Phase Engineering of Transition Metal Dichalcogenides

Abstract

Crystal phase has emerged as one of the important parameters, beyond size, morphology, composition, and facets, that determine the physicochemical properties of nanomaterials. As one group of materials with polymorphs, transition metal dichalcogenides (TMDs) have been widely explored for various applications owing to their intriguingly phase-dependent properties. Great efforts have been devoted to phase engineering of TMDs, including phase-controllable synthesis and phase transition of TMDs. For the phase transition of TMDs, though many phase transitions have been realized by various strategies, including laser irradiation, heat treatment, ion intercalation, and surface charge transfer, the unavoidable structural defects introduced during the phase transition process degrade the reversibility of these transitions. To realize the reversible phase transition of TMDs, we employed proton ions as the intercalators to drive the reversible phase transition of TMDs. For the phase-controllable synthesis of TMDs, although great efforts have been made in this research field, the general strategy for phase-controllable TMD monolayers has never been reported since the high synthetic temperature can easily transform the metastable-phase TMDs into the thermodynamically stable ones. To extend the applications of TMDs for various applications, a general strategy for the synthesis of TMD monolayers is urgently desired. In this thesis, we reported a general chemical vapor deposition (CVD) strategy for synthesizing group VIB TMD monolayers.

(1) For the phase transition of TMDs, with the aid of microcells, we successfully intercalate the proton ions into the van der Waals gas of 1T′-TMDs. Simultaneously, the electrons can be injected into the nanosheets, which can induce the phase transition of 1T′-TMDs to 1T′_d-TMDs. Interestingly, the phase transition can also induce the bandgap structure modulation from the metallic feature to a semiconducting feature. When the gate voltage was scanned back to zero voltage, the proton ions would be extracted from the van der Waals gaps of the TMD nanosheets. Simultaneously, the electrons would be extracted from the nanosheets, and the phase will transform from the semiconducting 1T′_d phase to the 1T′ phase. Impressively, an ultrahigh on/off ratio of up to 10⁶ can be realized during this phase transition process.

(2) For the phase-controllable synthesis of TMD monolayers, a general CVD method was developed to synthesize VIB TMD monolayers with selective 2H or 1T′ phase, i.e., MoS₂, WS₂, MoSe₂, WSe₂. Interestingly, compared to the 1T′-phase bulk crystals with phase transition temperatures ranging from 60 to 160 ºC, the as-synthesized 1T′-phase monolayers demonstrate much-improved phase transition temperatures ranging from 300 ºC to 500 ºC, paving the road map of metastable-phase TMDs for various applications. Comprehensive characterizations were employed on the monolayers, proving the high-phase purity of the as-synthesized TMDs. The optical and electrical measurements demonstrate the crystal-phase-dependent isotropy and anisotropy of the as-synthesized 2H and 1T′-phase TMDs monolayers, respectively, which further reveals the high-phase purity of the as-synthesized crystals. To further demonstrate the great potential of the as-synthesized monolayers for future electronics, the van der Waals integrated 2H and 1T′-MoS₂ homojunction was employed as a high-performance inverter with an ultrahigh rectifying ratio of over 10⁵.

Date of Award	7 Aug 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Hua ZHANG (Supervisor)