Simulation and Synthesis of Ultrathin Transition Metal Carbides and Transition Metal Dichalcogenides Heterostructures

Non-layered transition metal carbides (TMCs) and layered transition metal dichalcogenides (TMDs) are two extensively studied material families that have garnered significant attention over the past century. Recently, the focus on two-dimensional (2D) materials and heterostructures has given rise to a new field centered on TMC/TMD heterostructures. These heterostructures, formed through chemical conversion, display various configurations featuring coupled 2D–3D interfaces, which result in unique and exotic properties. Xu et al. [1] developed a liquid-metal-assisted chemical vapor deposition (LMCVD) technique to grow large-area ultrathin Mo₂C and investigated their robust superconducting properties down to a few nanometers in thickness. Through chemical conversion, Jeon et al. [2] synthesized the β-Mo₂C/MoS₂ lateral heterostructures exhibiting low contact resistance and a low Schottky barrier height, highlighting their potential as critical hybrid building blocks for future device applications. By combining computational methods, controlled synthesis, and advanced characterization, our collaborative team developed a comprehensive workflow to synthesize and analyze W_xC/WS₂ heterostructures in a series of consecutive steps.

The initial step involves synthesizing ultrathin tungsten carbide. Due to the material's 2D geometry, the surface effect alters the phase stability, which traditional phase diagrams cannot accurately describe. By utilizing density functional theory (DFT) calculations and thermodynamic analyses, we constructed a chemical potential diagram for W_xC in thin-film structures, revealing a different phase stability order compared to its bulk counterpart. Using LMCVD, we identified two phases of tungsten carbide: WC on copper substrates and metastable W₂C on gallium substrates. In the next step, we observed that high-temperature (800–900°C) heat treatment in hydrogen sulfide (H₂S) partially converted the tungsten carbide (WC and W₂C) nanoplates into crystalline WS₂. Characterization revealed that WS₂ formed on both the edge and basal surfaces of the tungsten carbide platelets. On the basal surface, distinct Moiré patterns indicated the presence of epitaxial and twisted WS₂. Using DFT calculations, we explored how W or C termination influences the adhesion work of the resulting heterostructure. The conversion of tungsten carbide to WS₂ was found to be more efficient at slightly lower temperatures and shorter durations for W₂C than for WC, with facets completely transforming into multilayer WS₂. This enhanced conversion for W₂C is attributed to a lower diffusion barrier for sulfur atoms into the tungsten carbide crystal, as quantified by nudged elastic band (NEB) calculations. In the final steps, we applied DFT calculations onto atomic models to study the electronic and transport properties across the WC/WS₂ and WC/WSe₂ interface [3].

The Moiré pattern heterostructure exhibited a greater energetic favorability when compared to the coherent epitaxial strain heterostructure. Our calculations revealed the formation of Schottky barriers within these systems, with the type (n-type or p-type) and height of the Schottky barriers being determined by the termination atoms of WC. We concluded that WC-c/WSe₂ has the smallest p-type Schottky barrier height (0.08 eV) among all other heterostructures. Transport properties were further assessed using the Simmons tunneling injection model [4]. These findings yielded valuable insights that can be leveraged in the design of high-performance nano-electronic devices built upon 2D materials.

1 Xu et al., Nature Mater. 2015;14:1135–41. doi:10.1038/nmat4374

2 Jeon et al., ACS Nano. 2018;12:338–46. doi:10.1021/acsnano.7b06417

3 Wang et al., Physical Review Materials 2024; 8:044004. https://doi.org/10.1103/PhysRevMaterials.8.044004

4 J.G. Simmons, Journal of Applied Physics 1963; 34:1793-1803. https://pubs.aip.org/aip/jap/article/34/6/1793/362794/Generalized-Formula-for-the-Electric-Tunnel-Effect