Luminescent Property Of NaGdF4:Er³⁺ Nanoparticles For Temperature Sensing

Rare earth ions（RE3+）possess superior physicochemical features such as narrow emission bands,long-lived luminescence and wide spectral band,because of its unique electronic configurations,which is in the dominant position in the field of luminescent materials.Temperature is probably the most fundamental parameter in industrial production,scientific research and daily life.With the rapid development of nanotechnology and biotechnology,a higher requirement to the thermometer are put forward.Recently,there has been growing interest in the use of RE3+ doped materials as optical temperature sensors.Compared with the traditional temperature sensor,optical thennometer has the advantages of non-contact,high sensitivity and high accuracy.The optical temperature sensors based on the fluorescence intensity ratio（FIR）is considered to be the most promising.FIR technology takes advantage of the temperature dependent emission intensities of two thermally coupled emitting levels of the RE3+ions.It can reduce the dependence of measurement condition,without affected by fluorescence loss and fluctuations in the excitation intensity.In this paper the choice has been an Er³⁺-doped NaGdF₄ Nanoparticles.The main contents include as follows.（1）The NaGdF₄:Er³⁺ nanoparticles were synthesized by coprecipitation and solvothermal method.The obtained nanoparticles were characterized by X-ray diffraction（XRD）and transmission electron microscopy（TEM）.The phase of all nanoparticles is hexagonal,NaGdF₄:Er³⁺.NPs prepared by coprecipitation methord has a good dispersion and uniform particle size.The average size is about 15nm.The water-soluble NaGdF₄:Er³⁺ NPs synthesized by solvothermal method have large particle size.When add PEG ligand,the aggregation is solved obviously.Characterize the NaGdF₄:Er³⁺ NPs luminescent properties by photoluminescence spectrum,the results showed the characteristic emission peaks of Er³⁺ ions.（2）To study the effect of Er³⁺ concentration on luminescent property and temperature sensing.The fluorescence intensity of NaGdF₄:5%Er³⁺ is the best.With the concentration further increasing,the concentration quenching effect is observed.While the temperature sensitivity of NaGdF₄:3%Er³⁺ is the best.Results show that increasing the Er³⁺concentration produces a reduction in the FIR of the thermalized levels and,therefore,in the sensitivity of the sensor.The radiative transfer process due to re-absorption of Er³⁺ions as a consequence of the large overlapping between the emission and the absorption green bands can explain this decrease.（3）The effect of core/shell structure NPs on luminescent property and temperature sensing was investigated.In order to reduce the surface defects of nano materials,different thickness of NaGdF₄ shell was coated,The fluorescence intensity of NaGdF₄:3%Er³⁺@ NaGdF₄ is stronger than the core nanoparticles,and the temperature sensing is consistent with the fluorescence spectrum.When the core-shell ratio is 1:4,the nanoparticles size is about 35nm.Better than NPs with core-shell ratio of 1:2.