Nanofiber structure, composite structure, method for forming a nanofiber film disk, and method for forming a conductive structure

Abstract

Arrays of discrete nanofiber films that are physically separated from one another are described. Techniques for forming nanofiber film arrays are also described. The technique for forming these structures involves placing a suspension of nanofibers and / or nanoparticles on a patterned substrate. A pressure differential is applied, drawing the solvent of the suspension through holes in the mask. The nanofibers collect in the impermeable layer of the mask in a pattern that corresponds to the negative features of the mask (grooves, holes, and channels) as the solvent flows through the holes.

Description

Technical Field

[0001] Related Applications This application claims priority to U.S. Provisional Patent Application No. 62 / 925,519, filed Oct. 24, 2019, and titled “PATTERNED NANOFIBER ARRAYS,” and U.S. Provisional Patent Application No. 62 / 962,532, filed Jan. 17, 2020, and titled “PATTERNED NANOFIBER ARRAYS ASSEMBLED THROUGH PATTERNED FILTRATION,” under 35 U.S.C. § 119(e), the entire disclosures of which are incorporated herein by reference in their entireties.

[0002] The present disclosure generally relates to nanofibers. Specifically, the present disclosure relates to techniques for forming patterned nanofiber arrays.

Background Art

[0003] Nanofibers are known to have disparate mechanical, optical, and electronic properties. However, due to the nanoscale dimensions of nanofibers, it has been difficult to devise configurations of nanofibers that can be integrated into commercially available products. An example of progress in the development of commercially useful embodiments of nanofibers is the production of “forests” of nanofibers. Such forests are arrays of parallel nanofibers grown perpendicular to a substrate surface. The forests can be drawn from the substrate into nanofiber sheets, where the nanofibers are parallel to each other within the plane of the sheet. The nanofiber sheets can then optionally be formed into nanofiber yarns.

Summary of the Invention

[0004] In a first example, a method of forming a plurality of discrete nanofiber films on a substrate includes providing a filter assembly having a mask and a porous layer on an opposite side of the mask, the filter assembly including a membrane adjacent to the mask, the mask defining a plurality of holes; disposing a nanofiber suspension including nanofibers suspended in a solvent on the membrane of the filter assembly; applying a pressure differential to the filter assembly, the pressure differential flowing the solvent through a hole of the plurality of holes of the mask; and forming a nanofiber structure on the membrane at a position corresponding to a hole of the plurality of holes in response to flowing the solvent through the hole. The membrane can be permeable to the solvent and not permeable to the nanofibers suspended in the solvent. The plurality of holes can form a regular pattern of geometric shapes, the nanofiber structure can include discrete nanofiber films, and each nanofiber film can include a plurality of nanofibers randomly oriented relative to each other within a plane of the nanofiber film. The nanofiber film can include conductive nanoparticles, and the method can include removing the nanofiber structure from the membrane and disposing the nanofiber structure on a second substrate.

[0005] In a second example, a method of forming a plurality of discrete conductive structures on a nanofiber substrate includes providing a filter assembly having a mask and a porous layer on the opposite side of the mask, the mask defining a plurality of holes; placing the nanofiber substrate on the mask; disposing a suspension of conductive nanoparticles in a solvent in fluid communication with the nanofiber substrate; applying a pressure differential to the filter assembly, the pressure differential forcing the solvent through holes among the plurality of holes of the mask; and forming conductive structures on the nanofiber substrate in a pattern corresponding to the holes among the plurality of holes. The nanofiber substrate can be a filtered nanofiber film including a plurality of nanofibers randomly oriented relative to each other within the plane of the filtered nanofiber film. The nanofiber substrate can be a drawn nanofiber sheet including a plurality of nanofibers aligned end-to-end within the plane of the drawn nanofiber sheet. The conductive nanoparticles can include silver (Ag) nanowires. The plurality of holes can form a regular pattern of geometric shapes, and the conductive structures can be a regular pattern of geometric shapes corresponding to the plurality of holes. The method can include removing the nanofiber substrate by immersing the filter assembly and the nanofiber substrate thereon in water, and can further include placing the nanofiber substrate and the conductive structures thereon on a final substrate.

[0006] In another example, a nanofiber structure includes a plurality of nanofiber films on a substrate, the nanofiber films including a plurality of nanofibers randomly oriented relative to each other within the plane of the film. The nanofiber films can be homogeneous or heterogeneous, can be discrete and separated from each other, and / or can form a regular array of films. At least one of the plurality of nanofiber films can be a geometric shape.

[0007] In another example, the composite structure includes a nanofiber substrate and a plurality of conductive structures within an array on the nanofiber substrate. The nanofiber substrate can be a drawn nanofiber sheet that includes a plurality of nanofibers aligned end-to-end within the plane of the drawn nanofiber sheet. It can be a filtered nanofiber film that includes a plurality of nanofibers randomly oriented with respect to each other within the plane of the filtered nanofiber film. The array can be in a regular pattern of geometric shapes.

[0008] In another example, the method includes assembling a filter stack that includes a porous layer and a mask on the porous layer, where the mask includes holes and / or a pattern of negative shapes below the surface level of the mask; fixing the porous layer; applying a pressure differential across the filter stack such that the pressure is higher above the stack than below the stack; disposing a suspension of a solvent and one or more of nanofibers and conductive nanoparticles on the filter stack; causing the solvent to flow through the holes of the mask in response to the applied pressure differential; and filtering out and removing an array of one or more of nanofibers and conductive nanoparticles, where the array forms one or more layers corresponding to the holes and / or the pattern of negative shapes of the mask. The method can include disposing a nanofiber sheet or nanofiber film on the mask and disposing the suspension in contact with the nanofiber sheet or nanofiber film, where one or more of nanofibers and conductive nanoparticles are formed in holes and / or a pattern of negative shapes on the nanofiber sheet. The pressure differential can be applied by reducing the pressure below the filter stack relative to the pressure above the filter stack or by increasing the pressure above the filter stack relative to the pressure below the filter stack.

