Solution-Dispersible Metal Nanorings with Deliberately Controllable

Letter pubs.acs.org/NanoLett

Solution-Dispersible Metal Nanorings with Deliberately Controllable Compositions and Architectural Parameters for Tunable Plasmonic Response Tuncay Ozel,† Michael J. Ashley,‡ Gilles R. Bourret,§ Michael B. Ross,§ George C. Schatz,*,§ and Chad A. Mirkin*,†,‡,§ †

Department of Materials Science and Engineering, ‡Department of Chemical and Biological Engineering, and §Department of Chemistry, and International Institute for Nanotechnology, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: We report a template-based technique for the preparation of solution-dispersible nanorings composed of Au, Ag, Pt, Ni, and Pd with control over outer diameter (60−400 nm), inner diameter (25−230 nm), and height (40 nm to a few microns). Systematic and independent control of these parameters enables fine-tuning of the three characteristic localized surface plasmon resonance modes of Au nanorings and the resulting solution-based extinction spectra from the visible to the nearinfrared. This synthetic approach provides a new pathway for solutionbased investigations of surfaces with negative curvature. KEYWORDS: Nanorings, nanotubes, coaxial lithography, plasmonics, metals, negative curvature

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one face, one in-plane, and one out-of-plane; see Figure S1 for a geometric scheme) and can focus incident light inside as well as outside their cavities.34 Interestingly, nanorings also have a surface of negative curvature. Such surfaces exhibit distinct physical and chemical properties compared to those with positive curvature as they demonstrate fundamentally different surface effects35 and an inverse orientation of surface-bound ligands,36 which can be exploited to induce thermodynamically or electronically preferential interactions in solution. To thoroughly study nanoring structures, one needs a solutionbased method with precise control over their composition and the three key architectural parameters that define them (i.e., outer diameter, inner diameter, and height), all of which affect their plasmonic characteristics. Although there have been advances focused on synthesizing solution-dispersible nanorings via galvanic replacement reactions37,38 and templatemediated syntheses,39,40 none allow one to systematically prepare structures with independent control over composition and nanoring outer diameter, inner diameter, and height. Herein, we describe a novel method for preparing such nanoring architectures based upon coaxial lithography (COAL)41 and show that the technique can be used to prepare Au, Ag, Pt, Ni, and Pd nanoring structures with outer diameters deliberately and independently varied over the 60−400 nm range, inner diameters over the 25−230 nm range, and height over the 40 nm to several micrometer range.

dvances in lithographic and synthetic techniques have enabled the realization of a variety of complex nanoscale structures.1,2 Indeed, the architectural control provided by such methods has led to the discovery of important structure− function relationships in the context of optics,3 electronics,4 and catalysis.5 This control has been especially important in developing an understanding of the plasmonic properties of nanostructures where the composition, size, and shape of a given structure can dramatically affect the number, type, and intensity of surface plasmon resonances.6 This fundamental understanding and structural control have led to the development of novel materials that have been used for surfaceenhanced spectroscopies,7−9 biological probes,10,11 nanoscale rulers,12 plasmonic metamaterials,13 anisotropic emitters,14 nanoscale lenses,15,16 and stimuli-responsive structures for controlling intracellular events.17−19 Although there are now reliable methods for synthesizing metallic structures with welldefined and in certain cases exotic geometries, such as triangular prisms,20,21 disks,22 rods,23,24 wires,25 numerous polyhedra,26 and stellated nanoparticles,27 there are other geometries, such as rings,28 split-rings,29 and bow-ties30 that often must be fabricated via site-directed top-down methods. Indeed, one can use techniques like colloidal lithography28,31 and e-beam lithography32,33 to make small quantities of certain structures that cannot be made via conventional solution-based chemistry. The nanoring architecture is one shape that is both interesting from a plasmonics standpoint and difficult to realize via solution-based synthetic methods. Metal nanorings have unique electromagnetic properties in that they can support three different localized surface plasmon resonances (LSPR, © XXXX American Chemical Society

Received: April 23, 2015 Revised: June 5, 2015

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DOI: 10.1021/acs.nanolett.5b01594 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Nanoring synthesis. Synthetic scheme (steps I−V, represented by the cross-section of a single anodized aluminum oxide template pore) showing the synthetic steps used to make the metal nanorings. (I) Electrodeposition of polymer nanorods onto Ni segments; (II) polymer contraction under vacuum drying to create space between the polymer nanorods and the pore walls; (III) pore-widening to adjust the diameter of the pores (corresponding to the outer diameter of the nanorings); (IV) polymer-thinning to decrease the diameter of the polymer nanorods (corresponding to the inner diameter of the nanorings); (V) deposition of the sacrificial and target ring segments; (VI) removal of the polymer nanorods and template with acetone and 0.5 M aqueous NaOH, respectively. A top view scanning electron microscopy (SEM) image shows the nanorings attached to the sacrificial metal segments (see Figure S4 for a cross-sectional image); and (VII) release of the nanorings into solution by etching the sacrificial metal segments, as shown by high-angle annular dark-field scanning transmission electron microscopy (STEM). Scale bars are 1 μm.

