Fabrication and characterization of suspended single-walled carbon nanotubes

Solid State Communications 139 (2006) 186–190 www.elsevier.com/locate/ssc

Fabrication and characterization of suspended single-walled carbon nanotubes Byung Seok Oh a , Yo-Sep Min b , Eun Ju Bae b , Wanjun Park b , Young Keun Kim a,∗ a Department of Materials Science and Engineering, Korea University, Anam-Dong 5-1, Seongbuk-Gu, Seoul 136-713, Republic of Korea b Materials & Devices Research Center, Samsung Advanced Institute of Technology, Yongin-City, Gyeonggi-Do 449-712, Republic of Korea

Received 12 January 2006; received in revised form 27 April 2006; accepted 16 May 2006 by B. Jusserand Available online 9 June 2006

Abstract Suspended single-walled carbon nanotubes (SWCNTs) between SiO2 pillars via a direct lithographic route using a simple mixture of catalyst precursor [Co(III) acetylacetonate, Co(acac)3 ] and conventional electron beam resist (ma-N2403) were fabricated. The catalytic electron beam resist (Cat-ER) layer plays dual roles as a catalyst and a resist layer for the growth and alignment of CNTs, respectively. The structure of the grown nanotube was characterized by Raman spectroscopy (633 nm laser excitation). Nanotubes grown from Cat-ER with Co(acac)3 show the typical Raman spectra of SWCNTs which are characterized by the strong tangential bands near to 1590 cm−1 and radial breathing modes (RBMs) in the low frequency region (<300 cm−1 ). The calculated diameter of the probed nanotubes individually corresponds to the range 0.86–1.77 nm. c 2006 Elsevier Ltd. All rights reserved.

PACS: 81.07.De; 81.16.Rf; 87.64.Je Keywords: A. Carbon nanotube; B. Suspended nanotube; C. Raman spectroscopy; D. Electron beam lithography

1. Introduction Carbon nanotubes (CNTs) are the most ideal and promising materials for future nanotechnology applications including field effect transistors (FETs) [1], single electron transistors (SETs) [2], field emission displays (FEDs) [3], memory devices [4], sensors and probes [5], which are anticipated to substitute conventional Si based devices [6]. Fundamentally, CNTs are one-dimensional nanostructures with either metallic or semiconducting behavior depending on their diameter and chirality [7]. In addition, they could exhibit ballistic electron transport, high current density, mechanical robustness and good thermal conductivity [8]. However, it is difficult to grow CNTs that possess uniform physical properties in a controllable manner. In particular, it is critical to control the chirality, orientation and growth position of the CNTs when considered as building blocks for electronic device applications Many researchers have been interested in controlling and analyzing individually suspended CNTs, not only from the ∗ Corresponding author. Tel.: +81 2 3290 3281; fax: +81 2 3290 3281.

E-mail address: [email protected] (Y.K. Kim). c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.05.028

standpoint of potential applications such as surround-gate field effect transistors but for fundamental characterization because they are free from van der Waals interactions [9]. Cassell et al. [10] demonstrated a self-directed growth of suspended single-walled CNTs (SWCNTs) on a 10 µm high silicon tower by contact printing a polydimethylsiloxane (PDMS) stamp with a liquid-phase catalyst precursor. They achieved suspended SWCNTs with a high degree of orientation and lengths up to 150 µm. Homma et al. [11] synthesized suspended SWCNTs on pillars with sputtered Fe or Co films after making an array of submicrometer scale silicon or silicon dioxide pillars using synchrotron-radiation lithography. Their suspended SWCNTs were in the form of either individuals or bundles depending on the catalyst particle size and growth temperature. However, it is thought that the fabrication method for suspended SWCNTs may require overcoming technological barriers to make them suited for nanoscale applications [12]. One example is that the suspended tubes should be selectively grown on the pillar regions without any growth on the substrate surface. Marty et al. demonstrated the possibility to self-assemble SWNTs between metallic electrodes for the fabrication of FETs without any

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post process by using hot-filament-assisted chemical vapour deposition (CVD) [13]. Recently, two groups reported a direct lithographic technique for site-selective growth of CNTs. Huang et al. [14] fabricated vertically aligned carbon fibers/nanotubes using a photoresist (PR) consisting of a polyvinyl alcohol, a crosslinking agent, a radical generator, and a metal salt. Patterned metal nanoparticles induced the growth of carbon fibers/nanotubes through the pyrolysis of C2 H2 which depended on the pyrolysis temperature and catalyst distribution. Ishida et al. [15] synthesized SWCNTs from resist dots with a diameter of 20 nm. The Raman spectra of SWCNTs grown under the ethanol CVD condition had radial breathing mode (RBM) peaks which were obtained from only three points through 100 different scans over the patterned area. Moreover, the intensity ratio of the D and G band (ID /IG ) was approximately one. This indicates that the grown SWCNT has low yield and defective structure. In this paper, we report the fabrication of suspended SWCNTs between SiO2 pillars via a direct lithographic route. The catalytic electron beam resist (Cat-ER) plays dual roles as a catalyst and an ER for growth and alignment of CNTs, respectively. Furthermore, the Cat-ER pattern can be used as an etch mask to obtain suspended CNTs between threedimensional structures. Our suspended SWCNTs grew only on pillar regions. This simple Cat-ER method is practical for growing individual CNTs on a defined region in the nanoscale, and can be completely compatible with conventional integrated circuit fabrication processes.

