Bridge- and Double-Bonded O and NH on Fully OH- and NH₂-Terminated Silicon Nanocrystals: Ground and Excited State Properties

The authors model fully hydroxyl- (OH-) and amino- (NH₂-) terminated silicon nanocrystals (Si-NCs) by time-dependent density functional theory (TD-DFT), and replace OH or NH₂ groups by respective double- (=) or bridge-bonded (>) groups  >/ = O or >/ = NH. Investigating ground state (GS) gaps and interface charge transfers (ICTs) from Si-NCs to anion groups, the authors show the impact of >/ = O and >/ = NH. Excited state (ES) calculations yielded transition energies E_trans, oscillator strengths f_osc and transition rates (Formula presented.). The exciton binding energy R* increases with ICT modulation in particular for >/ = O. Increase of (Formula presented.) is high for =O and comparatively low for >O which correlates with increased (decreased) ionisation of =O (>O), as compared to nominal OH termination. Findings are also met by >/=NH on Si-NCs, though the authors find the results there to be less apparent which is arguably originating from the specific anionic nature of N. As a result, Si-NCs with >O and in particular =O bonds show significantly increased optical activity, but also higher R* values. The latter hampers exciton dissociation, hence carrier transport, and results in an increased redshift in photoluminescence (PL). These statements apply also to Si₃N₄-embedded Si-NCs, though the differences there are less articulate.