Tuning the Bandgap Character of Quantum-Confined Si-Sn Alloyed Nanocrystals

Nanocrystals in the regime between molecules and bulk give rise to unique electronic properties. Here, a thorough study focusing on quantum-confined nanocrystals (NCs) is provided. At the level of density functional theory an approximate (quasi) band structure which addresses both the molecular and bulk aspects of finite-sized NCs is calculated. In particular, how band-like features emerge with increasing particle diameter is shown. The quasiband structure is used to discuss technological-relevant direct bandgap NCs. It is found that ultrasmall Sn NCs have a direct bandgap in their at-nanoscale-stable alpha-phase and for high enough Sn concentration (approximate to 41%) alloyed Si-Sn NCs transition from indirect to direct bandgap semiconductors. The calculations strongly support recent experiments suggesting a direct bandgap for these systems. For a quantitative comparison many-body GW + Bethe-Salpeter equation (BSE) calculations are performed. The predicted optical gaps are close to the experimental data and the calculated absorbance spectra compare well with the corresponding measurements.