Anti-Stokes Emission from Hot Carriers in Gold Nanorods - Nano

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Anti-Stokes Emission from Hot Carriers in Gold Nanorods Yi-Yu Cai, Eric Sung, Runmin Zhang, Lawrence J. Tauzin, Jun Liu, Behnaz Ostovar, Yue Zhang, Wei-Shun Chang, Peter Nordlander, and Stephan Link Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04359 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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Anti-Stokes Emission from Hot Carriers in Gold Nanorods Yi-Yu Cai,†∥ Eric Sung,†∥ #Runmin Zhang,‡∥ Lawrence J. Tauzin,†∥ Jun G. Liu,‡∥ Behnaz Ostovar, §∥

Yue Zhang,‡∥ Wei-Shun Chang,†∥¶ Peter Nordlander,‡§⊥∥ and Stephan Link*†§∥

†Department

of Chemistry, ‡Department of Physics and Astronomy, §Department of Electrical

and Computer Engineering, ⊥Department of Materials Science and NanoEngineering, and ∥Laboratory for Nanophotonics, Rice University, 6100 Main Street, Houston, Texas 77005, United States

KEYWORDS: gold nanoparticle; interband transition; intraband transition; anti-Stokes photoluminescence; hot electron temperature; surface plasmon resonance TOC graphic

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ABSTRACT: The origin of light emission from plasmonic nanoparticles has been strongly debated lately. It is present as the background of surface enhanced Raman scattering, and, despite the low yield, has been used for novel sensing and imaging application because of its photostability. While the role of surface plasmons as an enhancing antenna is widely accepted, the main controversy regarding the mechanism of the emission is its assignment to either radiative recombination of hot carriers (photoluminescence) or electronic Raman scattering (inelastic light scattering). We have previously interpreted the Stokes shifted emission from gold nanorods as the Purcell effect enhanced radiative recombination of hot carriers. Here we specifically focused on the anti-Stokes emission from single gold nanorods of varying aspect ratios with excitation wavelengths below and above the inter-band transitions threshold while still employing continuous wave lasers. Analysis of the intensity ratios between Stokes and antiStokes emission yields temperatures that can only be interpreted as originating from the excited electron distribution and not a thermally equilibrated phonon population despite not using pulsed laser excitation. Consistent with this result as well as previous emission studies using ultrafast lasers, the power-dependence of the upconverted emission is nonlinear and gives the average number of participating photons as a function of emission wavelength. Our findings thus show that hot carriers and photoluminescence play a major role in the upconverted emission.


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Emission from plasmonic nanoparticles has been the focus of considerable recent research,1-10 because of its contribution as a broad background to surface enhanced Raman spectra (SERS)11,12 and its possible applications in imaging13 and nanothermometry.14-16 Compared to semiconducting nanoparticles, light emission following photoexcitation of plasmonic nanostructures is typically more complex due to ultrafast electron-electron and electron-phonon interactions17 as well as the involvement of the collective oscillations of conduction band electrons,18 known as localized surface plasmons. Nanoparticle plasmons significantly enhance the extremely low emission quantum yield of bulk metals (QY ~ 10-10)19,20 and determine their spectral lineshape.6-9 However, the exact mechanism underlying plasmonic nanoparticle emission remains heavily debated. While the enhancement by surface plasmons is well accepted, the main point of contention is the interpretation of the emission as photoluminescence (PL) vs. electronic Raman scattering. PL occurs through radiative recombination of hot carriers during their interactions with electrons and phonons. The PL spectrum and intensity depend on the excitation wavelengths and on the localized surface plasmons which act as antennas for radiation into the far-field.6-9,20,21 We recently showed through experiments and theory that the Stokes-shifted emission of gold nanorods (AuNRs) at the plasmon resonance is consistent with a Purcell effect enhanced hot carrier22 recombination of inter- and intra-band transitions.10 The plasmon resonance of the AuNRs increases the photonic density of states (PDOS)23 and thus enhances the QY and shapes the emission spectrum. Our interpretation agrees with previous studies on various metallic nanostructures, focusing on the effect of electronic transitions and plasmonic enhancement.47,20,24,25

The second proposed mechanism for plasmonic nanoparticle emission is based on

electronic Raman scattering.2,11,26 Electronic Raman scattering is the coherent inelastic scattering 3 ACS Paragon Plus Environment

