controllable hydrothermal synthesis of zno nanowires arrays on al-doped zno

Applied Surface Science 257 (2011) 10134– 10140 Contents lists available at ScienceDirect Applied Surface Science jou rn al h om epa g e: www.elsevier.com/locate/apsusc Controllable hydrothermal synthesis of ZnO nanowires arrays on Al-doped ZnO seed layer and patterning of ZnO nanowires arrays via surface modification of substrate Jin Zhang a , Wenxiu Que a,∗ , Qiaoying Jia a , Xiangdong Ye b , Yucheng Ding b a Electronic Materials Research Laboratory, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, People’s Republic of China b State Key Laboratory of Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, Shaanxi, People’s Republic of China a r t i c l e i n f o Article history: Received 11 December 2010 Received in revised form 7 June 2011 Accepted 10 June 2011 Available online 7 July 2011 Keywords: Zinc oxide Nanowires Seed layer Fluorination Photoluminescence a b s t r a c t ZnO nanowire (NW) arrays are assembled on the Al-doped ZnO (AZO) seed layer by a hydrothermal process. Effects of the temperature and growth time of the hydrothermal process on morphological and photoluminescence properties of the as-assembled ZnO NW arrays are characterized and studied. Results indicate that the length and diameter of the ZnO NWs increase with a lengthening of the growth time at 80 ◦ C and the hydrothermal temperature has a significant effect on the growth rate and the photo- luminescence properties of the ZnO NW arrays. The patterned AZO seed layer is fabricated on a silicon substrate by combining a sol–gel process with an electron-beam lithography process, as well as a surface fluorination technique, and then the ZnO NW arrays are selectively grown on those patterned regions of the AZO seed layer by the hydrothermal process. Room-temperature photoluminescence spectra of the patterned ZnO NW arrays shows that only a strong UV emission at about 380 nm is observed, which implies that few crystal defects exist inside the as-grown ZnO NW arrays. © 2011 Elsevier B.V. All rights reserved. 1. Introduction ZnO is a semiconductor with exceptional electronic and pho- tonic properties as well as great thermal stability and oxidation resistance. Recent developments and capacity to synthesize ZnO nanostructures with different shapes [1–3] have led to novel and enhanced properties as compared to its bulk form, and thus enabling it to have many attractive applications. For example, it can be used as a potential material for nanodevice assembly and applications in blue-UV light emitters [4] and photodetectors [5], field emission devices [6], and dye-sensitized solar cells [7], etc. Indeed, well-aligned ZnO nanowire (NW) arrays have been formed on GaN, AlN, Al 1−x Ga x N, 6H–SiC, and ZnO buffer layers [8–10], but the optical properties of the NWs grown on buffer layers have been scarcely investigated, especially with respect to impurity and defect distribution, which can hinder the applications of the NW arrays. In recent years, a number of the ZnO thin films doped with various metallic ions have been extensively studied for the manipulation of their optical and electrical properties, the Al-doped ZnO (AZO) thin films are attractive due to their good conductivity, high trans- ∗ Corresponding author. Tel.: +86 29 82668679; fax: +86 29 82668794. E-mail address: wxque@mail.xjtu.edu.cn (W. Que). parency and relatively low cost [11,12]. In view of this, the use of a lattice-matched and conducting buffer layer may circumvent the problem and lead to potential integration with silicon micro- electronics [13–15]. Therefore, the luminescent and electron field emission properties of the ZnO NW arrays grown on the AZO seed layers are also reported by many research groups [16,17]. Further- more, in order to achieve an immense potential of the ZnO NW arrays, it is important and necessary to have a good control for the spatial arrangements and properties of the ZnO NW arrays [16]. In this paper, the ZnO NW arrays were grown on AZO seed layer, which was deposited by a sol–gel process, by a hydrother- mal method, and effects of the