Processes for growing carbon nanotubes using disordered carbon target

Abstract

Processes for producing single-wall carbon nanotubes without catalysts are provided. The nanotubes are produced by vaporizing silicon carbide and carbon.

Description

FIELD OF THE INVENTION

The present invention relates to processes for growing single-wall carbon nanotubes in the absence of a catalyst.

BACKGROUND OF THE INVENTION

In the field of molecular nanoelectronics, few materials show as much promise as nanotubes, and in particular carbon nanotubes, which comprise hollow cylinders of graphite. Nanotubes can be incorporated into electronic devices such as diodes and transistors, depending on the nanotube's electrical characteristics. Nanotubes are unique for their size, shape, and physical properties. Structurally, a carbon-nanotube resembles a hexagonal lattice of carbon rolled into a cylinder.

Besides exhibiting intriguing quantum behaviors at low temperature, carbon nanotubes exhibit the following important characteristics: a nanotube can be either metallic or semiconductor depending on its chirality (i.e., conformational geometry). Metallic nanotubes can carry extremely large current densities. Semiconducting nanotubes can be electrically switched on and off as field-effect transistors (FETs). The two types may be covalently joined (sharing electrons). These characteristics point to nanotubes as excellent materials for making nanometer-sized semiconductor circuits.

Nanotubes can be formed as single-wall carbon nanotube (SWNTs) or multi-wall carbon nanotube (MWNTs). SWNTs can be produced, for example, by arc-discharge and laser ablation of a carbon target (U.S. Pat. No. 6,183,714). Local growth of tubes on a surface can also be obtained by chemical vapor deposition (CVD). The growth of the nanotubes is made possible by the presence of metallic particles, such as Co, Fe, and/or Ni, acting as catalyst. The resultant carbon nanotubes comprise contaminants, e.g., catalyst particles. For most potential nanotube applications, the use of clean nanotubes can be important, for example, where nanotubes are incorporated as an active part of electric devices. The presence of contaminating atoms and particles can alter the electrical properties of the nanotubes. The metallic particles can be removed; however the process of cleaning or purifying the nanotubes can be complicated and can alter the quality of the nanotubes.

SWNTs have been identified as potential components of electronic devices. The quality of nanotubes, e.g., their ability to act as a semiconductor, can be affected by contaminants. Therefore, a need exists for a method of catalyst-free growth of single-wall carbon nanotubes.

U.S. Patent Application No. 2004/0035355 discloses a method for growing single-wall nanotubes comprising providing a silicon carbide semiconductor wafer comprising a silicon face and a carbon face, and annealing the silicon carbide semiconductor wafer in a vacuum at a temperature of at least about 1,350° C. and a pressure of 10⁻9 Torr thereby inducing formation of single-wall carbon nanotubes on the silicon face. The disclosed method utilizes relatively low pressures.

New and/or improved methods for making carbon nanotubes are desired.

SUMMARY OF THE INVENTION

One aspect of this invention is a process comprising:

• • a) providing a target comprising silicon carbide and carbon;
  • b) vaporizing the target in a catalyst-free environment in an inert atmosphere at a pressure from about 10⁻³ Torr to about 000 Torr; and
  • c) forming a product comprising at least one single-wall carbon nanotube.

Another aspect of the present invention is a single-wall carbon nanotube produced by a process comprising:

• • a) providing a target comprising silicon carbide and carbon;
  • b) vaporizing the target in a catalyst-free environment in an inert atmosphere at a pressure from about 10⁻³ Torr to about 000 Torr; and
  • c) forming a product comprising at least one single-wall carbon nanotube.

These and other aspects of the present invention will be apparent to those skilled in the art, in view of the following specification and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a transmission electron micrograph of agglomerates of single wall carbon nanotubes produced by one embodiment of the present process.

DETAILED DESCRIPTION OF THE INVENTION

All documents cited herein are expressly incorporated herein by reference in their entirety. Applicants also herein incorporate by reference the co-owned and concurrently filed application entitled “PROCESSES FOR GROWING CARBON NANOTUBES USING DISORDERED CARBON SOURCE” (Attorney Docket # CL 2627).

When an amount, concentration, or other value or parameter is given as either a range, preferred range, or a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of any upper range limit or preferred value and any lower range limit or preferred value, regardless of whether ranges are separately disclosed. Where a range of numerical values is recited herein, unless otherwise stated, the range is intended to include the endpoints thereof, and all integers and fractions within the range. It is not intended that the scope of the invention be limited to the specific values recited when defining a range.

The present invention provides a process for growing single-wall carbon nanotubes (SWNTs) in the absence of a catalyst. The process includes providing a target that is a mixture of silicon carbide and carbon, which comprises 50 weight percent silicon carbide or less, preferably about 45 weight percent or less, more preferably about 42 weight percent or less. In some embodiments, the amount of silicon carbide can be as low as 1 weight percent. Preferably, the amount of silicon carbide in the mixture is at least about 1 weight percent. The target comprising the mixture can be formed by, for example, forming a slurry of silicon carbide and carbon powders in a volatile solvent, allowing the solvent to evaporate, then compression molding the residual solid. The compression molded silicon carbide/carbon article can be optionally heated, preferably in an inert atmosphere, to substantially remove traces of the solvent and harden the target. Other methods for preparing such a target are known to those skilled in the art.

