Patent Publication Number: US-10773960-B2

Title: Low porosity nanotube fabric articles

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/578,691, entitled METHODS FOR CONTROLLING DENSITY, POROSITY, AND/OR GAP SIZE WITHIN NANOTUBE FABRIC LAYERS AND FILMS, which is a U.S. national phase application under 35 U.S.C. § 371 of International Patent Application No. PCT/US2011/024710 filed on Feb. 14, 2011, entitled METHODS FOR CONTROLLING DENSITY, POROSITY, AND/OR GAP SIZE WITHIN NANOTUBE FABRIC LAYERS AND FILMS, which claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 61/304,045, filed Feb. 12, 2010, which is incorporated by reference its entirety. This application is also related to the following U.S. patents, which are assigned to the assignee of the present application, and are hereby incorporated by reference in their entirety: 
     Methods of Nanotube Films and Articles (U.S. Pat. No. 6,835,591), filed Apr. 23, 2002; 
     Methods of Using Pre-Formed Nanotubes to Make Carbon Nanotube Films, Layers, Fabrics, Ribbons, Elements, and Articles (U.S. Pat. No. 7,335,395), filed Jan. 13, 2003; 
     Devices Having Horizontally-Disposed Nanofabric Articles and Methods of Making the Same (U.S. Pat. No. 7,259,410), filed Feb. 11, 2004; 
     Devices Having Vertically-Disposed Nanofabric Articles and Methods of Making Same (U.S. Pat. No. 6,924,538), filed Feb. 11, 2004; 
     Resistive Elements Using Carbon Nanotubes (U.S. Pat. No. 7,365,632), filed Sep. 20, 2005; and 
     Spin-Coatable Liquid for Formation of High Purity Nanotube Films (U.S. Pat. No. 7,375,369), filed Jun. 3, 2004. 
     This application is related to the following patent applications, which are assigned to the assignee of the application, and are hereby incorporated by reference in their entirety: 
     Methods of Making Carbon Nanotube Films, Layers, Fabrics, Ribbons, Elements, and Articles (U.S. patent application Ser. No. 10/341,005), filed Jan. 13, 2003; 
     High Purity Nanotube Fabrics and Films (U.S. patent application Ser. No. 10/860,332), filed Jun. 3, 2004; 
     Aqueous Carbon Nanotube applicator Liquids and Methods for Producing Applicator Liquids Thereof (U.S. patent application Ser. No. 11/304,315), filed Dec. 15, 2005; and 
     Anisotropic Nanotube Fabric Layers and Films and Methods of Forming Same (U.S. patent application Ser. No. 12/533,687), filed Jul. 31, 2009. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to nanotube fabric layers and films and, more specifically, to a method of controlling density, porosity and/or gap size within a nanotube fabric layers and films. 
     BACKGROUND 
     Any discussion of the related art throughout this specification should in no way be considered as an admission that such art is widely known or forms part of the common general knowledge in the field. 
     Nanotube fabric layers and films are used in a plurality of electronic structures, and devices. For example, U.S. patent application Ser. No. 11/835,856 to Bertin et al., incorporated herein by reference in its entirety, teaches methods of using nanotube fabric layers to realize nonvolatile devices such as, but not limited to, block switches, programmable resistive elements, and programmable logic devices. U.S. Pat. No. 7,365,632 to Bertin et al., incorporated herein by reference, teaches the use of such fabric layers and films within the fabrication of thin film nanotube based resistors. U.S. patent application Ser. No. 12/066,063 to Ward et al., incorporated herein by reference in its entirety, teaches the use of such nanotube fabrics and films to form heat transfer elements within electronic devices and systems. 
     Through a variety of previously known techniques (described in more detail within the incorporated references) nanotube elements can be rendered conducting, non-conducting, or semi-conducting before or after the formation of a nanotube fabric layer or film, allowing such nanotube fabric layers and films to serve a plurality of functions within an electronic device or system. Further, in some cases the electrical conductivity of a nanotube fabric layer or film can be adjusted between two or more non-volatile states as taught in U.S. patent application Ser. No. 11/280,786 to Bertin et al., incorporated herein by reference in its entirety, allowing for such nanotube fabric layers and films to be used as memory or logic elements within an electronic system. 
     U.S. Pat. No. 7,334,395 to Ward et al., incorporated herein by reference in its entirety, teaches a plurality of methods for forming nanotube fabric layers and films on a substrate element using preformed nanotubes. The methods include, but are not limited to, spin coating (wherein a solution of nanotubes is deposited on a substrate which is then spun to evenly distribute said solution across the surface of said substrate), spray coating (wherein a plurality of nanotube are suspended within an aerosol solution which is then dispersed over a substrate), and dip coating (wherein a plurality of nanotubes are suspended in a solution and a substrate element is lowered into the solution and then removed). Further, U.S. Pat. No. 7,375,369 to Sen et al., incorporated herein by reference in its entirety, and U.S. patent application Ser. No. 11/304,315 to Ghenciu et al., incorporated herein by reference in its entirety, teach nanotube solutions well suited for forming a nanotube fabric layer over a substrate element via a spin coating process. 
     SUMMARY OF THE DISCLOSURE 
     The current disclosure relates to a method for controlling density, porosity and/or gap size within nanotube fabric layers and films. 
     In particular, the present disclosure provides a nanotube fabric layer including a plurality of individual nanotube elements where open regions between said individual nanotube elements within the nanotube fabric layer define gaps within said nanotube fabric layer and where the gaps are limited in physical dimension to be smaller than a threshold. In some embodiments, a nanotube switching device includes such a nanotube fabric layer. 