temperature and growth time of the hydrothermal process on the morphological and photolumines- cence properties of the as-grown ZnO NW arrays were also studied and discussed. In addition, what we believe to be the first report on the fabrication of the patterned AZO seed layer on the silicon substrate by combining a sol–gel process with an electron-beam lithography process, as well as a surface fluorination technique, which can eliminate the effect of the electron-beam resist on the boundary of the patterned AZO seed layer, and thus the ZnO NW arrays could be successfully grown on the patterned regions of the AZO seed layer by employing the hydrothermal process. Further- more, the photoluminescene properties of the selectively grown ZnO NW arrays were also characterized and investigated. 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.06.163 J. Zhang et al. / Applied Surface Science 257 (2011) 10134– 10140 10135 Fig. 1. Schematic representation of the patterning process of the ZnO NW arrays on silicon substrate. 2. Experimental 2.1. Preparation of the AZO seed layer In order to compare the properties of the ZnO NW arrays grown on different seed layers, the ZnO and AZO seed layers were pre- pared by the sol–gel technique. Here, the Al-doped concentration of the AZO seed layer was 1.0 at.% with respect to Zn due to its relatively outstanding performance than other doping concentra- tions as shown in our previous report [18]. The ZnO and AZO sols were prepared as follows. In briefly, Zn(CH 3 COO) 2 ·2H 2 O was first dissolved in a 2-methoxyethanol monoethanolamine (MEA)- deionized water solution at room temperature. The molar ratio of the MEA and deionized water to zinc acetate was fixed at 1 and 0.5, respectively, and the concentration of the zinc acetate was 0.75 mol/L. For the AZO sol, an appropriate amount of alu- minum doping was obtained by adding AlCl 3 ·6H 2 O to above the as-prepared precursor solution. Then, the final solution was stirred at 60 ◦ C for 30 min until to yield a clear and homogeneous solution. The ZnO and AZO seed layers were deposited onto a quartz glass substrate by a multi-spin-coating process for 20 s at 3000 rpm. It should be mentioned that after spin-coating one layer, the coated sample should be preheated in the air at 200 ◦ C for 10 min and thus the one-layer thin film with about 50 nm thick can be eas- ily obtained. Finally, the sample with three layers was post-heated at a temperature of 500 ◦ C for 1 h in the air [19]. 2.2. Hydrothermal synthesis of the ZnO NWs on the ZnO and AZO seed layers Vertically aligned ZnO NW arrays were grown in a Teflon-lined stainless steel autoclave by immersing the substrates deposited with the ZnO or AZO seed layers into the mixed aqueous solu- tion, which includes Zn(NO 3 ) 2 (0.04 mol/L) and NaOH (0.8 mol/L), at 80–180 ◦ C for 1–3 h. The obtained samples were then washed by the deionized water and dried in the air at 80 ◦ C [20]. 2.3. Fabrication of the patterned ZnO NW arrays on silicon substrate Fig. 1 shows the fabrication process of the patterned ZnO NW arrays on the silicon substrate. Here, the electron-beam resist (ZEP520A from Zeon Corp.) was first spin-coated on the silicon sub- strate and followed the coated sample was put in an oven to prebake at 180 ◦ C for 30 min. Then, the prebaked sample was exposed for patterning at 30 kV under a high-resolution electron-beam lithog- raphy system (CABL-9000C Crestec Corp.). Subsequently, the silicon substrate with the patterned resist was immerged and rinsed in a ZMD-B (Methyl isobutyl ketone 89% and Isopropyl alcohol 11%) solution for 1 min to remove the exposed EB-resist. To make low surface energy coatings on the substrate, the silicon substrate with patterned resist was immerged into the solution, which consists of 2.0 vol.% (Heptadecafluoro-1,1,2,2-tetradecyl) trimethoxysilane (SC-1060F, from Sicong New Materials Corp.), 0.5 vol.% acetic acid and 97.5 vol.