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of light from electrons. In a single scattering event only one intermediate state is considered and the emission stops after dephasing. In contrast, for PL the initially created excited state can undergo momentum and energy changes through electron-electron and electron-phonon scattering before a photon is emitted.27,28 Upconverted emission or anti-Stokes emission has also been observed for the same nanoparticles and needs to be considered for a complete picture of light emission from plasmonic nanostructures.29-32 Because the anti-Stokes emission occurs at shorter wavelengths than the incident light, this process requires additional energy to be supplied either from multiple photon excitations or a thermal bath (phonons).14,32 Multi-photon excited emission from plasmonic nanostructures has routinely been observed using pulsed lasers. 33-37 Many hot carriers are generated here because of the large excitation power density during the short duration of femtosecond pulses. Electron-electron scattering creates pathways for radiative carrier recombinations at energies exceeding the incident photon energy, leading to anti-Stokes emssion.37 Under cw laser irradiation, the instantaneous excitation power is much lower, and electron-phonon coupling occurs on a similar time scale as successive photon absorption. In this case, anti-Stokes emission from plasmonic structures has been mainly attributed to interactions with phonons, either directly through phonon mediated charge carrier recombination (i.e. PL)14,29,30 or indirectly through a thermal activated electronic Raman scattering.11,15,16 In both models, a thermal equilibrium between electrons and phonons was assumed, and the effect of carrier-carrier interactions before reaching thermal equilibrium with the lattice was neglected. Neither model discussed the influence of the electronic band structure on anti-Stokes emission. While we recently reported on the Stokes emission from single AuNRs,10 this work provides insight into the mechanism of anti-Stokes emission. We examined the emission from single 4 ACS Paragon Plus Environment

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AuNRs following cw laser excitation at wavelengths overlapping inter- and intra-band transitions as well as the plasmon resonance (and hence the plasmon-enhanced region of the PDOS). Analyzing both the Stokes and anti-Stokes components of the emission spectra allowed us to extract AuNR temperatures based on either electronic or phononic occupations and to compare with results from a steady state model. The derived energy distributions of hot carriers cannot be obtained from the Stokes emission only. Furthermore, we measured the nonlinear power dependence of the emission, giving us further insight into the excitation, decay, and energy distribution of hot carriers.

Figure 1. Representative single particle emission spectra of four AuNRs for 405 (A,D), 633 (B) and 785 nm (E) excitation, and their dark-field scattering spectra (Sca) spectra (C, F); inset: corresponding SEM images with the scale bar for all SEM images corresponding to 50 nm. Each column shows the spectra from two particles. The shorter AuNRs having anti-Stokes emission were plotted with solid lines and the longer AuNRs emitting only Stokes emission are shown with dashed lines. The geometries and longitudinal surface plasmon resonance maxima are: (left


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column) solid line elastic scattering λmax 597 nm and AuNR dimensions of 30 x 63 nm; dashed line elastic scattering λmax 709 nm and AuNR dimensions of 23 x 75 nm; (right column) solid line elastic scattering λmax 734 nm and AuNR dimensions of 19 x 80 nm, dashed line elastic scattering λmax 861 nm and AuNR dimensions of 17 x 85 nm. The incident power densities for all excitation wavelengths are given in the Supporting Information. The laser light was blocked by a notch filter.

Using single particle spectroscopy to eliminate the effect of sample heterogeneity,38-41 we recorded the emission spectra resolving both Stokes and anti-Stokes parts for AuNRs with 633 and 785 nm excitation, i.e. above and below the inter-band threshold (1.7-1.8 eV),42,43 respectively (Figure 1). Three chemically synthesized AuNR samples44,45 were used covering a large range of average aspect ratios from 2.1 (sample 1, Figure S1), 3.5 (sample 2), and 5.3 (sample 3). By changing the aspect ratio and therefore tuning the longitudinal surface plasmon resonance wavelength between 568 and 883 nm, we were able to resolve both anti-Stokes and Stokes emission when the excitation light was close to the plasmon resonance. Example emission spectra for these excitation wavelengths are given in Figure 1B and 1E. More single particle emission spectra are shown in Figures S2 and S3. For comparison, as detailed further below, we also acquired emission spectra using a 405 nm laser, resulting in only Stokes shifted emission (Figure 1A and 1D), as well as the single particle dark-field scattering spectra (Sca), resulting from elastic light scattering, for each AuNR (Figure 1C and 1F). While the Stokes emission spectra of single AuNRs for 405 (Figure 1A and 1D), 633 (Figure 1B, dashed line) and 785 nm (Figure 1E, dashed line) excitations clearly resemble the Lorentzian


