CROSS REFERENCE TO RELATED APPLICATIONS
The present application claims priority to the following U.S. Provisional Patent Application, Serial No. 60/417,246.
- BACKGROUND INFORMATION
The present invention relates in general to nanostructured materials, and in particular, to using modified carbon nanotubes for field emission applications.
Carbon nanotubes (CNTs) are currently being investigated for use as cold electron sources in a variety of applications. These include displays, microwave sources, x-ray tubes, etc. For CNTs to be used as a cold cathode, they must be placed on a conductive surface (conductive substrate or conductive film on a non-conductive substrate). This has led some to place catalysts on the substrate surface and grow the carbon nanotubes in situ using CVD techniques (Kim et al., J. Appl. Phys., 90(5), 2591 (2001)). However, this has several draw-backs. This technique typically grows multi-wall carbon nanotubes (MWNTs). However, MWNTs have poorer field emission quality compared to single-wall carbon nanotubes (SWNTs). The substrate is subjected to high temperature, typically above 600° C., limiting the substrates that can be used. Uniformity is difficult to achieve because of the high temperature growth processes required. As a result, the manufacture of cathodes using this process will be very expensive due to the number and complexity of post-processing steps needed to generate a material capable of producing the desired level of field emission.
Other investigations have centered on processes for making CNT cathodes in a separate process, collecting them, and then dispensing them onto a substrate using a variety of techniques (Kim et al., Diamond and Related Materials, 9, 1184 (2000)). This has several advantages over the in situ method described above. First, the fabrication of the CNT material is decoupled from the fabrication of the cathode. This permits choosing the optimal CNT material for the application (single-wall, double-wall, multi-wall, purified, non-purified, etc.). Second, the dispensing process is carried out a relatively low-temperatures, permitting greater flexability in the choice of substrates. Third, uniform deposition over large area substrates is far more feasable using currently-available, low-cost equipment. Current dispensing processes, however, have their disadvantages. One of these is that the CNT fibers are often dispensed such that they clump together or are imbedded inside another material. These factors limit the performance of the CNT material. “Activation” processes are often employed after dispensing the CNT material, and these processes recover some of the performance of the virgin CNT (Chang et al., U.S. Pat. No. 6,436,221 B1). These “activation” process steps, however, can add cost to the product and may lead to non-uniform performance. Yet another disadvantage of current dispensing techniques is that the dispensed CNT fibers may not have sufficiently good contact to the substrate or the substrate's conductive layer such that this impedes their ability to supply the electrons needed for field emission.
It has been recently found that by mixing CNT material with other nanoparticle materials, the field emission properties of the CNT were improved (Mao et al., U.S. Provisional Application No. 60/417,246, incorporated herein by reference). Because neighboring nanotubes shield the extracted electric fields from each other (Bonard et al., Adv. Mat., 13, 184 (2001)), it is believed that this improvement is a result of induced separation of the CNT material by the nanoparticles. In situations where the CNT fibers are too close, they may electrically screen the applied electric field from each other. By increasing the separation between the fibers, the effective applied field strength at the emission sites is higher.
Many SWNT fibers are semiconducting with a bandgap that is dependent upon the chiral indices (n,m) of the SWNT. Choi et al. (U.S. Pat. No. 6,504,292 B1) teach that, for field emission applications, this bandgap can be overcome by depositing a metal film on CNT fibers that are already attached to a substrate. Choi et al. teach that the CNT fibers are coated after the fibers are grown using CVD techniques. This method has the inherent aforementioned disadvantages of growing CNTs on the substrate. Furthermore, were the CNT fibers to be dispensed onto the substrate and then coated, the problems of separating the CNT fibers for improved emission would still remain.
BRIEF DESCRIPTION OF THE DRAWINGS
Consequently, there is a demonstrated need for a method of dispensing SWNTs onto a substrate in such a way as to inhibit clumping, provide for sufficiently good contact to the substrate, overcome the limitations imposed by semiconducting CNTs, and which obviates the need for activation processes.
