A multistep diffusion-mediated process was developed to control the nucleation density, size, and lateral growth rate of WSe domains on c-plane sapphire for the epitaxial growth of large area monolayer films by gas source chemical vapor deposition (CVD). The process consists of an initial nucleation step followed by an annealing period in HSe to promote surface diffusion of tungsten-containing species to form oriented WSe islands with uniform size and controlled density. The growth conditions were then adjusted to suppress further nucleation and laterally grow the WSe islands to form a fully coalesced monolayer film in less than 1 h. Postgrowth structural characterization demonstrates that the WSe monolayers are single crystal and epitaxially oriented with respect to the sapphire and contain antiphase grain boundaries due to coalescence of 0° and 60° oriented WSe domains. The process also provides fundamental insights into the two-dimensional (2D) growth mechanism. For example, the evolution of domain size and cluster density with annealing time follows a 2D ripening process, enabling an estimate of the tungsten-species surface diffusivity. The lateral growth rate of domains was found to be relatively independent of substrate temperature over the range of 700-900 °C suggesting a mass transport limited process, however, the domain shape (triangular versus truncated triangular) varied with temperature over this same range due to local variations in the Se/W adatom ratio. The results provide an important step toward atomic level control of the epitaxial growth of WSe monolayers in a scalable process that is suitable for large area device fabrication.
Unlike the vast majority of transition metal dichalcogenides which are semiconductors, vanadium disulfide is metallic and conductive. This makes it particularly promising as an electrode material in lithium-ion batteries. However, vanadium disulfide exhibits poor stability due to large Peierls distortion during cycling. Here we report that vanadium disulfide flakes can be rendered stable in the electrochemical environment of a lithium-ion battery by conformally coating them with a ~2.5 nm thick titanium disulfide layer. Density functional theory calculations indicate that the titanium disulfide coating is far less susceptible to Peierls distortion during the lithiation-delithiation process, enabling it to stabilize the underlying vanadium disulfide material. The titanium disulfide coated vanadium disulfide cathode exhibits an operating voltage of ~2 V, high specific capacity (~180 mAh g
−1
@200 mA g
−1
current density) and rate capability (~70 mAh g
−1
@1000 mA g
−1
), while achieving capacity retention close to 100% after 400 charge−discharge steps.
Remarkable properties of layered metal dichalcogenides and their potential applications in various fields have raised intense interest worldwide. We report tens of microns-sized ultrathin single crystal SnS 2 flakes grown on amorphous substrates using a simple one-step thermal coevaporation process. X-ray pole figure analysis reveals that a majority of flakes are oriented with the (0001) plane parallel to the substrate and a preferred fiber texture. For few-layer-thick SnS 2 , Moire patterns of 6-fold and 12-fold symmetries are observed by transmission electron microscopy imaging and diffraction. These patterns result from the relative rotation between SnS 2 layers in the ultrathin flake. The 12-fold symmetry is consistent with a known quasicrystal pattern. The photoluminescence spectrum supports that these ultrathin flakes possess a direct bandgap. Carrier lifetime measured by time resolved photoluminescence of a single flake is a few nanoseconds. These results improve our understanding of the formation and shape of ultrathin SnS 2 flakes.
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