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lineshape of their elastic scattering spectra (Figure 1C and 1F), the spectral overlap between antiStokes emission and elastic scattering is less pronounced especially for 633 nm excitation (Figure 1B, solid line and Figure S2). We previously modeled the Stokes emission in AuNRs as the radiative recombination of excited charge carriers enhanced by the PDOS of the longitudinal plasmon resonance as the emission follows the scattering spectra lineshape and position except for a consistent blueshift.5-7,10,46 Anti-Stokes emission still occurs only at energies that coincide with the plasmon resonance and qualitatively follows shifts in the elastic scattering λmax (Figure S4). The corresponding emission and excitation polarizations furthermore follow the longitudinal AuNR mode (Figure S5). However, the same spectral similarities between Stokes emission and scattering are no longer observed, implying that factors in addition to enhancement by the PDOS play a role as well. This conclusion is further substantiated by examining the quantum efficiencies of anti-Stokes emission.


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Figure 2. (A) Emission QYs of Stokes emission (S (crosses) for AuNRs with resonance energies lower than excitation) and anti-Stokes emission (AS (circles) for AuNRs with resonance energies higher than excitation) with 405 (purple), 633 (green) and 785 nm (red) excitation. (B) Q-factors plotted vs. elastic scattering λmax of each AuNR.

Emission QYs can be used to compare the efficiencies of anti-Stokes to Stokes emission as a function of excitation wavelength when exciting inter- vs. intra-band transitions. The QY is defined as the number of emitted photons per absorbed photon. Because of the nonlinear power dependence of anti-Stokes emission, this definition is somewhat ambiguous. However, as we show below, the power-law exponent for the integrated anti-Stokes emission is significantly smaller than 2, causing only minor adjustments compared to the overall QY trends. QYs were calculated based on the incident laser intensities, the calculated absorption cross sections, and the number of collected photons, taking into account corrections for the spectral detection efficiencies.10 To obtain absorption cross sections, finite difference time domain simulations (FDTD) were performed for each AuNR using as input the particle dimensions from their scanning electron microscopy (SEM) images. Figure 2A summarizes the QYs of Stokes and antiStokes emission with 633 and 785 nm excitation for AuNRs plotted vs. their plasmon resonance maximum (elastic scattering λmax). We found that anti-Stokes emission is 10-100 times lower compared to Stokes emission for both excitation wavelengths of 633 and 785 nm. Anti-Stokes emission is mainly determined by the energy distribution of hot carriers. The QY of anti-Stokes emission for both 633 and 785 nm excitation decreases when the plasmon resonance maximum shifts to higher energies away from the excitation wavelength. The PDOS


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enhancement remains relatively constant for AuNRs with resonance energies below 1.8 eV, as concluded from the relatively constant QYs for Stokes emission with 405 nm excitation shown in Figure 2A and the Q-factors in Figure 2B (the Q-factor is inversely proportional to the PDOS).47 For AuNRs with resonance energies above 1.8 eV (in the case of 633 nm excitation), the Qfactor decreases as inter-band transitions broaden the surface plasmon. However, this dependence alone cannot explain the trend and magnitude of the QY for 633 nm excited antiStokes emission. We therefore conclude that the decreased population of hot carriers with increasing emission energy shift from the excitation wavelength (and not phonons as further shown below) is responsible for the decreasing QYs as the elastic scattering λmax blue-shifts. Interband excitation yields QYs that are an order of magnitude larger than intra-band excitation, again suggesting that hot electrons and holes are involved in the anti-Stokes emission and that the observed process should be PL. The AuNRs measured with 633 nm interband excitation have lower PDOS enhancement factors (Figure 2B) but ~10 times higher QYs compared to 785 nm intraband excitation (Figure 2A). We can explain this difference by considering that the recombination of d-band holes and sp-band electrons through inter-band transitions is momentum conserved and thus has higher radiative recombination rates than momentum forbidden transitions within the sp-band following 785 nm excitation.46 A similar difference between 633 and 785 nm excited Stokes emission is also seen in Figure 2A, and has been discussed and reported previously.10 If we assume an electronic Raman scattering mechanism, we can also compare emission efficiencies for these two excitation wavelengths by normalization with the maximum value of the simulated scattering cross sections.2 Figure S6 shows that this efficiency is still ten times larger for 633 nm excitation, implying that the observed difference would have to be due to tenfold different cross sections for electronic Raman scattering between 9 ACS Paragon Plus Environment

