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Growth Kinetics in a Large-Bore Vertically Aligned Carbon Nanotube Film Deposition Process† Ken Bosnick* and Lei Dai National Institute for Nanotechnology, National Research Council Canada, 11421 Saskatchewan DriVe, Edmonton, Alberta T6G 2M9, Canada ReceiVed: June 11, 2009; ReVised Manuscript ReceiVed: September 14, 2009
Vertically aligned films of multiwalled carbon nanotubes (VACNT) are synthesized in a large-bore chemical vapor deposition reactor by employing a Cr-Ni-Fe thin film catalyst stack predeposited on substrates. The kinetics of the growth process is studied by measuring the VACNT film thickness, the resistivity (indicative of the density), and the distribution of carbon nanotube (CNT) diameters as a function of pregrowth catalyst treatment time, growth time, and growth temperature. It is found that pregrowth treatment times of about 210 min are needed before reaching steady-state catalyst conditions. Shorter pregrowth treatment times produce a thicker but less dense film. The CNT diameters are only weakly affected by the pregrowth treatment time (for at least greater than 30 min.). A model is proposed to explain these results. The kinetics of the film growth are studied as a function of growth time and temperature under steady-state catalyst conditions. The CNT film thickness is well fit by the kinetic model H ) βτ(1 - e-t/τ) at temperatures between 625 and 750 °C, with a growth rate decay time τ of about 20 ( 5 min. The initial growth rate β peaks at 1.1 µm/min at 650 °C. Introduction Carbon nanotubes (CNT) have emerged as a central platform in nanotechnology.1 They represent a new class of materials with one microscale and two nanoscale dimensions. The nanoscale dimensions impart on these materials novel properties that are not seen in the bulk that can be tuned by adjusting the dimensions of the material. In addition to the tremendous scientific interest in these materials there is also an enormous technical interest. The microscale dimension permits these materials to be integrated into a host of devices and functional materials for a wide range of potential applications,2 including in electronics and photonics; sensors; multifunctional and structural composites; and high surface area support materials for catalysts. CNTs are synthesized by a chemical vapor deposition (CVD) process.3 Reactive gases are passed over catalytically active nanoparticles at elevated temperatures. The gases decompose at the catalyst, dissolve into it, and finally precipitate to form a solid. The nanoscale dimensions of the particle direct the precipitate into a tube. While success has been made employing pulsed lasers, plasmas, and electric arcs to provide the energy needed to make the reaction go, the most promising methods for commercialization of CNTs use a straight-up thermal process, either with catalyst particles generated in the gas phase (as in Unidym’s HiPCO process4) or with a premade, solidsupported catalyst material (as in Southwest Nanotechnologies’ CoMoCat process5). Currently, most commercial processes for the production of CNTs produce a bulk material that is highly entangled, disordered, and difficult to process. The vertically aligned carbon nanotube (VACNT) film is another common morphology of CNTs and may enable their technological exploitation while †
Part of the “Martin Moskovits Festschrift”. * To whom correspondence should be addressed. E-mail: ken.bosnick@ nrc.ca.
10.1021/jp905497x
avoiding many of the processing issues associated with the bulk materials. To date, however, these films have not been produced in large batches. The National Institute for Nanotechnology has installed an experimental CVD reactor designed for the largescale production of VACNT films and other CNT materials by thermal CVD. The reactor was built by Tystar, Inc. to our specifications and is based on their Tytan line of CVD reactors.6,7 The reactor is capable of processing batches up to 50 150-mm wafers with all steps between loading and unloading fully computer controlled. In this paper we report on the kinetics of the growth of vertically aligned carbon nanotube films in this reactor. Experimental Methods The basic steps in the process flow for the growth of VACNT films are load, heat and reduce, grow, cool, and unload. Thermally oxidized 100-mm silicon wafers are coated by e-beam evaporation with 3 nm Cr, 1 nm Ni, and 1 nm Fe to act as the catalyst for CNT growth. Four wafers are loaded into the reactor in each experiment, with two dummy wafers before and after the experiment wafers to simulate the conditions of a full load. After loading the catalyst coated wafers at 550 °C the reactor is purged with argon. The reactor is then heated to the growth temperature Tgr at a rate of about 10 °C/min under a flow of 25% hydrogen in argon (1500 sccm total) and held there for a total time (including heating time) of tpre. During this step the catalyst film stack alloys and breaks up into nanoscale particles on the surface of the wafers forming the final catalyst for the CNT growth. It is also possible that any metal oxide that formed on the wafers before being loaded into the reactor is reduced to elemental metals at this stage. During the growth stage 7% ethylene and 25% hydrogen in argon (1500 sccm total) is flowed into the reactor for a time tgr. The reactor is cooled to 550 °C under a flow of 25% hydrogen in argon, and the wafers are removed.
