US 20070218202 A1
A method of controllably aligning carbon nanotubes to a template structure to fabricate a variety of carbon nanotube containing structures and devices having desired characteristics is provided. The method allows simultaneous, selective growth of both vertically and horizontally controllably aligned nanotubes on the template structure but not on a substrate in a single process step.
1. A method of making carbon nanotubes, comprising:
providing a substrate containing a template structure having at least two surfaces;
providing a nanotube source gas onto the template structure; and
selectively and simultaneously growing the carbon nanotubes on the at least two surfaces of the template structure but not on exposed portions of the substrate such that the grown carbon nanotubes are controllably aligned in a direction perpendicular to a respective surface of the template structure.
2. The method of
3. The method of
the carbon nanotubes comprise multiwalled carbon nanotubes;
carbon nanotubes are grown at a temperature of 600 to 1100° C.;
the substrate comprises a silicon substrate;
the template structure comprises a silicon dioxide template structure that is not coated with a carbon nanotube growth catalyst; and
the carbon nanotubes simultaneously grow in orthogonal directions on orthogonal template structure surfaces.
4. The method of
5. The method of
6. The method of
7. A method of making carbon nanotubes, comprising:
providing a substrate containing a template structure, wherein the template structure comprises at least one inclined surface with an oblique inclination which is neither orthogonal nor parallel with respect to an upper plane of the substrate;
providing a nanotube source gas onto the template structure; and
selectively growing the carbon nanotubes on the inclined surface of the template structure but not on exposed portions of the substrate, such that the grown carbon nanotubes comprise a membrane having an open truncated cone shape.
8. The method of
the nanotube source gas comprises xylenes and ferrocene provided onto the template structure in a chemical vapor deposition apparatus at a temperature of 600 to 1100° C.;
the carbon nanotubes comprise multiwalled carbon nanotubes;
the substrate comprises a silicon substrate; and
the template structure comprises a silicon dioxide template structure.
9. The method of
10. The method of
The present application is a Divisional of U.S. application Ser. No. 10/361,640, filed Feb. 11, 2003, which claims benefit under 35 U.S.C. § 119(e) of U.S. provisional application 60/356,069, filed Feb. 11, 2002 and 60/385,393, filed Jun. 3, 2002, both of which are incorporated herein by reference in their entirety.
The U.S. Government may have certain rights in this invention pursuant to grant number N00014-00-1-2050 from the Office of Naval Research.
The present invention relates generally to carbon nanotubes and more particularly to selective growth of carbon nanotubes on template materials.
It is likely that future devices containing organized structures of various functional materials with new properties will be built from nanoscale building blocks. These nanoscale building blocks can be produced by a variety of synthesis routes. The novel properties of the nanoscale building blocks arising from their low dimensions are known for a wide range of materials.
Carbon nanotubes are a nanostructured material which promises to have a wide range of applications. However, the present techniques used to controllably build organized architectures of nanotubes with predetermined orientations have several deficiencies. For example, vertically aligned nanotubes has been fabricated on catalyst printed planar substrates by chemical vapor deposition. See, for example, W. Z. Li, et al. Science 274, 1701 (1996); R. Sen, A. Govindaraj, C. N. R. Rao, Chem. Phys. Lett. 267, 276 (1997); M. Terrones, et al. Nature 388, 52 (1997); Z. F. Ren, et al. Science 282, 1105 (1998); S. S. Fan, et al. Science 283, 512 (1999); H. Kind, et al. Adv. Mater. 11, 1285 (1999); R. R. Schlittler, et al. Science 292, 1136 (2001) and L. Dai, A. W. H. Mau, J. Phys. Chem. B 104, 1891 (2000). However, this technique requires deposition and patterning, usually in separate processing steps, of catalyst material, typically in nanoparticle assemblies or thin film forms, which complicates the nanotube fabrication method. This also does not allow growth of nanotubes in more than one preselected orientation at different locations in a controllable fashion.
