Frequency Upconversion in Core@shell Nanoparticles

Photoluminescence is a process of emission by absorption of light. There are two types of photoluminescence – downconversion and upconversion. In case of downconversion, the frequency of emitted light is less than that of absorbed light, whereas in the case of upconversion, the frequency of emitted light is more than that of absorbed light. The maximum efficiency can be obtained from the sequential energy transfer process. The highly activator-doped samples show concentration quenching in luminescence, and absorption takes place at resonance frequency ( f res ). Can upconverted luminescence reach a very high intensity in high concentration of activators as well as by absorption at non-resonance frequency ( f nonres )? It is possible to get when laser power is more than the critical value. This is called photon avalanche. There are many reports on the decrease of luminescence intensity when the size of the particle decreases to nanosize because of the association of surface defects, surface ligand, and solvent/medium, which act as the quencher. The luminescence intensity can be improved by core@shell formation. There are many ways of core@shell models such as active core@inactive shell, active core@inactive shell@active shell, active core@inactive shell@active shell@another shell, etc. This extends the utility of core active to multi-ways because of multi-functional properties, stability, easy surface functionalization, easy processing, etc. Here, synthesis, characterization, optical properties, and mechanism of upconverting core@shell nanoparticles and their applications are provided.