Patent Application
20230399487
Nanoscale materials, such as metal/metal oxide nanowires, quantum dots and carbon nanotubes (CNTs), attract significant research interest in the area of optics and electronics. Due to a fascinating combination of high electrical conductivity, unique tunable electronic properties, flexibility and transparency, percolating carbon nanotube networks (2-10 nm) and thin films (10-100 nm) have demonstrated high performance as electrodes and channel materials in a plethora of rigid and flexible electronic and optoelectronic devices such as, e.g., organic light emitting diodes (OLEDs), field effect transistors (FETs), sensors, touch screen panels and integrated circuits.
Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.
Disclosed herein are various examples related to self-assembled carbon nanotube (CNT) films. For example, CNT dispersion compositions, and methods of manufacturing thin carbon nanotube films by self-assembly on various substrates are presented. More specifically, carbon nanotube solutions can comprise aqueous dispersions of carbon nanotubes and a minimum quantity of amphiphilic pendant polymers. The CNT dispersion compositions can enable self-assembly of carbon nanotube films, e.g., on hydrophobic substrates.
Scalable fabrication of CNT thin film networks is important for widespread industrial implementation of CNT-based electronic devices. In principle, solution processing methods, including vacuum filtration/transfer, ultrasonic spraying, ink jet printing, dip-coating and slot-die coating, offer multiple advantages for scalability over direct growth of CNT films: lower temperature (plastic compatibility), speed and cost efficiency.
Successful solution-based deposition of uniform thin CNT films can require several important steps: 1) an efficient and stable dispersion of purified carbon nanotubes; 2) substrate functionalization to facilitate CNT adhesion; 3) uniform deposition and drying methods; and 4) in most cases, a washing step after CNT film coating to remove dispersant aids. These steps can be particularly stringent in the case of depositing very dilute, percolating CNT networks (2-10 nm films with carbon mass surface density of 142-710 ng/cm2) on slightly hydrophilic/semi-hydrophobic (e.g., most plastics, contact angles 6=70-90° and especially on hydrophobic (90-120°) substrates.
Uniform coating of low energy hydrophobic or semi-hydrophobic substrates with conducting transparent films is imperative for high performance electronic and optoelectronic devices. For example, for OLED display applications, a semiconducting layer uniformity of 5 to 15% for a high pixel-to-pixel homogeneity over the display area is demanded. Depending on the CNT coating method, the CNT solution viscosity and surface tension must be modified to be amenable with the surface energy of the substrate. Low energy substrates typically demand “surface activation”—functionalization by UV/oxygen or ozone treatment, corona discharge, and/or presence of high concentration surface wetting agents, binders and adhesion promoters—either on the substrate or in the CNT dispersion. These functionalities often change the surface properties irreversibly and/or are difficult to remove without introducing defects and non-uniformity in deposited CNT films. The activation agents can create water/oxygen redox couples or other charge traps in the vicinity of the electrode surface, which prove detrimental for electronic devices such as CNT-enabled TFTs. As such, forming uniform thin CNT film coatings on hydrophobic and semi-hydrophobic substrates remains a major challenge.
Self-assembly (SA) is a simple, fast and cost-effective method for the deposition of CNT networks, relying on interactions between CNT dispersions and the substrate. The SA method alleviates some of the most demanding rheology requirements for CNT dispersions by engineering the interactions between the carbon nanotube surface and the substrate. The Layer-by-Layer (LBL), Langmuir-Blodgett and Langmuir-Schaefer, evaporation driven dip-coating techniques have been utilized previously in attempts to form dilute CNT layers on hydrophilic substrates, but the deposited CNT coatings suffer from severe nonuniformity issues. Moreover, data on the performance of the SA-deposited CNT networks in electronic devices, scalability, and especially, data on the CNT film uniformity characterization, remain scarce. Selection of a suitable carbon nanotube dispersion formulation ensures the self-association of CNTs into homogenous films on a variety of hydrophilic and hydrophobic substrates. Therefore, CNT dispersion formulations, optimized for the scalable self-assembly methods, are highly desired.
Carbon nanotubes are bound into bundles due to strong van der Waals interactions. Typically, high power ultrasonication and/or dispersant agents are needed to de-bundle, solubilize, and stabilize the CNTs. Amphiphilic molecules are important stabilizers of dispersed systems. Commonly used CNT dispersant aids, such as ionic and non-ionic amphiphilic surfactants and small molecules (e.g., Triton-X, sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB)), associated with the carbon nanotube surface remain in a dynamic equilibrium with the surfactant molecules in the bulk. This dynamic equilibrium necessitates high surfactant concentrations, typically on the order of the surfactants' critical micelle concentration (CMC), for CNT stabilization.
