Patent Application
20170096339
A variety of methods have been proposed and/or demonstrated to align carbon nanotubes (CNTs), through either spontaneous or stimulated alignment, including both post-growth and direct-growth schemes. However, these methods may have issues with CNT densities incompatibility with standard microfabrication technology, a lack of chirality selectivity, scaling problems, impractically large magnetic fields, low degrees of alignment, etc. Hence, the current state of the art fails to provide methods for producing large-area single-domain films of highly-aligned, densely-packed, and chirality-enriched carbon nanotubes (e.g., SWCNTs).
Additionally, one of the challenges in nanoscience and nanotechnology is the creation of macroscopic devices by assembling nano-objects while preserving their rich variety of extraordinary properties. For example, the one-dimensional character of electrons, phonons, and excitons in individual single-wall carbon nanotubes (SWCNTs) leads to extremely anisotropic electronic, thermal, and optical phenomena that have stimulated much interest in diverse disciplines. However, their macroscopic manifestations have been limited. Therefore, a need exists for ways to produce large-scale architectures of aligned rod-like nanostructures or nanotubes (e.g., SWCNTs).
In one embodiment, a method for preparing a film of aligned rod-like nanostructures or nanotubes comprises preparing a solution that comprises rod-like nanostructures or nanotubes, wherein the rod-like nanostructures or the nanotubes are well-dispersed, and performing vacuum filtration of the solution through a filtration membrane, wherein the vacuum filtration produces a film on the filtration membrane where the rod-like nanostructures or the nanotubes are aligned. In some embodiments, the well-dispersed individual rod-like nanostructures or nanotubes are separately suspended in the solution. In some embodiments, the concentration of rod-like nanostructures or nanotubes may be below a threshold value and/or the filtration speed may be as slow as possible. In embodiments where a surfactant is utilized to aid dispersion, the surfactant concentration may be below a critical micelle concentration (CMC). In some embodiments, the speed of the vacuum filtration is 1-2 mL/hour or less. In some embodiments, the drying is performed by increasing a speed of the vacuum filtration to 10 mL/hour or greater. In some embodiments, the film is a wafer scale film with an area of 1 cm2 or greater. In some embodiments, the film may be for an electronic device.
The foregoing has outlined rather broadly various features of the present disclosure in order that the detailed description that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter.
For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions to be taken in conjunction with the accompanying drawings describing specific embodiments of the disclosure, wherein:
It is to be understood that both the foregoing general description and the following detailed description are illustrative and explanatory, and are not restrictive of the subject matter, as claimed. In this application, the use of the singular includes the plural, the word “a” or “an” means “at least one”, and the use of “or” means “and/or”, unless specifically stated otherwise. Furthermore, the use of the term “including”, as well as other forms, such as “includes” and “included”, is not limiting. Also, terms such as “element” or “component” encompass both elements or components comprising one unit and elements or components that comprise more than one unit unless specifically stated otherwise.
The section headings used herein are for organizational purposes and are not to be construed as limiting the subject matter described. All documents, or portions of documents, cited in this application, including, but not limited to, patents, patent applications, articles, books, and treatises, are hereby expressly incorporated herein by reference in their entirety for any purpose. In the event that one or more of the incorporated literature and similar materials defines a term in a manner that contradicts the definition of that term in this application, this application controls.
In some embodiments, methods of fabricating films of aligned rod-like nanostructures or nanotubes are performed on a large scale. In some embodiments, the methods include a step of dispersing carbon nanotubes to form a suspension (e.g., a solution) of well-dispersed rod-like nanostructures or nanotubes. Thereafter, the well-dispersed rod-like nanostructures or nanotubes are filtered onto a membrane (e.g., through vacuum filtration) to result in the formation of an aligned rod-like nanostructures or nanotube film on the membrane. In some embodiments, the rod-like nanostructures or nanotube film may be transferred onto a substrate. In some embodiments, the rod-like nanostructures or nanotube film may be patterned. In some embodiments, the rod-like nanostructures or nanotube film may be incorporated as a component of an electrical device.
The methods of the present disclosure can have numerous variations. As nonlimiting examples, the step of preparing a nanotube/nanostructure solution or dispersing rod-like nanostructures or nanotubes can occur in the presence or absence of surfactants. In some embodiments, surfactant concentrations are kept below the critical micelle concentration (CMC). In some embodiments, the step of preparing the solution may involve dispersing the rod-like nanostructures or nanotubes in any suitable solution, such as pure water, surfactant solutions, or organic solvents. In some embodiments, rod-like nanostructure or nanotube dispersion involves sonication. In some embodiments, rod-like nanostructure or nanotube dispersion occurs by dispersing a powder of rod-like nanostructures or nanotubes with surfactant in water to create an aqueous suspension (e.g., solution) using tip sonication and ultra-centrifugation (See, e.g., Journal of Physical Chemistry B 107 (2003), pp. 13357-13367). In some embodiments, a well-dispersed rod-like nanostructure or nanotube suspension (e.g., solution) can be made by directly dissolving a water-soluble functionalized rod-like nanostructure or nanotube powder (of any type of functionality) in water (See, e.g., Accounts of Chemical Research 35 (2002), pp. 1096-1104). In some embodiments, the nanotube/nanostructure solution may be considered to be well-dispersed when individual nanotubes/nanostructures are separately suspended in the solution. Any other methods that can produce well-dispersed rod-like nanostructure or nanotube suspensions or solutions can also be utilized. In some embodiments, a concentration of the nanotube/nanostructure in the solution may be 15 μg/mL or less.
The methods of the present disclosure can utilize various types of rod-like nanostructures or nanotubes to form films. As the methods discussed herein are applicable to any type of nanotubes or rod-like nanostructures, the term “nanotubes” or “carbon nanotubes” as utilized herein shall be understood to include rod-like nanostructures or the like. For instance, in some embodiments, the carbon nanotubes include, without limitation, single-wall carbon nanotubes (SWCNTs), multi-wall carbon nanotubes (MWCNTs), metallic carbon nanotubes, semiconducting carbon nanotubes, carbon nanotube analogs, and combinations thereof. In some embodiments, the carbon nanotubes include SWCNTs.
In some embodiments, the carbon nanotubes may include various diameters and chiralities. Moreover, the carbon nanotubes may be synthesized by various methods. Examples of such methods can include, without limitation, high-pressure carbon monoxide conversion (HiPco), the CoMoCAT® method, chemical vapor deposition (CVD) methods, arc discharge methods, and other similar methods.
In some embodiments, the carbon nanotubes may include carbon nanotube analogs. In some embodiments, the carbon nanotube analogs may include any species and/or any high aspect-ratio, rod-like nanostructures. In some embodiments, the carbon nanotube analogs include, without limitation, nanowires, semiconductor nanowires, boron nitride nanotubes, transition metal dichalcogenide nanotubes, and combinations thereof. In some embodiments, the degree of alignment may be independent on the size of the nanotubes or rod-like nanostructures, such as the diameters.
