Physics of inorganic upconverting nanophosphors and their relevance in applications

Upconversion luminescence involves absorption of one or more low energy (commonly infrared wavelength) photons leading to emission of a high energy (i.e., visible or ultraviolet wavelength) photon. It is widely exhibited from lanthanide-doped materials with low phonon energy whereby multiple electronic transitions may occur between the f-states of the luminescent centers. Since upconversion processes require metastable and long-lived intermediate energy levels acting as storage and reservoirs of the exciting photon energy, rare earth lanthanide ions are particularly suitable due to their rich scheme of 4f energy levels poorly affected by the crystal field. In this framework, inorganic upconverting nanoparticles (termed nano-upconverters herein), that is (sub- 20 to 100 nm) nano-sized inorganic materials exhibiting photon upconversion, have gained a lot of attention from the scientific community owing to their numerous applications (optoelectronics, nanophotonics, nanomedicine, biological and biomedical sensing and labeling, nano-thermometry in cells, photodynamic therapy, and amplified stimulated emission), advances in nanofabrication allowing nanostructured design and control on the surface composition as well as integration processing. Furthermore, remarkable differences as compared to the bulk counterpart materials due to scaled-down size, increased role played by surfaces and changes in the phonon density of states have prompted advances in the theoretical understanding of upconversion pathways and mechanisms. This chapter provides an introduction to the physics of nano-upconverters and discusses the relevance of the associated phenomena in applications. It is overviewed the basic processes related to and leading to upconversion (excited state absorption, energy transfer upconversion, cooperative transitions and photon avalanche), the role played by rare earth ions with their ladder-like scheme of energy levels, the critical choice of the host crystal, the factors affecting luminescence quantum yield and the impact on spectroscopy of upconversion of the size-dependent changes in the phonon density of states. In this respect, the advantages in using nano-upconverters with respect to materials exhibiting quantum-size effect, the role played by increasing surface-to-volume ratio and the relationship between peculiar aspects of nano-upconverters and applications are discussed. This chapter consists of two main sections: the first part focuses on basic knowledge on upconverters and the second one discusses the topics overviewed in the first part by pointing out the changes related to the spatial confinement. The extensive fundamental information provided in the first part makes the readership of this chapter open to non-specialist readers and lays the foundation of the needed knowledge to read the second part easily. Due to the impossibility of covering all the aspects of the physics and applications of upconversion at the nanoscale in this contribution, extensive literature sources are provided for the reader’s guidance. The fascinating and still not completely understood physics of nano-phosphors is expected to be an interesting research field with relevance in applications and fundamental knowledge in the upcoming years