- Email: [email protected]
Both-end opened nanostructure holes by embedded carbon nanotubes realized on thinned membranes on (1 0 0) silicon substrates
ARTICLE IN PRESS
Physica E 37 (2007) 226–230 www.elsevier.com/locate/physe
Both-end opened nanostructure holes by embedded carbon nanotubes realized on thinned membranes on (1 0 0) silicon substrates Y. Abdia,b, S. Mohajerzadeha,, S. Darbaria, E. Arzib a
Nano-Electronics Center of Excellence, Thin Film Lab, Department of Electrical and Computer Eng., University of Tehran, Tehran, Iran b Department of Physics, University of Tehran, Tehran, Iran Received 8 May 2006; received in revised form 24 June 2006; accepted 26 June 2006 Available online 10 August 2006
Abstract Vertically aligned carbon nanotubes (CNT) have been grown in a DC-PECVD apparatus on quartz membranes. /1 0 0S-oriented Si wafer has been anisotropically etched in a KOH solution. A mixture of acetylene and hydrogen gases is used to grow CNT while Ni acts as the catalyst layer. As-grown structures have been coated by titanium dioxide using chemical vapor deposition at atmospheric pressure. By means of a polishing and ashing process steps followed by total removing of the quartz membrane both ends of CNTs are opened and nano holes are obtained. SEM analysis is used to study the evolution of such nanostructures. r 2006 Elsevier B.V. All rights reserved. PACS: 68.65. k; 81.07.De; 81.15.Gh Keywords: Nanotubes; Membranes; Plasma ashing; Nano holes
1. Introduction Because of their unique properties, carbon nanotubes (CNTs) have found many applications in nanotechnology. Their sharp and pointed structure makes them suitable candidates for field emission devices and flat panel displays [1,2]. Also the possibility of ballistic transport though the tube allows realization of high-performance field effect transistors. The extraordinary mechanical properties of CNTs suits them for application as AFM tip or for direct writing on a resist-coated surface. The application of such nanostructures in nanolithography has also been recently reported [3]. Several techniques have been used for the growth of CNTs among which one can highlight thermal catalystbased CVD [4,5], laser-vaporization deposition [6] and arcdischarge growth [7,8], offering single-wall structures. However CNTs with suitable vertical alignment have been produced using a plasma-enhanced CVD approach [9,10]. The direction of the applied electric field of the plasma Corresponding author.
E-mail address: smohajer@tfl.ir (S. Mohajerzadeh). 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.06.008
governs the CNT’s alignment, yielding a vertical growth. A tip-plate configuration was found suitable for the growth of near-vertically aligned nanotubes at high pressures, not easily obtained with standard PECVD methods [11,12]. In this paper we report, for the first time, the growth of CNTs on a silicon-based membrane and its application for the creation of both-end opened nanostructures embedded in a titanium-oxide membrane. The growth of multi-wall nanotubes has been achieved in a DC-PECVD system for a vertical alignment configuration. 2. Experimental setup The fabrication process started with the cleaning of (1 0 0)-oriented silicon wafers using an RCA ]1 solution. Both sides of the silicon wafer were then coated by a thermally grown SiO2 at a temperature of 1050 1C in an oxygen environment. Subsequently a 5–10 mm thick quartz layer is deposited on the back and front sides of the process wafers to act as the masking layer for the etching step as well as the membrane material, respectively. The quartz layer was deposited using e-beam evaporation at a temperature of 350 1C and at a base pressure of
ARTICLE IN PRESS Y. Abdi et al. / Physica E 37 (2007) 226–230
227
Fig. 1. A schematic of the process flow for the formation of embedded nanotubes on silicon substrates; starting with the growth of CNTs (top-left), deposition of TiO2 (top-right) and polishing/ashing steps (bottom).
