DIRECT GROWTH OF SINGLE-WALLED CARBON NANOTUBE SCANNING PROBE MICROSCOPY TIPS

Herein, we report the first growth of single-walled carbon nanotubes (SWNTs) directly from conventional atomic force microscopy (AFM) cantilever assemblies to create very highresolution tips for scanning probe microscopies. Metal catalysts were deposited onto the pyramids of microfabricated AFM tips and chemical vapor deposition (CVD) was used to synthesize carbon nanotubes. Scanning and transmission electron microscopies show that nanotubes grow along the surface and reproducibly protrude from the tip apex in the optimal orientation for imaging, and that single SWNT, small SWNT bundle, or very small diameter multi-walled nanotube (MWNT) tips can be controllably made. AFM measurements demonstrate that these new nanotube tips are mechanically robust and can image with very high resolution. We believe that this straightforward growth method will make nanotube tips broadly accessible, and moreover, that the molecular size diameters of these SWNT tips creates unique opportunities for imaging in chemistry and biology. Carbon nanotubes are ideal structures for the tips used in scanning probe microscopies, such as AFM, since they (i) have intrinsically small diameters, which are comparable to molecules in the case of SWNTs, (ii) have high aspect ratios, (iii) can buckle elastically, and (iv) can be selectively modified at their ends with organic and biological species to create functional probes.1-6 Mechanical methods have been used to attach nanotube bundles in the fabrication of tips, although we believe that this timeconsuming approach has limited the development of nanotube tips.1-6 To overcome these limitations we have been exploring the direct catalytic growth of nanotubes from conventional tips and recently showed that individual MWNTs could be grown by CVD from the ends of Si tips with controlled orientation.7 In this first example of the direct growth of nanotube probes, we utilized selective etching of commercial tips to create nanopores, deposited catalyst within these pores, and used the pores to help orient the nanotubes in an ideal direction for imaging.7 Our present work extends this initial demonstration in two important ways: we show that (1) the pore etching step can be eliminated by using “surface growth” and (2) SWNT tips can be readily prepared by controlling the CVD growth conditions. Our overall strategy for the growth of SWNT tips is outlined in Figure 1. In this approach, catalyst is first deposited onto the pyramidal tip of a commercial cantilever assembly, and then CVD is used to grow the SWNT probe. The basis for this approach is our observation that SWNTs and small diameter MWNTs prefer to grow along a surface (due to the attractive nanotube-surface interaction8), and therefore, will generally bend to stay in contact rather than grow out from the surface when they encounter an edge. Nanotubes prepared from catalyst deposited on a pyramidal AFM tip will grow along the surfaces until they reach the pyramid edges, then some will be directed toward the tip apex along the edges. At the pyramid end the nanotubes will protrude straight from the apex (vs bending) to create an ideal tip, because the strain energy cost of bending the nanotube is not compensated by nanotube-surface interactions. We find that this approach is extremely robust and works readily with a wide range of catalysts. Well-defined SWNT tips are formed reproducibly after CVD growth with ethylene using electrophoretically deposited supported Fe-Mo9 and colloidal Fe-oxide10 catalysts.11,12 Representative electron microscopy images of a nanotube tip produced from the supported Fe-Mo catalyst after 3 min growth in 1:200:300 C2H4:H2:Ar at 800 °C are shown in Figure 2.12 These conditions were specifically chosen to favor the growth of SWNTs and very small diameter MWNTs (<10 nm), and it should be noted that well-defined changes in the ratio of C2H4:H2:Ar can be used to tune nanotube tips from SWNTs to large MWNTs.9 Fieldemission scanning electron microscopy (FE-SEM) images demonstrate that nanotube tips prepared in this way protrude from