LETTER pubs.acs.org/NanoLett
One-Dimensional Nanorod Arrays: Independent Control of Composition, Length, and Interparticle Spacing with Nanometer Precision Kyle D. Osberg,† Abrin L. Schmucker,‡ Andrew J. Senesi,‡ and Chad A. Mirkin*,†,‡ †
Department of Materials Science and Engineering, ‡Department of Chemistry, and International Institute of Nanotechnology, Northwestern University, Evanston, Illinois 60208, United States
bS Supporting Information ABSTRACT: We report the synthesis of solution dispersible, one-dimensional metal nanostructure arrays as small as 35 nm in diameter using on-wire lithography, wherein feature thickness and spacing in the arrays is tailorable down to approximately 6 and 1 nm, respectively. Using this unique level of control, we present solution-averaged extinction spectra of 35 nm diameter Au nanorod dimers with varying gap sizes to illustrate the effect of gap size on plasmon coupling between nanorods. Additionally, we demonstrate control over the composition of the arrays with Au, Ni, and Pt segments, representing important advances in controlling the ordering of sub-100 nm nanostructures that are not available with current synthesis or assembly methods. KEYWORDS: On-wire lithography, nanoparticle arrays, plasmon coupling, nanorods, multicomponent nanostructures
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hile methods to control the size and shape of metal nanostructures are extensive and highly tailorable for a variety of metals, assembly of these materials into controlled, ordered arrays remains an important synthetic challenge, particularly for one-dimensional (1D) materials. Current methods for fabricating 1D arrays of sub-100 nm metal nanoparticles typically rely on electron beam lithography,1-5 controlled aggregation,6-10 physical or chemical attachment to a preformed 1D structure,11-13 or anisotropic functionalization of the particles.14-18 However, these methods lack control over the number of assembled particles, the spacing between particles from 1 to 100 nm, the composition of each particle in the array, and are usually limited to specific conditions or surfaces to maintain their order. Indeed, it has proven to be exceedingly difficult to independently meet these requirements with current methods of preparing 1D metal nanostructure arrays, especially for solution dispersible ensembles. Consequently, researchers interested in the properties of coupled nanoparticle systems often rely on random drying of particles in the correct orientation to study these phenomena.19-23 Recently, our group reported the development of on-wire lithography (OWL), a novel method for synthesizing 1D arrays of nanostructures that could potentially overcome these limitations by allowing one to tailor the composition and distances between adjacent particles down to 2 nm.24-26 However, important limitations in the current OWL synthetic scheme have prevented the preparation of arrays with diameters smaller than 270 nm,27 limiting its impact to those interested in controlling the ordering and studying the properties of dispersed sub-100 nm coupled nanostructures. Herein, we report a new synthetic route for the preparation of solution dispersible, 1D arrays of Au nanostructures as small as r 2011 American Chemical Society
35 nm in diameter, leading to control in segment length and spacing down to approximately 6 and 1 nm, respectively. Importantly, we demonstrate an example of harnessing this high degree of structural tunability to study the effect of particle spacing on the plasmon coupling between closely spaced 35 nm diameter Au nanorods in solution. Interestingly, this represents an important example of studying this phenomenon with precise control over a range of gap sizes (up to ∼16 nm) in a solutionaveraged fashion, rather than at the single ensemble level. Furthermore, we illustrate unique versatility and compositional control at these dimensions by synthesizing multicomponent structures comprised of materials with disparate properties (Au-Ni and Au-Pt). These advances represent important steps toward controlling the synthesis and ordering of dispersible 1D nanostructure arrays to an extent that is not available with current synthesis or assembly methods. In OWL, a thin backing layer is deposited on one side of dropcast, template-synthesized, multisegmented nanowires using a line-of-sight method (usually 50 nm SiO2 by plasmaenhanced chemical vapor deposition, PECVD). The wires are then released into solution via sonication, and the more reactive material is selectively etched to produce particle arrays with highly controllable segment and gap lengths dictated by the original nanowire geometry, enabling important fundamental and applied studies in SERS-based biodetection27-30 and encoding27,28 and molecular electronics.31-33 Synthetically, it Received: November 29, 2010 Revised: January 3, 2011 Published: January 12, 2011 820
