Investigation Of Near Infrared Super Long Persistent Phosphors

This dissertation includes four chapters. Chapter 1 presents the background, history and current situation of the subject-the near infrared (NIR) long-lasting phosphors (LLP). Chapter 2 to chapter 4 describe the preparation of several kinds of new NIR LLP, investigate their crystal structure and luminescence properties, and show the NIR images of the phosphors taken by a night vision goggle coupled with a digital camera.In chapter 1, the basic principles of LLP are introduced at first. Then the past and current developments as well as the various mechanism models of LLP are presented. The formation and classification of traps are discussed. At last, the measuring methods and the new requirement of LLP are described.A series of NIR LLP, La3Ga5GeOi4:Cr3+,Zn2+ with or without other codopants, are introduced in chapter 2. There are three different kinds of sites occupied by Cr+ in this sample, which make the emission of Cr3+ complicated. Ions with different valences are adopted to study their influence on the long persistent properties. Yb3+, Nd3+ and Er3+ are also introduced into La3GasGeO14:Cr3+,Zn2+. By the persistent energy transfer from Cr3+ to Yb3+, Nd3+ and Er3+, new NIR LLP with different emissions are achieved. A mechanism model for the long persistent emission is presented based on the thermoluminescence curve. The NIR long afterglow images are taken, which prove an afterglow time more than 24 hours. The NIR phosphorescence can be achieved effectively under ultraviolet irradiation but is achieved barely by X-ray and blue light irradiations. NIR phosphorescence mechanisms corresponding to ultraviolet, X-ray and blue light irradiations were proposed by measuring the activation energy dependence of the persistence time. A new concept I10s/I100s is introduced to evaluate the phosphorescence time of a LLP.In Chapter 3, the synthesis and luminescent properties of LiGa5O8Cr3+ are discussed. This chapter consist of five parts:1. Cr3+ ions occupy a kind of octahedral sites in LiGa5O8, the crystal field strength around are calculated based on absorption spectrum by the way proposed by Tanabe-Sugano. The lifetime of excited states of Cr3+ ions decrease with the increasing of Cr3+ concentration.2. The NIR images suggest an afterglow time more than 138 hours, which is the longest up to now. Divalent and tetravalent ions are introduced into LiGa5O8:Cr3+ to create traps with positive or negative charge, Ca2+ and Sr2+ are proved to be suitable to enhance the afterglow time, however, Ti4+ and Ti4+/Mg2+ harms the long persistent properties. The influences of other divalent ions or trivalent ions on the thermoluminescent curves are analyzed.3. By the method of persistent energy transfer (PET), the energy are transferred to Ni2+, Yb3+, Nd3+ and Er3+ from Cr3+, and new emission locating from 985 nm to 1530 nm with long persistent properties are observed. However, the efficiency of the PET is not as high as expected.4. The influences on the particle size of heating temperature and chelant are investigated. Among the chelants used in our experiments, citric acid is found worse than glycerol and ethanol in achieving small size of nanophosphor.5. NaGa5O8:Cr3+ and KGasO8:Cr3+ are synthesized, the relative intensity of 4T2→4A2 and 2E-4A2 transition is higher than that in LiGa5O8:Cr3+ An reasonable explanation for this phenomenon is given, Na+ and K+ have larger radius than Li+, resulting in a weaker crystal field, so the enhancement of the emission of 4T2-4A2. The afterglow properties of NaGa5O8:Cr3+ and KGa5O8:Cr3+ are not as good as that of LiGa5O8:Cr3+.Three other kinds of NIR LLP are introduced in chapter 4. They are LiGaO2:Cr3+, LiGaxGeyO8:Cr3+ and LaAlO3:Mn4+/Cr3+。1. Cr3+ substitutes Ga3+ in tetrahedral site in LiGaO2:Cr3+, and is oxidized to Cr4+, producing a emission around 1222 nm by 3T2→3A2 transition, which has no afterglow.717nm emission results from 2E→44A2 transition of Cr+ in LiGa5O8 microlite impurity, whose afterglow time is very long.2. The emission of 2E→4A2 broadens with the increasing of GeO2 component in glassy LiGaGevyO8:Cr3+, which is caused by the different crystal circumstances and phonons. Intensity of 4T2→4A2 transition is enhanced by adopting appropriate amount of GeO2. The afterglow intensities and time varies with the value of x/y.3. The emission of LaAlO3:Mn4+ consists of two main parts, which are assigned to the 2E→4A2 transition and its phonon-assisted transition. The emission of LaAlO3:Cr3+is constituted by five parts, which also derive from 2E→4A2 transition and its phonon-assisted transition. These two samples are demonstrated to be good NIR LLP by the decay curves measured after the UV lamp irradiation.