Electronic and spectroscopic properties of Ge nanocrystals using diamondoid structures: A density functional theory study

Abstract

Energetically stable Ge diamondoids are used as building blocks to investigate Ge nanocrystal properties using density functional theory (DFT). Energy gap, HOMO and LUMO of Ge diamondoids vary according to confinement theory with shape fluctuations. Ge diamondoid vibrational force constants reach 2.18 mDyne/Å which is between that of bulk silicon and tin. Ge–Ge vibrational frequencies and reduced masses reach 357.47 cm[Formula: see text] and 41.93 amu, respectively, which are higher than the values of bulk Ge. Size variation of UV–Vis shows that the maximum optical peak moves from 163 nm to nearly 290 nm as the size of Ge diamondoids and molecules increases. The higher peak value approaches that of experimental Ge quantum dots at 300 nm. NMR spectra of Ge diamondoids are analyzed as a function of diamondoid sizes. Our results show that all investigated spectroscopic tools are sensitive to molecular or nanocrystals size. NMR is particularly good size indicator. Natural bond orbital (NBO) population analysis shows present diamondoid bondings differ from ideal [Formula: see text] bonding. The bonding for Ge valence orbitals is in the range [Formula: see text] depending on distance between Ge atom and diamondoid surface. Highest Ge diamondoids vibrational longitudinal optical (LO) mode is blue shifted with respect to experimental bulk value which is the opposite case for C and Si. H surface atom effects on electronic and vibrational properties are discussed.