WO2011022091A1

WO2011022091A1 - Method and apparatus for depositing a metal coating upon a nanotube structure

Info

Publication number: WO2011022091A1
Application number: PCT/US2010/030235
Authority: WO
Inventors: Brian Ruby; Cody Black
Original assignee: Carbon Nanoprobes, Inc.
Priority date: 2009-04-07
Filing date: 2010-04-07
Publication date: 2011-02-24

Abstract

A method and apparatus for depositing a coating upon a nanotube structure that does not damage or otherwise alter the nanotube structure. The invention can include placing a source metal within a container substantially adjacent to at least two electrodes; positioning the nanotube structure substantially adjacent to the container; and applying an electrical current to the electrodes sufficient to heat the container and the source metal, and cause a portion of the source metal to evaporate from the container and deposit on a portion of the nanotube structure.

Description

METHOD AND APPARATUS FOR

DEPOSITING A METAL COATING UPON A NANOTUBE STRUCTURE

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit under 35 U.S. C. § 119(e) of the earlier filing date of U.S. Provisional Application Serial Numbers 61/167,215 and 61/167,292 both filed on April 7, 2009, the entire disclosures of which are hereby incorporated by reference herein as if being set forth in their entirety.

FIELD OF THE INVENTION

[0002] The invention relates generally to a method and apparatus for depositing a metal, and more specifically to a method and apparatus for depositing a metal coating upon a nanotube structure.

BACKGROUND OF THE INVENTION

[0003] A nanotube is a hollow cylinder that is only a few nanometers wide and is made of one element, normally carbon. Carbon nanotubes are carbon-based molecular structures that can have lengths of several nanometers to several micrometers. Carbon nanotubes have unique mechanical and electrical properties that make them useful for a variety of applications, including use as probe tips in atomic force microscopy (AFM).

[0004] Atomic force microscopes use a probe tip to obtain images from a sample by dragging the probe tip over the surface of the specimen or within close proximity to the surface of the specimen. A probe tip that is attached to a cantilever can be dragged across the surface of the specimen and the displacement of the cantilever can be recorded as the probe tip tracks the topography of the specimen to generate an image of the sample. For example, a laser beam can be shined on the back surface of the cantilever while the probe tip is dragged across the surface of a sample, and the movement of the cantilever can be deduced from changes in the angle or position of the laser beam that is reflected from the surface of the cantilever. The resolution of images produced by AFM is related to the size and shape of the probe tip, and tips with small diameters and high aspect ratios of their length to their diameter can be used to resolve small lateral and vertical features. Carbon nanotubes are useful as AFM probe tips because they are extremely strong, and can be made of various lengths and aspect ratios to suit the user's needs.

[0005] The use of carbon nanotubes as AFM tips has been described, for example, by Dai, et al. in U.S. Pat. No. 6,346,189 and in U.S. Patent Application Publication US2002/0178846, both of which are incorporated herein by reference. But it has been difficult to mass produce such carbon nanotube tips.

[0006] Carbon nanotubes can be grown using chemical vapor deposition (CVD), but this process often causes the generation of an anti-reflective coating on the backside of the substrate. Such an anti-reflective coating can interfere with the feedback and control mechanisms in devices that have optical monitoring components, such as atomic force microscopes.

[0007] Thus, it is desirable to add a coating to the substrate to regain the substrate's reflective characteristics. It may also be desirable to add other types of coatings to nanotube structures, such as conductive coatings.

[0008] But many methods for depositing films or coatings onto a nanotube structure can damage or alter the commercially desirable features of the nanotube. As an example, conventional and known sputter deposition, plating deposition, as well as CVD techniques used to apply coatings or films onto a substrate have been found to damage or alter the nanotube structure. [0009] Accordingly, there is a need for a method for depositing a coating or film upon a nanotube structure that does not damage or otherwise alter the nanotube structure.

