Enhanced Light Extraction from SrGa2S4:Eu2+ Film Phosphors

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J. Phys. Chem. C 2008, 112, 7594–7598

Enhanced Light Extraction from SrGa2S4:Eu2+ Film Phosphors Coated with Various Sizes of Polystyrene Nanosphere Monolayers Ki Young Ko, Keyong Nam Lee, Young Kwang Lee, and Young Rag Do* Department of Chemistry, Kookmin UniVersity, Seoul 136-702, Korea ReceiVed: January 7, 2008

Hexagonally close-packed, two-dimensional (2D), polystyrene (PS) nanosphere photonic crystal (PC) monolayers were coated on sputtered SrGa2S4:Eu2+ films using a process involving the self-assembly of PS nanospheres and the simple transfer of the PS nanosphere monolayers to the phosphor surface. The overall aim of this study was to improve the efficiency of light extraction from the phosphor side of the SrGa2S4: Eu2+ films. The effects of using PS nanospheres of various diameters (330, 580, and 960 nm) on the extraction efficiency of the SrGa2S4:Eu2+ film phosphors were investigated. The integrated photoluminescence extraction was improved by a factor of 4.3 with respect to that of a conventional SrGa2S4:Eu2+ film phosphor through the addition of a 2D monolayer array of 580-nm PS nanospheres with highly crystalline PS monolayer domains. 1. Introduction :Eu2+

phosphors Various studies have focused on SrGa2S4 because Eu-doped thiogallate is an excellent green component in thin-film electroluminescent (TFEL) displays,1 field emission displays (FEDs),2 and tricolor white light-emitting diodes (LEDs).3 High quality and bright SrGa2S4:Eu2+ phosphors are essential for all these device applications. Thin-film SrGa2S4: Eu2+ phosphors have recently attracted attention on account of the many advantages of thin-film phosphors (TFPs) over the powder form.4–7 However, the low brightness of TFPs is the most significant drawback to their use in lighting and display devices. The low external efficiency of TFPs are mainly due to their poor light extraction efficiency, which results from the so-called “waveguide effect”. The waveguide modes of light emitted from the TFPs arise from the total internal reflection (TIR) at their interfaces, at which there are significant differences in refractive index (n).8 In the case of SrGa2S4:Eu2+ films (n ) 2.3),9 the fraction of light externally propagating from the phosphor side is approximately 4.7% (the fraction of escaping light is approximately 1/4n2).5 The remaining light generated inside the TFP is internally reflected at the interface and is eventually absorbed or emitted from the edge of the SrGa2S4: Eu2+ film-coated substrate. For the purpose of reducing the waveguide effect and, in turn, improving the light extraction efficiency from the phosphor side of the TFP layer, we previously proposed the addition of various 2D photonic crystal layers (PCLs) on top of the Y2O3:Eu3+ TFPs.10–12 2D square-lattice and hexagonal SiNx nanorod PCLs were fabricated on Y2O3:Eu3+ TFPs, which were found to enhance the extraction efficiency for phosphor side emission at the surface normal by a factor of 3.6-5.9.10,11 Recently, we developed a chemical approach for fabricating the 2D PCLs, such as hexagonally close-packed polystyrene (PS, n ) 1.59) nanosphere monolayers, for use as high-refractive index components in 2D PCLs to simplify the fabrication of 2D PCLs coated on TFPs.12 The use of PS-based nanosphere PC monolayers is one of the simplest chemical techniques for fabricating large area and periodic nanosized array patterns on phosphor * To whom correspondence should be addressed. Tel: +82-2-910-4893. Fax: +82-2-910-4415. E-mail: [email protected].

