Article pubs.acs.org/JPCC
Electron Energy Loss Spectroscopy of Surface Plasmon Resonances on Aberrant Gold Nanostructures Gregory T. Forcherio,† Drew DeJarnette,⊥,‡ Mourad Benamara,§ and D. Keith Roper*,†,∥ †
MicroElectronics-Photonics Program, §Institute for Nanoscience and Engineering, and ∥Ralph E. Martin Department of Chemical Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States ‡ Department of Mechanical Engineering, University of Tulsa, Tulsa, Oklahoma 74104, United States S Supporting Information *
ABSTRACT: Effects of structural elongation, annulation, and irregularity on emergent plasmonic modes were examined computationally and experimentally using low-loss electron energy loss spectroscopy (EELS). Resonant eigenstates were compared for multiple gold nanodisc shapes: circular, elongated, annulated, and acircular annulated. Losses of resonant eigenstates were mapped to distinguish discrete bright and dark resonance modes that emerged from morphological changes. Single bright and dark resonances exhibited by circular nanodiscs were supplemented by additional bright modes arising from axial asymmetry induced by radial aberrations in elongated discs. An antibonding bright mode appeared in annulated discs due to different dipole patterns arising from charge interactions on adjacent surfaces. Eccentricity on annulated discs hybridized resonant mode structures from discs, rings, and ellipses. Direct comparison between experimental measures and theory for a fabricated elongated nanoring with morphological distortions showed in vacuo discrete modes collapse into singular, red-shifted resonances with expanded bandwidths due to screening and interfacial damping with dielectric substrate and metallic adhesion interlayer. These results indicate effects of morphological aberrations and irregularities on the plasmon activity of deposited, self-assembled, and/or oxidized noble metal discs that may appear in natural environments.
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INTRODUCTION
To compute EELS modes, discrete dipole approximation (DDA) is preferred to boundary element method (BEM), as arbitrary structures may be examined with volume or surface discretization, substrate inclusion, and direct correlation with photon excitation.19 BEM is typically reserved for quasistatic structures whose volume may be approximated as a onedimensional surface contour.1 DDA solutions can be collated with related methods20−22 to predict nanometer-scale electron energy loss distributions with far-field optical extinction spectra and electric near-fields.23 Finite element method (FEM) is an alternative approach, but solution divergence below 8 eV energy loss24 precludes facile evaluation of noble metal plasmons. Correlation of computed photon and electron modal excitations with measured modes for acircular, annular, and aberrant nanostructures beyond quasistatic dimensions could support nonlinear four-wave mixing,25 on-chip antennae,26 lasing,27 and biological sensing.28 This work examined the effects of acircularity, annularity, and aberration on emergence of bright, dark, and hybrid modes in plasmonic nanostructures. Plasmon modes excited by electrons incident on circular, elongated (elliptical), and annulated (ringed) gold (Au) nanodiscs were computed at dimensions beyond the quasistatic limit using the open-source DDA code
Collective free electron oscillation at a metal−dielectric interface, i.e., a surface plasmon, can be induced by electrons1,2 in a scanning transmission electron microscope (STEM) to comprehensively characterize “bright” and “dark” plasmon modes of a nanostructure. “Bright” plasmon modes are excitable by photons3 and electrons, whereas “dark” plasmon modes with net zero dipole moment are only inducible by electrons. Engineered excitation and decay of discrete bright and dark plasmonic modes on nanostructures could optimize absorption, radiation, hot electron transport, or generation of electron−hole pairs for optoelectronics.4,5 Plasmonic activity of fabricated nanostructures evolves as a consequent of morphological/environmental aberrations due to deposition, selfassembly, or oxidization in natural environments.6,7 Bright and dark plasmon modes may be characterized by computation or direct measurement using low-loss electron energy loss spectroscopy (EELS).1,8 Mode structures have been computed for regular discs,9 split-ring resonators,10 triangular prims,11−13 nanorods,14 and nanobipyramids15 and a limited number of geometric aberrations: disc elongation,16 truncated prism edges,11 touching/asymmetric sphere dimers,14,17 and disc transformation into triangular prisms.18 The concomitant influence of acircularity, annularity, and aberrations on emergence of bright, dark, and hybrid modes on metallic discs has not been computed or measured by EELS. © 2016 American Chemical Society
Received: July 5, 2016 Revised: September 19, 2016 Published: October 3, 2016 24950
DOI: 10.1021/acs.jpcc.6b06724 J. Phys. Chem. C 2016, 120, 24950−24956
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Figure 1. Computational EELS spectra for a 60 nm radius (a) Au nanodisc subjected to (b) 1.5× aspect ratio elongation, (c) annulation, and (d) elongated annulation. Impacts points were at the center (blue), major edge/mid (red/purple), and minor edge/mid (green/teal). Structure schemes are inset and were 20 nm in height. “B” and “B*” indicate bonding and antibonding bright modes. “D” indicates dark mode.
