Research Progress of Rare Earth Doped Upconversion Luminescent Materials
Upconversion luminescence is a kind of anti-stokes shift luminescence.The upconversion luminescent materials which usually composed by rear earth elements and have some properties superior to ordinary organic materials,such as long luminous lifetime,photobleaching resistance,narrow emission bands and so on.Therefore,this unique phenomenon that absorbing low energy photons and emitting high energy photons has been attracted the attention of researchers in many fields.In this review,the most common mechanism of energy transfer upconversion was described in detail,two types of complexed lanthanide ions-mediated energy migration upconversion mechanism that realized though core-shell structure and other upconversion mechanisms were also introduced.The synthesis methods that most widely reported Co-precipitation method in recently for preparing core or core multi-shells upconversion nanoparticles and hydrothermal method that widely used for preparation bulk materials were introduced.Importantly,due to the abundant emission bands of these lanthanide doped nanoparticles and the strongly penetration of near-infrared laser,these nanoparticles have been used in biological domain,such as photodynamic therapy to eliminate tumors or some bacteria,cell and tissue imaging that achieved sub-centimeter depth.In addition,colorful upconverison luminescent materials with the feature of multi-mode color tuning were used in high-level anti-counterfeiting,optical information storage and transparent display.Furthermore,the upconverision nanoparticles are expected to be applied to the engineering projects based on the changes of emission characters that originated form the influence of the temperature or fluid rate to the population of lanthanide ions and this non-invasive optical method requires simple equipment with low cost that could obtain accurate results.Besides,the upconversion materials that excited by near-infrared light seems like more promissing to use the solar energy,more photocatalysts base on the upconversion materials that used the near-infrared light as the excitation resource rather than visible or ultraviolet light.So far,more and more nano-scale upconversion materials have been reported and used instead of bulk materials,because of their extraordinary modification potential though the construction of core-shell projects.However,the biggest obstacle limiting the applications of upconversion luminescent materials remains the extremely low quantum yield.In addition to the reason of the upconversion mechanism itself,the absorption cross-section of the lanthanide elements is too small and cannot be compared with organic dyes.The problem of low luminous efficiency needs to be solved urgently.On the premise of ensuring functionality,the structure of upconversion nanoparticles should also be simplified to reduce the difficulty of synthesis and elevate synthesis yield,and some new generation mechanisms for upconversion luminescence modulation needs to be found that requires the joint efforts of more relevant researchers to expanding the applications of upconversion luminescent materials.
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