5 10 6 Torr. Using standard photolithography proper openings on the back side of the wafer was made and the prepared sample was immersed in an 8 M KOH solution at a temperature of 60 1C to carry out the anisotropic etching step. After 20 h of etching all the silicon is removed in a rectangular or pyramidal shape and a quartz membrane is achieved on the opposite side of the sample which is not directly exposed to the etching solution. During the etching from the wafer back side, the front side of the sample must be protected to ensure that no damage happens to the front surface. After properly cleaning the membrane-carrying silicon sample, it was coated with a 5–10 nm thickness of Ni in an e-beam evaporation system and at a temperature of 300 1C. Ni is necessary for the CNT growth, acting as a catalyst. The specimen was then placed in the DC-PECVD chamber and was treated at the temperature of 650 1C with hydrogen blow at a flow rate of 30 sccm for 15 min. Immediately after this step, a 5.5 W/cm2 power density of plasma was applied to pre-condition the Ni seed and to form nanosized islands of nickel. After 5 min, acetylene (C2H2) was introduced into the chamber with a flow rate of 5 sccm for 20 min in order to initiate the growth of CNTs. Substrate temperature during the growth ranged from 550 to 650 1C and gas pressure was maintained at 1.6 Torr. The CNTs, grown on membrane, were coated by 100 nm thickness of titanium oxide using a CVD method. Coated structures were then mechanically polished to expose the nanotubes tips. By means of a 5% HF solution, the quartz membrane was removed and CNTs embedded in TiO2 were suspended. Using Oxygen plasma ashing at a temperature of 300 1C and 150 W plasma power from both front and back sides of sample, both ends of CNTs are burned. Oxygen plasma has strongly reactive oxygen species which can easily burn carbon and form carbon dioxide. This ashing process can be controlled to fully remove the whole CNTs and to form nanosize all-the-way-through holes in the titanium-oxide film. Finally we have both ends of
Fig. 2. An SEM image of the etched Silicon substrate from back side, evidencing a pyramidal crater formation and (b) the optical image of the sample taken in an angled view to show the back side of the sample.
nanotubes opened and exposed. By the aids of a plasmaashing step, the whole nanotube is burned out and a hole with a diameter equivalent to the outer diameter of CNTs is formed. Using this method hole diameter of such nanostructures is expected to be 50–100 nm. Fig. 1 schematically illustrates the fabrication steps, including the CNT growth and the sequence of deposition/ashing/polishing processes.
ARTICLE IN PRESS 228
Y. Abdi et al. / Physica E 37 (2007) 226–230
Fig. 3. SEM images corresponding to different samples prepared for this experiment. (a) A patterned growth of little lions as part of the university logo, and (b) the bare growth of CNTs on the silicon substrate. This sample has been obtained by patterning the nickel layer prior to the growth of CNTs. Features finer than 2 mm are achieved using this lithography and growth sequence.
Scanning electron microscopy (SEM) was used to study the structure of the nanostructures. The SEM was performed on a CAMSCAN2300 at an electron accelerating voltage of 25 kV and all images were generated using the secondary electron imaging. 3. Results and discussion Fig. 2 presents the SEM image of anisotropically etched /1 0 0S-oriented Si wafers from back side evidencing a pyramidal structure. Also part (b) in this figure corresponds to slanted optical view of the craters formed in the silicon substrate. Well-defined squares are observed in this image showing the formation of large and thinned membranes on the other side of the sample. Fig. 3 shows a set of SEM images of CNTs grown on silicon substrate and on the quartz membrane obtained at different temperature and plasma power density conditions. By increasing the plasma power during the growth and by increasing the deposited Ni thickness the diameter of asgrown nanotubes increases. Nanotubes obtained in this method have 50–100 nm diameter, as shown in Fig. 3. Also as seen from this figure, by properly patterning the nickel layer prior to the growth step, vertically aligned embossed features are obtained.
Fig. 4 presents the result of encapsulation of CNTs by deposition of TiO2 and the subsequent polishing of the embedded structures. As seen from Fig. (4.a), TiO2 was deposited conformal on all CNTs, forming a non-porous TiO2-based solid structure. Fig. (4.b) depicts an anomalous feature observed after the sample has been polished and prior to the completion of the plasma-ashing step. The round opening has been obtained only by polishing and with no extra lithography step. The diameter of the opening is much more than the one-standing nanotube and it is believed to be due to a cluster of some neighboring CNTs. The inner part which is located at the central part of the opened hole is the CNT cluster which is still not fully removed by RF plasma ashing. Fig. 4c shows the surface of sample after the ashing process has been accomplished. As can be observed from this image, the titanium-oxide film has become porous with holes of the order of 100–200 nm, corresponding to the outer diameter of the original CNTs. At some places, the packed nanotubes form a cluster where larger opening may be obtained, as observed in the previous image. The inset in this image shows a more magnified view of one of the holes in the TiO2 layer, evidencing a total removal of the inner CNT. It is worth mentioning that the SEM images presented in this figure correspond to a place where the titanium-oxide layer has
ARTICLE IN PRESS Y. Abdi et al. / Physica E 37 (2007) 226–230
229
Fig. 5. SEM images of a sample from its back side. (a) The pyramid crater is observed in this figure with the bottom membrane which has small holes in it, (b) the closer view of the membrane from back side and (c) a higher magnification image of the same sample.
