Preparation,optical Properties And Crystal Field Analysis Of Near-infrared Persistent Luminescence Material Cr³⁺ Doped CaGdAlO₄ Phosphors

Near-infrared?NIR?Persistent Luminescence materials wavelength is in the range of biological tissue transmission window?650-950 nm?,which has the advantages of less absorption of biological tissue and greater penetration depth in living applications.Through the afterglow imaging,the fluorescence background interference of the excitation light on the biological tissue can be avoided,which is beneficial to improve the imaging quality.In this paper,Cr³⁺doped CaGdAlO₄(CaGdAlO₄:x%Cr³⁺)powders with different Cr³⁺doping concentrations were prepared by combustion method.The influences of Cr³⁺doping,different replacement positions and heat-treatment conditions on the microstructure,morphology,particle size and luminescent properties of the samples were studied by means of X-ray diffraction,scanning electron microscopy,excitation and emission spectra.Research indicates:different doping concentrations of Cr³⁺ions replaces Gd³⁺ions CaGdAlO₄:x%Cr³⁺samples?x=0.5,1.0,1.5,2.0,2.5?were prepared by combustion method.A single pure phase powder was prepared.Under 610 nm excitation,NIR broadband emission peak with a maximum value of 744 nm appears in the range of 650-850 nm and several narrowband peaks overlap with it.Near-infrared Persistent Luminescence material(CaGdAlO₄:1.5%Cr³⁺)is the optimum concentration.It was heat-treated at 800°C for 2hours under vacuum,air and nitrogen atmospheres.The results show that the photoluminescence and near-infrared Persistent Luminescence of the sample after treatment with nitrogen and air atmosphere are small,while the CaGdAlO₄:1.5%Cr³⁺phosphor after heat-treatment under vacuum atmosphere has significant photoluminescence and near-infrared Persistent Luminescence.Near-infrared Persistent Luminescence is over 300seconds.Different doping concentrations of Cr³⁺ions replaces Al³⁺ions CaGdAlO₄:x%Cr³⁺samples?x=0.1,0.5,1.0,1.5,2.0?were prepared by combustion method.Under 592 nm excitation,NIR broadband emission peak with a maximum value of 744 nm appears in the range of 650-850 nm and several narrowband peaks overlap with it.The NIR emission intensity exhibits an initialrise and a subsequent decrease with the increase of Cr³⁺doping concentration and the optimum doping concentration is¹.0%.After the heat-treatment at800 ^oC in vacuum,the average grain size increases from 417 to 843 nm and the luminescent intensity increases by 2 times.It is found that Cr³⁺replaces Al³⁺site in the weak crystal fieldenvironment in CaGdAlO₄ host.The origin of NIR emission peaksof the samples was identified by the calculation of crystal field parameters and spectral analysis.It is found that the crystal field strength is¹.54 and smaller than 2.3,i.e.Dq/B=1.54<2.3,indicating Cr³⁺ions around a weak crystal field environment,which is consistent with the experimental result.The 670 nm broadband emission can be attributed to the zero phonon line?⁴T₂?⁴A₂?and 744 and 756 nm broadband emissions correspond to the phonon sideband transition⁴T₂?⁴A₂.After the heat treatment,the afterglow time of the samples exceeds 300seconds.In particular,compared with the case of Cr³⁺ions around medium and strong crystal field environments?the maximum value of NIR emission peak at 697 nm?,the maximum value of NIR emission peak of Cr³⁺ions around the weak crystal field environment moves to 744nm,which is closer to the center of the first biological window,suggesting that Cr³⁺doped CaGdAlO₄ has potential application in bioimaging.