Absence of redshift in the direct bandgap of silicon nanocrystals with reduced size

The high-energy Γ-Γ direct band gap transition in Si nanocrystals was recently argued to rapidly lower its energy (i.e. be ‘red-shifted’) with decreasing nanocrystal size, in contrast to what is expected from conventional quantum confinement (‘blue-shift’). This ‘anti-confinement’ trend, projected to lead at sufficiently small nano sizes to a truly directgap Si nanocrystal. Here, we combine predictive atomistic pseudopotential theory with single dot spectroscopy to investigate the origin of the red-shifted transition in Si nanocrystals. The measured and calculated absorption and emission bands are in good mutual accord, the direct band gap Γ-Γ transition does not show a redshift, nor does it appear in energy below the Γ-valley direct edge of bulk Si. We find the Γ-valley dominant conduction band state is slightly blue-shifted to higher energy instead of being rapidly red-shifted with reduced nanocrystal size. We conclude that the observed red-shifted band is not the intrinsic direct band edge and that the early theory that supported the previous assignment, based on assigning a negative effective mass to the Si electrons at Γ, is not supported by atomistic calculations.

Ref. J.W. Luo, S.S. Li, I. Sychugov, F. Pevere, J. Linnros, and A. Zunger, Nature Nano 12, 930(2017).