Fig. 4. SEM images corresponding to the steps in the fabrication process flow. (a) SEM image of coated CNTs with TiO2 evidencing a conformal coverage. (b) An SEM image of the sample after mechanical polishing step has been carried out and prior to the completion of the plasma-ashing step, and (c) the image of the sample after all processing steps have been accomplished. Inset shows similar hole after the oxygen plasma removes the inner CNTs.
been very thin so that its membrane nature is better highlighted. This phenomenon has been observed for all the places and holes are created in the thicker sides of the TiO2 layer as well.
With an extended-ashing step, the entire exposed nanotube is removed and in its place, narrow long standing holes are obtained. Fig. 5 shows the backside of a sample after removing the membrane followed by performing the ashing step. It is important to notice that the openings in TiO2 in this figure have diameters of the order of 100 to 200 nm, as expected from the initial diameter of the CNTs. Part (a) in this image depicts the overall view of the processed back side of the titanium-oxide membrane on the silicon substrate. Fig. 5b shows a closer look at the evolved holes in this sample. Since the removal of the back side of
ARTICLE IN PRESS 230
Y. Abdi et al. / Physica E 37 (2007) 226–230
the sample is made possible through chemical etching processes, less round structures are observed. Part (c) in this image yields a more magnified view of the holes in the titanium-oxide film as a result of total removal of CNT by plasma-ashing step. The dark area in this image corresponds to the regions where titanium oxide has been removed by extensive polishing. This image has been provided just to indicate that the small dark spots are allthe-way-through holes in the TiO2 membrane. 4. Summary and conclusion In summary we have successfully grown multi-wall CNTs on silicon-based membranes for the formation of nanoholes with both side opened. The etching of silicon wafers with (1 0 0) orientation is achieved in a standard KOH chemical solution and at a temperature of 55–60 1C. An e-beam-deposited quartz layer is used to act as the masking layer during the extended etching process. Quartz can also be used as the initial membrane film to perform the CNT growth step, although this initial membrane is finally removed. Once the CNTs are grown they are coated with an insulating layer (TiO2 here) and after several steps of mechanical polishing and RF plasma ashing, the exposed CNTs are fully removed and long and narrow holes are generated. SEM was used to investigate the growth of CNTs in desired areas, as well as to observe the process at every stage.
Acknowledgements This work has been supported with a grant from the Center of High-Tech, Ministry of Industry and Mines of Iran, Research Council of the Faculty of Engineering, University of Tehran. The authors are grateful to professor Kashani-Bozorg for SEM analyses. References [1] J.-M. Bonard, M. Croci, C. Klinke, F. Conus, I. Arfaoui, T. Stockli, A. Chatelaine, Carbon 40 (10) (2002) 1715. [2] M.I. Milne, K.B. Teo, G.A. Amaratunga, P. Legaganeux, et al., J. Mater. Chem. 14 (6) (2004) 933. [3] Y. Abdi, S. Mohajerzadeh, H. Hoseinzadegan, J. Koohsorkhi, Appl. Phys. Lett. 88 (2006) 053124. [4] J. Kong, A.M. Cassel, H. Dai, Chem. Phys. Lett. 292 (1998) 567. [5] J.H. Hafner, M.J. Bronikowski, B.R. Azamian, P. Nikolaev, A.G. Rinzler, D. Colbert, Chem. Phys. Lett. 296 (1998) 195. [6] T. Guo, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Chem. Phys. Lett. 243 (1995) 49. [7] D.S. Bethune, C.H. Kiang, M.S. de Vries, G. Gorman, R. Savoy, J. Vazquez, R. Beyers, Nature 363 (1993) 605. [8] S. Iijima, T. Ichihashi, Nature 363 (1993) 603. [9] L. Delzeit, C.V. Nguyen, R.M. Stevens, J. Han, M. Meyyappan, Nanotechnology 13 (2002) 280. [10] S. Hofmann, B. Kleinsorge, C. Ducati, A.C. Ferrari, J. Robertson, Diamond Relat. Mater. 13 (2004) 1171. [11] Ch. Taschner, F. Pacal, A. Leonhardt, P. Spatenka, K. Bartsch, A. Graff, R. Kaltofen, Surf. Coat. Technol. 174–175 (2003) 81. [12] S. Hofmann, C. Ducati, J. Robertson, B. Kleinsorge, Appl. Phys. Lett. 83 (2003) 135.
