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A study is conducted on surface structures on rutile TiO2(001) by using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density functional calculations. Microfaceting models are proposed that are in good agreement with atomically resolved STM images.

Monolayer transition metal dichalcogenides (TMDs) have been considered as promising materials for various next-generation semiconductor devices. However, carrier doping techniques for TMDs, which are important for device fabrication, have not been completely established yet. Here, we report a monolayer p–n junction formed using in situ substitutional doping during chemical vapor deposition (CVD). We synthesized monolayer MoS2–Nb-doped MoS2 lateral homojunctions using CVD and then characterized their physical and electrical properties. The optimized growth condition enabled us to obtain spatially selective and heavy Nb doping in the edge region of a single-crystalline MoS2, thus resulting in an obvious work function difference between the inner and edge regions of the crystal. The obtained monolayer crystal demonstrated n-type and degenerate p-type semiconducting behaviors in each region, and a clear rectifying behavior across the n-type and p-type interface was observed. We believe that the results obtained can expand the research field of exploring two-dimensional homo p–n junctions, which can be important for realizing various TMD-based devices, such as diodes and field-effect transistors, with low-contact resistance. [ABSTRACT FROM AUTHOR]

Scalable chemical vapor deposition (CVD) of two-dimensional semiconducting materials such as MoS2 and WS2 is a key technology for the application of these materials in real devices. In this work, we demonstrate the growth behavior of WS2 crystals from gaseous precursors, i.e. H2S and WF6, under alkali-metal-free conditions. The WS2 crystal growth exhibits layer-by-layer growth, and its behaviors, such as nucleation and lateral growth, are a thermally activated process: the temperature-dependent grain size and density are well fitted by the Arrhenius equation. The obtained WS2 crystal shows quality comparable to that obtained from metal oxides: the WS2 film shows sharp photoluminescence with a peak width of 54 meV and n-type field-effect transistor operation. Optimizing the growth conditions enabled us to obtain WS2 crystals with a grain size of ∼1.5 μm, which is the largest size ever reported for a transition-metal dichalcogenide grown by gas-source CVD without an alkali-metal promotor. [ABSTRACT FROM AUTHOR]

Graphene can lower spreading resistance owing to its anisotropic electrical resistivity. Although graphene is beneficial for collecting in-plane spreading currents, its low out-of-plane conductivity is a disadvantage. We theoretically study the spreading resistance of single- and multi-layer graphene films coated over a metal substrate. The effect of layer thickness and substrate conductivity on the spreading resistance of graphene is interpreted using electric field and equipotential lines. The results are compared to those obtained using conventional isotropic conducting films. We discuss the scenarios where graphene-coated layers are more beneficial than those coated using conventional materials. We also discuss the sensitivity in detecting the layer thickness of graphene by measuring spreading resistance. [ABSTRACT FROM AUTHOR]

Interlayer coupling in WS2–MoS2 van der Waals heterostructures was studied using low-frequency resonance Raman spectroscopy. The relationship between high-frequency Raman modes and a low-frequency Raman mode were investigated. The Raman intensity of interlayer shear-mode exhibited a linear correlation with the intensity of the two modes A1g and E2g1. The frequency difference between these two modes, A1g−E2g1, was also investigated. Microscopic measurements showed that the intensity ratio A1g/E2g1 is sensitively modulated by the spatially non-uniform electronic potential. On the other hand, A1g−E2g1 is stable and is determined by the number of layers and layer stacking configurations. Therefore, both parameters can be used independently to discuss the quality of vdW heterostructures. Our findings provide convenient markers for studying interlayer coupling of vdW heterostructures using Raman spectroscopy. [ABSTRACT FROM AUTHOR]

The surface structures of rutile Ti[O.sub.2] are examined by using noncontact atomic force microscopy (NC-AFM), scanning tunneling microscopy (STM) and density functional calculations. The decrease of the density of dangling bonds has stabilized the surface energy, thus resulting in the microfaceting missing-row reconstructions.

Two-dimensional (2D) transition-metal dichalcogenides have attracted a considerable amount of attention because of their potential for post-silicon device applications, as well as for exploring fundamental physics in an ideal 2D system. We tested the chemical vapour deposition (CVD) of WS2 using the gaseous precursors WF6 and H2S, augmented by the Na-assistance method. When Na was present during growth, the process created triangle-shaped WS2 crystals that were 10 μm in size and exhibited semiconducting characteristics. By contrast, the Na-free growth of WS2 resulted in a continuous film with metallic behaviour. These results clearly demonstrate that alkali-metal assistance is valid even in applications of gas-source CVD without oxygen-containing species, where intermediates comprising Na, W, and S can play an important role. We observed that the WS2 crystals grown by gas-source CVD exhibited a narrow size distribution when compared with crystals grown by conventional solid-source CVD, indicating that the crystal nucleation occurred almost simultaneously across the substrate, and that uniform lateral growth was dominant afterwards. This phenomenon was attributed to the suppression of inhomogeneous nucleation through the fast and uniform diffusion of the gas-phase precursors, supported by the Na-assisted suppression of the fast reactions between WF6 and H2S. [ABSTRACT FROM AUTHOR]