[0009] Another example is a method that includes flowing a fluid component of a nanofiber suspension through a membrane, a mask, and a porous layer to separate the fluid component from the nanofibers, forming a nanofiber film by dispersing the nanofibers on the membrane in a pattern corresponding to the pattern of holes in the mask, and separating the nanofiber film from the membrane.

[0010] Another example is a method of forming a conductive structure on a nanofiber membrane, the method including flowing a fluid component of a suspension of conductive nanoparticles through the nanofiber membrane, a mask, and a porous layer, and separating the conductive nanoparticles from the fluid component to form a pattern of the conductive structure on the nanofiber membrane. The fluid component can flow through the structures in the order of the membrane, the mask, and then the porous layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0011]

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[0012] The figures show various embodiments of the present disclosure for illustrative purposes only. Many variations, configurations, and other embodiments will become apparent from the following detailed description. Further, as will be understood, the drawings are not necessarily drawn to scale and are not intended to be limited to the specific configurations shown in the described embodiments. For example, although some of the figures generally show straight lines, right angles, and smooth surfaces, the actual implementations of the disclosed technology may have non-perfect straight lines and right angles, and some features may have surface topography or otherwise be non-smooth, considering the real-world limitations of the manufacturing process. Briefly, the figures are provided merely to illustrate exemplary structures.

[0013] Overview Nanofibers often have unique and interesting properties that are not present in similarly constructed bulk materials. However, due to the nanoscale dimensions of individual nanofibers, some nanofiber-based materials can be difficult to handle. For example, carbon nanofiber sheets have many interesting properties, but are physically delicate and can be torn, bent, or damaged by even the slightest forces during processing. Air currents caused by laboratory air handling equipment or the operator's breathing can damage the nanofiber sheet. Due to this physically delicate nature, some development efforts have focused not only on investigating and applying the unusual properties of nanofiber materials, but also on improving the handling of these materials.

[0014] The techniques described herein include a liquid-phase method for forming a plurality of discrete nanofiber structures (equivalently referred to as an "array") separated from each other on a substrate. Examples of nanofiber structures include a filtered film in which individual nanofibers are randomly oriented relative to each other within the plane of the film. The techniques described herein can also include a liquid-phase method for forming an array of other structures (e.g., silver nanowire dots or strips) on a nanofiber film.

[0015] Before describing these arrays and methods of making the arrays, nanofibers, nanofiber-filtered films, nanofiber forests, and sheets are described.

[0016] Nanofiber forest As used herein, the term "nanofiber" means a fiber having a diameter of less than 1 μm. While embodiments herein are primarily described as being made from carbon nanotubes, other carbon allotropes (regardless of graphene, micron or nano-sized graphite fibers and / or plates) as well as other compositions of nano-sized fibers (e.g., boron nitride) may be processed using the techniques described hereinafter. As used herein, the terms "nanofiber" and "nanotube" are used interchangeably and include both single-walled nanotubes, few-walled nanotubes and / or multi-walled carbon nanotubes in which atoms are bonded to each other to form a cylindrical structure. In some embodiments, the multi-walled nanotubes referred to herein have from 6 to 20 layers. As used herein, "nanofiber sheet" or simply "sheet" refers to a sheet of nanofibers aligned by a drawing process (described in PCT Publication No. WO2007 / 015710, which is hereby incorporated by reference in its entirety), so that the longitudinal axis of the nanofibers of the sheet is parallel to the major surface of the sheet rather than perpendicular to the major surface of the sheet (i.e., in many cases the sheet, often referred to as a "forest", remains in the as-deposited form). This is illustrated and shown in FIGS. 3 and 4, respectively.

[0017] The dimensions of the nanotubes can vary widely depending on the manufacturing method used. For example, the diameter of carbon nanotubes can be in the range of 0.4 nm to 100 nm, and their length can range from 10 μm to over 55.5 cm. Also, carbon nanotubes can have a very high aspect ratio (length to diameter ratio) of 132,000,000:1 or higher. Considering the various possible dimensions, the properties of carbon nanotubes are highly adjustable or "tunable". Many interesting properties of carbon nanotubes have been identified, but to utilize the properties of carbon nanotubes in practical applications, a scalable and controllable manufacturing method is needed that allows the characteristics of the carbon nanotubes to be maintained or enhanced.

[0018] Due to their unique structures, nanotubes have specific mechanical, electrical, chemical, thermal, and optical properties that make carbon nanotubes suitable for specific applications. In particular, carbon nanotubes exhibit excellent electrical conductivity, high mechanical strength, good thermal stability, and are also hydrophobic. In addition to these properties, carbon nanotubes can also exhibit useful optical properties. For example, carbon nanotubes can be used in light-emitting diodes (LEDs) and photodetectors to emit or detect light at narrowly selected wavelengths. Also, carbon nanotubes may prove useful for photon transport and / or phonon transport.

[0019] According to various embodiments of the present disclosure, nanofibers (including, but not limited to, carbon nanotubes) can be arranged in various configurations including a configuration referred to herein as a "forest". As used herein, a "forest" of nanofibers or carbon nanotubes refers to an array of nanofibers having substantially equivalent dimensions that are arranged substantially parallel to each other on a substrate. FIG. 1 shows an exemplary forest of nanofibers on a substrate. The substrate can be of any shape, but in some embodiments, the substrate has a plane on which the forest converges. As can be seen in FIG. 1, the nanofibers in the forest can have substantially equal heights and / or diameters.

[0020] Nanofiber forests as disclosed herein can be relatively high density. Specifically, the disclosed nanofiber forests can have a density of at least 1 billion nanofibers / cm 2 . In some specific embodiments, nanofiber forests as described herein can have a density between 10 billion / cm 2 and 30 billion / cm 2 . In other examples, nanofiber forests as described herein can have a density of 90 billion nanofibers / cm 2It can have a density within a range. This forest can include high-density or low-density regions, and certain regions may have no nanofibers. Also, the nanofibers within the forest can exhibit fiber-to-fiber connectivity. For example, adjacent nanofibers within the nanofiber forest can be attracted to each other by van der Waals forces. In any case, by applying the techniques described herein, the density of the nanofibers within the forest can be increased.

