Strong structural phase sensitive rare-earth photoluminescence color flips in KLaF4:RE3+ (RE3+ = Eu3+, Er3+/Yb3+) nanocrystals

Abstract

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 (²H_11/2 and ⁴S_3/2) to red (⁴F_9/2) 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.