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the follwing descriptions taken in conjunction with the accompanying drawings, in which:
FIG. 1 illustrates metallized carbon nanotubes on indium-tin-oxide (ITO)/glass, wherein the metal coating is not necessarily uniform over all of the carbon nanotubes (CNTs);
FIG. 2 illustrates an electroless plating bath used to coat carbon nanotubes with metal;
FIG. 3 illustrates an optical microscope image of metallized CNT material dispensed on a substrate surface;
FIG. 4 illustrates an optical microscope image of non-coated CNT material dispensed on a substrate surface;
FIG. 5 illustrates field emission current vs. electric field for cobalt-coated and non-coated carbon nanotubes; and
FIG. 6 illustrates a field emission display device incorporating the present invention.
The present invention is directed towards metallized carbon nanotubes, methods for making metallized carbon nanotubes, methods for dispensing metallized carbon nanotubes onto a substrate, cold cathode field emitting materials comprising metallized carbon nanotubes, and methods of using metallized carbon nanotubes as cold cathode field emitters.
Metallized carbon nanotubes, according to the present invention, are carbon nanotubes which have been at least partially coated with one or more metals. Carbon nanotubes, according to the present invention, include, but are not limited to, single-wall carbon nanotubes, multi-wall carbon nanotubes, buckytubes, carbon fibrils, derivatized carbon nanotubes, chemically-modified carbon nanotubes, metallic carbon nanotubes, semiconducting carbon nanotubes, and combinations thereof. Purity of the carbon nanotube reactant materials (i.e., the carbon nanotubes prior to being metallized) ranges generally from at least about 1 percent to at most about 100 percent, specifically from at least about 10 percent to at most about 100 percent, and more specifically from at least about 20 percent to at most about 100 percent. Carbon nanotubes, as described herein, can exist in bundles or as individual entities. Furthermore, the carbon nanotubes from which the metallized carbon nanotubes are derived can be produced by any process which suitably provides for carbon nanotubes according to the present invention.
Metal coatings (also termed “films”) on the carbon nanotubes comprise one or more metal layers and range generally in thickness from at least about 0.1 nanometer (nm) to at most about 10 micrometers (μm), specifically from at least about 0.1 nanometer to at most about 1 micrometer, and more specifically from at least about 0.5 nanometers to at most about 1 micrometer. Metal coatings on the carbon nanotubes include, but are not limited to nickel (Ni), iron (Fe), copper (Cu), silver (Ag), zinc (Zn), rhodium (Rh), tin (Sn), cadmium (Cd), chromium (Cr), beryllium (Be), palladium (Pd), indium (In), platinum (Pt), gold (Au), and combinations thereof. In some embodiments, the metal coating comprises an alloy of two or more metals. The weight percent of metal in the metallized carbon nanotube product ranges generally from at least about 0.1 percent to at most about 99 percent, specifically from at least about 1 percent to at most about 99 percent, and more specifically from at least about 5 percent to at most about 99 percent. In some embodiments of the present invention, these metal coatings are highly uniform over individual carbon nanotubes. In some embodiments, these metal coatings are non-uniform, non-continuous, and/or incomplete, as depicted in FIG. 1 wherein metal coating 105 is shown on carbon nanotubes 104 to form metallized carbon nanotubes 106. In some embodiments these metal coatings are deposited primarily on the exterior of carbon nanotube bundles. In some embodiments, bundles of carbon nanotubes are metallized within the interior of the bundle. In some embodiments, the carbon nanotubes are metallized endohedrally, inside the tube structure. Some embodiments comprise metallized carbon nanotubes with any combination(s) of the aforementioned metallized carbon nanotubes.
Exemplary methods of making metallized carbon nanotubes comprise the steps of: a) providing a plurality of carbon nanotubes; b) preparing an electroless metal plating solution; c) adding said carbon nanotubes to said electroless metal plating solution to form a reaction solution; d) subjecting said reaction solution to a reducing condition which causes metal ions in solution to be reduced to metal and nucleate on the carbon nanotubes to produce metallized carbon nanotubes; and e) removing said metallized carbon nanotubes from the reaction solution. In some embodiments of the present invention, the metallized carbon nanotubes are washed and dried after being removed from the reaction solution.
