Dr. Virginia M. Ayres
The Electronic and Biological Nanostructures Laboratory
Research - Fundamental Radiation Interactions
Fundamental Radiation Interactions in Reduced Dimensionalities (NASA, NSF)
Significance of Research
Despite intensive research, state-of-the-art transistor materials and architectures are still problematic in high radiation environments, resulting in the need for massive shielding, highly redundant electronic systems, or a combination of these design methods. Novel electronic nanomaterials and nanocircuits are a very recent research direction that offers promising alternatives to current technologies. It has been noted by a growing community of research groups that both nanomaterials: reduced dimensionality systems in which one or more sets of energy levels are quantized due to nanoscale spatial dimensions, and nanocircuits: which incorporate nanomaterials as key functional elements, display enhanced radiation resilience. Our group investigates energetic heavy ion radiation resilience of multiphase gallium nitride (GaN) nanowires and carbon nanomaterials, and their functionality in corresponding nanocircuits. We are currently developing the first fundamental physics description of primary radiation interactions, including damage propagation and self-healing mechanisms in nanomaterials and nanocircuit architectures. This description will put the observed inherent radiation resilience of nanomaterials and nanocircuits on the solid and predicable ground essential for novel device development. If a new enabling technology for space and other high radiation environments can be developed, it may have significant national impact. PICTURE+ CAPTION THIS PARAGRAPH-BOTTOM
Fundamental Radiation Interactions in Nanomaterials
Our group leads a team of international collaborative partners from the National Superconducting Cyclotron Laboratory at Michigan State University, Howard University, Tokyo Institute of Technology, NASA Goddard Space Flight Center and NASA Jet Propulsion Laboratory. This ambitious study is the first well-calibrated fundamental energetic heavy ion interaction investigation with reduced dimensionality systems, and is varied across a full spectrum of heavy to light ion species with different charge-to-mass-ratios, including Krypton-86, Krypton-78, Calcium-48 and Oxygen-16. Utilizing the multidisciplinary expertise of our research group we are developing a complete picture of the fundamental physics of radiation coupling in reduced dimensionalities.
In Multiphase GaN Nanowires Our group reported the first investigations of the radiation response of the novel multiphase GaN nanowires discussed above . In this investigation we observed an interesting radiation coupling effect, where highly localized plumes of amorphitization were observed near the nanowire surface. This differs from the conventional semiconductor material ionization mechanism of microns-long linear tracks. No long tracks were observed in any direction within the nanowire. Also, no propagation or accumulation of defects at the zinc-blende/wurtzite interface has been observed to date, which has positive implications for device development.
In SWCNTs To date our research in single walled carbon nanotubes confirms the high radiation resiliency phenomenon reported by other groups, for high and medium-Z heavy ions with energies comparable to galactic cosmic rays. We achieved the longest cumulative energy deposition in single walled carbon nanotubes reported to date: 170,000 Grey, > 2 hours of continuous heavy ion irradiation. Slight shifts in Raman spectra have been observed and reported that provide clues to actual radiation coupling in this very resilient system. We have made the first observation of C60 formation within single wall carbon nanotubes ("peapods") induced by heavy ion irradiation rather than electron irradiation . These results provide intriguing quantitative information about possible heavy ion interaction mechanisms with nanomaterials, which we are currently working to interpret.
In Carbon Onions Carbon onions are a less known carbon nanomaterial with very promising applications in tribology, including excellent performance in vacuum conditions. As discussed above, carbon nanotubes, a closely related form of nanofunctional carbon, have demonstrated inherent radiation resiliency and have been identified as a candidate material for space applications. Therefore, carbon onions may also have high resilience to heavy ion radiation, and their tribological performance can be scrutinized for use in space applications. We found that carbon onions retain structural integrity after heavy ion irradiation, but changes from smaller spherical to larger polygonal shapes were noted. This change exactly corresponds to the shape changes observed when the growth temperature is increased from mid-range (1700-1800°C) to high (2300-2400°C) in a non-radiative situation. This phenomenon is known to correlate with a reduction in sp3 defects in the sp2 lattice, which implies possible radiation-induced annealing of the graphene lattice. An improvement in tribological properties is also observed for carbon onions grown at the higher growth temperatures. The radiation interactions may actually change the carbon onion structure in ways that enhance rather than diminish their tribological performance .
The carbon onion research is the outgrowth of Professor Ayres' collaborative research while a Chair of International Cooperation at Tokyo Institute of Technology in Japan during sabbatical leave of absence in 2005-06. This collaboration has also been furthered through a NSF International Research in Engineering and Education grant that sent two of her students to Tokyo Institute of Technology for a three-month research visit from March-June 2007. japan slide showPICTURE+ CAPTION THIS PARAGRAPH-LEFT
Fundamental Radiation Interactions in Nanocircuits
Our group also reported the first real-time characterization of GaN nanowire-based NanoFETs during irradiation by Krypton-78 heavy ions, under high bias conditions . Normal real-time and post-irradiation electronic behavior was observed using a special remote site connection (NSCL SEETF Vault) that enables us to acquire continuous live electronic readings during heavy ion irradiation of nanocircuits. These results demonstrate the tremendous potential nanowire-based devices possess for applications in high radiation environments. These investigations are continuing. PICTURE+ CAPTION THIS PARAGRAPH-RIGHTCurrent Research
The next NSCL cyclotron run featuring Oxygen-16 heavy ions is tentatively scheduled for Spring 2008!PICTURE+ CAPTION THIS PARAGRAPH-RIGHT
References for Fundamental Radiation Interactions in Reduced Dimensionalities