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Molecular Electronics Local Probes of Nanoscale Systems Chemical and Biological Sensing Nanotube and Nanowire Electronics


Local Probes of Nanoscale Systems

    We use our Omicron-Zeiss Beetle STM/AFM for surface studies of nanotubes. Single Wall Nanotubes can be successfully imaged with both, atomic force and tunneling current feedback. In STM-Mode, we are also able to probe their local electronic density of states. In any case, careful sample preparation is crucial to obtain good imaging quality.
     Single nanotubes as well as ropes containing up to 100 tubes can be found dispersed over the substrate. In our STM-experiments, ropes are stable for many subsequent scans even with low tunneling resistances. Individual tubes can be distinguished within the ropes and their atomic structure can repeatedly be imaged with good tips and some care in vibration damping (we use Pt-Ir tips from DI). Single tubes tend to be less stable during scanning than ropes, since they are often pushed away by the imaging tip. They can be more easily imaged in AFM-(Tapping)-Mode where the tip sample distance is usually larger than in STM-Mode.
    Our STM studies reveal, that some tubes imbedded in ropes exhibit a distortion in their hexagonal lattice (article in PDF format). The angle between the armchair direction and the zigzag direction is no longer 90, but differs by some degrees from that value. A possible explanation for this phenomenon is a twist distortion. If a tube is twisted, the direction perpendicular to the tube axis stays the same, while the other directions are altered. Twists might be introduced into tubes in ropes by their neighbors during the growth process. They could have a strong impact on the electronic properties of intrinsically metallic tubes. (E.g. introducing isolator-like resistivity at low temperature). In this field we are working closely with our Theory Group.
    Our SPM is also capable of working in an AFM-Mode with a Needle-Sensor put in place of the STM tip. the Needle-Sensor consists of a quartz crystal that is driven at its resonant frequency of ~1MHz. Conventional AFM tips (with their cantilevers) are glued to the end of the crystal. We regulate by detecting the phase change of the oscillation rather than an amplitude change. AFM images of nanotubes obtained by Wilfried Clauss and David Bergeron reveal the fantastic resolution in AFM-Mode that can be obtained with our system.



Page last modified on November 5, 2005