     The present disclosure also provides a nanotube fabric layer including a plurality of individual nanotube elements where open regions between said individual nanotube elements within said nanotube fabric layer define a porosity of said nanotube fabric layer and where said porosity is selected to provide a uniform density of individual nanotube elements within the nanotube fabric layer. In some embodiments, the nanotube fabric layer has a high porosity. In some embodiments, the nanotube fabric layer has a low porosity. In some embodiments, a nanotube switching device includes such a nanotube fabric layer. 
     The present disclosure also provides a method of preparing a nanotube application solution. The method first includes forming a raw nanotube application solution, this raw nanotube application solution comprising a first plurality of nanotube elements at a first concentration level and a second plurality of ionic particles at a second concentration level dispersed in a liquid medium. The method further includes adjusting at least one of the first concentration level of the first plurality of nanotube elements and the second concentration level of the second quantity of ionic particles such as to control the degree of rafting realized within a nanotube fabric layer formed using the nanotube application solution. 
     According to one aspect of the present disclosure, the first plurality of nanotube elements are carbon nanotubes. 
     Under another aspect of the present disclosure, the first plurality of nanotube elements are single walled carbon nanotubes. 
     Under another aspect of the present disclosure, the first plurality of nanotube elements are multi-walled carbon nanotubes. 
     Under another aspect of the present disclosure, the second plurality of ionic particles include ammonium nitrate salts. 
     Under another aspect of the present disclosure, the second plurality of ionic particles include ammonium formate. 
     Under another aspect of the present disclosure, the second plurality of ionic particles include ammonium acetate. 
     Under another aspect of the present disclosure, the second plurality of ionic particles include ammonium carbonate. 
     Under another aspect of the present disclosure, the second plurality of ionic particles include ammonium bicarbonate, ionic organic species, and ionic polymers 
     Under another aspect of the present disclosure, the second plurality of ionic particles include ionic organic species. 
     Under another aspect of the present disclosure, the second plurality of ionic particles include ionic polymers. 
     Under another aspect of the present disclosure, the second plurality of ionic particles include inorganic salts. 
     Under another aspect of the present disclosure, the liquid medium is an aqueous solution. 
     Under another aspect of the present disclosure, the liquid medium is a nitric acid solution. 
     Under another aspect of the present disclosure, the liquid medium is a sulfuric acid solution. 
     Under another aspect of the present disclosure, the concentration level of nanotube elements within the nanotube application solution is increased in order to promote rafting in a nanotube fabric layer formed with such a solution. 
     Under another aspect of the present disclosure, the concentration level of nanotube elements within the nanotube application solution is decreased in order to discourage rafting in a nanotube fabric layer formed with such a solution. 
     Under another aspect of the present disclosure, the concentration level of ionic particles within the nanotube application solution is decreased in order to promote rafting in a nanotube fabric layer formed with such a solution. 
     Under another aspect of the present disclosure, the concentration level of ionic particles within the nanotube application solution is increased in order to discourage rafting in a nanotube fabric layer formed with such a solution. 
     Other features and advantages of the present invention will become apparent from the following description of the invention which is provided below in relation to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration depicting a nanotube fabric layer exhibiting essentially no rafting of the individual nanotube elements; 
         FIGS. 2A-2B  are SEM images (at different magnifications) of a nanotube fabric layer exhibiting essentially no rafting of the individual nanotube elements; 
         FIG. 3  is an illustration depicting a nanotube fabric layer exhibiting substantial rafting of the individual nanotube elements, according to one or more embodiments of the present disclosure; 
         FIGS. 4A-4B  are SEM images (at different magnifications) of a nanotube fabric layer exhibiting substantial rafting of the individual nanotube elements, according to one or more embodiments of the present disclosure; 
         FIG. 5  is a process diagram illustrating a method according to the present disclosure of preparing a nanotube application solution such as to form a highly rafted nanotube fabric layer; 
         FIG. 6  is a process diagram illustrating a method according to the present disclosure of preparing a nanotube application solution such as to form a substantially non-rafted nanotube fabric layer; 
         FIG. 7  is a graph plotting conductivity readings (measured in μS/cm) vs. ammonium nitrate salt levels (measure in ppm) taken on a plurality of nanotube application solutions; 
         FIGS. 8A-8C  are SEM images (at increasing magnifications) of an exemplary nanotube fabric layer formed according to the methods of the present disclosure which exhibits approximately 11.6% rafting; 
         FIGS. 9A-9C  are SEM images (at increasing magnifications) of an exemplary nanotube fabric layer formed according to the methods of the present disclosure which exhibits approximately 18.9% rafting; 
         FIGS. 10A-10C  are SEM images (at increasing magnifications) of an exemplary nanotube fabric layer formed according to the methods of the present disclosure which exhibits approximately 5.5% rafting; 
         FIGS. 11A-11C  are SEM images (at increasing magnifications) of an exemplary nanotube fabric layer formed according to the methods of the present disclosure which exhibits approximately 37.8% rafting; 
         FIGS. 12A-12C  are SEM images (at increasing magnifications) of an exemplary nanotube fabric layer formed according to the methods of the present disclosure which exhibits substantially no rafting; 
         FIGS. 13A-13C  are SEM images (at increasing magnifications) of an exemplary nanotube fabric layer formed according to the methods of the present disclosure which exhibits substantially no rafting; 
         FIGS. 14A-14C  are SEM images (at increasing magnifications) of an exemplary nanotube fabric layer formed according to the methods of the present disclosure which exhibits approximately 13.1% rafting; 
         FIGS. 15A-15C  are SEM images (at increasing magnifications) of an exemplary nanotube fabric layer formed according to the methods of the present disclosure which exhibits approximately 10.0% rafting; 
         FIGS. 16A-16C  are SEM images (at increasing magnifications) of an exemplary nanotube fabric layer formed according to the methods of the present disclosure which exhibits approximately 35.0% rafting; 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure teaches methods to increase or reduce the number of nanotube elements in a given area of nanotube fabric layers and films. These approaches can selectively create high density, low porosity nanotube fabrics in a controlled way. In this manner, for example, nanotube fabrics may be created in which essentially all gaps or pores between nanotubes within the fabric are no larger than a predetermined size. This is particularly useful for devices with extremely small circuit sizes in which a uniform dispersion of nanotubes is desired. For example, when a fabric with a high density and low porosity is patterned and etched, the remaining nanotube article is effectively assured of containing nanotubes as opposed to lacking nanotubes as a result of a large pore in the fabric. As the feature sizes decrease along with currently practiced lithography techniques, minimizing the porosity becomes more important to ensure a higher yield of functional circuit elements as the fabric is being etched. For example, the high density, low porosity fabrics can have nanotube free regions, i.e., pores that are less than the size of the small circuits according to the current lithography techniques (e.g., pores that are less than about 10 nm). Thus, the density or pore size is controlled such that the minimum number of nanotube elements required for operation can be utilized in a critical feature size of current lithography techniques, (e.g. 20 nm or less) which can be less than 20 nm. 
