# Patent

## Title
Method for transferring carbon nanotube array and method for forming carbon nanotube structure

## Abstract

A method for transferring a carbon nanotube array is provided. A substitute substrate, a growing substrate, and a carbon nanotube array grown on the growing substrate are provided. The carbon nanotube array has a bottom surface adjacent to the growing substrate and a top surface away from the growing substrate. The substitute substrate is placed on the top surface of the carbon nanotube array and water is sandwiched between the substitute substrate and the carbon nanotube array. The water is frozen between the substitute substrate and the carbon nanotube array. The substitute substrate is separated from the growing substrate to separate the bottom surface of the carbon nanotube array from the growing substrate. The ice is removed between the substitute substrate and the carbon nanotube array. A method for forming a carbon nanotube structure is also provided.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 201410124924.2, filed on Mar. 31, 2014 in the China Intellectual Property Office, the contents of which are hereby incorporated by reference. This application is related to application entitled, “METHOD FOR TRANSFERRING CARBON NANOTUBE ARRAY AND METHOD FOR FORMING CARBON NANOTUBE STRUCTURE”, Ser. No. 14/310,295, filed Jun. 20, 2014.
FIELD
The subject matter herein generally relates to methods for transferring carbon nanotube arrays and methods for forming carbon nanotube structures.
BACKGROUND
Carbon nanotube can be fabricated by drawing from a carbon nanotube array grown on a growing substrate (e.g., silicon wafer), as disclosed by U.S. Pat. No. 8,048,256 to Feng et al. The carbon nanotube film is free standing and includes a plurality of carbon nanotubes joined end-to-end by van der Waals attractive force therebetween. The carbon nanotubes in the carbon nanotube film are substantially aligned along the lengthwise direction of the carbon nanotube film, and thus, the carbon nanotube film has good thermal and electrical conductivity along the direction of the aligned carbon nanotubes. The carbon nanotube film is substantially transparent and can be used as a conductive thin film. Therefore, the carbon nanotube film can be used in many different fields, such as touch panels, liquid crystal displays, speakers, heating devices, thin film transistors, cables, and the like.



BRIEF DESCRIPTION OF THE DRAWINGS
Implementations of the present technology will now be described, by way of example only, with reference to the attached figures, wherein:
FIG. 1 is a schematic structural view of an embodiment of a method for transferring a carbon nanotube array.
FIG. 2 shows a scanning electron microscope (SEM) image of a carbon nanotube film drawn from the carbon nanotube array.
FIG. 3 shows carbon nanotubes joined end-to-end.
FIG. 4 is a schematic structural view of another embodiment of the method for transferring the carbon nanotube array.
FIG. 5 is a schematic structural view of yet another embodiment of the method for transferring the carbon nanotube array.
FIG. 6 is a schematic structural view of an embodiment of a method for forming a carbon nanotube structure.
FIG. 7 shows an optical photo of drawing the carbon nanotube film from the carbon nanotube array transferred to a substitute substrate.



ELEMENT LISTING


    10 carbon nanotube array
    102 bottom surface
    104 top surface
    20 growing substrate
    30 substitute substrate
    40 carbon nanotube structure
    50 drawing tool
    60 water
    60′ ice
    70 freezer