In contrast, polymer amphiphiles provide multi-point and therefore, more stable association. Importantly, amphiphilic polymers tend to self-assemble and form extended, highly organized hierarchal supramolecular structures (sheets, networks, fibers, ribbons) on air-liquid, liquid-liquid and solid-liquid interfaces. Self-assembly and rheological properties of such polymer systems are responsive to external stimuli, such as temperature, shear forces, pH, ionic strength, etc., which allows for a fast and precise property control. Among other amphiphilic polymers, water soluble, non-toxic, and biodegradable/biocompatible polyethylene glycol and polysaccharides, e.g., cellulose derivatives, starch, gelatin, chitosan, are known to effectively disperse carbon nanotubes due to non-covalent π-π interactions and entropic hydrophobic forces. Functionalization of amphiphilic polymers with pendant groups (e.g, polycyclic aromatic hydrocarbons, like pyrene, perylene etc.) that can associate with carbon nanotubes via strong van der Waals forces, provides an additional CNT association/dispersion stabilization mechanism.
In the present disclosure, solution processing, dip-coating, and Langmuir techniques can be used in a novel scalable approach to form self-assembled (SA) networks of water-soluble amphiphilic pendant polymer-functionalized carbon nanotubes (CNTs), including single walled carbon nanotubes (SWNTs), multi-walled, few-walled carbon nanotubes, and araphene. This approach allows for the fabrication of uniform thin SWNT films on a variety of flexible and rigid substrates. The homogenous SA SWNT films can be formed on hydrophilic, slightly hydrophobic and the most challenging low energy, highly hydrophobic substrates useful for various electronic and optoelectronic applications. The applications include but are not limited to thin film transistors (TFTs), vertical field effect transistors (VFETs), vertical organic light emitting transistors (VOLETs), organic light-emitting diode (OLED) and quantum-dot (QD) based displays, circuit boards and touch-screen panels in cellphone and automotive displays.
Various examples related to the carbon nanotube (CNT) solution compositions comprised of aqueous dispersions of carbon nanotubes and amphiphilic pendant polymers will now be disclosed. The CNT dispersion compositions can enable self-assembly of CNTs on a broad range of rigid and flexible hydrophobic and hydrophilic substrates. These self-assembly (SA) methods allow for scalable fabrication of uniform, percolating CNT networks and thin CNT films on a broad range of hydrophilic and more challenging low-energy, hydrophobic substrates, useful for various electronic applications, including but not limited to VFETs, VOLETs, OLEDs, QD based displays and automotive touch screen displays. Reference will now be made in detail to the description of the embodiments as illustrated in the drawings, wherein like reference numbers indicate like parts throughout the several views.
Referring to
Another example is directed to an aqueous amphiphilic pendant polymer-based carbon nanotube dispersion, containing a minimum quantity of dispersant. Excess dispersant can be removed from the CNT dispersion solution by dialysis, microfiltration, filtration/washing or centrifugation/washing/decanting cycles until the non-associated, CNT-free polymer concentration in solution is <10 μg/ml and preferably <1 μg/ml. Removal of polymer excess is important for the CNT thin film formation on the substrate to avoid preferential self-association of CNT-free polymer dispersant on the substrate, blocking the association of the CNT associated dispersant. This can also ensure ease of polymer residue elimination and/or removal after the CNT film is fabricated without introducing defects and non-uniformity, After the film is dried, the polymer dispersant residue can be cleaned by washing the CNT film on the substrate in water and/or common organic solvents, such as alcohols, acetone, mildly acidic or basic aqueous solutions, or by light irradiation, etc, Once the polymer dispersant residue is removed, the CNT film electrical conductivity and optical transmittance are increased.
An example can be directed to the self-assembly methods of forming thin and uniform carbon nanotube films on a variety of rigid and flexible substrates. The multitude of substrates include hydrophilic (θ=0-90°) and low-energy hydrophobic substrates (θ=90-120°. Examples of hydrophilic substrates include, but are not limited to, glass, metal, and metal oxides. Semi-hydrophobic or slightly hydrophilic substrates (θ=70-90°) include, but not limited to, plastics like PET, PVC, PVDF, polyimide, hydrophobized glass and Si/SiOx. Examples of the most challenging low-energy hydrophobic substrates include, but are not limited to, fluorinated polymers, such as PTFE, PFA, ETFE, Teflon-AF, Hyflon, Cytop; hydrophobic non-fluorinated polymers; fluorinated silanes; polysiloxanes (PDMS).
