Abstract:
Carbon nanotubes may be selectively opened and their exposed ends functionalized. Opposite ends of carbon nanotubes may be functionalized in different fashions to facilitate self-assembly and other applications.

Description:
BACKGROUND  
       [0001]     This invention relates generally to the formation and utilization of carbon nanotube structures.  
         [0002]     Carbon nanotubes are graphene cylinders whose ends are closed by caps including pentagonal rings. The nanotube is a hexagonal network of carbon atoms forming a seamless cylinder. These cylinders can be as little as a nanometer in diameter with lengths of tens of microns in some cases. Depending on how they are made, the tubes can be multiple walled or single walled.  
         [0003]     The carbon nanotubes may become the building blocks for mechanical, electronic, and biological structures. However, such applications require combining carbon nanotubes with one or more other elements. One way to combine these carbon nanotubes is to functionalize the nanotubes and then combine them with other chemicals or molecules.  
         [0004]     Thus, there is a need for better ways of functionalizing carbon nanotubes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a perspective view of one embodiment of the present invention at an early stage of manufacture;  
         [0006]      FIG. 2  is a perspective view of the embodiment shown in  FIG. 1  after further processing in accordance with one embodiment of the present invention;  
         [0007]      FIG. 3  is a perspective view of the embodiment shown in  FIG. 2  after further processing in accordance with one embodiment of the present invention;  
         [0008]      FIG. 4  is a perspective view of the embodiment shown in  FIG. 3  after further processing in accordance with one embodiment of the present invention;  
         [0009]      FIG. 5  is a perspective view of the embodiment shown in  FIG. 4  after further processing in accordance with one embodiment of the present invention;  
         [0010]      FIG. 6  is a vertical, cross-sectional view for an embodiment generally similar to  FIG. 1  or  FIG. 5  in accordance with one embodiment of the present invention;  
         [0011]      FIG. 7  is a cross-sectional view of the embodiment shown in  FIG. 6  after further processing in accordance with one embodiment of the present invention;  
         [0012]      FIG. 8  is a cross-sectional view corresponding to  FIG. 7  after further processing in accordance with one embodiment of the present invention;  
         [0013]      FIG. 9  is a cross-sectional view corresponding to  FIG. 8  after further processing in accordance with one embodiment of the present invention;  
         [0014]      FIG. 10  is a cross-sectional view of the embodiment shown in  FIG. 9  after further processing in accordance with one embodiment of the present invention;  
         [0015]      FIG. 11  is a cross-sectional view corresponding to  FIG. 10  after further processing in accordance with one embodiment of the present invention;  
         [0016]      FIG. 12  is a cross-sectional view further corresponding to  FIG. 6  in accordance with another embodiment of the present invention;  
         [0017]      FIG. 13  is a cross-sectional view corresponding to  FIG. 12  after further processing in accordance with one embodiment of the present invention;  
         [0018]      FIG. 14  is a cross-sectional view corresponding to  FIG. 13  after further processing in accordance with one embodiment of the present invention;  
         [0019]      FIG. 15  is a cross-sectional view corresponding to  FIG. 14  after further processing in accordance with one embodiment of the present invention;  
         [0020]      FIG. 16  is a cross-sectional view corresponding to  FIG. 15  after further processing in accordance with one embodiment of the present invention;  
         [0021]      FIG. 17  is a cross-sectional view corresponding to  FIG. 16  after further processing in accordance with one embodiment of the present invention;  
         [0022]      FIG. 18  is a cross-sectional view corresponding to  FIG. 6  in accordance with another embodiment of the present invention;  
         [0023]      FIG. 19  is a cross-sectional view of the embodiment shown in  FIG. 18  after further processing in accordance with one embodiment of the present invention;  
         [0024]      FIG. 20  is a cross-sectional view corresponding to  FIG. 19  after further processing in accordance with one embodiment of the present invention;  
         [0025]      FIG. 21  is a cross-sectional view corresponding to  FIG. 20  after further processing in accordance with one embodiment of the present invention;  
         [0026]      FIG. 22  is a cross-sectional view of the embodiment shown in  FIG. 21  after further processing in accordance with one embodiment of the present invention;  
         [0027]      FIG. 23  is a cross-sectional view corresponding to  FIG. 22  after further processing in accordance with one embodiment of the present invention;  
         [0028]      FIG. 24  is a cross-sectional view corresponding to  FIG. 23  after further processing in accordance with one embodiment of the present invention;  
         [0029]      FIG. 25  is a cross-sectional view corresponding to  FIG. 24  after further processing in accordance with one embodiment of the present invention;  
         [0030]      FIG. 26  is a cross-sectional view of another embodiment of the present invention;  
         [0031]      FIG. 27  is a top plan view of another embodiment of the present invention; and  
         [0032]      FIG. 28  is a side elevational view of the embodiment shown in  FIG. 27  after further processing in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0033]     Referring to  FIG. 1 , carbon nanotubes  10  may be aligned on a substrate  12 . The alignment may be accomplished using electric fields or molecular combining as two examples.  
