Plasmonic-Enhanced Luminescence Characteristics of Microscale

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Functional Inorganic Materials and Devices

Plasmonic-Enhanced Luminescence Characteristics of MicroScale Phosphor Layers on a ZnO Nanorod-Arrayed Glass Substrate Oh Hyeon Kwon, Jin Woo Jang, Sun-Joo Park, Jun Sik Kim, Sung Jun Hong, Ye Seul Jung, Heesun Yang, Young-Joo Kim, and Yong Soo Cho ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13767 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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ACS Applied Materials & Interfaces

Plasmonic-Enhanced Luminescence Characteristics of Micro-Scale Phosphor Layers on a ZnO Nanorod-Arrayed Glass Substrate Oh Hyeon Kwon,†,# Jin Woo Jang,†,# Sun-Joo Park,∥,# Jun Sik Kim,†,‡ Sung Jun Hong,† Ye Seul Jung,† Heesun Yang,§ Young Joo Kim,*,∥ Yong Soo Cho*,† †Department

of Materials Science and Engineering, Yonsei University, Seoul 03722, Korea

∥Department

of Mechanical Engineering, Yonsei University, Seoul 03722, Korea

‡R&D §Department

Center, LG Display Co., Ltd, Gyeonggi-do 10843, Korea

of Materials Science & Engineering, Hongik University, Seoul 04006, Korea

ABSTRACT. We present a planar luminescent layer for glare-free, long life-span white lightemitting diodes (LEDs), with attractive light outputs. The novel and facile remote phosphor approach proposed in this work enhances luminescence properties by combining a waveguiding ZnO-based nanostructure with plasmonic Au nanoparticles. The system comprised a micro-scale yellow phosphor layer that is applied by simple printing onto an Au nanoparticle-dispersed ZnO nanorod array. This architecture resulted in a considerable enhancement in luminous efficacy of approximately 18% due to the combination of waveguide effects from the nanorod structure and plasmonic effects from the Au nanoparticles. Performance was optimized according to the length of the Zn nanorods and the concentration of Au. An optimal efficiency of ~84.26 lm/W for a silicate phosphor-converted LED was achieved using long ZnO nanorods and an Au concentration of 12.5 ppm. The finite-difference time-domain (FDTD) method was successfully used to verify the luminous efficacy improvements in the Au nanoparticle-intervened nano-structures via the wave-guiding and plasmonic effects.

KEYWORDS: light emitting diodes, remote phosphor, printing, ZnO nanorods, plasmonic 1 ACS Paragon Plus Environment

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INTRODUCTION Solid state white-light sources based on light-emitting diodes (LEDs) have received significant attention in recent years.1-5 Among these, the usage of phosphor-converted white LEDs (pcWLEDs) increases everyday due to their competitive advantages, which include relatively low power consumption, high-efficiency, longevity, improved physical strength, fast switching and environmental friendliness.1,6 A popular recent trend is the fabrication of white LEDs by the combination of a blue-emitting diode chip and a yellow phosphor-embedded epoxy resin.7,8 However, pc-WLEDs still face technical challenges, including light-energy losses and thermal degradation.9,10 Upon increasing forward bias current, for example, the dissipation of thermal sources leads to unstable fluctuations in the correlated color temperature (CCT).11 Therefore, to overcome these specified shortcomings, recently, research into the fabrication of pc-WLEDs has shifted to a remote phosphor approach, where the phosphor layer is physically separated from the blue light-emitting chip.11-13 This methodology provides improved light output with an extended lifetime and less wasted energy by glare reduction (uniform hue), as well as less color shifts and thermal quenching.14 Although the light that is produced is attractive, the device suffers from a loss of trapped light due to back-scattering from the emitter and total internal reflection (TIR).9,15 Thus, several approaches have been reported to enhance the light extraction by minimizing TIR.15-20 In this context, our group has recently reported costeffective and reliable methods based on the printing of a remote phosphor particles-embedded layer.21-24 The merit of the printing technology is that it provides a controllable thickness in the range of only a few tens of micrometers, as well as provides the potential for large-scale surface coverage in a single processing step. Hence, a tailor-made and affordable device can be achieved by adjusting the thickness of the phosphor layer to produce a very thin, micrometerscale LED device. In addition, simultaneous efforts to improve light extraction by adopting various

