Wafer-scale single-crystal monolayer graphene grown on sapphire substrate.

The growth of inch-scale high-quality graphene on insulating substrates is desirable for electronic and optoelectronic applications, but remains challenging due to the lack of metal catalysis. Here we demonstrate the wafer-scale synthesis of adlayer-free ultra-flat single-crystal monolayer graphene on sapphire substrates. We converted polycrystalline Cu foil placed on Al2O3(0001) into single-crystal Cu(111) film via annealing, and then achieved epitaxial growth of graphene at the interface between Cu(111) and Al2O3(0001) by multi-cycle plasma etching-assisted-chemical vapour deposition. Immersion in liquid nitrogen followed by rapid heating causes the Cu(111) film to bulge and peel off easily, while the graphene film remains on the sapphire substrate without degradation. Field-effect transistors fabricated on as-grown graphene exhibited good electronic transport properties with high carrier mobilities. This work breaks a bottleneck of synthesizing wafer-scale single-crystal monolayer graphene on insulating substrates and could contribute to next-generation graphene-based nanodevices.

ZnSe and ZnTe as tunnel barriers for Fe-based spin valves.

Owing to their use in the optoelectronic industry, we investigate whether ZnSe and ZnTe can be utilised as tunnel barrier materials in magnetic spin valves. We perform ab initio electronic structure and linear response transport calculations based on self-interaction-corrected density functional theory for both Fe/ZnSe/Fe and Fe/ZnTe/Fe junctions. In the Fe/ZnSe/Fe junction the transport is tunneling-like and a symmetry-filtering mechanism is at play, implying that only the majority spin electrons with Δ1 symmetry are transmitted with large probability, resulting in a potentially large tunneling magnetoresistance (TMR) ratio. As such, the transport characteristics are similar to those of the Fe/MgO/Fe junction, although the TMR ratio is lower for tunnel barriers of similar thickness due to the smaller bandgap of ZnSe as compared to that of MgO. In the Fe/ZnTe/Fe junction the Fermi level is pinned at the bottom of the conduction band of ZnTe and only a giant magnetoresistance effect is found. Our results provide evidence that chalcogenide-based tunnel barriers can be utilised in spintronics devices.

Theoretical Insights into the Hydrogen Evolution Reaction on VGe2N4 and NbGe2N4 Monolayers.

Catalytically active sites at the basal plane of two-dimensional monolayers for hydrogen evolution reaction (HER) are important for the mass production of hydrogen. The structural, electronic, and catalytic properties of two-dimensional VGe2N4 and NbGe2N4 monolayers are demonstrated using the first-principles calculations. The dynamical stability is confirmed through phonon calculations, followed by computation of the electronic structure employing the hybrid functional HSE06 and PBE+U. Here, we introduced two strategies, strain and doping, to tune their catalytic properties toward HER. Our results show that the HER activity of VGe2N4 and NbGe2N4 monolayers are sensitive to the applied strain. A 3% tensile strain results in the adsorption Gibbs free energy (ΔG H*) of hydrogen for the NbGe2N4 monolayer of 0.015 eV, indicating better activity than Pt (-0.09 eV). At the compressive strain of 3%, the ΔG H* value is -0.09 eV for the VGe2N4 monolayer, which is comparable to that of Pt. The exchange current density for the P doping at the N site of the NbGe2N4 monolayer makes it a promising electrocatalyst for HER (ΔG H* = 0.11 eV). Our findings imply the great potential of the VGe2N4 and NbGe2N4 monolayers as electrocatalysts for HER activity.

VS2 wrapped Si nanowires as core-shell heterostructure photocathode for highly efficient photoelectrochemical water reduction performance.

Interfacing an electrocatalyst with photoactive semiconductor surfaces is an emerging strategy to enhance the photocathode performance for the solar water reduction reaction. Herein, a core-shell heterostructure photocathode consisting of vanadium disulfide (VS2) as a 2D layered electrocatalyst directly deposited on silicon nanowire (Si NWs) surface is realized via single-step chemical vapor deposition towards efficient hydrogen evolution under solar irradiation. In an electrochemical study, 2D VS2/Si NWs photocathode exhibits a saturated photocurrent density (17 mA cm-2) with a maximal photoconversion efficiency of 10.8% at -0.53 V vs. RHE in neutral electrolyte condition (pH∼7). Under stimulated irradiation, the heterostructure photocathode produces a hydrogen gas evolution around 23 µmol cm-2 h-1 (at 0 V vs. RHE). Further, electrochemical impedance spectroscopy (EIS) analysis reveals that the high performance of the core-shell photocathode is associated with the generation of the high density of electron-hole pairs and the separation of photocarriers with an extended lifetime. Density functional theory calculations substantiate that core-shell photocathodes are active at very low Gibbs free energy (ΔGH*) with abundant hydrogen evolution reaction (HER) active sulphur sites. The charge density difference plot with Bader analysis of heterostructure reveals the accumulation of electrons on the sulphur sites via modulating the electronic band structure near the interface. Thus, facilitates the barrier-free charge transport owing to the synergistic effect of Si NWs@2D-VS2 core-shell hybrid photocatalyst for enhanced solar water reduction performance.

