Transformation of nanostructure arrays

A method and apparatus for transforming vertically-aligned nanostructures into densified, horizontally-aligned arrays. A contact element such as a roller is used to topple an array of carbon nanotubes or other nanostructures by drawing or rolling the contact element across the surface of the substrate such that the vertically-aligned nanostructures are forced into at least partial horizontal-alignment while being densified to give the transformed array enhanced properties. The contact element engages the nanostructures at a location below their upper distal end to topple and densify the array without disrupting the relative alignment of the individual nanostructures in the array. Transfer printing of the nanostructures is also provided.

TECHNICAL FIELD

This invention relates generally to nanostructure arrays and, in particular, to the processing of nanostructure arrays to improve their utility in electrical, mechanical, thermal, fluidic, and other applications.

BACKGROUND OF THE INVENTION

Studies have shown that at least one type of nanostructure, carbon nanotubes (CNTs), may be able to replace copper in both vertical and horizontal microelectronic interconnects (Naeemi and Meindl,IEEE Trans. Electron Devices54(1):26-37, 2007). In particular, it has been theoretically shown that for global interconnects, bundles of single-wall nanotubes (SWNTs) or multi-wall nanotubes (MWNTs) can potentially replace copper wires by allowing the use of smaller interconnect dimensions while keeping delay and crosstalk noise constant, thus increasing the bandwidth density of global interconnects.

In addition, while copper is prone to electromigration and boundary scattering at emerging linewidths of less than 100 nm, CNTs are resistant to electromigration, have μm electron mean free path, and can handle substantially higher current densities up to 109A/cm2. Calculations have also shown that CNTs can decrease switching energy consumption, and GHz operation of a single large diameter multi-wall CNT (MWCNT) as a horizontal interconnect has been experimentally realized (Close et al.,Nano Lett.8(2):706-709, 2008).

However, various challenges can arise when applying CNTs in interconnect technology. Some of these challenges include: 1) providing for horizontal orientation of CNT bundles on a chip; 2) providing a high packing fraction or density of CNTs; 3) providing CNT growth conditions such as temperature, pressure, and gas composition that are compatible with CMOS processing over wafer-scale areas; and 4) providing low contact resistance by assuring contact to all graphene shells (walls) of all tubes. Further, as key performance parameters (such as mean free path, number of conduction channels, etc.) depend on CNT length and diameter, a fabrication strategy would preferably facilitate tunability of CNT diameter as well as build interconnects from continuous parallel CNTs.

Researchers have sought to fabricate horizontally-aligned CNTs by direct growth on substrates (e.g., alignment by crystallographic interactions or gas flows), possibly followed by transfer printing. But sufficiently high CNT densities have not been achieved using these methods, and multi-layer approaches such as repeated transfer printing of single layers of CNTs require an impractical number of steps.

One method of attempting to obtain high density horizontally-aligned CNTs is capillarity-driven densification by controlled dipping of patterned sections of vertically-aligned CNTs (VA-CNTs) in solvents such as IPA or acetone (Hayamizu et al.,Nature Nanotechnology3:289-294, 2008). By engineering the catalyst and the dipping/drawing motion, “CNT wafers” consisting of horizontally aligned overlapping arrays of CNTs have been manufactured and used in device fabrication. The density that can be achieved using this method is limited by the zipping force of the solvent that results from the liquid surface tension and the contact angle between the solvent used and the CNTs.

Another method includes obtaining a CNT film from a “CNT carpet” by shearing the top of VA-CNT arrays, using a thin sheet of foil to lay the arrays down without disturbing their alignment, and compressing the CNT film covered by the foil using a roller. Finally, the CNT film may be detached from the foil and the growth substrate and transferred to different materials host substrates using a dry peel and place method (Pint et al.,ACS Nano2(9):1871-1878, 2008).

Another method includes manufacturing “CNT papers” by pushing a microporous membrane against a CNT forest by means of a cylinder having diameter much larger than the CNT forest height. The effect of the rolling motion of the cylinder on the CNT forest is compared to dominos pushing one another over where it is hypothesized that CNTs are sliding on each other to achieve the final aligned CNT film structure. The porous membrane (with the CNTs sticking to it) is peeled off of the growth substrate and ethanol is spread on the membrane to release the CNT paper (Wang et al.,Nanotechnology19:1-6, 2008).

