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We report on the large-scale synthesis of highly oriented ultrathin MoO3 layers using a simple and low-cost atmospheric pressure, van der Waals epitay growth on muscovite mica substrates. By this method, we are able to synthesize 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 be etensively characterized to demonstrate the high quality of the resulting crystal. We also study the electronic band structure of the material by density functional 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 µm x 110 µm in lateral dimensions) and find responsivities of 30 mA W--1 for a laser power density of 13 mW cm--2 in the UV region of the spectrum and also as an electron acceptor in a MoS2-based field-effect transistor. [ABSTRACT FROM AUTHOR]

Controlling the bandgap through local-strain engineering is an exciting avenue for tailoring optoelectronic materials. Two-dimensional crystals are particularly suited for this purpose because they can withstand unprecedented nonhomogeneous deformations before rupture; one can literally bend them and fold them up almost like a piece of paper. Here, we study multilayer black phosphorus sheets subjected to periodic stress to modulate their optoelectronic properties. We find a remarkable shift of the optical absorption band-edge of up to ∼0.7 eV between the regions under tensile and compressive stress, greatly exceeding the strain tunability reported for transition metal dichalcogenides. This observation is supported by theoretical models that also predict that this periodic stress modulation can yield to quantum confinement of carriers at low temperatures. The possibility of generating large strain-induced variations in the local density of charge carriers opens the door for a variety of applications including photovoltaics, quantum optics, and two-dimensional optoelectronic devices.