surfaces. This simplified self-assembly process can be applied to PS nanospheres of various sizes as well as other types of film phosphors, such as green SrGa2S4:Eu2+ TFPs. However, there is little information on the effects of the different sizes of nanosphere coated on the phosphor film and the effect of PS nanosphere PC on the sulfide film phosphor. This article discusses a possible mechanism for the formation of the PS nanosphere monolayer in solution and the technique for transferring the nanospheres from the water surface to the substrate. The use of PS nanosphere monolayers as PCLs was also investigated as a means of enhancing the extraction efficiency of green emission from the SrGa2S4:Eu2+ TFPs. Moreover, the suitability of applying this simplified self-assembly process to PS nanospheres of various sizes (330, 580, and 960 nm) for fabricating a variety of lattice constants of the 2D photonic crystal layer was tested. Finally, the effects of varying the diameter of the PS nanospheres on the extraction efficiency of the sputtered SrGa2S4:Eu2+ TFPs were investigated with the aim of optimizing the extraction efficiency of green emission from the TFPs. 2. Experimental Section The SrGa2S4:Eu2+ film phosphors were deposited on quartz (JMC Glass) substrates by radio frequency (rf) magnetron sputtering. The depositions were carried out under the following conditions: mixed Ar atmosphere with 10 wt % H2S, a background pressure of 1.0 × 10-6 Torr, a working pressure of 1.0 × 10-2 Torr, an rf power of 150 W, 4-in. SrGa2S4:Eu2+ pellet targets (High Purity Co. Ltd., 99.9%), and a substrate temperature of 300 °C. The SrGa2S4:Eu2+ films were postannealed in a vacuum for 2 h at 700 °C. The Eu2+ content (Eu/ (Sr+Eu) atomic ratio) of the SrGa2S4:Eu2+ thin films was fixed at 2 atom % SrGa2S4:Eu2+ films with a thickness of 1.0 ( 0.05 µm. These optimum conditions have become established as the standard phosphor layers for assessing the effects of PS nanosphere monolayers on the green emission of SrGa2S4:Eu2+ film phosphors. Our method for fabricating the close-packed PS nanosphere monolayers is based on the self-assembly of three different sizes of PS nanospheres (Interfacial Dynamics Co., 330, 580, and

10.1021/jp800115r CCC: $40.75  2008 American Chemical Society Published on Web 04/11/2008

Light Extraction from SrGa2S4:Eu2+ Film Phosphors

J. Phys. Chem. C, Vol. 112, No. 20, 2008 7595

Figure 1. Schematic diagram showing the method for fabricating the PS nanosphere monolayer on a SrGa2S4:Eu2+ thin film: Dispersed nanospheres floating on the water surface f crystallization by surfactant f transfer to the film surface.

960 nm). Figure 1 shows a schematic diagram of the formation of a monolayer of PS nanospheres on a SrGa2S4:Eu2+ thin film. As previously reported,13 the close-packing process also requires the PS nanospheres to be able to diffuse freely across the water surface and seek sites with the lowest energy configuration. Ordering on a scale of several square millimeters can be achieved because the PS nanospheres floating on the water surface have sufficient mobility to form large crystals from the small irregular PS crystal domains. The growth of large closepacked domains with a size of several square millimeters can be promoted by creating gentle waves in the water medium through the slow and careful agitation of the beaker.14 As previously reported,12 a few drops of a sodium dodecyl sulfate (SDS) solution can be applied, resulting in the formation of a rigid and close-packed PS monolayer due to modification of the surface tension of water. The method for depositing a PS nanosphere monolayer onto a SrGa2S4:Eu2+ coated quartz substrate involves transferring the monolayer from the water surface to the phosphor surface by scooping it up using the SrGa2S4:Eu2+ film coated quartz substrate. After the water is evaporated, the PS nanospheres with three different sizes are drawn together by capillary forces and crystallize in a hexagonal close-packed monolayer array on the SrGa2S4:Eu2+ surface. The crystal structures of the SrGa2S4:Eu2+ TFPs were determined by X-ray diffraction (XRD). The XRD patterns of the SrGa2S4:Eu2+ TFPs were obtained using a Philips model PW1800 X-ray diffractometer with Cu KR1 radiation. The diffraction patterns were obtained over the range 10–80° 2θ at a scan rate of 1° 2θ/min. The thicknesses and surface images of the films and PS nanosphere-coated films were determined by field emission-type scanning electron microscopy (FE-SEM) (JSM 7401F, JEOL) operated at 10 kV. The surface morphology of the flat SrGa2S4:Eu2+ films was determined by atomic force microscopy (AFM) (Seiko instrument model SPA 400) operated in contact mode (Si Cantilever). The room temperature and transmitted photoluminescence (PL) excitation and emission spectra of both the flat and 2D PS nanosphere-coated SrGa2S4: Eu2+ films were measured using a spectrophotometer (PSI Co. Ltd., Darsar). 3. Results and Discussion Polycrystalline films of the SrGa2S4:Eu2+ phosphors were prepared by rf-sputter deposition and postannealing, and their XRD patterns coincided with those reported in the corresponding JCPDS cards (Figure 2). The XRD patterns show that the asgrown SrGa2S4:Eu2+ TFPs annealed at 700 °C crystallize in the pure orthorhombic PbGa2Se4 phase.15 In addition to the effects of the crystal structure on the emission properties of the film phosphors, it is well-known that the properties of the surface morphology of SrGa2S4:Eu2+ film phosphors such as their grain size have strong effects on their extraction efficiencies. The 2D AFM image shown in the left inset in Figure 2 shows a grain size and root mean square (rms) roughness of the SrGa2S4:Eu2+ TFPs on quartz substrates of approximately 58.8 and 5.27 nm, respectively. The surfaces of the sputtered SrGa2S4:Eu2+ TFPs