SUMMARY OF THE INVENTION

[0010] The present invention provides a method and apparatus for depositing a coating upon a nanotube structure that does not damage or otherwise alter the nanotube structure. The invention can include placing a source metal within a container substantially adjacent to at least two electrodes; positioning the nanotube structure substantially adjacent to the container; and applying an electrical current to the electrodes sufficient to heat the container and the source metal, and cause a portion of the source metal to evaporate from the container and deposit on a portion of the nanotube structure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a top view of a patterned substrate.

[0012] FIG. 2 A is a side view of a portion of the patterned substrate showing an array of precursor tips.

[0013] FIG. 2B is a side view of a portion of the patterned substrate showing an array of precursor tips with a layer of resist.

[0014] FIG. 2C is a side view of a portion of the patterned substrate showing an array of precursor tips with a layer of catalytic material on top of the layer of resist.

[0015] FIG. 2D is a side view of a portion of the patterned substrate showing an array of precursor tips with the resist layer removed and catalytic material on the apexes of the precursor tips.

[0016] FIG. 3 A is a top view of a flat, doped substrate with perimeter rails.

[0017] FIG. 3B is a side view of a sandwich of the patterned substrate and the doped substrate. [0018] FIG. 4 is a schematic view of a processing furnace arrangement.

[0019] FIG. 5A and FIG. 5B are schematic side views of a portion of the patterned substrate showing an array of precursor tips.

[0020] FIGS. 6 A and 6B are scanning electron micrograph images of a nanotube grown on a precursor tip.

[0021] FIGS. 7A and 7B are front and cross-sectional views of a nanotube structure.

[0022] FIG. 8 shows a thermal evaporation apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The following detailed written description of embodiments of the present invention, in conjunction with particular embodiments as described in the accompanying figures, will facilitate understanding of the present invention. It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clear understanding of the present invention.

[0024] Referring generally now to FIGS. 1-8 in which like reference characters refer to like parts throughout the several figures, one embodiment of a method and apparatus for depositing metal coatings upon nanotube structures is provided according to the present invention, along with a method for generating nanotube structures. This exemplary nanotube-structure-generating method is described to better understand the invention, which can be used to deposit metal coatings on nanotube structures regardless of how such structures are generated.

[0025] Referring to FIG. 1 and the method for generating nanotube structures, a commercially available wafer substrate 100 (such as those available from Veeco Probes) can have a top surface 102 with an unpatterned perimeter 106 and a central patterned portion 104. In this embodiment, the central patterned portion 104 has 375 prefabricated cantilevers with a pyramidal tip on each cantilever. The wafer substrate 100 can be made of silicon or other suitable materials for fabricating an array of cantilevers. The wafer substrate 100 can be attached to a thicker substrate to provide support and flatness to the wafer substrate 100 while chemical vapor deposition (CVD) steps are performed on the substrate 100.

[0026] Referring to FIG. 2A, the pyramidal tips 202 can protrude about 1-20 microns above the top surface 102 of the substrate 100. On any given wafer substrate 100 the pyramidal tips 202 can have a uniform height to within about 5% of the specified average height of the pyramidal tips 202 on the substrate 100, but different wafers from the same manufacturer can have pyramidal tips 202 with different average heights.

[0027] The wafer surface 102 containing the array of pyramidal tips 202 is spin coated with a resist layer 204 to cover the surface 102 at the base of the pyramidal tips 202, as shown in FIG. 2B. In a preferred embodiment, a polymethlymethacrylate (PMMA) material is used for the resist layer 204, but other materials can also be used. The resist layer 204 tends to pool around the bases of the pyramidal tips 202, leaving the apexes of the pyramidal tips 202 uncoated by the resist layer. The resist can be applied to the top surface 102 several times to reach a desired thickness of the resist layer 204 that covers most of the pyramidal tips 202 but leaves the apexes 206 of the tips exposed. After an adequate amount of resist has been applied to cover all but the apexes 206 of the pyramidal tips 202, the resist is hardened by baking it on a hot plate at about 90⁰C for about one minute.