% isopropyl alcohol, for 2 h and then picked out. Fol- lowed that the immerged sample was heated at 150 ◦ C in an oven for 1 h and cooled down to room temperature, the residual resist was then removed from the silicon substrate by rinsing it with chlorobenzene, thus, the template was obtained. Finally, the AZO sol was spin-coated on the patterned silicon substrate and the ZnO NW arrays were selectively grown on the patterned regions of the AZO seed layer by the hydrothermal process. The structural properties of the ZnO NW arrays were char- acterized by using a D/max-2400 X-ray diffraction spectrometer (Rigaku) with Cu K␣ radiation and operated at 40 kV and 100 mA from 20 to 70 ◦ in 2Â, and the scanning speed was 15 ◦ min −1 at a step of 0.02 ◦ . The morphological properties of the ZnO NW arrays were observed by a JEOL JSM-7000F field-emission scanning electron 10136 J. Zhang et al. / Applied Surface Science 257 (2011) 10134– 10140 Fig. 2. (a) SEM image of the ZnO seed layer, (b) SEM image of the AZO seed layer. microscopy (FE-SEM). The UV–vis absorption spectra and transmit- tance spectra of the ZnO NW array films were characterized by a JASCO V-570 UV/VIS/NIR spectrometer and the photoluminescence spectra of the ZnO NW arrays were measured at room temperature by a FLUOROMAX-4 spectrometer. 3. Results and discussion SEM images and XRD patterns of both the ZnO and AZO seed layer, which are deposited on the quartz glass substrate, are showed in Figs. 2 and 3, respectively. It can be observed from Fig. 2 that the AZO seed layer has a smaller grain size as compared to that of the ZnO seed layer, and the (0 0 2) diffraction peak of the AZO seed layer in intensity is higher than that of the ZnO seed layer as seen in Fig. 3. It is indicated that the AZO seed layer has a better crystalline orientation (0 0 2) than that of the ZnO seed layer, which coincides with those reported in our previous work [18]. Fig. 4(a) shows the transmittance spectra of the ZnO and AZO seed layers as well as the corresponding ZnO NW arrays grown on these seed layers. Results indicate that all the ZnO and AZO thin films exhibit a transmittance of higher than 85% in the visible region. However, it is worthy to note that the transmittance of the AZO thin film layer is obviously higher (90%) than that of the ZnO thin film layer, which is probably related to the optimized crystalline orientation of the (0 0 2) and the seed layer with smaller grain size. Furthermore, the transmittance of the ZnO NW arrays, which are grown on the ZnO 20 30 40 50 60 70 Intensity (a.u.) 2 Th eta / de gree ZnO AZO (002) Fig. 3. XRD patterns of the ZnO and AZO seed layers. or AZO seed layer at 80 ◦ C for 1 h, is still above 40% in the visible region. In addition, the transmittance of the ZnO NW arrays grown on the AZO seed layer is lower than that grown on ZnO seed layer owing to its high light scattering and decrease light transmittance [18]. Fig. 4(b) shows the absorption spectra of the obtained samples. It is found that the absorption peak of the AZO thin film layer has a blue shift as compared to that of the ZnO thin film layer due to the Burstein–Moss effect [21,22]. 300 400 500 600 700 800 30 40 50 60 70 80 90 100 (a) (b) Trans (%) Wavelength (nm) 1 2 3 4 5 325 350 375 400 425 0.0 0.1 0.2 0.3 0.4 0.5 0.6 ZnO AZO Abs (a.u.) Wavelength / nm Fig. 4. (a) Transmittance spectra of the quartz glass (curve 1), AZO thin film (curve 2), ZnO thin film (curve 3), ZnO NW arrays grown on the ZnO seed layer at 80 ◦ C for 1 h (curve 4) and ZnO NW arrays grown on the AZO seed layer at 80 ◦ C for 1 h (curve 5). (b) Absorption spectra of the ZnO and AZO thin films. J. Zhang et al. / Applied Surface Science 257 (2011) 10134– 10140 10137 Fig. 5. SEM images of the ZnO NWs grown on the ZnO and AZO seed layers: (a), (b) and (c) are SEM images of the ZnO NWs grown on the ZnO seed layer at 80 ◦ C for 1 h, 2 h and 3 h, respectively, (d), (e) and (f) are SEM images of the ZnO NWs grown on the AZO seed layer at 80 ◦ C for 1 h, 2 h and 3 h, respectively, (g) and (h) are SEM images of the ZnO NWs grown on the ZnO seed layer for 1 h at 130 and 180 ◦ C, respectively, (i) and (j) are SEM images of the ZnO NWs grown on the AZO seed layer for 1 h at 130 and 180 ◦ C, respectively. Fig. 5 shows the SEM images of the ZnO NW arrays grown on the ZnO and