Therefore, the weak ultraviolet emission and dominant blue band in the PL spectrum demonstrate the existence of ZnO and a large number of oxygen vacancies in the as-grown specimen. A comparison

of the hemispherical reflectance of the branched ZnO/Si nanowire arrays and a flat silicon wafer is provided in Figure 3d. The reflectance of the arrays is less than 15% over the wavelength range from ultraviolet to the mid-infrared CBL-0137 supplier region, which is drastically decreased relative to that of the silicon wafer. This significant property suggests that the nanotrees might be a promising candidate of antireflective surfaces or photoelectronics and photocatalysis for sunlight harvest. The ultralow reflectance of the specimen may result from the enhanced light-trapping and scattering for rough surface and large surface area of the nanotree arrays, multiple scattering of light within the Selleck P5091 hierarchical structure, as well as an effective SB-715992 cost refractive index (RI) gradient from air (RI ≈ 1.0) through ZnO nanowire array (RI ≈ 2.0) to Si nanowire array and substrate (RI ≈ 3.5) [18]. In addition, the abrupt drop in reflection is originated from band-edge absorption of the specimen [27]. The direct and

indirect bandgaps of the components can thus be estimated by the onset points, which are 397 nm (equal to 3.123 eV) for the direct bandgap of ZnO nanowire branches and 1,221 nm (equal to 1.015 eV) for the indirect bandgap of Si nanowire backbones. In contrast to the Si wafer value 1,213 nm (equal to 1.022 eV) or to the general value of bulk materials, 3.37 eV

for ZnO [7] and 1.12 eV for Si [5], the bandgaps of the as-grown specimen are found to be faintly narrowed down, suggesting Tobramycin ideal components of the object. The small difference may be due to the presence of ionic vacancies and structural defects in the nanotrees, as testified in the PL spectrum. The above results and analysis confirm that branched ZnO/Si nanowire arrays with hierarchical structure can be facilely grown on the silicon substrate in a wafer scale by the cost-effective methods. However, as the procedure includes chemical etching for the silicon backbones and hydrothermal growth of the ZnO branches, different synthesis parameters may cause serious influences on the structure and performance of the ZnO/Si nanowire arrays. For this reason, we systematically study crucial influences of the key parameters on the structure of the objects. First, the influence of etching solution on the silicon backbones is investigated, and the results are shown in Figure 4. We can see in Figure 4a that the Si nanobelt or nanowire arrays orient vertically on the Si substrate when the substrate was immersed into aqueous solution of HF/AgNO3 (5.25/0.02 M) at room temperature for 20 min.