Strong structural phase sensitive rare-earth photoluminescence color flips in KLaF:RE (RE = Eu, Er/Yb) nanocrystals.

Systematic and strong rare-earth photoluminescence (PL) color flips that are highly sensitive to structural phase transformation in KLaF:RE (RE = Eu, Er/Yb) nanocrystals are demonstrated. Room-temperature wet chemical synthesis at various reaction times involves a systematic conversion from cubic (α, space group Fm3[combining macron]m) to hexagonal (β, space group P6[combining macron]2m) polymorph nanocrystals of 4 to 8 nm sizes. The unusual down-conversion photoluminescence (DC-PL) asymmetric ratio of the hypersensitive transition (D→F, Red) to that of nearly invariant transition (D→F, Orange) (R/O from 1.2 to 3.0) of KLaF:Eu is substantially affected by the site-selective occupancy and local symmetry around the Eu ions, according to crystal packing transformation. The NIR excited up-conversion photoluminescence (UC-PL) of Er ions produces a strong color flip from green (H and S) to red (F) dominated emissions based on their cubic or hexagonal crystal packing. The site occupancy and phonon energies strongly influence various nonlinear energy transfer mechanisms within RE ion energies and the results are explained accordingly. The present study substantially reveals the local host effects and these two distinct polymorph nanoparticles can be potentially utilized for color-specific studies related to applications such as color-specific biological in vitro and in vivo imaging and other optoelectronic device applications.