Epitaxial growth of ZnO nanorod arrays via a self-assembled microspheres lithography

Accepted Manuscript Title: Epitaxial growth of ZnO nanorod arrays via a self-assembled microspheres lithography Authors: Bo-Cheng Lin, Ching-Shun Ku, Hsin-Yi Lee, Albert T. Wu PII: DOI: Reference:

S0169-4332(17)31057-7 http://dx.doi.org/doi:10.1016/j.apsusc.2017.04.042 APSUSC 35717

To appear in:

APSUSC

Received date: Revised date: Accepted date:

2-2-2017 28-3-2017 5-4-2017

Please cite this article as: Bo-Cheng Lin, Ching-Shun Ku, Hsin-Yi Lee, Albert T.Wu, Epitaxial growth of ZnO nanorod arrays via a self-assembled microspheres lithography, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.04.042 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Epitaxial Growth of ZnO Nanorod Arrays via a Self-Assembled Microspheres Lithography Bo-Cheng Lina, Ching-Shun Ku b, Hsin-Yi Lee b*, Albert T. Wua* a

Department of Chemical and Material Engineering, National Central University, Jhongli City

320, Taiwan b

National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park,

Hsinchu 30076, Taiwan

E-mail: [email protected] (Albert T. Wua*), [email protected] (Hsin-Yi Lee b*) GRAPHICAL ABSTRACT

Highlights 

Only one rod grows in the confined space under optimized growth conditions.

  

-scan shows six-fold symmetry implying a good epitaxial relationship. Well aligned ZnO nanorods were fabricated by a simple hydrothermal method. AZO buffer layers relax the strain from lattice mismatch between ZnO and substrate.

Abstract Through a simple hydrothermal method, well-aligned and periodic honeycomblike ZnO nanorod arrays were fabricated on a c-plane sapphire with an aluminum-doped ZnO (AZO) seed layer. Vertical and highly ordered ZnO nanostructures with the <0002> orientation were synthesized by employing a self-assembled monolayer of polystyrene (PS) microspheres as a mask. The optimal growth conditions allowed the

growth of only one rod in the confined space between the microspheres. A -scan exhibited six-fold symmetry, which indicates a favorable epitaxial relationship between the ZnO nanorods, seed layer, and c-plane sapphire substrate. The epitaxial relation is as follows: [0001]ZnO∥[0001]AZO∥[0001]c-plane sapphire. These results indicate that the AZO seed layer acts as a buffer layer that can relax the strain between ZnO and c-plane sapphire generated by the large lattice mismatch of 18%. The size of the resulting ZnO nanorods of diameter 20–90 nm could be tuned by varying the concentration of the solution, pH, and duration of reaction. The large aspect ratio of the ZnO nanorod arrays can serve as a template for high-surface-area applications. Keywords: Zinc oxide nanorod arrays, Hydrothermal growth, Epitaxial growth, Nanosphere lithography

Introduction Zinc oxide (ZnO) materials have a direct wide band gap of 3.37 eV at room temperature and a large excitation binding energy (60 meV) compared with gallium nitride (GaN) (approximately 24 meV). These materials have attracted considerable attention and have been employed in various devices because of their excellent performance in many fields such as optoelectronics, electronics, piezoelectrics, photonics, and sensor applications [1–8]. The wurtzite hexagonal crystal structure of ZnO has been used to synthesize a large family of nanostructures, including nanowires, nanorods, nanobelts, and nanocombs [1]. Among nanorods, one-dimensional (1D) ZnO nanorods, which can form the (0002) crystallographic planes most easily, are the most reactive and have polarized surfaces that are terminated with O or Zn atoms [9]. Several approaches have been proposed to fabricate ZnO nanostructures. Sugunan et al. reported the use of a sol–gel seed layer to synthesize ZnO nanorods through a hydrothermal method; however, the orientation of ZnO was highly dependent on the quality of the seed layer [10]. Vapor–liquid–solid [11, 12] and metal–organic chemical vapor deposition (MOCVD) [13] can be used to fabricate well-aligned and patterned ZnO nanorods; however, both approaches require a high temperature and expensive equipment and precursors. Electrodeposition has been reported to yield large-scale ZnO nanorods through one-step electrochemical deposition with or without template assistance [5–7]. Nevertheless, it requires an additional metal layer to serve as the electrode and saturated O2 in the electrolytic solution as the precursor. The hydrothermal method is a simple process that can be implemented at low temperatures; moreover, this method is cost-effective for large-scale manufacturing [14, 15]. Many attempts have been made to fabricate 1D vertical ZnO array structures

