J. Phys. Chem. C 2010, 114, 12925–12930
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Orthogonal Growth of Horizontally Aligned Single-Walled Carbon Nanotube Arrays Hiroki Ago,*,†,‡,§ Tetsushi Nishi,‡ Kenta Imamoto,‡ Naoki Ishigami,‡ Masaharu Tsuji,†,‡ Tatsuya Ikuta,⊥ and Koji Takahashi⊥ Institute for Materials Chemistry and Engineering, Kyushu UniVersity, Fukuoka 816-8580, Japan; Graduate School of Engineering Sciences, Kyushu UniVersity, Fukuoka 816-8580, Japan; PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan; and Graduate School of Engineering, Kyushu UniVersity, Fukuoka 819-0395, Japan ReceiVed: April 13, 2010; ReVised Manuscript ReceiVed: June 3, 2010
Direction-controlled growth of horizontally aligned single-walled carbon nanotubes (SWNTs) on r-plane sapphire substrates and their alignment mechanisms are demonstrated. On a flat r-plane substrate, anisotropic nanotube-substrate interaction is known to align SWNTs parallel to the [11j01j] direction of the sapphire. We find that the introduction of a slight miscut (-1° inclined to the [11j01j] direction) on the substrate changed the SWNT growth direction by 90°, aligning perpendicular to the [11j01j] direction. This dramatic change of the growth direction is explained by the contribution of newly proposed one-dimensional surface atomic rows and/or atomic steps appeared on the r-plane. Annealing the substrate in hydrogen atmosphere prior to SWNT growth recovers the original nanotube growth direction, while annealing in air deteriorates the alignment. The direct growth of an orthogonally aligned SWNT array is achieved through optimized surface treatment. Site-selective directional control of aligned SWNTs is also demonstrated by applying hydrogen annealing to the miscut substrate whose surface is partially covered with SiO2. Our study gives insights into the alignment mechanism on single crystal substrates and offers a new means to assemble SWNTs for advanced integrated structures. Introduction Organized architectures of nanomaterials have attracted great interest due to their fundamental importance in nanoscience and applications in nanotechnology. Single-walled carbon nanotubes (SWNTs) are one of the most important nanomaterials as a building block of such architectures because of their unique electrical, optical, mechanical, and thermal properties.1 Horizontal alignment of SWNTs on substrate surface is essential to bring out their excellent transport properties via building welldefined device structures.2 In addition, the nanotube alignment can avoid intertube hopping which deteriorates the transport in random network of nanotubes,3 thus enabling high current and high frequency operation. The development of synthetic methods of horizontally aligned arrays of SWNTs serves as a model for the integration of other nanomaterials, such as metal and metal oxide nanowires.4,5 The direct growth of horizontally aligned SWNT array offers several advantages as compared with postgrowth deposition methods utilizing solution of dispersed SWNT. The direct growth can provide aligned nanotubes which are mostly free from defects and contaminants because the dispersion process is not required in the direct growth. Also, the positions of aligned tubes are defined by patterning the metal catalyst. For the alignment of SWNTs, external forces, such as electric field6 and laminar gas flow,7,8 were used to align tubes during the chemical vapor deposition (CVD) growth, but there are limitations in the degree of alignment as well as the nanotube density. Horizontal * To whom correspondence should be addressed. E-mail: ago@ cm.kyushu-u.ac.jp. † Institute for Materials Chemistry and Engineering. ‡ Graduate School of Engineering Sciences, Kyushu University. § PRESTO-JST. ⊥ Graduate School of Engineering, Kyushu University.
alignment on single crystal sapphire (R-Al2O3)9-13 and quartz (SiO2)14-18 offers a new route to self-assemble nanostructures based on weak van der Waals force. These single crystals enable the growth of high density, highly aligned SWNT arrays. Recently, the base-growth mode (the catalyst stays at the root of the nanotube during its growth) has been experimentally observed.19,20 The observation of the base growth mode indicates that the SWNT-substrate interaction determines the SWNT orientation. This infers that surface states of substrates, such as chemical states, anisotropic atomic arrangements, and nanoscale geometry, play a crucial role in the growth of nanotubes. Mainly two models are proposed for the aligned SWNT growth on single crystal surfaces: step-templated growth9,11 and lattice-oriented growth.10-13,18 In the former, (i) enhanced van der Waals interaction due to higher contact area at the step edge and (ii) electrostatic interaction induced by uncompensated surface dipoles are proposed to align nanotubes along the step edges.9 We created the artificial step structures on r-plane sapphire and demonstrated direct growth of bent SWNTs by changing the nanotube growth direction at the step edges.21 Recently, this step-templated growth approach has been developed to SWNTs grown on SiO2/Si wafers.22-24 In the latter, lattice-oriented growth, SWNTs usually align along one unique direction determined by the atomic arrangement of the crystalline plane; the SWNT growth is guided by the presence of a one-dimensional array of surface atoms.10,12,18 We found the unique unidirectional SWNT growth on r-plane sapphire that reflects the asymmetric surface atomic arrangements in the [11j01j] direction and the opposite direction.25 Theoretical calculations suggested that attractive van der Waals force guides the nanotube growth and that there are several possible growth orientations with different interaction energies: the most energetically stable orientation matched with the experimentally
10.1021/jp1032993 2010 American Chemical Society Published on Web 06/21/2010
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J. Phys. Chem. C, Vol. 114, No. 30, 2010
Ago et al.
observed SWNT direction.26,27 If one can align SWNTs in multiple directions utilizing these substable directions, more complex and sophisticated SWNT architectures can be done on a single substrate, which widens the possible design of device structures. Here, we report that new SWNT growth direction, normal to the original growth direction, was obtained on r-plane sapphire by introducing miscut with an inclined angle of ∼1°. Pretreatment in hydrogen was found to recover the growth direction to the original direction. These two growth directions were used to realize orthogonal arrays of SWNT by single CVD. Experimental Section The flat and miscut r-plane substrates were purchased from Kyocera Co., Japan. The flat (