Rolling out of vertical CNTs using a large diameter roller to obtain horizontally aligned CNT structures is also disclosed in U.S. Pat. No. 7,514,116 B2.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, there is provided a method of densifying a nanostructure array. The method includes the steps of providing a nanostructure device that includes a substrate having a planar surface and an array of aligned nanostructures extending away from the substrate, placing a contact element adjacent the substrate such that the contact element has a central axis oriented parallel to the substrate surface, and moving the contact element such that it initially contacts at least some of the nanostructures at a location below a distal end of the nanostructures and re-orients the nanostructures into at least partial alignment with the substrate surface.

In accordance with another aspect of the invention there is provided an apparatus for densifying a nanostructure array. The apparatus includes a base for supporting a nanostructure device having a substrate and an array of aligned nanostructures extending away from the substrate, a contact element for moving over the surface of the substrate, and a loading mechanism operatively connected to the contact element for applying a load to the contact element to force it towards the substrate, wherein the contact element is configured to have a first contact point with the nanostructures that is above the substrate and below a vertical midpoint of the nanostructures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Nanostructures of the type contemplated herein are structures having at least two dimensions in the nanoscale; for example, elongate structures having a diameter or plural cross-sectional dimensions within the general range of 0.1 to 100 nanometers. This includes, for example, nanotubes, nanowires, nanorods, nanocolumns, and nanofibers. A nanostructure array comprises a plurality of nanostructures having their elongate dimensions generally aligned with one another or in the same general direction. For example, a vertically-aligned nanostructure array comprises a plurality of nanostructures generally aligned in a direction perpendicular to a growth substrate. The height of the nanostructure array can be in the nanoscale range, or can be larger; for example, in the micron or millimeter range. Among the many types of nanotubes and nanowires, choice of materials for a nanostructure array is subject to a number of considerations and constraints, including suitability of the materials for the desired application and compatibility of the nanostructures and related processing conditions (e.g., temperature, catalyst, precursors) with the fabrication process for the surrounding device. A nanostructure array can comprise nanostructures formed from one material, or can comprise nanostructures formed from two or more materials to create hybrid arrays. Similarly a nanostructure array can comprise various nanostructures such as single-wall nanotubes, multi-wall nanotubes, and nanowires, all in the same array.

FIG. 1depicts a side view of an exemplary embodiment of a nanostructure array densifying apparatus32as it might be used for densifying a nanostructure device10that includes one or more arrays12of aligned nanostructures extending away from the upper, planar surface of a substrate14. As shown inFIG. 1, the apparatus32includes a base34for supporting the nanostructure substrate14, a contact element such as a roller18for contacting the nanostructures12, and a loading mechanism36for applying a load to the roller18. In general, the method used in this embodiment includes placing roller18adjacent the substrate14such that the roller has its central axis oriented parallel to the substrate surface, and then moving the roller such that it directly contacts at least some of the nanostructures and re-orients the nanostructures into at least partial alignment with the substrate surface. The roller18illustrated in this embodiment is generally cylindrical or round in cross-section and can be moved along the substrate14so that it directly engages and topples the nanostructure arrays12. The movement along the substrate14can be a rolling movement (shown best inFIGS. 2 and 3), such that the roller18rotates about a central axis and comes into contact with the nano structures. The roller18directly contacts the nanostructures and is configured to have a first contact point28(shown best inFIG. 2) with the nanostructure array12that is above the substrate14and below a vertical midpoint30of the array12that is to be densified.

When using roller18, two limiting geometric cases relating the roller size to the dimensions of the nanostructure array should be considered. In the first case, shown inFIG. 4a, the roller can have a diameter smaller than the height of the array. For example, a roller diameter of 0.6 mm is generally suitable for processing nanostructures having height greater than 0.7 mm, or preferably greater than 1.0 mm. In this case, the surface of roller may or may not slip relative to the surface of the nanostructure array, when the roller and the array are in contact. In the second case, shown inFIG. 4b, the roller can have a diameter equal to or greater than the height of the array. In this case, the surface of the roller will slip relative to the surface of the nanostructure array, when the roller and the array are in contact. In both cases, the thickness t of the nanostructure array is preferably much less than the height h of the nanostructure array.