Physica E 37 (2007) 226–230 www.elsevier.com/locate/physe
Both-end opened nanostructure holes by embedded carbon nanotubes realized on thinned membranes on (1 0 0) silicon substrates Y. Abdia,b, S. Mohajerzadeha,, S. Darbaria, E. Arzib a
Nano-Electronics Center of Excellence, Thin Film Lab, Department of Electrical and Computer Eng., University of Tehran, Tehran, Iran b Department of Physics, University of Tehran, Tehran, Iran Received 8 May 2006; received in revised form 24 June 2006; accepted 26 June 2006 Available online 10 August 2006
Abstract Vertically aligned carbon nanotubes (CNT) have been grown in a DC-PECVD apparatus on quartz membranes. /1 0 0S-oriented Si wafer has been anisotropically etched in a KOH solution. A mixture of acetylene and hydrogen gases is used to grow CNT while Ni acts as the catalyst layer. As-grown structures have been coated by titanium dioxide using chemical vapor deposition at atmospheric pressure. By means of a polishing and ashing process steps followed by total removing of the quartz membrane both ends of CNTs are opened and nano holes are obtained. SEM analysis is used to study the evolution of such nanostructures. r 2006 Elsevier B.V. All rights reserved. PACS: 68.65. k; 81.07.De; 81.15.Gh Keywords: Nanotubes; Membranes; Plasma ashing; Nano holes
1. Introduction Because of their unique properties, carbon nanotubes (CNTs) have found many applications in nanotechnology. Their sharp and pointed structure makes them suitable candidates for field emission devices and flat panel displays [1,2]. Also the possibility of ballistic transport though the tube allows realization of high-performance field effect transistors. The extraordinary mechanical properties of CNTs suits them for application as AFM tip or for direct writing on a resist-coated surface. The application of such nanostructures in nanolithography has also been recently reported [3]. Several techniques have been used for the growth of CNTs among which one can highlight thermal catalystbased CVD [4,5], laser-vaporization deposition [6] and arcdischarge growth [7,8], offering single-wall structures. However CNTs with suitable vertical alignment have been produced using a plasma-enhanced CVD approach [9,10]. The direction of the applied electric field of the plasma Corresponding author.
E-mail address: smohajer@tfl.ir (S. Mohajerzadeh). 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.06.008
governs the CNT’s alignment, yielding a vertical growth. A tip-plate configuration was found suitable for the growth of near-vertically aligned nanotubes at high pressures, not easily obtained with standard PECVD methods [11,12]. In this paper we report, for the first time, the growth of CNTs on a silicon-based membrane and its application for the creation of both-end opened nanostructures embedded in a titanium-oxide membrane. The growth of multi-wall nanotubes has been achieved in a DC-PECVD system for a vertical alignment configuration. 2. Experimental setup The fabrication process started with the cleaning of (1 0 0)-oriented silicon wafers using an RCA ]1 solution. Both sides of the silicon wafer were then coated by a thermally grown SiO2 at a temperature of 1050 1C in an oxygen environment. Subsequently a 5–10 mm thick quartz layer is deposited on the back and front sides of the process wafers to act as the masking layer for the etching step as well as the membrane material, respectively. The quartz layer was deposited using e-beam evaporation at a temperature of 350 1C and at a base pressure of
ARTICLE IN PRESS Y. Abdi et al. / Physica E 37 (2007) 226–230
227
Fig. 1. A schematic of the process flow for the formation of embedded nanotubes on silicon substrates; starting with the growth of CNTs (top-left), deposition of TiO2 (top-right) and polishing/ashing steps (bottom).
