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Scalable Inkjet-Based Structural Color Printing by Molding Transparent Gratings on Multilayer Nanostructured Surfaces Hao Jiang* and Bozena Kaminska School of Engineering Science, Simon Fraser University, Burnaby, British Columbia V5A1S6, Canada S Supporting Information *
ABSTRACT: To enable customized manufacturing of structural colors for commercial applications, up-scalable, low-cost, rapid, and versatile printing techniques are highly demanded. In this paper, we introduce a viable strategy for scaling up production of custom-input images by patterning individual structural colors on separate layers, which are then vertically stacked and recombined into full-color images. By applying this strategy on molded-ink-onnanostructured-surface printing, we present an industryapplicable inkjet structural color printing technique termed multilayer molded-ink-on-nanostructured-surface (MMIONS) printing, in which structural color pixels are molded on multiple layers of nanostructured surfaces. Transparent colorless titanium dioxide nanoparticles were inkjet-printed onto three separate transparent polymer substrates, and each substrate surface has one specific subwavelength grating pattern for molding the deposited nanoparticles into structural color pixels of red, green, or blue primary color. After index-matching lamination, the three layers were vertically stacked and bonded to display a color image. Each primary color can be printed into a range of different shades controlled through a half-tone process, and full colors were achieved by mixing primary colors from three layers. In our experiments, an image size as big as 10 cm by 10 cm was effortlessly achieved, and even larger images can potentially be printed on recombined grating surfaces. In one application example, the M-MIONS technique was used for printing customizable transparent color optical variable devices for protecting personalized security documents. In another example, a transparent diffractive color image printed with the M-MIONS technique was pasted onto a transparent panel for overlaying colorful information onto one’s view of reality. KEYWORDS: inkjet printing, structural color, titanium dioxide nanoparticle, subwavelength grating, diffractive color, optical variable device, transparent display
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generated from physical structures, structural colors are usually very resistant to fading.10,39 The benefits of structural colors can be exploited toward a broad range of industrial applications, such as security and authentication,44 colored solar cells,45,46 product packaging,9 and optical archival storage.39,47 For practical industrial use, production techniques of structural colors must be economical and scalable, and such viable techniques may be mainly categorized into two types: large-quantity replication and customized printing. In techniques for large-quantity replication of the same color pattern, a master stamp with surface-relief nanostructures for the desired color pattern is first originated,
ecent years have witnessed tremendous progress in generating colors from artificial nanostructures, a field known as structural color printing. Various structural coloration mechanisms based on interaction of light with optical nanostructures have been intensively studied and demonstrated, for example, plasmonic colors from metal nanostructures,1−20 colors originating from optical resonances in dielectric metasurfaces,21−25 reflective colors from photonic crystals,26−35 diffractive colors from nanostructure arrays,36−39 and interference colors in thin films.40−43 Compared to conventional coloration approaches based on dyes or pigments, structural color printing has appealing advantages: structural colors can be patterned in ultrahigh resolution, with the smallest pixels in subwavelength scale;4,5 structural color printing usually implements metals and dielectric materials that are environmentally friendly;9 since the colors are © 2018 American Chemical Society
Received: December 3, 2017 Accepted: February 14, 2018 Published: February 14, 2018 3112
DOI: 10.1021/acsnano.7b08580 ACS Nano 2018, 12, 3112−3125
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Cite This: ACS Nano 2018, 12, 3112−3125
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Figure 1. Concepts of M-MIONS printing on subwavelength grating surfaces. (a) Schematic of molding one diffractive pixel from a polymer subwavelength grating surface. The grating surface is first replicated from a stamp using nanoimprint lithography. Dielectric TiO2 NPs are inkjet printed on the grating surface, and the formed TiO2 thin film is molded by the underlying polymer grating structures to gain diffractive colors. Index-matching lamination deactivates polymer gratings that are not covered by NPs to display a transparent background. (b) Schematic of diffractive colors from one molded grating pixel. (c) Side-view schematic of diffractive colors viewed from the back side of molded grating pixels with light illuminated from the same side of the viewer (reflection mode). (d) Side-view schematic of diffractive colors viewed from the back side of molded grating pixels with light illuminated from the opposite side of the viewer (transmission mode). (e) Schematic of molding red, green, and blue grating pixels on three different subwavelength grating surfaces. (f) Schematic of stacking and laminating three layers with alignment of the patterns. (g) Schematic of the diffractive full-color pattern viewed from the stacked layers.
color pattern could vary broadly, from a single piece to a very large volume. Therefore, techniques that can enable customized printing of structural color patterns are in demand. Such techniques must be scalable and inexpensive and can rapidly print any custom-input image with good appearance. Optical techniques have been implemented to pattern excellent structural colors, but their up-scalability in terms of production speed remains a question.11,20,26,36,39,41,47 Inkjet-based structural color printing techniques can meet the overall demands of customized printing for industrial applications. Inkjet printing of optically variable inks, composed of flakes of multilayer dielectric materials to display interference colors, has already been popularly used in the security industry for decades.53 Special inks of colloidal nanospheres that can self-assemble into photonic crystals have been used to print structural color patterns.27,29,30,33−35 Ordinary liquids can also be printed onto
and then the pattern is replicated into a massive quantity, usually through low-cost roll-to-roll nanoimprinting.48−51 The origination process usually implements time-consuming nanopatterning techniques such as electron beam lithography9,52 and laser interference lithography,44 but the upfront cost in origination is significantly diluted by the large number of replications. For example, in the security industry, an anticounterfeiting feature of a certain structural color pattern may be replicated into billions of pieces to be inserted into banknotes. In the current manufacturing industry, there is an accelerating trend of customized manufacturing of commercial products. For industrial applications, structural colors are mainly used to establish a certain appearance or to store information, and either application often requires customization of the color pattern. Furthermore, the target quantity for each 3113
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gratings is deactivated due to index-matching, whereas the molded TiO2 grating structure remains diffractive because of the contrast in refractive index between TiO2 and the polymer. The molded TiO2 grating structure in fact becomes a diffractive color pixel encapsulated inside a transparent polymer, as shown in Figure 1b. TiO2 is a dielectric material with low optical loss in the visible spectrum, and the molded grating pixel is therefore transparent and allows most light to transmit through it. Figure 1c,d show two side-view schematics of the diffraction of light from the molded grating pixel, under reflection mode and transmission mode, respectively. Consider a beam of white light is incident from an angle α from a medium of refractive index n (usually air, n = 1), and the grating has a grating period Λ, light of wavelength λ carried in the mth grating order is diffracted into the angle β. The relationship of Λ and λ is given by eq 1.
