8718
J. Phys. Chem. C 2009, 113, 8718–8723
Controlled Nanostructuration of Catalyst Particles for Carbon Nanotubes Growth Sandra Rizk, Badreddine M. Assouar,* Ludovic De Poucques, Patrick Alnot, and Jamal Bougdira Institut Jean Lamour, Nancy UniVersity, CNRS, BouleVard des Aiguillettes, BP 239, F-54506 VandoeuVre-le`s-Nancy Ce´dex, France ReceiVed: January 21, 2009; ReVised Manuscript ReceiVed: April 6, 2009
We report on a novel method based on a plasma pretreatment for controlled nanostructuration and patterning of iron catalyst particles from a continuous film in view of carbon nanotube growth. The effects of the hydrogen plasma conditions on the diameter and the density of the catalyst nanoparticles was studied and discussed. We were then able to propose a comprehensive mechanism for the metallic nanostructuration. We showed that as the plasma power density increases, first a reduction of the iron nanoparticle size is observed followed by for the highest plasma powers a phenomenon of alteration in the deposited film. A better control of the nucleation process and the nanostructuration were observed for low hydrogen pressures. The correlation between the plasma parameters and the obtained iron nanoparticles was established. The growth of carbon nanotubes (CNTs) was carried out on the patterned catalyst nanoparticles under CH4/H2 microwave plasma. High quality double-walled and multiwalled CNTs of a diameter of about 5 nm have be obtained. 1. Introduction In recent years, we have noted many significant developments around different types of carbon structures such as diamonds, diamond-like carbon (DLC) thin films, nanoclusters, buck balls or fullerenes, and carbon nanotubes (CNTs). While there are many potential industrial applications for diamonds, DLCs (carbon nanostructures like nanodiamonds and carbon nanotubes) are expected to find applications in numerous fields, as presented recently by Melechko et al.1 Martel et al.2 have shown some results on field effect transistors based on single or multiwalled CNTs. P. Gro¨ning et al. published a review paper demonstrating that CNTs can be clearly considered as the materials taking us from nanoscience to nanotechnology in the field of electron emission technology.3 Another application, which is interesting due to the biocompatibility of the CNTs, is using them as electrochemical biosensors4 or as gas sensors.5 The control of structures and morphologies of produced CNTs is of critical interest for the achievement of practical applications. Among the synthesis methods, PECVD (plasma enhanced chemical vapor deposition) is recognized to be the most promising process because it offers the possibility to well-control the experimental parameters throughout the growth process including the preparation of the catalyst. This is the reason why, there is growing interest in the study of the nanostructuration of the catalyst particles in view of CNT growth.6,7The control of the nanoparticles diameter and their distribution in the surface is a crucial step to control the growth and the patterning of CNTs. The nanostructuration for selective growth of CNT with a specific pattern can be made using electron beam lithography for instance. However, this technological step does not allow obtaining large nanostructure surfaces and requires an ex-situ process which induces a nondesired oxidation of the catalyst. The in situ nanostructuration of catalyst film can be carried out as we will show in this study by H2 plasma pretreatment. Optimization of the experimental parameters has led to a well* Corresponding author. E-mail:
[email protected]. Telephone: + 33 3 83 68 49 05. Fax: + 33 3 83 68 49 33.
Figure 1. Nanostructuration of a continuous film: (a) continuous iron film deposited on a silicon substrate; (b) nanostructured film after a hydrogen plasma pretreatment. The inset shows a tilted substrate.
controlled and tunable process of catalyst nanostructuration for the preparation of substrates showing a homogeneous particle density over centimeter squares. As mentioned above, the mechanism of preparation of the catalyst nanoparticles is crucial in order to control the diameter of CNTs and then their morphology. The mechanism of the homogeneous nanostructuration of iron catalyst film, used in this study, under various
10.1021/jp9006334 CCC: $40.75 2009 American Chemical Society Published on Web 04/23/2009
Controlled Nanostructuration
Figure 2. Effect of pressure on nanostructuration: increasing pressure slows down the nucleation process. These samples were treated for 2 min under a hydrogen plasma.
J. Phys. Chem. C, Vol. 113, No. 20, 2009 8719
Figure 4. Effect of pretreatment time on catalyst nanostructuration under a hydrogen plasma: increasing time helps the film nanostructuration.
