J. Phys. Chem. C 2009, 113, 13121–13124
13121
Direct Growth of Bent Carbon Nanotubes on Surface Engineered Sapphire Hiroki Ago,*,†,‡,§ Kenta Imamoto,‡ Tetsushi Nishi,† Masaharu Tsuji,†,‡ Tatsuya Ikuta,⊥ Koji Takahashi,⊥ and Munetoshi Fukui# Institute for Materials Chemistry and Engineering, Graduate School of Engineering Sciences, Kyushu UniVersity, Fukuoka 816-8580, Japan, PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan, Graduate School of Engineering, Kyushu UniVersity, Fukuoka 819-0395, Japan, and Hitachi High-Technologies Corporation, Ibaraki 312-0057, Japan ReceiVed: March 18, 2009; ReVised Manuscript ReceiVed: May 24, 2009
Bending of horizontally aligned single-walled carbon nanotubes (SWNTs) was achieved on surface engineered single-crystal sapphire (R-Al2O3). The SWNTs grown on the r-plane sapphire are aligned along the specific crystallographic [11j01j] direction due to the lattice-oriented growth, and we created artificial step structures perpendicular to this SWNT growth direction. These steps changed the nanotube growth direction from the [11j01j] to the step direction with the bending angle of nearly 90°. Effects of the bending structure on electron transport property were studied. Our approach to combine the lattice-oriented growth with the step-templated growth will offer a new route toward the growth of two-dimensionally controlled SWNT architectures for future nanoelectronics. Introduction Single-walled carbon nanotubes (SWNTs) have promising properties for future nanoelectronic applications, such as high carrier mobility, high current-carrying capacity, mechanical flexibility, and thermal stability.1-3 Integration of SWNTs on a substrate through the control of their direction and position is of great importance together with the controlling chirality and/ or metallic-semiconducting property. Previously, the horizontally aligned SWNT growth was achieved on single-crystal substrates, such as sapphire (R-Al2O3),4-8 quartz (SiO2),9-11 and MgO.12 These single crystal surfaces realized the growth of highly aligned SWNT array with a high density. On the single-crystal substrates, two different mechanisms, lattice-oriented growth4,7 and step-templated growth,6 have been proposed for the observed nanotube alignment. Using a-plane sapphire substrates with controlled miscut angle and direction, it was revealed that the SWNT orientation strongly depends on the step height; the relatively high steps gave the step-templated nanotubes, while the low, single atomic, steps (∼0.3 nm) gave the lattice-oriented nanotubes.13 This finding implies a possibility of controlling the SWNT growth direction uniquely by combining these growth modes. Growth of a SWNT serpentine with multiple round turn structure was demonstrated by utilizing both the step-templated growth and gas-flow induced alignment, but density of the serpentines was relatively low, and the bending position was difficult to control.14 In the transfer process of aligned SWNTs using a polymer film, long SWNTs were bent by folding the transferring polymer matrix.15 This method is only applicable to very long SWNTs,15 and direct growth of bent SWNTs is desired for microscale integration applicable to electronic applications. * 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. § PRESTO-JST. ⊥ Graduate School of Engineering, Kyushu University. # Hitachi High-Technologies Corporation.
