Home > Press > Magnetism flicks switch on "dark excitons"
Tests At Leading Magnetic Labs Shed Light On Nanotube Mystery
Magnetic Transistor Could "Dial In" Quantum Effects
Houston, TX | Posted on January 10, 2006
In new experimental research appearing in this
week's issue of Physical Review Letters, a Rice University-led team of
nanoscientists and electrical engineers has flipped the switch on "dark
excitons" in carbon nanotubes by placing them inside a strong magnetic
The research offers new insight into the fundamental optical properties of
semiconducting nanotubes, hollow straw-like molecules of pure carbon.
Leading computing companies would like to use nanotubes as optical
components in next-generation microchips that are faster, more powerful and
more energy efficient.
"Single-walled carbon nanotubes offer engineers the intriguing possibility
of building chips where electrical inputs can be converted into light and
moved about the chip as optical signals rather than electrical signals,"
said lead researcher Junichiro Kono, associate professor of electrical and
computer engineering at Rice. "Thus far, the poor optical performance of
nanotubes - in some cases as few as one in 100,000 incoming photons causes a
fluorescent emission - has prevented engineers from developing the
technology for applications."
Kono said the new research may help scientists formulate new tests to answer
some of the most perplexing questions about the optical properties of
nanotubes. For example, scientists are currently debating whether low
fluorescence efficiencies in nanotubes arise from the intrinsic physical
structure of nanotubes or from external factors like structural defects and
impurities. Some of the leading theories have the missing light disappearing
into "dark" excitons - odd quantum pairings of electrons and electron
"holes" that are forbidden by quantum rules from fluorescing. The new
magnetic method of overcoming this dark exciton effect could be used to
probe the intrinsic properties of nanotubes and help settle the debate.
The team tested materials in some of the world's most powerful magnetic
fields. Experiments were conducted at both the Laboratoire National des
Champs Magnétiques Pulsés in Toulouse, France, and at the National High
Magnetic Field Laboratory at New Mexico's Los Alamos National Laboratory.
"We hope that our experimental methods will help better inform theorists and
ultimately aid in the development of new devices with far superior functions
than those based on existing technology," said Sasa Zaric, whose doctoral
dissertation will be based on the work.
Nanotubes are a fraction of the size of transistors used in today's best
microchips. As electronic components, nanotubes could reduce power demands
and heating in next-generation chips. But as optical components they offer
far more. The replacement of copper cables with fiberoptics revolutionized
the volume and speed of data transmission in the telecom industry 20 years
ago, and the parallels in microchips are tantalizing.
The research was funded by the Robert A. Welch Foundation and the National
Science Foundation. Rice co-authors include electrical and computer
engineering's Sasa Zaric, Gordana Ostojic and Jonah Shaver, and chemistry's
Valerie Moore, Robert Hauge and Richard Smalley. Other co-authors include
Oliver Portugall, Paul Frings and Geert Rikken, all of the Laboratoire
National des Champs Magnétiques Pulsés in Toulouse, France; Madalina Furis
and Scott Crooker of the National High Magnetic Field Laboratory at Los
Alamos National Laboratory; and Xing Wei of the National High Magnetic Field
Laboratory at Florida State University.
About Rice University:
For more information, please click here
Rice University is consistently ranked one of America's best teaching and research universities. It is distinguished by its: size: 2,850 undergraduates and 1,950 graduate students; selectivity: 10 applicants for each place in the freshman class; resources: an undergraduate student-to-faculty ratio of 6-to-1, and the fifth largest endowment per student among American universities; residential college system, which builds communities that are both close-knit and diverse; and collaborative culture, which crosses disciplines, integrates teaching and research, and intermingles undergraduate and graduate work. Rice's wooded campus is located in the nation's fourth largest city and on America's South Coast.
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