Direct Observation of the Epitaxial Growth of Bismuth Telluride Topological Insulators from One-Dimensional Heterostructured Nanowires.

As extraordinary topological insulators, 2D bismuth telluride (BiTe) nanosheets have been synthesized and controlled with a few-layer structure by a facile and fast solvothermal process. The detail-oriented growth evolution of 2D BiTe in an ethylene glycol reducing solution is discovered and recorded for direct observation of the liquid-solid interactions through the use of environmental SEM. At the initial synthesis stage, Te nanowires are rapidly synthesized and observed in solution. In the next stage, Bi nanoclusters slowly adhere to the Te nanowires and react to form hierarchical Te-BiTe nanostructured materials. Additionally, the Te nanowires shorten in-plane in an orderly manner, while the BiTe nanosheets exhibit directional out-of-plane epitaxial growth. In the last procedure, BiTe nanosheets with a clear hexagonal appearance can be largely obtained. Experiments performed under these rigorous conditions require careful consideration of the temperature, time, and alkaline environment for each reaction process. In addition, the yield of a wider and thinner BiTe nanosheet is synthesized by manipulating the crystal growth with an optimal alkaline concentration, which is found through statistical analysis of the AFM results. In the UV-Vis-NIR spectroscopy results, the main peak in the spectrum tends to redshift, while the other peak in the ultraviolet range decreases during BiTe nanosheet synthesis, facilitating a rapid understanding of the trends in the morphological evolution of the BiTe materials in solution. By rationalizing the above observations, we are the first to report the success of environmental SEM, HAADF-STEM, and UV-Vis-NIR spectroscopy for confirming the BiTe nanosheet formation mechanism and the physical properties in the solvent media. This research promotes the future optimization of promising BiTe nanomaterials that can be used in the fabrication of thermoelectric and topological components.