Molding Inkjetted Silver on Nanostructured Surfaces for High-Throughput Structural Color Printing Hao Jiang,* Sheida Alan, Haleh Shahbazbegian, Jasbir N. Patel, and Bozena Kaminska School of Engineering Science, Simon Fraser University, Burnaby, British Columbia V5A1S6, Canada S Supporting Information *
ABSTRACT: Inkjet printing of silver ink has been widely used to print conductive patterns in flexible electronic devices, and the printed patterns are commonly known to be colorless. We demonstrate that by printing a single type of ordinary silver nanoparticle ink on top of a substrate patterned with polymer nanostructures, the printed silver is molded by the nanostructures and gains robust structural colors. The colors are tunable by varying the geometries of nanostructures, and a broad range of visual colors can be achieved by mixing the red, green, and blue colors displayed from silver dots printed on different nanostructures. Such mechanism can enable full-color, scalable, high-throughput, versatile, and cost-effective printing of structural color images for regular publishing and displaying purposes. In experiments, we implemented a transparent polymer substrate patterned with diffractive nanostructure arrays to print fullcolor images. The printed images display color-shifting optically variable effects useful for security and authentication applications that demand customizable anticounterfeiting features. KEYWORDS: inkjet printing, structural color, silver nanoparticle, nanostructure array, diffractive color, plasmonic color, optically variable device color printing are mainly limited to security features3,4 and microscale nano-optical displays.1,5 In order to precisely tailor the optical responses and to print colors in ultrahigh resolution, nanoscale lithography techniques are usually used to pattern nanostructures, such as electron beam lithography (EBL)1−4,16,17 and focused ion beam (FIB) milling.11,18 In general, due to the very slow writing speed of nanoscale lithography, the reasonable pattern size is often limited to a few square millimeters, and patterning a color image in large scale is very time-consuming and costly. One up-scalable and economical approach of manufacturing large-scale structural color patterns is originating a master stamp and replicating the patterns.4,32,33 The origination of master stamp usually implements nanoscale lithography for nanostructures or laser interference lithography for periodic structures. The replication of the patterns is usually based on nanoimprint lithography (NIL), which relies on mechanical molding of polymer materials through hot embossing,34 UV
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tructural color printing, implementing micro- or nanostructures to display colors, has generated considerable interest in recent years thanks to the explosive advancement in nanotechnology. Unlike traditional color printing techniques which are based on pigments or inks, structural colors rely on the interaction of light with physical structures and have appealing advantages in terms of resolution,1 color chromaticity,2 chemical stability,3 and being environmental friendly.4 Generating structural colors from precisely engineered optical nanostructures has become an increasingly important branch of nano-optics in recent years, and many types of structural color pixels have been demonstrated, such as transmissive plasmonic color filters used in color image sensor and display devices,2,5−15 reflective plasmonic pixels for patterning color images with resolution at optical diffraction limit,1,3,4,16−19 angle-dependent diffractive colors in nanostructure arrays,15,20−23 interference colors in multiple layers,24−28 etc. Nanostructures with exotic optical effects and color properties have also been discovered.29−31 Despite the rapid development of structural color printing techniques, the bottleneck of low throughput and poor versatility in manufacturing large-scale color patterns is still unresolved. As a result, the practical applications of structural © 2016 American Chemical Society
Received: September 28, 2016 Accepted: November 8, 2016 Published: November 8, 2016 10544
DOI: 10.1021/acsnano.6b06531 ACS Nano 2016, 10, 10544−10554
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Figure 1. Structural coloration based on MIONS. (a) Schematic of the coloration process. Inkjet-printed silver is molded by the underlying polymer nanostructures and gains structural colors. Index-matching lamination deactivates unprinted polymer nanostructures to display a transparent background and also protects the sample surface for ambient use. (b) Schematic of the structural colors viewed from the top side of the printed silver dot. (c) Side-view schematic illustrates the diffraction of light from the silver nanocone array viewed from the top side. (d) Schematic of the structural colors viewed from the back side of the printed silver dot. (e) Side-view schematic illustrates the diffraction of light from the silver nanowell array viewed from the back side.
