High Quality 2D α-MoO3 Microcrystals Produced by Laser Processing

2D-Transition Metal oxides (TMOs), although less studied than the well-known 2D transition metal dichalcogenides(2D-TMCs) are appealing due to some specific advantages. Like the TMCs they are semiconductors, however they are less contaminant and offer a large potential for tuning their electro-optical properties by varying their stoichiometry. In particular, MoO₃ is transparent and shows a wide bandgap (&gt;3 eV) and a high dielectric constant k ~ 500. Additionally, orthorhombic α-MoO₃ possesses the well-known layered crystal structure of MoO₃ which offers the possibility to create two dimensional (2D) morphologies.[1] In this context, 2D MoO₃ has shown extraordinary properties such as anisotropic polariton propagation,[2] and is an ideal material for electronic applications for high power electronics and short wavelength optoelectronics.<br/><br/>Currently most of the work in 2D-semiconductors is still made using small flakes obtained from mechanically exfoliated single crystals, however there is no doubt that it will be advantageous to deposit large 2D-strucutres as this will have tremendous impact both for the manipulation an integration of these materials.[3] In context we show the successful preparation of nanometer thick thin films formed by 2D MoO₃ crystals by a pulsed laser deposition-based process that starts with the deposition in vacuum from a MoO₃ target of dense, amorphous and continuous substoichimetric MoO_3-Xlayers. Subsequently, the films are annealed in air up to 300 C while following the evolution of their optical properties in-situ in the UV-VIS wavelength region by spectroscopic ellipsometry. When the temperature reaches 250 C a clear change in the optical properties starts that is related to the films crystallization. Analysis of the optical properties shows how the initially absorbing films with a metallic-like behavior after the annealing become transparent in the NIR and VIS regions, and shows a band gap &gt;3 eV. This optical change is related to the formation of large rectangular micron size α-MoO₃ single crystals with a thickness of the order 10 nm easily observable by optical microscopy. A full characterization of the morphology, structure and stoichiometry has been confirmed by X-ray diffraction analysis, Raman spectroscopy, AFM and transmission electron microscopy. We will show the distinct fetures of the measured dielectric constant of the 2D MoO₃ microcrystals and compare them to that of the bulk in terms of defects and lattice stress. Finally, we analyze the implications for different optoelectronic applications.<br/><br/>[1] I. A. de Castro, et al., . Molybdenum Oxides – From Fundamentals to Functionality. Adv. Mater. 29, 1-31 (2017).<br/>[2] W. Ma, et al In-plane anisotropic and ultra-low-loss polaritons in a natural van der Waals crystal. <i>Nature</i>. <b>562</b>, 557–562 (2018).<br/>[3] J-H Kim, et al. Van der Waals epitaxial growth of single crystal α-MoO3 layers on layered materials growth templates. <i>2D Mater.</i> <b>6</b> (2019).