Controlled deposition and alignment of carbon nanotubes

A carbon nanotube (CNT) attraction material is deposited on a substrate in the gap region between two electrodes on the substrate. An electric potential is applied to the two electrodes. The CNT attraction material is wetted with a solution defined by a carrier liquid having carbon nanotubes (CNTs) suspended therein. A portion of the CNTs align with the electric field and adhere to The CNT attraction material. The carrier liquid and any CNTs not adhered to the CNT attraction material are then removed.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to methods of assembling carbon nanotube devices. More specifically, the invention is a method for the controlled deposition and alignment of carbon nanotubes.

SUMMARY OF THE INVENTION

The present invention is a method for the deposition and alignment of carbon nanotubes. The method uses an assembly comprising a substrate having at least two electrodes supported thereon and opposing one another with a gap region being defined therebetween. A carbon nanotube (CNT) attraction material is deposited on the substrate in at least the gap region. An electric potential is applied to the two electrodes so that an electric field is generated across the gap region between the electrodes. The CNT attraction material is wetted with a solution defined by a carrier liquid having carbon nanotubes (CNTs) suspended therein. As a result, a first portion of the CNTs are aligned with the electric field and adhered to the CNT attraction material, while a second portion of the CNTs are not adhered to the CNT attraction material. The second portion of the CNTs that are not adhered to the CNT attraction material, along with the carrier fluid, are then removed from the assembly.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, and more particularly toFIG. 1, an assembly used to demonstrate the deposition and alignment of CNTs in accordance with the present invention is shown and referenced generally by numeral10. Assembly10is shown and will be described herein for purposes of demonstrating the methodology of the present invention. However, it is to be understood that the particular structure and construction of assembly10are not limitations of the present invention.

Assembly10includes a substrate12with spaced-apart electrical contact pads14and16deposited thereon. For example, in terms of many microcircuit applications, substrate12is a silicon wafer and contact pads14and16are any highly conductive material such as gold. Typically, each of contact pads14and16has a respective electrode contact leg14A and16A extending therefrom such that legs14A and16A oppose one another as shown. The particular size and shape of the contact pads and legs can be adapted for a particular application as would be understood by one of ordinary skill in the art. Contact pads14and16are coupled to a voltage source18capable of applying an electrical potential thereto. Voltage source18can be an alternating current (AC) or direct current (DC) source without departing from the scope of the present invention.

Electrically coupled to leg14A is an electrode20and electrically coupled to leg16A is an electrode22. Electrodes20and22are deposited on substrate12such that portions thereof oppose one another with a gap24being defined therebetween. Electrodes20and22can be, but are not required to be, parallel to one another. Additional opposing pairs of electrodes can be provided without departing from the scope of the present invention.

In general, the present invention modifies assembly10by (i) specific placement thereon of a material that attracts CNTs thereto, and (ii) deposition and alignment of CNTs on the specifically-placed CNT attraction material such that the CNTs provide good electrical conductivity between aligned CNTs. At a minimum, and as will be explained with reference toFIG. 2, the CNT attraction material is positioned between electrodes20and22, i.e., in gap24. However, the CNT attraction material can further be deposited on and between electrodes20and22(and beyond the electrodes if so desired) as will be explained later below with reference toFIG. 4. The CNTs deposited and aligned by the present invention can be single or multi-wall CNTs. However, because of their remarkable strength, single-wall CNTs (SWCNTs) will be preferred for most applications.

Referring additionally now toFIG. 2, a perspective view of a portion of substrate12with electrodes20and22deposited thereon is shown. In accordance with the present invention, a CNT attraction material30is deposited in the gap between opposing portions of electrodes20and22. At least one CNT32is coupled to material30and aligned such that each tube axis32A is substantially perpendicular to electrodes20and22to define an electrical conduction path between aligned ones of CNTs32. If the ultimate application of aligned CNTs is to use the electrodes20and22along with aligned CNTs in an electrical conduction path, the aligned ones of CNTs32must contact each of electrodes20and22. However, it is to be understood that the present invention does not require that aligned ones of CNTs32contact one or both of electrodes20and22. That is, the electrical conduction path defined by aligned ones of CNTs32could be used to conduct between elements (not shown) deposited on and/or across aligned ones of CNTs32.