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633 and 785 nm. This scenario, while possible, appears unlikely because enhancement by the plasmon resonance is similar for both 633 and 785 nm since we tuned the tuned the plasmon resonance across these excitation wavelengths within the dispersion of AuNR aspect ratios. Enhancement by resonant interband transitions at 633 nm also cannot explain our data following a study that related resonance Raman enhancement to the imaginary part of the dielectric function,48 which for gold is actually larger at 785 nm.49 By analyzing the ratio of anti-Stokes to Stokes emission next we further demonstrate that for AuNRs under these excitation conditions their emission is originating from hot carriers.

Figure 3. Temperatures extracted from emission spectra excited with 633 nm (green) and 785 nm (red) light. (A) Ratio of integrated anti-Stokes to Stokes emission intensity across the detectable spectral region as a function of resonance elastic scattering λmax for only those AuNRs with resonance energies larger than excitation. Ratios normalized by the scattering spectra are shown in Figure S7. AuNR temperatures extracted from the emission spectra based on (B) Bose– Einstein and (D) Boltzmann statistics.14 (C) Calculated steady state temperatures of single AuNRs based on their absorption cross sections.50 The variance in calculated temperatures for 10 ACS Paragon Plus Environment

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AuNRs with similar λmax is due to the fact that AuNRs with different sizes and hence different absorption cross sections can have the same resonance maximum. The absorbed power (Figure S8) indeed varies similarly with the scattering λmax.

The ratio between Stokes and anti-Stokes emission can be exploited to assess the temperature of the system. For molecules this temperature corresponds to the occupation of vibrational modes.51 For AuNRs, Carattino et al. recently showed how the Stokes and anti-Stokes emission for 633 nm excitation can be used to extract the particle temperature. The emission spectra were fitted using the plasmon spectrum and a Bose–Einstein (BE) statistical model for the phonon distribution.14 We followed this model to analyze our measured ratios of Stokes to anti-Stokes emission for 633 and 785 nm excitation (Figure 3A). As input for the plasmon spectra, the measured elastic scattering spectra were used. We used two different statistical models for the excitations: A Bose–Einstein distribution which would be appropriate for phonons and a Boltzmann distribution for plasmon-generated hot carriers. Although electron-hole occupations are governed by Fermi-Dirac statistics, as an approximation of only the excited electrons, we used Boltzmann statistics instead following previous work.11,29,52 In the limit of large temperatures, the three distributions are equivalent. Detailed information about the modeling is described in the Supporting Information (Figures S9 and S10). The calculated temperatures using both statistical models are shown in Figure 3B and 3D. The results from the two models are similar for 633 nm excitation, while for 785 nm excitation the mean absolute percentage error between the experimental anti-Stokes/Stokes ratios and calculated values obtained using the statistical models is three times larger for the Bose–Einstein statistics (Figure S10).


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The Bose–Einstein statistics represent the phonon distribution and the calculated temperature would correspond to an effective particle temperature. For 633 nm excitation, the resulting temperature is around 500 K (Figure 3B), and for 785 nm close to 2000 K (Figure 3B). These temperatures are unrealistic for two reasons: the calculated steady-state temperatures are below 400 K (Figure 3C);50 and no reshaping of the AuNR occurred. Reshaping, which is not observed (Figure S11), would have occurred at temperatures above 400 K.53 We therefore assign the extracted temperatures to the hot carrier distribution rather than the lattice temperature. The temperatures that best describe our experimental results using Boltzmann statistics are shown in Figure 3D. These temperatures represent a non-equilibrium state between electrons and phonons. In a dynamical picture, energy from previous excitations is added to the excited electron distribution and an upconverted photon is emitted before thermalization.17 Because the electronic heat capacity for gold is two orders of magnitude lower than the bulk heat capacity in the temperature range considered here,54 the high electron temperatures in Figure 3D will not result in significant heating of the nanoparticle. The dynamics of hot carrier-phonon equilibration can be modeled using the two-temperature model.55 The resulting lattice temperatures are presented in Figure S12A and are even lower than those corresponding to the steady-state case (Figure 3C). On the other hand, electron temperatures calculated using the two-temperature model based on the steady-state lattice temperatures (Figure 3C) are several thousand Kelvin (Figure S12B). This can be explained by the fact that in the two-temperature model all photons are absorbed first in a short pulse and then allowed to relax, while under cw laser conditions excitations and decay processes are not separated in time. Energy relaxation to the environment is also not considered and would further reduce the expected lattice temperatures. The two-temperature model is therefore only a simple approximation for cw excitation, which, under our excitation powers,


