Bilayer Lateral Heterostructures of Transition Metal Dichalcogenides and their Optoelectronic Response.

Two-dimensional lateral heterojunctions based on monolayer transition metal dichalcogenides (TMDs) have received increasing attention given that their direct band gap makes them very attractive for optoelectronic applications. Although bilayer TMDs present an indirect band gap, their electrical properties are expected to be less susceptible to ambient conditions, with higher mobilities and density of states when compared to monolayers. Bilayers and few-layers single domain devices have already demonstrated higher performance in radio frequency and photo-sensing applications. Despite these advantages, lateral heterostructures based on bilayer domains have been less explored. Here, we report the controlled synthesis of multi-junction bilayer lateral heterostructures based on MoS-WS and MoSe-WSe mono-domains. The hetero-junctions are created via sequential lateral edge-epitaxy that happens simultaneously in both the first and the second layer. A phenomenological mechanism is proposed to explain the growth mode with self-limited thickness that happens within a certain window of growth conditions. With respect to their as-grown monolayer counterparts, bilayer lateral heterostructures yield nearly one order of magnitude higher rectification currents. They also display a clear photovoltaic response, with short circuit currents ~10 times larger than those extracted from the as-grown monolayers, in addition to room-temperature electroluminescence. The improved performance of bilayer heterostructures significantly expands the potential of 2D materials for optoelectronics.