Nanotube Probe Microscopy Tips

In Atomic Force Microscopy (AFM), the probe is typically a sharp pyramidal tip on a micron-scale cantilever that allows measurement of the tip-sample force interactions and surface topography. Since the resulting image is a convolution of the structure of the sample and tip, sample features are either broadened by the tip, or narrowed in the case of trenches (Fig. 19). A well-characterized tip is therefore essential for accurately interpreting an image, and moreover, the size of the tip will define the resolution of the image. Well-characterized nanostructures, such as a carbon nanotube, may prove to be the ideal AFM tip.

Integrated AFM cantilever-tip assemblies are fabricated from silicon (Si) or silicon nitride (Si3N4) [70]. The tips on these assemblies are pyramidal in shape, have cone angles of 20-30 degrees and radii of curvature of 5-10 nm (Si) or 20-60nm (Si3N4). Several techniques have been developed to improve these geometrical factors such as oxide sharpening, focused ion beam (FIB) milling, electron beam deposition of carbon, as well as improvements in the original tip formation processes. Despite these technological advances, there remain important limitations. Variation in tip-to-tip properties can be quite large, and will always be difficult to control at the scale relevant to highresolution structural imaging. In addition, tips wear during scanning [71,72], making it quite difficult to account for tip contributions to image broadening. The problems of tip wear increase for sharper tips due to the higher pressure at the tip-sample interface.

Consider the ideal AFM tip. It should have a high aspect ratio with 0° cone angle, have a radius as small as possible with well-defined and reproducible molecular structure, and be mechanically and chemically robust such that it retains its structure while imaging in air or fluid environments. Carbon nanotubes are the only known material that can satisfy all of these criteria, and thus have the potential to create ideal probes for AFM imaging. For example, nanotubes have exceptional mechanical properties. Recent calculations [73] and experimental measurements [4,5,63] of SWNT and MWNT Young's moduli yield values ranging 1-2 TPa, demonstrating that nanotubes are stiffer than any other known material. The extremely high Young's modu-

Fig. 19. Tip-sample convolution effect. The black line represents the path of the tip as it scans over the sample. The finite size of the imaging tip will broaden raised features and restrict access to recessed features lus of nanotubes is critical to the creation of high aspect ratio, sub-nanometer radius tips with high resolution- if the modulus were significantly smaller, the amplitude of thermal vibrations would degrade the resolution of tips. In addition, carbon nanotubes buckle elastically under large loads unlike conventional materials which either fracture or plastically deform. Experimental studies in which nanotubes were used as AFM tips [74] and others which measured the deflection of nanotubes pinned at one end to a surface [5] revealed that the buckling is elastic. Both types of experiments demonstrated that nanotubes can be bent close to 90° many times without observable damage, and thus should be highly robust probes for AFM imaging.

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