     Conversely, the methods can be used to create highly porous, low density fabrics, if so desired. For example, it may desirable to have a nanotube fabric where the nanotubes are dispersed to increase the optical transparency of the nanofabric. In other applications, where a thicker fabric, formed of multiple layers of nanotube fabrics, is desired, it may be preferable to limit the concentration of nanotubes to reduce cost and the electrical resistance of the fabric. Further, for low density and high porosity fabrics, it is also important that the nanotubes be dispersed in a uniform manner across the fabric. 
     Fabric porosity and density may be controlled in a variety of ways including, but not limited to, techniques for controlling rafting within the nanotube fabric. These fabrics can then be used in nanotube switching devices. 
     As described within U.S. Pat. No. 7,375,369 to Sen et al. and U.S. patent application Ser. No. 11/304,315 to Ghenciu et al., both incorporated herein by reference in their entirety, nanotube fabrics and films can be formed by applying a nanotube application solution (for example, but not limited to, a plurality of individual nanotube elements suspended within an aqueous solution) over a substrate element. A spin coating process, for example, can be used to evenly distribute the nanotube elements over the substrate element, creating a substantially uniform layer of nanotube elements. In other cases, other processes (such as, but not limited to, spray coating processes or dip coating processes) can be used to apply and distribute the nanotube elements over the substrate element. 
     It should be noted that nanotube elements used and referenced within the embodiments of the present disclosure may be single walled nanotubes, multi-walled nanotubes, or mixtures thereof and may be of varying lengths. Further, the nanotubes may be conductive, semiconductive, or combinations thereof. 
     Within many applications it is desirable to control the porosity of a nanotube fabric layer as it is formed—that is, to control how closely packed together or sparsely distributed the individual nanotube elements within the fabric layer are. In one example, a high porosity uniform nanotube fabric may have voids—that is gaps in the fabric between individual nanotube elements—on the order of 50 nm in size. In another example, a low porosity uniform nanotube fabric layer may have voids on the order of 10 nm in size. 
     In some applications the sheet resistance of a nanotube fabric layer may be controlled by controlling the porosity of the nanotube fabric layer, or a density of nanotubes in the fabric, along with other variables (such as, but not limited to, the length of the individual nanotube elements within the fabric and the thickness of the nanotube fabric layer). By controlling the porosity of a nanotube fabric layer, such a fabric layer can be reliably tuned to have a sheet resistance from about 1 k-Ohm/square to about 1 M-Ohm/square. 
     In another applications by limiting the porosity of a nanotube fabric layer the density of an array of nanotube switching devices may be increased. U.S. patent application Ser. No. 11/280,786 to Bertin et al., incorporated herein by reference in its entirety, teaches a nonvolatile two terminal nanotube switch structure having (in at least one embodiment) a nanotube fabric article deposited between two electrically isolated electrode elements. As Bertin teaches, by placing different voltages across said electrode elements, the resistive state of the nanotube fabric article can be switched between a plurality of nonvolatile states. That is, in some embodiments the nanotube fabric article can be repeatedly switched between a relatively high resistive state (resulting in, essentially, an open circuit between the two electrode elements) and a relatively low resistive state (resulting in, essentially, a short circuit between the two electrode elements). 
     The fabrication of an array of such nanotube switching devices can include patterning of a nanotube fabric layer to realize a plurality of these nanotube fabric articles. The porosity of a nanotube fabric layer—or more specifically the size of the voids within a nanotube fabric layer—can limit the feature size to which these nanotube fabric articles can be patterned. For example, to fabricate a nanotube switching device array wherein the individual nanotube switching devices are on the order of 20 nm square (that is, the nanotube fabric article within each device is essentially 20 nm by 20 nm), the porosity of the nanotube fabric array may need to be such that voids within the nanotube fabric layer are on the order of 10 nm. In this way, the fabrication of highly dense nanotube memory arrays (wherein the individual nanotube switching elements within the array are patterned at a sub 20 nm geometry, for example) can require highly dense (that is, less porous with void sizes on the order of 10 nm or less) nanotube fabric layers. 
