| International Data Corporation predicts that by 2025,the total amount of data generated globally will reach 175ZB(1ZB=1012GB),of which cold data(important data that is accessed less frequently,such as statistics of cities,patient health data of hospitals,confidential national defense data,etc.)accounts for about 80%.Optical information storage is considered as one of the most promising cold data storage methods due to its unique advantages of high security,long life and low energy consumption.Due to the limitation of two-dimensional spatial resolution,traditional optical information storage technologies,such as optical discs,digital video discs or Blu-ray discs,are facing increasing challenges.Currently,commercial optical information storage media are mainly composed of photothermal plastics,photorefractive crystals and photoinduced polymers,which are prone to aging and failure.Therefore,it is urgent to develop a kind of new durable super-resolution recording materials.Inorganic electron-trapping materials have good chemical stability and high temperature stability,highlighting the characteristics of long-life time,and have become a new generation of storage media after traditional dyes and phase change materials.Its unique optical storage mechanism transforms the slow photothermal effect into the faster photo-quantum effect,which can reduce energy consumption of writing and improve writing rate.Combined with advanced optical storage technology,the current two-dimensional plane can be extended to multidimensional space,so that the storage density is greatly improved.However,there are still many problems to realize highdensity multidimensional optical information storage.Firstly,in rare earth doped electron-trapping materials,the induction of trap levels is the basis.Secondly,the effective regulation of trap levels is the key to ensure that the mechanism of optical information writing and readout can occur.From the perspective of energy band engineering,it is the synergistic effect of doped non-luminescent rare earth ions or transition metal ions with rich electronic structure that can introduce local trap levels into matrix band gap.Those trap levels play a role of “intercepting” or binding the excited-transition luminescent carriers in energy band.Additionally,the number of carriers captured by traps is closely related to the density and depth of trap levels: the more and deeper the trap levels are,the larger the storage capacity of carriers,which restricts the reading times and stability of stored information.The critical problem of material research is the design of band structure,which is embodied in the mechanism of matrix selection(band gap),doping ions and doping amount(impurity level introduction).On the basis of summarizing previous studies,yttrium germanate(Y2Ge O5)with wide band gap as the host of electron-trapping materials was selected in this work,intending to introduce rare earth ions to induce separate trap levels in the band gap.A novel wideband gap bifunctional rare earth ions(luminescent/manufacturing defect)co-doped germanate phosphor Y1.989 Ge O5 :0.01Pr3+,0.001Tb3+(YGO:Pr,Tb)was designed and optimized.The important discovery of scientific experiments is that this phosphor has triple discrete trap levels and can be used as optical information storage medium.The main results are as follows:1)based on the TL spectrum to analyse the distribution of trap levels,it's revealed that YGO:Pr,Tb has three narrow distribution(peak width: 0.063,0.082 and 0.154 e V)trap energy levels(peak position: 0.69,0.93,and 1.21 e V),these trap levels can effectively trap carriers,thus providing an effective energy barrier for storing optical information and a mechanism for resisting thermal perturbation at room temperature.2)first principles calculation reveals the nature and origin of trap levels.At first,both Pr3+ and Tb3+ were more inclined to occupy Y site in the hexagonal coordination environment with lower substitution energy.Then,co-doping of Tb3+ ions transforms the band structure from a matrix wide-band gap(4.0 e V)semiconductor into a metallike characteristic due to the large number of charge transfer layers(or electron storage layers),which are mainly contributed by O 2p orbitals.Moreover,the doping of Tb3+ ions can not only lead to the change of oxygen coordination environment and introduce the impurity level,the synergistic effect of Tb3+ and Pr3+ co-doping can also enhance the trap levels.Finally,combined with electron paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy analysis,it is further proved that the triple trap levels originated from oxygen vacancy.3)the aging study of thermoluminescence spectra illustrate that the shallow trap level shows a faster charging speed of carriers and a faster emptiness rate.And bare powders of YGO:Pr,Tb with excellent information retention rate over 50% after 7 days at room temperature shows great advantages in long-term storage of information.4)the simulation demonstration of writing and reading of optical information patterns is completed with the developed material in macroscopic scale.Optical information can be effectively encoded by a commercially available 254 nm Hg lamp and stably stored in deep trap levels.The pre-written information then can be decoded by thermal stimulation(250 oC)or optical-stimulation(lasers of 515 and 650,and 980 nm).Remarkably,relying on the experimental setup of a super-resolution configuration(developed by Shanghai Institute of Optics and Fine Mechanics,Chinese Academy of Sciences),a 10×10 array of optical information points(radius: 0.5 ?m,interval: 1 ?m)is automatically recorded in a 2D progressive scan bit-by-bit mode by utilizing a 515 nm femtosecond laser and read out by laser linear continuous scanning.This further confirms the optical information writing and reading characteristics of this material.The recorded images of optical information points also indicate that photoluminescence is a highly localized photoelectronic process,in which electrons at specific spots on the "analog disc" are captured and released by laser beam excitation,so as to realize the writing and reading of tiny optical information points.This work not only shows the possibility and potential of YGO:Pr,Tb materials in optical information storage,but also reveals the structure-activity relationship and internal influence mechanism among the complex doping mechanism,the induction and distribution of trap levels and optical storage performance from the perspective of scientific basis.The recorded images of optical information points also indicate that photoluminescence is a highly localized photoelectronic process,in which electrons at specific sites on the "analog disc" are captured and released by laser beam excitation,thus high density micro-nano optical information points can be written-in and read-out on two-dimensional or even three-dimensional space. |
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