Abstract
The formation of high-density zinc oxide (ZnO) nanorods on porous silicon (PS) substrates at growth temperatures of 600–1000 °C by a simple thermal evaporation of zinc (Zn) powder in the presence of oxygen (O2) gas was systematically investigated. The high-density growth of ZnO nanorods with (0002) orientation over a large area was attributed to the rough surface of PS, which provides appropriate planes to promote deposition of Zn or ZnOx seeds as nucleation sites for the subsequent growth of ZnO nanorods. The geometrical morphologies of ZnO nanorods are determined by the ZnOx seed structures, i.e., cluster or layer structures. The flower-like hexagonal-faceted ZnO nanorods grown at 600 °C seem to be generated from the sparsely distributed ZnOx nanoclusters. Vertically aligned hexagonal-faceted ZnO nanorods grown at 800 °C may be inferred from the formation of dense arrays of ZnOx clusters. The formation of disordered ZnO nanorods formed at 1000 °C may due to the formation of a ZnOx seed layer. The growth mechanism involved has been described by a combination of self-catalyzed vapor-liquid-solid (VLS) and vapor-solid (VS) mechanism. The results suggest that for a more precise study on the growth of ZnO nanostructures involving the introduction of seeds, the initial seed structures must be taken into account given their significant effects.
Highlights
Zinc oxide (ZnO) with a wide direct bandgap of 3.37 eV and a large exciton binding energy of60 meV, is one of the most extensively studied semiconducting materials for use in various types of applications [1,2,3]
It can be clearly observed that the growth of high-density ZnO nanorods on Si(100) bulk substrates was not achieved at all tested temperatures
We have demonstrated the formation of high-density ZnO nanorods on porous silicon (PS) substrates at growth temperatures of 600–1000 °C by a simple thermal evaporation of Zn powder in the presence of O2 gas
Summary
Zinc oxide (ZnO) with a wide direct bandgap of 3.37 eV and a large exciton binding energy of. ZnO nanostructures such as nanorods, nanowires, nanonails, nanotubes, nanocombs, nanosheets, nanobelts and nanoribbons have been synthesized by various physical and chemical fabrication techniques These fabrication methods include thermal evaporation [3,9,10,11,12,13,14,15,16], chemical vapor deposition [17,18], hydrothermal [19], sol-gel [20,21,22] and electrodeposition [1,23]. If high purity metal and oxygen (O2) are used as the source materials and the synthesis is carried out in a properly sealed high vacuum chamber, extremely high purity and high crystallinity of ZnO nanostructures can be expected These are the reasons why the use of a vapor-phase transport method is inevitable. We report the formation of large-area high-density ZnO nanorods on PS substrates at growth temperatures of 600, 800 and 1000 °C by a thermal evaporation process without any catalyst. 514.5 nm line of an Ar+ laser at incident power of 20 mW/a 325 nm line of a continuous He-Cd laser at room temperature)
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