Article pubs.acs.org/JPCC
Plasmon-Mediated Whispering-Gallery-Mode Emission from Quantum-Dot-Coated Gold Nanosphere Xianguang Yang, Dinghua Bao, and Baojun Li* State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen University, Guangzhou 510275, China ABSTRACT: Plasmons of gold nanostructures provide a powerful tool to enhance exciton−exciton coupling and energy transfer at spherical geometry, while quantum dots have confined excitons with strong optical oscillator strength and gain. Here, we report a plasmon-mediated whispering-gallery-mode emission from CdSe-ZnS quantum dot-coated gold nanosphere at room temperature. The whispering-gallerymode emissions with different gold nanosphere sizes have been studied. Polarizationdependent emission is observed and explained. The decay dynamics of radiative emission is investigated by analyzing the photoluminescence decay spectrum. The underlying physical mechanism of the measured interference rings is further investigated by spatial distribution of photoluminescence intensity.
1. INTRODUCTION Composite nanostructures with semiconductor and metallic nanoparticles have been fabricated by different techniques.1−4 Nanointegration of semiconductors with metals is particularly important and fascinating because such composite nanostructures combine the advantages of these two promising materials. The localized surface plasmon polaritons of metallic nanoparticles capacitates optical manipulation and concentration on a subwavelength scale well below the diffraction limit.5−9 Semiconductor quantum dots (QDs) act as active components with large emission/absorption and strong optical gain, which can compensate the loss of the plasmonic cavity.4 Ultracompact photonic devices based on metal−semiconductor composite nanostructures have already been reported covering photodetectors,3,10,11 optical modulators,12 photovoltaic devices,13,14 and nanolasers.15−18 The confinements of light into submicrometer scale nanocavites have been intriguing a wide variety of interests, resulting from their capability of greatly enhancing light−matter interactions. Compared with dielectric cavities, plasmonic cavities enable light confinement into the sub micrometer mode volume.19,20 A single-nanoparticle plasmonic cavity is of particular interest as it supports ultrahigh Purcell factor F resulting from ultrasmall mode volume V.16,21 Optical studies based on circular cavities are fascinating because of their whispering gallery modes (WGMs), resulting from the total internal reflection of light at the dielectric interface around the circumference of a sphere. Nevertheless, when the diameter of a cavity decreases, radiation loss of the WGM will increase because of the weakness of the total internal reflection.22−24 Thus, dielectric cavities based WGM has limited the reduction of the cavity diameter, which extremely hinders the miniaturization of photonic devices. Fortunately, the plasmonic WGM cavity with a small diameter can support WGM © 2015 American Chemical Society
properties in submicrometer size, where the surface plasmon propagates along the equator, which makes it act as a promising candidate to integrate photonic devices with relatively high density. In this work, we report a submicrometer-sized gold nanosphere for QD WGM emission. The QD-coated gold nanosphere composite nanostructure was synthesized by utilizing electrostatic interaction. The size- and polarizationdependent emission properties have been systematically studied. The mechanism of dynamics behind the emission process was explored by time-resolved fluorescence spectroscopy. In addition, the interference pattern was also explained.
2. METHODS Amine groups (−NH2) appended gold nanospheres and carboxy groups (−COOH) appended QDs (quantum yield >30%) were purchased from NanoSeedz and Zkwy Bio-Tech companies, respectively. To coat QDs on a gold nanosphere, a feasible synthesis of QDs coated on the surface of gold nanosphere is as follows: First, the amine-terminated gold nanosphere (concentration 9.5 × 109 per mL) and carboxyterminated QD (concentration 4 × 10−6 M/L) solutions were adjusted to pH ∼ 4 and pH ∼ 10 to prepare polycationic and polyanionic nanoparticles. Second, the gold nanosphere solution was slowly dropped in the QD solution with gentle shaking for QDs assembling on gold nanospheres. Once pale haziness occurred, the dropwise addition of gold nanosphere solution was finished. At this moment, the pH of the mix suspension was 7.5−7.8, and relatively homogeneous coating of QDs on gold nanospheres was achieved under a series of Received: June 15, 2015 Revised: October 22, 2015 Published: October 29, 2015 25476
DOI: 10.1021/acs.jpcc.5b07475 J. Phys. Chem. C 2015, 119, 25476−25481
Article
The Journal of Physical Chemistry C
Figure 1. TEM images and EDS analysis. (a) TEM image of a representative 470 nm diameter gold nanosphere. (b) TEM image of QD-coated gold nanosphere with diameter of about 490 nm. QDs are indicated by red arrows. (c) The high resolution TEM image of QDs. (d) EDS spectrum of QD-coated gold nanosphere shown in panel b. Inset: The absorption and emission spectrum of CdSe-ZnS QDs.
