Reaching Excitonic Limit in 2D Janus Monolayers by In-situ Deterministic Growth.

Named after the two-faced Roman god of transitions, transition metal dichalcogenide (TMDs) Janus monolayers have two different chalcogen surfaces, inherently breaking the out of plane mirror symmetry.[ 4] The broken mirror symmetry and the resulting potential gradient lead to the emergence of quantum properties[ 7] such as the Rashba effect[ 8] and the formation of dipolar excitons.[ 11] Experimental access to these quantum properties, however, hinges on the ability to produce high-quality 2D Janus monolayers. Here, our results introduce a holistic 2D Janus synthesis technique that allows real-time monitoring of the growth process. Our prototype chamber integrates in-situ spectroscopy, offering fundamental insights into the structural evolution and growth kinetics, that allow us to evaluate and optimize the quality of Janus monolayers. We demonstrate the versatility of this method by synthesizing and monitoring the conversion of SWSe, SNbSe, and SMoSe Janus monolayers. Deterministic conversion and real-time data collection further aid us to convert exfoliated TMDs to Janus monolayers for the first time and reach unparalleled exciton linewidth values compared to the current best standard. The results offer an insight into the process kinetics and aid in the development of new Janus monolayers with high optical quality, which is much needed to access their exotic properties. This article is protected by copyright. All rights reserved.