[0028] Next, a catalyst suspension layer 208 can be spin coated onto the apexes 206 of the pyramidal tips 202 and on top of the resist layer 204, as shown in FIG. 2C. The catalyst layer 208 contains materials that catalyze the growth of nanotubes on the apexes 206 of the tips 202. For example, catalyst suspension layer 208 containing Fe-Mo nanoparticles for catalyzing the growth of nanotubes can be prepared according to a procedure described in Li and Liu, "Preparation of Monodispersed Fe-Mo Nanoparticles as the Catalyst for CVD Synthesis of Carbon Nanotubes," Chem. Matter, Vol. 13, pp. 1008-14 (2001), which is incorporated herein by reference. A catalyst suspension solution can be created by dissolving 1.00 mmol (0.196 g) Fe(CO)₅, 0.020 mmol (0.053 g) Mo(CO)₆, 0.100 mmol (0.144 g) octanoic acid and 0.100 mmol (0.242 g) bis-2-ethylhexylamine in 5.00 mL octyl ether and refluxing the solution under an N₂ atmosphere for 30 minutes at high temperature (e.g., about 300⁰C). The formation of Fe-Mo catalytic nanoparticles in this solution is indicated by the solution turning black. Other catalytic materials can also be used. For example, a Co-Mo catalyst embedded in a silica matrix can be prepared according to a procedure described in "Long and Oriented Single- Walled Carbon Nanotubes Grown by Ethanol Chemical Vapor Deposition," Limin Huang, Xiaodong Cui, Brian White, and Stephen P. O'Brien, J. Phys. Chem. B, vol. 108, pp. 16451-56 (2004) (hereinafter "Huang"), which is incorporated herein by reference.

[0029] In an alternative method, catalyst material can be applied to the entire surface 102 of the substrate 100 without first applying a resist layer. In this case, the catalyst material is not limited to the apex 206 of a precursor tip 202, but is applied over the entire surface of the tip 202.

[0030] After the catalytic material solution is prepared according to Huang, it can be poured onto the central patterned portion 104. The first substrate 100 is preheated so that when the catalyst solution is applied it evaporates very quickly, leaving the apexes 206 of the pyramidal tips 202 coated with a catalytic material 208, as shown in FIG. 2D. Because the pyramidal tips 202 provide a precursor base upon which nanotube tips are grown, the pyramidal tips 202 may also be known as precursor tips. The precursor tips 202 need not be pyramidal in shape but can be of any shape that provides a base upon which a catalytic material 208 may be deposited. The catalyst-coated apexes 206 are then calcinated according to the procedure described in Huang.

[0031] Although a method of applying catalyst material to the apexes 202 of precursor tips 202 has been described, catalyst material may be applied to localized islands at the end of individual cantilevers by other methods, and the presence of a precursor tip 202 is not necessary for locating catalyst material at the ends of the cantilevers. For example, known masking and lithography techniques can be used to deposit small catalyst islands directly onto the ends of flat cantilevers without using precursor tips 202 that protrude from the cantilevers.

