  Silicon nanocrystals (SiNCs) have been under active investigation in the last
decades and have been considered as a promising candidate for many
optoelectronic applications including highly-efficient solar cells. Some of the
fundamental properties of interest in these nanostructures is the temperature
dependence of their optical absorption onset, and how this is controlled by
different passivation regimes. In the present work we employ first-principles
calculations in conjunction with the special displacement method to study the
temperature dependence of the band gap renormalization of free-standing
hydrogen-terminated, and oxidized SiNCs, as well as matrix-embedded SiNCs in
amorphous silica, and we obtain good agreement with experimental
photoluminescence data. We also provide strong evidence that the
electron-phonon interplay at the surface of the nanocrystal is suppressed by
oxidation and the surrounding amorphous matrix. For the matrix-embedded SiNCs,
we show a high correlation between the temperature dependence of the band gap
and the Si-Si strained bonds. This result emphasizes the immanent relationship
of electron-phonon coupling and thermal structural distortions. We also
demonstrate that, apart from quantum confinement, Si- Si strained bonds are the
major cause of zero-phonon quasidirect transitions in matrix-embedded SiNCs. As
a final point, we clarify that, unlike optical absorption in bulk Si,
phonon-assisted electronic transitions play a secondary role in SiNCs.
