High-density semiconductor nanorod arrays (NRAs) with one-dimensional (1D) structures have been extensively studied for their application in photonic and electronic devices. [1,2] Especially, 1D periodic NRAs of GaN, ZnO, and ZnS have attracted considerable interest in application to ultraviolet (UV) laser devices due to their direct wide bandgaps of DE g ³ 3.0 eV.[1±4] Among them, ZnO (DE g = 3.37 eV) is thought to be the most suitable material for UV laser devices because of its large exciton binding energy of 60 meV compared to the thermal energy (26 meV) of room temperature.[5]Recently, the room-temperature UV lasing emission from a directionally grown ZnO nanoarray was demonstrated with a threshold power density below 100 kW cm ±2. [1,6] Such NRAs with high-quality UV lasing properties were fabricated only by physical techniques like molecular beam epitaxy (MBE), metal±organic chemical vapor deposition (MOCVD), and gold-catalyzed vapor-phase transport (VPT) techniques; those are, however, expensive and energy consuming processes since they are operated under extreme conditions. [2,7,8] For example, the high-quality ZnO NRA with the best UV-lasing properties was realized by a gold-catalyzed VPT process at 925 C. [8] At this high temperature, however, one can hardly use a silicon (Si) wafer as a substrate, and it is necessary to use a sapphire (Al 2 O 3 ) wafer, even though the Si substrate would be advantageous in terms of easy transformation into electronic devices and low price. It is worth pointing out here that a perfectly and directionally grown ZnO NRA on a Si wafer has been rarely achieved due to the thermal instability of the Si substrate and the large lattice mismatch (~40 %) between the substrate and the ZnO NRA.[9±11]In the present study, a high-quality ZnO NRA was successfully grown on a Si wafer by a wet-chemical process at 95 C for 6 h, where the Si wafer was dip-coated with 4 nm sized ZnO nanoparticles as a buffer and seed layer prior to the crystal growth. To summarize the result in advance, we found that the product ZnO NRA's threshold power density of 70 kW cm ±2 is comparable to the lowest one of 40 kW cm ±2 determined for ZnO NRAs on Al 2 O 3 substrates.[1]ZnO nanoparticles as a starting precursor for the ZnO NRA were prepared according to the previously reported method.[12] As shown in Figure 1a, the particle size of monodispersed ZnO nanoparticles with quasi-spherical shape is determined to be approximately 4 nm. A single particle is observed in detail in the inset of Figure 1a, where the lattice distance between adjacent lattice planes is measured as 5.2 corresponding to the d-spacing between the (0001) planes. The surface image of the present ZnO NRA shows that the nanorod has a well-defined hexagonal plane with a homogeneous diameter of approximately 100 nm due to the uniform growth rate (Fig. 1b). The cross-sectional image in Figure 1c indicates that the nanorods with a uniform length of 1.5 lm are directionally and densely grown over the entire seeded surface of the Si wafer. In add...

ZnO nanocoral reefs and nanofibers are synthesized on the glass substrate dip coated with ZnO seed with nanoparticles with an average size of 5 nm under a hydrothermal reaction. The ratios of length to diameter for the former and the latter are determined to be 100 and 1000, respectively. In addition, we found that a threshold power density for UV lasing action could be remarkably reduced from 40 kW/cm2 for the nanocoral reefs to 8 kW/cm2 for the nanofibers by increasing the cavity length of ZnO nanowires.

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ZnO nanowall array (ZNWA) has been directionally grown on the buffer layer of ZnO nanoparticles dip-coated on Si-wafer under a soft solution process. Nanowalls on substrate are in most suitable shape and orientation not only as an optical trap but also as an optical waveguide due to their unique growth habit, V[011¯0]⪢V[0001]≈V[0001¯]. Consequently, the stimulated emission at 384 nm through nanowalls is generated by the threshold power density of only 25 kW/cm2. Such UV lasing properties are superior to those of previously reported ZnO nanorod arrays. Moreover, there is no green (defect) emission due to the mild procedure to synthesize ZNWA.