Various carbon nanotube concentrations may be utilized in a solution or suspension. For instance, in some embodiments, the carbon nanotube concentration in a suspension or a solution is below the minimal value required for driving a three-dimensional liquid crystal phase transition, which is widely believed to be able to align carbon nanotubes in a suspension.
The methods discussed herein can also utilize various vacuum filtration methods to produce a film of aligned rod-like nanostructures or nanotubes, such as on the filtration membrane. See, e.g., Science 305 (2004), pp. 1273-1276. For instance, in some embodiments, the carbon nanotube suspension may be poured through a funnel (e.g., a filtration funnel) with a membrane. In some embodiments, the membrane may be hydrophilic or have a hydrophilic coating. In some embodiments, the membrane may have nanopores, micropores, macropores, and combinations thereof. In some embodiments, the membrane is a porous membrane with pore sizes of tens of nanometers. In some embodiments, the membrane has pores sized from 50 nm to 200 nm. In some embodiments, the membrane has the shape of a paper, such as a filter paper. In some embodiments, the membrane has a negative charge. In some embodiments, a speed of the vacuum filtration may be 1-2 mL/hour or less. In some embodiments, drying the film may be performed by increasing a speed of the vacuum filtration to 10 mL/hour or greater.
In some embodiments, the carbon nanotube dispersion is added to a membrane in a single aliquot. In some embodiments, the carbon nanotube dispersion is added to a membrane in multiple aliquots. In some embodiments, the filtration speed is adjusted in order to maximize carbon nanotube alignment.
In some embodiments, a differential pressure across a membrane pushes the solution slowly through the pores, leaving carbon nanotubes on the membrane. In some embodiments, the filtration speed is kept low. In some embodiments, the vacuum filtration results in the spontaneous alignment of carbon nanotubes. In some embodiments, the spontaneous carbon nanotube alignment occurs in a two-dimensional manner. As a nonlimiting example, after filtration via the methods discussed herein, the rod-like nanostructures or nanotubes may be aligned so that the central axes passing through the center of the nanotubes are approximately parallel (e.g.
In some embodiments, spontaneous alignment of rod-like nanostructures or nanotubes based on a liquid-crystal phase transition (i.e., into the nematic phase) is obtained during vacuum filtration. In some embodiments, such alignment is attained once one or more of the following conditions are met: (i) the carbon nanotube solution needs to be well-dispersed to the single-tube level (i.e., individual carbon nanotubes have to be separately suspended in the solution); (ii) if a surfactant-assisted method is used to disperse carbon nanotubes, then the surfactant concentration needs to be below the critical micelle concentration (CMC); (iii) the concentration of carbon nanotubes needs to be below a threshold value; (iv) the filtration speed needs to be as slow as possible; and/or (v) the drying process of the film needs to be fast in order to avoid any damage. When the above conditions are met, a wafer-scale, uniform carbon nanotube film with a readily controllable thickness can be formed on a membrane (e.g., a filter paper). In some embodiments, the film may have an area of 1 cm2 or greater.
In some embodiments, the methods can control the thickness of the formed films by varying one or more parameters. In some embodiments, the thickness of the film can be controlled by the filtration volume, nanotube concentration in the nanotube solution, and combinations thereof. For instance, in some embodiments, the film thickness is controllable from about 1 nm to about 100 nm. In some embodiments, the methods can be utilized to precisely control the thickness of the formed films from atomic to macroscopic scale.
In some embodiments, vacuum filtration occurs by a “controlled differential pressure filtration (CDPF)” method that is illustrated in
Moreover, different diameter carbon nanotubes may require different filter paper pore sizes, different filtration speeds, and different carbon nanotube concentrations. As such, wafer•scale semitransparent films of aligned carbon nanotubes can be produced when these parameters are optimized, as shown in
In some embodiments, the formed carbon nanotube films of the present disclosure can be transferred onto another substrate. In some embodiments, formed carbon nanotube films can be transferred to an arbitrary substrate for specific applications by dissolving the membrane (e.g., filter paper) in a proper solvent.
In some embodiments, the formed carbon nanotube films can be patterned. In some embodiments, patterning occurs by using conventional photolithography techniques.
The methods of the present disclosure can be utilized to form various types of large scale films of aligned carbon nanotubes. Additional embodiments of the present disclosure pertain to the large scale films of aligned carbon nanotubes (also referred to as films).
In some embodiments, the films of the present disclosure are in an inch-size scale. In some embodiments, the films of the present disclosure have surface areas that range from about 1 mm2 to about 1 cm2. In some embodiments, the films of the present disclosure have surface areas that are more than about 1 cm2. In some embodiments, the films of the present disclosure are macroscopic. In some embodiments, at least one of the dimensions of the films of the present disclosure is one centimeter or larger (i.e., “wafer-scale”, as used herein).
In some embodiments, the films of the present disclosure have a strong carbon nanotube alignment and a high carbon nanotube density. For instance, in some embodiments, the carbon nanotubes in the films of the present disclosure contain at least about 106 nanotubes in a cross-sectional area of at least about 1 μm2. In some embodiments, the carbon nanotubes in the films of the present disclosure have an S alignment value of about 1.
In some embodiments, the films of the present disclosure have a thickness ranging from about 1 nm to several micrometers. In some embodiments, the films of the present disclosure have a thickness ranging from about 1 nm to about 100 nm. In some embodiments, the films of the present disclosure are transparent or semi-transparent. In some embodiments, the films of the present disclosure are in the form of three-dimensional crystals that include single-chirality carbon nanotubes.
In some embodiments, the carbon nanotubes in the films of the present disclosure are in monodomain or single-domain form or the CNT film has approximately all the CNTs aligned the same way throughout the film. In some embodiments, the carbon nanotubes in the films of the present disclosure lack any surfactants. In some embodiments, the carbon nanotubes in the films of the present disclosure lack any residual metallic tubes, which are known to be efficient photoluminescence quenchers when in contact with semiconducting nanotubes.
In some embodiments, the carbon nanotubes in the films of the present disclosure are selected by type, by chirality, or combinations thereof. For instance, in some embodiments, the films of the present disclosure contain large-area single-domain films of highly-aligned, densely-packed, and chirality-enriched carbon nanotubes (e.g., SWCNTs).
