Photoluminescence Quantum Yield from Gold Nanorods: Dependence

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Photoluminescence Quantum Yield from Gold Nanorods: Dependence on Excitation Polarization Weidong Zhang, Yuqing Cheng, Jingyi Zhao, Te Wen, Aiqin Hu, Qihuang Gong, and Guowei Lu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00206 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 14, 2019

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The Journal of Physical Chemistry

Photoluminescence Quantum Yield from Gold Nanorods: Dependence on Excitation Polarization Weidong Zhang,1 Yuqing Cheng,1 Jingyi Zhao,1 Te Wen,1 Aiqin Hu,1 Qihuang Gong,1,2 and Guowei Lu1,2,* 1 State

Key Laboratory for Mesoscopic Physics & Collaborative Innovation Center

of Quantum Matter, School of Physics, Peking University, Beijing 100871, China 2

Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi 030006, China

Abstract We demonstrated that the luminescence quantum yield of single gold nanorods illuminated by a continuous-wave laser at a wavelength of 532 nm depends on the angle of excitation polarization, while that excited by a 633 nm laser does not. The electrons in the sp-band dominate the luminescence process under the 633 nm laser, resulting in a constant quantum yield irrespective of whether the excitation polarization is perpendicular or parallel. Under the 532 nm laser, the electrons in both the d-band (interband transition) and sp-band (surface plasmon) are involved in luminescence. The variation of quantum yield under the 532 nm laser results from the different conversion efficiencies of the d-band interband transition and the sp-band plasmons into luminescence. Furthermore, we found that the plasmon mode coupling effect strongly modulated the plasmon emission efficiency by comparing the luminescence of two sets of nanorods with different sizes. Generally, smaller-size gold nanorods result in higher quantum yields of the interband transition. These findings pave a way to understanding the luminescence process of plasmonic nanostructures and establishing principles to control such luminescence through plasmon mode coupling.

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Introduction The photoluminescence (PL) of noble metallic nanostructures has desirable optical properties, such as high photostability without bleaching or blinking.1-3 Here, PL is defined as any secondary light emission that differs in energy from the excitation energy. This phenomenon has attracted considerable attention, partly because it has many potential applications in biosensing,4-5 imaging tags,6-7 plasmon mode mapping,8-9 plasmonic catalysis,10 and thermometers.11-13 Since the first observation of PL from bulk gold by Mooradian in 1969,14 many mechanisms, such as interband transition, Auger relaxation, electronic Raman scattering,15-17 intraband transition, plasmon emission, and inelastic decay of plasmon resonances, have been proposed to understand the PL process.18-19 PL was first proposed to arise from interband transitions of d-band electrons into the conduction band and subsequent radiative recombination, and this process is enhanced by localized surface plasmon (LSP) resonance.14, 20 To explain strong infrared emission, which is much lower in energy than the width of the energy gap, intraband transitions were proposed, and the breakdown of symmetry and momentum selection rules in metal nanostructures was presumed to allow efficient intraband radiative recombination.2, 21-22 In addition, according to a suggested microscopic mechanism based on the radiative decay of surface plasmons, the excited d-band holes recombine non-radiatively with sp-band electrons, then emit particle plasmons.23 Although there is no consensus on their physical origin, all researchers acknowledge the pivotal role of the LSPs in the PL process. The LSPs strongly modify both the excitation and emission processes, which provides an effective way to control and optimize the PL of metallic nanostructures. Because the LSP modes of metallic nanostructures are dependent on their sizes, shapes, constituent materials, and environment, many efforts have been devoted to investigating the PL properties of a wide range of nanostructures, such as chemically synthesized nanoparticles,24 nanorods,1,

4, 25

nanocubes,26 or lithographically fabricated structures.27 Another strategy to

control the PL properties is based on LSP mode coupling, which can strongly modify the spectral shape and the PL quantum yield (QY).16, 28-31 The plasmon mode coupling effect often results in an


