arXiv subjects

Hui-Ming Cheng

Publications and source records attributed to Hui-Ming Cheng.

23 records · Page 2Linked to original sources

arXiv2020

Vertical CVD Growth of Highly Uniform Transition Metal Dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted great attention due to their physical and chemical properties that make them promising in electronics and optoelectronics. Because of the difficulties in controlling concentrations of solid precursors and spatially non-uniform growth dynamics, it is challenging to grow wafer-scale 2D TMDCs with good uniformity and reproducibility so far, which significantly hinders their practical use. Here we report a vertical chemical vapor deposition (VCVD) design to grow monolayer TMDCs with a uniform density and high quality over the whole wafer, and with excellent reproducibility. The use of such VCVD design can easily control the three key growth parameters of precursor concentration, gas flow and temperature, which cannot be done in currently widely-used horizontal CVD system. Statistical results show that VCVD-grown monolayer TMDCs including MoS2 and WS2 are of high uniformity and quality on substrates over centimeter size. We also fabricated multiple van der Waals heterostructures by the one-step transfer of VCVD-grown TMDC samples, owning to its good uniformity. This work opens a way to grow 2D materials with high uniformity and reproducibility on the wafer scale, which can be used for the scalable fabrication of 2D materials and their heterostructures.

physics.app-ph↗

arXiv2020

Unsaturated Single Atoms on Monolayer Transition Metal Dichalcogenides for Ultrafast Hydrogen Evolution

Large scale implementation of electrochemical water splitting for hydrogen evolution requires cheap and efficient catalysts to replace expensive platinum. Molybdenum disulfide is one of the most promising alternative catalysts but its intrinsic activity is still inferior to platinum. There is therefore a need to explore new active site origins in molybdenum disulfide with ultrafast reaction kinetics and to understand their mechanisms. Here, we report a universal cold hydrogen plasma reduction method for synthesizing different single atoms sitting on two-dimensional monolayers. In case of molybdenum disulfide, we design and identify a new type of active site, i.e., unsaturated Mo single atoms on cogenetic monolayer molybdenum disulfide. The catalyst shows exceptional intrinsic activity with a Tafel slope of 35.1 mV dec-1 and a turnover frequency of ~10^3 s-1 at 100 mV, based on single flake microcell measurements. Theoretical studies indicate that coordinately unsaturated Mo single atoms sitting on molybdenum disulfide increase the bond strength between adsorbed hydrogen atoms and the substrates through hybridization, leading to fast hydrogen adsorption/desorption kinetics and superior hydrogen evolution activity. This work shines fresh light on preparing highly-efficient electrocatalysts for water splitting and other electrochemical processes, as well as provides a general method to synthesize single atoms on two-dimensional monolayers.

cond-mat.mtrl-sci↗

arXiv2020

Dissolution-Precipitation Growth of Uniform and Clean Two Dimensional Transition Metal Dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have attracted much interest and shown promise in many applications. However, it is challenging to obtain uniform TMDCs with clean surfaces, because of the difficulties in controlling the way the reactants are supplied to the reaction in the current chemical vapor deposition (CVD) growth process. Here, we report a new growth approach called dissolution-precipitation (DP) growth, where the metal sources are sealed inside glass substrates to control their feeding to the reaction. Noteworthy, the diffusion of metal source inside glass to its surface provides a uniform metal source on the glass surface, and restricts the TMDC growth to only a surface reaction while eliminates unwanted gas-phase reaction. This feature gives rise to highly-uniform monolayer TMDCs with a clean surface on centimeter-scale substrates. The DP growth works well for a large variety of TMDCs and their alloys, providing a solid foundation for the controlled growth of clean TMDCs by the fine control of the metal source.

cond-mat.mtrl-sci↗

arXiv2020

Mass Production of Two-Dimensional Materials by Intermediate-Assisted Grinding Exfoliation

The scalable and high-efficiency production of two-dimensional (2D) materials is a prerequisite to their commercial use. Currently, only graphene and graphene oxide can be produced on a ton scale, and the inability to produce other 2D materials on such a large scale hinders their technological applications. Here we report a grinding exfoliation method that uses micro-particles as force intermediates to resolve applied compressive forces into a multitude of small shear forces, inducing the highly-efficient exfoliation of layer materials. The method, referred to as intermediate-assisted grinding exfoliation (iMAGE), can be used for the large-scale production of many 2D materials. As an example, we have exfoliated bulk h-BN into 2D h-BN with large flake sizes, high quality and structural integrity, with a high exfoliation yield of 67%, a high production rate of 0.3 g h-1 and a low energy consumption of 3.01x10^6 J g-1. The production rate and energy consumption are one to two orders of magnitude better than previous results. Besides h-BN, this iMAGE technology has been used to exfoliate various layer materials such as graphite, black phosphorus, transition metal dichalcogenides, and metal oxides, proving its universality. Molybdenite concentrate, a natural low-cost and abundant mineral, was used as a demo for the large-scale exfoliation production of 2D MoS2 flakes. Our work indicates the huge potential of the iMAGE method to produce large amounts of various 2D materials, which paves the way for their commercial application.

physics.app-ph↗

arXiv2013

Interstitial-Boron Solution Strengthened WB$_{3+x}$

By means of variable-composition evolutionary algorithm coupled with density functional theory and in combination with aberration-corrected high-resolution transmission electron microscopy experiments, we have studied and characterized the composition, structure and hardness properties of WB$_{3+x}$ ($x$ $<$ 0.5). We provide robust evidence for the occurrence of stoichiometric WB$_3$ and non-stoichiometric WB$_{3+x}$ both crystallizing in the metastable $hP$16 ($P6_3/mmc$) structure. No signs for the formation of the highly debated WB$_4$ (both $hP$20 and $hP$10) phases were found. Our results rationalize the seemingly contradictory high-pressure experimental findings and suggest that the interstitial boron atom is located in the tungsten layer and vertically interconnect with four boron atoms, thus forming a typical three-center boron net with the upper and lower boron layers in a three-dimensional covalent network, which thereby strengthen the hardness.

cond-mat.mtrl-sci↗