Figure 2. XRD pattern of a sputtered SrGa2S4:Eu2+ film phosphor grown at 300 °C under H2S gas and annealed for 2 h at 700 °C in a vacuum. The left inset shows a two-dimensional AFM image of the sputtered SrGa2S4:Eu2+ film phosphor (scale bar: 500 nm), and the right inset shows a side-view SEM image of the SrGa2S4:Eu2+ film deposited on a quartz substrate (scale bar: 500 nm).

were sufficiently smooth from the standpoint of the amount of anomalous scattering of the film phosphor through the surface grains.5 It was speculated that the grains in the sputtered SrGa2S4:Eu2+ TFPs will be sufficiently small that they will perturb the guided modes of emitted light only to a small degree and will only have a minor positive effect on the extraction efficiency. The right inset in Figure 2 shows a cross-sectional view of an SEM image of a SrGa2S4:Eu2+ TFP deposited on a quartz substrate for 1.5 h and annealed at 700 °C for 2 h in a vacuum. The thicknesses of both SrGa2S4:Eu2+ TFPs deposited on the quartz substrates were approximately 1.0 µm. The crystallinity, surface roughness, and thickness of the SrGa2S4: Eu2+ film phosphors allow the sputtered SrGa2S4:Eu2+ TFPs to be used as reference samples for comparing the nanospherecoated SrGa2S4:Eu2+ TFPs. As shown in Figure 1, PS nanospheres were formed on the water surface through two consecutive steps: floating and crystallization.12 At first, PS nanospheres with various diameters of 330, 580, and 960 nm, of which the surface was modified with chloromethyl and sulfate functional groups, can float on the water surface. This results because the surface of the PS nanospheres is hydrophobic in nature. When hydrophobic large, heavy colloidal particles float on the water surface, they can cause the water surface to have a concave meniscus. In addition, the attractive floatation capillary force driven by the gravityinduced concave meniscuses among the neighboring colloidal particles enables close packing. However, the floating PS nanospheres are so light that they do not deform the planar water surface sufficiently to cause the close packing phenomenon by an attractive floatation capillary force. Hence, at this stage, they float on the water surface with significant mobility and interparticle space. Regularly arrayed PS nanospheres with some interparticle space are present because of their modest surface polarity originating from proper contents of surface functional groups (chloromethyl and sulfate).16 However, they contain very small crystallites and are easily broken during the successive process. Second, gentle waves of the liquid medium were made by slow and careful vessel tilting to promote the growth of large crystals. After this agitation step, hexagonally close-packed PS nanosphere monolayers of various sizes with approximately 1 × 1 cm2 crystallites were created. The domain size of the PS monolayer on the water surface was controlled by several factors, including the diameter and size homogeneity of the nanospheres, the agitation time, the applied force, and the frequency of this agitation treatment. Therefore, a domain size larger than 1 × 1 cm2 for each PS nanosphere size was obtained by changing the agitation time. However, the resulting relatively ordered PS crystal domains are easily broken during transfer

