Intraband Hot-Electron Photoluminescence from Single Silver Nanorods

May 31, 2016 - lasers with single-nanoparticle spectroscopy. We attribute the. PL of silver nanorods to the intraband transition excited hot- electron...
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Intraband Hot-Electron Photoluminescence from Single Silver Nanorods Kai-Qiang Lin,†,‡ Jun Yi,‡ Shu Hu,‡ Juan-Juan Sun,†,‡ Jue-Ting Zheng,‡ Xiang Wang,‡ and Bin Ren*,†,‡,§ †

Collaborative Innovation Center of Chemistry for Energy Materials, ‡State Key Laboratory of Physical Chemistry of Solid Surfaces, and §The MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China S Supporting Information *

ABSTRACT: We report the observation of the undocumented visible one-photon photoluminescence (PL) of single silver nanorods excited by 532 and 633 nm continuous wave lasers with single-nanoparticle spectroscopy. We attribute the PL of silver nanorods to the intraband transition excited hotelectron radiative decay. The PL of silver nanorods closely resembles the corresponding LSPR scattering spectrum, and both are dependent on their aspect ratio. The good correlation between the quantitative PL intensity and the absorption cross section at the excitation wavelength of each nanorod leads to an aspect ratio independent PL quantum yield. The PL quantum yield of silver nanorods is similar to that of gold nanorods (10−6), indicating an efficient intraband excitation of hot electrons. The understanding of the PL mechanism of Ag nanorods points to the high-energy nature of the hot electrons excited via intraband transition, which has important indications in utilizing hot electrons for energy harvesting and photocatalysis. KEYWORDS: photoluminescence, plasmonics, silver nanorod, hot electron

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pair is transferred to the plasmon excitation, which then emits a photon.11,15−17 The third mechanism argues that the PL observed is actually the electronic inelastic light scattering or plasmon-enhanced electronic Raman scattering.18−20 The main argument on the mechanisms is whether the PL comes from the interband or intraband process. Due to the low interband transition band gap of gold (1.8 eV around the X symmetric point),21 it is difficult to distinguish the different PL components between the interband transition and intraband transition in the visible range. By tuning the LSPR of the nanorod to near-infrared, Fang et al. successfully demonstrated that interband transition is not necessary for PL by using 785 nm excitation, whose energy is well below the interband transition band gap.17 Hugall et al. observed a temperaturedependent continuum background in the surface-enhanced Raman scattering (SERS) of a gold plasmonic substrate under 785 nm excitation but assigned them to the electronic inelastic scattering.20 Thus, to further understand the PL, it would be highly valuable to explore the excitation and emission wavelength dependent quantum yield without the interference of interband transitions. However, the near-infrared scattering is invisible in a dark-field microscope, which made the controllable and quantitative single-nanoparticle measurement difficult. An LSPR-dependent PL quantum yield of single gold

lasmonic nanoparticles have received considerable attention due to the significant promise for solar energy harvesting through hot-electron-assisted photocatalysis.1,2 Interestingly, the hot-electron-assisted catalysis efficiency has been recently correlated with the intensity of the photoluminescence (PL) of gold nanoparticles.3,4 The PL of a bulk metal surface was first observed in 1969 with a low quantum yield of about 10−10,5 which was attributed to the radiative recombination of electrons in the conduction band and holes in the d bands generated by optical excitation (interband transition). Later, Boyd et al. found that roughened metal surfaces exhibited much higher single-photon-induced luminescence efficiency than smooth surfaces due to the local field enhancement originated from rough surface protrusions.6 In 2000, the PL of gold nanorods with more than 4 orders of magnitude higher quantum yield was observed,7 which impelled the metal PL into applications.8,9 Such a huge enhancement is originated from the localized surface plasmon resonance (LSPR), which provides a strong local electric field enhancement inside nanoparticles.7,10−12 However, the PL mechanism is still under debate.13 Three mechanisms have been proposed for the PL of gold nanostructures in the literature. The first mechanism attributes PL to the electron−hole interband radiative recombination, which can be enhanced by the incoming and outgoing electric field via coupling to the surface plasmon resonance.7,10,14 The second mechanism proposes that PL is the plasmon radiative decay, in which the energy of the thermalized electron−hole © XXXX American Chemical Society

