Dong Hoon Keum

Phase patterning for ohmic homojunction contact in MoTe2

Making better contacts A key issue in fabricating transistors is making a good electrical contact to the semiconductor gate material. For two-dimensional materials, one route is through a phase transition that converts a hexagonally packed semiconductor phase into a distorted octahedrally packed metallic phase. Cho et al. show that laser heating of molybdenum telluride (MoTe2) achieves this conversion through the creation of Te vacancies. The phase transition improves charge carrier mobility while maintaining the low resistance necessary for improved transistor function. Science, this issue p. 625 A laser-heating method creates a metallic phase on semiconducting molybdenum telluride. Artificial van der Waals heterostructures with two-dimensional (2D) atomic crystals are promising as an active channel or as a buffer contact layer for next-generation devices. However, genuine 2D heterostructure devices remain limited because of impurity-involved transfer process and metastable and inhomogeneous heterostructure formation. We used laser-induced phase patterning, a polymorph engineering, to fabricate an ohmic heterophase homojunction between semiconducting hexagonal (2H) and metallic monoclinic (1T’) molybdenum ditelluride (MoTe2) that is stable up to 300°C and increases the carrier mobility of the MoTe2 transistor by a factor of about 50, while retaining a high on/off current ratio of 106. In situ scanning transmission electron microscopy results combined with theoretical calculations reveal that the Te vacancy triggers the local phase transition in MoTe2, achieving a true 2D device with an ohmic contact.

A Van Der Waals Homojunction: Ideal p–n Diode Behavior in MoSe2

A MoSe2 p-n diode with a van der Waals homojunction is demonstrated by stacking undoped (n-type) and Nb-doped (p-type) semiconducting MoSe2 synthesized by chemical vapor transport for Nb substitutional doping. The p-n diode reveals an ideality factor of ≈1.0 and a high external quantum efficiency (≈52%), which increases in response to light intensity due to the negligible recombination rate at the clean homojunction interface.

Heterogeneous Defect Domains in Single-Crystalline Hexagonal WS2

Single-crystalline monolayer hexagonal WS2 is segmented into alternating triangular domains: sulfur-vacancy (SV)-rich and tungsten-vacancy (WV)-rich domains. The WV-rich domain with deep-trap states reveals an electron-dedoping effect, and the electron mobility and photoluminescence are lower than those of the SV-rich domain with shallow-donor states by one order of magnitude. The vacancy-induced strain and doping effects are investigated via Raman and scanning photoelectron microscopy.

Role of alkali metal promoter in enhancing lateral growth of monolayer transition metal dichalcogenides

Synthesis of monolayer transition metal dichalcogenides (TMDs) via chemical vapor deposition relies on several factors such as precursor, promoter, substrate, and surface treatment of substrate. Among them, the use of promoter is crucial for obtaining uniform and large-area monolayer TMDs. Although promoters have been speculated to enhance adhesion of precursors to the substrate, their precise role in the growth mechanism has rarely been discussed. Here, we report the role of alkali metal promoter in growing monolayer TMDs. The growth occurred via the formation of sodium metal oxides which prevent the evaporation of metal precursor. Furthermore, the silicon oxide substrate helped to decrease the Gibbs free energy by forming sodium silicon oxide compounds. The resulting sodium metal oxide was anchored within such concavities created by corrosion of silicon oxide. Consequently, the wettability of the precursors to silicon oxide was improved, leading to enhance lateral growth of monolayer TMDs.

van der Waals Metallic Transition Metal Dichalcogenides

Transition metal dichalcogenides are layered materials which are composed of transition metals and chalcogens of the group VIA in a 1:2 ratio. These layered materials have been extensively investigated over synthesis and optical and electrical properties for several decades. It can be insulators, semiconductors, or metals revealing all types of condensed matter properties from a magnetic lattice distorted to superconducting characteristics. Some of these also feature the topological manner. Instead of covering the semiconducting properties of transition metal dichalcogenides, which have been extensively revisited and reviewed elsewhere, here we present the structures of metallic transition metal dichalcogenides and their synthetic approaches for not only high-quality wafer-scale samples using conventional methods (e.g., chemical vapor transport, chemical vapor deposition) but also local small areas by a modification of the materials using Li intercalation, electron beam irradiation, light illumination, pressures, and strains. Some representative band structures of metallic transition metal dichalcogenides and their strong layer-dependence are reviewed and updated, both in theoretical calculations and experiments. In addition, we discuss the physical properties of metallic transition metal dichalcogenides such as periodic lattice distortion, magnetoresistance, superconductivity, topological insulator, and Weyl semimetal. Approaches to overcome current challenges related to these materials are also proposed.

Intragranular Dispersion of Carbon Nanotubes Comprehensively Improves Aluminum Alloys

Abstract The room‐temperature tensile strength, toughness, and high‐temperature creep strength of 2000, 6000, and 7000 series aluminum alloys can be improved significantly by dispersing up to 1 wt% carbon nanotubes (CNTs) into the alloys without sacrificing tensile ductility, electrical conductivity, or thermal conductivity. CNTs act like forest dislocations, except mobile dislocations cannot annihilate with them. Dislocations cannot climb over 1D CNTs unlike 0D dispersoids/precipitates. Also, unlike 2D grain boundaries, even if some debonding happens along 1D CNT/alloy interface, it will be less damaging because fracture intrinsically favors 2D percolating flaws. Good intragranular dispersion of these 1D strengtheners is critical for comprehensive enhancement of composite properties, which entails change of wetting properties and encapsulation of CNTs inside Al grains via surface diffusion‐driven cold welding. In situ transmission electron microscopy demonstrates liquid‐like envelopment of CNTs into Al nanoparticles by cold welding.