Mudar Ahmed Abdulsattar, A. J. Hashim, Mohammed E. Ismael

Abstract

Stable Si diamondoids are used to investigate Si nanocrystals and as an approach to Si bulk properties using local spin density approximation (LSDA). HOMO, LUMO, energy gap of Si diamondoids, size variation, and the effect of nanocrystals shape are all investigated. Vibrational force constant of Si diamondoids reaches 0.95 mDyne/Å. This value is lower than bulk silicon. Si-Si vibrational frequency reaches 502.9 cm^-1, and vibrational reduced mass reaches 14.2 amu. These two quantities are higher than the values of bulk Si. UV-Vis spectra show the movement of the peak from 114 nm to nearly 281.5 nm with increasing size of Si diamondoids. This trend of values is compared with experimental Si quantum dots at 440 nm. NMR spectra are shown that it can differentiate between different Si molecules and diamondoids as a function of size. All Si diamondoids properties are sensitive to Si diamondoids size. NBO population analysis displays that investigated molecular bonding deviates from sp³ bonding. The bonding of Si valence orbitals is in the extent ([core]3s^1.173p^2.893d ^0.024p^0.01 to [core]3s^1.163p^2.683d^0.01) depending on the location of Si atom‎.