because of their high surface area, high aspect ratio, and controllable nanowire density. Well-aligned ZnO arrays have been fabricated through electron beam lithography, laser interference lithography, and microsphere lithography [16–21], among which microsphere lithography is the most promising. First, because of the self-assembly property of 2D colloidal microspheres, the morphology can be controlled by varying the self-assembly conditions such as temperature, solvent composition, and humidity [22]. Second, large-scale patterns can be prepared using the Langmuir–Blodgett technique and transferred onto several types of substrates [23]. Third, the nanowire density and the space between each 1D nanostructure can be adjusted using various sizes of microspheres. To synthesize vertical ZnO arrays, c-plane sapphire wafers are typically used as substrates. The lattice constants of ZnO crystals are a = 3.249 and c = 5.206 Å and those of c-plane sapphire are a = 4.754 and c = 12.991 Å; they have a large lattice mismatch of approximately 18% with the sapphire substrate, which may generate defects and dislocations in the ZnO materials and cause a rotation of 30° or 90° on the basal plane [24, 25]. The insertion of a buffer layer with materials such as GaN [26], MgO [27], 6H-SiC [28], and ZnO [13, 25, 29, 30] deposited at a high temperature can relax the strain between the ZnO material and sapphire substrate. Some studies have reported the deposition of ZnO thin films as buffer layers through pulsed laser deposition, molecular beam epitaxy, MOCVD, and radio frequency magnetron sputtering (RF sputtering). ZnO thin films have been synthesized on ZnO layers at least 200 °C [13, 25, 29, 31]. Chung et al. used the rocking curve results to demonstrate that ZnO nanorods synthesized on aluminum-doped ZnO (AZO)/Si at 500 °C improved the alignment [32]. In this study, a cost-effective RF sputtering method was adopted to deposit AZO thin films with native defects on sapphire substrates at room temperature. Furthermore, polystyrene (PS) microspheres of diameter 762 nm were adopted to serve as a lithography mask to synthesize arrayed ZnO nanorods. ZnO was synthesized in areas between microspheres by using a hydrothermal process on a c-plane sapphire substrate with an AZO seed layer. Synthesizing a single rod in each confined space while maintaining well-aligned arrays of ZnO nanostructure is difficult. Zhang et al. grew ZnO nanorod arrays on Si and GaN substrates by using the hydrothermal method. They employed a PS microsphere self-assembled monolayer (SAM) and a TiO2 assistant inverted SAM (ISAM) as a mask to grow a single nanorod in each space. However, when the SAM mask was used, only nanocone-like structures were observed, and the size of the ZnO nanorods was uncontrollable [20, 33]. In this study, we determined the optimal conditions to synthesize only a single rod in the spaces confined by PS microspheres. The solution concentration, pH, and duration of reaction during the hydrothermal

process were investigated, and the optimal growth conditions were determined. These ZnO nanorods provide the largest surface area for further applications. Detailed -scan analyses through synchrotron radiation X-ray clearly revealed that the ZnO, AZO, and c-plane sapphire layers were well aligned and had a strong epitaxial relationship. The results suggested that AZO helped relax the strain that might have been induced by the large lattice misfit.

Experimental An AZO (2 wt% Al) seed layer of thickness 100 nm was deposited on a cplane sapphire substrate through RF sputtering at approximately 298 K (RF power, 100 W). Pure PS microspheres (diameter, 762 nm) were diluted in a solution (5% w/v aqueous suspension, micro particles GmbH) by adding 95% ethanol at an equal volume. A PS monolayer was formed by spreading the microspheres on a microscope slide, slowly immersing the slide into a glass vessel filled with deionized water, and transferring them onto a c-plane sapphire substrate with an AZO seed layer. A hydrothermal aqueous solution (Zn(NO3)2·6 H2O, 99.5% purity, ALFA; hexamethylenetetramine, HMTA, 99.5% purity, ALFA) served as a precursor with the pH adjusted from 2 to 11.5, and the concentration was varied as 10, 25, 50, and 100 mM. The solution was heated to 90 °C and maintained at a constant temperature in the reaction vessel. The substrates with a monolayer of PS spheres were placed in the reaction vessel, and the reaction vessel was sealed for 0.5, 1, 2, and 4 h. Subsequently, the substrate was removed and immersed in a solution of tetrahydrofuran and deionized water to eliminate the PS microspheres, and was subsequently dried in air. A schematic of the fabrication of ZnO nanorod arrays is presented in Figure 1. The morphology of the ZnO nanorods was characterized through field-emission scanning electron microscopy (FESEM, Hitachi S-4300); the crystal structure was identified through Xray diffraction (XRD) measurements at the Taiwan Light Source beamline BL-17B1 in the National Synchrotron Radiation Research Center, Hsinchu, Taiwan. The grain size along the direction in the plane was measured by vertically tilting the samples (). The epitaxial relation was confirmed through a -scan in which the sample was rotated 360° to derive the azimuthal symmetry. Photoluminescence (PL, Protrustech Corporation Limited) was measured at 298 K using a 226-nm diode-pumped solid-state laser. The ZnO composition was determined through X-ray photoelectron spectroscopy (XPS, Thermo VG-Scientific Sigma Probe). The O 1s peak was fitted using the Lorentzian function with a confidence of 97.7%.

