Phase-controlled synthesis and electronic transport of ultrathin WC and W2C platelets

The transition metal carbide and nitride (TMC, TMN) families have historically been studied and applied for their high hardness, chemical stability, and electrocatalytic activity. Initially studied in bulk, non-layered morphologies, TMC/Ns have received renewed interest with the development of novel top-down and bottom-up approaches to isolate layered TMC/Ns (MXenes) and ultrathin, non-layered TMC/Ns (UThTMC/Ns), respectively. The MXene family is large, but challenges persist in phase control and isolation of tungsten carbide owing to the lack of appropriate precursor material, as well as crystalline phase control beyond the MAX phase parent material. Following recent works in bottom-up synthesis of UThTMCs, we show the isolation of two tungsten carbide phases using a liquid-metal-assisted chemical vapor deposition (LMCVD). Moreover, in the tungsten carbide system, little attention has been given to the influence of diffusion barrier and reactive gas ratios on the thickness, morphology, and phase of the crystalline products. In this work, we report the synthesis of WC with copper/tungsten foil stacks, and W₂C from gallium/tungsten substrates. We identify the phase of these compounds using a combination of X-ray diffraction and planar/cross sectional selected area electron diffraction. The chemical compositions of WC and W₂C are investigated by planar energy dispersive X-ray spectroscopy (EDX) in a scanning transmission electron microscope. To explain how these distinct phases are isolated, we carried out a density functional theory study to probe thermodynamic properties of the WC and W2C systems. We find that the carbon concentration and atomic terminations at the surface of WC and W2C are critical to the preferential isolation of W2C on Ga/W substrates. Finally, we report the electronic transport measurements of UThTMCs single crystals of WC and W2C. In particular, we find that semimetallic WC does not enter a superconducting state down to 10 mK, while W2C enters a superconducting state below 2.85 K. We then compare in-plane and out-of-plane magnetic field measurements to assess the dimensionality of this superconducting state.