Two-Dimensional Inverse Opal ZnO Nanorod Networks with Photonic

J. Phys. Chem. C , 2009, 113 (33), pp 14778–14782. DOI: 10.1021/jp904824q. Publication Date (Web): July 9, 2009. Copyright © 2009 American Chemical...
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J. Phys. Chem. C 2009, 113, 14778–14782

Two-Dimensional Inverse Opal ZnO Nanorod Networks with Photonic Band Gap Yu-Cheng Chang,† Han-Wei Wu,† Hsuen-Li Chen,‡ Wen-Yun Wang,‡ and Lih-Juann Chen*,† Department of Materials Science and Engineering, National Tsing Hua UniVersity, Hsinchu, Taiwan 30043, Republic of China, and Department of Materials Science and Engineering, National Taiwan UniVersity, Taipei, Taiwan 10617, Republic of China ReceiVed: May 23, 2009; ReVised Manuscript ReceiVed: June 21, 2009

Two-dimensional (2D) inverse opal ZnO nanorod networks exhibiting a photonic crystal band gap have been grown by an aqueous chemical method with nanosphere lithography at low temperatures. The 2D ZnO photonic crystals with band gap at the green light emission region have been successfully fabricated for the first time. The presence of the photonic band gap was inferred from the reflection spectra and confirmed by the photonic band calculation. The ZnO nanorod networks exhibit prominent blue shift at UV emission and almost no green emission attributed to singly ionized oxygen vacancies. The combination of aqueous chemical growth and nanosphere lithography provides a large-scale, facile, and low-cost fabrication method at low temperatures, which shall be of significant value for practical applications of the grown photonic crystals. Introduction The development of novel nanostructured materials with controlled shape and ordered morphology has stimulated wide interest for exploiting their unusual properties and unique functions.1 The ability to develop nonclassical lithographic techniques to pattern, to assemble, and to integrate nanostructures as functional two-dimensional (2D) networks is a challenge for fabricating functional and practical nanodevices.2,3 Nanosphere lithography (NSL) is an innovation of the technique originally known as “natural lithography”, where a monodisperse or multidisperse nanosphere template acts as a deposition mask.4-7 NSL is an inexpensive, inherently parallel, simple fabrication process and materials-general nanofabrication technique that is now being widely employed.4,8 Photonic crystals are ordered nanostructures in which two media with different dielectric constants or refractive indices are arranged in a periodic form.9,10 The periodic structures can be used to control light propagation in a way analogous to semiconductor lattices with electrons.11 For the past decade, the regular structures of photonic crystals have been extensively fabricated by nanosphere lithography. Colloidal self-assembly seems to be the most efficient method for fabrication of 2D or three-dimensional (3D) photonic crystals by the regular arrangement of monolayer or multilayer nanospheres.8-14 2D photonic crystal slabs have attracted much interest due to the relative ease of achieving a full photonic band gap.13 Chemical vapor deposition (CVD) and atomic layer deposition (ALD) with good step coverage are widely used to fabricate the inverse opal photonic crystals by nanosphere lithography.15,16 ZnO with a high refractive index can provide enough contrast in the inverted structure to open an additional pseudogap in the high-energy regime.15-18 Zinc oxide is one of the most promising functional materials owing to its wide direct band gap (3.37 eV) and large exciton binding energy (60 meV).13,19,20 The ZnO nanostructure has attracted much attention for its applications in nanodevices such * Corresponding author. Telephone number: 886-3-5731166. Fax number: 886-3-5718328. E-mail: [email protected]. † National Tsing Hua University. ‡ National Taiwan University.

