Plasmonic Probe of the Semiconductor to Metal Phase Transition in

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Plasmonic probe of the semiconductor to metal phase transition in vanadium dioxide Davon W. Ferrara, Joyeeta Nag, Evan R. MacQuarrie, Anthony B. Kaye, and Richard F. Haglund Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl401823r • Publication Date (Web): 05 Aug 2013 Downloaded from http://pubs.acs.org on August 7, 2013

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Plasmonic probe of the semiconductor to metal phase transition in vanadium dioxide Davon W. Ferrara,∗,†,‡ Joyeeta Nag,‡ Evan R. MacQuarrie,‡ Anthony B. Kaye,‡,¶ and Richard F. Haglund, Jr.‡ Department of Chemistry and Physics, Belmont University, 1900 Belmont Boulevard, Nashville, TN 37212-3757, Department of Physics and Astronomy, Vanderbilt University, 6301 Stevenson Center, VU Station B #351807, Nashville, TN 37235-1807, and Department of Physics, Texas Tech University, Box 41051, Lubbock, TX 79409-1051 E-mail: [email protected]

KEYWORDS: plasmon, nanoantenna, gold, vanadium dioxide, thermochromic, metamaterial Abstract An array of 180-nm diameter gold nanoparticles (NPs) embedded in a thin vanadium dioxide film was used as a nanoscale probe of the thermochromic semiconductor-to-metal transition (SMT) in the VO2 . The observed 30% reduction in plasmon dephasing time resulted from the interaction between the localized surface plasmon resonance of the NPs with the 1.4 eV electronic transitions in VO2 . The NPs act as nanoantennas probing the SMT, and homogeneous broadening of the gold plasmon resonance is observed at the temperaturess where electron correlations are strongest in VO2 . ∗ To

whom correspondence should be addressed of Chemistry and Physics, Belmont University, 1900 Belmont Boulevard, Nashville, TN 37212-3757 ‡ Department of Physics and Astronomy, Vanderbilt University, 6301 Stevenson Center, VU Station B #351807, Nashville, TN 37235-1807 ¶ Department of Physics, Texas Tech University, Box 41051, Lubbock, TX 79409-1051 † Department

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Nanocomposites consisting of noble metal nanostructures and metal oxides with solid-solid phase transitions have garnered much attention in recent years as a class of active plasmonic metamaterials. 1–3 When the phase transition modulates the optical properties of the metal oxide in the vicinity of the localized surface-plasmon resonance (LSPR) of the metal, it can lead to a shift in resonance spanning hundreds of nanometers in wavelength. 4–7 In nanocomposites consisting of vanadium dioxide (VO2 ) films and gold nanostructures, the semiconductor-to-metal phase transition (SMT) of the VO2 leads to plasmon-enhanced absorption 8–14 that can be tuned thermally, 15 optically, 16 electrically, 17 or mechanically. 18 Most previous studies of Au::VO2 structures have focused on modulated or enhanced absorption of macroscale films for applications ranging from optical switches to “smart” window coatings over a broad range of frequencies and employing a variety of nanostructures. 2,4,6–14,19,20 However, Au::VO2 nanocomposites also present an opportunity to probe the fundamental physics of the SMT at the nanoscale. If the sizes of the Au nanostructures are close to those of the VO2 grains within the nanocomposite, the metal nanostructure can function as a nanoantenna, 21 transmitting information into the far field about the several VO2 grains located within the plasmon near field. 22,23 For Au nanoparticles (NPs) with LSPRs in the near-infrared, Au::VO2 nanocomposites exhibit a unique, and so-far unexplored, feature: the Au LSPR can be made to coincide in energy with the VO2 interband transitions from the occupied vanadium 3d|| band to the empty 3dπ band centered at approximately 1.4 eV (Fig. 1a–b). During the SMT, these split 3d|| bands merge and, in conjunction with the 3dπ band, form the conduction band. 16,24 Using Au NPs with LSPR tuned to coincide with this interband transition presents a singular opportunity to observe the interaction between the delocalizing, strongly correlated electrons of the embedding medium and the collective excitation of free-electrons in the Au NPs. The interaction, mediated by the plasmon near-field, results in an energy transfer from the Au NPs to the VO2 that damps the plasma oscillation on a time scale comparable to the photochromic switching time of adjoining VO2 grains. Recent studies using Au nanocomposites as a local probe of VO2 films have shown that the statistical nature of the SMT 25,26 can alter the hysteretic optical response of the nanostructure