In some embodiments, the films of the present disclosure include single-chirality ensembles of carbon nanotubes. In some embodiments, the films of the present disclosure include single crystals of carbon nanotubes. In some embodiments, the films of the present disclosure include wafer•scale single crystals of single-chirality carbon nanotubes. In some embodiments, “single crystals” refers to the existence of a continuous and unbroken periodic array with well•defined translational symmetry in which carbon nanotubes are perfectly aligned and packed in all three spatial directions. In some embodiments, “single-chirality” indicates that the crystal is comprised of only one species of carbon nanotubes having the same chiral indices, (n,m). In some embodiments, the carbon nanotubes in the films of the present disclosure include, without limitation, single crystals of metallic carbon nanotubes, single crystals of semiconducting carbon nanotubes, single crystals of single-chirality semiconducting carbon nanotubes with desired bandgaps, and combinations thereof.
In some embodiments, the films of the present disclosure include carbon nanotubes with a hexagonal crystalline pattern. In some embodiments, the carbon nanotubes in the films of the present disclosure are in “crystalline” form. In some embodiments, the term “crystalline” requires perfect periodicity with a well•defined aligned pattern. In some embodiments, the carbon nanotubes in the films of the present disclosure are enriched to a single chirality.
In some embodiments, the films of the present disclosure can be fabricated into electronic devices. Further embodiments of the present disclosure pertain to electronic devices that contain the films of the present disclosure. In some embodiments, the electronic devices of the present disclosure include, without limitation, optoelectronic devices, photodetectors, Hall-bar devices, field effect transistors, terahertz polarizers, terahertz/infrared polarizers using metal-semiconductor mixed carbon nanotubes, thin-film transistors, thin-film transistors using semiconducting carbon nanotubes, polarized photoluminescence devices, polarized light emission devices (e.g., light emitting diodes and laser diodes) using semiconducting carbon nanotubes, polarization-sensitive electro-optic modulators, polarization-sensitive carbon nanotubes detectors (e.g., in a wide wavelength range from the far-infrared to the ultraviolet), electrical cables (e.g., electrical cables made of aligned ‘armchair quantum wires’ with high strength and conductivity for power transmission using single crystals of armchair carbon nanotubes), polarization-sensitive photodetectors (e.g., photodetectors in an ultrawide wavelength range from the terahertz to the ultraviolet using single crystals of semiconducting carbon nanotubes and their heterostructures), saturable absorbers for ultrafast lasers (e.g., from the visible to the infrared range using single crystals of single-chirality carbon nanotubes, high mechanical strength carbon nanotube sheets, fibers, and composites using single crystals of carbon nanotubes), high performance electrodes (e.g., for use as fuel cells, solar cells, and high•performance materials for hydrogen storage using single crystals of carbon nanotubes), and combinations thereof.
In some embodiments, the present disclosure provides a new process to produce a wafer-scale (e.g., inch-sized) film with an adjustable thickness of aligned carbon nanotubes. The methods of the present disclosure are universal. Furthermore, the methods of the present disclosure are applicable to any carbon nanotube species and/or any high aspect-ratio, rod-like nanostructures, including semiconductor nanowires and boron nitride nanotubes.
Since the methods of the present disclosure are applicable to all species of carbon nanotubes, both metallic and semiconducting, with any diameter or chirality, and irrespective of the synthesis method, the methods of the present disclosure have a diverse range of potential applications in various electronic devices (as previously described). For instance, one can create the most appropriate single crystals of carbon nanotubes with tailored optical absorption and emission properties desired for specific optoelectronic applications.
In some embodiments, the methods of the present disclosure solve a problem of aligning carbon nanotubes on a large scale in a simple and universal way. Individual carbon nanotubes have been shown to possess many unique properties that are promising for various applications in nanotechnology. However, macroscopic assemblies of carbon nanotubes have so far shown only poor properties because of the lack of alignment of carbon nanotubes. In contrast, the methods discussed herein based on vacuum filtration provide a uniform, large-scale carbon nanotube film with a high degree of alignment in a well-controlled, simple, and reproducible manner, regardless of the type, diameter, or chirality of the carbon nanotubes used.
Another advantage of the methods of the present disclosure over the conventional vacuum filtration methods is the ability to align carbon nanotubes while (i) being independent of carbon nanotube types, chiralities and growth methods; (ii) producing wafer-scale carbon nanotube films; (iii) providing a simple method of use; (iv) providing high density carbon nanotubes; (v) providing control over thickness; and/or (vi) providing ease with which to perform photolithography to fabricate devices. As set forth previously, such advantages can be obtained through the following three steps: (1) preparation of a well-dispersed carbon nanotube solution or suspension; (2) vacuum filtration to produce an aligned carbon nanotube film on filter paper; and/or (3) transfer of the carbon nanotube film onto arbitrary substrates.
In some embodiments, the methods of the present disclosure can be easily scaled up for industrial manufacturing. In more specific embodiments, the methods of the present disclosure provide uniform, wafer-scale (e.g., >cm2) carbon nanotube (e.g., SWCNT) films of an arbitrarily and precisely controllable thickness (e.g., from 1 nm to ˜100 nm) with a high degree of alignment (e.g., S˜1) and packing (e.g., ˜106 nanotubes in a cross-sectional area of 1 μm2) in a well-controlled, simple, and reproducible manner, regardless of the synthesis method, metallicity, or chirality of the carbon nanotubes used. Furthermore, the produced films are compatible with standard microfabrication processes to fabricate various electronic and photonic devices.
In some embodiments, the methods of the present disclosure allow one to have crystals of, for example, semiconducting (6,5), metallic (6,6), and quasi-metallic (7,4) carbon nanotubes, as illustrated in
Reference will now be made to more specific embodiments of the present disclosure and experimental results that provide support for such embodiments. However, the disclosure below is for illustrative purposes only and is not intended to limit the scope of the claimed subject matter in any way. The following examples are included to demonstrate particular aspects of the present disclosure. It should be appreciated by those of ordinary skill in the art that the methods described in the examples that follow merely represent illustrative embodiments of the disclosure. Those of ordinary skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments described and still obtain a like or similar result without departing from the spirit and scope of the present disclosure.
In this Example, a simple and robust method is reported for preparing exceptionally large (>cm2) monodomain films of aligned SWCNTs based on spontaneous alignment that occurs during slow vacuum filtration. The produced films are globally aligned within ±1.5° (the nematic order parameter ˜1) and are highly packed, containing ˜106 nanotubes in a cross-sectional area of 1 μm2. The film thickness is controllable from 1 nm to ˜100 nm. Sufficiently thick films act as ideal polarizers in the terahertz frequency range. This method is universally applicable to SWCNTs synthesized by various methods. Moreover, by combining this method with recently-developed sorting techniques, highly-aligned and chirality-enriched SWCNT thin-film devices were fabricated. Semiconductor-enriched devices exhibited polarized light emission, polarization-dependent photocurrent, and anisotropic conductivities and transistor action with high on/off ratios. These results significantly advance the frontier of research toward scalable carbon-based electronics and photonics.