Results and discussion

1. ZnO nanorods synthesized using hydrothermal method ZnO nanorods were easily synthesized on an appropriate seed layer, such as AZO, which served as the nucleation site for nanorod growth. However, the density of ZnO nanorods was difficult to control by simply adjusting the hydrothermal conditions, and the alignment of the rods was greatly affected by the microstructure of the AZO seed layer because of its crystallographic relation with the c-plane sapphire. To fabricate ZnO nanorod arrays, we used PS microspheres with the self-assembly property as a mask to define the growth position. Because ZnO nanorods grow in the spaces between three neighboring PS microspheres, selecting PS microspheres of an appropriate size is important. Dong et al. reported that achieving hexagonal close packing by using PS microspheres of diameter <200 nm is difficult [20]. To achieve hexagonal close-packed patterns and to synthesize a single nanorod on each confined space, PS microspheres of diameter 762 nm were selected as the pattered mask. A PS microsphere array is illustrated in Figure 2(a). A hydrothermal process is defined as a heterogeneous reaction in an aqueous solution at a high temperature or pressure [34]. The hydrothermal growth condition of ZnO is sensitive to pH, concentration of precursors, reaction time, and growth temperature [35–37]. Wahab et al. and Singh et al. suggested that the variation in pH influences the hydrolysis and condensation properties of the solution during ZnO formation and affects the morphology of the nanorods [38, 39]. He et al. reported that the length of the ZnO nanorod increases as the precursor concentration is varied [40]. Li et al. indicated that the diameter of ZnO nanopillars could be efficiently controlled by varying the reaction time. These conditions are essential for controlling the morphology of the nanorods. [10]. HMTA might dissociate to NH4+ and OH− at high temperatures, and the hydroxide ions react with Zn2+ to form ZnO nanorods. Several studies have demonstrated that the optimal growth temperature is 90 °C. Hence, we selected 90 °C as the reaction temperature in this study. The solution was adjusted for pH, concentration, and duration of reaction. In the growth of materials in aqueous solutions, pH invariably plays an important role.

1.1 Influence of pH on ZnO nanorods To elucidate the effect of pH on the growth of ZnO nanorods, the solution pH was carefully adjusted by adding nitric acid (HNO3) and ammonia (aqueous NH3). In addition to varying pH, all samples were fixed at a temperature of 90 °C for 1 h by using the precursor at a concentration of 25 mM. The growth mechanism of ZnO is indicated by the following chemical reactions [41]:

