Chapter 6 - Oxide-based upconversion nanophosphors: Synthesis, characterization, and applications

Abstract

Upconversion is an optical process that involves the conversion of lower-energy photons into higher-energy photons. Upconversion photoluminescence is a unique property of mostly certain inorganic materials, which are capable of converting low-energy infrared radiation into a higher energy emission at visible wavelengths. It has unique luminescent properties, such as high penetration depth into tissues, low background signals, large Stokes shifts, sharp emission bands, and high resistance to photo-bleaching. These properties make upconversion nanophosphors an attractive alternative source for overcoming current limitations in traditional fluorescent probes. The aim of this chapter is to study the unique properties and the applicability of nanosized upconverting nanophosphors as optoelectronic device applications. The upconversion photoluminescence efficiency is dependent on the crystallite size because the increasing surface-to-volume ratio of nano-sized upconversion nanophosphors renders them more susceptible to quenching effects of the environment than their bulk counterpart. Oxide based upconversion nanophosphors have been synthesized by chemical precipitation method. The SEM image shows the nanophosphors are in the shape of rods. The diameter of rods has been found that 1.52 µm to 1.61 µm and the length of about 4.89 µm. It has also been found that at room temperature ultraviolet visible (UV-VIS) absorption band is around 355 nm (blue shifted as compared to bulk). Electroluminescence (EL) studies show that emission of light is possible at very small threshold voltage and increases rapidly with increasing applied voltage. It is seen that smaller nanophosphors give higher EL brightness starting at lower threshold voltage. The brightness is also affected by increasing the frequency of AC signal. EL spectra also show that the peak positions have shifted due to increasing the doping concentration.