The relative roller and nanostructure array dimensional relationships noted above are useful where it is desirable to maintain the relative orientation of the individual nanostructures (e.g., so that after densification they continue to extend generally linearly and in parallel with respect to each other). By initially contacting the nanostructure array at a location that is below the distal (free) end of the individual nanostructures, and by preventing adhesion of the nanostructures to the roller18or other contact element, the array can be toppled in a controlled manner that substantially maintains the relative orientation of the individual structures. This can help increase the packing factor (discussed below) which can impart beneficial properties to the densified array. The result of undesirable adhesion can be understood by reference toFIGS. 5-7. For arrays of carbon nanotubes (CNTs), there is a tendency of the CNTs to stick to the roller as soon as they come in contact with it. This creates shear in the CNT arrays and results in the formation of buckles in the otherwise resulting horizontally aligned nanostructures. This is shown diagrammatically inFIG. 5. Moreover, due to the inherent lateral entanglement in CNT arrays resulting from the synthesis process, the slip between individual CNTs within the array is minimal. Therefore, if the thickness t of the array is substantially large enough (for example, as shown inFIG. 6), the same shear and buckling deformation occur resulting in high defects in the resulting horizontally aligned CNTs.

The trajectory taken by a point on the roller surface during rolling is represented inFIG. 7aby the cycloid curve. At contact, the CNTs stick to the roller and the point of contact takes the cycloid trajectory thus creating shear and deformation in the part of the nanostructure array below that point. The deformation length is estimated to be the difference between the cycloid trajectory and a circular trajectory having the bottom of the CNTs as the circle center. The larger the deformation length, the lower the quality of the horizontally-aligned array. As shown inFIG. 7b, when the roller diameter (d) is larger than twice the CNT array height (h), the top of the array will stick to the roller as they get in contact, resulting in that point taking the cycloid trajectory (d=2 h) shown above. The entire length of the array is subject to deformation and the deformation length is shown above as length (b). Note that (b) is larger than (a) corresponding to roller diameter equal to array height (d=h). Moreover, as the diameter of the roller get larger the deformation is larger thus inducing more defects in the array as depicted by trajectory (d=4 h) and the corresponding deformation length c. Thus, as described above, this problem with adhesion of the individual nanostructures to the roller18can be reduced or avoided in various ways, such as by minimizing the roller diameter or by creating slip between the roller and nanostructures. Slip can be enhanced by, for example, proper selection of the roller surface material, by maintaining a small diameter roller, by relative rotation (forward or backspin) of the roller, or by any combination of these or by any other suitable means. As another example, a small diameter roller can be used for which the rotation and translation are independently controllable, thereby permitting more precise control of slipping between the roller and nanostructure array.

An advantage of using a small roller diameter can be further understood by consideration of the van der Waals forces between the CNTs and roller. Since the resultant force from the van der Waals attraction between the CNTs and roller is directly proportional to the contact area, it is desirable to minimize the local contact area between the CNTs and the roller. From Hertzian contact mechanics, the width of the contact area between a cylinder (diameter d, length l, modulus E1, poisson's ratio ν1) and a plane (E2, ν2), is

Therefore, a small diameter roller made of a material having high elastic modulus gives relatively weak adhesion to the CNTs. This simple formula agrees with observations made that the CNTs tend to stick to rollers having a substantially larger diameter than described above, and/or to rollers made of a soft material (e.g., PDMS or Nylon).

Apart from a rolling movement using roller18, transformation of the array (e.g., toppling) and densification can be carried out in other ways. For example, instead of using roller18, a different contact element can be used that does not involve rotation of the element as it is moved across the substrate. Thus, it will be appreciated that toppling can be carried out using only translation of the contact element relative to the arrays12such as by drawing a wire across the array. It is also contemplated that a non-cylindrical contact element may be moved in translation to topple and densify a nanostructure array. For example, the contact element may have a teardrop cross-section having a radius on a leading edge with a flat bottom. Various shapes are possible and can be used to vary the densification of the array, the load distribution on the array during densification, the adhesion of the nanostructures to the contact element, and the localized stresses on the nanostructures, among other things.

Referring back toFIG. 1, the loading mechanism36depicted in the exemplary embodiment includes an upper plate38having a surface40that contacts roller18. The upper plate38is generally parallel to the base34. The upper plate38and the base34are movable in relation to one another in a direction parallel to the base34or the surface40, shown horizontal inFIG. 1. The base34and the upper plate38are also movable in relation to one another in a direction perpendicular to the base34or the surface40to accommodate various size rollers18and to facilitate application of a load on the roller18. The loading mechanism36may also include a load applicator42. The load applicator42inFIG. 1is a spring that acts on the base34to apply a load to the roller18in a direction perpendicular to the surface40. Though shown as a spring here, other load applicators42are possible, such as one or more pneumatic or hydraulic cylinders, servo motor devices, power screws, or other devices. The load applicator42may alternatively act on the upper plate38or additional load applicators may be used to act on both the upper plate38and the base34. The load applied by the load applicator42can be variable. Varying the compression of the spring42with an adjustment screw as shown inFIG. 1is one way of varying the load, but other methods and devices are possible.