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during the thermochromic phase change, such as increased scattering efficiency in the case of single gold nanoparticles 22 and a sensitive dependence of the extinction hysteresis width resulting from the split-ring resonator geometry. 23 These studies relied on the gold nanostructure to probe one or more surrounding VO2 grains, and both the observed effects derive from the defect-driven phase transition of VO2 nanoparticulates making up the film. 27 In this study, to study the delocalization of 3d-band electrons of VO2 during the SMT, we measured the far-field extinction of lithographically prepared arrays of circular Au NPs (Fig. 1c) that exhibit an LSPR at 1.4 eV in semiconducting VO2 . We originally hypothesized that inhomogenous broadening would dominate the SMT since the size of the NPs was comparable with the VO2 grains, of order 30–100 nm in diameter (Fig. 1c), thus maximizing single-grain effects. However, our results show that inhomogenous broadening is minimized because of the averaging effect of the nanoantenna array, and that an effective-medium model containing only homogenous broadening is sufficient to describe our observations. Apparently, the symmetry and geometrical regularity of the NP array and coherent excitation of the nanocomposite averages out fluctuations due to the single grains as reported in [ 22,23], reducing inhomogeneous broadening of the LSPR due to the statistical nature of the SMT. This averaging effect allows us to conclude that as the LSPR increases in energy by 0.33 eV at the completion of transition, the plasmon dephasing time τ2 descreases by as much as 30% at a specific point during the transition. To make this measurement, a square array of 180-nm diameter Au NPs, with 500 µm on a side and a grating constant of 450 nm, was fabricated on an indium-tin-oxide coated glass substrate by electron-beam lithography using a poly(methyl methacrylate) resist. Gold was subsequently deposited by electron-beam evaporation to a thickness of 20 nm, as measured by a quartz crystal microbalance. A 60 nm film of VO2 was deposited over the array by pulsed laser ablation of a vanadium target in 10 mTorr O2 background gas (KrF laser pulsed at 25 Hz, fluence 1.2 J/cm2 ) followed by annealing for 45 minutes in 250 mTorr O2 at 450◦ C; 15 the thickness was confirmed by profilometry. The sample was mounted on a copper block heated by a Peltier element; a thermocouple, surface-mounted to a witness substrate, monitored the temperature. To characterize the


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Figure 1: (a) VO2 band structure in semiconducting and metallic states. Arrows indicate the transition peak energies. 16 (b) The imaginary and (inset) real parts of the dielectric function for semiconducting (blue solid) and metallic (red dash) VO2 and Au (black dash-dotted). 28,29 (c) Scanning electron microscope images of Au array before (upper left) and after (lower right) VO2 deposition.


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sample, white-light transmission hysteresis measurements as described in [ 25] were made on the plain VO2 film and on the VO2 -coated array using a tungsten lamp and InGaAs detector. The optical hysteresis in both measurements showed critical temperatures for heating (72◦ C) and cooling (64◦ C) consistent with a stochiometric VO2 film. 25,30 Spectrally resolved extinction measurements at temperature increments of approximately 1◦ C in the vicinity of the SMT were acquired using a broadband tungsten lamp. At each temperature, a reference spectrum of the plain VO2 film and an extinction measurement on the array were acquired. The temperature was recorded to the nearest 0.1◦ C and averaged between extinction and corresponding reference spectra. The extinction spectra were smoothed using a standard SavitzkyGolay filter over an interval of 101 of 1340 data points. 31 Figure 2 shows typical smoothed spectra. The LSPR position was extracted by finding the wavelength of the maximum extinction. To determine the linewidth, the full-width at 90% of this maximum extinction was first determined since the LSPR is located near the edge of the spectrometer range. Assuming Lorentzian resonances, this corresponds to one third of the full-width half-maximum Γ50 . Error bars were calculated by taking the standard deviation of measurements below 45◦ C where the film does not exhibit significant switching.

