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Functionalized CNT as ultrahigh resolution local probes


The small diameter and very high aspect ratio of CNT, combined with their chemical stability, structural robustness, and electrical conductivity, makes them extremely attractive as nanoscale probes of physical and biological samples. Particularly promising is the attachment to or growth of CNT on standard atomic force microscopy tips. Such CNT-functionalized probes have previously been used for topographical imaging, and, when functionalized with a ferromagnetic coating, for magnetic force imaging. Optimizing molecular beam epitaxy cobalt deposition on single walled carbon nanotubes, we have demonstrated sub-10 nm resolution and non-perturbative imaging of domains and spin nanostructures in hard and soft magnetic materials, all at ambient conditions with no special vacuum, temperature or humidity controls.

CNT tips Figure 1.
High-field electronic transport of dielectric-encapsulated MWNT probes. (a) SEM image of a dielectric-coated MWNT probe, exposed at the tip apex by focused ion beam etching, allowing electrical contact with the sample surface. The inset shows the MWNT tip prior to SiOx deposition. (b) The time trace of the current I(t), showing no measurable change in resistance at current densities of 1012 A/m2 over a time scale of 100 s. (c) Simulations of the resistive heating of dielectric-encapsulated MWNT probes show a strong variation of the temperature along the CNT, and a more uniform distribution in the transverse direction.

We have also shown that biased CNT-based probes can also be used to switch the ferroelectric polarization in epitaxial thin films. A recently developed rigidification process with a SiO2 coating by Intel allows buckling of the CNT to be avoided during this process, giving full control over sub 5 nm domains. In such rigidified tips, we have demonstrated enhanced carrying current capacity, with the dielectric coating acting as both a sink for Joule heating, and a protective barrier against oxidation. In contact with gold surfaces, these probes can carry currents of 0.12 mA at a power of 1.5 mW, and show no measurable change in resistance at current densities of 10 12 A/m2 over a time scale of 1000 s.

CNT tips Figure 2.
Ultra-high resolution achieved with Co-coated MWNT used as magnetic probes. MFM images of a high density hard drive obtained with (a) commercial MFM probes, (b)–(d) tn 5, 10 and 15 nm thick Co-coated MFM–CNT tips, respectively. All scans were 1 μm in length. In each case, the cross-sectional profile corresponds to the line indicated on each image, showing the resolution as a function of the Co thickness. (e) Zoom on the area indicated in (d), showing the intersection of several bits, and sub-10 nm resolution for the tn = 15 nm Co-coated MFM–CNT tip. The profile is a 10 line average cross section.