Thin
layers of inorganic halide perovskites A
n+1M
n
X3n+1 (n = 1–6, A= Cs, M = Pb and Sn, and X =
Cl, Br, and I) have been studied in orthorhombic and cubic phases
along with layers of monoclinic CsSnCl3. It is found that
one-unit-cell-thick layers have low stability except the monoclinic
CsSnCl3 for which formation energy is slighly
less than the bulk value. However, Cs2PbI4 is
unstable in both cubic and orthorhombic phases. The formation energy
for n > 3 becomes comparable to bulk, but the
inclusion
of spin–orbit coupling is found to be important for the stabilization
particularly for layers with Pb. Importantly, layers of environmentally
friendly Sn-based systems have similar values of formation energy
in the orthorhombic and cubic phases as well as similar band gaps
which make them good materials for solar cell applications as temperature
range changes during their operation. The studied 66 cubic and orthorhombic
nanosystems have direct band gap (0.6–2.9 eV) using generalized
gradient approximation for the exchange-correlation functional, but
the use of the HSE06 method increases the band gap. The reduced dimensionality
leads to elongation (contraction) of MX6 octahedra perpendicular
(parallel) to the plane of the layers and an increase in the band
gap. The presence of surface makes the hybridization between M s and
X p orbitals near the valence band maximum stronger than in bulk which
is good for light absorption. The effective mass of the electrons
and holes is very light which augers well for the transport properties.
Lead-based systems have larger band gap, and these can be useful in
applications such as light-emitting diodes.
We
study the stability of the rare-earth dopant, Eu, in metal-halide
perovskite CsPbBr3 with cubic and orthorhombic structures
from first-principles calculations. In these perovskites, Eu is substitutionally
doped on Pb sites because of their comparable ionic sizes which lead
to only a small strain in the doped system. Accordingly, our results
show that the cost of Eu doping is quite small compared to the values
in other common hosts such as GaN. This makes these perovskites excellent
candidates to develop rare-earth doped semiconducting materials for
bright luminescence as it has also been observed recently in the case
of thin films of CH3NH3Pb1–x
Eu
x
I3, Eu-doped
quantum dots of CsPbBr3, and nanocrystals of CsPbCl3 in addition to their outstanding properties for applications
in solar cells.
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