Figure 2: Typical extinction spectra of the nanocomposite vertically offset by 0.75; ‘×’ (‘◦’) indicates the LSPR peak dipole (quadrupole) energy as the nanocomposite heats (solid curves) and cools (dashed) through the phase transition. The temperature at which the measurement was taken is recorded on the right of the plot. As seen in Fig. 2, the extinction spectra reveal large dipole and smaller quadrupole resonances 5 ACS Paragon Plus Environment

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that blueshift smoothly as the VO2 transitions from the semiconducting to the metallic state and back. Our analysis focuses on the LSPR dipole resonance, located at ωLSPR = 1.24 eV (λLSPR = 1000 nm) in semiconducting VO2 that moves to 1.57 eV (785 nm) in the metallic state. The LSPR hysteresis, characteristic of the first-order transition and shown in Fig. 3a, was fitted using a fourparameter sigmoid to determine the critical temperature Tc and the width of the transition Tw for both the heating and cooling curves. The maximal change in LSPR approximately occurs during the narrow temperature range where the correlation between VO2 3d electrons is strongest. 32 The LSPR hysteresis reveals two salient features of the interaction between plasmon and VO2 electrons. First, there is no appreciable difference in transition characteristics between the LSPR hysteresis of the Au::VO2 array and the VO2 film; that is, the critical temperatures Tc are 72◦ C and 64◦ C for heating and cooling, and Tw ≈ 2.75◦ C for both heating and cooling, whether on the array or the plain film. This indicates that the nanoantennas are probing the medium without impacting the VO2 phase transition temperature, contrary to the report in [ 6]. We also see no indication of doping effects that could occur from Au diffusion into the VO2 . 33 Second, since the Au NPs are comparable in size to the VO2 grains, and given the statistical nature of the SMT, it is important to determine whether each NP probes the average dielectric changes in several switching domains within the plasmon near field, or whether each NP experiences a sudden shift in LSPR as individual VO2 grains turn metallic. The smooth transitions measured in this experiment, with no evidence of “avalanche” effects, 26 suggest that each NP is probing a sufficient number of VO2 grains that an effective-medium model 34–36 with a temperature-dependent metallic fraction f can be used. 36 Taken together, these two features suggest that the NP array is sensitive to the effective medium of the VO2 , rather than the switching of individual VO2 grains as in previous studies. 22,23 The nanoantennas can be modeled phenomenologically in a simple dipole approximation by treating the NPs as oblate spheroids in the electrostatic limit. 4,34 Accordingly, the polarizability α at a temperature T of a single, oblate spheroid in the electrostatic limit is given by

α (ω, T ) = V

ε − εm , εm + L (ε − εm ) 6

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(1)

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Figure 3: (a) The LSPR energy versus temperature and (inset) metallic fraction determined from extinction spectra; ‘•’ (‘◦’) indicate experimental data and solid (dashed) lines indicate fits as the film heats (cools). (b) The real part of the effective dielectric function with increasing temperature. 0 as predicted using Eq. 4 and [ 28]; blue solid, red dashed, and black dot-dashed ‘•’ indicate εm 0 of semiconducting and metallic VO and ε 0 of Au, respectively. Arrows indicate the curves are εm 2 direction of temperature change.


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0 + iε 00 are the complex diwhere V is the particle volume, ε (ω) = ε 0 + iε 00 and εm (ω, T ) = εm m

electric functions of the Au and surrounding effective medium, respectively, and L is the particle geometrical factor. Explicit expressions for L are given in many texts, including [ 34]; for the NPs in this study, we calculate L = 0.12. The effective medium comprises not only VO2 , but also the ITO-coated glass substrate and defects within the film such as non-transitioning VOx or voids; however, we are interested only in the phenomenological changes to εm due to the VO2 component of the film. Determining accurate dielectric function data of the Au::VO2 nanocomposite is part of a separate, ongoing study. In Eq. 1, the real and imaginary parts of the denominator determine the physical behavior of the LSPR. The LSPR energy ωLSPR is determined by the vanishing of the real part of the denominator in Eq. 1; that is, where 0 ε 0 (ωLSPR ) = κεm (ωLSPR , T ) ,

(2)

