Luminescent And Temperature Characteristics Of Rare Earth Doped Upconversion Materials

In recent years,the up-conversion materials based on matrixes of vanadates,molybdates,and phosphates have been paid more and more attentions.The molybdate and tungstate with scheelite structure have been studied experimentally owing to the lower phonon energy,high thermal stability and chemical thermal stability.In this work,the scheelite structured molybdates and tungstates were selected as the substrates for the Ho³⁺,Er³⁺,and Tm³⁺ions doped up-conversion materials and their luminescence and heat sensing features have been studied as well.The NaY_0.98-x?MoO₄?₂:0.02Ho³⁺,xYb³⁺?x=0.1,0.2,0.25,0.3,0.35,0.37?and NaY_0.68-x?MoO₄?₂:0.02Ho³⁺,0.30 Yb³⁺,x Nd³⁺?x=0.27,0.29,0.32?series of phosphors were synthesized by solid-state reaction method.The X-ray diffraction patterns illustrated that the as-prepared samples were all in pure phase.The emission spectra excited by 980 nm wavelength indicate the optimum doping concentration of the sensitizer Yb³⁺ion is 30%.The addition of Nd³⁺can achieve light color control.When the Nd³⁺ion is 0.32,the ratio of green light to red light is 1.79,which means that the ratio of ⁵S₂/⁵F₄?⁵I₈ and ⁵F₅?⁵F₈radiation transition is increased.By changing the power of the excitation,it was found that the ground state population of LiY_0.648Yb_0.3Ho_0.02Nd_0.032?MoO₄?₂ powder was exhausted when the excitation power was 100mW.However,the fluorescence spot is slightly larger than the laser spot,which is possibly due to the energy migration caused by the Yb³⁺-Nd³⁺resonance.By varying the temperature,the fluorescence intensity ratios of the ⁵F₅ splitting peaks of the Ho³⁺ion in LiY_0.68Yb_0.3Ho_0.02?MoO₄?₂ and LiY_0.648Yb_0.3Ho_0.02Nd_0.032?MoO₄?₂phosphor were studied.The fitting line shows that population number in ⁵F₅ splitting levels follows the Boltzmann type distribution.The result of data calculation indicated that the splitting levels energy gap values were 366.71 cm^-11 and 268.43 cm^-1,respectively.In the measurement range,LiY_0.68Yb_0.3Ho_0.02?MoO₄?₂ had the maximum absolute sensitivity and maximum relative sensitivity at 298 K,which were 31.10×10^-44 K^-11 and 0.437%K^-1,respectively.LiY_0.648Yb_0.3Ho_0.02Nd_0.032?MoO₄?₂ at 302 K have the maximum absolute sensitivity and the maximum relative sensitivity,which were 15.5×10^-44 K^-11 and 0.215%K^-1,respectively.LiY?MoO₄?₂:Yb³⁺/Er³⁺phosphors were successfully prepared by solid-state method at medium temperature.The phosphor emits green light?528⁵50 nm?and relatively weak red light?648⁶75 nm?under the excitation of 980 nm laser,and it is derived from the²H_11/2/⁴S_3/2?⁴I_15/25/2 and ⁴F_9/2?⁴I_15/25/2 energy level radiation transitions,respectively.By changing the pump power,it is concluded that the up-conversion luminescence is a two-photon process.When the pump power increases from 10 mW to 100 mW,the tuning of the emitted light from near white light to green light is detected.The fluorescence intensity ratio I₅₄₂/I₅₅₂52 shows a continued rise in the temperature range from 298 to 513 K,and the population of the ²H_11/21/2 energy level and the ⁴S_3/2/2 energy level follow the Boltzmann distribution.The calculated gap between the²H_11/2?1?level and the⁴S_3/2?1?level is 756 cm^-1,which is very close to the theoretical value.The sensitivity can reach a maximum of263.2×10^-4K^-1at 473 K.LiLa?WO₄?₂ phosphors were successfully prepared by high-temperature solid-state method and the XRD spectra showed the prepared sample is pure phase.Under excitation of980 nm,LiLa?WO₄?₂:Yb³⁺,Er³⁺,Tm³⁺phosphors showed the clear emission peak of 510⁵70nm Er³⁺and the emission band of 640⁶80 nm.In addition,phosphor in room temperature with 280 mW high power pumping?or sample is heated to 545 K with 200 mW power pumping?,showed the emission band of Er³⁺ions at 680⁷20 nm,which is derived from⁴F_7/2?⁴I_13/23/2 energy level radiation transitions of Er³⁺ions.The two-photon and three-photon processes of green emission of Er³⁺ions in LiLa?WO₄?₂:Yb³⁺,Er³⁺,and Tm³⁺were studied.This was due to the cross-relaxation between Tm³⁺ions and Er³⁺ions.Furthermore,Tm³⁺ions have a significant enhancement effect on the radiative transitions of the²H_11/2?⁴I_15/2levels of Er³⁺.The fluorescence intensity ratio I_510⁵39/I_539⁵7039⁵70 continued to rise with increasing temperature.Ln(I_510⁵39/I_539⁵70)fit well with 1/T into a straight line,indicating that the population number of ⁴H_11/2,⁴S_3/2/2 energy levels follows the Boltzmann distribution.The energy gap calculated from the fitted straight line is 557.6 cm^-1,and the maximum absolute sensitivity is 39.1×10^-44 K^-11 at 443 K,and the maximum relative sensitivity is0.879%K^-1at 302 K.