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Fig. 1. (a) 1 H-nuclear magnetic resonance (400 MHz, CDCl3) spectra of Co(acac)3 , ma-N2403, as-dissolved Cat-ER and 40 day old Cat-ER. The arrows indicate methine proton peaks of acetylacetonate ligands. (b) Thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) data of a 0.1 M mixture solution of Co(acac)3 and ma-N2403.

2. Experiments Preparation of the Cat-ER: 0.178 g (0.05 mol) of Co(acac)3 (Aldrich, USA) were dissolved in 10 mL of a negative tone electron beam resist ma-N 2403 (Micro Resist Technology, Germany). The catalyst/e-beam resist mixture was stirred and sonicated for 5 min, respectively. Fabrication of the pillar pattern by direct e-beam lithography: A thin layer (∼1 µm) of resist was prepared by spin coating the 0.05 M Cat-ER solution on a SiO2 /Si substrate at 5000 rpm for 30 s. After baking on the hot plate at 90 ◦ C for 1 min, the substrate was exposed to the electron beam. To make a pattern of square arrays, we used an e-beam lithography system consisting of a Raith ELPHYTM Quantum based JEOL JXA-840A SEM, which was operated at Vaccel = 15 keV and I = 10 pA. The exposed sample was developed in ma-D 532 for 70–100 s and rinsed in distilled water. Subsequently, for SiO2 pillar arrays, the resist pattern was etched using a reactive ion etcher. As a result of etching, a pillar with 300 nm diameter and 400 nm height was completed. Removal of e-beam resist: Catalyst patterns for CNT growth can be obtained by two steps of O2 burning and plasma ashing. First, the etched sample was loaded in a thermal chamber and the O2 burning process was carried out in a quartz tube with an O2 flow rate of 500 sccm at 550 ◦ C for 30 min. Additionally ashing was executed for 1 min under 300 W plasma with an O2 flow rate of 500 sccm to remove residual ER.

Growth of suspended SWCNTs: The substrate was heated up to 900 ◦ C in a thermal CVD with a N2 carrier gas (900 sccm) and kept there for 10 min. For CNT growth, C2 H2 (100 sccm)/H2 (100 sccm) gases were simultaneously injected into a quartz tube at 900 ◦ C for 30 min. After growth, the substrate was cooled at room temperature, immediately. Finally, suspended SWCNTs were grown between the patterned pillar arrays by pyrolysis of acetylene. Analytical method: For thermal gravimetry and differential scanning calorimetry (TG-DSC) analysis, an Al crucible filled with 0.05 M Cat-ER was heated up to 600 ◦ C (10 ◦ C min−1 ) under air conditions. The FE-SEM images were obtained by a Hitachi S-4700, operated at 5–20 kV. To make the SEM observation easier, Pt was deposited onto the specimen. Raman spectra with excitation laser wavelength of 633 nm were measured using a Renishaw system 3000. The spot size of the focused laser beam was 1 µm. 3. Results and discussion The addition of Co(III) acetylacetonate to the ma N-2403 does not deteriorate the Cat-ER significantly. This chemical stability is supported by proton nuclear magnetic resonance spectroscopy as shown in Fig. 1(a). Mixtures of Co(acac)3 with ma-N2403 were reasonably stable so that methine proton peaks of acetylacetonate ligands in the proton nuclear magnetic

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Fig. 2. Schematic diagram of the fabrication procedure for suspended SWCNTs by direct electron beam lithography using the Cat-ER.

