- Email: [email protected]
A study of the effect of different catalysts for the efficient CVD growth of carbon nanotubes on silicon substrates
ARTICLE IN PRESS
Physica E 37 (2007) 6–10 www.elsevier.com/locate/physe
A study of the effect of different catalysts for the efficient CVD growth of carbon nanotubes on silicon substrates Alberto Ansaldoa,, Miroslav Halusˇ kac, Jirˇ ı´ Cˇechc, Jannik C. Meyerc, Davide Riccia, Flavio Gattib, Ermanno Di Zittia, Silvano Cincottia, Siegmar Rothc a
Dipartimento di Ingegneria Biofisica ed Elettronica, Universita` di Genova, Via Opera Pia 11a, I-16145 Genova, Italy b Dipartimento di Fisica Universita` di Genova, Via Dodecanneso 33, I-16146 Genova, Italy c Max-Planck Institut fu¨r Festko¨rperforschung, HeisenbergstraX e 1, D-70569 Stuttgart, Germany Available online 13 November 2006
Abstract We report on the identification of efficient combinations of catalyst, carbon feedstock, and temperature for the ethanol chemical vapour deposition (CVD) growth of single-wall carbon nanotubes (SWCNTs) onto silicon substrates. Different catalyst preparations, based on organometallic salts (Co, Fe, Mo, Ni acetate, and bimetallic mixtures), have been spin coated onto thermally grown silicon dioxide on silicon chips to perform tests in a temperature range between 500 and 900 1C. The samples have been then characterized by Raman spectroscopy, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. Assuming the growth of high-quality isolated nanotubes as target, the ratio in Raman spectra between the intensity of the G peak and of the D peak has been used as the main parameter to evaluate the performance of the catalytic process. A comparison made for both single metals and bimetallic mixtures points out best conditions to achieve efficient CVD growth of SWCNTs. r 2006 Elsevier B.V. All rights reserved. PACS: 81.07.De Keywords: Fabrication of single-wall carbon nanotubes, SWCNT; Chemical vapour deposition; Fabrication and characterization of nanoscale materials
1. Introduction The fascinating properties of carbon nanotubes (CNTs) [1] make them suitable elements for the development of beyond-the-roadmap applications in future microelectronics [2,3]. Silicon-based nanoscale devices could be wired by CNT interconnections [4,5] outperforming conventional materials, or single-wall CNTs (SWCNTs) could be used within device structures [6], as demonstrated by many different schemes of CNT field effect transistors (CNTFETs) [7–10]. One of the essential issues in these applications is the controlled growth of SWCNTs at specific locations and in specific directions. To this aim, in-situ chemical vapour deposition (CVD) CNT growth has been investigated, and many different CVD Corresponding author. Tel.: +39 010 353 2382; fax: +39 010 353 2020.
E-mail address: [email protected] (A. Ansaldo). 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.09.008
processes were developed [11–15]. However, several issues, such as size, helicity, and orientation of SWCNTs grown onto typical microelectronic substrates—and among them the widely present silicon dioxide—still have to be addressed [3,16]. This work aims to study the influence of different catalyst preparations in an ethanol-CVD process for SWCNT growth onto a thermally grown silicon dioxide on silicon substrate. Such a process offers potential advantages such as low synthesis temperature, simplicity, good yield, and allows obtaining high-quality nanotubes. We performed a detailed comparison among different catalyst preparations while varying the temperature. 2. Experimental The reactor is a quartz tube of 120 cm length and 25 mm inner diameter that is fixed and goes through a Cabolite
ARTICLE IN PRESS A. Ansaldo et al. / Physica E 37 (2007) 6–10
