Phase-Controlled Colloidal Synthesis of Monolayer Transition Metal Dichalcogenides Quantum Dots and lateral Heterostructures

Abstract

Two-dimensional group-VI transition-metal dichalcogenide (TMDs) quantum dots exhibit attractive optical, electronic, and catalytic properties, yet these functionalities are highly sensitive to crystal phase. Achieving selective phase control in colloidal synthesis is therefore essential but remains technically challenging. Although colloidal methods allow scalable production of monodisperse nanostructures, most reported syntheses depend on system-specific precursor–ligand chemistries, and a broadly applicable phase-control framework has yet to be established. In Mo-based TMDs such as MoS₂ and MoSe₂, the 2H phase is consistently favored because it is thermodynamically stable and separated from the 1T` phase by a large energy barrier. Consequently, few colloidal studies have reproducibly obtained the 1T` phase of MoS₂, even when reaction conditions are extensively modified. By contrast, W-based TMDs exhibit an opposing trend in which the 1T` phase forms readily under kinetic conditions. This behavior arises from the smaller 2H–1T` energetic offset, the more spatially extended 5d orbitals of W, and strong ligand interactions that stabilize distorted W–W chains. Nevertheless, producing high-quality monolayer 2H-phase W-based quantum dots remains difficult, creating an inverted stability challenge relative to Mo systems. Taken together, these contrasting phase preferences underscore the lack of a universal principle for crystal-phase control in colloidal TMD quantum dots. To address this limitation, we developed a ligand-modulated colloidal strategy that enables controlled synthesis of monolayer TMD quantum dots with tunable 2H and 1T` phases. Additionally, we extended the colloidal approach to the synthesis of nanoscale TMD lateral heterostructures, achieving the first synthesis of 2H-phase TMD lateral heterostructures. We further investigated how the interactions between 1T`- and 2H-phase lateral domains influence the overall structure. In the first part of this thesis, we report a colloidal synthesis strategy for phase-controlled fabrication of monolayer TMDs quantum dots (QDs), enabling selective formation of semiconducting 2H and metallic 1T` phases. By adjusting the oleic acid (OLAC) to oleylamine (OLAm) ratio during hot-injection, oleylammonium (OLAm⁺) species are formed, which interact with [MX₂]⁻ units in the lattice to stabilize the 1T` phase. Moreover, the introduction of oleic acid decreases the effective Lewis basicity of oleylamine, thereby lowering the concentration of free amines and promoting the release of more reactive S²⁻ or Se²⁻ species, which generates a burst of supersaturation. This drives the system into a kinetically controlled regime that preferentially facilitates nucleation of the 1T` phase. Systematic syntheses of MoS₂, MoSe₂, WS₂, and WSe₂ show that the absence of OLAC yields 2H phases, while higher OLAC content favors the 1T` structure. Atomic-level identification of these phases was confirmed by HAADFSTEM analysis. In the second part, we reduced the size of TMDs lateral heterostructures to the nanoscale via colloidal synthesis for the first time. Photoluminescence measurements showed clear peak shifts, indicating a pronounced quantum confinement effect in these nanoscale heterostructures. We further investigated lateral growth between the 1T` and 2H phases and demonstrated a lattice-mismatch-driven strategy for phase engineering in colloidal WS₂–MoSe₂ and WS₂–MoS₂ core–shell lateral heterostructures. Epitaxial growth of 2H-MoSe₂ shells on 1T-WS₂ cores introduces interfacial lattice mismatch, generating compressive strain that drives the 1T`-WS₂ core to transition into the 2H phase. Atomic-resolution HAADFSTEM analysis reveals that the extent of 2H-MoSe₂ shell coverage directly controls the degree of the phase transition. Initially, 2H-MoSe₂ forms at the edges, partially converting the 1T`-WS₂ core, and as the shell grows laterally, accumulated strain induces complete conversion to 2H-WS₂. HRSTEM imaging further visualizes the atomic-scale structural evolution and phase boundaries. A similar lattice-mismatch-induced transition is observed in WS₂–MoS₂ core–shell lateral heterostructures. Importantly, both WS₂–MoSe₂ and WS₂–MoS₂ heterostructures remain monodisperse after the phase transition.