0 decreases (Fig. 1b), which where κ ≡ 1−L−1 = −12.4. As VO2 transitions to the metallic state, εm

causes ωLSPR to blueshift towards larger ε 0 . 4 The shape of the NPs determines the magnitude of the resonance shift. According to Eq. 2, the NPs used in this study are more than six times as sensitive to changes in εm as spherical NPs with κ = −2. This, along with the steep slope of ε 0 , 0 as seen schematically leads to a relatively large shift in LSPR for a relatively small change in εm

in Fig. 3b. 4 Because of the large magnitude in LSPR shift, if the NPs were switching one-by-one as a result of single-grain effects, we would expect to see two dipole LSPR modes at the midpoint ( f = 0.5) of the phase transition, rather than the single, smoothly shifting dipole resonance. The VO2 metallic fraction f , was modeled as

f (T ) = 1 −

1 , T −Tc 1 + exp Tw

(3)

where Tc and Tw were determined from the sigmoidal fit to Fig. 3a. 36 The change in ωLSPR (T ) is linear with increasing f , Fig. 3a (inset), with slope proportional to κ. The linear shift of the LSPR wavelength with increasing film metallicity is probably due to the limited number of grains probed 8 ACS Paragon Plus Environment

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by the plasmon field and suggests that the VO2 effective medium can be described by

εm = (1 − f ) εsemi + f εmet ,

(4)

where εsemi and εmet are the dielectric functions of semiconducting and metallic VO2 . In Fig. 3a (inset), Eq. 4 was used to fit the data; fits based on Bruggeman and Maxwell-Garnett effectivemedium theories 25,36,37 were indistinguishable from the linear model. 34,35 Although we are not able to determine the dielectric functions of VO2 from our data, to illustrate the changes in the real part of the effective medium, the “trajectory” of the plasmon as it experiences both a blueshift in 0 is shown in Fig. 3b using the dielectric functions reported in [ 28] and energy and decrease in εm

the experimentally determined LSPR energy. While the LSPR energy blushifts linearly with increasing metallic fraction, the LSPR width reveals more complex features. In particular, near the threshold temperature of the SMT a spike in the LSPR width Γ50 is observed (Fig. 4a). In the heating cycle, the spike occurs just as the transition to the metallic state begins; during the cooling cycle, the spike occurs just as the equilibrium semiconducting state is reached. In general, the LSPR linewidth results from a combination of inhomogenous and homogeneous broadening. Inhomogeneous broadening is due to variations in the ensemble of particles within the array geometry. 35 In lithographically prepared arrays, variations among Au NPs are minimal, 38 but variations in grain size and crystal orientation in the VO2 film may contribute to inhomogeneous broadening. However, since we are concerned with effects of the transition on the linewidth, a static inhomogeneous contribution to the linewidth is not a factor. Since the statistical nature of the SMT leads to spatial variations in the effective medium, 32 however, it is reasonable to ask whether there is evidence of inhomogeneous broadening during the transition. For a first-order transition, this broadening should be greatest at T = Tc , where the fluctuations in the statistical distribution of the VO2 states ( f = 0.5) are maximum. However, Fig. 4b shows that the LSPR width for the NPs has a maximum around f = 0.43 as determined by averaging the fits between the heating and


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Figure 4: The LSPR full-width half-maximum versus (a) temperature and (b) metallic fraction determined from extinction spectra. ‘•’ (‘◦’) indicate experimental data and red solid (blue dashed) lines indicate fits using an electrostatic dipole model and (Eq. 5) as the film heats (cools). The solid black line indicates an average between heating and cooling fits using the model. Vertical lines in (b) coincide with f = 0.5 (solid) and the peak LSPR width, f = 0.43 (dashed).


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cooling data. This suggests that the spikes in LSPR width (Fig. 4a) are dominated by homogeneous broadening, although we cannot rule out a modest role for inhomogeneous broadening. The homogeneous linewidth Γ50 , determined by the imaginary part of the denominator in Eq. 2, is related to the plasmon dephasing time τ2 by Γ50 = 2¯h/τ2 and is a measure of the damping mechanism that degrades electronic excitation into thermal energy. 39 From Eq. 2, Γ50 ∝ εm00 + L (ε 00 − εm00 ). For particles in a nonabsorbing medium (εm00 = 0, Γ50 ∝ Lε 00 ) with dimensions small compared to the wavelength but larger than the Fermi scattering length, non-radiative damping of the plasmon oscillation occurs predominantly through inelastic scattering between the excited free electrons and the Au lattice, other electrons, and Coulomb interactions with the environment. 35 However, in an absorbing medium, the second term of the denominator of Eq. 1 is small compared to εm00 , and thus Γ50 (T ) is dominated by changes in the the VO2 effective medium. Plasmon damping is therefore controlled by electromagnetic energy losses through the evanescent near-field and far-field scattering. 40,41 00 Studies of the dielectric functions of VO2 thin films have reported differences in εsemi and 00 based on the fabrication method and thickness, but with essentially the same major features, εmet

