Upconverting Nanoparticles

Abstract

Upconversion (UC) refers to nonlinear optical processes in which the sequential absorption of two or more photons leads to the emission of light at shorter wavelength than the excitation wavelength (anti-Stokes type emission). In contrast to other emission processes based on multiphoton absorption, upconversion can be efficiently excited even at low excitation densities. The most efficient UC mechanisms are present in solid-state materials doped with rare-earth ions. The development of nanocrystal research has evoked increasing interest in the development of synthesis routes which allow the synthesis of highly efficient, small UC particles with narrow size distribution able to form transparent solutions in a wide range of solvents. Meanwhile, high-quality UC nanocrystals can be routinely synthesized and their solubility, particle size, crystallographic phase, optical properties and shape can be controlled. In recent years, these particles have been discussed as promising alternatives to organic fluorophosphors and quantum dots in the field of medical imaging.

Synthesis of β-Phase NaYF4:Yb,Er Upconversion Nanocrystals and Nanorods by Hot-Injection of Small Particles of the α-Phase

“…As expected for the two-photon excitation process, all data sets are in accord with a slope of two [64]. Similar to results published by other groups, the upconversion intensity of NaYF 4 :Yb,Er nanoparticles strongly depends on the particle size, crystal phase and the existence of a passivation shell over the optically active particle core [9,31,38,[65][66][67][68]. Smaller particles are expected to show a weaker upconversion efficiency because of their large surface-to-bulk ratio.…”

“…During the last years, a large number of research papers was pub-lished on the synthesis of NaREF 4 nanocrystals (RE = rare-earth), since NaREF 4 and LiREF 4 are attractive host lattices for a variety of functional optical materials such as upconversion phosphors, [6][7][8][9][10][11][12][13][14] quantum-cutting phosphors, [15][16][17][18][19] laser materials, [20,21] and materials for solar cell applications [22][23][24][25][26]. A variety of synthetic methods has been described for producing solvent-dispersible sodium rare-earth tetrafluoride nanocrystals with controlled size, shape and capping ligand, including thermal decomposition, [7,[27][28][29][30][31][32] co-precipitation, [6,11,[33][34][35][36][37][38][39][40] or solvothermal methods [41][42][43][44][45][46][47]…”

Excitation energy migration dynamics in upconversion nanomaterials

“…Relevant progress has already been discussed and reviewed. 35 Here we only aim to update our comprehension of the relevant dynamics. As mentioned in the introduction, unmodified surfaces act as a quenching factor for upconversion emission because they contain charged defects and/or high vibrational modes of solvents or surface-bound ligands.…”

Multi-photon excitation in uncaging the small molecule bioregulator nitric oxide

“…Thus, doping Ln 3+ cations into a sodium yttrium fluoride matrix NaYF 4 , especially one in the β-phase, leads to strong emissions from the f-f states of these ions owing to the longer lifetimes under such circumstances. These properties are especially well suited for the NIR to UV/visible upconversion nanoparticles (UCNPs) [69][70][71][72] that will be discussed in this section.…”