WO2008066952A2

WO2008066952A2 - Selective destruction of metallic nanostructures via electromagnetic radiation

Info

Publication number: WO2008066952A2
Application number: PCT/US2007/069345
Authority: WO
Inventors: Ruth Yu-Ai Y. Zhang; Timothy P. Burgin; Larry A. Nagahara
Original assignee: Motorola, Inc.
Priority date: 2006-07-31
Filing date: 2007-05-21
Publication date: 2008-06-05
Also published as: WO2008066952A3; WO2008066952A4

Abstract

A method is provided for eliminating carbon nanotubes (26) having a metallic characteristic from grown carbon nanotubes (24). The method comprises growing at least one carbon nanotube (26) having metallic qualities and at least one carbon nanotube (28) having semiconductor qualities, and applying microwave radiation to destroy at least a portion of each of the at least one carbon nanotube (26) having metallic qualities. Electrodes (12, 14) may be formed at opposed ends of the at least one carbon nanotubes (28) having semiconductor qualities for forming an electronic device such as a sensor or a transistor.

Description

SELECTIVE DESTRUCTION OF METALLIC NANOSTRUCTURES VIA ELECTROMAGNETIC RADIATION

FIELD OF THE INVENTION

[0001] The present invention generally relates to forming a nanostructure device, and more particularly to selectively eliminating carbon nanotubes having a metallic characteristic from an assembly of carbon nanotubes containing both metallic and semiconducting properties.

BACKGROUND OF THE INVENTION

[0002] One-dimensional nanostructures, such as belts, rods, tubes and wires, have become the latest focus of intensive research with their own unique applications. One-dimensional nanostructures are model systems to investigate the dependence of electrical and thermal transport or mechanical properties as a function of size reduction. In contrast with zero-dimensional nanostructures, such as quantum dots, and two-dimensional nanostructures, (e.g., GaAs/AlGaAs superlattice) direct synthesis and growth of one-dimensional nanostructures has been relatively slow due to difficulties associated with controlling the chemical composition, dimensions, and morphology.

[0003] Carbon nanotubes are one of the most important species of one- dimensional nanostructures. Carbon nanotubes are one of four unique crystalline structures for carbon, the other three being diamond, graphite, and fullerene. In particular, carbon nanotubes refer to a helical tubular structure grown with a single wall (single-walled nanotubes) or multiple wall (multi-walled nanotubes). These types of structures are obtained by rolling a sheet formed of a plurality of hexagons. The sheet is formed by combining each carbon atom thereof with three neighboring carbon atoms to form a helical tube. Carbon nanotubes typically have a diameter on the order of a fraction of a nanometer to a few hundred nanometers. As used herein, a "carbon nanotube" is any elongated carbon structure. [0004] Another class of one-dimensional nanostructures is nanowires. Nano wires of inorganic materials have been grown from metal (e.g., Ag,and Au), elemental semiconductors (e.g., Si, and Ge), III- V semiconductors (e.g., GaAs, GaN, GaP, InAs, and InP), II-VI semiconductors (e.g., CdS, CdSe, ZnS, and ZnSe) and oxides (e.g., SiC^ and ZnO). Similar to carbon nanotubes, inorganic nanowires can be synthesized with various diameters and length, depending on the synthesis technique and/or desired application needs.

[0005] Both carbon nanotubes and inorganic nanowires have been demonstrated as field effect transistors (FETs) and other basic components in nanoscale electronics such as p-n junctions, bipolar junction transistors, inverters, etc. The motivation behind the development of such nanoscale components is that "bottom- up" approach to nanoelectronics has the potential to go beyond the limits of the traditional "top-down" manufacturing techniques.

[0006] Another major application for one-dimensional nanostructures is chemical and biological sensing. The extremely high surface-to-volume ratios associated with these nanostructures make their electrical properties extremely sensitive to species adsorbed on their surface. For example, the surfaces of semiconductor nanowires have been modified and implemented as highly sensitive, real-time sensors for pH and biological species.

[0007] A carbon nanotube is also known to be useful for providing electron emission in a vacuum device, such as a field emission display. The use of a carbon nanotube as an electron emitter has reduced the cost of vacuum devices, including the cost of a field emission display. The reduction in cost of the field emission display has been obtained with the carbon nanotube replacing other electron emitters (e.g., a Spindt tip), which generally have higher fabrication costs as compared to a carbon nanotube based electron emitter.

