van der Waals epitaxial growth of single crystal α -MoO ₃ layers on layered materials growth templates

Abstract

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.

Optical-Based Thickness Measurement of MoO3 Nanosheets

“…Among them, wide bandgap semiconductor materials have attracted a great deal of attention due to their potential in optoelectronic applications [ 10 ] requiring electrically conductive materials that are transparent to visible light. Molybdenum trioxide (MoO 3 ) in its α-phase is a van der Waals material with a monolayer thickness of ~0.7 nm [ 11 , 12 ] and a direct bandgap of approximately 3 eV [ 13 , 14 , 15 ], suitable for such applications [ 16 ]. It has been used in thin films to enhance the injection of holes in organic light-emitting diodes as a buffer layer [ 17 , 18 ], in organic photovoltaics [ 19 ], perovskite solar cells [ 20 ] and silicon solar cells [ 21 ], furthermore it can be used in gas sensors [ 15 , 22 , 23 ].…”

2D Layered Materials for Ultraviolet Photodetection: A Review

“…However, the monolayer was found to be mobile during the AFM scan and is therefore highly unstable. [ 93 ]…”

“…[ 61,94 ] Several research groups have investigated the bandgap of few‐layer (3.25 [ 95 ] , 2.9 96 ] and 3.2 eV [ 97 ] ), bilayer (3 eV [ 98 ] ) and monolayer (3 [ 98 ] and 3.2 eV [ 93 ] ) MoO 3 . On reducing the thickness to a monolayer, the bandgap increases [ 93 ] and remains indirect for all the thicknesses of MoO 3 . [ 93,98 ]…”

“…On reducing the thickness to a monolayer, the bandgap increases [ 93 ] and remains indirect for all the thicknesses of MoO 3 . [ 93,98 ]…”

Advances in the designs and mechanisms of MoO3 nanostructures for gas sensors: a holistic review

“…The two sublayers then bind together with van der Waals forces by sharing the edges of the octahedra along the [001] direction to form MoO6 octahedra layers 44,45 . These layers then alternately stack along the [010] direction to form an α-MoO3 structure [46][47][48] . This unique layered structure of α-MoO3 has increases content of pentavalent Mo 5+ ions, which possesses strong affinity to oxygen 49,50 .…”