The process starts with preparation of a well-dispersed CNT suspension. A CNT powder with surfactant was dispersed in water to create an aqueous CNT suspension using tip sonication and ultra-centrifugation (See Example 1.1).
The second step utilizes vacuum filtration method (
To obtain spontaneous CNT alignment in this nonlimiting example, one has to satisfy three conditions: i) the surfactant concentration may be below the critical micelle concentration (CMC); ii) the CNT concentration may be below a threshold value, and/or iii) the filtration process may be well controlled at a low speed. When these conditions are met, a wafer-scale, uniform, and aligned CNT film is formed on the filter membrane (
The film can then be transferred to any substrate in a straightforward manner after dissolving the filter membrane in a proper solvent (see Example 1.3). The result is a large-area, semi-transparent film of aligned CNTs, as shown in the optical (
As the cross-sectional TEM image in
Using cross-polarized microscopy, strong optical anisotropy can be demonstrated both on a macroscopic (cm) scale (
Due to the exceptionally high degree of CNT alignment, these peaks are completely absent for the perpendicular polarization, and instead, a broad absorption feature is observed in an intermediate energy region between the (E11S and E22S) peaks. Without being bound by theory, this feature appears to be attributed to the cross-polarized and depolarization-suppressed absorption peak, predicted more than 20 years ago and detected in polarized photoluminescence excitation spectroscopy experiments in individualized CNTs. This is the first time that this transition has been directly observed in standard absorption spectroscopy because of macroscopic alignment.
As noted previously, the exceptionally strong polarization-dependence of THz transmission through aligned CNT films can be utilized to form an ideal THz polarizer with extremely large extinction ratios (ER). ER=T|/T⊥, where T| (T⊥) is the transmittance for the parallel (perpendicular) polarization.
The data for the perpendicular case completely coincides with the reference trace (i.e., no attenuation occurs within the SWCNT film). On the other hand, there is significant attenuation for the parallel case. Note that the THz beam had a mm2 size, thus probing a macroscopic area.
Due to the ultrahigh density of the films, the ER per thickness calculated from
The methods of making aligned films is universally applicable to different types of SWCNTs. Table 1 lists seven representative suspensions used, which contained SWCNTs synthesized by the arc-discharge, CVD, CoMoCAT, and HiPco methods.
†Suspension #3 contained arc-discharge-synthesized CNTs that were functionalized by polyaminobenzene sulfonic (PABS) acid.
‡Suspension #4 contained TUBALL ™ nanotubes from OCSiAl (http://ocsial.com/en/product/tuball/).
τSuspension #7 was enriched in (7,6), (8,6), and (10,5) SWCNTs.
⊥Suspension #5, #6, and #7 did not have high enough carrier densities to show strong enough THz attenuation to determine STHz.
In Table 1, dt is the average nanotube diameter, and lt is the average nanotube length measured by atomic force microscopy. For dispersing CNTs, sodium deoxycholate (DOC) was used for Suspensions #1 and #4-#7 and sodium dodecylbenzenesulfonate (SDBS) for #2, as surfactant. Pre-functionalized, water-soluble SWCNTs were used in Suspension #3.
Successful formation of aligned films made from suspension #3 indicates that surfactant is not a crucial element for spontaneous CNT alignment, as long as the CNTs are well dispersed in the suspension. However, CNTs in all these suspensions were negatively charged. Thus, it is currently not clear whether CNT alignment can be achieved with positively charged or neutral surfactants.
Also listed in Table 1 are nematic order parameters, STHz and SRaman, as determined through THz and Raman measurements, respectively. Only SRaman is shown for Suspensions #5, #6, and #7 because the films from these suspensions did not have sufficiently high carrier densities to show strong enough THz attenuation for determining STHz. There is no apparent relationship observed between the structure parameters of SWCNTs and the achieved values of STHz and SRaman. Arc-discharge SWCNTs tend to align more strongly than other types of nanotubes, but further systematic studies using diameter- and length-sorted samples are needed to clarify whether this difference comes from their differences in dt, lt, or dt/lt.
The strikingly high values of S achieved precludes the possibility that the alignment mechanism is based on three-dimensional (3D) nematic ordering of rigid rods, for which Onsager's theory predicts an upper limit of S=0.79. Formation of a 3D nematic liquid-crystal phase of CNTs would require a high CNT concentration8, with a threshold value of ˜5 mg/mL for CNTs with lt/dt˜103. This condition was not met in this case (typical CNT concentration ˜15 μg/mL, and lt/dt=150-550), suggesting that a different alignment mechanism is at work.
A clue for the mechanism comes from observations that the degree of alignment was sensitive to the hydrophobicity of the filter membrane surface, similar to a prior report on 2D nematic ordering of DNA-wrapped CNTs. Alignment was achieved only when the filter membrane had a hydrophilic coating layer (PVP). Additionally, control of the flow rate, CNT concentration, and surfactant concentration was crucial.
Based on these observations, and without being bound by theory, it is proposed that CNT alignment occurs in a 2D manner. Hydrophilic PVP coating makes the filter paper surface negatively charged during filtration, and as a result, negatively charged CNTs in the suspension are repelled from the surface. At the same time, CNTs feel van der Waals attraction from uncoated regions of the membrane surface. The competition of these two forces creates a potential minimum near the surface, where CNTs accumulate, interact with each other, and form an ordered 2D phase. Because the formation of ordered structure requires horizontal (i.e., in-plane) rotation of CNTs in a finite time period to arrange themselves within the 2D layer, an appropriate filtration speed and CNT concentration are important. Furthermore, the surfactant concentration affects the charge density on the PVP layer, which in turn influences the electrostatic repulsion potential, while at the same time the suspension viscosity depends on the surfactant concentration, which influences the rotational motion of CNTs in the 2D layer.
2D surface confinement is a novel route towards fabricating ordered nanostructures. In the Langmuir-Schaefer method for example, nano-objects initially float on the surface of water (i.e., confined at the interface between air and water), and eventually form an ordered phase. However, CNT concentrations used in previous studies were extremely low (e.g., 0.1-1 μg/mL). The obtained films of aligned CNTs remained small and thin (limited to a monolayer or several layers), and the degree of alignment was low (e.g., the Raman G-peak intensity anisotropy ≈10, compared to ˜160).
In addition to stronger alignment, the films have other distinct advantages. First, unlike the previous 2D ordering studies, the method allows continued accumulation of aligned CNTs to achieve optically thin films (˜100 nm). The fact that S values higher than the rigid-rod limit to such thicknesses suggests that some attractive interaction is present between the tubes when the surfactant concentration is low. Second, the cross-sectional areal density is as high as 106 nanotubes/μm2, orders of magnitude larger than any previous reports.