7596 J. Phys. Chem. C, Vol. 112, No. 20, 2008

Figure 3. Schematic diagram of the light paths in a SrGa2S4:Eu2+ TFP coated with a 2D nanosphere PC array.

Figure 4. Top-view SEM images of the arrays of PS nanospheres with three different diameters (330, 580, and 960 nm) coated onto SrGa2S4:Eu2+ thin films on quartz substrates (scale bar: 1 µm).

of the monolayer onto the SrGa2S4:Eu2+ phosphor surface. A few drops of SDS (NaC12H25SO4) were introduced to form rigid PS nanosphere monolayers and to maintain the rigidity of the PS domain. SDS interferes with or modifies the hydrogenbonding network of water. Since this reduces the cohesive forces in water, it leads to a decrease in surface tension. Consequently, the addition of SDS deforms the water surface onto which the PS nanospheres float more significantly than before and increases the attractive floatation capillary, which makes it possible to form a rigid close-packed PS nanosphere monolayer floating on the water surface. Finally, the rigid close-packed PS nanosphere monolayer was transferred from the water surface to the surface of the SrGa2S4:Eu2+ film using simple scooping technology. Scooping hexagonally close-packed nanosphere monolayers of various sizes with the SrGa2S4:Eu2+ film coated quartz substrate involves transferring the monolayer from the water surface to the phosphor surface. After the water evaporates from the PS nanosphere monolayer coating on the phosphor surface, the PS nanospheres are drawn together by capillary forces and crystallize on the SrGa2S4:Eu2+ surface in a hexagonal close-packed monolayer array. To reduce the strong waveguide effect and, in turn, improve the light extraction efficiency from the phosphor side of the SrGa2S4:Eu2+ TFPs, the top of the SrGa2S4:Eu2+ TFPs were coated with 2D nanosphere PC layers as mentioned above. The use of PS nanosphere monolayers has been reported to be a film phosphor-independent technique for producing well-ordered PC array structures with wavelength scale spacing. Figure 3 shows a schematic diagram of the scattered modes of the guided light within the SrGa2S4:Eu2+ films produced by the 2D PS nanosphere PC monolayer, which was designed to enhance the extraction of light from the phosphor side. Improvements in the extraction efficiency due to leaky and/or Bragg scattering produced by the 2D periodic arrays have been reported in other publications.17–19 A process involving the self-assembly of the PS nanospheres was used to fabricate such nanosphere monolayer-coated SrGa2S4:Eu2+ films. Figure 4 shows plan-view SEM images of the arrays of 2D hexagonally distributed closepacked PS nanospheres with three different diameters coated onto the SrGa2S4:Eu2+ thin films on quartz substrates. The longrange ordering achieved by controlling the growth step and adding a surfactant is clearly evident in the SEM images. Indeed, these SEM images and other low-magnification images show

