Patent Application
20080014443
The present invention relates generally to the field of nanotechnology and, more particularly, to a method and apparatus for transferring an array of oriented carbon nanotubes.
Aligned multiwalled carbon nanotubes (MWNT) are most commonly grown using chemical vapor deposition (CVD). This method, however, has a serious drawback: aligned MWNTs can only be grown only on certain materials because of the high temperature at which the growth takes place (about 700° C.). As a result, current technology does not allow patterned aligned MWNT to be grown on any type of substrate. For example, it is not possible to grow nanotubes by an acetylene-based CVD process on metals that have low melting temperatures (e.g., Al, Pb, etc.) or metals that react with acetylene (e.g., Cu, etc.).
Another known method of growing aligned MWNT is the electrophoretic method, which is a cold cathode fabrication process further described in Y. Nakayam, S. Akita, Synth. Met. 117 (2001) 207-210. MWNTs produced in this manner have limited application because of the weak adhesion of the carbon powder to the substrate. Other processes may include the arc discharge method or microwave plasma deposition method.
Ink-jet or screen printing of patterned cold emitters is another fabrication method, which involves the printing of specially formulated inks having conducting or semiconducting particles in an insulating matrix. The application of this method is hindered because the printable field emitters produced by this method have high threshold voltages (˜20 V μm−1) and require special conditioning procedures to obtain uniform electron emission at sufficiently low fields.
Field electron emission (FE) from MWNT has become a widely known phenomenon since the discovery of nanotubes in 1991. Field emitters comprising aligned arrays of MWNTs have higher current densities and a better stability over long duration of time as compared to conventional field emitters. Moreover, there are no methods and processes capable to transfer matter in high vacuum while retaining the pattern and preferential orientation of the matter without using complicated multi-step lithographic processes involving masks, stamps and/or pre-patterned substrate preparations.
Different techniques have been proposed for improvement of mechanical properties of carbon nanotube emitters, such as embedding the bottom part of vertically aligned carbon nanotubes into a SiO2 film or burying nanotubes in a polymer matrix. However, these techniques only improve mechanical contact, but they do not decrease electrical and thermal resistance of the contacts, which are the critical parameters for applications based on vertical geometry such as cold field emitter, nanoelectrode arrays for chemical and biosensors and vertical nanotransistors.
There is, therefore, a need for a method and apparatus for transferring an array of oriented carbon nanotubes having a certain pattern, and improved thermal and electrical characteristics.
The present invention transfers arrays and patterned arrays of aligned multiwalled carbon nanotubes under high electric field in a high vacuum from one surface to another. Such a transfer of matter in high vacuum over macroscopic distances and subsequent deposition on surfaces while still retaining the pattern and orientation without using mask and pre-patterned substrate was heretofore unknown. Note that the matter transfer described herein is not merely the transfer of single ions or molecules, but is the transfer of nano size structures made of huge number of atoms. Such sublimation is accompanied by field electron emission and spark light emission from nanotubes. Since the tubes fly through the separation between the substrate (cathode) and the anode through high electric field, the tubes carry a lot of kinetic energy and when the tubes are re-deposited onto the anode, the adhesion between the re-deposited tubes and the anode is stronger than the original adhesion of the tubes and the cathode substrate. In addition, sublimation onto anodes made of different materials can be achieved. Thus, the present invention's method and apparatus to transfer nanotubes onto any material or substrate by the application of an electric field is not only simple but is also highly effective.
Furthermore, the present invention allows the making of more sharp and precise patterns or creating new patterns (different from original pattern on the cathode) on the anode and thus obtaining clean patterns on the anode by coating an insulating anode surface with a conductive material in the specific places. For example, the pattern can be reinforced by making the conductive material on the anode (gold, ITO, etc. on glass) in patterns. Since the tubes will only stick on the surface that has a layer of conducting material, this method can be used to make clean patterns on the anode.
