Prediction of Ln(3+)-4f energy levels in beta-NaYF4:Ln(3+) and understanding of absorption behaviors

Abstract

In lanthanide ion (Ln)-doped luminescent systems, electron transfer between 4f-4f states in Ln(3+) ions is critical for luminescing specific colors. Therefore, understanding the 4f orbital energy levels of Ln(3+) ions is a prerequisite for elucidating the specific light absorption - emission process in luminescent particles and for designing superior luminescent materials. In this study, we first predict the ground energy states of Ln(3+) - 4f (E(Ln(3+) - 4f)) in hexagonal (beta)-NaYF4:Ln(3+) model system relative to the valence band maximum of the host material using first principles density functional theory calculations. We obtain E(Ln(3+) 4f) values comparable to the empirical values and find that they are dependent on both the dopant and host material, which directly affect the optical absorption behaviors. Based on the absorption behaviors of the singly Ln(3+)-doped system, we discuss the mechanism by which lanthanide-codoped systems (e.g., beta-NaYF4:Ln((1))(3+),Ln((2))(3+)) react to incident near infrared light with respect to the dopant distribution. This study can serve as a guideline for designing efficient luminescent systems that absorb light in the desired energy range and finely tune the luminescence color of the systems.