Ko et al. that typical single-crystal domain sizes of nanospheres of three different sizes are in the order of several square millimeters. As shown in Figure 4, the self-assembly of the monodisperse nanospheres yields triangularly shaped arrays with a P6mm symmetry, which is also a size-independent technique for producing well-ordered PC array structures within this range of nanosphere diameters from 330 to 960 nm. Therefore, the use of a nanosphere coating is a simple but effective method for fabricating 2D PC arrays of nanospheres of various sizes on SrGa2S4:Eu2+ TFPs, irrespective of the type and surface morphology of the film phosphor.20 To make a systematic comparison of the SrGa2S4:Eu2+ TFPs with and without nanosphere PC arrays, the PL transmitted from the phosphor side was measured in a direction normal to the surface (normal mode) and in all directions with respect to the surface using an integrated sphere (integrated mode). This is because the luminescence has strong angular dependence.21 Figure 5 shows the integrated PL emission and excitation spectra for a conventional flat TFP and SrGa2S4:Eu2+ TFPs coated with 330-, 580-, and 960-nm spheres. The emission spectra show that the broad green emission in these systems emanates from the 4f65d1(a1) f 4f7 transitions of the Eu2+ ions. This figure indicates that the emission intensity of the 2D PS nanosphere array-coated samples is significantly higher than that of the conventional flat sample. The inset in Figure 5a shows photographs of both the flat and the nanosphere-coated SrGa2S4: Eu2+ TFPs under similar excitation conditions. This photograph also confirms that the PL intensities of the green emission of SrGa2S4:Eu2+ TFPs coated with the 2D PS nanospheres are much higher than those of the flat TFPs. The excitation spectra of both the flat and nanosphere-coated film phosphors are similar and quite broad and intense up to 500 nm. The excitation below 280 nm is essentially the result of a valence-to-conduction band transition of the phosphors. The broadband in the visible region arises from a transition between the 4f65d1 and 4f7 configurations of Eu2+.22 This figure also shows that the excitation intensity of the 2D PS nanosphere array-coated samples is significantly higher than that of the conventional flat sample. Therefore, there is significant improvement in the PL emission and excitation intensity, as a result of the introduction of the symmetrical nanosphere PC scattering layers at the interfaces between the air and the SrGa2S4:Eu2+ thin films. These enhancements in extraction efficiency can be attributed to the perturbation effect of the leaky and/or Bragg scattering modes produced by the PS nanosphere PC array.10–12 The degree to which the introduction of the 2D nanosphere PC arrays onto the SrGa2S4:Eu2+ TFPs enhances the amount of extracted light was determined by examining the effects of varying several parameters. One of the most important parameters controlling the PC scattering effect is the diameter of the regularly arranged PS nanospheres. Samples of monodisperse PS nanospheres with different diameters but the same type and level of surface functionalization were purchased. The enhancement ratio of a SrGa2S4:Eu2+ thin film coated with a 2D nanosphere PC is defined as the ratio of its light output to that of the equivalent flat SrGa2S4:Eu2+ film phosphor. Figure 6 shows the changes in the enhancement ratios of the normally directed and integrated PL as a function of the diameter of the PS nanospheres. These plots show that the amount of extracted light depends on the size of the 2D nanospheres; the enhancement ratio of the extraction efficiency varies with the nanosphere diameter in both the normally directed and integrated graphs shown in Figure 6. This suggests that samples with nanospheres of different sizes produce PC arrays with different lattice

Light Extraction from SrGa2S4:Eu2+ Film Phosphors

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Figure 5. Plots of (a) the integrated PL emission (λex ) 365 nm) and (b) excitation spectra (λem ) 529 nm) of a conventional flat SrGa2S4:Eu2+ TFP and of sputtered SrGa2S4:Eu2+ TFPs coated with nanospheres (330, 580, and 960 nm). The inset in Figure 5a shows actual photographs (taken with a Samsung camera, model No. 11) of a conventional film and a film coated with a highly crystalline 2D PS nanosphere PCL under illumination at 365 nm (Spectroline mercury lamp, model ENF-2400C/FE).

