Upconversion Luminescent Materials: Properties and Luminescence Mechanisms

Abstract

The upconversion (UC) luminescent materials typically refer to the materials doped with rare earth (RE) ions or transition metal ions as the luminescence centers, among which the lanthanides ions possess superior UC efficiency. UC luminescence (UCL) is a kind of anti-Stokes process, which absorbs two or more photons with a low energy while emits one photon with a high energy. The energy difference between the absorbed and emitted photons are typically much larger than kT (k is the Boltzmann constant and T is the Kelvin temperature). At present, the UCL covers the whole visible spectrum range, which can be applied in many fields including the solid laser, the multicolor display technology, the optical data storage, the biological probe, and the bio-imaging. Especially in the biological field, the UC materials can penetrate a much depth in the body benefiting from its infrared light excitation, and meanwhile, the florescence emission from the organic molecules can be avoided. Therefore, the UC materials yield many unique features in the detection and the identification of the biomolecules. Here, in this chapter, a fundamental description of the UC processes and the properties of the UC materials is provided. Section 1.1 provides a brief introduction to the UCL materials and the main UCL processes. Section 1.2 describes the necessary mechanisms and the processes involved in the UCL. Section 1.3 introduces the methods for improving the luminescent performance of the UCL materials.