Figure 3. Evaluation of power density as function of total gas pressure and microwave power.
plasma parameters was established and explained. Compared to the existing empirical methods of catalyst nanostructuration based, for instance, on thermal annealing,8 our method allows controlling both the homogeneity of nanostructuration over large surfaces and the size of catalyst nanoparticles. The physical interpretation of such mechanism by plasma diagnostic and obtained nanoparticles analyses were carried out. Plasma diagnostic using laser induced fluorescence (LIF) technique and nanoparticles analyses using scanning electron microscopy and energy dispersive X-ray spectroscopy were investigated. The advantages of the nanostructuration using plasma pretreatment compared to a common lithography technique are demonstrated. The growth of CNT under optimized experimental conditions was achieved and their structure was analyzed using transmission electron microscopy. 2. Experimental Methods We pointed out in this study the mechanism of the iron catalyst nanostructuration under plasma experimental parameters. The effect of the H2 plasma pretreatment parameters, including microwave power (varying from 200 to 450 W) and total gas pressure (15-40 mbar) and the effect of the catalyst thickness (10, 5, and 2 nm) on the diameter and the density of iron catalyst nanoparticles were investigated. The carbon
Figure 5. Effect of microwave power on nanostructuration (pressure )25 mbar): increasing microwave power helps nanostrucuration but creates defects when exceeding a certain power. The time pretreatment is set to 2 min.
nanotubes were grown with a CH4-H2 gas mixture after the optimization of the pretreatment step allowing the achievement of the diameter and the homogeneity of the nanoparticles on silicon substrate in a controlled process. The size of silicon substrate is about 1.5 × 1.5 cm2. The reactor used for growing the carbon nanotubes is composed of a cylindrical quartz tube (50 mm in diameter, 350 mm in length) which intersected a rectangular waveguide, the dimensions of which were chosen to drive the TE10 mode of a 2.45 GHz microwave provided by a 0-1200 W power supply. The plasma was generated in the cylindrical tube. The temperature on the surface of the samples was measured by means of an infrared bicolor pyrometer. The
8720
J. Phys. Chem. C, Vol. 113, No. 20, 2009
Figure 6. Catalyst particle mean diameter and the sample’s temperature variation with microwave power.
Figure 7. Variation of H-atom density, using LIF measurements, above the sample’s surface with input microwave power.
gas mixture composition was ensured by mass flow-meters that were computer-controlled in order to maintain both the CH4-H2 ratio and the total pressure at constant values. For the CNT growth, the CH4 concentration was varied from 10% to 40% in the total CH4-H2 gas mixture. The substrates used in this study were silicon (100) covered by different Fe catalyst thicknesses, which we deposited in a ultrahigh vacuum thermal deposition chamber. 3. Results and Discussion 3.1. Pretreatment. The control of the nucleation sites is of crucial interest for the synthesis of CNTs. Many techniques exist to prepare nucleation sites such as lithography and thermal treatment.9-11 The novelty of our work is to propose a simple, controlled and relatively low-cost technique which is based on a plasma treatment. The interest of this process lies in avoiding the heavy technological steps necessary for lithography processes and patterning large surfaces at shorter time. Our treatment process allows the synthesis of highpurity catalyst particles with a well-defined size distribution in a single step. The mean particle size can be controlled by the total gas pressure, microwave power, catalyst thickness and time treatment. Studying the plasma and its effect on the catalyst iron deposited thin film allows us to propose a model explaining the nanostructuration mechanism of the
Rizk et al. deposited catalyst. Actually, we established a model based on the variation of the plasma parameters. Next parts report on the effects on the catalyst structuration of each of these parameters on both the size of catalyst nanoparticles and their homogeneity. We will see that low gas pressure and moderate power are requisite for homogeneous distribution of catalyst nanoparticles. The size of these nanoparticles is directly linked to the thickness of catalyst film. The produced CNTs on a structured 2-nm-thick iron catalyst film show a diameter of 5 nm. A direct correlation between catalyst particle size and carbon nanotubes diameter was observed. 