Diverse nanostructures of bc8 silicon mediated by controllable transition kinetics of beta-Sn silicon

Metastable polymorphs of silicon exhibit direct and narrow band gaps, offering broad application potential. The transition kinetics in the formation of metastable Si have been extensively studied, but the relationship between transition conditions and nanostructures of product phases remains unclear. Here, we report diverse nanostructures of body-centered cubic (bc8) Si formed by rapid decompression of beta Sn Si at different rates. High-resolution transmission electron microscopy images show that slow decompression results in the formation of bc8 nanocrystals with long-range order and abundant interfacial defects, including dislocation, twin, stacking fault, and internal lattice strain. In contrast, rapid decompression generates bc8 nanoclusters with a macroscopic amorphous characteristic, as evidenced by the broad diffuse halo in fast Fourier transform pattern. Statistical analysis of the size distribution shows that the grain size decreases monotonically and significantly with increasing decompression rate, indicating a clear rate-dependent behavior. These findings provide structural evidence and mechanistic insights into the kinetically-controlled formation of the metastable nanostructures for promising applications.