prefabricated photonic crystal substrates to tune the structural colors.28,32 Colorless dielectric titanium dioxide (TiO2) nanoparticle ink has been inkjet-printed onto a holographic paper to preserve the diffractive colors.54 The same type of ink was also inkjet-printed into optical-grade thin films to generate thickness-dependent interference colors for custom-input images.43 Our previous work has introduced an inkjet-based molded-inkon-nanostructured-surface (MIONS) printing, which implements a nanostructured surface composed of pixelated nanocone arrays for printing full-color images.37,38 Silver nanoparticle ink droplets were accurately printed onto red, green, and blue bands according to the desired color pattern. The printed silver thin-film dots were molded by the underlying nanostructured surface into structural color pixels. The substrate is generic for printing all different patterns and can be replicated using roll-to-roll nanoimprinting. The MIONS technique can print any custom-input full-color image inexpensively and can enable both diffractive colors and plasmonic colors. One technical challenge in scaling up MIONS printing for industrial use is the stringent requirement on the accuracy and consistency of alignment during continuous operation of the inkjet printer. Alignment can be conveniently calibrated for a small printable area, but it is difficult to be maintained over a large area. For example, when printing a big color pattern, alignment can be lost in some subregions due to inhomogeneous warping of the polymer substrate surface. Misalignment between the printed ink pattern and the pixel bands can cause wrong colors, referred to as “color leaking”.38 One solution in terms of equipment that is currently under development is a smart printing head that can optically detect pixel bands in real time and dynamically compensate alignment errors in all subregions.
n Λ(sin α − sin β) = mλ
(1)
For a given configuration of lighting and viewing angle, diffractive colors in red, green, and blue can be displayed from three different gratings of appropriate grating periods calculated from the grating equation, eq 1. Figure 1e−g schematically show the inkjet printing of full-color images using three layers of grating surfaces. Detailed experimental procedures in printing are provided in the Materials and Methods section. Subwavelength grating for red, green, and blue color is patterned onto three separate layers, and each layer is inkjet printed with TiO2 NPs. After printing and drying, the three layers are laminated and stacked vertically with alignment of the printed patterns. When observing from the back side, a viewer sees a full-color diffractive color pattern, as shown in Figure 1g. A wide range of colors are mixed by red, green, and blue diffractive colors from grating pixels that are molded on corresponding layers. Compared to MIONS printing based on tiled pixel bands, MMIONS printing on multiple layers has four major advantages. First, the printing on three layers is completely isolated from each other, and each layer has only one grating pattern. Problems of “color leaking” and a bottleneck of image size are completely eliminated in M-MIONS printing. The printable image size can be easily scaled up to enable customized manufacturing of structural colors for commercial applications. Second, although alignment between layers is still needed, it merely requires overlaying the patterns for good visual effects, and the alignment error tolerance is at least 250 μm. In comparison, for pixel-band-based MIONS printing, the alignment error must be smaller than 10 μm and be kept consistent over the entire printable area. Third, the alignment between layers is carried out after printing and therefore does not interrupt the continuous operation of the printer. Fourth, although the M-MIONS technique requires three separate master stamps (one stamp for each layer), the origination of each master stamp is straightforward using laser interference lithography because the pattern is uniform on the stamp. In comparison, the origination of a master stamp with a layout of tiled pixel bands is more challenging, as it requires microscopic alignment between pixel bands when implementing laser interference lithography.37
CONCEPTS OF MULTILAYER MIONS PRINTING In this paper, we present a multilayer color separation strategy that can fundamentally solve the problem of “color leaking” caused by inconsistency of alignment in MIONS printing. Based on this strategy, we developed a technique, referred to as multilayer molded-ink-on-nanostructured-surfaces (MMIONS) printing, which implements a multilayer configuration of nanostructured surfaces for scalable inkjet printing of transparent full-color images. The concept is schematically shown in Figure 1. Each layer of nanostructured surface has only one homogeneous nanostructure for molding one type of structural color pixel structure, and different colors are mixed from vertically stacked red, green, and blue layers. To elucidate the concept in detail, Figure 1a first shows a 3-D schematic for molding one diffractive color pixel on a nanostructured surface patterned with 1-D subwavelength grating. The grating surface is nanoimprinted into a transparent UV polymer from a stamp, and the grating structures uniformly cover the entire printable area of the substrate. Ink droplets containing titanium dioxide nanoparticles (TiO2 NPs) are inkjet-printed onto the polymer grating surface according to a custom-input pattern. After drying the ink droplets, the TiO2 NPs are molded by the underlying polymer grating surface into dielectric grating structures composed of close-packed TiO2 NPs. Then the entire surface is laminated with a transparent index-matching cover film. The refractive index of the cover film is around 1.5, the same as the polymer of the grating surface. When the cover material fills the grooves of the gratings that are not covered with NPs, diffraction from those
RESULTS Principles of Color Mixing from Multilayer Grating Pixels. M-MIONS printing relies on mixing red, green, and blue diffractive pixels from vertically stacked layers to display 3114
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Figure 2. Replicated polymer nanostructured surfaces with subwavelength grating patterns. (a, b, and c) SEM image of subwavelength gratings replicated in UV polymer for red, green, and blue layers, respectively. (d) Geometrical parameters of the fabricated gratings (side view).