While growth of vertically aligned nanotubes on planar substrates by CVD has been reported extensively, obtaining nanotubes that are exclusively oriented parallel to the substrate in predetermined orientations has been more difficult. Suspended nanotubes across elevated structures have been produced recently by several different methods. One method involves adjusting the gas flow during CVD. See N. R. Franklin, H. Dai, Adv. Mater. 2000, 12, 890; N. R. Franklin, Q. Wang, T. W. Tombler, A. Javey, M. Shim, H. Dai, Appl. Phys. Lett. 2002, 81, 913; and Y. Homma, Y. Kobayashi, T. Ogino, T. Yamashita, Appl. Phys. Lett. 2002, 81, 2261. Another method involves applying an electrical field during CVD. See Y. Zhang, A. Chang, J. Cao, Q. Wang, W. Kim, Y. Li, N. Morris, E. Yenilmez, J. Kong, H. Dai, Appl. Phys. Lett. 2001, 79, 3155; and A. Ural, Y. Li, H. Dai, Appl. Phys. Lett. 2002, 81, 3464. However, these approaches require pre-deposition and pre-patterning of nanoscale catalyst particle assemblies. Also, the probability of nanotubes bridging across different catalyst islands is difficult to predict and control. Several articles also note that nanotubes can be aligned in horizontal configurations through electric fields or microfluidic forces. See A. Star, et al., Angewandte Chem. International Edition 40, 1721 (2001) and T. Rueckes, et al., Science 289, 94 (2000). However, these methods are also complicated, and are difficult to scale (e.g., create them reproducibly on an 8 inch Si wafer) and to control, for developing devices for applications. Moreover, in these cases the nanotubes are not rooted to the substrate (i.e. they are just lying on them, and hence not very robust).
Several methods have also been suggested for controlled placement of carbon nanotubes onto electrode pairs, including AC bias-enhanced deposition and chemically modified adsorption. See L. A. Nagahara, I. Amiani, J. Lewenstein, R. K. Tsui, Appi. Phys. Lett. 2002, 80, 3826 and M. Burgard, G. Buesberg, G. Philipp, J. Muster, S. Roth, Adv. Mater. 1998, 10, 584.
Some of the present inventors have also previously suggested to selectively grow carbon nanotubes on silica templates located on a silicon substrate without growing the nanotubes on the silicon substrate. See Z. J. Zhang, B. Q. Wei, G. Ramanath, P. M. Ajayan, Appl. Phys. Lett. 77, 3764 (2000). The use of this template structure is advantageous in that it does not require the deposition and patterning of the catalyst material. However, as can be seen in
A preferred embodiment of the invention provides a method of making carbon nanotubes, comprising providing a substrate containing a template structure having at least two surfaces and providing a nanotube source gas onto the template structure. The nanotube source gas preferably comprises a mixture of nanotube forming precursor gas, such as xylenes, and a catalyst gas, such as ferrocene. However, other suitable gases or a single source gas may be used instead. The method further provides for selectively and simultaneously growing the carbon nanotubes on the at least two surfaces of the template structure but not on exposed portions of the substrate (i.e., in exclusion to the substrate material which supports the template structure). The grown carbon nanotubes are controllably aligned in a direction perpendicular to the respective surfaces of the template structure.
Another preferred embodiment of the invention provides a structure, comprising a substrate and a template structure located on the substrate, wherein the template structure comprises at least two surfaces. The structure also comprises a first plurality of carbon nanotubes disposed on a first surface of the template structure, wherein the first plurality of carbon nanotubes are controllably aligned in a first direction perpendicular to the first surface of the template structure, and a second plurality of carbon nanotubes disposed on a second surface of the template structure, wherein the second plurality of carbon nanotubes are controllably aligned in a second direction perpendicular to the second surface of the template structure, such that the first direction is different than the second direction.