AZO seed layer at 80–180 ◦ C for 1–3 h, respectively. The insets as shown in Fig. 5 are the cross-section of the ZnO NWs arrays. Fig. 6 shows that the TEM images and the selected area electron diffraction (SAED) pattern of the ZnO NWs which grown on the AZO seed layer at 80 ◦ C for 1 h. Fig. 6(a) is a typical low- magnification image of the synthesized ZnO NW. The diameter of the tip is slightly smaller than the bottom. The atomic arrangements 10138 J. Zhang et al. / Applied Surface Science 257 (2011) 10134– 10140 Fig. 6. TEM images of the ZnO NWs grown on the AZO seed layer (a) low-magnification image, (b) high-magnification image, (c) corresponding selected area electron diffraction pattern (SAED). of the ZnO NW are seen in Fig. 6(b). It clearly shows the ZnO (0 0 2) fringes perpendicular to the wire axis are on average separated by 0.26 nm, indicating the crystalline ZnO NWs growth along the ZnO (0 0 2) direction. Also, the diffraction pattern confirms that the ZnO NWs have a single crystalline growth along ZnO (0 0 2) as shown in Fig. 6(c). In addition, the similar results are also observed for the ZnO NWs grown on the ZnO seed layer at 80 ◦ C for 1 h. The values of the length and diameter of the ZnO NWs, which is obtained from Fig. 5 are summarized in Fig. 7. Corresponding to the different seed layers (ZnO, AZO), the ZnO NWs grown on the ZnO seed layer are labeled as ZnO-NWZ, and the ZnO NWs grown on the AZO seed layer are labeled as ZnO-NWA. It can be seen from Fig. 5 that all the ZnO NW arrays obtained under different hydrothermal conditions are vertically aligned on the seed layers. For the hydrothermal tem- perature at 80 ◦ C and the growth time between 1 and 3 h, the length and diameter of the ZnO-NWZ and the ZnO-NWA increase with a lengthening of the growth time as shown in Fig. 7. However, it is also interesting to note for the same growth time that the length of the ZnO-NWA is much longer that of the ZnO-NWZ and the diam- eter of the ZnO-NWA is much smaller than that of the ZnO-NWZ. These results are probably related to the crystal grain size of the seed layer, that is to say, the bigger the crystal grain size is, the shorter and wider the grown ZnO NW is as shown in Fig. 5(a)–(f). In other words, the aspect ratio of the ZnO-NWA is higher than that of the ZnO-NWZ. Moreover, the distance among the ZnO-NWA is bigger than that among the ZnO-NWZ. That is because the distance among the ZnO NWs is also dependent on the crystal interspaces and the crystal grain size of the seed layer. As can be seen in Fig. 2, the quantity of the crystal grains within the unit area of the AZO seed layer is more than that of the ZnO seed layer, which leads to a larger number of the ZnO NWs can be grown on the AZO seed layer as shown in Fig. 5. Furthermore, it is also observed for the same growth time (1 h) but different hydrothermal temperatures that the length of the ZnO-NWZ increases and the diameter of the ZnO-NWZ enlarge extremely with the increase of the hydrothermal tempera- ture as compared to that of the ZnO-NWA. A reasonable explanation is that the small crystal grain of the AZO seed layer restricts the cross-growth of the ZnO NWs. The length of the ZnO-NWA also increases with the raising of the hydrothermal temperature from 80 ◦ C to 130 ◦ C, but when the hydrothermal temperature is further up to 180 ◦ C, the length of the ZnO-NWA is shorter than that of the ZnO-NWA grown at 130 ◦ C. A possible explanation is as follows. As reported in Refs. [23,24], the hydrothermal synthesis of the ZnO NWs is a dynamic balance process as follows. [Zn(OH) n ] n−2− → ZnO + H 2 O + [OH] − , (n = 2, 4) (1) ZnO + 2[OH] − → [ZnO 2 ] 2− + H 2 O (2) Thus, the [Zn(OH) n ] n−2− groups dehydrate at the surface of the ZnO seed layer to form the ZnO molecules, H 2 O molecules and [OH] − , and the formed [OH] − dissolves the ZnO molecules to form [ZnO 2 ] 2− groups at the same time. While the supersaturation of the [Zn(OH) n ] n−2− groups is high enough, the growth rate of the ZnO NWs will be much higher than the dissolution rate. While the hydrothermal temperature increased to 130 ◦ C, the