eDDA.29 Computed modes were compared with modes excited by electrons via EELS for an aberrant, annulated disc in a STEM with a subnanometer electron beam.1,8 Electrons transmitted with characteristic amounts of lost energy were binned to give EELS spectra. EELS permits analysis of nonradiative plasmon decay, such as hot electron transfer,30 without confounding interactions arising from continuous-wave spectral characterization over a large geometric area. Measured EELS data from an irregular, acircular nanoring was compared with predictions to account for structural distortions that created energy loss hotspots.
the structure. eDDA was used instead of DDEELS19 because the former can be integrated into the widely accessible lightexcitation DDSCAT. DDSCAT (v7.3) 31 was used to distinguish bright modes from eDDA-computed mode structures, a technique used by Sigle et al. in characterizing silver nanohole clusters.32 Modes not observed in DDSCAT simulations were deemed dark modes. Experimental. Au nanorings and nanoellipses were fabricated and characterized following ref 30. Briefly, lattices of nanostructures were thermally evaporated onto a 50 nm thick silicon nitride (SiN) membrane using an electron resist patterned by electron beam lithography. Deposited metal nanostructures consisted of 8 nm (15 nm) of Au for nanorings (nanoellipses), along with a 2 nm chromium (Cr) base. Samples were subjected to oxygen plasma prior to EELS to prevent carbon buildup. EELS was performed at 120 kV with 0.05 eV energy binning in a STEM (FEI Tecnai F-20, FEI, Hillsboro, OR, USA) outfitted with a GATAN post-column imaging filter (GIF Quantum 963, GATAN, Pleasanton, CA, USA). Energy resolution was approximately 0.50 eV, as defined by the full width at half-maximum (fwhm) of the zero-loss peak (ZLP). The ZLP was extracted using the power law approximation; reflected tail and polynomial fits were also considered but were less consistent in resultant eigenstates. Spectra were captured with the ZLP maxima and negative side
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METHODS Numerical. Regular nanodisc, nanoellipse, nanoring, and elliptical nanoring structures were discretized into a cubic lattice of point dipoles and probed with a point electron source using the discrete dipole approximation package eDDA v2.0.29 A 2.5 nm interdipole spacing was used to provide solution convergence without excessive computation time and memory expense. Previous electron-excitation DDA work has examined impact points outside the nanostructure. This work examined electron impact within the nanostructure but between grid points to avoid incidence at dipole locations causing computational divergence. EELS spectra were generated at energy losses between 1 and 3 eV at a single location (i.e., impact point) on 24951
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possible geometric distributions for induced charge densities in the ELDOS to accommodate higher-order multipole LSP resonances. Angular rotational symmetry of a regular disc precluded resonant energy dependence on the angular coordinate. The dark mode at 2.45 eV represents the bulk plasma energy of Au.41 Bulk Au mode observation has been reported for other structures.34,41,42 Interpretation of mode-types from EELS spectra requires coordinated consideration of (i) loss states supported at a given impact point and (ii) spatially adjacent points which support the same loss state. The 2.03 eV low-energy resonance was only supported by excitation outside the centroid (i.e., purple and red impact points). Positive charge would build up around the impact point in a lab-frame to maintain charge neutrality, and the incident electron energy would be preferentially lost to the disc edges, as shown in Figure S1(a) of Supporting Information. Within the context of polarized light, a dipole ⎯⇀ ⎯ moment (∑ p ≠ 0) would be formed between positive and negative charge on opposing sides of the disc, hence its denotion as a bright mode. At 2.45 eV energy loss, dipole moments equal in magnitude would be formed from the disc edges (negative charges) to the disc center (built up positive charge at blue impact point), as shown in Figure