Figure 1 XRD patterns of ZnO NWs grown at 550°C for 60, 90, and 120 min, respectively. Figure 2a,b,c,d,e,f shows the cross-sectional and plane-view FESEM images of the ZnO NWs for different growth durations. It is notable that both the average length and diameter of the NWs increase as the growth time is increased. In addition, the areal Selleck CB-839 densities of ZnO NWs are 5.2 × 109, 2.9 × 109, and 1.8 × 109/cm2 with growth time of 60, 90, and 120 min, respectively. By varying the growth time from 60 to 120 min, the diameters of ZnO NWs increased from several tens to several hundreds of nanometers, and the lengths increased from 200 nm to 1.5 μm accordingly. It is also noteworthy
that the ZnO NWs were almost aligned to the substrate surface. These observations are consistent with AR-13324 chemical structure the XRD results. In a typical metal-catalyzed VLS mechanism, nanosized metal clusters play a critical role in forming liquid droplets that adsorb the gas-phase reactants where nanorod growth occurs. Hence, metallic nanoparticles with spherical
shape are commonly found at the end of nanorods grown by the metal-catalyzed VLS method. Since no metallic particle was observed on the top of the ZnO NWs, we could rule out the possibility of a VLS-like mechanism and claim that the VS model dominates the nanowire growth. Figure Selleck JIB04 2 Cross-sectional and top-view FESEM images of ZnO NWs grown at different growth times. (a, d) NWs grown for 60 min, (b, c, e, f) from a Zn source at 550C for (a) 60 min (b) 90 min, and (c) 120 min of reaction times. Photoluminescence of the obtained ZnO NWs synthesized at different growth times was also investigated
at room temperature, and the results are shown in Figure 3. The PL spectra consist of a sharp and strong UV emission peak centered at about 380 nm and a weak green emission centered at about 500 nm. The UV emission is attributed to the near-band-edge (NBE) emission, and the green emission is related to the intrinsic defects in the ZnO samples. When the growth time increased, the intensity of NBE emission (I NBE) also increased while the green emission (I green) decreased. Since ZnO NWs were fabricated under a fixed growth temperature, the improvement of crystal quality might play a minor role. Thus, an increase in the NBE-to-green emission ratio with increasing growth time could PIK3C2G result from the reduced concentration of surface defects. Generally, the green emission is attributed to single ionized oxygen vacancies (V o) [16]. Recently, it has been recognized that the surface states that originated from large surface-to-volume ratios seriously influence the PL features in nanomaterials. As manifested in the SEM images, the average diameter becomes smaller with decreasing growth time. Having a larger surface-to-volume ratio in nanostructures means a larger density of surface states. Therefore, higher surface states of ZnO NWs with a smaller diameter can be responsible for the origin of the enhanced green emission.