[0032] Growing the nanostructures to a predetermined maximum length using the CVD process disclosed herein can be accomplished as follows. Referring to FIGS. 3A and 3B, a second substrate 300 having an etched top surface 302 with perimeter rails 304 that are higher than a central flat portion 306 can be placed over the first substrate 100 in a position that allows multiple nanotubes having predetermined lengths to be grown simultaneously on the apexes 206 of the precursor tips 202. The rails 304 of the second substrate 300 can be of equal height or a different height. The rails can be created on a flat, doped silicon substrate 300 by well-known lithography and etching techniques. For example, lithography and etching techniques can be used to remove a layer from the surface of the substrate except in the regions of the rails 304, which are not etched, such that rails 304 that extend above the surface 306 of the substrate 300 remain after the etching process. Referring to FIG. 3B, when the second substrate 300 is placed on top of the patterned wafer 100, the perimeter rails 304 rest on the perimeter 106 of the patterned wafer 100 so that the central flat portion 306 of the second substrate 300 is disposed at a distance, d, from the top surface 102 of the patterned wafer 100 to form a sandwich having a hollow central tunnel between the patterned wafer 100 and the second wafer 300. Although FIG. 3B shows the precursor tips 202 facing up, the orientation of the first wafer 100 and the second wafer 300 can be reversed, such that the first wafer 100 is above the second wafer 300 and the precursor tips 202 face down. A layer of conductive and material can be deposited on the surface 306 of the second wafer 300 to enhance the conductivity of the wafer when used to shorten the nanotubes grown on the precursor tips 202, as described in more detail below. For example, a conductive material as described in Maruyama et al., "Direct Synthesis of High-Quality Single- Walled Carbon Nanotubes on Silicon and Quartz Substrates," Chem. Phys. Lett., Vol. 377, p. 49-54, which is incorporated herein by reference, can be applied to the surface 306 of the wafer. The layer of conductive material can also include catalytic material for catalyzing the growth of nanotubes on the surface or for attracting nanotubes grown on the precursor tips 202 to grow directly towards the surface 306.

[0033] Nanotubes can be grown on the apexes 206 of the precursor tips 202 by flowing carbon-containing gasses through the hollow central tunnel and over the apexes 206. In the tunnel between the patterned wafer 100 and the second wafer 300, the apexes 206 are located at a distance from the central flat portion 306 of the second substrate 300, where the distance between the central flat portion 306 and the apexes 206 of the precursor tips 202 is equal to the distance, d, minus the height of the precursor tips 202. When the rails 304 are of equal height, this distance is substantially constant across the entire surface of the flat portion 306. When the rails are of different heights, the distance varies across the central flat portion 306. The distance between the apexes 206 of the precursor tips 202 and the surface 306 of the doped substrate 300 determines the maximum length to which nanotubes can grow on the precursor tips 202. This maximum length can be chosen to be between 5 nanometers and 500 micrometers. The second substrate 300 can be made of doped silicon (e.g., doped with 10¹⁷ boron atoms per cm³) so that the substrate 300 is conductive. An electrical contact 312 can be disposed on a back surface 310 of the second substrate 300 such that an electrical potential can be applied to the substrate 300.

[0034] After the wafer sandwich is constructed, a carbon-containing gas is flowed through the central tunnel to grow nanotubes on the apexes 206 of the precursor tips 202 through a CVD process. Referring to FIG. 4, as described in Zheng, et al, "Efficient CVD Growth of Single- Walled Carbon Nanotubes on Surfaces Using Carbon Monoxide Precursor," Nano Lett., Vol. 2, pp. 895-898 (2002), which is incorporated herein by reference, in one process for growing the nanotubes, a two furnace arrangement can be used. The wafer sandwich is positioned in a second furnace 404, and H₂ is input from a first gas source 400 at a rate of 400 standard cubic centimeters per minute (SCCM) into a first furnace 402 until the first furnace reaches a temperature of about 500⁰C, while H₂ is input from a second gas source 406 at a rate of 400 SCCM into a second furnace 404 until the second furnace reaches a temperature of about 800- 900⁰C. The first furnace 402 and the second furnace 404 are coupled by a small conduit 408 such that gas can flow from one furnace to the other. When both furnaces 402 and 404 reach their intended temperatures, the gas flow into the first furnace 402 is changed to CO at a rate of 400 SCCM, and the flow of H₂ into the second furnace 404 is increased to 800 SCCM. While CO flows through the central tunnel of the wafer sandwich and over the surface 102 of the patterned wafer, CO is dissociated and carbon nanotubes grow on the catalyst covered apexes 206 of the precursor tips 202. Nanotubes grow on the precursor tips 202 perpendicular to the surface 102 of the wafer until they reach the flat central portion 306 of the second wafer 300. These gas flows are maintained for about 15 minutes and then the entire system is cooled under a flow of H₂. Growth of nanotubes on the catalyst islands at the apexes 206 of the precursor tips progresses until the nanotubes reach their maximum length near to or in contact with the surface 306 of the doped silicon wafer 300.