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waveguiding nanostructures, particularly on the surface of GaN-based active chips, are being pursued.25-28 Kim et al.25 reported a significant enhancement in light extraction efficiency that was attributed to the waveguide effect of ZnO sub-microrods in an InGaN blue LED. Hsaio et al.26 also reported an approximately 10.5% light extraction enhancement by the growth of syringe-like ZnO nanorods on the surface of an LED chip. Park et al.28 used ZnO nanostructures to improve light extraction efficiency by radiative recombination and to tune color by creating defect states by doping Zn sites with indium. However, no previous reports discuss the direct application of waveguiding nanostructures to the phosphor component of WLEDs. Furthermore, the interaction between metal nanoparticles (Au, Ag, or Pt) and luminescent materials is interesting because the luminous efficacy enhancement relies on the electromagnetic field associated with the localized surface plasmon (LSP) effect.29-34 As such, nanostructures decorated with metal nanoparticles have attracted attention for the enhancement of the luminous efficacy of LEDs and optoelectronic devices.31,35 Among the metal nanoparticles, Au nanoparticles are known to exhibit the strongest LSP effect at the metal/semiconductor interface.36 This LSP effect increases the density of states and accelerates the radiative recombination rate, which results in an enhancement of luminous efficacy.37 Evidently, the above-mentioned strategies are utilized to enhance the luminous efficacy of LED sources, such as blue or near-ultraviolet LEDs or core/shell (metal nanoparticle/luminescent particle)-based fluorescent materials but similar strategies have not yet been applied to phosphor technology-based WLEDs. Herein, we introduce a novel and facile approach of fabricating a light-converting section consisting of a plasmon-assisted waveguiding layer and a planar yellow phosphor-printed layer in order to improve luminous efficacy ultimately from the combined waveguiding/plasmonic effects. The planar phosphor layer was prepared by printing/UV-curing the phosphor-paste to have a thin thickness of a few tens of micro-meter. Yellow-emitting (Ba,Sr,Ca)2SiO4:Eu2+ was



chosen as a model phosphor for screen-printing on a cost-effective soda lime silicate (SLS) glass substrate. A gold nanoparticle-incorporated ZnO nanorod-array was optimized with the length and width of nanorods for better luminescence performance. Since the Au nanoparticles are applied directly into the ZnO nanorods-structure, the content of Au can be substantially reduced compared to the reported cases where the Au nanoparticles were incorporated into thick phosphor-resin composite specimens.38,39 As a result, we report a substantial improvement of total ~18% in luminous efficacy from both the waveguiding and plasmonic contributions. The packing density of ZnO nanorods and the relative concentration of Au nanoparticles are dealt here as main parameters to define the optimal condition. The enhanced electric field distribution of effective resonance coupling between the LSPs and the emitted photons is also verified theoretically by using the FDTD simulation.

EXPERIMENTAL SECTION A unique, integrated remote phosphor structure consisting of phosphor-polymer composite layer/Au-dispersed ZnO nanorods/glass substrate was prepared for this study. First, ZnO thin films were deposited onto a regular SLS glass substrate by spin coating a ZnO precursor solution. The solution was prepared by dissolving 0.5 M zinc acetate Zn(CH3COO)2∙2H2O (99%, Aldrich) and 0.75 M monoethanolamine C2H7NO (90%, Aldrich) in 2-methoxyethanol (C3H8O2). Spin coating was performed twice at room temperature to obtain a seed layer with a thickness of approximately 30 nm. The as-deposited films were calcined at 350oC for 10 min at ambient atmospheric conditions. Next, the supported seed layer was dipped vertically into an aqueous solution of zinc acetate with 0.05 M hexamethylenetetramine (HMT, C6H12N4) (90%, Aldrich) for nanorod growth. Different concentrations of Zn acetate (0.005–0.03 M) were used in the same growth condition (2 h at 95oC). After 2h, the reacted samples were rinsed with distilled water and dried under a N2 flow. For the incorporation of Au nanoparticles into