nanoimprint,35 or a combination of both. NIL can be implemented in different configurations such as full-wafer parallel imprint,34 step and repeat NIL,36 and roller NIL.37 During industrial manufacturing processes, full-wafer parallel imprint and step and repeat NIL are usually used for producing large-area nanoimprint molds from originated master stamps,36 and roller NIL is implemented for replicating patterns in huge volumes, owing to its continuous and high-speed processing, simple system construction, low cost, and low energy consumption.38−40 Such an origination and replication approach enables cost-effective manufacturing of structural color patterns but is only practical for printing the same color pattern in a big volume. Printing different color images in a small quantity for daily use with the image size sufficiently large enough to be viewed by naked eyes remains a daunting challenge when considering the economical affordability. Other than nanoscale lithography, novel structural color printing techniques with higher throughput have also been reported in recent years. Microscale photolithography, which enables much faster writing speed than nanoscale lithography, has been implemented to pattern structural colors.22,25,41 The colors of a superparamagnetic nanoparticle ink were tuned by the applied magnetic field and fixed by photolithography.41 Photolithography followed by wet etching was used to pattern reflective interference colors from anodic alumina oxide templates.25 A reusable generic substrate comprising pixelated structural pixels was patterned using photolithography to selectively imprint nanostructures onto a separate substrate surface to form a structural color image.22,23 Large-scale structural colors, mimicking the wings of Morpho butterflies, have been patterned using thin-film deposition of alternating dielectric layers24 or laser interference lithography.27 Other techniques based on patterning colloidal plasmonic nanoparticles have also been introduced, such as photothermal patterning of gold nanorods using a laser beam42 and precise assembly of plasmonic nanoparticles onto predefined binding
sites.43 Although these techniques are generally more efficient than nanoscale lithography techniques, the throughput is still not sufficient to allow color images to be printed conveniently. We aspired to innovate structural color printing into a scalable technique that can potentially be comparable to popularly used desktop printers in terms of throughput and versatility. Such a technique must possess at least three important properties. First, large-scale, full-color images can be rapidly printed according to any user-input digital color image. Furthermore, the resolution needs to be high enough for good viewing effects to be sensed by human eyes. Finally, the structural colors on the printed images must be robust enough for daily use in ambient environment. Considering these requirements, structural colors patterned with inkjet printing are one most promising candidates, as inkjet printing allows rapid deposition of wide range of materials with microscale resolution according to any user-defined pattern and quantity.44−47 Recently, nanoscale resolution has been achieved using electrostatic autofocusing of ink nanodroplets to directly print plasmonic nanoantenna.48 Conventional inkjet color printing implements color inks based on dyes or pigments to print color patterns. In contrast, inkjet-based structural color printing relies on either printing specially synthesized inks composed of nanostructured materials onto regular substrates28,49−51 or printing regular inks onto specially engineered substrates.52,53 Optically variable inks composed of multilayer flakes have been applied in the security printing industry for decades.49 Colloidal nanospheres have been inkjet-printed to assemble into photonic crystals to display color patterns.50,51 Other similar inks based on assembly of nanospheres can also be potentially inkjet-printed into structural color patterns.54−56 Recently, a titania-based colloidal ink has been inkjet-printed to generate thickness-dependent interference colors.28 Ordinary liquids have been used as inks to tune colors on photonic crystal substrates based on controllable swelling of the periodic lattices caused by liquids.52,53 Colorless 10545
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Figure 2. Experimental results of printing silver nanoparticles ink on a polymer nanocone array for red diffractive color. (a) SEM images (tilted angle at 30°) of one silver dot printed on the polymer nanocone array. The printed silver dot has an average diameter of around 18 μm. At the edge of the dot, polymer nanocones collapsed due to the retraction of the ink droplet when the solvent evaporated. (b) Cross-sectional SEM image (tilted angle at 52°) of silver nanoparticles printed onto the polymer nanocones. FIB was used to mill the sample along the center of the nanocones. The thick platinum (Pt) layer was deposited on top of the nanostructures before FIB to protect the nanostructures during milling. The holes in platinum layer are artifacts of Pt deposition. Measured diffraction spectrum of (c) the printed silver nanocone array and (d) the printed silver nanowell array viewed from the top and back side, respectively, after the sample was laminated. The inset optical microscope images show the diffractive color of one dot. The lighting and viewing angles used to measure diffraction efficiency are schematically shown in the inset as well.