3.2. Pressure Effect. We have observed the surface of an as-deposited of 10nm-thick Fe film thermally evaporated onto a silicon substrate using scanning electron microscopy (SEM). In Figure 1a, we can see that that the film is continuous and has no defect. As this film is subjected to a hydrogen plasma pretreatment for 2min, its surface topography becomes discontinuous and some nanoparticles of about 150nm in diameter are formed (Figure 1b). A tilted image, by 85°, is presented in the inset of the same figure. This sintering is driven by a surface and elastic minimization, enabled by the increase of the surface mobility of the metal atoms at the substrate surface during the plasma pretreatment.12 Figure 2 shows SEM images of iron surface after the nucleation step at a pressure of 25, 30, and 35 mbar, for 2 min and for a fixed microwave power of 250 W. A large number of nuclei were distributed over the surface. We can see that the size and the heterogeneity of nuclei increased with the increase of the gas pressure: the nucleation process is slowed down as the pressure is increased. Theprincipal reason in the slowdown of the nucleation process is the diminishing of the hydrogen atom energy and density as gas pressure is increased. According to previous models established13 on plasma behavior, as total gas pressure is raised, the majority of the electron-neutral reactions become vibrational. Indeed, as pressure increases more, neutral particles that are present collide with the electrons hence the mean free path for the electrons is shorter. More frequent collisions result in a decrease in electron temperature which shifts electron-neutral reactions toward processes with low threshold energy hence vibrational transfer. Since input power is lost primarily to vibrational excitation, the fraction of input power that leads to dissociation is significantly reduced. Regarding the sample surface temperature variation, we find that it increases with microwave power but stayed almost constant with total gas pressure. The evolution of the plasma density in the reactor (Figure 3) shows that for a fixed microwave power, the plasma density increases with pressure whereas it remains almost unchanged for a fixed pressure when changing the microwave power. 3.3. Time Factor. The exposure time of the pretreatment has an effect on the iron catalyst nucleation as can be seen in Figure 4. As the pressure is modified, pretreatment times have to be varied at the meantime in order to the same nucleation density. When increasing total gas pressure from 25 to 30 mbar, one has to increase the pretreatment time from 2 to 4 min in order to complete the nucleation of a fixed density. For the same nucleation density one needs 10 min of pretreatment for a 35mbar total gas pressure. As explained in the previous section, the energy and the density of the atomic hydrogen that arrive at the sample surface decrease with pressure, so for high pressures more time is needed to reduce the continuous iron film into nanoparticles.
Controlled Nanostructuration
J. Phys. Chem. C, Vol. 113, No. 20, 2009 8721
Figure 8. SEM micrograph of iron catalyst pretreated using high power: (a) present EDSX spectrum obtained on the etched part; (b) EDSX spectrum obtained on the metallic particle.
that the plasma volume increases with microwave power. To go further we have measured the atomic hydrogen concentration above the substrate surface for different microwave power using the laser induced fluorescence technique.14 As expected, the results shown in Figure 7 reveal that atomic hydrogen concentration increases with microwave power and saturates at 450 W. In fact, as the microwave power is increased, more and more electrons are generated through electron-impact ionization which produces more hydrogen atoms. Besides, the interaction between hot electrons and gaseous species increases, causing the rise of the gas temperature. All these factors contribute for the sintering of the film into smaller nanoparticles.
Figure 9. SEM micrographs of pretreated iron catalyst as a function of catalyst thickness.
3.4. Plasma Power Effect. We have studied the effect of plasma microwave power on the nucleation of the iron catalyst. First, as microwave power was increased from 200 to 450 W for a fixed pressure (25 mbar in the case of Figure 5), the average size of the nanoparticles decreases from 200 to 100 nm and slightly increases at higher powers. The surface sample temperature was increased from 840 to 1085 °C (Figure 6). As mentioned previously, the power density remains unchanged when changing the microwave power. This is due to the fact
However, when the microwave power exceeds a certain value (this value depends on pressure), as can be seen on the SEM micrographs in Figure 5, we observe an alteration of the iron surface and a reduction in the density (iron volume) compared to the previous plasma conditions. Under these conditions, the etching effect seems to be dominant. A typical Electron dispersive X-ray spectroscopy spectrum taken from a spherical iron particle is shown in Figure 8b. The strong peak corresponding to Fe is clearly revealed. In addition, peaks corresponding to C and O had much smaller intensities than that of Fe, though the C peak originated from the contamination of SEM observation (carbon probably comes from a contamination during SEM observations). The spectrum showed in Figure 8a reveals that the etched surface corresponds well to the silicon.