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should result in a steady-state condition with highly energetic charge carriers present in addition to a thermal equilibrium between the AuNRs (i.e. lattice) and their surroundings. An analysis of the time intervals between incident absorbed photons provides further support for our interpretation of the extracted temperature reflecting the hot carrier distribution rather than phonons. Based on the absorption cross sections of the AuNRs and the excitation power, the average time intervals between absorption events range from 0.12 - 0.36 ps for 633 nm and 0.04 - 0.14 ps for 785 nm excitation, which is much smaller than the time scale for electron-phonon coupling.18 A non-equilibrium hot carrier distribution is therefore maintained continuously during illumination and clearly plays a role in the emission. Our work is also in agreement with studies reported by He et al.29 and Szczerbiński et al.52 who also fitted the 633 nm excited antiStokes emission with a Boltzmann distribution. We note that for very low excitation power and hence long time intervals between successive photon absorptions, a distinction between a non-equilibrium electronic and equilibrated lattice temperature is no longer possible. In this limit, our approach cannot distinguish between emission from hot carrier PL and emission from electronic Raman scattering as then enough time is available for electrons and phonons to reach a thermal equilibrium before the next photon absorption. For 633 nm and incident power of 0.01 MW/cm2 corresponding to a photon spacing of 3.6 ps, our results are in excellent agreement with the work by Carattino et al.14 The difference between Boltzmann and Bose-Einstein statistics almost vanishes and the calculated values in both cases can be interpreted as the lattice temperature.


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Figure 4. (A) Excitation power dependence of the integrated Stokes (crosses) and anti-Stokes (circles) emission of representative single AuNRs for 405 (purple), 633 (green) and 785 nm (red) excitation. Power-law exponents p of each particle are given next to the data. (B) Spectrally resolved power-law exponents for 633 and 785 nm excited emission of two single AuNRs (green: 633 nm, AuNR dimensions of 31 x 59 nm; red: 785 nm, AuNR dimensions of 25 x 79 nm) plotted vs. the energy shift of the emission from the excitation wavelengths. The error bars of the power-law exponents were obtained by measuring the emission at every excitation power three times in random order. (C,D) Excitation power dependent normalized emission spectra of the two single AuNRs with 633 (C) and 785 nm (D) excitations.

Pump-power dependent measurements illustrate that the experimentally observed anti-Stokes emission requires energy from more than one excitation event. The integrated Stokes and antiStokes emission intensities were plotted against the excitation power for AuNRs with different excitation wavelengths in Figure 4A. The Stokes emission intensities always show a linear


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dependence on excitation power as expected for a one-photon emission process.1 On the other hand, the anti-Stokes emission for both 633 and 785 nm excitations have power-law exponents larger than 1. This nonlinear power dependence is further analyzed by plotting these power-law exponents as a function of emission energy shift from the excitation wavelength (Figure 4B).31,32 The corresponding spectral changes of the emission for two power densities are plotted in Figure 4C and 4D for 633 and 785 nm excitation, respectively. While the power-law exponents for the Stokes emission remain constant across the entire emission region, for the anti-Stokes emission they increase with the emission energy (Figure 4B), a trend that is well reproduced for additional AuNRs as illustrated in Figure S13. We interpret the power-law exponent as the average number of photons needed for emitting anti-Stokes emission at a certain wavelength. The power-law exponent always remains smaller than 2, consistent with our discussion above that under cw irradiation successive photons are absorbed during the time scale of hot carrier relaxation. With larger anti-Stokes shift a larger number of incident photons is required, also because hot carriers with higher energies decay faster.17,56,57 Differences in the slopes of the observed power-law exponents as a function of emission energy shift between 633 and 785 nm excitations are likely due to a combination of factors: higher photon energy of 633 nm light requiring fewer photons, differences in excitation power density; and different hot carrier dynamics following inter- and intra-band excitation. The power-law exponent as a function of photon energy for anti-Stokes emission was first reported for plasmonic nanostructures using pulsed 770 nm laser excitation and assigned to intraband PL.31,32 The results and interpretation are very similar to ours for excitation with cw 785 nm light. Unlike the cw case (Figure 4B), all power-law exponents for gold nanostructures were found to be larger than 2 for pulsed excitation.32 Furthermore, their anti-Stokes emission spectra 15 ACS Paragon Plus Environment

