Carbon nanotube atomic force microscopy tips: direct growth by chemical vapor deposition and application to high-resolution imaging

Abstract

Carbon nanotubes are potentially ideal atomic force microscopy probes because they can have diameters as small as one nanometer, have robust mechanical properties, and can be specifically functionalized with chemical and biological probes at the tip ends. This communication describes methods for the direct growth of carbon nanotube tips by chemical vapor deposition (CVD) using ethylene and iron catalysts deposited on commercial silicon-cantilever-tip assemblies. Scanning electron microscopy and transmission electron microscopy measurements demonstrate that multiwalled nanotube and single-walled nanotube tips can be grown by predictable variations in the CVD growth conditions. Force-displacement measurements made on the tips show that they buckle elastically and have very small (</= 100 pN) nonspecific adhesion on mica surfaces in air. Analysis of images recorded on gold nanoparticle standards shows that these multi- and single-walled carbon nanotube tips have radii of curvature of 3-6 and 2-4 nm, respectively. Moreover, the nanotube tip radii determined from the nanoparticle images are consistent with those determined directly by transmission electron microscopy imaging of the nanotube ends. These molecular-scale CVD nanotube probes have been used to image isolated IgG and GroES proteins at high-resolution.

Figure 1

Schematic of the CVD nanotube tip preparation approach.

Figure 2

Electron microscopy of CVD nanotube tips. (a) FE-SEM image of MWNT tubes grown from a Si cantilever/tip assembly. (b) TEM of a CVD MWNT tip. (c) Field emission/SEM image of SWNT bundles grown from a Si cantilever/tip assembly. (d) TEM of several CVD SWNTs comprising a bundle tip.

Figure 3

Force calibration plots for a CVD nanotube tip. (a) Tip amplitude oscillation vs. height above the sample. (b) Tip deflection vs. height above the sample. Regions A and B highlight the pre- and postbuckling areas.

Figure 4

IgG imaged by a CVD nanotube tip. Large scan area in a shows several molecules that exhibit the characteristic Y shape. (b) High resolution images of IgG reveal very little tip-induced broadening when compared with the crystal structure in c. [Bars = 50 nm (a), 20 nm (Inset), 10 nm (b), and 10 nm (c).]

Figure 5

GroES imaged by a CVD nanotube tip. Large scan area in a shows both “dome” and “pore” conformations, representing the two sides of GroES facing up. (b) A higher resolution image of the pore side shows the heptameric symmetry, which matches well with the crystal structure (c).