  The phonon properties, electronic structures and optical properties of novel
carbon allotropes, such as monolayer penta-graphene (PG), double-layer PG and
T12-carbon, were explored by means of first-principles calculations. Results of
phonon calculations demonstrate that these exotic carbon allotropes are
dynamically stable. In addition, the bulk T12 phase is an indirect-gap
semiconductor having a bandgap of ~4.89 eV. Whereas the bulk material
transforms to a 2D phase, the monolayer and double-layer PG become quasi-direct
gap semiconductors with smaller band gaps of ~2.64 eV and ~3.27eV,
respectively. Furthermore, the partial charge density analysis indicates that
the 2D phases retain part of the electronic characteristics of the T12 phase.
The linear photon energy-dependent dielectric functions and related optical
properties including refractive index, extinction coefficient, absorption
spectrum, reflectivity, and energy loss spectrum were also computed and
discussed. The structural estimation obtained as well as other findings are in
agreement with existing theoretical data. The calculated results are beneficial
to the practical applications of these exotic carbon allotropes in
optoelectronics and electronics.