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the nanorod structure, a commercially available dispersion solution of Au nanoparticles in ethanol (AUS-EM1000, Nano High Tech) was spin-coated on top of the nanostructures. Different Au concentrations (12.5–100 ppm) were evaluated in order to optimize the plasmonic effect in this system. As

a

reference

phosphor,

commercially

available

yellow

silicate

phosphor

(Ba,Sr,Ca)2SiO4:Eu2+ (PA556, Force4 Co.) was used. The phosphor powder was intimately mixed with a commercial UV-curable organic vehicle (Loctite 3321, Henkel Co.) using a paste mixer. The amount of the phosphor relative to the organic vehicle was fixed as 70 wt.%. The paste was carefully screen-printed onto the Au-dispersed ZnO nanorods/glass substrate using a 325-mesh screen. Finally, the printed paste was cured at room temperature for 10 min by direct exposure to a UV light source having a 500 W intensity. Surface and cross-sectional microstructures were observed by field emission scanning electron microscopy (FESEM: JSM-5400, JEOL). X-ray diffraction (XRD) patterns of ZnO nanorods were examined using a Rigaku Ultima IV diffractometer with CuK radiation. The morphology and elemental mapping of the nanostructures were also studied by transmission electron microscopy (TEM: JEM-ARM 200F, JEOL). Optical absorbance were measured with a UV-visible spectrophotometer (UV 3101, Shimadzu). Emission spectra and the CIE chromaticity coordinates of the resultant devices were measured using a spectrum analyzer (DARSA-5200, PSI) with an integrating sphere system and a 443 nm blue diode excitation source under an input forward bias current of 200 mA. The printed phosphor layer was directed toward the blue diode such that the light first passed through the phosphor layer and then through the Au-ZnO nanorod section. To validate the LSP effect of Au-dispersed ZnO nanorod with different emission wavelength on white LED performance, the three-dimensional FDTD method was used to theoretically analyze the electric field profiles. The electric field distribution enhancement inside the sample



was calculated using a commercially available FDTD solution (Lumerical, Inc., Canada). The simulations were performed using the typical values of refractive index of glass, Au, ZnO and UV curable polymer reported in literature. The diameter of Au nanoparticles was set to 20 nm, and the diameter and length of ZnO nanorods were 60 and 600 nm, respectively. The Au nanoparticles were randomly dispersed on ZnO nanorods to make the situation analogous to the real sample. The peak wavelength of blue light from blue diode was set to 443 nm while the peak wavelength of yellow light excited by yellow phosphor was 560 nm.

RESULTS AND DISCUSSION Figure 1a shows the schematic procedure employed to prepare the remote phosphor component, which comprises a phosphor-polymer composite layer on an Au-dispersed ZnO nanorods/glass substrate. An approximately 30-nm-thick ZnO seed layer was first deposited onto a bare sodalime silicate glass substrate by spin coating and pre-firing at 350 °C. A ZnO nanorod array was formed from the seed layer by immersing the substrate into an acetate-based solution as has been reported elsewhere.40,41 ZnO nanorods are known to grow anisotropically at polar facets and to easily convert into wurtzite structures upon drying or dehydration.40-42 The Au nanoparticle-dispersed solution was then spin coated onto the ZnO nanostructure. Next, a separately prepared (Ba,Sr,Ca)2SiO4:Eu2+ silicate phosphor/UV-polymer paste was screen printed onto the Au-incorporated nanorod-array structures. A UV-curing process was followed at room temperature to ensure a dense structure and good adhesion. Figure 1b demonstrates the final structure of the remote phosphor component, including the actual dimensions of each layer. Prior to the incorporation of the Au nanoparticles, the effect of the ZnO nanorods on the luminescence characteristics of the printed remote phosphor layer was investigated. Figure 2 presents the microstructures of the ZnO nanorod-array and the printed phosphor layer. The 6 ACS Paragon Plus Environment