[0008] Unlike other inorganic one-dimensional nanostructures, carbon nanotubes can function as either a conductor, or a semiconductor, according to the chirality and the diameter of the helical tubes. With metallic-like nanotubes, a one- dimensional carbon-based structure can conduct a current at room temperature with essentially no resistance. Further, electrons can be considered as moving freely through the structure, so that metallic-like nanotubes can be used as ideal interconnects. When semiconductor nanotubes are connected to two metal electrodes, the structure can function as a field effect transistor wherein the nanotubes can be switched from a conducting to an insulating state by applying a voltage to a gate electrode. Therefore, carbon nanotubes are potential building blocks for nanoelectronic and sensor devices because of their unique structural, physical, and chemical properties.

[0009] In these applications using semiconductor nanotubes, e.g., transistors or sensors, it is desired to minimize the number of metallic nanotubes since they would short-out or reduce the semiconductor effect of the other nanotubes. One important parameter in a field effect transistor device is the current flowing through the device while in it's 'off state. A large amount of leakage current results in, for example, increase power usage for the device, significant generation of waste heat, and lower sensitivity in sensor devices.

[0010] Accordingly, it is desirable to provide a simple yet reliable technique to disable metallic nanotube(s) selectively in desired locations for device applications. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.

BRIEF SUMMARY OF THE INVENTION

[0011] A method is provided for eliminating carbon nanotubes having a metallic characteristic from grown carbon nanotubes. The method comprises growing at least one carbon nanotube having metallic qualities and at least one carbon nanotube having semiconductor qualities, and applying microwave radiation to destroy at least a portion of each of the at least one carbon nanotube having metallic qualities.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and

[0013] FIG. 1 is a simplified cross-sectional view of a structure on which the exemplary method may be applied;

[0014] FIG. 2 is a simplified top view of the apparatus of FIGS. 1;

[0015] FIG. 3 is a simplified top view of an apparatus on which an exemplary embodiment of the method has been applied; and

[0016] FIG. 4 is a simplified flow chart of the steps of an exemplary embodiment.

DETAILED DESCRIPTION OF THE INVENTION

[0017] The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.

[0018] One dimensional nanostructures such as nanotubes and nanowires show promise for the development of molecular-scale sensors, resonators, field emission displays, and logic/memory elements. One dimensional nanostructures are herein defined as a material having a high aspect ratio of greater than 10 to 1 (length to diameter). [0019] A growth technique is disclosed wherein one dimensional nanostructures having metallic qualities may be eliminated, for example, from a plurality of carbon nanotubes, some of which have semiconducting qualities and some of which have metallic qualities. The structure comprising the plurality of nanostructures is submitted to microwave radiation. The nanostructures having the metallic qualities partially absorb the microwave energy and convert the energy into heat. In the presence of oxygen, the nanostructures are destroyed, thereby preventing any current carrying capability. By destroyed, it is meant that the nanostructures are at least partially "burned" through, resulting in preventing electron flow therethrough or modifying its metallic properties such as increasing its resistivity.

[0020] Though the present invention may be applied to nanostructures as defined herein, the exemplary embodiment illustrates the treatment of carbon nanotubes; however, the invention should not be limited to carbon nanotubes. Referring now to FIG. 1, illustrated in simplified cross-sectional views, and in FIG. 2 in a simplified top view, is an assembled structure utilized for growth of carbon nanotubes according to an exemplary embodiment of the present invention. More specifically, illustrated in FIG. 1 is a structure 10 including two or more electrodes 12, 14. Although electrodes 12, 14 are shown as positioned on insulating layer 18, they could be recessed or buried. The structure 10 in this particular embodiment includes a substrate 17, comprising a semiconductor material 16 which has been coated with an insulating material 18. It should be understood that anticipated by this disclosure is an alternate embodiment in which substrate 17 is formed as a single layer of insulating material, such as glass, plastic, ceramic, or any dielectric material that would provide insulating properties. By forming substrate 17 of an insulating material, the need for a separate insulating layer formed on top of a semiconducting layer, or conductive layer, such as layer 18 of FIG. 1, is eliminated. [0021] The semiconductor material 16 comprises any semiconductor material well known in the art, for example, silicon (Si), gallium arsenide (GaAs), germanium (Ge), silicon carbide (SiC), indium arsenide (InAs), or the like. Insulating material 18 is disclosed as comprising any material that provides insulative properties such silicon oxide (SiC^), silicon nitride (SiN), or the like. The insulating material 18 comprises a thickness of between 2 nanometers and 10 microns. Semiconductor material 16 and insulating material 18 form substrate 17 as illustrated in FIGS. 1 and 2. Alternatively, the substrate 17 may comprise a non-semiconductor material such as glass or plastic. In this specific example, assembly 10 includes a first electrode 12 and a second electrode 14 formed on an uppermost surface of insulating material 18. Fabrication of metal electrodes 12 and 14 is carried out using any form of lithography, for example, photolithography, electron beam lithography, and imprint lithography on a substrate 17. In some embodiments, electrodes 12, 14 may comprise highly doped semiconductor material. Electrodes 12 and 14 comprise a thickness in the range of 1 nanometer to 5000 nanometers. Electrodes 12 and 14 are formed to define therebetween a gap 20. The gap 20 between electrodes 12 and 14 may be between 1 nanometer and several hundred microns.