S1(b) of the Supporting Information. Thus, no net dipole is formed due to ⎯⇀ ⎯ dipole moment vector cancellation, i.e., ∑ p = 0, which precludes photonic induction. The symmetric charge distribution of dark modes in discs are sometimes referred to as “breathing” modes due to their analogy to acoustic vibrations.9 Though not observable with photons, a bright dipole LSP induced by photons can decay via coupled excitation of a dark mode in an adjacent particle.43,44 Detailed discussion of modetype attribution and corresponding EELS maps of discs can be found in sections S.1 and S.2 of the Supporting Information and in refs 9 and 34. Impact of Acircularity in Nanodiscs. Radial elongation of the nanodisc to a 1.5× aspect ratio caused a second bright mode at 1.67 eV to emerge while blue-shifting the original bright mode by 0.17 eV due to radial asymmetry. Corresponding maps supporting mode attribution may be found in refs 34 and 42. Nanorods analogously support two bright modes in the ELDOS, observed in polarized optical excitation along transverse (rod diameter) and longitudinal (rod length) axes.45 Figure 1(b) shows simulated EELS spectra for a Au ellipse with 90 nm (major) and 60 nm (minor) radii (20 nm thickness) impacted at the center (blue), mid/edge major radius (purple/red), and mid/edge minor radius (teal/ green). Major axis excitation induced a 1.67 eV bright LSP. Minor axis excitation induced a 2.21 eV bright LSP. Dipole LSP are known to red-shift from increasing the radial dimension of an axis according to Mie theory of oblate spheroids. The 2.45 eV dark “bulk” mode was preserved despite the structural asymmetry. Photon excitation of an ellipse can controllably decay into a dark mode on an adjacent, rotated ellipse by controlling the relative physical orientation between the two.5 Impact of Annularity in Nanodiscs. Annulation of the nanodisc created distinct “bonding” and “anti-bonding” bright modes by charge interactions between adjacent surfaces, in this case, the inner and outer walls. Figure 1(c) shows computed EELS spectra for a Au nanoring with 60 nm outer radius and 40 nm inner radius (20 nm thickness) impacted at the mid(purple) and far edge-points (red) of the ring wall. Resonances observed at 1.41, 2.11, and 2.36 eV corresponded to bright
cutoff to accommodate higher exposure times for improved signal:noise; thus, Richardson−Lucy deconvolution33 or direct subtractions of a Gaussian- or Lorentzian-fitted ZLP were precluded. The diameter of the STEM probe was approximately 2−3 nm. EELS maps visualize resonant energy losses on a nanostructure;10−13,34,35 however, acquisition of EELS mapping herein was precluded by STEM probe energy/ intensity drift and environmental factors.
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RESULTS AND DISCUSSION Low-loss EELS identifies where and how incident energy is lost on a metal nanostructure through resonant eigenstates, thus describing the electromagnetic local density of states (ELDOS) and plasmon decay path. EELS is useful to understand the evolution of ELDOS due to morphological aberrations of fabricated metal nanostructures functioning in natural environments. Reviews by Kociak and Garcia de Abajo et al. provide comprehensive discussions of plasmon excitation using EELS.1,2 Electrons with particular energies lost to the structure from the incident STEM beam (120 keV herein) correspond to plasmon modes and qualitatively describe the ELDOS36 by identification of the electric dipole, quadrupole, and higher multipoles. Despite relative agreement between EELS spectra and the ELDOS, direct quantitative correlation between the two is precluded by differences in respective excitation electricfield distributions.2,16 EELS can be correlated to optical spectroscopy results using modal decomposition.37 The bandwidth of each EELS plasmon mode comprises plasmonexcited electron decay pathways including carrier−carrier scattering and carrier−phonon scattering. For example, EELS has been used to infer efficiencies of hot carrier transfer from localized surface plasmons (LSP) excited on metal nanostructures to supporting graphene30 and molybdenum difsulfide38 by accounting for alternate contributions