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increased local electric field,16, 30-32 new super-radiant mode,29 and modifications of the plasmon decay channels.28 Regarding the PL QY of metallic nanostructures, Dulkeith et al. found that the QY of gold nanospheres was independent of their particle size.23 Yorulmaz et al. observed an increase of QY by about an order of magnitude for gold nanorods compared with nanospheres.33 Cheng et al. found that the QYs of gold nanospheres, nanorods and nanobipyramids were essentially dependent on the excitation wavelength, and all three morphologies showed roughly equivalent QYs when the excitation wavelength was blue-detuned and close to their LSP resonances.34 To date, despite the widespread study of excitation polarization-dependent PL spectra, few reports have focused on the QY as a function of excitation polarization. In this work, we measure the PL spectra of individual gold nanorods (GNRs) illuminated by continuous-wave (CW) lasers with different excitation polarization angles (perpendicular or parallel). The PL intensity of the GNRs is dependent on the excitation polarization, and is highest when the excitation light polarization is parallel with the GNR’s longitudinal axis. We find that the PL QY of GNRs excited by a 633 nm laser is independent of the excitation polarization, while the QY under excitation at 532 nm depends on the polarization angle. We propose that the polarization dependence of the QY originates from the different efficiencies of conversion of interband transitions and plasmon emissions into luminescence. Moreover, we compare two sets of different-size GNRs with similar LSP resonant peaks, and find that the mode coupling effect plays a non-negligible role in the PL process.

Methods and Results GNRs with different sizes were synthesized by a seed-mediated method.35-36 We obtained two series of GNRs with different sizes. Thick GNRs with dimensions of ~D (62 ± 6 nm ) × L (135 ± 10 nm) were defined as series GNR-A, and thin GNRs with dimensions of ~D (36.5 ± 6 nm) × L (96 ± 10 nm) were defined as series GNR-B. Then, the GNRs were immobilized onto silane-functionalized glass coverslips with an average interparticle spacing of several micrometers for single nanoparticle measurements. The method of preparing samples has been described in detail in



previous reports.37 To measure the spectra of single GNRs, dark-field scattering and PL spectroscopy were integrated into a microspectroscopy system (NTEGRA Spectra, NT-MDT). We measured the scattering spectra of the GNRs using a white light dark-field scattering (DFS) method. Meanwhile, the microspectroscopy system also allowed us to measure the PL spectra of the same GNRs. For PL observations, the samples were excited by CW lasers at λ = 532 or 633 nm at room temperature. A half-wave plate and Glan-polarizer were utilized to control excitation polarization and perform emission polarization analysis. Figure 1(a) is a schematic of the single particle optical experiment. The polarization of excitation light is defined as 0° or 90° when the light polarization is parallel with or perpendicular to the longitudinal axis of the GNRs. Figure 1(b) shows an optical confocal scanning image. Figure 1(c) and (d) are the PL scanning images under excitation by a 532 nm CW laser with different excitation polarization angles, indicating that the PL intensities vary with this angle. As previously reported, the relationship between PL intensity and excitation polarization can be well fitted with a trigonometric function. We measured the PL intensity of individual GNRs under different polarization angles and fitted the data by the equation Ipl = 𝑎 + 𝑏𝑐𝑜𝑠2(𝜃 ― 𝜑),37 where 𝜃 is the polarization angle of the excitation light, and 𝜑 is the relative orientation of the GNRs. We performed PL measurements on representative GNRs of both series, illuminated with both 532 nm and 633 nm lasers. Figure 2(a) shows representative DFS and PL spectra of single GNRs. Figure 2(b) and 2(c) show the PL spectra of the same single GNR-A excited by 532 nm and 633 nm lasers with different excitation polarization angles. Figure 2(d) shows the PL intensities as a function of the excitation polarization angle. The PL excited by the 633 nm laser exhibits pure dipole behavior, in good agreement with previous reports.4, 33, 38 This polarization-dependent feature allows us to determine the relative orientation of the GNRs. The PL emission is very weak when the 633 nm laser’s polarization is set perpendicular to the GNR-A’s longitudinal axis, which implies that the interband transition at 633 nm (1.96 eV) is negligible. Therefore, for the PL excited by the 633 nm laser, both the excitation and emission processes can be attributed solely to the sp-band electrons. On the other hand, regarding the PL excited by the 532 nm laser, both d-band electrons (interband