     One method of controlling the porosity of a nanotube fabric layer is to control the degree of rafting—that is, the percentage of nanotube elements within the fabric layer which tend to bundle together along their sidewalls—within the nanotube fabric layer. By controlling certain parameters during the formation of a nanotube fabric layer, a nanotube fabric layer can be formed which is highly rafted (and, consequently, highly dense—for example, with voids on the order of 10 nm), moderately rafted (and, consequently, marginally dense—for example, with voids on the order of 25 nm), or substantially free from rafts (and consequently, highly porous—for example with voids on the order of 50 nm). 
       FIG. 1  depicts a nanotube fabric layer  100  which is substantially free of rafts. As described above, within such a fabric layer individual nanotube elements  110  are formed into a uniform highly porous fabric wherein the individual nanotube elements are arranged in substantially random orientations. For example, the voids within such a fabric layer  100  might range between 25 nm and 50 nm, corresponding to a sheet resistance between about 1000 k-Ohm/square to 1 M-Ohm/square within a single fabric layer. A thicker fabric layer may be formed with substantially the same porosity by applying (through multiple spin coating operations, for example) multiple fabric layers over the nanotube fabric layer  100  illustrated in  FIG. 1 . 
       FIGS. 2A and 2B  are SEM images depicting an exemplary nanotube fabric layer ( 201  and  202 , respectively) substantially free of rafts and analogous to the nanotube fabric layer  100  depicted in  FIG. 1 .  FIG. 2A  shows the nanotube fabric layer  201  at a 10,000× magnification, and  FIG. 2B  shows the nanotube fabric layer  202  at a 75,000× magnification. Within both images, the random orientation—and essentially complete lack of rafting—is evident within the exemplary nanotube fabric layer. 
       FIG. 3  depicts a nanotube fabric layer  300  which includes a moderate number of rafted nanotube bundles  320  as well as a number of unbundled nanotube elements  310 . Within such a fabric layer, individual nanotube elements within the rafted bundles  320  are packed tightly together such as to minimize the porosity within that region of the nanotube fabric layer  300 . In this way, the nanotube fabric layer  300  is significantly denser as compared to the fabric layer  100  illustrated in  FIG. 1 . For example, the voids within such a fabric layer  300  might range between 10 nm and 20 nm, corresponding to a sheet resistance between about 10 k-Ohm/square to 100 k-Ohm/square within a single fabric layer. A thicker fabric layer may be formed with substantially the same porosity by applying (through multiple spin coating operations, for example) multiple fabric layers over the nanotube fabric layer  300  illustrated in  FIG. 3 . 
       FIGS. 4A and 4B  are SEM images depicting an exemplary nanotube fabric layer ( 401  and  402 , respectively) which exhibits a moderate amount of rafting and is analogous to the nanotube fabric layer  300  depicted in  FIG. 3 .  FIG. 4A  shows the nanotube fabric layer  401  at a 10,000× magnification, and  FIG. 4B  shows the nanotube fabric layer  402  at a 75,000× magnification. Within both images, the randomly oriented bundles of rafted nanotube elements ( 410  and  420 , respectively) are evident within the exemplary nanotube fabric layer. 
     In some cases, rafting of individual nanotube elements can occur because during the formation of a nanotube fabric layer groups of nanotube elements bundle together along their sidewalls due to van der Waals interactions (atomic level forces between the individual nanotube elements) or through JI-JI interactions (a stacking effect due to the presence of a free electrons in the JI-orbitals along the nanotube structure). Within an application solution—that is, a dispersion of individual nanotube elements within a liquid medium—the van der Waals and JI-JI interactions can be promoted or discouraged by the presence of certain ionic species within the solution. Such ionic species include, but are not limited to, ammonium salts, nitrate salts, ammonium nitrate salts, ammonium formate, ammonium acetate, ammonium carbonate, ammonium bicarbonate, ionic organic species, ionic polymers, and inorganic salts. A high concentration of such ionic species within the application solution (for example, on the order of 20 ppm or more ammonium nitrate salts within an aqueous nanotube application solution) will tend to interfere with these interactions and thereby reduce the degree of rafting within a nanotube fabric layer formed with such an application solution. Conversely, a low concentration of such ionic species within the application solution (for example, on the order of 10 ppm or less ammonium nitrate salts within an aqueous nanotube application solution) will tend to allow a plurality of these rafted bundles to form within a nanotube fabric layer. 
     It should be noted that this rafting effect—wherein a plurality of nanotube elements bundle together along their sidewalls to realize an orderly raft like structure—is different from the so-called clumping defects described within U.S. patent application Ser. No. 11/304,315 to Ghenciu et al., the entire disclosure of which is hereby incorporated by reference. The clumping defects described by Ghenciu are the result of precipitation or aggregation of the individual nanotube elements within the solution and are characterized by individual nanotube elements twisting around each other and bundling into clump like structures within the application solution. Such undesirable clumping defects can result in non-uniform and non-planar nanotube fabric layers. Conversely, as described by the present disclosure, a rafted nanotube fabric can remain, in most cases, substantially uniform and thus can be employed to control the density of a nanotube fabric layer. Further, the rafts described herein are essentially two dimensional nanotube structures, i.e., the height of the raft is generally one nanotube thick. The clumping defects referenced in Ghenciu generally result in three dimensional nanotube clumps. 
