Density Functional Theory Study of Small SiC Nanoclusters

A comparative study of the results obtained in the density functional theory (DFT) using a localized and that a plane-wave basic sets has been performed. The systems chosen are the small semiconductor binary SimCn nanoclusters mainly having m + n <= 5 and the results are the various physical properties i.e., the stability, structural, electronic, vibrational, optical and Raman scattering properties. The localized basis set employed consists of the contracted Gaussian orbitals in the B3LYP-DFT method and the chosen method using the plane-wave set is the usual pseudopotential method in generalized gradient approximation. The time-dependent density functional theory is employed for predicting the optical properties for the nanoclusters. The results predicted by both the above formalisms are quite similar e.g., the most stable structures but are marginally different for some properties. The binding energies obtained in the B3LYP-DFT method are approximately 5% smaller as compared to those in the pseudopotential method. In the case of the SiC cluster, the reported experimental value of 2.30 eV is very close to the pseudopotential value of 2.35 eV but far from the B3LYP-DFT value of 2.12 eV. The vibrational frequencies obtained in the pseudopotential method are in better agreement with the experimental data as compared to those calculated in the B3LYP-DFT method. The behaviour seen earlier in the isolated C and Si nanoclusters is preserved in the SiC nanoclusters. In the most stable configurations, the structures containing the carbon atoms in majority are linear chains whereas the clusters having silicon atoms in majority are either planar or three-dimensional ones. In Si2C formula, both the linear and triangular configurations are equally stable. The results for the various physical properties for the various structures are seen to be in agreement with the available experimental data. Also, a number of new cluster configurations have been predicted which are highly stable but not reported in the literature. The growth of these most stable structures should be possible in the experiments.