Atomistic Observation of the Local Phase Transition in MoTe ₂ for Application in Homojunction Photodetectors

Abstract

Direct atomic‐scale observation of the local phase transition in transition metal dichalcogenides (TMDCs) is critically required to carry out in‐depth studies of their atomic structures and electronic features. However, the structural aspects including crystal symmetries tend to be unclear and unintuitive in real‐time monitoring of the phase transition process. Herein, by using in situ transmission electron microscopy, information about the phase transition mechanism of MoTe ₂ from hexagonal structure (2H phase) to monoclinic structure (1T′ phase) driven by sublimation of Te atoms after a spike annealing is obtained directly. Furthermore, with the control of Te atom sublimation by modulating the hexagonal boron nitride (h‐BN) coverage in the desired area, the lateral 1T′‐enriched MoTe ₂/2H MoTe ₂ homojunction can be one‐step constructed via an annealing treatment. Owing to the gradient bandgap provided by 1T′‐enriched MoTe ₂ and 2H MoTe ₂, the photodetector composed of the 1T′‐enriched MoTe ₂/2H MoTe ₂ homojunction shows fast photoresponse and ten times larger photocurrents than that consisting of a pure 2H MoTe ₂ channel. The study reveals a route to improve the performance of optoelectronic and electronic devices based on TMDCs with both semiconducting and semimetallic phases.