Chemistry Reference
In-Depth Information
their sharp emission bandwidths, tunable emission color, long
luminescent lifetime, and low toxicity. RE ions exhibit up- and down-
conversion 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 20
th
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.
