Abstract
Based on ab initio molecular dynamic simulations, we have theoretically investigated the structural stabilities and electronic properties of X22H28 (X=C, Si, and Ge) nanocrystals, as a function of temperature with consideration of vibrational entropy effects. To compare the relative stabilities of X22H28 isomers, the vibration free energies are obtained according to the calculated phonon spectrum, where the typical modes are shown to be dominant to the structural stabilities. In addition, there is a significant gap reduction as the temperature increases from 0 K to 300 K, where the decrements are 0.2 /0.5 /0.6eV for C/Si/Ge nanocrystals, respectively. The dependence of energy gap on the variance of bond length is also analyzed according to the corresponding atomic attributions to the HOMO and LUMO levels.
Highlights
There has been a great interest in hydrogenated diamond nanocrystals,[1,2] where hydrogenated diamond nanocrystals were isolated and synthesized.[3]
The first-principle calculations of X22H28 nanocrystals were based on density functional theory (DFT) method implemented in the Vienna ab initio simulation package[34,35] (VASP)
IIIA, we compare the relative stabilities for the three isomers of X22H28 according to the vibration free energies and the total energies, where the low frequency vibrational modes are shown to be crucial to the structural stabilities
Summary
There has been a great interest in hydrogenated diamond nanocrystals,[1,2] where hydrogenated diamond nanocrystals were isolated and synthesized.[3]. The simulated optical adsorption by combining first-principles calculations and Important Sampling Monte Carlo methods in the basic diamond nanocrystals is in quantitative agreement with the experiment, demonstrating compelling evidence for the role of quantum nuclear dynamics in the photophysics.[10]. The indirect band gap of silicon (Si) limits its applications on optoelectronics, while Si nanostructures (such as porous silicon,[11] Si nanoparticles,[12] Si nanocrystals,[13] and Si nanocrystals embedded in Si oxide14,15) have exhibited visible photoluminescence at room temperature[14] due to the quantum confinement effect. Franceschetti[20] theoretically calculated temperature dependence of the gap of Si nanocrystals using constant temperature molecular dynamics (MD) methods. Hartel et al.[21] investigated the temperature-dependent gap of the Si nanocrystals, which were embedded in Si substrates. Germanium (Ge) nanocrystals have stimulated extensive researches about the preparative technique[22,23] and the fundamental principles since the photoluminescence of Ge quantum dot.[24]
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