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transition) and sp-band free electrons (intraband transition or plasmon) respond to the excitation light field. The polarization-dependent PL intensity does not behave as a pure single dipole, showing a low modulation depth as a function of the excitation polarization angle. We obtained the normalized QY of GNR-A by the equation 𝑄𝑌 = 𝑁𝑝𝑙/𝑁𝑎𝑏𝑠 , where 𝑁𝑎𝑏𝑠 is linearly proportional to the absorption cross-section. Hence, the QY can be simplified as: 𝑄𝑌 ∝ 𝑁𝑝𝑙 /𝜎𝑎𝑏𝑠. We calculated the absorption cross-section of GNR-A under different polarization angles with the finite-difference time-domain (FDTD) method.39-40 Experimentally, the height of GNR-A was obtained by atomic force microscopy, and the scattering spectrum was measured using white light DFS. Then, we numerically tuned the lengths of the GNRs to match the experimental scattering spectrum. Figure 3(a) shows the absorption cross-section of GNR-A under different polarization angles. Figure 3(b) shows the absorption cross-section (solid curves) at wavelengths of 532 nm and 633 nm, and the PL intensities (dots) excited by 532 nm and 633 nm lasers under different polarization angles. The modulation depth of the absorption cross-section at 532 nm is less than that of the PL intensity excited by the 532 nm laser. In contrast, the modulation depth of the absorption cross-section at 633 nm is the same as the PL intensity. This result implies that the PL QY excited by the 633 nm laser should be a constant. Conversely, as shown in Figure 3(c), the obtained PL QYs excited by the 532 nm laser are dependent on the angle of excitation polarization. The QY of GNR-A is highest when the excitation polarization is parallel with the longitudinal axis. Additionally, we note that the PL QYs under the 532 nm laser are also different for each excitation polarization mode irrespective of whether the GNR is immersed in water or oil, which have different refractive indices. In addition, we also measured the PL properties of GNR-B, which are thinner than GNR-A. For a reasonable comparison, we chose a specific GNR-B that possessed the same longitudinal LSP resonance as the GNR-A, at a wavelength of around 642 nm. The full width at half-maximum (FWHM) of GNR-B scattering was about 50 nm, in good agreement with the results obtained by atomic force microscopy. Figure 4(a) shows representative DFS and PL spectra of GNR-B



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illuminated with both 532 nm and 633 nm lasers. Figure 4(b) shows the PL spectra excited by the 532 nm laser under different excitation polarization angles. Figure 4(c) shows the numerical absorption cross-section of GNR-B under different excitation polarization angles. Figure 4(d) shows the QYs of GNR-A and GNR-B under different polarization angles. Like for GNR-A, the obtained QYs of GNR-B under the 532 nm laser differ as a function of the excitation polarization angle. However, the QY is highest when the excitation polarization is perpendicular, which is opposite to the case of GNR-A. Under the 633 nm laser, the PL results of GNR-B follow the same trend as that of GNRA, and the data are not shown here. It should be noted that the maximum QY of GNR-A is lower than that of GNR-B.

Discussion Usually, the PL QY is assumed to be an intrinsic constant for luminescent materials regardless of the excitation wavelength.41-42 In addition, semiconductor quantum dots often possess isotropic degenerate excitation transition dipoles, which likewise leads to constant PL intensity and QY regardless of the excitation polarization.43 However, recent studies demonstrate that the PL QYs of gold nanostructures are essentially dependent on the excitation wavelength.34,