Published 2010 by the American Chemical Society Published on Web 11/04/2009
Growth Kinetics in Film Deposition Process The basic metrology used to monitor the results of the runs consists of profilometry for film thickness, resistivity for film density, and field-emission scanning electron microscopy (FESEM) for CNT diameters. The profilometry is performed on a KLA-Tencor P-10 profilometer (scan length 1000 µm, scan speed 200 µm/s, sampling rate 50 Hz, stylus force 3.0 mg, vertical range 65 µm). The sheet resistance is measured using a Lucas Laboratories four point prober with a Keithley 2400 source meter (probe SP4-40085TFY, spacing between tips 0.04 in., tip radius 0.005 in., spring pressure 85 g, tip material tungsten carbide). Film thicknesses and sheet resistances are measured three times at the center, bottom, top, left, and right sides of each wafer and averaged for the location to form a five point map of the wafer for each property. Resistivity is calculated as the product of the film thickness and the sheet resistance for each location. The mean thickness and resistivity are calculated for each wafer from the maps with the standard deviation indicating cross-wafer uniformity. A final thickness and resistivity value and cross-wafer uniformity for each run is calculated by averaging the results for the four wafers used in that run. Note that cross-wafer uniformity is generally much worse than wafer-to-wafer or measurement uniformity and therefore dominates the uniformity metric. Error bars in this paper for thickness and resistivity indicate (1σ in cross-wafer uniformity. CNT diameter distributions are measured using a Hitachi S4800 FESEM (1280 × 960 pixel resolution, 300 000× magnification, 15 kV accelerating voltage, ∼20 µA emission current, ∼4 mm working distance). The microscope has an imaging resolution of about 1 nm under these conditions. The images are collected by dispersing ∼0.05 mg of the CNT film (sampled from the center of the first experiment wafer in the run) in 10 mL of isopropanol in a small vial by sonicating for 30 min. A total of 1 mL of this solution is dropped onto a 200mesh, lacey-carbon, copper transmission electron microscope (TEM) grid and allowed to dry. Twenty images are collected in various regions on the TEM grid where 5-15 diameters can be clearly distinguished in each image. The diameters are measured using the ruler tool in the Quartz PCI software package (175-225 measurements per sample). The mean and standard deviation are calculated from the measured diameters. Error bars in this paper for diameter distributions indicate (1σ. High-resolution TEM (HRTEM) images were collected on a JEOL JEM 2200FS microscope operating at 200 kV on the same grids as for the diameter measurements. The lattice image resolution is about 0.1 nm. Thermal gravimetric analysis (TGA) was performed on a TA Instruments Q500 TGA by scraping the VACNT film from one wafer into the TGA pan (2-10 mg of material, 25-900 °C heating range, 10 °C/min ramp, 45 mL/ min air sample gas, 40 mL/min nitrogen balance gas, platinum pan). Results and Discussion FESEM micrographs of a typical film at the edge of a cleaved wafer and of the CNTs dispersed on a TEM grid are shown in Figure 1, as well as a HRTEM micrograph of an individual CNT from a typical film. The HRTEM micrograph shows that the films consist of multiwalled CNTs, with about twenty walls and a small amount of amorphous carbon evident on the surface. The CNTs in the films have a generally surface-normal orientation.7 A typical TGA thermogram is shown in Figure 2. The material begins burning around 550 °C and finishes around 700 °C with about 7% noncombustible residue. Two peaks are seen in the derivative, a major peak (Tburn) at 635 °C and a minor one at 665 °C.
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Figure 1. (Top) FESEM micrograph of a typical VACNT film produced in the reactor (2 µm scale bar). (Bottom, left) Typical FESEM micrograph of the CNTs dispersed on a TEM grid as used for diameter measurements (100 nm scale bar). (Bottom, right) HRTEM micrograph of an individual CNT from the film (5 nm scale bar).
