Pengfei Yang

Van der Waals Epitaxial Growth of 2D Metallic Vanadium Diselenide Single Crystals and their Extra‐High Electrical Conductivity

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 VSe2 nanosheets on an atomically flat mica substrate via a one-step chemical vapor deposition method is reported. The thickness of the VSe2 nanosheets is precisely tuned from several nanometers to several tenths of nanometers. More significantly, the 2D VSe2 single crystals are found to present an excellent metallic feature, as evidenced by the extra-high electrical conductivity of up to 106 S m-1 , 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-VSe2 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.

Direct Chemical Vapor Deposition Growth and Band-Gap Characterization of MoS2/h-BN van der Waals Heterostructures on Au Foils

Stacked transition-metal dichalcogenides on hexagonal boron nitride (h-BN) are platforms for high-performance electronic devices. However, such vertical stacks are usually constructed by the layer-by-layer polymer-assisted transfer of mechanically exfoliated layers. This inevitably causes interfacial contamination and device performance degradation. Herein, we develop a two-step, low-pressure chemical vapor deposition synthetic strategy incorporating the direct growth of monolayer h-BN on Au foil with the subsequent growth of MoS2. In such vertical stacks, the interactions between MoS2 and Au are diminished by the intervening h-BN layer, as evidenced by the appearance of photoluminescence in MoS2. The weakened interfacial interactions facilitate the transfer of the MoS2/h-BN stacks from Au to arbitrary substrates by an electrochemical bubbling method. Scanning tunneling microscope/spectroscopy characterization shows that the central h-BN layer partially blocks the metal-induced gap states in MoS2/h-BN/Au foils. The work offers insight into the synthesis, transfer, and device performance optimization of such vertically stacked heterostructures.

Batch production of 6-inch uniform monolayer molybdenum disulfide catalyzed by sodium in glass

Monolayer transition metal dichalcogenides (TMDs) have become essential two-dimensional materials for their perspectives in engineering next-generation electronics. For related applications, the controlled growth of large-area uniform monolayer TMDs is crucial, while it remains challenging. Herein, we report the direct synthesis of 6-inch uniform monolayer molybdenum disulfide on the solid soda-lime glass, through a designed face-to-face metal-precursor supply route in a facile chemical vapor deposition process. We find that the highly uniform monolayer film, with the composite domains possessing an edge length larger than 400u2009µm, can be achieved within a quite short time of 8u2009min. This highly efficient growth is proven to be facilitated by sodium catalysts that are homogenously distributed in glass, according to our experimental facts and density functional theory calculations. This work provides insights into the batch production of highly uniform TMD films on the functional glass substrate with the advantages of low cost, easily transferrable, and compatible with direct applications.Growth of large-area monolayer transition metal dichalcogenides is critical for their application but remains challenging. Here Yang et al. report rapid chemical vapor deposition of 6-inch monolayer molybdenum disulfide by sufficiently uniformly supplying the precursors and catalysts.

Application of chemical vapor–deposited monolayer ReSe2 in the electrocatalytic hydrogen evolution reaction

Controlled synthesis of structurally anisotropic rhenium diselenide (ReSe2) with macroscopically uniform and strictly monolayer thickness as well as tunable domain shape/size is of great interest for electronics-, optoelectronics-, and electrocatalysis-related applications. Herein, we describe the controlled synthesis of uniform monolayer ReSe2 flakes with variable morphology (sunflower- or truncated-triangle-shaped) on SiO2/Si substrates using different ambient-pressure chemical vapor deposition (CVD) setups. The prepared polycrystalline ReSe2 flakes were transferred intact onto Au foil electrodes and tested for activity in the hydrogen evolution reaction (HER). Interestingly, compared to the compact truncated-triangle-shaped ReSe2 flakes, their edge-abundant sunflower-shaped counterparts exhibited superior electrocatalytic HER activity, featuring a relatively low Tafel slope of ∼76 mV/dec and an exchange current density of 10.5 μA/cm2. Thus, our work demonstrates that CVD-grown ReSe2 is a promising two-dimensional anisotropic material for applications in the electrocatalytic HER.

Surface Plasmon Enhanced Strong Exciton–Photon Coupling in Hybrid Inorganic–Organic Perovskite Nanowires

Manipulating strong light-matter interaction in semiconductor microcavities is crucial for developing high-performance exciton polariton devices with great potential in next-generation all-solid state quantum technologies. In this work, we report surface plasmon enhanced strong exciton-photon interaction in CH3NH3PbBr3 perovskite nanowires. Characteristic anticrossing behaviors, indicating a Rabi splitting energy up to ∼564 meV, are observed near exciton resonance in hybrid perovskite nanowire/SiO2/Ag cavity at room temperature. The exciton-photon coupling strength is enhanced by ∼35% on average, which is mainly attributed to surface plasmon induced localized excitation field redistribution. Further, systematic studies on SiO2 thickness and nanowire dimension dependence of exciton-photon interaction are presented. These results provide new avenues to achieve extremely high coupling strengths and push forward the development of electrically pumped and ultralow threshold small lasers.