Figure 2. Characterization of nanorings composed of different materials. STEM images (left) and elemental maps via energy-dispersive X-ray spectroscopy (right) of single component (a) Au, (b) Ag, (c) Pt, (d) Ni, and (e) Pd nanorings and a multicomponent (f) Au−Ni−Pt nanotube. STEM images in a, c, d, and e are false-colored for clarity. Refer to Figure S5 for large-area STEM images of single component nanorings. Scale bars are 100 nm.

formed by successive electrodeposition of a sacrificial metal segment and a target material around the PANI nanorod core with outer diameters that can be controlled by pore-widening with 0.5 M NaOH (III). Moreover, nanoring inner diameters were controlled by polymer-thinning with ethanol/water mixtures (IV, see Figure S3 for details), and their heights were dictated by the number of Coulombs passed during deposition (V). Sequential deposition of sacrificial and target shell segments can be repeated to generate multiple nanorings within each pore. The nanorings were then dispersed in solution by dissolving the polymer in acetone, removing the AAO template in aqueous NaOH (VI), and then selectively dissolving the sacrificial Ni and Ag metal segments through oxidation with appropriate aqueous etchants (VII). The synthetic process presented above is a universal approach for the synthesis of nanorings composed of many electrodepositable materials, as long as they are stable with

The synthesis of metal nanorings using COAL [scheme depicted in Figure 1 and explained in detail in the Supporting Information (SI)] was performed by sequential electrodeposition of various target and sacrificial materials into porous anodized aluminum oxide (AAO) templates using a conventional three-electrode electrochemical setup (see Figure S2 for details regarding an optional “pore refinement” step).25,41−43 In a typical experiment, a silver segment was first electrodeposited within each pore to ensure a consistent conductive interface between the evaporated silver film25,41−43 and the subsequently deposited segments. A sacrificial nickel segment was then electrodeposited, functioning as a stable base for the subsequent potentiostatic electropolymerization of aniline to prepare polyaniline (PANI) nanorods inside the AAO template (Figure 1, step I). Vacuum drying of the PANI nanorods induced contraction, creating space between the nanorods and the pore walls of the template (II).41,44−46 Nanorings were B

DOI: 10.1021/acs.nanolett.5b01594 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters respect to the etching conditions used for the sacrificial segments. To illustrate this point, we synthesized single component metal nanorings made of Au, Ag, Pt, Ni, and Pd and a multicomponent nanotube composed of Au, Ni, and Pt and then characterized them using energy-dispersive X-ray (EDX) spectroscopy (Figure 2). Nanorings composed of different materials were prepared by slight variations to the synthetic protocol; namely, the sacrificial shell segments and metal etching solutions were changed (Figure 1, V and VII, see SI for more details). Au and Pt nanorings were electrodeposited on a sacrificial Ni shell segment which was subsequently dissolved in aqueous 3% FeCl3 for 20 min. Ag nanorings were synthesized by electrodeposition of a Ni−Ag−Ni shell, with the Ni cap segment serving to protect the Ag during the chemical removal of the Ag backing layer. Ni segments were then etched with 15% H3PO4 in water for 15 min. Ni and Pd nanorings were electrodeposited on a sacrificial Ag shell segment, which was etched by a 1:1 H2O2:NH4OH mixture that is also used to dissolve the Ag backing layer. Finally, the multicomponent nanotube was synthesized by sequential electrodeposition of a Au−Ni−Pt shell on a sacrificial Ni segment, which was etched by 3% FeCl3 exposure for 20 min while still inside the AAO template. The target Ni segment of the nanotube remained intact as it was physically protected from FeCl3 by PANI, the AAO template, and the Au and Pt shell segments. Importantly, COAL provides the ability to control nanoring architecture in addition to its compatibility with a variety of metals. In order to investigate the structural tailorability of the synthetic process described in Figure 1, Au rings with various dimensions were synthesized. Though the techniques described below can be applied to tune the dimensions of nanorings composed of any of the materials shown in Figure 2, Au was chosen for its chemical stability and measurable plasmonic response to architectural changes. We studied this response by preparing solution-dispersible Au nanorings in templates with different nominal pore sizes (35, 55, and 100 nm from Synkera Technologies). Synthesis in these templates yields Au nanorings with outer diameters of 63 ± 5 nm, 95 ± 10 nm, and 179 ± 17 nm, respectively (see Figure 3a for top view SEM images of Au nanorings after PANI and template dissolution and Table S1 for complete statistical analysis of Au nanoring samples). As a result, we found that wide-range tuning of the nanoring LSPR wavelength can be achieved simply by starting with templates with different pore diameters (Figure 3b). This synthetic flexibility results in the ability to tune resonances from the visible (590 nm) to the near-infrared (1150 nm) by increasing overall nanoring size (both outer and inner diameters). In order to achieve a greater understanding of how changes in nanoring outer diameter, inner diameter, and height affect the LSPR, we varied each dimension independently (Figure 4). Changes in the experimental extinction spectra were modeled with discrete dipole approximation simulations (Figure 4, right column, and Figures S7 and S8) and are in reasonable agreement with the experimentally observed spectra (Figure 4).47 Notably, the experimental spectra exhibit broader LSPRs in all cases when compared to the calculated spectra; we attribute this to variation in the nanoring size, shape (circular vs. elliptical, see Figure S9), position of the cavity (concentric vs. noncentered),48 and crystallinity.49 While broader peaks are nearly always observed in experiment compared to simulation, when using 100 nm pore diameter templates (Figure 3, green trace and Figure 4), our experimental data also demonstrate a