Received: April 6, 2016

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Figure 1. (a and b) Schematic configurations for the single-nanoparticle dark-field scattering measurement and photoluminescence measurement, respectively. (c) SEM image of silver nanorods synthesized. (d) TEM image of silver nanorods. (e and f) Typical pair of a correlated dark-field image (e) and SEM image (f). The insets show a magnified SEM image of each nanoparticle nearby (appearing as white dots in low-magnification images). The scale bar for the insets is 100 nm.

mechanism with the well-known interband transition. A singleparticle PL investigation of 35 silver nanorods and 25 gold nanorods with different aspect ratios allows us to further analyze the PL mechanism. Combining simulated absorption cross sections of nanorods, we obtain the luminescence quantum yield of both silver and gold nanorods as a function of LSPR.

nanoparticles under 785 nm excitation would be very challenging. Compared with gold, silver has a large separation between the Fermi edge and the d-band of approximately 3.9 eV.21−23 Thus, if a visible or near-infrared PL of the silver nanoparticle can be observed, the interband radiative recombination contribution can be ruled out for further investigation. For example, an excitation and emission wavelength dependent quantum yield of PL from a singular intraband contribution can be possibly explored in such a platform. Moreover, silver nanoparticles generally provide higher detection sensitivity than gold nanoparticles due to the low loss of silver, which would be valuable for sensing and cell imaging.24,25 However, the investigations on PL of silver nanoparticles are very limited. To our best knowledge, the PL of both silver films and colloids previously reported were around 340 nm (3.6 eV), and a clear visible one-photon PL of silver nanoparticles has not been reported yet.6,26−28 Moreover, the recent experimental PL investigations related to silver nanowires show no detectable PL in the visible range for pure silver nanowires,29,30 which questions the existence of the visible PL of silver nanoparticles. A continuum in the visible and near-infrared range has recently been observed by Haug et al. when the electron gas of a silver nanoparticle film reaches temperatures of thousands of degrees Kelvin under 86 fs ultrafast laser illumination.31 Whether visible PL of silver exists under continuous wave laser illumination is still an open question, since the electron gas temperature can only imperceptibly increase. Herein, we report the observation of the undocumented visible one-photon PL of a single silver nanoparticle. We synthesize silver nanorods to obtain LSPRs in the visible and near-infrared range. By using a dark-field microscope combined with a Raman instrument, we perform a detailed single nanoparticle study on the correlation of the PL and dark-field scattering of silver nanorods supporting different LSPRs. The 532 nm (2.33 eV) and 633 nm (1.96 eV) continuous wave (CW) laser used for PL excitation is well below the 3.9 eV interband transition band gap in silver, suggesting a different

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RESULTS AND DISCUSSIONS The connection between the morphology of the individual silver nanorods and their single-nanoparticle optical response was accomplished using a quartz substrate with microfabricated markers (Figure S1 in the Supporting Information) for colocalization in the dark-field microscope and the scanning electron microscope (SEM). As shown in Figure 1a, a transmission dark-field microscope was used to perform darkfield scattering measurement, and the PL was measured right after using the same objective (Figure 1b). A typical pair of a correlated dark-field image and SEM image of silver nanorods are shown in Figure 1e and f. Silver nanorods of different aspect ratios show rich LSPR scattering color covering the whole visible range, which is different from the red color of gold nanorods (Figure S2). Although it is still a great challenge to synthesize silver nanorods with a high yield, the unavoidable inhomogeneity can be overcome through a single nanoparticle study using a dark-field microscope. In particular, as shown in Figure 1e and f, the small silver nanospheres, which are the main product, can be seen from the SEM image but are invisible in the dark-field image. In fact, the broad distribution of size and aspect ratio of Ag nanorods provides a wide LSPR range for a detailed analysis. We chose a first silver nanorod with a red color in the darkfield microscope (like the nanorod on the left in Figure 1e and f) for the following experiment. The scattering spectrum of the silver nanorod was first collected through a microscope objective using a home-built combined dark-field microscope and Raman microscope system (see Figure 1 and schematic diagram of the system in Figure S3 in the Supporting B