to resonant bandwidth. Figure 1 shows the evolution of plasmon mode structure within the ELDOS of a 60 nm radius Au nanodisc (upper left) as a result of elongation and annulation. The plasmonic mode structure for a given geometry is comprised of bright, dark, and hybrid resonance modes. Bright modes (labeled B and B* for bonding and antibonding, respectively) are resonances which ⎯⇀ ⎯ exhibit a finite dipole moment (∑ p ≠ 0), thus enabling both photon and electron excitation. Electrons in EELS are also capable of exciting dark modes (labeled D) with net zero dipole ⎯⇀ ⎯ moment (∑ p = 0) that cannot be directly optically induced. In the quasistatic limit for Au, the bright and dark modes converge into a single mode at 2.45 eV where the real component of its dielectric function crosses zero, i.e., the bulk plasma frequency.39 The two modes split and shift when the radially symmetric form is elongated (upper right), annulated (lower left), and concomitantly elongated and annulated (lower right). Regular Nanodiscs. Resonant excitation of centrosymmetric nanostructures just beyond the quasistatic regime yields distinct bright and dark plasmonic resonances. This is observed in Figure 1(a), which shows pointwise EELS data of a 60 nm radius disc computed using eDDA at the particle center (blue), half radius (purple), and edge (red). Modal eigenstates at 2.03 and 2.45 eV corresponded to bright and dark plasmon resonances, respectively. The bright plasmon mode at 2.03 eV corresponded to a dipole LSP, as observed previously via optical excitation.40 Expansion of the radial dimension would (i) red-shift the dipole LSP and (ii) increase the quantity of 24952
DOI: 10.1021/acs.jpcc.6b06724 J. Phys. Chem. C 2016, 120, 24950−24956
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The Journal of Physical Chemistry C bonding, dark, and bright antibonding modes,46 respectively. Corresponding EELS maps corroborating mode attribution may be found in Figure S2 in Supporting Information section S.3. These EELS results match reported experimental and theoretical results within 0.17−0.03 eV for comparably sized rings under photon excitation.20,21,47 Higher energy antibonding exhibits dipole moments on each wall, anticipated to occur near 2.4 eV according to a Drude-type dielectric model.47 Antibonding modes were not observed in EELS experiments on smaller nanorings.48 Lower energy bonding exhibits collective negative and positive charge on opposing walls, anticipated to occur near 1.2 eV according to a Drude-type dielectric model.47 Increasing the ring aspect ratio (wall thickness to radius) blueshifts the bonding mode resonance to that of a disc LSP (i.e., mode convergence)47 as described earlier. Inter-wall coupling and charge transfer in nanorings could be explained by LCcircuit models.49 Anticipated charge distributions for each bright mode type are characterized in ref 47. The 2.11 eV dark ⎯⇀ ⎯ resonance would create net zero dipole moments (∑ p = 0) along the ring wall. In other words, positive charge would build up in the center of the wall with negative charge distributed along the inner and outer edges (see higher loss intensity at 2.11 eV for edge impact). Such a charge distribution would not be possible with photons because each subwavelength wall experiences a static excitation field.2 Dark modes were not observed in EELS experiment of rings with 7−10 nm wall thicknesses.48 Concomitant Impact of Acircularity and Annularity. Simultaneous radial and annular aberrations resulted in a mode structure hybridized between a nanoring [Figure 1(c)] and a nanoellipse [Figure 1(b)]. The EELS graph in Figure 1(d) shows the mode structure for this elliptical Au nanoring 90 nm (60 nm) across the outer (inner) major axis and 60 nm (40 nm) across the outer (inner) minor axis; the 1.5× aspect ratio was held constant from the nanoellipse simulations. Resonance modes were 1.16 eV bright minor axis bonding, 1.44 eV bright major axis bonding, 1.94 eV dark plasmon, and 2.42 eV bright antibonding. Occurrence of bonding along each axis, emergence of antibonding, and a single dark mode are consistent with hybridization of the ring and ellipse mode structures observed in