propagate across those small uncovered areas as surface plasmon. Importantly, the enhanced QD emission by plasmon provides enough optical signals to achieve WGM emission. Corresponding energy-dispersive X-ray spectroscopy (EDS) was also measured and shown in Figure 1d. The EDS analysis confirms the existence of Au (30.25 wt %), Cd (0.06 wt %), Se (0.43 wt %), Zn (0.96 wt %), and S (0.41 wt %) elements. The number of coated-QDs per gold nanosphere was estimated to be 2.5 × 103. The inset of Figure 1d shows the absorption and emission spectrum of CdSe-ZnS QDs. 3.2. Optical Characterization. Optical characterization of the QD-coated gold nanospheres was carried out under an optical microscopy by using micro spectrophotometer (CRAIC 20/20 PV). A 365 nm continuous work laser was used to excite the QD-coated gold nanospheres. The photoluminescence (PL) signals were collected by a 100× objective (numerical aperture NA = 0.95) and directed through a dichroic mirror and a 365 nm notch filter. The filtered light was split by a beam splitter and directed to a spectrometer and a charge-coupled device (CCD) camera for spectrum and image measurement,
repetitive experiments. The number of QDs coated to a single gold nanosphere can be controlled via adjusting the concentration of CdSe-ZnS QDs and the time of chemical reaction.
3. RESULTS AND DISCUSSION 3.1. TEM and EDS Characterization. Figure 1a shows a representative transmission electron microscope (TEM) image of gold nanosphere with diameter of 470 nm. Figure 1b shows the TEM image of corresponding QD-coated gold nanosphere with diameter of 490 nm. Figure 1c gives the high resolution TEM image of QDs. The average diameter of coated QDs (red arrows indicate ensembles of QDs) is 5 nm. It is estimated that about 2 layers of QDs were assembled on the gold nanospheres, and the innermost layer can act as a dielectric space to avoid emission quenching. Although, the QD-coating is not very uniform, and some small areas are not covered with QDs, which results in the nonuniform enhancement of emission and even no emission from small uncovered areas. However, the enhanced emission from covered areas would 25477
DOI: 10.1021/acs.jpcc.5b07475 J. Phys. Chem. C 2015, 119, 25476−25481
Article
The Journal of Physical Chemistry C
The PL spectra of both the 800 and 550 nm diameter QDcoated gold nanosphere also occurs the separated peaks because of the WGMs, while the peak positions are blueshifted with diameter increasing as compared with that of the 490 nm diameter QD-coated gold nanosphere. As can be seen from Figure 3c (green line), it is particularly surprising that upon the diameter increased to 550 nm, the intensity profile of the WGM already deviates the PL profile of QDs solution. It can also be seen from Figure 3c (red line) that the 800 nm diameter QD-coated gold nanosphere occur a more blue-shift in the emission profile of the peak intensity. These differences were measured directly for various QD-coated gold nanospheres, which indicate that the observed blue-shifts are general within the sample set of gold nanosphere and not due to the pick out of specific subsets. Theoretically, the occur of PL from the QD-coated gold nanospheres require the electron−hole recombination process generates photon with energy matching one WGM, and the energy density of WGM along the QD spatial position should not be equal to zero.27 Photon transition resulting in PL emission must be both “electronically” satisfied and “photonically” satisfied. The electronic density of states and electron−hole recombination are strongly