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surface (Figures 2a–c) and cross-sectional (Figures 2d–f) SEM images of the as-grown ZnO samples processed using different concentrations of zinc acetate in the nanorod growth solution suggest a well-defined nanorod morphology with fairly good alignment. The length of nanorods was ~85, ~450 and ~600 nm for Zn acetate concentrations of 0.005, 0.01, and 0.03 M, respectively. The diameter of the corresponding nanorods was found to be ~32, ~41, and ~60 nm. These results indicate that the concentration of Zn is a critical factor in determining the dimensions of the nanorods. Furthermore, XRD results presented in Figure S1 of the Supporting Information indicate ZnO nanorod crystallinity with preferable orientation along the z-axis, which becomes stronger with increasing Zn concentration. In addition, the effects of Zn concentration in the solution used for the seed layer growth are demonstrated in Figure S2, in terms of morphology evolution of the resultant nanorods. A typical cross-sectional SEM image of the phosphor layer printed on the ZnO nanorod/glass substrate is shown in Figure 2g. Figures 2h,i show the energy dispersive spectroscopy (EDS) color-mapping for strontium (phosphor) and carbon (polymer matrix), respectively, for the dotted area indicated in Figure 2g. It is evident from the EDS color mappings that the micrometer-sized phosphor particles are well-distributed within the UV-cured polymer matrix. Although the curing process is relatively short, the particles are arranged uniformly while the polymer matrix begins to form. The UV-curing process can be easily adjusted as the curing occurs near room temperature within a few tens of minutes.21 Additional cross-sectional images showing phosphor layers printed onto nanostructures prepared from different Zn concentrations are shown in Figure S3. It is well known that the measure of merit for light sources is their luminous efficacy (.43 The luminescence ability of the phosphor layer on the ZnO nanorods/glass substrate was examined by applying the light emission of a blue LED chip (exc ≈ 443 nm). Figure 3a shows the emission spectra of the white LED under an input-forward bias current of approximately 7 ACS Paragon Plus Environment


200 mA for the samples processed on different nanorod-length substrates. The emission spectra exhibit two different emission bands that correspond to a short-wavelength emission peak located at approximately 441 nm (originating directly from the blue LED chip) and a broad emission band with a band maximum at ~560 nm (emitted from the yellow (Ba,Sr,Ca)2SiO4:Eu2+ orthosilicate phosphor). The emission band is due to the 4f65d1→4f7 (5d– 4f) transition of Eu2+ ions followed by the absorption of blue light. In the electroluminescence (EL) spectra, the 5d–4f emission band intensity increased with increasing nanorod length. As such, the actual values of luminescent parameters such as the Commission Internationale de l’Eclairage (CIE), the correlated color temperature (CCT) and , also depended on the length of the nanorods (see Figure 3 table inset). The  values were calculated using the simple relation, L/P input, where L is the luminous flux and P input is the inputted forward bias current. At a forward-bias current of 200 mA and an operating voltage of 3.6 V, the luminous efficacy of the phosphor layer printed onto the ZnO nanorods increased significantly to ~81.57 lm/W relative to the flat substrate reference value of ~71.58 lm/W (Figure 3b). This enhancement in luminous efficacy is due to the waveguiding effect of the ZnO nanorods, which minimizes total internal reflection depending on the length and distribution of the nanorods. The emitted photons from the phosphor layer may potentially undergo multiple scatterings from the sidewalls of the nanorods. The luminous efficacy can be increased through modulation of the packing density of the nanorods.44 Theoretical estimates have determined that longer nanorods may be useful to optimize photon propagation and, therefore, light extraction.27 Additionally, CCT values decreased from 9636 K to 6039 K, indicating a tuning of the white-light from cool to warm white, which was further confirmed by the CIE chromaticity coordinates plotted in the 1931 CIE color-space diagram, as shown in Figure 3c. The shift toward the yellow spectral region by lengthening the nanorods is evident from the CIE coordinates. We incorporated different amounts of colloidal Au nanoparticles into the ZnO nanorods 8 ACS Paragon Plus Environment

