Broadband Omnidirectional Light Trapping in Gold-Decorated ZnO

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Broadband omnidirectional light trapping in gold-decorated ZnO nanopillar arrays Yuan-Ming Chang, Man-Ling Lin, Tung-Yen Lai, Chang-Hung Chen, HsinYi Lee, Chih-Ming Lin, YewChung Sermon Wu, Yen-Fu Lin, and Jenh-Yih Juang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16412 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 17, 2017

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ACS Applied Materials & Interfaces

Broadband Omnidirectional Light Trapping in Gold-Decorated ZnO Nanopillar Arrays Yuan-Ming Chang,† Man-Ling Lin,‡ Tung-Yen Lai,§ Chang-Hung Chen,† Hsin-Yi Lee,§,⊥ Chih-Ming Lin,# Yew-Chung Sermon Wu,§ Yen-Fu Lin,†,* and Jenh-Yih Juang‡,* † ‡ §

Department of Physics, National Chung Hsing University, Taichung 402, Taiwan Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan Department of Materials and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan

⊥

National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan

#

Department of Physics, National Tsing Hua University, Hsinchu 300, Taiwan

ABSTRACT The Photoluminescence (PL) and reflectivity characteristics of zinc oxide nanopillars (ZnO-NPs) grown on indium-tin-oxide (ITO)-coated glasses were investigated. The room temperature PL showed bright white-light emission for the undoped ZnO-NPs grown at 600 °C, suggesting the close relation between the optical characteristic and the growth conditions being carried out for obtaining the present ZnO-NPs. The reflectivity of the as-grown ZnO-NPs array was about ~29 % with the wavelength of the incident light ranging from 200-1800 nm. Nevertheless, the reflectance reduced significantly to less than 9.9 % when a layer of gold (Au) was deposited on ZnO-NPs by sputtering for 5 minutes, corresponding to more than 65% reduction in Au-coated ZnO-NPs (Au/ZnO-NPs). Moreover, the angle-resolved reflectance measurements on the present Au/ZnO-NPs array show an omnidirectional light-trapping characteristic. These remarkable characteristics, broadband and omnidirectional light-trapping of 1

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Au/ZnO-NPs, are attributed to the extended effective optical path of the incident light due to sub-wavelength scattering resulting from the presence of Au nanoparticles. Key words: photoluminescence, ZnO, white-light emission, light-trapping, broadband, omnidirectional 1. INTRODUCTION Due to the transparency in the visible range and direct wide band gap (3.37 eV), zinc oxide (ZnO) has been attracting great interest in nanomaterial-related field in recent years.1-3 Among these fascinating characteristics, high efficiency exciton emission at room temperature (RT) resulted from large exciton binding energy (60 meV)4 is another unique feature of ZnO, which promises ZnO great application potential for ultraviolet or blue emitting devices. Moreover, based on energy transfer to longer wavelength emitters, ZnO can in principle give rise to green, yellow, or red emission. Thus, tremendous efforts have been devoted toward manufacturing white-light emission element directly by engineering the growth processes of ZnO-related materials.5-11 Nevertheless, most of the reports have been either on diode structures involving doping5, dispersed quantum dots6-8 and nanocrystallites9 of ZnO or resorting to rather complicated solution synthesis routes in fabricating ZnO nanostructures.10,11 In this study, we report the first observation of bright white photoluminescence (PL) emission from undoped ZnO nanopillars (ZnO-NPs) by 2


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merely controlling the growth temperature in a tube furnace. The ZnO-NPs were synthesized by vapor transport deposition on indium-tin-oxide (ITO)-coated glass without any catalyst. Comparing to the various abovementioned processes, the present method has the following advantages. First, the manufacturing process to grow the ZnO-NPs array is catalyst-free. The catalyst-free growth of ZnO provides excellent opportunities to be applied in many optoelectronic devices. Moreover, the ZnO-NPs were grown on relatively lower cost ITO-coated glass as compared to sapphire or Si substrates ubiquitously used for growing ZnO-related structures. Perhaps, even more importantly, in the present method no dopant was used during the growth of ZnO-NPs array in the furnace. In addition to the light-emitting characteristics, it has also been well documented that ZnO is an excellent candidate for manufacturing solar cells.12-14 Except for pursuing materials with more intrinsic conversion efficiency, one of the important issues in photovoltaic applications is to maximize the effectiveness of solar energy harvesting. In this respect, how to reduce the loss of incident light has become equally important for enhancing the ultimate performance of ZnO-based photovoltaic cells. Previously, in order to improve the conversion efficiency, antireflective coatings (ARC) or nanostructures were often added on the surface of the photovoltaic devices to decrease the loss of incident light. Unfortunately, most of the ARC layers exhibited undesirable 3