Where upper plate38is included as a part of apparatus32, it may also include a cutout having a width greater than or equal to a width of the nanostructure substrate14to allow the nanostructure array12to extend past a plane defined by the surface40. The cutout can allow clearance for the nanostructure array12so that the upper plate38does not contact the array12. An example of such a cutout is best shown inFIG. 3.

Loading mechanisms36other than those shown inFIG. 1are contemplated. For example, the roller18may have an axle extending through its central axis and the load could be applied to the axle from above or from below without contacting the outer surface of the roller. Many other loading mechanisms36are possible.

FIGS. 2 and 3depict an exemplary embodiment of a method of processing nanostructure arrays in accordance with the invention. The method may include one or more steps in addition to those described here. The method includes providing an array12or multiple arrays of vertically aligned nanostructures on a substrate surface16, providing a roller18having a central axis, placing the roller18on or in proximity to the substrate surface16such that the central axis is substantially parallel to the substrate surface16, moving the roller18with reference to the substrate surface16such that it directly contacts at least some of the nanostructures and topples the array and aligns at least a portion of the nanostructures with the substrate surface16. After toppling the nanostructures, continued contact between the roller and nanostructures densifies the nanostructures, while the densified nanostructure array is in contact with both the roller and the substrate surface. In this example, the roller18used in this exemplary method can be constructed according to the exemplary apparatus32already described and has a diameter less than a height of the array and a first contact point28with the nanostructures that is above substrate surface16and below a midpoint30of the nanostructures. In this embodiment, the central axis of the roller18is substantially parallel to a width of the array12and perpendicular to a thickness of the array12. The width w and thickness t are best shown inFIG. 8a. The axis of the roller18need not be parallel with the width dimension of the array. Some arrays may not have discernable widths and thicknesses or may have irregular cross-sections.

As shown inFIGS. 2-4, the process can be carried out using nanostructure arrays12in which the thickness t of the array is less than the height of the array above the substrate. Preferably, this is done using arrays12in which the ratio of the thickness to height is less than 0.1. Also, the substrate can include a plurality of such arrays12in which the spacing between arrays (in the direction of motion of the roller or other contact element) is less than the height of the arrays in which case the adjacent arrays of nanostructures may overlie each other to some extent after toppling. An example is shown inFIG. 9wherein wide strips of small thickness nanostructure arrays (FIG. 9a) are toppled to form a sheet of overlapping strips (FIG. 9b). Alternatively, the substrate can have some or all of its arrays12with spacing between adjacent arrays greater than or equal to the height of the arrays so that no overlap occurs.

When overlapping lines of nanostructures as shown inFIG. 9, the overlapped portions are not fully merged together after mechanical rolling. This can be seen inFIG. 10awhich shows the distal (free) end of one array lying on top of another array, as well as inFIG. 10cwhich shows one array lying over the proximal (attached) end of another array. After using liquid capillary forces the overlapping carbon nanotube bundles densify and merge together creating interpenetrating bundles. This is shown inFIGS. 10band 10dwhich depict the areas shown inFIGS. 10aand 10c, respectively, after densification using capillary forces. To achieve the capillary densification shown, the toppled arrays are infiltrated with an organic solvent such as isopropanol or acetone, such as by condensation of a solvent vapor onto the substrate. The solvent is then subsequently evaporated. This evaporation of the solvent draws the CNTs into a more tightly packed arrangement. The final packing fraction depends on the initial spacing (packing fraction) of the CNTs after the rolling process. Therefore, maximal packing of CNTs is achieved by combining mechanical and capillary densification methods.

The array12can have a first thickness prior to moving the roller18and a second thickness after the roller18topples the array12, wherein the second thickness is less than the first thickness. The array12can also define a first cross-sectional area prior to moving the roller18and a second cross-sectional area after the roller18topples the array12, wherein the second cross-sectional area is less than the first cross-sectional area. These dimensional changes are indicative of the densification of the array, where the individual nanostructures within the array are forced closer to one another. The amount of densification can be controlled by the magnitude of the load applied to the roller18, as will be described in further detail. The forces applied to the roller, and the trajectory of the roller, can be varied continuously during the process.