Another example can be directed to methods of fabricating thin CNT films as illustrated in
The pre-formed self-associated CNT network can then be picked-up from the interface using an inverted hydrophilic or hydrophobic substrate 202, allowing for deposition of the uniform thin SA CNT film 203. When the substrate 202 is placed face down onto the pre-formed thin homogenous CNT film at the air-liquid interface the functionalized CNTs adhere to the substrate surface. If an ordered directional CNT film formation is needed for a specific application, the CNT network pre-formed at the air-CNT dispersion interface could be compressed in a certain direction using a Langmuir trough.
One feature of the first SA method is the preliminary self-assembly of the CNTs into a thin-film layer at the air-liquid interface of the CNT dispersion. Then the pre-formed CNT film can adhere to the substrate surface by contact. Self-assembly at the air-liquid interface and in the bulk of solution occurs predominantly via hydrophobic interactions between hydrophobic segments, building blocks or functionalities of the amphiphilic polymer backbone. Self-assembly can be assisted by hydrogen bonding between hydrophilic segments or polar functional groups of the amphiphilic polymer backbone.
In the three SA methods, the carbon nanotube film density could be reliably controlled by the functionalized CNT concentration in the dispersion, coating time and/or by the number of coating steps. For example, the functionalized CNT dispersions can be diluted to a concentration needed to deposit a single layer CNT film of the desired CNT density (so called “single-use” dispersions). After a single-layer or multilayer CNT film deposition using one of the three SA methods is complete, and the layer dried on the substrate surface, the formed coating can be soaked in water, common organic solvents such as alcohols, acetone, or mild acidic or basic aqueous solutions to remove the minimal polymer dispersant residue.
Uniformity of the SA-formed CNT films is a prerequisite to achieve high performance electronic and optoelectronic devices. The uniformity of the self-assembled SWNT films can be visualized by making multi-point AFM images of different areas of the film.
As self-assembly in the bulk of solution (the second SA method) occurs predominantly via hydrophobic forces, the CNTs association with hydrophobic and semi-hydrophobic substrates is quite fast (10 s-30 min coating step, depending on the solution CNT concentration and the desired film density). The association of the CNTs with hydrophilic substrates (e.g., metal/metal oxides, glass) in this case has been observed to proceed more slowly, needing longer times (60 s-2 h, depending on the desired film density). Thus, the second SA method can be used for a selective CNT deposition on predominantly hydrophobic substrates, when deposition is allowed to proceed in a short coating time regime. The second SA method can be utilized for the CNT deposition simultaneously on both hydrophobic and hydrophilic substrates in a longer time regime. The CNT self-assembly on hydrophilic substrates can be facilitated by substrate treatment with adhesion promoters or hydrophobizing agents such as, e.g., aminopropyltriethoxysilane (APTES), fluorinated silanes, and/or fluorinated surfactants. The ability to control the self-assembly on different energy substrates is important for patterning processes of different electronic and optoelectronic device structures and/or architectures.
The uniformity of the self-assembled SWNT films can be evaluated by taking a series of AFM images of the film. Another method to assess homogeneity of the self-assembled SWNT films, is to deposit SWNTs onto so called resistance uniformity test devices in either a top-contact configuration 400 as illustrated in
The metal contacts 404 can first be deposited onto the substrate with subsequent deposition of the hydrophobic layer 402. The CNT film can then be self-assembled simultaneously on hydrophobic layer and hydrophilic metal contacts, e.g., using the three SA methods. For a successful CNT film deposition, using the second SA method, the coating process should be sufficiently long (60 s-2 h, depending on the desired film density) or the substrate can be treated with the adhesion promoters and/or hydrophobizing agents to facilitate self-assembly on hydrophilic metal contacts.
