The present disclosure generally relates to methods and compositions for gravure printing.
Printed electronics has drawn tremendous interest in the past few decades (Refs. 1-6), as it offers an attractive alternative to conventional silicon-based fabrication technologies by enabling low-cost, large-area, flexible devices for many applications such as energy storage (Ref. 7), thin film transistor (Ref. 8), light-emitting diodes (Ref. 9) and wearable sensors for health monitoring (Ref. 10). Central to this technology are high-performance functional inks and high-throughput printing methods, such as screen (Ref. 11-15), inkjet (Ref. 16-18), gravure (Ref. 19, 20) and flexographic printing (Ref. 21). Among the existing printing methods, gravure printing, which utilizes direct transfer of functional inks through physical contact of engraved structures with a substrate, is a promising option for large-scale applications due to its high-speed, high-resolution deposition of functional materials and compatibility with roll-to-roll processes.
One important application of gravure printing is the fabrication of conductive elements as electrodes and conductors. The scope of gravure-printed electronic materials has been previously limited to polymers (e.g. PEDOT:PSS) (Ref. 22) and nanoparticles (Ref. 23, 24). Recently, one-dimensional nanomaterials (e.g. metal nanowires (Ref. 25, 26) and carbon nanotubes (Ref. 27, 28)) and two-dimensional nanomaterials (e.g. graphene) (Ref. 29, 30) have been gravure-printed on flexible substrates as flexible and transparent electrodes and interconnects. Among these nanomaterials, silver nanowires (AgNWs) have emerged with promising potential in electronic applications (Ref. 31-43). Compared to the silver particles used in the traditional silver inks, AgNWs offer better electrical conductivity and flexibility, which are key to flexible electronics applications. There have been recent studies on gravure printing of AgNWs as transparent conductive films (Ref. 25, 26), However, the best reported resolution was limited to 230 μm (Ref. 25). The printing resolution of gravure printing is mainly dependent on ink properties (e.g. surface tension and viscosity) (Ref. 44) in addition to trench resolution (Ref. 24). It remains challenging to realize high-resolution gravure printing of AgNWs as a result of their large length-to-diameter aspect ratio.
In one aspect, the present disclosure provides an ink composition including a plurality of metal nanowires, one or more water soluble polymers, and an aqueous liquid carrier. In one embodiment, the plurality of metal nanowires includes silver nanowires.
In another aspect, the present disclosure provides a method of gravure printing a metal nanowire structure onto a substrate including the steps of depositing a first quantity of the ink composition described herein into one or more cavities in a gravure plate; transferring the ink composition from the one or more cavities onto a substrate; and removing at least a portion of the liquid carrier and at least a portion of the one or more water soluble polymers from the ink composition to form a metal nanowire structure comprising a plurality of metal nanowires disposed on the substrate.
The present disclosure may be better understood with reference to the following figures.
Many modifications and other embodiments disclosed herein will come to mind to one skilled in the art to which the disclosed compositions and methods pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the disclosures are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. The skilled artisan will recognize many variants and adaptations of the aspects described herein. These variants and adaptations are intended to be included in the teachings of this disclosure and to be encompassed by the claims herein.
Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any of the other several embodiments without departing from the scope or spirit of the present disclosure.
Any recited method can be carried out in the order of events recited or in any other order that is logically possible. That is, unless otherwise expressly stated, it is in no way intended that any method or aspect set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not specifically state in the claims or descriptions that the steps are to be limited to a specific order, it is no way intended that an order be inferred, in any respect. This holds for any possible non-express basis for interpretation, including matters of logic with respect to arrangement of steps or operational flow, plain meaning derived from grammatical organization or punctuation, or the number or type of aspects described in the specification.
All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited. The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the present invention is not entitled to antedate such publication by virtue of prior invention. Further, the dates of publication provided herein can be different from the actual publication dates, which can require independent confirmation.
While aspects of the present disclosure can be described and claimed in a particular statutory class, such as the system statutory class, this is for convenience only and one of skill in the art will understand that each aspect of the present disclosure can be described and claimed in any statutory class.
It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosed compositions and methods belong. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealized or overly formal sense unless expressly defined herein.
Prior to describing the various aspects of the present disclosure, the following definitions are provided and should be used unless otherwise indicated. Additional terms may be defined elsewhere in the present disclosure.