Here, we report the direct growth of bent SWNTs on the sapphire substrate with artificial step structures. At the plain sapphire r-plane (11j02), a SWNT was aligned along the specific crystallographic [11j01j] orientation, but when the SWNT reached a step, it changed its growth direction and aligned along the step edge. This method, in principle, enables the directional control at the position defined by photolithography and wetetching. The transport property of the bent SWNT was studied by a nanoprobing system inside the scanning electron microscope (SEM). Experimental Section The r-plane substrates with a miscut angle of less than 0.3° were purchased from Kyocera Co., Japan. First, the sapphire surface was covered with a thin film of amorphous SiO2 by plasma-enhanced chemical vapor deposition (CVD) using tetraethoxysilane (Si(OC2H5)4). Photolithography was performed on the SiO2 surface to leave stripe patterns of photoresist. Then, the exposed SiO2 surface was etched with buffered hydrofluoric acid (BHF) followed by removal of the photoresist by oxygen plasma treatment. By use of the patterned SiO2 film as a mask, the exposed sapphire surface was etched by immersing the substrate into hot solution of sulfuric acid (H2SO4) and phosphoric acid (H3PO4).16 After thoroughly washing with water, the remaining SiO2 film was removed by BHF. The depth of the etching pattern was measured by atomic force microscopy (AFM, Veeco Nanoscope IIIa). Either size-controlled colloidal iron oxide nanoparticles with 4 nm diameter4 or alcohol solution of iron-molybdenum salts (Fe(NO3)3 · 9H2O and MoO2(acac)2)17 were deposited on the etched substrate to use as catalyst. SWNTs were grown by thermal CVD with methane (CH4) and hydrogen (H2) gases at 900 °C for 10 min, and the resulting nanotubes were characterized by SEM (HITACHI S-4800). Transport properties were measured with a nanoprobing SEM system (HITACHI N-6000) having six piezo-controlled tungsten probes. Results and Discussion As-received, plain r-plane sapphire gives horizontally aligned SWNTs grown along the [11j01j] direction due to the anisotropic
10.1021/jp902409w CCC: $40.75 2009 American Chemical Society Published on Web 07/01/2009
13122
J. Phys. Chem. C, Vol. 113, No. 30, 2009
Figure 1. Schematic illustration of the fabrication process of bent SWNTs on the surface engineered r-plane sapphire. (1) Preparation of stripe pattern of photoresist on a SiO2/sapphire. The SiO2 layer was predeposited by the plasma-enhanced CVD. (2) Removal of SiO2 layer by BHF, followed by photoresist lifting-off. (3) Wet etching of the exposed sapphire surface using SiO2 as a mask. (4) Deposition of the catalyst and growth of bent SWNTs by CVD.
atomic arrangement of the sapphire surface.4 We fabricated stripe patterns of artificial steps perpendicular to this [11j01j] direction based on the processes shown in Figure 1. The sapphire surface covered with the patterned SiO2 mask was etched by soaking in the mixed solution of H2SO4 and H3PO4,16 and the step height (i.e., etching depth) was controlled by the etching time and temperature. The etching rates determined for the r-plane sapphire at 280 and 300 °C were 17 and 58 nm/min, respectively (Supporting Information). We mainly used the etching temperature of 280 °C, because of the higher controllability due to the slower etching rate. It should be noted that the artificial steps are not identical to the atomic steps usually observed on single-crystal substrates with a certain miscut angle.13 Because the artificial steps were shallow, it was difficult to observe detailed structure of the step edge by AFM and SEM measurements. Figure 2 shows the SEM images of the SWNTs grown on the sapphire substrates with patterned artificial steps. The bright white lines represent SWNTs as already confirmed for the present CVD condition.5,17 In Figure 2a, one can see that the most of SWNTs were aligned along the [11j01j] direction, but some SWNTs (marked 1, 2, 3) were bent at the step edges with the angle of nearly 90 degrees. The magnified image (Figure 2b) clearly indicates that two SWNTs were bent and changed their growth direction when they encountered the artificial step edges. This indicates that the SWNT growth direction was modified by the artificial patterns created on the sapphire surface, through the combination of lattice-oriented growth and steptemplated growth. The advantage of our method is that we can define the bending position by top-down lithographic approach. This is in contrast to the SWNT serpentine formed on a stepped quartz substrate, on which the bending of SWNTs occurred randomly.14 However, several SWNTs were found to terminate their growth when they reached the step edge (marked 4 and 5 in Figure 2a). This is understood by both the base-growth and tip-growth modes. In the former, the pushing force provided from the catalyst particle may not be strong enough to mechanically bend the nanotubes to align along the step edge.