dependent on the composition, shape, and size of used QD. Meanwhile, the photonic density of states is successively decided by the size, and refractive-index difference of the gold nanosphere. Experimentally, the QD-coated gold nanosphere PL is tested under the same QD composition and average size, excitation wavelength, excitation intensity, excitation polarization, temperature, and surrounding medium, it can be supposed that the measured blue-shifts of emission peak are resulted from altering in both the WGM density of states and the coupling strengths of near-field interaction between coated QDs and gold nanosphere. Actually, they are the results of altering in the size and refractive-index of the gold nanosphere, as the same as altering in the spatial location of the coated-QDs with respect to the gold nanosphere. Both alterations are because of the QDs adsorption onto the surface of QD-coated gold nanosphere. The adsorption of a certain number of QDs onto the surface of QD-coated gold nanosphere gives rise to alterations in the boundary conditions for light wave propagation within the nanocavity. Thus, the blue-shifts of emission peak positions occurred in the WGM spectrum.26 At the same time, the distribution of adsorptive-QDs alters the spatial location of the photonic emitters (QDs) with respect to the nanocavity along the radial direction. These alterations could account for the measured blue-shifts in the emission profile of intensity. It could be supposed that in the new spatial location along radial direction, those band-edge photon transitions of QDs are intensely prohibited, while photon transitions with higher energy are more efficiently coupled to the optical field within the nanocavity. The photon transition of QDs is sensitive to the local electromagnetic field, the distribution of electromagnetic field around the gold nanosphere is spatial dependent, and thus, the transition of QDs in different spatial location is correspondingly changed. 3.5. Decay Dynamics of Emission. To precisely study the decay dynamics of radiative emission of QD-coated gold nanosphere and the influence of the gold nanosphere on the PL lifetime of the QDs, the PL decay spectra of the QDs were measured without and with 470 nm diameter gold nanosphere (excitation with a pulsed laser of 405 nm, 130 mW). The repetition rate of 1 MHz (corresponding to a pulse-to-pulse interval of 1000 ns) was used to ensure complete PL decay
respectively. In addition, polarization controller was inserted in the light path for polarization measurement. 3.3. Polarization-Dependent Emission. To explore the polarization of emission light from the QD-coated gold nanosphere, we insert a linear polarizer between the microscope objective and the CCD camera for polarization selection, and use a well-positioned 490 nm diameter QD-coated gold nanosphere for test. The direction of emission light from individual coated-QDs is perpendicular to the surface of gold nanosphere. When the polarization angle θ (defined as the cross angle between the direction of emission light and the optical axis direction of polarizer) changes, the PL intensity of emission light exhibits an oscillation behavior between the maximum and the minimum, as can be demonstrated experimentally. For reference, Figure 2 shows dark-field PL
Figure 2. Optical microscope images of QD-coated gold nanosphere excited by 365 nm laser with an optical power of 10 mW under different polarization angle θ of −90° (a), −70° (b), −50° (c), −30° (d), −10° (e), and 10° (f). Scale bar in panel f is applicable to panels a−e.