The present invention can be used to produce inexpensive gated field emission arrays by depositing aligned carbon nanotubes onto patterned substrate with gates. The invention solves the serious technological problem of growing CNT in deep wells of field emitter arrays or avoids the difficulties of anisotropic etching of conventional tip geometry. In addition, the present invention produces patterned aligned carbon nanotube field emitters having order-of-magnitude higher electrical, thermal and mechanical properties. For example, the following characteristics can be achieved: (1) increase the thermal conductivity from 10 W/mK to 200 W/mK, which is close to the thermal conductivity of metals (KCu=398 W/mK); and (2) increase the electrical conductivity from 10 S/cm to 100+ S/cm. The resulting nanotubes can be used in displays, atomic/ionic microscopes, lasers, etc.
More specifically, the present invention provides a method for transferring an array of oriented carbon nanotubes from a first surface to a second surface by providing the array of oriented carbon nanotubes on the first surface within a vacuum chamber, providing the second surface within the vacuum chamber separate from the first surface, and applying an electric potential between the first surface and the second surface such that the array of oriented carbon nanotubes are sublimed from the first surface and re-deposited on the second surface accompanied by field emission.
Accordingly, the present invention provides an array of oriented carbon nanotubes on a substrate made in accordance with the method described above. Likewise, the present invention provides an array of oriented carbon nanotubes disposed on a metallic substrate having a low melting point or that is reactive with acetylene or other reactive gases using in CVD processes. The present invention also provides an array of oriented carbon nanotubes disposed on a substrate, having a resistance on the order of 3-4Ω, an electrical conductivity greater than 100 S/cm, and a thermal conductivity greater than 100 W/mK.
Alternatively, the method may include providing a third surface such that the second surface is disposed between the first surface and the third surface and wherein the second surface is porous such that the array of oriented carbon nanotubes are sublimed from the first surface, pass substantially through the second surface and are re-deposited on the third surface. The third surface can be in physical contact with the second surface or separate from the second surface. The second surface can be a wire mesh, one or more wires, one or more strips, a solid surface having one or more apertures, or other conducting material with windows that will produce the desired pattern of oriented carbon nanotubes on the third surface.
In addition, the present invention provides an apparatus for creating a patterned array of oriented carbon nanotubes on a substrate that includes a vacuum chamber, a first surface and second surface disposed within the vacuum chamber, a power source external to the vacuum chamber for providing an electric potential to the first surface and the second surface, the first surface having an array of oriented carbon nanotubes disposed thereon, the second surface mounted on the anode separate from and parallel with the first surface, and a controller connected to the vacuum chamber and the power source for applying the electric potential between the first surface and the second surface such that the array of oriented carbon nanotubes are sublimed and re-deposited on the second surface.
Alternatively, the apparatus may include a third surface such that the second surface is disposed between the first surface and the third surface and wherein the second surface is porous such that the array of oriented carbon nanotubes are sublimed from the first surface, pass substantially through the second surface and are re-deposited on the third surface. The third surface can be in physical contact with the second surface or separate from the second surface. The second surface can be a wire mesh, one or more wires, one or more strips, a solid surface having one or more apertures, or other conducting material that will produce the desired pattern of oriented carbon nanotubes on the third surface.
Other features and advantages of the present invention will be apparent to those of ordinary skill in the art upon reference to the following detailed description taken in conjunction with the accompanying drawings.
The above and further advantages of the invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which:
While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.
The present invention transfers arrays and patterned arrays of aligned multiwalled carbon nanotubes under high electric field in a high vacuum from one surface to another. Such a transfer of matter in high vacuum over macroscopic distances and subsequent deposition on surfaces while still retaining the pattern and orientation without using mask and pre-patterned substrate was heretofore unknown. Note that the matter transfer described herein is not merely the transfer of ions or molecules, but is the transfer of nano size structures. Such sublimation is accompanied by field electron emission and caused by spark light emission as shown in
Furthermore, the present invention allows the making of more sharp and precise patterns or creating new patterns (different from original pattern on the cathode) on the anode and thus obtaining clean patterns on the anode by coating an insulating anode surface with a conductive material in the specific places. For example, the pattern can be reinforced by making the conductive material on the anode (gold, ITO, etc. on glass) in patterns. Since the tubes will only stick on the surface that has a layer of conducting material, this method can be used to make clean patterns on the anode.