[0022] A catalyst, such as nanoparticles 22, is typically formed in contact with the electrodes 12, 14 to initiate the growth of carbon nanotubes 24. It should be understood that the carbon nanotubes 24 may be formed in any manner known or hereinafter developed.

[0023] After catalytic nanoparticles 22 positioning, one dimensional nanotubes 24 are then grown from the catalytic nanoparticles 22 in a manner known to those skilled in the art, e.g., applying a gas comprising hydrogen and carbon for carbon nanotube growth. Although only a few catalytic nanoparticles 22 and carbon nanotubes 24 are shown, those skilled in the art understand that any number of catalytic nanoparticles 22 and carbon nanotubes 24 could be formed. It should further be noted that the electrodes 12, 14 may be formed either before or after the formation of the carbon nanotubes 24. Furthermore, in yet another embodiment, the electrodes 12, 14 may be omitted altogether.

[0024] The semiconductor nanostructures 28 may be prepared, for example, as a field effect transistor for use in sensors or electronic circuits, or as conductive elements, in which case a carbon nanotube 24 will be grown from one catalytic nanoparticle 22 to an electrode or to another carbon nanotube 24 to form a electrical connection between electrodes as shown in FIGS. 1-3.

[0025] In another exemplary embodiment, the carbon nanotubes 24 may be grown in bulk and deposited from a liquid suspension onto a substrate prior to applying the microwave radiation to destroy the metallic nanotubes 26.

[0026] When used for a display device, the carbon nanotubes 24 may be grown in a vertical direction. It should be understood that any one dimensional nanostructure having a height to radius ratio of greater than 10, for example, would function equally well with some embodiments of the present invention.

[0027] Carbon nanotubes 24 can function as either a conductor, like metal, or a semiconductor, according to the chirality and the diameter of the helical tubes. Referring to FIGS. 1 and 2, carbon nanotubes 24 comprise both nanotubes 26 with metallic qualities and nanotubes 28 with semiconducting qualities. With metallic - like nanotubes 26, a one-dimensional carbon-based structure can conduct a current at room temperature with essentially no resistance. Further, electrons can be considered as moving freely through the structure, so that metallic-like nanotubes 26 can be used as ideal interconnects. When semiconductor nanotubes 28 are connected to two metal electrodes 12, 14, the structure can function as a field effect transistor wherein the nanotubes 28 can be switched from a conducting to an insulating state by applying a voltage to a gate electrode. Therefore, carbon nanotubes 28 are potential building blocks for nanoelectronic and sensor devices because of their unique structural, physical, and chemical properties. The growth of carbon nanotubes 24 typically results in the growth of both metallic nanotubes 26 and semiconductor nanotubes 28. When the use of semiconductor nanotubes 28 are intended, metallic nanotubes 26 are undesired since they "short out" the semiconductor nanotubes 28 (electrons follow the path of least resistance through the metallic nanotubes 26).

[0028] In order to "destroy" or render non-metallic, the metallic nanotubes 26, while leaving the semiconductor nanotubes 28 in place, the structure 10, with the carbon nanotubes 24 grown thereon, is subjected to microwave radiation of an intensity and duration sufficient to cause the metallic nanotubes 26 to destruct, but insufficient to cause the semiconductor nanotubes 28 to destruct. The result may be seen in FIG. 3 wherein the metallic nanotubes 26 have been "burned" through (destroyed), while the semiconductor nanotubes 28 remain in their original condition (as in FIG. T). If the burn through is not complete, the metallic properties of the metallic nanotube 26 may be rendered non-metallic, e.g., resistive.

