The effect of catalytic thin film thickness on the diameter control of individual carbon nanotubes grown by plasma enhanced chemical vapor deposition was investigated. Individual carbon nanotubes were grown on catalytic nanodot arrays, which were fabricated by e-beam lithography and e-beam evaporation. During e-beam evaporation of the nanodot pattern, more catalytic metal was deposited at the edge of the nanodots than the desired catalyst thickness. Because of this phenomenon, carbon atoms diffused faster near the center of the dots than at the edge of the dots. The carbon atoms, which were gathered at the interface between the catalytic nanodot and the diffusion barrier, accumulated near the center of the dot and lifted the catalyst off. From the experiments, an individual carbon nanotube with the same diameter as that of the catalytic nanodot was obtained from a 5 nm thick catalytic nanodot; however, an individual carbon nanotube with a smaller diameter (~40% reduction) was obtained from a 50 nm thick nanodot. We found that the thicker the catalytic layer, the greater the reduction in diameter of the carbon nanotubes. The diameter-controlled carbon nanotubes could have applications in bio- and nanomaterial scanning and as a contrast medium for magnetic resonance imaging.
Recently, various nanomaterials have been widely used in nanoscience, engineering, and technology. In particular, carbon nanotubes (CNTs) are the most popular material in the nanotechnology field. Since discovered in the soot of an arc discharge apparatus [
In the last decade, chemical vapor deposition (CVD) techniques, particularly plasma enhanced chemical vapor deposition (PECVD), have been widely used to grow CNTs [
In this paper, we report the effect of the catalytic layer thickness on the diameter of individual carbon nanotubes grown using direct current PECVD (DC-PECVD). Specifically, we show that the thickness of the catalytic nanodots affects the diameter of the resulting vertically aligned individual carbon nanotubes. From the field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) images, we verified the relation between the carbon nanotube diameter and the catalytic layer thickness of nanodots.
Co, Fe, Mo, and Ni are generally used as catalysts for carbon nanotube growth. In this study, we used Ni as the catalytic metal. For growing CNTs at determined locations, we patterned nanodots with diameters of 400 nm on a p-type, boron-doped silicon (100) wafer. Titanium nitride (TiN) with 200 nm thickness was deposited before catalytic layer deposition to provide a buffer layer on the SiO2/Si substrate in order to prevent catalyst silicide formation under high growth temperature conditions. To make small nanodots, electron beam lithography (Raith 150) was used on an e-beam resist (3% 950,000-MW (molecular weight) PMMA (polymethyl methacrylate) in chlorobenzene) deposited on the titanium nitride/silicon dioxide/silicon wafer. Through a series of e-beam exposures, we found that an e-beam area dose of 150
Schematic diagram showing microfabrication processes for nanodot arrays and carbon nanotube growth by plasma enhanced chemical vapor deposition (PECVD).
Vertically aligned CNTs were grown on the patterned metal catalyst (in this case, Ni) by PECVD. The PECVD instrument had a DC plasma chamber consisting of a pair of electrodes in which one electrode was grounded and the other was connected directly to a power supply. The negative DC bias voltage applied to the cathode dissociates the feedstock hydrocarbon gas and generates many carbon-bearing radicals for CNT growth.
Figure
Schematic diagram of plasma enhanced chemical vapor deposition and generated plasma (inset).
A patterned sample was loaded in the PECVD chamber evacuated by rotary and turbo-molecular pumps to a base pressure (<10−6 Torr), which was needed for eliminating the impurities and water vapor. When the pressure reached the desired value, the temperature of the heater beneath the cathode was gradually increased to 580°C. When the temperature became 580°C, ammonia (NH3) gas was introduced into the chamber for 5~15 min without igniting the plasma to etch the catalyst. After subjecting the catalyst to preetching without plasma, acetylene (C2H2) was injected into the chamber and the plasma was ignited to initiate CNT growth. After the CNT growth time, the plasma was turned off and the heater was cooled down slowly to prevent CNT damage from a sudden change in temperature. During the CNT growth process, amorphous carbon was deposited on the cathode and heater, which may have caused fluctuations in the plasma. To form stable plasma, the heater and cathode were cleaned after several experiments. Figure
Field emission scanning probe microscopy (FESEM) images of (a) catalytic nanodot array and (b) vertically aligned individual carbon nanotube array.
Nanodots were fabricated by e-beam lithography and e-beam evaporation as a catalyst for vertically aligned carbon nanotube growth. In the e-beam process, the edge bead effect generally appears with very small patterns. The edge bead effect occurs when evaporated material is preferentially deposited toward the edge of a pattern compared to the center. Figure
(a) Scanning electron microscopy (SEM) image and (b) atomic force microscopy (AFM) scanning profile of catalytic nanodot with a thickness of 5 nm; (c) SEM image and (d) AFM scanning profile of catalytic nanodot with a thickness of 50 nm.
A carbon atom of particle is diffused onto or into a metal layer. These atoms of particles are diffused according to the Brownian diffusion distance [
CNTs can be grown by following a three-step procedure as follows: (1) decomposition of the hydrocarbon gas (in this study, acetylene (C2H2)) over a catalytic metal (Ni), (2) diffusion of the carbon atoms through the bulk and surface of the catalyst, and (3) subsequent precipitation of the carbon atoms beneath the catalyst [
(a) Three-dimensional (3D) atomic force microscopy (AFM) scanning image of 50 nm thick nanodot and (b) schematic diagram of carbon atoms’ diffusion.
The CNT’s electric dipole, accruing from the electric field exerted on the CNT, interacts with the DC electric field and experiences an alignment torque, developing stresses at the interface between the CNT and the catalyst particle. Because the diffusion flux depends on the stress gradient as well as the concentration gradient, stress-induced fluctuations in the diffusion rate can be primary mechanism for the vertical alignment of the CNTs. Figures
Scanning electron microscopy (SEM) images of vertically aligned carbon nanotubes from catalytic nanodots with thicknesses of (a) 5 nm, (b) 30 nm, and (c) 50 nm; (d) relation between thickness of catalytic nanodots and diameter of carbon nanotubes.
In this work, the effect of catalyst layer thickness on the diameter of vertically aligned individual carbon nanotubes produced by PECVD was investigated. For the growth of individual carbon nanotubes, catalytic nanodot arrays were fabricated by e-beam lithography and e-beam evaporation. During e-beam evaporation, the edge effect was observed in catalytic nanodots, because the diameters of the dots were very small. Because of the edge effect, the catalytic metal was deposited preferentially toward the edge of the dot resulting in a larger thickness than desired. When growing vertically aligned individual carbon nanotubes by PECVD, thick catalytic nanodots reduced the diameter of the synthesized carbon nanotubes. The reduction occurred because carbon atoms diffused faster near the center of dots than at the edges. The carbon atoms diffused to the interface between the catalytic nanodot and the diffusion barrier and accumulated near the center, causing the center of the catalytic nanodot to lift off faster than the edge of the dot.
From the experiments, individual carbon nanotubes with the same diameter as that of the catalytic nanodots (400 nm) were obtained when the catalyst thickness was 5 nm. For 50 nm thick catalytic nanodots, a carbon nanotube with diameter of 260 nm (reduction by
Diameter control of vertically aligned individual carbon nanotubes is very important and useful for the application of CNTs in electric and energy-based devices and for scanning bio- and nanomaterials. Furthermore, the diameter-controlled carbon nanotubes could be applied to a contrast medium for magnetic resonance imaging.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was supported by the 2012 Inje University research Grant.