Referring to
Recently, a fundamentally new CNT-enabled transistor architecture has been introduced, that promises to greatly lower active-matrix light-emitting diode (AMOLED) display manufacturing costs by dramatically reducing the backplane circuit complexity. At minimum, the conventional active-matrix backplane circuit needed to independently light each pixel of an AMOLED display utilizes a switching transistor, a drive transistor, and a capacitor in the so called 2T+1C circuit. The drive TFT must be able to source large and stable on-state currents at reasonable drive voltages. This requires that the TFT have a very low on-state channel resistance, which can be achieved in one of two ways: (1) employing a relatively high-mobility semiconductor, or with (2) a short channel length. The first option limits the choice of channel semiconductors to a handful of inorganic materials (IGZO and LTPS) which are difficult to process on large substrate sizes. The second option—in the conventional TFT architecture—is limited by the patterning resolution used to define the source-to-drain spacing.
A dilute, transparent carbon nanotube film is a device component and acts as a planar source electrode 704 with a contact 705, on which in the VFET device the semiconductor 706 and drain 707 are stacked. The gate field modulates the charge injection barrier at the interface of the CNT film and the semiconducting channel. In a VOLET device a hole transport layer 708, an emissive layer 709 and an electron transport layer 710 (the organic light emitting diode components) can be added between the channel layer and the drain electrode. As proof of principle, the SWNT films used in these devices were prepared by a vacuum filtration/transfer method. The vacuum filtration/transfer method produces highly uniform SWNT films with a precise film density control but is hardly scalable. Self-assembly deposition methods described in this disclosure can provide a scalable alternative for the uniform thin CNT film deposition.
Performance of the CNT-VOLETs rely on the high electrical conductivity and optical transmittance of the CNT films. In these devices the magnitude of the source-drain current is dictated by the gate-modulated charge injection barrier at the interface between the CNT film and the organic semiconductor (OSC). The SWNT film should be sufficiently dilute (CNT film with carbon mass surface densities of 150-1000 ng/cm2) to allow the gate-field access to the CNT-OSC interface (through the naturally occurring pores in the CNT film) to electrostatically control the height and width of the injection barrier.
The JLV graphs in
The disclosed methodology allows for the fabrication of highly uniform, dilute, percolating CNT networks and thin CNT films onto low energy substrates by facile, scalable self-assembly methods that do not require substrate surface functionalization or CNT dispersion rheology adjustment. This can facilitate widespread CNT film-based device implementation in the area of optics and electronic, including OLED displays, touch screen panels, TFTs, FETs and sensor industries.
An example of the present disclosure is directed to the amphiphilic pendant polymer CNT dispersion formulation comprising aqueous dispersion of SWNTs and pyrene-substituted hydroxypropylcellulose (Py-HPC, 1 pyrene functional group per 130 polymer repeat units, Mw=60,000 Da). Although Py-HPC is a known CNT dispersant, no self-assembly CNT film deposition using Py-HPC-based CNT dispersions has been reported. Minimization of the excess Py-HPC polymer in the dispersion is one of the factors enabling SWNTs self-assembly on the substrates. Excess dispersant polymer results in a preferential polymer deposition on the substrate. The functionalization of HPC with Py-groups allowed for the additional CNT association (via Py-CNT sidewall pi-stacking) and thus for the utilization of much lower Py-HPC concentrations to obtain stable CNT dispersions. First, a (1:1 weight ratio) PyHPC: SWNT solution (Cpy-HPC=0.001-0.0014 wt % or 10-15 μg/ml) was prepared and then the non-associated, CNT-free polymer excess was removed by dialysis until the Py-HPC concentration reached <0.3 μg/ml. No SWNT flocculation was observed in the dialyzed Py-HPC/SWNT dispersions for over a year. Such minimum excess SWNT dispersion enabled carbon nanotube self-assembly and the uniform SWNT thin film formation preferentially on hydrophobic substrates, which is advantageous for various electronic applications.
It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.
The term “substantially” is meant to permit deviations from the descriptive term that don't negatively impact the intended purpose. Descriptive terms are implicitly understood to be modified by the word substantially, even if the term is not explicitly modified by the word substantially.
It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a concentration range of “about 0.1% to about 5%” should be interpreted to include not only the explicitly recited concentration of about 0.1 wt % to about 5 wt %, but also include individual concentrations (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within the indicated range. The term “about” can include traditional rounding according to significant figures of numerical values. In addition, the phrase “about ‘x’ to ‘y’” includes “about ‘x’ to about y”.
This application claims priority to, and the benefit of, co-pending U.S. provisional application entitled “Self-Assembled Thin Carbon Nanotube Films Using Amphiphilic Pendant Polymer Dispersants” having Ser. No. 63/350,678, filed Jun. 9, 2022, which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 63350678 | Jun 2022 | US |