as ultraviolet laser,21 photonic crystals,13 chemical sensors,22 solar cells,23 piezoelectric gated diodes,24 and piezoelectric nanogenerators.25 A facile fabrication technique of aqueous chemical growth (ACG) has been demonstrated to grow ZnO nanowires, oriented ZnO nanorod arrays, ZnO microtubes, and ferric oxide nanorods.26-28 Such a method can be used to grow ZnO nanostructures on various substrates (e.g., amorphous, single-crystalline, polycrystalline, and flexible substrates). In addition, the low-cost growth is conducted at low temperatures. The present work has successfully grown uniform ZnO nanorods at the bordering regions of self-assembled monolayer PS nanospheres at low temperature using a downward ACG method. The removal of the PS nanospheres has produced 2D inverse opal ZnO nanorod networks. The presence of the photonic band gap was inferred from the reflection spectra and confirmed by the photonic band calculation. The ZnO nanorod network exhibits prominent blue shift at UV emission and almost no green emission attributed to singly ionized oxygen vacancies. The fabrication of large-scale 2D ZnO photonic crystals at low temperature is facile, of low cost, and highly efficient, which shall be conductive to practical device applications. Experimental Section Silicon (100) substrates were ultrasonically cleaned in acetone and DI water sequentially for 1 h each. Colloidal crystal monolayer was synthesized by applying 15 µL of a solution of 5% suspension of 488 nm polystyrene (PS) spheres on acclivitous substrates. The uniform domain size of PS spheres can be as large as 1 cm × 1 cm. Au buffer layers of different thicknesses were deposited on the monolayer structure by electron-beam evaporation. The substrate with PS nanosphere monolayer and Au buffer layer was placed on a hot plate at 50 °C, and 20 µL of 5 mM zinc acetate in ethanol solution was dripped on the substrate. ZnO nanocrystals were formed between nanospheres after heating to 90 °C for 20 min. The ZnO nanocrystals between nanospheres were used as seeds to grow ZnO nanorods and produce ZnO nanorod networks between nanospheres. ZnO nanorods were grown by an aqueous chemical method in 150 mL aqueous solution containing 10 mM zinc acetate dihydrate (98%, Aldrich) and 10 mM hexamethylene-

10.1021/jp904824q CCC: $40.75  2009 American Chemical Society Published on Web 07/09/2009

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Figure 1. (a) Top-view and (b) cross-sectional SEM images of ZnO nanorod arrays grown on the Au film substrate by the downward growth method. (c) TEM image of a ZnO nanorod. (d) HRTEM image and corresponding SAED pattern confirming the single crystallinity of the ZnO nanorod in (c).

tetramine (99%, Aldrich) (HMTA) each. The substrate was positioned downward and nearly flush with the surface of the reaction solution and heated to about T ) 90 °C for 3 h.19 Finally, the nanospheres were uniformly removed by sonicating in toluene for 2 min. The morphology of nanostructures was examined with a field emission scanning electron microscope (FESEM) using a JEOL JSM-6500F SEM operating at 10 kV accelerating voltage. A JEOL-2010 transmission electron microscope (TEM) operating at 200 kV was used to examine the microstructures. The reflection spectra were obtained with a Hitachi Model U-4100 spectrophotometer. The incident angle of the UV-near IR beam was adjusted relative to the normal direction. The cathodoluminescence (CL) spectra were acquired with an electron probe microanalyzer (Shimadzu EPMA-1500) attached to a SEM. CL spectra were accumulated in a single-shot mode with an exposure rate of 1 nm/s. All the CL spectra were taken at room temperature. Results and Discussion Well-aligned and epitaxial ZnO nanorod arrays were grown on [0001]-oriented seed layer by the hydrothermal growth method. Twenty microliter 5 mM zinc acetate in ethanol solution was dripped on the preevaporated 15 nm thick Au film of Si substrate at 50 °C and annealed in air for 20 min at 90 °C. Figure 1a shows a SEM image depicting the ZnO nanorod arrays grown from equimolar (10 mM) zinc acetate and HMTA reaction solution by the downward growth method with a total growth time of 3 h. A cross-sectional SEM image is shown in Figure 1b. The lengths and diameters of the nanorods are about 1.0-1.6 µm and 25-80 nm, respectively. A previous work showed that the zinc acetate precursor provides more alkaline condition relative to zinc nitrate to change the ZnO morphology from nanoneedles to nanorods in a downward growth process.19 Figure 1c shows the TEM image of a ZnO nanorod (in Figure 1a) with a length of 820 nm and diameter of about 47 nm. Figure 1d and the inset show the HRTEM image taken from part of an individual nanorod and the corresponding selected area electron diffraction (SAED) pattern, respectively. Both the TEM image and diffraction pattern reveal that the nanorod is single crystalline and grown in the [0001] direction.