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Figure 2. (a) Normalized PL excited by a 532 nm laser (solid lines) and dark-field scattering (dotted lines) spectra of silver nanorods with different aspect ratios. (b) PL spectra of a single silver nanorod excited at 532 nm taken by varying the orientations of excitation. The PL can be observed when the polarization of the laser is parallel to the long axis of the nanorod (black line). In contrast, the emission spectrum excited by a laser perpendicular to the nanorod (gray line) is nearly the same as the background spectrum (red line). The inset shows the linear dependence of the PL intensity on the excitation power, which indicates a one-photon process. (c) Normalized PL excited by a 633 nm laser (solid lines) and dark-field scattering (dotted lines) spectra of silver nanorods with different aspect ratios. (d) PL spectra of a single silver nanorod excited at 633 nm taken at different excitation polarization. The inset shows the linear dependence of the PL intensity on the excitation power.

the 560−700 nm range that follows the LSPR scattering shape (dotted lines in Figure 2c) in a similar manner as gold nanorods reported in the literature.13,17 Notably, the PL peaks are slightly blue-shifted with respect to the scattering peaks as previously reported.16,17 To further investigate the generality of this shift, we change the excitation laser from 532 nm to 633 nm. As shown in Figure 2d, the PL of the silver nanorod excited at 633 nm shows the same polarization dependency and laser power dependency as the PL excited at 532 nm. We analyze now the mechanism of the PL of the silver nanorods. Since both the 532 nm (2.33 eV) and 633 nm (1.96 eV) excitation are well below the 3.9 eV band gap in silver, the interband transition process cannot be excited efficiently.21,31−33 Thus, the PL we observed should be attributed to the response of the carriers in the conduction band. As shown in Figure 3, two models have been proposed in the literature for the interaction of photon and electron in the conduction band. The first model, analogous to Raman

Information). A 532 nm CW laser was then used for excitation to verify the existence of the visible PL for silver nanorods. The polarization of the laser was kept parallel to the longitudinal axis of the nanorod to ensure the highest signal-to-noise ratio (SNR). Surprisingly, a strong PL peak was observed at the same wavelength as the LSPR scattering peak even at a relatively low laser power (29.1 μW), as shown in Figure 2b. The PL peak disappeared when the polarization of the laser is perpendicular to the nanorod, which shows a clear correlation of the PL and the longitudinal LSPR mode of the silver nanorod and also excludes the possible contribution of photoluminescence of the small silver clusters. It should be pointed out that both the peak position and the intensity of the PL of gold nanorods do not change as the polarization was rotated by 90° with a green laser,16 because the laser energy is able to excite the transverse LSPR mode of gold nanorods but not that of silver nanorods. The absorption of the gold nanorods around 532 nm is nearly excitation polarization independent, whereas the absorption of the silver nanorods around 532 nm is strongly dependent on the excitation polarization, as shown in Figure S4. Since the nanorod behaves like a dipole antenna, the PL emission polarization of silver nanorods is parallel to the long axis of the nanorods, which is the same as that of gold nanorods.16 The PL intensity increases linearly with the increase of the excitation power until the morphology of the nanorods starts to change due to the laser heating effect, which indicates a one-photon luminescence process. To further explore the correlation of the LSPR scattering and PL, a set of silver nanorods with different aspect ratios were chosen for PL and scattering measurements. The white light polarization was also kept parallel to the longitudinal axis of the nanorods. As shown in Figure 2a, we find that the 532 nm laser excited PL spectra (solid lines in Figure 2a) of the six silver nanorods show a prominent peak in

Figure 3. Simplified band diagram of silver, showing the interband transition, intraband transition, and electronic Raman process. C