FIGS. 1A-B, 2A-D, 4A-C, 5, 6A-G, 8A-F, 9C, 11A-B and 12C are SEM images of carbon nanotube structures according to preferred embodiments of the present invention.
FIGS. 7A-B are side cross sectional schematic views of a method of making carbon nanotube structures according to a preferred embodiment of the present invention.
The present inventors have developed a method of controllably aligning carbon nanotubes to a template structure to fabricate a variety of carbon nanotube containing structures and devices having desired characteristics. For example, by selecting a template structure with a sufficient thickness allows a simultaneous growth of both vertically and horizontally controllably aligned nanotubes in a single process step. In fact, the nanotubes may be aligned in any set of in-plane and/or out of plane orientations and grown in a single process step. The level of control provided by this method provides construction of complex, nanotube based, highly organized 1-to-3-D architectures for building or use in nanotube based devices and systems in a scalable fashion.
A template structure, pattern or material is a structure, pattern or material which allows selective growth of carbon nanotubes on it without growing any detectable amount of carbon nanotubes on exposed portions of the substrate which supports the template. Nanotubes grow normal to, and selectively on the template structure, inheriting the topography of the template structure. Thus, the nanotubes are controllably aligned in a direction perpendicular to the surface of the template structure from which they grow such that all the nanotubes which grow from a particular template structure surface are oriented in the same direction. The precise control of nanotube orientation allows the fabrication of a wide variety of organized architectures of differing complexities, shapes, densities, dimensions and orientation. The bottom-up fabrication approach is easy, scalable, and compatible with silicon microfabrication techniques and processes.
In a preferred aspect of the present invention, the template structure comprises a silicon dioxide structure located on a silicon substrate. However, other suitable template and substrate materials may be used instead. For example, other silicon oxide and metal oxide template structures materials, such as silicon oxynitride, magnesium oxide, aluminum oxide or indium tin oxide, may be used instead. Thus, a carbon nanotube growth catalyst material, such as a metal nanoparticle layer, is not necessary to selectively grow carbon nanotubes, and is preferably omitted to simplify processing. In alternative aspects of the present invention, a metal that does not catalyze nanotube growth, such as gold or copper, can also be used to mask part of the template structure material on the substrate. The substrate material can be any material that does not catalyze nanotube growth. Examples of preferred substrate materials are a single crystal silicon wafer, epitaxial silicon and polysilicon layers. Furthermore, other semiconductor materials, such as II-VI and III-V semiconductor materials, and non-semiconductor materials, such as ceramics, metals, glasses and plastics that do not catalyze nanotube growth may be used as a substrate by appropriately selecting the template material which allows selective growth of carbon nanotubes.
In another alternative aspect of the present invention, the substrate may be omitted entirely, and the nanotubes may be fabricated on free standing template structures. For example carbon nanotubes may be formed on the surfaces of oxide particles, such as silica, alumina, MgO and other silicon and metal oxide particles. Preferably, the particles are substantially spherical oxide particles, such as spherical, oval and roughly spherical (i.e., spherical particles with chiseled surfaces) particles. The particles may be micro sized (preferably 1 to 1,000 micron diameter) or macro sized (preferably 1,000 microns to 1 cm diameter). The carbon nanotubes are aligned perpendicular to the oxide particle surfaces.
In preferred embodiments of the present invention, controllably aligned multiwalled carbon nanotubes are selectively and simultaneously grown in patterns and in multiple directions on lithographically patterned silica templates in a single process. This process is preferably carried out through a CVD method that delivers the nanotube-forming precursor and the catalyst material (in compound or elemental form) from the gas phase either simultaneously or sequentially.
The specific examples of nanotube structures of the present invention shown are illustrated in SEM images in the Figures. However, the present invention should not be considered limited by the structures and methods of the specific examples, which are provided for illustration of the present invention.
