented surfaces with a cubic structure. The substrate must also have alow surface energy relative to Au, should be able to withstandthe processing temperatures needed for dewetting, and shouldnot allow Au to interdiffuse. Having a well-defined number ofAu atoms deposited in each of the openings in the resist leadsto a high degree of size uniformity (see ESI, Fig. S4†). While itis possible to vary the size of the structure by adjusting theamount of Au deposited, there are limits on the degree towhich this can be done. If the thickness of the lithographically-defined Au disk is too thin, then it gives rise to instabilities in the dewetting process that cause the Au disc to breakup into multiple islands at each array site (see ESI, Fig. S5†). Atthis point, arrayed structures with a smaller diameter can onlybe formed through the use of a stamp with smaller featuresizes or the use of a modified assembly process that accelerates dewetting through the incorporation of a sacrificial Sblayer.24 The templated dewetting process is also imperfect inthat it leads to variability in the nanostructure center-to-centerdistances (see ESI, Fig. S4†). Asserting control over the nano-This journal is The Royal Society of Chemistry 2018
View Article OnlineNanoscalePaperstructure shape is also difficult because the high temperaturesused in the dewetting process drive the nanostructure geometry toward that which is thermodynamically favorable, wherelittle can be done to alter it.25Published on 19 September 2018. Downloaded by University of Notre Dame on 10/9/2019 5:17:58 PM.2.3.Solution-based growth modesOnce fabricated, the Au seed arrays were subjected to solutionbased syntheses adapted from one of the many seed-mediatedprotocols that have been devised to generate colloidal nanostructures. Each of the three examples presented herein werecarried out in aqueous solutions heated to 95 C using only abeaker and hot plate. While such syntheses require little in theway of instrumentation, architectural control is afforded by thechemical environment in which the seeds are immersed. Withthe use of Au seeds that are well-bound to the substrate comesthe added advantage that they can be transferred from onechemical environment to another in rapid succession, a capability that is unavailable to colloidal growth modes. In thissection, such chemical controls are used to transform Au seedarrays into core–shell, core–void–nanoshell, and core–void–nanoframe nanostructures.2.3.1. Arrays of core–shell structures. The synthesis,characterization, and application of colloidal bimetallicnanostructures has demonstrated a versatility and utilitythat greatly exceeds their monometallic counterparts.26 Suchstructures derive benefit from the greater range of physicochemical properties accessible when two metals form ananoscale interface or are combined to obtain an alloy orintermetallic compound. One of the most fundamental bimetallic architectures is the core–shell structure (abbreviatedas core@shell) in which one metal is heterogeneously deposited on a second metal. Such syntheses can lead to growthmodes that promote specific facets and where the two metalsform a heteroepitaxial relationship with each other. Such reactions typically proceed by adding metal seeds and a reducingagent to an aqueous solution of metal ions of a second metal.The ensuing reaction sees the ions transformed into a neutralspecies that readily deposits on the seed material (e.g., Ag being reduced to Ag). Fig. 2 shows a schematic representationof the shape transformation, SEM images, and elementalmapping of a periodic array of Au@Ag core–shell structuresobtained when carrying out a liquid-phase synthesis in whichAg ions, derived from AgNO3, are reduced onto Au seeds byascorbic acid. The product of the reaction is a highly facetedstructure that expresses square (100) and hexagonal (111) Agfacets. This facet pattern, along with the fact that the structuresshow in-plane alignment with each other (Fig. 2c), reveal thatthe underlying Au seed is [111]-oriented and has a heteroepitaxial relationship with the substrate. Also noteworthy is that theso-formed core–shell structure is fundamentally different fromits colloidal counterpart in that the shell does not completelyencapsulate the core since the substrate prevents the growthsolution from coming into contact with the underside of the Auseed. Nonetheless, the ability to fundamentally transform theAu seed array in a reaction lasting 12 min demonstrates thepotency of this wet-chemistry approach.This journal is The Royal Society of Chemistry 2018Fig. 2 (a) Schematic showing the transformation that occurs when Auseeds are transformed into faceted Au@Ag core–shell structures usingsolution-based chemistry. SEM images of the Au@Ag structures takenfrom a (b) tilted- and (c) top-view. (d) EDS maps and line scans of anindividual core–shell structure (scale bar 100 nm).2.3.2. Arrays of core–void–shell structures. Some of themost sophisticated nanostructures have emerged from reactions that incorporate chemistries that hollow part of an existing structure through the use of a preferential etch, theKirkendall effect, or galvanic replacement reactions.27Associated with this complexity are properties that continuously change as the structure advances through a hollowingprocess that alters both the morphology and composition andwhich can be halted at any intermediate point in the reaction.Galvanic replacement reactions are particularly intriguing inthis regard in that hollowing is accompanied by the depositionof a secondary metal. Such reactions occur spontaneouslywhen a solid metal, often referred to as the template, isexposed to ions of a second metal with a higher electrochemical potential. The ensuing reaction sees the reductionand deposition of the aqueous metal onto the template as thesolid metal template is oxidized to form ions that enter theliquid phase. The product of the reaction is typically ahollow shell with a composition that is an alloy of the twometals.Nanoscale, 2018, 10, 18186–18194 18189
View Article OnlinePublished on 19 September 2018. Downloaded by University of Notre
surfaces of engineered nanostructures in a manner that is responsive to scalability, throughput, and cost-effectiveness. 2. Results 2.1. Fabrication strategy The strategy used to fabricate periodic arrays of complex metal nanostructures relies upon a three-stage processing route. The first stage utilizes NIL as a means to impose a periodicity over
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