Selective patterned growth of ZnO nanowires/nanosheets and their photoluminescence properties

Zinc oxide (ZnO) demonstrates promising applications in photocatalysis, photodetectors and light-emitting devices. The shape of the ZnO nanoparticles plays an important role in determining their mechanical and optical properties. Here we report facile and controllable synthesis of ZnO nanowires and nanosheets with hydrothermal synthesis on patterned aluminum substrates prepared by colloidal lithography. The nanowire areas and nanosheet areas were well-defined in micron scale and they displayed dramatically different photoluminescence properties: while nanowires showed strong band-edge emission at 382 nm with a moderate orange emission at 600 nm, nanosheets only showed broad and intense green emission in the 547 nm range. The method demonstrated here enables periodic ZnO nanowire/nanosheet patterning with well-controlled morphology and luminescence property, which would provide a large-scale facile fabrication technique to tailor the substrate’s optical properties by using ZnO as the only constituent oxide component. ©2015 Optical Society of America OCIS codes: (160.4670) Optical materials; (220.4241) Nanostructure fabrication; (250.5230) Photoluminescence. References and links 1. T. D. Dao, C. T. T. Dang, G. Han, C. V. Hoang, W. Yi, V. Narayanamurti, and T. Nagao, “Chemically synthesized nanowire TiO2/ZnO core-shell p-n junction array for high sensitivity ultraviolet photodetector,” Appl. Phys. Lett. 103(19), 193119 (2013). 2. L. T. Singh, R. P. Sugavaneshwar, and K. K. Nanda, “Carbon nanotube-ZnO nanowire hybrid architectures as multifunctional devices,” AIP Advances 3(8), 082106 (2013). 3. S. L. Shinde and K. K. 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Introduction
ZnO is an attractive semiconductor oxide with direct band-gap of 3.3 eV and large exciton binding energy of 60 meV. Many promising applications for ZnO have been proposed as UV photodetectors [1,2], sensors [3][4][5], photocatalytic materials [6][7][8], piezoelectric devices [9,10], and light-emitting devices [11,12]. In particular, low-dimensional ZnO nanoparticles have received considerable attentions due to their large surface-to-volume ratios. The optical, electrical and mechanical properties of ZnO nanoparticles are highly dependent on their dimensionality, shape and surface morphology. A great variety of ZnO nanoparticles with different morphologies, including nanowire, nanotube and nanoplate, have been fabricated [13][14][15][16]. A large number of studies have been devoted to their optical property since it can be varied dramatically through the incorporation of surface defects which are largely affected by the crystal growth procedures. To control the crystal growth, methods like template-assisted or surfactant-mediated growth are often adopted for tuning the morphologies in physical growth methods such as molecular beam epitaxy and vapor-liquid-solid phase (VLS) growth. But few studies have been reported so far on wet chemical synthesis combined with templateassisted growth [17,18]. In particular, hydrothermal synthesis of ZnO nanostructures has seen rising popularity in recent years as it is a low-cost and environment-friendly wet chemical technique. Combination with selective growth would give rise to big advantage for controlling the morphologies as well as tailoring the optical and mechanical properties of the ZnO nanostructures.
Besides the particle shapes, it is also of great importance to be able to fabricate large-scale periodic patterns of the ZnO nanostructures adaptable for a range of applications [19,20]. Colloidal lithography (it is also generally referred as nanosphere lithography) [21][22][23], which provides outstanding control to create large-area periodic patterns, has been used to create ZnO nanorod/nanopillar array [24][25][26], where only one type of nanostructure was fabricated. Herein, combining hydrothermal synthesis with colloidal lithography, we demonstrate nanopatterning of periodic ZnO nanowire/nanosheet hybrid nanostructures. Al nanohole array was fabricated by colloidal lithography on indium tin oxide (ITO) substrates and subsequently ZnO nanowires grew inside the holes while nanosheets exclusively grew on the Al-covered area in the same growth solution. The nanostructures were characterized by photoluminescence (PL), energy-dispersive x-ray spectroscopy (EDS) and TEM. The method presented here can be applied in a number of applications, from widening the active spectral region of ZnO-based light-emitting devices to down-conversion for photovoltaics and photocatalyst applications.