For clarity of illustration, the size of CNTs32is greatly exaggerated and only two sets of aligned CNTs are shown. However, as would be understood by one of ordinary skill in the art, many more sets of aligned CNTs would be present in the actual device. Furthermore, if spacing between electrodes20and22is small (e.g., less than one micron), it is possible for a single one of CNTs32to span between electrodes20and22.

To achieve the structure illustrated inFIG. 2, assembly10is first processed to specifically place CNT attraction material30in its desired location(s). While a variety of methods can be used to deposit CNT attraction material30, one method will be described herein by way of an illustrative example. The area of assembly10to receive CNT attraction material30can be spin coated with a resist material (e.g., poly(methylmethacrylate) or PMMA, polymethylglutarimide, etc.) and then patterned with an electron beam to define the desired “receive” location(s) (e.g., gap24). After cleaning (e.g., in an oxygen plasma), CNT attraction material30is deposited on the surface of assembly10. The resist material (as well as the portion of CNT attraction material30deposited thereon) is then removed (e.g., using standard cleaning procedures) thereby leaving CNT attraction material only in the receive location(s) such as gap24.

CNT attraction material30can be any material that suitably attracts and adheres CNTs thereto. Such a material can have an amino-terminated surface that will form a hydrogen bond with one or more hydrogen molecules found on the sidewall of a CNT. Accordingly, CNT attraction material30can be a monolayer material such as a self-assembled monolayer (SAM) of amino-terminated moieties. In terms of the structure shown inFIG. 2, wherein CNT attraction material30adheres only to the substrate12between electrodes20and22, an example of a commercially-available CNT attraction material is aminopropyltriethoxysilane or APTES. APTES does not bond to metal, which electrodes20and22may be made of. However, other suitable monolayers can be used without departing from the scope of the present invention. For example, if CNT attraction material30is also to be deposited and adhered to electrodes20and22(as is the case with the structure shown inFIG. 4), a thiol-type of SAM can be used.

In terms of the APTES monolayer, when it comes into contact with a silicon oxide surface (i.e., the surface of a typical substrate12), it orients itself through a self-assembly process so that the amino (—NH2) head group is pointing away from the surface of the substrate. Several different reactions resulting in different anchoring mechanisms can occur when APTES comes into contact with carboxyl (—COO) and hydroxyl (—OH) groups on the sidewall surface of CNTs. For example, with the correct selection of CNT processing and monolayer selection, a hydrogen bond forms between the monolayer and the carboxyl/hydroxyl group in the sidewall of the CNT. The carboxyl and hydroxyl groups on the nanotube surface contain a partially negative charge, while the amino head-group on the APTES is partially positive. Thus, the charges will attract, and an electrostatic bond can form. Specifically, the electron from the APTES headgroup is partially shared with the carboxyl and/or hydroxyl group on the CNT's surface. Covalent bonds could also be created by performing an aminolysis reaction so that the carboxyl groups will form an amide (—COONH—) linkage with the monolayer, although this reaction would require the use of a catalyst.

As mentioned above, the monolayer does not need to be APTES. Any monolayer that would react with the carboxyl/hydroxyl groups on the CNT sidewall could be selected. Examples include monolayers that have a hydroxyl head-group (e.g., hydrogen bonding with the carboxyl groups and some with the hydroxyl groups) or a carboxyl head-group (e.g., more hydrogen bonding and esterification with the hydroxyl side groups could be performed to create covalent bonds, i.e., a—COOC—bond). Also, choosing monolayers that have no reactive headgroups (e.g., octadecyltrichlorosilane or OTS) can be used to “shield” the surface from nanotube attachment. Finally, the carboxyl/hydroxyl groups on the CNT sidewalls can be modified directly to enhance or prohibit their attachment to surfaces. For example, modifying a CNT so that the sidewall thereof is functionalized with a thiol group (—SH) would cause it to attach to a gold surface.

