Iso-oriented α-MoO 3(010) films with a monolayer thickness can be grown on SrTiO 3(001) substrate by molecular beam epitaxy viaa self-limiting mechanism.
The ability to synthesize well-ordered two-dimensional materials under ultra-high vacuum and directly characterize them by other techniques in situcan greatly advance our current understanding on their physical and chemical properties. In this paper, we demonstrate that iso-oriented α-MoO 3films with as low as single monolayer thickness can be reproducibly grown on SrTiO 3(001) substrates by molecular beam epitaxy ((010) MoO3‖(001) STO, [100] MoO3‖[100] STOor [010] STO) through a self-limiting process. While one in-plane lattice parameter of the MoO 3is very close to that of the SrTiO 3( a MoO3= 3.96 Å, a STO= 3.905 Å), the lattice mismatch along other direction is large (∼5%, c MoO3= 3.70 Å), which leads to relaxation as clearly observed from the splitting of streaks in reflection high-energy electron diffraction (RHEED) patterns. A narrow range in the growth temperature is found to be optimal for the growth of monolayer α-MoO 3films. Increasing deposition time will not lead to further increase in thickness, which is explained by a balance between deposition and thermal desorption due to the weak van der Waals force between α-MoO 3layers. Lowering growth temperature after the initial iso-oriented α-MoO 3monolayer leads to thicker α-MoO 3(010) films with excellent crystallinity.