Large-area growth of Mo2C thin films and MXene structures via the PVD–CVD hybrid technique

Abstract

MXenes are a family of two-dimensional (2D) materials known for their exceptional properties, such as high electrical conductivity and mechanical strength. Typically, they are obtained through an etching process from their MAX phase precursors, which introduces functional groups on the surface, leading to instability and variability in their properties, such as lower electrical conductivity. In this research, we present a study focused on the synthesis of molybdenum carbide (Mo₂C) thin films and MXene structures with controlled thickness and substrate coverage, using physical vapor deposition (PVD) and chemical vapor deposition (CVD). Unlike previous vapor-deposited studies, where copper foil is used as a catalyst in the CVD process, we investigate the deposition of thin copper films via thermal evaporation (PVD) onto molybdenum precursor substrates. This approach results in significantly improved Mo₂C surface coverage and film thickness control. Characterization techniques such as μ-Raman spectroscopy, X-ray diffraction, and energy-dispersive spectroscopy (EDX) confirm the formation of α-Mo₂C thin films and MXene structures. Moreover, elevated growth temperatures facilitate the synthesis of MXene-graphene hybrids. The results demonstrate that the catalyst thickness plays a crucial role in determining the nucleation and growth morphology of the Mo₂C structures. Thinner copper catalysts promote faster Mo atom transition to the surface, resulting in higher Mo₂C nuclei density and dendritic growth, while thicker catalysts lead to fewer nucleation sites and larger grain formations. Reducing the thermally evaporated copper film thickness to as little as 50 nm enables the transformation of these Mo₂C films into MXene structures, whereas thinner copper films reduce the substrate coverage significantly. This research highlights the critical role of catalyst thickness in optimizing the properties of Mo₂C thin films for potential electronic applications, offering valuable insights to improve the performance and functionality of MXene structures.