Novel atomic force microscope cantilevers and piezoresistance of carbon nanotubes and germanium nanowires

There has been great interest in recent years in nanometer-scale materials and tools for fabricating and characterizing them. One such tool is the atomic force microscope (AFM), which has developed rapidly since its invention in 1986. Imaging soft or fragile samples with AFM requires low-spring-constant cantilevers to minimize the force on the sample. Silicon nitride is well suited to making such soft cantilevers, but it is not ideal for making sharp tips. We combined a silicon nitride cantilever with a sharp silicon tip to achieve a hybrid with both a low spring constant and a sharp tip.; Carbon nanotubes have received great attention for their remarkable mechanical and electrical properties. Their electromechanical properties are also interesting, as a few groups have determined by deforming suspended nanotubes. Their change in resistance under strain is stronger than that of silicon, which is commonly used in mechanical sensors. However, the fragility of suspended tubes makes them impractical for mass-produced sensors. Tubes on surfaces are more robust. We fabricated micromachined pressure sensors using a thin silicon nitride membrane with metal-contacted carbon nanotubes on the surface. Deforming the membrane with gas pressure stretched the nanotube, and we measured the resistance changes. They were stronger than those of suspended nanotubes, probably because of local deformations in the nanotube caused by interaction with the surface.; Nanowires of semiconductors such as silicon and germanium have also evoked much interest for electronics and optics applications. The piezoresistive effect in nanowires should also be larger than that of the bulk material, which could be useful for increasing the sensitivity of mechanical sensors or for enhancing the performance of nanowire transistors. We have fabricated pressure sensors with germanium nanowires as the sensing elements, and we have found the piezoresistance to be much stronger than in the bulk. We understand this to be partly the result of the one-dimensionality, but there may be other contributing factors. We have also measured the mechanical properties of germanium nanowires suspended across trenches by pressing on them with AFM cantilevers. We found that their behavior matched what we expected for a simple mechanical model.