ORNL scientists Thomas Thundat and Adosh Mehta have collaborated with Ramesh Bhargava of Nanocrystals Technology in Briarcliff, N.Y., to cage single atoms in nanocrystals not much larger than the atoms themselves. Previous attempts to catch atoms have been difficult because of the unpredictable nature of atoms, as dictated by the rules of quantum mechanics.
Though the process that was used to catch the atom was not disclosed, the scientists claim that it enables them to study the properties of a single atom at room temperature using conventional microscopy techniques. This is far more practical than methods that require cooling the atoms to cryogenic -- or extremely cold -- levels or trapping atoms in the gas phase using ion trap mass spectrometers.
The researchers found that when the atoms are trapped in a specially fabricated nanocrystal host structure, the atoms can be excited with a laser into four distinct levels of brightness, what scientists are calling a blinking phenomenon. This occurs as the crystal structure around the atom changes.
This newly discovered technique could have potential applications for optical sensing, display systems, and computing, including a possible four-bit optical storage system.
The trapping process is complex and was done by Bhargava at Nanocrystals Technology. By confining a single atom inside a nanocrystal, Bhargava has devised a material with potential uses ranging from bio-sensors to optical computing and just about anything optical in between. Perhaps the immediate applications lie in computing -- optical storage systems, optical sensing and display systems.
Wherever their work leads, the researchers are excited about achieving a feat that had only been achieved with single molecules rather than individual atoms.
"The problem has been that we have had to study ensembles of atoms instead of a single atom," said Bhargava, president of Nanocrystals. "Now we can look at the properties of a single atom at room temperature. It makes it much more feasible."
The blinking phenomenon occurs as the crystal structure around the trapped atom changes. A laser supplies energy for this process, which has scientists speculating about potential applications.
"We´re looking perhaps at a new class of nano-scale materials with novel optical properties," said Mike Barnes, a member of ORNL´s Chemical and Analytical Chemistry Division. "The challenge we face will be in controlling this process and fully understanding the mechanism." Which is where the scientists are at present.
QUANTUM CONFINED ATOMS (QCA)
What Bhargava found in his 10-year quest to isolate single atoms for study is quantum confined atoms (QCA), or atoms caged inside nanocrystals. The atoms are important in their own right since researchers can use QCAs to study individual atoms, but a QCA is also special because it makes very energy-efficient phosphor, a material that emits visible light.
"What I´m trying to do is look at nanotechnology at the highest level where I can have control based on single atoms yet the control is so powerful I can make devices," Bhargava told NanotechPlanet. "I don´t want to say I have everything resolved. I just want to give the impression that this is real. I have products."
Because of their unique structure, QCAs are an ideal material for use in all manner of optical devices. To exploit the commercial opportunities QCAs afford, Bhargava quit his director´s job at Philips Research Labs in Briarcliff Manor, N.Y., in 1993 to found his current venture, Nanocrystals Technology. Since that time, he has spent upwards of $9 million uncovering the intricacies and uses of QCAs.
Exactly how and why QCAs do what they do is still somewhat of a mystery, but the mechanics of how to make and control them are well enough understood to develop commercial applications based on their properties.
"What I have seen with my own eyes in my own lab is really, truly amazing," said Mike Barnes, an Oak Ridge National Labs researcher who has been studying the physics of Bhargava´s QCAs. "And we´re still trying to understand it."
HOW QCAs WORK
Basically, QCAs work by absorbing photons of ultraviolet (UV) light and re-emitting that light at a lower frequency, or color. As the size of the nanocrystal cage approaches two nanometers (basically the size of the atom itself), the conversion of light energy approaches 100 percent. Unlike their close cousins quantum dots (which are also phosphors), color is not dictated by the size of the nanocrystal, but by the atom confined within. This means QCAs can be made and sold in bulk, by color - a definite advantage in the world of commercial manufacturing.
QCAs have a number of advantages over today´s commercial bulk phosphors, which reabsorb almost as much light as they emit when excited by the UV light. Bulk phosphors also scatter light in all directions, making them unsuitable for use in the next generation of energy-efficient solid state lighting (SSL). QCA nanophosphors, on the other hand, do not have these drawbacks, making them ideal for SSL use.
Because QCA nanophosphors are up to five-times more efficient at converting UV light to white light, developing QCA-based SSL could save billions in energy costs worldwide and, since less energy will be needed, initiate a massive reduction of the pollution associated with the generation of electricity. This because most of the world outside of the United States uses between 60 percent and 80 percent of its electricity to power light bulbs.
"His technology can potentially provide a very efficient phosphor for these devices, which means an extremely efficient light bulb," Steve Johnson, group leader of the Lighting Research Group at Lawrence Berkley National Labs, said. "In the wildest dreams we could get 80 percent or 90 percent efficient devices."
This makes the advancement of QCA-based SSL perhaps the most exciting and profitable near term application.
Although all of the major lighting manufacturers are working on SSL, Bhargava believes QCAs are the breakthrough that will finally enable commercialization of the technology. That is why he is talking with major players in the lighting market, including General Electric, Philips and Siemens, about partnering in a market estimated by the U.S. Department of Energy to be worth $3.2 billion by 2005.
"The way I see it is I have the unique phosphor technology which they don´t. If they use their own phosphor they get less light output," Bhargava said. Up to 20 percent less, he added.
A QCA-based SSL lamp will work something like this: since QCAs can be mixed with polymers they can be formed into any shape imaginable. Once mixed and formed into the shape of a light bulb, for example, all that is missing is a source of UV light to excite the atoms and cause them to luminesce. Because the color output of an individual QCA is controllable, mixing a batch of red, green and blue (the primary colors from which all other colors are derived) QCAs is easy. Add UV light and out comes white light. SSL light can be easily tweaked to mimic warm incandescent light or made bright white to light a stadium, for example.
As an added benefit, QCA-based SSL lighting runs so cool it can be used almost anywhere and will work under the most demanding conditions, lasting hundreds of times longer than conventional lighting. Finally, QCA nanophosphors can be used to improve the efficiency of existing LEDs, halogen lamps, arc lamps and fluorescent bulbs.
Bhargava is hoping to have the last hurdles to commercialization overcome within the next year.
Even closer to fruition is a way to digitize and dramatically improve the resolution of diagnostic X-ray images using QCAs. With a little more work and a little more money from investors, this technology should be ready for the market early next year. In fact, Bhargava has formed a company under the umbrella of Nanocystals Technology around the idea - Nanocrystal Imaging Corp. - and is actively visiting with interested venture capital firms to acquire the capital needed to complete the transition from the lab to the open market.
With this technology Bhargava expects to dominate the X-ray imaging business while fundamentally changing it forever. Gone will be films and developing chemicals. With Bhargava´s technology, the images go right from the receiver plate to a display device. No time will be lost waiting for images and ultra fine resolution.
Other uses include nanomagnets, nanomemory chips for optical computers and bio-tags for molecular research; the list is long and growing. Using QCAs to make clear glass windows or reading glasses that block UV light, for example, is already a near-term application being explored by manufacturers.
After 10 years of research and development what Bhargava has discovered may not be so much a new way to look at single atoms in the laboratory (his original goal) but, more to the point, a way to look at existing technologies in a new light.
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