Atomic nanotube tips

Schnitzler, G.R., Cheung, C.L., Hafner, J.H., Saurin, A.J., Kingston, R.E., and Lieber, C.M. (2001) Direct imaging of human SWI/SNF-remodeled mono- and polynucleosomes by atomic force microscopy employing carbon nanotube tips. Mol. Cell. Biol. 21, 8504-8511. 

Chemical and Genetic Probes—Nanotube-tipped atomic force microscopes can trace a strand of DNA and identify chemical markers that reveal DNA fine structure. A miniaturized sensor has been constructed based on coupling the electronic properties of nanotubes with the specific recognition properties of immobilized biomolecules by attaching organic molecules handles to these tubular nanostructures. In one study, the pi-electron network on the CNT is used to anchor a molecule that irreversibly adsorbs to the surface of the SWNT. The anchored molecules have a tail to which proteins, or a variety of other... 

Wilson NR, Macpherson JV (2009) Carbon nanotube tips for atomic force microscopy. Nat Nanotechnol 4 483 91... 

Atomic Force Microscopy Atomic force microscopy is a direct descendant of STM and was first described in 1986 [254], The basic principle behind AFM is straightforward. An atomically sharp tip extending down from the end of a cantilever is scanned over the sample surface using a piezoelectric scanner. Built-in feedback mechanisms enable the tip to be maintained above the sample surface either at constant force (which allows height information to be obtained) or at constant height (to enable force information to be obtained). The detection system is usually optical whereby the upper surface of the cantilever is reflective, upon which a laser is focused which then reflects off into a dual-element photodiode, according to the motion of the cantilever as the tip is scanned across the sample surface. The tip is usually constructed from silicon or silicon nitride, and more recently carbon nanotubes have been used as very effective and highly sensitive tips. 

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

Many properties of nanotubes are highly dependent on the conditions given. While this may pose problems in the characterization, it can still be employed to study those very environmetal conditions. The nanotubes are thus used as probe or sensor. One of these applications, namely that as a tip for atomic force microscopy, has already been described in Section 3.6.1.1 as after all, these nanotube tips, too, act as sensors that detect variations of the surface structure on a substrate. The sensor applications hitherto presented in the hterature may be divided into two groups physical sensors on the one hand, and chemical ones on the other. These differ in the kind of properties that they serve to examine. 

U2. Umemura, K., Komatsu, J., Uchihashi, T., et al. Atomic force microscopy of RecA-DNA complexes using a carbon nanotube tip. Biochem. Biophys. Res. Comm. 281, 390-395 (2001). 

In 1995, FE from a multiwalled CNT (MWCNT) film was reported by De Heer et al. [14], and the FE property of an isolated MWCNT was reported by Rinzler et al. [15]. FE of electrons from individually mounted CNTs was found to be dramatically increased when the nanotube tips were opened by laser evaporation or oxidative etching. Emission currents of 0.1-1 (xA were readily obtained at room temperature with bias voltages of less than 80 V. Dai et al. [16] attached individual nanotubes with a length of several micrometers to the silicon cantilever of a conventional atomic force microscope (AFM) using a soft acrylic adhesive. Because of their flexibility, the tips are resistant to damage from tip crashes, whereas their slenderness permits the imaging of sharp recesses in surface topography. 

Fig. I. Field emission dala from a mounted nanotube. An activated nanotube emits a higher current when heated by the laser than when the laser beam is bloeked (a). When aetivated by exposing the nanotube to oxygen while heating the tip, this behavior is reversed, and the emission current increases dramatically when the laser is blocked. The activated state can also be achieved by laser heating while maintaining a bias voltage of —75 V. Note that the scale of the two plots is different the activated current is always higher than the inactivated current. As discussed in the text, these dala led to the conclusion that the emitting feature is a chain of carbon atoms pulled from a single layer of the nanotube —an atomic wire.

Fig. 2. A graphic of a nanotube showing a pulled-out atomic wire and several stabilizing spot-welds. Only two layers have been shown for clarity, although typical multiwalled nanotubes have I0-I5 layers. The spot-weld adatoms shown between layers stabilize the open tip conformation against closure. The atomic wire shown was previously part of the hexagonal lattice of the inner layer. It is prevented from pulling out further by the spot-weld at its base.

Nanotubes are being used as points in some SPM units. The ends of these nanotubes can be closed or functionalized offering even finer tips and tips that interact with specific sites allowing manipulation on an atom-by-atom basis. These nanotubes are typically smaller than silicon tips and are generally more robust. 

Figure 7 Examples of nanotribology on dry carbon surfaces for atomic force microscopy (AFM) (a) schematic description of the out-of-plane graphene deformation with the sliding AFM (Lee et al., 2010), (b) nanotube without tip (left) and tip-nanotube interaction under 2.5 nN normal force (right) (Lucas et al., 2009), (c) stick-slip rolling model with a step rotation of a C60 molecule (Miura et al., 2003), and (d) ballistic sliding of gold nanocluster on graphite (Schirmeisen, 2010).

C.L. Cheung, J.H. Hafner, C.M. Lieber, Carbon Nanotube Atomic Force Microscopy Tips Direct Growth by Chemical Vapor Deposition and Application to High-Resolution Imaging , Proc. Natl. Acad. Sci. USA, 97, 3809 (2000)... 

Field emission from carbon nanotubes may be more complex than implied by the analysis above since, in contrast to metal tips, the Fermi wavelengths are comparable to the tip size and consequently electronic states exist, which extend over the entire tip [152,168]. Emission is therefore not from individual atoms but from these tip states such that the emission is coherent. The field emission patterns from individual MWNTs in fact carry the signature of coherent emission from the tip states [168] (see Fig. 36). SWNTs have been imaged as well and the patterns correspond with calculated nanotube charge densities [152]. 

Fullerene C(,o adsorbed onto STM tips has been reported to enhance atomic resolution images of highly oriented pyrolytic graphite [110]. Recently, Dai et al. [Ill] have demonstrated that multiwalled carbon nanotubes (MWNTs) attached to the silicon cantilever of a conventional atomic force microscope (AFM) can be used as well-defined tips with exceptionally high resistance to damage from tip crashes. The MWNT were attached by first coating the bottom 1 -2 mm section of the silicon tip with an acrylic adhesive by inserting them... 

---