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Encyclopedia of Physical Science and Technology EN014A-653 July 28, 2001 20:55

Rare Earth Elements and Materials 17

computer monitors, fluorescent lighting, and medical maticity (high color purity), high luminance, high effi-
X-ray photography. ciency, low saturation, and good aging properties while
it is operated at the low voltages and high current den-
sities on portable displays. The materials currently used
1. Luminescent Lighting
are sulfur-free rare earth compounds that are more stable
A prediction was made in early 1970s that a luminescent than conventional CRT phosphors. FED devices are pro-
lamp with a high efficiency and high light quality could jected to become commercially significant around 2004
be obtained by combining three phosphors (luminescent and to experience a strong growth rate for the rest of the
materials) which emit in narrow wavelength intervals cen- decade and beyond. Initially, FEDs are likely to replace
tered around 450 nm (blue), 550 nm (green), and 610 nm small LCDs in digital cameras and camcorders, but have
(red). Immediately, it was clear that phosphors doped with the promise, eventually, to be deployed in laptop comput-
lanthanide ions were the most obvious choice to construct ers and other larger-area display applications.
this tricolor luminescent lamp.
The perfection of phosphors using blue emissions from
3. Phosphors for Organic Light-Emitting
divalent europium, green emissions from trivalent terbium
Devices (OLEDs)
coactivatedbycerium,andred fromtrivalenteuropiumhas
made it possible to make trichromatic fluorescent tubes There is a strong demand to replace the cumbersome cath-
recently miniaturized in the form of compact lights for ode ray tubes on televisions and computers with a large
use in the home. dimensional flat screen. This goal may be achieved by the
Giving off a color very close to that of incandescent FED technology as briefed above using more advanced
lights, the trichromatic system of fluorescent light bulbs RE-phosphors. It may also be achieved by the newly
has a five to eight times greater light output than that of emerging organic light-emitting technology. An organic
traditional light bulbs using an off-white band emitter. The light-emitting diode (OLED) consists of a light-emitting
life span for rare earth-based bulbs is more than a 1000 hr junction layer sandwiched between two electrodes, one
longer, resulting in significant savings to the user. of which is transparent (Fig. 13). When an electrical cur-
There has also been a marked increase in the use of rent is passed through the OLED, the electrical energy is
phosphors in signs and signals. Numerous safety signs converted into light (electroluminescence), which passes
(exit lights, reflective safety bands, and highway mark- out of the device through the transparent electrode. This
ings, etc.) require the use of phosphorescence. Rare earths conceptually simple design paves the way for thin, poten-
are part of the mineral phosphorescent product with the tially flexible displays on plastic substrates and represents
longest known phosphorescence duration. the future for flat-panel display technology. The properties
of the light-emitting layer are critical to the performance
of the OLED. The improvements in efficiency, lifetime,
2. Phosphors for Field Emission Display (FED)
and ease of manufacturing that will be necessary for the
In color television, the image is reproduced by selective realization of a low-cost, high-volume solution for flat
excitation of three RE-phosphors (blue, green, and red) panel displays, can only arise by careful optimization of
deposited on the internal face of the screen by a highly the light-emitting layer.
powerful electron beam originating from a metal elec- In the lanthanide elements, f electrons residing are
trode (cathode). Such an excitation technique is termed buried deep within the metal ion. The lack of any signifi-
Cathode Ray Tube (CRT) technology. It is estimated that cant interaction between them and the surrounding ligands
over 80% of the total worldwide display phosphor market gives rise to two important features of rare earth phosphors
will be utilized in the CRT industry (TVs and desktop PC
monitors).
A new technology called Field Emission Display (FED)
works on a principle similar to that of CRT, but instead
of using just one “gun” spraying electrons against the
phosphor-coated inside of the screen face, there are as
many as 500 million of them (microtips). The main driver
of this technology is the quest for brighter displays with
less viewing angle dependence than liquid crystal displays
(such as those currently used in laptop computers).
Any phosphor that is to be used in a FED must posses FIGURE 13 Schematic representation of an organic light-
stringent properties in order to be successful: good chro- emitting diode.