Carbon nanotubes, as described herein, can be carbon nanotubes of any dimension, chirality, and number of walls that suitably provides for carbon nanotubes of the present invention and include, but are not limited to, single-wall carbon nanotubes (SWNTs), multi-wall carbon nanotubes (MWNTs), buckytubes, carbon fibrils, derivatized carbon nanotubes, chemically-modified carbon nanotubes, metallic carbon nanotubes, semiconducting carbon nanotubes, and combinations thereof. In some embodiments of the present invention, the carbon nanotubes are treated with hydrochloric acid prior to the metallization step.
An electroless plating solution (commonly referred to as a plating bath), according to the present invention, comprises a solvent, a metal salt, and a reducing agent (See Ranney et al., Electroless Plating and Coating of Metals,” Noyes, Park Ridge, N.J. (1972), incorporated herein by reference, for a detailed description of electroless plating techniques). In some embodiments of the present invention, there is a promoter species which helps to dissolve the metal salt. In some embodiments, there may be a balancing agent to control the pH. The solvent can be any solvent which suitably provides for the solvation of the electroless plating solution components. An exemplary solvent is water. The metal salt can be any metal salt that suitably provides for electroless metal plating according to the present invention and includes, but is not limited to, salts of the following: nickel, iron, copper, silver, zinc, rhodium, tin, cadmium, chromium, beryllium, palladium, indium, platinum, and combinations thereof. In some embodiments, alloys of two or more metals are plated on the carbon nanotubes with this process. The reducing agent can be any reducing agent that suitably provides for the reduction of the metal salt according to the present invention and includes, but is not limited to NaH2PO2.H2O, N2H4.2HCl, N2H4.xH2O, and combinations thereof. The optional promoter species can be any species which suitably promotes the electroless metal plating process of the present invention by facilitating the dissolution of the metal salt in the solution. Suitable promoter species include, but are not limited to C4H4O6KNa.4H2O, Na2C4H4O6, Na3C6H5O7.2H2O, and combinations thereof. The optional balancing agent can be any species which suitably provides for the control of pH according to the present invention. Suitable balancing agents include, but are not limited to NaOH, KOH, NH4OH, and combinations thereof.
In some embodiments of the present invention, the process of adding the carbon nanotubes to the electroless plating solution is carried out by first ultrasonicating the carbon nanotubes in a suitable solvent just prior to addition. This enhances their dispersal in the electroless plating solution to form a reaction solution. This reaction solution is subjected to a reducing condition which causes metal ions in solution to be reduced to metal and nucleate on the carbon nanotubes to produce metallized carbon nanotubes. Reducing conditions, according to the present invention, are any conditions which suitably provide for a reduction of the metal ions in solution. Such reducing conditions induce this reduction and include, but are not limited to, heating, irradiation, chemical activation, and combinations thereof. In some embodiments, the electroless plating solution is subjected to the reducing condition prior to the addition of the carbon nanotubes.
In some embodiments of the present invention, the degree of carbon nanotube metallation (i.e., the amount of metal coated on the carbon nanotubes) is modulated by the amount of carbon nanotubes present in the reaction solution. In other embodiments, the degree of carbon nanotube metallation is modulated by the concentration of metal salts and reducing agents present in the reaction solution. In other embodiments, the degree of carbon nanotube metallation is modulated by the time the carbon nanotubes spend in the reaction solution. In still other embodiments, a combination of one or more of the aforementioned methods of modulating the degree of carbon nanotube metallation is used to produce a metallized carbon nanotube product with certain desired characteristics dependent upon the degree in which is has been metallized.