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incident angle dependent characteristics15 that often led to serious degradation in conversion efficiency of photovoltaic cells. Thus, it is necessary to create omnidirectional and broadband antireflection structures for enhancing conversion efficiency of solar cells. In this paper, an efficient and simple approach is proposed to realize much enhanced incident light trapping in an omnidirectional manner. To modify the antireflective features of the ZnO-NPs’ surface, gold (Au) thin film was deposited on the top surfaces of ZnO-NPs to form Au/ZnO-NPs. The obtained Au/ZnO-NPs array not only exhibits broadband (from 200 to 1800 nm) antireflection characteristic but also unprecedented omnidirectional (from -60 to 60°) light-trapping performance. 2. EXPERIMENTAL SECTION The ITO-coated glasses were cut into 2×2 cm2 pieces and used as substrates for growing ZnO-NPs. The ZnO-NPs were grown on ITO-coated glass by vapour phase transport method. Briefly, the process was carried out as follows: about 0.5 g of zinc powder inside a quartz boat was heated at 600 or 700 °C; the evaporated Zn vapour was then carried toward the ITO-coated glass substrate situated at the other end of the furnace by a flowing mixed gas comprising of 500 sccm argon gas and 30 sccm oxygen gas for 30 minutes. Since the substrate temperature was much lower comparing to evaporated Zn and carrying gases, the resultant ZnO vapor condensated 4


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on the substrate and grew into ZnO-NPs. Different thicknesses of Au layers were then deposited onto the obtained ZnO-NPs at ambient temperature to form Au/ZnO-NPs hetero-structures using a sputter coater (TED Pella 108) by varying the sputtering time. In this work, the as-fabricated ZnO-NPs obtained at growth temperature of 600 °C was denoted as sample A. Whereas those treated with sputtering time of 0.5, 1, 3, and 5 minutes were labelled as samples B, C, D, and E, respectively. The morphology and microstructure of the Au/ZnO-NPs were examined by using field emission scanning electron microscope (FESEM) (JEOL JSM-6700F) and transmission electron microscope (TEM) (JEOL JEM-2010F) with an operation voltage of 200 kV, respectively. In addition, the energy dispersive x-ray spectroscopy (EDS) analyses were performed to reveal the composition distribution of the samples. The existent phases in films and the crystallographic orientations of all samples were identified by high-resolution x-ray diffraction (HRXRD) (PANalytical X'Pert Pro Singapore). The measurements of photoluminescence (PL) were carried out at room temperature with a 325 nm He-Cd laser (IK3252R-E, Kimmon).2 The reflectivity measurements for all samples were conducted using a spectrophotometer (Jasco V-670). The unpolarized light with wavelength in the range of 200 to 1800 nm was used. To gain more precise information on the optical characteristics of the all samples, an integrating sphere was adopted in the spectrophotometer to determine the 5



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total reflectance.16-18 Moreover, the collected reflectance was resolved by a spectrometer (SPM-002-ET, Photoncontrol Inc.) to obtain the angle-resolved reflective maps with the angle of incidence (AOI) ranging from -60° to 60°. 3. RESULTS AND DISCUSSION Figure 1a shows the typical morphology of the ZnO-NPs grown on ITO-coated glass substrate at 700 °C. The image clearly indicates that the as-grown ZnO-NPs are more or less aligned vertically with the dimension of 200~500 nm in diameter and 1.5~2.7 µm in height. Moreover, from Figure 1b, it is evident that, although most of the ZnO-NPs appeared to remain isolated, coalescence of two or more nanopillars has already taken place within the growth time of 30 minutes. Nevertheless, it is interesting to note that even with the pillar coalescence the top surface of the as-fabricated ZnO-NPs remains flat and smooth with the symbolic hexagonal facet feature. It is worthwhile to note that the morphology, including diameter and length, of the ZnO-NPs grown at 600 °C are almost the same with those displayed here for the ZnO-NPs grown at 700 °C. It has been pointed out19,20 that the primary growth mechanisms for obtaining 1D semiconductor nanostructures are dominated by routes of vapor-solid (VS), vapor-liquid-solid (VLS), or a combination of both (VS and VLS). Since the present ZnO-NPs all displaying clear hexagonal facet feature without any residual catalyst remaining on the pillar tips (Figure 1b), it is suggestive that the 6


