UC Berkeley

Atomically thin van der Waals (vdW) materials are sensitive to their surrounding environment, and the properties they exhibit can be significantly affected by changing this environment. By placing two atomically thin layers together with different work functions, charge will transfer between the two materials. Appreciable hole doping (on the order of 1013 holes/cm2) is realized in graphene, MoS2, and WSe2 interfaced with α-RuCl3 as a result of this charge transfer. α-RuCl3 has a relatively large work function that allows it to accept a large density of electrons. The charge transfer to α-RuCl3 can be modified by increasing the distance between the vdW material and α-RuCl3 with a hBN spacer layer.The electrochemical intercalation of lithium ions in graphene and TMD heterostructures is shown to change significantly in the presence of α-RuCl3. Raman spectroscopy and electrical transport measurements are used to monitor the intercalation reactions in situ. The main finding is that α-RuCl3 significantly lowers the driving force needed to electrochemically insert lithium ions into vdW heterostructures. It is hypothesized that the hole doping induced by α-RuCl3 increases the favorability of electron reduction in the other layer that is concomitant with lithium intercalation. In addition, the intercalation driving force and the intercalation amount can be tuned with the doping in these heterostructures using hBN spacers. The control α-RuCl3 offers for ionic charge accumulation could potentially inform future battery technologies. The control over electronic charge accumulation in α-RuCl3 heterostructures also has interesting applications. The transport of graphene interfaced with monolayer α-RuCl3 is shown to exhibit an anomalous, dynamic response to gate voltages when observed by low temperature magnetotransport measurements. While the origin of this behavior is unknown, the characteristics of the transport in the presented graphene/α-RuCl3 devices could be useful for next generation electronic devices. When TMDs are interfaced with α-RuCl3, their trans- port properties are also significantly impacted. WSe2 doped with α-RuCl3 is metallic and its pristine photoluminescence (PL) response is quenched, which could have significant implications if utilized in optoelectronic devices. In addition, lateral heterojunctions of pristine WSe2 and WSe2 on α-RuCl3 exhibit pP type diode junction behavior, with direction dependent current responses. This study demonstrates the versatility of α-RuCl3 heterostructures, and documents the progress we have made towards honing our ability to control electronic and ionic charge in vdW heterostructures.