8722
J. Phys. Chem. C, Vol. 113, No. 20, 2009
Rizk et al.
Figure 10. Sketch map of catalyst nanostructuration: (a) Fe continuous film deposited at room temperature on a silicon substrate; (b) formation of nanostructured particles after a hydrogen plasma pretreatmen; (c) effect of pressure on nanostructuration, where high total gas pressures lead to a decrease of the energy brought to the surface by hydrogen atoms, and then to a bad nanostructuration and decreasing gas pressure allows the formation of homogeneous nanoparticles as desired for the growth of carbon nanotubes; (d) effect of microwave power. This is the second parameter that acts significantly in the nanostructuration process since increasing it leads to a better nanostructuration but exceeding a certain value brings undesired effects such as substrate damage and defects.
Figure 11. Carbon nanotube’s growth on a 2 nm catalyst thickness: (a) TEM micrograph showing the CNTs (b) TEM micrograph showing the tube superposition. The inset gives an idea about the tube’s diameter.
3.5. Film Thickness Effect. The effect of the modifications of the thicknesses of the catalyst film is shown in Figure 9. As expected, we have observed that the size of the Fe islands depends on the initial thickness of the deposited catalyst film. Under the same plasma conditions, a film 10 nm thick gives
rise to iron islands having an average size of about 100 nm, while a film 2 nm thick results in smaller islands having a size of 10 nm. For intermediate thickness, for example, a 5 nm iron film leads to a nanoparticle diameter of about 20 nm. These results are in agreement with the established correlation between catalyst film thickness and the nanoparticles size in several studies.15-17 On the basis of the obtained results and as we mentioned in the beginning of section 3, a model of controlled catalyst nanostructuration was established. Figure 10 illustrates indeed the effect of the used pressures and the plasma powers on the nanostructuration mechanism. Working at a high total gas pressure leads to a decrease of the energy brought to the surface by hydrogen atoms, and then to a noncontrolled nanostructuration. A diminishing of the gas pressure allows the formation of homogeneous nanoparticles as desired for the growth of carbon nanotubes. Microwave power is the second main parameter that acts significantly for the nanostructuration process; high power are required to guarantee a high-quality nanostructuration but exceeding a certain value induces undesired effects such as substrate damage and defects. 3.6. CNT Growth. Following the optimization of the pretreatment parameters, carbon nanotubes (CNTs) were grown on the patterned iron substrates. After the pretreatment step and without stopping the plasma, a mixture of CH4/H2 is introduced
Controlled Nanostructuration
J. Phys. Chem. C, Vol. 113, No. 20, 2009 8723
Figure 12. TEM images showing the correlation between catalyst nanoparticles (catalyst layer) and CNT diameters.
in the chamber. On the basis of our previous work,18 a “hard” pretreatment leads the formation of silicon carbide. Using high microwave powers and pressures during pretreatment, the silicon substrate is attacked and serves as silicon source for the formation of SiC during the growth step. As shown above, a soft pretreatment is required in order to achieve a controlled nucleation and structuration of the catalyst. Consequently, the chosen pretreatment parameters for the growth of CNTs, are a microwave power of 300W and a total gas pressure of 25 mbar. Figure 11 shows a typical TEM micrograph of the observed carbon nanotubes on a 2nm-thick iron catalyst film. The produced CNT have double walled (inset of Figure 11) and multiwalled