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prior to the printing of the phosphor layer to further enhance the luminous efficacy. Figure 4a presents a TEM image of the Au nanoparticle-incorporated ZnO nanorod substrate. The identities of each component (i.e., substrate, ZnO nanorods, and Au particles) are easily recognizable in the elemental mapping images shown in Figures 4b–d. EDS elemental mapping of the area within the dotted lines of Figure 4a is shown in Figure 4e–g. Elemental mapping of the low magnification TEM image indicates the presence of Si in the K-shell at 1.740 keV from the SLS glass substrate (Figure 4b). Elemental mapping of low and high magnification TEM images suggests the presence of Zn from the ZnO nanorods in the K-shell at 8.637 keV and Au from the Au nanoparticles, which occupied the L-shell at 9.712 keV. Figures 4h and 4i present high resolution TEM images of an Au nanoparticle-ZnO nanorod with well-defined lattice fringes with interplanar distances of 0.235 nm and 0.276 nm, which correspond to the Au (111) and ZnO (1ī00) planes, respectively. Figure S4 presents additional TEM images with electronic diffraction (SAED) patterns of selected areas. The SAED patterns indicate that the Au nanoparticles are polycrystalline while ZnO nanorods are single crystal. Absorbance and transmission spectra of the Au nanoparticle-ZnO nanorod substrates were obtained. The absorbance spectra presented in Figure 5a shows the Au surface plasmon resonance (SPR) band, with the band maximum at approximately 540 nm, due to the collective oscillations of the surface electrons in the conduction band.45 A pronounced intensity enhancement without peak shift for the absorbance of the SPR band was observed as the mass of Au nanoparticles increased from 0 to 50 ppm. This observation is anticipated from the similar size/morphology of the Au nanoparticles across all samples. As reported, different size and shape of Au nanoparticles result in a notable shift in the peak wavelength and changes to the breadth of the SPR absorption band.46 The absorption band maxima at approximately 540 nm overlap with the emission spectra of the (Ba,Sr,Ca)2SiO4:Eu2+ phosphor. This spectral overlap likely results in the effective energy transfer by the resonance between the phosphor9 ACS Paragon Plus Environment


generated photons and the local surface plasmons (LSPs) excited by the Au nanoparticles, leading to an enhancement in the emission intensity. Previous studies have indicated the increase of light extraction efficiency due to the resonant coupling between semiconductor photons and metal LSPs, which creates additional extraction paths.35 With the dispersion of Au nanoparticles on the ZnO nanorods, the interactions between the photons and the Au nanoparticles results in the LSP resonance coupling, which contributes to the enhanced light extraction of the photons. The white light entering into the ZnO nanorod structure (after passing through the phosphor layer) undergoes multiple scattering from the sidewalls of the nanorods as if it is confined in air gap between the nanorods. The luminous efficacy of white light is improved by scattered resonances of ZnO nanorods. The light passing through the ZnO nanorod meets the Au nanoparticles and generates a plasmonic effect. Particularly, yellow light is involved in the plasmonic-photoluminescence enhancement since the wavelengths of the Au surface plasmon resonance band and yellow light are coincident. The Au nanoparticles form the dipoles composed of photoexcited d-band holes and upper-lying s-band electrons by light passing through the ZnO nanorods, which causes photoluminescence by radiative recombination.47,48 As a result, the photoluminescence of light overlapping with the plasmonic resonance frequency is enhanced Figure 5b shows the emission spectra of the fabricated white LEDs, which is based on the Au nanoparticles-incorporated nanorod structure. The emission spectra were measured under an input-forward bias current of approximately 200 mA as a function of the Au nanoparticle concentration. They show similar emission bands to those observed in Figure 3a for the device with only Zn nanorods. In the emission spectra, the yellow band emission intensity increased with the incorporation of Au nanoparticles to the maximum at the concentration of 12.5 ppm. As a result, the luminous efficacy was enhanced by up to 4% compared with the device without Au nanoparticles (Figure 5c). This result suggests that Au nanoparticles improve the


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conversion efficiency of the yellow phosphor when combined with waveguiding nanostructures. The contribution of ~4% seems far less than the reported enhancement of 1320% by Au nanoparticles in pc-WLEDs where the Au nanoparticles were directly incorporated into the phosphor specimens.38,39 Here, we did apply the Au nanoparticles into the ZnO nanorods-layer, not into the phosphor layer. Because of this unique approach, we could save a lot of Au content. According to our estimation, for example, the 55-370 mg Au nanoparticles per 1 g phosphor was needed (to demonstrate the 13-20% improvements in literature38,39) while our system required only ~1.5 mg Au per 1 g phosphor for the average 4% improvement. The luminous efficacy gradually decreased with further increases in the concentration of Au above 12.5 ppm. The enhancement and quenching of the emission are related to the plasmonic effect of Au nanoparticles. Specifically, the luminescence enhancement from the coupling of Au nanoparticles is due to an increase in the radiative recombination rate by SPR while quenching is known due to non-radiative recombination.49,50 Non-radiative recombination is possible when the metal ion is in close proximity to the emissive layer from the sequential destructive interference of the SPR.49 A similar quenching effect was observed in other studies, where either yttrium iron garnet or CdS was mixed with excessive amounts of Au.49,51 Distance-engineering of the Ag nanoparticles was reportedly critical to effective plasmonic effects in the case of the luminescence of CdSe quantum dots.52 The aggregation of Au nanoparticles with a high concentration was reported to delocalize free electrons inside the particles resulting in the suppression of the localized SPR by Au.53 The calculated  values were found to increase from 81.57 lm/W to 84.26 lm/W with the addition of 12.5 ppm Au, which is meaningful since the total improvement of approximately 18% is achieved by combining the waveguiding nanostructure with the plasmon effect. Figure 5d shows a CIE color-space diagram that displays the color coordinates of the emitted light at different concentrations of Au nanoparticles. No significant variation in the color coordinates was