𝑍𝑛2+ + 2𝑂𝐻 − ↔ 𝑍𝑛𝑂(𝑠) + 𝐻2 𝑂

(1) (2) 𝑍𝑛(𝑂𝐻)2 ↔ 𝑍𝑛𝑂(𝑠) + 𝐻2 𝑂 Figure 2(b–f) presents FESEM images, captured at 45°, of ZnO nanorod arrays at pH of 2, 4.6, 6.7, 10.8, and 11.5. The initial pH of the solution for the precursor Zn(NO3)2·6 H2O and HMTA was 6.7. A honeycomb-like rod structure was observed; however, several rods grew in a single open space between the PS microspheres [Figure 2(d)]. When the pH was reduced to 4.6, a single ZnO nanorod grew from each space, and the rods were straight and well aligned [Figure 2(c)]. Moreover, the rod structure disappeared when pH was further reduced to 2 [Figure 2(b)] because ZnO was dissolved in an acidic solution environment. Excess HNO3 yielded more Zn2+ ions in the hydrothermal solution, which retarded the growth rate of ZnO. By contrast, as the pH increased beyond 6.7, the density of the ZnO nanorods increased and the morphology of ZnO was affected; the solution at a pH of 10.8 yielded nanorods of shorter length and longer diameter. Several nanorods were connected, resulting in the loss of the alignment [Figure 2(e)]. When pH reached 11.5, only a bowl-like structure was observed on the surface, and the seed layer was etched by the additional OH− ions from the aqueous NH3 [Figure 2(f)]. 1.2 Effect of precursor concentration on ZnO nanorods To synthesize well-aligned ZnO nanorods in spaces between the PS spheres, the concentrations of chemical reactants and the duration of reaction during the hydrothermal process must be considered. Figure 2(g–j) presents the FESEM images of ZnO nanorods synthesized at concentrations of 10, 25, 50, and 100 mM, respectively, of the aqueous solution that served as a precursor at 90 °C and a fixed pH of 4.6 for 1 h. The molar ratio of Zn(NO3)2·6 H2O to HMTA was maintained at 1:1 irrespective of the solution concentration. The results evidenced that the length of the ZnO nanorods was closely related and linearly proportional to the precursor concentration. A higher Zn2+ concentration yields longer nanorods [Figure 2(g–i)]. After the chemical reaction (1), synthesizing ZnO balances the chemical potential of the elevated Zn ions. Because the growth rate varies between a polar and nonpolar surface, ZnO nanorods tend to grow longer along crystallographic planes (0002) with increasing precursor concentration. Moreover, Figure 2(i) shows that more than one rod grew in one defined space when the precursor concentration reached 50 mM. Increasing the concentration of Zn ions in the solution was reported to increase the number of nuclei on the substrate and thus yield a high density of nanorods [42]. At a concentration of 100 mM, the nucleation sites were abundant; the rods merged and formed a continuous film. The FESEM images indicated that the optimal concentration of the precursor solution is 25 mM.

1.3 Influence of reaction time on ZnO nanorods Figures 2(k–n) present the plane-view FESEM images of ZnO nanorods at 0.5, 1, 2, and 4 h, respectively. The duration of growth influenced the diameter of the ZnO nanorods. When this duration was less than 1 h, only a web-like structure was visible on the surface [Figure 2(k)], indicating that the rods began to form on the nucleation sites but that the duration was insufficient for ZnO to grow into a rod. When the growth duration was increased to 1 h, a rod-like morphology was observed [Figure 2(l)]. As the duration of growth was increased from 1 to 4 h, the diameter of the rods increased from 200 to 600 nm. When the duration of growth exceeded 4 h, no further change in the ZnO diameter was observed, indicating that the precursor was exhausted after the hydrothermal process for 4 h. According to these results, the optimal conditions of synthesizing well-aligned ZnO nanorod arrays were determined. Figure 3(a) presents a well-aligned ZnO nanorod array grown at a pH of 4.6, precursor concentration of 25 mM, and duration of 1 h of hydrothermal growth. The density and length of the ZnO nanorods were 9.4 × 107 nanorods and 737.2 nm, respectively. Consequently, the additional surface area contributed by the ZnO nanorods was estimated to be approximately 2.62 times higher than that of the thin film structure, suggesting that the gas sensor properties of these nanorods increased and that they can serve as templates for advanced high-surface-area applications. 2. Characteristics of nanorods The crystal structure of the ZnO nanorod array was confirmed through XRD () measurements using synchrotron radiation [Figure 3(b)]. The XRD pattern of ZnO nanorods on the c-plane sapphire substrate exhibited three peaks at 34.64°, 41.94°, and 73.04°, which correspond to ZnO (0002), Al2O3 (0006), and ZnO (0004), respectively. No other signal was observed for any ZnO sample. This result indicated that the nanorods grew vertically on the substrate and preferentially orientated along the c-axis. Figure 3(c) presents the in-plane XRD pattern of both the AZO seed layer and the ZnO nanorods, which clearly depicts the formation of single ZnO nanorods on the defined spaces. Only one peak of (101̅0) was observed, which was in good agreement with the X-ray results showing well-aligned ZnO nanorods. The grain sizes of the AZO seed layer and ZnO nanorods were 9.7 and 66.7 nm, respectively, calculated using the Scherrer equation in the direction in-plane. The grains of the AZO seed layer served as nucleation sites for the growth of nanorods. In the FESEM images [Figure 2(k)], in the area within the circle, several embryo nanorods were observed at 0.5 h, indicating that the growth of nanorods was still controlled by the AZO seed layer. As the duration of growth increased, the embryo nanorods merged together to form a single