STF 15/–/180 tube furnace. The furnace is placed onto a trolley and can be moved on a rail. This solution allows the samples to heat and cool in a few minutes, virtually independently from the behaviour of the oven itself. A 5 mM solution of a metal acetate (Co, Ni, Fe, Mo) or a mix of two solutions, is spin coated on a thermally grown silicon dioxide surface on p-doped silicon chips. The CVD growth is performed by bubbling an argon mixture (95% Ar, 5% H2) through ethanol (room temperature) at atmospheric pressure (gas flow 230 sccm). Decomposition and reduction of the organic salts to activate the catalyst are performed directly in the reactor by annealing for 15 min in the argon mixture. After CNT growth, the samples have been characterized by Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Raman spectra were measured using microscope laser Raman spectroscopy with a Jobin Yvon–LabRam spectrometer. The laser excitation wavelength was 633 nm. SEM was performed using Zeiss ULTRA 55 FESEM and LEO 1530 VP FESEM. AFM imaging was performed using a PSIA XE-100 instrument in non-contact mode equipped with ultra-sharp MikroMasch silicon tips. Experiments have been devised with the aim of selecting the most promising combinations of catalysts and thermodynamic conditions for growing isolated CNTs and sparse networks. The ratio of the intensity of the G and D peaks in the Raman spectra has been used to estimate and compare the quality of the nanotubes on the sample. These data can be plotted as a function of metal ratios in the catalysts and of temperature, thus giving a compact, quantitative and easy to interpret representation that could drive further investigations. The first experiment was aimed at comparing the single metal preparations. Four different metal salt solutions (Ni, Co, Fe, Mo) have been spin coated on the substrates (4 4 mm2 sample, p-doped silicon, 100 nm thick wet thermally grown oxide). Different temperatures between 500 and 900 1C have been used (Fig. 1). The growth has been performed at the same time on the whole batch of four chips (one for each metal), to ensure the closest growing conditions. In a second step we tested the binary alloys of most promising metals—namely cobalt–iron and cobalt–nickel—in the temperature range of 7001–9001C.
7
nanotubes (approximately 585 1C in our experiments) is in agreement with previous results in Ref. [17]. Following these results, two families of bimetallic compounds have been tested: iron–cobalt (Fe–Co) and nickel–cobalt (Ni–Co). For each metal a 5 mmol solution has been prepared. The different bimetallic preparations have then been mixed in the following ratios: 1:9, 3:7, 5:5, 7:3, and 9:1. The growth has been performed on each batch of seven chips (the two pure metals, plus the five mixtures) at the same time. The activity of cobalt appears to be strongly reduced by mixing it with iron (Fig. 2), both in
Fig. 1. Comparison of the behaviour of the different pure metals (cobalt, nickel, iron, molybdenum) while varying the temperature (from 500 to 900 1C).
3. Results and discussion Iron preparation shows a very weak activity for nanotube growth, and molybdenum does not seem to work at all. The behaviour of cobalt and nickel is more interesting: while cobalt is very efficient at high temperature and its activity drops with decreasing temperature, nickel shows an increment in the ratio of the intensity of G and D peaks by reducing the temperature down to 600 1C. Below this temperature the activity decreases dramatically. The activation temperature of nickel as catalyst for
Fig. 2. Comparison of the behaviour of Fe–Co mixtures (from pure cobalt to pure iron, 10:0, 9:1, 7:3, 5:5, 3:7, 1:9, 0:10) at different temperatures (900, 800, and 700 1C): plot of the ratio between the intensity of G and D peaks while varying the parameters.
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A. Ansaldo et al. / Physica E 37 (2007) 6–10
quality and yield. The ratio between the intensity of G and D peaks gets lower and lower by adding iron for all the temperatures tested (900, 800, 700 1C). The best Raman spectrum at each temperature is always obtained for pure cobalt (Fig. 3). The scenario changes completely for Ni–Co mixtures. For these compounds, at least one mixture always shows nanotubes of higher quality than those obtained with pure metals (Fig. 4). The optimal ratio is different at each temperature (Fig. 5). At 900 1C, adding 10% of nickel to pure cobalt (1:9 ratio) enhances our quality index of 90% (intensity of G/D peak ratio from 71 to 133). When the
Fig. 5. Raman spectra of the samples prepared with the best catalyst Ni– Co mixture for each temperature.
Fig. 3. Raman spectra of the samples prepared with the best catalyst Fe–Co mixture for each temperature.
