Motility of Metal Nanoparticles in Silicon and Induced Anisotropic Silicon Etching

The autonomous motion behavior of metal particles in Si, and the consequential anisotropic etching of silicon and production of Si nanostructures, in particular, Si nanowire arrays in oxidizing hydrofluoric acid solution, has been systematically investigated. It is found that the autonomous motion of metal particles (Ag and Au) in Si is highly uniform, yet directional and preferential along the [100] crystallographic orientation of Si, rather than always being normal to the silicon surface. An electrokinetic model has been formulated, which, for the first time, satisfactorily explains the microscopic dynamic origin of motility of metal particles in Si. According to this model, the power generated in the bipolar electrochemical reaction at a metal particle's surface can be directly converted into mechanical work to propel the tunneling motion of metal particles in Si. The mechanism of pore and wire formation and their dependence on the crystal orientation are discussed. These models not only provide fundamental interpretation of metal-induced formation of pits, porous silicon, and silicon nanowires and nanopores, they also reveal that metal particles in the metal/Si system could work as a self-propelled nanomotor. Significantly, it provides a facile approach to produce various Si nanostructures, especially ordered Si nanowire arrays from Si wafers of desired properties.