Quantum confinement and size resolved modeling of electronic and optical properties of small soot particles

Abstract

Electronic and optical properties of flame-generated carbon nanoparticles were modeled based on quantum confinement and amorphous semiconductor theory of monodispersed particles. The ionization energies and optical band gaps of polydispersed particles 4–23 nm in volume median diameter (Liu et al., Proc. Natl. Acad. Sci. U.S.A. 116 (2019) 12692–12697) were re-analyzed by deconvolution to obtain particle-size specific properties. The complex refractive index was derived from optical absorption and the Kramers–Krönig relations over the wavelength range of 185–1400 nm. Dependence of the refractive index on the primary particle size is shown for the first time. The real component of the refractive index is weakly dependent on the particle size, but the imaginary component is found to be particularly sensitive to the particle size. The refractive indices of relatively large soot particles are found to be in close agreement with literature values in the visible spectrum; those of smaller particles (〈 ∼ 15 nm in diameter) show a significant difference from the literature values. The current result highlights the need for accounting for the size effect in the refractive index in laser diagnostics of soot in flames. It also suggests that some of the earlier extinction and scattering measurements of flame soot may have to be re-evaluated, especially for lightly sooting flames and during the early stage of soot growth.