[0035] Although a two furnace setup using CO as a source of carbon can be used to grow nanotubes, nanotubes can be grown using other methods as well. For example, a single furnace can be used with methane as the source of carbon, as described by Dai in U.S. Pat. No.

6,346,189, which is incorporated herein by reference. In another process, the wafer sandwich can be loaded into a single furnace chamber, whereupon argon and hydrogen gas is flowed through the furnace over the wafer sandwich at flow rates of 480 and 100 SCCM, respectively. The temperature of the wafer sandwich is raised to about 85O⁰C while argon and hydrogen gas is introduced to the chamber to reduce catalyst material from an oxide form to a metallic form. After the furnace temperature reaches about 85O⁰C₅ the argon and hydrogen gas flows are maintained for about 10 minutes, and then ethanol is introduced to the furnace at a flow rate of about 10 SCCM, while the hydrogen flow rate is reduced to about 40 SCCM and the argon flow rate is increased to about 740 SCCM. The ethanol, argon, hydrogen mixture is flowed into the furnace for about 15 minutes, and the carbon in the ethanol gas provides the source of carbon for the nanotubes to grow on the apexes 206 of the tips 202. After nanotubes have been grown, the ethanol and hydrogen gas flows are terminated and the furnace is cooled to room temperature under an argon gas flow. After the nanotubes have been grown on the apexes 206 of the precursor tips 202 and the system has been cooled, the nanotubes grown on the apexes are shortened to desired lengths.

[0036] In several preferred embodiments, the desired length can be between about 5 nanometers and about 500 micrometers, between about 5 nanometers and about 50 micrometers, between about 5 nanometers and about 5 micrometers, between about 100 nanometers and about 500 micrometers, between about 100 nanometers and 50 micrometers, between about 100 nanometers and 5 micrometers, between about 500 nanometers and about 500 micrometers, between about 500 nanometers and 50 micrometers, between about 500 nanometers and 5 micrometers, or between about 3 micrometers and 5 micrometers.

[0037] In one process, after the growth furnace(s) has cooled, the furnace containing the wafer sandwich is flushed with a non-reactive gas (e.g., Ar, He, Xe, Kr, N₂) to remove impurities. Then, a constant voltage is applied to the doped wafer 300, while the first wafer 100 is held at ground. The potential difference between the wafers 100 and 300 can be about 1.5 volts or can be higher, for example, about 20-50 volts. The application of the voltage to the doped wafer 300 breaks connections between the nanotubes and the surface 306 of the doped wafer 300 and cleaves the ends of the nanotubes that are distal from the precursor tips from the surface of the second substrate. As the voltage is ramped up, the nanotubes are shortened. The nanotubes grown on the precursor tips 202 are all shortened at the same time, such that their free ends become located at substantially the same distance from the surface 306 of the doped wafer 300. In addition to cleaving and shortening by the application of a voltage, the nanotube tips can be shortened and cleaved in other ways, for example, by the application of a liquid or a gas phase chemical, which causes cleaving or shortening of the nanotubes. After the nanotubes have been shortened, the individual cantilevers on the patterned wafer 100 can be separated from each other and used in individual atomic force microscopes.

[0038] Referring to FIG. 5A, as an alternative to the precursor tips 202 protruding above the surface 102 of the substrate 100, the precursor tips 202 can be located in between rails 502 that are created on the surface of the substrate 100. The rails 502 can be created by etching the surface of the substrate 100, such that the precursor tips 202 are formed in channels 500 between the rails 502, which protect the tip 202 from damage. Referring to FIG. 5B, to form nanotubes on the precursor tips 202 located between rails 502, a flat substrate 504 having a surface 506 is placed on top of the rails 502, and carbon-containing gas is flowed through the channels 500 to deliver material for nanotube growth on the precursor tips 202. The length of nanotubes grown on a precursor tip 202 in such a configuration depends on the distance between the apex of the tip 202 and the surface 506 of the substrate 504.

