Controllable Growth of Few-Layered MoS2 Films and Their Memristive Behaviours

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as promising candidates for future electronic materials, enabling data-centred energyeffective applications. 2D MoS2 is the archetypical member of TMDs, offering exceptional electrical, optoelectronic, and mechanical properties. Despite extensive research efforts, there are still significant hurdles to realise their application potential. Achieving controllable and scalable thesis of 2D MoS2 remains a critical challenge. While alkali halide assisted chemical vapour deposition (CVD) enables wafer-scale growth of polycrystalline 2D MoS2 on SiO2 substrate, the residual alkali atoms are detrimental to device performance, and the uncontrolled grain structures lead to high device variability and poor reliability. This thesis introduces a novel contamination-free growth promoter, photoresist S1813, which enables the clean and controlled synthesis of 2D MoS2 with desirable grain structures. At a sulfur temperature of 250 �C, the promoter-enhanced region shows over threefold coverage, a sixfold increase in the monolayer ratio, and a 60-fold enhancement in the proportion of large flakes (� 10 �m) compared to pristine regions. Systematical investigation under varing sulfur temperature reveal a transition from homogeneous to heterogeneous nucleation and a strong correlation between sulfur temperature and sulfur vacancy density. Using photoresist patternability, a scalable and highly controllable polymer-assisted CVD method is developed for the selective growth of MoS2. By strategically incorporating polymer mesas patterned via laser direct writing (LDW) and optical lithography (OL), we demonstrate precise spatial control over nucleation and film morphology. Systematic analyses across the substrate revealed a clear correlation among MoOx particle density, mesa thickness, and surrounding film dimensions, highlighting the critical influence of reactant concentrations, particularly MoOx and sulfur, on MoS2 growth outcomes. These findings elucidate the diffusion-controlled growth mechanisms responsible for patterned MoS2 formation. Importantly, this method was successfully scaled to achieve milimeter-scale patterned growth, demonstrating significant potential for industrial adoption. By carefully controlling the initial polymer thickness and pattern geometry, we consistently produce monolayer MoS2 films with lateral dimensions exceeding 5 mm. To demonstrate the versatility of the growth method, we have fabricated planar memristors based on the 2D MoS2 films. A forming process enables bipolar switching in the Au/MoS2/Au memristor, where a low operating voltage (< 1 V) drives defect migration and modulates the Schottky barrier at the film-metal interface. The tuning of the Schottky barrier regulates the resistance states of the channel as well as the dynamics of the switching. The growth strategy is intrinsically simple, scalable, controllable, and compatible with current semiconductor technology. The research can be extended to other TMDs and enable a wide range of 2D electronic devices. Specifically, the controllability and versatility of the method enable us to explore (1) selective stable unipolar non-degenerated p-type doping in 2D MoS2 films, (2) patterned 2D TMD functional blocks with desirable grain structures, and (3) the potential of the 2D memristors as an artificial synapse in neuromorphic applications.