Fig. 4. Comparison of the behaviour of Ni–Co mixtures (from pure cobalt to pure nickel, 10:0, 9:1, 7:3, 5:5, 3:7, 1:9, 0:10) at different temperatures (9001, 8001, and 700 1C): plot of the ratio between the intensity of G and D peaks while varying the parameters.
temperature is lowered to 800 1C cobalt starts to lose its effectiveness, but it is still possible to achieve a higher quality (intensity of G/D peak ratio equal to 77) by adding 50% of nickel (1:1 ratio). Finally, the 7:3 ratio seems to be the best choice at 700 1C: again the performance of the selected mixture is better than the best pure metal tested. The G/D intensity is 22 for pure nickel, while it is close to 55 for the 7:3 Ni–Co catalyst. Such values are consistent with the behaviour of single metals, but it is remarkable that these particular mixtures show higher activity than pure metals themselves. Concerning the alloying nature of the catalyst, it is noteworthy that the crystalline structure of the bulk material in the equilibrium—in all the ratios and temperatures tested—should be monophase and BCC, according to the phase diagram for iron–cobalt and nickel–cobalt [18]. This fact could also explain the low activity of iron and iron–cobalt alloys in our experiments as the catalytically active form of iron for CNTs is referred to be the gammairon (FCC) [19]. AFM and SEM imaging confirmed the presence of single-wall nanotubes and a quite homogeneous distribution of catalysts and nanotubes on the whole surface, except the rim of the chip, where a higher concentration of catalyst is present, due to the deposition method (spincoating). A correlation between Raman measurements and these images is quite clear. On samples prepared with iron, molybdenum, and cobalt–iron mixtures, the nanotubes are hard to find and of poor quality (short multi-wall nanotubes and other bent fibres). Cobalt forms a continuous carpet-like network, pretty dense, but the quality drop is immediately evident by adding only 10% of iron. Pure nickel shows behaviour again coherent with prevalence of isolated tubes or very sparse networks. Finally, cobalt–nickel compounds present the best behaviour, allowing the production of relatively long
ARTICLE IN PRESS A. Ansaldo et al. / Physica E 37 (2007) 6–10
and high-quality nanotubes, with the possibility of tuning the production from ‘‘carpet-like’’ structures to isolated nanotubes. One of the problems with such kinds of samples is that dense networks are quite difficult to be characterized because of nanotube crossings, and also the presence of a thick insulating support that makes both SEM a nd TEM imaging challenging. Atomic force microscopy of a ‘‘carpet-like’’ network of SWCNTs grown with pure cobalt at 900 1C, for example, gives us information on diameters but it is impossible to estimate nanotube length. SEM imaging (Fig. 6) confirms the presence of this dense network and demonstrates the remarkable length of the obtained nanotubes. The nanotubes are straight, i.e. there is a low number of defects, in agreement with the very high G/D ratio (low defect level in nanotubes) measured in Raman spectra. The AFM characterization of a sample like Ni:Co 1:1, 900 1C is easier as it shows some quite long isolated SWCNTs. In this case, SEM imaging is useful to understand the network structure (Fig. 7): nanotubes are long but less straight as compared to pure cobalt, due
9
Fig. 8. HR-TEM image of a bundle of two nanotubes prepared with pure cobalt at 900 1C.
Fig. 6. SEM image of a sample prepared with pure cobalt at 900 1C. Fig. 9. (a) Low magnification dark field TEM of a selected sample. The arrow points to the position where we measured electron diffraction of one individual freestanding SWNT. (b) Electron diffraction pattern of an individual SWNT. It is identified as (10,6) by comparison with simulations [18].
Fig. 7. SEM image of a sample prepared with a Ni–Co acetate mixture (Ni:Co 1:1) at 900 1C.
probably to the presence of a larger number of defects, again in agreement with Raman measurements. The length and diameter of the tubes changes with both catalyst and temperature. For the samples showing high G/D intensity, the nanotubes length ranges from 1 to 20 mm (typically 10 and 15 mm), while the nanotubes diameter ranges between 0.9 and 2.1 nm. A further, though preliminary, characterization by HR-TEM, bright and dark field TEM, and by electron diffraction from individual SWCNTs has been accomplished. The results confirmed the high quality of the nanotube production. Samples have been prepared by evaporating gold grids, then the silicon dioxide was
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completely etched away by an HF 1% aqueous solution. The first HR-TEM images taken on a sample prepared with cobalt at 900 1C show nice SWCNTs, very often in bundles of two (Fig. 8). No double-wall or multi-wall nanotubes have been found. Using a different etching process [20], it has been possible to prepare specimens from the samples grown using Ni:Co 1:1 at 900 1C, for electron diffraction and dark field imaging. The dark field TEM lets us select the nanotube to be measured (Fig. 9(a)). Having the electron diffraction pattern, it is possible to identify the nanotube by a comparison with simulations [20]. The diffraction patterns show a high crystallinity of the nanotubes. Fig. 9(b) shows the pattern of a (10,6) SWNT, which is semiconducting and has a diameter of 1.096 nm, thus confirming the presence of rather small diameters in our CVD production. 4. Conclusions A variety of different metals and mixtures have been tested as catalysts for thermal ethanol-CVD growth of SWCNTs on thermally grown silicon dioxide on silicon substrates. Pure cobalt and pure nickel are more efficient than iron and molybdenum. Ni–Co mixtures are particularly promising for obtaining an efficient growth process at low temperatures. For some temperatures the optimum Ni/ Co ratios have been determined. Acknowledgement Work supported by Fondazione CARIGE, DAAD, and the European Commission’s Sixth Framework Programme (SPANG Project Contract No. NMP4-CT-2003-505483).
