Phase Evolution of Bulk Nanocrystalline Silicon At High Pressure

Diamond cubic Si is a widely used semiconductor; however, its optical properties still need to be improved to meet industrial demands. There are several Si metastable phases obtained from high-pressure synthesis, including the hexagonal diamond and bc8 phases. Nanostructured bc8-Si is considered as a promising solar-absorbing material owing to its direct band gap and multiple exciton generation but it is hard to produce through high-pressure methods due to amorphization during unloading. In this work, bulk nanocrystalline diamond cubic Si was compressed up to ∼20 GPa in a diamond anvil cell and decompressed at ambient conditions. The phase transitions during compression resemble those occured in previous nanostructured Si, while upon decompression, the phase transitions are similar to those observed in bulk Si, leading to the recovery of bulk nanocrystalline bc8/r8 phase. Annealing the bulk nanocrystalline bc8/r8-Si led to a gradual transformation from the r8 to bc8 phase. Grain boundaries are considered important for r8-Si nucleation in bulk nanocrystalline Si during decompression. The high-pressure behaviors of Si nanopowders under different pressure environments were also explored to verify the significance of grain boundaries in pressure-induced phase transitions. This study offers a new pathway toward the nanocrystalline bc8/r8-Si phase synthesis for Si-based optical applications.