namely, resonant structures near 3.5 eV and 1.4 eV in the semiconducting state. 28,30,42–45 Kakiuchida et al. model these structures using a single Lorentzian oscillator for each resonance. 42 During the SMT, the 1.4 eV oscillator resonance energy redshifts – that is, the energy approaches zero as the electrons delocalize – and broadens to become the metallic conduction band. This change corresponds to the convergence of the strongly correlated split vanadium 3d bands responsible for the phase transition. 42 To fit the changes in Γ50 during the SMT, we combine the Lorentzian oscillator model described by Kakiuchida et al. 42 with the effective medium given by Eq. 4. In our model, the imaginary part 00 surrounding each NP is of the effective dielectric function εm

00 εm (ω) =

Aeff γe f f ω , 2 − ω2 2 + γ2 ω2 ω3d eff


(5)

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where A is the effective oscillator amplitude, ω3d is the energy at the VO2 3d-band center, and γeff is the effective damping constant. As the VO2 transitions from semiconducting to metallic, these three parameters are taken to vary linearly with the metallic fraction. That is,

ω3d = (1 − f ) ωsemi + f ωmet ,

(6)

Aeff = (1 − f ) Asemi + f Amet ,

(7)

γeff = (1 − f ) γsemi + f γmet ,

(8)

The subscripts ”semi” and ”met” denote the VO2 semiconducting and metallic states. Since Γ50 is 00 according to the denominator of Eq. 1, the experimental values approximately proportional to εm

of Γ50 (T) was fitted to Eq. 5 by using Asemi as a scale factor to normalize the fit to the LSPR width at near-room temperatures. The energies ωsemi and ωmet were determined from [ 42] to be 1.4 eV amd 0 eV, respectively, while ω = ωLSPR was experimentally determined from Fig. 3a. The only three parameters varied in the least squares fit were: (1) Amet (average results: 1.8 eV2 ); (2) γsemi 00 based on di(2.6 eV); (3) γmet (0.9 eV). Our model makes no assumptions about the values of εm

electric function measurements and is strictly a single-particle model without any inhomogeneous broadening. As seen in Fig. 4, this simple model fits the results well, justifying the neglect of inhomogeneous broadening and thus supporting the approximation of homogenous broadening. 00 probed by the LSPR is As was done for ε 0 in Fig. 3b, the trajectory showing the changes in εm

shown in Fig. 5. First, we used the Verleur dielectric function data, Eq. 4, and our experimentally 00 – and therefore the predicted determined values for ωLSPR to calculate predicted trajectory of εm

behavior of Γ50 – without fitting to the measured LSPR width. Then, to futher verify the results of our model, the experimental fit results for Γ50 was linearly rescaled so that the shape of the experimentally determined trajectory could be compared to that predicted by the Verleur dielectric function data. The results, shown in Fig. 5, show that both trajectories are in remarkable agreement, and strongly support the hypothesis of homogeneous broadening. From the model, the nanoscale physics of the interaction between the plasmon and strongly 12 ACS Paragon Plus Environment

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Figure 5: The imaginary part of the effective dielectric function with increasing temperature. 00 (ω ‘•’ indicates εm LSPR ) predicted from [ 28] using Eq. 4, ‘◦’ the effective medium calculated from the electrostatic dipole model and (Eq. 5) and scaled to fit the prediction. Arrow indicates the direction of temperature increase. Blue (red) solid curve is the VO2 dielectric function in the semiconducting (metallic) state, and each dashed curve corresponds to the effective medium calculated for the indicated metallic fraction. correlated electrons of the VO2 can be deduced. During the phase transition, the 1.4 eV interband resonance in Fig. 1c arising from 3d|| → 3dπ transitions corresponds to the region of strong correlation. Since the intitial LSPR resonance at 1.24 eV overlaps with this transition, the plasmon response seen in Figs. 3 and 4 allows us to connect the resonant electronic structure with the optical response of the strongly correlated electrons. Upon inducing the SMT, both the upper V 3d|| and 3dπ bands move toward the Fermi level and the lower V 3d|| band (Fig. 1a). As the strongly correlated 3d electrons delocalize, increased carrier mobility and electron damping hinder the external 0 begins electromagnetic field from inducing a polarization in the film. The resulting decrease in εm

blueshifting the LSPR because it takes more energy to polarize the VO2 grains within the plasmon field (Eq. 2). Simultaneously, the increased scattering of the delocalizing electrons in the film leads to higher energy losses as the plasmon field-enhancement around the NPs contributes to the electronic excitations within the metal VO2 conduction band. As the LSPR continues to blueshift through the region of strong correlation, the 1.4 eV Lorentz oscillator characterizing the VO2 transitions in the effective medium redshifts to zero, 42 corresponding to a Drude free-electron gas and completing 13 ACS Paragon Plus Environment