Figure 2. Schematic plots illustrating the steps to fabricate the ZnO nanorod networks: (a) self-assembly of monolayer PS nanospheres, (b) deposition of gold buffer layer, (c) deposition of ZnO seed layer, (d) growth of ZnO nanorods, and (e) side view and top view after removal of PS nanospheres.

Figure 2 depicts schematically the method used to fabricate the regular structures of 2D photonic crystal slabs. The fabrication processes can be briefly divided into five steps; selfassembly of monolayer PS nanospheres, deposition of Au buffer layer, catalytic growth of ZnO seed layer on Au, growth of ZnO nanorods on ZnO seed layer, and removal of PS nanospheres. In the first step, large-scale monolayer PS nanospheres were deposited on acclivitous substrates. In the second step, the evaporated Au film was used as a buffer layer to avoid direct deposition of ZnO nanocrystals to the bottom of PS nanospheres. In the third step, ZnO nanocrystals can be deposited by dripping 20 µL 5 mM zinc acetate ethanol solution at 50 °C. Heating the substrate and dripped zinc acetate would quickly result in the transformation into ZnO nanocrystals between PS nanospheres. In the fourth step, downward growth of ZnO nanorods was carried out. In the fifth step, the PS nanospheres were removed by sonicating in toluene solution for 2 min. Figure 3a shows a low-magnification SEM image of a sample with 15 nm thick Au layer evaporated on PS nanosphere monolayer. Yang et al. demonstrated the decomposition of zinc acetate at 200-350 °C to provide nucleation sites of ZnO nanocrystals for vertical growth of nanowires.29 The glass transition temperature (Tg) is the temperature at which the amorphous phase of the polymer is converted between rubbery and glassy states. The nanoscale materials have a lower Tg relative to bulk materials. Bulk polystyrene has a low glass transition temperature at 99.1 °C and decreases to 91.0 °C for the 17 nm thick film.30 To produce uniform ZnO seed layer between the PS nanospheres, the dripping of zinc acetate in ethanol solution was first carried out with the substrate maintained at 50 °C to avoid the vestige of ethanol solvent. Subsequently, ZnO seed layer was formed between the PS nanospheres after annealing at 90 °C for 20 min. Following steps 4 and 5 described above, 2D inverse opal ZnO nanorod networks were formed as shown in the low- and highmagnification SEM images in parts b and c, respectively, of

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Figure 3. (a) Top-view low-magnification SEM image of the monolayer PS nanospheres and evaporated 15 nm Au film, (b) top-view lowmagnification and (c) high-magnification SEM images of the ZnO nanorod networks grown in-between PS nanospheres by the downward growth method, (d) top-view SEM image of the bowl structure of ZnO nanorods grown by the upward growth method, (e) TEM image of a ZnO nanorod in (b), and (f) HRTEM image and corresponding SAED pattern confirming the single crystallinity of the ZnO nanorod in (e).

Figure 3. The ZnO nanorods were measured to be 8-25 and 180-190 nm in diameter and length, respectively. The small sizes arise from the difficulty of precursors to infiltrate the restricted space between PS nanospheres to participate in the reaction process. The PS nanospheres were slightly detached off the substrate in the downward growth process and allowed ZnO nanorods with enough space to grow laterally. On the other hand, the reaction was so confined that the central regions of nanopores are free from ZnO nanorods. For the upward growth, short nanorods were grown not only in the space between but also at the bottoms of nanospheres as shown in the top-view SEM image in Figure 3d. The growth of ZnO nanorods at the bottoms of nanospheres without the seed layer is attributed to the gravitational pull of precursors to facilitate the infiltration toward the bottoms of nanospheres and induce the growth. Figure 3e shows the TEM image of a downward grown ZnO nanorod with a length of 178 nm and diameter of about 11 nm. Figure 3f and the inset show the HRTEM image taken from part of a nanorod and the corresponding SAED pattern, respectively. Both the TEM image and SAED pattern indicate that the nanorod is single crystalline and grown in the [0001] direction. The metal buffer layers play an important role in growing regular 2D ZnO nanorod networks. If no metal buffer layers were involved, two kinds of morphological features appeared. One is similar to the case of upward growth seen in Figure 3d, and the other is composed of ZnO nanorods and nanotubes distributed at the bordering regions and the bottoms of nanospheres, respectively. Parts a and b of Figure 4 show schematic diagrams and top-view SEM images of the two kinds of

