Synthesis and Electronic Transport of Ultrathin Single Crystal WC and W₂C

The transition metal carbide (TMC) family has historically been studied and applied for their high hardness, chemical stability, and electrocatalytic activity. Initially studied in bulk, non-layered morphologies, TMCs have received renewed interest with the development of novel top-down and bottom-up approaches to isolate layered TMCs (MXenes)¹ and ultrathin, non-layered TMCs (UThTMCs)², 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.³ Following recent works in bottom-up synthesis of UThTMCs^4,5, 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.<br/>In this work, we report the synthesis of WC (P-6m2) with copper/tungsten foil stacks, and W₂C (Pbcn) 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 the scanning transmission electron microscope. Moreover, we probe the dependence of crystal morphology with changes in growth temperature as well as methane/hydrogen (CH₄/H₂) ratios. Another aspect of the synthesis that we have investigated are byproducts in the synthesis of WC/Cu/W and W₂C/Ga/W by combination of scanning electron microscopy, EDX, and Raman spectroscopy. We find that an increase in the CH₄/H₂ ratio promotes the formation of graphene in both systems, yielding irregular shaped W₂C nanoplates with large thickness, and polycrystalline WC nanoplates. In the W₂C/Ga/W system, we also find that crystalline Ga₂O₃ is formed during the synthesis and can be limited using higher H₂ concentrations.<br/>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.<br/>References<br/>1: Naguib et al. <i>Advanced Materials</i> <b>23</b>, 4248-4253 (2011)<br/>2: Xu et al. <i>Nature Materials</i> <b>14</b>, 1335-1141 (2015)<br/>3: Y. Gogotsi and Q. Huang <i>ACS Nano</i> <b>15</b>, 5775-5780 (2021)<br/>4: Wang et al. <i>Advanced Electronic Materials</i> <b>5</b>, 1-7 (2019)<br/>5: Zeng et al. <i>Nano Energy</i> <b>33</b>, 356-362 (2017)