Study of electronic and optical properties of ZnO clusters using TDDFT method

We present a systematic computational study of the electronic and optical properties of (ZnO)_n clusters with $n=1$–16, 24 and 36 using time dependent density functional calculations (TDDFT). The calculations bring out the role of dimensionality of clusters, multiple connectivity of Zn–O network and origin of blue-shift in the absorption spectra. This is accompanied by large value of optical gap in 2D clusters as compared with 3D structures. The observed optical gap are strongly related to the size, geometrical shape, symmetry and dimensionality of the structures. Our calculations also shows the blue shift of the spectral lines from planar and ringlike structures to 3D spheroidal clusters due to the occurrence of the dipole-forbidden transitions in the optical gap for highly symmetric structures. A microscopic analysis of the molecular orbitals and eigenvalue spectra show that the electronic transitions occur between the non-bonding p-states of Oxygen and unoccupied sp-hybrid orbitals of Zinc atoms. We additionally use many body perturbation theory (MBPT) method to validate the TDDFT results. (paper)