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

• Published in: physica status solidi (b) 2019-05, Vol.256 (5), p.n/a
• Main Authors: König, Dirk, Yao, Yao, Smith, Sean
• Format: Article
• Language: English
• Subjects: density functional theory; excited state; ground state; interfaces; silicon nanocrystals
• ISSN: 0370-1972; 1521-3951
• DOI: 10.1002/pssb.201800336

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. Ground and excited state properties of silicon nanocrystals with complete OH‐ or NH2‐termination are calculated by TD‐DFT as function of interface bond modification, replacing —OH (—NH2) groups by O (NH) having a double‐ (=) or bridge‐bond (>). The increase in optical activity is considerable for =O and notable for >O modifications, while the situation is more balanced for NH2 termination. Findings affect the absorption edge in both energy and transition rates.