Abstract
Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been the subject of sustained research interest due to their extraordinary electronic and optical properties. They also exhibit a wide range of structural phases because of the different orientations that the atoms can have within a single layer, or due to the ways that different layers can stack. Here we report a unique study involving direct visualization of structural transformations in atomically thin layers under highly non-equilibrium thermodynamic conditions. We probe these transformations at the atomic scale using real-time, aberration-corrected scanning transmission electron microscopy and observe strong dependence of the resulting structures and phases on both heating rate and temperature. A fast heating rate (25 °C/sec) yields highly ordered crystalline hexagonal islands of sizes of less than 20 nm which are composed of a mixture of 2H and 3R phases. However, a slow heating rate (25 °C/min) yields nanocrystalline and sub-stoichiometric amorphous regions. These differences are explained by different rates of sulfur evaporation and redeposition. The use of non-equilibrium heating rates to achieve highly crystalline and quantum-confined features from 2D atomic layers present a new route to synthesize atomically thin, laterally confined nanostructures and opens new avenues for investigating fundamental electronic phenomena in confined dimensions.
Highlights
Two-dimensional nanomaterials have received significant attention due to the wide range of exciting properties that they exhibit[1]
We find that the effect of heating rate on the morphology of bilayer/few-layer MoS2 is consistent with the equilibrium bulk binary phase diagram of Mo and S32,33
During rapid in situ heating, the samples are heated at 25 °C/sec from 500 to 700 °C (Fig. 6a), while during the ex situ experiment shown in Fig. 6b the sample was heated over the same temperature range at a rate of 25 °C/min
Summary
Two-dimensional nanomaterials have received significant attention due to the wide range of exciting properties that they exhibit[1]. There have been several studies investigating the effect of heat treatments on the structural properties of these systems, but to date these have largely focused on amorphous to crystalline transformation kinetics, point defect kinetics, and dislocation kinetics under equilibrium conditions from ranging ambient to elevated temperatures[4,5]. We have studied the effect of equilibrium vs nonequilibrium diffusion conditions during the heating of atomically thin 2D MoS2 sheets. We find that the effect of heating rate on the morphology of bilayer/few-layer MoS2 is consistent with the equilibrium bulk binary phase diagram of Mo and S32,33.
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