In some embodiments of the present invention, prior to the step of removing the metallized carbon nanotubes from the reaction solution, a stabilizing agent is added to slow the reduction of the metal ions. A stabilizing agent can be any species which suitably provides for the slowing of the reduction process of the present invention and includes, but is not limited to, H3BO3, C3H6O3, and combinations thereof. Such slowing of the reaction facilitates greater control over the nature of the end product. Suitable methods of removing the metallized carbon nanotubes from the reaction solution include, but are not limited to, centrifugation (and subsequent decantation), filtration, and combinations thereof. In some embodiments of the present invention, after the step of removing the metallized carbon nanotubes from the reaction solution, there is a step of washing the metallized carbon nanotube product. Suitable washing solvents include any solvent which suitably removes unwanted reactants or reaction products from the final product. Suitable solvents include, but are not limited to, water, isopropyl alcohol, acetone, and combinations thereof. Optional drying of the metallized carbon nanotube product can be carried out by any drying process which suitably provides for the drying of the metallized carbon nanotubes according to the present invention and includes, but is not limited to, heating, exposure to vacuum, vacuum heating, irradiation, and combinations thereof.
Exemplary methods of dispensing metallized carbon nanotubes onto a substrate comprise: a) dispersing the metallized carbon nanotubes in a solvent to form a suspension; and b) applying the suspension to a substrate using an “applicator means.” Solvents into which the metallized carbon nanotubes are dispersed include, but are not limited to, isopropanol, methanol, acetone, water, ethanol, and combinations thereof. Methods of dispersing the metallized carbon nanotubes in the solvent include, but are not limited to, stirring, shaking, ultrasonic assistance, and combinations thereof. FIG. 1 illustrates one embodiment of metallized carbon nanotubes 106 on a substrate 103.
An applicator means, according to the present invention, can be any method which suitably dispenses the suspension of metallized carbon nanotubes onto a substrate in a controlled manner. Such application can be uniform or non-uniform, and can vary considerably in terms of the thickness of the resulting film, or layer, of metallized carbon nanotubes on the substrate. An exemplary applicator means comprises a spraying technique whereby the suspension of metallized carbon nanotubes is sprayed onto a surface using a sprayer. While not intending to be bound by theory, a sprayer, according to the present invention, can be a pump sprayer which rapidly pushes the suspension through a small orifice and, upon exiting said orifice, the suspension becomes an aerosol of small suspension droplets which are directed toward the substrate surface. Optionally, the substrate can be heated during the application process to prevent the running of excess solvent. Typically, the substrate, after having applied the metallized carbon nanotubes to its surface, is dried to remove any excess solvent. A substrate, as described herein, can be any substrate which suitably provides for a surface on which to dispense metallized carbon nanotubes according to the present invention and includes, but is not limited to, metals, ceramics, glass, semiconductors, coated surfaces, layered materials, and combinations thereof.
In some embodiments of the present invention, the metallized carbon nanotubes are used for field-emission application. In some embodiments, these metallized carbon nanotubes are more suitable for field-emission applications than carbon nanotubes without a metal coating. While not intending to be bound by theory, it is likely that, when incorporated into a device for field-emission applications, the metallized carbon nanotubes are better separated from one another, creating a carbon nanotube arrangement of lower density that reduces the shielding effects contributed by neighboring carbon nanotubes. Furthermore, said metal coatings likely enhance the flow of electrons in semiconducting carbon nanotubes and at the nanotube-substrate junction. In some embodiments of the present invention involving field emission applications, the metallized carbon nanotubes are dispensed onto a substrate using an applicator means, and the resulting substrate (with the metallized carbon nanotubes) is used as the cathode in, for example, a field emission display. Other field emission applications in which metallized carbon nanotubes can be used include, but are not limited to X-ray sources, electron sources, rf arrays, microwave tubes, and combinations thereof.
