| Mechanoluminescencet(ML)materials are a new and developing kind of trap-controlled energy-saving phosphors which respond to nondestructure mechanical stimulate and produce light emission,and enabling the luminescence of electron-hole pairs in the "exciting-trapping-releasing-recombination" mechanism cyclic.It shows tremendous potential applications in the visualization of stress distribution,structural health monitoring,life sciences,displays and lightings.Trap centers,luminescent centers and piezoelectricity are the essential features of most recoverable ML materials.Accordingly,the traditional method to search for recoverable ML materials is primarily based on scereening the existing persistent phosphors with piezoelectricity.Due to the limited number of available candidate persistent phosphors,the family of recoverable ML materials alrealy known is relatively small,which seriously hampers its practical application.On the one hand,in order to break the limitations of the traditional method for preparing elastically-stressed luminescent materials,our group recently proposed the "doping activator in piezoelectric matrix"(extrinsic deficiency)strategy,which introduced luminescent ions and extrinsic defects by doping activator.Pr3+ activator is used to substitute non-intrinsic Sr2+ in Sr2Nb2O7 piezoelectric matrix to introduce luminescence center,and a new type of ML material Sr2Nb2O7:Pr3+ is developed.The material exhibits bright red ML under compression and friction.Compared to the isomorphic Ca2Nb2O7:Pr3+ which requires a long-time delay to achieve high-contrast ML due to the strong long-lasting afterglow,Sr2Nb2O7:Pr3+ enables to obtain high-contrast ML in a short-time delay because of the weak short-lasting afterglow,which greatly simplifies the imaging process to facilitate the further application of ML.The investigations of structural,optical characteristics and trap properties between Sr2Nb2O7:Pr3+ and Ca2Nb2O7:Pr3+ reveal that the unique ML of Sr2Nb2O7:Pr3+ should result from its lower trap concentration and shallower trap depth relative to Ca2Nb2O7:Pr3+.In addition,comparing with the photoluminescent emission of the isomorphous Ca2Nb2O7:Pr3+ displays red color,the emission of Sr2Nb2O7:Pr3+ shows orange-red.And the different photoluminescent colors of the two materials are due to different piezoelectric substrates Sr2Nb2O7 and Ca2Nb2O7.The difference in distance between Pr3+ and Nb5+ results in highly different energy levels of the internal valence charge transfer state(IVCT).The study is expected to establish new design strategy and largely expand the horizons of existing recoverable ML materials,thereby paving the way for the fundamental research and proming applications of recoverable ML.On the other hand is an" un-doping in piezoelectric matrix" method to form intrinsic defects which serve as combination levels and trap levels.Our research on argon-sintered Ca Zn OS piezoelectric semiconductor shows a multi-mechano-responsive bright emitting piezoluminescence,which is sensitive to ultrasonic vibration,friction,impact and compression.Our experimental and density functional theory investigations indicate that the intrinsic oxygen vacancies play the leading role of luminescent centers and trap states in multi-colored components of photoluminescence and piezoluminescence.And we propose a mechanism of intrinsic oxygen vacancies mediated luminescence to describe the processes of photoluminescence and piezoluminescence.In addition to saving resources and protecting environment,our research is expected to open a new door for design and development of novel self-piezoluminescent materials... |
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