Figure 3. Experimental results of printing TiO2 NPs on red, green, and blue grating surfaces. (a) Schematic of printing 11 shades of gray on a grating surface. The density of printed ink increases as the shade of gray becomes darker. (b) Side-view schematic of the lighting angle and viewing angle for capturing color photos and measuring diffraction efficiency. (c) Photos of the red, green, and blue grating surface after being printed with 11 different levels and laminated. (d−f) Optical microscope image of blue grating surface printed with ink density level 10, 8, and 5, respectively. (g) Measured normalized diffraction efficiency of molded red, green, and blue grating pixels. (h) Brightness of diffractive pixels vs printed ink density level for red, green, and blue layers. The brightness was calculated by integrating the measured diffraction spectra in the red, green, and blue spectral range, respectively.
different colors. The displayed color is determined by the diffraction efficiency of the grating pixels that are separately molded on three layers, given by [ηR, ηG, ηB] where ηR, ηG, and ηB are the diffraction efficiency of a red, green, and blue pixel, respectively. The diffraction efficiency of each grating is further
determined by the normalized diffraction efficiency of the molded grating structure and its filling ratio. The normalized diffraction efficiency is denoted as ηnr, ηng, and ηnb for grating structures molded on a red, green, and blue grating surface, respectively. The filling ratio is defined as the percentage of the 3115
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ACS Nano area covered by TiO2 NPs, denoted as Ar, Ag, and Ab for ink printed on a red, green, and blue grating, respectively. The mixed color from three layers can be described by eqs 2−4, by neglecting the transmission loss from the layers stacked in front. In fact, the M-MIONS printing process is essentially to modulate the filling ratio of ink on each layer of the grating surface, according to the input digital color image. ηR = ηnr A r
(2)
ηG = ηng Ag
(3)
ηB = ηnbAb
(4)
three grating surfaces. After drying and index-matching lamination, optical properties of the samples were characterized. Figure 3b shows a side-view schematic of the laminated sample with specified default lighting and viewing angles used in optical characterizations. Figure 3c shows photos of the printed diffractive color patterns on red, green, and blue gratings. Microscope images of blue colors from selected ink density levels are shown in Figure 3d−f. The irregular shapes of the blue dots are due to merging of printed ink droplets. Measured normalized diffraction spectra in Figure 3g show diffraction peaks centered around 640, 550, and 460 nm wavelengths, for red, green, and blue pixels, respectively. These measured peaks match closely with theoretical calculations listed in supplementary Figure S2. The peak normalized diffraction efficiency is in the range of about 0.3−0.6%. It should be noted that the normalized diffraction efficiency is the measured diffraction efficiency from the spot probed through the microscope normalized with respect to the filling ratio and therefore measures the property of the pixel structures themselves regardless of the printed sizes. Figure 3h shows quantitative analysis on the printed grayscale patterns to evaluate the brightness of diffractive colors vs printed ink density levels. Diffraction spectra were measured from all rectangles shown in Figure 3c, and the pixel brightness of each rectangle was calculated by integrating the diffraction efficiency within the spectral range corresponding to that pixel color. The pixel brightness increases monotonically with the printed ink density; however, the dependence is not linear because of merging of the printed dots. As determined from digital image processing, when printed at full ink density, the actual filling ratio on the red, green, and blue grating surface is about 29%, 40%, and 39%, respectively. As studied in our previous work, dot merging can cause randomness in the filling ratio and thus affects the accuracy of the pixel brightness.38 When printed with denser ink droplets, the negative effect of merging is more pronounced. As controlled through the inkjet printer’s half-tone process, each grayscale in the print pattern is translated into a specific dot array pattern with clearly defined drop spacings and droplet sizes. The drop spacings and droplet sizes are automatically determined by the printer for achieving a specific density of printed ink, which can then be optically mixed into the wanted pixel brightness. The inkjet printer used in this work (Epson Stylus C88+) implements a special variable size droplet technology, where the printer can produce a number of different ink droplet sizes adjusted according to the input grayscale value. The smallest droplet size is 3 pL, and the smallest drop spacing is 18 μm × 5 μm. It should be noted that the default half-tone process of the printer is optimized for printing conventional black and color inks on plain paper substrates whose surfaces tend to absorb inks. In contrast, in this work, TiO2 NPs dispersed in a mixture of water and ethylene glycol were used as the ink and the polymer grating surface is a solid nonabsorbing surface with low wettability for ink. When ink droplets are jetted into a well-defined array of droplets of appropriate droplet sizes, the droplets impact the solid substrate surface and some droplets randomly migrate and merge into larger dots due to the low wettability. After the ink dries, TiO2 NPs are formed into a relatively random pattern due to the merging of dots, which can cause inaccuracy in the filling ratio and the brightness of diffractive colors. Despite the inaccuracy, the pixel brightness for different printed ink density levels is sufficiently differentiable to display at least 11 different
Molding Grating Pixels on Single Layers. Since the colors are determined by the property of the molded grating structure and the filling ratio (ink coverage) together, in order to achieve full control of colors, the properties of molded diffractive pixels must be studied first. In the experiments, we printed TiO2 NPs on single layers of polymer grating surfaces, which were replicated from master stamps. The master stamps were fabricated using laser interference lithography and dry etching. Detailed fabrication procedures are provided in the Materials and Methods section. Scanning electron microscope (SEM) images of the replicated subwavelength grating structures on red, green, and blue layers are shown in Figure 2a, b, and c, respectively. Due to laser interference lithography over a large area (8-in. wafers), there are certain variations of the fabricated structures. The geometrical parameters of the gratings are schematically defined in Figure 2d, with average parameters summarized in the inset table. The average grating period is 544, 470, and 397 nm for red, green, and blue gratings, respectively. The variations in grating periods were controlled to be smaller than 25 nm. Supplementary Figure S2 shows the calculated diffraction wavelengths from three different gratings for various angles. It should be noted that, in this work, we applied subwavelength gratings (i.e., grating periods smaller than the wavelength of light) such that correct diffractive colors are viewed with a steep light incidence angle, and the visual color in negative first grating order is more angle-robust than diffractive colors from larger grating periods. In our experiments, we printed TiO2 NP ink on each type of grating surface with varying filling ratios of molded gratings. In the printing process, a desktop digital inkjet color printer was used and the prepared ink material was filled into the cartridge for the black channel, as will be presented in detail in the Materials and Methods section. All input patterns are grayscale images because the ink is jetted from the black channel only. The inkjet printer is a commercial product for regular office use for printing on mainly paper substrates. The half-tone process, used by the printer, translates each shade of gray into a dot array pattern for controlling the density of printed ink droplets on the substrate. When printing on polymer grating surfaces, the resultant filling ratio of molded gratings is different from the density of printed ink droplets because the hydrophobic polymer substrate surface can cause merging of ink droplets, which will be investigated in detail later. Nonetheless, the filling ratio still depends on the printed ink density and is therefore controlled by the shade of gray in the input image. As shown in Figure 3a, 11 shades of gray, equally spaced from full white to full black, were used as the print pattern to print rectangles on the grating surfaces. A darker shade will be translated into a higher density of printed ink. Therefore, 11 different levels in the printed ink density were tested on the 3116
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Figure 4. SEM images of the printed TiO2 NPs on the blue grating surface for an ink density level of 8. (a) Overview of the printed ink dots. (b) SEM image of one ink dot. (c−e) Zoom-in SEM images of (b) to illustrate the coverage of NPs on the grating surface. (f) SEM image of TiO2 NPs inside a groove.
