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

Abstract

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 Etrans, oscillator strengths fosc and transition rates AAbs. The exciton binding energy R* increases with ICT modulation in particular for >/ = O. Increase of AAbs 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.

Absorption and Photoluminescence of Silicon Nanocrystals Investigated by Excited State DFT: Role of Embedding Dielectric and Defects

“…The claim that stretched Si-Si bonds and Si-O bridge bonds present nonradiative recombination defects could not be reproduced by more sophisticated ES calculations involving hybrid DFT on albeit smaller Si-NCs. [4,5] We therefore concur that DBs present the dominant nonradiative recombination path for embedded Si-NCs. [41,49] This finding is corroborated by D it values of SiO 2 -embedded Si-NCs being reduced down to d NC % 2.5-10% for passivated Si-NCs relative to their unpassivated counterparts; see the discussion of D it values below.…”

“…Zhou et al [3] reported on excited-state (ES) time-dependent (TD-) DFT calculations of Si 35 NCs fully covered by OH groups, equivalent to 1 ML SiO 2 . The impact of modified O bonds, namely, bridge-bonded (>O) and double-bonded (═O) species to otherwise fully OH-covered Si-NCs, was carried out, [4,5] confirming and further detailing Zhou's results. Guerra et al calculated absorption spectra of SiO 2 -embedded ultrasmall Si-NCs (Si 10 [7 Å size], and Si 35 [11 Å size]), [6,7] whereby the energies of the fundamental absorption transitions E abs gap were suspiciously low and Si-Si bond lengths of 3.1 Å are a massive 32% "bigger" as compared with the value of bulk Si.…”

“…We calculate the absorption rate following ref. [5], making use of Fermi's golden rule for optical transitions. [74,75] Extending this approach to the N ¼ 8 lowest-energy transitions of the fundamental gap with normalization to exactly one exciton yields…”

“…In accordance with experimental findings, [71,72] we consider the fundamental transitions to be direct (or quasidirect), whereby the phononic contribution to the fundamental transition is not required (or can be ignored), rendering the associated Kronecker delta for momentum conservation to be δ k ðk fin À k ini þ k½ℏωÞ∶ ¼ 1. [5] As we have an ensemble of transition energies, it is straightforward to calculate the standard deviation of E abs gap , that is…”

“…The termination of Si‐NCs with OH and groups has been shown to accurately emulate and embedding. [ 4,5,9–11,14,15 ] We describe the real‐space DFT and TD‐DFT methods we used in Section 2. In Section 3, we investigate the GS electronic properties of Si‐DBs ( defects) at corner, edge, and facet positions of the NC/dielectric interface.…”