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present ZnO-NPs might have been grown primarily via the VS mechanism.21 To unveil the detailed microstructural features of the ZnO-NPs samples, the as-grown ZnO-NPs were further examined by TEM analyses. Figure 1c shows a lower magnification TEM photograph, displaying the generally straight and uniform feature of the as-grown ZnO-NPs. The high-resolution TEM (HRTEM) image and the diffraction pattern displayed in Figure 1d and its inset further reveal that the obtained ZnO-NPs are essentially single crystals. Furthermore, as indicated in Figure 1d, the lattice spacing is ca. 0.52 nm, in agreement with that of the (0002) planes in wurtzite structured bulk ZnO. Previously, it had been pointed out that the optical properties exhibited by the ZnO nanostructures are very much dependent on the detailed process parameters and can be very different from the their bulk and thin film counterparts.22-24 It is thus worthwhile to explore the optical features of the ZnO-NPs prepared at the growth temperature of 600 and 700 °C. In this work, the room-temperature PL measurements were carried out to delineate the possible effects resulted from growth temperature. As shown in Figure 2, the PL spectrum (blue solid triangles) for ZnO-NPs grown at 700 °C displays only a sharp and strong peak at approximately 378 nm (3.28 eV), which corresponds to the intrinsic near-band edge (NBE) emission of undoped ZnO.25 In contrast, the PL curve (red solid circles) of ZnO-NPs grown at 600 °C appears to 7



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be much richer in structure. That is, except for the NBE peak (which is also much more broadened as compared to that of the 700 °C ones), emissions distributed over the entire visible region can also be clearly observed. As shown in the insets of Figure 2, the pictures taken by using a digital camera clearly illustrate a purely bluish emission for ZnO-NPs grown at 700 °C (inset b) and a bright white emission for ZnO-NPs grown at 600 °C (inset c). This can be further confirmed from inset a of Figure 2 by plotting the intensity in the logarithm scale, which clearly indicates that the intensities of blue emission (wavelength from 476-495 nm), green emission (495-570 nm) and red emission (620-750 nm) are very close to each other for the 600 °C case. Among the visible emissions observed in ZnO, it is generally conceived that the blue emission close to NBE transition is arisen from the transition process between the shallow donor level associated with the valence band and the oxygen vacancies.26 Whereas, the green and yellow emissions are primarily due to oxygen-related defects and deep levels associated with oxygen interstitials, repectively.27 Finally, the red emission is primarily associated with oxygen and zinc vacancy complexes. In any case, from the PL results shown in Figure 2, it is beyond any doubt that the crystalline quality of ZnO-NPs grown at 700 °C is much better than those grown at 600 °C.28 Although it is not clear at present why by merely lower the growth temperature by 100 °C (from 700 °C to 600 °C) would result in such a 8


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dramatic difference in the resultant defect structures, the fact that the present as-grown ZnO-NPs can result in such bright white-emission at RT is, nevertheless, quite interesting. Comparing with other ZnO-related nanostructures exhibiting quasi-white PL emissions, which were either derived by catalytic solution processes10 or with microwave-assisted solution growth,11 the present method not only is much simpler but also is capable of controlling the emission of the as-grown ZnO nanostructure from purely bluish or bright-white by merely adjusting the furnace temperature. To the best of our knowledge, this perhaps is the first observation of white light-emission from undoped ZnO nanostructures at room temperature and may turn out to be a viable alternative method for developing white light emitters based on ZnO nanostructures. On the other hand, it has been pointed out that the existence of various defects, such as point, line and planar defect states might have been responsible for the enhanced light-trapping effect observed in various nanostructures.29,30 Thus, the abundance of defects capable emitting lights covering essentially the entire visible regime existing in ZnO-NPs grown at 600 °C implies that it might be also beneficial in enhancing broadband light trapping. To further explore the feasibility of using the 600 °C ZnO-NPs as antireflective nanostructures, we performed a systematic measurements along this line. However, since the flatness of the top surface of individual ZnO-NP 9



