Growth of Single-Walled Carbon Nanotubes without a Metal Catalyst-A Surprising DiscoveryWe thank the DFG (Hi-468/17-1) and the Cluster of Excellence "Engineering of Advanced Materials" for financial support

Carbon nanotubes are characterized by outstanding and unprecedented electronic and mechanical properties. Singlewalled carbon nanotubes (SWNTs), in particular, are very attractive materials with a series of potential applications as sources for electron field emission and scanning probe microscopy, in nanoelectronic devices, and as components of electrochemical energy storage systems. They represent the ultimate carbon fiber, with the highest thermal conductivity and the highest tensile strength of any material; SWNTs have Young s moduli of around 1 TPa and are thus up to 100 times stronger than steel. In contrast to the synthesis of multiwalled carbon nanotubes (MWNTs), however, the inexpensive mass production of SWNTs in multikilogram quantities is still an elusive goal. The techniques for the productions of SWNTs can be classified roughly into three main categories: a) laser ablation, b) arc discharge, and c) chemical vapor deposition (CVD). Whereas methods a and b use graphite as the starting material, the CVD techniques are based on small precursor molecules such as methane, acetylene, alcohols, and carbon monoxide. Common to all of these techniques is the use of transition-metal catalysts, typically elements of the iron (Fe, Co, Ni, and their alloys). It was believed that the application of these carbide-forming metals is indispensable for the growth of SWNTs. However, these metal catalysts have a major drawback for both the characterization of the SWNTs as well as their application; namely, it is difficult if not impossible to remove them completely after the tube production. Owing to the chemical, redox, and magnetic properties of the metal nanoparticles, interference with the corresponding tube properties cannot be avoided. Moreover, the performance of SWNT-based materials as catalyst supports as well as in applications in molecular electronics, biology, and medicine is obscured by the presence of metal particle impurities. In many cases the catalyst particles are covered by a carbon shell, which imposes additional problems for the non-destructive purification of the SWNTs such as by treatment with non-oxidizing acids. In Very recently, two research groups have independently discovered methods for the production of SWNTs, which do not require the use of iron-group catalysts. As a consequence, the field of carbon nanotube research has made significant advances. These developments were stimulated by the fact that in the past three years substantial efforts have been invested in replacing the iron-group catalysts with other metal nanoparticles such as Au, Cu, Pd, Rh, Mg, Mn, Cr, Sn, and Al as well as semiconductor nanoparticles like Si, Ge, and SiC and Fe3C. All of these systems have been demonstrated to be active for SWNT growth, although they were originally considered to be inactive based on the traditional thinking for SWNT formation. As a consequence, new questions concerning the growth mechanism have been posed. Ren and Cheng et al. could now show that in a CVD procedure, in which a SiO2 film 30 nm thick deposited onto a Si or Si/SiO2 (1 mm thick thermally grown SiO2 layer) wafer served as substrate, the formation of dense and uniform SWNT networks was observed after a flow of CH4 and H2 at 900 8C was applied. Interestingly, almost no impurities were found by AFM, SEM, and HRTEM measurements. AFM experiments on the deposited SiO2 film further indicated that SiO2 nanoparticles with an average size of 1.9 nm formed after H2 treatment at 900 8C. The authors suggest that the presence of these SiO2 nanoparticles is crucial for the catalytic activity of these substrates (Scheme 1). In another experiment the same researchers found that it is possible to promote a patterned growth of SWNTs on a Si/SiO2 wafer at any desired position by scratching the wafer with a sharp tip. After CVD growth SWNTs were found only in the scratched areas of the