Ultra-broad spectral photo-response in FePS3 air-stable devices

Van der Waals materials with narrow energy gaps and efficient response over a broadband optical spectral range are key to widen the energy window of nanoscale optoelectronic devices. Here, we characterize FePS as an appealing narrow-gap p-type semiconductor with an efficient broadband photo-response, a high refractive index, and a remarkable resilience against air and light exposure. To enable fast prototyping, we provide a straightforward guideline to determine the thickness of few-layered FePS nanosheets extracted from the optical transmission characteristics of several flakes. The analysis of the electrical photo-response of FePS devices as a function of the excitation energy confirms a narrow gap suitable for near IR detection (1.23 eV) and, more interestingly, reveals a broad spectral responsivity up to the ultraviolet region. The experimental estimate for the gap energy is corroborated by ab-initio calculations. An analysis of photocurrent as a function of gate voltage and incident power reveals a photo-response dominated by photogating effects. Finally, aging studies of FePS nanosheets under ambient conditions show a limited reactivity of the outermost layers of flakes in long exposures to air.
The present work has received funding from the European Union’s Horizon 2020 research and innovation program through the European Research Council grants ERC-2017-StG-755655 2D-TOPSENSE, ERC-2018-AdG-788222 MOL-2D, under the Graphene Flagship through projects GrapheneCore2 (grant nr. 785219), and GrapheneCore3 (grant nr. 881603) and through the COST Action CA15128 MOLSPIN. The authors also acknowledge funding from the Spanish Government through the Unit of Excellence “María de Maeztu” MDM-2015-0538, through projects MAT2017- 88377-C2-2-R (AEI/FEDER), MAT2017-88377, MAT2017-89528 (AEI/FEDER), and R.F.’sJuan de la Cierva-formación fellowship FJCI-2017-32919. This work has been supported by the Generalitat Valenciana, through grants CIDEGENT/2018/004, CIDEGENT/2018/005, CDEIGENT/2019/022, the Prometeo Program of Excellence, and M.R.’s APOSTD/2020/249 fellowship.