Nanoscale thermal properties from time-resolved optical investigations – OPTHERMAL

With the fast development of micro- and nano-size devices, heat transfer at a submicron scale has become a central problem in nanosciences and nanotechnology, fostering a fast emergence of this new field. Actually, heat transfer at a nanoscale raises many fundamental questions on the nature of the involved mechanisms and on their evolution with size reduction. For instance, when the dimensions of the system become smaller or comparable to phonon mean free paths, the traditional approaches developed in solid state physics become inapplicable. The same problem is encountered when in presence of interfaces, or when heat has to be transferred across thin fluid layers or 1-D junctions. The interfacial resistance between two materials, its modification at the nanoscale, and the interactions mechanism between nanoparticles are also key points, but still little studied, for understanding the enhanced thermal conductivity of 'nanofluids' (colloidal suspensions of oxide or metal nanoparticles). Our proposal aims at developing original experiments and simulations to investigate selectively different fundamental aspects of heat transfer at a submicron scale. The experimental strategy is based on time-resolved optical spectroscopy of metal nanoparticles used as local heaters and heat detectors. This technique has been very little exploited to analyse nanoparticle-environment heat transfers and the underlying mechanisms, though, as compared to contact-based methods, it has the great advantage of being non intrusive and permits the study of various environments. Furthermore, the improved control of nanoparticle synthesis offers large, and unexplored, possibilities for adaptation of the nanoparticles to the study of a specific problem. In our project, metal nanospheres with well controlled interfaces (presence and nature of the surfactants) and new bimetallic particles, i.e., Au-Ag nanodumbbells with adjustable particle sizes and controlled distance, will be synthesised to address selectively three key aspects of heat transfer at a nanoscale: (i) The thermal conductance across solid-liquid or solid-amorphous interfaces, and its possible use as a probe of the interface: investigations will be performed in gold or silver nanospheres dispersed in a fluid or an amorphous matrix. The linear and nonlinear thermal regimes will be studied in different fluids. (ii) The thermal conductance of molecular junctions: it will be based on investigating heat transfer between the Ag and Au spheres of nanodumbbells deposited on a surface. (iii) The heat transfer mechanism across very thin fluid films: using the same approach as for point ii) but in solution. The experimental approach will be backed-up by a theoretical and numerical effort to model heat transfer at the molecular level in the relevant geometry. This will permit detailed connection of the measured nanoparticle thermal relaxation kinetics with the underlying heat transfer and heat conduction processes, necessary to extract the relevant parameters. The three groups involved have a well established expertise covering all three necessary aspects (time-resolved experiments at LASIM, nanoparticle synthesis and functionalisation at LPCML, and modelling at LPMCN). Their location on the same campus will ensure a tight collaboration and an excellent synergy.