Preferential Growth of Semiconducting Single-Walled Carbon Nanotubes by a Plasma Enhanced CVD Method

Single-walled carbon nanotubes (SWNT) are grown by a plasma enhanced chemical vapor deposition (PECVD) method at 600 °C. The nanotubes are of high quality as characterized by microscopy, Raman spectroscopy, and electrical transport measurements. High performance field effect transistors are obtained with the PECVD nanotubes. Interestingly, electrical characterization reveals that nearly 90% of the nanotubes are semiconductors and thus highly preferential growth of semiconducting over metallic tubes in the PECVD process. Control experiments with other nanotube materials find that HiPco nanotubes consist of ∼61% semiconductors, while laser ablation preferentially grows metallic SWNTs (∼70%). The characterization method used here should also be applicable to assessing the degree of chemical separation of metallic and semiconducting nanotubes. Single-walled carbon nanotubes (SWNTs) have been established as ballistic metallic and semiconducting molecular wires potentially useful for future high performance electronics.1-4 To realize this potential, it is necessary to achieve preferential growth of semiconducting versus metallic nanotubes or enable high degrees of separation5-8 of the two types of nanotubes. Here, we present synthesis of high quality SWNTs by a plasma enhanced CVD method at 600 °C, and an unexpected result that the PECVD method preferentially grows semiconducting nanotubes at a high percentage of ∼90%. The preferential growth has prompted us to investigate the percentages of semiconducting (sSWNT) and metallic SWNTs (m-SWNT) in materials grown by other methods, both as control experiments and to elucidate these previously unknown parameters for some of the widely used nanotube materials. We conclude that the relative abundances of semiconducting and metallic nanotubes grown by various methods are different and do not necessarily follow the 2:1 ratio expected for random chirality distribution. Highly preferential growth of a certain type of SWNT can occur depending on the growth method. The results and characterization method presented here should also have implications to chemical separation of nanotubes. A home-built radio frequency (RF, 13.56 MHz) 4-in. remote PECVD system9 was used for nanotube growth (Figure 1). The plasma discharge source consisted of a copper coil wound around the outside of the 4-in. quartz tube near the feed-gas entrance. We operated the plasma in capacitive mode with the interior furnace wall acting as an electrode and the coil acting as the counter electrode. This created a low-density plasma that propagated down the interior of the quartz tube and reached the sample placed at the center of the tube reactor, 40 cm away from the plasma coil. The * Corresponding author: [email protected]. † Department of Chemistry and Laboratory for Advanced Materials. ‡ Department of Electrical Engineering. § These authors contributed equally. Figure 1. Schematic drawing of the PECVD reactor used for the synthesis of SWNTs. NANO LETTERS 2004 Vol. 4, No. 2 317-321 10.1021/nl035097c CCC: $27.50 © 2004 American Chemical Society Published on Web 01/22/2004 sample underwent no local heating due to the plasma, as verified by a thermocouple located directly under the sampleholder. The design of the PECVD system is such that the plasma can be operated in inductively coupled mode as well, though the transition from capacitive to inductive mode occurs at far higher input power than is used for the current