grating lines (Figure 4c). No coffee-ring effects were observed because we added ethylene glycol into the water-based ink, which can induce an additional inward Marangoni flow within evaporating droplets to suppress coffee-ring effects.55,56 As shown in Figure 4f, the sizes of NPs are distributed in the range of 15−25 nm in diameter. As shown in supplementary Figure S3a, for the red grating surface, TiO2 NPs can barely cover the grating lines because the red grating has much deeper grooves than the blue grating. At the center of the dot, TiO2 NPs were deposited into a film of about 280 to 320 nm thickness embedded inside the grooves. The results on the green grating surface, as shown in supplementary Figure S3b, are similar to those from the blue grating, because these two gratings have comparable depths of grooves. Full-Color Images Printed Using the M-MIONS Technique. Figure 5 shows a full-color pattern printed with the M-MIONS technique. For a target color image shown in Figure 5a, grayscale print patterns for red, green, and blue layers are generated using custom-written MATLAB codes. As described above, an ink density level ≤ 8 yields good color patterns. Therefore, the pattern generation process considers the ink density level of 8 as the maximum density, and the shades of gray on the print patterns were linearly scaled accordingly. Under such a configuration, nine different shades of gray can be printed in red, green, and blue grating surfaces, and the actual number of printable colors is 729. All the print patterns have mirrored orientations relative to the target image because the printed colors are to be viewed from the back side. After printing the patterns on the corresponding grating layers and drying the ink, the printed layers were laminated layer by layer while applying visual alignment between layers. Details are presented in the Materials and Methods section. The bold square outlining borders on the print patterns serve as alignment marks. A photo of the produced full-color pattern is given in Figure 5b. The photo was captured by placing the printed transparent color image in front of a dark background. The size of the pattern is about 1 in. × 1 in. The colors of the target image have been presented quite accurately on the produced sample.
shades of colors, as shown in the diffraction measurement results in Figure 3h. In addition, the visual effects of colors printed with different ink density levels have also been evaluated with the eye. The colors printed with ink density levels 9 and 10 show big grains due to the merging of very densely printed ink droplets. For an ink density level ≤ 8, the visual effects are good despite some small grains and inhomogeneities visible in the printed image. The merging of dots can certainly be avoided by modifying the half-tone process. For example, if a grayscale value is translated into a dot array pattern with large enough drop spacings and small enough droplets, the merging of dots can be eliminated. Such a half-tone process specifically optimized for M-MIONS printing with special considerations on inks and substrates can potentially be achieved by reprogramming the printer’s driver software or implementing a professional industrial or research-grade inkjet printer with full control of drop positions, drop spacings, and droplet sizes.37,38 Nonetheless, one goal of this work is to prove that even regular low-cost desktop inkjet printers for office use can be used for printing photosize structural color patterns. Advanced modifications on the inkjet printer or implementation of other professional inkjet printers will be investigated in our future work for achieving more accurate control of the filling ratio. The printed TiO2 NPs on grating surfaces have been characterized with SEM imaging. For the sake of brevity, only SEM images of the printed TiO2 NPs on a blue grating surface are shown in Figure 4. Results on red and green grating surfaces are provided in supplementary Figure S3. The irregular sizes and shapes of the merged dots can be clearly seen in Figure 4a. The printed dots tend to be in elliptical shapes (for example, the dot shown in Figure 4b) because the ink has higher wettability along the direction parallel with the grating lines than the perpendicular direction. As can be seen from Figure 4c−e, NPs are mainly deposited inside the grooves to form into dielectric structures made of close-packed NPs. At the edge of the printed dot (Figure 4d), TiO2 NPs are completely embedded inside the 317-nm-deep grooves, without any NPs on top of the grating lines. TiO2 NPs gradually thicken toward the center of the dot (Figure 4e) and eventually cover all 3117
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Figure 5. Full-color image printed using M-MIONS printing. (a) Process of generating print patterns for red, green, and blue layers, according to the target image. (b) Photo of the printed sample captured under default lighting and viewing angle. (c−h) Optical characterization results on the three color spots marked in (b). (c, e, and g) Microscope images of the diffractive color pattern. (d, f, and h) Diffraction spectra.