[0021] A method for manufacturing a nanofiber forest is described, for example, in PCT No. WO2007 / 015710, which is hereby incorporated by reference in its entirety.

[0022] A variety of methods can be used to generate a nanofiber precursor forest. For example, in some embodiments, the nanofibers can grow within a high-temperature furnace schematically shown in FIG. 2. In some embodiments, the catalyst can be deposited on a substrate, placed in a reactor, and then exposed to a fuel compound supplied to the reactor. The substrate can withstand temperatures of 800 °C or even higher than 1000 °C and can be an inert material. Si wafers (e.g., alumina, zirconia, SiO 2Although other ceramic substrates may be used instead of (glass ceramics), the substrate may include stainless steel or aluminum disposed on a silicon (Si) wafer below. In an example where the nanofibers of the precursor forest are carbon nanotubes, a carbon-based compound such as acetylene may be used as the fuel compound. After being introduced into the reactor, the fuel compound(s) may then begin to accumulate on the catalyst and aggregate by growing upward from the substrate to form a forest of nanofibers. The reactor may also include a gas inlet through which the fuel compound(s) and carrier gas may be supplied to the reactor and a gas outlet through which the consumed fuel compound and carrier gas may be discharged from the reactor. Examples of carrier gases include hydrogen, argon, and helium. These gases (especially hydrogen) can also be introduced into the reactor to promote the growth of the nanofiber forest. In addition, dopants to be incorporated into the nanofibers can be added to the gas stream.

[0023] In the process used to manufacture a multi-layer nanofiber forest, one nanofiber forest is formed on a substrate, and then the growth of a second nanofiber forest that contacts the first nanofiber forest continues. The multi-layer nanofiber forest can be formed by many suitable methods, such as forming the first nanofiber forest on a substrate, depositing a catalyst on the first nanofiber forest, and then introducing an additional fuel compound into the reactor to promote the growth of the second nanofiber forest from the catalyst positioned on the first nanofiber forest. Depending on the growth method applied, the type of catalyst, and the location of the catalyst, the second nanofiber layer can grow on top of the first nanofiber layer or, for example, after refreshing the catalyst with hydrogen gas or the like, can grow directly on the substrate and grow under the first nanofiber layer. In any case, the second nanofiber forest can be aligned almost end-to-end with the nanofibers of the first nanofiber forest, but there is an interface that is easily detectable between the first forest and the second forest. The multi-layer nanofiber forest can include any number of forests. For example, the multi-layer precursor forest can include 2, 3, 4, 5, or more forests.

[0024] Nanofiber sheet In addition to being arranged in a forest configuration, the nanofibers of the present application can also be arranged in a sheet configuration. As used herein, the terms "nanofiber sheet", "nanotube sheet", or simply "sheet" refer to an arrangement of nanofibers in which the nanofibers are aligned end-to-end in a plane. An exemplary figure of a nanofiber sheet is shown in FIG. 3 with dimensional labels. In some embodiments, the sheet has a length and / or width that is more than 100 times greater than the thickness of the sheet. In some embodiments, the length, width, or both are 10 3 times, 10 6 times, or 10 9It is extremely large. The nanofiber sheet can have a thickness, for example, between about 5 nm and 30 μm, as well as any length and width suitable for the intended application. In some embodiments, the nanofiber sheet can have a length between 1 cm and 10 meters, and a width between 1 cm and 1 meter. These lengths are provided for illustrative purposes only. The length and width of the nanofiber sheet are not determined by any physical or chemical properties of the nanotubes, forest, or nanofiber sheet itself, but are restricted by the configuration of the manufacturing equipment. For example, a continuous process can produce sheets of any length. These sheets can be wound onto rolls during production.

[0025] As shown in FIG. 3, the axis along which the nanofibers are aligned from end to end is referred to as the direction of nanofiber alignment. In some embodiments, the direction of nanofiber alignment can be continuous across the entire nanofiber sheet. The nanofibers are not necessarily perfectly parallel to each other, and it is understood that the direction of nanofiber alignment is an average or general measure of the alignment direction of the nanofibers.

[0026] The nanofiber sheet can be assembled using any suitable process of any type that can produce the sheet. In some exemplary embodiments, the nanofiber sheet can be drawn from a nanofiber forest. An example of a nanofiber sheet drawn from a nanofiber forest is shown in FIG. 4.

[0027] As shown in FIG. 4, the nanofibers can be drawn laterally from the forest and then aligned end-to-end to form a nanofiber sheet. In embodiments where the nanofiber sheet is drawn from the nanofiber forest, the dimensions of the forest can be controlled to form a nanofiber sheet having a specific dimension. For example, the width of the nanofiber sheet may be approximately equal to the width of the nanofiber forest from which the sheet is drawn. Further, the length of the sheet can be controlled, for example, by ending the drawing process when the desired sheet length is achieved.

[0028] The nanofiber sheet has many properties that can be utilized for various applications. For example, the nanofiber sheet can have adjustable opacity, high mechanical strength and flexibility, thermal conductivity and electrical conductivity, and can also exhibit hydrophobicity. Considering the high degree of alignment of the nanofibers within the sheet, the nanofiber sheet can be extremely thin. In some examples, the nanofiber sheet is on the order of about 10 nm thick (when measured within normal measurement tolerances), and its thickness is represented almost two-dimensionally. In other examples, the thickness of the nanofiber sheet can be 200 nm or 300 nm. Therefore, the nanofiber sheet can add a minimal additional thickness to the component.

[0029] Similar to the nanofiber forest, the nanofibers in the nanofiber sheet can be made to function as a treatment agent by adding chemical groups or elements to the surface of the nanofibers of the sheet, which provides a different chemical activity from the nanofibers alone. The functionalization of the nanofiber sheet can be carried out with nanofibers that have been previously functionalized or can be carried out with nanofibers that have not been previously functionalized. The functionalization can be carried out using any of the techniques described herein, including but not limited to CVD and various doping techniques.