44-45

In the

present study, we further demonstrate that the PL QY of GNRs is also dependent on the angle of excitation polarization. In our experiments, for the PL excited by a 532 nm laser, the photon energy of the excitation light is around the threshold of the interband transition of gold materials. Therefore, we consider that the electrons in both the d-band and sp-band can be excited by the 532 nm laser to induce interband transition and collective electron oscillation (i.e. a surface plasmon). The interband transition is often assumed to be polarization-independent. To understand the observed polarization-dependent behaviors, we approximate the absorption cross-section as σ𝑒𝑥𝑐 ≃ σ𝑑 + σ𝑑𝛾𝐿𝑆𝑃(𝜃) + 𝜎𝐿𝑆𝑃(𝜃), where σ𝑑 is the interband absorption crosssection arising from the d-band, 𝛾𝐿𝑆𝑃(𝜃) is the local field enhancement factor affecting interband transition, and 𝜎𝐿𝑆𝑃(𝜃) is the pure absorption cross-section arising from the LSP. If only the first term, σ𝑑, responds to the excitation, then the absorption is independent of the excitation


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polarization, which is not in agreement with the experimental results. Secondly, if only the first and second terms (σ𝑑 + σ𝑑𝛾𝐿𝑆𝑃(𝜃)) are involved in the excitation process, the polarization dependence of absorption can apparently be explained by the polarization-dependent 𝛾𝐿𝑆𝑃(𝜃) term. A greater enhancement of the LSP resonant mode would result in higher absorption. However, even though the decay process of electron–hole pairs is dependent on the initial excitation polarization, such that higher absorption should result in higher emission, this should not affect the QY. This conflicts with the experimental results, which show that the PL QY is excitation polarization-dependent. Hence, the free electrons in the sp-band must be involved in the excitation and emission process. This conclusion is in agreement with recent studies, which found that the interband transitions and LSP modes both contribute to the excitation process.22, 44 The PL excited by the 633 nm laser mainly originates from the response of the sp-band electrons, i.e. by directly exciting the longitudinal plasmon modes. As shown above, we found that the PL QY excited

by

the

633

nm

laser

was

polarization-independent,

so

we

obtain

𝑃𝐿633(𝜃) = 𝜂𝐿𝑆𝑃 ― 633𝜎𝐿𝑆𝑃(𝜃), where 𝜂𝐿𝑆𝑃 ― 633 is the plasmon emission efficiency arising from the inelastic radiative decay of the LSP mode. The luminescence of the longitudinal plasmon band under the 532 nm laser can be produced in two ways: first, by an energy transfer inelastically from the transversal to the longitudinal plasmon mode; secondly, by conversion of directly excited electron–hole pairs into the longitudinal plasmon mode.38 Hence, the PL induced by the 532 nm laser originates from two sources: LSP mode (sp-band) and interband transition (d-band). For this PL, 𝑃𝐿532(𝜃) ∝ 𝜂𝑑𝜎𝑑(1 + 𝛾𝐿𝑆𝑃(𝜃)) + 𝜂𝐿𝑆𝑃 ― 532𝜎𝐿𝑆𝑃(𝜃), where 𝜂𝑑 and 𝜂𝐿𝑆𝑃 are assumed to be constant (they are polarization-independent for a specific GNR). For example, the decay of an electron–hole pair results in light emission with a constant efficiency 𝜂𝑑, which is independent of the excitation polarization. Then, we can write the following PL QY relation: 𝑄𝑌532 ∝ 𝜂𝑑𝜎𝑑(1 + 𝛾𝐿𝑆𝑃(𝜃)) + 𝜂𝐿𝑆𝑃 ― 532𝜎𝐿𝑆𝑃(𝜃) 𝜎𝑑(1 + 𝛾𝐿𝑆𝑃(𝜃)) + 𝜎𝐿𝑆𝑃(𝜃)

. The difference between 𝜂𝑑 and 𝜂𝐿𝑆𝑃 ― 532 results in the polarization

dependence of 𝑄𝑌532 . For the thick GNR-A, the PL QY excited along the transverse axis is lower than that along the longitudinal axis, which implies that 𝜂𝑑 is lower than 𝜂𝐿𝑆𝑃. For the thin GNR-B,



