Crystal-Plane Dependence of Critical Concentration for Nucleation on Hydrothermal ZnO Nanowires

Hydrothermal ZnO nanowires have shown great potential for various nanoscale device applications due to their fascinating properties and low-temperature processing. A preferential crystal growth of ZnO (0001) polar plane is essential and fundamental to realize the anisotropic nanowire growth. Here we demonstrate that a critical concentration for a nucleation strongly depends on a crystal plane, which plays an important role on an anisotropic growth of hydrothermal ZnO nanowires. We measure a growth rate of each crystal plane when varying a concentration of Zn ionic species by using a regular array structure. Selective anisotropic growth on (0001) plane emerges within a certain concentration range. Above the concentration range, a crystal growth on (101̅0) plane tends to simultaneously occur. This strong concentration dependence on the crystal plane is understood in terms of a critical concentration difference between (0001) plane and (101̅0) plane, which is related to the surface energy difference between the crystal planes.