Vaporization of the target can be carried out by laser ablation or other suitable methods known to those skilled in the art, such as, for example, rf induction heating and sputtering. The vaporization can be carried out at temperatures between about 100° C. and 1500° C. and pressures of vacuum (e.g., about 10⁻³ Torr) to above atmospheric pressure.

In some preferred embodiments, the vaporization is carried out at a temperature from about 1000° C. to about 1200° C. Preferably, the vaporization is carried out in the presence of a non-oxidizing gas, such as argon, neon, helium, nitrogen or mixtures thereof. By “non-oxidizing”, as used herein, is meant an atmosphere in which oxygen content is minimized. Minimization of oxygen in the atmosphere during nanotube production is desirable because oxygen can oxidize the carbon, thereby reducing the production of the desired nanotubes. However, the total absence of oxygen is not required. Thus, in one illustrative embodiment, the target material is mixed, pressed and heated in an inert atmosphere at 1150° C. to harden the target before it is placed into a laser ablation system, wherein the oxygen content is minimized. Generally it is preferred that the atmosphere comprise no more than about 100 ppm oxygen, preferably about 50 ppm or less, more preferably about 25 ppm or less. Carbon nanotubes can desirably be formed in the presence of a non-oxidizing gas, such as argon, neon, helium, nitrogen or mixtures thereof. Commercially available tanks of gases, such as 99.9% pure argon, are suitable for the process of forming carbon nanotubes.

While the selection of the gas under which vaporization is carried out is not critical, the nature of the gas can affect the amount of nanotubes produced. While it is not intended that the invention be bound by any particular theory, it is believed that the thermal conductivity of the gas can affect the formation of nanotubes. For example, the use of helium may result in the formation of fewer nanotubes than would the use of nitrogen, because the higher degree of cooling expected to occur with helium can result in a cooler, and therefore less active, growth zone.

In one embodiment of this invention, a SWNT-containing product produced using the processes disclosed herein can serve as a target for one or more additional cycles of vaporization and SWNT-formation.

The process can further comprise an annealing step. Annealing does not require substantial additional processing, and can be accomplished by allowing the newly formed nanotubes to remain undisturbed and cool following ablation. The annealing can be performed in an ultra-high vacuum (UHV) (e.g., at a pressure less than about 10⁻⁹ Torr), or at higher pressures, even above atmospheric pressure (760 torr). Generally, a pressure of about 500 torr is suitable.

It is generally desirable to grow the tubes at pressures of at least 1 millitor, and preferably at 500 Torr or above. In some preferred embodiments, the pressure is about 1000 Torr. It is generally not desirable that the pressure be greater than about 1000 Torr. Although a reduction in pressure below about 500 Torr has not been observed to undesirably affect the rate of growth of nanotubes, pressures of about 500 Torr or greater are often practical. In embodiments, the pressure is from about 300 Torr to about 600 Torr. The nanotubes that are formed are predominately SWNTs as shown in the Figure. The SWNTs can be very long and have a good crystalline quality. “Good crystalline quality” means substantially free of observable defects under transmission electron microscopy.

All of the compositions and processes disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and processes have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations can be applied to the processes and methods and in the steps or in the sequence of steps of the processes described herein without departing from the concept, spirit, and scope of the invention. All substitutions 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.

EXAMPLE

The present invention is further defined in the following Example. It should be understood that this Example, while indicating a preferred embodiment of the invention, is given by way of illustration only. From the above discussion and this Example, one skilled in the art can ascertain the preferred features of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications to adapt it to various uses and conditions. This Example shows the production of single wall nanotubes by vaporizing silicon carbide with no added catalyst.

A silicon carbide target was made by mixing 36.1 grams (g) of silicon carbide powder (Third Millennium Technologies, Inc., Knoxville, Tenn.) with 51.5 g of Dylon®graphite cement (Dylon Industries, Inc., Cleveland, Ohio) in 80 ml of methanol. The methanol was allowed to evaporate overnight. The remaining solid was broken into small pieces in a mortar and pestle and compression molded at 130° C. for 1 hour. The molded article was then baked at 1150° C. for 10 hours in flowing Ar, and then inserted into a furnace at 1100° C. The target was ablated with Nd—Yag lasers running at 30 Hz with a pulse width of 10 nanoseconds. The pressure in the furnace was maintained at 500 torr. The target was rotated during ablation to achieve even ablation, and 1.12 g of product were collected after 1 h of run time. The micrographs in FIG. 1 show the presence of single wall carbon nanotubes.

Claims

1. A process comprising:

a) providing a target comprising silicon carbide and carbon;

b) vaporizing the target in a catalyst-free environment in an inert atmosphere at a pressure from about 10−3 Torr to 1000 Torr); and

c) forming a product comprising at least one single-wall carbon nanotube.

2. The process of claim 1, wherein the vaporization step is carried out by laser ablation.

3. The process of claim 2, wherein the laser ablation is performed at a temperature from about 100° C. to about 1500° C.

4. The process of claim 2, wherein the laser ablation is performed at a temperature from about 1000° C. to about 1200° C.

5. The process of claim 1, wherein the pressure is about 500 Torr or greater.

6. The process of claim 1, further comprising an annealing step after the formation of the at least one single-wall carbon nanotube.

7. The process of claim 1, wherein the vaporization is carried out in the presence of an inert gas selected from argon, neon, helium, nitrogen and mixtures thereof.

8. A single-wall carbon nanotube produced by the process of claim 1.