microscope images of emission light from QD-coated gold nanosphere excited by 365 nm laser with an optical power of 10 mW under polarization angles of −90°, −70°, −50°, −30°, −10°, and 10°, respectively. The PL intensity of the emission light increases with θ changing from −90° to 10°. Figure 3a shows θ-dependent PL intensity of the emission light. The PL intensity profile can be described by a sinusoidal dependence I = I1 cos(2θ) + I0, where I0 and I1 are fit parameters. The minimum (θ = −90°) and the maximum (θ = 10°) PL intensities correspond to perpendicular and parallel to the light extinction of the gold nanosphere, respectively. The oscillation period of 180° is regardless of the size of gold nanosphere. This is similar to those reported in plasmonic structure of QDs on gold nanodisc.25 The optical microscopy equipped with a liquid-nitrogen-cooled spectrometer was used to measure the emission spectra of the QD-coated gold nanosphere. Figure 3b shows that the emission spectrum contains 5 narrow-bands, which is the characteristic of the WGMs. The highest peak of WGM is well matched with the peak of the solution PL. The results obtained are consistent with previously published work.26 At the same time, the WGM emission spectrum of Figure 3b indicates that the optical feedback is good enough to compensate the loss of plasmon. 3.4. Size-Dependent Emission. To study the sizedependent WGM emissions, two more different sized QDcoated gold nanospheres have been synthesized and studied. 25478
DOI: 10.1021/acs.jpcc.5b07475 J. Phys. Chem. C 2015, 119, 25476−25481
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Figure 3. Spectroscopy analysis. (a) Polarization-dependent PL intensity. The red solid line is the best-fit sine curve. Insets: Optical microscope images of QD-coated gold nanosphere with polarization angles of θ = −90° and θ = 10°, respectively. (b) WGM emission spectrum of a QD-coated gold nanosphere (blue solid line) and the respective solution QD emission spectrum (red dotted line). The inset gives a microscope image of the QD-coated gold nanosphere with polarization angle of θ = −10°. The peak intensity distribution of WGM spectrum matches well with the envelope of solution emission. (c) WGM emission spectra of QD-coated gold nanosphere with diameters of 800 nm (red), 550 nm (green), and 490 nm (blue). Insets show the corresponding TEM images of QD-coated gold nanospheres. The scale bar in blue box is applicable to green and red boxes. (d) PL decay spectra of QD-coated gold nanosphere (blue line) and the respective QDs solution (red line). The lifetime of QD-coated gold nanosphere and QDs solution are 9 and 13 ns, respectively.
F = τfree/τcav = (13 ns)/(9 ns) = 1.44. The difference is because the lifetimes are simply 1/e values. Theoretical calculations of the electromagnetic field distribution have already indicated that high-Q whispering gallery modes can be concentrated in circular rings.30 As the light wave was confined around the surface of gold nanosphere by surface plasmon, the propagation of the light wave is in the QDs layer. So the free spectral range of plasmon-mediated WGMs can be calculated by λfree = (λc/ n)2/2nπR = 1/n2 × λc2/2nπR, where λc and λc/n are the wavelength of light in vacuum and QDs layer, respectively.31 The calculated λfree is 30 nm, which is in agreement with the measured data from the WGM spectrum of Figure 3b. 3.6. Interference Rings from Emission. Additionally, we investigate the detailed spatial distribution of PL intensity emission from QD-coated gold nanosphere. Under excitation, bright PL spots are observed and shown in Figure 4a,d, with clear interference patterns manifesting good spatial coherence of the emitted light. The PL spot size of the QD-coated gold nanosphere (even with the limitation of spatial resolution) is ∼500 nm, indicating much tighter confinement of the surface plasmon polariton radiation. To precisely study the detailed spatial distribution