[0030] In addition, the nanofiber sheet can have high purity when drawn from the nanofiber forest. In some examples, more than 90%, more than 95%, or more than 99% of the weight percentage of the nanofiber sheet is attributable to the nanofibers. Similarly, the nanofiber sheet can contain more than 90 wt%, more than 95 wt%, more than 99 wt% or more than 99.9 wt% carbon.

[0031] Filtered nanofiber film Another planar form of the assembled nanofibers is a "filtered film" in which one or more multi-walled nanotubes, few-walled nanotubes, and / or single-walled nanotubes are dispersed in a solvent (i.e., most of the nanotubes are individually suspended and not adsorbed to other nanotubes), and then formed into a sheet of randomly oriented carbon nanotubes. This ability of individual nanotubes to disperse uniformly in a solvent can then be used to produce dimensionally uniform (i.e., uniform thickness) nanotube films by removing the solvent from the suspended nanofibers. This physical uniformity (which is further improved by stacking multiple filtered films on top of each other) can also improve the uniformity of the properties of the entire film (e.g., permeability to radiation).

[0032] For clarity, multi-walled nanotubes are considered to have 4 to 20 concentric layers and a diameter of 4 nm to 100 nm, few-walled nanotubes are considered to have 2 to 3 concentric layers and a diameter of 2 nm to 6 nm, and single-walled carbon nanotubes are considered to have 1 layer and a tube diameter of 0.2 nm to 4 nm.

[0033] These three different types of nanotubes can each have different properties. In one example, few-layer carbon nanotubes and single-layer carbon nanotubes can be more conveniently dispersed in a solvent (i.e., most of the nanotubes are individually suspended and not adsorbed to other nanotubes), and then formed into a sheet of randomly oriented carbon nanotubes. This ability of individual nanotubes to disperse uniformly in a solvent can then be used to produce dimensionally uniform nanotube-filtered films formed by removing the solvent from the suspended nanofibers. The strength of the van der Waals attractive forces between the nanofibers also differs between single-layer / few-layer nanofibers and multi-layer nanofibers. Generally, single-layer / few-layer nanofibers have a greater van der Waals attractive force between each other than that observed in multi-layer nanofibers. This increase in the attractive force between single-layer / few-layer nanofibers can improve the ability of single-layer / few-layer carbon nanotubes to adhere to each other and form a coherent nanofiber structure such as a filtered film. Sheets or films formed from single-layer carbon nanotubes and few-layer carbon nanotubes can conform to the topography of the underlying surface in smaller dimensions than sheets or films formed from multi-layer carbon nanotubes. In some examples, sheets or films formed from single-layer carbon nanotubes and / or few-layer carbon nanotubes can conform to a small underlying substrate topography of 10 nm that is at least 50% smaller than the feature size to which a multi-layer carbon nanotube film can conform. In some cases, multi-layer carbon nanotubes produce a structurally non-uniform film where the single-layer / few-layer nanotubes aggregate together, thereby being less likely to conform and / or adhere to the underlying surface.

[0034] The preparation of the filtered film can begin by preparing a dry mixture of one or more multi-walled nanotubes, few-walled nanotubes, and / or single-walled nanotubes in the desired proportions. Next, one or more of this mixture of different types of nanotubes can be suspended in a solvent. In another example, separate suspensions of nanotubes at known concentrations in a solvent are prepared. For example, separate suspensions of multi-walled carbon nanotubes, few-walled carbon nanotubes, and single-walled carbon nanotubes can be prepared. Next, the suspensions can be mixed in the desired ratios to reach the desired relative ratios of multi-walled and single-wall / few-walled nanotubes in the combined suspension and ultimately in the final filtered film.

[0035] In some examples, the solvents used to prepare the nanotube suspensions can include water, isopropyl alcohol (IPA), N-methyl-2-pyrrolidone (NMP), dimethyl sulfide (DMS), and combinations thereof. In some examples, surfactants can also be included to aid in the uniform dispersion of carbon nanofibers in the solvent. Examples of surfactants include, but are not limited to, sodium cholate, sodium dodecyl sulfate (SDS), and sodium dodecylbenzene sulfonate (SDBS). The weight percent of surfactant in the solvent can be any effective concentration, for example, from 0.1 wt% to 10 wt% of the solvent. In one set of embodiments, a mixture of multi-walled carbon nanotubes and few-wall / single-wall carbon nanotubes can be prepared and suspended in water and an SDS surfactant. The ratio of multi-walled carbon nanotubes to single-wall (or few-wall) carbon nanotubes by weight can vary from 100% multi-walled to 100% single-wall, including 0% multi-walled, 20% multi-walled, 40% multi-walled, 60% multi-walled, or 80% multi-walled, and all ratios in between. The mixing and suspension of the nanotubes in the solvent can include mechanical mixing (e.g., using a magnetic stir bar and stir plate), ultrasonic agitation (e.g., using an immersion ultrasonic probe), or other means.

[0036] As described above, the examples described in this specification include nanofiber films that can be formed from one type of nanofiber (e.g., single layer, few layer, multi-layer) or a combination of these different types of nanofibers. Examples composed of multiple types of nanofibers can be described as "composite films" by combinations or mixtures of various types of nanofibers. In some examples of this specification, multi-walled carbon nanotubes can have a median length of about 300 μm (+ / - 10%). As will be understood in light of the following description, multi-walled carbon nanotubes having a length of at least 250 μm or more can be included in the filtered film, and generally short (e.g., 0.5 μm to 30 μm) single layer and / or few layer carbon nanotubes can also be included to improve the mechanical stability of the filtered film. Films formed from only either longer multi-walled nanotubes or shorter single layer / few layer carbon nanotubes generally have less durability (i.e., are less resistant to mechanical failures such as cracking and disintegration) than those containing a mixture of multi-layer and single layer / few layer carbon nanotubes.

