NANOSCALE HEAT TRANSFER IN SYNTHETIC METALLIC OPALS

Heat transport at nanoscale is of importance for many nanotechnology applications [1]. Considerable efforts are nowadays to discover new geometries with a huge number of internal interfaces and thermal barriers in order to increase the thermal resistance and/or manipulate the heat flow. A common material used to achieve these requirements is the synthetic opal, a special 3D Photonic Crystal (PhC) widely used in nanophotonics [2,3,4]. One of the most intriguing and relevant open question is how can heat be diffused in these nanostructures. In principle the heat flow should be reduced due to the huge numbers of internal interfaces and thermal barriers, but an exact answer is not trivial because the heat transport at nanoscale may differ substantially from that at macroscale [5]. This reason motivates our research on Nickel and Palladium inverse opals. The internal structure of the inverted opal is made of close-packed submicron air spheres (200nm÷500nm) regularly placed in the metal. Each metallic nanostructure has been thermally characterized by using photothermal radiometry [6] by a Ar laser @ 488 nm modulated by an acousto-optical modulator at a frequency ranging from 1 Hz up to 100 kHz so to change the penetration (i.e. the spatial resolution) of the induced thermal waves from 100nm to 1mm. The modulated infrared emission from the surface is collected by a Germanium lens and focused onto an infrared detector HgCdTe. The signal allows to measure the effective thermal diffusivity and the porosity of the whole structure. The calculated porosity via photothermal radiometry is about 50-60% for opals with spheres of 320nm, while reaches 60-70% for larger spheres of 450nm. These values of thermal porosity (obtained via photothermal measurements) well fit with the expected geometrical porosity of the opaline structure [7] This research demonstrates how photothermal radiometry may be particularly suitable to detect the thermal porosity and the effective thermal diffusivity in these nanostructures. This work has been done in the framework of the PhOREMOST Network of Excellenc