Ago et al. In the latter case, the catalyst nanoparticle located at the nanotube tip may lose the catalytic activity or mobility when it touches the sidewall of a step. This issue will be discussed later. Interestingly, as shown in Figure 2c, a few SWNTs bent twice and return to the backward direction (marked 6 and 7). This detachment of the SWNTs from the step edge is important for further directional control, because this will give us the opportunity to grow SWNTs with multiple bends. We infer that the doubly bent SWNT structure observed in Figure 2c stems from irregular edge structure. The step edge created by the wet etching is not atomically smooth, being different from the steps formed on inclined single-crystal substrates. Therefore, at a nanoscale level, a part of the SWNT would locally orient in the near [11j01j] direction, even though the SWNT is attached on the sidewall of the artificial steps. We think that this partial misalignment induced the detachment of the SWNT from the step edge and alignment along the [11j01j] direction once again. Other SWNTs were not affected by the steps (marked by 8), probably because the height of the etched step was relatively low (∼1 nm). Bent SWNTs were also observed when they climbed down the step edges, as shown in Figure 3. This phenomenon cannot be explained by the steric hindrance suffered from the high artificial steps. We speculate that the van der Waals interaction between a SWNT and the substrate is essential for both the lattice-oriented growth and the step-templated growth. At step edge, a SWNT attaches not only to the top surface of the sapphire but also to the side wall of the step,18 and we think this is why SWNTs shown in Figure 3 was bent even when the SWNT climb down the step. We patterned the iron oxide nanoparticle catalyst by a microcontacting printing method using a poly(dimethoxylsiloxane) (PDMS) stamp and synthesized nanotubes. In Figure 3b, the SWNT grown from the catalyst pattern changed the growth direction when it climbed down the artificial step. Apparently, a part of the SWNT was extruded from the step edge. This image suggests that the SWNT growth occurred with a base-growth mode, because the aligned SWNT is pushed off from the catalyst pattern so that the part of SWNT is supposed to go over the step edge with keeping the nanotube tip attached to the step edge. Thus, the growth termination observed at the step edges shown in Figure 2a (marks 4 and 5) may be ascribed to the base-growth mode. We previously studied the growth mechanism of the aligned SWNTs by using the carbon isotopes.19 During the CVD, the carbon feedstock was switched from 13CH4 to 12CH4, and isotope distribution in the resulting SWNT was analyzed by a Raman mapping technique. For many SWNTs, the initially introduced 13C was observed at the nanotube tip, suggesting the base-growth mode.19 Thus, the bent SEM image shown in Figure 3b is consistent with our previous result. The SWNTs observed by the SEM showed the bending angle of nearly 90° (Figures 2 and 3), but it seems to be difficult for the bent SWNT to continue to grow. SWNTs look white and thick in the SEM images due to the charging effect on an electrically insulating sapphire substrate, but the actual SWNT diameter (1-2 nm) is much smaller than that expected from the SEM images.17 Hence, we think that the real curvature of the bent SWNT is not so high as expected from the SEM images and the SWNT bent smoothly at the step edges. This is why a SWNT can grow even after the bending occurs. Assuming the base-growth mode, chirality of the SWNT does not change at the bending point, because the SWNT is extended from the catalyst. A growing SWNT is supposed to follow the same path which the tip passed through. Thus, introducing a pentagon-
Bent Growth of Aligned SWNTs
J. Phys. Chem. C, Vol. 113, No. 30, 2009 13123
Figure 2. SEM images of bent SWNTs observed at the patterned steps. The step height was ∼8 nm (a, b) and ∼1 nm (c). The patterned structures are highlighted with light blue. Insets show the schematic images of the bent nanotubes with the patterned structures.
Figure 3. SEM image of the bent SWNT at the step edges. The step height was ∼13 nm (a) and ∼2 nm (b). The square pattern in (b), highlighted with brown, represents the catalyst pattern prepared by a microcontact printing method.
heptagon pair20 at the step edge is unlikely to occur. Since we could not observe AFM images at the bending point, we cannot exclude the possibility of buckling of the SWNT, which is dependent on the nanotube diameter.22,23 It is important to discuss on effects of the step height. We studied the sapphire substrates with several step heights and found that the steps with the height around 5-10 nm were the most effective to bend the aligned SWNTs. On these steps, the relative abundance of the single-bent, straight (climbed), and stopped nanotubes was about 10-25%, 30-50%, and 20-40%, respectively. A few SWNTs showed the double-bent structure shown in Figure 2c (