ZnO nanoparticle synthesis
The ZnO nanowire/nanosheet array was synthesized on ITO glass substrates by hydrothermal method combined with colloidal lithography as illustrated in Fig. 1. The ITO glass substrates were cleaned using acetone, ethanol and de-ionized water (5 min sonication for each cleaning step). Mixed solution of zinc acetate dihydrate [Zn(C 2 H 3 O 2 ) 2 ·2H 2 O] (0.3 M) and monoethanolamine (MEA) solution (0.3 M) in ethanol was spin-coated onto the substrates three times at 3000 rpm for 30s. A ZnO seed layer was then formed on the ITO glass substrates. The seed layer was dried at room temperature and annealed in ambient air at 300 °C for one hour.
Colloidal lithography was then performed on the substrates. Polystyrene spheres (PS) with diameter of 4.4 µm were chosen as the original lithography masks to create the patterned Al nanohole arrays (We use the term "nanohole" because the thickness of the nanohole is 50 nm even though its lateral dimension is in μm range). Firstly, a monolayer of PS spheres was prepared on top of the ZnO seeds/ITO glass substrates. To shrink the PS spheres, reactive ion etching (RIE) was employed using O 2 gas with an etching rate of 3.1 nm/second. The designed lithography masks with different sizes were obtained by controlling the etching time. Then an Al film of 50 nm was deposited onto the substrates using electron beam deposition. The patterned ZnO seed layer was finally achieved by removing the etched PS arrays.

Characterization
Photoluminescence spectra of the samples were collected using a micro PL system (Horiba LabRam HR) with 325 nm He-Cd laser as the excitation source. SEM imaging and EDS were carried out inside a Hitachi FE-SEM (SU8000). High resolution TEM (HRTEM) imaging was done in a JEOL TEM (JEM 2100F) working at 200 kV.  Figure 3(a) shows the representative photoluminescence spectra of the synthesized nanowires and nanosheets from Fig. 2. The ZnO nanowires exhibit the band-edge emission at 382 nm together with a broad emission band centered around 600 nm, which is attributed to the oxygen defects in the ZnO nanowires as in earlier reports [27][28][29]. By contrast, the ZnO nanosheets show dramatically different PL spectra with a broad green band centered at 547 nm. Furthermore, no band-edge emission is observed from the nanosheets. Instead, a weak PL shoulder at 430 nm appears on the spectrum.  A PL line-scan was performed across the nanowire-nanosheet boundary. The 382 nm and 547 peak intensity were recorded and displayed in Fig. 3(b). It is clearly shown that the bandedge emission at 382 nm (black squares) experienced a sharp intensity drop, from 3000~4000 to 100, at the boundary (~-7 μm) when the excitation laser moved from the nanowire region to nanosheet region. By contrast, the intensity of the green emission at 547 nm (red squares) increases by 3-4 times concurrently. The changes of peak intensity show a good agreement with the spectra in Fig. 3(a). It is noted that, for the 547 nm peak intensity, there is a partial contribution from the 600 nm broad band in the nanowire region (< −7 μm), which results in the relatively smaller intensity contrast compared with 382 nm peak intensity (from ~3000 to ~100).

