Caged Atoms 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 sources."

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 conventional nanoparticles.

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.

Adosh 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."