Observation of Bismuth Nanoparticle Premelting by Slow and Ultrafast Laser Heating

Abstract

The melting of Bi nanoparticles (NPs), prepared by thermal evaporation, was studied by electron diffraction for slow thermal heating and for ultrafast laser heating. For thermal heating at a rate of ∼1 K/min, the onset of melting of the NPs was at ∼44 K below the Bi bulk melting point, T m . Lattice contraction was observed as the NPs were heated near their melting point. For ultrafast heating with a 800 nm, 110 fs laser pulse at a rate of ∼10 15 K/s, the melting point of the NPs was almost similar to that for slow heating, showing no evidence of superheating. This result differs from that of previous observations of superheating of Bi and is attributed to the rounded shape of the Bi NPs which are bounded by surfaces that melt below or at T m . The dependence of the diffraction intensity on temperature for the (110) order gives a larger slope for laser heating compared to thermal heating. This observation reflects the difference in the meansquare atomic displacement between thermal and laser heating as a result of the generation of directional-dependent coherent acoustic phonons from the decay of laser-excited optical phonons.