Chang et al. morphological features. A recent work has correlated the enhanced formation of ZnO nanotubes to the decrease in precursor solution.31 Based on the observation, the two morphological features are correlated with the difference in the infiltration of ZnO seed layer and zinc acetate precursor between PS nanospheres. During the dripping of ethanol solution of zinc acetate and the evaporation of ethanol, the bonding of PS nanospheres and substrate may become loosened. As a result, the ZnO seed layer is grown at the bottoms of PS nanospheres. The loosened PS nanospheres may detach from the substrate during the downward growth process and facilitate the growth of the ZnO nanorods at the borders and bottoms. The corresponding morphological feature is shown in Figure 4a. In some cases, the PS nanospheres were still loosely attached to the substrate during the downward growth process. The difficulty in the infiltration of reaction precursors toward the bottoms led to the growth of nanotubes at the bottoms as shown in Figure 4b. The thickness of the Au buffer layer was also found to influence the growth morphology of nanoporous structures. For the 5 nm thick Au buffer layer, ZnO nanorods and nanotubes were distributed at the borders and bottoms of the PS nanospheres, respectively. It appeared that the thin Au layer was not able to suppress the infiltration of the ZnO seed layer to the bottoms of PS nanospheres. Compared to the case with the 15 nm thick Au buffer layer, as shown in Figure 3c, the thinness of the Au layer may lead to the loose bonding between the PS nanospheres and the substrate. As a result, limited deposition of the ZnO seed layer at the bottoms of nanospheres led to the growth of nanotubes. A SEM image and corresponding schematic are shown in Figure 4c. Vayssieres et al. have rationalized ZnO microtube formation in terms of selective dissolution of the metastable polar [0001] face of ZnO microrod.27 Chang et al. showed that a high concentration of ZnO nanorods have a nitrogen-containing compound which was at the core during initial growth steps, but the core eventually dissolved into the solution leading to hollow structures after long reaction time.32 Therefore, the growth of ZnO nanotubes is influenced by many factors. Nevertheless, the observations in the present study appeared to strengthen the point that enhanced formation of ZnO nanotubes is facilitated by the diminished supply of precursors. The regular 2D ZnO nanorod networks are of inverse opal structures, which shall be useful as 2D photonic crystals. The photonic band structure can be determined by theoretical calculation and reflection spectra.13,33,34 The photonic band structure was calculated following the scheme described in ref 13. The 2D photonic crystal with triangular lattice of truncated air holes was assumed in the calculation process. The average distance of two neighboring air holes is about 488 nm, and the average internal diameter of air holes is about 200 nm. The refractive index of ZnO is 1.99.13 The slab thickness h was taken to be 330 nm as determined by cross-sectional SEM images. The calculated photonic band structure of the 2D photonic crystal slab is shown in Figure 5a. The pseudoband gap region is predicted in the frequency range of ωa/(2πc) from 1.065 to 1.124, which corresponds to wavelengths from 458 to 434 nm. The photonic band gap of 2D ZnO nanorod networks was measured by UV-near IR spectroscopy, with fixed incident angle of 85° to the normal direction, as shown in Figure 5b. The reflection spectra have two prominent peaks at 437 and 662 nm, respectively. The 437 nm peak is in excellent match with the calculated photonic crystal band gap. Both the reflection spectra and the photonic band structure confirmed the photonic crystal properties of the inverse opal 2D ZnO nanorod networks.

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Figure 4. Schematic plots and top-view SEM images of nanoporous structures grown by the downward growth method with (a) PS nanospheres slightly floated off from the substrate to grow ZnO nanorods, (b) PS nanospheres not floated off from the substrate to grow ZnO nanotubes, and (c) 5 nm thick Au on PS nanospheres for the growth of nanorods and nanotubes at the bottoms and bordering regions of PS nanospheres, respectively.

Figure 6. Reflection spectra of ZnO nanorod networks measured by the adjustable incident angles of UV-near IR spectrometry. Figure 5. (a) Calculated photonic band structure of the 2D ZnO nanorod networks. (b) Reflection spectrum of ZnO nanorod networks measured at the 85° incident angle.

