PROBING SOOT STRUCTURE AND ELECTRONIC PROPERTIES BY OPTICAL AND ELECTRICAL MEASUREMENTS

In this work, the physicochemical transformation of soot particles at the initial stages of formation and growth was investigated in a laminar premixed flame of ethylene and air. The selected flame condition allowed producing two classes of carbon nanoparticles that can be defined as incipient soot and primary soot particles, on the basis of their size distribution. The two classes of particles have been selectively collected in well distinct flame zones for further chemico-physical characterization. The optical band gap decreases as particle size grow, and the same behavior is also observed for the electronic band gap measured by scanning tunneling spectroscopy. Particle structure at molecular level and medium range has been investigated by Fluorescence spectroscopy and Raman spectroscopy. Slight changes in particle composition were observed for incipient and primary soot. Both kinds of particles are made of a similar ensemble of fluorescence centers, which produce excitation dependent emission irrespective of particle sizes. In agreement with the Raman spectra, the molecular constituents of the particles are polyaromatic compounds with an average size of the order of ovalene molecules that only slightly increase from incipient to primary soot particles. The band gap closure, observed as particle size increases following the evolution from incipient soot to primary soot, does not seem to be correlated to changes in particle composition at molecular level. Nevertheless, we cannot exclude that the band gap is also affected by the supramolecular organization of the particles and that a structural rearrangement of the molecules constituting the particles occurs as they grow.