These two advantages indicate that a transition from 2D-like to 3D-like ordering occurs as CNTs gradually accumulate on the surface. Once there is an aligned layer, the CNTs that follow tend to align with the already-existing alignment direction. Finally, there is in principle no limitation on the achievable area of aligned films. Practically, it is only limited by the size of the filter membrane.
With the ultimate aim of developing optoelectronic technologies based on aligned single-chirality SWCNTs, CNT suspensions enriched in specific types and chiralities were used in making aligned CNT films (e.g., Suspension #6 in Table 1) and fabricated light emission and detection devices (
Recent advances in post-growth separation and sorting techniques allow suspensions of specific chirality, (n,m), and SWCNTs at large enough quantities to be prepared for making macroscopic films. Here, the aqueous two-phase extraction method may be used to enrich (6,5) SWCNTs.
Finally, the electronic devices were fabricated out of aligned CNT films using standard microfabrication techniques and tested their conductivities and transistor performance (
The conductivity along the alignment direction was 2500 S/cm. In conventional, randomly-aligned CNT films made by vacuum filtration, conductivity values as high as this value are obtainable only with heavy doping while the improved films are not intentionally doped. In addition, transistor behaviors of a (6,5)-enriched aligned film (
As shown previously, the on-current density can also be enhanced by using larger-diameter nanotubes, which is also demonstrated by the transistor based on semiconductor-enriched arc-discharge CNTs with an average diameter of 1.4 nm (
In summary, a new process to produce a wafer-scale (i.e., inch-size) film of aligned CNTs of an adjustable thickness is reported. This method works for CNTs synthesized by various methods and is expected to be applicable to other high aspect-ratio, rod-like nanostructures that can be suspended in the same way as used in the experiments. Terahertz/infrared polarizers have been demonstrated using metal-semiconductor mixed CNTs, thin-film transistors using semiconducting CNTs, polarized light emission devices using semiconducting CNTs, and polarization-sensitive CNT detectors.
Two batches of arc-discharge SWCNTs (P2-SWNT and P8-SWNT) with an average diameter of 1.4 nm were purchased from Carbon Solutions, Inc. Two types of Co-MoCAT SWCNTs (CG 200 and SG 65i) were purchased from Sigma-Aldrich. The former had a wide diameter distribution (0.7-1.4 nm), whereas the latter was enriched in (6,5) SWCNTs with ˜90% purity. CVD-grown TUBALL™ CNTs, which were 75% SWCNTs, were obtained from OCSiAl and had an average diameter of ˜1.8 nm. HiPco SWCNTs (batch #195.5), with a diameter range of 0.9-1 nm, were used after purification and diameter sorting.
As surfactants for CNT dispersion, sodium deoxycholate (DOC, Sigma-Aldrich) and sodium dodecylbenzenesulfonate (SDBS, Sigma-Aldrich) were used. All CNTs (except for P8-SWNT) were initially dispersed in either 1% (wt./vol.) DOC or 0.4% (wt./vol) SDBS by bath sonication (Cole-Parmer 60-W ultrasonic cleaner, model #08849-00) for 5-10 minutes at a starting concentration of 0.2-0.8 mg/mL. The obtained suspension was then further sonicated with a tip sonicator (XL-2000 Sonicator, Qsonica, LLC., ¼″ probe, ˜30 watts) for 45-60 minutes. The suspension was cooled in an ice water bath during the sonication. Next, the suspension was centrifuged for 1 hour at 38000 rμm (Sorvall Discovery 100SE Ultracentrifuge using a Beckman SW-41 Ti swing bucket rotor) to remove large bundles of CNTs. After centrifugation, the upper 60% of the supernatant was collected and then diluted with Nanopure water. The concentration of surfactant was diluted below its critical micelle concentration (CMC). Depending on the type of CNTs used, the final concentration of DOC before film making varied from 0.02% to 0.1%, and the final concentration of SDBS was ˜0.02%.
Well-dispersed CNT suspensions with dilute CNTs (˜1-15 μg/mL) and dilute surfactant (below CMC) was filtered through a vacuum filtration system in a well-controlled manner. Polycarbonate filter membranes (Nuclepore Track-Etched Polycarbonate Hydrophilic Membranes) with different pore sizes from 50 to 200 nm were used. The filtration speed was kept low and controlled in a range of 1-2 mL/hour. The pressure applied to the system by vacuum pumping was monitored through sensitive pressure gauges. A speeding-up procedure was performed at the end of the filtration process to dry the film quickly. For this fast-dry procedure, the filtration speed was increased from ˜1 mL/hour to ˜10 mL/hour. Before transferring the film onto another substrate, the film was pumped on for an additional 15-30 minutes.
For characterization and device fabrication, films were transferred onto solid substrates (e.g., SiO2/Si and quartz) using a wet transfer process, in which the polycarbonate filter membrane was dissolved away with organic solvent (N-methyl-2-pyrrolidone or chloroform). The films were then thoroughly washed by acetone and nanopure water. Subsequent X-ray photoelectron spectroscopy experiments did not detect any residual surfactants. Properties of fabricated CNT films were characterized by various methods. Anisotropic optical transmission on a large scale was characterized by polarized optical microscopy in both co-polarized and cross-polarized configurations. Alignment structure on micro- and nano-meter scales was examined by scanning electron microscopy (JEOL 6500F Scanning Electron Microscope) and transmission electron microscopy (JEOL 2100 Field Emission Gun Transmission Electron Microscope). The quality of CNT alignment was characterized in terms of local and global nematic order parameters based on polarized Raman spectroscopy (RENISHAW inVia Raman Microscope) and terahertz transmission spectroscopy (a home-made time-domain THz spectroscopy system). Film thicknesses were measured by atomic force microscopy (Bruker Multimode 8). The average lengths of CNTs in suspensions were measured by atomic force microscopy. Photoluminescence excitation spectra of the single-chirality (6,5) CNT suspension and polarized photoluminescence of the single-chirality (6,5) films were measured with a home-made photoluminescence excitation spectroscopy setup.
Aligned (6,5) nanotube films were cut into ribbons and then transferred to glass substrates. Two electrodes were formed by sputtering 50-nm gold with a shadow mask at the two ends of a ribbon. A 660-nm laser beam with a maximum power of ˜50 mW was incident on and near a gold-CNT junction. A half-wave plate was used to rotate the polarization of the incident light beam without changing the incident power. The generated photo-voltage was amplified by a Stanford Research SR560 voltage preamplifier and then fed into a Stanford Research SR830 lock-in amplifier.