Exploring the Superior Anchoring Performance of the Two-Dimensional Nanosheets B2C4P2 and B3C2P3 for Lithium-Sulfur Batteries.

Potential anchoring materials in lithium-sulfur batteries help overcome the shuttle effect and achieve long-term cycling stability and high-rate efficiency. The present study investigates the two-dimensional nanosheets B2C4P2 and B3C2P3 by employing density functional theory calculations for their promise as anchoring materials. The nanosheets B2C4P2 and B3C2P3 bind polysulfides with adsorption energies in the range from -2.22 to -0.75 and -2.43 to -0.74 eV, respectively. A significant charge transfer occurs from the polysulfides, varying from -0.74 to -0.02e and -0.55 to -0.02e for B2C4P2 and B3C2P3, respectively. Upon anchoring the polysulfides, the band gap of B3C2P3 reduces, leading to enhanced electrical conductivity of the sulfur cathode. Finally, the calculated barrier energies of B2C4P2 and B3C2P3 for Li2S indicate fast diffusion of Li when recharged. These enthralling characteristics propose that the nanosheets B2C4P2 and B3C2P3 could reduce the shuttle effect in Li-S batteries and significantly improve their cycle performance, suggesting their promise as anchoring materials.

Inducing Half-Metallicity in Monolayer MoSi2N4.

First-principles calculations are performed for the recently synthesized monolayer MoSi2N4 [Science 369, 670-674 (2020)]. We show that N vacancies are energetically favorable over Si vacancies, except for Fermi energies close to the conduction band edge in the N-rich environment, and induce half-metallicity. N and Si vacancies generate magnetic moments of 1.0 and 2.0 µB, respectively, with potential applications in spintronics. We also demonstrate that N and Si vacancies can be used to effectively engineer the work function.

Electronic structure of Pr2MnNiO6 from x-ray photoemission, absorption and density functional theory.

The electronic structure of double perovskite Pr2MnNiO6 was studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 4+ and 2+ states respectively. Based on charge transfer multiplet analysis of the 2p XPS spectra of both ions, we find charge transfer energies [Formula: see text] of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni2+ and Mn4+ ions reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the ionic values. The partial density of states clearly show a charge transfer character of the system for U - J [Formula: see text] 2 eV. The O 1s edge absorption spectra reveal a band gap of 0.9 eV, which is close to the value estimated from analysis of Ni and Mn 2p photoemission and absorption spectra. The combined analysis of nature of spectroscopic data and first principles calculations reveal that the material is a p - d type charge transfer insulator with an intermediate covalent character according to the Zannen-Sawatzy-Allen phase diagram.

Nature of transport gap and magnetic order in zircon and scheelite type DyCrO4 from first principles.

Our first principles density functional theory calculations within GGA + U approximation reveal that the nature of transport gaps in the zircon and scheelite phases of DyCrO(4) are quite different. While in the scheelite phase the origin of the gap is more like that of the Mott-Hubbard systems, in the zircon phase the origin is not strictly a Mott-Hubbard or a charge transfer type. In the framework of the Zaanen-Sawatsky-Allen phase diagram, the DyCrO(4) in its zircon phase could be placed in the intermediate regime between the charge transfer and Mott-Hubbard insulators. On the issue of ground state magnetic order in these two phases, where no consensus exists so far from experimental observations, we have performed GGA and GGA + U calculations on various possible magnetic configurations. We clearly establish from our theoretical calculations that the ferrimagnetic order, where ferromagnetic Dy and Cr sublattice are aligned antiparallel to each other, is the ground state in the zircon phase, while in the scheelite phase competing long-range antiferromagnetic orders are observed. Our estimation of various superexchange interactions indicate that competing ferro- and antiferro-magnetic interactions exist which would explain the experimental observation of metamagnetic transitions on application of a small external magnetic field in these systems.