2D metallic transition-metal dichalcogenides (MTMDs) have recently emerged as a new class of materials for the engineering of novel electronic phases, 2D superconductors, magnets, as well as novel electronic applications. However, the mechanical exfoliation route is predominantly used to obtain such metallic 2D flakes, but the batch production remains challenging. Herein, the van der Waals epitaxial growth of monocrystalline, 1T-phase, few-layer metallic VSe nanosheets on an atomically flat mica substrate via a "one-step" chemical vapor deposition method is reported. The thickness of the VSe nanosheets is precisely tuned from several nanometers to several tenths of nanometers. More significantly, the 2D VSe single crystals are found to present an excellent metallic feature, as evidenced by the extra-high electrical conductivity of up to 10 S m , 1-4 orders of magnitude higher than that of various conductive 2D materials. The thickness-dependent charge-density-wave phase transitions are also examined through low-temperature transport measurements, which reveal that the synthesized 2D metallic 1T-VSe nanosheets should serve as good research platforms for the detecting novel many-body states. These results open a new path for the synthesis and property investigations of nanoscale-thickness 2D MTMDs crystals.

2D metallic TaS is acting as an ideal platform for exploring fundamental physical issues (superconductivity, charge-density wave, etc.) and for engineering novel applications in energy-related fields. The batch synthesis of high-quality TaS nanosheets with a specific phase is crucial for such issues. Herein, the successful synthesis of novel vertically oriented 1T-TaS nanosheets on nanoporous gold substrates is reported, via a facile chemical vapor deposition route. By virtue of the abundant edge sites and excellent electrical transport property, such vertical 1T-TaS is employed as high-efficiency electrocatalysts in the hydrogen evolution reaction, featured with rather low Tafel slopes ≈67-82 mV dec and an ultrahigh exchange current density ≈67.61 µA cm . The influence of phase states of 1T- and 2H-TaS on the catalytic activity is also discussed with the combination of density functional theory calculations. This work hereby provides fundamental insights into the controllable syntheses and electrocatalytic applications of vertical 1T-TaS nanosheets achieved through the substrate engineering.

Dendritic patterns mediated by non-equilibrium growth processes are ubiquitous in all length scales from atomic metal islands [ 1 ] to macroscopic snowfl akes. [ 2 ] These fractal species have attracted great research interests regarding their complex morphologies, unique formation mechanisms, [ 3 ] and more importantly intriguing physical and chemical properties. [ 4 ] A model fractal growth system toward these is metal-on-metal epitaxy [ 5 ] which is usually mediated by a limited diffusion of adatoms along island edges with respect to that on free surfaces, following a diffusion-limited aggregation (DLA) mechanism. [ 6 ] Notably, adatoms in this model have to hop among predefi ned lattice sites of the substrate to achieve effi cient bonding, [ 7 ] and the resultant epilayer usually follows the same lattice symmetry as that of the substrate. However, it remains an open question what would happen if the symmetry of the overlayer differs from that of the substrate. Recently, it is intriguing to see that van der Waals epitaxial growth of compact 2D atomic crystals can be realized on symmetry-different metal substrates (e.g., graphene grown on Cu(001) [ 8 ] ). In this regard, it is imperative to know if the fl ake shape of similar 2D layered materials could be tailored from compact to fractal by the deliberately introduced adlayer-substrate symmetry disparity.Monolayer molybdenum disulfi de (MoS 2 ) has emerged as a star material due to its exceptional electronic [ 9 ] and optoelectronic [ 10 ] properties among the 2D crystal family. Particularly, in recent works, MoS 2 with rich active edges was proved to be an effi cient catalyst in electrochemical hydrogen evolution, [ 11 ] petrochemical hydrodesulfurization, [ 12 ] etc. However, dendritic MoS 2 samples were rarely achieved through chemical vapor deposition (CVD) or any other bottom-up synthesis methods, [ 13 ] with respect to that of the universally derived compact MoS 2 triangles.Herein, we realized the controllable synthesis of 2H-phase monolayer MoS 2 dendrites on a symmetry-disparate SrTiO 3 (STO) (001) substrate via a facile CVD method. With a combination of various characterization techniques, we tried to uncover the coverage-dependent fractal dimension, the crystallinity inside a dendritic fl ake, and fi nally the key factors that induce the dendritic growth behavior. It is expected that the Dendritic patterns generated in non-equilibrium growth processes are prevalent in nature while their formation mechanisms are far from fully understood. Here, we report a coverage-dependent fractal degree evolution of monolayer 2H-MoS 2 dendrites synthesized on a symmetry-disparate substrate of SrTiO 3 (001). Surprisingly, various characterizations have revealed that the monolayer dendrites featured with orthogonal backbones are single crystalline, possessing both peculiar adlayer-substrate interaction and abnormal indirect bandgap on SrTiO 3 (001). Further theoretical calculations indicate that a prominent diffusion anisotropy of monomer precursors, combined w...

Rhenium diselenide (ReSe2) has recently garnered great research interest due to its distorted 1T structure, anisotropic physical properties, and applications in polarization-sensitive photodetectors. However, ReSe2 synthesized by chemical vapor deposition (CVD) is usually a multilayer/polycrystalline material containing numerous grain boundaries, thereby hindering its further applications. Here we describe the direct CVD growth of high-quality monolayer ReSe2 single crystals with a parallelogram shape arising from its anisotropic structure on a gold foil substrate. In particular, we use low-energy electron microscopy/diffraction combined with scanning tunneling microscopy/spectroscopy to determine the atomic-scale structure, domain orientation/boundaries, and band features of monolayer ReSe2 flakes grown directly on gold foils. This work may open new opportunities for the direct synthesis and in situ characterization of CVD-grown monolayer ReSe2.