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could significantly reduce the effectiveness of coupling between the incident light and the ZnO-NPs,17 small amount of Au was thus deposited onto the ZnO-NPs to improve the coupling. Figure 3a-d are the SEM images displaying the typical morphology and distribution of the Au nanoparticles obtained with sputtering time of 0.5, 1, 3 and 5 minutes, respectively. The density of the Au nanoparticles (see the insets of Figures 3a-d) is estimated to be around 0-1, 3-7, 20-45 and 30-55 on the top surface of individual ZnO-NP for sputtering time of 0.5, 1, 3 and 5 minutes, respectively. (See Supporting Information Table S1) As is evident from Figures 3a-d, initially the sputtered Au appeared as a continuous layer on the top surface of ZnO-NPs. Nonetheless, due to the dominance of surface diffusion, the growth mechanism of Au film immediately switched to the three-dimensional nucleation and growth (Volmer-Weber) mode. This is clearly seen in Figures 3b-d that even with only 1 minute of sputtering time coalescence of Au nanoparticles has already been actively engaged, leading to the appearance of isolated island-like particles. The density of Au islands (or nanoparticles) and the overall thickness of the Au layer, as judged from Table S1 and the SEM images shown in Figure 3 both increases rather rapidly with increasing sputtering time. To further delineate and analyse the structural properties in a more quantitative manner for the all ZnO-NPs samples grown on ITO-coated glass substrates, XRD 10


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measurements were performed. Figure 4 shows the XRD results of the all samples, with samples A, B, C, D, and E being presented in brown, blue, green, pink, and red lines, respectively. The results indicate that all the diffraction peaks belonging to ZnO are corresponding to the wurzite structure, indicating that the obtained ZnO-NPs are indeed of single-phase characteristic with the (002) and (103) crystallographic orientation being the main growth directions normal to the film surface. The appearance of diffraction peaks corresponding to other orientations indicates that the ZnO-NPs might not be perfectly aligned along the normal direction of the substrate surface, which, in fact, is in good agreement with the results shown in Figure 1a. Interestingly, the full-width at half-maximum (FWHM) of the (002) diffraction peak (inset b) of the as-grown sample (sample A) is measured to be only ∼0.3°, indicating the highly crystalline nature of the obtained ZnO-NPs, which is also consistent with the HRTEM images shown in Figure 1d. Finally, as depicted in the inset a of Figure 4, the intensity of the representative diffraction peak of Au(111) grows with increasing sputtering time, which is understandable as more Au has been deposited onto the surface of the ZnO-NPs. As shown in Figure 5, the TEM images clearly illustrate that the Au nanoparticles are dispersing not only on the top surface but also on the sidewalls of the pillars, as indicated by the arrows shown in Figure 5a. Moreover, as revealed by the HRTEM image shown in Figure 5b, the distinct crystal 11



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structure of the Au nanoparticles and sharp interfaces between the Au nanoparticle and ZnO-NPs strongly suggest that the Au nanoparticles are, in fact, only “decorating” on the NPs’ surface instead of “alloying with” or “embedding into” the NPs. As will be shown below, these “decorating” Au nanoparticles are playing prominent roles in both coupling and trapping the incident light into the ZnO-NPs. Due to its unique features in incorporating with other materials, ZnO nanostructures have been demonstrated to have tremendous potential for solar cell applications.31-33 Thus, how to reduce the reflectivity of these nanostructures has been one of the keys for improving the conversion efficiency. In this work, comprehensive measurements on the reflectance for all samples were carried out to delineate the effects of Au nanoparticle decoration on the ZnO-NPs. The total reflectance spectra, including the specular reflections, were collected over the wavelength range of 200-1800 nm. Figure 6 displays the results obtained for polished Si substrate, as-grown ZnO-NPs (grown at 600 °C), as well as the Au/ZnO-NPs obtained with various Au deposition durations. The inset of Figure 6 displays the visual appearance of bared polished Si, and sample A-E, denoted as (a)-(e) in the insets, respectively. The result for the polished Si substrate (black line) clearly displays the typical wavelength-dependence of reflectance18 with a mean reflectance reaching 42.7% within the range of measurements. For the as-grown ZnO-NPs (sample A; brown line), a mean 12