The nanotube structures shown in the SEM images in the Figures were selectively grown on non-planar patterns composed of SiO2 and Si surfaces. The substrates were Si (100) wafers capped with thermally grown or plasma-enhanced chemical vapor deposition (PECVD) deposited SiO2 template structures having a thickness of 100 nm to several microns, such as 2 to 8.5 microns. For example, thick silica layers (up to about 8.5 microns) were deposited by PECVD to create high-aspect-ratio silica features. Patterns of Si/SiO2 of various shapes were generated by photolithography followed by a combination of wet and/or dry etching.
Patterned nanotube growth was achieved without metal catalyst predeposition and patterning, thereby simplifying the template preparation by eliminating 2 processing steps. Instead, CVD nanotube growth was stimulated by exposing the substrate and the template structure on the substrate to vapor mixtures comprising xylenes (C8H10), a nanotube-forming precursor, and ferrocene (Fe(C5H5)2), a nanotube catalyst, at about 600 to 1100° C., preferably at about 800 to 900° C. However, other suitable source gases and temperatures may be used instead. Ferrocene was dissolved in xylenes (which preferably contains different isomers) at concentrations of about 0.01 g/ml, the mixture was pre-heated, co-evaporated and fed into the CVD chamber. Ferrocene preferably comprises 0.001 to 1 percent of the ferrocene/xylenes mixture. Prolonged growth in the temperature range of 600-1100° C., produced films of densely packed multiwalled carbon nanotubes on the template structures but not on the substrate. Uniform, vertically aligned nanotube films having a thickness of a few microns to several tens of microns were produced in few minutes at growth rates of about 10 microns/minute. The nanotubes in the nanotube films were about 30 nm diameter multiwalled carbon nanotubes. No nanotube growth was observed on pristine Si surfaces, or on the native oxide layer formed on the silicon substrate. Aligned nanotubes grew readily on SiO2 templates in a direction normal to the template growth surface, and the selectivity was retained down to micron size SiO2 templates.
Using the substrate-selective and normal direction growth process, the nanotubes structures of various shapes and orientations with respect to the Si surface were fabricated on active SiO2 template surfaces. Thus, carbon nanotubes were selectively grown in desired patterns and architectures without further photolithographic patterning.
The method and structure of the first preferred embodiment will now be described. In the first preferred embodiment, the carbon nanotubes were selectively grown in one direction on various template structure shapes to produce various nanotube structure shapes having the footprint of the template surfaces. These nanotube structures can then be optionally removed from over the substrate and placed into a suitable device.
FIGS. 1A-B show a striking example of aligned nanotube pillars placed on specific sites on the substrate according to one aspect of the first embodiment.
The structures shown in FIGS. 1A-C were made by the following method. The SiO2 template structures or patterns 12 were formed on a silicon substrate 10 by conventional photolithography. The template structures 12 were exposed to xylenes/ferrocene gas mixtures at temperatures ranging from 600 to 1100° C., preferably 800 to 900° C. in a CVD tube furnace. The nanotubes 14 grew extremely selectively on the SiO2 patterns 12, but did not grow on the silicon substrate 10, leaving the exposed silicon substrate blank. Alignment of nanotubes can be easily identified from the sidewalls, which separate the micro pillar arrays of nanotubes. The template structures had a diameter of about 10 microns to form cylindrical nanotube pillars having a diameter of about 10 microns in diameter and separations between individual pillars 14 of 10, 5 and 2 microns for the top, bottom and middle pillar sets, respectively. In addition to the cylindrical pillars 14 and cylindrical template structures 12, other suitable pillar 14 shapes, such as polygonal (triangular, rectangular, trapezoidal, etc.), oval or irregular shapes, can be fabricated by shaping the template structures 12 accordingly. Furthermore, while SiO2 template structures 12 on a silicon 10 substrate were illustrated, other suitable materials for the template structures 12 and the substrate 10 may be used instead, as described herein.