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the PL QY excited along the transverse axis is higher than that along the longitudinal axis, which indicates that 𝜂𝑑 is higher than 𝜂𝐿𝑆𝑃. In brief, the observed polarization dependence of PL 𝑄𝑌532 results from the different conversion efficiencies into luminescence for the d-band and sp-band electrons after photonic excitation. Now we turn to understanding why 𝜂𝐿𝑆𝑃 is different for the different-size GNRs. We propose that the coupling effect between the LSP modes strongly modulates 𝜂𝐿𝑆𝑃. For mode coupling, the coupling strength is dependent on the energy detuning between the two modes. Comparing the absorption and PL spectra of the two series of GNRs, for GNR-A, the LSP resonance frequency near the corresponding frequency of the 532 nm excitation source light is closer to the longitudinal emission band. This may result in stronger coupling between the surface plasmon at the excitation frequency and the emission LSP modes. To verify this assumption, we used a phenomenological theoretical model to simulate the interaction of modes and calculated the PL spectra of a single GNR.34, 46 The GNR was modeled as an optical nano-resonator with two LSP modes possessing different resonance frequencies ω and a decay rate 𝜅. We set the mode 𝑎 near the excitation frequency, and mode 𝑏 as the longitudinal LSP band of the GNR. The Hamiltonian for individual modes is written as Hmodes = ωt𝑎 + 𝑎 + 𝜔𝑙𝑏 + 𝑏. We considered a non-reversion mode coupling, i.e. mode 𝑎 can transfer to mode 𝑏, while mode 𝑏 cannot transfer to mode 𝑎. Hence, the interaction Hamiltonian of the mode coupling is written as Hint = 𝐽 ∗ 𝑏 + 𝑎, where J is a coupling coefficient. We note that while these two modes cannot be directly interconverted, inelastic processes such as the excitation of electron–hole pairs, phonons, and Landau damping could enable the transfer of energy between the modes.22-23 For instance, Wackenhut et al. and Fang et al. proposed that when plasmon modes are excited directly, a transversal plasmon mode can be converted to a longitudinal plasmon mode via an electron–hole pair.38, 41 The interaction Hamiltonian between the light and LSP modes can be given as Hlight = 𝐸 ∗ (𝑎 + 𝑒

―𝑖𝜔𝑒𝑥𝑡

+ 𝑎𝑒𝑖𝜔𝑒𝑥𝑡), where 𝜔𝑒𝑥 is the

excitation light frequency. Hence, the system Hamiltonian is given by H = Hmodes + 𝐻𝑖𝑛𝑡 + 𝐻𝑙𝑖𝑔ℎ𝑡. The dynamics of these modes can be solved by the equations 𝑎 = 𝑖[𝐻,𝑎] ― 𝜅𝑎𝑎 and 𝑏 = 𝑖[𝐻,𝑏] ―


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𝜅𝑏𝑏. By using the input-output relation, < 𝑎𝑜𝑢𝑡 >= 2𝜅𝑒𝑥𝑎 < 𝑎 > , where < 𝑎 > is the quantum average of the operator 𝑎 and 𝜅𝑒𝑥𝑎 is the outgoing coupling rate of the GNR, the detected intensity of light emission from the nano-resonator can be evaluated by IPL ― a(𝜔) = 𝑅𝑒[ ∞ 1 𝑇

∞

+ + ∫0 < 𝑎𝑜𝑢𝑡 (𝑡 + 𝜏)𝑎𝑜𝑢𝑡(𝑡) > 𝑒 ―𝑖𝜔𝜏𝑑𝜏] = 𝑅𝑒[∫0 [𝑇∫0𝑎𝑜𝑢𝑡 (𝑡 + 𝜏)𝑎𝑜𝑢𝑡(𝑡)𝑑𝑡]𝑒 ―𝑖𝜔𝜏𝑑𝜏]. Hence, the total

