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CPM Seminar
Viola Barwich
Institute of Physics Due to their remarkable properties carbon nanotubes are well suited as probing tips for Scanning Force Microscopy [1]. These properties include a small diameter and a high aspect ratio, besides their stiffness and high elastic bending limit which ensures that their original shape is resumed after stress release [2]. To prepare carbon nanotube tips we previously used a carbon adhesive to attach multiwalled carbon nanotubes bundles to microfabricated silicon Scanning Force Microscopy tips, using an optical microscope and micrometer screws. Due to the limited resolution of the optical microscope we could observe and attach only thick multiwalled carbon nanotube bundles (diameters about 50 to 80 nm) [3]. Since then we learned to grow thinner individual carbon nanotubes on top of an SFM tip by means of a CVD process [1]. In this approach, catalyst particles are first deposited onto the pyramidal tip of a microfabricated silicon cantilever, and then CVD is used to grow carbon nanotubes. The particles used were alumina enclosed by iron and molybdenum or a thin film of iron [4]. In a first step we determined the optimal conditions for growing thin and straight carbon nanotubes. Therefore we systematically studied the role of the following process parameters: catalyst, growth temperature, growth time and gas flow. The characterisation of each resulting carbon nanotubes was done by Scanning Electron Microscopy. We found that the optimal growth temperature is about 750C and the optimal growth time is about 10 min. The ratio of the flow of ethylene, hydrogen and argon is about 10: 700:1050 cm3/min. After growth, the nanotubes are shortened to less than 200 nm by electrical etching in order to reduce the lateral thermal vibration amplitude. Topographic images of ionic surfaces have been recorded with similar tips in UHV in the non-contact mode. Resonance frequency vs. distance curves reveal a reduced long-range force for the nanotube tip compared to conventional silicon tips [3]. The latest results will be presented and compared with theoretical estimates. References
[1] J. H. Hafner et al,
Nature 378, 761 (1999)
Thursday, August 2nd 2001, 15:30 |