Dual Role of Adsorbent and Non-monotonic Transfer p-Doping of Diamond.

Exposure to oxygen is usually detrimental for materials and devices as it leads to undesirable surface oxidation or even deeper corrosion. However, experiments with hydrogen-terminated H-diamond show that oxygen adsorption plays an instrumental role in inducing the p-type surface conductivity. Using first-principles calculations, we explore how the surface-physisorbed molecular O2 serves as an electron acceptor in the transfer doping of diamond. On the other hand, calculations reveal that in a chemisorbed state, oxygen groups substitute H, which lowers the bands in diamond and inhibits the transfer doping. This explains the non-monotonic carrier density dependence on the exposure to oxygen (or, similarly, other adsorbent-acceptor). We further find that ozone can be more efficient for p-type doping of H-diamond due to O3 having lower LUMO energy levels than in molecular O2.

Atomic-scale determination of spontaneous magnetic reversal in oxide heterostructures.

Interfaces between transition metal oxides are known to exhibit emerging electronic and magnetic properties. Here we report intriguing magnetic phenomena for La2/3Sr1/3MnO3 films on an SrTiO3 (001) substrate (LSMO/STO), where the interface governs the macroscopic properties of the entire monolithic thin film. The interface is characterized on the atomic level utilizing scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS), and density functional theory (DFT) is employed to elucidate the physics. STEM-EELS reveals mixed interfacial stoichiometry, subtle lattice distortions, and oxidation-state changes. Magnetic measurements combined with DFT calculations demonstrate that a unique form of antiferromagnetic exchange coupling appears at the interface, generating a novel exchange spring-type interaction that results in a remarkable spontaneous magnetic reversal of the entire ferromagnetic film, and an inverted magnetic hysteresis, persisting above room temperature. Formal oxidation states derived from electron spectroscopy data expose the fact that interfacial oxidation states are not consistent with nominal charge counting. The present work demonstrates the necessity of atomically resolved electron microscopy and spectroscopy for interface studies. Theory demonstrates that interfacial nonstoichiometry is an essential ingredient, responsible for the observed physical properties. The DFT-calculated electrostatic potential is flat in both the LSMO and STO sides (no internal electric field) for both Sr-rich and stoichiometric interfaces, while the DFT-calculated charge density reveals no charge transfer/accumulation at the interface, indicating that oxidation-state changes do not necessarily reflect charge transfer and that the concept of polar mismatch is not applicable in metal-insulator polar-nonpolar interfaces.

Nobler than the Noblest: Noncubic Gold Microcrystallites.

Conventional gold comprising the cubic lattice is universally known for its stability. However, well known to chemists and metallurgists, this nobility is challenged by reagents such as aqua regia, which dissolve gold to form a salt solution. Among metals, mercury blends with gold to form amalgam, otherwise transition metals such as copper tend to interact with gold surfaces in electrochemical media. Herein, we report a combined experimental and theoretical investigation of the stability of Au microcrystallites bearing unconventional crystal lattices that exhibit enhanced stability towards Hg and aqua regia and practically no interaction with Cu during electroless plating. The unconventional gold is undoubtedly nobler.

Effect of Nitrogen Substitution in V2 O3 on the Metal-Insulator Transition.

The effect of N-doping on the paramagnetic-antiferromagnetic transition associated with the metal-insulator (M-I) transition of V2 O3 at 150 K has been studied in bulk samples as well as in nanosheets. The magnetic transition temperature of V2 O3 is lowered to ∼120 K in the N-doped samples. Electrical resistivity data also indicate a similar lowering of the M-I transition temperature. First-principles DFT calculations reveal that anionic (N) substitution and the accompanying oxygen vacancies reduce the energy of the high-temperature metallic corundum phase relative to the monoclinic one leading to the observed reduction in Nèel temperature. In the electronic structure of N-substituted V2 O3 , a sub-band of 2p states of trivalent anion (N) associated with its strong bond with the vanadium cation appears at the top of the band of O(2p) states, the 3d-states of V being slightly higher in energy. Its band gap is thus due to crystal field splitting of the degenerate d-orbitals of vanadium and superexchange interaction, which reduces notably (ΔEg =-0.4 eV) due to their hybridization with the 2p states of nitrogen. A weak magnetic moment arises in the monoclinic phase of N-substituted V2 O3 with O-vacancies, with a moment of -1 µB /N localized on vanadium atoms in the vicinity of oxygen vacancies.