[0039] Referring to FIGS. 6 A and 6B, nanotubes 600 grown on precursor tips 602 with the above described methods are shown in scanning electron micrograph images.

[0040] Although growth of carbon nanotubes has been described, the methods and systems described herein can be used to grow other nanostructures (e.g., nanotubes, nanowires, nanofibers) of other chemical compositions as well. For example, cobalt oxide (Co₃O₄) wires can be grown.

[0041] FIGS. 7A and 7B show, respectively, a front and a cross-sectional view of a nanotube structure 610. As shown, a nanotube structure 610 can include an AFM holder chip 606 for use with an atomic force microscope, a cantilever 604 held by the AFM holder chip 606, a precursor tip 602 at the end of a cantilever 604, and a nanotube 600 grown on the precursor tip 602. Unless otherwise specified, the term nanotube structure refers to any structure that includes a nanotube.

[0042] Many types of films and coatings may be desirable to provide certain attributes to nanotube structures 610, including but not limited to reflective, conductive, physical, magnetic, adhesive, structural, and supportive attributes.

[0043] For example, when growing carbon nanotubes 600 via chemical vapor deposition

(CVD) onto a catalyst coated substrate as described above, it is common to generate a carbonaceous coating on the backside of the substrate, making the substrate anti-reflective. Anti- reflective coatings can interfere with the feedback and control mechanisms in devices that have optical monitoring components, such as atomic force microscopes. For example, where carbon nanotubes 600 are used as AFM probes, optically reflective cantilevers 702 are required to generate high signal-to-noise ratios. As such, one or more reflective films can be applied to the backside of cantilevers 702 to boost data signals. These films are critical with respect to cantilevers 702 that have an anti-reflective coating deposited as a byproduct of a CVD process.

[0044] A conductive film or coating to the nanotube structure 610 may also be desirable. In many nanotechnology applications, it is useful to have a conductive and sharp probe tip with nanometer scale dimensions. This is a highly desirable trait particularly in the field of scanning probe microscopy. In many applications, it is desirable or even necessary to have a conductive pathway from the apex of the probe tip to the terminal end(s) of the device.

[0045] In one embodiment, a carbon nanotube 600 can be affixed to or grown via CVD onto a precursor structure 602, such as a silicon pyramid. To ensure a conductive pathway from the apex of the nanotube 600 to the terminal end(s) of the AFM holder chip 606, a conductive metal film/coating can be applied to one or more sides of a carbon nanotube probe tip.

[0046] Because the conductivity of a nanotube structure 610 does not depend on the use or presence of a precursor structure, in another embodiment one or more carbon nanotubes 600 can be vertically aligned on one or more cantilevers 604 and/or substrates with no precursor structures. In such an embodiment, a conductive coating is applied to one or more sides of the cantilevers 604 and/or substrates to create a conductive pathway.

[0047] Whatever film or coating is applied, it is critical that the method for depositing the film or coating onto the nanotube structure 610 does not damage or alter the commercially desirable features of the nanotube 602. As an example, conventional and known sputter deposition, plating deposition, as well as CVD techniques used to apply coatings or films onto a substrate have been found to damage and/or alter the nanotube structure 610.

[0048] It has been found that certain physical methods of deposition, particularly those that do not generate an electrical field or an electromagnetic effect, are particularly effective in depositing reflective, conductive and other films with minimal or no damage or alteration to the nanotube structure 610. As non-limiting examples, thermal and electron beam evaporation deposition techniques are effective methods of depositing reflective, conductive, and other films onto nanotube structures 610.

[0049] In one preferred embodiment, thermal evaporation is used to deposit a reflective or conductive coating onto a nanotube structure. Examples of reflective or conductive coatings include, but are not limited to, aluminum, chrome, copper, gold, nickel, platinum, silver, titanium, and tungsten, and alloys of these metals.

