To optical properties of carbon nanoparticles: A need in comprehending Urbach energy

In this work, light absorption spectra of carbon nanoparticles sampled at different heights from an ethy- lene/air premixed flame were studied. The spectral behavior of the light absorption coefficient was ana- lyzed in the high energy (Tauc) and low energy (Urbach) regions. In the Tauc region, information on the optical band gap can be retrieved, while in the Urbach region the value of the Urbach energy can be esti- mated. The variation of the spectral behavior of the light absorption coefficient in the low energy region was ascribed to different Urbach energies of the material depending on the sampling height. The Urbach energy is related to the disorder of the particles under analysis. The conclusion was made that due to the strong tail absorption, the traditional optical band gap concept is not an accurate approach in describing properties of studied nanocarbons, which can be better emphasized with the Urbach parameter. More- over, the high value of the Urbach energy obtained is hypothesized to occur due to the fast nanoparticle formation in a flame that leads to a strong structural disorder. A thorough study of the Urbach energy and its relation to the nanoparticle growth looks an essential step in comprehending optical properties of carbonaceous particulate generated in flames.