structures with an averaged diameter of about 5 nm. These CNTs are of high quality confirmed by Raman spectroscopy, not shown in this paper. Concerning the CNT growth, we have established a direct correlation between catalyst particle size (directly linked to the catalyst thickness) and carbon nanotubes diameter. Figure 12 gives an example showing this correlation. We have observed that the CNT diameter for the 2nm catalyst layer (Figure 12a) is around 2-10 nm and the one for 5 nm catalyst layer (Figure 12b) is around 5-20 nm. 4. Conclusion In this work we propose a simple, controlled and relatively low-cost technique, based on hydrogen plasma pretreatment for thin catalyst film patterning. A nanostructuration mechanism of film catalyst as function of the plasma parameters was proposed. The effects of the plasma parameters such as pressure and microwave power, on the catalyst nanostructuration were separately studied and discussed regarding the plasma behavior. High pressures induced a slow down in the nucleation process whereas high powers lead to a film etching. The hydrogen atoms energy and density were found to play an essential role in structuration process. Long pretreatment times favor film nanostructuration. Furthermore, we show that catalyst thickness controls the diameter of the formed nanoparticles. Following the catalyst patterning, carbon nanotubes were grown under a
CH4/H2 plasma. TEM analysis showed carbon nanotubes having doublewalled and multiwalled structure. Acknowledgment. The authors would like to thank gratefully Dr. Brigitte Vigolo for here fruitful and helpful discussions and Dr. J. Ghanbaja for TEM analysis. References and Notes (1) Melechko, A. V.; Merkulov, V. I.; McKnight, T. E.; Guillorn, M. A.; Klein, K. L.; Lowndes, D. H.; Simpson, M. L. J. Appl. Phys. 2005, 97, 041301. (2) Martel, R.; Schmidt, T.; Shea, H. R.; Hertel, T.; Avouris, P. Appl. Phys. Lett. 1998, 73, 2447. (3) Gro¨ning, P.; Ruffieux, P.; Schlapbach, L.; Gro¨ning, O. Adv. Eng. Mater. 2003, 5, 541. (4) Wang, J. Electroanalysis 2005, 17, 7. (5) Cantalini, C.; Valentini, L.; Lozzi, L.; Armentano, I.; Kenny, J. M.; Santucci, S. Sensors Actuators B 2003, 93, 333. (6) Esconjauregui, S.; Whelan, C. M.; Maex, K. Carbon 2009, 47, 659. (7) Jeong, G.-H.; Olofsson, N.; Falk, L. K. L.; Campbell, E. E. B. Carbon 2009, 47, 696. (8) Chhowalla, M.; Teo, K. B. K.; Ducati, C.; Rupesinghe, N. L.; Amaratunga, G. A. J.; Ferrari, A. C.; Roy, D.; Roberstson, J.; Milne, W. I. J. Appl. Phys. 2001, 90, 5308. (9) Hofmann, S.; Cantoro, M.; Kaempgen, M.; Kang, D.-J.; Golovko, V. B.; Li, H. W.; Yang, Z.; Geng, J.; Huck, W. T. S.; Johnson, B. F. G.; Roth, S. Robertson. J. Appl. Phys. A. 2005, 81, 1559. (10) Wang, S.; Wang, P.; Zhou, O. Diam. Relat. Mater. 2006, 15, 361. (11) Tseng, S.-C.; Tsai, C.-H.; Lee, C.-H.; Tsai, C.-H.; Chen, S.-P.; Liang, C.-C.; Hsieh, G. W.; Lin, W.-C. J. Phys.: Conf. Ser. 2005, 10, 186. (12) Jiran, E.; Thompson, C. V. J. Electron. Mater. 1990, 19, 1153. (13) Chen, C.-K.; Wei, T.-C.; Collins, L.-R.; Phillips, J. J. Phys. D: Appl. Phys. 1999, 32, 688. (14) De Poucques, L.; Bougdira, J.; Hugon, R.; Henrion, G.; Alnot, P. J. Phys. D: Appl. Phys. 2001, 34, 896. (15) Rizzo, A.; Rossi, R.; Signore, M. A.; Piscopiello, E.; Capodieci, L.; Pentassuglia, R.; Dikonimos, T.; Giorgi, R. Diam. Relat. Mater. 2008, 17, 1502. (16) Garg, R. K.; Kim, S. S.; Hash, D. B.; Gore, J. P.; Fisher, T. S. J. Nanosci. Nanotechnol. 2008, 8, 3068. (17) Choi, J. H.; Lee, T. Y.; Choi, S. H.; Han, J. H.; Yoo, J.-b.; Park, C.-Y.; Jung, T.; Yu, S. G.; Yi, W.; Han, I.-T.; Kim, J. M. Diam. Relat. Mater 2003, 12, 794. (18) Rizk, S.; Assouar, M. B.; Belmahi, M.; Le Brizoual, L.; Bougdira, J. Phys. Status Solidi A. 2007, 204, 3085.
JP9006334