As used herein, “comprising” is to be interpreted as specifying the presence of the stated features, integers, steps, or components as referred to, but does not preclude the presence or addition of one or more features, integers, steps, or components, or groups thereof. Moreover, each of the terms “by”, “comprising,” “comprises”, “comprised of,” “including,” “includes,” “included,” “involving,” “involves,” “involved,” and “such as” are used in their open, non-limiting sense and may be used interchangeably. Further, the term “comprising” is intended to include examples and aspects encompassed by the terms “consisting essentially of” and “consisting of.” Similarly, the term “consisting essentially of” is intended to include examples encompassed by the term “consisting of.
As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise.
It should be noted that ratios, concentrations, amounts, and other numerical data can be expressed herein in a range format. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as “about” that particular value in addition to the value itself. For example, if the value “10” is disclosed, then “about 10” is also disclosed. Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms a further aspect. For example, if the value “about 10” is disclosed, then “10” is also disclosed.
When a range is expressed, a further aspect includes from the one particular value and/or to the other particular value. For example, where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure, e.g. the phrase “x to y” includes the range from ‘x’ to ‘y’ as well as the range greater than ‘x’ and less than ‘y’. The range can also be expressed as an upper limit, e.g. ‘about x, y, z, or less’ and should be interpreted to include the specific ranges of ‘about x’, ‘about y’, and ‘about z’ as well as the ranges of ‘less than x’, less than y’, and ‘less than z’. Likewise, the phrase ‘about x, y, z, or greater’ should be interpreted to include the specific ranges of ‘about x’, ‘about y’, and ‘about z’ as well as the ranges of ‘greater than x’, greater than y’, and ‘greater than z’. In addition, the phrase “about ‘x’ to ‘y’”, where ‘x’ and ‘y’ are numerical values, includes “about ‘x’ to about ‘y’”.
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 numerical range of “about 0.1% to 5%” should be interpreted to include not only the explicitly recited values of about 0.1% to about 5%, but also include individual values (e.g., about 1%, about 2%, about 3%, and about 4%) and the sub-ranges (e.g., about 0.5% to about 1.1%; about 0.5% to about 2.4%; about 0.5% to about 3.2%, and about 0.5% to about 4.4%, and other possible sub-ranges) within the indicated range.
As used herein, the terms “about,” “approximate,” “at or about,” and “substantially” mean that the amount or value in question can be the exact value or a value that provides equivalent results or effects as recited in the claims or taught herein. That is, it is understood that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art such that equivalent results or effects are obtained. In some circumstances, the value that provides equivalent results or effects cannot be reasonably determined. In such cases, it is generally understood, as used herein, that “about” and “at or about” mean the nominal value indicated ±10% variation unless otherwise indicated or inferred. In general, an amount, size, formulation, parameter or other quantity or characteristic is “about,” “approximate,” or “at or about” whether or not expressly stated to be such. It is understood that where “about,” “approximate,” or “at or about” is used before a quantitative value, the parameter also includes the specific quantitative value itself, unless specifically stated otherwise.
As used herein, the term “effective amount” refers to an amount that is sufficient to achieve the desired modification of a physical property of the composition or material.
As used herein, the terms “optional” or “optionally” means that the subsequently described event or circumstance can or cannot occur, and that the description includes instances where said event or circumstance occurs and instances where it does not.
Unless otherwise specified, temperatures referred to herein are based on atmospheric pressure (i.e. one atmosphere).
The term “gravure printing” used herein refers to a scheme of filling ink in a groove formed on a surface of a printing plate and removing ink in portions other than the groove to thereby transfer only the ink filled in the groove to an object to be printed.
The term “gravure plate” used herein refers to plates used in gravure printing. The gravure plate may be etched or engraved. During printing process, the gravure plate may be smeared with ink, the higher surface is wiped clean, and the ink left in recess area of the gravure plate will make the print.
The term “gravure cylinder” used herein refers to a cylinder with engraving on the surface of the cylinder. In one embodiment, a gravure cylinder includes a steel cylinder base, or an underlying metal structure that supports the engraved image-carrying layer.
The term “thixotropic behavior” used herein refers to fluids that are non-Newtonian fluids, i.e. which can show a time-dependent change in viscosity. The term “non-Newtonian” refers to fluid having a viscosity that is dependent on an applied force such as shear or thermal forces. For example, shear thinning fluids decrease in viscosity with increasing rate of shear. The greater chemical fluid of the water barrier layer undergoes shear stress, the lower its viscosity will be. When the share stress is removed, the viscosity can be re-built up.
