Theoretical and Experimental Study on Exciton Properties of TT-, T-, and H-Nb<sub>2</sub>O<sub>5</sub>
Yongfang Jia, Mi Zhong, Feng Yang, Chuanhui Liang, Huanxin Ren, Biao Hu, Qijun Liu, Hui Zhao, Yong Zhang, Yong Zhao
Abstract
We have theoretically and experimentally studied exciton properties of three normal Nb2O5 polymorphs, including pseudo-hexagonal (TT-), orthorhombic (T-), and monoclinic (H-) phases. Theoretical work shows that the exciton binding energy for H-Nb2O5 is 73.553 meV, much higher than that for TT-Nb2O5 (3.515 meV) and T-Nb2O5 (24.071 meV). The H-Nb2O5 material, as characterized by field-induced surface photovoltage spectroscopy, uniquely presents bound exciton states near the bottom of the conduction band. Thermogravimetric analysis and photoluminescence (PL) lifetime demonstrate that the bound exciton states in H-Nb2O5 originate from interactions between the exciton pairs and oxygen vacancy sites. Such excitons in H-Nb2O5 can be effectively dissociated into free electrons and holes with the incorporation of the oxygen vacancies, leading to the longer PL lifetime. This work allows one to find correlations between material structures and exciton properties, which will provide some basic knowledge to optimize materials for optoelectronic and photonic performance.