The monolayer WO3-based UV-A photodetectors, fabricated by atomic layer deposition (ALD) technique at the large area of SiO2/Si wafer, have demonstrated vastly improved functional capabilities: extremely fast response time of less than 40 mu s and photoresponsivity reaching of similar to 0.329 AW(-1). Their ultrafast photoresponse time is at least 400-fold improvement over the previous reports for any other WO3-based UV photodetectors that have ever been fabricated, and significantly faster than most of other photodetectors based on two-dimensional (2D) nanomaterials reported-to-date. Moreover, their measured long-term stability exceeds more than 200 cycles without any visible degradation. The ALD-eposited WO3 monolayer has also exhibited wider bandgap of 3.53 eV and the UV-A photodetector based on it is environmentally friendly, highly reliable, with excellent reproducibility and long-term stability. Thus, the shift to mono-layered semiconductors, which possess completely new quantum confined effects, has the greatest potential in creating a new class of nano-materials, which in return windows new functional opportunities for various opto-electronic instruments built on semiconductor monolayer and, more importantly, can result in new strategy for fabrication highly-flexible, inexpensive and extremely-sensitive devices. This strategy also opens up the great opportunities for industrialization and commercialization of the photodetectors and other optoelectronic devices based on monolayer or few-layered 2D nanomaterials. (C) 2017 Elsevier B.V. All rights reserved.