DETAILED DESCRIPTION
The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “another,” “an,” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean “at least one.”
It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures and components have not been described in detail so as not to obscure the related relevant feature being described. Also, the description is not to be considered as limiting the scope of the embodiments described herein. The drawings are not necessarily to scale and the proportions of certain parts have been exaggerated to better illustrate details and features of the present disclosure.
Several definitions that apply throughout this disclosure will now be presented.
The term “contact” is defined as a direct and physical contact. The term “substantially” is defined to be essentially conforming to the particular dimension, shape, or other description that is described, such that the component need not be exactly conforming to the description. The term “comprising,” when utilized, means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in the so-described combination, group, series, and the like.
Referring to FIG. 1, the present disclosure is described in relation to a method for transferring a carbon nanotube array 10.
In block S1, a substitute substrate 30 and a growing substrate 20 are provided. The growing substrate 20 has a carbon nanotube array 10 grown thereon, and the carbon nanotube array 10 is in a state that is capable of having the carbon nanotube structure 40 drawn therefrom. The carbon nanotube array 10 includes a bottom surface 102 adjacent to the growing substrate 20 and a top surface 104 away from the growing substrate 20.
In block S2, the substitute substrate 30 is placed on the top surface 104 of the carbon nanotube array 10, and water 60 is sandwiched between a surface of the substitute substrate 30 and the top surface 104 of the carbon nanotube array 10.
In block S3, the water 60 between the substitute substrate 30 and the carbon nanotube array 10 is frozen into ice 60′.
In block S4, the substitute substrate 30 and the growing substrate 20 are moved away from each other, thereby separating the carbon nanotube array 10 from the growing substrate 20.
In block S5, the ice 60′ between the substitute substrate 30 and the carbon nanotube array 10 is removed by heating. The carbon nanotube array 10 is transferred from the growing substrate 20 onto the substitute substrate 30. The state of the carbon nanotube array 10, before, during, and after the transfer onto the substitute substrate 30, and after the removal of the ice 60′, is still capable of having the carbon nanotube structure 40 drawn therefrom.
The carbon nanotube structure 40 can be a free-standing structure including a plurality of carbon nanotubes joined end-to-end by van der Waals attractive force therebetween. The carbon nanotube structure 40 can be a carbon nanotube film or a carbon nanotube wire.
The carbon nanotube array 10 is grown on the growing substrate 20 by a chemical vapor deposition (CVD) method. The carbon nanotube array 10 includes a plurality of carbon nanotubes oriented substantially perpendicular to a growing surface of the growing substrate 20. The carbon nanotubes in the carbon nanotube array 10 are closely bonded together side-by-side by van der Waals attractive forces. By controlling growing conditions, the carbon nanotube array 10 can be essentially free of impurities such as carbonaceous or residual catalyst particles. Accordingly, the carbon nanotubes in the carbon nanotube array 10 are closely contacting each other, and a relatively large van der Waals attractive force exists between adjacent carbon nanotubes. The van der Waals attractive force is so large that when drawing a carbon nanotube segment (e.g., a few carbon nanotubes arranged side-by-side), adjacent carbon nanotube segments can be drawn out end-to-end from the carbon nanotube array 10 due to the van der Waals attractive forces between the carbon nanotubes. The carbon nanotubes are continuously drawn to form a free-standing and macroscopic carbon nanotube structure 40, which can be in the shape of a film or a wire. The carbon nanotube array 10 that can have the carbon nanotube structure 40 drawn therefrom can be a super aligned carbon nanotube array. A material of the growing substrate 20 can be P-type silicon, N-type silicon, or other materials that are suitable for growing the super aligned carbon nanotube array.
The carbon nanotube structure 40 drawn from the carbon nanotube array 10 includes a plurality of carbon nanotubes joined end-to-end and can be a free-standing carbon nanotube film. The carbon nanotube film includes a plurality of carbon nanotubes substantially aligned along the same direction.
