Near-infrared Energy Transfer From Nd³⁺ To Yb³⁺ In NaYF₄

To convert ultraviolet (UV) light into near-infrared (NIR) light in phosphors is demanded for the development of solar cells. In recent years, much work has been conducted on searching rare earth luminescent materials, and improving the luminescence efficiency. Fluoride, aluminate or borate are always selected as the host materials. Fluoride has a low maximum phonon energy which can reduce energy loss caused by multi-phonon relaxation. Meanwhile, its thermal stability and chemical stability are very important for the practical application of designed materials. In this paper, NaYF4 is selected as the host materials, a series of NaYF4:Nd3+, Yb3+samples were prepared via the hydrothermal method. The excitation and emission spectra of NaYF4:Nd3+, Yb3+ samples and the luminescence decay curves of Nd3+ and Yb3+ were measured and analyzed. The energy transfer process from Nd3+ to Yb3+ was demonstrated. The concentration dependence of NIR emission and the decay lifetimes of Nd3+and Yb3+was also discussed.The main researches are as follows:1. The excitation and emission spectra of NaYF4:Nd3+, Yb3+ samples revealed an efficient energy transfer process from Nd3+ to Yb3+. This process results in the Yb3+ NIR fluorescent emission at 980 nm. The fluorescence intensities of Nd3+and Yb3+ varied with different Yb3+ doping concentrations under excitation at 354 nm. There is no emission band observed in the visible region under excitation at 520 nm, which could be explained by the multiphonon relaxation of the Nd3+ ions. Multiphonon relaxation will be also occurred in YBO3:Nd3+, Yb3+ samples upon excitation at 354 nm.2. Luminescence decay curves of Nd3+ 4F3/2 emission (866 nm) and Yb3+ 2F5/2 emission (986 nm) were recorded upon excitation at Nd3+ 4F5/2 level. It provides direct evidence of energy transfer process from Nd3+to Yb3+, Nd3+:4F3/2+Yb3+:2F7/2â†'Nd3+: 4I9/2+Yb3+:2F5/2 and energy transfer efficiency was also calculated.3. Luminescence decay curves of Nd3+ 4F3/2 emission (866 nm) and Yb3+ 2F5/2 emission (986 nm) were recorded upon excitation at Nd3+ 4D1/2 level (355 nm) and 4G5/2 level (574 nm). It shows the build-up time of the decay curves upon excitation at 355 nm is shorter than the build-up time upon excitation at 574 nm. It proved down conversion exists in NaYF4: Nd3+, Yb3+ samples upon excitation at 355 nm.4. When the concentration of Yb3+is 2% and 5%, the decay curves of the co-doped samples were fitted. The excitation wavelength was 355 nm, monitoring wavelength was 986 nm. The possible first step can be that the energy corresponding to Nd3+4D3/2â†'4G7/2 or 2G9/2â†'4F3/2 is transferred to the Yb3+2F5/2 level through a cross relaxation process, followed by the second step Nd3+:4F3/2+Yb3+:2F7/2â†'Nd3+: 4I9/2+Yb3+:2F5/2. In other words, down conversion exists in the NaYF4 co-doped samples.