ZnO nanorod. The growth of the nanorod likely yielded a larger grain size than the AZO seed layer, and these two layers exhibited little correlation after a prolonged growth period. In this study, the unique crystallographic relationship between the seed layer and the c-plane sapphire was noteworthy because the seed layer was prepared by sputtering at room temperature, which is a rather simple method of deposition. A sputtered seed layer is expected to be polycrystalline with little crystallinity, as shown in Figure 3(b); however, the c-plane sapphire affected the preferred orientation of grains in the seed layer and thus influenced the effective alignment of axes a⃑ and ⃑b of the AZO unit cells in addition to the sapphire substrate. To further assess the orientation inplane of ZnO nanorod arrays, -scans were performed on ZnO (101̅1), AZO (101̅1), and c-plane sapphire (112̅3) [Figure 3(d)]. Previous studies have reported that ZnO materials exhibit domain matching epitaxy (DME) when the lattice mismatch is larger than 7%–8%, and the nanorods rotate 30° or 90° on the basal plane with respect to the sapphire substrate because of the large lattice mismatch of approximately 18% between ZnO and the sapphire substrate [31, 43]. Strain relaxation possibly generates the defects and dislocations. As previously mentioned, a thin buffer layer of various materials is required to relax the strain. However, in this study, the -scan results of ZnO nanorods revealed that the AZO seed layer and c-plane sapphire evidently exhibited a six-fold symmetry with a 60° interval, and the peaks of ZnO (101̅1) and AZO (101̅1) exhibited favorable alignment with the c-plane sapphire (112̅3). This result indicates that DME was not present and that the AZO seed layer served not only as a nucleation site for the growth of nanorods but also as the buffer layer that relaxed the strain between ZnO and sapphire substrate. The AZO layer doped with 2 wt% Al atoms could likely induce more defects and dislocations than those in GaN, MgO, or ZnO when deposited at room temperature through RF sputtering. These natural defects facilitate the relaxing of the strain between ZnO and the sapphire substrate without additional annealing. Based on these results, the epitaxial relation is as follows: [0001]ZnO ∥ [0001]AZO∥[0001]c-plane sapphire. The crystalline nature of the ZnO nanorod array was confirmed through PL measurements at nearly 298 K. The PL spectrum of the ZnO nanorod structure on the c-plane sapphire substrate exhibited only one emission line at 376.75 nm [Figure 4(a)]. The absence of other broad visible features indicates the lack of intrinsic defects such as vacancies of zinc (VZn) in the ZnO nanorods [44], that ZnO nanorods synthesized through the hydrothermal process have high crystallinity, and that a strong epitaxial relationship exists between ZnO, AZO, and the c-plane sapphire substrate because no defect-related emission was observed. The composition of the ZnO nanorod array was determined through XPS [Figure 4(b) and (c)]. The peaks at 1023.7 and 1046.9 eV were

contributed by Zn 2p3/2 and Zn 2p1/2, respectively [Figure 4(b)]. Both peaks are sharp and evidence the presence of Zn2+ ions. The O 1s spectrum is presented in Figure 4(c). After fitting the signal, a peak was observed near 531.3 eV, which corresponds to the Zn–O bond. A shoulder peak at approximately 532.6 eV was observed because of the chemisorption of oxygen on the surface [45].

Conclusion The density and morphology of ZnO nanorod arrays were controlled by adjusting pH, concentration of precursors, and duration of reaction for optimized growth conditions. XRD patterns demonstrated that ZnO nanorods, the AZO seed layer, and the c-plane sapphire have the preferred orientation <0002>, which indicate wellaligned ZnO nanorod arrays. The results of the -scan revealed a six-fold symmetry of the structure. The AZO seed layer served not only as a nucleation site for the growth of nanorods but also as a buffer layer that helped relax the strain between ZnO and the sapphire substrate. The unique crystallographic relation between the seed layer and cplane sapphire indicates that ZnO nanorods grew epitaxially at the initial stage. The formation of only one ZnO nanorod on a defined area was confirmed through XRD in the plane, which indicated that the growth of the nanorods yielded grain sizes larger than those of the AZO seed layer. The two layers exhibited little correlation after a prolonged period of growth. The PL spectrum exhibited only a UV line with no broad visible emission, indicating that the ZnO nanorods synthesized through the hydrothermal process have high crystallinity and a strong epitaxial relationship between ZnO, AZO, and the c-sapphire substrate. Thus, well-aligned and patterned ZnO nanorod arrays are suitable for use as templates for high-surface-area applications.