Referring to FIG. 2 and FIG. 3, the carbon nanotube film can include or consist of a plurality of carbon nanotubes. In the carbon nanotube film, the overall aligned direction of a majority of the carbon nanotubes is substantially aligned along the same direction parallel to a surface of the carbon nanotube film. A majority of the carbon nanotubes are substantially aligned along the same direction in the carbon nanotube film. Along the aligned direction of the majority of carbon nanotubes, each carbon nanotube is joined to adjacent carbon nanotubes end to end by van der Waals attractive force therebetween, whereby the carbon nanotube film is capable of being free-standing structure. There may be a minority of carbon nanotubes in the carbon nanotube film that are randomly aligned. However, the number of the randomly aligned carbon nanotubes is very small and does not affect the overall oriented alignment of the majority of carbon nanotubes in the carbon nanotube film. Some of the majority of the carbon nanotubes in the carbon nanotube film that are substantially aligned along the same direction may not be exactly straight, and can be curved at a certain degree, or not exactly aligned along the overall aligned direction by a certain degree. Therefore, partial contacts can exist between the juxtaposed carbon nanotubes in the majority of the carbon nanotubes aligned along the same direction in the carbon nanotube film. The carbon nanotube film can include a plurality of successive and oriented carbon nanotube segments. The plurality of carbon nanotube segments are joined end to end by van der Waals attractive force. Each carbon nanotube segment includes a plurality of carbon nanotubes substantially parallel to each other, and the plurality of paralleled carbon nanotubes are in contact with each other and combined by van der Waals attractive force therebetween. The carbon nanotube segment has a desired length, thickness, uniformity, and shape. There can be clearances between adjacent and juxtaposed carbon nanotubes in the carbon nanotube film. A thickness of the carbon nanotube film at the thickest location is about 0.5 nanometers to about 100 microns (e.g., in a range from 0.5 nanometers to about 10 microns). When the carbon nanotube structure 40 has a small width, the carbon nanotube structure 40 can be a free-standing carbon nanotube wire.
The term “free-standing” includes, but is not limited to, a carbon nanotube structure 40 (e.g., film or wire) that does not need to be supported by a substrate. For example, a free-standing carbon nanotube structure 40 can sustain the weight of itself when it is hoisted by a portion thereof without any significant damage to its structural integrity. If the free-standing carbon nanotube structure 40 is placed between two separate supporters, a portion of the free-standing carbon nanotube structure 40 suspended between the two supporters can maintain structural integrity. The free-standing carbon nanotube structure 40 is realized by the successive carbon nanotubes joined end to end by van der Waals attractive force.
In the present disclosure, the growing of the carbon nanotube array 10 and the drawing of the carbon nanotube structure 40 are processed on different structures (i.e., the growing substrate 20 and the substitute substrate 30). The substitute substrate 30 for drawing the carbon nanotube structure 40 can be made of low-price materials, and the growing substrate 20 can be recycled quickly. Thus, production of the carbon nanotube structure 40 can be optimized.
The substitute substrate 30 can be a soft, elastic, or rigid solid substrate. The substitute substrate 30 has a surface to accept the carbon nanotube array 10 thereon. The surface of the substitute substrate 30 can be flat when the carbon nanotube array 10 is grown on a flat growing surface of the growing substrate 20. During transferring of the carbon nanotube array 10 from the growing substrate 20 to the substitute substrate 30, the state of the carbon nanotube array 10 is still capable of drawing the carbon nanotube structure 40 from the carbon nanotube array 10 on the substitute substrate 30. That is, the carbon nanotube array 10 transferred to the substitute substrate 30 is still a super aligned carbon nanotube array.
The carbon nanotube array 10 is arranged upside down on the surface of the substitute substrate 30. The carbon nanotubes are grown from the growing surface of the growing substrate 20 to form the carbon nanotube array 10. The carbon nanotube includes a bottom end adjacent or contacting the growing substrate 20 and a top end away from the growing substrate 20. The bottom ends of the carbon nanotubes form the bottom surface 102 of the carbon nanotube array 10, and the top ends of the carbon nanotubes form the top surface 104 of the carbon nanotube array 10. After the carbon nanotube array 10 is transferred to the substitute substrate 30, the top surface 104 of the carbon nanotube array 10 is now adjacent to or contacting the substitute substrate 30, and the bottom surface 102 of the carbon nanotube array 10 is now away from the substitute substrate 30.
