| The low temperature conductance measurements of Carbon Nanotubes (CNT), and other nanostructures, are commonly conducted on samples with fixed leads, which provide the global information about the nanostructure's electronic properties. To measure local electronic properties, we have built a conductive tip Atomic Force Microscope (AFM). This AFM allows us to place a mobile contact anywhere along the tube and carry out local transport investigation, including measurements taken while we radially deform the nanotube with the tip.;Our low temperature conductive tip AFM is suitable for operation in a helium refrigerator with a bore 1.25" or more, at temperatures of 4K or less. It uses a thermally compensated, nonmagnetic design based on the "Besocke Beetle". All movements are achieved by piezotubes, so heat load at low temperatures is small. For tip-surface distance control, we use a homemade frequency detection system based on a commercially available quartz tuning fork. The scanning tip is mounted on the tuning fork. We use a platinum/iridium tip rather than the more commonly used tungsten, as it does not form a native oxide. The tip is independently wired which allows us to measure the current flow along the tube. Previous low temperature AFMs used the conductive tip only to gate the nanotube. Our samples are CVD grown carbon nanotubes upon a silicon dioxide substrate, contacted by a palladium/gold grid.;With our AFM, we have carried out the first low temperature scanning of conductance along nanotubes on insulating substrates. While radially deforming a metallic carbon nanotube with our tip, we have observed a reversible gap opening, indicating a metallic to semi-conducting transition. This result qualitatively matches theoretical predictions. We have observed Coulomb blockade in transport between carbon nanotubes and the tip by AFM for the first time. By gently perturbing the nanotube with the tip, we have increased the strength of some single electron conductance peaks while diminishing others. This result is probably due the perturbation of the probability density functions inside the nanotube. This novel instrument will allow us to study the local properties of a broad class of conductive nanostructures positioned on insulating substrates. |