Figure 5c−h show optical microscope images of the diffractive colors and the associated diffraction spectra, which were collected from three marked spots in Figure 5b. The red-color letter ‘N’ was displayed by gratings molded on the red layer only (Figure 5c,d). The white-color letter ‘O’ was displayed from all three layers to mix into a white color (Figure 5e,f). The magenta-color letter ‘F’ was displayed from the red and blue layers (Figure 5g,h). The small spikes in the measured diffraction spectra in the red range are artifacts caused by the light source, which has a spike around 650 nm. Although optical microscope images show inhomogeneous colors on individual layers, the overall viewing effects are still good despite some small noticeable grains of colors. One concern in the multilayer configuration is the transmission loss of light from the layer in front. For example, as shown in Figure 1f,g, when the color pattern from the stacked layers is to be viewed from back side under reflection mode, the transmission loss through the blue layer will definitely affect the brightness of the green and red layer. The transmission spectra through individual printed layers are shown in supplementary
Figure S4. The transmission measurements were carried out from regions printed with the highest ink density (level 10). The red layer and green layer show more than 90% transmission, and the blue layer gives more than 85% transmission, across the entire visible spectrum. The transmission microscope images in supplementary Figure S4b−d also confirm that the molded gratings are very transparent. Each layer is thus considered sufficiently transparent, and the effects of transmission loss can be neglected. The transmission spectra show different transmission rates for red, green, and blue layers. We attribute the differences in transmission to the coverage of TiO2 NPs on the grating surfaces. On the red layer, TiO2 NPs are almost completely embedded inside the grooves of the grating lines, as shown in supplementary Figure S3a. On the green layer, a significant portion of TiO2 NPs are deposited on top of the grating lines into a rough top layer, as shown in supplementary Figure S3b. On the blue layer, the coverage of TiO2 NPs on top of the grating lines is further increased into a rougher layer, as shown in Figure 4. The close-packed NPs molded inside the grooves yield dielectric grating structures 3118
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Figure 6. Customizable transparent color OVDs manufactured using the M-MIONS printing process. (a) Schematic of the customizable transparent OVDs laminated onto a personal ID card to prevent alteration of information. (b, c, d) Photo of a printed color image captured from different angles. The angles are defined in the side-view schematics.
color OVDs are manufactured using the above-mentioned large-quantity replications, based on master stamp origination and roll-to-roll nanoimprinting. The color patterns are fixed to the master stamps, and each attempt of customization on the pattern requires originating a new master stamp. Due to the high cost in master stamp origination, it is not practical to include customized features through such traditional manufacturing approaches. In comparison, inkjet-based structural color printing techniques can conveniently enable customization on the OVDs.27,29,33,37,43 In this work, M-MIONS printing can be used for manufacturing color OVDs with customized color patterns and features. The printed images appear iridescent, and the colors shift vibrantly when the images are tilted, because the colors are based on angledependent diffraction from grating pixels. Since the printed color patterns are solely determined by the input digital images, customization of the OVDs can be achieved effortlessly by including customized features into the custom-input digital image. Figure 6a shows the concept of customizable transparent color OVDs for protecting ID cards. A transparent color OVD, with a color pattern specifically customized according to the owner of the ID card, can be inexpensively produced using MMIONS printing and laminated onto the card surface. Any attempt to alter the information on the card will leave obvious traces on the laminated OVDs. A face photo and signature of the owner can also be included on the printed OVDs to enhance the complexity of the security features. The color
with smooth sidewalls, while the rough top layer scatters light and reduces the transmission. The blue layer has the roughest top layer and therefore the lowest transmission. In contrast, the red layer has almost no rough top layer and therefore the highest transmission. In addition, the transmission spectra of red, green, and blue layers in supplementary Figure S4a all show lower transmission in the shorter wavelength range because the scattering of light caused by surface roughness is more severe for shorter wavelengths. Another concern is the stacking order of the red, green, and blue layers. In our experiments, we have tested different stacking orders. Two sets of red, green, and blue layers were printed with the same image under the same printer configuration. These two sets were stacked and bonded in different stacking orders into two samples. Supplementary Figure S5 elucidates the stacking orders of the two sets of printed layers and shows the photographs captured from two samples under the same conditions. The results show that the stacking order makes very little difference in the colors, because all layers have high transparency. Customizable Transparent Color Optical Variable Devices (OVDs). Optical variable devices are security devices that exhibit distinctive color patterns with optical variable effects such as color shifting and movement effects. Since the appealing visual effects from an OVD are very difficult to mimic or duplicate, OVDs are widely used as anticounterfeiting and authentication features to protect important documents including banknotes, ID cards, and passports.44 Traditional 3119
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Figure 7. Transparent color image display panel implementing color images printed with the M-MIONS technique. (a) Schematic of the function of the transparent color image display panel. A printed transparent diffractive color image is pasted onto the front panel, and a lighting bar is installed on top. With the light off, the color image appears transparent and the viewer sees through the panel. With the light on, the viewer sees the color patterns floating on the panel. (b) Side-view optical diagram of the transparent display panel. (c) Photo of a custom-assembled toy model. (d) Photo after placing a glass panel pasted with a printed image in front of the toy model while the light is off. The existence of the color image is barely noticeable. (e) Photo of the setup after turning on the light. Information related to the toy model appears colorfully on the panel. The blue circular pattern at the bottom left corner is the logo of NanoMedia Solutions Inc. (printed with permission).