Acknowledgements Ministry of Science and Technology of Taiwan (104-2221-E-008-014-MY3) and National Synchrotron Radiation Research Center of Taiwan provided financial support.

References [1] S. Baruah, J. Dutta, Hydrothermal growth of ZnO nanostructures, Science and Technology of Advanced Materials, 10 (2016) 013001. [2] J.B. Baxter, A.M. Walker, K.v. Ommering, E.S. Aydil, Synthesis and characterization of ZnO nanowires and their integration into dye-sensitized solar cells,

Nanotechnology, 17 (2006) S304-S312. [3] N. Saito, H. Haneda, T. Sekiguchi, N. Ohashi, I. Sakaguchi, K. Koumoto, LowTemperature Fabrication of Light-Emitting Zinc Oxide Micropatterns Using SelfAssembled Monolayers Adv. Mater., 14 (2002). [4] K.Y. Ko, H. Kang, J. Park, B.-W. Min, H.S. Lee, S. Im, J.Y. Kang, J.-M. Myoung, J.-H. Jung, S.-H. Kim, H. Kim, ZnO homojunction core–shell nanorods ultraviolet photo-detecting diodes prepared by atomic layer deposition, Sensors and Actuators A: Physical, 210 (2014) 197-204. [5] A.P. D. Pullini, S. Zanin, and D. Busquets Mataix High Efficiency Electrodeposition of Large Scale ZnO Nanorod Arrays for Thin Transparent Electrodes, Journal of The Electrochemical Society, 159 (2012) E45-E51. [6] D. Tamvakos, S. Lepadatu, V.-A. Antohe, A. Tamvakos, P.M. Weaver, L. Piraux, M.G. Cain, D. Pullini, Piezoelectric properties of template-free electrochemically grown ZnO nanorod arrays, Applied Surface Science, 356 (2015) 1214-1220. [7] D.P. Alina Prun˘a, and David Busquets Mataix, Influence of Deposition Potential on Structure of ZnO Nanowires Synthesized in TrackEtched Membranes, Journal of The Electrochemical Society, 159 (2012) E92-E98. [8] O. Singh, N. Kohli, R.C. Singh, Precursor controlled morphology of zinc oxide and its sensing behaviour, Sensors and Actuators B: Chemical, 178 (2013) 149-154. [9] V.F. Rivera, F. Auras, P. Motto, S. Stassi, G. Canavese, E. Celasco, T. Bein, B. Onida, V. Cauda, Length-dependent charge generation from vertical arrays of high-aspect-ratio ZnO nanowires, Chemistry, 19 (2013) 14665-14674. [10] L. Spanhel, Colloidal ZnO nanostructures and functional coatings: A survey, Journal of Sol-Gel Science and Technology, 39 (2006) 7-24. [11] G.H. Weibing Wu, Shougang Cui, Ying Zhou, and Haitao Wu, Epitaxy of Vertical ZnO Nanorod Arrays on Highly (001)-Oriented ZnO Seed Monolayer by a Hydrothermal Route, Crystal Growth & Design, 8 (2008). [12] F.M. R. Ayouchi , D. Leinen , J.R. Ramos-Barrado Growth of pure ZnO thin films prepared by chemical spray pyrolysis on silicon, Journal of Crystal Growth 247 (2003) 497-504. [13] M. Guo, P. Diao, S. Cai, Hydrothermal growth of well-aligned ZnO nanorod arrays: Dependence of morphology and alignment ordering upon preparing conditions, Journal of Solid State Chemistry, 178 (2005) 1864-1873. [14] J.M. Downing, M.P. Ryan, M.A. McLachlan, Hydrothermal growth of ZnO nanorods: The role of KCl in controlling rod morphology, Thin Solid Films, 539 (2013) 18-22. [15] S.M. Michael H. Huang, Henning Feick, Haoquan Yan, Yiying Wu, Hannes Kind,

