Observation of linear solid-solid phase transformation in silicon nanoparticles

In bulk single-crystal silicon, the semiconducting diamond-to-metallic ß-Sn phase transformation nucleates on defects, and is manifested by a sharp uptake in light absorption at a threshold of ∼11 Gpa, accompanied with the creation of nanosized (20-50 nm) fragmentation domains. We report on the observation of linear uptakes in the absorption and in the luminescence and with insignificant spectral change in ultrasmall 1-nm Si particles. We associate the gradual absorption uptake and luminescence yield with silicon-metal transformation on the surface. The insignificant change in the spectral content of the luminescence points to surface stability for particles, which are smaller than the bulk fragmentation domain. First-principles atomistic calculations yield absorption behavior that exhibits gradual uptake followed by sharp uptake at ∼9-11 Gpa. The results point to the conclusion that two-dimensional surface-like phase transformations are manifested by linear uptake in absorption and luminescence.