References [1] M.S. Dresselhaus, G. Dresselhaus, P. Avouris (Eds.), Carbon Nanotubes Synthesis, Structure, Properties, and Applications, Springer, Berlin, 2001. [2] International Technology Roadmap for Semiconductors, 2005, version, http://public.itrs.net. [3] A.P. Graham, G.S. Duesberg, W. Hoenlein, F. Kreupl, M. Liebau, R. Martin, B. Rajasekharan, W. Pamler, R. Seidel, W. Steinhoegl, E. Unger, Appl. Phys. A 80 (2005) 1141. [4] Y. Homma, T. Yamashita, Y. Kobayashi, T. Ogino, Physica B 323 (2002) 122. [5] Y. Kobayashi, D. Takagi, Y. Ueno, Y. Homma, Physica E 24 (2004) 26. [6] V. Derycke, R. Martel, J. Appenzeller, P. Avouris, Nano Lett. 1 (2001) 453. [7] R.V. Seidel, A.P. Graham, J. Kretz, B. Rajasekharan, G.S. Duesberg, M. Liebau, E. Unger, F. Kreupl, W. Hoenlein, Nano Lett. 5 (2005) 147. [8] A. Javey, R. Tu, D. Farmer, J. Guo, R. Gordon, H. Dai, Nano Lett. 5 (2005) 345. [9] Y. Lin, J. Appenzeller, J. Knoch, P. Avouris, IEEE Trans. Nanotechnol. 4 (2005) 481. [10] J. Appenzeller, Y. Lin, J. Knoch, Z. Chen, P. Avouris, IEEE Trans. Electron. Dev. 52 (2005) 2568. [11] A. Ural, Y. Li, H. Dai, Appl. Phys. Lett. 81 (2002) 3464. [12] G.S. Duesberg, A.P. Graham, M. Liebau, R. Seidel, E. Unger, F. Kreupl, W. Hoenlein, Nano Lett. 3 (2003) 257. [13] X. Wang, A. Volodin, C. Van Haesendonck, N. Moreau, A. Fonseca, J.B. Nagy, Physica E 25 (2005) 597. [14] S. Maruyama, R. Kojima, Y. Miyauchi, S. Chiashi, M. Kohno, Chem. Phys. Lett. 360 (2002) 229. [15] H.E. Unalan, M. Chhowalla, Nanotechnology 16 (2005) 2153. [16] H. Dai, Surf. Sci. 500 (2002) 218. [17] R. Seidel, G.S. Duesberg, E. Unger, A.P. Graham, M. Liebau, F. Kreupl, J. Phys. Chem. B 108 (2004) 1888. [18] Massalski, Tadeusz B., [Hrsg.], ASM International, Binary Alloy Phase Diagrams, Metals Park, OH, 1990. [19] Y.J. Jung, B. Wei, R. Vajtai, P.M. Ajayan, Y. Homma, K. Prabhakaran, T. Ogino, Nano Lett 3 (2003) 561–564. [20] J.C. Meyer, M. Paillet, G.S. Duesberg, S. Roth, Ultramicroscopy 106 (2006) 176.