     Rafting also can be promoted (or discouraged) by controlling the concentration of nanotube elements with an application solution—that is, by controlling the number of individual nanotube elements per unit volume present within the applicator liquid. Van der Waals interactions between closely situated nanotube elements within a highly concentrated application solution (for example, an application solution with an optical density on the order of 35) can tend to increase the incidence of rafting within a nanotube fabric layer formed with such a solution. Conversely, an application solution with a relatively low concentration of nanotube elements (for example, an application solution with an optical density on the order of 10) can significantly reduce the opportunity for these van der Waals interactions and result in less rafting. It should be noted that optical density (a spectrographic technique well known to those skilled in the art) is typically used to characterize the density of nanotube elements within an application solution. The technique relies on measuring the amount of light absorbed by a nanotube application solution—essentially the light absorbed by the individual nanotube elements within such a solution—to determine the concentration of nanotube elements dispersed in the solution. For example, a solution with an optical density of 30 corresponds to approximately 0.1% concentration (by weight) of nanotube elements within the solution. 
     The use of these two parameters (the concentration of an ionic species within the application solution and nanotube concentration within the application solution) to control the degree of rafting within a nanotube fabric layer is illustrated in the exemplary nanotube fabric layers depicted in  FIGS. 7A-7C, 8A-8C, 9A-9C, 10A-10C, and 11A-11C  and described in detail within the discussion of those figures. 
     In addition, within certain applications other parameters of a nanotube application solution may contribute to the degree to which a nanotube fabric layer formed with that solution will raft. Such parameters include, but are not limited to, the presence of other carbon allotropes (for example, amorphous carbon), the temperature of the application solution as it is applied to the surface of a wafer or other substrate, the chemical composition of the liquid medium used, the method used for depositing the application solution to the surface of a wafer or other substrate, and the acidity of the solution. 
       FIG. 5  illustrates an exemplary application solution preparation process according to the methods of the present disclosure which is well suited for forming a rafted nanotube fabric layer. 
     At the start of the application solution preparation process illustrated in  FIG. 5 , a plurality of individual nanotube elements  505  is dispersed into a liquid medium  510  (such as, but not limited to, an aqueous solution, a sulfuric acid solution, or a nitric acid solution) to form raw nanotube application solution  515 . Raw nanotube application solution  515  includes a plurality of individual nanotube elements  560 , a plurality of impurities (such as, but not limited to, residual metallic catalyst particles, amorphous carbon particles, and other carbonaceous impurities)  565 , and a concentration of an ionic particles  570  (such as, but not limited to, ammonium salts, nitrate salts, ammonium nitrate salts, ammonium formate, ammonium acetate, ammonium carbonate, ammonium bicarbonate, ionic organic species, ionic polymers, and inorganic salts). The raw nanotube application solution  515  is then passed through a filtration/purification process  520  which removes a significant percentage of impurities  565  to realize purified nanotube application solution  525 . A typical filtration/purification process  520  also can remove a percentage of the ionic particles  570  within raw nanotube application solution  515 , as is shown in the graphic representation of the purified nanotube application solution  525  in  FIG. 5 . 
     A typical purified nanotube application solution  525  can include less than 1×10 18  atoms/cm 3  of impurities  565  and be substantially free of particle impurities  565  having a diameter of greater than about 500 nm. It also can include a nanotube concentration of 100 mg/l (a nanotube concentration well suited for memory and logic applications). This typical purified nanotube application solution  525  might also include an ionic species of ammonium nitrate salt at a concentration of about 15 ppm. 
     The formation and purification of nanotube application solutions (such as described above) is taught in U.S. Pat. No. 7,375,369 to Sen et al. as well as U.S. patent application Ser. No. 11/304,315 to Ghenciu et al. Within both references, a plurality of filtration/purification processes are detailed, including cross flow filtration, nitric acid treatment, hydrochloric acid treatment, and high speed centrifugation. 
     Within the exemplary process detailed in  FIG. 5 , the purified nanotube application solution  525  is then passed through an ionic particle concentration level adjustment process  530  which further reduces the concentration of ionic particles  570  within the purified application solution  530  resulting in intermediate application solution  535 . For an exemplary ionic species of ammonium nitrate salts, this intermediate application solution  535  can have an ionic particle concentration level of less than 10 ppm. This ionic particle concentration level adjustment process  530  may be realized through an additional filtering process (such as, but not limited to, a cross flow filtration process, a sonication filtration process, and a centrifugation filtration process). 
     In a next process step, intermediate application solution  535  is passed through a nanotube concentration adjustment process  540 , which increases the concentration of nanotube elements within the intermediate application solution  535  resulting in a final application solution  545 , which is well suited for forming a rafted nanotube fabric layer. For example, the nanotube application solution can be adjusted such that final application solution  545  possesses an optical density on the order of 35. Typically such a nanotube concentration adjustment process  540  is realized by removing a volume of the liquid medium  510  from the solution, though the methods of the present disclosure are not limited in this regard. 
     Within the exemplary process detailed in  FIG. 5 , a spin coating process  550  is used to apply final nanotube application solution  545  over a silicon wafer to realize rafted nanotube fabric layer  555  wherein a plurality of rafted bundles of nanotube elements  575  are distributed throughout the fabric layer. 
     In this way a purified nanotube application solution  525  (prepared according to the methods taught by Sen and Ghenciu) is rendered into an application solution  545  well suited for forming a rafted nanotube fabric layer by reducing the concentration of ionic particles within the original purified solution in one operation and by increasing the concentration of nanotube elements within the solution in a second operation. 