Eicke Weber, Richard Russo, Peidong Yang, Room-Temperature Ultraviolet Nanowire Nanolasers, SCIENCE, 292 (2001). [16] L. Miao, Y. Ieda, S. Tanemura, Y.G. Cao, M. Tanemura, Y. Hayashi, S. Toh, K. Kaneko, Synthesis, microstructure and photoluminescence of well-aligned ZnO nanorods on Si substrate, Science and Technology of Advanced Materials, 8 (2016) 443-447. [17] H.-G. Chen, Z.-W. Li, Seed-assisted growth of epitaxial ZnO nanorod arrays with self-organized periodicity and directional alignment, Applied Surface Science, 258 (2011) 556-564. [18] S. Xu, Y. Wei, M. Kirkham, J. Liu, W. Mai, D. Davidovic, R.L. Snyder, Z.L. Wang, Patterned Growth of Vertically Aligned ZnO Nanowire Arrays on Inorganic Substrates at Low Temperature without Catelyst, J. AM. CHEM. SOC. , 130 (2008) 14958–14959. [19] D.S. Kim, R. Ji, H.J. Fan, F. Bertram, R. Scholz, A. Dadgar, K. Nielsch, A. Krost, J. Christen, U. Gosele, M. Zacharias, Laser-interference lithography tailored for highly symmetrically arranged ZnO nanowire arrays, Small, 3 (2007) 76-80. [20] A. Kosiorek, W. Kandulski, P. Chudzinski, K. Kempa, M. Giersig, Shadow Nanosphere Lithography simulation and experiment, Nano Lett., 4 (2004). [21] X. Wang, C.J. Summers, Z.L. Wang, Large Scale Hexagonal-Patterned growth of aligned ZnO nanorods for nano-optoelectronics and nanosenseor arrays, Nano Lett., 4 (2004). [22] J.J. Dong, X.W. Zhang, Z.G. Yin, S.G. Zhang, J.X. Wang, H.R. Tan, Y. Gao, F.T. Si, H.L. Gao, Controllable growth of highly ordered ZnO nanorod arrays via inverted self-assembled monolayer template, ACS Appl Mater Interfaces, 3 (2011) 4388-4395. [23] D.F. Liu, Y.J. Xiang, X. C. Wu, Z.X. Zhang, L.F. Liu, L. Song, X.W. Zhao, S.D. Luo, W.J. Ma, J. Shen, W.Y. Zhou, G. Wang, C.Y. Wang, S.S. Xie, Periodic ZnO Nanorod Arrays Defined by Polystyrene Microsphere, Nano Lett, 6 (2006). [24] J. Zhang, Y. Li, X. Zhang, B. Yang, Colloidal self-assembly meets nanofabrication: from two-dimensional colloidal crystals to nanostructure arrays, Adv Mater, 22 (2010) 4249-4269. [25] M. Kondo, K. Shinozaki, L. Bergstroem, N. Mizutani, Preparation of Colloidal Monolayers of Alkoxylated Silica Particles at the Air-Liquid Interface, Langmuir, 11 (1995) 394–397. [26] J. Narayan, B.C. Larson, Domain epitaxy: A unified paradigm for thin film growth, Journal of Applied Physics, 93 (2003) 278-285. [27] Y.Y. Toshiyuki Nakamura , Takeshi Kusumori , Hideki Minouraa, Hachizo Muto, Improvement in the crystallinity of ZnO thin films by introduction of a buffer layer, Thin Solid Films 411 (2002) 60-64.

