2D Materials Beyond Graphene: Optoelectronic Properties of Transition Metal Dichalcogenides

Several transition metal dichalcogenides are semiconductors which, similarly to graphene, are composed of van der Waals coupled atomic layers. When exfoliated down to a single atomic layer, a transition from an indirect to a direct band gap is observed suggesting their suitability for electronics and optoelectronics. In fact, several classes of high-performance devices have already been reported along with significant progress in understanding their physical properties. Here, we will provide a very brief overview on the field and discuss experimental studies which further unravel their potential for optoelectronic applications. In particular, we will discuss initial results on multilayered WSe₂ field-effect transistors which indicate extremely high photoresponsivities, i.e. in excess of ~7 A/W, along with photo response times ranging from τ ~5 μs to 40 μs, which are promising values for photo detecting applications. We will also discuss the observation of a hitherto unreported electro-optical effect, namely light-induced diode like response in MoSe₂ field-effect transistors whose sense of current rectification can be controlled by a gate voltage. This effect could lead to a new type of optoelectronic switch. Finally, we will present results from electrostatically gated MoSe₂ PN junctions, unveiling photovoltaic conversion efficiencies exceeding 10% under white light illumination from devices composed of approximately 10 atomic layers. All these observations underline the particularly strong interaction between light and charge carriers in these systems.