Single-step route to diamond-nanotube composite
© Varshney et al.; licensee Springer. 2012
Received: 19 July 2012
Accepted: 18 September 2012
Published: 27 September 2012
Candle wax was used as a precursor for the production of a diamond-nanotube composite in a single step. The composite films were fabricated by sulfur-assisted hot-filament chemical vapor deposition technique. The morphology of the composite films was analyzed by scanning electron microscopy and transmission electron microscopy. Raman spectra of the films show characteristic diamond band at 1,332 cm−1, D-band around 1,342 cm−1, and graphitic G-band around 1,582 cm−1. The electron energy-loss spectroscopy recorded at the carbon K-edge region shows signature features of diamond and carbon nanotube in the fabricated material. The ability to synthesize diamond-nanotube composites at relatively low temperatures by a single-step process opens up new possibilities for the fabrication of nanoelectronic devices.
Since the documented discovery of carbon nanotubes (CNTs) in 1991 by Iijima  and the realization of their unique physical properties, including mechanical, thermal, and electrical, many investigators have endeavored to fabricate advanced CNT composite materials that exhibit one or more of these properties. Integration of CNTs with various kinds of materials, leading to the fabrication of composites possessing properties of the individual components with a synergistic effect, has gained growing interest. Carbon composites, such as diamond-nanotube, have various industrial applications because of their outstanding physical and chemical properties. CNTs are tiny rolled-up tubes of sp2-hybridized carbon that possess unique electronic transport properties and are the strongest known material, while diamond, consisting of sp3-hybridized carbon, exhibits high hardness, stable surface chemistry, and good biocompatibility [2, 3]. The exceptional mechanical and electronic properties of CNTs and diamond make them promising candidates for use in microelectronic devices. A diamond-nanotube composite has excellent thermal conductivity and field emission characteristics and finds applications in various fields that require a combination of good thermal and electrical properties such as wear-resistant coatings, thermal management of integrated circuits, field emission devices, and electrical field shielding in MEMS and microelectronics .
There are several recent reports on the development of hybrid materials comprising of diamond and CNTs [4–6]. However, the reported methods have either two distinct seeding sources or a tedious two-step fabrication route which requires the use of a catalyst to provide initial nucleation sites for growth of both CNTs and diamond. To enhance the nucleation rate during diamond film growth, various techniques such as ultrasonic seeding , surface scratching, and bias-enhanced nucleation have been applied [8, 9]. All the above techniques are mechanically abrasive and adversely affect the use of substrate for electronic applications, thus fueling the need for precursors that are cheap and handy and do not harm the substrate surface. Also, the transition metal catalysts, viz. Fe, Co, and Ni, that are used for CNT growth [10–12] react unfavorably with materials found in circuits and composites. Moreover, catalyst nanopowder can be toxic and cause problems in clean room environments. The present paper reports the catalyst-free growth of CNTs that is seeded by paraffin wax.
There are many reports on the fabrication of single-walled carbon nanotubes from polymers using metal catalyst [13, 14]. In all previously reported techniques, the hybrid material was fabricated in two separate steps, each for metal-catalyzed CNT growth and diamond nanoparticle seeded diamond growth, but a single-step fabrication of diamond-nanotube composite using a single seeding source has been difficult. Here, we report the fabrication of diamond-nanotube composite by hot-filament chemical vapor deposition (HFCVD) in a single-step process, where paraffin wax is utilized as a seeding material for both diamond and carbon nanotubes. In the present report, trace amounts of sulfur have been used in the fabrication process to aid the formation of CNTs. The role of sulfur is explained in the ‘Results and discussion’ section. This paper demonstrates a new synthetic pathway for diamond-nanotube composite utilizing candle wax (paraffin wax), a cheap and widely used hydrocarbon.
Polycrystalline copper substrates (99.9% pure, 0.5-mm thick, and 14-mm disk diameter) were hand polished with 600-grit sandpaper on both sides to make them flat. One side was further polished with 1,000-, 1,500-, and 2,000-grit sandpaper to smoothen the surface. The substrates were then cleaned in an ultrasonic bath with 2-propanol for 15 min and then dried with nitrogen. About 5 to 10 gm of candle wax (commercially available) was melted on a hot plate in a glass beaker by heating it to a temperature of 120°C at a rate of 15°C/min. A small portion of this melt was transferred onto a copper disk substrate with a thickness of 500 to 700 nm and allowed to cool to room temperature. The disks were then placed in the HFCVD chamber and exposed to a gas mixture of 0.3% methane and 99.7% of hydrogen (consisting of 500 ppm of H2S) at a constant pressure of 20 Torr and a total gas flow of 100 sccm. The reaction was activated for 3 h by a rhenium filament (8 cm in length and 0.5 mm in diameter) positioned 8 mm above the substrate. The temperature of the substrate and the filament was at approximately 550°C and 2,500°C, respectively.
The Raman scattering spectra were obtained using a triple monochromator (ISAJ-Y Model T 64000, HORIBA Ltd., Kyoto, Japan) with around 1 cm−1 resolution using the 514.5-nm line of Ar laser. The morphologies of the as-deposited materials were determined using a JEOL JEM-7500 F scanning electron microscope (SEM; JEOL Ltd., Tokyo, Japan).
The samples were also analyzed using a Carl Zeiss LEO 922 energy-filtered transmission electron microscope (TEM; Carl Zeiss AG, Oberkochen, Germany) operated at 200 kV, including an omega-type energy filter to study the electron energy-loss spectroscopy (EELS). The TEM samples were prepared by immersing an ultrathin carbon-coated copper grid into an ultrasonicated suspension of the fabricated material in ethanol. The Fourier transform infrared spectroscopy (FTIR) was carried out using a Bruker Tensor 27 instrument (Bruker Optik GmbH, Ettlingen, Germany). The samples were prepared by melting 0.5 g of wax onto the substrate and allowing it to cool to room temperature.
Results and discussion
Candle wax is generally composed of paraffin, which is made of heavy straight-chain hydrocarbons obtained from crude petroleum oil . Crystalline paraffin waxes are solid and crystalline mixtures of hydrocarbons consisting of linear n-alkane and branched iso- and cyclo-alkanes with carbon lengths ranging from C16 to C30 and higher [16, 17]. The paraffin crystallites seem to act as nucleation sites for diamond and carbon nanotube growth in the presence of hydrocarbon radicals and atomic hydrogen in the chemical vapor deposition (CVD) system, resulting in microcrystalline diamond-carbon nanotube films .
A simple and reproducible single-step method for the fabrication of diamond-nanotube composite films was reported. The characteristic features of both carbon nanotubes and diamond in the composite film were evident from the Raman spectroscopy, EELS, SEM, and TEM results. To our knowledge, this is the first example of a sulfur-based growth of diamond-nanotube composite. The low-temperature fabrication of the present diamond-nanotube composite without the use of a metal catalyst and in a single-step process opens up new possibilities for the fabrication of composite-based nanoelectronic devices.
DV and MA are PhD students at the University of Puerto Rico (UPR). MJFG is an assistant professor at the Department of Physics and Department of Chemistry at UPR. BRW is a dean and professor at UPR. GM is a professor and the director of the Department of Physics at UPR.
This research project was carried out under the auspices of the Institute for Functional Nanomaterials (NSF grant no. 1002410). This research was also supported in part by PR NASA EPSCoR (NNX07AO30A) and PR NASA Space Grant (NNX10AM80H). The authors gratefully acknowledge the instrumentation and technical support of the Raman Facility (Dr. R.S. Katiyar).
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