Temperature-dependent vibrational spectra and melting behavior of small silicon clusters basedon ab initio molecular dynamics simulations

摘要

Vibrational spectra of Si_n (n=4-7) clusters are obtained by using Fourier transformation of the velocityautocorrelation function of the atoms calculated based on Car-Parrinello molecular dynamics. By comparingthe calculated vibrational spectra with experimental results, the ground-state atomic structures of the clusterscan be determined. Changes in the vibrational spectra and the bond-length root-mean-square fluctuation withtemperature are obtained. It is shown that the characteristic structures of the vibrational spectra with ground-state configuration start to disappear before the melting start points indicated by sharp increase in the bond-length root-mean-square fluctuation with temperature. Therefore the vibrational spectra cannot be used toidentify the existence of these clusters in gases before the temperature going down well below their actualformation temperatures. Melting of these small silicon clusters starts at a temperature lower than the bulkmaterial and undergoes transition states over a finite temperature range. The trajectories of the atoms inconfiguration space are used to show that the structure changes during the melting process depend closely onthe ground-state structures. The calculated electronic energy levels of Si4 cluster, as an example, indicate thatduring melting process there are considerable energy-level broadenings caused by atomic thermal motions.