On the luminescence of Ti4+and Eu3+ in monoclinic ZrO2: High performance of optical thermometry derived from energy transfer
Monoclinic ZrO2 (m-ZrO2) is an attractive material for photonic applications due to the low site symmetry and phonon energy. In this paper, we presented the detailed studies on the luminescence properties of Ti4+ and Eu3+ in m-ZrO2. In view of the remaining controversy on the origin of the “intrinsic” white-blue luminescence in pristine m-ZrO2, more spectroscopic information by characterizing the m-ZrO2 intentionally doped with Ti4+, Ti3+ and other metal ions as well as extensive discussions on the general characteristics of different luminescent processes were provided to evidence the luminescence as Ti3+→O- charge transfer transition in [Ti(IV)O7]10- complex due to the trace amount of Ti4+ impurity contamination in ZrO2 reagent. The competitive absorption was observed between host exciton and oxygen-metal charge transfer process (O2--Ti4+, O2--Eu3+ ), even between the two types of charge transfer process because of the sharing of electrons at the top of the valance band, which greatly influenced the absorption and excitation processes. The spectral components comprising the broad excitation band of O2--Ti4+ white-blue or Eu3+ red emissions in different phosphors were identified, and their evolutions with doping concentration were explained. The energy state locations of oxygen defects produced by intrinsic charge compensation in m-ZrO2:Eu3+ were proposed to be ~2.41 eV above the top of valance band by using Eu3+ as luminescence probe. Efficient energy transfer from [Ti(IV)O7]10- complex to Eu3+ was first observed in m-ZrO2:Ti4+,Eu3+ phosphors, which were demonstrated to be high performance of ratiometric self-referencing optical thermometric materials based on the dual-emitting combination strategy, with relative sensitivity amounting to ~3.84%K-1. Optical thermometry covering the physiological temperature range but also with high relative sensitivity could be achieved upon appropriate doping.