Strong structural phase sensitive rare-earth photoluminescence color flips in KLaF4:RE3+ (RE3+ = Eu3+, Er3+/Yb3+) nanocrystals†
Systematic and strong rare-earth photoluminescence (PL) color flips that are highly sensitive to structural phase transformation in KLaF4:RE3+ (RE3+ = Eu3+, Er3+/Yb3+) nanocrystals are demonstrated. Room-temperature wet chemical synthesis at various reaction times involves a systematic conversion from cubic (α, space group Fmm) to hexagonal (β, space group P2m) polymorph nanocrystals of 4 to 8 nm sizes. The unusual down-conversion photoluminescence (DC-PL) asymmetric ratio of the hypersensitive transition (5D0 → 7F2, Red) to that of nearly invariant transition (5D0 → 7F1, Orange) (R/O from 1.2 to 3.0) of KLaF4:Eu3+ is substantially affected by the site-selective occupancy and local symmetry around the Eu3+ ions, according to crystal packing transformation. The NIR excited up-conversion photoluminescence (UC-PL) of Er3+ ions produces a strong color flip from green (2H11/2 and 4S3/2) to red (4F9/2) dominated emissions based on their cubic or hexagonal crystal packing. The site occupancy and phonon energies strongly influence various nonlinear energy transfer mechanisms within RE3+ 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.