TiO2 ink has been inkjet-printed onto a holographic paper to form into high refractive index film for preserving the diffractive colors of the holographic pattern.57 These mentioned inkjetbased structural color printing techniques already showed success for certain applications, but there are still inherent limitations in terms of material availability and cost, robustness of the printed images, and spectral tunability of colors. In this paper, we report a convenient and powerful structural coloration technique, termed as molded ink on nanostructured surfaces (MIONS), based on molding the shape of printed ink material on top of surface-supported transparent polymer nanostructures to display structural colors. The process is schematically shown in Figure 1. A transparent nanostructured substrate is first prepared by imprinting polymer nanostructures from a prefabricated stamp onto a plastic sheet. The nanostructures cover all of the printable area of the substrate. Silver nanoparticle ink droplets are then printed onto the nanostructured surface using inkjet printing according to a userdefined pattern. It should be noted that such silver inks have been widely used in printing conductive electrodes for flexible electronic devices, and the printed patterns were commonly known to be “colorless”.58−60 In contrast, in our MIONS technique, after a silver ink droplet dries into a solid silver thinfilm, the silver is molded by the underlying polymer nanostructures and gains structural colors. The colors are determined by the geometries and shapes of polymer nanostructures as well as the rheological properties of ink. After printing, the entire sample surface is laminated with a transparent index-matching cover film to deactivate the unprinted polymer nanostructures to display a transparent background. MIONS printing can enable full-color, scalable, highthroughput, versatile, and cost-effective printing of structural color images and possesses five major favorable characteristics. First, the printing process is fast, and the resolution is high enough for good viewing effects, owing to the inkjet-based
patterning technique. Second, all related materials are ordinary and accessible at low cost, and the equipment is inexpensive, too. Third, the nanostructures on the substrates cover all of the printable area and are generic for different color images. That means any user-input color image can be printed onto one replica of same nanostructured substrate. Using inexpensive roll-to-roll embossing, a huge volume of such replicas can be manufactured from a nickel stamp onto rolls of thin foils, making the cost of each piece of substrate comparable to regular plastic or paper substrates. Fourth, the printed colors are very robust because the index-matching cover film also serves as a protective coating to prevent silver from being tarnished in ambient environment. Finally, MIONS printing is essentially based on metalizing dielectric nanostructures using inkjet printing. Similarly, many plasmonic printing techniques are based on physical vapor deposition of metal film on lithography-patterned nanostructures.1,17 Therefore, MIONS printing can print high-performance plasmonic pixels and can also benefit from the rapid development in the field of plasmonic pixels.
