Our group uses a new approach to measure the shear stress between elastically deformed nanowires and a surface. The wires are bent in a controlled way with the tip of an atomic force microscope (AFM). After the tip has passed, the wires relax to a ‘most bent state’, in which there is an equilibrium between elastic restoring forces within the wire and friction between the wire and the surface. From measurements of the radius of curvature of the bent wire, standard elasticity theory allows the maximum static friction force to be calculated if we assume the bulk value for the Young’s modulus. From the friction force a shear stress can also be determined.
We have studied InAs nanowires on Si/SiO2 and Si3N4 substrates. The wires were typically from 0.5 to a few microns long, with diameters varying between 20 and 50 nm. They were grown by chemical beam epitaxy at 430°C and transferred onto the patterned substrate by a dry deposition technique. AFM imaging of the samples is performed in tapping mode. Manipulation is done in a ‘Retrace Lift’ mode, where feedback is turned off for the reverse scan and the tip follows a nominal path. The effective manipulation force during the reverse scan can be changed by varying an offset in the height of the tip over the surface.
We have investigated friction between wires and the substrate, as well as interactions between wires and gold features patterned onto the substrates. We have also studied how nanowires deposited soon after growth oxidize in situ, allowing us to deduce their orientation on the surface.
This work is part of the PARNASS project under the EU Sixth Framework Programme.
|