In block S2, the water 60 can be in a shape of fine droplets, mist, or film. The water 60 can spread on the entire top surface 104. Water has a poor wettability for carbon nanotubes. Thus, when a small amount of water 60 is on the top surface 104 of the carbon nanotube array 10, it cannot infiltrate inside the carbon nanotube array 10 and will not affect the state of the carbon nanotube array 10. A diameter of the water droplet and a thickness of the water film can be in a range from about 10 nanometers to about 300 microns. The substitute substrate 30 and the top surface 104 of the carbon nanotube array 10 are both in contact with the water 60.
In block S2, to maintain the state of the carbon nanotube array 10 of being capable of drawing the carbon nanotube structure 40, the substitute substrate 30 may apply a pressing force as small as possible to the carbon nanotube array 10. The pressing force can satisfy 0<f<2N/cm2. The pressing force does not press the carbon nanotubes down or vary the length direction of the carbon nanotubes in the carbon nanotube array 10. The carbon nanotubes in the carbon nanotube array 10 between the substitute substrate 30 and the growing substrate 20 are always substantially perpendicular to the growing surface of the growing substrate 20.
In one embodiment, block S2 includes blocks S21 and S22.
In block S21, the water 60 is formed on the top surface 104 of the carbon nanotube array 10. The water 60 can be formed into fine droplets or a mist in the air and drop or collect onto the top surface 104 of the carbon nanotube array 10.
In block S22, the substitute substrate 30 and the carbon nanotube array 10 on the growing substrate 20 are brought together such that the surface of the substitute substrate 30 and the water 60 on the top surface 104 are contacting each other.
In another embodiment, block S2 includes blocks S21′ and S22′.
In block S21′, the water 60 is formed on the surface of the substitute substrate 30. The water 60 can be formed into fine droplets or a mist in the air and drop or collect onto the surface of the substitute substrate 30.
In block S22′, the substitute substrate 30 and the carbon nanotube array 10 on the growing substrate 20 are brought together such that the top surface 104 of the carbon nanotube array 10 and the water 60 on the surface of the substitute substrate 30 are contacting each other.
In block S3, after the freezing of the water 60, the substitute substrate 30 and the carbon nanotube array 10 can be firmly bonded together by the ice 60′ therebetween.
Referring to FIG. 4, in one embodiment, the lamination of the growing substrate 20, the carbon nanotube array 10, the water 60, and the substitute substrate 30 can be put into a freezer 70 with a temperature below 0° C. to freeze the water 60.
Referring to FIG. 5, in another embodiment, when block S2 includes blocks S21 and S22, a temperature of the substitute substrate 30 can be decreased to below 0° C. before block S22. For example, before block S22, the substitute substrate 30 can be kept in the freezer 70 for a period of time until the substitute substrate 30 reaches a temperature below 0° C. Thus, when the substitute substrate 30 contacts the water 60 on the top surface 104 of the carbon nanotube array 10, the water 60 can be directly frozen into ice 60′.
In block S4, due to the bonding between the carbon nanotube array 10 and the substitute substrate 30 by the ice 60′, the separating of the two substrates can separate the carbon nanotube array 10 from the growing substrate 20. During the separating, a majority of the carbon nanotubes in the carbon nanotube array 10 can be detached from the growing substrate 20 at the same time by cutting means, or moving either the substitute substrate 30 or the growing substrate 20, or both, away from each other along a direction substantially perpendicular to the growing surface of the growing substrate 20. The carbon nanotubes of the carbon nanotube array 10 are detached from the growing substrate 20 along the growing direction of the carbon nanotubes. When both the substitute substrate 30 and the growing substrate 20 separate, the two substrates both moves along the direction perpendicular to the growing surface of the growing substrate 20 and depart from each other.
In block S5, the heating can melt the ice 60′ into water and dry the water between the substitute substrate 30 and the carbon nanotube array 10. Or, the heating can directly sublimate the ice 60′. The removal of the ice 60′ does not affect the state of the carbon nanotube array 10. Due to the thickness of the ice 60′ being small, after the removal of the ice 60′, the top surface 104 of the carbon nanotube array 10 can be in contact with the surface of the substitute substrate 30 and bonded by van der Waals attractive forces.