resonance (NMR) spectra did not disappear or shift from the as-dissolved for the 40 day old solution. This is an evidence that the Cat-ER is a simple mixture of Co(acac)3 and the ER without any chemical reaction. Before the growth of CNTs, we burned the substrate at 550 ◦ C for 30 min with an oxygen flow of 500 sccm (1.5 Torr) in order to remove organic substances in the Cat-ER patterns and deposit iron catalysts on the defined region. Thermal properties of Cat-ER in air were investigated with simultaneous TG-DSC. The burning temperature was determined by considering the positions of the strongest exothermic peak in the DSC thermogram as depicted in Fig. 1(b). The residual masses (∼1.2%) at the strongest exothermic peaks were also low enough to expect complete removal of organics. In a previous report [10], the fabrication of suspended SWCNTs required complex work between pillar fabrication and CNT growth, which included a series of catalyst preparation, surface treatment of the stamp, catalyst coating, printing, and long heat treatment. However, our simple method needs an O2 burning process after the pillar fabrication. Fig. 2 outlines the procedures of generating suspended SWCNTs by direct electron beam lithography. A square shape pattern was formed on a SiO2 substrate with Cat-ER as illustrated in Fig. 2. The oxide layer was subsequently etched by reactive ion etching (RIE). Since the burning process leaves cobalt catalyst on the surface after the removal of organics, CNTs can be grown only on the deposited catalyst in the pre-patterned region. After burning the organics from the pattern and growing CNTs, we successfully obtained suspended SWCNTs on the SiO2 pillars (see Fig. 3(a) and (b)). The suspended SWCNTs grew only on the pillar region, while none of the CNTs grew on the substrate surface. Most nanotubes were nearest-neighbor connections, but some second nearest-neighbor connections and y-junction shaped CNTs were observed. A considerable number of the grown SWNTs were so short that they could not reach the neighboring pillars and remained suspended with a free end. The density of suspended SWCNTs was about 0.7 CNTs/pillar (2.5 CNTs/µm2 ). When the spacing between pillars increased from 300 to 1000 nm, the density of suspended CNTs decreased remarkably. We could not find the correlation of a direction

Fig. 3. (a) SEM image (top view) of suspended SWCNTs grown on SiO2 pillars, and (b) tilt view. The gas flow direction during the CNT growth is denoted with an arrow.

between CNTs growth with the gas flow but the directionality was mainly determined by the pattern of the pillars. We employed Raman spectroscopy to analyze the suspended SWCNTs. The Raman spectra were obtained from five different point scans over the patterned area using a 1 µm laser spot with excitation wavelength of 633 nm (1.92 eV). The Raman spectra of the sample using 0.05 M Co(acac)3 Cat-ER showed the formation of typical SWCNTs, as characterized by the strong G band of tangential modes and sharp RBM (radial breathing mode). The diameter (dt , nm) of CNTs can be calculated from the frequency (ωRBM , cm−1 ) of the RBM with the equation ωRBM = α/dt , where α is experimentally found to be 248 cm−1 nm for isolated SWCNTs on SiO2 /Si substrates [16]. In the RBM mode (Fig. 4(a)), several peaks were observed in a wide range of 140–287 cm−1 in order of 287 (1st), 140 (2nd), and 219 cm−1 (3rd). These intensities are much stronger than the SiO2 /Si substrate peak at 303 cm−1 . The corresponding diameters of the observed peaks were determined to be 0.86–1.77 nm with the above-mentioned equation. The higher frequency components ($G+ ) near to 1590 cm−1 and the lower frequency components ($G− ) near to 1552 cm−1 in Fig. 4(b) are

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Fig. 4. Raman spectra of grown suspended SWCNTs using 0.05 M Cat-ER with Co(acac)3 . (a) Radial breathing mode, and (b) tangential and disorder-induced mode from 633 nm laser excitation. Raman spectra were calibrated with the peak at 303 cm−1 due to the SiO2 /Si substrate denoted with an asterisk.

attributed to vibrations along the direction of the CNT axis and along the circumferential direction, respectively. For metallic SWCNTs, the lower frequency component generally shows a very broad Breit–Wigner–Fano line. The G band components, $G+ and $G− , are related with the equation $G− = $G+ − c/dt2 , where c = 47.7 cm−1 nm2 for the semiconducting and c = 79.5 cm−1 nm2 for the metallic CNT. The $G− band is generally widened by the presence of metallic tubes among the probed tubes. The observed wide bands of $G− reveals that the grown nanotubes are a mixture of semiconducting and metallic tubes, although the metal-to-semiconducting tube ratio cannot be evaluated from the Raman spectra. From the Kataura diagram, and considering the 1.96 eV laser excitation, the RBM lines at 284, 287 and 140 cm−1 correspond to semiconducting SWNTs, while the RBM line at 219 cm−1 corresponds to a metallic tube [17]. A weak peak at 1303 cm−1 of the D band is related to disordered graphite and amorphous carbon. The value of ID /IG is 0.64, averaged from five points. 4. Summary Single-walled carbon nanotubes (SWCNTs) suspended between pillars were grown only on a predefined surface of three-dimensional structures, which were fabricated via a direct e-beam lithographic method using a mixture of Co(III) acetylacetonate and a conventional e-beam resist. The catalytic e-beam resist (Cat-ER) plays dual roles as a catalyst and a resist for growth and alignment of SWCNTs, respectively. This method may enable fundamental characterization of SWCNTs and industrial applications of various nanotube-based electronic devices. Acknowledgement This work was supported by the Korean Ministry of Science and Technology through the Tera-level Nano Devices (TND) Program.

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