The present invention can be used to produce inexpensive gated field emission arrays by depositing aligned carbon nanotubes onto patterned substrate with gates. The invention solves the serious technological problem of growing CNT in deep wells of field emitter arrays or avoids the difficulties of anisotropic etching of conventional tip geometry. In addition, the present invention produces patterned aligned carbon nanotube field emitters having order-of-magnitude higher electrical, thermal and mechanical properties. For example, the following characteristics can be achieved: (1) increase the thermal conductivity from 10 W/mK to 200 W/mK, which is close to the thermal conductivity of metals (KCu=398 W/mK); and (2) increase the electrical conductivity from 10 S/cm to 100+ S/cm. The resulting nanotubes can be used in displays, atomic/ionic microscopes, lasers, etc.
As shown in
Now referring to
Accordingly, the present invention provides an array of oriented carbon nanotubes on a substrate made in accordance with the method described above. Likewise, the present invention provides an array of oriented carbon nanotubes disposed on a metallic substrate having a low melting point or that is reactive with acetylene. The present invention also provides an array of oriented carbon nanotubes disposed on a substrate, having a resistance on the order of 3-4Ω, an electrical conductivity greater than 100 S/cm, and a thermal conductivity greater than 100 W/mK.
The array of oriented carbon nanotubes are sublimed when the field emitted electron flow through the carbon nanotubes reaches a threshold current, which can be between 0.3 mA/cm2 to 10 mA/cm2, depending on the cathode-anode spacing and other parameters. The threshold current can be an alternating current (AC), which can be adjusted, or a direct current (DC). The electric field is created by the electric potential between the first surface and the second surface, which is pulsed when AC is used. For example,
The electric potential is applied to form an electrical field that causes the field electron emission from nanotubes on a first surface, creates a force that moves departed tubes and assists the array of oriented carbon nanotubes to form a pattern on the second surface. The adhesion of the re-deposited carbon nanotubes on the second surface typically exceeds the adhesion of the carbon nanotubes on the first surface. Typically, the electrical resistance of the re-deposited carbon nanotubes on the second surface is on the order of 3-4Ω.
The first surface and the second surface can be oriented vertically, substantially planar, non-planar, substantially perpendicular to one another, not perpendicular to one another, or any desirable configuration. The first surface and the second surface are typically separated by a distance of between 10 and 500 microns. The separation between the first surface and the second surface can be adjusted to fit the application. In addition, the surfaces are not limited to planar surfaces, e.g., the first surface is a cylinder and the second surface is a target disposed within the cylinder.
The first surface is typically a metallic cathode and the second surface is typically a metallic anode, or other materials that can be electrically conductive, such as aluminum. The second surface may include gold, indium tin oxide coated glass or fluorinated tin oxide coated glass. The second surface may also be a metallic substrate or film having a low melting temperature (less than or equal to 700° C.), such as aluminum or lead. In fact, the second surface can have a melting temperature of less than 200 to 300° C. The second surface can also be a metallic substrate that is reactive with acetylene, such as bronze, copper or a metal alloy. In addition, the second surface may include one or more areas of conductive material and one or more areas of non-conductive material such that the areas of conductive material form a pattern. The first surface may include a first pattern while the second surface includes a second pattern. The first pattern can differ from the second pattern.
The array of oriented carbon nanotubes can be grown on the first surface using any desired process, such as a catalytic thermal chemical vapor deposition process. The array of oriented carbon nanotubes can be re-deposited on more than one surface of the target, such as a wire, a second cylinder or a circuit board. As a result, the present invention can be used to form one or more interconnects in an electrical circuit, a portion of a video display, a conductor, a transistor, or a one or more gated field emission arrays, etc. For example, flexible displays can be created without the use of binders is possible because the transfer of nanotubes does not require any adhesive. In addition, the array of oriented carbon nanotubes can be re-deposited to form a dense array of nanotubes from which one or more twist yarns can be spinned, or a dense and long array of nanotubes from which one or more transparent nanotube ribbons and sheets can be drawn. The method of the present invention can be performed a part of a continuous production process.