Figure 6. Plots of the enhancement ratio for the integrated and normal directional PL intensities of nanosphere-coated, sputtered SrGa2S4:Eu2+ TFPs as a function of the PS nanosphere diameter (330, 580, and 960 nm).

constants and different angular-dependent light patterns because of the extracted modes. As previously reported,23 regardless of the height of the PC layer, there is a cutoff for the lattice constant of approximately 200 nm (Λcutoff ) λ/(neff + 1), where Λcutoff is the cutoff lattice constant (the diameter of the PS nanospheres), λ is the wavelength of emitted light (529 nm), and neff is the effective refractive index of the PC layer) below which leaky waves remain trapped in the TFP-coated substrate. The enhanced extraction efficiency of the TFPs is due to the leaky modes produced by the 2D nanosphere PCLs as a result of phase matching for nanosphere diameters above the cutoff lattice constant. In this study, PS nanospheres of three different diameters (330, 580, and 960 nm), which are larger than the cutoff lattice constant, were selected to determine the effects of the nanosphere diameter on the extraction efficiency of SrGa2S4:Eu2+ films coated with PS nanosphere arrays. The extraction efficiency has its largest value when the diameter of the 2D nanospheres in the PC is similar to the vacuum wavelength. Samples with a nanosphere diameter of 580 nm exhibited the largest enhancement ratios, 4.3 and 6.0, in the integrated and normal modes, respectively, which were calculated with respect to the properties of the flat SrGa2S4:Eu2+ sample at the same emission wavelength. The improvement in PL intensity of the green SrGa2S4:Eu2+ films is clear. This intensity is due to diffraction scattering resulting from the suppression by the hexagonally close-packed 2D PS nanosphere PC structure of the guided modes of the SrGa2S4:Eu2+/air interface. The enhanced extraction efficiency and angular dependence of the out-coupled light confirm that the far-field emission patterns of the light extracted from the samples coated with nanospheres of different sizes are modified from Lambertian patterns to radiation patterns, which are dependent on the structural parameters of the 2D periodic PCL arrays. The results

of this study and a previous publication12 suggest that the enhanced extraction efficiency (4.3-fold) of the SrGa2S4:Eu2+ thin films coated with 2D nanosphere PCLs with a diameter of 580 nm is associated with a PL extraction efficiency higher than that of Y2O3:Eu3+ films (3.7-fold) coated with the same PS nanosphere PC. The 2D nanosphere PCLs perturb the guided light of SrGa2S4:Eu2+ thin films more effectively than the Y2O3: Eu3+ films because of the larger difference in refractive index between the SrGa2S4:Eu2+ film (n ) 2.3) and PS nanosphere (n ) 1.59) PCLs than that between the Y2O3:Eu3+ film (n ) 1.86) and PS nanosphere PCLs. 4. Conclusions In an attempt to increase the extraction efficiency of the phosphor-side light emission of the TFPs, 2D PS nanosphere/ air PCLs on SrGa2S4:Eu2+ TFPs were fabricated using a simple self-assembly process involving the crystallization and transfer of PS nanospheres. This floating and scooping technique of the nanosphere coating is a simple but effective method for fabricating 2D PC arrays of nanospheres with various sizes (330-960 nm) on SrGa2S4:Eu2+ TFPs, irrespective of the type and surface morphology of the film phosphor. The maximum enhancements of the integrated and normal direction PL extraction efficiencies achieved in this study were improved approximately 4.3 and 6.0, respectively, after coating with 580nm nanospheres. Consistent with the enhanced extraction efficiency found for the red emission of 2D PS nanospherecoated Y2O3:Eu3+ in a previous study,12 these improvements in the extraction efficiency of green emission from the phosphorside of SrGa2S4:Eu2+ TFPs were attributed to the strong perturbation of the wave-guided light in the film phosphors that is produced by the addition of the 2D PS nanosphere/air PC monolayer arrays. It is expected that this self-assembly and film phosphor-independent coating technique, which does not alter the crystalline characteristics of the base phosphor films, can be used to enhance the extraction efficiency of films of almost all types of luminescent materials, including inorganic and organic films or quantum dot films.24 Acknowledgment. This work was supported by Grant No. 2007-02397 of the Nano R&D Program and Grant No. R112005-048-00000-0 of the ERC program of the Ministry of Science and Technology in Korea. References and Notes (1) Benalloul, P.; Barthou, C.; Benoit, J.; Eichenauer, L.; Zeinert, A. Appl. Phys. Lett. 1993, 63, 1954.

