Direct growth of monolayer 1T–2H MoS2 heterostructures using KCl-assisted CVD process

Abstract

Accessing the metastable phases in a controlled fashion can further expand the applications of atomically thin transition metal dichalcogenides (TMDs). Although top-down approaches based on ion intercalation exfoliation have shown to be an effective route to transform 2H phase into 1T and/or 1T′ polytype phases, a bottom-up growth strategy could be more suitable for device integration. Herein, we show that by assisting the atmospheric pressure chemical vapor deposition (APCVD) growth with a specific alkali metal halide (AMH), it possible to induce the direct synthesis of 1T phase domains coexisting with 2H phase structure in micrometer-sized MoS2 monolayer flakes. The photoluminescence emission and structural properties of three different AMH (NaCl, KBr and KCl) MoS2 crystals are compared. Both NaCl and KBr assisted MoS2 monolayers displayed the semiconducting 2H-phase. On the other hand, we demonstrate that KCl promotes the formation of a 1T–2H phase mixture. X-ray photoemission spectroscopy and resonant Raman studies performed on KCl–MoS2 monolayers show the emergence of a second chemical state and 1T Raman bands compared to the rest of the samples. High-resolution scanning transmission electron microscope imaging revealed important changes in the atomic arrangement between 2H and 1T domains, providing clear evidence of the presence of the 1T metastable phase in the lattice. Moreover, the growth 1T domains can also be controlled by modifying the deposition temperature. Our experiments show that the introduction of KCl during the APCVD growth result in stable 1T-MoS2 domains, providing a simple and reproducible route towards the polymorphism phase engineering of layered TMDs using a direct bottom-up approach.