Thus, as disclosed herein, the present invention is also directed towards an improved field emission cathode using carbon nanotube emitters that are first coated with a metal film and then dispensed onto the cathode. This field emission cathode is illustrated in FIG. 1. Referring to FIG. 1, metallized carbon nanotubes 106 are shown on a substrate 103 which comprises a conductive layer 102 and an optional layer 101, which can be either conductive or non-conductive. Collectively, this forms field-emission cathode 100. This cathode has advantages over the current art in that: a) the metal layer provides a high level of electrical conductivity along the length of the CNT fiber even if the fiber is semiconducting; b) the metal layer provides an additional means of separating the CNT fibers from each other, decreasing the mutual electrical shielding and eliminating the need for post-deposition activation steps; c) metal-coated carbon fibers adhere to metal layers on the substrate much more strongly than do bare carbon nanotubes (adhesion forces between metals are much stronger than the adhesion forces between the substrate and the un-metallized carbon nanotubes); and the metal coatings can be applied to SWNTs and MWNTs, semiconducting or metallic CNTs, purified or non-purified CNTs—all using standard electrolytic techniques permitting selection from a large variety of available CNT fibers.
Referring to FIG. 6, the field emission cathode described above can be incorporated into field emission display 600. On substrate 601, conductive layer 602 is deposited and metallized carbon nanotube layer 603 is deposited on top thereof. The anode includes substrate 604, which may be a glass substrate, conductive layer 605, which may be indium-tin-oxide, and a phosphor layer 606 for receiving electrons emitted from metallized carbon nanotube layer 603. Electrons are emitted from layer 603 in response to an appropriate electric field between the anode and the cathode.
The following examples are provided to more fully illustrate some of the embodiments of the present invention. The examples illustrate methods by which metal-coated (metallized) CNTs can be made and prepared for field emission applications. It should be appreciated by those of skill in the art that the techniques disclosed in the examples which follow represent techniques discovered by the inventor to function well in the practice of the invention, and thus can be considered to constitute exemplary modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.
Coating Single-Wall Carbon Nanotubes with a Cobalt Thin Film
This process provides a way of depositing a metal thin film or coating on the surface of carbon nanotubes using an electroless plating technique. Using this relatively inexpensive and simple process, metallized carbon nanotubes can be made efficiently in relatively large amounts.
The single-wall carbon nanotube (SWNT) material used here was purchased from Iljin Nanotech, Inc. (Korea). The length of the SWNTs ranged from approximately several micrometers to approximately 20 micrometers, and the diameters were generally less than about 2 nanometers.
Referring to FIG. 2, electroless plating apparatus 200 comprises an electroless plating solution 204 contained in a beaker 203 which in turn is immersed in a water bath 202. Water bath 202 is heated by a magnetic stirring hotplate 201 and temperature is monitored by thermometer 206. Stirring is accomplished with stir bars 205 activated by the magnetic stirring hotplate 201 and the stirring motor 207. In the present example, electroless plating solution 204 comprises water and the following chemicals:
1. A cobalt (Co) salt (CoSO4.7H2O) to provide Co ions (Note that other salts may be used, e.g., CoCl2.6H2O). Concentration of this component is approximately 20-28 grams per liter.
2. A reducing agent (NaH2PO2.H2O) to reduce Co ions to Co(0). Concentration of this component is approximately 18-25 grams per liter.
3. A promoter species to facilitate dissolution of the Co salt into the solution (C4H4O6KNa.4H2O). Concentration of this component is approximately 140-160 grams per liter.
4. A stabilizing agent (H3BO3), to slow the reducing reaction. Concentration of this component is 27-35 grams per liter.
5. A balancing agent (NaOH). This is used to control the pH value of the solution. The amount of this material that is used is that needed to maintain a pH of 8-10 for the metal plating solution.
The above chemicals were dissolved in deionized water up to 900 milliliters.
The cobalt ions in this solution undergo reduction under a reducing condition of approximately 85-95° C. The pH of the solution needs to be controlled before and during the reaction. In this example, the pH value was maintained at about 9. NaOH was added during the plating process to control the pH of the solution.