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the SMT. By then, the LSPR is no longer resonant with the 3d transitions of the VO2 , and thus the nanoantennas are no longer sensitive to changes in the band structure. Although the interaction between the Au plasmon and the strongly correlated electrons of the VO2 3d bands is relatively brief as the LSPR shifts above the interband transition energy, the effects on the Au plasmon lead to a 30% decrease in dephasing time (τ2 goes from 3.1 fs at f = 0 to 2.2 fs at f = 0.43). The competition between free-electron excitations of the Au plasmon and the interband transitions of the VO2 increases as those transitions become more accessible during the SMT. This suggests the possibility of designing an experimental geometry that would lead to even stronger coupling between Au nanoantenna structures and VO2 electrons during the phase transition. 46 In conclusion, our results show that Au::VO2 nanocomposites consisting of Au NPs comparable to or larger than the VO2 grain size can be understood using a single-particle model and an appropriate effective-medium theory. Although we cannot rule out a relatively small contribution from inhomogenous broadening of the plasmon linewidth during the SMT, we conclude that the “spikes” in plasmon linewidth seen in Fig. 4a are mostly due to changes in the dielectric function of the VO2 , and thus to homogenous broadening of the LSPR, since the peaks occur well below f = 0.5 and the single-particle model can completely describe our results. A better understanding of the effective medium and an integrated model accounting for ensemble and single-particle effects would be needed to quantify any inhomogeneous contribution, which may lead to a slight shift in the position (towards f = 0.5) and width of the peak. This is the subject of future work. Three different experiments building on the present results suggest themselves. First, spherical 00 within a smaller energy band because they have been NPs may be used to probe changes to εm

estimated to have the minimum shift in LSPR energy. This would allow for continuous monitoring of a specific electronic band. A second experiment probing the nanoatenna arrays at a large angle with respect to the surface normal may allow probing of chemical interface damping and increased coupling between the plasmon and the strongly correlated electrons of the VO2 by exciting the plasmonic mode along the 20 nm axis of the NP, thus probing within the Fermi length. 47 Alternatively, NPs of mean diameter smaller than 20 nm might be used. However, as the nanoparticles’


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diameter decreases, single VO2 grain effects should become more apparent and increase the inhomogenous contributions during the SMT. Further modeling will be needed to account for both inhomogenous and homogeneous broadening. Third, this work and evidence of electron injection from the Au to the VO2 reported in [ 6] invite femtosecond pump-probe measurements on Au::VO2 nanocomposites. Generally, the plasmon damping by the VO2 leads to lower extinction efficiency and resonance quality factors in Au::VO2 nanocomposites with LSPR energies above the VO2 bandgap. Although this limits applications at optical and NIR energies that require strong absorption, our results and the geometry-dependent shift in LSPR (Eq. 1) suggests that spherical NPs with resonances between the 1.4 eV and 3.5 eV transitions will have more stable optical characteristics. On the other hand, applications that rely on the modulation of the LSPR rather than enhanced absorption would benefit from the incorporation of Au::VO2 nanorods.

Acknowledgement DWF was supported as a research assistant by the U.S. Department of Energy, Office of Science (DE-FG02-01ER45916) and the ITT Corporation’s National Security Technology Applications Division. JN was supported by a research assistantship funded by the National Science Foundation (ECCS-0801985). ERM received a William and Nancy McMinn Honor Scholarship for the Natural Sciences at Vanderbilt University. Samples were fabricated and characterized in facilities of the Vanderbilt Institute of Nanoscale Science and Engineering, which were renovated by the National Science Foundation (NSF ARI-R2 DMR-0963361).

References (1) Nishi, H.; Asahi, T.; Kobatake, S. Journal of Physical Chemistry C 2009, 113, 17359–17366. (2) Cortie, M. B.; McDonagh, A. M. Chemical Reviews 2011, 111, 3713–3735.


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