In all cases of the nanotube pillars shown in FIGS. 1A-C, the nanotube platelets shown in FIGS. 2A-B and the nanotube fibers shown in FIGS. 2C-D, the present inventors observed good adhesion between individual nanotubes within each geometrical block, and between the nanotubes and the substrate. The nanotube blocks, however, can be detached from the substrates by ultrasonic agitation, and can be manipulated individually using any suitable nanotube manipulation method. For example, the removed nanotube pillars, platelets and/or fibers may be selectively placed into a desired device, such as an electronic device.
The method and structure of the second preferred embodiment will now be described. In the second preferred embodiment, porous carbon nanotube films were fabricated. These porous films were fabricated with a high degree of control over pore sizes, shapes and separations. This is illustrated in FIGS. 3A-C, which show nanotube films with regular arrays of pores. Three different pore features imprinted in the nanotube film are shown in FIGS. 3A-C. For example, nanotube films with square and rectangular pores arranged in a rectangular grid are shown in
The porous nanotube films were obtained by using a porous template structure or layer formed over a substrate. For example, a porous silica template layer was obtained by forming a silica layer on the silicon substrate and then photolithographically masking the layer and etching pores or holes in this layer. Since the pores or holes are formed by photolithography and etching, they have a controlled size. The pores or holes extend down to the silicon substrate and may extend into the silicon substrate if desired. Thus, portions of the substrate are exposed through pores in the porous template layer. Nanotube-forming gases or mixtures were provided onto the porous template layer and the carbon nanotubes were selectively grown on the porous template layer. However, the nanotubes were not formed on portions of the substrate exposed through pores in the porous template layer. Furthermore, the nanotubes did not form on the template layer pore sidewalls because the template layer thickness is not sufficient to allow nanotube growth on side surfaces of the pores in the template layer. For example, the template layer may have a thickness of less than about 200 nm, depending on the exact processing conditions and the template material used, to avoid nanotube growth on the side surfaces of the pores. If desired, after the nanotube film is grown, the substrate may be selectively removed, such as by polishing or by silicon selective etching, to form a free standing porous nanotube film. In an alternative aspect of the second embodiment, the porous nanotube film is produced by a different method. The template material can be selectively covered by non-catalytic masking materials (e.g., gold) through lithography and deposition, at locations where pores are desired. The template material has a plurality of first regions masked by a masking material which does not catalyze nanotubes, such as gold or copper masking material. The ends of the nanotubes are attached to the template material such that the plurality of pores in the nanotube film have a controlled size and are aligned with the plurality of first regions on the template material masked by the masking material. When the nanotubes are grown on the masked template material, the nanotubes selectively grow on the exposed template material but not on the masked first regions of the template material.
Thus, the template layer may be considered to be the negative pattern of the one used to make free-standing nanotube blocks shown in
The method and structure of the third preferred embodiment will now be described. In the third embodiment, selective nanotube growth on the template structure having at least one, and preferably two or more surfaces. The nanotube growth occurs in a direction normal to the respective surfaces and hence this approach can be harnessed to simultaneously grow nanotubes in several predetermined directions. For example, nanotube growth in mutually orthogonal directions may be carried out by using template structures comprising of deep etched trenches, drilled all the way to silicon substrate, separating several thick SiO2 template structure towers. Preferably, the nanotubes are grown on side surfaces of the template structures, which are preferably thicker than about 200 nm, preferably thicker than 2 microns. For example, the template structure may be several microns thick, such as 2 to 10 microns thick, preferably 5 to 8.5 microns thick. However, thickness of the template structure for growing controllably aligned nanotubes on the side surfaces of the template structure varies based on the template material and the nanotube growth conditions. The preference of nanotubes to grow normal to, and selectively on, silica surfaces, forces the nanotubes to inherit the topography of the substrate templates, enabling the premeditation of both nucleation sites and growth direction.