The term “aspect ratio” used herein refers to the ratio of its longer dimension to its shorter dimension.
Reference throughout this specification to “one embodiment”, “an embodiment”, “another embodiment”, “some embodiment,” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” “in another embodiment”, or “in some embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention. For example, in the appended claims, any of the claimed embodiments can be used in any combination.
Gravure printing is a promising technique for large-scale printed electronics. However, gravure printing of silver nanowires (AgNWs) so far has been limited in terms of resolution and electrical conductivity. In one aspect, gravure printing of water-based AgNW ink on a flexible substrate is demonstrated. By tailoring the ink properties, printing conditions and post-printing treatment, gravure printing enables printing of high-resolution, highly conductive AgNW patterns in large areas, with resolution as fine as 50 μm and conductivity as high as 5.34×104S cm−1. The printed AgNW patterns on the flexible substrate show excellent flexibility under repeated bending. All of these characteristics demonstrate the excellent potential of gravure printing of AgNWs for developing large-area flexible electronics.
In one aspect, the present disclosure provides large-scale, high-resolution patterning of AgNWs by gravure printing. A new type of water-based AgNW ink was developed. Rheological behavior of the ink was investigated to correlate the ink compositions, the rheological properties and the printing results. By tailoring the ink properties and printing conditions, continuous lines with resolution as fine as ˜50 μm were achieved over large areas with notable reliability and uniformity. The conductivity of the printed AgNW lines was measured to be as high as 5.34×104 S cm−1. In addition, the printed AgNW lines on a flexible polyethylene terephthalate (PET) film showed excellent flexibility under repeated bending.
In one aspect, the present disclosure provides an ink composition including a plurality of metal nanowires, one or more water soluble polymers, and an aqueous liquid carrier. In one embodiment, the plurality of metal nanowires includes silver nanowires. In one embodiment, the plurality of metal nanowires has an average longitudinal dimension of from about 10 micrometers to about 100 micrometers (μm), from about 10 μm to about 90 μm, from about 10 μm to about 80 μm, from about 10 μm to about 70 μm, from about 10 μm to about 60 μm, from about 10 μm to about 50 μm, from about 10 μm to about 40 μm, from about 10 μm to about 30 μm, or from about 10 μm to about 20 μm. In one embodiment, the plurality of metal nanowires has an average longitudinal dimension of about 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, or 100 μm.
In one embodiment, the plurality of metal nanowires has an average diameter of from about 10 nm to about 200 nm, about 20 nm to about 100 nm, about 30 nm to about 100 nm, about 40 nm to about 100 nm, about 50 nm to about 100 nm, about 60 nm to about 100 nm, about 70 nm to about 100 nm, or about 80 nm to about 100 nm. In one embodiment, the plurality of metal nanowires has an average diameter of about 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm, or 100 nm. In one embodiment, the plurality of metal nanowires has an average aspect ratio of greater than or equal to about 50:1, or greater than or equal to about 100:1, or greater than or equal to about 150:1, or greater than or equal to about 200:1, or greater than or equal to about 300:1. In one embodiment, the plurality of metal nanowires has an average aspect ratio of from about 50:1 to about 1000:1, from about 100:1 to about 1000:1, from about 150:1 to about 1000:1, from about 200:1 to about 1000:1, from about 300:1 to about 1000:1, or from about 400:1 to about 1000:1. In one embodiment, the plurality of metal nanowires has an average aspect ratio of about 50:1, 100:1, 150:1, 200:1, 250:1, 300:1, 350:1, 400:1, 450:1, 500:1, or 550:1.
In one embodiment, the ink composition includes from about 1 weight percent (wt %) to about 10 wt % metal nanowires, from about 2 wt % to about 9 wt % metal nanowires, 2.5 wt % to about 7.5 wt % metal nanowires, from about 3 wt % to about 7 wt % metal nanowires, or from about 4 wt % to about 6 wt % metal nanowires, based on the weight of the entire ink composition. In one embodiment, the ink composition includes about 1 wt %, 2 wt %, 3 wt %, 4 wt %, 5 wt %, 6 wt %, 7 wt %, 8 wt %, 9 wt %, or 10 wt % metal nanowires based on the weight of the entire ink composition.