     It should be noted that while the exemplary process detailed in  FIG. 5  describes a specific nanotube application solution process in order to illustrate the methods of the present disclosure, the methods of the present disclosure are not limited to this specific example. For example, within some applications the order of the ionic particle concentration level adjustment process  530  and the nanotube concentration adjustment process  540  can be reversed (that is, the application solution first increased in nanotube concentration and then reduced in ionic particle concentration). Further, within some applications the ionic particle concentration level adjustment process  530  may be removed altogether and the nanotube concentration adjustment process  540  used alone to sufficiently render purified nanotube application solution  525  into a solution well suited for forming a rafted nanotube fabric layer. Within still other applications, the nanotube concentration adjustment process  540  may be removed altogether and the ionic particle concentration level adjustment process used alone to sufficiently render purified nanotube application solution  525  into a solution well suited for forming a rafted nanotube fabric layer. 
       FIG. 6  illustrates an exemplary application solution preparation process according to the methods of the present disclosure which is well suited for forming a nanotube fabric layer substantially free of rafted bundles of nanotube elements. 
     At the start of the application solution preparation process illustrated in  FIG. 6 , a plurality of individual nanotube elements  605  is dispersed into a liquid medium  610  (such as, but not limited to an aqueous solution, a sulfuric acid solution, or a nitric acid solution) to form raw nanotube application solution  615 . Raw nanotube application solution  615  includes a plurality of individual nanotube elements  660 , a plurality of impurities (such as, but not limited to, residual metallic catalyst particles, amorphous carbon particles, and other carbonaceous impurities)  665 , and a concentration of ionic particles  670  (such as, but not limited to, ammonium salts, nitrate salts, ammonium nitrate salts, ammonium formate, ammonium acetate, ammonium carbonate, ammonium bicarbonate, ionic organic species, ionic polymers, and inorganic salts). The raw nanotube application solution  615  is then passed through a filtration/purification process  620  which removes a significant percentage of impurities  665  to realize purified nanotube application solution  625 . A typical filtration/purification process  620  can remove a percentage of the ionic particles  670  within raw nanotube application solution  615 , as is shown in the graphic representation of the purified nanotube application solution  625  in  FIG. 6 . 
     A typical purified nanotube application solution  625  can include less than 1×10 18  atoms/cm 3  of impurities  665  and be substantially free of particle impurities  665  having a diameter of greater than about 500 nm. It also can have a nanotube concentration of 100 mg/l (a nanotube concentration well suited for memory and logic applications). This typical purified nanotube application solution  625  also can include an ionic species of ammonium nitrate salt at a concentration of about 15 ppm. 
     The formation and purification of nanotube application solutions (such as described above) is taught in U.S. Pat. No. 7,375,369 to Sen et al. as well as U.S. patent application Ser. No. 11/304,315 to Ghenciu et al. Within both references, a plurality of filtration/purification processes are detailed, including cross flow filtration, nitric acid treatment, hydrochloric acid treatment, and high speed centrifugation. 
     Within the exemplary process detailed in  FIG. 6 , the purified nanotube application solution  625  is then passed through an ionic particle concentration level adjustment process  630  which increases the concentration of ionic particles  670  within the purified application solution  625  resulting in intermediate application solution  635 . For an exemplary ionic species of ammonium nitrate salts, this intermediate application solution  535  can include an ionic particle concentration level of greater than 30 ppm. This ionic particle concentration level adjustment process  625  can be realized through the introduction of an additional quantity of ionic particles  670  into the purified application solution  625 . 
     In a next process step, intermediate application solution  635  is passed through a nanotube concentration adjustment process  640 , which decreases the concentration of nanotube elements within intermediate application solution  635  resulting in a final application solution  645 , which is well suited for forming a nanotube fabric layer substantially free of rafted bundles of nanotube elements. For example the nanotube application solution can be adjusted such that final application solution  645  possesses an optical density on the order of 10. Typically, such a nanotube concentration adjustment process  640  can be realized by adding an additional volume of the liquid medium  610  into the solution, though the methods of the present disclosure are not limited in this regard. 
     Within the exemplary process detailed in  FIG. 6 , a spin coating process  650  is used to apply final nanotube application solution  645  over a silicon wafer to realize nanotube fabric layer  655  which is substantially free of rafted bundles of nanotube elements. 
     In this way a purified nanotube application solution  625  (prepared according to the methods taught by Sen and Ghenciu) is rendered into an application solution  645  well suited for forming a nanotube fabric layer substantially free of rafted bundles of nanotube elements by increasing the concentration of ionic particles within the original purified solution in one operation and by decreasing the concentration of nanotube elements within the solution in a second operation. 
     It should be noted that while the exemplary process detailed in  FIG. 6  describes a specific nanotube application solution process in order to illustrate the methods of the present invention, the methods of the present invention are not limited to this specific example. Indeed, within some applications the order of the ionic particle concentration level adjustment process  630  and the nanotube concentration adjustment process  640  can be reversed (that is the application solution first decreased in nanotube concentration and then increased in ionic particle concentration). Further, within some applications the ionic particle concentration level adjustment process  630  may be removed altogether and the nanotube concentration adjustment process  640  used alone to sufficiently render purified nanotube application solution  625  into a solution well suited for forming a nanotube fabric layer substantially free from rafted bundles of nanotube elements. Within still other applications, the nanotube concentration adjustment process  640  may be removed altogether and the ionic particle concentration level adjustment process used alone to sufficiently render purified nanotube application solution  625  into a solution well suited for forming a nanotube fabric layer substantially free from rafted bundles of nanotube elements. 
