Crystal-field Stark effect on the upconversion light emission spectrum of α-NaYF4 nanoparticles doped with Dy3+, Er3+, or Yb3+

Abstract

NaYF4 nanoparticles (NPs) form in two crystal structures, cubic (α-phase) and hexagonal (β-phase), each one presenting a different crystal electric field (CEF) Stark effect, that affects the upconversion (UC) light emission of the NPs when doped with rare-earth elements. Therefore, the knowledge of the CEF parameters, the wave functions, and energy levels of the rare earth (RE) J-multiplet is expected to be of great help for the understanding and improvement of the UC light emission. In this work, α-phase NaYF4 NPs doped with Dy3+, Er3+, or Yb3+ were investigated by means of magnetization, electron spin resonance (ESR), and optical spectroscopy techniques. Fittings of the temperature- and magnetic-field-dependent magnetization were performed to determine the fourth- and sixth-order cubic CEF parameters, B4 and B6. The ground state of Er3+, Yb3+, and Dy3+ in these α-NaYF4 NPs was confirmed by low-temperature ESR experiments. The obtained CEF parameters were used to write down a total Hamiltonian that allows to determine the CEF Stark splitting for all energy levels of the REs 4f unfilled shell. We give details of how the Stark effect affects the overall energy splitting of the various J-multiplets and may explain the fine structure of the UC light emission in these cubic NaY1-δREδF4 NPs.