for Flat-Panel Displays
Flat-screen, high-definition televisions and flat-panel
displays to replace bulky computer monitors consume lots of power. Thinner
screens with reduced power appetites may someday become available as
a result of an emerging ORNL technology involving nanoparticles doped
with light-emitting atoms. This technology could make possible flat-panel
displays that are sharper than laptop computer screens. It should also
lead to higher-resolution detectors of gamma rays, X-rays, and other
ionizing radiation, as well as of fluorescent tags on DNA fragments.
Highly efficient laser diodes for optical circuits may also be made
using this technology.
Thomas Thundat of LSD, SSD's Doug Lowndes, Gilbert M.
Brown of CASD, postdoctoral scientist Adosh Mehta, Arun Majumdar of
the University of California at Berkeley, and Ramesh Bhargava of Nanocrystals
Technology, Inc., are developing a new class of nanosensors and nanodevices
using caged atoms in doped nanoparticles. Each particle is doped with
a few atoms of a rare-earth element that emits light of a specific color
when excited by an electric field, ultraviolet light, or mechanical
action. The dopants are europium (emitter of red light), thulium (blue
light), and terbium (green light).
"The dopant atoms caged inside the nanoparticles
show interesting properties as a result of quantum confinement,"
Thundat says. "Because of the overlap of the electron shells of
the dopant and host atoms, the nanoparticles are more efficient photon
The researchers are building a flat-panel display that
uses low voltage for operation. "In a clean room in the Solid State
Division, we are making silicon devices with microchannels and demonstrating
electroluminescence," Thundat says. "When the device is under
an electrical potential, the electrons excite the doped nanoparticles
sandwiched between the silicon base and a conducting glass electrode
on the top. The light from these doped particles should be sufficiently
intense for a flat-panel display because they have a much higher quantum
efficiency than undoped nanoparticles."
Some scientists are trying to find ways to control the
sizes of nanoparticles because size determines the color of the light
they emit. Doping the nanoparticles with rare-earth atoms to get the
desired color removes the need for stringent control of the size of
In a TV set, each tiny square, or pixel, on the screen
contains emitters of the different colors red, blue, and green
behind which are sources of electrons that stimulate the emission.
If the pixel for the image being shown must be red, then only the voltage
source for the red emitter will be turned on for that pixel. In the
ORNL device, each microchannel represents one pixel. The nanoparticles
are arranged so that only one type of doped particle is electrically
activated if light of only one color is desired.
Mehta holds a glass plate that glows green in ultraviolet light.
The plate's tiny channels are packed with gadolinium oxide nanoparticles
doped with terbium.
Because of the smaller sizes of the nanoparticle light
emitters, very little power will be used and less space required for
the power sources. Thus, the doped nanoparticle approach could enable
the development of thinner flat panel displays and could extend the
lifetime of laptop computer batteries.
In a radiation detector made from the ORNL nanodevice,
the gamma rays excite the caged atoms in the nanoparticles. The radiation
detector consists of hundreds of vertical channels 10 microns in diameter
and 300 microns deep. Packed inside each channel are thousands of 2-3-nm
particles of gadolinium oxide. The emitted light from the doped nanoparticles
travels through the channels and reaches a photodetector at the base.
"Today, it takes more than five incoming photons
to produce an outgoing photon that can be detected in a photodetector,"
Mehta says. "Our device will have a much higher quantum efficiency
because it will produce a photon for every photon coming in, providing
a much sharper image or giving a more precise reading."