[0037] FIG. 5 is a schematic diagram of a composite nanotube filtered film 500 in an example of the present disclosure. As shown, the composite nanotube filtered film 500 includes single-layer / few-layer nanotubes 504 interdispersed with multi-layer carbon nanotubes 508. In this exemplary film 500, the single-layer / few-layer carbon nanotubes 504 can generally provide at least two beneficial effects to the structure of the film 500. For example, the single-layer / few-layer carbon nanotubes 508 can increase the number of indirect connections between adjacent multi-layer carbon nanotubes 508 by filling the gaps between adjacent multi-layer carbon nanotubes 504. The interconnectivity between short nanofibers and long nanofibers can improve the transmission and dispersion of forces applied to the film, and thus improve durability. In a second example of a beneficial effect, the single-layer / few-layer carbon nanotubes 504 can reduce the median and / or average size of the gaps between adjacent and / or overlapping multi-layer carbon nanotubes 508, which can be advantageous for some embodiments. Additionally, if there are too many longer multi-layer carbon nanotubes, they may aggregate when dispersed in a solvent. This can cause the film to become non-uniform. Shorter nanotubes are more easily dispersed in a solvent and thus are more likely to form a dimensionally uniform film with a uniform density of nanotubes per unit volume or area.

[0038] Properties of Nanofiber Filtered Films Filtered films, particularly films made of single-layer and / or few-layer carbon nanotubes, also generally have higher permeability to radiation of several wavelengths. In some examples, the transmittance of the incident radiation can reach 90% or 97% at 550 nm. In some cases, this transmittance is significantly higher than that of a drawn sheet of multi-layer carbon nanotubes (such as a sheet drawn from a carbon nanotube forest described below). Without wishing to be bound by theory, the aligned orientation of the nanotubes in the drawn sheet is thought to increase the scattering of radiation compared to the filtered film. In part, due to the high permeability of the filtered film (comprising randomly oriented nanotubes), there is interest in forming transparent filters and pellicles from the filtered carbon nanotube film for various applications.

[0039] Despite the advantages of the above monolayer and few-layer carbon nanotubes, multi-layer carbon nanotubes also have advantages that are not necessarily observed to the same extent in nanotube structures formed from single-layer or few-layer nanotubes. For example, structures formed from multi-layer carbon nanotubes have generally been observed to have a higher emissivity than structures formed from few-layer / single-layer carbon nanotubes. Without wishing to be bound by theory, it is thought that the large number of layers and the large diameter of the multi-layer carbon nanotubes are factors contributing to the increase in emissivity. For example, multi-layer carbon nanotube structures (e.g., nanotube forests, nanotube sheets) have a higher thermal emissivity than nanotube structures formed from single-layer / few-layer nanotubes. In one comparative example, the emissivity of a nanofiber structure containing multi-layer carbon nanotubes is on the order of 0.275 (+ / - 15%), while a nanofiber structure containing single-layer carbon nanotubes can have a significantly lower emissivity of 0.05 (+ / - 15%). The high emissivity can be particularly advantageous in technical applications where the process can cause heating within the nanofiber structure and the mechanisms of conductive or convective cooling of the nanofiber structure are limited or not technically feasible.

[0040] Nanofiber structures that are transmissive to radiation of a specific wavelength (e.g., extreme ultraviolet or "EUV" in the range from 10 nm to 124 nm) are expected to be used as filters (also called "pellicles") in EUV lithography devices. The pellicle can function as a particle filter, preventing foreign objects from adhering to the surface of the patterned material and / or preventing foreign objects from adhering to the surface of the lithography mask used to pattern the photoactive surface. This reduces the proportion of defects introduced by lithography and improves the manufacturing yield of the patterned device.

[0041] Despite the high transmissivity in the EUV radiation wavelength range, there are still challenges in the adoption of nanofiber EUV pellicles. For example, cooling of the nanofiber pellicle can be important to prevent overheating of the pellicle due to absorption of EUV energy during lithography patterning. As the temperature of the pellicle rises, the integrity of the nanofiber structure can degrade, and since EUV lithography is performed in a vacuum and the pellicle is mostly suspended (with the peripheral edges attached to a frame), the opportunity for convective or conductive cooling of the nanofiber structure in this environment is reduced. For this reason, thermal radiation is a preferred mechanism for cooling the nanofiber pellicles used in EUV applications.

[0042] Multi-layer carbon nanotube structures generally have high emissivity and address the problem of cooling of EUV pellicles, but multi-layer carbon nanotubes also become less transmissive than single-layer / few-layer carbon nanofibers of randomly oriented filtered films when aligned in a drawn sheet. The more transparent (but less emissive) few-layer / single-layer nanofiber films are often too mechanically delicate to be used as pellicles. In some cases, films and sheets made of few-layer / single-layer nanofibers are brittle because of their relatively short length (e.g., less than 100 μm) and can collapse when exposed to the pressure cycles commonly used in EUV lithography equipment (e.g., changes from + / −1 atmosphere to 2 atmospheres (from atmosphere to vacuum)).

[0043] Patterned nanofiber array The foregoing configuration of the nanofibers generally has a planar continuous configuration (sheet or forest), but the embodiments described below use the solvent-based techniques above to form a patterned array of discrete nanofiber-filtered films (of any of a variety of shapes and patterns) on a substrate. For example, discrete circular, square, linear, columnar, or other nanofiber-filtered film patterns separated from each other can be formed on a substrate. In some examples, the resulting array is conductive and / or transmissive to extreme UV radiation, infrared radiation, or other wavelengths within the optical spectrum. This patterned array of nanofiber structures can be transferred to another substrate if desired.

[0044] FIG. 6 shows a cross-sectional view of one embodiment of a patterned nanofiber array 600 and a processing structure 604 on which the patterned array is formed. The patterned nanofiber array 600 of this cross-sectional view includes nanofiber film disks 602A and 602B. The processing structure 604 includes a filter assembly 606 and a suction fitting 608.