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aggregation of the wires upon dropcoating and drying on a substrate, deposition of high quality thin (sub-10 nm) SiO2 films to serve as backing layers on the wires, and subsequent dispersion of the wires into solution by sonication. Chief among these obstacles, it was determined that the smaller nanowires could not be dispersed by sonication after drying and subsequent deposition of the thin backing layer. With their increased surface area, we hypothesized that sonication was not providing enough energy to overcome the increased van der Waals attraction between the nanowires and the substrate. To test this hypothesis, a porous substrate was employed in order to limit the interaction of the nanowires with the substrate surface. This concept is illustrated schematically in Figure 1B; when the nanowires sediment from solution to bridge the pores on the surface, the total interaction area between the wires and the underlying surface is greatly reduced, limiting the points of contact to the solid regions between the pores (solid lines in Figure 1B). Consequently, the nanowires can be recovered more easily from the surface during sonication for the following two reasons: a decrease in the van der Waals attraction between the surfaces and an increase in the exposed area available for the ultrasonic waves in solution to affect the nanowires. Similar to those used to synthesize the nanowires, AAO membranes are an ideal material to meet these criteria and were chosen to test this hypothesis and serve as the support substrate for the deposition of the SiO2 backing layer (Figure 1A,D). After electrochemically synthesizing the nanowires in smaller diameter pore AAO membranes (35-100 nm) and releasing them into solution, the nanowires were vacuum filtered onto larger diameter pore AAO membranes (100-200 nm diameter) for the subsequent backing deposition step. Since their axial length is larger than the pores in the AAO, the nanowires collect on the surface of the membranes (Figure 1D), landing as bridges across the pores and fulfilling the conditions shown in Figure 1B. Furthermore, this strategy served to overcome the problems observed with capillary-induced aggregation of the thinner nanowires upon drying. Vacuum filtration forces the nanowires to dry in place on the substrate much more rapidly than unassisted drying methods and limits capillary forces that cause the wires to aggregate (Supporting Information Figure S1). Confirming our hypothesis, use of the porous AAO surface allowed recovery of sub-100 nm nanowires in high yield upon sonication in ethanol (Figure 1E), providing a good substrate on which to disperse the nanowires and deposit the SiO2 film. Finally, it was necessary to study the deposition of very thin (sub-10 nm) SiO2 support films using line-of-sight growth methods. Previously, the 360 nm diameter OWL structures were supported by a 50 nm SiO2 layer deposited by PECVD, but the smaller (sub-80 nm diameter) structures presented here require significantly thinner support films (5-15 nm). Initial attempts at scaling down the PECVD deposition time to deposit thinner films produced highly irregular and irreproducible films on the nanorods, leading to large variations in structure quality and yield (Supporting Information Figure S2A). We hypothesized that this lack of reproducibility was a result of two factors that are inherent to PECVD: complex reactions on the surface that are difficult to control for very thin films and imperfect line-of-sight depositions that connect the nanowires to the substrate instead of only depositing on the exposed side. As an alternative, we studied deposition of thin SiO2 films with electron beam (e-beam) evaporation. Since it is a physical vapor deposition technique that does not require the complex reactions of PECVD and
Figure 1. Synthetic strategy for preparing sub-100 nm diameter nanostructure arrays with OWL. (A) Schematic depiction of method beginning with (1a) release of template-synthesized multisegmented metal nanowires from AAO membranes into solution by dissolving the membrane and metal backing. (1b) Vacuum filtration of nanowires on to larger pore AAO membranes. (2) Electron beam evaporation of a thin (5-10 nm) SiO2 backing layer (blue coating). (3) Sonication in ethanol to release half-coated wires into solution. (4) Selective etching of sacrificial material (typically Ni, black segments) to produce 1D nanostructure arrays with geometry matching the original nanowire. (B) Schematic showing the decreased interaction area (solid black lines) between the nanowires and the porous substrate, decreasing the van der Waals attraction between the surfaces. (C-E) SEM images of 55 nm Au-Ni nanowires after steps 1, 2, and 4 (C, D, and E, respectively). (C) The 55 nm diameter Au-Ni nanowires codeposited with 270 nm Au-Ni nanowires, highlighting the increased resolution available with this new method compared to the previous smallest OWL structure. Scale bar is 350 nm. (D) Au-Ni nanowires filtered on to a porous AAO substrate, depicting the decreased interaction area shown schematically in B. Scale bar is 300 nm. (E) SEM image of 55 nm Au nanorods arrays with varying gap sizes after etching away the Ni segments. Scale bar is 165 nm.