In one embodiment, the one or more water soluble polymers includes a poly(ethylene oxide) polymer, optionally having a molecular weight of about 1,000,000 Dalton. In one embodiment, the one or more water soluble polymers includes a polyvinylpyrrolidone polymer. In one embodiment, the aqueous liquid carrier includes water, and optionally a co-solvent including an alcohol; optionally the alcohol includes methanol, ethanol, n-propanol, or n-butanol.
In one embodiment, the ink composition includes up to about 96 wt % aqueous liquid carrier, based on the weight of the entire ink composition. In one embodiment, the ink composition includes up to about 60 wt % water, based on the weight of the entire ink composition. In one embodiment, the ink composition has thixotropic behavior. In one embodiment, the ink composition has a capillary number (Ca) of about 0.8 to about 1.2, optionally from about 0.9 to about 1.1, optionally about 1.0. In one embodiment, the ink composition further includes one or more additives comprising a surfactant, a dispersant, a corrosion inhibitor, a stabilizer, an adhesion promoter, an antioxidant, a viscosity modifier, or a combination or mixture thereof.
In one aspect, the present disclosure provides a method of gravure printing a metal nanowire structure onto a substrate including the steps of depositing a first quantity of the ink composition described herein into one or more cavities in a gravure plate; transferring the ink composition from the one or more cavities onto a substrate; and removing at least a portion of the liquid carrier and at least a portion of the one or more water soluble polymers from the ink composition to form a metal nanowire structure comprising a plurality of metal nanowires disposed on the substrate.
In one embodiment, the gravure plate is a gravure cylinder. In one embodiment, the step of depositing the ink composition comprises applying a second quantity of ink to the gravure plate that is in excess of the first quantity, and removing the excess quantity of the ink composition from the gravure plate with a doctor blade. In one embodiment, the step of transferring the ink composition includes directly transferring the ink composition to the substrate by directly contacting the substrate with the surface of the gravure plate, resulting in the transfer of the ink composition from the one or more cavities to the substrate. In one embodiment, the step of transferring the ink composition includes transferring the ink composition to a transfer surface, and contacting the substrate with the transfer surface resulting in a transfer of the ink composition to the substrate.
In one embodiment, the step of removing the one or more water soluble polymers includes one or more iterations of: increasing the temperature of the ink composition to a first temperature; and rinsing the ink composition with water. In one embodiment, the method includes from 1 to 10 iterations of increasing the temperature and rinsing; optionally from 1 to 8 iterations, from 1 to 7 iterations, or from 1 to 6 iterations, from 1 to 5 iterations, from 1 to 4 iterations or from 1 to 3 iterations. In one embodiment, the first temperature is at or above a glass transition temperature of one of the water soluble polymers. In one embodiment, the increasing the temperature of the ink composition further includes thermal annealing of one or more of the metal nanowires. In one embodiment, the substrate is a flexible substrate.
In one embodiment, the method forms a metal nanowire structure having a smallest dimension of less than about 100 μm, optionally less than about 90 μm, or less than about 80 μm or less than about 70 μm, or less than about 60 μm, or about 50 μm. In one embodiment, the metal nanowire structure has a smallest dimension of about 1-100 μm, about 5-90 μm, about 5-80 μm, about 5-70 μm, about 5-60 μm, about 5-50 μm, about 5-40 μm, or about 5-30 μm. In one embodiment, the metal nanowire structure has a smallest dimension of about 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, or 100 μm. In one embodiment, the metal nanowire structure includes a structure formed by the method described herein. In one embodiment, the metal nanowire structure includes a one-dimensional pattern, a two-dimensional pattern, or a combination thereof.
In one embodiment, the metal nanowire structure includes one or more of a line, a curve, a fractal pattern, a grid, or a combination thereof. In one embodiment, the metal nanowire structure includes two or more interconnected metal nanowire structures. In one embodiment, the metal nanowire structure includes a three-dimensional pattern. In one embodiment, the plurality of metal nanowires in the metal nanowire structure are oriented along a first direction. In one embodiment, at least 30% of the metal nanowires have a longitudinal dimension within plus or minus 15 degrees the first direction.
