Controlled growth and applications of carbon nanotube tips for scanning probe microscopy

• Main Author: Yenilmez, Erhan
• Format: Dissertation
• Language: English
• Subjects: chemistry, physical; physics, condensed matter
• ISBN: 9780542571589; 0542571587

Carbon nanotubes that are cylindrical structures of carbon with a few nanometers in width and micrometers in length have been introduced as ultra sharp and long lasting probes for scanning probe microscopy (SPM) compared to commonly used sharp crystal probes etched on silicon.
We investigate a process where we can grow carbon nanotube tips on a wafer of crystal silicon tips using chemical vapor deposition (CVD). We would like to have one individual nanotube protruding from the apex and pointing within a narrow solid angle along the axis of each silicon tip. We identify the key elements to reach this yield as a 10nm layer of silicon oxide on the surface of the cantilever, a nominally 2 Angstroms thick film of cobalt catalyst and a reduction of the catalyst before CVD. We use ethanol as a carbon feedstock gas for CVD. This wafer scale process has reproducibly been shown to work with over 90% yield. There is still room for improvement of the orientation of the grown nanotube tips.
The force calibration mode of an atomic force microscope is used to controllably bend and buckle an individual nanotube between the probe cantilever and the surface. An external voltage is applied to cut the end of the mechanically deformed nanotube. This shortening process enables us to make nanotube tips with desired length especially in the 100nm to 500nm range.
We introduce magnetic force microscopy (MFM) capability for carbon nanotube tips by coating them with cobalt. Our metal-coated nanotube tips confines the magnetic material at the tip in a cylindrical volume, which increases the resolution compared to commercially available MFM tips that are pyramid shaped silicon tips coated with a magnetic thin film. We have imaged features as small as 20nm are imaged using these tips on an experimental magnetic recording media.
We make high aspect ratio tips by coating micrometers long as grown nanotube tips with metal and aligning them to a desired direction using focused ion beam. We perform high-aspect ratio imaging using these tips.