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reflectance of ~29.0% is observed in the same wavelength range. It is, nevertheless, very interesting to notice that with merely 3 minutes of Au deposition, the reflectance, especially for long-wavelength regime, has been substantially reduced. For the sample E (5 min Au deposition), the measured reflectance is reduced to below 10 % over the same wavelength range, exceeding 65 % reduction in reflectance as comparing to that of the as-grown ZnO-NPs. The significant reduction in reflectance is believed to arise from the fact that the Au nanoparticles decorated on the ZnO-NPs may have acted as the subwavelength scattering components, resulting in increased effective optical path length of the incident light by coupling and trapping freely propagating plane waves. In particular, as pointed out by Atwater and Polman,34 the hemispherical shape Au nanoparticles seen in Figure 5b can have near 40-fold of optical path length enhancement. Moreover, sample E also exhibits the highest absorption over the entire wavelength range (200-1800 nm) among all samples tested (Supporting Information Figure S1), which agrees well with the reflectance results. It is noted that from the TEM micrograph shown in Figure 5a, the sputtered Au-NPs do not exactly cover the entire side surface of the ZnO-NPs conformably. Nevertheless, they do distribute rather “uniformly” over the longitudinal surface of the ZnO-NPs. Such structure might lead to some variations in the effective refractive index along the longitudinal axis of the ZnO-NPs, especially in between the adjacent nano-pillars. The gradient of 13



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refractive index in various nanostructures is known to have substantial influences in antireflection effect.35 Thus, the possible variations in the present Au-NP-decorated ZnO-NPs might be also beneficial in enhancing the antireflection or light trapping effects. In any case, the combination of increasing the effective length of the incident light optical path and the variations of the effective refractive index resulted from Au-NPs decoration is believed to result in the significant reduction of reflection loss of the incident light. Comparing with the results reported in other ZnO-related antireflective nanostructures,36,37 the present results not only exhibited comparable performance, but also over a wider spectrum range. In addition to low reflectivity over a wide wavelength range, the capability of exhibiting the incident angle-independent reflection (omnidirectional) characteristic represents yet another important aspect required for more efficient solar energy harvesting. The light trapping of previous photovoltaic cells made of antireflective coating nanostructures, however, generally lack of this important feature and displaying apparent angle-dependence on incident light.15 To the aspect, we have also investigated the spectral reflectance as a function of angle of incidence (AOI; from -60 to 60°) for the present Au/ZnO-NPs. Figures 7a-e illustrate the angle-resolved reflectance maps (R(θ,λ); where R is the reflectance, θ and λ is the angle and wavelength of incident light, respectively.) measured as a function of AOI and 14


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wavelength of incident light of all samples. It is evident from Figures 7a-e that, for any particular wavelength within the range of 300 to 1000 nm, varying the incident angle of the light does not result in any noticeable difference in reflectance. This is especially obvious in Figure 7d (sample D) and 7e (sample E)), wherein the reflectance has been significantly reduced by the incorporation of Au-NPs. The measured results evidently indicate that the reflectance curves are incidence angle-independent (from -60 to 60°) over the entire wavelength range. Moreover, the reflectance spectra of all samples are in totally agreement with the reflectance curves shown in Figure 6. It is noted that by comparing the angle-resolved map of the as-grown ZnO-NPs (Figure 7a) with that of sample E (Figure 7e), an enormous reduction in reflectance over a wavelength range 200-1800 nm and incident angle of ± 60° is clearly observed, indicating that the Au-nanoparticle decoration is indeed an efficient way to improve the broadband antireflective and omnidirectional light trapping for the ZnO-NPs nanostructures. In particular, comparing to most antireflective coating technologies ubiquitously adopted in current solar cell industry, the fabrication processes of the present Au-decorated ZnO-NPs are much simpler and more efficient, thus may have significant implications in solar energy harvest applications. 4. CONCLUSION 15