Physica E 37 (2007) 6–10 www.elsevier.com/locate/physe
A study of the effect of different catalysts for the efficient CVD growth of carbon nanotubes on silicon substrates Alberto Ansaldoa,, Miroslav Halusˇ kac, Jirˇ ı´ Cˇechc, Jannik C. Meyerc, Davide Riccia, Flavio Gattib, Ermanno Di Zittia, Silvano Cincottia, Siegmar Rothc a
Dipartimento di Ingegneria Biofisica ed Elettronica, Universita` di Genova, Via Opera Pia 11a, I-16145 Genova, Italy b Dipartimento di Fisica Universita` di Genova, Via Dodecanneso 33, I-16146 Genova, Italy c Max-Planck Institut fu¨r Festko¨rperforschung, HeisenbergstraX e 1, D-70569 Stuttgart, Germany Available online 13 November 2006
Abstract We report on the identification of efficient combinations of catalyst, carbon feedstock, and temperature for the ethanol chemical vapour deposition (CVD) growth of single-wall carbon nanotubes (SWCNTs) onto silicon substrates. Different catalyst preparations, based on organometallic salts (Co, Fe, Mo, Ni acetate, and bimetallic mixtures), have been spin coated onto thermally grown silicon dioxide on silicon chips to perform tests in a temperature range between 500 and 900 1C. The samples have been then characterized by Raman spectroscopy, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. Assuming the growth of high-quality isolated nanotubes as target, the ratio in Raman spectra between the intensity of the G peak and of the D peak has been used as the main parameter to evaluate the performance of the catalytic process. A comparison made for both single metals and bimetallic mixtures points out best conditions to achieve efficient CVD growth of SWCNTs. r 2006 Elsevier B.V. All rights reserved. PACS: 81.07.De Keywords: Fabrication of single-wall carbon nanotubes, SWCNT; Chemical vapour deposition; Fabrication and characterization of nanoscale materials
1. Introduction The fascinating properties of carbon nanotubes (CNTs) [1] make them suitable elements for the development of beyond-the-roadmap applications in future microelectronics [2,3]. Silicon-based nanoscale devices could be wired by CNT interconnections [4,5] outperforming conventional materials, or single-wall CNTs (SWCNTs) could be used within device structures [6], as demonstrated by many different schemes of CNT field effect transistors (CNTFETs) [7–10]. One of the essential issues in these applications is the controlled growth of SWCNTs at specific locations and in specific directions. To this aim, in-situ chemical vapour deposition (CVD) CNT growth has been investigated, and many different CVD Corresponding author. Tel.: +39 010 353 2382; fax: +39 010 353 2020.
E-mail address: [email protected] (A. Ansaldo). 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.09.008
processes were developed [11–15]. However, several issues, such as size, helicity, and orientation of SWCNTs grown onto typical microelectronic substrates—and among them the widely present silicon dioxide—still have to be addressed [3,16]. This work aims to study the influence of different catalyst preparations in an ethanol-CVD process for SWCNT growth onto a thermally grown silicon dioxide on silicon substrate. Such a process offers potential advantages such as low synthesis temperature, simplicity, good yield, and allows obtaining high-quality nanotubes. We performed a detailed comparison among different catalyst preparations while varying the temperature. 2. Experimental The reactor is a quartz tube of 120 cm length and 25 mm inner diameter that is fixed and goes through a Cabolite
ARTICLE IN PRESS A. Ansaldo et al. / Physica E 37 (2007) 6–10
STF 15/–/180 tube furnace. The furnace is placed onto a trolley and can be moved on a rail. This solution allows the samples to heat and cool in a few minutes, virtually independently from the behaviour of the oven itself. A 5 mM solution of a metal acetate (Co, Ni, Fe, Mo) or a mix of two solutions, is spin coated on a thermally grown silicon dioxide surface on p-doped silicon chips. The CVD growth is performed by bubbling an argon mixture (95% Ar, 5% H2) through ethanol (room temperature) at atmospheric pressure (gas flow 230 sccm). Decomposition and reduction of the organic salts to activate the catalyst are performed directly in the reactor by annealing for 15 min in the argon mixture. After CNT growth, the samples have been characterized by Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM). Raman spectra were measured using microscope laser Raman spectroscopy with a Jobin Yvon–LabRam spectrometer. The laser excitation wavelength was 633 nm. SEM was performed using Zeiss ULTRA 55 FESEM and LEO 1530 VP FESEM. AFM imaging was performed using a PSIA XE-100 instrument in non-contact mode equipped with ultra-sharp MikroMasch silicon tips. Experiments have been devised with the aim of selecting the most promising combinations of catalysts and thermodynamic conditions for growing isolated CNTs and sparse networks. The ratio of the intensity of the G and D peaks in the Raman spectra has been used to estimate and compare the quality of the nanotubes on the sample. These data can be plotted as a function of metal ratios in the catalysts and of temperature, thus giving a compact, quantitative and easy to interpret representation that could drive further investigations. The first experiment was aimed at comparing the single metal preparations. Four different metal salt solutions (Ni, Co, Fe, Mo) have been spin coated on the substrates (4 4 mm2 sample, p-doped silicon, 100 nm thick wet thermally grown oxide). Different temperatures between 500 and 900 1C have been used (Fig. 1). The growth has been performed at the same time on the whole batch of four chips (one for each metal), to ensure the closest growing conditions. In a second step we tested the binary alloys of most promising metals—namely cobalt–iron and cobalt–nickel—in the temperature range of 7001–9001C.