In addition, the present invention provides an apparatus for creating a patterned array of oriented carbon nanotubes on a substrate that includes a vacuum chamber, a first surface and second surface disposed within the vacuum chamber, a power source external to the vacuum chamber for providing an electric potential to the first surface and the second surface, the first surface having an array of oriented carbon nanotubes disposed thereon, the second surface mounted on the anode separate from and parallel with the first surface, and a controller connected to the vacuum chamber and the power source for applying the electric potential between the first surface and the second surface such that the array of oriented carbon nanotubes are sublimed and re-deposited on the second surface
Alternatively, the apparatus may include a third surface such that the second surface is disposed between the first surface and the third surface and wherein the second surface is porous such that the array of oriented carbon nanotubes are sublimed from the first surface, pass substantially through the second surface and are re-deposited on the third surface. The third surface can be in physical contact with the second surface or separate from the second surface. The second surface can be a wire mesh, one or more wires, one or more strips, a solid surface having one or more apertures, or other conducting material that will produce the desired pattern of oriented carbon nanotubes on the third surface.
As demonstrated by the present invention, it is possible to transfer patterned carbon nanotubes under high vacuum from the substrate that they are grown on (the cathode) to the opposite electrode: the plus charged anode. This process takes place only for nanoscale size objects, which have electrical conduction as a result of applying a negative bias to the nanotubes. Upon this voltage the electrons are emitted by electric field enhanced on the tips of tubes and fly in vacuum towards positively charged anode. This electronic phenomenon is well known as field emission. What is new is that above certain threshold current, the carbon nanotubes themselves are sublimed and are re-deposited on the anode. These transferred nanotubes retain the original pattern of nanotubes and their alignment even after being transferred to the anode over the distances of 10 to 500 microns.
Various tests and examples of the present invention will now be described in reference with
The field emission measurements of MWNT arrays were performed in 10−6 Torr vacuum chamber equipped by turbo-molecular pump. A diode setup was used for performing the experiments. An image of the experimental setup, as well as a schematic representation are shown in
Under high vacuum it is possible to transfer patterned carbon nanotubes from the substrate that they are grown on (the cathode) to the opposite electrode: the plus charged anode. This process takes place only for nanoscale size objects, which have electrical conduction as a result of applying a negative bias to the nanotubes. Upon this voltage the electrons are emitted by electric field enhanced on the tips of tubes and fly in vacuum towards positively charged anode. This electronic phenomena is well known as field emission. What is new, is that above certain threshold current, the carbon nanotubes themselves are sublimed and are deposited on the anode. These transferred nanotubes retain the original pattern of nanotubes and their alignment even after being transferred to the anode over the distances of 10 to 500 microns.
The transfer of the vertically aligned multiwalled carbon nanotubes occurs as a result of heating and melting of the Fe catalyst on the substrate, which is necessary for growth. The anode cathode spacing for the transfer is in the range of 10-500 μm depending on the size and height of the nanotubes. The transfer occurs during the emission current in the range of 0.32 mA/cm2 to 10 mA/cm2. The transfer of the nanotubes is accompanied by spark emission occurring as a result I of the melting of the Fe catalyst.
The present invention can be used to fabricate cold electron emitters that can work at low power and low electric fields, particularly for mass spectrometers to be used in space. The usual electron energies required are in the range of 20 to 90 eV and the emission currents should be in the range of 100 to 300 micro amps.
These re-deposited tubes are found to be able to emit electrons in reversed electric fields, which proves, that tubes did not transform to amorphous carbon upon field-induced sublimation. The optical spectra of emitted light are recorded by OMA at different electric fields in the range of 200 V-10 kV (both AC and DC) and have been found to have more complicated form than recently reported blackbody radiation shape for low field DC-field emission. The spectra induced by AC pulsed currents show clear discrete lines, which imply a molecular or atomic origin. The temperature of nanotubes overheated by emission currents can be estimated from light emission spectra. The SEM showed clear signatures of melting of the Si substrate under the spark emitting tubes.
