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 alpha-MoO3 nanosheets on various 2D growth templates are characterized. Monolayer and multilayer alpha-MoO3 nanosheets are successfully grown on a 2D substrate by simply evaporating amorphous molybdenum oxide thin film in ambient conditions. A single-crystal alpha-MoO3 nanosheet without grain boundary is epitaxially grown on various 2D substrates despite a large lattice mismatch. During growth, the quasi-stable monolayer alpha-MoO3 first covers the 2D substrate, then additional layers are continuously grown on the first monolayer alpha-MoO3. The band gap of the alpha-MoO3 increases from 2.9 to 3.2 eV as the thickness decreases. Furthermore, due to oxygen vacancies and surface adsorbates, the synthesized alpha-MoO3 is highly n-doped with a small work function. Therefore, alpha-MoO3 field-effect transistors (FETs) exhibit a typical n-type conductance. This work shows the great potential of ultra-thin alpha-MoO3 in 2D-material-based electronics.c