usually about 500 μm thick. In future work, the grating surfaces will be replicated using a roll-to-roll nanoimprint process onto plastic foils thinner than 50 μm, such that the total thickness of the produced OVDs will be reduced to about 150 μm to enable practical integration with ID cards. “Print and Paste” Transparent Color Image Display Panel. Transparent displays or see-through displays can display images with attractive appearances on transparent screens and interactively overlay live information onto a viewer’s perception of the real world to enable an important application known as augmented reality. Commercially available transparent displays have already been realized through various digital display technologies including liquid crystal displays (LCD),57 organic light emitting diode (OLED) displays,58 and on-screen projection.59,60 The images printed using the M-MIONS technique possess three noteworthy characteristics. First, the images have very high transparency. Second, once illuminated with a directional beam from a steep angle, the printed colors become very bright owing to the angle-specific diffraction of light. Third, any custom-input image can be directly printed and then pasted onto various transparent panels. These characteristics can enable a very low-cost transparent color image display panel for displaying static colorful information to a viewer. Figure 7a shows the concepts of a “print and paste” transparent color image display panel. A simplified side-view schematic with emphasis on the optical configuration is given in Figure 7b. In these schematics, a model, mimicking the Academic Quadrangle Building of Simon Fraser University, is a physical object positioned inside a display box. A transparent color image is printed with the M-MIONS technique and pasted on the surface of the front glass panel. From the top, a white LED lighting bar is installed, and the LED emits directional white light with an incidence angle between 60° and 80°. The on/off status of the LED is controlled in response to a viewer’s motion. When the LED is off, the viewer directly sees
shifting effects of the OVDs are shown in Figure 6b−d. We kept the angle between the light source and viewer constant, i.e., 45°, and rotated the sample. As the sample is rotated clockwise, the diffractive colors blue-shift, displaying eyecatching optical variable effects. The diffraction spectra of the molded red, green, and blue grating pixels under these angles are provided in supplementary Figure S6. Besides these configurations of angles, the color-shifting effects are also obvious when rotating the angles in other reasonable fashions. If we fix the angle between the sample and the viewer while rotating the light source, with increasing light incidence angle, the colors red-shift. If we fix the angle between the sample and the light source while rotating the viewing direction, the colors red-shift when the viewer rotates toward the light source. If the light source and the viewer are both fixed while rotating the sample on an axis perpendicular to the grating lines, the diffractive colors turn off after the sample is rotated a few degrees. The above-mentioned optical characterizations were carried out under directed light beam to accurately relate the colors and diffraction spectra to the configuration of angles. Under lighting conditions in normal environments, such as sunlight or regular room light, the light comes from wider angles and has effects on both the viewing angles and the brightness. Under such conditions, the colors can be viewed from slightly broader angles but appear a little fainter than being illuminated with a directed light beam. Nonetheless, when a viewer rotates the sample by hand and examines the image with the naked eye, the iridescent bright colors with color-shifting effects can be clearly observed. Such a behavior can also be seen from the diffractive colors from other commercially available color OVDs. M-MIONS printing requires stacking three layers of polymer substrates, and the produced OVDs may be too thick. In this work, each substrate is about 250 μm thick, and the total thickness of the produced OVD is around 750 μm. Such an OVD would be too thick for laminating ID cards, which are 3120
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viewer’s motion. The printed transparent image was pasted onto the front glass and a simple ultrasonic motion sensor was installed below the front glass. This sensor can detect a viewer’s motion to control the white LED lighting bar installed on the ceiling of the box. The LED is switched on to illuminate the transparent color image once a waving hand is detected by the ultrasonic sensor. In this work, three layers are used in printing full-color transparent color images. In fact, more layers can be stacked together to include more layers of color information. For example, stereoscopic 3-D images can be printed using the MMIONS technique on two sets of red, green, and blue grating layers (six layers in total). One printed set diffracts a color pattern into a viewer’s left eye and another set into the right eye, by adjusting the light diffraction angle from the grating pixels. Through such 3-D effects, a viewer can potentially see virtual colorful objects superimposed onto the physical objects or scenes, and many interesting application ideas can be explored. Another potential benefit of printing more layers is to improve the viewing angle of the color patterns. Due to the diffractive nature of the colors from grating pixels, the colors appear brightest when a viewer perpendicularly faces the panel and become fainter as the viewer moves sideways. The limit in the angle can potentially be improved by printing images on more layers of grating surfaces with grating lines on each surface being intentionally oriented to cover a wider range of diffraction angles.
through the panel to watch the object only, owing to the high transparency of the printed image; when the LED is switched on, the printed color pattern appears brightly on the panel because of the diffraction of light into the viewer’s eyes. In experiments, we printed a full-color pattern with a size of 10 cm × 10 cm using the same process as described in Figure 5. Then, we pasted the printed image (about 750 μm thick) onto a transparent polymer glass (6 in. × 6 in. size, 0.5 in. thick) and placed the glass in front of a custom-assembled toy model. A white LED lighting bar with directional lighting was installed according to the diagram in Figure 7b. Visual effects of the transparent color image display were tested, and the results are shown in Figure 7c−e. A photo of the toy model alone (the physical object only, without any glass panel or image in front) is shown in Figure 7c, to be used as a reference. Figure 7d shows the photo of the scene after the glass panel (attached with the printed transparent image) is placed in front of the object while the LED is off. The existence of the printed image can barely be noticed, when comparing Figure 7c and d. The printed transparent color image does not block or disturb the view of the object. Once the LED is switched on, the printed diffractive color pattern is illuminated to display bright and colorful information, as