was necessary to reconsider each of these steps to devise a method for preparing smaller diameter structures (Figure 1). Beginning with the synthesis of sub-100 nm multisegmented nanowires, the deposition of segments of each metal was carefully studied in anodic aluminum oxide (AAO) templates with pore diameters ranging from 35 to 100 nm. In each case, the electrochemical deposition conditions were finely tuned by controlling the applied potential and calculated charge to produce high quality, monodisperse metal segments with strong junctions between neighboring materials in the nanowires. For each of the metals studied (Au, Ag, Ni, and Pt), this series of optimization and calibration experiments led to well-formed and controllable segments with variations matching those reported for previous OWL structures but with dimensions that are significantly smaller. Figure 1C depicts an SEM image of 55 nm diameter Au-Ni nanowires codeposited with a 270 nm Au-Ni nanowire, which represents the smallest previously reported diameter for OWL, illustrating the striking decrease in size available with structures at these length scales. However, in attempting to manipulate these wires with OWL, we found it necessary to overcome three important obstacles that had not been encountered with the larger structures: capillary-induced 821
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Figure 3. Normalized solution-averaged extinction spectra collected in water of 35 ( 5 nm diameter dimers comprised of 68 ( 10 nm long Au nanorods with decreasing gap sizes (g, see inset) from approximately 16 to 0 nm (left to right). The transverse plasmon mode (around 550 nm) is mostly unchanged, whereas the longitudinal mode (between 800 and 1200 nm) redshifts with decreasing gap sizes of 16 ( 3 nm (blue), 10.3 ( 1.7 nm (red), 3.3 ( 1.2 nm (purple), and 0 nm (144 ( 16 nm long Au rod with no gap, gray).
Figure 2. Sub-80 nm OWL nanostructure arrays with independently tailorable diameters, lengths, and gap sizes. (A) Representative SEM image of 71 ( 8 nm diameter structures with varying gap sizes between approximately 20 and 4 nm. (B) Representative TEM image of 54 ( 6 nm diameter structures with varying disk thicknesses between approximately 150 and 6 nm (6.4 ( 2.1 nm). (C) Representative TEM image of 35 ( 5 nm diameter structures with gap thicknesses of 5.4 ( 1.9 nm (left) and 2.6 ( 1.6 nm (right). All scale bars equal to 100 nm. (D-F) Schematic images of structures from A-C drawn to scale to illustrate the range of sizes that have been synthesized. In each structure, a thin (sub15 nm) SiO2 layer is holding the nanorod segments together (apparent in SEM images in Supporting Information Figure S2).
10-20% variation in nanorod lengths and diameters observed for the 35-100 nm structures would hold for these structures. Note that the segment and gap lengths are measured at the narrowest point across for each structure, and the averages and standard deviations are calculated from measurements of approximately 30 structures for each sample. The roughness and nonuniformity of the nanorod segments can be attributed to limitations in the diffusion of metal ions into the pores of the AAO membranes and could be mitigated through pulsed electrochemical depositions of the metals as has been demonstrated for larger AAO-synthesized Au nanorods.34 Coupled with the ability to easily fabricate larger rod segments and gaps, these resolution limits provide one with the unique ability to fabricate structures that vary in length, diameter, and spacing by hundreds of nanometers. Furthermore, one can controllably create a large number of identical asymmetric arrays where the feature sizes and gap thicknesses can be individually varied at each feature location (Figures 1E and 2), which is particularly difficult to achieve by current methods. To illustrate one example of using this method to study the properties of coupled nanoparticles, we prepared 35 ( 5 nm diameter nanorod dimers comprised of 68 ( 10 nm long Au rod segments (aspect ratio of approximately 2:1) spaced by gap thicknesses between 16 and 0 nm to study the effect of interparticle spacing on plasmon coupling between nanorods (Figure 3). As was demonstrated previously,15,18,19 the transverse plasmon mode is relatively