APPLIED PHYSICS LETTERS
VOLUME 77, NUMBER 19
6 NOVEMBER 2000
Lift-up growth of aligned carbon nanotube patterns Bingqing Wei,a) Z. J. Zhang, G. Ramanath, and P. M. Ajayan Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3590
共Received 28 July 2000; accepted for publication 18 September 2000兲 The fabrication of three-dimensional networks of carbon nanotubes with controlled orientation will be essential for building large-scale functional devices integrated with microelectronics circuits. We describe here our recent work on the controlled synthesis of vertically aligned carbon-nanotube patterns, grown under patterned metal layers on Si substrates by combining chemical-vapor deposition and conventional lithography. We show that metal patterns are lifted up by vertically aligned nanotubes during growth. This lift-up growth links the thin-film metal patterns and the Si substrate via nanotube assemblies, giving the possibility of creating nanotube architectures in three dimensions. The possible scenarios of the growth of aligned nanotube films leading to the lift up of the metal films are discussed. © 2000 American Institute of Physics. 关S0003-6951共00兲00845-7兴 There is widespread interest in growing controllably aligned carbon nanotubes, and significant progress has been made in this direction in recent years.1–3 Chemical-vapor deposition 共CVD兲 is widely used for this purpose due to its advantages of growing nanotubes on prepatterned catalysts4–12 on planar substrates such as Si. One- and twodimensional connections and/or junctions with nanotubes have been fabricated by in situ growth processing and subsequent nanofabrication.13–19 However, interconnecting nanotubes with the substrate and/or metal films—crucial for realizing three-dimensional nanotube-based devices—have not been realized yet. In this letter, we demonstrate a way of growing vertically aligned nanotubes, which connect patterned Ni films and the Si substrates. The vertically aligned multiwalled nanotubes grow underneath the Ni layers, resulting in the lift-off of the Ni patterns from the Si substrates. Thus, a forest of nanotubes grows and links the Ni layers with the Si共001兲 substrate. 15–100-nm-thick Ni layers were lithographically patterned on Si共001兲 substrates with or without a thermally oxidized SiO2 layer. Carbon nanotubes were produced using the CVD method by exposing the substrates to a mixture of xylene and ferrocene mixtures at 800 °C for 15–30 min.4,8 Xylene provides the carbon source while Fe from ferrocene serves as the catalyst for nanotube growth. Scanning electron microscopy 共SEM兲 and transmission electron microscopy 共TEM兲 were employed to characterize the growth features of carbon nanotubes on the planar Si substrates. Figure 1共a兲 shows an optical image of a Si substrate covered with thin-film Ni patterns 共bright areas兲 for reference. The SEM image of the exposed sample in Fig. 1共b兲 shows that vertically aligned multiwalled nanotubes grow only underneath the Ni layers, and result in the lifting off of the Ni patterns from the substrate. There is no observable nanotube growth on the Ni film surfaces. Parts of the Ni film have peeled off and flipped over without detaching from the well-aligned layer of nanotubes 关arrows in Fig. 1共b兲兴, indicating strong bonding between the film and the nanotubes. Many nanotubes also remain rooted to the substrate. These a兲
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results suggest that nanotubes can simultaneously connect the Ni film and the substrate, which could be useful for generating three-dimensional device architectures. TEM observations reveal that the average diameter of the nanotubes produced by this method ranges from 30 to 50 nm 共inset of Fig. 1兲. Figure 2共a兲 shows a typical SEM image of a layer of vertically aligned multiwalled carbon nanotubes, which connect the Si共001兲 substrate with a thin layer of porous Ni film that is lifted up by the growth of nanotubes. There is no growth of nanotube layers on the Si substrate surface without Ni films, indicating that the nanotube growth is site selective and can be tailored by conventional lithography. Spatially
FIG. 1. 共a兲 Optical image of a Si substrate covered with a thin-film Ni pattern; and 共b兲 SEM image showing the growth of nanotubes under the Ni film, parts of the Ni film are flipped over and attached to a well-aligned layer of nanotubes 共arrows兲. Inset is a TEM image of a typical multiwalled nanotube grown by CVD.
0003-6951/2000/77(19)/2985/3/$17.00 2985 © 2000 American Institute of Physics Downloaded 09 Nov 2006 to 128.113.37.3. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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Wei et al.
