Rice physicist Junichiro Kono and his team have been studying the Aharonov-Bohm effect -- the interaction between electrically charged particles and magnetic fields -- and how it relates to carbon nanotubes. While doing so, they came to the unexpected conclusion that magnetic fields can turn highly conductive nanotubes into semiconductors.
Their findings are published online this month in Physical Review Letters.
"When you apply a magnetic field, a band gap opens up and it becomes an insulator," said Kono, a Rice professor in electrical and computer engineering and in physics and astronomy. "You are changing a conductor into a semiconductor, and you can switch between the two. So this experiment explores both an important aspect of the results of the Aharonov-Bohm effect and the novel magnetic properties of carbon nanotubes."
Kono, graduate student Thomas Searles and their colleagues at the National Institute of Standards and Technology (NIST) and in Japan successfully measured the magnetic susceptibility of a variety of nanotubes for the first time; they confirmed that metallics are far more susceptible to magnetic fields than semiconducting nanotubes, depending upon the orientation and strength of the field.
Single-walled nanotubes (SWNTs) -- rolled-up sheets of graphene -- would all look the same to the naked eye if one could see them. But a closer look reveals nanotubes come in many forms, or chiralities, depending on how they're rolled. Some are semiconducting; some are highly conductive metallics. The gold standard for conductivity is the armchair nanotube, so-called because the open ends form a pattern that looks like armchairs.
Not just any magnet would do for their experiments. Kono and Searles traveled to the Tsukuba Magnet Laboratory at the National Institute for Materials Science (NIMS) in Japan, where the world's second-largest electromagnet was used to tease a refined ensemble of 10 chiralities of SWNTs, some metallic and some semiconducting, into giving up their secrets.
By ramping the big magnet up to 35 tesla, they found that the nanotubes would begin to align themselves in parallel and that the metallics reacted far more strongly than the semiconductors. (For comparison, the average MRI machine for medical imaging has electromagnets rated at 0.5 to 3 tesla.) Spectroscopic analysis confirmed the metallics, particularly armchair nanotubes, were two to four times more susceptible to the magnetic field than semiconductors and that each chirality reacted differently.
The nanotubes were all about 0.7 to 0.8 nanometers (or billionths of a meter) wide and 500 nanometers long, so variations in size were not a factor in results by Searles. He spent a week last fall running experiments at the Tsukuba facility's "hybrid," a large-bore superconducting magnet that contains a water-cooled resistive magnet.
Kono said the work would continue on purified batches of nanotubes produced by ultracentrifugation at Rice. That should yield more specific information about their susceptibility to magnetic fields, though he suspects the effect should be even stronger in longer metallics. "This work clearly shows that metallic tubes and semiconducting tubes are different, but now that we have metallic-enriched samples, we can compare different chiralities within the metallic family," he said.
Co-authors of the paper include Yasutaka Imanaka and Tadashi Takamasu of NIMS, Tsukuba, Japan; Hiroshi Ajiki of the Photon Pioneers Center at Osaka University, Japan; and Jeffrey Fagan and Erik Hobbie, researchers at NIST, Gaithersburg, Md.
Searles conducted the majority of the research during a visit to NIMS supported in part by a National Science Foundation Partnerships for International Research and Education grant to Kono and his co-principal investigators. Other funding came from the Department of Energy Office of Basic Energy Sciences, the Robert A. Welch Foundation and the Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
David Ruth | EurekAlert!
When helium behaves like a black hole
22.03.2017 | University of Vermont
Astronomers hazard a ride in a 'drifting carousel' to understand pulsating stars
22.03.2017 | International Centre for Radio Astronomy Research
Astronomers from Bonn and Tautenburg in Thuringia (Germany) used the 100-m radio telescope at Effelsberg to observe several galaxy clusters. At the edges of these large accumulations of dark matter, stellar systems (galaxies), hot gas, and charged particles, they found magnetic fields that are exceptionally ordered over distances of many million light years. This makes them the most extended magnetic fields in the universe known so far.
The results will be published on March 22 in the journal „Astronomy & Astrophysics“.
Galaxy clusters are the largest gravitationally bound structures in the universe. With a typical extent of about 10 million light years, i.e. 100 times the...
Researchers at the Goethe University Frankfurt, together with partners from the University of Tübingen in Germany and Queen Mary University as well as Francis Crick Institute from London (UK) have developed a novel technology to decipher the secret ubiquitin code.
Ubiquitin is a small protein that can be linked to other cellular proteins, thereby controlling and modulating their functions. The attachment occurs in many...
In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are creating...
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are...
Enzymes behave differently in a test tube compared with the molecular scrum of a living cell. Chemists from the University of Basel have now been able to simulate these confined natural conditions in artificial vesicles for the first time. As reported in the academic journal Small, the results are offering better insight into the development of nanoreactors and artificial organelles.
Enzymes behave differently in a test tube compared with the molecular scrum of a living cell. Chemists from the University of Basel have now been able to...
20.03.2017 | Event News
14.03.2017 | Event News
07.03.2017 | Event News
22.03.2017 | Materials Sciences
22.03.2017 | Physics and Astronomy
22.03.2017 | Materials Sciences