ABSTRACT

Rare earth doped materials are vital in many optoelectronics applications such as solid-state lasers [1], light-emitting diodes (LEDs) [2], optical fiber amplifiers [3], and photovoltaic devices [4]. Majority of earlier works focus on the rare earth doped materials of bulk nature. However, with the development of nanotechnology over the past decades, research and applications of rare earth materials in optoelectronics have moved from bulk to nanoscale. In this chapter, recent developments of rare earth nanophosphors in LEDs, and the effects of various rare earth dopants and particle size on luminescence properties are discussed. 6.1 Introduction and Overview

Over the past few decades, rare earth materials have been widely studied due to their unique luminescent properties arisen from

their sharp emission bandwidths, tunable emission color, long luminescent lifetime, and low toxicity. RE ions exhibit up-and downconversion luminescence [5]. Up-conversion is a process whereby photons are pumped by a longer wavelength excitation, which produces emission of shorter wavelength. This is applicable in the field of biomedical research where the up-conversion processes of RE doped materials proved to be promising candidates for biological assays and biomedical imaging [6-9]. However, the down-conversion process, the reverse of up-conversion, is the typical mechanism of light emission in LEDs [10-12]. Rare earth phosphors are under extensive investigation during the past decades due to the increasing demand of higher energy efficient light sources and luminescent panels [13]. It has driven developments targeted to create optoelectronic devices such as white light emitting diodes [2, 14, 15] and display panels with enhanced efficiency [16, 17]. Traditional light bulbs, which are widely used in incandescent lamps, are very energy inefficient, where only 3-5% of electrical energy is converted into visible light. Conventional incandescent lamps rely on either heat or discharge of gases. Both processes involve large energy losses due to high temperatures and large Stokes shifts. In order to make energy efficient systems, efforts have been made to replace conventional incandescent lamps with more efficient white LEDs, which reduce energy consumption and greenhouse gas emissions [18]. The investigation of rare earth materials in optoelectronics started in the beginning of the 20th century with the emphasis on bulk material [19-21]. Due to higher surface area-to-volume ratio, nanomaterials are usually more reactive and sensitive than their bulk counterparts. In order to enhance efficiency, most recent development in rare earth doped phosphors for optoelectronics applications are moving toward nanosize particle [22, 23]. Various rare earth based nanophosphors used in white LED, primarily yellow, red, green, and blue emitting phosphors will be discussed. This chapter will begin with a brief explanation on the working principles of LED and how white light is produced in LEDs using rare earth doped nanomaterials. This is followed by an overview of the various synthesis methods of the nanophosphors and key factors affecting luminescence efficiency. The bulk of the chapter will concentrate on the effect of dopant, structure, host materials and particle size on the luminescence properties of the various rare earth nanophosphors.