In one embodiment, a metal nanowire structure having a length of up to 3 centimeters has an electrical conductivity of greater than about 3.5×104 S cm−1, optionally greater than 4.0×104 S cm−1, or greater than 4.5×104 S cm−1, or greater than 5.0×104 S cm−1. In one embodiment, a metal nanowire structure having a length of up to 3 centimeters has an electrical conductivity from about 3.5×104 S cm−1 to about 3.5×105 S cm−1, from about 4.0×104 S cm−1 to about 4.0×105 S cm−1, from about 4.5×104 S cm−1 to about 4.5×105 S cm−1, from about 5.0×104 S cm−1 to about 5.0×105 S cm−1, from about 5.5×104 S cm−1 to about 5.5×105 S cm−1, In one embodiment, a metal nanowire structure having a length of up to 3 centimeters has a resistance of less than about 260Ω, optionally less than about 200Ω, or less than about 150Ω, or less than about 100Ω. In one embodiment, a metal nanowire structure having a length of up to 3 centimeters has a resistance of about 1-500Ω, 1-400Ω, 1-300Ω, 1-260Ω, 1-200Ω, 1-150Ω, 1-100Ω, or 1-50Ω. In one embodiment, a metal nanowire structure having a length of up to 3 centimeters has a resistance of about 50Ω, 100Ω, 150Ω, 200Ω, 250Ω, or 300Ω.
In one embodiment, the metal nanowire structure is bendable. In one embodiment, the metal nanowire structure maintains a constant resistance (R/R0=1) up to a tensile bending strain about 1.25%. In one embodiment, the metal nanowire structure maintains a constant resistance (R/R0=1) over 500 cycles up to a tensile bending strain of about 1.25%.
Materials. The reagents used in this study included silver nitrate (AgNO3), poly(vinylpyrrolidone) (PVP, K-30), sodium chloride (NaCl), ethylene glycol (EG), acetone, ethanol, Poly(ethylene oxide) (PEO, average Mv ˜1,000,000). All reagents were of analytical grade and purchased from Sigma Aldrich. All chemicals were used as received without further purification.
Synthesis of AgNWs. AgNWs were fabricated using a modified polyol reduction method (Ref. 55). PVP solution (50 mL 0.09 M in EG) was heated to 170° C. in a three-neck flask with stirring at 300 rpm for 1 h. NaCl solution (150 μL 0.1 M in EG) was then added and stirred for 10 min. AgNO3 solution (50 mL 0.06 M in EG) was subsequently added to the flask dropwise at a rate of 2.5 mL min−1. After the AgNO3 solution was added to the flask, the oil bath reaction lasted another 20 min. The clear solution changed color to glistening gray, which indicated the formation of AgNWs. Then the solution was cooled to room temperature and was centrifuged at 2000 rpm for 10 min with acetone and ethanol, respectively, to remove solvent (EG), surfactant (PVP) and other impurities (e.g. small amount of Ag nanoparticles) in the supernatant.
Preparation of AgNW Inks. Poly(ethylene oxide) (PEO, average My ˜1,000,000) was purchased from Sigma Aldrich and used as received. 4 wt % PEO solution was prepared by dissolving PEO powder in a mixed solvent of ˜50 wt % deionized water (DI water) and ˜46 wt % ethanol. AgNWs with different weight content were added into the PEO solution and the as-prepared inks were stirred with magnetic stir bar the speed of 1000 rpm for 60 min to ensure uniform AgNW inks.
Gravure Printing and Post-Printing Treatment of AgNW Patterns. Inverse direct gravure printing system, which used a flat printing plate to transfer patterns to a substrate on a roll, was set up. The flat printing plate, which contained engraved intaglio trenches, was fabricated by laser cutting of flat clear cast acrylic sheet (McMaster-Carr) using a laser cutter (VLS 6.60, Universal Laser Systems). Trenches with different width were obtained by controlling the power of the laser beam and the depths of all trenches were ˜60 μm. A stainless steel cylinder with diameter of 25 mm was used as an impression roller. Polyethylene terephthalate film (PET, MELINEX® 454, Dupont) with thickness of 125 μm was first treated with plasma cleaner (PDC-32G, Harris Plasma) for 1 min to enhance the hydrophilic property and then wrapped around the printing roller by an elastic double-sided tape. The angle between the printing plate and the doctor blade was fixed at 75°. The printing speed was ˜1.5 mm·s−1. The printed AgNW patterns were first annealed on a hot plate at 150° C. for 2 min to evaporate the water and ethanol, followed by washing with DI water at 70° C. for 10 min to remove part of PVP and PEO. The thermal annealing and water washing process were repeated several times. Finally, the coating was annealed at 150° C. for 5 min to help fuse the AgNW junctions, and the final conductive AgNW patterns were obtained.