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Figure 4. (a) PL intensity of 35 Ag (dark gray spheres) nanorods and 23 Au nanorods (orange spheres) excited at 532 nm as a function of LSPR wavelength of nanorods. Simulated absorption cross section (Abs.) at 530 nm of Ag (red line with gray star) and Au nanorods (red line with orange cross) with 23 nm diameter as a function of LSPR wavelength. (b) Intensity of PL of 17 Ag (gray hexagons) and 23 Au nanorods (orange hexagons) excited at 633 nm as a function of LSPR wavelength of nanorods. Simulated absorption cross section at 630 nm of Ag (red line with gray star) and Au nanorods (red line with orange cross) with 23 nm diameter as a function of LSPR wavelength. (c) Quantum yield of 532 nm excited PL of individual silver and gold nanorods as a function of LSPR wavelength. (d) Quantum yield of 633 nm excited PL of individual silver and gold nanorods as a function of LSPR wavelength.

to achieve different thermally excited electron distributions (detailed in Supporting Information S1.7). Hence, it seems that the PL of silver nanorods does not follow the first model. We now discuss the possibility of the second model, intraband transition, which has recently been considered to be the origin of the hot electrons with a high energy.35,37 The intraband transition has been proposed to explain the observation of infrared emission of the rough gold film excited at 780 nm (1.59 eV), which is well below the band gap for interband transition in gold (1.8 eV near the X symmetric point and 2.4 eV near the L symmetric point).34 Although the intraband transition on smooth metal surfaces is forbidden, the breakdown of symmetry and momentum selection rules in metal nanoparticles were presumed to allow efficient intraband radiative recombination.31,34,38 Several kinds of processes including phonon coupling, the Umklapp process, and Landau damping assisted by the SPP momentum have been discussed to provide enough momentum,35 whereas, the radiative recombination of the electron−hole pair in the conduction band has to overcome the fast nonradiative electron−electron (10−100 fs) and electron−phonon (100−1000 fs) scattering to be observable.39,40 Through the average bandwidth (145 meV) of the PL of single silver nanorods, we can estimate the radiative relaxation rate to be around 9 fs, which is reasonable to compete with the fast intraband nonradiative relaxation process. While intraband transitions should be possible within the optical near field inside silver, the energy must be coupled to the far-field radiation to be detected. This coupling has been

scattering, postulates that the electron can be transiently excited to a virtual state (similar to that of the molecule) and then relax to a different energy level around the Fermi energy.19,20 Thus, the PL should actually be the plasmon-enhanced electronic Raman scattering (also called electronic inelastic light scattering20). The second model proposes that the intraband transition of the electron within the conduction band may happen.34,35 Thus, the PL should originate from the radiative recombination of the electron−hole pair following the intraband transition. Notably, the electronic Raman should be excitation wavelength independent, whereas the intraband transition PL should be excitation wavelength dependent. As shown in Figure 2a and c, the PL was strongly shaped by the plasmon resonance response through emission enhancements. While both the 532 and 633 nm excited PL peaks follow the dark-field scattering peaks from the same nanorods, there are reproducible shifts between the 532 nm excited PL and the dark-field scattering, which was not observable under 633 nm excitation. To further subtract the plasmon resonance from the PL, we divided the PL spectra of nanorods (solid lines in Figure 2a and c) by their own scattering spectra (dotted lines in Figure 2a and c). As shown in Figure S5, the ratio spectra of PL to scattering show convincing consistency for different nanorods using either 532 nm excitation or 633 nm excitation.36 However, the ratio spectra are quite different with 532 nm excitation from that with 633 nm excitation, which implies an excitation wavelength dependent PL response of silver nanorods. Moreover, the temperature variations due to the excitation lasers are too small D