[0050] FIG. 8 shows a thermal evaporation apparatus 700 for depositing a coating according to one embodiment of the invention. A nanotube structure holder 702 holds a nanotube structure 610 in a position adjacent to the source metal 712 such that the nanotube structure 610 can receive the evaporated source metal 712. FIG. 8 shows the nanotube structure 610 above the source metal 712. The figure also shows the nanotube 600 facing the source metal 712, as is appropriate for applying a conductive coating. The nanotube structure 610 can also be oriented such that the backside of the nanotube structure 610 faces the source metal 712, as is appropriate when applying a reflective coating. Note also that the thermal evaporation apparatus 700 can be configured to apply a conductive coating in one step and a reflective coating in a another step, where both steps apply the described method of thermal evaporation. [0051] FIG. 8 also shows that a container 710, such as an evaporation boat, holds the source metal 712 adjacent to two electrodes 708. In this embodiment, the container 710 is between and in contact with the two electrodes 708, though the container 710 need only be positioned such that it can receive heat from the electrodes 708. A current source 714 can apply a current to the electrodes 708, causing the container 710 to heat up until the source metal 712 begins evaporating and at least a portion of the evaporated source metal 712 is received by the nanotube structure 610, as depicted by the illustrated source metal pathway 716. The thermal evaporation apparatus 700 can also include a shutter 706 for controlling the exposure of the nanotube structure 610 to the evaporated source metal 712. As with the other figures, FIG. 8 is a simplified illustration of the depicted apparatus 700, and accordingly only illustrates those elements relevant for a clear understanding of the invention. Other elements necessary for operation of the apparatus 700 are well known to one of skill in the art. Further, the illustrated elements are not necessarily drawn to scale.

[0052] The thermal evaporation can occur in a vacuum chamber 704 and under low pressure in a preferred range of about 10^"6 to 10^'5 Torr. A pump or other appropriate device can be used to adjust the pressure within the vacuum chamber 704.

[0053] The current can be adjusted such that the source metal 712 evaporates at an optimal or desired deposition rate or deposition thickness, depending on the application for which the carbon nanotube structure 610 is used. In different embodiments, a strong enough current is applied to the electrodes 708 such that the source metal 712 evaporates at a deposition rate of about 0.5 to about 5 angstroms/second, or about 0.5 to about 4 angstroms/second, or about 0.5 to about 3 angstroms/second, or about 0.5 to about 2 angstroms/second, or to about 0.5 to about 1 angstrom/second, and to a thickness of about 20 to about 100 nanometers, or about 20 to about 80 nanometers, or about 20 to about 60 nanometers, or about 20 to about 40 nanometers, or about 25 to about 35 nanometers, or about 25 to about 30 nanometers. Alternatively, the source metal 712 can deposit at a thickness of about 15 to about 150 nanometers, or about 15 to about 130 nanometers, or about 15 to about 110 nanometers, or about 15 to about 90 nanometers, or about 15 to about 70 nanometers, or about 15 to about 50 nanometers, or about 25 to about 50 nanometers. A strong enough current should be applied to raise the temperature of the container 710 to the boiling point of the source metal 712 within the preferred pressure range. The evaporation of the source metal 712 under the low pressure will cause the metal particles to travel at a mean free path depositing onto the nanotube structure.

[0054] In a preferred embodiment, thermal evaporation is used to deposit gold as a reflective coating onto a silicon cantilever 604 comprising one or more freestanding carbon nanotubes 600. Using a Varian NRC Thermal Evaporator, a tungsten evaporation boat 710 containing gold as the source metal 712 to be evaporated is positioned between two copper electrodes 708 under the pressure of 10^"6 Torr. In different embodiments, a current is applied to the electrodes 708 such that the gold evaporates at a preferred deposition rate of about 0.5 to about 1 angstrom/second, and to a preferred thickness of about 25 to about 30 nanometers.