5 10 6 Torr. Using standard photolithography proper openings on the back side of the wafer was made and the prepared sample was immersed in an 8 M KOH solution at a temperature of 60 1C to carry out the anisotropic etching step. After 20 h of etching all the silicon is removed in a rectangular or pyramidal shape and a quartz membrane is achieved on the opposite side of the sample which is not directly exposed to the etching solution. During the etching from the wafer back side, the front side of the sample must be protected to ensure that no damage happens to the front surface. After properly cleaning the membrane-carrying silicon sample, it was coated with a 5–10 nm thickness of Ni in an e-beam evaporation system and at a temperature of 300 1C. Ni is necessary for the CNT growth, acting as a catalyst. The specimen was then placed in the DC-PECVD chamber and was treated at the temperature of 650 1C with hydrogen blow at a flow rate of 30 sccm for 15 min. Immediately after this step, a 5.5 W/cm2 power density of plasma was applied to pre-condition the Ni seed and to form nanosized islands of nickel. After 5 min, acetylene (C2H2) was introduced into the chamber with a flow rate of 5 sccm for 20 min in order to initiate the growth of CNTs. Substrate temperature during the growth ranged from 550 to 650 1C and gas pressure was maintained at 1.6 Torr. The CNTs, grown on membrane, were coated by 100 nm thickness of titanium oxide using a CVD method. Coated structures were then mechanically polished to expose the nanotubes tips. By means of a 5% HF solution, the quartz membrane was removed and CNTs embedded in TiO2 were suspended. Using Oxygen plasma ashing at a temperature of 300 1C and 150 W plasma power from both front and back sides of sample, both ends of CNTs are burned. Oxygen plasma has strongly reactive oxygen species which can easily burn carbon and form carbon dioxide. This ashing process can be controlled to fully remove the whole CNTs and to form nanosize all-the-way-through holes in the titanium-oxide film. Finally we have both ends of
Fig. 2. An SEM image of the etched Silicon substrate from back side, evidencing a pyramidal crater formation and (b) the optical image of the sample taken in an angled view to show the back side of the sample.
nanotubes opened and exposed. By the aids of a plasmaashing step, the whole nanotube is burned out and a hole with a diameter equivalent to the outer diameter of CNTs is formed. Using this method hole diameter of such nanostructures is expected to be 50–100 nm. Fig. 1 schematically illustrates the fabrication steps, including the CNT growth and the sequence of deposition/ashing/polishing processes.
ARTICLE IN PRESS 228
Y. Abdi et al. / Physica E 37 (2007) 226–230
Fig. 3. SEM images corresponding to different samples prepared for this experiment. (a) A patterned growth of little lions as part of the university logo, and (b) the bare growth of CNTs on the silicon substrate. This sample has been obtained by patterning the nickel layer prior to the growth of CNTs. Features finer than 2 mm are achieved using this lithography and growth sequence.
Scanning electron microscopy (SEM) was used to study the structure of the nanostructures. The SEM was performed on a CAMSCAN2300 at an electron accelerating voltage of 25 kV and all images were generated using the secondary electron imaging. 3. Results and discussion Fig. 2 presents the SEM image of anisotropically etched /1 0 0S-oriented Si wafers from back side evidencing a pyramidal structure. Also part (b) in this figure corresponds to slanted optical view of the craters formed in the silicon substrate. Well-defined squares are observed in this image showing the formation of large and thinned membranes on the other side of the sample. Fig. 3 shows a set of SEM images of CNTs grown on silicon substrate and on the quartz membrane obtained at different temperature and plasma power density conditions. By increasing the plasma power during the growth and by increasing the deposited Ni thickness the diameter of asgrown nanotubes increases. Nanotubes obtained in this method have 50–100 nm diameter, as shown in Fig. 3. Also as seen from this figure, by properly patterning the nickel layer prior to the growth step, vertically aligned embossed features are obtained.
Fig. 4 presents the result of encapsulation of CNTs by deposition of TiO2 and the subsequent polishing of the embedded structures. As seen from Fig. (4.a), TiO2 was deposited conformal on all CNTs, forming a non-porous TiO2-based solid structure. Fig. (4.b) depicts an anomalous feature observed after the sample has been polished and prior to the completion of the plasma-ashing step. The round opening has been obtained only by polishing and with no extra lithography step. The diameter of the opening is much more than the one-standing nanotube and it is believed to be due to a cluster of some neighboring CNTs. The inner part which is located at the central part of the opened hole is the CNT cluster which is still not fully removed by RF plasma ashing. Fig. 4c shows the surface of sample after the ashing process has been accomplished. As can be observed from this image, the titanium-oxide film has become porous with holes of the order of 100–200 nm, corresponding to the outer diameter of the original CNTs. At some places, the packed nanotubes form a cluster where larger opening may be obtained, as observed in the previous image. The inset in this image shows a more magnified view of one of the holes in the TiO2 layer, evidencing a total removal of the inner CNT. It is worth mentioning that the SEM images presented in this figure correspond to a place where the titanium-oxide layer has
ARTICLE IN PRESS Y. Abdi et al. / Physica E 37 (2007) 226–230
229
Fig. 5. SEM images of a sample from its back side. (a) The pyramid crater is observed in this figure with the bottom membrane which has small holes in it, (b) the closer view of the membrane from back side and (c) a higher magnification image of the same sample.