The reflection peak of 662 nm comes from thin film diffraction. For thin film diffraction

2d sin θ ) nλ

(1)

where d is thin film thickness, θ is the angle of incidence, n is an integer, and λ is wavelength. If the incident angle was changed to 75° and 65°, the photonic band gap peak disappeared and the 662 nm peak shifted to 640 and 599 nm, respectively, as shown in Figure 6. If the average thickness of ZnO nanorod networks is about 330 nm and n is 1, wavelengths of diffraction at different incident angles are 658 nm (85°), 638 nm (75°), and 598 nm (65°). The calculated wavelengths match rather well with those of the measured values. It can therefore be concluded that thin film diffraction indeed led to the peaks near 600 nm in the reflection spectra.

Cathodoluminescence (CL) is a useful technique for characterizing the optical properties of nanostructures. Since CL uses an electron beam for excitation, it is feasible to excite only a single or a small group of nanostructures. All the CL spectra were taken at room temperature.19 For nanorod arrays, the strong UV emission occurs at about 377 nm (3.29 eV), which comes from recombination of exciton and almost no emission from defects, as shown in Figure 7. The emission from defects is not evident. For comparison, the CL spectra from nanorod networks with a small diameter of about 8-25 nm exhibit a 7 nm (60 meV) shift in the emission peak. The blue shift is attributed to the quantum confinement effect arising from the reduced size of the diameters. In the CL emission spectra, the energy shift of nanorods with diameters far beyond the quantum confinement regime is ascribed to the surface effect due to the increased surface-to-volume ratio.35 The enhancement in deep-UV optical properties shall facilitate the applications of nanorods for nanoscale light-emitting devices. Three-dimensional inverse opal ZnO photonic crystals have previously been fabricated with chemical vapor deposition and

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Figure 7. Cathodoluminescence spectra of the ZnO nanostructures grown on Si wafer precoated with 15 nm Au film: (a) nanorod arrays and (b) nanorod networks.

atomic layer deposition of ZnO in conjunction with nanosphere lithography.15,16 2-D periodic structures in ZnO films were fabricated with focused ion beam etching.18 2D photonic crystal slabs of ZnO/TiO2 matrix were fabricated from ALD of TiO2 layer on 2D ZnO nanorod arrays.13 Infiltration of CdSe nanocrystals on 2D ZnO inverse opals has resulted in the demonstration of photonic effect.17 On the other hand, the present work achieved for the first time the growth of 2D inverse opal ZnO nanorod networks which exhibit a photonic band gap. The large-scale fabrication of ZnO photonic crystals with the facile and low-cost method at low temperatures shall be conductive to their practical applications. Summary and Conclusions In summary, 2D inverse opal ZnO nanorod networks have been grown on silicon. The 488 nm PS nanospheres monolayer was used as the mold to grow the ZnO networks through the steps of self-assembly of monolayer PS nanospheres, deposition of Au buffer layer, catalytic growth of ZnO seed layer on Au, growth of ZnO nanorods on ZnO seed layer, and removal of PS nanospheres. The presence and thickness of Au buffer layer were found to be critical to obtain the desired inverse opal ZnO nanorod networks through a downward growth process. A peak at 437 nm in the reflection spectrum is in excellent agreement with the calculated photonic crystal band gap. In addition, the CL spectra of ZnO nanorod networks exhibit significant blue shift at UV emission and almost no green emission attributed to singly ionized oxygen vacancies. The work represents the successful growth of 2D ZnO photonic crystal with band gap at the green light emission region for the first time. The combination of aqueous chemical growth and nanosphere lithography provides a facile, large-scale, and low-cost fabrication method at low temperatures, which shall be of significant value for practical applications of the grown photonic crystals. Acknowledgment. The research was supported by the National Science Council through grants No. NSC 96-2221-E007-151 and NSC 97-2120-M-007-003. References and Notes (1) Fu, M.; Zhou, J.; Xiao, Q.; Li, B.; Zong, R.; Chen, W.; Zhang, J. AdV. Mater. 2006, 18, 1001–1004.

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