supersatura- tion and the molecule energy of the [Zn(OH) n ] n−2− groups achieve the best values, which leads to a fast growth rate of the ZnO NWs. While the hydrothermal temperature further increases up 1 2 3 4 5 1 2 3 (a) (b) Length of th e Zn O NWs / µm Growth time / hour 80 100 120 140 160 180 Temperature of chemical bath / o C 0 50 100 150 200 250 300 350 1 2 3 Diameter of th e Zn O NWs / n m Growth time / hour 80 100 120 140 160 180 Temperature of chemical bath / o C Fig. 7. Effect of the hydrothermal growth time and temperature on the length and diameter of the ZnO NWs: (a) a relationship between the length of the ZnO NWs and the hydrothermal growth time and temperature; (b) a relationship between the diameter of the ZnO NWs and the hydrothermal growth time and temperature. J. Zhang et al. / Applied Surface Science 257 (2011) 10134– 10140 10139 to 180 ◦ C, the growth rate of the ZnO NWs decreases due to the decrease of the supersaturation of the [Zn(OH) n ] n−2− groups. It can be concluded based on above results and discussion that the growth rate of the ZnO NWs is determined by the supersaturation of the [Zn(OH) n ] n−2− groups and the concentration of the [OH] − in the solution and the fastest growth rate of the ZnO-NWA can be obtained at 130 ◦ C. Due to the effect of the Al-doping on the seed layer, the ZnO-NWA has a higher aspect ratio than that of the ZnO- NWZ, which is more suitable for those potential applications in the dye-sensitized solar cells, luminescent and electron field emission devices. Fig. 8 shows room temperature photoluminescence (PL) spectra (excite at 365 nm, 1 nm slit width) of the ZnO NW arrays grown on the AZO seed layers at different temperatures of 80 ◦ C, 130 ◦ C and 180 ◦ C for 1 h, respectively. The inset shows the partial enlarged PL spectra from 390 to 420 nm. The intensities of these PL spectra are also normalized by the thickness of the ZnO NW array films. It can be seen that with the increase the hydrothermal temperature, the position of the peaks occurred red-shift and the intensity of the peaks decreases gradually. It is probably related to an increase of the crystal defects in the ZnO NW arrays due to higher growth tem- perature [25–27]. When the ZnO NWs grow at 80 ◦ C, a sharp and strong UV peak at 380 nm, which can be assigned to the intrinsic excitation of ZnO, dominates the PL spectra and no other peaks are observed in the spectrum curve, indicating that few crystal defects exist in the ZnO NW arrays grown on the AZO seed layer at 80 ◦ C for 1 h. It should be mentioned here that the similar results are also observed for the ZnO NW arrays grown on the AZO seed layer at 80 ◦ C for longer growth time. However, with the increase the hydrothermal temperature to 130 ◦ C or above, a relative weak band 380 390 400 410 420 430 440 450 0.0 5.0x10 5 1.0x10 6 1.5x10 6 2.0x10 6 Normalized Intensity (a.u.) Wavelength / nm ZnO NWs gro wn at 80 o C for 1h ZnO NWs gro wn at 130 o C for 1h ZnO NWs gro wn at 180 o C for 1h 390 400 410 420 Intensity (a.u.) Wavelength / nm Fig. 8. Normalized PL spectra of the ZnO NW arrays grown on the AZO seed layer for 1 h at 80, 130 and 180 ◦ C, respectively. between 390 and 420 nm can be clearly observed as shown in the inset of Fig. 7, indicating that some crystal defects start to occur due to higher growth temperature, which may lead to the red-shift of the PL peaks and the decrease of the PL peaks in intensity. That is to say, with the increase the hydrothermal temperature, the dissolu- tion rate of the [OH] − will be intensified on the surface of the ZnO NWs, and it is probably to lead to more crystal defects in the ZnO NWs due to the corrosiveness of the aqueous solution. As shown in Fig. 1, the silicon substrate is first patterned with EB resist and EB exposal, then the low surface energy coating over the outside of the patterned region is derived by the surface fluorizated process on the silicon substrate by using fluoric organic solvents. Fig. 9. SEM images of the patterned ZnO NW arrays (a) top-view of the ZnO NW arrays, (b) line width of 1 ␮m, (c) line width of 500 nm, (d) line width of 200 nm, (e) line width of 100 nm and (f) line width of 50 nm, respectively. 