Theoretical study on structural, excitonic and electronic properties of Nb₂O₅ with five crystal structures

Using the first-principles density functional theory, we have studied the structural, excitonic and electronic properties of Nb₂O₅ with five different crystal structures. In this paper, the structural characteristics of monoclinic, orthorhombic and tetragonal systems have been studied. Different from the classical layered structure, Nb₂O₅ is a layered shear structure composed of a certain number of NbO₆ octahedrons. Exciton effect plays an important role in the physical process of semiconductor. The absorption and recombination of excitons directly affect the light absorption and luminescence of semiconductors. In order to explore the strength of exciton effect, the binding energy of exciton is calculated. By calculating the exciton binding energies, we found that the exciton binding energy for monoclinic Nb₂O₅ (the space group is C2/c) and orthorhombic Nb₂O₅ were as high as 69.611 and 75.548 meV. The high exciton binding energy is mainly due to the weak dielectric shielding ability and effective electron hole coulomb interaction. The microcosmic reasons for the difference of exciton binding energies in different crystal structures were analyzed by electrical properties. By analyzing the energy band diagram, it is found that the band gap is closely related to the excitonic binding energy. In general, the position of the atom, the shape of the energy band and the strength of the bond are all factors that contribute to the differences in exciton binding energy.