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Abstract: Luminescence-based techniques continue to attract considerable attention due not only to their current range of applications but also to their wide potential in the fields of optical devices and biomedicine. To date, numerous luminescent materials, such as fluorescent proteins, organic dyes, metal complexes, semiconductors, noble metal nanoparticles, as well as lanthanide-doped inorganic phosphors  have been developed for use in various applications. Most of these conventional materials exhibit luminescent emission with a Stokes shift .But upconversion emission is an anti-stoke process. To date, two main processes have been devised for achieving up-conversion  luminescence emission under  CW  low-energy  excitation. These are the up-conversion luminescence emission of lanthanide ions (such as Er3+, Ho3+, and Tm3+) and the so called triplet-triplet annihilation (TTA)-based upconversion. Photon upconversion is a process in which the sequential absorption of two or more photons leads to the emission of light at shorter wave length than the excitation wavelength. There are three basic mechanism for photon upconversion in inorganic materials and at least two distinct mechanisms in organic materials. In inorganic material photon up conversion occur through an energy transfer upconversion, photon avalanche and excited state absorption. In case of organic material it is occur by a triplet-triplet annihilaton and energy pooling.  The lanthanide ions are very suitable to be used for upconversion because they have abundant energy levels that can be used for spectral conversion. Among all lanthanide ions, the Er3+−Yb3+ couple is commonly used for realizing efficient upconversion. On the other hand, endowed with the advantages of both fluorides and oxides, i.e., low phonon energy and good chemical stability, rare earth oxyfluorides are expected to be promising host materials to achieve upconversion luminescence. Up-conversion material has wide application , such as UCNPs for Bioimaging , Lanthanide UCNPs for Therapies, Upconversion Materials for Photocatalysis, Upconversion Materials for Solar Fuels, UCNPs for Sensing etc.

References:
(1) Chem. Rev. 2015, 115, 395-465
(2) ACS Appl. Energy Mater. 2018, 1, 447-454
(3) Nano Lett. 2018, 18, 2964−2969