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J. Phys. Chem. C 2008, 112, 18350–18354
Hole Doping to Aligned Single-Walled Carbon Nanotubes from Sapphire Substrate Induced by Heat Treatment Hiroki Ago,*,†,‡,§ Izumi Tanaka,† Masaharu Tsuji,†,‡ Ken-ichi Ikeda,‡ and Seigi Mizuno‡ Institute for Materials Chemistry and Engineering and Graduate School of Engineering Sciences, Kyushu UniVersity, Kasuga, Fukuoka 816-8580, Japan, and PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan ReceiVed: July 8, 2008; ReVised Manuscript ReceiVed: September 11, 2008
We studied the effects of heat treatment on single-walled carbon nanotubes (SWNTs) aligned on a sapphire (R-Al2O3) substrate to understand the interaction between the SWNTs and sapphire. The Raman measurements showed a clear upshift of the G-band after heat treatment at 1000 °C in a high vacuum. Furthermore, Auger spectroscopy showed an increase of the [Al]/[O] atomic ratio of the sapphire surface upon heat treatment, indicating the removal of oxygen atoms from the sapphire surface. The observed upshift of the G-band is accounted for by the hole doping to the aligned SWNTs from the oxygen-deficient sapphire substrate. This annealing-induced carrier doping from the underlying substrate would offer a new and unique approach to modify the electronic structure of SWNTs. Introduction Single-walled carbon nanotubes (SWNTs) have unique electronic properties originating in their one-dimensional structure rolling up of a single layer of graphene. Because of their well-defined π-conjugated network and low dimensionality, SWNTs show high carrier mobility, high current density, and mechanical flexibility, offering a strong potential for nanoelectronics applications.1 Recent progress on horizontally aligned SWNT growth on single-crystal sapphire (R-Al2O3)2-5 and quartz (SiO2)6 has stimulated these electronic applications, such as field-effect transistors (FETs)7 and high-frequency transistors, because the aligned array greatly improved device productivity and enabled high current operation. There are mainly two mechanisms of the aligned growth of SWNTs: step-templated growth2,6,8 and lattice-oriented growth.3-5,8 In the latter, SWNTs are oriented to specific crystallographic axes of sapphire, and their growth direction is predetermined by the surface structure so that we call it “atomic arrangement programmed (AAP) growth”.3,4 This AAP growth is mainly explained by the anisotropic van der Waals interaction between a SWNT and the crystal surface, which might be related to the anisotropic charge distribution or the local dipole moment on the sapphire surfaces consisting of cationic Al and anionic O atoms. The observation of the base-growth mode supports the importance of the SWNT-sapphire interaction rather than the catalystsapphire interaction,9 but a detailed mechanism of the aligned growth is still unclear. The heat treatment on the powdered SWNTs or double-walled carbon nanotubes (DWNTs) in a vacuum increases the diameters of nanotubes at temperatures higher than 1000 °C.10-12 This is accounted for by the heat-induced coalescence of carbon nanotubes: a nanotube in a bundle is deformed at high temperature, forming chemical bonds with a neighboring nanotube, finally leading to a fused large-diameter nanotube.11,12 * To whom correspondence should be addressed. E-mail: ago@ cm.kyushu-u.ac.jp. † Institute for Materials Chemistry and Engineering, Kyushu University. ‡ Graduate School of Engineering Sciences, Kyushu University. § PRESTOsJST.
In the case of the AAP-grown SWNTs, the nanotubes are mostly isolated and the full length of the nanotube adheres to the sapphire surface. Therefore, it is interesting to study effects of the heat treatment of the aligned SWNTs in relation to the interaction with the underlying substrate surface. In this paper, we heat treated a SWNT array aligned on a sapphire substrate in a high vacuum and studied their geometric and electronic changes by means of a scanning electron microscope (SEM) and Raman spectroscopic measurements. The annealing at 800 and 1000 °C shifted the tangential vibrational mode, G-band, of the SWNTs to higher frequencies. Based on the Auger measurements and the observed oxidation behavior of the nanotubes, we propose that heat treatment induced the p-type doping to the aligned SWNTs from the sapphire substrate. This is the first demonstration of substantial carrier doping from a substrate to SWNTs without applying voltage and dopant molecules. Experimental Section The r-plane sapphire substrates (11j02) with a miscut angle of below 0.3° were purchased from Kyocera Co., Japan. The sapphire substrate was spin-coated with a photoresist, followed by ultraviolet exposure through a photomask for the catalyst patterning. A thin film of Fe metal catalyst with thickness of