Preferably, the template structure that is located on the substrate has at least one, and preferably two or more surfaces. A first plurality of carbon nanotubes are disposed on a first surface of the template structure. These first plurality of carbon nanotubes are controllably aligned in a first direction perpendicular to the first surface of the template structure. Furthermore, a second plurality of carbon nanotubes are disposed on a second surface of the template structure. This second plurality of carbon nanotubes are controllably aligned in a second direction perpendicular to the second surface of the template structure, such that the first direction is different than the second direction. The same applies for other surfaces of the template structures.
FIGS. 4A-D illustrate vertically and horizontally aligned nanotube arrays adjacent to each other produced in a single-step growth process.
Another preferred aspect of the third embodiment is illustrated in
The method and structure of the fourth preferred embodiment will now be described. In the fourth embodiment, the structure contains nanotubes packed with oblique inclinations, neither orthogonal nor planar or parallel with respect to the upper substrate plane, by using deep-trench template structures, such as silica structures, with inclined side surfaces.
The term “cone” includes the right circular cone with a circular base or directrix as well as structures having a curved (open or closed curve) or polygonal base or directrix to form pyramidal type structures with irregular or trapezoidal faces extending toward a common vertex. Thus, “cone” includes any structure having a surface generated by a straight line passing though a fixed point and moving along the intersection with a fixed curve.
The free standing membrane structures 18 made of aligned nanotubes created in the shapes of open truncated cones may have any desired size. The width of the membrane or the size of the opening can be controlled by the pattern diameter, the angle of trench wall and the time of growth. Typical examples of different diameter, cone angle, size of opening of the nanotube membrane are shown in FIGS. 6A-F. The diameter of the pattern structures are 300, 250, 200, 150, 100 and 50 microns, respectively.
This cone shaped membrane is formed the same way as the previous nanotube structures. The nanotube source gas is provided onto the template structure 12 and the carbon nanotubes 18 are selectively grown on the inclined surface 16 of the template structure but not on exposed portions of the substrate 10, such that the grown carbon nanotubes comprise a membrane having an open truncated cone shape. The nanotubes 14 may also be grown on the upper surface of the template structure 12 or the upper surface of the template structure may be masked, as will be described below, to form only the nanotube membrane 18 on the template structure 12.
FIGS. 6A-G also show the flexibility of the method of the fourth embodiment to obtain radially oriented nanotubes with the entire spectrum of in-plane orientations relative to the substrate plane. The present inventors produced structures with different diameter, cone angle, size of opening and film thickness by tailoring the pattern dimensions, and growth time to control nanotube length. Similarly, complex shapes of nanotube structures can be generated by altering the trench geometry and depth.
The method and structure of the fifth preferred embodiment will now be described. In the fifth embodiment, a masking layer or material is formed on a portion of the template structure to selectively grow nanotubes only on an exposed portion of the template structure. A masking layer is a layer which inhibits growth of nanotubes on the template structure. For example, a 2 to 20-nm-thick gold layer can be used as a masking material for silica template structures. Other examples of masking materials include polysilicon, copper, or any other material that does not catalyze nanotube growth. The masking material preferably has a thickness at which it forms a continuous layer. Thus, in a preferred aspect of the fifth embodiment, a template structure is located on the substrate. A masking material covers a first portion of the template structure. A plurality of carbon nanotubes are located on a second portion of the template structure not covered by the masking material. The carbon nanotubes are not located on the masking material or on exposed portions of the substrate. Preferably, a plurality of template structures are located on the substrate.
The method of the fifth preferred embodiment includes providing a substrate containing a template structure having at least a portion covered by a masking material and providing a nanotube source gas onto the template structure. The method also includes selectively growing the carbon nanotubes on an exposed portion of the template structure but not on exposed portions of the substrate and not on portions of the template structure covered by the masking material.
The method provides selective growth of nanotube bundles in controlled directions parallel to the substrate surface, by inhibiting nanotube growth from certain template structure portions or surfaces, such as the upper and/or some of the side surfaces. The lengths of CNTs growing in different directions can be tuned by adjusting the masking material coverage and thickness respective template structure surfaces, the deposition time, and the SiO2 pattern spacing. This allows fabrication of arrays of low-electrical resistance nanotube-metal contact structures that could be used to create nanotube-based electrical devices.