Figure 2. Typical TGA thermogram of a VACNT film (solid line is the normalized weight, dotted line is the derivative, tpre ) 210 min, Tgr ) 700 °C, tgr ) 10 min).
The results of varying the pregrowth catalyst treatment time tpre on film thickness, film resistivity, and CNT diameter distributions are shown in Figure 3 (with Tgr ) 700 °C and tgr ) 10 min.). The large-bore reactor takes about 20 min to heat from 550 to 700 °C and stabilize, and therefore the runs start at 30 min. The runs were generally performed in random order. For lower pregrowth treatment times, the films are thicker but have a higher resistivity. A higher resistivity in these films indicates a lower mass density, since a lower density film will have less conduction paths in parallel and will therefore be more resistive. As tpre increases the films become thinner and more dense until about 210 min, after which time further pregrowth catalyst treatment has no effect on the film. The diameter distributions show a slight decrease in the mean diameter with increasing tpre but are generally not very sensitive to pregrowth catalyst treatment over the times considered here. The insensitivity of the diameter distributions suggests that the catalyst particle size is not changing significantly in the
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Figure 5. Effect of growth temperature Tgr on film thickness and diameter distribution. Lines are included as an aid to the eye. Film resistivity did not show a consistent trend with growth temperature (tpre ) 210 min, tgr ) 10 min).
Figure 3. Effect of catalyst pretreatment time tpre on film thickness, resistivity, and diameter distributions. The catalyst film alloys, breaks up into particles, and reduces for about 210 min after which time further pretreatment has no effect on the final film (Tgr ) 700 °C, tgr ) 10 min).
Figure 4. Schematic of the mechanism for catalyst pretreatment.
window of pregrowth treatment times investigated here. Since the catalyst starts as a uniform film, this would suggest that breaking up into catalyst particles occurs entirely within the first 30 min of catalyst pretreatment time. It is proposed that further pregrowth treatment serves to “activate” more of these catalyst particles.8 This model is shown schematically in Figure 4 and can be understood by assuming that the growth rate of a CNT is given by R ) k [C]n, where k ) R for an activated particle and 0 for an nonactivated particle, [C] is the local concentration of carbon at the catalyst particle, and n is an arbitrary exponent.
In this model it is assumed that the only effect of pregrowth catalyst treatment (beyond the first 30 min) is to convert more of the catalyst particles from the k ) 0 nonactivated state to the k ) R activated state. More activated catalyst particles will produce more CNTs and directly lead to a more dense film. The decreased thickness indicates a decreased growth rate and is caused by a decrease in [C]. One possible cause for this decrease in [C] is an increased demand on the ethylene feed gas at the catalyst particle by a greater number of nearby activated catalyst particles. A second possible cause is a decrease in the diffusion rate of the ethylene through the CNT film with increased film density. This explanation depends explicitly on a base-growth mode which has only been implicitly assumed until now.9 While the effects of gas diffusion through the film is often cited in the literature, others have claimed that this argument has no merit for typical conditions in CNT film growth.10,11 The experimental results presented here do not explain the nature of the “activation” process; however, it is likely due to alloying and oxide reduction. This hypothesized activation process should be verifiable by an in situ TEM experiment, perhaps performed by evaporating the catalyst stack onto a Si3N4 membrane, but is beyond the scope of this paper. At a growth temperature of 700 °C, the catalyst reaches a steady state after about 210 min of pregrowth treatment. Conducting growth kinetics experiments under these steady state conditions allows for the separation of the effects of catalyst activation and CNT growth. Kinetics studies carried out with shorter pregrowth treatment times will produce results that reflect a convolution of a changing catalyst and the true CNT growth kinetics. Figure 5 shows the thickness and diameter distributions of the CNT film produced as a function of growth temperature after 210 min of pregrowth treatment. The resistivity did not show a consistent trend with growth temperature in these experiments. The thickness and resistivity are plotted in Figure 6 as a function of growth time for the 700 °C growth temperature. The thickness increases monotonically with growth time but at an exponentially decreasing rate. The resistivity of the film
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Figure 6. Effect of growth time tgr on film thickness and resistivity. The curve on the thickness plot is a fit of the kinetics model H ) βτ(1 - e-t/τ) to the thickness data. Diameter distributions were not significantly affected by growth time (tpre ) 210 min, Tgr ) 700 °C).