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ZnO-NPs have been successfully grown on the ITO-coated glass without using any catalyst. To enhance the light-trapping feature for the obtained ZnO-NPs, we further deposited Au nanoparticles on the surface of the obtained ZnO-NPs by using sputtering process to form Au/ZnO-NPs heterostructures. The morphology of the Au nanoparticles indicated that the growth was governed by surface diffusion dominated Volmer-Weber growth mode. More significantly, the present Au/ZnO-NPs has evidently exhibited omnidirectional low reflectivity over an unprecedented wavelength range, namely from 200 to 1800 nm with an incident angle span of ± 60°. The lithography-free and simple fabrication processes adopted in obtaining the present results undoubtedly has pointed to an approach with significant application potentials in photovoltaic devices. ■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: The related parameters of Au nanoparticles for sample B-E, absorption spectra of sample A (brown line), sample B (blue line), sample C (green line), sample D (pink line) and sample E (red line). ■ AUTHOR INFORMATION 16


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Corresponding Author *E-mail: [email protected] (Y.-F. Lin). *E-mail: [email protected] (J.-Y. Juang). ORCID Yen-Fu Lin: 0000-0002-1545-9143 Jenh-Yih Juang: 0000-0002-8654-9015 ■ ACKNOWLEDGMENTS This work was partially supported by the Ministry of Science and Technology (MOST) of Taiwan, under Grant No.: MOST 105-2811-M-005-018. Prof. Jenh-Yih Juang is supported in part by the MOST of Taiwan and the MOE-ATP program operated at NCTU. The authors would like to thank Prof. Kaung-Hsiung Wu (Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan) for optical measurements. The authors also would like to thank Dr. Che-Yi Lin (Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan) and Dr. Shih-Hsie Yang (Department of Electrical Engineering & Institute of Electronic Engineering, National Tsing Hua University, Hsinchu 300, Taiwan) for useful discussion.

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(25) Chang, Y.-M.; Kao, P.-H.; Liu, M.-C.; Lin, C.-M.; Lee, H.-Y.; Juang, J.-Y. Fabrication and Optoelectronic Properties of Core-Shell Biomimetic ZnO/Si Nanoball Arrays. RSC Adv. 2012, 2, 11089-11094. (26) Zhang, D. H.; Xue, Z. Y.; Wang, Q. P. The Mechanisms of Blue emission from ZnO Films Deposited on Glass Substrate by R.F. Magnetron Sputtering. J. Phys. D: Appl. Phys. 2002, 35, 2837-2840. (27) Chang, Y.-M.; Jian, S.-R.; Lee, H.-Y.; Lin, C.-M.; Juang, J.-Y. Enhanced Visible Photoluminescence from Ultrathin ZnO Films Grown on Si-Nanowires by Atomic Layer Deposition. Nanotechnology 2010, 21, 385705. (28) Chang, Y.-M.; Lin, M.-L.; Lai, T.-Y.; Lee, H.-Y.; Lin, C.-M.; Wu, Y.-C. Sermon; Juang, J.-Y. Field Emission Properties of Gold Nanoparticle-Decorated ZnO Nanopillars. ACS Appl. Mater. Interfaces 2012, 4, 6676-6682. (29) Li, X.; Li, J.; Chen, T.; Tay, B. K.; Wang, J.; Yu, H. Periodically Aligned Si Nanopillar Arrays as Efficient Antireflection Layers for Solar Cell Applications. Nanoscale Res. Lett. 2010, 5, 1721-1726. (30) To, W.-K.; Fu, J.; Yang, X.; Roy, V. A. L.; Huang, Z. Porosification-Reduced Optical Trapping of Silicon Nanostructures. Nanoscale 2012, 4, 5835-5839.

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Figure caption Figure

1. (a) The cross-sectional and (b) top view SEM image of the as-grown

ZnO-NPs/ITO-glass (sample A) synthesized in the conventional furnace at 700 °C. The inset is the EDS analysis of as-grown ZnO-NPs/ITO-glass. (c) The TEM and (d) high-resolution TEM (HRTEM) image of individual ZnO-NP. The inset exhibits the corresponding selected-area electron diffraction pattern from the individual ZnO-NP. Figure

2. The PL emission ZnO samples grown at 700 (blue trianles) or 600 °C (red

circles) at RT. The inset (a) shows the of PL measurements (in logarithm) of ZnO-NPs grown at 600 °C from 400 to 700 nm for wavelength. The insets (b) and (c) show the purely bluish or bright white emission from as-grown ZnO-NPs grown by 700 or 600 °C when irradiated with a 325-nm-He-Cd laser, respectively. Figure