An aligned nanotube film was transferred onto a substrate comprised of heavily doped silicon (acting as a global back gate for the field effect transistors) and 285-nm thick layer of thermal silicon oxide (acting as the insulating layer). The first photolithography step was to define the electrode area with Shipley Microposit S1813 photoresist. The electrodes were defined and fabricated by lifting off electron-beam-evaporated titanium (1 nm)/palladium (10 nm)/gold (20 nm). The next step was to define a Hall bar structure with one channel parallel and the others perpendicular to the CNT alignment direction, as shown in
For the CoMoCAT SG-65i sample, after dispersion and centrifugation (see Methods), the aqueous two-phase extraction (ATPE) method was adopted to perform chirality separation. After chirality separation, the dextran and polyethylene glycol (PEG) used in the ATPE method were removed by ultrafiltration. The concentration of surfactant was then readjusted to the value required for making films.
Arc-discharge P8-SWNTs are water-soluble because of functionalization by mpolyaminobenzene sulfonic (PABS) acid. The CNT powder was directly dissolved into nanopure water with an initial concentration of ˜0.5 mg/mL. In order to get better dispersion, the suspension was tip-sonicated for 45 minutes and then centrifuged, as described in the Methods section; the upper ˜60% of the supernatant was collected for making films.
HiPco single-wall carbon nanotubes (SWCNTs), batch #195.5, were dispersed by DOC (1% wt./vol.) with an initial concentration of ˜0.6 mg/mL of SWCNTs. After centrifugation, the supernatant (˜0.3 mg/mL) was collected and further purified and sorted by the ATPE method. The purified sample was enriched in the chiralities (10,5), (8,6), and (7,6), with an average diameter was 0.9-1 nm.
Fabrication of Aligned CNT Films: Filtration System
The experimental system comprises a filtration setup (filter membranes, filtration funnel, etc.), a vacuum pump, and multiple pressure gauges. The vacuum pumping rate was controlled by valves, and the applied pressure was monitored by pressure gauges mounted on the vacuum line.
The Nuclepore track-etched polycarbonate (hydrophilic) filter membranes used were purchased from GE Healthcare Life Sciences. The pore sizes included 80 nm, 100 nm, and 200 nm.
The filtration setup (Millipore® XX1002500 glass microanalysis 25 mm vacuum filter holder with 15 mL funnel and fritted glass filter support) was purchased from Fisher Scientific Company, LLC, including a 15 mL glass funnel, a fritted glass filter support, and a silicone stopper for the attachment to a vacuum filtering flask. The vacuum pump (Fisher Scientific™ Maimadry™ Oil-Free Vacuum Pump) was purchased from Fisher Scientific Company, LLC. Pressure gauges (Magnehelic Differential Pressure Gauge) with different ranges were purchased from Dwyer Instruments, Inc.
Filtration Procedure
The filtration procedure generally included the following three steps for the purpose of controlling the filtration speed at a slow and steady rate.
Step I:
Put a gentle or zero pressure on the filtration system at the beginning so that the filtration speed of the CNT suspension is as slow as 1-2.5 mL/hour. This can be achieved by controlling multiple valves in the vacuum line to adjust the vacuum pumping rate and choosing filter membranes with different pore sizes.
Step II:
Monitor the filtration speed during the filtration process. As CNTs are gradually deposited on the filter membrane, the flow rate slows down. Try to keep the speed above 0.5 mL/hour in order to avoid inhomogeneous deposition of CNTs.
Step III:
Increase the filtration rate at the end of the filtration process. The CNT layer deposited on the filter membrane can be disturbed by the residual solution at the end of the filtration if the speed is slow (˜1-2.5 mL/hour). The increase in filtration rate at the end is important to achieve a uniform CNT layer and protect the alignment structures from being destroyed by the residual solution. Usually, the increase is begun when there is still a certain distance (˜3-5 millimeters) between the upper surface of the filtering suspension and the surface of the filter membrane. Before the two surfaces touch each other, a final steady speed is reached around 1 drop/20 seconds, namely ˜10 mL/hour.
Filtration Details of Different Types of CNTs
Arc-Discharge SWCNTs:
The original P2-SWNT concentration after tip sonication was 0.4 mg/mL. After centrifugation, a P2-SWNT suspension of ˜0.15 mg/mL in DOC (1% wt./vol.) or SDBS (0.4% wt./vol.) was obtained, which was then diluted by nanopure water. The concentration of CNTs after dilution was ˜15 μg/mL, and the DOC concentration was ˜0.1% for DOC-dispersed samples, while the SDBS concentration was ˜0.02% for SDBS-dispersed samples. For P8-SWNTs, the concentration of CNTs was ˜10 μg/mL in water. Polycarbonate filter membranes with a pore size of 200 nm were used for the filtration of arc-discharge CNT suspensions. No pressure was applied on the system at the beginning of filtration as the pore size of filter membranes was large enough to establish a desired filtration speed. The speeding up started when there was ˜15%-20% suspension left. The final speed was stabilized at around 20 seconds per drop. The time for the entire filtration of 4-5 mL sample was 1.5-2 hours. After the filtration was finished, the film was kept under pressure for an additional 15-30 minutes, allowing the film to dry completely.
TUBALL CNTs:
The original CNT concentration was 0.4 mg/mL, A CNT suspension of ˜0.14 mg/mL was obtained in DOC surfactant (1% wt./vol.) after centrifugation. After dilution with nanopure water, the concentration of CNTs and DOC surfactant were ˜14 μg/mL and 0.1% wt./vol., respectively. The filtration procedure was the same as for the arc-discharge CNTs described above.
CoMoCAT SWCNTs (CG200):
The original CNT suspension of ˜0.4 mg/mL was dispersed in DOC surfactant (1%). After centrifugation, the sample was diluted by either Nanopure water or DOC solution with a certain concentration, the final concentration of CNT and DOC surfactant for film making were in the range from 4 μg/mL-30 μg/mL and 0.03-0.5%, respectively. Polycarbonate filter membranes with a pore size of 80 nm were used for sample filtration. A weak pressure of 10-30 Pascal was applied to the system at the beginning, which was measured by pressure meters mounted in the vacuum pipeline. The filtration speed was around 5-6 minutes per drop in the beginning. The final speed was increased up to 20 seconds per drop in order to fix and dry the film fast. For a sample with ˜1 mL volume, it took around one hour to finish the entire filtration process.
CoMoCAT SWCNTs (SG65i):
The original CNT solution of ˜0.4 mg/mL was dispersed in DOC surfactant (1%). After type separation (ATPE) and sample cleaning via ultrafiltration, the concentration of CNTs was adjusted to ˜4 μg/mL determined by its optical density from UV-Vis-NIR absorption spectroscopy. The concentration of DOC surfactant was readjusted to 0.025%. Polycarbonate filter membrane with a pore size of 80 nm was used for filtration, and the procedure was the same as that for CoMoCAT CG200 described above.
