Vectorial Growth of Metallic and Semiconducting Single-Wall Carbon Nanotubes

A new approach for vectorial growth of single-wall carbon nanotube arrays is presented. The origin of growth is defined by patterning the catalyst nanoparticles, while the direction of growth is defined by a local electric field parallel to the substrate. Statistical analysis of the nanotube angular distribution indicates that field-directed growth can discriminate between metallic and semiconducting nanotubes during their formation. Vectorial growth could be used to produce nanotube-based circuitry for molecular electronics. Carbon nanotubes1 exhibit outstanding structural, mechanical, and electronic properties, which make them promising building blocks for molecular electronics.2,3 Recently, we have presented a new approach for the realization of a highly integrated, ultrafast, nonvolatile random access memory for molecular computing based on carbon nanotubes, where the nanotubes act as both interconnect wires and functional devices,4 and other groups have been exploring nanotube field-effect transistors as components for nanoelectronics.5-7 To achieve the integration of large arrays of such devices, however, two critical issues must be effectively addressed. First, to produce large arrays of SWNTs on surfaces, it is necessary to develop reliable synthetic methods for the production of SWNTs in defined locations and directions. Several approaches have been reported to address this issue. SWNTs have been selectively deposited from liquid suspensions on chemically functionalized patterns,8 or using microfluidics and electric fields.9 The success of this approach for the assembly of isolated SWNTs has been limited due to the tendency of SWNTs to aggregate into ropes and tangles. Suspended nanotube networks have been produced by chemical vapor deposition (CVD) from microfabricated pillars,10-12 where directionality was attributed to a mechanism of selective pinning of the growing nanotubes on the tops of nearest neighbor pillars. Recently, directional growth of suspended SWNTs was further enhanced using electric fields.13 SWNTs have also been found to grow on welldefined silicon surfaces in four or six possible directions that are defined by the lattice of Si(100) or Si(111) substrates, respectively.14 A second critical issue for the application of carbon nanotubes as building blocks in molecular electronics is the need to organize selectively metallic or semiconducting nanotubes, since semiconductors and metals play very different roles in electronic circuits and devices. SWNTs can be either metallic or semiconducting, depending on both their diameter and chirality. Assuming a random distribution of nanotube chiralities, one-third of the nanotubes should be metallic and two-thirds should be semiconducting. Unfortunately, no methods are yet available for the production of (or separation of heterogeneous mixtures to yield) relatively pure samples consisting of either metallic or semiconducting carbon nanotubes. Recently, a method for the selective destruction of metallic SWNTs from mixed ropes has been achieved using electrical breakdown.5 Although these developments are certainly significant, both the production of ordered SWNT arrays on surfaces and the selection of metallic vs semiconducting SWNTs remain two major obstacles toward nanotube-based electronic technology. Vectorial growth of SWNTs is a new approach whereby the growth of these one-dimensional nanostructures is defined in the form of a vector, i.e., having an origin (x,y), a direction (φ), and a length (L), as represented in Figure 1a. Ideally, one would also like to gain control over the diameter (d) and the chirality (θ) of the SWNTs, which determine their electronic type, i.e., metallic or semiconducting. Our working * Corresponding authors. E-mail: [email protected]; [email protected]. NANO LETTERS 2002 Vol. 2, No. 1