Formation and size dependence of germanium nanoparticles at different helium pressures

We report the formation of germanium nanoparticles and threadlike nanostructures by the process of thermal vapor deposition. Germanium was thermally evaporated and condensed on graphite substrates under varied ambient helium pressures of 100, 200, 300, and 400 torr. The deposited films were characterized through transmission electron microscopy. These investigations revealed the occurrence of germanium particles. The size of these particles varied inversely with ambient pressure. Whereas for an ambient pressure of 100 torr the average particle size was approximately 0.3-0.5 micron, for 400 torr the average size was approximately 30-60 nm. At high He pressure (e.g., approximately 400 torr), the nanoparticles that formed when annealed for 30 min at 200 degrees C in vacuum adopted threadlike nanostructures. A qualitative explanation has been put forward for the formation of germanium particles and variation of size of these particles with helium pressure in terms of condensation, collision, and coalescence.