HiPco SWCNTs (Batch #195.5):
After the purification and sorting, the sample was diluted by nanopure water, and the concentration of SWCNTs was readjusted to ˜12 μg/mL. The concentration of DOC surfactant was readjusted to 0.04%. Polycarbonate filter membrane with a pore size of 80 nm was used for the filtration, and the procedure was the same as that for CoMoCAT CG200 described above.
Characterization of Aligned CNT Films
Before characterization measurements, the CNT films were transferred to different solid substrates (silicon, SiO2/Si, quartz, etc.) by a wet transfer technique, thoroughly cleaned by nanopure water, and dried by nitrogen gas. During the transfer process, the filter membrane was dissolved by N-methyl-2-pyrrolidone (NMP) or chloroform.
Polarized Optical Microscopy:
A Zeiss Axioplan 2 microscope was used to optically characterize the CNT film on a macroscopic scale. Two modes (cross-polarized and co-polarized) were used. For the former case, the polarizer and analyzer were placed orthogonally. When the CNT alignment direction was along the polarization direction of either the polarizer or analyzer, the film was opaque. When the CNT alignment direction was 45 degrees with respective to the polarization direction, the film was bright. The optical images for arc-discharge CNTs (P2-SWNT) under cross-polarized mode are shown in
Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM):
SEM and TEM were used to characterize the alignment of CNTs on a microscopic scale. The SEM images in
Preparation of a cross-sectional CNT sample was made as follows. An aligned arc-discharge SWCNT film was transferred onto a SiO2/Si substrate and cut perpendicular to the overall alignment direction with a focused ion beam (FIB) using a FEI Helios 660 SEM/FIB. The slab thickness was reduced to ˜70 nm, as shown in
Polarized Raman Spectroscopy:
A Renishaw inVia Raman microscope was used to quantitatively characterize the degree of alignment of CNT films. The Raman spectra of aligned films made of other different types (Suspension #2-Suspension #7 in Table 1) besides arc-discharge (P2-SWNT) CNTs dispersed in DOC were also measured under different polarization configurations (IVV, IVH, and IHH) with an excitation laser wavelength of 514 nm, as shown in
Terahertz (THz) Time-Domain Spectroscopy (THz-TDS):
THz-TDS was used to quantitatively characterize the degree of alignment on a macroscopic scale (˜1 mm2). The experimental setup was a transmission THz-TDS system using an ultrafast Ti:Sapphire laser. Already linearly-polarized THz radiation went through a wire-grid polarizer before being focused onto the sample. The CNT films were transferred onto intrinsic silicon substrates for the transmission measurements. The thickness of the films ranged from 10-100 nm, and their areas were on the order of cm2 to fully cover the THz beam size ˜mm2. Polarization-dependent THz transmission data were obtained by rotating the sample, changing the direction of CNT alignment with respect to the polarization direction of the incident beam. The polarization-dependent THz transmission frequency-domain spectra of aligned films (Suspension #1-Suspension #4 in Table 1) are shown in
Atomic Force Microscopy (AFM):
A Bruker Multimode 8 AFM was used to characterize the average length of CNTs and the thicknesses of the CNT films.
Photoluminescence Excitation (PLE) Spectroscopy:
PLE was used to characterize the purity of the (6,5)-enriched sample. A homemade PLE setup had excitation and emission spectral ranges from 450 to 800 nm and from 850 to 1400 nm, respectively. The setup comprised a white light source (xenon lamp), a monochromator, a cold mirror, and an InGaAs CCD detector. The PL of (6,5)-enriched film was measured directly on the filter membrane after the filtration and drying procedures at fixed excitation wavelength 570 nm, on resonance with E22 of (6,5)-enriched film, while the suspension sample was put in a quartz cuvette for the PLE measurements. Both PL and PLE data were taken in reflection geometry at room temperature. In order to check the polarization of PL from the aligned CNT film, a polarizer was inserted between the sample and the InGaAs detector.
X-Ray Photoelectron Spectroscopy (XPS):
Survey spectra of an aligned CNT thin film and a polycarbonate filter membrane were taken using an X-ray photoelectron spectrometer (PHI Quantera XPS). The monochromatic X-ray source was A1 Kα with an energy of 1486.7 eV, a beam spot size of 200 μm, and a power of 50 W. The pass energy was set to 140 eV. Shown in
Semiconductor-metal separation was performed on arc-discharge (P2-SWNT) CNTs by redox sorting method (
Factors Affecting the Degree of CNT Alignment
There are several important factors that have a strong influence on the degree of alignment for all types of CNTs. These factors include, but are not limited to, the filter membrane surface properties, the concentration of surfactant, the concentration of CNTs, and/or the filtration speed. The polarization-dependent absorption spectra of the fabricated CNT films were measured using a 660 nm diode laser. The LDr defined as LDr=3(A∥−A⊥)/(A∥+2A⊥) at this wavelength was then calculated and used as a measure of the degree of alignment, together with SEM images, to evaluate the alignment quality under different conditions.
Filter Membrane Surface Properties
For this study, two types of polycarbonate filter membranes were used. Type A is coated with a layer of poly (vinylpyrrolidone) (PVP) on the surface and therefore is hydrophilic, Type B is hydrophobic without PVP coating on the surface. The presence of PVP coating can be detected by nitrogen is peak in X-ray photoelectron spectroscopy (XPS).
It is known that PVP molecules are polar and have strong interaction with water molecules through hydrogen bonds. This property makes a PVP-coated surfacehydrophilic. When PVP molecules interact with water, hydration shells form around the carbonyl groups of PVP. The outside of the hydration shells are covered by negatively polarized oxygen atoms. In addition, PVP molecules have strong interaction with anionic surfactants; its flexible polymer chain can effectively trap surfactants in a solution. Both the formation of hydration shells and the trap of anionic surfactants will lead to an accumulation of negative charges in the PVP layer. When negatively charged CNTs, due to the wrapping of negatively charged surfactants approaching the surface, they will face an energy barrier due to the electrostatic repulsion from the PVP layer. In the mean time, they will also feel an attraction due van der Waals forces from the surface. It is believed that the competition between these two forces can create a potential well near the surface where CNTs are confined in a narrow layer, and an ordered 2D phase appears. Without being bound by theory, it is believe the CNT alignment occurs with the same mechanism. The importance of hydrophilic PVP coating is that it makes the filter membrane surface negatively charged, allowing the formation of a potential well near the surface, which confines CNTs in a narrow layer. When CNTs can rotate in-plane and rearrange among themselves in this confined layer, a 2D nematic phase will be developed at an appropriate surface concentration (see the diagram in
Additionally, the surface morphology of filter membrane was checked using AFM. Small shallow grooves with depths of 5 to 10 nm, widths of a few μm, and spacing of a few to tens of μm mostly along one direction on the membrane surface (
Surfactant Concentration
CoMoCAT CG200 CNTs were used to systematically investigate the influence of surfactant concentration, CNT concentration and filtration speed as discussed further herein. For this study, the concentration of CNTs was fixed at ˜15 μg/mL, and the concentration of DOC surfactant was varied from 0.03 to 0.5%. The samples were filtered at a low filtration speed as described previously.