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wavelength independent intraband transition efficiency implies a sufficient momentum supported in the silver nanorods for the PL process. By comparing the quantum yield of 532 nm excited PL of silver (dark gray spheres) and gold nanorods (orange spheres) in Figure 4c, we found that the 532 nm excited PL quantum yield of silver nanorods is at the magnitude of 10−6, which is the same as that of gold nanorods. The PL quantum yield of gold nanorods we obtained agree well with the quantum yield reported in refs 13 and 17. In contrast, the quantum yield of 633 nm excited PL of gold nanorods is slightly stronger than that of silver nanorods. It may be explained by the better match of the excitation laser wavelength (633 nm) and the interband transition band gap around the X symmetry point in gold. Since the band structure and field confinement of gold nanoparticles are quite similar to those of silver nanoparticles, the intraband transition in gold nanoparticles should not be negligible. Moreover, supposing the intraband luminescence quantum yield of gold nanorods is similar to that of silver nanorods, the observed PL of gold nanorods may contain a large component of intraband luminescence. However, further distinguishing the contribution of interband and intraband transition to the PL of gold nanoparticles would become a great challenge. There has been a conflict between the high energy in demand for hot-electron-assisted reactions and the relatively low energy of the “lukewarm” electrons excited through interband transitions.35 However, the hot-electron hole pairs excited by the intraband transition will produce hot electrons with an energy sufficient for the reported plasmon-assisted reactions.42,43 With this understanding, it is not surprising that Ag nanoparticles showed a better activity for plasmon-assisted reactions.43−49 The intraband luminescence can provide a spectrum of the intraband transition excited hot electron−hole pairs. Nonetheless, it would be challenging to distinguish the different contribution of interband and intraband transition excited hot electron−hole pairs in hot-electron-assisted reactions.

suggested to be mediated by the excitation of localized surface plasmons.15,17,34,36 To estimate the efficiency of such intraband luminescence of silver nanorods, we first compare the PL intensity of silver nanorods with the gold nanorods whose quantum yield has previously been quantified to be around 10−6.13,17 We measured the PL of 35 Ag nanorods (dark gray spheres) and 23 Au nanorods (red spheres) with different LSPRs (aspect ratio) for the same laser power density and accumulation time. As shown in Figure 4a, the LSPR of silver nanorods (gray spheres) ranges from 540 to 730 nm, whereas the LSPR of Au nanorods (orange spheres) is limited to longer than 630 nm. Unexpectedly, the PL intensity of individual Ag nanorods can be comparable to that of gold nanorods, indicating an efficient intraband transition inside silver nanorods. Notably, the 532 nm excited PL intensity of Ag nanorods decreases as the LSPR red shifts. A similar relation between PL intensity and LSPR peak position of silver nanorods was found in the 633 nm excitation case, as shown in Figure 4b. In general, the PL intensity of gold nanorods is nearly twice that of silver nanorods, supporting the same LSPR in both the 532 and 633 nm excitation cases. However, the PL quantum yield is related not only to the PL intensity but also to the absorption cross section at the excitation wavelength and can be written as13,17 ηPL =

Nem NPL hνexc = Nabs σNR Πsetup Iexc

(2)

where ηPL is the PL quantum yield, Nem is the number of emitted photons, Nabs is the number of absorbed photons, NPL is the measured luminescence intensity after correcting the wavelength response of the setup, Πsetup is the detection efficiency of the setup, estimated to be 5%, σNR is the absorption cross section of nanorods at the excitation wavelength, Iexc is the excitation laser intensity, and hvexc is the photon energy of the excitation light. Since the measurement of the absorption cross section of nanorods at the singlenanoparticle level is still challenging,13,17 we simulated the absorption cross section of the nanorods based on the average diameter and the dark field scattering peak position. By tuning the length of the silver and gold nanorods with a diameter of 23 nm, we obtain the LSPR-dependent absorption cross section curves shown in Figure 4a and b (red and light red lines). Surprisingly, the LSPR-dependent absorption cross section curves show good agreement with the LSPR-dependent PL intensity of both silver and gold nanorods especially for 532 nm excitation. Thus, the decrease of the PL intensity as the LSPR red shifts can be well understood by the decrease of the absorption cross section at the excitation wavelength. We further calculate the PL quantum yield according to eq 2 based on the ratio of the PL intensity to the absorption cross section of nanorods. Indeed, as shown in Figure 4c and d, differing from the PL intensity, the quantum yield of silver nanorods is no longer LSPR dependent in both 532 and 633 nm excitation cases. Since the wavelength-dependent response of the setup has already been corrected in the NPL term, there will be no wavelength-dependent response of setup including lens and CCD in the quantum yield results. The fact that the quantum yield is independent of both excitation and emission wavelength further supports the intraband transition model, because the interband transition efficiency will vary with both the excitation and emission wavelength.41 Moreover, the emission