of PL intensity, the PL intensity profiles were measured using the histogram tool in Adobe Photoshop. The corresponding resolution is not the real spatial resolution, it is the resolution of pixel. Figure 4b shows the PL intensity profile along the x axis indicated in panel a. Further, the enlarged view of green shadow area indicated in panel b is shown in Figure 4c, which shows that the PL intensity exhibits an oscillation behavior between the maximum and the minimum from 200 to 300 nm along the x axis. For comparison, Figure 4e shows the PL intensity profile along the x axis indicated in panel d. Further, the enlarged view of
before the next pulse excites the QD samples and to prevent multiple excitations. The pump intensity of per-pulse is 130 nJ/ cm2. Figure 3d shows the PL decay spectra of QD-coated gold nanosphere (blue line) and the respective QDs solution (red line). Actually, it is a multiple-exponential process. The emission decay at least includes the intrinsic, plasmonic quenching, and plasmonic enhancement. The lifetime of QDcoated gold nanosphere and QDs solution, which were extracted from 1/e values, are 9 and 13 ns, respectively. Compared to the lifetime of QDs solution, the lifetime decreases from 13 to 9 ns with the presence of gold nanosphere, suggesting that an increase of the radiative recombination rate. This implies that the intensity of local optical field increases with the presence of gold nanosphere. The enhancement of the spontaneous emission rate was demonstrated because the radiative lifetime of the QDs is larger than the lifetime of photons coupled to the nanocavity. Therefore, the decrease of PL lifetime confirms the near-field interaction between the surface plasmon enhanced local optical field by gold nanoshperes and the radiative decay of QDs. The modification of the spontaneous emission rate is presented by the Purcell factor F = τfree/τcav = 3Q(λc/n)3/(4π2Veff),25,28 where the effective volume Veff = (4/3)π × (500/2)3 nm3 − (4/ 3)π × (470/2)3 nm3 ≈ 2233 nm3 was estimated from the size of the light spot and gold nanosphere. The quality factor Q was calculated as follows: the center wavelength λc = 625 nm with a line width of 7.8 nm was measured from Figure 3b, thus, quality factor Q = λc/Δλ = (625 nm)/(7.8 nm) = 80. The refractive index n ∼ 2.07 was estimated from the volume ratio of CdSe and ZnS in the core−shell structure.29 The calculated Purcell factor F = 3Q(λc/n)3/(4π2Veff) = 1.51. On the other hand, according to lifetime, it can also be obtained that Purcell factor 25479
DOI: 10.1021/acs.jpcc.5b07475 J. Phys. Chem. C 2015, 119, 25476−25481
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The Journal of Physical Chemistry C
Figure 4. Interference patterns and spatial distribution of PL intensity. (a) Optical microscope image of QD-coated gold nanosphere with polarization angle θ = −50°. (b) PL intensity profile along the x axis indicated in panel a. (c) Enlarged view of green shadow area indicated in panel b. (d) Optical microscope image of QD-coated gold nanosphere with polarization angle θ = −10°. The scale bar is applicable to panel a. (e) PL intensity profile along the x axis indicated in panel d. (f) Enlarged view of green shadow area indicated in panel e. The PL intensity profiles were measured along the x axis using the histogram tool in Adobe Photoshop.
4. CONCLUSIONS
green shadow area indicated in panel e is shown in Figure 4f, which shows that the PL intensity exhibits an oscillation behavior between the maximum and the minimum from 400 to 600 nm along the x axis. Interestingly, it is observed that the spatial region of oscillation behavior is dependent on the polarization angle. When the polarization angle θ changes from −50° to −10°, the corresponding spatial region of oscillation behavior changes from 200−300 nm to 400−600 nm. We propose that this may be because the orientation of the coatedQDs is susceptible to the polarization direction. Once the polarization direction changes, the spatial orientations of coated-QDs will change, thus, changing the spatial distribution of coherence light from each coated-QD. Therefore, the region of oscillation resulted from interference of coherence light will change correspondingly in experiment. Anyway, the measured PL intensity profile matches well with the optical microscope image, and consistent with the theoretical emission pattern reported by previous work.32
