Study On Vibrational Properties And Melting Behavior Of Small Silicon Clusters

The ground state atomic configurations, vibrational spectra and melting behavior of small silicon clusters Si_n(n=4-7) have been investigated by using ab initio molecular dynamics simulation based on the density functional theory (DFT) combined with the local density approximation (LDA). The changes of the electron structure of Si₄ during melting process has been calculated using augmented plane wave plus local orbital (APW+lo) method.The motion of atoms in silicon clusters has been simulated with microcanonical ensemble. The eigenvibration frequencies are obtained by Fourier transforming the velocity autocorrelation function (VAF). The ground state atomic configurations of Si_n(n=4-7) are determined by comparing calculated vibration spectra with the experimental values. The ground state atomic configurations of Si₄-Si₇ have D2h, D3h, D4h and D5h symmetry respectively. The normal modes of Si_n(n=4-7) and their symmetry have been shown using group theory. Our results are in good agreement with experimental values and theoretical values in references.The thermal motion of the atoms in silicon clusters has also been simulated with canonical ensemble. The calculated bond length root-mean-square fluctuation show that Si₄-Si₇ undergo a transition state with a long temperature region during the transition process from solidlike state to liquidlike state. At the transition temperature, silicon clusters undergo a dynamics coexistence of the two states. The mean value of temperatures in transition state is defined melting point. According to this definition, the melting points of Si₄-Si₇7 are 1250K, 950K, 1250K and 1400K. The vibrational spectra and the atom trajectories at different temperatures show that silicon clusters in solidlike state have shell structures. As the temperature is rising, the shell structure is broken and the region of atomic vibration extends gradually. Under the anharmonic effect brought by thermal motion, the eigenvibration modes of silicon clusters are gradually disappeared as the temperature rises. The electronic density of states (DOS) of Si₄ for different structures at different temperatures has been investigated and the results show that there are energy level broadening caused by thermal motion.