Nanotubes as Tips in Atomic Force Microscopy

Atomic force microscopy is a powerful method for surface characterization. It is based on an interaction between a tip mounted to a cantilever and the substrate. The latter is systematically scanned to obtain a three - dimensional picture of its surface (Figure 3.105). Contrary to other methods in high-resolution microscopy, the samples can be examined at ambient conditions, and even nonconducting materials do not require coating with a metallic conductor, so the effort for sample preparation is markedly reduced.

The efficiency ofAFM depends on several parameters pertaining to apparatus. Essential prerequisites for a good z-resolution include the flexibility ofthe cantilever and a sensitive detection of its movement. An accurate positioning and the sharpness of the microscopic tip, on the other hand, are substantial contributions to the x,y-resolution. Upon optimal adjustment of all parameters, the method is able to provide pictures with atomic resolution. The size and the shape of the microscopic tip constitute the limiting factor to the resolution of small objects, and especially of narrow, deep slits. Normally, tips made of silicon are used in atomic force microscopy. They are pyramidal in shape and exhibit a diameter of ca. 10 nm on their apex. These tips are unsuitable to examining deep grooves and, due to their brittleness, their mechanical resistance leaves much to be desired (Figure 3.106). Moreover, the resolution decreases when the tip breaks. Carbon

, . debundling, opening and functionalization by acid treatment (b) modification of the COOH groups

, . debundling, opening and functionalization by acid treatment (b) modification of the COOH groups

Figure 3.105 Scheme showing a CNT tip for atomic force microscopy. For chemoselective applications it might be even functionalized with suitable groups.

silicon tip i CNT-tip silicon tip i CNT-tip

Figure 3.106 One of the advantages of carbon nanotubes is their large length : diameter ratio. Thus even deeply trenched structures may be correctly imaged.

nanotubes, on the other hand, offer a number of favorable characteristics predestining them for a use as microscopic tip. Firstly, they exhibit a suitable shape due to the large proportion of length to diameter, and secondly, they feature large mechanical resistance against the bending strain effective in AFM.

Both single- or multiwalled nanotubes may be used to prepare tips for atomic force microscopy. Several examples in the literature show that in comparison to conventional tips made of silicon or silicon nitride, a better resolution can be achieved. The first report on a nanotube tip describes a multiwalled tube stuck to a pyramid of silicon and trimmed to the desired length by a current impulse. It is crucial that exactly one MWNT protrudes at the tip. The resolution attainable with this setup is about 100 nm for deep slits and ca. 10 nm in lateral direction.

Single -walled tubes are promising too as due to their smaller diameter they should allow for a further increase in resolution. The values obtained to date are about 5 nm. SWNTs feature yet another advantage: they may be grown directly on the silicon substrate after preparing the latter with a suitable catalyst. In principle, this would enable a mass production of such tips. However, it is still a problem that not only one tube is generated, and that the nanotubes do not necessarily protrude at right angles from the tip.

Moreover, the tips may be chemically modified so in addition to a high resolution, also a chemical selectivity is achieved. Thus, it is possible to visualize not only the topology, but also the chemical structure of the respective surface. Scanning, for instance, a hydroxylated surface with an AFM tip that is functionalized with an acid amide allows for detecting the distribution of hydroxyl groups on this surface (Figure 3.105b).

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