Ionic Polymer Gated WS2-Graphene Photodetectors

Layer by layer assembly of two-dimensional materials enables the tailored fabrication of atomically thin electronic devices. By using different material combinations, these van der Waals heterostructures can possess novel properties not found in the constituent materials. For example, combining graphene with semiconductor materials in heterostructure photodetectors, allows the detection of light signals down to powers in the femtowatt range. Currently, the operational speed of these detectors is limited by the presence of long-lived charge traps to a few tens of Hz. In this work, we fabricate and fully characterize atomically thin graphene-WS
₂
heterostructure photodetectors, encapsulated in an ionic polymer. we demonstrate an internal gain exceeding 10
⁶
electrons per photon due to the high mobility graphene channel and the strong light-matter interaction of WS
₂
. However, unique to our devices is the expansion of the operational bandwidth into the kHz range, whilst simultaneously observing responsivities as high as 10
⁶
A/W. Highly mobile ions and the nanometer Debye length of the ionic polymer are used to screen charge traps and tune the Fermi level of graphene over an unprecedented range at the graphene-WS
₂
interface. This combination of both high responsivity and kHz bandwidth makes these photodetectors suitable for video-frame-rate imaging applications. Furthermore, to extend the spectral range into the near-infrared the use of alternative semiconductor materials to replace WS
₂
will also be discussed.