The Electronic Structure of Amorphous Carbon Nanodots

We
have studied hydrogen-passivated amorphous carbon nanostructures
with semiempirical molecular orbital theory in order to provide an
understanding of the factors that affect their electronic properties.
Amorphous structures were first constructed using periodic calculations
in a melt/quench protocol. Pure periodic amorphous carbon structures
and their counterparts doped with nitrogen and/or oxygen feature large
electronic band gaps. Surprisingly, descriptors such as the elemental
composition and the number of sp3-atoms only influence
the electronic structure weakly. Instead, the exact topology of the
sp2-network in terms of effective conjugation defines the
band gap. Amorphous carbon nanodots of different structures and sizes
were cut out of the periodic structures. Our calculations predict
the occurrence of localized electronic surface states, which give
rise to interesting effects such as amphoteric reactivity and predicted
optical band gaps in the near-UV/visible range. Optical and electronic
gaps display a dependence on particle size similar to that of inorganic
colloidal quantum dots.