Characterization. The morphologies of the as-synthesized AgNWs and the gravure-printed AgNWs lines were tested by field-emission scanning electron microscopy (SEM, FEI Quanta 3D FEG) operated at 5 kV. The transmission electron microscope (TEM) image of the as-synthesized AgNWs was obtained by field emission ultra-high resolution scanning transmission electron microscope (Field Emission STEM, JEOL 2010F). The structural characterization of the AgNW films was performed using X-ray diffraction (XRD, Rigaku SmartLab X-Ray Diffractometer) with Cu Kα radiation (λ=0.1542 nm). Rheological behavior of the AgNW inks was measured using a Haake VT500 viscotester system. A pre-conditioning step at a shear rate of 0.1 s−1 for 10 s was applied before each test to assure uniformity of the fluids and all of the tests were done at room temperature (25° C.). Surface tension of the AgNW inks was tested by Ramé-Hart contact angle goniometer at room temperature (25° C.). The dimensions of the gravure-printed AgNW lines were obtained by using an optical microscope (Nikon Eclipse). Alignment of AgNWs in the gravure-printed line patterns was analyzed with ImageJ software and at least 100 AgNWs for each line were analyzed. Thickness of the gravure-printed AgNW patterns was measured by a Dektak profilometer. A Fluke 115 true RMS multimeter was used to measure the resistance of the printed AgNW lines. The mechanical stability test was performed using a lab-made bending test machine. Scotch tape was used to evaluate the adhesion of the printed AgNW line after post-printing treatment. Scotch tape was applied on the samples and after pressing them on the substrate by hands the tape was peeled off slowly.
Calculation of sheet resistance of the gravure-printed AgNW lines. According to the formula
where ρ is the electrical resistivity of the printed lines, A is the cross-sectional area of the printed lines, L is the length of the printed lines, Rs is the sheet resistance of the printed lines and h is the thickness of the printed lines. Thus, sheet resistance can be calculated by
The length of the printed lines was 3 cm for all samples. The resistance of the printed lines can be measured by a multimeter, while the cross section and thickness of the printed lines can be measured by a Dektak profilometer. The cross section area and the thickness of the printed lines were integral area and average thickness, respectively.
The AgNWs synthesized by the modified polyol reduction method were characterized by SEM and TEM, as shown in
Viscosity is the most important ink parameter to tailor. PEO, a flexible, non-ionic water-soluble polymer used in a wide variety of applications, was used to assist ink formulation for the following reasons (Ref. 45, 46). First, PEO has a high molecular weight (average Mv ˜1,000,000), which can increase the viscosity of the ink dramatically, and provides thixotropic behavior for the AgNW ink; second, PEO can function as a dispersive agent to improve dispersion of AgNWs as the hydroxy groups can bond with the surface of the AgNWs; third, PEO is also alcohol-soluble and can precipitate together with AgNWs to generate solid composite sediments with good redispersion capability, which is conducive to forming a uniform, continuous pattern. In a typical formulation, DI water, ethanol and PEO were mixed by stirring at a weight ratio of 12.5:11.5:1 for 24 h to make homogeneous solution. Then, AgNWs were added into the solution to make the gravure-printing inks with three different AgNW solid contents: 3.0 wt % (AgNW Ink-L), 3.7 wt % (AgNW Ink-M) and 5.0 wt % (AgNW Ink-H). The inks were stirred for 10 min to obtain the final stable AgNW inks. As-prepared viscous AgNW Ink-H is shown in
The rheological behavior of the AgNW inks was investigated to determine how the AgNW solid content affected the ink properties.
where η is the ink's viscosity, γ is the ink's surface tension and U is the printing speed (˜1.5 mm·s−1 in this work). After calculation, the Ca for the AgNW Ink-L, AgNW Ink-M, and AgNW Ink-H were 0.62, 0.84, and 1.09, respectively. According to Ref. 20, at very low capillary number (Ca<1), pattern fidelity was deteriorated by ink drag-out from the cells; at very high capillary number (Ca>1), inefficient doctoring left ink in non-patterned areas. Optimal printing can be achieved by adjusting the printing speed and the ink parameters to make Ca≈1 (Ref. 20). Accordingly, the ink with a concentration of 5.0 wt % (AgNW Ink-H) was selected for printing in the rest of this work, as it possessed the rheological properties that best meet the printability.