Experimental Details:
Observations and Results:
Discussion: The following three cases can be seen from the OMA images:
Growth process of vertically aligned forests of multi walled carbon nanotubes. The well-ordered aligned MWNTs were synthesized by the catalytic thermal chemical vapor deposition (CVD) at the atmospheric pressure. The substrates were n++ Silicon (Si) (100) wafers with a resistivity of 0.005-ohm cm. A 5 nm thick Iron (Fe) film was deposited on the Si wafers by electron beam evaporation or photolithography using a lift-off process. The Fe deposited substrates were placed in the center of a quartz tube (I.D. 45 mm) located in a horizontal tubular electric furnace. The temperature of the substrates was raised at a rate of 15° C./min in a Helium atmosphere. Helium gas of 550 sccm flowed into the quartz tube while the furnace was heated in order to prevent the oxidation of transition metal. After reaching the reaction temperature 680° C., Acetylene gas was introduced into the chamber at the flow rate of 40 sccm. The deposition time was varied from 3 min to 12 min. The length of the carbon nanotubes was adjusted by the variation of the growth time. After the growth process was done, the tube furnace was cooled down to room temperature in Helium ambient. Both pulsed AC and DC fields were used in transfer of these nanotubes onto the anode. The transfer took place as a result of the Fe catalyst melting and vaporizing due to the heat produced as a result of very high currents.
Growth of vertically aligned and patterned carbon multi walled nanotubes. In order to grow patterned multi walled carbon nanotubes, the Fe catalyst film deposited on the on the Si substrate was patterned using e-beam photolithography. The size of the pattern could be changed as necessary. The transfer in the case of patterned nanotubes is similar to that of unpatterned aligned nanotubes.
Deposition on metallic surfaces. The transfer of the nanotubes can be achieved onto surfaces of conductive materials such as Al or Cu. Since the transfer takes place when the nanotube tips are very hot, they penetrate into the metal surface and lead to a better electrical contact.
Deposition on conductive polymers. The transfer of NT can also be done onto a thin layer of polymer and organic matter such as (Alq3), or ITO on metal on plastic. Since the organic layers are very soft, the nanotubes are able to penetrate deep and form excellent electrical and thermal contact.
Instead of making strips on the non-conducting material, a mesh or grid can be placed in front of the non-conducting material (either touching the substrate or not touching it). Potential is applied to the mesh and current is also collected on the mesh. If the size of the openings in the mesh are made up of large squares, then the nanotubes can fly through the mesh and get deposited on the non-conducting material. See
It is also possible to transfer the nanotubes onto nonconducting materials by making thin strips of the conducting material on the top of the non-conducting material. The positive potential will be applied to the strips and current will be collected only at the strips but the nanotubes will be transferred and will cover the complete area between strips.
Deposition of carbon nanotube arrays which can be spinned into yarns of sheets. This example shows that if the original nanotube array on the first surface is high enough and dense enough, then the deposited nanotubes array is also dense enough for spinning from it the yarns and sheets as described by inventors in their recent papers (M. Zhang, et al., Science, 306, 1358 (2004); M. Zhang et al., Science 309, 1215 (2005)), which are hereby incorporated by reference in their entirety. In this example the original MWCNTs forest of 5 by 5 mm area is prepared unpatterned with the height of forest of 300 micron. Deposition is performed on the anode of Al, separated by 500 micron spacing from cathode. The deposited tubes are then tested for drawing a yarn from side of it, and applying twist during drawing following the described technique. Twisted yarn of 1 sm length and 2 micron diameter has been obtained.
Although preferred embodiments of the present invention have been described in detail, it will be understood by those skilled in the art that various modifications can be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.
This application claims priority to U.S. Provisional Patent Application No. 60/626,789 filed Nov. 11, 2004 entitled Method and Apparatus for Transferring an Array of Oriented Carbon Nanotubes, which is herein incorporated by reference in its entirety.
The U.S. Government may own certain rights in this invention pursuant to the terms of the U.S. Air Force Office of Scientific Research Grant No. F 49620-03-1-0164.
| Number | Date | Country | |
|---|---|---|---|
| 60626789 | Nov 2004 | US |
