Synthesis And Luminescence Properties Of Novel Ultra-Broadband Near-infrared Phosphor Materials

With the rapid development of compact spectrometers,it is highly expected that mobile phones and other portable devices will be capable to perform food analysis and health monitoring in daily life by applying near-infrared spectroscopy.The implementation of this technology requires a broadband near-infrared phosphorconverted LED(NIR pc-LED)light source.However,current near-infrared phosphors generally suffer from narrow full width at half maximum(FWHM),low quantum efficiency(QE),and mismatch with blue LED chips,thus it is necessary to develop phosphors that can be well pumped by blue LED light and cover the emission range in 700～1400 nm.Inspired by the broadband NIR emission and adjustability of Cr3+activator,we mainly focus on three oxides which can provide crystallographic sites with relatively weak crystal field environments,and their phase structure,luminescence properties,spectral tunability and synthesis optimization are comprehensively investigated,the results are summarized as below:(1)A series of La2MgZrO6:Cr phosphors,which exhibited an ultra-broadband NIR emission centered at 825 nm with a high FWHM of 210 nm and 58%IQE,were successfully synthesized by high temperature solid-state reaction in reducing atmosphere.The host structure belongs to the double-perovskite oxide with two octahedral moieties suitable for Cr3+.By comprehensive analysis of structural refinement,DFT calculation of doping energy and luminescence properties,etc.,the ultra-broadband emission was proved to be originated from two distinct octahedral sites.The critical Cr3+ concentration was 3%,and the low thermal quenching was revealed by that the emission intensity remained only 53%in 400 K compared to that at room temperature.The broadband NIR emission can be varied by cation substitution,whereas the emission intensity declined because of structural defects or quenching centers are introduced.The phosphors showed a good stability.Finally,two multifunctional NIR pc-LED devices were demonstrated as novel light sources for NIR spectroscopy applications.Taking a view of the crystal structure with similar size of octahedral moieties,oxides like La2MgZrO6 may be an alternative choice and provide a new perspective for exploring broadband NIR phosphors.(2)A series of La2Hf2O7:Cr,Ga phosphors were successfully synthesized by high temperature solid-state reaction.The excitation maximum of this phosphor was well matched with the 450 nm LED chip.Under such an excitation a broadband NIR emission centered at 1230 nm with a high FWHM of 180 nm and a decay time of 1 ms was obtained,which can serve as an extension for a wider spectral coverage.By the analysis of crystal structure and phase purity,the luminescent La2Hf2O7:Cr,Ga was identified to be crystalized in a cubic pyrochlore structure.The experimental results showed that the NIR emission was only observed in Cr,Ga co-doping samples.Additionally,the emission intensity can be largely enhanced by optimizing the Ga content to 5%and sintering atmosphere.However,the emission of 1230 nm is difficult to explain in primary investigation,we assumed that because of the unique oxygen defects in pyrochlore hosts,the co-doping of Ga3+ may induce a structural distortion or provide a distinct site for Cr3+ to generate a lower exited state.(3)A series of La3Ga5HfO14:Cr phosphors were successfully synthesized by high temperature solid-state reaction.Under blue light excitation,a broadband NIR emission of Cr3+(4T2 level)centered at 950 nm is typically attributed to a weak crystal filed environment in A3BC3D2O14 structure.The 950 nm broadband emission was dominant when Cr doping concentration was low,while another emission band centered at 1230 nm belonging to an impurity(La2Hf2O7:Cr,Ga)was seen.The spectral range 700～1400 nm can be fully covered by tuning the Cr doping concentration to alter the relative content of two-phase mixtures.By cation substitution of Si→Hf to form a solid-solution,the ultra-broadband and evenly distributed NIR emission spectra can be further tuned to achieve an ideal spectral band shape.