7
nanotubes (approximately 585 1C in our experiments) is in agreement with previous results in Ref. [17]. Following these results, two families of bimetallic compounds have been tested: iron–cobalt (Fe–Co) and nickel–cobalt (Ni–Co). For each metal a 5 mmol solution has been prepared. The different bimetallic preparations have then been mixed in the following ratios: 1:9, 3:7, 5:5, 7:3, and 9:1. The growth has been performed on each batch of seven chips (the two pure metals, plus the five mixtures) at the same time. The activity of cobalt appears to be strongly reduced by mixing it with iron (Fig. 2), both in
Fig. 1. Comparison of the behaviour of the different pure metals (cobalt, nickel, iron, molybdenum) while varying the temperature (from 500 to 900 1C).
3. Results and discussion Iron preparation shows a very weak activity for nanotube growth, and molybdenum does not seem to work at all. The behaviour of cobalt and nickel is more interesting: while cobalt is very efficient at high temperature and its activity drops with decreasing temperature, nickel shows an increment in the ratio of the intensity of G and D peaks by reducing the temperature down to 600 1C. Below this temperature the activity decreases dramatically. The activation temperature of nickel as catalyst for
Fig. 2. Comparison of the behaviour of Fe–Co mixtures (from pure cobalt to pure iron, 10:0, 9:1, 7:3, 5:5, 3:7, 1:9, 0:10) at different temperatures (900, 800, and 700 1C): plot of the ratio between the intensity of G and D peaks while varying the parameters.
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A. Ansaldo et al. / Physica E 37 (2007) 6–10
quality and yield. The ratio between the intensity of G and D peaks gets lower and lower by adding iron for all the temperatures tested (900, 800, 700 1C). The best Raman spectrum at each temperature is always obtained for pure cobalt (Fig. 3). The scenario changes completely for Ni–Co mixtures. For these compounds, at least one mixture always shows nanotubes of higher quality than those obtained with pure metals (Fig. 4). The optimal ratio is different at each temperature (Fig. 5). At 900 1C, adding 10% of nickel to pure cobalt (1:9 ratio) enhances our quality index of 90% (intensity of G/D peak ratio from 71 to 133). When the
Fig. 5. Raman spectra of the samples prepared with the best catalyst Ni– Co mixture for each temperature.
Fig. 3. Raman spectra of the samples prepared with the best catalyst Fe–Co mixture for each temperature.
Fig. 4. Comparison of the behaviour of Ni–Co mixtures (from pure cobalt to pure nickel, 10:0, 9:1, 7:3, 5:5, 3:7, 1:9, 0:10) at different temperatures (9001, 8001, and 700 1C): plot of the ratio between the intensity of G and D peaks while varying the parameters.
