Photo-induced structural evolution in metal films and nano-ceramics

David Wang Auditorium, 3rd floor Dalia Maydan Bldg.
Prof. Alexander Shluger, Invited Speaker

Prof. Alexander Shluger, Invited Speaker

Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK.

I will present the experimental results and computational modelling of laser-induced structural transformations of gold nano-films and ceramic nano-particles. The structural dynamics of photo-excited gold nano-films was studied using relativistic ultrafast electron diffraction measurements and modelled using a two-temperature molecular dynamics (2T-MD) method. This allows us to make a direct comparison of the time evolutions of measured and calculated Bragg peaks. The quantitative agreement between the temporal evolutions of the experimental and theoretical Bragg peaks at all photon fluences suggests that the 2T-MD method provides a faithful atomistic representation of their structural evolution. The results reveal the transition between slow heterogeneous melting at low absorbed photon fluence to rapid homogeneous melting at higher fluence and nonthermally driven melting at very high fluence [1]. Nanoparticles of wide band gap materials MgO and CaO, subjected to low-intensity ultraviolet irradiation with 266 nm (4.66 eV) photons, emit hyperthermal oxygen atoms with kinetic energies of up to ∼0.4 eV. Using an ab initio embedded cluster method we unravel a variety of elementary photo-induced processes at both ideal and defect-containing surfaces of these nanoparticles and develop a mechanism for the desorption process [2]. The proposed mechanism includes multiple local photo-excitations resulting in sequential formation of localized excitons, their ionization, and further excitations. We demonstrate how judicious choice of sub-bandgap photon energies can be used to selectively modify surfaces of nanomaterials.
[1] S. L. Daraszewicz et al. Phys. Rev. B 88, 184101 (2013)[2] P. E. Trevisanutto et al. J. Phys. Chem. C 113, 1274–1279 (2009)

Host: Eugene Rabkin