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Abstract:
A central goal of nanotechnology is the precise construction of nanoscale heterostructures with optimized chemical, physical or biological functionalities. It is known that function stems from structure but, in addition, function always involves nonequilibrium conditions and energy flow. The central topic of this thesis is the ultrafast energy flow in nanoscale heterostructures and how this energy flow drives ultrafast structural changes. The main experimental technique of this work is femtosecond electron diffraction, which probes the lattice response to electronic excitations. The nanoscale heterostructures contain metallic (Au) nanostructures of well-defined 0D or 2D morphology, supported on 2D substrates. In photoexcited heterostructures, thermal equilibrium is restored by electron-lattice interactions, within each component, and electronic and vibrational coupling across their interface. A newly developed model of ultrafast energy flow is used to measure the microscopic couplings, like electron-phonon coupling and interfacial vibrational coupling in nanoscale heterostructures using the observed Debye-Waller dynamics. Ultrafast energy flow in supported metallic nanostructures can initiate a rich variety of real-space motions like anharmonic lattice expansion and surface premelting, which manifest as distinct and quantifiable observables in reciprocal-space. These phenomena have been studied for Au nanoclusters on amorphous thin-film substrates. Au nanoclusters are found to exhibit ultrafast surface premelting at atypically low lattice temperatures and pronounced electron-lattice nonequilibrium conditions. Femtosecond electron diffraction is mostly used to study ultrafast motions related with phonons but in ultrasmall nanocrystals a new observable arises: the motion of the phonons’ frame of reference, meaning the crystal itself. This has been demonstrated for Au nanoclusters attached on graphene using femtosecond electron diffraction experiments, molecular dynamics and electron diffraction simulations. The substrate has a significant effect on the energy flow and the structural motions of ultrasmall, adsorbed nanostructures and, inversely, metallic nanostructures can alter fundamental properties of semiconducting substrates. Surface decoration with plasmonic, quasi-2D nanoislands of Au sensitizes WSe2 to sub-band-gap photons, causes nonlinear lattice heating and accelerates electron-phonon equilibration times. Conclusively, nanoscale heterostructures have a rich variety of nonequilibrium phenomena that affect their structure at ultrafast timescales. Ultrafast diffractive probes, like femtosecond electron diffraction, can provide a detailed, quantitative understanding of this relationship.