By controlling the adhesion of the nanostructures to the growth substrate and to the roller, the nanostructures can adhere to the growth substrate or to the surface of the roller. Therefore, the nanostructures can remain on the growth substrate after the process is complete, as in the embodiments above, or can be transferred to a second substrate, as shown inFIG. 11, where the nanostructures adhere to the surface of the roller, and then are released upon contact with a second substrate. Strong adhesion between the individual CNTs and the substrate is created by rapid cooling in the reaction atmosphere, so as to form a carbide interface between the nanostructures and catalyst particles. Alternatively, when He flow is used during the cooling step, the CNTs are easily delaminated from the substrate because the adhesion strength between the CNTs and roller exceeds the strength between the CNTs and the substrate. Strong adhesion is useful when the CNTs remain on the growth substrate, whereas weak adhesion is useful when the CNTs are printed to a secondary substrate.

To carry out the transfer printing, the roller is placed between the growth substrate and the host (transfer substrate). This facilitates transformation, densification, and transfer of the nanostructures to the host substrate, which can be another silicon wafer, metal foil, or polymer such as Kapton or PDMS (polydimethylsiloxane). Elastomeric substrates can also be used as the transfer substrate. The contact force between the roller and the host substrate, and the surface properties of the roller and the host substrate can be controlled to facilitate detachment of the nanostructures from the roller upon contact with the host substrate. In another embodiment, the nanostructure arrays can be rolled and transferred to a roller, and then subsequently transferred from the roller to another substrate as a secondary operation.

FIG. 12depicts yet another means of transfer printing. In it, the CNT arrays can be rolled to form horizontally aligned arrays on the growth substrate. Subsequently, the arrays can be transferred without the need of a roller to an arbitrary substrate by controlling the carbon nanotube-growth substrate adhesion relative to the carbon nanotubes-receiving substrate adhesion. Again, this can be done, for example, by use of He flow during the cooling step of the CNT growth process. As one example of a transference process, the horizontally-aligned CNT arrays can be robustly transfer printed to substrates such as flexible films of PDMS and Polyimide (Kapton) as shown inFIGS. 13aandb, respectively. A smooth PDMS carrier substrate, made by curing and then delaminating the PDMS from a template silicon substrate, is laminated onto the original horizontally-aligned CNT substrate after rolling and capillary densification. Taking advantage of kinematically controlled adhesion of PDMS to SiO2, the CNTs stick to the PDMS when the carrier substrate is peeled quickly from the growth substrate. By laminating the PDMS carrying the CNTs onto a polymide film and then peeling it off slowly the CNT arrays are uniformly transferred to polyimide as shown inFIG. 13. In contrast to previous PDMS transfer of crystallographically-aligned CNTs, the high packing density and uniform texture of our CNT arrays facilitates their direct printing without need to infiltrate the CNTs with a polymer as a carrier medium. Raman spectroscopy measurements again show no considerable change in the IG/IDratio, suggesting the printing process does not damage the CNTs.

FIG. 14shows various determined electrical conductivities versus packing fraction for various individual CNTs, CNT bundles, and copper traces. Conductivity of the horizontally-aligned nanostructures described herein is anisotropic. This is shown inFIG. 15wherein conductivity is substantially greater in the direction of the alignment of the CNTs than in a direction perpendicular to them. Differences in directional conductivity can be controlled so that, for example, a relative conductivity in the parallel direction can be at least ten times greater than the conductivity perpendicular to a nanotube direction. Or as another example, the parallel direction conductivity can be at least one hundred times greater than the perpendicular conductivity. Further conductivity properties exhibited by the nanostructure arrays processed according to the teachings herein are described in S. Tawfick et al.,Flexible High-Conductivity Carbon-Nanotube Interconnects Made by Rolling and Printing, small 2009, 5, No. 21, 2467-2473 (2009), the complete contents of which are hereby incorporated by reference.

EXAMPLE

Arrays of densely packed, horizontally-aligned CNTs were manufactured by a method as disclosed herein. Lithographically patterned films of 1/10 nm Fe/Al2O3were deposited by electron beam evaporation on a Si substrate. Arrays of vertically-aligned CNTs were grown by heating the substrate to 775 C in a 1″ diameter quartz tube furnace, under a flow of 100/400 sccm He/H2, followed by 100/400/100 sccm C2H4/H2/He for 20 minutes.

