Evidence of liquid state for small bismuth particles

Small bismuth particles have been formed on amorphous carbon films by molecular beam deposition. The pressure during the deposition was less than 1 × 10^?4 Pa. At low thicknesses (<1.5 nm) most of Bi particles are small (2 to 10 nm) and isolated. Electron diffraction and dark field transmission electron microscopy observations (dark field T.E.M.) show that these particles are not crystallized. Increasing the thickness of the deposit, the diameter of aggregates and also the number of crystallized particles increase. Then there is coexistence between non-crystallized and crystallized particles. At thicknesses higher than 2 nm, electron diffractions show rings (indicating the crystallization of particles) which can be indexed in the normal rhombohedral structure of bismuth. In situ low temperature T.E.M. observations of low (or intermediate) thickness Bi deposits performed using a cooling stage show the crystallization of particles. Returning at room temperature, many particles which were not crystallized at the begining of the experiment retain the crystallized structure. It is then necessary to warm up the sample to melt these particles which crystallize again at room temperature. This behaviour agrees with a liquid state for particles after deposition which can be explained by a supercooling phenomenon.