2D-GaS as a Photocatalyst for Water Splitting to Produce H2.

Using first-principles local and hybrid density functional theoretical calculations, a thickness-dependent electronic structure of layered GaS is determined, and it is shown that 2D GaS has an electronic structure with valence and conduction bands that straddle the redox potentials of hydrogen evolution reaction and oxygen evolution reaction up to a critical thickness (<5.5 nm). Here, simulations of adsorption of H2O on nanoscale GaS reveal that localized electronic states at its edges appear in the gap and strengthen the interaction with H2O, further activating the surface atomic sites. It is thus predicted that GaS synthesized with a controlled thickness and preferred edges may be an efficient catalyst for photocatalytic splitting of water. Experiments that verify some of the predictions in this study are presented, and it is shown that GaS is effective in absorption of light and evolution of H2 (887 µmol h(-1) g(-1)) in the presence of aqueous solution of hydrazine (1% v/v). This study should open up the use of nanoscale GaS in conversion of solar energy into environment-friendly chemical energy in the form of hydrogen.

Extraordinary Changes in the Electronic Structure and Properties of CdS and ZnS by Anionic Substitution: Cosubstitution of P and Cl in Place of S.

Unlike cation substitution, anion substitution in inorganic materials such as metal oxides and sulfides would be expected to bring about major changes in the electronic structure and properties. In order to explore this important aspect, we have carried out first-principles DFT calculations to determine the effects of substitution of P and Cl on the properties of CdS and ZnS in hexagonal and cubic structures and show that a sub-band of the trivalent phosphorus with strong bonding with the cation appears in the gap just above the valence band, causing a reduction in the gap and enhancement of dielectric properties. Experimentally, it has been possible to substitute P and Cl in hexagonal CdS and ZnS. The doping reduces the band gap significantly as predicted by theory. A similar decrease in the band gap is observed in N and F co-substituted in cubic ZnS. Such anionic substitution helps to improve hydrogen evolution from CdS semiconductor structures and may give rise to other applications as well.

Adsorption and splitting of H2S on 2D-ZnO(1-x)N(y): first-principles analysis.

We present a thorough analysis of molecular adsorption of a toxic gas, H2S, on pristine, defective and N-substituted 2D-ZnO using first-principles simulations within density functional theory and the parameterized form of van der Waals (vdW) interaction. We find that the binding of H2S with pristine 2D-ZnO is relatively weak (adsorption energy EA = -29 to -36 kJ mol(-1)) as it is mainly through the vdW interaction. However, substitutional nitrogen doping in 2D-ZnO leads to a drastic increase in the adsorption energy (EA = -152 kJ mol(-1)) resulting in dissociation of H2S molecules. This originates fundamentally from a strong covalent bonding interaction between an unpaired electron in the p-orbital of nitrogen and an electron in the s-orbital of H. While O-vacancy in 2D-ZnO has little effect on its interaction with H2S at lower coverages, a strong interaction at higher coverages leads to splitting of H2S and formation of H2 molecules. Our work shows that 2D-ZnO is a promising material to facilitate capturing of toxic H2S from the environment and at the same time converting it to a green source of energy.

Graphene: a reusable substrate for unprecedented adsorption of pesticides.

Unprecedented adsorption of chlorpyrifos (CP), endosulfan (ES), and malathion (ML) onto graphene oxide (GO) and reduced graphene oxide (RGO) from water is reported. The observed adsorption capacities of CP, ES, and ML are as high as ~1200, 1100, and 800 mg g(-1) , respectively. Adsorption is found to be insensitive to pH or background ions. The adsorbent is reusable and can be applied in the field with suitable modifications. A first-principles pseudopotential-based density functional analysis of graphene-water-pesticide interactions showed that the adsorption is mediated through water, while direct interactions between graphene and the pesticides is rather weak or unlikely.