[0045] At a high level, the technique for forming the patterned nanofiber array 600 is initiated, as shown in FIG. 6, by placing the filter assembly 606 on the suction fitting 608. Next, a pressure difference can be applied across the membrane 624, with the pressure being greater on the upper side than the lower side. For example, a negative pressure can be applied to the filter assembly 606 via the suction fitting 608 through a vacuum pump or other similar device. A nanofiber suspension (including one or more types of nanofibers as described above) can be applied to the exposed surface of the filter assembly 606. The negative pressure applied to the surface of the filter assembly 606 on the opposite side of the applied nanofiber suspension can draw the suspension towards the holes in one layer of the filter assembly. The solvent can pass through the various layers of the filter assembly, but the nanofibers cannot, so the nanofibers remain on the exposed surface of the filter assembly 606 in a pattern. A more detailed description of the process and the various elements shown in FIG. 6 continues.

[0046] The filter assembly 606 includes a support frame 612, a porous layer 616, a mask 620, and a membrane 624.

[0047] The porous layer 616 is composed of one or more porous materials (i.e., materials through which fluid can pass) that can be in contact with one side of the mask 620 and held between the mask 620 and a negative pressure source (e.g., a vacuum pump). The porosity and gas / fluid permeation characteristics of the porous layer 616 help provide a more uniform distribution of negative pressure across the entire area of the mask 620. Examples of porous and / or gas / fluid permeable materials that can be used to form the porous layer 616 include, among others, woven fabrics (e.g., nylon woven fabric, acrylic woven fabric, synthetic fiber woven fabric, natural fiber woven fabric), non-woven fabrics (felt, wool, cotton), extruded woven fabrics with a permeable barrier layer (e.g., PTFE such as GORETEX®), sintered glass, sintered stainless steel, glass frit, etc., but are not limited thereto.

[0048] The support frame 612 holds the porous layer 616 in contact with the mask 620 and is used to prevent the porous layer 616 from separating from the filter assembly 606 during the application of negative pressure. If the porous layer has sufficient rigidity, it can serve both the purposes of the porous layer and the support frame.

[0049] The mask 620 can be disposed between the porous layer 616 and the membrane 624 and on the side of the porous layer 616 opposite to the negative pressure source. The mask 620 includes a pattern of negative spaces 622 (e.g., holes) corresponding to the pattern in which the array 600 of nanofiber films is formed. Through this pattern of holes 622 with the porous layer 616 in the middle, a negative pressure is applied to the suspension of nanofibers on the exposed surface of the filter assembly 606. The mask 620 can be made of a polymer film in which the pattern of holes 622 is formed. The mask 620 can also be formed from glass or woven fabric, and either of these can be processed to include the holes 622. Other materials that can be used for the mask 620 will be understood in light of the present disclosure.

[0050] The membrane 624 is a filtration structure that allows the solvent (used to suspend the nanofibers) to be drawn into the holes 622 of the mask 620 in response to the negative pressure applied via the suction fitting 608. Nanofibers that cannot pass through the membrane 624 accumulate on the surface of the membrane 624 at positions corresponding to the underlying holes 622, and thus are formed into an array of discrete nanofiber-filtered films corresponding to the pattern of holes 622 in the mask 620. Examples of materials that can be used for the membrane 624 include, but are not limited to, nitrocellulose, cellulose (i.e., filter paper), polytetrafluoroethylene (PTFE), nylon, or any other material commonly used as a filtration membrane or filter paper.

[0051] The suction fitting 608 can be any structure (e.g., a funnel) that can form a seal around or with a part of the filter assembly 606. This seal enables the negative pressure applied to the underside of the filter assembly 606 through the suction fitting 608 to draw the solvent through the filter assembly 606. As described above, this results in the formation of an array of filtered nanofiber films 600 on the exposed surface of the filter assembly 606. In some examples, the applied negative pressure is -1 atmosphere (atm) or between -0.1 atm and -1.0 atm.

[0052] The nanofiber suspension can be stirred during the process to ensure a uniform distribution of nanofibers of different sizes or properties. In other cases, the suspension can be sedimented to partially separate nanofibers of different sizes or properties. For example, denser or larger multi-layer nanofibers can accumulate near the bottom of the suspension, while single-layer or less dense nanofibers remain in the uniform suspension. This separation can be utilized to deposit primarily a layer of multi-wall nanotubes first and then primarily a layer of few-wall or single-wall nanotubes in a single process, resulting in a heterogeneous layer.

[0053] Figures 7(A)-(G) show various plan views of exemplary masks and corresponding nanofiber arrays that can be manufactured using the techniques described herein. Figure 7(A) shows a mask 704 having various circular, elliptical, and rectangular holes 708 therein. This shows that in some examples, mask 704 can include any of various hole shapes including any of regular polygons, irregular polygons, and various other shapes. Figure 7(B) shows the corresponding pattern of the filtered nanofiber film 712. Figure 7(C) shows a mask 716 having a plurality of parallel and elongated rectangular holes, each having a width β and a separation distance α between adjacent side surfaces (as shown in the enlarged inset 7(D)). The values of α and β can be anywhere between a few microns and several hundred centimeters. In some examples, the values of α and β are such that the corresponding pattern 724 of the filtered nanofiber film (having the same (+ / -5%) spacing and width as in mask 716) can be used as a diffraction grating for extreme UV radiation, infrared radiation, or radiation in the optical spectrum. Figures 7(F) and (G) show a mask 728 having a pattern of circular holes 732 within the array that forms the corresponding nanofiber film array 736.

[0054] In any of the foregoing examples, each structure of the array can have an average thickness of from 0.02 microns to 100 microns (depending on the concentration of nanofibers in the solvent and the amount of suspension flowing through the mask 620).