Of course, other masking layer fabrication methods may be used instead. For example, the masking layer may be deposited by evaporation, sputtering, CVD or plating on the entire surface of the substrate and the template structures. The masking layer may then be photolithographically patterned to cover only the desired portions of the template structures 12. Alternatively the masking layer can be deposited only at desired locations by using a shadow mask, or through lift-off lithography techniques.
The Au-coated SiO2 template structures were placed into a CVD tube furnace to grow carbon nanotubes selectively on exposed SiO2 surfaces using a xylenes-ferrocene mixture. Substrates without Au coating were also loaded in the CVD furnace. A solution of 0.4 g ferrocene dissolved in 40 ml xylenes was fed into the CVD furnace at temperatures between 600 to 1100° C. for time intervals ranging from 5 to 30 minutes. This method produced aligned nanotubes, which were typically multiwalled nanotubes with diameters of about 20 to 50 nm.
In contrast, FIGS. 8C-F illustrate selective CVD in-plane growth of aligned CNT bundles on exposed portions of template structures whose remaining portions were covered by a masking material. The selective growth is clearly in
The CNTs grow straight, in a direction perpendicular to the silica surface seeding them, until they arrive at the next template structure, as shown in
The process of selectively coating Au on different SiO2 surfaces can be harnessed to controllably grow CNTs along any sets of in-plane directions. For example, the nanotube growth direction was reversed by coating Au on the upper surface and left sidewall of each template structure, as shown in
The CNT length is solely controlled by deposition time in the absence of physical obstacles in the growth direction. The CNTs grown from the last template structure in
Thus, the nanotube source gas is provided for a sufficient time to allow the carbon nanotubes growing on the exposed portions of a one template structure to contact the masking material covering the covered portion of an adjacent template structure to form a nanotube bridge between these template structures. Furthermore, the nanotube source gas is provided for a sufficient time to simultaneously grow carbon nanotubes of a different length to allow the carbon nanotubes growing on the exposed portions of template structures to contact the masking material covering the covered portions of adjacent template structures. Thus, the growth time is selected to allow the nanotubes to bridge the longest desired structure spacing or distance. Thus, by varying the growth time of nanotubes on an array of differently spaced template structures, only the template structures with the desired maximum inter structure spacing are connected by the bridges.
In order to examine this correlation, the present inventors measured the length of CNTs grown on SiO2 surfaces with different Au coverages. The Au coverages were measured by RBS and varied between 0 and 1.1×1017 atoms/cm2.
The plot in
Thus, in a method of a preferred aspect of the fifth embodiment, a first material which facilitates growth of carbon nanotubes, such as a silica or other suitable material layer or structure is provided. At least a portion of the first material is covered with a masking material, such as Au or another material which inhibits CNT growth. A nanotube source gas is provided onto the first material. The carbon nanotubes of a first length are selectively grown on an exposed portion of the first material. If the masking material is thicker than a critical thickness at which it comprises a continuous layer (e.g., Au layer with nominal thickness of greater than about 20 nm), then no CNTs are grown on the masking material. If the masking material is thinner than a critical thickness at which it comprises a continuous layer (i.e., such as an about 20 nm or thinner Au layer), then shorter CNTs of a second length less than the first length are growth on the masking material. Thus, if plurality of different masking materials and/or a plurality of masking material portions of different thickness are formed over the first material, then a plurality of CNTs with different lengths may be grown in the same growth step.
Therefore, by varying the masking material thickness and/or type and/or by varying the spacing the template structure spacing, a plurality of CNTs of different length may be controllably grown during the same deposition step. The growth method includes providing a growth surface which facilitates growth of carbon nanotubes, providing a nanotube source gas onto the growth surface, and controllably growing the carbon nanotubes of different length during the same deposition step.