unaffected by coupling in the axial dimension, but there is a significant redshift in the longitudinal mode (peaks at 800-1200 nm) with decreasing gap thicknesses of 16 ( 3, 10.3 ( 1.7, 3.3 ( 1.2, and 0 nm (TEM images in Supporting Information Figure S3). The 0 nm gap used in these studies corresponds to a nanorod with no gap region and a length that is twice as long as the nanorods comprising the dimer structures (144 ( 16 nm). Importantly, this trend and the approximate magnitudes of the longitudinal shifts are consistent with theoretical calculations from Mulvaney and co-workers on similar structures (Supporting Information Figure S3).19 This is a significant demonstration of how one can use this method to rationally and precisely probe the solution-averaged optical properties of coupled plasmonic nanostructures over a large range of gap sizes (up to approximately 16 nm). Unlike other methods to
instead deposits material from a source in a true line-of-sight process, we hypothesized that e-beam evaporation would lead to more uniform and easier to control films, greatly increasing the reproducibility and yield. Indeed, this hypothesis was confirmed by comparing recovered nanowires with SiO2 films created by e-beam to those created by PECVD. Coating the nanowires by PECVD led to highly variable yields and films that are irregular and rough, whereas the SiO2 films deposited by e-beam evaporation led to more reproducible and homogeneous results (Supporting Information Figure S2). Combined with the use of AAO as the SiO2 deposition substrate, this innovation allowed us to overcome the synthetic challenges to produce monodisperse Au nanostructure arrays in high yield with finely tunable 1D architectures, such as the 55 nm diameter array structures with varying gaps shown in Figure 1E. Following these synthetic methods (Figure 1A), arrays with varying geometries, dimensions, and compositions were synthesized to highlight the unique ability to independently control each of these important parameters. Beginning with geometry, asymmetric Au nanostructure arrays with diameters of 71 ( 8, 54 ( 6, and 35 ( 5 nm were prepared with metal segment lengths between approximately 150 and 6 nm and gap thicknesses between approximately 40 and 3 nm (Figure 2), demonstrating control and versatility that is difficult, if not impossible, to produce with other methods for ordering sub-100 nm structures. Furthermore, the 35 nm diameter structures are an order of magnitude smaller than previous OWL arrays and extend the unique tailorability of the larger structures to colloidal length scales. Figure 2A,C depicts 71 and 35 nm diameter structures, respectively, which are comprised of nanorod segments spaced by varying gap sizes down to 2.6 ( 1.6 nm. Similarly, Figure 2B presents 54 nm diameter arrays with varying segment thicknesses down to 6.4 ( 2.1 nm, which is more than three times smaller than the previous limit for OWL. In theory, this method could be used to create sub-30 nm diameter gapped nanorod arrays, provided one could produce multisegmented nanowires in high yield at these length scales; however, it is not known whether the 822
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control that leads to complex materials made up of components that are individually addressable in length, spacing, diameter, and composition at each location in the arrays, providing a powerful strategy for synthesizing one-dimensional nanostructure arrays with controlled properties.
’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental methods, synthetic optimization results, additional plasmon coupling data, and representative EDX spectra of the heterostructures. This material is available free of charge via the Internet at http://pubs. acs.org.
Figure 4. Materials control and generality. (A) Representative TEM image from sample of 35 ( 6 nm diameter Pt-Au heterostructures with gap thicknesses of 1.4 ( 1.1 nm. (B) EDX map of structure shown in (A) displaying spatial distribution of Pt (green) and Au (orange). (C) Representative TEM image from sample of 54 ( 7 nm diameter Ni-Au heterostructures with gap thicknesses of 10 ( 3 nm. (D) EDX map of structure shown in (C) displaying spatial distribution of Ni (blue) and Au (red). All scale bars are 25 nm.
