New iron-filled, light-blocking fullerenes.
Like eyeglasses that darken in sunlight to shield the eyes, fullerenes block bright light. But unlike the lenses, whose molecules rearrange in less than a second in the presence of bright light, fullerenes act much faster and via a much different mechanism.
"It's purely electronic rearrangement on the order of nanoseconds," says Lee W. Tutt, a chemist at Hughes Research Laboratories in Malibu, Calif. This lightning-fast response means that fullerene materials may one day guard sensors -- or perhaps eyes -- against damage by lasers. Such protection becomes more necessary as lasers proliferate in a variety of applications, Tutt notes.
Most materials do not handle light this way. Like old-fashioned sunglasses, they allow a constant percentage of light energy to pass through them. Thus, if a light is bright enough, enough gets through to do damage. But fullerenes and a few other substances show nonlinear optical (NLO) properties. Thus, Tutt says, when light intensity gets high enough, NLO materials change their light-transmitting properties, becoming ever more opaque as intensity increases. Thus, NLO materials transmit a maximum amount, rather than simply a percentage, of incoming light.
Tutt and Alan Kost, also at Hughes, tested the light-limiting properties of the bucyball and its 70-carbon cousin in separate solutions. They subjected each to 8-nanosecond-long pulses from a green laser and sampled the laser's energy entering and exiting the solution.
The buckyball solution, which at lower light levels transmits 80 percent of the energy, becomes more opaque when the laser's energy exceeds 240 millijoules per square centimeter ([cm.sup.2]). It keeps transmitted light to 80 percent of the 240 level even when its energy reaches 3 joules per [cm.sup.2], they report in the March 19 NATURE.
The Hughes researchers also assessed other light-limiting materials. "If they're really comparing apples to apples, then they show that [C.sub.60] is superior to other materials that have been studied in the past," comments Zakya H. Kafafi, a chemist at the Naval Research Laboratory in Washington, D.C. She and her colleagues have studied NLO properties of fullerenes using a different color light.
Tutt and Kost think fullerenes limit light because of the way their electrons react to light energy. The laser causes the electrons to jump to a slightly higher energy state; while excited, the electrons are much more likely to absorb energy and thus temporarily exhibit a greater ability to block even intense light.
However, the Navy lab's work indicates that the laser energy alters optical properties by heating the fullerenes. Thus thermal, not electronic, effects may lead to optical limiting, Kafafi notes.
Instead of using fullerenes to limit light, T. Pradeep and colleagues at the Indian Institute of Science in Bangalore have used them as cages. By trapping atoms inside fullerenes, scientists hope to create fullerenes with useful electronic or magnetic properties.
The Indian chemists added an iron compound to the reaction chamber in which they vaporize graphite to make fullerenes. From the soot, they extracted 60-, 58-and 56-carbon fullerenes containing single iron atoms, they report in the March 11 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY.
"If this bears out, this will be the first report of a [C.sub.60] cage with a metal atom that has been isolated," notes Richard E. Smalley, a chemist at Rice University in Houston. Like others, Smalley has trapped atoms inside fullerenes but has been unable to purify enough of the resulting material to study it in detail. Pradeep's group made enough iron-fortified fullerenes to contrast their character with that of fullerenes with iron attached to the outside of the carbon cages. Their results indicate the iron inside binds tightly to the carbon atoms and retains its electrons.
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|Title Annotation:||all-carbon molecules|
|Date:||Mar 28, 1992|
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