Figure 3. Au nanorings synthesized in templates with different pore diameters. (a) Top view SEM images of Au nanorings with outer diameters of ∼63, 95, and 179 nm, and inner diameters of ∼25, 37, and 95 nm (top to bottom, respectively). Scale bars are 100 nm. (b) Extinction spectra of Au nanorings with different diameters (shown in (a) and STEM images in top row, scale bars are 50 nm) show plasmon resonances at ∼593, 715, and 1150 nm (smallest to largest template, respectively). Electron microscopy images of nanorings synthesized in a 270 nm pore diameter template are shown in Figure S6.

consistent ∼200 nm red-shift compared to our discrete dipole approximation calculations. We attribute this discrepancy largely to dispersity in nanoring shape, as more elliptical nanorings exhibit significantly red-shifted SPR peaks compared to perfectly circular nanorings. Simulations suggest that maintaining fixed inner and outer diameters in one axis of a nanoring while stretching them in the other axis can account for a significant red-shift, even as large as 200 nm (Figure S9). The presence of elliptical nanorings is explained by the shape of the AAO pores into which the metals are electrodeposited. Since not all of the AAO pores are perfectly circular, we observe this variation in nanoring shape. Notably, we also observe excellent agreement in peak position between experiment and simulation for nanorings synthesized in the 35 and 55 nm pore diameter templates (Figure 3b, Figure S7). When using 100 nm pore diameter templates, our base case nanoring sample was synthesized using an 18 min porewidening step with no polymer-thinning, and nanorings had an aspect ratio (height−outer diameter) of approximately 0.4. We found that NaOH exposure beyond 18 min led to the merging of some pores, compromising the homogeneity of the resulting structures. Conversely, pore-widening must be performed for at least 3 min in order to generate a solution of fully formed rings without the formation of crescent-like structures. Samples prepared at each of these extremes allowed for the demonstration of outer diameter tunability using AAO templates with equivalent nominal diameters; this control then allows one to deliberately tune nanoring LSPR (Figure 4a). For example, an average outer diameter increase from 155 to 180 nm (with inner diameter and aspect ratio remaining constant) results in a 50 nm blue-shift of the most intense LSPR wavelength (from 1200 to 1150 nm), which we attribute to an increase in the nanoring wall thickness.28 We also used the technique to investigate the relationship between nanoring inner diameter and LSPR; control over inner diameter was effected by slowly dissolving the PANI nanorod core that defines the inner diameter of the resulting nanorings. To achieve homogeneous and reproducible PANI dissolution, C

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Figure 4. Control over all three architectural parameters of Au nanorings. STEM images (left), measured extinction spectra (middle), and calculated extinction spectra (right) of the nanorings are presented in (a), (b), and (c). (a) The outer diameter was enlarged by increasing the duration of the pore-widening step from 3 to 18 min. This increased the outer diameter from 156 to 179 nm (maintaining constant inner diameter and aspect ratio), which corresponds to a 50 nm blue-shift in the most intense LSPR wavelength. (b) The inner diameter was reduced from 95 to 64 nm (maintaining constant outer diameter and aspect ratio) through the introduction of the polymer-thinning step; this corresponds to a 192 nm blue-shift in the LSPR wavelength. (c) The height was increased with the amount of charge passed through the electrodes during the electrodeposition step. By increasing the applied charge from 350 to 1750 mC, the aspect ratio was increased from ∼0.4 to ∼1.8 (maintaining constant outer and inner diameters), which corresponds to a significant red-shift in the lowest frequency plasmon resonance wavelength by 478 nm. Scale bars are 100 nm in all of the electron microscopy images. The blue curves in all of the extinction spectra correspond to the same nanoring sample that has been porewidened for 18 min.

