Ultrafast thermal response of bismuth and indium nanoparticles near their melting points

The thermal response of Bi and In nanoparticles on carbon film and In nanoparticles embedded in Al are studied by ultrafast electron diffraction (UED) with a temporal resolution up to ~1.5 ps. The results show that the thermal response of nanoparticles near their melting points upon excitation by the femtosecond laser pulses deviates from that by heating under equilibrium conditions. A subset of the Bi nanoparticles and the In nanoparticles embedded in Al shows crystal integrity above their bulk melting points, consistent with transient superheating. For the Bi nanoparticles, this may be attributed to faceting of some of the nanoparticles, while for the embedded In nanoparticles the In/Al interface may suppress the growth of the In surface molten layer. We also investigated the anisotropic thermal expansion of the nanoparticles by measuring the angular shift of the diffraction peaks with UED.