Fig. 4. SEM images corresponding to the steps in the fabrication process flow. (a) SEM image of coated CNTs with TiO2 evidencing a conformal coverage. (b) An SEM image of the sample after mechanical polishing step has been carried out and prior to the completion of the plasma-ashing step, and (c) the image of the sample after all processing steps have been accomplished. Inset shows similar hole after the oxygen plasma removes the inner CNTs.
been very thin so that its membrane nature is better highlighted. This phenomenon has been observed for all the places and holes are created in the thicker sides of the TiO2 layer as well.
With an extended-ashing step, the entire exposed nanotube is removed and in its place, narrow long standing holes are obtained. Fig. 5 shows the backside of a sample after removing the membrane followed by performing the ashing step. It is important to notice that the openings in TiO2 in this figure have diameters of the order of 100 to 200 nm, as expected from the initial diameter of the CNTs. Part (a) in this image depicts the overall view of the processed back side of the titanium-oxide membrane on the silicon substrate. Fig. 5b shows a closer look at the evolved holes in this sample. Since the removal of the back side of
ARTICLE IN PRESS 230
Y. Abdi et al. / Physica E 37 (2007) 226–230
the sample is made possible through chemical etching processes, less round structures are observed. Part (c) in this image yields a more magnified view of the holes in the titanium-oxide film as a result of total removal of CNT by plasma-ashing step. The dark area in this image corresponds to the regions where titanium oxide has been removed by extensive polishing. This image has been provided just to indicate that the small dark spots are allthe-way-through holes in the TiO2 membrane. 4. Summary and conclusion In summary we have successfully grown multi-wall CNTs on silicon-based membranes for the formation of nanoholes with both side opened. The etching of silicon wafers with (1 0 0) orientation is achieved in a standard KOH chemical solution and at a temperature of 55–60 1C. An e-beam-deposited quartz layer is used to act as the masking layer during the extended etching process. Quartz can also be used as the initial membrane film to perform the CNT growth step, although this initial membrane is finally removed. Once the CNTs are grown they are coated with an insulating layer (TiO2 here) and after several steps of mechanical polishing and RF plasma ashing, the exposed CNTs are fully removed and long and narrow holes are generated. SEM was used to investigate the growth of CNTs in desired areas, as well as to observe the process at every stage.
Acknowledgements This work has been supported with a grant from the Center of High-Tech, Ministry of Industry and Mines of Iran, Research Council of the Faculty of Engineering, University of Tehran. The authors are grateful to professor Kashani-Bozorg for SEM analyses. References [1] J.-M. Bonard, M. Croci, C. Klinke, F. Conus, I. Arfaoui, T. Stockli, A. Chatelaine, Carbon 40 (10) (2002) 1715. [2] M.I. Milne, K.B. Teo, G.A. Amaratunga, P. Legaganeux, et al., J. Mater. Chem. 14 (6) (2004) 933. [3] Y. Abdi, S. Mohajerzadeh, H. Hoseinzadegan, J. Koohsorkhi, Appl. Phys. Lett. 88 (2006) 053124. [4] J. Kong, A.M. Cassel, H. Dai, Chem. Phys. Lett. 292 (1998) 567. [5] J.H. Hafner, M.J. Bronikowski, B.R. Azamian, P. Nikolaev, A.G. Rinzler, D. Colbert, Chem. Phys. Lett. 296 (1998) 195. [6] T. Guo, P. Nikolaev, A. Thess, D.T. Colbert, R.E. Smalley, Chem. Phys. Lett. 243 (1995) 49. [7] D.S. Bethune, C.H. Kiang, M.S. de Vries, G. Gorman, R. Savoy, J. Vazquez, R. Beyers, Nature 363 (1993) 605. [8] S. Iijima, T. Ichihashi, Nature 363 (1993) 603. [9] L. Delzeit, C.V. Nguyen, R.M. Stevens, J. Han, M. Meyyappan, Nanotechnology 13 (2002) 280. [10] S. Hofmann, B. Kleinsorge, C. Ducati, A.C. Ferrari, J. Robertson, Diamond Relat. Mater. 13 (2004) 1171. [11] Ch. Taschner, F. Pacal, A. Leonhardt, P. Spatenka, K. Bartsch, A. Graff, R. Kaltofen, Surf. Coat. Technol. 174–175 (2003) 81. [12] S. Hofmann, C. Ducati, J. Robertson, B. Kleinsorge, Appl. Phys. Lett. 83 (2003) 135.