[0055] Once formed, an array of nanofiber films (e.g., as shown by Examples 712, 724, 736) can be placed on another (final) substrate. Examples of final substrates include, but are not limited to, nanofiber sheets, graphene sheets, polymer sheets, polymer sheets coated with an adhesive layer, silicon wafers, glass substrates (e.g., silicate glass). In some examples, the array of nanofiber films can be removed from the membrane 624 by immersing the membrane 624 in water. Since the nanofiber films are hydrophobic, they are generally lifted from the membrane 624 during immersion. Next, the final substrate can be brought into contact with the floating array, adhered to the array, and the array can be removed from the water. In other examples, the array of nanofiber films can be placed directly on the final substrate by simply placing the final substrate in direct contact with the exposed surface of the nanofiber sheet of the array. Depending on the relative attraction / adhesion between the final substrate and the sheet of the array, the sheet of the array can adhere to the final substrate and be removed from the membrane 624. In yet other embodiments, a soluble membrane can be used and removed (dissolved) from the nanofiber film by adding a suitable solvent.

[0056] Conductive Array on Nanofiber Filtered Film Figure 8 shows an alternative configuration for preparing an embodiment in which an array of discrete nanofiber / nanoparticle / microparticle regions can be formed in a pattern and / or array on a filtered nanofiber film (or alternatively, a nanofiber sheet drawn from a nanoforest of nanofibers). The processing structure 804 shown in Figure 8 includes a filter assembly 806 and a suction fixture 808. The filter assembly 806 includes a support frame 812, a porous layer 816, a filter membrane 820, and a patterned non-permeable layer 824.

[0057] The nanofiber sheet and / or nanofiber-filtered film 830 is disposed on the patterned non-permeable layer 824. The nanofiber sheet and / or filtered film 830 corresponds to the above-described embodiments and thus does not require further explanation.

[0058] The support frame 812, the porous layer 816, and the suction fixture 808 are all the same as or similar to those described above in the context of FIG. 6. Therefore, there is no need to explain these elements further.

[0059] The filter membrane 820 can be formed from a material such as a woven fabric that helps to smooth out any pattern that may be created by the rigid support frame 812. It can be similar to the porous layer 616 described above.

[0060] The patterned non-permeable layer 824 is manufactured to include holes 822 (of any shape and any pattern), such that the negative pressure applied through the suction fixture 808 can draw the solvent from the nanofiber or nanoparticle suspension through the various elements of the filter stack 806, thereby leaving the corresponding pattern of nanofibers / nanoparticles / microparticles 802A, 802B on the exposed surface of the nanofiber film / nanofiber sheet 830. The pattern of the holes 822 can be formed in the patterned non-permeable layer 824 by etching (e.g., photolithography, laser), or mechanical processing (e.g., blade, milling).

[0061] In some examples, the patterned non-permeable layer 824 can be formed from, among other things, graphene oxide, polymers, glass (e.g., borosilicate glass), sheets of silicon wafers.

[0062] Unlike the example presented above in the context of FIG. 6, the filtered nanofiber film 830 (or nanofiber sheet drawn from a nanofiber forest) is disposed on the filter stack 806. Next, a suspension of nanofibers and / or conductive nanoparticles (or both) can be placed on top of the nanofiber film 830. When a pressure differential is applied across the filter stack, the solvent is drawn through the filter stack via the holes 822. The result is an array 800 of nanofiber / nanoparticle / microparticle films 802A, 802B on the exposed surface of the nanofiber film 830 corresponding to the holes of the patterned non-permeable layer 824 and other negatively shaped patterns. These patterns can be of any shape or distribution, such as those previously described in the context of FIGS. 7(A)-(G). In some examples, the suspension can be of conductive nanoparticles such as silver (Ag) nanowires. Regardless of the composition, the array 800 can generally be referred to as an array 800 of conductive structures 802.

[0063] The limitations on the magnitude of the negative pressure are the same as those described in the context of FIG. 6.

[0064] In this example, and in FIG. 6, a positive pressure can be applied to the surface where the suspension is added (opposite the surface of the suction filter) such that the solvent is pushed through the filter stack.

[0065] In some examples, the nanofiber film 830 having a pattern of nanofibers and / or the nanoparticles 802A, 802B on the surface can be removed from the filter stack 806 by immersion in deionized water. Since the nanofiber film 830 is hydrophobic, this layer is naturally lifted from the filter stack 806 and floats on the water surface. Next, a frame can be used to lift the film from the water surface and deposit the filtered film on the frame. Optionally, the surface tension of the water (or other solvent) can be changed by adding a surfactant or other solvent. This configuration of the array of conductive structures 802A, 802B on the nanofiber film 830 on the frame (or other substrate) 834 is shown in FIG. 9. Next, the composite film can be dried (e.g., using a low humidity environment, heat, vacuum). This process can be repeated to optionally form a stack film of different compositions of multi-layer, few-layer, and / or single-layer nanotubes, as well as non-carbon materials such as metal particles.

[0066] This exemplary process can be repeated multiple times to produce multiple films of carbon nanotubes and / or multiple patterns of nanofiber arrays. In some examples, individual films (each having the same or different ratios of multi-layer and few-layer / single-layer carbon nanotubes) are stacked on top of each other to form a multi-layer composite film. Stacking two or more films can produce a more uniform stack with more uniform properties. For example, if there is a local defect (e.g., a hole or a tear) in one film within the stack, the adjacent film within the stack can provide physical continuity and uniformity of properties that would not otherwise exist at the location of the defect. In some embodiments, the stack can include any number from 2 to 10 individual films, each of which can have the same or different composition (i.e., different relative ratios of single-layer / few-layer carbon nanotubes) as the other films within the stack.

[0067] Further Considerations The foregoing description of the embodiments of the present disclosure has been presented for purposes of illustration and is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Those skilled in the art will recognize that many modifications and variations are possible in light of the above disclosure.

[0068] The words used in this specification are primarily selected for readability and for the purpose of instruction, and may not have been selected to describe or limit the subject matter of the invention. Accordingly, the scope of the present disclosure is intended to be limited not by this “Description of Embodiments of the Invention,” but rather by any claims that arise in an application based on this specification. Accordingly, the disclosure of this embodiment is intended, not as a limitation, but rather to exemplify the scope of the invention as set forth in the claims that follow.

