  We report on the large-scale synthesis of highly oriented ultrathin MoO3
layers using a simple and low-cost atmospheric pressure by van der Waals
epitaxy growth on muscovite mica substrates. By this method we are able to
synthetize high quality centimeter-scale MoO3 crystals with thicknesses ranging
from 1.4 nm (two layers) up to a few nanometers. The crystals can be easily
transferred to an arbitrary substrate (such as SiO2) by a deterministic
transfer method and extensively characterized to demonstrate the high quality
of the resulting crystal. We also study the electronic band structure of the
material by density functional theory calculations. Interestingly, the
calculations demonstrate that bulk MoO3 has a rather weak electronic interlayer
interaction and thus it presents a monolayer-like band structure. Finally, we
demonstrate the potential of this synthesis method for optoelectronic
applications by fabricating large-area field-effect devices (10 micrometers by
110 micrometers in lateral dimensions), finding responsivities of 30 mA/W for a
laser power density of 13 mW/cm2 in the UV region of the spectrum and also as
an electron acceptor in a MoS2-based field-effect transistor.
