The Effect of Particle Size on the Optical and Electronic Properties of Hydrogenated Silicon Nanoparticles

18 December 2023, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

We use a combination of GW-BSE and time-dependent DFT to study the optical and electronic properties of hydrogen terminated silicon nanoparticles. We predict that the lowest excited states of these silicon nanoparticles are excitonic in character and that the corresponding excitons are completely delocalised over the volume of the particle. The size of the excitons is precited to increase proportionally with the particle size. Conversely, we predict that the fundamental gap, the optical gap, and the exciton binding energy increase with decreasing particle size. The exciton binding energy is predicted to counter-act the variation in the fundamental gap and hence to reduce the variation of the optical gap with particle size. The variation in the exciton binding energy itself is probably caused by a reduction in the dielectric screening with decreasing particle size. The intensity of the excited state corresponding to the optical gap and other low energy excitations are predicted to increase with decreasing particle size. We explain this increase in terms of the ‘band structure’ becoming smeared out in reciprocal space with decreasing particle size, increasing the ‘overlap’ between the occupied and unoccupied quasiparticle states and thus, the oscillator strength. Fourier transforms of the lowest excitons show that they inherit the periodicity of the frontier quasiparticle states. This combined with the delocalisation of the exciton and the large exciton binding energy means that the excitons in silicon nanoparticles combine aspects of Wannier-Mott, delocalisation and effect of periodicity of the underlying structure, and Frenkel, large exciton binding energy, excitons.

Keywords

silicon
nanoparticles
quantom dots
GW
Bethe–Salpeter

Supplementary materials

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Description
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Electronic Supporting Information
Description
Tables of G0W0(-BSE), evGW(-BSE) and qsGW(-BSE) results and DFT predicted photoluminescence energies and Stokes shifts, and plots of the fundamental gap versus particle size, optical gap versus particle size using the lowest bright excited state for Si10H16 and of the natural transition orbitals obtained with TDDFT and additional Fourier transforms
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Electronic Supporting Information - DFT optimised structures
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DFT optimised structures of all relevant particles.
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