Power density dependent upconversion properties of NaYbF4: Er3+@NaYbF4: Tm3+@NaYF4 nanoparticles and their application in white-light emission LED

Abstract

White light sources play a vital role in our daily lives, which were usually generated by optimizing the proportions of individual emitters with different colors. Benefiting from the abundant energy levels of rare earth ions (Re3+), Re3+ doped upconversion nanoparticles (UCNPs) with multicolor luminescence output in one fixed structure have attracted broad attention. Here, we constructed a NaYbF4: Er3+@NaYbF4: Tm3+@NaYF4 core-shell-shell structure with Er3+ and Tm3+ in different layers to avoid the cross-relaxation. In the structure, red and green emissions were originated from Er3+, and blue emission was generated by Tm3+ through the upconversion process. The simultaneous red, green and blue emissions in the designed structure made it possible to realize white light emission. More importantly, the NaYbF4 host leads to three photons upconversion pathway and result in tunable color output under moderate varying power density. The results showed that warm white color was achieved at the excitation power of 20.9 W/cm2, meanwhile, tunable output colors were also realized by changing the external excited power density. Finally, the high luminescence intensity of NaYbF4: Er3+@NaYbF4: Tm3+@NaYF4 makes it possible to be excited by a near infrared-light emitting diode (NIR-LED), and a white emission LED was prepared by encapsulating the prepared UCNPs on a NIR chip.