RESULTS To demonstrate the MIONS printing technique in experiments, we implemented polymer nanocone arrays (NCAs) on a transparent polyethylene terephthalate (PET) sheet as substrates. After printing and lamination, the printed silver dot is essentially a silver nanostructure immersed inside the transparent polymer background. The silver nanostructure has different surface profiles on the two sides, specifically, a silver NCA when viewed from top, whereas a silver nanowell array (NWA) when viewed from the bottom. The silver NCA and NWA both function as 2-D gratings to diffract light of a certain spectral band into the directions determined the grating order. As shown in Figure 1c,e, consider that a white light is incident at an angle α from a medium of refractive index n (usually air), the NCA or NWA has a grating period Λ, and light of 10546
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Figure 3. Full-color MIONS printing on a substrate with pixelated nanostructures. (a) Schematic of the full-color printing process. The substrate surface is imprinted with pixelated nanostructures which are periodically repeated R, G, and B pixel bands that can display red, green, and blue colors, respectively, after being printed with silver. The colors on the bands in the schematic are a guide for eyes to identify the pixel bands. Given a color image, a digital pattern was first generated and then printed onto the pixel bands with accurate registration relative to the pixel layout using inkjet printing. After printing and lamination, the printed color pattern is to be viewed from the back side. Different colors are achieved by mixing the R, G, and B colors at appropriate ratios. (b−d) SEM images (tilted angle at 30°) of the polymer nanostructures in R, G, and B pixel bands, respectively. (e) Measured diffraction spectra of the printed silver NWAs on R, G, and B bands. (f) CIE-1931 chromaticity map of the diffractive colors of three printed silver NWAs. The circles are the colors of the printed silver NWAs, and the triangle encloses all colors that can be mixed from the three silver NWAs. The inset panel shows the lighting and viewing angle in diffraction measurement. Scale bar: (b−d) 500 nm.
was laminated with a transparent index-matching cover film. Since the cured cover film has the same refractive index as the polymer NCAs, the unprinted polymer NCAs are deactivated and function as black pixels for diffractive colors. Figure 2c,d shows the optical characterization results of silver NCA and NWA when viewed from the top and back sides, respectively. For light incident from the top, it is the silver NCA immersed inside the UV polymer that diffracts light. Similarly, for light incident from the bottom, the silver NWA diffracts light. The lighting and viewing angles are specified in the inset panels of the figures. The diffraction efficiency was measured using collimated light beam, microscope, and spectrometer (Supporting Information). The normalized diffraction efficiency is the diffraction efficiency of the probed spot normalized with the coverage ratio of the printed silver. It is therefore a property of the printed silver nanostructure and is independent from the size or density of the printed silver dots. It can be observed that the silver NWAs are brighter than silver NCAs, as confirmed by the inset microscope images of diffractive colors. We attribute the origins of the differences to the collapsing of the polymer nanocones when the solvent evaporates. The collapse of polymer nanocones can significantly disrupt the periodicity of the silver NCAs to dim the diffraction. In comparison, such an effect on the silver NWA is weaker because the surface of silver NWA that interacts with light is determined by the base of the polymer nanocones which are
wavelength λ carried in the mth grating order is diffracted into the angle β. The relationship of Λ and λ is given by nΛ(sin α − sin β) = mλ. Figure 2 shows exemplary experimental results of one silver dot printed on a polymer NCA. Prior to printing, the substrate surface was coated with a self-assembled monolayer of perfluorodecyltrichlorosilane (FDTS) to achieve low wettability of the ink on the nanostructures (contact angle around 114°). An alcohol-based silver nanoparticles ink with a particle size ≤50 nm is used. Figure 2a shows the scanning electron microscope (SEM) images of the silver printed onto the polymer NCAs. The polymer NCA has square lattice with period Λ = 640 nm. Bare polymer nanocones have an average height of about 750 nm and base diameter of about 520 nm. The printed silver dot has an average diameter around 18 μm. At the edge of the printed silver dot, polymer nanocones collapsed with tips pointing toward the center of the dot, due to the retraction of the ink droplet when the solvent evaporated. Figure 2b shows the cross-sectional SEM image of the printed NCAs after milling the sample using FIB. Due to the low wettability of the polymer NCA surface, the silver nanoparticles from the ink mainly precipitated surrounding the bases of the cones forming into a dense film of about 300 nm thick after the solvent evaporated. The average grain size is around 50 nm. On the tips and sidewalls of the nanocones, silver grains formed into a very thin layer,