     The following examples describe the formation of several nanotube fabric layers (with varying degrees of rafting) according to the methods of the present disclosure. Within each example, a purified nanotube application solution was first realized through the methods taught by Ghenciu in U.S. patent application Ser. No. 11/304,315 (and described in the discussions of  FIGS. 5 and 6 ). This purified nanotube application solution was then adjusted as specified in each example to realize a specific nanotube concentration and ionic particle concentration level. Within each example the ionic species adjusted was ammonium nitrate salts. The resulting solution was then deposited on a four inch Si/SiO 2  wafer via a spin coating operation. For all examples, the nanotube concentration is measured in terms of optical density (a spectrographic technique well known to those skilled in the art), and the ammonium nitrate salt concentration is measured in parts-per-million (ppm) with respect to the solution. 
     It should be noted that while the following examples specify the level of ammonium nitrate salt (the exemplary ionic species used in each of the examples) in terms of ppm, another methods of tracking an ionic species concentration level may prove more convenient for some applications.  FIG. 7  is a graph plotting the conductivity (measured in μS/cm) of a plurality of nanotube application solutions against the concentration level (measured in ppm) of ammonium nitrate salts in each application solution. As can be observed in  FIG. 7 , the conductivity of these application solutions will tend to track the concentration of ammonium nitrate salts dispersed in each. For example, within the application solutions used in the following example, a conductivity reading of approximately 700 μS/cm or higher would indicate that an application solution would be likely to promote rafting. Conversely, a conductivity reading of approximately 500 μS/cm or lower would indicate that an application solution would be likely to discourage rafting. As such, it may be convenient within some applications of the methods of the present disclosure to track and adjust the conductivity of a nanotube application solution instead of the concentration level of a particular ionic species within that application solution. 
     For all examples, the spin coating operation was as follows. A raw wafer was pre-baked on a 300° C. hot plate for five minutes. Approximately 3 ml of the adjusted solution was dispensed onto the wafer via a plastic pipette while the wafer was rotated at 60 rpm. After thirty seconds, the spin speed was increased to 500 rpm for two seconds, then subsequently reduced to fifty rpm for 180 seconds, and finally increased to 2000 rpm for twenty seconds. The wafer (now coated with the application solution) was then placed on a 300° C. hot plate for two minutes. After a cool down cycle, the entire process was repeated again twice such as to apply three coats of the application solution over the wafer. 
     For the application solutions used in the following examples it was found that generally an ammonium nitrate salt concentration level of 10 ppm or lower would tend to result in a highly rafted fabric. It was further found that generally an ammonium nitrate salt concentration level of 20 ppm or more would tend to result in fabric layers with lower incidences of rafting. Applications solutions with ammonium nitrate salt concentration levels between these ranges were found to result in fabric layers with moderate rafting. 
     Further, for the application solutions used in the following examples it was found that generally an optical density of approximately 10 or lower would tend to result in fabric layers with low incidences of rafting. It was further found that generally an optical density of 30 or more would tend to result in fabric layers with very high incidences of rafting. Applications solutions with optical densities between these ranges were found to result in fabric layers with moderate rafting. 
     Example 1 
       FIGS. 8A-8C  are SEM images of an exemplary nanotube fabric layer at different magnifications ( 801 ,  802 , and  803  respectively) prepared according to the methods of the present disclosure. The nanotube fabric layer depicted in  FIGS. 8A-8C  was rendered from an application solution with an optical density of 19.11 and an ammonium nitrate salt concentration of 16 ppm. These parameters resulted in a moderate amount of rafting within the nanotube fabric layer ( 801 ,  802 ,  803 ). Analysis of the entire nanotube fabric layer showed that approximately 11.6% of the fabric (by area) was comprised of rafted bundles of nanotube elements. These rafted bundles of nanotube elements are evident in  FIG. 8A  ( 810 ),  FIG. 8B  ( 820 ), and  FIG. 8C  ( 830 ). 
     Example 2 
       FIGS. 9A-9C  are SEM images of an exemplary nanotube fabric layer at different magnifications ( 901 ,  902 , and  903  respectively) prepared according to the methods of the present disclosure. The nanotube fabric layer depicted in  FIGS. 9A-9C  was rendered from an application solution with an optical density of 34.35 and an ammonium nitrate salt concentration of 12 ppm. These parameters resulted in a high degree of rafting within the nanotube fabric layer ( 901 ,  902 ,  903 ). Analysis of the entire nanotube fabric layer showed that approximately 18.9% of the fabric (by area) was comprised of rafted bundles of nanotube elements. These rafted bundles of nanotube elements are evident in  FIG. 9A  ( 910 ),  FIG. 9B  ( 920 ), and  FIG. 9C  ( 930 ). 
     Example 3 
       FIGS. 10A-10C  are SEM images of an exemplary nanotube fabric layer at different magnifications ( 1001 ,  1002 , and  1003  respectively) prepared according to the methods of the present disclosure. The nanotube fabric layer depicted in  FIGS. 10A-10C  was rendered from an application solution with an optical density of 10.02 and an ammonium nitrate salt concentration of 11 ppm. These parameters resulted in a low degree of rafting within the nanotube fabric layer ( 1001 ,  1002 ,  1003 ). Analysis of the entire nanotube fabric layer showed that approximately 5.5% of the fabric (by area) was comprised of rafted bundles of nanotube elements. These rafted bundles of nanotube elements are evident in  FIG. 10A  ( 1010 ),  FIG. 10B  ( 1020 ), and  FIG. 10C  ( 1030 ). 
