Madden Eimear, Zwijnenburg Martijn A
Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK.
Phys Chem Chem Phys. 2024 Apr 17;26(15):11695-11707. doi: 10.1039/d4cp00119b.
We use a combination of many-body perturbation theory and time-dependent density functional theory 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 predicted 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.
我们运用多体微扰理论和含时密度泛函理论相结合的方法，来研究氢终止硅纳米颗粒的光学和电子性质。我们预测这些硅纳米颗粒的最低激发态具有激子特性，并且相应的激子在颗粒体积内完全离域。预计激子的大小会与颗粒大小成比例增加。相反，我们预测基本能隙、光学能隙和激子结合能会随着颗粒尺寸的减小而增加。预计激子结合能会抵消基本能隙的变化，从而减小光学能隙随颗粒尺寸的变化。激子结合能本身的变化可能是由于随着颗粒尺寸减小，介电屏蔽作用减弱所致。预计与光学能隙及其他低能激发相对应的激发态强度会随着颗粒尺寸的减小而增加。我们从“能带结构”在倒易空间中随着颗粒尺寸减小而变得模糊这一角度来解释这种增加，这增加了占据和未占据准粒子态之间的“重叠”，进而增加了振子强度。最低激子的傅里叶变换表明它们继承了前沿准粒子态的周期性。这与激子的离域以及大的激子结合能相结合，意味着硅纳米颗粒中的激子兼具了万尼尔 - 莫特激子、离域以及底层结构周期性效应，以及弗伦克尔激子、大激子结合能的特点。