There are two possible reasons for the CNT misalignment with high surfactant concentration. Firstly, the increase of surfactants will lead to the increase of charge density in the PVP layer. This is because there will be more surfactant molecules trapped in the PVP layer, forming negatively charged micelle structures. When the electrostatic repulsion is too strong compared to van der Waals force, the potential well becomes shallow and then disappears, leading to the absence of 2D confinement. Secondly, the formation of an aligned CNT film requires the rotation of CNTs in the confined layer. The formation of micelle structures will increase the viscosity of the suspension affecting the in-plane movement of CNTs.
In addition, more insights can be obtained by comparing the results from suspension#1, 2 and 3 in table 1. In Suspension #1 (DOC) and Suspension #2 (SDBS), arc-discharge CNTs were dispersed by DOC and SDBS, respectively. Both of these surfactants are anionic, which can be trapped by the polymer chains of PVP. However in Suspension #3, CNTs are acid-functionalized; there were no anionic surfactants in the suspension. Therefore, for Suspension #3, the negative charge density on the surface of the filter paper should be smaller compared to Suspension #1 and 2, leading to a weaker electrostatic repulsion. This is likely the reason why the alignment degree was decreased in Suspension #3.
Filtration Speed
Two samples were prepared with the same CNT concentration of ˜15 μg/mL and the same DOC concentration of ˜0.045% but at two different filtration speeds (1 mL/hour and 5 mL/hour).
where V is the filtration speed, C is the CNT concentration, NA is Avogadro constant, MCNT is the molar mass of CNTs, L is the length of CNTs and R is the radius of the overall filtration area on the filter paper surface. For CNTs (CG200) used here, the length L is around 166 nm and diameter is around 1 nm, so the MCNT is estimated around 2.4×105 g/mol based on the total number of carbon atoms in one CNT. With a CNT concentration of 15 μg/mL, the estimated delay time ΔT is around 0.19 s (0.038 s) for the filtration speed of 1 mL/hour (5 mL/hour). Based on a previous report, the rotational diffusion coefficient (DR) of CNTs with diameter of ˜1 nm is estimated to be ˜8 rad2/s when they are confined in a narrow layer. Therefore, a typical rotational diffusion time is around 0.125 s (˜1/DR). It can be seen when the speed is too fast (5 mL/hour), the delay time ΔT (0.038 s) between two CNTs depositing on the surface becomes much shorter than the CNT rotational diffusion time (0.125 s). It indicates that CNTs on the surface do not have enough time to fully align before the next nanotube deposits on top with such high filtration speed. The result is a decrease of the degree of CNT alignment, as evidenced by the experiment result.
CNT Concentration
CNT suspensions with different concentrations were prepared in the range from 4 to 30 μg/mL by diluting the parent high-concentration solution with nanopure water while keeping the concentration of DOC the same (0.05%) and below its CMC. Experimental results showed that alignment started degrading when the CNT concentration increases to a certain value. Specifically, for the CG 200 sample being used here, the transition concentration was ˜20 μg/mL as shown in
The phenomenon observed here is different from general 3-D liquid crystal phase transitions where a much higher concentration of CNTs (˜5 mg/mL) is required to drive the system from an isotropic phase into an anisotropic phase. However, our observation can be understood in the context of 2D nematic ordering. When an ordered phase occurs in a 2D manner, the bulk concentration does not need to be very high because of the natural accumulation of CNTs in a confined layer. In these experiments, under a steady filtration speed, individual CNTs sequentially fall into a confined layer near the surface, leading to an increase of surface density of CNTs in the confined region. Furthermore, as discussed before, the formation of alignment requires the rearrangement of CNTs. When the CNT concentration is higher, there will be more CNTs falling onto a given position in a given time interval (i.e., AT in Equation (3) decreased), which means that they cannot fully align among themselves, leading to a decrease of alignment degree.
CNT Diameters
When the above conditions are met, global alignment appears for all CNTs independent of growth methods and species, used for dispersion. However, the alignment quality varies for different species of CNTs. Within existing experimental results, the nematic order parameter, S, was more sensitive to the CNT diameter than to the aspect ratio. The SG65i sample, with the smallest diameter (0.73 nm) and the largest aspect ratio (˜525), showed a value of S around 0.75. This is much smaller than the S of the arc-discharge P2-SWNT (˜1), which had an average diameter of 1.4 nm and average aspect ratio of ˜161. Generally, large-diameter CNTs (arc-discharge and TUBALL) tended to align better than small-diameter CNTs (CoMoCAT and HiPco).
In order to further check the diameter influence on the degree of CNT alignment, the following test on the same batch of HiPco CNTs (batch #195.5) was performed. CNTs with diameters smaller than ˜0.8 nm were removed from the original sample by using the diameter-sorting capability of the ATPE method. Shown in
Without being bound by theory, the diameter influence on CNT alignment is probably due to the following two reasons. Firstly, CNT diameter will influence the strength of van der Waals attraction. The smaller the diameter of an individual CNT, the weaker the van der Waals force between the CNT and the substrate is. The shape of the potential well is dependent on the subtle competition of the van der Waals attraction and the electrostatic repulsion. If the van der Waals force is too weak, the potential well will become shallow. As a result, the degree of CNT alignment will decrease due to the loss of the 2D confinement. Secondly, the stiffness of CNTs is different for different diameters, which is proportional to (CNT diameter)3. So CNTs of small diameters are much less stiff compared to CNTs of large diameters. As a result, they tend to be bent and entangled more easily in the presence of external disturbances, leading to less ideal alignment structures.
Without further elaboration, it is believed that one skilled in the art can, using the description herein, utilize the present disclosure to its fullest extent. The embodiments described herein are to be construed as illustrative and not as constraining the remainder of the disclosure in any way whatsoever. While the embodiments have been shown and described, many variations and modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims, including all equivalents of the subject matter of the claims. The disclosures of all patents, patent applications and publications cited herein are hereby incorporated herein by reference, to the extent that they provide procedural or other details consistent with and supplementary to those set forth herein.
This application claims the benefit of U.S. Provisional Patent Application No. 62/233,901 filed on Sep. 28, 2015, which is incorporated herein by reference.
This invention was made with government support under Grant No. DE-FG02-06ER46308 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 62233901 | Sep 2015 | US |
