Physical models for carbonaceous cosmic nanoparticles are constrained by the observational (see Motivation) as well as by the cosmic abundance constraints. They rely on the optical properties measured for specific carbonaceous materials available in the laboratory.
There are two approaches to characterize the extinction properties of these analogue particles:
Nano-sized carbon grains (soot) as analogue material can be produced by different techniques. For all these techniques, the generation of a supersaturated vapour is the key feature in the production process of nanoscale particles. This vapour is either generated by evaporation of bulk material (e.g. laser ablation, resistive heating, arc sputtering) or the decomposition of a precursor gas (e.g. laser or flame pyrolysis, radio frequency and microwave plasma decomposition). The generated vapour is quenched in an inert gas to achieve supersaturation and to force condensation.
Carbon nano-grains usually possess a disordered structure which is characterized by the nearest neighbour bonding types (i.e. by the sp2/sp3-ratio) and by the range, type, and order of present microcrystallinity. These structural properties somehow define the electronic density of states (DOS) and, therefore, the optical behaviour of the grains. As already mentioned there is an additional shape and size effect on the optical properties of nano-particles which can be explained classically on the basis of the scattering theory and which sometimes strongly masks the structural effects.
In the standard model of disordered carbonaceous material, the electronic band structure near the band edges is determined by the size distribution of stacked plane polyaromatic (sp2 ) sheets, often called basic structural units (BSU). This is due to the fact that pi states are more weakly bound and, therefore, lie closer to the Fermi energy surface EF than the sigma states. Furthermore, it has been shown theoretically that for the pi electrons of the sp2 sites, it is energetically more favourable to be organized in compact clusters of fused sixfold rings than to be dispersed homogeneously in a surrounding sp3 matrix. These BSUs with sizes between 6 and 40 A determine the transition between the localized and delocalized pi states and, therefore, the size of the energy gap Eg between the filled valence and the empty conduction bands. The localized pi states of the smaller BSUs are found energetically in the band gap, where they define, due to their size distribution, the slope of the band edges (tail states).
In addition to the BSU structure, it is known from recent investigations, including my own work, that bent polyaromatic layers play an important role for the structure of carbon nano-particles. This structure is related to the classical paracrystalline model and includes the totally closed and ordered modifications, well-known as bucky-onions and bucky-tubes.
From the discussion of the disordered carbon structures, it becomes clear that the principal requirements for a structural study are to find the sp2/sp3 ratio and the hydrogen content on one hand as well as the degree, size, and type of medium range order on the other. Therefore, I use high-resolution transmission electron microscopy (HRTEM) as an appropriate analytical method to characterize the structural order of the nano-grains. Electron energy loss (EEL) and optical spectroscopy from the ultraviolet to the infrared I apply to characterize the sp2/sp3 ratio, the hydrogen content and the band structure.