Ultrafast relaxation of laser-excited flat nanoislands and nanoparticles investigated by ultrafast electron diffraction*

Ultrafast electron diffraction is used to probe the electron-phonon energy relaxation and phonon dynamics in Bi flat nanoislands and nanoparticles. The sample was excited with 110-fs laser pulses. By controlling the temperature and laser fluence during the annealing of thin Bi films, we prepare polycrystalline flat nanoislands and nanoparticles and investigate their relaxation dynamics. The temporal development of the various relaxation pathways is probed with a temporal resolution up to ~1.5 ps. The results show dependence on direction and varies between the nanoislands and nanoparticles. The response of the (110) diffraction peak for the 5-nm thick flat nanoislands is found to be faster than ~16 nm diameter nanoparticles. The response of diffraction for both particle shapes is found to be anisotropic. We also investigate the superheating of flat nanoislands and nanoparticles by monitoring the long-range order with excitation at various levels.