probe plasmon coupling, such as end-to-end assembly15,18 or random drying19,23 of solution-synthesized nanorods on a surface, this method has the advantage of allowing one to rationally prepare structures with highly programmable geometries (Figure 2; diameter, gap thickness, segment length, and number of coupled particles) and simply measure their extinction spectra in solution to determine the orientationally averaged optical properties of the entire population. In addition to these Au structures, there are a host of other materials with interesting coupling and transport properties at sub-100 nm length scales. Multicomponent arrays with sub-10 nm spacing and control over the composition at each segment are particularly interesting and difficult to prepare using other methods. Two examples of these hybrid materials are plasmoniccatalytic and plasmonic-magnetic arrays, both of which can be rationally prepared in high yield using our method. As a proof-ofconcept, 35 ( 6 nm diameter Pt-Au (Figure 4A,B) and 54 ( 7 nm diameter Au-Ni (Figure 4C,D) heterostructure arrays were synthesized with 1.4 ( 1.1 nm gaps and 10 ( 3 nm gaps between adjacent metal segments, respectively. In each case, energy dispersive X-ray spectroscopy (EDX) clearly confirms the distinct regions of each element (Figure 4B,D for elemental maps and Supporting Information Figure S4 for spectra). While Ni was used as the sacrificial segments in all other structures, Ag was used in the Au-Ni structures. The resulting arrays are similar in quality to the Au-only structures and demonstrate a unique control over composition that is difficult to achieve with other synthesis or assembly methods, enabling potential studies and applications based on the interaction and coupling between these materials with disparate properties. In conclusion, we report a unique and powerful method to control the geometry and composition of sub-100 nm diameter metal nanostructure arrays with OWL. Using this new synthetic procedure, structures can be prepared with diameters as small 35 nm, enabling metal segments and gap thicknesses down to approximately 6 and 1 nm, respectively. Importantly, we illustrate how this high degree of control can be used to probe the effects of particle spacing on the solution-averaged plasmon coupling properties of Au nanorods. By extending this to heterostructures containing Ni and Pt, we demonstrate geometry and composition
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The authors thank Matthew R. Jones for helpful discussions. C.A.M. gratefully acknowledges an NSSEF Fellowship from the DoD, DARPA, and the AFOSR for supporting this work. C.A.M. is also grateful for NSF MRSEC support and acknowledges the DOE Office (Award No. DE-SC0000989) for support via the NU Non-equilibrium Energy Research Center. K.D.O. acknowledges Northwestern University for a Ryan Fellowship and the National Science Foundation for a Graduate Research Fellowship. Electron microscopy was performed in the EPIC facility of the NUANCE center at Northwestern University. ’ REFERENCES (1) Corbierre, M. K.; Beerens, J.; Beauvais, J.; Lennox, R. B. Chem. Mater. 2006, 18, 2628–2631. (2) Wei, Q. H.; Su, K. H.; Durant, S.; Zhang, X. Nano Lett. 2004, 4, 1067–1071. (3) Vieu, C.; Carcenac, F.; Pepin, A.; Chen, Y.; Mejias, M.; Lebib, A.; Manin-Ferlazzo, L.; Couraud, L.; Launois, H. Appl. Surf. Sci. 2000, 164, 111–117. (4) Maier, S. A.; Kik, P. G.; Atwater, H. A.; Meltzer, S.; Harel, E.; Koel, B. E.; Requicha, A. A. G. Nat. Mater. 2003, 2, 229–232. (5) Krenn, J. R.; Dereux, A.; Weeber, J. C.; Bourillot, E.; Lacroute, Y.; Goudonnet, J. P.; Schider, G.; Gotschy, W.; Leitner, A.; Aussenegg, F. R.; Girard, C. Phys. Rev. Lett. 1999, 82, 2590. (6) Chen, G.; Wang, Y.; Tan, L. H.; Yang, M.; Tan, L. S.; Chen, Y.; Chen, H. J. Am. Chem. Soc. 2009, 131, 4218–4219. (7) Chen, G.; Wang, Y.; Yang, M.; Xu, J.; Goh, S. J.; Pan, M.; Chen, H. J. Am. Chem. Soc. 2010, 132, 3644–3645. (8) Lin, S.; Li, M.; Dujardin, E.; Girard, C.; Mann, S. Adv. Mater. 2005, 17, 2553–2559. (9) Liao, J.; Zhang, Y.; Yu, W.; Xu, L.; Ge, C.; Liu, J.; Gu, N. Colloids Surf., A 2003, 223, 177–183. (10) Tang, Z. Y.; Kotov, N. A. Adv. Mater. 2005, 17, 951–962. (11) Katz, E.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 6042–6108. (12) Dujardin, E.; Peet, C.; Stubbs, G.; Culver, J. N.; Mann, S. Nano Lett. 2003, 3, 413–417. (13) Warner, M. G.; Hutchison, J. E. Nat. Mater. 2003, 2, 272–277. (14) Caswell, K. K.; Wilson, J. N.; Bunz, U. H. F.; Murphy, C. J. J. Am. Chem. Soc. 2003, 125, 13914–13915. (15) Thomas, K. G.; Barazzouk, S.; Ipe, B. I.; Joseph, S. T. S.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 13066–13068. (16) Murphy, C. J.; San, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J. X.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857–13870. 823
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