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CONCLUSIONS In summary, we reported the visible one-photon PL of single silver nanoantennas with visible excitation. The PL of single silver nanorods excited at either 532 or 633 nm, following LSPR scattering, shows good tunability in the visible and nearinfrared range. Because the excitation energy is not sufficient to excite the interband transition, we assigned the PL of silver nanorods to the hot-electron radiative recombination following the intraband transition. Quantitatively comparing the PL intensity of silver and gold nanorods, we found that the quantum yield of silver nanorods can be even higher than gold nanorods under 532 nm excitation but slightly lower than gold nanorods under 633 nm excitation. In contrast with “lukewarm” carriers excited by an interband transition, the intraband transition was predicted to provide hot electrons with a much higher energy. Our investigation toward PL of single silver nanorods indicates a sufficient intraband excitation of hot electrons that may be relevant to the hot-electron-related energy harvesting and photocatalysis.

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METHODS Sample Preparation. Silver nanorods were synthesized based on two reported method.50,51 To prepare Ag seeds, 5 mL E

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of ethylene glycol (EG) in a 25 mL round-bottom flask was first heated under strong magnetic stirring in an oil bath set at 150 °C. Then 0.06 mL of NaHS (3 mM in EG) was injected. After 2 min, 0.5 mL of HCl (3 mM in EG) and 1.25 mL of PVP (20 mg/mL, in EG) were injected. After another 2 min, 0.4 mL of CF3COOAg (282 mM in EG) was injected into the solution. The reaction were terminated with an ice−water bath after 1.5 h. To synthesize the Ag nanorods, 1.25 mL of EG was held in a 20 mL glass flask and heated under magnetic stirring in an oil bath set at 150 °C. After 10 min, 80 μL of NaBr (6 mM) in EG was introduced. After another 1.5 min, 0.3 mL of PVP solution in EG (20 mg/mL) was injected, followed by the addition of 40 μL of the as-prepared single-crystal Ag seeds (in EG) and 40 μL of AgNO3 solution in EG (48 mg/mL). After 3 h, the growth was terminated by immersing the reaction vial in an ice−water bath. The SEM and TEM images of synthesized silver nanorods are shown in Figure S6. The average width of the silver nanorods was characterized to be 23 nm, which is the same value used for COMSOL simulation. Gold nanorods were synthesized following a reported method.52 Briefly, to prepare the seed solution first, 5 mL of 0.5 mM HAuCl 4 was mixed with 5 mL of 0.2 M hexadecyltrimethylammonium bromide (CTAB) solution in a 25 mL round-bottom flask. Then, 0.6 mL of fresh 0.01 M sodium borohydride (NaBH4) was diluted to 1 mL with water and was injected into the HAuCl4−CTAB solution under vigorous stirring. To synthesize gold nanorods, 0.90 g of CTAB and 0.15 g of sodium oleate (NaOL) were first dissolved in 25 mL of 50 °C water in a 50 mL cuvette. After cooling to 30 °C, 1.8 mL of 4 mM silver nitrate (AgNO3) solution and 25 mL of 1 mM HAuCl4 solution were then added. A 150 μL amount of hydrochloric acid (37 wt % in water) was added to adjust the pH. Right after, 125 μL of ascorbic acid was added followed by the addition of 200 μL of prepared seed solution. The synthesis finished by keeping the solution undisturbed at 30 °C overnight after being gently shaken. The amounts of CTAB, NaOL, AgNO3, HCl, and seed solution were adjusted to control the average localized surface plasmon resonance (LSPR) peak of the nanorods. The aspect ratio of gold nanorods used was characterized through SEM to be 85.6 ± 5.5 nm/22.4 ± 2.1 nm. The particles were centrifuged and washed two times with Milli-Q water. Then the nanorods were diluted and dropped onto a quartz substrate with co-localization markers. The sample was put in a vacuum desiccator and dried under vacuum conditions using a water circulation vacuum pump. Fabrication of Markers on a Quartz Substrate and SEM Characterization. The co-localization markers on the quartz plate were fabricated by microfabrication techniques. The quartz plate was cleaned in a piranha solution and then rinsed in chromic acid to remove surface impurities and residues. Photoresist was then coated on the quartz substrates. Using conventional photolithography, the patterns of a selfdesigned photolithographic mask were transferred to the photoresist surface. After aluminum evaporation and the liftoff process, the photoresist was removed and only the aluminum pattern was left on the substrate. As shown in Figure S1, one pattern consists of 5 × 5 units, and each unit consists of 22 × 11 squares of 60 μm2. To identify every single square, each row and column was labeled with Chinese coordinates. We can thus locate the nanoparticles inside one square through these labels quickly. A photo of a one-inch