FIG. 2. 共a兲 SEM image of a layer of vertically aligned multiwalled carbon nanotubes connecting the Si substrate and a thin layer of porous Ni film lifted up during nanotube growth. 共b兲 SEM micrograph of a Ni substrate exposed to ferrocene only, i.e., without xylene. Note that micron-size catalyst particles aggregate at both the edge areas of the Ni film.
resolved Auger spectroscopy and energy-dispersive x-ray 共EDX兲 spectrometer of substrates exposed to only ferrocene 共without xylene兲 revealed that micron-size particles aggregate at both edge areas of the Ni film 关see Fig. 2共b兲兴. The growth of vertically aligned carbon nanotubes underneath the Ni film separates the film from the substrate. Inhomogeneous growth rates for the nanotubes can sometimes produce nonuniform lengths of nanotubes in local areas causing the Ni film to bend in order to accommodate this length difference 关see the arrows in Fig. 3共a兲兴. For the same reason, Ni films also develop strain and break, as seen in Fig. 3共b兲. However, the nanotubes are clearly attached to the Ni films even when they break off from the substrate. This is clearly seen in Fig. 3共c兲, which shows three parallel columns of well-aligned nanotubes attached to delaminated Ni films. Note that the nanotubes connect the substrate and the Ni film uniformly along the length of the Ni lines for more than 100 m. The line of bright contrast 共shown by the arrows兲 within each column indicates the aggregation of catalyst particles, and is confirmed by EDX. The aggregation of Fe particles to the nanotube arrays suggests that the prepatterned thin Ni films may not act as real catalysts but provide a secondary role in promoting the nucleation and the growth of nanotubes in specific locations from underneath the films. To confirm this, another experiment was carried out under the same conditions, but without using the ferrocene catalyst in the feedstock. An amorphouscarbon layer was found instead of nanotubes, highlighting the importance of ferrocene for nanotube growth. The TEM images revealed iron particles inside several nanotubes, confirming that Fe is the catalyst for nanotube growth in our experiments. The oxidation of the Ni surface during high-
FIG. 3. SEM micrograph showing the delaminating and lift up of Ni films from the Si substrate during nanotube growth. The Ni film 共a兲 bends or 共b兲 cracks, as indicated by the arrows. 共c兲 Three columns of well-aligned nanotubes on Ni films which have broken from the substrate and flipped over to the left side. The lines of bright contrast 共indicated by the arrows兲 within each column are due to the aggregation of Fe particles.
FIG. 4. Schematic sketch illustrating the growth of aligned nanotube films, leading to the lift up of the metal films: 共a兲 Fe particles serve as catalysts and the carbon atoms deposit at the step corners of the Ni film and the substrate, and 共b兲 the Ni film lifts up during the growth of carbon-nanotube arrays. Downloaded 09 Nov 2006 to 128.113.37.3. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
Appl. Phys. Lett., Vol. 77, No. 19, 6 November 2000
temperature exposure is the plausible reason for the relative inefficiency of Ni as a catalyst.20 A simple schematic growth model for the aligned nanotube layer, which leads to the lift up of the Ni films is shown in Fig. 4. We suggest that ferrocene dissociates preferentially at the Ni sites, providing iron particles, which catalyzes the growth of nanotubes from the carbon precursor. Growth occurs via the substrate, forcing the Ni films to lift up with preferential growth occurring at the bottom surface of Ni. Thus, a vertically aligned carbon nanotube layer grows with the two ends contacting the Ni films and the Si substrate, respectively. In conclusion, it is crucial to evolve strategies to interconnect nanotubes with metal catalyst films, and with the substrate, for realizing nanotube-based three-dimensional device architectures. We have demonstrated a way of growing vertically aligned nanotubes, which connect patterned Ni films and the Si substrates. The vertically aligned multiwalled nanotubes grow only underneath the Ni layers, and result in the lifting off of the Ni patterns from the substrate. The results offer insights into the possible mechanisms of catalyst-driven growth of nanotubes and the possible tailoring of nanotube networks using combinations of catalysts, some of them active and some passive, during growth. The authors gratefully acknowledge funding from the Office of Naval Research under Grant No. N00014-00-10250. The authors also acknowledge support from the Interconnect Focused Center at RPI, funded by MARCO and New York State. 1
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