The origin of the
self-activated luminescence in the apatite-type
M5(PO4)3X (MPOX; M = Sr or Ba; X
= Cl or Br) samples and the spectral assignment for cerium-doped Sr5(PO4)3Cl (SPOC) phosphors are determined
from first-principles methods combined with hybrid density functional
theory (DFT) calculations, using the standard PBE0 hybrid functional,
with wave function-based embedded-cluster ab initio calculations (at the CASSCF/CASPT2/RASSI–SO level). Electronic
structure calculations are performed in order to accurately derive
the band gaps of the hosts, the locations of impurity states in the
energy bands that are caused by native defects and doped Ce3+ ions, and the charge-compensation mechanisms of aliovalent doping.
The calculations of defect formation energies under O-poor conditions
demonstrate that the native defects are easily generated in the undoped
MPOX samples prepared under reducing atmospheres, from which thermodynamic
and optical transition energy levels, as well as the corresponding
energies, are derived in order to interpret the luminescence mechanisms
of the undoped MPOX as previously reported. Our calculations reveal
that the self-activated luminescence is mainly attributed to the optical
transitions of the excitons bound to the oxygen vacancies (VO), along with their transformation of the charge states 0 ↔
1+. Furthermore, the eight excitation bands observed in the synchrotron
radiation excitation spectra of SPOC: Ce3+, Na+ phosphors are successfully assigned according to the ab
initio calculated energies and relative oscillator strengths
of the 4f1 → 5d1–5 transitions
for the Ce3+ ions at both the Sr(1) and Sr(2) sites in
the host. It is hoped that the feasible first-principles approaches
in this work are applied in order to explore the origins of the luminescence
in undoped and lanthanide-doped phosphors, complementing the experiments
from the perspective of chemical compositions and the microstructures
of materials.
Atmosphere pressure chemical vapor deposition (CVD) is one of the most powerful methods of synthesizing high quality and large area MoS2 films with a reasonable cost. In our work, the large-scale and high crystalline quality monolayer MoS2 nanosheets were synthesized on Silicon substrate with a 300 nm oxide layer using MoO3 and S powders as precursors by an atmosphere pressure CVD. The results suggest that the surface morphology, crystalline quality and luminescence of CVD-grown MoS2 nanosheets can be tunable by controlling the precursor ratio (the effective Mo: S ratio). Excessive S-rich atmosphere is favor to synthesize large-size and high crystalline quality monolayer MoS2 nanosheets with sharp corners and straight edges. This study may provide insight into the synthesis of large-scale and high crystalline quality MoS2 films.
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