Approximately 3-4 grams of carbon nanotube powder is ultrasonicated in a beaker containing approximately 100 milliliters of water for several minutes before being introduced into the electroless plating solution (after addition, total solution is 1000 milliliters). After the solution is prepared, it is heated in a water bath to 85-95° C. and the ultrasonicated SWNTs are then added to the electroless plating solution quickly while the solution was stirred. Because the carbon nanotubes easily clump together, the water+CNT mixture should be ultrasonicated immediately before adding it to the plating solution. The typical reaction time in the plating solution is about 5-10 minutes. Longer times do not appear to affect the results greatly. During the reaction, gas is evolved from the solution. The solution is pink at the beginning but gradually turns colorless. At the end of the reaction, little or no gas is evolved from the solution.
- Example 2
After reaction/deposition of metal, the reaction beaker is taken out of the water bath and allowed to cool down to room temperature. After several minutes, the metallized carbon nanotube powders collect at the bottom of the beaker and the solution is decanted from the powder. The powder is washed several times, each time being careful to not disturb the powder. Washing dilutes the concentration of any electroless plating reactants still remaining on the powder after the reaction. The powder is then removed and dried in a furnace at about 60° C.-100° C. for several hours. The carbon nanotube powder is now coated with a thin layer or film of metal.
Dispensing Carbon Nanotubes onto a Substrate
In this example, cobalt-metallized SWNT powder was mixed with isopropyl alcohol (IPA) to form a suspension. The suspension comprised approximately 1 gram of metallized SWNTs in 1000 ml IPA. Because the SWNTs clump together readily, ultrasonic agitation was used to disperse the nanotubes in the IPA before spraying the solution onto cathode substrates. The SWNT/IPA suspension was sprayed onto conductive indium-tin-oxide (ITO)/glass substrate with an area of 2×2 cm2. In order to prevent the IPA from flowing uncontrollably, the substrate was heated up to approximately 30-70° C. on both the front side and back side during the spraying process. The substrate was sprayed back and forth several to tens of times until the carbon nanotubes covered on the entire surface. The thickness of the carbon nanotube layer was about 1-20 μm. The film was then dried in air.
FIGS. 3 and 4 show optical images of CNT material after having been dispensed onto a substrate surface (FIG. 3: metal-coated CNTs, FIG. 4: non-coated CNTs). Isolated, metallized CNT islands/clusters are formed during the dispensing spray process. The size of these islands is approximately 10-30 micrometers in diameter. A density of approximately 40,000 islands per square centimeter was achieved by the spaying process, and a considerable number of small clusters (approximately 0.5-5 μm in diameter) between these islands. If it is assumed that every island and cluster provides one emission site each, this would provide 1,000,000 emission sites per square centimeter.
3. Field Emission Test of the Samples
Substrates with metallized SWNT material coated on them were prepared as cathodes and tested for field emission properties as illustrated in FIGS. 1 and 6. Non-metallized SWNT coated substrates were also prepared in an identical fashion by the spray process for comparison purposes. The cathodes were tested by mounting them with a phosphor screen in a diode configuration with a gap of about 0.5 mm. The test assembly was placed in a vacuum chamber and pumped to 10−7 torr. The electrical properties of the cathodes were then measured by applying a negative, pulsed voltage to the cathode and holding the anode at ground potential and measuring the current at the anode. A pulsed voltage was used to prevent damage to the phosphor screen at the high current levels (duty factor: 2%). FIG. 5 illustrates the results of these tests. In each case, the cathodes were not “activated,” they were tested as they were deposited. It was found that the metallized CNT cathodes were very stable and very uniform. The non-metallized cathodes typically were unstable during the turn-on process (several arcing events occurred). From FIG. 5 it can be seen that metallized SWNTs yield much better field emission properties than the non-metallized SWNTs. Tests on the cathodes show threshold extraction fields of about 2 V/μm and emission current of 30 mA at 4 V/μm for Co-coated CNT compared with extraction fields of 3.5 V/μm and emission current of 30 mA at 6.5 V/μm for non-metallized CNTs.
All of the compositions and methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and methods and in the steps or in th esequence of steps of the methods described herein without departing from the concept, spirit, and scope of the invention. More specifically, it will be apparent that certain agents which are both chemically and physiologically related may be substituted for the agents described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope, and concept of the invention as defined by the appended claims.