Design,Synthesis And Spectroscopy Of Manganese Doped Near-infrared Luminescent Materials

Accompanying with the speedy development of technology in the 21^st century,luminescent materials have been being widely applied into people's daily life.Near-infrared luminescent material is a kind of fluorescent material which can transfer ultraviolet light or visible light to near-infrared light.An influential application of fluorescent material is the frequent usage of near-infrared luminescent material in many fields,such as anti-counterfeit identification,laser communication,medical diagnosis,information storage and so on.Mn doped fluorescent materials exhibit many different valance states,and their emission wavelengths can be changed into the visible or near-infrared region due to different ionic valence states.Emission wavelengths of fluorescent materials with high valence Mn,like Mn⁴⁺,Mn⁵⁺or Mn⁶⁺can cover the whole field of near-infrared?650-1400 nm?region.These fluorescent materials with high valence Mn really are near-infrared luminescent materials with excellent luminescence properties,and these fluorescence materials do have significant application prospects in terms of tunable laser,spectral modulation and fluorescence imaging.This disertation systematically describes luminous kinds and luminescent mechanisms of Mn with different valence.Meanwhile,studies and researches about Mn doped near-infrared luminescent materials in the past decades are also reviewed.Moreover,we seriously discuss some current problems and challenges of Mn doped near-infrared luminescent materials.Thus,in order to improve the properties of Mn doped near-infrared luminescent materials,it makes great sense for this disertation to put an emphasis on designing new type of Mn doped near-infrared luminescent materials,as well as focusing on explore new methods and new ideas to synthesize Mn doped near-infrared luminescent nano-material.Detailed research results can be briefly concluded as follows:?1?So far,there are merely few hosts of Mn⁴⁺doped long persistent phosphorescence materials,thus by selecting and comparing aluminate hosts,we confirm that the perovskite type LaAlO₃ is the very host with strongest long persistent luminescence.Moreover,we systematically investigate the influences of doping Si,Ge,Sn into LaAlO₃:Mn⁴⁺phosphor on its long-lasting afterglow property.In addition,we also adjust preparation conditions to enhance the long persistent luminescence of LaAlO₃:Mn⁴⁺,and eventually we find that LaAlO₃:Mn⁴⁺phosphor synthesized under air atmosphere presents strongest long persistent luminescence when doping with 1 mol%Sn⁴⁺.Apart from this,we explore a new approach to improve the long persistence and fluorescence properties of GdAlO₃:Mn⁴⁺,a material with relative weak luminescence performance:we use Mg²⁺-Si⁴⁺to replace Al³⁺-Al³⁺.According to photoluminescence spectra and photoluminescence excitation spectra,we discuss the effect of structural replacement on luminescence property of GdAlO₃:Mn⁴⁺phosphor.Meanwhile,we also explore the influence of structural replacement on long persistence property of GdAlO₃:Mn⁴⁺phosphor by analyzing thermoluminescence curve and long afterglow decay curve.?2?We first put forward a new methodology to realize the long persistent luminescence of Mn⁴⁺:through reasonably selecting matrix compositions.We designed and successfully synthesized double perovskite type La₂MgGeO₆:Mn⁴⁺phosphor,a near-infrared luminescent material.Under 254 nm ultraviolet light excitation,a deep-red emission with central wavelength at 708 nm is observed,which is originated from the ²E_g?⁴A_2g transition of Mn⁴⁺.After stopping the irradiation of ultraviolet light,La₂MgGeO₆:Mn⁴⁺phosphor can still produce long persistent luminescence with same wavelength.Basing on analyzing thermoluminescence curve,long afterglow decay curve and electron paramagnetic resonance?EPR?spectrum,we can infer that the long persistent phosphorescence mainly comes from the intrinsic defects of host lattice.Through codoping Al³⁺ion,the long persistent luminescence of La₂MgGeO₆:Mn⁴⁺is improved by the formation of new defects.Moreover,we also conducted a simple imaging demonstration.After 1 hour of attenuation,the signal of long persistent luminescence of La₂MgGeO₆:Mn⁴⁺phosphor can still be caught by a sensitive near-infrared camera,which proves that this kind of material has potential application in fluorescent imaging.?3?We successfully synthesized a series of Mn⁵⁺doped M₂SiO₄?M=Ca,Sr,Ba?near-infrared fluorescent phosphors by using high temperature solid state method.We systematically investigate the influence of changing crystal structure and chemical compositions of M₂SiO₄:Mn⁵⁺?M=Ca,Sr,Ba?phosphors on luminescence property.When the radius of alkaline earth metal ion changes,the emission wavelength of Mn⁵⁺shifts accordingly.The energy level splitting and nephelauxetic effect are responsible for the changes of peak positions of emission spectra.The red shift of emission peak is originated from the distortion of O-Mn-O bond angle,which leads to energy level splitting of ¹E;while the blue shift of emission peak is mainly caused by nephelanxetic effect.We also discuss the effects of Ca's partial substitution on the luminescence property of Ba₂SiO₄:Mn⁵⁺phosphor.Replacing Ba²⁺with a small quantity of Ca²⁺can break the symmetry of local crystal field,which is able to increase the photoluminescence intensity of Mn⁵⁺.On the contrary,replacing Ba²⁺with a large quantity of Ca²⁺can shorten the ionic distance between Mn⁵⁺-Mn⁵⁺,resulting in nonradiative loss of energy transfer,which is the main reason for the decrease of fluorescence intensity and fluorescence lifetime.?4?In order to solve current problems of traditional high temperature solid state method in synthesizing special valence state ion doped nanomaterials,we propose a simple chemical method to dope Mn⁶⁺into BaSO₄ nanoparticles under room temperature.Due to efficient anion exchange in nanoscale,not only the valence state of Mn⁶⁺can be stabilized,but also the morphologies and diameters of the nanoparticles are controlled by using this novel method.This new developed method provides a broader view to design and synthesize nanomaterials doped with special valence ions.Basing on X-ray diffraction?XRD?pattern,Raman spectra and energy dispersion spectra?EDS?,we confirm that Mn⁶⁺is successfully doped into the crystal lattice of BaSO₄,which proves that the facile chemical approach does have its effectiveness and validity.Under the excitation of 808 nm laser diode,BaSO₄:Mn⁶⁺nanoparticles exhibits a broadband emission from 1000 nm to 1400 nm,which covers the whole second near infrared window.The result indicates that BaSO₄:Mn⁶⁺may become a new type of near infrared fluorescence phosphors in the application of fluorescent imaging.?5?In this chapter,we further extend the efficient anion exchange method to prepare Mn⁵⁺doped nanomaterials.Thus,Mn⁵⁺doped Ba₃?VO₄?₂ nanoparticles have been successfully obtained using the above method.Adding 0.1 wt%hydrogen peroxide can turn Mn⁶⁺into Mn⁵⁺,and adding KOH ice-cold solution can stabilize the valence state of Mn⁵⁺.We have proved the universality and feasibility of this general approach by employing XRD pattern,scanning electron microscopy,transmission electron microscope and EDS elemental analysis.When excited by 808 nm laser diode,Ba₃?VO₄?₂:Mn⁵⁺nanoparticles present a near infrared emission with wavelength at 1190 nm.In addition,we also carried out a simple biological imaging demonstration experiment to verify that Ba₃?VO₄?₂:Mn⁵⁺nanoparticles with its excitation band at first near infrared window and emission band at second near infrared window is an ideal material for the potential application in the bio-imaging.