Two-dimensional nanoplates of Bi₂Te₃ and Bi₂Se₃ with reduced thermal stability

Free-standing thin nanoplates of Bi₂Te₃ and Bi₂Se₃ were synthesized by solvothermal method. It was demonstrated that the thickness of the nanoplates can be controlled by introducing a controlled amount of polyvinylpyrrolidone (PVP) in the synthesis reaction. PVP bonds to the polar basal planes of hexagonal crystal structure of Bi₂Te₃ and Bi₂Se₃, and they suppress the growth (speed) of the hexagonal crystals in the c-axis direction. Highly anisotropic growth yielded the formation of 2-dimensional nanostructures of nanoplates. The plates were examined directly with transmission electron microscopy (TEM) with in-situ heating. These crystalline nanoplates with extremely high width to thickness ratios were found to exhibit much lower thermal stability compared to the bulk counterpart or the conventional nanoparticles as represented by the reduced melting temperature. The melting temperature of a nanoplate decreased by more than 100°C compared to the melting temperature of the bulk material. While it is widely known that the meting temperature decreases for nanoparticles with reduced sizees in all three spatial dimensions, we demonstrate that the reduction in one dimension, i.e. thickness of the platelets in the present study, is effective enough to induce much greater decrease of the melting point than the decrease as observed for the case of nanoparticles.