Figures 1(b) and 1(c). For example, consider the 1.5× aspect ratio nanoellipse in Figure 1(b) subjected to annulation to result in the elliptical nanoring. As described for the nanoring, annulation at the considered dimensions (i) red-shifts the bright mode by 0.62 eV, (ii) redshifts the dark mode, and (iii) causes a bright antibonding mode to emerge at 2.36 eV. Annulating the nanoellipse [transition from (b) to (d) in Figure 1] caused both bonding modes to red-shift, red-shifted the dark mode down to 1.94 eV, and induced a bright antibonding mode at 2.42 eV. The energy of the bright bonding mode for the 0.6× aspect ratio major axis was lower than the bright bonding mode for the 0.7× aspect ratio minor axis, consistent with previous work under light excitation.20,21 A similar analysis can be performed by elongating the nanoring to a 1.5× aspect ratio to arrive at the plasmon mode structure exhibited by the elliptical nanoring. Morphological Distortions in Fabricated Structures. Radial aberrations, morphological distortions, and interfacial damping by substrates present in fabricated nanostructures alter plasmonic resonance modes relative to corresponding simulations of homogeneous structures in vacuo. A radially aberrant nanoellipse and nanoring, each with ca. 1.5× aspect ratios, were
fabricated using electron beam lithography and thermal evaporation. The fabricated ellipse and ring were each examined using EELS. Primary features in spectra from the nanoellipse were consistent with those simulated in Figure 1(b) after considering screening-induced red-shifts. However, overlap between high-energy bright and dark modes prevented clear resolution of the features supported by minor axis edge excitation. These spectra are shown and described in section 4 of the Supporting Information. Complexity inherent to the plasmon mode structure of the elliptical nanoring simulated in Figure 1(d) (e.g., distinct, well-separated bright and dark modes), the presence of Cr and SiN membrane, and the degree of morphological distortions present in the fabricated sample warranted further examination. The fabricated nanoring was directly modeled using eDDA, as shown in Figure 2(a) with a target generated from a high-
Figure 2. EELS spectra for a fabricated, aberrant nanoring with an approximate 1.5× radial aspect ratio according to (a) eDDA in a vacuum, (b) eDDA with a Cr adhesion layer and SiN membrane, and (c) experiment taken at the major (purple) and minor (teal) axis impact points. A HAADF-STEM image of the ring is inset to (c) from which the eDDA target was directly generated, along with corresponding impact points. The interdipole spacing was 5.55 nm in eDDA. “B” indicates a bright mode, and “D” indicates a dark mode. 24953
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minor axis, consistent with the results from Figure 1(d) and previous observations.20,21 These maxima appeared to correspond to theoretical, vacuum bright 1.88 eV and dark 2.07 eV resonances in Figure 2(a) due to close energy proximity in experiment and common ca. 0.15 eV energy separation. A resonant energy differential between theory and experiment of about 0.025 eV has been reported for smaller quasistatic nanorings.48 Energy differences between simulated and observed modes in Figure 2(b) and 2(c), respectively, may result from (i) inhomogeneities in Au/Cr thickness and crystal structure; (ii) increased ring diameter relative to reported structures; (iii) finite EELS energy resolution (0.50 eV); and/or (iv) possible beam damage accrued from consecutive spectral acquisitions.17 Peak width differences between simulated and observed modes could arise from these influences as well as localization of electron charge from adsorbed elemental defects on the Au surface reported to split modal peaks in EELS.55 Absent Richardson−Lucy deconvolution,33 0.50 eV resolution as measured by ZLP fwhm may be inadequate to resolve close peak splitting, resulting in broadened measured peaks. Inclusion of additional effects into simulations to improve correspondence of EELS simulation and experiment of electron decay routes in nanostructures supported by dielectric substrates is the subject of future work.