observed at the different Au concentrations. The (0.313, 0.313) coordinate with a CCT value of 6625 K was obtained for the Au concentration of 12.5 ppm, which is similar to the true daylight CCT of 6500 K.54 We additionally simulated the electrical field intensity distribution to visualize the effect of the LSP resonance by Au nanoparticles on the extraction of wave-guided optical modes, which propagate in the vertical direction along the side walls of ZnO nanorods of the white LED. Figure 6 shows the FDTD simulation results of the electric field intensity distribution (|E2|) for the ZnO nanorod structure (Figure 6a) and the 12.5 ppm Au nanoparticle decorated ZnO nanorod structure (Figure 6b). Herein, the electric field intensity distributions were computed for the 443 nm wavelength of blue LED emission and the 560 nm wavelength of phosphorconverted yellow emission. To realize the similar emission condition on pc-LED, the oscillating dipole source was used for a spontaneous isotropic and incoherent emission by Purcell effects of the simulation.55 The dipole source was assumed to be distributed uniformly at the position below ZnO nanorod structure.37 As expected, the ZnO nanostructure alone in Figure 6a demonstrates the wave-guiding effects along the nanorods for both emissions at 443 and 560 nm. The stronger electric field distribution by resonance was observed along the vertical gaps between the nanorods, with the contributions from different refractive index between the ZnO and air. Once the light proceeds into the ZnO nanorod structure, the light is susceptible to be refracted from the nanorod surface to the air, which follows the Snell’s Law.26,55 The ZnO nanorods showed the scattered resonances which resulted in localized light enhancement as optical resonator-based waveguides.56,57 According to the resonance theory, the degree of enhancement is related to the length of resonator and the half-wavelength of incident light. In our system, the length of ZnO nanorod is 600 nm, which is more matched with the wavelength of yellow light (560 nm) than that of blue light (443 nm). Accordingly, the ZnO nanorod waveguide is more effective on the yellow light source.


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In addition, we validate the contribution of Au nanoparticles for further increase of luminous efficacy by LSP effect as shown in Figure 6b. Both lights of the 443 nm and 560 nm wavelengths were resonated locally around Au nanoparticles. It is advantageous of this system to enhance the electric field by creating plasmon coupling as the photons meet with Au nanoparticles on the ZnO nanorod sidewall. The LSP effect indicates stronger resonance coupling with the 560 nm yellow light than the 443 nm blue light. The FDTD result is coincident with the plasmon band of Au nanoparticles and the wavelength of yellow light as observed in Figure 5a. The electric field intensity distribution clearly demonstrates that the LSP effect was generated near the Au nanoparticles and became more effective on the yellow light than the blue light (See Figure S7). Eventually, the relative intensity of the 560 nm wavelength of yellow light was improved by ~14% (See Figure S8). This means that the white light generated in our system results in not only the enhanced light intensity but also the warmer color temperature of white light due to the improved yellow light by the Au nanoparticles.

CONCLUSIONS We have successfully introduced a novel and facile remote phosphor approach to enhance the luminous efficacy of white LEDs by utilizing both waveguiding nanostructures and plasmonic Au nanoparticles. The presence of a (002) reflection confirmed the growth of highly dense ZnO nanorods aligned along the c-direction. The SAED patterns revealed single crystal ZnO nanorods and polycrystalline Au nanoparticles. The optical absorption spectra exhibited the Au nanoparticle plasmonic band. Using this approach, ZnO nanorods increased the excitation energy due to the multiple scatterings of photons from the sidewalls of nanorods, which resulted in enhancement of the luminous efficacy. Further enhancements of the luminous efficacy were obtained in the Au-ZnO nanorod structures by the photon-LSP resonance coupling effect that transfers the waveguided light into LSPs for efficient light scattering. The



effect was verified by the FDTD simulation of electric field distribution for Au nanoparticles dispersed on the ZnO nanorod structure. Overall, the increase in the luminous efficacy of approximately 18% represents a significant improvement in the field of white light LEDs. With the combined micro-scale phosphor approach, the current technique suggests a potentially promising method for the fabrication of planar LED devices.