[28] O. Lupan, T. Pauporté, B. Viana, I.M. Tiginyanu, V.V. Ursaki, R. Cortès, Epitaxial Electrodeposition of ZnO Nanowire Arrays on p-GaN for Efficient UV-Light-Emitting Diode Fabrication, ACS Applied Materials & Interfaces, 2 (2010) 2083-2090. [29] Y. Chen, S.-k. Hong, H.-j. Ko, V. Kirshner, H. Wenisch, T. Yao, K. Inaba, Y. Segawa, Effects of an extremely thin buffer on heteroepitaxy with large lattice mismatch, Applied Physics Letters, 78 (2001) 3352-3354. [30] H.-M. Cheng, H.-C. Hsu, S. Yang, C.-Y. Wu, Y.-C. Lee, L.-J. Lin, W.-F. Hsieh, The substrate effect on the in-plane orientation of vertically well-aligned ZnO nanorods grown on ZnO buffer layers, Nanotechnology, 16 (2005) 2882-2886. [31] D.-K.H. Kyu-Hyun Bang, Jae-Min Myoung, Effects of ZnO buffer layer thickness on properties of ZnO thin films deposited by radio-frequency magnetron sputtering, Applied Surface Science, 207 (2003) 359-364. [32] Y. Chen, D.M. Bagnall, H.-j. Koh, K.-t. Park, K. Hiraga, Z. Zhu, T. Yao, Plasma assisted molecular beam epitaxy of ZnO on c -plane sapphire: Growth and characterization, Journal of Applied Physics, 84 (1998) 3912-3918. [33] J.A.Z. Ting Fung Chung, and Shuit-Tong Lee, Luminescent Properties of ZnO Nanorod Arrays Grown on AlZnO Buffer Layer, J. Phys. Chem. C, 112 (2008) 820-824. [34] K. Byrappa, T. Adschiri, Hydrothermal technology for nanotechnology, Progress in Crystal Growth and Characterization of Materials, 53 (2007) 117-166. [35] G. Amin, M.H. Asif, A. Zainelabdin, S. Zaman, O. Nur, M. Willander, Influence of pH, Precursor Concentration, Growth Time, and Temperature on the Morphology of ZnO Nanostructures Grown by the Hydrothermal Method, Journal of Nanomaterials, 2011 (2011) 1-9. [36] Z. Li, X. Huang, J. Liu, Y. Li, G. Li, Morphology control and transition of ZnO nanorod arrays by a simple hydrothermal method, Materials Letters, 62 (2008) 15031506. [37] L.L. Yang, Q.X. Zhao, M. Willander, Size-controlled growth of well-aligned ZnO nanorod arrays with two-step chemical bath deposition method, Journal of Alloys and Compounds, 469 (2009) 623-629. [38] A. Sugunan, H.C. Warad, M. Boman, J. Dutta, Zinc oxide nanowires in chemical bath on seeded substrates: Role of hexamine, Journal of Sol-Gel Science and Technology, 39 (2006) 49-56. [39] R. Wahab, S.G. Ansari, Y.S. Kim, M. Song, H.-S. Shin, The role of pH variation on the growth of zinc oxide nanostructures, Applied Surface Science, 255 (2009) 48914896. [40] O. Singh, M.P. Singh, N. Kohli, R.C. Singh, Effect of pH on the morphology and gas sensing properties of ZnO nanostructures, Sensors and Actuators B: Chemical, 166167 (2012) 438-443.

[41] M.N.R. Ashfold, R.P. Doherty, N.G. Ndifor-Angwafor, D.J. Riley, Y. Sun, The kinetics of the hydrothermal growth of ZnO nanostructures, Thin Solid Films, 515 (2007) 8679-8683. [42] S. Xu, C. Lao, B. Weintraub, Z.L. Wang, Density-controlled growth of aligned ZnO nanowire arrays by seedless chemical approach on smooth surfaces, Journal of Materials Research, 23 (2011) 2072-2077. [43] P. Fons, K. Iwata, S. Niki, A. Yamada, K. Matsubara, Growth of high-quality epitaxial ZnOlms on a-Al 2 O 3, Journal of Crystal Growth, 201/202 (1999) 627-632. [44] K.H. Tam, C.K. Cheung, Y.H. Leung, A.B. Djurisic, C.C. Ling, C.D. Beling, S. Fung, W.M. Kwok, W.K. Chan, D.L. Phillips, L. Ding, W.K. Ge, Defects in ZnO Nanorods Prepared by a Hydrothermal Method, J. Phys. Chem. B, 110 (2006) 2086520871. [45] G.X. Congkang Xu, Yingkai Liu, Guanghou Wang, A simple and novel route for the preparation of ZnO nanorods, solid state communications, 122 (2002) 175-179.

Figure 1. (The fabrication of ZnO nanorod arrays.)

Figure 2. ((a) The 762 nm PS microsphere array, (b-f) the FESEM images at 45o view of ZnO nanorod arrays at pH 2, 4.6, 6.7, 10.8 and 11.5, (g-j) the FESEM images of ZnO nanorods grown at concentrations 10, 25, 50 and 100 mM, (k-n) the plane view FESEM images of ZnO nanorods at 0.5, 1, 2 and 4 h)

Figure 3. ((a) the well aligned ZnO nanorod array at pH 4.6, precursor concentration 25 mM and duration 1 h of hydrothermal growth, (b) the XRD pattern of ZnO nanorods on c-plane sapphire, 3(c) the XRD pattern in plane of both the AZO seed layer and the ZnO nanorod, (d) the XRD -scan,)

Figure 4. ((a) the PL spectrum of the ZnO nanorod. The XPS analysis of ZnO nanorod (b) Zn region, (c) O region)