Revealing Electron–Phonon Interactions and Lattice Dynamics in Nanocrystal Films by Combining <i>in Situ</i> Thermal Heating and Femtosecond Laser Excitations in 4D Transmission Electron Microscopy

We present a comparative investigation on static equilibrium and transient structural dynamics of nanocrystalline gold films on silicon nitride supports performed at various in situ temperatures and by ultrafast laser excitations in a four-dimensional ultrafast transmission electron microscope (4D-UTEM). The change of relative diffraction intensity and lattice spacing with rising temperatures was systematically measured for {220} Debye–Scherrer rings via the in situ heating technique, which leads to a precise determination of the actual Debye temperature and a finding of significant depression of lattice expansions in the films. The diffraction intensity/lattice spacing–temperature relationship calibrated by the static, thermally equilibrium observations was then employed for investigating ultrafast transient dynamics on the same specimen region. The electron–phonon coupling constant g was determined to be 7.2 × 1015 W/m3 K in combination with simple two-temperature model analysis. We found a marked variation of temperature rise maximum (at quasi-equilibrium states) in between the temporal evolutions of lattice spacing and diffraction intensity, a phenomenon which may only be explained by the effect of nonthermal equilibrium relaxation dynamics following femtosecond laser excitations. The method demonstrated here can thus be applied to quantitative evaluations of nonthermal equilibrium contributions during the electron–lattice thermalization.