An apparatus was built wherein a smooth stainless steel roller was placed between a parallel base and upper plate. Opposite motion of the base and upper plate caused the roller to rotate about a fixed virtual pivot. Growth substrates including various sizes of VA-CNT arrays were affixed to the base. When the substrates advanced, the roller first toppled the CNT arrays, then densified the CNT arrays using the contact stresses between the roller and the substrate. The applied force can determine the thickness of the densified arrays. Different patterns of catalyst were designed and fabricated to investigate the rolling mechanism and densification factor. The conservation of the CNT quality and alignment after densifying was characterized using Raman spectroscopy and SEM.

The steps used to obtain densified horizontal-aligned CNT arrays are as follows:1. Patterned VA-CNT arrays were grown on Si—SiO2substrates, to a height of 1-2 millimeters, using thermal CVD as discussed previously.2. The substrate with the VA-CNT arrays was affixed to the lower spring loaded base of the apparatus using a vacuum chuck.3. The 0.6 mm diameter stainless steel roller was aligned parallel to the CNT arrays.4. The upper plate, in the form of parallel rails defining a cutout in the plate, were moved down to contact and apply force to the substrate through the roller via the spring-loaded base until the desired compression force was reached.5. The base and upper plate were moved simultaneously in directions opposite to one another such that friction caused the pin to roll in the direction of the VA-CNT arrays.

Various cross-sectional shapes of VA-arrays were studied using this densification method. The effect of different loads on the final dimensions of the arrays was also studied. As used here, the dimension perpendicular to the axis of the roller in the direction of rolling will be called the thickness, and the dimension parallel to the axis of the roller will be called the width. The rectangular dimensions of the original cross-sections of the arrays that were studied are as follows:

For each of these cross sections, three sets of normal forces were applied to the roller by adjusting the deflection of the spring loaded base, and the dimensional changes were measured from SEM micrographs.

The width and the height of the CNT arrays did not substantially change. These dimensions are insensitive to the applied force. In fact, the change in thickness of an array can be regarded with great accuracy as the change in the total volume of the array, and hence the change in bulk density. Also, the decrease in the thickness of the array is approximately linear with the increase of the normal force, as shown for example inFIGS. 16aand 16b, until a limit governed by tight packing among the CNTs is reached.FIG. 16plots the densification ratio (w/t) and packing fraction of horizontally-aligned CNT arrays created from vertically-aligned CNT blades—a 300 μm×20 μm array forFIG. 16aand a 300 μm×100 μm array forFIG. 16b. The densification factor is calculated by dividing the original thickness of the array by the final thickness of the array. This linear behavior may be due to the low packing fraction of the CNTs in the as-grown array. The average as-grown bulk density of the VA-CNT arrays were estimated to be 0.028 g/cm3, which corresponds to 2.5×1010CNTs/cm2for the case of MWCNTs having an average of 10 nm outer diameters and 6 nm inner diameters. Thus, the packing fraction was approximately 2% for the as-grown arrays. The rolling and the subsequent densification resulted in CNT arrays with a 30× decrease in thickness which corresponds to a 60% packing fraction.

Raman spectra measured on the CNT arrays before and after densification and the G/D ratio showed that the quality of the resultant horizontally-aligned CNT arrays are preserved.FIGS. 17aand 17billustrate the general vertical and horizontal alignment, respectively, of CNTs before and after densification according to the method described. Note thatFIG. 17ais approximately twice the magnification ofFIG. 17b. These images are indicative of the preservation of alignment before and after densification. Additional thickness measurements were taken and densification factors calculated after exposing densified arrays to acetone vapor. This additional step further increased the densification factors of the arrays, as shown inFIGS. 16aand16b.

It is to be understood that the foregoing is a description of one or more preferred exemplary embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but rather is defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to particular embodiments and are not to be construed as limitations on the scope of the invention or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other embodiments and various changes and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art. For example, in another embodiment, the adhesion between the CNTs and the roller (contact element) is controlled by applying a voltage between the contact element and the CNTs. This is shown inFIG. 18. In one example, a cylindrical roller is made from a metal and coated with a thin electrical insulating layer. A voltage is applied to both the metal core of the roller and the CNTs. If an equal voltage is applied to both the roller and CNTs, with respect to a common reference (ground), electrostatic repulsion between the roller and CNTs exerts an additional force between the roller and CNTs, promoting toppling and densification. If there is a voltage difference between the roller and CNTs, electrostatic attraction draws the CNTs toward the roller and increases the adhesion force. The voltage may be changed to achieve control of the adhesion and CNT-roller interaction during the process. Furthermore, other means such as a fluid flow can be used to topple and densify the nanostructure arrays, and all such other embodiments, changes, and modifications are intended to come within the scope of the appended claims.