OPTICAL AND THERMAL CHARACTERIZATION OF NANOSTRUCTURES BY PHOTOTHERMAL TECHNIQUES

Heat transport at nanoscale is of importance for many nanotechnology applications. The request to reduce the size of electronic devices and integrated micro/nano-electro-mechanical systems provides the main driving force behind the scientific research and technological advancement in nanotechnology. It is now widely accepted that the thermal management in nanosize devices becomes fundamental as the size of the device reduces. Thermal conduction in nanostructures plays a critical role in controlling the performances and stability of nanodevices. In this paper we show how photothermal techniques represent useful non-destructive techniques to study the optical and thermal properties in nanostructures. The photothermal deflection spectroscopy (PDS) may be used to measure the absorbance spectrum in nanostructures so to detect absorption lines, or photonic band gap in photonic crystals, or to evaluate the entity of the scattering phenomena in carbon nanotubes. We have applied PDS in the UV/VIS/IR range from 250 to 1200 nm and by using a low modulation frequency from 1Hz to 100Hz which limits the spatial resolution so that the optical and thermal measurements are averaged over the volume of the nanostructure. The Photothermal radiometry (PTR) may be used to localize the internal heat sources induced by a pump beam partially absorbed in the volume, and to measure the effective thermal diffusivity of the entire structure. The modulation frequency obtained by an acousto-optical modulator ranges from 1 Hz up to 100 kHz, allowing the improvement of the thermal resolution till to submicron range. We have applied PTR to synthetic opals as SiO2/GaN and SiO2/VO2. The Ar laser is used as pump beam partially absorbed in the whole structure. Assuming that the heat is generated proportionally to the light intensity flowing in the structure, one may visualize the internal light propagation by performing the heat depth profile reconstruction by inverting the photothermal radiometric data. PTR technique has been applied also to carbon nanotube film deposited onto a silicon substrate, In this case PTR allows to detect the thermal wave interference in the film and to measure the effective thermal diffusivity of the CNT, the thermal resistance with the substrate, and the infrared emission property of the CNT. Applications and limits of the different photothermal techniques are also deeply discussed for different specific nanostructures.