shown in Figure 7e. Information related to the toy model, including the name, makers’ names, QR code, and colorful test patterns, is clearly overlaid in front of the physical object to enhance a viewer’s visual experience. On our transparent display panel, the printed color information is a static color pattern. To display different information, a new color image must be printed and pasted. In comparison, other mainstream transparent displays can all play motion pictures and the live information can be interactively updated. Nonetheless, our transparent display panel has advantages in its intrinsic simplicity and is sufficient for applications that can be satisfied with a single frame of information. Example applications include shop cabinets, shop windows, signages, and museum display boxes. In a museum display box, one transparent diffractive color image can be printed for the object being exhibited and the produced image can be effortlessly pasted onto the glass. It should be noted that the pasted image does not compromise the security or safety of the display box itself, which is very important for exhibiting objects of high value. If the exhibition object is changed, a new image will be printed to replace the previous image. For ideal performance of our transparent display panel, the light emitted from an LED lighting bar will only illuminate the diffractive color image, and the disturbance on the physical object or the viewer should be minimized. It is achieved by using subwavelength grating pixels and directional LEDs. Subwavelength grating pixels require illumination from a very steep angle (60−80° in this case) for the negative first order, as shown in Figure 7b. With the directional LED light source set up for that angle, the viewer and the object will not be illuminated as long as they keep a sufficient distance from the lighting bar. The viewer and object are slightly affected by the diffused light from the light source, but those effects are negligible. The visual experiences from our transparent color image displays are overall excellent, although the information contains only one frame. Furthermore, if a simple motion sensor is installed to control the LED, a viewer can actively turn on or off the information on the panel to gain some simple interactive experiences. A video in the Supporting Information shows the function of one such prototype display box in response to a
DISCUSSION M-MIONS printing is based on a multilayer color separation strategy in which structural colors are separately printed on individual layers, which are then vertically stacked and recombined into full-color images. Similarly, Jalali et al. have demonstrated stacking of plasmonic colors in multilayer 3D nanostructures in which the colors can drastically change when the layers are exfoliated.17 In M-MIONS printing, each layer is responsible for only one structural color and is patterned in a top-down fashion, i.e., from a large-area uniform nanoimprinted grating surface into molded grating pixels of a certain pattern. Although our multilayer strategy increases the complexity of printing from a single layer into multiple layers, the patterning on each layer is significantly simplified such that the overall throughput and printable image size can both be improved. Such a multilayer strategy combined with top-down patterning can also potentially be applied to scale up other structural color printing techniques, for example, structural colors from alldielectric metasurfaces. Furthermore, owing to separating colors in different layers, each primary color can be printed and displayed without interfering with other primary colors. Rough alignment for vertically overlapping the patterns is sufficient for displaying high-quality visual colors. In comparison, in structural color printing techniques that print all pixels on a single layer, the arrangement of pixels in different colors always requires special considerations and precise controls for generating correct colors. In our previous work on pixel-band-based MIONS printing, the image is printed onto a single layer of tiled pixel bands with microscopic alignment between the printed ink pattern and the pixel bands. The alignment accuracy is the main factor that limits the image size and production speed. The printed image displays colors in a fashion resembling color LCD displays with tiled red, green, and blue pixels. In this paper, M-MIONS printing implements vertically stacked multiple layers of 3121
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dielectric and plasmonic metasurfaces, are also attainable through the M-MIONS printing technique using specially designed nanostructured surfaces. In M-MIONS printing, a broad range of visual colors was attained by simply mixing the three primary colors, which were printed in different shades. The shades of colors were automatically controlled through the half-tone process of the digital inkjet printer according to the generated grayscale print patterns. In this work, 11 different shades for each primary color were printed. In the future, much finer shades can be achieved through more accurate control of ink droplets and substrate surface properties to enable high-quality color printing. For inkjet printing, the spreading of ink droplets and the shape of dried dots are mainly determined by the wettability of inks on the substrate surface. In our work, we coated a perfluorooctyltriethoxysilane (PFOTS) monolayer on the grating surfaces before printing to reduce the wettability. The function of the PFOTS monolayer is elucidated in supplementary Figure S7. Two identical blue grating surfaces, one without surface treatment and the other coated with a PFOTS monolayer, were characterized using contact angle measurements. As schematically shown in supplementary Figure S7a, an ink droplet tends to spread preferentially along the direction parallel with the grating lines. For the blue grating surface without surface treatment (supplementary Figure S7b,c), the measured contact angle is about 13° along the direction parallel with the grating lines and about 19° along the direction perpendicular to grating lines. For the blue grating surface coated with a PFOTS monolayer (supplementary Figure S7d,e), the measured contact angle is about 87° along the direction parallel with the grating lines and about 97° along the direction perpendicular to the grating lines. Supplementary Figure S8 shows the SEM images of the TiO2 NPs printed on a blue grating surface without surface treatment. The ink droplets significantly spread on the grating surface and merge into very big dots of irregular shapes, as shown in supplementary Figure S8a. The TiO2 NPs are deposited into a very thin layer inside the grooves of the grating lines, which results in very dim diffractive colors after index matching lamination. In this work, TiO2 NPs are used as the high-refractive-index ink material. In fact, other transparent inks can also be potentially used to print full-color diffractive images as long as the refractive index of the solute of the ink is sufficiently different from that of the polymer background. Good candidates include transparent lacquers, silica nanoparticle ink, and polystyrene nanoparticle ink.
