Anion-Substitution-Induced Nonrigid Variation of Band Structure in SrNbO_3–xN_x (0 ≤ x ≤ 1) Epitaxial Thin Films

Abstract

Pervoskite oxynitrides exhibit rich functionalities such as colossal magnetoresistance and high photocatalytic activity. The wide tunability of physical properties by the N/O ratio makes perovskite oxynitrides promising as optical and electrical materials. However, composition-dependent variation of the band structure, especially under partially substituted composition, is not yet well understood. In this study, we quantitatively analyzed the composition-dependent variation of band structure of a series of SrNbON (0 ≤ x ≤ 1.02) epitaxial thin films. Electrical conductivity decreased along with the increase of N content x as a result of an increase in Nb valence from 4+ to 5+. Optical measurements revealed that the N 2p band is formed at a critical composition between 0.07< x < 0.38, which induces charge-transfer transition (CTT) in the visible-light region. These variations in the band structure were explained by first-principles calculations. However, the CTT energy slightly increased at higher N contents (i.e., lower carrier density) on contrary to the expectation based on the rigid-band-like shift of the Fermi level, which suggests a complex combination of the following band-shifting effects induced by N-substitution: whereas (1) reduction of the Burstein-Moss effect causes CTT energy reduction, (2) enhancement of hybridization between Nb 4d and N 2p orbitals and/or (3) suppression of many-body effects enlarge the band gap energy at larger N content. The band structure variation in perovskite oxynitride as presently elucidated would be a guidepost for future material design.

Modifying Metastable Sr1–xBO3−δ (B = Nb, Ta, and Mo) Perovskites for Electrode Materials

“…These changes in the absorption profile coincide well with experimental and theoretical reports. 1 , 2 , 34 − 36 The first noticeable color change (from white to light blue, t < 0.5 h) is ascribed to the removal of surface defects that are periodically arranged within the lattice ( Figure 2 a). These defect sites are eliminated during the H 2 /Ar treatment, transforming the initially white powder (insulating) to a light blue (conductive).…”

“…As displayed in Table 1 , the calculated ρ at the Fermi level is consistent with reported values. 36 , 65 , 66 Then, to mirror the measured SBO stoichiometries ( Tables S3 and S5 ), the following models were utilized: (i) for SMO, no Sr (V Sr ) and O (V O ) vacancies were incorporated into the corresponding defect-free model, (ii) for SNO, 2 V Sr and 2 V O vacancies were introduced into the corresponding defect-free model, and (iii) for STaO, 10 Na atoms (7 A-site, 3 B-site) and 5 V O were introduced into the corresponding defect-free model. The calculated band structures and density of states (DOS) for defective SBO NPs show that the Fermi level sits in the respective bands ( Figure S12 a–c, dashed red line), revealing that the observed electronic conductivity of these metastable crystals is a result of n-type (SNO and SMO) and p-type (STaO) behavior.…”

Evaluation of Band Alignment of SrNbO2N Using Hard X-ray Photoelectron Spectroscopy

“…This interpretation was supported by the result that the N 1s spectrum had a single component. Because the amount of the second component was ∼5% of the main component and the oxide phase led to a more negative valence band energy, 23 the influence of the defect component on the evaluation of the VBM energy was negligible. To discuss the difference of the VBM position between SrNbO 2 N and SrTiO 3 , the VB spectra were also evaluated by HAXPES.…”

Defect Engineering for Photocatalysis: From Ternary to Perovskite Oxynitrides

“…Using a EuTiO 3 precursor this exchange resulted in a maximum N stoichiometry of 0.75, whereas a Eu 2 Ti 2 O 7 precursor decomposed into Eu 2 O 3 and uncharacterized phases . Oka et al . revealed subtle effects of nitrogen content on the electrical conductivity and optical properties of SrNbO 3−x N x (0≤x≤1.02) thin films.…”

“…This in turn reduced the valence band maximum energy and thus increased the bandgap. The work of Oka et al . suggests that for SrNbO 2 N no enhancement in optical properties is gained beyond this threshold nitrogen content, since the N 2p valence band is fully formed at this composition.…”