Claims

1. comprising a plurality of nanofiber films disposed on a nanofiber filter membrane, wherein the nanofiber films include a plurality of nanofibers randomly oriented with respect to each other within the plane orientation of at least one of the nanofiber films, wherein the at least one nanofiber film has at least one geometric shape, wherein the plurality of nanofiber films form a pattern, wherein the nanofiber filter membrane is composed of a plurality of nanofibers, has solvent permeability, and has non-permeability with respect to the nanofibers of the nanofiber film, a nanofiber structure.

2. The nanofiber structure according to claim 1, wherein each of the plurality of nanofiber films is discrete and separated from each other.

3. The nanofiber structure according to claim 1, wherein the plurality of nanofiber films form a regular array.

4. The nanofiber structure according to claim 1, wherein the plurality of nanofiber films have a homogeneous shape or a heterogeneous shape.

5. The nanofiber structure according to claim 1, wherein the nanofiber filter membrane is a drawn nanofiber sheet including a plurality of aligned nanofibers, and the plurality of aligned nanofibers exist from end to end within the plane orientation of the drawn nanofiber sheet.

6. The nanofiber structure according to claim 1, wherein the nanofiber filter membrane is a filtered nanofiber film including a plurality of randomly oriented nanofibers, wherein the plurality of randomly oriented nanofibers have a plane orientation with respect to the surface of the filtered nanofiber film.

7. The nanofiber structure according to claim 1, wherein the plurality of nanofiber films further include a plurality of conductive nanoparticles.

8. comprising at least one nanofiber film disposed on a nanofiber filter membrane, wherein the at least one nanofiber film includes a plurality of nanofibers randomly oriented with respect to each other within the at least one nanofiber film, wherein the at least one nanofiber film has at least a first geometric shape, The first geometric shape includes at least one enclosed second geometric shape, and the at least one enclosed second geometric shape does not include the nanofibers. The nanofiber filter membrane includes the filtered nanofiber film including a plurality of nanofibers randomly oriented with respect to each other within the plane orientation of the filtered nanofiber film, or includes the drawn nanofiber sheet including a plurality of nanofibers aligned in the drawing direction from end to end within the plane orientation of the drawn nanofiber sheet. The nanofiber filter membrane is a nanofiber structure having solvent permeability and non-permeability to the nanofibers.

9. A composite structure including a nanofiber filter membrane and a plurality of conductive structures, wherein the nanofiber filter membrane is composed of a plurality of nanofibers, has solvent permeability, and has non-permeability to conductive nanoparticles, the plurality of conductive structures are arranged in an array on the nanofiber filter membrane, and at least one of the plurality of conductive structures has at least one geometric shape.

10. The composite structure according to claim 9, wherein the nanofiber filter membrane is the drawn nanofiber sheet including a plurality of nanofibers aligned from end to end within the plane of the drawn nanofiber sheet.

11. The composite structure according to claim 9, wherein the nanofiber filter membrane is the filtered nanofiber film including a plurality of nanofibers randomly oriented with respect to each other within the plane of the filtered nanofiber film.

12. The composite structure according to claim 9, wherein the array is a regular pattern of the at least one geometric shape.

13. A method of forming a plurality of discrete nanofiber film disks, comprising: providing a filter assembly having a membrane, a mask, and a porous layer, wherein the membrane is permeable to a solvent and impermeable to nanofibers, the mask has a plurality of holes, and is disposed in direct face-to-face contact between the membrane and the porous layer; said providing; disposing a nanofiber suspension containing nanofibers suspended in a solvent on the membrane of the filter assembly. Applying a pressure difference across the entire filter assembly, with a higher pressure above the membrane and flowing the nanofiber suspension through the filter assembly, said applying; Forming a plurality of discrete nanofiber film disks on the membrane corresponding to the plurality of holes of the mask, said nanofiber film disks including a plurality of nanofibers randomly oriented with respect to each other within the plane of the nanofiber film disk and being discrete and separated from each other, said forming, including;

14. The method according to claim 13, wherein the plurality of holes of the mask form a pattern of at least one geometric shape.

15. The method according to claim 13, further comprising conductive nanoparticles in the nanofiber film disk.

16. Removing the nanofiber film disk from the membrane; Placing the nanofiber film disk on a substrate; The method according to claim 13, further comprising.

17. The method according to claim 13, further comprising conductive nanoparticles.

18. A method of forming a plurality of discrete conductive structures on a nanofiber substrate, comprising: Providing a filter assembly having a nanofiber substrate, a mask, and a porous layer, said mask being disposed in direct face-to-face contact between said nanofiber substrate and said porous layer, said mask having a plurality of holes, said plurality of holes defining a geometric shape; said providing; Placing a suspension of conductive nanoparticles in a solvent in fluid communication with the nanofiber substrate, said filter assembly being permeable to said solvent and not permeable to said conductive nanoparticles, said placing; Applying a pressure difference across the entire filter assembly, with a higher pressure above the nanofiber substrate, said pressure difference forcing said solvent through said nanofiber substrate, said plurality of holes of said mask, and then said porous layer, said applying; Forming a conductive structure on the nanofiber substrate in a pattern corresponding to said plurality of holes of said mask, including.

19. The method according to claim 18, wherein the nanofiber substrate is the filtered nanofiber film including a plurality of nanofibers randomly oriented with respect to each other within the plane of the filtered nanofiber film.

20. The method according to claim 18, wherein the nanofiber substrate is the drawn nanofiber sheet including a plurality of nanofibers aligned in the drawing direction from end to end within the plane of the drawn nanofiber sheet.

21. The method according to claim 18, wherein the conductive nanoparticles include silver (Ag) nanowires.

22. The plurality of holes form a regular pattern of the geometric shape, The method according to claim 18, wherein the conductive structure has the regular pattern of the geometric shape corresponding to the plurality of holes.

23. The method according to claim 18, further comprising removing the conductive structure and the nanofiber substrate from the mask by immersing the conductive structure and the filter assembly thereon in water.

24. The method according to claim 23, further comprising disposing the nanofiber substrate and the conductive structure thereon on a final substrate.

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