7598 J. Phys. Chem. C, Vol. 112, No. 20, 2008 (2) Stoffers, C.; Yang, S.; Zhang, F.; Jacobsen, S. M.; Wagner, B. K.; Summers, C. J. Appl. Phys. Lett. 1997, 71, 1759. (3) Huh, Y. D.; Shim, J. H.; Kim, Y.; Do, Y. R. J. Electrochem. Soc. 2003, 150, H57. (4) Benalloul, P.; Barthou, C.; Benoit, J. J. Alloys Compd. 1998, 275– 277, 709. (5) Jones, S. L.; Kumar, D.; Cho, K. G.; Singh, R.; Holloway, P. H. Displays 1999, 19, 151. (6) Jones, S. L.; Kumar, D.; Singh, R. K.; Holloway, P. H. Appl. Phys. Lett. 1997, 71, 404. (7) Singh, R. K.; Chen, Z.; Kumar, D.; Cho, K.; Ollinger, M. Appl. Surf. Sci. 2002, 197–198, 321. (8) Do, Y. R.; Park, D. H.; Kim, Y. C.; Huh, Y. D. J. Electrochem. Soc. 2003, 150, H260. (9) Chartier, C.; Jabbarov, R.; Jouanne, M.; Morhange, J. F.; Benalloul, P.; Barthou, C.; Frigerio, J. M.; Tagiev, B.; Gambarov, E. J. Phys.: Condens. Matter 2002, 14, 13693. (10) Lee, Y. K.; Cho, J. Y.; Park, C. R.; Huh, Y. D.; Kim, Y. C.; Do, Y. R. Electrochem. Solid-State Lett. 2007, 10, H82. (11) Lee, Y. K.; Oh, J. R.; Huh, Y. D.; Do, Y. R. Appl. Phys. Lett. 2007, 91, 232908. (12) Lee, Y. K.; Oh, J. R.; Do, Y. R. Appl. Phys. Lett. 2007, 91, 041907. (13) Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2001, 105, 5599.

Ko et al. (14) Kosiorek, A.; Kandulski, W.; Chudzinski., P.; Kempa, K.; Giersig, M. Nano Lett. 2004, 4, 1359. (15) Peters, T. E.; Baglio, J. A. J. Electrochem. Soc. 1972, 119, 230. (16) Here we purchased negatively charged PS nanospheres with chloromethyl and sulfate groups on the surface from Interfacial Dynamics Co. (17) Fan, S.; Villeneuve, P. R.; Joannopoulos, J. D. Phys. ReV. Lett. 1997, 78, 3294. (18) David, A.; Meier, C.; Sharma, R.; Diana, F. S.; DenBaars, S. P.; Hu, E.; Nakamura, S.; Weisbuch, C. Appl. Phys. Lett. 2005, 87, 101107. (19) Do, Y. R.; Kim, Y. C.; Song, Y. W.; Cho, C. O.; Jeon, H.; Lee, Y. J.; Kim, S. H.; Lee, Y. H. AdV. Mater. 2003, 15, 1214. (20) Whitney, A. V.; Meyer, B. D.; Van Duyne, R. P. Nano Lett. 2004, 4, 1507. (21) Lee, Y. J.; Kim, S. H.; Kim, G. H.; Lee, Y. H.; Cho, S. H.; Song, Y. W.; Kim, Y. C.; Do, Y. R. Opt. Express 2005, 13, 5864. (22) Chartier, C.; Barthou, C.; Benalloul, P.; Frigerio, J. M. J. Lumin. 2005, 111, 147. (23) Lee, Y. J.; Kim, S. H.; Huh, J.; Kim, G. H.; Lee, Y. H.; Cho, S. H.; Kim, Y. C.; Do, Y. R. Appl. Phys. Lett. 2003, 82, 3779. (24) Ganesh, N.; Zhang, W.; Mathias, P. C.; Chow, E.; Soares, J. A. N. T.; Malyarchuk, V.; Smith, A. D.; Cunningham, B. T. Nat. Nanotechnol. 2007, 2, 515.

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