[0055] While preferred embodiments of the present invention have been shown and described, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the invention in its broader aspects. The appended claims are intended to cover, therefore, all such changes and modifications as fall within the true spirit and scope of the invention.

Claims

We claim:

1. A method of depositing a coating on a nanotube structure, the method comprising the

steps of:

placing a source metal within a container, wherein the container is substantially adjacent to at least two electrodes;

positioning the nanotube structure substantially adjacent to the container; and

applying an electrical current to the electrodes sufficient to:

heat the container and the source metal, and

cause a portion of the source metal to evaporate from the container and deposit on a portion of the nanotube structure.

2. The method of claim 1, wherein the container is an evaporation boat.

3. The method of claim 1, wherein the container is in contact with two of the at least two

electrodes.

4. The method of claim 1, wherein the nanotube structure is positioned above the source

metal.

5. The method of claim 1, wherein the nanotube structure comprises a carbon nanotube.

6. The method of claim 1, wherein the portion of the nanotube structure receiving the

source metal includes a cantilever.

7. The method of claim 1, wherein the portion of the nanotube structure receiving the

source metal includes a precursor tip.

8. The method of claim 1, wherein the source metal is reflective.

9. The method of claim 1, wherein the source metal is conductive.

10. The method of claim 1, wherein the method further comprises the following steps: placing a second source metal within the container;

applying a second electrical current to the electrodes sufficient to:

heat the container and the second source metal, and

cause at least a portion of the second source metal to evaporate from the container and deposit on the nanotube structure.

11. The method of claim 10, wherein the source metal is reflective and the second source

metal is conductive.

12. The method of claim 1, wherein the source metal is selected from one of the

following: aluminum, chrome, copper, gold, nickel, platinum, silver, titanium, and tungsten.

13. The method of claim 1, further comprising the step of placing the source metal under

a pressure of about 10^" to 10^" Torr.

14. The method of claim 1, wherein the step of applying an electrical current to the

electrodes further includes adjusting the applied current to cause a desired deposition rate or a desired deposition thickness of the source metal.

15. The method of claim 14, wherein the desired deposition rate is about 0.5 to 1

angstrom per second.

16. The method of claim 14, wherein the desired deposition thickness is about 25 to 30

nanometers.

17. A nanotube structure produced according to the process of claim 1.

18. A nanotube structure produced according to the process of claim 4.

19. A nanotube structure produced according to the process of claim 6.

20. An apparatus comprising:

a container configured to receive a source metal;

a nanotube structure holder positioned substantially adjacent to the container and

configured to receive a nanotube structure; and

at least two electrodes positioned substantially adjacent to the container, wherein the electrodes are configured to receive electrical current, heat the container and the source metal, and cause a portion of the source metal to evaporate from the container and deposit on a portion of the nanotube structure.

21. The apparatus of claim 20, wherein the container is an evaporation boat.

22. The apparatus of claim 24, wherein the container is in contact with two of the at least

two electrodes.

23. The apparatus of claim 20, wherein the nanotube structure is positioned above the

source metal.

24. The apparatus of claim 21, wherein the nanotube structure comprises a carbon

nanotube.

25. The apparatus of claim 20, wherein the source metal is reflective.

26. The apparatus of claim 20, wherein the source metal is conductive.

27. The apparatus of claim 21, wherein the source metal is selected from one of the

28. The apparatus of claim 20, further comprising a device configured to place the source metal under a pressure of about 10^"6 to 10^'5 Torr.

29. The apparatus of claim 20, wherein adjustment of the electrical current received by the electrodes adjusts a deposition rate or a deposition thickness of the source metal.

30. The apparatus of claim 29, wherein adjustment of the electrical current adjusts the deposition rate to about 0.5 to 1 angstrom per second.

31. The apparatus of claim 29, wherein adjustment of the electrical current adjusts the deposition thickness to about 25 to 30 nanometers.