In summary, QDs and gold nanosphere composite nanostructures have been synthesized through electrostatic interaction between gold nanospheres and QDs. Such structures enable the systemic investigations of the WGM emission as a function of the size and polarization. The results show that the largest PL emission can be achieved when the polarization angle of about 10°, where the WGM is maximized, and the peak positions of the WGM emissions are blue-shift as the size increasing. The time-resolved fluorescence spectroscopy of QDs without and with gold nanosphere indicates that the lifetime drops from 13 to 9 ns, resulting in an increase of the radiative recombination rate. Moreover, the measured interference rings are explained with the detailed spatial distribution of PL intensity. These results offer a feasible strategy to prepare active plasmonic hybrid nanocavities for extreme light concentration and whispering-gallery-mode emission nanophotonic devices using quantum dots. 25480
DOI: 10.1021/acs.jpcc.5b07475 J. Phys. Chem. C 2015, 119, 25476−25481
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(20) Kang, J.-H.; No, Y.-S.; Kwon, S.-H.; Park, H.-G. Ultrasmall Subwavelength Nanorod Plasmonic Cavity. Opt. Lett. 2011, 36, 2011− 2013. (21) Meng, X.; Kildishev, A. V.; Fujita, K.; Tanaka, K.; Shalaev, V. M. Wavelength-Tunable Spasing in the Visible. Nano Lett. 2013, 13, 4106−4112. (22) Kwon, S.-H. Deep Subwavelength Plasmonic WhisperingGallery-Mode Cavity. Opt. Express 2012, 20, 24918−24924. (23) Ratchford, D.; Shafiei, F.; Kim, S.; Gray, S. K.; Li, X. Manipulating Coupling Between a Single Semiconductor Quantum Dot and Single Gold Nanoparticle. Nano Lett. 2011, 11, 1049−1054. (24) Ward, J.; Benson, O. WGM Microresonators: Sensing, Lasing and Fundamental Optics with Microspheres. Laser Photonics Rev. 2011, 5, 553−570. (25) Zhu, Q. Z.; Zheng, S. P.; Lin, S. J.; Liu, T. R.; Jin, C. J. Polarization-Dependent Enhanced Photoluminescence and Polarization-Independent Emission Rate of Quantum Dots on Gold Elliptical Nanodisc Arrays. Nanoscale 2014, 6, 7237−7242. (26) Gomez, D. E.; Pastoriza-Santos, I.; Mulvaney, P. Tunable Whispering Gallery Mode Emission from Quantum-Dot-Doped Microspheres. Small 2005, 1, 238−241. (27) Chiasera, A.; Dumeige, Y.; Feron, P.; Ferrari, M.; Jestin, Y.; Nunzi Conti, G.; Pelli, S.; Soria, S.; Righini, G. C. Spherical Whispering-Gallery-Mode Microresonators. Laser Photonics Rev. 2010, 4, 457−482. (28) Gu, Y.; Wang, L.; Ren, P.; Zhang, J.; Zhang, T.; Martin, O. J.; Gong, Q. Surface-Plasmon-Induced Modification on the Spontaneous Emission Spectrum via Subwavelength-Confined Anisotropic Purcell Factor. Nano Lett. 2012, 12, 2488−2493. (29) Xu, J.; Xia, J.; Wang, J. Quantum Dots Confined in Nanoporous Alumina Membranes. Appl. Phys. Lett. 2006, 89, 133110. (30) Neuhauser, R. G.; Shimizu, K. T.; Woo, W. K.; Empedocles, S. A.; Bawendi, M. G. Correlation between Fluorescence Intermittency and Spectral Diffusion in Single Semiconductor Quantum Dots. Phys. Rev. Lett. 2000, 85, 3301−3304. (31) Glazov, M.; Ivchenko, E.; Poddubny, A.; Khitrova, G. Purcell Factor in Small Metallic Cavities. Phys. Solid State 2011, 53, 1753− 1760. (32) Artemyev, M. V.; Woggon, U.; Wannemacher, R.; Jaschinski, H.; Langbein, W. Light Trapped in a Photonic Dot: Microspheres Act as a Cavity for Quantum Dot Emission. Nano Lett. 2001, 1, 309−314.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (11274395) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13042).