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are broad spanning to more than 1 eV above the excitation energy,32 with emission line shapes tracking the plasmonic feature more clearly compared to cw laser excitation.35,36,58,59 With an incident photon flux that is 2 orders of magnitude larger compared to cw excitation and because all the incident energy is concentrated within the short pulse, the hot carrier distribution will be broader and skewed to higher energies.10,17 Indeed, it has been observed that higher excitation powers60 and shorter excitation pulses33 can lead to higher order power-law anti-Stokes emission. Previous ensemble emission studies have also shown that under the same excitation power, the anti-Stokes emission spectra with cw excitation are much narrower than with pulsed laser excitation.26 In conclusion, our study presents a unified picture of anti-Stokes and Stokes emission from single gold nanorods under cw laser excitation as originating from the Purcell effect enhanced radiative recombination of hot carriers, i.e. PL. While the surface plasmon enhances both sides of the spectra, the hot carrier distribution has a stronger influence on the lineshape of anti-Stokes emission. The effective temperature of the hot electrons can be extracted from the single particle emission spectra, and support this mechanistic view of PL in contrast to previous explanations focusing on the phonon occupation and lattice temperature.11,14,30 Upconverted emission is possible following both inter- and intra-band excitation, and the nonlinear power dependence of anti-Stokes emission under cw laser excitation yields the average number of incident photons within the electron-phonon thermalization time necessary to produce upconverted photons. As the emission energy increases, more photons are needed. A mechanistic understanding of light emission from plasmonic nanoparticles and its relationship to hot carriers provides important insight into hot carrier generation and relaxation61-64 which is crucial for applications such as plasmon-enhanced photocatalysis.65-70 16 ACS Paragon Plus Environment

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While we conclude that the emission from isolated AuNRs on dielectric supports is due to PL, it is not necessary or even justified to generalize the origin of emission to all plasmonic systems including coupled nanostructures and other plasmonic metals having different band structures. In fact, it has recently been suggested that the emission from gold and silver nanoparticles have different origins.70 Further studies on other metals are therefore needed. In addition to single particle emission spectroscopy, time-resolved measurements based on femtosecond luminescence upconversion would help to further distinguish between PL and electronic Raman scattering. If the emission lifetime were similar to electron-phonon coupling (~ 1 ps), PL involving competing carrier scattering events would be the better description, while electronic Raman scattering should occur only during the duration of the femtosecond excitation pulse. So far, both of these scenarios have been reported for ensembles of colloidal gold nanoparticles.71,72 Although more challenging because of the reduced photon count rate, luminescence upconversion on a single nanoparticle or a pristine nanoparticle array is therefore needed to avoid any possible contaminations that could be present in ensemble solutions.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. The Supporting Information contains experimental methods, sample characterization, and further data analysis (PDF). Corresponding Author 17 ACS Paragon Plus Environment

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*Email: [email protected] Present Addresses #(E.S.)

Department of Chemistry, Massachusetts Institute of Technology, Cambridge,

Massachusetts 02139, United States ¶(W.-S.C.)

Department of Chemistry and Biochemistry, University of Massachusetts Dartmouth,

285 Old Westport Rd, North Dartmouth, Massachusetts 02747, United States Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge financial support from the Robert A. Welch Foundation (C-1222 to P.N. and C-1664 to S.L.) and the Air Force Office of Scientific Research via the Department of Defense Multidisciplinary University Research Initiative, under Award FA9550-15-1-0022. S.L. acknowledges support from the National Science Foundation (ECCS- 1608917). We thank Dr. Bruce R. Johnson, Jacob Pettine, and Dr. Chang Yan for useful discussions. ABBREVIATIONS PL, photoluminescence; PDOS, photonic density of states; QY, quantum yield; AuNRs, gold nanorods; Sca, dark-field scattering; FDTD, finite difference time domain; SEM, scanning electron microscopy. 18 ACS Paragon Plus Environment

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