Growth of Large-Scale, Large-Size, Few-Layered α-MoO on SiO and Its Photoresponse Mechanism.

Layered α-MoO is a multifunctional material that has significant application in optoelectronic devices. In this study, we show the growth of large-scale, large-size, few-layered (FL) α-MoO nanosheet directly on technical substrates (SiO and Si) by physical vapor deposition. We suggest that the growth is self-limiting in the [010] direction because of the re-evaporation and high diffusion capacity of MoO species at high temperature. As-prepared FL α-MoO is nonconductive and shows poor response to photoillumination with wavelength of 405 and 630 nm. Its work function is strongly altered by the substrate. Improvement of conductivity and photoresponse is observed after the FL device is annealed in vacuum. Line defects along the [001], [100], and [101] directions belonging to the generation of O and O vacancy states appear, and the interfacial effect is suppressed. Scanning near-field optical microscope shows that the defects are absorption sites. Kelvin probe force microscope reveals decrease of apparent work function under illumination, which confirms that electrons are excited from defects states. Our findings show that intense studies on defect engineering are required to push forward the application of two-dimensional metal oxides.