detected PL spectrum is IPL(𝜔) = IPL ― a(𝜔) + IPL ― b(𝜔). By adjusting the resonant frequency of mode 𝑎 to approach the frequency of mode 𝑏, we can obtain PL spectra with different amounts of detuning. Figure 5 shows two representative normalized PL spectra with different amounts of detuning corresponding to GNR-A and GNR-B. We find that the PL of GNR-A is stronger than for GNR-B, which means that the former has a higher plasmon emission efficiency. The plasmon emission arising from the decay of the LSP mode in the sp-band electrons overwhelms the contribution to the total emission from the d-band electrons. Tuning the PL conversion efficiency through the LSP coupling is an efficient way to control the PL process. For instance, to improve the PL QY of a metallic nanostructure, the bandwidths of the LSP modes should overlap, and the bandwidth of at least one LSP mode should cover the excitation frequency. We note that the total QY of GNR-A is lower than that of GNR-B. This can most likely be attributed to the lower efficiency 𝜂𝑑 of the d-band electrons of GNR-A, because the near-field enhancement effect is weaker for larger nanoparticles.47 This near-field “lightning” effect strongly modifies the quantum efficiency of the interband transition via the Purcell effect.1-2, 22, 44 Hence, to optimize the PL QY, there is a tradeoff between the contributions of d-band and sp-band electrons when the excitation photon energy is beyond the interband energy gap.

Conclusions We experimentally studied the PL properties of single GNRs under different excitation polarization angles with CW laser light of wavelengths 633 nm and 532 nm. The PL QY of the single GNRs illuminated by the 532 nm laser depended on the excitation polarization angle, while that excited by the 633 nm laser did not. There are two main contributions to the PL when excited by



the 532 nm laser: interband transition and plasmon emission. The different conversion efficiency of those processes results in the observed polarization-dependent QY. In addition, based on a comparison of two sets of GNRs with different size, we propose that the plasmon emission efficiency is modulated by LSP mode coupling, such that stronger mode coupling results in higher efficiency of plasmon emission. Smaller-size GNRs tend to produce a higher interband transition QY because of the Purcell effect. These findings provide new insights into the PL process of plasmonic nanostructures and show us a way to control the PL process through plasmon mode coupling.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Notes The authors declare no competing financial interest. Acknowledgment This work was supported by the National Natural Science Foundation of China (grant nos. 61521004, 11527901).


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Figures:

Figure 1. (a) Schematic of single nanoparticle measurements with different excitation polarization angles. Optical confocal scanning image (b), and scanning PL images excited by 532 nm laser with parallel (c) and perpendicular (d) polarizations.


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Figure 2. (a) PL and DFS spectra of the same gold nanorod. (b) PL spectra excited by 532 nm laser under different excitation polarization angles. (c) PL spectra excited by 633 nm laser under different excitation polarization angles. (d) PL intensity as function of polarization angle. Red and green points are PL intensities excited by 633 nm and 532 nm lasers, connected by the corresponding fitting curves (dashed lines) according to ∝ 𝑎 + 𝑏𝑐𝑜𝑠2(𝜃 ― 𝜑).



Figure 3. (a) Normalized absorption cross-section of GNR with different excitation polarization angles. (b) Normalized PL intensity and absorption cross-section as function of excitation polarization angle. Green points and lines are absorptions at 532 nm and the corresponding PL intensities under laser excitation at 532 nm. Red points and lines are the corresponding data under laser excitation at 633 nm. (c) Normalized QY as function of excitation polarization angle, calculated by the relation: QY ∝ 𝑁𝑝𝑙/𝑁𝑎𝑏𝑠.


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Figure 4. (a) PL and DFS spectra of the same gold nanorod (D ~ 36.5 ± 6nm × L ~ 96 ± 10nm). (b) PL spectra excited by 532 nm laser under different excitation polarization angles. (c) Normalized absorption cross-section of GNR with different polarization angles of incident light. (d) Black line is QY of GNR-B, red line is QY of GNR-A (the same as in Figure 3c). Red arrow shows the longitudinal axis of GNRs.



Figure 5. Normalized PL spectra simulated with the phenomenological theoretical model, including two representative spectra with different amounts of detuning between two LSP modes to simulate GNR-A (red) and GNR-B (black).


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Photoluminescence Quantum Yield from Gold Nanorods: Dependence

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