10140 J. Zhang et al. / Applied Surface Science 257 (2011) 10134– 10140 380 390 400 410 420 430 440 450 0.0 2.0x10 5 4.0x10 5 6.0x10 5 8.0x10 5 1.0x10 6 1.2x10 6 Intensity (a.u.) Wavelength / nm 390 40 041 042 0 Intensity (a.u.) Wavelength / nm Fig. 10. PL spectra of the patterned ZnO NW arrays grown on the AZO seed layer at 80 ◦ C for 1 h. It should be mentioned here that this kind of surface fluorination technique is a universal method for patterning sol–gel thin films. Thus, when the AZO sol is spin-coated on the silicon substrate, the AZO sol cannot be deposited on the low surface energy region due to its low adhesion, but the AZO sol can be firmly deposited on those patterned regions. In addition to, this process can eliminate the effect of the resist on the boundary of the patterned AZO seed layer. In order to achieve a good photoluminescence property, the patterned ZnO NW arrays are only grown at 80 ◦ C for 1 h. Images of the patterned ZnO NW arrays are shown in Fig. 9. It can be seen that the ZnO NW arrays can be selectively and sharply grown on the patterned regions of the AZO seed layer. Fig. 9(a) shows the patterned ZnO NW arrays at a large feature size area. Fig. 9(b)–(g) show the as-grown ZnO NW arrays on the patterned regions with a width of 1 ␮m, 500 nm, 200 nm, 100 nm, and 50 nm, respectively. It can be clearly observed from Fig. 9 that vertically aligned ZnO NW arrays can be easily grown on the patterned regions of the AZO seed layer. The as-grown ZnO NW arrays show an acicular morphology, and the average length and diameter of the ZnO NWs are around 1 ␮m and 50 nm, respectively. Fig. 10 shows the room temperature PL spectrum (excite at 365 nm, 1 nm slit width) of the patterned ZnO NW arrays grown on the AZO seed layers. It is found that only a sharp and strong UV peak at 380 nm dominates the PL spectrum. The inset shows the partial enlarged PL spectrum from 390 to 420 nm, and no other peaks are observed in the curve. These results indicate that there are few crystal defects to exist in the patterned ZnO NW arrays grown on the sol–gel derived AZO seed layer at the hydrothermal temperature of 80 ◦ C for 1 h. 4. Conclusions The vertically aligned ZnO NW arrays have been successfully grown on the AZO seed layer by the hydrothermal method. Effects of the hydrothermal parameters on the morphological and pho- toluminescence properties of the ZnO NW arrays have been also studied. Results indicate that the fastest growth rate of the ZnO- NWA can be obtained at 130 ◦ C and the ZnO-NWA has the higher aspect ratio than that of the ZnO-NWZ due to the effect of the Al- doping on the seed layer. Furthermore, the patterned ZnO-NWA arrays with strong PL emission and few crystal defects have been obtained by combining the sol–gel process with the electron-beam lithography process, as well as the surface fluorination technique, which is probably suitable for the applications in the luminescent and electron field emission devices. Acknowledgments This work was supported by the Ministry of Science and Tech- nology of China through 863-project under grant 2009AA03Z218, the Major Program of the National Natural Science Foundation of China under grant no. 90923012, and Xi’an Applied Materials Inno- vation Fund (XA-AM-200909). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apsusc.2011.06.163. References [1] Z.W. Pan, Z.R. Dai, Z.L. Wang, Nanobelts of semiconducting oxides , Science 291 (2001) 1947–1949. [2] M.H. Huang, Y.Y. Wu, H. Feick, N. Tran, E. Weber, P. 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Chem. 45 (2006) 3256–3260. . hydrothermal synthesis of ZnO nanowires arrays on Al-doped ZnO seed layer and patterning of ZnO nanowires arrays via surface modification of substrate Jin Zhang a , . W.X. Que, Preparation and characterization of sol–gel Al-doped ZnO thin films and ZnO nanowire arrays grown on Al-doped ZnO seed layer by hydrothermal method . air [19]. 2.2. Hydrothermal synthesis of the ZnO NWs on the ZnO and AZO seed layers Vertically aligned ZnO NW arrays were grown in a Te on- lined stainless