The present inventors carried out electrical resistance measurements with a two-point probe on Au-coated SiO2 template structures with and without CNT bridges to explore their use in meso-scale circuits.
For all three samples, the resistance increases linearly with the number of patterns. The circuit contact resistance between the probe and the Au layer on top of SiO2 patterns (=2Rprobe-Au), obtained from the ordinate intercept of each plot, is also nearly constant at about 200-300 ohms for all samples. However, the average resistance between two adjacent patterns, denoted by the slope of the plots, is more than a factor of two smaller for patterns bridged by CNTs (Rbridged=54±2Ω) compared with that of the unbridged patterns (Runbridged=120Ω). This result indicates that the CNT bridges provide a low-resistance electronic transport pathway between the SiO2 patterns. The low resistance of the pattern-CNT-pattern units in the prepared samples is consistent with the good contact between CNT tips and Au layers observed in
The fifth preferred embodiment provides in-plane growth of CNTs in predefined directions and with tunable lengths by selective masking of SiO2 template structures with a metal that does not catalyze nanotube growth compared to silica. This method can be used to create nanotube bridges that interconnect SiO2 patterns and provide low-resistance pathways for electrical transport. The methods which combine CVD with topographical masking of patterned substrates may be used for controllably growing nanotube-metal architectures for electronic switching, memory storage, sensing and actuation devices.
The method and structure of the sixth preferred embodiment will now be described. A structure of the sixth preferred embodiment includes at least one suspended template material layer. A first aligned carbon nanotube layer is located on a first surface of the template material layer. A second aligned carbon nanotube layer is located on a second surface of the template material layer, located opposite to the first surface. Thus, a bilayer of aligned carbon nanotubes contains a suspended template layer between them. The nanotubes extend away from the respective surface of the template layer. If desired, three or more layers of aligned carbon nanotubes may be formed on two or more template layers.
The suspended template material layer may be a template material cantilever or membrane supported on a portion of a substrate that does not catalyze carbon nanotube growth. If the template layer has sufficient thickness, then a third aligned carbon nanotube layer is located on an edge surface of the template material layer, such that the third aligned carbon nanotube layer is located perpendicular to the first and the second aligned carbon nanotube layers.
The two growth directions from the SiO2 surfaces (inset) and a schematic illustration of nanotube bilayer growth are shown in
FIGS. 12A-B are schematic sketches showing simultaneous multilayer and multidirectional growth of oriented nanotubes from thick (e.g., 2 microns or more) SiO2 layer suspended on deep etched Si pillars.
Simultaneous multidirectional growth of highly oriented nanotubes with tunable lengths of the preferred embodiments allows formation of complex three dimensional CNT networks, where vertically and horizontally aligned nanotube arrays enable the construction of hierarchical multilevel architectures. Specifically, the nanotube networks may be used in diverse applications such as nanotube-based electronic devices, micro and nano-electromechanical systems, micro- and nano-size porous supports and membranes for catalysis, fluidics and separation, and skeletal reinforcements for composites.
The present inventors have achieved excellent control and flexibility in designing and fabricating a wide variety of carbon nanotube architectures, as discussed in the preferred embodiments. Several ordered nanotube based structural elements of different orientations may be integrated onto one substrate by combining standard lithography techniques with a substrate-selective CNT CVD growth process effected by gas-phase delivery of catalysts. Structures such as micro-fibers and membranes containing highly aligned nanotubes can fabricated and could find use in nanocomposites and electrode systems. The fabrication method described here is far easier and more versatile than the prior art methods. The preferred method does not require a metal catalyst patterning step, thus simplifying the method to a great degree and providing better control, particularly for the catalyst particle size, for the growth process. The preferred method is also scalable over large areas for commercial production with the aid of fabrication techniques commonly used in silicon microfabrication technology.
The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The description was chosen in order to explain the principles of the invention and its practical application. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.