For drawing the carbon nanotube structure 40, the bonding force between the carbon nanotube array 10 and the substitute substrate 30 should be small. In blocks S3 to S5, the bonding force is increased by the ice 60′ to separate the carbon nanotube array 10 from the growing substrate 20 and decreased by removing the ice 60′ before drawing the carbon nanotube structure 40. Thus, the material of the substitute substrate 30 is not limited and can be at least one of metal, glass, crystal, ceramic, silicon, silicon dioxide, plastic, and resin, such as polymethyl methacrylate and polyethylene terephthalate.
Referring to FIG. 5, the present disclosure is described in relation to a method for forming a carbon nanotube structure 40 including the previously described blocks S1 to S5, and further including block S6. In block S6, the carbon nanotube structure 40 is drawn from the carbon nanotube array 10 on the substitute substrate 30.
Referring to FIG. 6, in the block S6, the carbon nanotube structure 40 is drawn from the carbon nanotube array 10 that was transferred to the substitute substrate 30, not from the carbon nanotube array 10 located on the growing substrate 20. In one embodiment of block S6, the carbon nanotube structure 40 can be drawn from the carbon nanotube array 10 upside down on the surface of the substitute substrate 30 (i.e., drawn from the bottom surface 102 of the carbon nanotube array 10).
Block S6 can include block S61 and S62:
In block S61, a carbon nanotube segment having a predetermined width is drawn from the carbon nanotube array 10 on the substitute substrate 30. The segment is selected using a drawing tool 50 (e.g., adhesive tape, pliers, tweezers, or other tool allowing multiple carbon nanotubes to be gripped and pulled simultaneously).
In block S62, a plurality of carbon nanotube segments joined end to end by van der Waals attractive force is drawn by moving the drawing tool 50, thereby forming a continuous carbon nanotube structure 40.
In block S61, the carbon nanotube segment includes a single carbon nanotube or a plurality of carbon nanotubes substantially parallel to each other. The drawing tool 50 such as adhesive tape can be used for selecting and drawing the carbon nanotube segment. The adhesive tape may contact with the carbon nanotubes in the carbon nanotube array to select the carbon nanotube segment. The drawing tool 50 can select a large width of carbon nanotube segments to form the carbon nanotube film, or a small width of the carbon nanotube segments to form the carbon nanotube wire.
In block S62, an angle between a drawing direction of the carbon nanotube segments and the growing direction of the carbon nanotubes in the carbon nanotube array 10 can be larger than 0 degrees (e.g., 30° to 90°).
Block S4 is different from block S6. The purpose of block S4 is to separate the carbon nanotube array 10 as a whole from the growing substrate 20. The carbon nanotube array 10 separated from the growing substrate 20 still in the array shape. The purpose of block S6 is to draw out carbon nanotubes one by one or segment by segment to form a carbon nanotube film or wire from the carbon nanotube array 10 on the substitute substrate 30.
In the present method for making the carbon nanotube structure 40, the growing of the carbon nanotube array 10 and the drawing of the carbon nanotube structure 40 can be processed on different substrates. The substitute substrate 30 can be made of cheap material, and the expensive growing substrate 20 can be recycled quickly and used again for growing new carbon nanotube arrays 10, thus speeding up the production of the carbon nanotube arrays.
Depending on the embodiment, certain of the blocks of the methods described may be removed, others may be added, and the sequence of blocks may be altered. It is also to be understood that the description and the claims drawn to a method may include some indication in reference to certain blocks. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the blocks.
The embodiments shown and described above are only examples. Even though numerous characteristics and advantages of the present technology have been set forth in the foregoing description, together with details of the structure and function of the present disclosure, the disclosure is illustrative only, and changes may be made in the detail, especially in matters of shape, size and arrangement of the parts within the principles of the present disclosure up to, and including the full extent established by the broad general meaning of the terms used in the claims. It will therefore be appreciated that the embodiments described above may be modified within the scope of the claims.

