V. Radhika Devi

DFT calculation of vibrational frequencies of clusters in GaAs and the Raman spectra.

We have calculated the vibrational frequencies of clusters of Ga and As atoms from the first principles using the density-functional theory (DFT) method and the local-density approximation (LDA). We find that the calculated value of 286.2 cm(-1) for a linear cluster of Ga(2)As(2) is very near the experimental value of 292 ± 4 cm(-1). The calculated value of 289.4 cm(-1) for Ga(2)As(6) (dumb bell) cluster is indeed very near the experimental value. There are strong phonon correlations so that the cluster frequency is within the dispersion relation of the crystal LO value. There is a weak line in the experimental Raman spectrum at 268 cm(-1) which is very near the value of 267.3 cm(-1) calculated for the Ga(2)As (triangular) cluster. The weak lines corresponding to the linear bonds provide the strength to the amorphous samples. There are clusters of atoms in the glassy state of GaAs.

The Raman Spectra of Nanocomposite Clusters of Atoms in Phosphorous-Selenium Glassy State

We make clusters of atoms of the size of less than 1 nanometer by using the density functional theory and from that we obtain the bond lengths corresponding to the minimum energy configuration. We are able to optimize large clusters of atoms and find the vibrational frequencies for each cluster. This calculation provides us with a method to identify the clusters present in an unknown sample of a glass by comparing the experimental Raman frequency with the calculated value. We start with the experimental values of the Raman frequencies of PSe (Phosphorous-Selenium) glass. We calculate the structural parameters of PSe, P4Se, P2Se2, P4Se5, PSe4, P4Se3 clusters of atoms and tabulate the vibrational frequencies. We compare the calculated values with those measured. In this way we find the clusters of atoms present in the glass. Some times, the same number of atoms can be rearranged in a different symmetry. Hence we learn the symmetries of molecules. We find that certain symmetries are broken due to self-organization in the glassy state.