Inverse direct gravure printing was used to print AgNW patterns, as shown schematically in
Two types of water-soluble polymers existed in the AgNW inks—poly(vinylpyrrolidone) (PVP) coating that was introduced to control the growth of AgNWs during the synthesis (also serving as a surfactant that helps disperse AgNWs in solutions) and PEO that was used as the additive for formulating the AgNW inks. These non-conductive polymers can be seen as barriers to electron transport, so the printed AgNWs had relatively high electrical resistance. After gravure printing, post-printing treatment of the AgNW patterns, including thermal annealing and water washing, was developed to improve the electrical conductivity.
The mechanism of the post-printing treatment was further investigated. As shown in
To evaluate the electrical properties of the printed AgNWs, 3 cm-length AgNW lines with widths of 50, 75, 100, 125 and 150 μm were gravure-printed on a PET film and treated with thermal annealing and water washing.
A clear linear relationship, with the correlation factors over 0.998 in all cases, can be seen, which indicates excellent uniformity of the printed AgNW lines of different widths. It can also be seen that the resistance decreases with increasing width of the AgNW lines. For example, the resistances were measured to be 262.2±9.0, 179.2±8.4, 130.1±5.9, 98.1±1.1 and 55.2±0.4 0 for the line widths of 50, 75, 100, 125 and 150 μm, respectively, with the corresponding sheet resistances calculated to be 0.468±0.016, 0.408±0.014, 0.324±0.015, 0.279±0.014 and 0.229±0.006 Ωsq−1 (
where σ, R, L, and A are conductivity, resistance, the length and the cross-sectional area of the AgNW lines, respectively. Based on the measured resistances and the line geometries, the conductivities were calculated to be (5.34±0.35)×104, (4.91±0.23)×104, (4.41±0.27)×104, (3.98±0.16)×104and (3.64±0.14)×104 S cm−1 for the line widths of 50, 75, 100, 125 and 150 μm, respectively. It can be seen that the electrical conductivity decreases slightly as the printed line width increases. A similar trend was observed in screen-printed AgNW lines, which was attributed to the presence of voids in the AgNW patterns with larger line widths (Ref. 13). In this work, however, only a few voids were observed and this trend is more likely due to the NW alignment. The AgNWs showed better alignment along the printing direction for the narrower line, as compared to the wider line where a significant proportion of AgNWs were randomly oriented (
In addition to being highly conductive, the printed AgNW lines exhibited robust mechanical responses under tensile bending condition, which is of critical relevance to flexible electronics. The bending test (
To demonstrate the general applicability of the developed gravure printing method, large-area and complicated AgNW patterns (e.g. lines, curves, Greek cross fractal patterns and grids) with different line widths and shapes were printed on PET films (
Water-based AgNW inks were developed and gravure printed on flexible PET films. The AgNW ink, which contains a low solid content of 5.0 wt %, had a viscosity as high as 20.9 Pa s at 10 s-1 shear rate and appropriate rheological behavior suitable for gravure printing. Uniform and sharp-edged lines with resolution of 50 μm were obtained by gravure printing of the AgNW ink. Moreover, post-printing treatment with a low thermal annealing temperature of 150° C. and water washing was developed, which improved the electrical conductivity of the printed patterns to as high as 5.34×104 S cm−1. In addition, gravure-printed large-area AgNW grids indicates that the integration of AgNWs with gravure printing holds promising potential for commercially relevant, highly scalable applications in printed and flexible electronics.
The present disclosure further includes the following embodiments/paragraphs.
depositing a first quantity of the ink composition according to any one of claims Error! Reference source not found. to Error! Reference source not found. into one or more cavities in a gravure plate;
transferring the ink composition from the one or more cavities onto a substrate; and
removing at least a portion of the liquid carrier and at least a portion of the one or more water soluble polymers from the ink composition to form a metal nanowire structure comprising a plurality of metal nanowires disposed on the substrate.
The following references are incorporated herein in their entirety:
It will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. Other embodiments of the disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the disclosure disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the disclosure being indicated by the following claims.
This application claims the benefit of U.S. Provisional Application Ser. No. 62/908,708, filed Oct. 1, 2019, which is incorporated herein by reference in its entirety.
This invention was made with U.S. Government support under grant number 1160483, awarded by the National Science Foundation. The U.S. government has certain rights in the invention.
Number | Date | Country | |
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62908708 | Oct 2019 | US |