         [0034]     The substrate  12  and carbon nanotubes  10  are then covered with a photoresist layer  14  as shown in  FIG. 2 . The photoresist layer  14  is patterned by lithography as shown in  FIG. 3  to form a mask  14   a  over the carbon nanotubes  10 . Plasma etching, indicated as O 2  etching in  FIG. 4 , may be applied to cut nanotubes  10  into uniform length as shown in  FIG. 5 . The lithography may include photolithography, e-beam lithography, or other lithography. While an oxygen plasma etching process is illustrated, other techniques are possible as well.  
         [0035]     As shown in  FIG. 6 , a carbon nanotube  10  may be aligned on a substrate  12 . A number of other carbon nanotubes  10  aligned generally parallel to the illustrated nanotube  10  may be arranged extending into the page in  FIG. 6 .  
         [0036]     Thereafter, the nanotubes  10  and the substrate  12  may be coated with photoresist  16  as shown in  FIG. 7 . Lithography may be utilized to expose the end portions of the carbon nanotubes as shown in  FIG. 8 . Oxygen plasma etching ( FIG. 9 ) may then burn out the exposed end portions of the carbon nanotubes  10 . The carbon nanotubes  10  are then cut to the length defined by the lithography.  
         [0037]     The cut nanotubes  10  have open ends. A solution of chemical agents (layer  18 ) is then applied to the ends of nanotubes. The sidewalls of the nanotubes are still protected by photoresist  16 . As a result, chemicals in the layer  18  ( FIG. 10 ) can only access the open ends of the carbon nanotubes  10 . One or more functional groups from the layer  18  may be attached to the open ends of the carbon nanotubes  10  from the chemical laden layer  18 . Without limiting the scope of the present invention, the layer  18  may include carboxylic or amine groups. The layer  18  containing different chemicals can be applied more than once to attach multiple functional groups to the ends of nanotubes.  
         [0038]     As shown in  FIG. 11 , the photoresist  16  and the chemical laden layer  18  may be removed to obtain the functionalized carbon nanotube  10 . The ends A and B may both be functionalized in one embodiment.  
         [0039]     Referring to  FIGS. 12 through 17 , a single end functionalization technique is illustrated. The end to be functionalized is illustrated as B in  FIG. 12 . The carbon nanotube  10  may be covered with photoresist  16  as shown in  FIG. 13 . Lithography is utilized to expose only end B of the carbon nanotube  10 , as indicated in  FIG. 14 . The carbon nanotube  10  may be cut off to length using oxygen plasma etching as shown in  FIG. 15 . The exposed, open-ended tube  10  may then be coated with a chemical laden layer  18  to end functionalize the end B of the carbon nanotube  10 , as shown in  FIG. 16 . In  FIG. 17 , the chemical laden layer  18  and the photoresist  16  may be removed. At this point, only the end B of the carbon nanotube  10  is functionalized. Applying the same process to end A may functionalize end A with a different molecule. The tube length may be defined by the lithography in the two-step end functionalization process.  
         [0040]     Referring to  FIGS. 18 through 25 , the end A is to be functionalized with a chemical that may not be compatible with photoresist. The carbon nanotube  10  may be covered by a layer of photoresist  16  as indicated in  FIG. 19 . The end B of the carbon nanotube  10  may then be exposed ( FIG. 20 ) using conventional photolithography process to remove a portion of the photoresist  16 . Thereafter, a layer of silicon dioxide or another protection material  20  may be deposited over the structure as shown in  FIG. 21 . The deposition may be done using conventional chemical vapor deposition and lithography in one embodiment.  
