Surface Structure and Silicon Nanocrystal Photoluminescence: The Role of Hypervalent Silyl Groups

We report a combined experimental and theoretical study of the relationship between the surface structure of silicon nanocrystals synthesized in a nonthermal plasma reactor and their photoluminescence (PL) yields. Upon heating to 160 °C, a significant change in the SiH stretch region of the vibrational spectrum is observed indicating a decrease in surface SiH₃ groups, which correlates with an increase in the PL yield. Effusion of SiH_x and Si₂H_2x from the material is detected by residual gas analysis upon heating to temperatures below 200 °C, suggesting a weakly bound species. Analysis of electron paramagnetic resonance spectra before and after heating points to a small reduction in the density of dangling bonds upon heating but this reduction does not correlate with the increase in PL yield. Electronic structure calculations indicate that SiH₃^- groups may hypervalently bond to fully coordinated surface silicon atoms, resulting in a relatively weak (0.70 eV) bond that is consistent with the experimentally observed effusion at low temperature. Furthermore, nonadiabatic molecular dynamics simulations indicate that such hypervalent silyl defects provide efficient pathways for nonradiative recombination via conical intersections that are energetically accessible after near-infrared excitation.