Mechanical Assembly of Nanotube Tips

In the first demonstration of carbon nanotube AFM tips, Dai et al. [74] manually attached MWNTs to the pyramids of conventional tips. In this process, micromanipulators are used to control the positions of a commercial cantilever-tip assembly and nanotubes while viewing in an optical microscope. Micromanipulators allow the fabrication of nanotube tips that are well aligned for AFM imaging; that is, they are parallel to the tip axis and therefore perpendicular to the sample surface. However, the attached nanotube tips are typically too long to permit high-resolution imaging. The vibration amplitude at the end of the tip, Xtip, can be readily estimated by equating Tjk.BT with the potential energy of the fundamental bending mode. This yields where kB is Boltzmann's constant, T is the temperature in Kelvin, L is the length, E is the Young's modulus and r is the radius of the nanotube tip. The length of a nanotube tip can be decreased to reduce the amplitude of vibration to a level where it does not affect resolution by electrical etching on a conductive surface [74,75].

Mechanically assembled MWNT tips have demonstrated several important features. First, the high aspect ratio of the tips enabled more accurate images of structures with steep sidewalls such as silicon trenches [74]. Second, these studies revealed that tip-sample adhesion could be greatly reduced due to the small size and cylindrical geometry of the nanotube [74,75], which allows imaging at lower cantilever energies. Third, they clearly demonstrated the elastic buckling property of nanotubes, and thus their robustness.

However, MWNT tips were found to provide only a modest improvement in resolution compared with standard silicon tips when imaging isolated amyloid fibrils [76]. The clear route to higher resolution is to use SWNTs, since they typically have 0.5-2 nm radii. Unfortunately, bulk SWNT material consists of bundles approximately 10 nm wide containing up to hundreds of nano-tubes each, and thus cannot provide enhanced resolution unless single tubes

or small numbers of tubes are exposed at the bundle ends. Wong et al. [77,78] attached these bundles to silicon AFM tips, and adjusted their lengths by the electrical etching procedure described above for optimal imaging. This etching process was found to occasionally produce very high-resolution tips that likely resulted from the exposure of only a small number of SWNTs at the apex. It was not possible in these studies to prepare individual SWNT tips for imaging.

The mechanical assembly production method is conceptually straightforward but also has several limitations. First, it inherently leads to the selection of thick bundles of nanotubes since these are easiest to observe in the optical microscope. Recently, Nishijima et al. [79] mechanically assembled nanotube tips inside a Scanning Electron Microscope (SEM). The use of a SEM still limits assembly to nanotube bundles or individual tubes with diameters greater than 5-10 nm, and moreover, increases greatly the time required to make one tip. Second, well-defined and reproducible tip etching procedures designed to expose individual SWNTs at the tip apex do not exist. Third, a relatively long time is required to attach each nanotube to an existing cantilever. This not only inhibits carrying out the research needed to develop these tips, but also precludes mass production required for general usage.

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