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REFERENCES
(1) Costi, R.; Saunders, A. E.; Banin, U. Colloidal Hybrid Nanostructures: A New Type of Functional Materials. Angew. Chem., Int. Ed. 2010, 49, 4878−4897. (2) Khanal, B. P.; Pandey, A.; Li, L.; Lin, Q.; Bae, W. K.; Luo, H.; Klimov, V. I.; Pietryga, J. M. Generalized Synthesis of Hybrid MetalSemiconductor Nanostructures Tunable from the Visible to the Infrared. ACS Nano 2012, 6, 3832−3840. (3) Fan, P.; Chettiar, U. K.; Cao, L.; Afshinmanesh, F.; Engheta, N.; Brongersma, M. L. An Invisible Metal-Semiconductor Photodetector. Nat. Photonics 2012, 6, 380−385. (4) Banin, U.; Ben-Shahar, Y.; Vinokurov, K. Hybrid SemiconductorMetal Nanoparticles: From Architecture to Function. Chem. Mater. 2014, 26, 97−110. (5) Pelton, M.; Aizpurua, J.; Bryant, G. Metal-Nanoparticle Plasmonics. Laser Photonics Rev. 2008, 2, 136−159. (6) Schuller, J. A.; Barnard, E. S.; Cai, W. S.; Jun, Y. C.; White, J. S.; Brongersma, M. L. Plasmonics for Extreme Light Concentration and Manipulation. Nat. Mater. 2010, 9, 193−204. (7) Gjonaj, B.; Aulbach, J.; Johnson, P. M.; Mosk, A. P.; Kuipers, L.; Lagendijk, A. Active Spatial Control of Plasmonic Fields. Nat. Photonics 2011, 5, 360−363. (8) Hess, O.; Pendry, J. B.; Maier, S. A.; Oulton, R. F.; Hamm, J. M.; Tsakmakidis, K. L. Active Nanoplasmonic Metamaterials. Nat. Mater. 2012, 11, 573−584. (9) Fang, Z.; Zhu, X. Plasmonics in Nanostructures. Adv. Mater. 2013, 25, 3840−3856. (10) Konstantatos, G.; Sargent, E. H. Nanostructured Materials for Photon Detection. Nat. Nanotechnol. 2010, 5, 391−400. (11) Berini, P. Surface Plasmon Photodetectors and Their Applications. Laser Photonics Rev. 2014, 8, 197−220. (12) Pacifici, D.; Lezec, H. J.; Atwater, H. A. All-Optical Modulation by Plasmonic Excitation of CdSe Quantum Dots. Nat. Photonics 2007, 1, 402−406. (13) Atwater, H. A.; Polman, A. Plasmonics for Improved Photovoltaic Devices. Nat. Mater. 2010, 9, 205−213. (14) Gan, Q.; Bartoli, F. J.; Kafafi, Z. H. Plasmonic-Enhanced Organic Photovoltaics: Breaking the 10% Efficiency Barrier. Adv. Mater. 2013, 25, 2385−2396. (15) Oulton, R. F.; Sorger, V. J.; Zentgraf, T.; Ma, R.-M.; Gladden, C.; Dai, L.; Bartal, G.; Zhang, X. Plasmon Lasers at Deep Subwavelength Scale. Nature 2009, 461, 629−632. (16) Noginov, M.; Zhu, G.; Belgrave, A.; Bakker, R.; Shalaev, V.; Narimanov, E.; Stout, S.; Herz, E.; Suteewong, T.; Wiesner, U. Demonstration of a Spaser-Based Nanolaser. Nature 2009, 460, 1110− 1112. (17) Wu, C.-Y.; Kuo, C.-T.; Wang, C.-Y.; He, C.-L.; Lin, M.-H.; Ahn, H.; Gwo, S. Plasmonic Green Nanolaser Based on a Metal-OxideSemiconductor Structure. Nano Lett. 2011, 11, 4256−4260. (18) Yin, Y.; Qiu, T.; Li, J.; Chu, P. K. Plasmonic Nano-Lasers. Nano Energy 2012, 1, 25−41. (19) Seo, M.-K.; Kwon, S.-H.; Ee, H.-S.; Park, H.-G. Full ThreeDimensional Subwavelength High-Q Surface-Plasmon-Polariton Cavity. Nano Lett. 2009, 9, 4078−4082. 25481
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