Direct visualization of out-of-equilibrium structural transformations in atomically thin chalcogenides

Two-dimensional (2D) nanomaterials, particularly transition metal dichalcogenides (TMDCs) like MoS₂, are of significant interest due to their unique electronic and optical properties, making them promising for various device applications. While previous research has investigated the effects of heat treatments on TMDCs, focusing primarily on equilibrium conditions, this study explores the impact of non-equilibrium heating rates on their structural transformations at the atomic scale. TMDCs, including MoS₂, can exist in various phases (e.g., 2H, 1T, 3R), influenced by atomic distortions and layer stacking. The 2H phase is thermodynamically stable, while others are metastable, reverting to 2H over time. Despite extensive research on phase identification and evolution, the nucleation and formation of metastable phases remain debated. In situ electron microscopy studies of TMDCs have largely focused on individual point defects, with limited large-area, multi-scale observations of structural transformations under non-equilibrium conditions. This gap in understanding motivates the current research, which aims to investigate how processing conditions impact atomic-scale structure and phase evolution in confined dimensions, leading to the formation of new, metastable phases. The study hypothesizes that varying the heating rate of bilayer MoS₂ can be used to tune the sulfur concentration and consequently alter its structure and morphology, exploiting the well-known phenomena of sulfur vacancy formation, melting point depression in quantum-confined materials, and the Mo-S binary phase diagram.

The literature review extensively cites prior works on 2D nanomaterials, TMDCs, and the effects of heat treatments on their structural properties. It highlights studies focusing on amorphous-to-crystalline transformations, point defect kinetics, and dislocation kinetics under equilibrium conditions. The review also covers research on various phases of Mo and W TMDCs (2H, 1T, 1T', 2H', 3R), their formation mechanisms, and the metastable nature of some phases. It acknowledges existing in situ electron microscopy studies of TMDCs, primarily focusing on point defect kinetics and heterojunction formation. However, it emphasizes the lack of understanding regarding the influence of processing conditions, especially non-equilibrium heating rates, on the atomic-scale structure and phase evolution, including the formation of metastable phases. The existing literature provides the basis for the current study's hypothesis that varying the heating rate can be used to control the sulfur concentration and thus the material's structure.

The study utilizes mechanically exfoliated MoS₂ layers transferred onto heater-embedded TEM grids. Two distinct heating methods are employed: (1) localized, fast heating (25 °C/sec) using embedded heating elements, creating highly non-equilibrium conditions, and (2) global, slow heating (25 °C/min) in an equilibrium environment (hot-walled reactor). Aberration-corrected scanning transmission electron microscopy (STEM) is used for real-time atomic-scale visualization of structural transformations. To address potential beam-induced transformations, the researchers analyze regions unexposed to the electron beam, confirming that the observed changes are primarily due to the thermal stimulus. Energy-dispersive X-ray spectroscopy (EDS) is used for compositional analysis. Bright-field transmission electron microscopy (BF-TEM) with a high-frame-rate camera is used to capture real-time coalescence of nanostructures. Ex situ slow heating experiments in a quartz tube furnace (vacuum and Ar environments) are conducted for comparison. Image simulations (QSTEM) are used to correlate experimental observations with different MoS₂ polyphases. Image analysis techniques, including morphological image filtering and connected component analysis, are employed to characterize the shape and size distribution of nanocrystalline islands formed during fast heating.

Fast heating leads to the disintegration of continuous MoS₂ layers into highly crystalline nanoscale islands (<20 nm) exhibiting hexagonal symmetry and composed of a mixture of 2H and 3R phases. In contrast, slow heating results in decomposition into amorphous structures. High-magnification STEM images reveal atomic-level details of the phase transformations, including the formation of atomically sharp heterojunctions between 2H and 3R phases. Real-time BF-TEM imaging shows the coalescence of these nanoscale islands, with the islands exhibiting a shape between a circle and a hexagon, suggesting a rounded hexagonal morphology. Ex situ slow heating experiments confirm the formation of sulfur vacancies, leading to non-stoichiometric decomposition and the formation of nanocrystalline or amorphous MoₓS₂₋ₓ, and eventually Mo nanocrystals due to sulfur evaporation. The epitaxial relationship between fully and partially disintegrated regions is observed, with islands in partially disintegrated regions showing preferential alignment with the underlying MoS₂ layer, while fully disintegrated regions exhibit randomly oriented islands. Statistical analysis of the island shape factors shows a distribution consistent with the dominance of hexagonal shapes indicative of a stoichiometric balance.

The findings demonstrate that heating rate significantly impacts the structural evolution of atomically thin MoS₂. Fast heating promotes rapid sulfur vacancy formation, leading to the formation of volatile sulfur compounds that leave the surface. Before those compounds leave, Mo adatoms begin to form which recombine with sulfur to form stoichiometric MoS2 nanocrystals. This, along with the sulfur conservation on the surface, leads to both void formation and a CVD-type environment for stoichiometric nucleation. Slow heating, however, allows sufficient time for sulfur evaporation, resulting in non-stoichiometric decomposition. This explains the contrasting results of highly crystalline islands in fast heating versus amorphous regions in slow heating. The observation of 3R phase formation during fast heating suggests that rapid layer restacking and vertical mass transport play a role in stabilizing this metastable phase. The epitaxial alignment observed in partially disintegrated regions suggests that the initial disintegration process preserves some structural order. The dominant hexagonal shape of the nanocrystals formed under fast heating further supports the preservation of the initial stoichiometric ratio.

This study reveals the critical role of heating rate in determining the structural transformations of atomically thin MoS₂. Fast heating yields highly crystalline, quantum-confined nanostructures, while slow heating leads to non-stoichiometric decomposition. The findings highlight a novel top-down approach for creating atomically thin, highly ordered nanostructures with potential for optoelectronic and catalytic applications. Future research could explore the detailed electronic and optical properties of these newly synthesized nanostructures, and investigate the applicability of this technique to other 2D materials.

The study primarily focuses on bilayer and few-layer MoS₂. The generalizability of the findings to monolayer or thicker MoS₂ needs further investigation. The influence of the substrate on the structural transformations could also be explored in more detail. While efforts were made to minimize the effect of electron beam irradiation, potential beam-induced changes cannot be entirely ruled out. Future studies with different substrates could help to eliminate those effects.