Conclusions

Scanning probe microscopies have contributed significantly to understanding a wide range of properties of carbon nanotubes. STM and STS have been used to characterize the atomic structure and tunneling density of states of individual SWNTs and SWNT ropes. Studies of defect-free SWNTs have demonstrated semiconducting and metallic behavior that depends predictably on helicity and diameter. STM and spectroscopy measurements have defined the 1D VHS in the DOS for both metallic and semiconducting tubes, and comparisons to tight-binding calculations have shown good agreement with ^-only calculations. Deviations from "simple" ^-only models also suggest that further work will be necessary to understand fully how tube-tube interactions, which can produce broken symmetry, and curvature effects perturb the electronic structure of SWNTs. STM has also been used to characterize local structure and electronic properties of SWNT bends and ends. These studies have shown the presence of sharp spectroscopic features that in many cases can be understood well using n-only models, although more subtle features, which may reflect electron scattering, will require more detailed experimental and theoretical focus to unravel. The characterization of electronic features at SWNT ends also has implications to understanding and developing the chemical reactivity of this material and to efficiently couple nanotubes for electron transport. In addition, STM has been used to probe the electronic properties of finite length nanotubes. These studies show that it is possible to access readily a regime of "0D" behavior-where finite length produces quantization along the tube axis. These results suggest a number of future opportunities to probe, for example, connections between extended and molecular systems.

AFM has also proven to be a valuable tool for assessing the mechanical properties of nanotubes and for manipulating nanotubes into new structures. AFM has been used to assess fundamental energetics of nanotube-surface interactions and the frictional properties as nanotubes are slid and/or rolled on surfaces. The high force sensitivity of AFM has also been exploited to assess the Young's modulus of nanotubes and to determine the bending and tensile strengths of MWNTs and SWNTs, respectively. In addition, AFM has been exploited as a tool for positioning nanotube precisely to form nanoscale electronic devices. Lastly, carbon nanotubes have been used as novel probe microscopy tips. CVD methods have been demonstrated to produce well-defined MWNT and SWNT tips in an orientation optimal for imaging. AFM studies using these nanotube tips have demonstrated their robustness and high resolution. This new generation of probes has demonstrated ultrahigh resolution and chemically sensitive imaging capabilities, and is expected to have a significant impact on nanoscale research in Biology, Chemistry and Physics. As one looks to the future, we believe that probe microscopy studies of and with nanotubes will be rewarded with answers to many fundamental scientific problems, and moreover, will push many emerging concepts in nanotechnologies.

0 0

Post a comment