Figure 7. Effect of growth temperature Tgr on initial growth rate β and growth rate decay time τ from the kinetics model H ) βτ(1 e-t/τ). The error bars indicate (1σ in the fitting coefficient (tpre ) 210 min).
decreases with growth time, suggesting that either the average density is increasing as the film grows or that the CNTs are becoming more entangled as they get longer, creating more conduction paths. The mean CNT diameter does not show a significant effect as a function of growth time, indicating that the resistivity decrease with growth time is not due to an increase in mass density caused by increasing CNT diameters during growth. However, the mass density may still be increasing by an increase in the total number of CNTs in the film, that is, by an increase in number density. An increase such as this could be caused by delayed nucleation at catalyst particles but should be minimal under the steady-state catalyst conditions employed here since the maximum number of particles should already be activated. TGA analysis on the films shows an increase in Tburn with tgr from about 635 °C at 3 min to about 685 °C at 100 min. Amorphous carbon likely burns at a lower temperature than the CNTs, and therefore the decrease in resistivity appears not to be due to an increase in density caused by deposition of amorphous carbon during extended growth times. An increase in entanglement is therefore the likely explanation for the decrease in resistivity with growth time. Qualitative imaging by SEM does not give adequate information to confirm this directly; however, small-angle X-ray scattering (SAXS) might.12 SAXS gives information directly on average film morphology and CNT orientation. A SAXS study of these films is underway but is beyond the scope of this paper. This study should be able to resolve any ambiguity in the interpretation of resistivity with film morphology (i.e., density vs entanglement). Following the work of others, the CNT film thickness is fit in Figure 6 by the kinetic model H ) βτ(1 - e-t/τ), where H is the thickness of the film, β is the initial growth rate, and τ is the growth rate decay time.13-15 This model fits the kinetic data well for all of the temperatures investigated here (except 600 °C where the data begins to show more scatter). The fitting parameters β and τ are plotted in Figure 7 for all growth
temperatures investigated. The fits indicate that the growth rate decay time τ is about 20 ( 5 min and shows no trend with growth temperature (ignoring the 600 °C data due to the poor fit of the kinetic model). The initial growth rate β peaks with a value of 1.1 µm/min at 650 °C, as the thickness curve in Figure 5 does. Above this temperature it decreases with an apparent linear dependence on growth temperature. There are three general processes that contribute to the dependence of the kinetic parameters on growth temperature: thermal activation of an elementary CNT growth reaction, catalyst activation, and catalyst deactivation. Thermal activation covers many possible elementary chemical reactions (e.g., the decomposition of ethylene) and is characterized by an exp(-Ea/ kbT) temperature dependence, where Ea is the activation energy for the reaction and kb is the Boltzmann constant. Thermal activation shows a distinct monotonic increase in the rates with increasing temperature. The effect of catalyst activation has been dealt with previously in the pregrowth catalyst treatment experiments and should be minimized under the steady state catalyst conditions employed here. However, the pregrowth catalyst treatment experiments were conducted with a fixed growth temperature of 700 °C and it is possible that the catalyst comes to a different steady-state at different growth temperatures. The tpre experiment was extended to longer times at a growth temperature of 650 °C and it was confirmed that the catalyst had indeed reached a steady-state at this lower temperature by 210 min of catalyst pretreatment. The diameter distributions shown in Figure 5 show a slightly increasing trend in mean diameter with increasing growth temperature. This may indicate an increase in steady-state catalyst particle size with increasing growth temperature or it may be due to a thermal activation effect. The in situ TEM experiment proposed earlier should be able to distinguish between these two explanations. Catalyst deactivation is caused by such processes as poisoning with amorphous carbon. These deactivation processes may also be thermally activated but will contribute a decrease in CNT
