  Soot nucleation is one of the most complex and debated steps of the soot
formation process in combustion. In this work, we used scanning tunneling
microscopy (STM) and spectroscopy (STS) to probe morphological and electronic
properties of incipient soot particles formed right behind the flame front of a
lightly sooting laminar premixed flame of ethylene and air. Particles were
thermophoretically sampled on an atomically flat gold film on a mica substrate.
High-resolution STM images of incipient soot particles were obtained for the
first time showing the morphology of sub-5 nm incipient soot particles.
High-resolution single-particle spectroscopic properties were measured
confirming the semiconductor behavior of incipient soot particles with an
electronic band gap ranging from 1.5 to 2 eV, consistent with earlier optical
and spectroscopic observations.
