Formation and Characterization of 2D Mo₂C Crystals via Biased CVD

2D ultrathin transition-metal carbides (TMCs), nitrides (TMNs), and carbonitrides are a family of important functional nanomaterials for various applications due to their properties that are directly dependent on the structure of the crystals, which can be controlled by processing. MXenes formed by wet etching which may consist of defects and surface functional groups (due to the use of acids) are especially useful for applications like composites, energy storage; however, for advanced electronics it is necessary to develop a versatile technique to produce high quality MXenes with controlled thickness, area, and high reproducibility.<br/> <br/>Biased CVD is a promising technique to produce large area MXenes. In Biased CVD, a stack of metal substrate (e.g. Mo and Cu) is exposed to a volatile precursor (e.g., CH₄) which decomposes on the substrate surface (e.g., Cu) and reacts with the Metal (e.g., Mo) that is biased towards the surface to produce the desired deposit (e.g., Mo₂C). It is called biased because the catalyst layer (Cu) is used like a diode: Cu lets Mo atoms to diffuse through Cu to the surface but does not let carbon atoms to diffuse through Cu (due to the difference in solubilities). Hence the diffusion is simply called biased.<br/> <br/>Studies in literature showed that the morphology and the thickness of Mo₂C crystals are strongly affected by catalyst type, thickness, composition, and the simultaneous graphene growth during CVD. To produce controlled morphology (thin, large area, high quality MXene crystals), the understanding of the growth mechanism via biasing reaction is crucial. However, it is still not clear and reproducible control of thickness and morphology is still a problem. Although it is revealed that simultaneous graphene growth affects the morphology and the thickness of Mo₂C crystals; there are conflicts in literature and the effect of its presence on the Mo₂C formation is not well understood. Moreover, fundamental physico-chemical and kinetic parameters related to the bulk and surface diffusion (associated with graphene, M and X) are missing in the literature and there is lack of data on the nucleation and growth kinetics of MXenes for physico/chemical modelling.<br/> <br/>In this presentation, we present an investigation on the controlled growth of thin Mo2C crystals on both Cu and In substrates <i>via</i> CVD using CH4. SEM, EDS, Raman spectroscopy, XPS, and XRD studies show that hexagonal Mo2C crystals, which are orthorhombic, grow along the [100] direction together with graphene and an amorphous carbon thin film on Cu and In, respectively. The growth mechanism is examined and discussed in detail, and a model is proposed. AFM studies agree well with the proposed model.<br/> <br/>This material is based on the work supported by the Air Force Office of Scientific Research (Award number: FA9550-19-1-7048).