Direct synthesis and in situ characterization of monolayer parallelogrammic rhenium diselenide on gold foil

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.Improving the synthesis of crystalline monolayer transition metal dichalcogenides requires insight into domain and boundary structures. Here, the authors produce monolayer rhenium diselenide by chemical vapour deposition onto gold foil, allowing in situ analysis of domain and defect structure.

Temperature-dependent Raman spectroscopy studies of the interface coupling effect of monolayer ReSe2 single crystals on Au foils

Rhenium diselenide (ReSe2), which bears in-plane anisotropic optical and electrical properties, is of considerable interest for its excellent applications in novel devices, such as polarization-sensitive photodetectors and integrated polarization-controllers. However, great challenges to date in the controllable synthesis of high-quality ReSe2 have hindered its in-depth investigations and practical applications. Herein, we report a feasible synthesis of monolayer single-crystal ReSe2 flakes on the Au foil substrate by using a chemical vapor deposition route. Particularly, we focus on the temperature-dependent Raman spectroscopy investigations of monolayer ReSe2 grown on Au foils, which present concurrent red shifts of Eg-like and Ag-like modes with increasing measurement temperature from 77-290 K. Linear temperature dependences of both modes are revealed and explained from the anharmonic vibration of the ReSe2 lattice. More importantly, the strong interaction of ReSe2 with Au, with respect to that with SiO2/Si, is further confirmed by temperature-dependent Raman characterization. This work is thus proposed to shed light on the optical and thermal properties of such anisotropic two-dimensional three-atom-thick materials.

Ultrafast Charge Transfer in Perovskite Nanowire/2D Transition Metal Dichalcogenide Heterostructures

Mixed-dimensional van der Waals (vdW) heterostructures between one-dimensional (1D) perovskite nanowires and two-dimensional (2D) transition metal dichalcogenides (TMDCs) hold great potential for novel optoelectronics and light-harvesting applications. However, the ultrafast carrier dynamics between the 1D perovskite nanowires and 2D TMDCs are currently not well understood, which is critical for related optoelectronic applications. Here we demonstrate vdW heterostructures of CsPbBr3 nanowire/monolayer MoS2 and CsPbBr3 nanowire/monolayer WSe2 and further present systematic investigations on their charge transfer dynamics. We show that CsPbBr3/MoS2 and CsPbBr3/WSe2 are type-I and type-II heterostructures, respectively. Both electrons and holes transfer from CsPbBr3 to MoS2 with an efficiency of 71%. As a contrast, holes transfer from CsPbBr3 to WSe2 with a carrier transfer efficiency of 70% and electrons transfer inversely within 7 ps. The ultrafast and efficient charge transfer in the 1D/2D perovskite-TMDC heterostructures suggest great promise in light emission, photodetector, and photovoltaic devices.

Na-assisted fast growth of large single-crystal MoS 2 on sapphire

Monolayer molybdenum sulfide (MoS2), a typical semiconducting transition metal dichalcogenide, has emerged as a perfect platform for next-generation electronics and optoelectronics due to its sizeable band gap and strong light-matter interactions. Nevertheless, the controlled growth of a monolayer MoS2 single-crystal with a large-domain size and high crystal quality still faces great challenges. Herein, we demonstrate the fast growth of a large-domain monolayer MoS2 on the c-plane sapphire substrate with the assistance of sodium chloride (NaCl) crystals as the intermediate promoter. Particularly, the volatilization temperature of the NaCl crystal and the growth temperature of MoS2 are established to be the key parameters that influence the growth efficiency of MoS2 at an optimized growth condition. Monolayer triangular MoS2 domain with an edge length ∼300 μm is obtained within 1 min, featured with a growth rate ∼5 μm s-1. The Na element from the NaCl crystal is found to be able to facilitate the two dimensional growth of monolayer MoS2. This work thus offers novel insights into the high-efficiency production of large-domain monolayer MoS2 on insulating growth substrates.

Space-confined growth of monolayer ReSe2 under a graphene layer on Au foils

Vertical heterostructures based on two-dimensional (2D) materials have attracted widespread interest for their numerous applications in electronic and optoelectronic devices. Herein, we report the direct construction of an abnormal graphene/ReSe2 stack on Au foils by a two-step chemical vapor deposition (CVD) strategy. During the second growth stage, monolayer ReSe2 is found to preferentially evolve at the interface between the first-grown graphene layer and the Au substrate. The unusual stacking behavior is unraveled by in-situ “cutting open” the upper graphene from the defects to expose the lower ReSe2 using scanning tunneling microscopy (STM). From combination of these results with density functional theory calculations, the domain boundaries and edge sites of graphene are proposed to be adsorption sites for Re and Se precursors, further facilitating the growth of ReSe2 at the van der Waals gap of graphene/Au. This work hereby offers an intriguing strategy for obtaining vertical 2D heterostructures featured with an ultra-clean interface and a designed stacking geometry.