quartz substrate with markers and a dark-field image of part of one unit are shown in Figure S1. Correlation between singleparticle spectra and morphology of the nanorods was accomplished using this substrate for identification in the dark-field microscope and the scanning electron microscope (Hitachi S-4800). Since the quartz substrate is not conductive, 3 nm platinum was deposited after the optical measurement and before SEM imaging. Single-Nanoparticle Spectroscopy. Single-nanoparticle spectroscopic measurements were carried out on a Renishaw inVia Raman spectrometer with a home-built dark-field microscope (inverted, Leica). A motorized stage (Prior ProScan II) was used to trace the nanoparticle position. A schematic diagram of the experimental setup is shown in Figure S3. The same objective lens (NA = 0.64, 50×, Leica) was used to collect the elastic scattering and PL spectra. The transmission darkfield scattering experiment was conducted with 100 W halogen lamp illumination through an air condenser (NA = 0.80−0.95, Leica). The dark-field image was recorded by an EXi Aqua camera from Q-Imaging. A 30 μm slit was set in the detection path, and a 150 grooves/mm grating was used to disperse the scattering and PL spectra. To measure the PL spectra in Figure 2a−c and Figure 4, the powers of the 532 and 633 nm laser were set at 29.1 and 3.9 μW, respectively. The spectra were acquired for 10 s with three accumulations, without an observable difference between each acquisition. The PL spectra in Figure 2d were collected for 1 s under 397 μW 633 nm excitation. The laser polarization was kept parallel to the longitudinal axis of the nanorods in every acquisition through a half-wave plate. The white light polarization was also kept parallel to the longitudinal axis of the nanorods for every scattering spectrum using a high transmission polarizer. To make sure no morphology change happened during the laser illumination, the scattering spectra were measured before and after PL spectra acquisitions of every nanorod. All spectra have been calibrated for the wavelength-dependent response of the system. An AvaLight-HAL-CAL calibrated light source was used for calibration. The scattering spectra were given by Iscattering =

Iparticle − Isubstrate Isubstrate

where Iscattering is the real scattering spectrum from a single nanoparticle, Iparticle is the original scattering spectrum collected by CCD, and Isubstrate is the background spectrum collected in the dark region nearby without nanoparticles. Theoretical Simulations. A commercial finite-element method simulation software (COMSOL multiphysics package 4.4) was used to model the nanorods. We created a rectangular domain around a single nanorod, and perfectly matched layers were employed to simulate an open boundary. The permittivity of the silver and gold nanorod were taken from Johnson and Christy,53 and the quartz substrate that is placed beneath the nanorod was assumed to be semi-infinite, with a refractive index n = 1.4525. The medium over this substrate is air. The rod geometry was modeled as a cylinder with a semisphere attached at the two ends. The diameters of both gold and silver nanorods were set at 23 nm. The lengths of the silver nanorods were set from 68 to 110 nm. The lengths of the gold nanorods were set from 69 to 92 nm. F

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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/acsphotonics.6b00238. Experimental and theoretical details and additional data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail (B. Ren): [email protected]. Author Contributions

Experiments were designed and executed by K.Q.L. Theoretical simulations were performed by J.Y. The theoretical analysis and data interpretation were performed by K.Q.L., J.Y., and B.R. 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.

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ACKNOWLEDGMENTS We acknowledge support from MOST (2011YQ03012400 and 2013CB933703), NSFC (21227004, 21321062, and J1310024), and MOE (IRT13036).

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REFERENCES

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DOI: 10.1021/acsphotonics.6b00238 ACS Photonics XXXX, XXX, XXX−XXX