         [0041]     Thereafter, the underlying photoresist  16  may be removed, resulting in the structure shown in  FIG. 22 . The exposed portion of the carbon nanotube  10  (not covered by the silicon dioxide  20 ) may then be removed using oxygen plasma etching as shown in  FIG. 23 . The resulting structure may then be covered with an end functionalizing chemical  18  to end functionalize the open ended carbon nanotube  10  in  FIG. 24 . Thereafter, the chemical  18  and the silicon dioxide  20  may be removed as shown in  FIG. 25 .  
         [0042]     Referring next to  FIG. 26 , the end functionalized carbon nanotubes  10  may be utilized for self-assembly of carbon nanotube arrays. The functionalized ends A and B are arranged so that the end B extends vertically and the end A is attached to a structure  24  to which it is attracted. The structure  24  is also functionalized with molecules that specifically bind to the functional groups on the ends A of the carbon nanotubes  10  and not the functional groups on the ends B. The end functionalized carbon nanotubes  10  already have one end A attached to the structure  24 . The other end B may stay in solution. The resulting structure may form a self-aligned vertical array with uniform thickness. An example of one application is for thermal interface material (TIM) fabrication.  
         [0043]     Referring to  FIG. 27 , end functionalized carbon nanotubes  10  may be utilized for self-assembly of an organized carbon nanotube array at a specific location and orientation on the substrate  26 . An area  30  of the substrate  26  is functionalized with molecules that specifically bind with functional group on the end A. Another area  28  is functionalized with molecules that specifically bind with functional group B on the opposite end B of the carbon nanotubes  10 . If the distance between the attachment points is equal to the length of the end functionalized carbon nanotubes  10 , the first end A of the functionalized carbon nanotubes  10  binds to the area  30  and the second end B binds to the area  28 .  
         [0044]     Referring to  FIG. 28 , the structure shown in  FIG. 27  may be further processed to include a gate dielectric layer  32  and a gate electrode  34 . The use of an array of nanotubes  10  may increase the current drive of a transistor formed using the nanotubes  10  as an effective channel. The transistor may include a source  30  and a drain  28  that are functionalized to attach to specific carbon nanotube functionalized ends A and B.  
         [0045]     In one embodiment, a deoxyribonucleic acid (DNA) molecule may include the information to drive the self-assembly process. A single stranded DNA molecule may be attached to the end of a carbon nanotube using the method described above. The single stranded DNA may have a sequence complementary to another single stranded DNA molecule or to a linker of a double stranded DNA at desired locations. The two DNA molecules may be bound to each other according to sequence matching between the two types of DNA molecules, and thus immobilize the end of the nanotube to the desired location.  
         [0046]     The carbon nanotubes  10  may be functionalized with a protein streptavidin. That protein may bind to an antibody that attaches to a specific location, locating the nanotube  10  at the correct address. The nanotube assembly may be placed on a passivated, oxidized silicon wafer before metallization.  
         [0047]     In some embodiments of the present invention it is possible to select to functionalize only one or more ends of a carbon nanotube. In some embodiments different ends may be functionalized with different molecules. Lithography and etching methods may be utilized to selectively expose one end of a carbon nanotube. The exposed end can then be chemically functionalized and may be connected with one or more other functional groups of available molecules. The second end of the carbon nanotube can be exposed by repeating the lithography and etching process. The second end of the carbon nanotube may then be functionalized with a second functional group or be connected with one or more available molecules.  
         [0048]     In addition, end functionalized carbon nanotubes of uniform length may be utilized for these procedures. The different functionalizations at the two ends of the nanotubes may be useful in self-assembly and pattern formation for building components of carbon nanotubes. For example, it may be useful for biosensors.  
         [0049]     Carbon nanotubes may be made either polar or amphiphilic by appropriate modification. One end of the resulting carbon molecule may then be immobilized and alignment may be achieved through molecular combing. One end only may be functionalized with biomolecules and the structure may then be utilized for a biosensor in one embodiment.  
         [0050]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.