Resolution of Nanotube Probes

The high-resolution capabilities of nanotube tips have been evaluated using 5.7 nm diameter gold nanoparticle imaging standards. These are good standards due to their well-defined shape and size, and their incompressibility [83]. The effective tip radius can be calculated from particle images using a two-sphere model [84]. Mechanically assembled MWNT tips typically have shown radii as small as 6 nm, which is expected for the size of arc-produced MWNT material [76]. Manually assembled SWNT tips are composed of thick SWNT bundles made from 1.4 nm nanotubes. The etching procedure can produce high resolution on gold nanoparticles, down to 3.4 nm radius [76,77], although more often the resolution is lower. Pore-growth CVD MWNT have radii ranging 3-6 nm when measured from gold nanoparticle images [68], which is better resolution than manually assembled tips because they are relatively thin and always consist of individual nanotubes. Both the pore-growth [82] and surface growth [69,67] SWNT bundles have shown tip radii of less than 4nm, reflecting the very thin SWNT bundle or individual SWNT tip structures possible with these methods. These results are summarized in Table 1.

Table 1. Radius of curvature for differently prepared nanotube tips

Nanotube Type

Tip Radius

Mech. Assembled MWNT

> 6nm

Mech. Assembled SWNT

> 3.5 nm

Pore Growth MWNT

3.5-6 nm

Pore Growth SWNT

2-4 nm

Surface Growth SWNT

2-4 nm

Low Density Surface Growth

< 2nm

Another significant point regarding nanotube tips is that the range of tip radii measured by AFM has agreed with the range measured by TEM. These results demonstrate that the imaging has been carried out with well-defined probes, which should be amenable to tip deconvolution techniques. Lastly, the levels of resolution achieved with the nanotubes obtained on gold nanoparticles nearly 6 nm in height, rather than on 1-2 nm features which at times indicate high resolution with tenuous asperities on microfabricated Si and Si3N4 tips.

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