     Example 4 
       FIGS. 11A-11C  are SEM images of an exemplary nanotube fabric layer at different magnifications ( 1101 ,  1102 , and  1103  respectively) prepared according to the methods of the present disclosure. The nanotube fabric layer depicted in  FIGS. 11A-11C  was rendered from an application solution with an optical density of 19.69 and an ammonium nitrate salt concentration of 1.5 ppm. These parameters resulted in a high degree of rafting within the nanotube fabric layer ( 1101 ,  1102 ,  1103 ). Analysis of the entire nanotube fabric layer showed that approximately 37.8% of the fabric (by area) was comprised of rafted bundles of nanotube elements. These rafted bundles of nanotube elements are evident in  FIG. 11A  ( 1110 ),  FIG. 11B  ( 1120 ), and  FIG. 11C  ( 1130 ). 
     Example 5 
       FIGS. 12A-12C  are SEM images of an exemplary nanotube fabric layer at different magnifications ( 1201 ,  1202 , and  1203  respectively) prepared according to the methods of the present disclosure. The nanotube fabric layer depicted in  FIGS. 12A-12C  was rendered from an application solution with an optical density of 19.71 and an ammonium nitrate salt concentration of 25 ppm. These parameters resulted in substantially no rafting within the nanotube fabric layer ( 1201 ,  1202 ,  1203 ). Analysis of the entire nanotube fabric layer showed that the fabric layer was substantially free of rafted bundles of nanotube elements. 
     Example 6 
       FIGS. 13A-13C  are SEM images of an exemplary nanotube fabric layer at different magnifications ( 1301 ,  1302 , and  1303  respectively) prepared according to the methods of the present disclosure. The nanotube fabric layer depicted in  FIGS. 13A-13C  was rendered from an application solution with an optical density of 10.02 and an ammonium nitrate salt concentration of 27 ppm. These parameters resulted in substantially no rafting within the nanotube fabric layer ( 1301 ,  1302 ,  1303 ). Analysis of the entire nanotube fabric layer showed that the fabric layer was substantially free of rafted bundles of nanotube elements. 
     Example 7 
       FIGS. 14A-14C  are SEM images of an exemplary nanotube fabric layer at different magnifications ( 1401 ,  1402 , and  1403  respectively) prepared according to the methods of the present disclosure. The nanotube fabric layer depicted in  FIGS. 14A-14C  was rendered from an application solution with an optical density of 9.4 and an ammonium nitrate salt concentration of 2.5 ppm. These parameters resulted in a moderate degree of rafting within the nanotube fabric layer ( 1401 ,  1402 ,  1403 ). Analysis of the entire nanotube fabric layer showed that approximately 13.1% of the fabric (by area) was comprised of rafted bundles of nanotube elements. These rafted bundles of nanotube elements are evident in  FIG. 14A  ( 1410 ),  FIG. 14B  ( 1420 ), and  FIG. 14C  ( 1430 ). 
     Example 8 
       FIGS. 15A-15C  are SEM images of an exemplary nanotube fabric layer at different magnifications ( 1501 ,  1502 , and  1503  respectively) prepared according to the methods of the present disclosure. The nanotube fabric layer depicted in  FIGS. 15A-15C  was rendered from an application solution with an optical density of 33.9 and an ammonium nitrate salt concentration of 33 ppm. These parameters resulted in a moderate degree of rafting within the nanotube fabric layer ( 1501 ,  1502 ,  1503 ). Analysis of the entire nanotube fabric layer showed that approximately 10.0% of the fabric (by area) was comprised of rafted bundles of nanotube elements. These rafted bundles of nanotube elements are evident in  FIG. 15A  ( 1510 ),  FIG. 15B  ( 1520 ), and  FIG. 15C  ( 1530 ). 
     Example 9 
       FIGS. 16A-16C  are SEM images of an exemplary nanotube fabric layer at different magnifications ( 1601 ,  1602 , and  1603  respectively) prepared according to the methods of the present disclosure. The nanotube fabric layer depicted in  FIGS. 16A-16C  was rendered from an application solution with an optical density of 33.9 and an ammonium nitrate salt concentration of 7.5 ppm. These parameters resulted in a high degree of rafting within the nanotube fabric layer ( 1601 ,  1602 ,  1603 ). Analysis of the entire nanotube fabric layer showed that approximately 35.0% of the fabric (by area) was comprised of rafted bundles of nanotube elements. These rafted bundles of nanotube elements are evident in  FIG. 16A  ( 1610 ),  FIG. 16B  ( 1620 ), and  FIG. 16C  ( 1630 ). 
     We have described multiple techniques to control the porosity and/or density of a nanotube fabric. The techniques also can be said to control the positioning of the nanotubes within the fabric, to control the positions of gaps within the nanotube fabric, and to control the concentration of the nanotubes within the fabric. For example, these techniques can provide low porosity, high density fabrics. Further, the techniques can be described as controlling the gaps of nanotubes within the nanotube fabric. Thus, we have disclosed techniques to create devices sized to and smaller than the current lithography limits (for example, less than or equal to about 20 nm). Low porosity, high density fabrics also can be created by, for example, filling gaps in the nanotube film with additional nanotube elements. In other embodiments, a high density fabric can be created by applying a physical force to the fabric. Further, the density or porosity of the fabric can be controlled to create low density and highly porous nanotube fabrics, if so desired. 
     Further, the methods of the present disclosure are useful for any application using nanotube fabrics wherein the concentration of the individual nanotube elements within the fabric or the dimensions of gaps within the fabric are required to fit within a preselected tolerance. 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention not be limited by the specific disclosure herein.