Si(100) wafers were subjected to severe plastic deformation under a pressure of 24 GPa using high-pressure torsion (HPT). Si wafers were plastically deformed at room temperature. HPT-processed samples were composed of metastable body centered cubic Si-III and rhombohedral Si-XII phases in the initial cubic diamond Si-I. The volume fraction of metastable phases increased with increasing plastic strain. Successive annealing at 873 K led to the reverse transformation of metastable phases. A broad photoluminescence peak centered at about 650 nm appears due to the reverse transformation of Si-III/Si-XII nanograins and the reduction of number of defects in Si-I nanograins.

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