Min Hong

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.

Two-dimensional metallic tantalum disulfide as a hydrogen evolution catalyst

Two-dimensional metallic transition metal dichalcogenides are emerging as prototypes for uncovering fundamental physical phenomena, such as superconductivity and charge-density waves, as well as for engineering-related applications. However, the batch production of such envisioned transition metal dichalcogenides remains challenging, which has hindered the aforementioned explorations. Herein, we fabricate thickness-tunable tantalum disulfide flakes and centimetre-sized ultrathin films on an electrode material of gold foil via a facile chemical vapour deposition route. Through temperature-dependent Raman characterization, we observe the transition from nearly commensurate to commensurate charge-density wave phases with our ultrathin tantalum disulfide flakes. We have obtained high hydrogen evolution reaction efficiency with the as-grown tantalum disulfide flakes directly synthesized on gold foils comparable to traditional platinum catalysts. This work could promote further efforts for exploring new efficient catalysts in the large materials family of metallic transition metal dichalcogenides, as well as exploiting their applications towards more versatile applications.Metallic transition metal dichalcogenides are important materials for catalysis, but scalable and controllable preparation methods are scarce. Here, the authors synthesize 2H-TaS2 as centimetre-scale films of tunable thickness and show they are an efficient catalyst for hydrogen evolution.

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 400 µm, can be achieved within a quite short time of 8 min. 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.

Modulating the Electronic Properties of Monolayer Graphene Using a Periodic Quasi-One-Dimensional Potential Generated by Hex-Reconstructed Au(001)

The structural and electronic properties of monolayer graphene synthesized on a periodically reconstructed substrate can be widely modulated by the generation of superstructure patterns, thereby producing interesting physical properties, such as magnetism and superconductivity. Herein, using a facile chemical vapor deposition method, we successfully synthesized high-quality monolayer graphene with a uniform thickness on Au foils. The hex-reconstruction of Au(001), which is characterized by striped patterns with a periodicity of 1.44 nm, promoted the formation of a quasi-one-dimensional (1D) graphene superlattice, which served as a periodic quasi-1D modulator for the graphene overlayer, as evidenced by scanning tunneling microscopy/spectroscopy. Intriguingly, two new Dirac points were generated for the quasi-1D graphene superlattice located at -1.73 ± 0.02 and 1.12 ± 0.12 eV. Briefly, this work demonstrates that the periodic modulation effect of reconstructed metal substrates can dramatically alter the electronic properties of graphene and provides insight into the modulation of these properties using 1D potentials.

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.

Vertical 1T-TaS2 Synthesis on Nanoporous Gold for High-Performance Electrocatalytic Applications

2D metallic TaS2 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 TaS2 nanosheets with a specific phase is crucial for such issues. Herein, the successful synthesis of novel vertically oriented 1T-TaS2 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-TaS2 is employed as high-efficiency electrocatalysts in the hydrogen evolution reaction, featured with rather low Tafel slopes ≈67-82 mV dec-1 and an ultrahigh exchange current density ≈67.61 µA cm-2 . The influence of phase states of 1T- and 2H-TaS2 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-TaS2 nanosheets achieved through the substrate engineering.

Unique Transformation from Graphene to Carbide on Re(0001) Induced by Strong Carbon–Metal Interaction

During graphene growth on various transition metals in the periodic table, metal carbides always emerge to behave as complex intermediates. On VIII metals, metastable carbides usually evolve and then transform into graphene along the phase interfaces, and even no metal carbides can form on IB-IIB metals. In contrast, during graphene growth on group IVB-VIB metals, carbides are usually generated even before the evolution of graphene and stably exist throughout the whole growth process. However, for the remaining transition metals, e.g., group VIIB, located in between IVB-VIB and VIII, the interplay between graphene and carbide is still vague. Herein, on Re(0001) (VIIB), we have revealed a novel transition from graphene to metal carbide (reverse to that on VIII metals) for the first time. This transition experienced graphene decomposition, dissolution, and carbon segregation processes, as evidenced by scanning tunneling microscopy (STM) and on-site, variable-temperature low electron energy diffraction (LEED) characterizations. This work thus completes the picture about the interplay between graphene and carbide on/in transition metals in the periodic table, as well as discloses a new territory for the growth of carbon-related materials, especially the metal carbide.

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.