[0029] When microwave energy penetrates through a material with complex permittivity, the microwave energy is absorbed partially by the material and converted into heat. The power that is absorbed can be described by P = σE², where σ is the total effective conductivity, and E is the magnitude of the internal electric field. E and σ are interdependent, and functions of frequency. Single wall nanotube polymer composites exhibit high electric loss in the 0.5 to 2.0 GHz range. And the loss is caused by the presence of metallic single wall nanotubes. When single wall nanotubes are irradiated with microwave of appropriate frequency and conditions in a resonant cavity that enhances the microwave absorption of the single wall nanotubes, such that the semiconducting single wall nanotubes are more transparent to the microwave, but the metallic nanotubes absorb more microwave energy due to their high effective conductivity (or electric loss). Microwave generated heat will selectively destroy the metallic nanotubes in the presence of oxygen at a critical temperature. If metallic carbon nanotubes absorb more microwaves of a certain frequency than the semiconducting nanotubes, the metallic nanotubes arrive at the critical temperature sooner than the semiconducting nanotubes. Metallic carbon nanotube can therefore be selectively destroyed by controlling the irradiation time. As a result, a device comprising only semiconducting carbon nanotubes may be formed.

[0030] The process preferably may be conducted at a frequency of 0.5 to 500.0 GHz, although the frequency range may be much larger, and for generally less than five minutes and at a temperature of less than 1000° C.

[0031] The process is further illustrated by the flow chart in FIG. 4 wherein a material 16 is provided 42 to form a substrate 17. The material 16 may be coated 44 with an insulating material 18. At least one carbon nanotube 28 having semiconducting qualities and at least one carbon nanotube 26 having metallic qualities are grown 46 above the substrate 17. Microwave radiation is applied 48 to the structure 10 to at least partially burn off the at least one carbon nanotube 26 having metallic qualities. Electrodes 12, 14 may be formed 50 either prior to or subsequent to the growth of the carbon nanotubes 26, 28.

[0032] While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.

Claims

CLAIMS What is claimed is:

1. A method of forming an electronic device, comprising: growing at least one one-dimensional nanostructure having metallic qualities and at least one one-dimensional nanostructure having semiconductor qualities; and applying microwave radiation to destroy at least a portion of the at least one one-dimensional nanostructure having metallic qualities.

2. The method of claim 1 further comprising forming electrodes on opposed ends of the at least one one-dimensional nanostructure having semiconductor qualities.

3. The method of claim 1 wherein the growing step comprises growing carbon nanotubes.

4. The method of claim 1 wherein the applying step comprises applying microwave radiation at a frequency in the range of 0.5 to 500.0 GHz.

5. The method of claim 1 wherein the applying step comprises applying microwave radiation for less than five minutes.

6. The method of claim 1 wherein the applying step comprises applying microwave radiation at a temperature of less than 1000° C.

7. A method of forming a structure, comprising: forming a plurality of carbon nanotubes, the carbon nanotubes including nanotubes having semiconducting properties and nanotubes having metallic properties; and subjecting the carbon nanotubes to microwaves, whereby the nanotubes having metallic properties are at least partially destroyed.

8. The method of claim 7 further comprising forming electrodes on opposed ends of the carbon nanotubes having semiconductor properties.

9. The method of claim 7 wherein the applying step comprises applying microwave radiation at a frequency in the range of 0.5 to 500.0 GHz

10. The method of claim 7 wherein the applying step comprises applying microwave radiation for less than five minutes.

11. The method of claim 7 wherein the applying step comprises applying microwave radiation at a temperature of less than 1000° C.

12. The method of claim 8 wherein the electrodes comprise two electrodes positioned one of on, within a recess, or buried on a substrate.

13. A method of forming an electronic device, comprising: forming a first plurality of carbon nanotubes having semiconducting properties and a second plurality of carbon nanotubes having metallic properties; and subjecting the first and second plurality of carbon nanotubes to microwave radiation, whereby the second plurality of carbon nanotubes have their metallic properties modified.

14. The method of claim 13 further comprising forming electrodes on opposed ends of the first plurality of carbon nanotubes having semiconducting properties.

15. The method of claim 13 wherein the subjecting step comprises applying microwave radiation at a frequency in the range of 0.5 to 500.0 GHz.

16. The method of claim 13 wherein the subjecting step comprises applying microwave radiation for less than five minutes.

17. The method of claim 13 wherein the subjecting step comprises applying microwave radiation at a temperature of less than 1000° C.