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Posted: Apr 13, 2007
Why defects can be a useful thing for nanotechnology engineers
(Nanowerk Spotlight) Research into the subject of radiation damage in graphite began in the early 1940s as a part of the development of nuclear weapons and nuclear power. Some designs of nuclear power reactors, such as the Chernobyl reactors, use graphite as moderator (the material which slows down the neutrons released from fission so that they cause more fission). The damage to the graphite moderators caused by radiation has been one of the major concerns of the nuclear power industry and radiation defects, i.e. structural irregularities, in graphite produced upon irradiation, their structure, properties and formation mechanisms, have been subject of intense research. Several years ago, defects in carbon materials became a hot topic again but now in the context of carbon layered nanostructures, such as multiwalled and bundled carbon nanotubes, which closely resemble graphite in their structure. The formation of irradiation-induced defects in graphite like layered carbon nanostructures, multiwalled and bundled carbon nanotubes, nanoonions, etc. changes their mechanical and electronic properties and may even trigger dramatic structural changes. While the terms "radiation damage" and "defect" are perceived negatively by people, the nanoengineering research community is trying to make use of defect structures for the deliberate modification of carbon nanomaterials, which can eventually be used in the manufacturing of nanoelectromechanical systems (NEMS). This process is sometimes called "defect-assisted engineering."
The structure and dynamics of defects in graphite and carbon nanostructures as well as the mechanisms underlying their creation and transformation remain elusive. However, this knowledge is crucial for a defect-assisted engineering of nanostructures. Radiation damage of matter is governed by the displacement of atoms from their equilibrium positions due to electronic excitations and direct collisions of high-energy particles with the nuclei. If the kinetic energy transferred from a
high-energy electron or ion to the nucleus is higher than the displacement threshold, a carbon atom can leave its initial position to form a metastable defect structure on a sub-picosecond (< 10-12 s) time scale. Such events are called knock-on displacements. Gaining control over the early stages of defect formation by tuning the irradiation conditions will make the paradigm of defect-assisted engineering feasible.
Artistic impressions of the atomic structure of graphite after irradiation. Different types of the irradiation induced defects (interstitial atoms and vacancies, intimate Frenkel pairs) are present. Defects bridging adjacent layers of carbon atoms with strong covalent bonds can be recognized. Displaced carbon atoms are shown in red. (Image: EPFL)
"Our theoretical work is a major advance in understanding the structure and formation mechanisms of these defects for the irradiation conditions close to the displacement threshold, i.e. when the energy of bombarding particles like electrons is just enough to create defects" Oleg V. Yazyev explains to Nanowerk. " In our work we use a state-of-the-art first principles molecular dynamics approach in order to systematically investigate the formation of defects which happen within the first picosecond after the initial collision of a bombarding particle with one of the carbon atoms."
In this paper, Yazyev and his collaborators do not only predict new defect structures and describe mechanisms of their formation, but also draw an important conclusion concerning the viability of defect-assisted engineering: low-energy interlayer defect structure, so-called intimate Frenkel pairs, can be created selectively and do not contribute to the structural deterioration of a material or a nanostructure.
"Many defect structures, their properties and formation mechanisms, reproduced in our work have been proposed before" says Yazyev. "However, we developed a systematic methodology to study structures and formation mechanisms which is necessary for drawing up conclusions about the possibility of creating certain defect structures selectively."
The project of the EPFL researchers was motivated by experimental work performed in the group of their colleague Professor Laszlo Forro. This group was able to achieve a 30-fold increase of the bending modulus of carbon nanotube bundles by irradiating the material with electrons of predetermined energy (80 keV). Such modification eliminates the major obstacle in the fabrication of macroscopic fibers
from nanotube bundles. Unknown covalent cross-links bridging carbon nanotubes were postulated to be responsible for the reinforcement effect ("Reinforcement of single-walled carbon nanotube bundles by intertube bridging").
"Our research elucidated their structure and predicted the range of electron beam energies at which such purposeful reinforcement can be achieved" says Yazyev.
This new work shows the crucial role played by the early stage dynamics in the defect formation process and identifies the conditions at which selective creation of defects can be achieved. The results are of practical importance for radiation-assisted manufacturing of carbon materials and nanostructures with new desired properties and functions.
Specifically, defect-assisted engineering may find an application in manufacturing NEMS based on carbon nanostructures. One of the basic ideas is to use multiwalled carbon nanotubes as bearings in such devices. Creating interlayer defects between individual carbon nanotube components one can weld them together. Since such defects (intimate Frenkel pairs) can be thermally annealed, such welding can in principle be reverted which adds an extra degree of freedom to the manufacturing processes.