Electronic band gap of flame-formed carbon nanoparticles by scanning tunneling spectroscopy

The electronic band gap of flame-formed carbon nanoparticles (CNPs) is determined for the first time using scanning tunneling spectroscopy (STS). CNPs are sampled from a laminar, burner-stabilized stagnation ethylene-air flame through in-situ insertion of gold-coated mica substrates, on which CNPs are collected as thin films. Optical band gap measurements using UV-Vis absorption spectroscopy are also performed to compare with the STS measurements. The suitability of STS in measuring electronic band gaps is first demonstrated through measurements using reference materials: the electronic band gaps of naphthalene, pyrene, and coronene are found to be slightly larger than the respective optical band gaps, as expected, because of the over potential in the presence of a current. Flame-formed CNPs of three different particle size distributions were sampled and tested. The electronic band gap of CNPs shows a clear size-dependency that supports the earlier conclusion (Liu et al. Proc. Natl. Acad. Sci. U.S.A. 116 (2019) 12,692-12,697) that flame-formed CNPs exhibit apparent quantum confinement or quantum dot behaviors. Furthermore, the measured electronic band gaps matches the respective optical results within the experimental uncertainty of 0.1-0.2 eV. The over potential observed in PAH samples is not present in CNP samples, which may indicate stronger intermolecular interactions among the PAHs constituting the CNPs than those of crystalline PAHs.