With additional reference now toFIG. 3, the sequence of steps used in the present invention (to create the structure shown inFIG. 2) are characterized in schematic form with a brief description thereof being provided in the corresponding box of the flowchart that is beside the description. For simplicity, a side view of only the relevant portion of assembly10is shown at each step of the sequence.

At step100, assembly10is prepared for processing such that electrodes20and22are placed on substrate12with gap24defined therebetween. Once CNT attraction material30has been deposited in its desired location(s) at step102, voltage source18is activated at step104so that an electric field is generated between electrodes20and22and across CNT attraction material (in gap24) as indicated by arrow40. To insure good alignment of CNTs32falling between electrodes20and22, it is preferred that voltage source18is activated before the deposition of the solution-suspended CNTs32at step106. However, there may be some applications where it is desirable to activate voltage source18at the same time as, or just after, the deposition of the solution-suspended CNTs32. Note that the direction of electric field40depends on the polarity of the electric potentials applied to electrodes20and22.

Next, at step106, a quantity of CNTs32suspended in a carrier liquid solution34are deposited on assembly10on and around CNT attraction material30. Carrier liquid34is chosen so that the CNTs do not clump together. CNTs tend to clump together in solution due to strong van der Waal's forces between individual CNTs. These forces are directly related to the size of the CNTs as well as the distance therebetween. The best solvent to disperse particular CNTs also depends on the origin of the CNTs (e.g., vendor, batch or lot, etc.) and how the CNTs have been processed (e.g., cut with nitric acid to form functionalized sidewalls, purified, etc.). Because of these variables, several different solvents may be used, such as toluene, n-methylprolidone (NMP), dichloromethane (DCM), dimethylforamide (DMF), and even water that contains various surfactants (e.g., Triton X-100, sodium dodecylsulfate, and others as would be well understood in the art). In general, the carrier liquid should minimize van der Waal forces between the CNTs suspended therein. Furthermore, when mixing the CNTs in the carrier liquid, ultrasonic energy can be used to help disperse the CNTS therein.

By virtue of this process, those of the solution-suspended CNTs that come into contact with CNT attraction material30(i) already have their tube axis32A substantially aligned with the direction of electric field40as illustrated inFIG. 2, and (ii) adhere thereto in an aligned fashion by means of hydrogen bonding with the sidewall of CNTs32. After a brief period of time (e.g., ranging from tens of seconds to several minutes with CNT densities being proportional to exposure time), electric field40is removed as well as any remaining liquid solution and CNTs not adhered to CNT attraction material30, thereby leaving CNTs32aligned and adhered on CNT attraction material30as shown inFIG. 2.

Removal of the liquid carrier and CNTs suspended therein can simply involve the blowing (as indicated by arrow50in step108) of an inert gas such as nitrogen across the surface of assembly10(with CNT attraction material30and CNTs32deposited thereon) until dry. To assure the removal of any CNTs32left in areas other than on CNT attraction material30, additional processing can be implemented at step110. Specifically, a rinse liquid60(e.g., n-methylyrolidone) is washed over the assembly as it is vibrated (e.g., sonification by acoustic wave energy62) thereby causing the non-adhered ones of CNTs32to become suspended in rinse liquid60. An inert gas (e.g., nitrogen) is then used to blow off the rinse liquid and suspended CNTs as indicated by arrow64. As a result, the structure shown inFIG. 2is achieved. The method provides for the controlled deposition and alignment of CNTs such that their electrical conductive properties can be exploited.

As mentioned above, and as shown inFIG. 4, the final product produced by the present invention could have CNT attraction material30deposited on electrodes20and22as well as therebetween. This approach is achieved by proper selection of CNT attraction material30for adherence to (metal) electrodes20and22. In this example, CNTs32will be adhered to and aligned on CNT attraction material30both over and between electrodes20and22. The processing steps for achieving this structure are identical to that described above. If electrical contact is desired between electrodes20and22and ones of CNTs32positioned thereover, an additional step of ultraviolet ozone cleaning can be applied to the appropriate areas on electrodes20and22after removal of the carrier liquid and excess CNTs.

Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, means-plus-function and step-plus-function clauses are intended to cover the structures or acts described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.