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

Bürkle, Marius and Lozac’h, Mickaël and McDonald, Calum and Macias-Montero, Manuel and Alessi, Bruno and Mariotti, Davide and Svrcek, Vladimir (2020) Tuning the Bandgap Character of Quantum-Confined Si–Sn Alloyed Nanocrystals. Advanced Functional Materials, 30 (22). 1907210. ISSN 1616-301X (https://doi.org/10.1002/adfm.201907210)

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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 α-phase and for high enough Sn concentration (≈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.