the AAO pores were widened to maximize space for the ethanol/water solution to travel deep into the pores, increasing PANI dissolution from its sides. After testing the effect of different concentrations of ethanol in water (60−90%) for varied periods of time (10−60 min), we found that the rate of polymer-thinning slows with exposure time (see Figure S3). Moreover, we determined that using aqueous solutions with ethanol concentrations of 70% was optimal, since these thinned the polymer nearly as much as more concentrated solutions (>80% ethanol) without dissolving the polymer as much from the top (which, if done excessively, would introduce disks into solution). Thus, we observed the smallest nanoring inner diameters, while maintaining pure nanoring solutions (i.e., no disks), by exposing samples to 70% ethanol for 30 min. The average inner diameter can be reduced by more than 30% (from 95 to 64 nm, while maintaining consistent outer diameter and aspect ratio) through this polymer-thinning step, which

results in an almost 200 nm blue-shift in the LSPR wavelength (from 1150 to 958 nm). We also attribute this shift to an increase in the nanoring wall thickness (in this case, a smaller cavity), leading to more disk-like resonance behavior.28,50 This technique also provides control over nanoring height. To effect such control, one simply adjusts the amount of charge passed through the electrodes according to the desired length, a technique used routinely to control rod length in templatecontrolled nanowire synthesis.25,42,43,51−56 The aspect ratios of the samples discussed so far were deliberately maintained around 0.4 to eliminate the convoluting effects that an inconsistent aspect ratio would have on the nanoring LSPR. Note that in only one case involving the smallest rings (63 nm outer diameter), the aspect ratio was required to be 0.75 to maintain structural stability; structures with smaller aspect ratios could be generated but exhibited fragility in solution as evidenced by STEM (i.e., broken and incompletely formed D

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acknowledges DoD for a National Defense Science and Engineering Graduate (NDSEG) fellowship.

rings, see Figure S10). By maintaining a consistent aspect ratio, we ensured that LSPR wavelength shifts would be solely a consequence of the varied architectural parameter. For comparison purposes, by adjusting the nanoring aspect ratio to 1.8 (while maintaining the outer and inner diameters of the base case sample), a nearly 500 nm red-shift of the most intense LSPR wavelength (from 1150 to 1628 nm) is observed (Figure 4c). This shift is due to the longer oscillation pathway for the electrons along the extended height of the nanorings, leading to rod-like resonance behavior.6,51 Note that electrodeposition also allows one to synthesize nanotubes that are microns in length, but such structures are not the focus of this study. In this work, we have shown that COAL can be used for the synthesis of solution-dispersible metal nanorings with unprecedented control over composition and architectural parameters. In particular, Au nanorings prepared with a wide range of dimensions led to the tunability of their LSPRs from the visible to the near-infrared. Our synthetic approach opens a new pathway for solution-based investigations of surfaces with negative curvature, whose chemical and physical behavior remain poorly understood. Such surfaces have potential applications in catalysis57 and site-specific nanomaterial interactions.36 Moreover, nanorings prepared using COAL can be used to explore the possibilities of selective functionalization of negatively curved surfaces and nanoparticle assembly within tailorable cavities for fundamental light−matter interaction studies.

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ASSOCIATED CONTENT

S Supporting Information *

Materials, instruments, detailed experimental procedures, nanoring measurement statistics, and supplementary simulations, STEM, and SEM images referenced in the text. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b01594.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

T.O. and M.J.A. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the following awards: AFOSR FA9550-09-1-0294, AOARD FA2386-13-14124; the Office of the Asst. Secr. of Defense for Research and Engineering, DoD/NSSEFF Program/NPS N00244-09-1-0012 and N00244-09-1-0071; National Science Foundation Graduate Research Fellowship under Grant No. DGE-1324585. This work made use of the EPIC facility (NUANCE CenterNorthwestern University), which has received support from the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); and the State of Illinois, through the IIN. T.O. acknowledges 3M for a science and technology fellowship, ECS for a summer fellowship, and SPIE for an optics and photonics education scholarship. M.J.A. acknowledges NSF and Patrick G. and Shirley W. Ryan for graduate research fellowships. M.B.R. E

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