Electronic Structures of Oxygen-deficient Ta2O5

We provide a first-principles description of the crystalline and oxygen-deficient Ta2O5 using refined computational methods and models. By performing calculations on a number of candidate structures, we determined the low-temperature phase and several stable oxygen vacancy configurations, which are notably different from the previous results. The most stable charge-neutral vacancy site induces a shallow level near the bottom of conduction band. Stability of different charge states is studied. Based on the results, we discuss the implications of the level structures on experiments, including the leakage current in Ta2O5-based electronic devices and catalysts.Comment: 23 pages, 4 figures and 2 tables in the body texts plus 4 figures in the supplementary materia

Theoretical Scanning Tunneling Microscopy Images of the Ga-rich GaAs(001)-(4×2) Surface(STM-GaAs)

application/pdfTotal energy calculations have been performed for the Ga-rich GaAs(001)-(4×2) surface using first-principles plane-wave pseudopotential techniques. There are two distinctly different structural models proposed for the Ga-rich (4×2) surface. The Ga-model proposed by Biegelsen et al, consists of two Ga dimers on the top layer and another Ga dimer at the third layer, whereas the As-model by Skala et al. consists of two As dimers on the top layer and two Ga dimers in the second layer. Calculated results show that the As-model can be safely ruled out since the As-model is energetically unstable with respect to the Ga-model. The Ga-model, on the other hand, has a problem that there is apparent discrepancy between the STM image and the surface geometry of the Ga-model. It is found that the local density of states near the Fermi level are significantly affected by the existence of the third layer Ga dimers and that the Ga-dimer atoms on the top layer asymmetrically contribute to the STM image. The calculated charge distributions for the Ga-model can explain the observed STM image quite well.紀要類(bulletin)493224 bytesdepartmental bulletin pape

Possible Magnetic Behavior in Oxygen-deficient {\beta}-PtO2

We studied the electronic properties of beta-platinum dioxide ({\beta}-PtO2), a catalytic material, based on density functional theory. Using the GGA+U method which reproduces the GW band structures and the experimental structural parameters, we found that the creation of an oxygen vacancy will induce local magnetic moment on the neighboring Pt and O atoms. The magnetism originates not only from the unpaired electrons that occupy the vacancy induced gap state, but also from the itinerant valence electrons. Because of antiferromagnetic (AF) coupling and the localized nature of gap states, the total magnetic moment is zero for charge-neutral state (V_o^0) and is ~ 1 \mu B for singly-charged states (V_o^\mu). Calculation of grand potential shows that, the three charge states (V_o^0, V_o^\pm) are of the same stability within a small region, and the negatively charged state (V_o^-) is energetically favored within a wide range of the band gap. On this basis we discussed the implication on catalytic behavior.Comment: 45 pages, 11 figures, 3 table