Abstract: We present electronic band structures of transparent oxides calculated using the Tran-Blaha modified Becke-Johnson (TB-mBJ) potential. We studied the basic n-type conducting binary oxides In2O3, ZnO, CdO and SnO2 along with the p-type conducting ternary oxides delafossite CuXO2 (X = Al, Ga, In) and spinel ZnX2O4 (X = Co, Rh, Ir). The results are presented for calculated band gaps and effective electron masses. We discuss the improvements in the band gap determination using TB-mBJ compared to the standard generalized gradient approximation (GGA) in density functional theory (DFT) and also compare the electronic band structure with available results from the quasiparticle GW method. It is shown that the calculated band gaps compare well with the experimental and GW results, although the electron effective mass is generally overestimated.

Abstract: We examine the electronic structure of the family of ternary zinc spinel oxides ZnX2O4 (X=Al, Ga and In). The band gap of ZnAl2O4 calculated using density functional theory (DFT) is 4.25 eV and is overestimated compared with the experimental value of 3.83.9 eV. The DFT band gap of ZnGa2O4 is 2.82 eV and is underestimated compared with the experimental value of 4.45.0 eV. Since DFT typically underestimates the band gap in the oxide system, the experimental measurements for ZnAl2O4 probably require a correction. We use two first-principles techniques capable of describing accurately the excited states of semiconductors, namely the GW approximation and the modified BeckeJohnson (MBJ) potential approximation, to calculate the band gap of ZnX2O4. The GW and MBJ band gaps are in good agreement with each other. In the case of ZnAl2O4, the predicted band gap values are >6 eV, i.e. ~2 eV larger than the only reported experimental value. We expect future experimental work to confirm our results. Our calculations of the electron effective masses and the second band gap indicate that these compounds are very good candidates to act as transparent conducting host materials.

Abstract: It has been theoretically proposed that the spin textures of surface states in a topological insulator can be directly transferred to graphene by means of the proximity effect, which is very important for realizing the two-dimensional topological insulator based on graphene. Here we report the anomalous magnetotransport properties of graphene-topological insulator Bi2Se3 heterojunctions, which are sensitive to the electronic coupling between graphene and the topological surface state. The coupling between the p_z orbitals of graphene and the p orbitals of the surface states on the Bi2Se3 bottom surface can be enhanced by applying a perpendicular negative magnetic field, resulting in a giant negative magnetoresistance at the Dirac point up to about -91%. Obvious resistances dip in the transfer curve at the Dirac point is also observed in the hybrid devices, which is consistent with theoretical predictions of the distorted Dirac bands with nontrivial spin textures inherited from the Bi2Se3 surface states.

Abstract: Although largely studied, contradictory results on nickel oxide (NiO) properties can be found in the literature. We herein propose a comprehensive study that aims at leveling contradictions related to NiO materials with a focus on its conductivity, surface properties, and the intrinsic charge defects compensation mechanism with regards to the conditions preparation. The experiments were performed by in situ photo-electron spectroscopy, electron energy loss spectroscopy, and optical as well as electrical measurements on polycrystalline NiO thin films prepared under various preparation conditions by reactive sputtering. The results show that surface and bulk properties were strongly related to the deposition temperature with in particular the observation of Fermi level pinning, high work function, and unstable oxygen-rich grain boundaries for the thin films produced at room temperature but not at high temperature (>200 degrees C). Finally, this study provides substantial information about surface and bulk NiO properties enabling to unveil the origin of the high electrical conductivity of room temperature NiO thin films and also for supporting a general electronic charge compensation mechanism of intrinsic defects according to the deposition temperature.

Abstract: Twin boundaries have been exploited to stabilize ultrafine grains and improve mechanical properties of nanomaterials. The production of the twin boundaries and nanotwins is however prohibitively challenging in carbide ceramics. Using a scanning transmission electron microscope as a unique platform for atomic-scale structure engineering, we demonstrate that twin platelets could be produced in carbides by engineering antisite defects. The antisite defects at metal sites in various layered ternary carbides are collectively and controllably generated, and the metal elements are homogenized by electron irradiation, which transforms a twin-like lamellae into nanotwin platelets. Accompanying chemical homogenization, alpha-Ti3AlC2 transforms to unconventional beta-Ti3AlC2. The chemical homogeneity and the width of the twin platelets can be tuned by dose and energy of bombarding electrons. Chemically homogenized nanotwins can boost hardness by similar to 45%. Our results provide a new way to produce ultrathin (< 5 nm) nanotwin platelets in scientifically and technologically important carbide materials and showcase feasibility of defect engineering by an angstrom-sized electron probe.

Abstract: We examine the propagation of the recently discovered electron vortex beams in a longitudinal magnetic field. We consider both the Aharonov-Bohm configuration with a single flux line and the Landau case of a uniform magnetic field. While stationary Aharonov-Bohm modes represent Bessel beams with flux- and vortex-dependent probability distributions, stationary Landau states manifest themselves as nondiffracting Laguerre-Gaussian beams. Furthermore, the Landau-state beams possess field- and vortex-dependent phases: (i) the Zeeman phase from coupling the quantized angular momentum to the magnetic field and (ii) the Gouy phase, known from optical Laguerre-Gaussian beams. Remarkably, together these phases determine the structure of Landau energy levels. This unified Zeeman-Landau-Gouy phase manifests itself in a nontrivial evolution of images formed by various superpositions of modes. We demonstrate that, depending on the chosen superposition, the image can rotate in a magnetic field with either (i) Larmor, (ii) cyclotron (double-Larmor), or (iii) zero frequency. At the same time, its centroid always follows the classical cyclotron trajectory, in agreement with the Ehrenfest theorem. Interestingly, the nonrotating superpositions reproduce stable multivortex configurations that appear in rotating superfluids. Our results open an avenue for the direct electron-microscopy observation of fundamental properties of free quantum-electron states in magnetic fields.

Abstract: The major mechanism responsible for plasmonic enhancement of titanium dioxide photocatalysis using gold nanoparticles is still under contention. This work introduces an experimental strategy to disentangle the significance of the charge transfer and near-field mechanisms in plasmonic photocatalysis. By controlling the thickness and conductive nature of a nanoparticle shell that acts as a spacer layer separating the plasmonic metal core from the TiO2 surface, field enhancement or charge transfer effects can be selectively repressed or evoked. Layer-by-layer and in situ polymerization methods are used to synthesize gold core–polymer shell nanoparticles with shell thickness control up to the sub-nanometer level. Detailed optical and electrical characterization supported by near-field simulation models corroborate the trends in photocatalytic activity of the different systems. This approach mainly points at an important contribution of the enhanced near field.

Abstract: The 3D reconstruction of a tilt series for electron tomography is mostly carried out using the weighted backprojection (WBP) algorithm or using one of the iterative algorithms such as the simultaneous iterative reconstruction technique (SIRT). However, it is known that these reconstruction algorithms cannot compensate for the missing wedge. Here, we apply a new reconstruction algorithm for electron tomography, which is based on compressive sensing. This is a field in image processing specialized in finding a sparse solution or a solution with a sparse gradient to a set of ill-posed linear equations. Therefore, it can be applied to electron tomography where the reconstructed objects often have a sparse gradient at the nanoscale. Using a combination of different simulated and experimental datasets, it is shown that missing wedge artefacts are reduced in the final reconstruction. Moreover, it seems that the reconstructed datasets have a higher fidelity and are easier to segment in comparison to reconstructions obtained by more conventional iterative algorithms.