Electronic and Optical Properties of Double- and Bridge-Bonded O and N at Fully OH- and NH2-Terminated Silicon Nanocrystals (poster)

We model fully OH- and NH2-terminated silicon nanocrystals (SiNCs) - Si35(OH)36 and Si35(NH2)36 - by hybrid-DFT [1,2]. We substitute OH [NH2] groups by double-bonded (=) O [=NH] segments and corner Si(OH)2 [Si(NH2)2] segments for bridge-bonded (>) O [>NH] groups. Correlating ground state (GS) gaps and charge transfers from SiNCs to anion groups with atomic ratios of Si to O [N] atoms, we show the specific impact of = and > anion bonds. Excited state (ES) calculations yielded the three lowest singlet transition energies (ES gap, absorption) and oscillator strengths (optical transition probabilities P_ot). ES energies closely follow the trend of GS gaps for OH-terminated SiNCs with =O, while those for SiNCs with >O show an increasing deviation from GS gaps to lower energies with rising numbers of >O. An increased modulation of charge transfer occurs due to low negative ionization of >O atoms which appears to distort the exciton field and thus increases exciton binding energies. For SiNCs with =O atoms, stronger SiNC ionization seems to improve carrier separation, decreasing exciton binding energies and lifting ES gaps. Thus, P_ot of SiNCs rise hyperlinear with the number of =O atoms, while a saturation occurs for SiNCs with >O atoms. Similar results were found for NH2-terminated SiNCs with =NH vs. >NH, with less influence on the electronic and optical SiNC behaviour due to lower SiNC ionization.
[1] D. König et al., Phys. Rev. B 78, 035339 (2008)
[2] D. König et al., Adv. Mater. Interf. 1, 201400359 (2014)