Functionally graded nanocrystalline silicon powders by mechanical alloying

Functionally graded nanocrystalline silicon powder, having varied bandgap. has been prepared using mechanical alloying technique. The solar grade (99.999% pure) nanocrystalline silicon (nc-Si) powder was ball milled in argon atmosphere in an attrition mill for 4 h to 20 h (4 h, 12 h and 20 h) to reduce crystallite size in it. The milled powder samples were then degassed, to remove entrapped gases, in a vacuum of 10−2 torr at 200 °C for 1 h. Transmission electron microscopy and X-ray diffraction studies show the crystallite size of all the powders are in the nanometer range and decrease in crystallite size was seen with increasing milling time. Raman spectroscopy shows nanocrystalline phases in the milled powder particles with 100% crystalline volume fraction. UV-Vis-IR spectroscopy was used to determine bandgap of the powder samples using Tauc formula. The bandgap and crystallite size in silicon powder is found to be the function of milling duration, without addition of any alloying element. The nc-Si powder with different bandgaps may find potential applications in the field of functionally graded solar cells.