Fundamental Resolution of Difficulties in the Theory of Charged Point Defects in Semiconductors

Optically active vibrations of extrinsic iron defects in zinc oxide

In the paper, using Fe3+ ions in a non-centrosymmetric ZnO lattice as an example, we present a theoretical study of impurity-induced vibrations. The modeling was carried out within the framework of density functional theory using the generalized gradient approximation and the potential-based method. The C3v lattice distortions around a trivalent impurity were computed. Independent calculation methods give similar results, which indicates their reliability. We calculated local symmetrized phonon densities of states in Fe-doped ZnO and determined the frequencies of the impurity vibrations of different symmetry types induced by charged Fe ions. The results of lattice-dynamic calculations were used to interpret the structure of the phonon sideband that accompanies the zero-phonon line in the polarized emission spectra associated with intracenter transitions of Fe3+. We believe that the approach used allows us to objectively evaluate the contribution of charged impurities with a weak electron-phonon coupling and surrounding ions to the formation of the main peaks observed in the vibronic spectrum of crystals.

Electronic and magnetic properties of charged point defects in monolayer CrI3

The two-dimensional magnetic material CrI₃ has gained considerable attention owing to its promising applications in photoelectric and spin-related devices. Recently, various structural defects in CrI₃ have been identified; however, the charge states of these defects have been mainly ignored. Here, we report on an investigation of the charged defects in monolayer CrI₃, focused on the electronic and magnetic properties of the five most stable point defects using first-principles calculations. For positively charged I vacancies and negatively charged Cr vacancies, a blue- and red-shift of defect states near the Fermi level can be observed because of the atom relaxation. Our results also indicate that, among the five defects, the Cr interstitial defect has the smallest ionization energy of 0.34 eV, which makes its ionization easiest. Furthermore, a 0.2 μ_B enhancement of the magnetic moment on the nearest Cr atom can be found for the I vacancy and Cr interstitial defect. The investigation contributes to the atomic-scale comparison and understanding of the charged defects of monolayer CrI₃.

Oxygen vacancy regulation strategy in V-Nb mixed oxides catalyst for enhanced aerobic oxidative desulfurization performance

Bimetallic oxide is a potential catalyst for oxidative desulfurization of fuel. Thus, an appropriate method is needed to improve its catalytic performance. Manufacturing defect is an effective means. In this contribution, an oxygen vacancies (OVs) regulation strategy for enhancing the catalytic activity of bimetallic oxide is proposed. Density functional theory (DFT) calculations show that the crystal phase has a huge influence on the generation energy of oxygen vacancies, so a series of V-Nb mixed oxide with different crystal phases are synthesized. Detailed characterizations show that the as-prepared tetragonal V-Nb mixed oxide (T-VNbO_x) has lower OVs formation energy and larger OVs concentration (compared to orthorhombic V-Nb mixed oxides, O-VNbO_x). Owing to the activation of OVs, the catalytic activity of T-VNbO_x was significantly enhanced to form ultra-deep oxidative desulfurization. In addition, T-VNbO_x can be cycled eight times without significantly degrading the desulfurization performance.