Small Polarons in Two-Dimensional Pnictogens: A First-Principles Study

We
report the first-principles study of small polarons in the most
stable two-dimensional pnictogen allotropes: blue and black phosphorene
and arsenene. While both cations and anions of small hydrogen-passivated
clusters show charge localization and local lattice distortions, only
the hole polaron in the blue allotrope is stable in the infinite size
cluster limit. The adiabatic polaron relaxation energy is found to
be 0.1 eV for phosphorene and 0.15 eV for arsenene. The polaron is
localized on lone-pair orbitals with half of the extra charge distributed
among 13 atoms. In the blue phosphorene, these orbitals form the valence
band’s top with a relatively flat band dispersion. However,
in the black phosphorene, lone-pair orbitals hybridize with bonding
orbitals, which explains the difference in hole localization strength
between the two topologically equivalent allotropes. The polaron’s
adiabatic barriers for motion are small compared to the most strongly
coupled phonon frequency, implying the polaron barrierless motion.