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growth rates with increasing temperature. The temperature dependence of the thickness of the CNT films in our experiments is determined by the dependence of the initial growth rate β, which peaks at 650 °C. Increasing β up to this temperature is dominated by thermal activation processes. Above this temperature catalyst deactivation begins to dominate and the initial growth rate begins to decrease with increasing temperature. Interestingly, this implies that catalyst deactivation at higher temperatures shows a more pronounced effect on the initial growth rate β than on the growth rate decay time τ, suggesting that the growth rate decay is not dominated by catalyst deactivation processes such as poisoning by amorphous carbon. This observation is consistent with the mechano-chemical model for growth termination proposed by Han and co-workers.10 Conclusions The effects of pregrowth catalyst treatment time and general growth kinetics for a VACNT film process in a large-bore CVD reactor were studied by measuring film thickness, resistivity, and CNT diameter distributions. Pregrowth treatment times of about 210 min are needed before reaching steady-state catalyst conditions. Shorter pregrowth treatment times produce a thicker but less dense film. The CNT diameters are only weakly affected by the pregrowth treatment time (for at least greater than 30 min). A model is proposed to explain these results whereby the catalyst film quickly breaks up into catalyst particles but further pregrowth treatment is needed to activate these particles. More activated particles lead to a more dense film but due to increased demand on feedstock also produce a thinner film. The kinetics of the film growth were studied as function of growth time and temperature under steady-state catalyst conditions. The CNT film thickness is well fit by the kinetic model H ) βτ(1 e-t/τ) at temperatures between 625 and 750 °C, with a growth rate decay time τ of about 20 ( 5 min. The initial growth rate β peaks at 1.1 µm/min at 650 °C and decreases above this temperature due to catalyst deactivation processes. Small-angle X-ray scattering and in situ TEM experiments are proposed to further the understanding of the growth mechanism of VACNT films.
Bosnick and Dai Acknowledgment. Funding for this work was provided by the National Research Council of Canada and laboratory space by the University of Alberta. Most of the data in this paper were collected by hard-working Technical Officer Ryan Lister. We gratefully acknowledge the technical support from the NINT Cleanroom staff in keeping the reactors and other key equipment operational. Electron microscopy was performed in the NINT Electron Microscopy Facilities, with HRTEM images collected by Jian Chen. TGA analysis was performed in the NINT Analytical Facilities. We thank Frank Hegmann for helpful comments concerning the resistivity measurements. References and Notes (1) Toma´nek, D.; Jorio, A.; Dresselhaus, M. S.; Dresselhaus, G. Top. Appl. Phys. 2008, 111, 1–12. (2) Endo, M.; Strano, M. S.; Ajayan, P. M. Top. Appl. Phys. 2008, 111, 13–61. (3) Joselevich, E.; Dai, H.; Liu, J.; Hata, K.; Windle, A. H. Top. Appl. Phys. 2008, 111, 101–164. (4) See http://www.unidym.com/technology/cnt_manufacture.html. (5) See http://www.swnano.com/tech/what_is_comocat.php. (6) See http://www.tystar.com/additional_carbon.php. (7) Dai, L.; Wang, P.; Bosnick, K. J. Vac. Sci. Technol. A 2009, 27, 1071–1075. (8) Cantoro, M.; Hofmann, S.; Mattevi, C.; Pisana, S.; Parvez, A.; Fasoli, A.; Ducati, C.; Scardaci, V.; Ferrari, A. C.; Robertson, J. J. Appl. Phys. 2009, 105, 064304. (9) Gohier, A.; Ewels, C. P.; Minea, T. M.; Djouadi, M. A. Carbon 2008, 46, 1331–1338. (10) Han, J. H.; Graff, R. A.; Welch, B.; Marsh, C. P.; Franks, R.; Strano, M. S. ACS Nano 2008, 2, 53–60. (11) Xiang, R.; Yang, Z.; Zhang, Q.; Luo, G.; Qian, W.; Wei, F.; Kadowaki, M.; Einarsson, E.; Maruyama, S. J. Phys. Chem. C 2008, 112, 4892–4896. (12) Wang, B. N.; Bennet, R. D.; Verploegen, E.; Hart, A. J.; Cohen, R. E. J. Phys. Chem. C 2007, 111, 5859–5865. (13) Futaba, D. N.; Hata, K.; Yamada, T.; Mizuno, K.; Yumara, M.; Iijima, S. Phys. ReV. Lett. 2005, 95, 056104. (14) Einarsson, E.; Murakami, Y.; Kadowaki, M.; Maruyama, S. Carbon 2008, 46, 923–930. (15) Vinten, P.; Lefebvre, J.; Finnie, P. Chem. Phys. Lett. 2009, 469, 293–297.
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