SHAPE TRANSFORMATION AND SURFACE MELTING OF CUBIC AND TETRAHEDRAL PLATINUM NANOCRYSTALS

We report transmission electron microscopic studies of in-situ tempera ture-induced shape transformation and melting behavior of polymer-capp ed cubic and tetrahedral nanocrystals. Our results indicate that the s urface-capping polymer is removed by annealing the specimen at tempera tures between 180 and 250 degrees C. The particle shapes show no chang e up to similar to 350 degrees C. In the temperature range between 350 and 450 degrees C, a small truncation occurs in the particle shapes b ut no major shape transformation is observed. The particle shapes expe rience a dramatic transformation into spherical-like shapes when the t emperature is raised above similar to 500 degrees C, where surface dif fusion or surface premelting (softening) takes place. Above 600 degree s C, surface melting becomes obvious leading to coalescence of the sur faces of neighboring nanocrystals and a decrease in the volume occupie d by the assembled nanocrystals. The surface melting forms a liquid la yer a few atomic layers deep around the still solid core of the nanocr ystal. This temperature is much lower than the melting point of bulk m etallic platinum (1769 degrees C). The reduction in the melting temper ature is discussed in terms of the surface tension of the solid-liquid interface (gamma(SL)). For an 8 nm diameter Pt nanocrystal, gamma(SL) is calculated to be 2.0 N m(-1) at 650 degrees C, which is smaller th an that of the bulk solid-vapor metal surface tension (gamma(sv)). Thi s reduction is proposed to be due to the compensation of the increase in gamma(sv) of the nanocrystal by the wetting effect at the solid-liq uid interface.