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 which are highly sensitive to the structural phase transformation in KLaF4:RE3+ (RE3+ = Eu3+ , Er3+ /Yb3+ ) nanocrystals are demonstrated. A room-temperature wet chemical synthesis at various reaction times produce systematic conversion from cubic (α, space group Fm-3m) to hexagonal (β, space group P6 @#x0305;2m) polymorph nanocrystals of 4 to 8 nm sizes. The unusual down-conversion photoluminescence (DC-PL) asymmetric ratio between 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 Eu3+ ions, according to the crystal packing transformation. The NIR excited up-conversion photoluminescence (UC-PL) of Er3+ ions produce 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 the RE3+ ion energies and the results are explained accordingly. The presented study substantially explains the local host effects and these two distinct polymorph nanoparticles can be potentially utilized to color-specific studies related to applications such as color-specific biological in-vitro and in-vivo imaging and other optoelectronic device applications.