Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Physics

Abstract

The optimal methodology to determine the thermal properties of solids is particularly influenced by their thinness and morphology. As far as highly thermally conducting and ultrathin layered materials on transparent and insulating substrates are concerned, contactless measurements that deposit a modest amount of heat on the sample are essential. In our thesis a combination of analytical and numerical solutions of Fourier’s equation of heat is used, with the appropriate boundary conditions, for modelling the thermally modulated optical response of solids, in phase and amplitude, as a function of a their periodical illumination by a light beam that generates a heating of the thin-film substrate system at the same periodicity. Inverse solutions of Fourier's heat equation are required to extract the thermal conductivity and specific heat of the system in these cases. Two configurations are specifically considered. In the first configuration, in which the thermal properties are uniform at the macroscopic level in the directions orthogonal to the light beam, a photothermal deflection (PTD) configuration is considered and modelled under the assumption that the thermo-optical properties can be measured both from the thin film-side and the substrate-side of the system. We find that, on both sides, the phases of the PTD principally depend on the thermal diffusivity of the thin film, while the amplitudes also depend on the specific heat. In a second configuration, in which the thermal conductivity changes from point to point of the surface at the mesoscopic level, we developed a numerical method to solve Fourier's equation in configurations in which the thermo-optical properties are measured by scanning near-field optical techniques.

This configuration has been used graphene thin films decorated with a copper nanoparticle (Cu-NP) layer, before and after the deposition of of Cu-NPs and after Cu-NP removal. In this system, we have been able to show that the decrease of thermal conductivity of graphene in contact to metal nanoparticles is due to phonon scattering by Dirac electrons in graphene, and not to metal-graphene interfacial thermal resistance, solving a longstanding debate in the literature. ‎A description of this phenomenon in terms of diagrammatic quantum field theory will be offered.

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