Double-walled carbon nanotubes (DWNTs) have been a key focus material of research in recent years owing to their unique electronic structure and properties. However, the incorporation of DWNTs with quantum dots (QDs) into nanocomposites via a covalent chemical approach as well as the optical properties of the composites have rarely been explored. In particular, the investigation based on CNT-QD systems, one of well-studied nanomaterial heterostructures, provides insights into a fundamental understanding of efficient charge separation within heterostructures via charge transfer relative to other relaxation pathways such as exciton recombination. In this specific work, DWNT-CdSe heterostructures were obtained by covalently conjugating 2-aminoethanethiol (AET)-modified CdSe QDs with terminal amino groups onto the surfaces of oxidized DWNTs via the formation of amide bonds. For AET-CdSe, a characteristic emission in NIR was observed due to the trap states induced by the presence of AET capping ligands. Besides, the magnitude of the trap emission was found to be associated with the concentration of AET added into systems. Interestingly, the observed trap emission is effectively quenched upon conjugation with the DWNT as a result of the charge transfer from trap states of CdSe to DWNTs. More strikingly, the time-resolved photoluminescence studies showed the exciton decay of DWNT-CdSe composites was recovered from multi-exponential to nearly mono-exponential behavior, thereby suggesting that a unique exciton dynamic occurred in the DWNT-CdSe heterostructure. More careful kinetic studies are in progress.

High power THz sources and amplifiers hold the potential to revolutionize the remote sensing and communication industries. Traveling wave tubes (TWT) are the most promising technology for compact high power amplifiers at THz frequencies. High frequency applications necessitate the need for small diameter beam tunnels and thus small diameter high current electron beams. Small diameter beams are typically achieved by emitting electrons from a thermionic cathode surface much larger than the THz TWT beam tunnel and focusing the beam using either electrostatic or magnetic field electron beam optics. To avoid using focusing optics, a micron sized high aspect ratio field emission (FE) cathode can be immersed in the confining magnetic field and used to generate an electron beam with a smaller diameter than the THz TWT beam tunnel. The key technical challenge with using a micron sized FE cathode is achieving the high current densities, long life time, and uniform current densities within the electron beam. The Air Force Research Laboratory (AFRL) began research on small diameter (<100 micron) single fiber DC cathodes for this application. Two types of fibers were tested, 1) those made from carbon with graphitized outer shells, and 2) fibers made solely from single walled carbon nanotubes (SWNT) that are densely packed and highly aligned along the axial direction of the fiber. Carbon fiber cathodes suffered serious degradation due to joule heating after only 25 hours of operation while only producing up to 200 microamps of current. A single SWNT fiber cathode has emitted approximately 3 milliamps of current for hundreds of hours while suffering minimal damage, as well as an 8x reduction in turn-on voltage over carbon fiber cathodes. Efforts are currently underway to optimize these SWNT fiber FE cathodes by tip shaping and applying low work function coatings.

Carbon nanostructures such as nanotube (CNT) and nanofiber (CNF) are the most promising materials for applications in next-generation silicon integrated circuits. Knowledge of the temperature dependence of these materials is critically important as it relates directly to circuit performance. However, in practice, it is extremely difficult to measure and control the temperature of each test device and maintain thermal equilibrium because of its small thermal capacity. Therefore, one must determine the temperature of test devices by other means. Here we report results of such a study on vertical via and horizontal CNF test devices. CNF can potentially be a replacement for copper in on-chip via interconnects [1], as well as in through-silicon-vias (TSVs) in three-dimensional chips [2,3 ]. The horizontal CNF test device can be used as a prototype of interconnect lines between adjacent transistors in the same silicon layer. In the horizontal structure, the temperature of the CNF is extracted from current stress measurements using our heat transport model [4] . For the via test device, the CNF temperature is estimated from that of the temperature-controlled measurement system [1]. In both cases, the conductivity of CNF is determined from the measured current-voltage characteristics. We find that in either case, the conductivity increases with increasing temperature as expected. However, the measured resistance of the test device in each case contains a very different contact resistance component, due to the much higher contact resistance in the horizontal structure [5]. From the conductivity versus temperature behavior, we extract the activation energy, which turns out to be about 30 meV in each case. This finding suggests that the change in conductivity in CNF, regardless of device configuration, is due to electron trapping and detrapping at defect sites within the carbon nanostructure.

[1] Q. Ngo, T. Yamada, M. Suzuki, Y. Ominami, A. M. Cassell, J. Li, M. Meyyappan, and C. Y. Yang, IEEE Trans. Nanotech, 6 (2007) 688.

[2] T. Wang, K. Jeppson, N. Olofsson, E. E. B. Campbell and J. Liu, Nanotechnology, 20 (2009) 485203.

[3] J. U. Knickerbocker, C. S. Patel, P. S. Andry, C. K. Tsang, L. P. Buchwalter, E. J. Sprogis, H. Gan, R. R. Horton, R. J. Polastre, S. L. Wright, and J. M. Cotte, IEEE J. of Solid State Circuits, 41 (2006) 1718.

[4] T. Yamada, T. Saito, D. Fabris, and C. Y. Yang, IEEE Elect. Dev. Lett., 30 (2009) 469.

Multilayered dense single-walled carbon nanotubes (SWCNTs) were deposited and aligned for fabrication of carbon nanotube field-effect transistors (CNTFETs) using the alternating electric field-directed dielectrophoresis (DEP) method. Ultrapurified high-pressure carbon monoxide (HiPCO)-grown SWCNTs were ultrasonically dispersed in n-methyl pyrrolidone (NMP) for deposition and alignment. High-performance CNTFETs with high on/off drain-source current ratios and good saturation of drain-source current were fabricated using semiconductors as the source/drain contact materials. The current-voltage (IV) electrical property of the fabricated CNTFETs was measured. The processes for alignment of multilayered dense SWCNTs and fabrication of CNTFETs and the electrical property of the fabricated CNTFETs will be reported in the conference.