## Claims

1. A method for transferring a carbon nanotube array, the method comprising:
providing a substitute substrate, a growing substrate, and the carbon nanotube array grown on the growing substrate, the carbon nanotube array having a bottom surface adjacent to the growing substrate and a top surface away from the growing substrate, the carbon nanotube array being configured for drawing a carbon nanotube structure therefrom;
placing the substitute substrate on the top surface of the carbon nanotube array and sandwiching water between the substitute substrate and the carbon nanotube array;
freezing the water between the substitute substrate and the carbon nanotube array into ice;
separating the substitute substrate from the growing substrate, thereby separating the bottom surface of the carbon nanotube array from the growing substrate; and
removing the ice between the substitute substrate and the carbon nanotube array, the carbon nanotube array still being configured for drawing the carbon nanotube structure from the substitute substrate.

2. The method of claim 1, wherein the carbon nanotube structure is a carbon nanotube film or a carbon nanotube wire.

3. The method of claim 1, wherein the carbon nanotube structure comprises a plurality of carbon nanotubes joined end to end.

4. The method of claim 1, wherein the water is in a shape of a plurality of droplets, mist, or film.

5. The method of claim 4, wherein a diameter of a droplet and a thickness of the film is in a range from about 10 nanometers to about 300 microns.

6. The method of claim 1, wherein the sandwiching the water between the substitute substrate and the carbon nanotube array comprises:
forming the water on the top surface of the carbon nanotube array; and
contacting a surface of the substitute substrate and the water on the top surface with each other.

7. The method of claim 6, wherein the freezing the water between the substitute substrate and the carbon nanotube array comprises contacting the substitute substrate having a temperature below 0° C. with the water on the top surface of the carbon nanotube array.

8. The method of claim 1, wherein the sandwiching the water between the substitute substrate and the carbon nanotube array comprises:
forming the water on a surface of the substitute substrate; and
contacting the top surface of the carbon nanotube array and the water on the surface of the substitute substrate with each other.

9. The method of claim 8, wherein the freezing the water between the substitute substrate and the carbon nanotube array comprises placing a lamination of the growing substrate, the carbon nanotube array, the water, and the substitute substrate into a freezer, the freezer having an internal temperature below 0° C.

10. The method of claim 1, wherein the carbon nanotube array comprises a plurality of carbon nanotubes, and during the separating the carbon nanotube array from the growing substrate, substantially all carbon nanotubes are simultaneously detached from the growing substrate.

11. The method of claim 1, wherein the carbon nanotube array comprises a plurality of carbon nanotubes, and the carbon nanotubes of the carbon nanotube array are detached from the growing substrate along a growing direction of the carbon nanotubes.

12. The method of claim 1, wherein the growing substrate comprises a growing surface for growing the carbon nanotube array, and a moving direction of at least one of the substitute substrate and the growing substrate is substantially perpendicular to the growing surface of the growing substrate during the moving of the at least one of the substitute substrate and the growing substrate.

13. A method for transferring a carbon nanotube array, the method comprising:
providing a first substrate, a second substrate, and the carbon nanotube array grown on the first substrate, the carbon nanotube array having a bottom surface adjacent to the first substrate and a top surface away from the first substrate, the carbon nanotube array being configured for drawing a carbon nanotube structure therefrom;
placing the second substrate on the top surface of the carbon nanotube array and sandwiching water between the second substrate and the carbon nanotube array;
freezing the water between the second substrate and the carbon nanotube array into ice;
separating the second substrate from the first substrate, thereby separating the bottom surface of the carbon nanotube array from the first substrate; and
removing the ice between the second substrate and the carbon nanotube array, the carbon nanotube array still being configured for drawing the carbon nanotube structure from the second substrate.

14. A method for forming a carbon nanotube structure, the method comprising:
providing a substitute substrate, a growing substrate, and a carbon nanotube array grown on the growing substrate, the carbon nanotube array having a bottom surface adjacent to the growing substrate and a top surface away from the growing substrate, the carbon nanotube array being configured for drawing a carbon nanotube structure therefrom;
placing the substitute substrate on the top surface of the carbon nanotube array and sandwiching water between the substitute substrate and the carbon nanotube array;
freezing the water between the substitute substrate and the carbon nanotube array into ice;
separating the substitute substrate from the growing substrate, thereby separating the bottom surface of the carbon nanotube array from the growing substrate;
removing the ice between the substitute substrate and the carbon nanotube array, the carbon nanotube array still being configured for drawing the carbon nanotube structure from the substitute substrate; and
drawing the carbon nanotube structure from the carbon nanotube array on the substitute substrate.

15. The method of claim 1, wherein the removing the ice between the substitute substrate and the carbon nanotube array comprises heating the ice to melt the ice into water and dry the water or to sublimate the ice between the substitute substrate and the carbon nanotube array.

16. The method of claim 13, wherein the removing the ice between the second substrate and the carbon nanotube array comprises heating the ice to melt the ice into water and dry the water or to sublimate the ice between the second substrate and the carbon nanotube array.

17. The method of claim 14, wherein the removing the ice between the substitute substrate and the carbon nanotube array comprises heating the ice to melt the ice into water and dry the water or to sublimate the ice between the substitute substrate and the carbon nanotube array.