3. The top view SEM image of the Au nanoparticles on ZnO-NPs. The Au

elements were depositited on ZnO-NPs for (a) 0.5 (sample B), (b) 1 (sample C), (c) 3 (sample D), and (d) 5 minutes (sample E), respectively. The insets are the high-magnification SEM image of corresponding Au/ZnO-NPs. Figure

4. The XRD spectra of ITO-glass (black line), sample A (wine line), sample

B (blue line), sample C (green line), sample D (pink line) and sample E (red line), respectively. The inset (a) is the intensity of the representative diffraction peak of

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Au(111) of sample A-E. The inset (b) is the full-width at half-maximum (FWHM) of the (002) diffraction peak of the sample A (as-grown ZnO-NPs). Figure

5. (a) TEM image of the Au-decorated ZnO-NPs (sample E). (b) HRTEM

image of the interface of Au nanoparticle and ZnO-NP (sample E). Figure

6. The reflectance curves of polished Si and all ZnO samples at normal

incidence. The insets are the pictures of (a) sample A (as-grown ZnO-NPs), (b) sample B, (c) sample C, (d) sample D and (e) sample E on 6-inch wafer, respectively. Figure

7. Angle-resolved reflectance spectra of the (a) sample A, (b) sample B, (c)

sample C, (d) sample D and (e) sample E, respectively.

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Figure

1

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Figure

2

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Figure

3

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Figure

4

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Figure

5

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Figure

6

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Figure

7

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Table Of Contents (TOC) graphic

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Figure 1. (a) The cross-sectional and (b) top view SEM image of the as-grown ZnO-NPs/ITO-glass (sample A) synthesized in the conventional furnace at 700 °C. The inset is the EDS analysis of as-grown ZnONPs/ITO-glass. (c) The TEM and (d) high-resolution TEM (HRTEM) image of individual ZnO-NP. The inset exhibits the corresponding selected-area electron diffraction pattern from the individual ZnO-NP. 200x200mm (200 x 200 DPI)



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Figure 2. The PL emission ZnO samples grown at 700 (blue trianles) or 600 °C (red circles) at RT. The inset (a) shows the of PL measurements (in logarithm) of ZnO-NPs grown at 600 °C from 400 to 700 nm for wavelength. The insets (b) and (c) show the purely bluish or bright white emission from as-grown ZnO-NPs grown by 700 or 600 °C when irradiated with a 325-nm-He-Cd laser, respectively. 202x150mm (200 x 200 DPI)


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Figure 3. The top view SEM image of the Au nanoparticles on ZnO-NPs. The Au elements were depositited on ZnO-NPs for (a) 0.5 (sample B), (b) 1 (sample C), (c) 3 (sample D), and (d) 5 minutes (sample E), respectively. The insets are the high-magnification SEM image of corresponding Au/ZnO-NPs. 200x200mm (200 x 200 DPI)



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Figure 4. The XRD spectra of ITO-glass (black line), sample A (wine line), sample B (blue line), sample C (green line), sample D (pink line) and sample E (red line), respectively. The inset (a) is the intensity of the representative diffraction peak of Au(111) of sample A-E. The inset (b) is the full-width at half-maximum (FWHM) of the (002) diffraction peak of the sample A (as-grown ZnO-NPs). 150x124mm (200 x 200 DPI)


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Figure 5. (a) TEM image of the Au-decorated ZnO-NPs (sample E). (b) HRTEM image of the interface of Au nanoparticle and ZnO-NP (sample E). 200x100mm (200 x 200 DPI)



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Figure 6. The reflectance curves of polished Si and all ZnO samples at normal incidence. The insets are the pictures of (a) sample A (as-grown ZnO-NPs), (b) sample B, (c) sample C, (d) sample D and (e) sample E on 6-inch wafer, respectively. 179x134mm (200 x 200 DPI)


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Figure 7. Angle-resolved reflectance spectra of the (a) sample A, (b) sample B, (c) sample C, (d) sample D and (e) sample E, respectively. 246x365mm (200 x 200 DPI)


Broadband Omnidirectional Light Trapping in Gold-Decorated ZnO

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