The Interplay of Interstitial and Substitutional Copper in Zinc Oxide.

Cu impurities are reported to have significant effects on the electrical and optical properties of bulk ZnO. In this work, we study the defect properties of Cu in ZnO using hybrid quantum mechanical/molecular mechanical (QM/MM)-embedded cluster calculations based on a multi-region approach that allows us to model defects at the true dilute limit, with polarization effects described in an accurate and consistent manner. We compute the electronic structure, energetics, and geometries of Cu impurities, including substitutional and interstitial configurations, and analyze their effects on the electronic structure. Under ambient conditions, CuZn is the dominant defect in the d9 state and remains electronically passive. We find that, however, as we approach typical vacuum conditions, the interstitial Cu defect becomes significant and can act as an electron trap.

High Anisotropic Optoelectronics in Two Dimensional Layered PbSnX2 (X = S/Se).

We systematically study the giant anisotropic optoelectronics in layered PbSnX2 (X = S/Se). The highly anisotropic optoelectronics mainly originates from the asymmetric sublattices SnX, resulting in the anisotropy of photoelectronic properties with fascinating visible light absorption range in single-layer and bilayer PbSnX2. We employ uniaxial strain in both the x and y directions and find an indirect-to-direct band gap transition, while the quasiparticle indirect band gap presents excellent linear scaling with biaxial strain in monolayer PbSnX2. We also demonstrate ultrahigh anisotropic mobilities of electrons (µy > µx) and holes (µx > µy) in both single-layer and bilayer PbSnX2 (X = S/Se), and spin-orbit coupling effects and the increase of layer number significantly reduce exciton binding energies and band gaps. Finally, the strong layer dependence of the band structure is clearly seen when the film thickness is less than 4 layers. Our results provide a fundamental understanding of highly anisotropic PbSnX2 (X = S/Se) and show two potential candidates in photoelectric applications.

Stability and Catalytic Performance of Single-Atom Supported on Ti2 CO2 for Low-Temperature CO Oxidation: A First-Principles Study.

Based on first-principles calculations, the potential of Ti2 CO2 monolayer (MXene) as a single-atom catalyst (SAC) support for 3d transition metal (TM) atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) is studied for CO oxidation. We first screen the support effect according to the stability of a single metal atom and find that Sc and Ti supported on Ti2 CO2 have stronger adsorption energy than the cohesive energy of their bulk counterparts and therefore, we selected Sc and Ti supported on Ti2 CO2 for further catalytic reactions. The stability and the potential catalytic reactivity are verified by electronic structure and charge transfer analysis. Both Eley-Rideal (ER) and Langmuir-Hinshelwood (LH) mechanisms are considered in this study, and lower energy barriers of 0.002 and 0.37 eV were found in the ER mechanism compared to the LH mechanism, which are 0.25 and 0.34 eV for Sc and Ti catalysts, respectively. Moreover, kinetic ER and LH mechanisms are favorable for both Sc- and Ti/Ti2 CO2 because of the comparable energy barrier to other metals and SAC supported on 2D materials. However, Ti/Ti2 CO2 catalyst is thermodynamically unfavorable. Based on these calculations, we propose that Sc supported on Ti2 CO2 is the best catalyst for CO-oxidation. The current study not only broadens the scope of the single-atom Sc catalyst but also extends the consideration of MXene support for catalyst optimization.

Identification of electronic descriptors for catalytic activity of transition-metal and non-metal doped MoS2.

While the d-band theory offers successful electronic descriptors for catalytic activity of transition metals, transition metal compounds still need substantial theoretical input for the identification of reactivity descriptors for fast screening of earth-abundant catalysts. We study transition metal (TM) and non-metal doped MoS2, a promising substitute for noble metals as catalysts for multiple reactions, to clarify how doping modifies the reactivity by regulating the electronic structure of the host. We find that doping can significantly change the density of states (DOS) at band edges and the position of the Fermi level, which renders the S p-band center εp a good descriptor for H adsorption on both TM and non-metal doped MoS2. Dopants to the left of the host elements in the periodic table that have fewer electrons pin the Fermi level into the valence band, and those to the right that have more electrons pin the Fermi level into the conduction band, resulting in separated linear relationships between H binding energy and S p-band center. Moreover, by a close examination of the electronic structure of late TM doped systems, we identify the position of the late TM dopant induced DOS peak near the conduction band minimum (CBM), εTM, as a refined descriptor which shows a linear relationship for H as well as C, N, O adsorption. Finally, we generalize our descriptor to MoSe2 and MoTe2 to include all three anions in transition metal dichalcogenides (TMDs) and show a universal scaling relationship between the H binding energy and anion p-band center. Together we further enhance our understanding on identifying electronic descriptors for TM compounds.

UV-vis spectroscopic studies of low-energy argon-ion-bombarded ion-exchanged glasses.

The ion-exchanged glasses bombarded with 80 eV, 100 eV, and 120 eV argon ions at room temperature are investigated. The optical and structural properties of ion-exchanged glasses before and after bombardment were analyzed by means of a UV-vis spectrometer, x-ray photoelectron spectroscopy, and electron probe micro analysis, respectively. The optical absorption and transmittance spectra of ion-exchanged glasses appear as obvious changes in the UV-visible region after bombardment. The optical absorption band and transmittance properties of ion-exchanged glasses at about 369-900 nm are less sensitive to the ion bombardment energy than that at about 200-280 nm. The changes in binding energy shift and peak area ratios of non-bridging oxygen and bridging oxygen contributions to the O 1s lines were observed with increasing ion beam bombardment energy. Accompanied with out-diffusion of potassium cations during argon ion bombardment, the peak of potassium cations concentration in the exchanged region decreases and moves into the interior of glasses in different degrees. The results show that variation of structure and optical properties of the ion-exchanged glasses are indicative of alterations of the silicate network structure induced by argon ion bombardment, which provide important information for application of the ion-exchanged glasses.