Van der Waals epitaxial growth of single crystal α-MoO₃ layers on layered materials growth templates

Since the isolation of graphene, various two-dimensional (2D) materials have been extensively investigated. Nevertheless, only few 2D oxides have been reported to date due to difficulties in their synthesis. However, it is expected that the layered transition-metal oxides (TMOs) could be missing blocks for van der Waals heterostructures and essential elements for 2D electronics. Herein, the crystal structure and band structure of van der Waals epitaxially grown α-MoO₃ nanosheets on various 2D growth templates are characterized. Monolayer and multilayer α-MoO₃ nanosheets are successfully grown on a 2D substrate by simply evaporating amorphous molybdenum oxide thin film in ambient conditions. A single-crystal α-MoO₃ nanosheet without grain boundary is epitaxially grown on various 2D substrates despite a large lattice mismatch. During growth, the quasi-stable monolayer α-MoO₃ first covers the 2D substrate, then additional layers are continuously grown on the first monolayer α-MoO₃. The band gap of the α-MoO₃ increases from 2.9 to 3.2 eV as the thickness decreases. Furthermore, due to oxygen vacancies and surface adsorbates, the synthesized α-MoO₃ is highly n-doped with a small work function. Therefore, α-MoO₃ field-effect transistors (FETs) exhibit a typical n-type conductance. This work shows the great potential of ultra-thin α-MoO₃ in 2D-material-based electronics.