angle annular dark-field (HAADF) STEM image that is inset in Figure 2(c). Dipole locations for eDDA were designated using gray scale pixel intensity; intensity >50 (0 to 255 scale) classified a pixel for inclusion in the target structure. Three bright (“B”) modes (1.42, 1.76, and 1.88 eV) and one dark (“D”) mode (2.07 eV) were simulated by eDDA in vacuo in Figure 2(a). Inclusion of the 2 nm Cr layer and SiN membrane in the eDDA simulation suppressed bright resonances in the ELDOS to a single mode due to substrate-induced interfacial damping as shown in Figure 2(b). EELS spectra measured from the fabricated structure in Figure 2(c) exhibited additional redshifting due to ring composition inhomogeneity unaccounted for in the eDDA description as well as metrological deficiencies. Impact points in the experiment and computation were the center of the ring wall along the major axis (purple) and minor axis (teal). Predicting plasmon excitation for nanostructures of this size (approximately 600 nm major diameter) is difficult because quasi-static approximations become convoluted with dynamic effects21 such as dynamic depolarization from phase retardations across the particle50 and spontaneous light emission from excitons. Therefore, data from the elliptical ring in Figure 1 (common 1.5× aspect ratio) and results under light excitation (DDSCAT) were used to interpret resonant modes in Figure 2. Energy losses of 1.42, 1.76, and 1.88 eV [Figure 2(a)] corresponded to bright dipolar plasmons. These were not suspected to exhibit bonding or antibonding behavior as observed for the annular structures of Figure 1 due to the extent of acircularity and mass distortions in the fabricated structure. Energy loss hotspots at these fabrication imperfections can be advantageous in some applications, such as sensing or surface-enhanced Raman spectroscopy, as illustrated by EELS analysis on nanoshurikens51 and “spiky” octahedrons.52 Furthermore, bright modes in the 1.5−2.0 eV range would not be characteristic of bonding/antibonding behavior in a ring of this size.20,21,47 The energy loss eigenstate at 2.07 eV in Figure 2(a) was deemed dark, consistent with the ca. 2 eV dark mode for the elliptical ring in Figure 1(d). A 2.07 eV resonance was not observed using photon excitation (i.e., DDSCAT) polarized across the minor axis. Screening of the 2 nm Cr adhesion layer and SiN membrane in experiments was expected to reduce the net energy required for plasmon excitation by acting against the Coulombic restoring force of the induced charge distribution in the ring. Inclusion of Cr and SiN in the eDDA simulation, shown in Figure 2(b), compressed the two respective modes supported by each impact point in vacuo into single, red-shifted resonances. The vacuum bright (dark) 1.88 eV (1.73 eV) mode peak from the major (minor) axis apparently red-shifted to yield a 1.64 eV (2.07 eV) mode. Decoherence and subsequent damping of each plasmon mode due to Au interfacial damping with Cr and SiN was captured by eDDA in expanded resonance bandwidths.30,53,54 Bright modes simulated in vacuo at 1.42 and 1.76 eV were not apparent in the substrate-containing simulation. Experimental EELS spectra for the fabricated nanoring shown in Figure 2(c) exhibited a single, broad resonance peak at each impact point, similar to eDDA simulations that included the adhesion layer and substrate in Figure 2(b). Experimental resonance peak maxima at 1.20 and 1.35 eV were red-shifted an additional 20%+ relative to simulated values in Figure 2(b). The plasmon mode excited for the major axis appeared at a lower energy than the plasmon mode for the
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CONCLUSIONS Electron energy losses were discretized in Au nanodiscs, nanoellipses, and nanorings using an open-source DDA model, allowing discrete bright and dark plasmonic resonances to be distinguished. Introducing acircularity, annularity, and morphological distortions to a regular disc resulted in emergence of complex plasmonic modes. Centrosymmetric (i.e., disc and ring) nanostructures supported bright and dark modes. Introduction of radial aberrations to a formerly centrosymmetric structure (i.e., ellipse) caused two distinct bright plasmon resonances to emerge. Annularity (i.e., rings) introduced an antibonding bright mode and red-shifted the existing bright mode due to charge density interactions between adjacent interfaces of the ring wall. Introducing radial aberrations and distortions to the ring structure in eDDA in vacuo compressed the observable mode structure, while including substrate of a lithographed nanoring in eDDA suppressed, red-shifted, and dampened resonances observed in vacuo. Experimental EELS resonances red-shifted beyond simulations that included dielectric substrate and a metallic adhesion interlayer due to accrued physical and metrological deficiencies. Tandem simulation and classification of plasmon eigenstates using EELS is useful to guide design of metamaterials with anticipated irregularities for improved optoplasmonic activity.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b06724. An illustrative, in-depth description of interpreting EELS spectra and maps for resonance mode identification and experimental EELS measurements on an aberrant nanoellipse (PDF) 24954
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected], phone: (479) 575-6691. Present Address ⊥
U.S. Department of Energy, Washington D.C. 20585, United States. Author Contributions
G.T.F. and D.D. contributed equally. G.T.F. performed numerical modeling and prepared text and figures for the manuscript. D.D. performed numerical modeling, fabrication, and characterization, and drafted the initial text. M.B. performed characterization. D.K.R. directed the work and refinement of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by NSF CBET-1134222, NSF ECCS-1006927, NSF EEC-1260301, NSF Graduate Research Fellowship awarded to G.T.F., the University of Arkansas Foundation, and the Walton Family Charitable Foundation. The authors are grateful to the Arkansas Nano-Bio Materials Characterization Facility, supported by the NSF. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
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