“…3,6,8,9 The decoupled
[PbX 6 ] 4– octahedra in Cs 4 PbX 6 , compared to the halide-coupled [PbX 6 ] 4– octahedra in LHP, cause the bandgap of Cs 4 PbX 6 to shift toward values of free [PbX 6 ] 4– clusters in solutions and essentially lead to single molecule-like,
excitonic absorption bands. 13,16,29 The fact that Cs 4 PbX 6 phases have bandgap
values close to their respective free [PbX 6 ] 4– clusters is also evident from the optical properties of a series
of alkali metal halides (AX, A = Na, K, Rb, Cs; X = Cl, Br, I) doped
with Pb 2+ . 47 For these materials,
the spectral position of the excitonic absorption is almost independent
of the alkali metal (NaBr:Pb 2+ = 304 nm, CsBr:Pb 2+ , = 313 nm Cs 4 PbBr 6 = 314 nm), 47,48 while it is strongly dependent on the halide ion.…”