Results and discussions
The distinctive photoluminescence properties of nanowires and nanosheets could be of great importance for a variety of applications. The above sample only displayed two regions with ZnO nanowires and nanosheets. Periodic patterns can be designed to create novel twodimensional structures with highly tunable optical properties, which may find applications in light-emitting devices. Here, we employed another facile and widely-used technique -colloidal lithography -to create periodic hexagonal pattern of the ZnO nanowires with nanosheets in between. In general, colloidal lithography produces monolayers of close-packed latex nanospheres/microspheres on substrates. With RIE, we can further reduce the size of the sphere while keep the periodicity intact. Figure 4 shows the SEM images of the synthesized nanowire/nanosheet patterns. The dry etching time increases from panel a to f leading to decreased size of the Al nanoholes. In panels a-c, it can be seen that hexagonal patterns of the ZnO nanowire bunches are achieved with decreasing diameter of the bunch due to the shrinking Al nanoholes underneath while the periodicity is kept the same. Further reducing the size of the Al nanohole results in termination of the ZnO nanowire growth and the substrates were totally covered with ZnO nanosheets. However, it is clearly shown in the SEM images (d-f) that the morphology of the nanosheets inside the hole (on the ITO substrate) is different from those on the Al thin film. In general, the size of the former is smaller than that of the latter. In addition, more needlelike ZnO nanoobjects are accumulated in the hole region. Apparently, the Al film plays a critical role determining the morphologies of the grown ZnO nanostructures. It is reported that the formation of a boehmite layer is responsible for the growth of sheet-like ZnO nanostructures [15]. Thus, with this simple technique, not only are we able to create periodic ZnO nanowire patterns, but also we can control the morphology of the ZnO nanosheets and even realize periodic patterning of different ZnO nanosheets. Such a ZnO growth technique with high tunability could be applied to a wide range of applications for complex patterning and advanced optical property control. We investigated the PL properties of the periodic ZnO nanowire/nanosheet patterns. Figure 5 shows the optical image ( Fig. 5(a), marked region) and the corresponding PL map (Fig. 5(b)). The darker circular areas in Fig. 5(a) are the ZnO nanowire bunches arranged in hexagonal pattern as in Fig. 4(b). We scanned the marked region and selected the band-edge emission 382 nm for PL mapping. As shown in Fig. 5(b), the large bright spots are the bandedge emissions from the bunched ZnO nanowires matching well with the optical image. Hence, a good correlation is achieved between the PL map and the marked optical image. The demonstrated technique here can provide simple and effective means to engineer the optical properties of the samples. Now we will try to explain the difference between the PL spectra of ZnO nanowires and nanosheets. It is known that various deep level native defects, such as oxygen and zinc vacancies, in ZnO nanostructures account for the broad visible PL bands observed from samples prepared with different techniques. However, the exact origins of these PL bands are still under debate [29][30][31][32]. It is believed that oxygen interstitials or the surface OH groups are responsible for the yellow-orange emission at 600 nm as we observed in the ZnO nanowires. For the most controversial green emission, latest study suggests that zinc vacancies are a more feasible source for the green PL band [30,33]. This is qualitatively confirmed by mapping the distribution of element Zn and O on the sample by EDS as shown in Fig. 6. In Fig. 6(b), it is shown that there are more Zn elements in the nanowire regions than in the nanosheet regions. However, Fig. 6(c) reveals that more O elements exist in the ZnO nanosheet areas. Therefore, the O/Zn atom ratio is higher in the ZnO nanosheets than that in the ZnO nanowires. Closer examination reveals that the O/Zn atom ratio is about 2 in the ZnO nanosheet region indicating that Zn vacancies are abundant in the nanosheets giving rise to that strong green emission as observed in Fig. 3(a). The crystalline structures of the ZnO nanowire/nanosheet were further evaluated with high-resolution TEM as shown in Fig. 7. ZnO nanowire (Fig. 7(a)) shows single crystalline feature while nanosheet ( Fig. 7(b)) exhibits polycrystalline nature, which makes it prone to defect growth and explains the absence of band edge emission at 382 nm.

Conclusion
In conclusion, we have demonstrated well-controlled patterning of ZnO nanowires and nanosheets on ITO glass substrates modified with Al nanohole arrays, which were patterned by colloidal lithography. From the same growth solution, ZnO nanowires grow inside the Al nanohole (on the glass substrate) while ZnO nanosheets grow exclusively on the Al thin film. The size of the Al nanohole and thus the size of the nanowire bundle can be accurately controlled by etching time of the polystyrene spheres. The ZnO nanowires and nanosheets exhibit distinctive photoluminescence properties. The nanowires show the band-edge emission at 382 nm together with a broad yellow-orange band while the nanosheets only show a broad and strong green emission. This is attributed to the zinc vacancies in the nanosheets as confirmed by EDS mapping. The proposed fabrication technique here provides a facile means to control the optical properties of ZnO nanostructures over a large area, and can be of use for flexibly tailoring the active spectral region in ZnO-based light-emitting devices and for energy harvesting applications.