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Figure 1. (a) Schematic diagram depicting the entire processing steps of forming ZnO seed layer on a soda-lime silicate glass substrate, growing ZnO nanorod, incorporating Au nanoparticles into the ZnO nanorods, and printing a UV-curable yellow silicate phosphor paste, with (b) the final planar phosphor structure on the ZnO nanorod-arrayed glass substrate (right side).



Figure 2. (a–c) Surface and (d–f) cross-sectional SEM images of ZnO nanorods prepared with different Zn acetate concentrations of 0.005, 0.01 and 0.03M in growth solution, respectively, but processed in the identical condition except the Zn concentration, (g) a typical crosssectional SEM image of the printed and UV-cured phosphor layer on the ZnO nanorods-glass substrate, in which the well-dispersed silicate phosphor particles are seen, and (h,i) EDS elemental mapping images of Sr and C in the phosphor section, which represents the phosphor particle and UV-polymer matrix, respectively.


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Figure 3. (a) Emission spectra, (b) luminous efficacy and (c) CIE coordinates of the resultant phosphor-LED devices with the variations of the average length of ZnO nanorods, which obtained by changing the concentration of Zn in the growth solution. All corresponding values are listed in a table, relative to those of the reference sample without ZnO nanorods.



Figure 4. (a) A TEM image of Au nanoparticles-incorporated ZnO nanorods-glass substrate with the EDS elemental mapping images of (b) Si K1, (c) Zn K1, and (d) Au L1 to identify each part, (e) an enlarged TEM image of the black-line box with the EDS elemental mapping images of (f) Zn K1 and (g) Au L1, and (h) HRTEM and (i) magnified images of an interface between the Zn nanorod and Au nanoparticle.


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Figure 5. (a) UV-visible absorbance spectra of Au nanoparticle-incorporated ZnO nanorod substrates with different Au concentrations from 12.5 to 50 ppm, and (b) emission spectra, (c) relative luminescence intensity and (d) CIE coordinates of the resultant phosphor-LED devices, all compared to the reference samples without Au incorporation.



Figure 6. Schematics of ZnO nanorod-structures used for the FDTD simulation and the resultant distributions of electric field intensity over the structure based on (a) ZnO nanorods and (b) Au nanoparticle-dispersed ZnO nanorods for the two emitted wavelength of 443 and 560 nm, which correspond to the peak wavelength of emitted blue light and excited yellow light, respectively.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: 10.1021/acsami.xxxxxx. XRD pattern of ZnO nanorods processed with different Zn concentration; additional microstructures of ZnO nanorods; cross-sectional images of phosphor layers on ZnO nanorods; additional TEM images of Au and ZnO nanorods; additional information on FDTD simulation (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]. ORCID Heesun Yang: 0000-0002-2827-2070 Yong Soo Cho: 0000-0002-1601-6395 Author Contributions #The

authors contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of Korea (20173010013340) and the Creative Materials Discovery Program by Ministry of Science and ICT (2018M3D1A1058536). We also acknowledge the initial contributions by Dr. G. Seeta Rama Raju in preparing the manuscript. . REFERENCES (1) Guan, N.; Dai, X.; Messanvi, A.; Zhang, H.; Yan, J.; Gautier, E.; Bougerol, C.; Julien, F. H.; Durand, C.; Eymery, J.; Tchernycheva, M. Flexible White Light Emitting Diodes Based on Nitride Nanowires and Nanophosphors. ACS Photonics 2016, 3, 597-603. (2) Zhang, F.; Zhong, H.; Chen, C.; Wu, X.; Hu, X.; Huang, H.; Han, J.; Zou, B.; Dong, Y. Brightly Luminescent and Color Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology. ACS Nano. 2015, 9, 4533-4542. 21 ACS Paragon Plus Environment


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Plasmonic-Enhanced Luminescence Characteristics of Microscale

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