temperature is lowered to 800 1C cobalt starts to lose its effectiveness, but it is still possible to achieve a higher quality (intensity of G/D peak ratio equal to 77) by adding 50% of nickel (1:1 ratio). Finally, the 7:3 ratio seems to be the best choice at 700 1C: again the performance of the selected mixture is better than the best pure metal tested. The G/D intensity is 22 for pure nickel, while it is close to 55 for the 7:3 Ni–Co catalyst. Such values are consistent with the behaviour of single metals, but it is remarkable that these particular mixtures show higher activity than pure metals themselves. Concerning the alloying nature of the catalyst, it is noteworthy that the crystalline structure of the bulk material in the equilibrium—in all the ratios and temperatures tested—should be monophase and BCC, according to the phase diagram for iron–cobalt and nickel–cobalt [18]. This fact could also explain the low activity of iron and iron–cobalt alloys in our experiments as the catalytically active form of iron for CNTs is referred to be the gammairon (FCC) [19]. AFM and SEM imaging confirmed the presence of single-wall nanotubes and a quite homogeneous distribution of catalysts and nanotubes on the whole surface, except the rim of the chip, where a higher concentration of catalyst is present, due to the deposition method (spincoating). A correlation between Raman measurements and these images is quite clear. On samples prepared with iron, molybdenum, and cobalt–iron mixtures, the nanotubes are hard to find and of poor quality (short multi-wall nanotubes and other bent fibres). Cobalt forms a continuous carpet-like network, pretty dense, but the quality drop is immediately evident by adding only 10% of iron. Pure nickel shows behaviour again coherent with prevalence of isolated tubes or very sparse networks. Finally, cobalt–nickel compounds present the best behaviour, allowing the production of relatively long
ARTICLE IN PRESS A. Ansaldo et al. / Physica E 37 (2007) 6–10
and high-quality nanotubes, with the possibility of tuning the production from ‘‘carpet-like’’ structures to isolated nanotubes. One of the problems with such kinds of samples is that dense networks are quite difficult to be characterized because of nanotube crossings, and also the presence of a thick insulating support that makes both SEM a nd TEM imaging challenging. Atomic force microscopy of a ‘‘carpet-like’’ network of SWCNTs grown with pure cobalt at 900 1C, for example, gives us information on diameters but it is impossible to estimate nanotube length. SEM imaging (Fig. 6) confirms the presence of this dense network and demonstrates the remarkable length of the obtained nanotubes. The nanotubes are straight, i.e. there is a low number of defects, in agreement with the very high G/D ratio (low defect level in nanotubes) measured in Raman spectra. The AFM characterization of a sample like Ni:Co 1:1, 900 1C is easier as it shows some quite long isolated SWCNTs. In this case, SEM imaging is useful to understand the network structure (Fig. 7): nanotubes are long but less straight as compared to pure cobalt, due
9
Fig. 8. HR-TEM image of a bundle of two nanotubes prepared with pure cobalt at 900 1C.
Fig. 6. SEM image of a sample prepared with pure cobalt at 900 1C. Fig. 9. (a) Low magnification dark field TEM of a selected sample. The arrow points to the position where we measured electron diffraction of one individual freestanding SWNT. (b) Electron diffraction pattern of an individual SWNT. It is identified as (10,6) by comparison with simulations [18].
Fig. 7. SEM image of a sample prepared with a Ni–Co acetate mixture (Ni:Co 1:1) at 900 1C.
probably to the presence of a larger number of defects, again in agreement with Raman measurements. The length and diameter of the tubes changes with both catalyst and temperature. For the samples showing high G/D intensity, the nanotubes length ranges from 1 to 20 mm (typically 10 and 15 mm), while the nanotubes diameter ranges between 0.9 and 2.1 nm. A further, though preliminary, characterization by HR-TEM, bright and dark field TEM, and by electron diffraction from individual SWCNTs has been accomplished. The results confirmed the high quality of the nanotube production. Samples have been prepared by evaporating gold grids, then the silicon dioxide was
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completely etched away by an HF 1% aqueous solution. The first HR-TEM images taken on a sample prepared with cobalt at 900 1C show nice SWCNTs, very often in bundles of two (Fig. 8). No double-wall or multi-wall nanotubes have been found. Using a different etching process [20], it has been possible to prepare specimens from the samples grown using Ni:Co 1:1 at 900 1C, for electron diffraction and dark field imaging. The dark field TEM lets us select the nanotube to be measured (Fig. 9(a)). Having the electron diffraction pattern, it is possible to identify the nanotube by a comparison with simulations [20]. The diffraction patterns show a high crystallinity of the nanotubes. Fig. 9(b) shows the pattern of a (10,6) SWNT, which is semiconducting and has a diameter of 1.096 nm, thus confirming the presence of rather small diameters in our CVD production. 4. Conclusions A variety of different metals and mixtures have been tested as catalysts for thermal ethanol-CVD growth of SWCNTs on thermally grown silicon dioxide on silicon substrates. Pure cobalt and pure nickel are more efficient than iron and molybdenum. Ni–Co mixtures are particularly promising for obtaining an efficient growth process at low temperatures. For some temperatures the optimum Ni/ Co ratios have been determined. Acknowledgement Work supported by Fondazione CARIGE, DAAD, and the European Commission’s Sixth Framework Programme (SPANG Project Contract No. NMP4-CT-2003-505483).
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