nanostructured surfaces to scale up the image size and the production speed toward practical industrial applications. Alignment between patterns printed on different layers with macroscopic accuracy is sufficient, which fundamentally breaks the bottleneck imposed by alignment accuracy in pixel-bandbased MIONS printing. Using the M-MIONS technique, the inkjet printing process can be carried out rapidly and continuously, and there is no stringent requirement on the printer. The produced image displays colors similar to a conventional color photographic film, which separates colors into vertically stacked layers of primary colors. In this work, one full-color image of 10 cm × 10 cm can be printed on three substrates within 1 min using a desktop inkjet printer. The printer has a printing resolution of about 180 lines per inch (LPI) for printing regular color inks on paper substrates. In our experiments, due to the merging of printed dots on polymer grating surfaces, the effective resolution is lower. The thinnest fine line printed on our polymer substrates is about 500 μm wide, equivalent to a printing resolution of around 50 LPI. It should be noted that, using professional industrial inkjet printers, the merging of dots can be avoided and much higher resolution is achievable. One distinctive advantage of M-MIONS printing is that every production step can be carried out using commercially available, low-cost, and up-scalable production techniques. Based on the M-MIONS technique, industrial-scale printing of transparent diffractive colors for custom-input images can be realized by combining roll-to-roll replication of nanostructured substrates and roll-to-roll inkjet printing. There is no fundamental limit on the practical image size. The grating patterns on master stamps can be recombined into large-area uniform patterns and then replicated onto plastic thin foils using roll-to-roll nanoimprinting. Large-area images can be directly printed onto such recombined grating patterns. In addition, the image size can be further multiplied by printing images into sections, which are then combined into one image. In our experiments, we applied an antiadhesive monolayer on the replicated grating surfaces before printing to tune the wettability of the ink and removed this monolayer using oxygen plasma after printing for tight bonding of the cover film. The treatment of this monolayer complicated the printing process and must be avoided for future industrial production. One simple solution is to replicate the grating surfaces using a UV polymer with appropriate wettability of ink droplets. In terms of image size, roll-to-roll nanoimprinting can produce large images; however, it is restricted to replicating the same patterns in large quantities.51 Plasmonic colors can be patterned into a large area according to custom-input images using laser-induced coloration on metal surfaces.20 Compared to laser coloration, M-MIONS printing has appealing advantages in patterning speed and costs of equipment and substrates, which are key attributes for commercial applications that require large image size and low manufacturing cost. In other inkjet-based techniques capable of low-cost printing in large sizes, the structural colors and other optical properties of the pixels are determined by and limited to the synthesized ink materials.27,29,30,33−35,43,53,54 In M-MIONS printing, the structural coloration mechanism is mainly controlled through the nanostructured surfaces, which can be precisely engineered for different applications. In this work, diffractive colors from subwavelength gratings were used for applications in optical variable devices and transparent color display panels. Other structural coloration mechanisms, such as resonant colors from
CONCLUSIONS We demonstrated the concepts and experimental results of MMIONS printing based on inkjet printing of TiO2 nanoparticles on multiple layers of subwavelength grating surfaces. Transparent full-color diffractive images were rapidly printed according to custom-input images. Different colors were achieved by mixing diffractive colors from vertically stacked transparent dielectric gratings that were molded on red, green, and blue layers. In experiments, photosize full-color images were printed with 729 different printable colors and 50 LPI resolution. Experimental results proved that the problems of “color leaking” and limited image size in pixel-band-based MIONS printing were both solved using the M-MIONS technique. In addition, M-MIONS printing can be implemented through a broad range of inkjet printers, and even a 3122
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Printing and Drying. According to the target color image, the print patterns for red, green, and blue layers were generated using custom-written MATLAB codes. Each type of substrate was loaded into the printer and printed with the corresponding pattern. After printing, the substrates were baked on a hot plate surface at 35 °C for 20 min to dry the ink. Photographs of the printing process and configuration parameters of the inkjet printer are provided in supplementary Figure S9. Index-Matching Lamination. After printing and drying, the sample surface has polymer gratings and a printed TiO2 film. Before lamination, the sample surface was treated with a gentle oxygen plasma etching for 5 s to remove the PFOTS monolayer and to activate the polymer surface for bonding. To laminate a single layer (red, green, or blue), a transparent, lowviscosity UV-curable glue (UV CURE 60-7159, Epoxies, Etc., refractive index about 1.5) was dispensed on the sample surface, and a thin PET sheet was placed on top of the UV glue. After removing air bubbles with pressure, the UV glue was cured under UV exposure from both sides (10 min exposure). For laminating three layers to produce a full-color image, the printed red layer was first laminated on a PET sheet using the same lamination process as described above. Next, the printed green layer was laminated onto the back side of the red layer using the same procedure, with visual alignment of the patterns between the two layers. Similarly, a blue layer was then aligned and laminated onto the back side of green layer using the same procedure. Optical Characterization. After printing and lamination, the samples were optically characterized for spectral measurement and capturing photos. Supplementary Figure S10 shows a schematic of the optical setup for measuring diffraction efficiency, which was used in our previous work.37,39 The sample is mounted on a rotation stage to control the light incidence angle and light collection angle. Broad-band white light from a fiber-connected light source is collimated by a C1 collimator. The output parallel beam is then incident onto the sample surface, and the diffracted light is collected by a microscope objective lens (4× Olympus plan achromat objective, numerical aperture = 0.10, working distance = 18.5 mm). Behind the objective lens, an iris I1 reduces the numerical aperture to about 0.06. The diffracted light is split by a beam splitter (50:50 split ratio). Half of the diffracted light is recorded by a digital color camera to give a microscope color image of the diffractive colors. The other half is first focused onto an I2 iris for selecting a circular spot from the sample surface, and then the transmitted light is coupled into a fiber-connected spectrometer to record an intensity spectrum, IS(λ). The reference spectrum, Iref(λ), is measured by loading a flat mirror under the objective lens and recording the mirror reflection of the source beam. The diffraction efficiency from a spot selected by iris I2 is given by
low-cost desktop color inkjet printer for regular office use was proven capable of printing large structural color images. The printed images were applied as transparent OVDs for authenticating personalized security documents and “print and paste” transparent color image display panels for overlaying colorful information in front of physical objects. M-MIONS printing can potentially enable industrial-scale customized manufacturing of structural colors with high speed, low cost, and very large image size.
MATERIALS AND METHODS The process in printing a custom-input color image is mainly composed of four steps: (1) replicating grating surfaces in a UV polymer and applying an appropriate surface treatment if necessary; (2) preparing the TiO2 NP ink and the inkjet printer; (3) generating print patterns according to the target color image, printing the patterns on the corresponding substrates, and drying the ink with applied heating; (4) index-matching lamination (oxygen plasma before lamination if the polymer surface is antiadhesive). This section provides details about the relevant steps. Replication of Subwavelength Grating Surfaces. A schematic of the fabrication process is shown in supplementary Figure S1. Three master stamps with desired subwavelength grating patterns on quartz wafers were first fabricated using laser interference lithography and reactive ion etching. A quartz wafer was cleaned and evaporated with an aluminum (Al) thin film. A photoresist layer was spin-coated on top of the Al layer (step (i)) and then exposed with periodic nanopatterns using laser interference lithography (step (ii)). After developing the photoresist (step (iii)), the patterned photoresist layer was used as a mask to etch the pattern into the Al layer using reactive ion etching (step (iv)). After stripping the photoresist using oxygen plasma (step (v)), the patterned Al layer was used as a mask to etch into the quartz surface using reactive ion etching (step (vi)). Vertical side walls with a taper angle larger than 80° were obtained from the etching. After etching, the quartz stamps were thoroughly cleaned with chemicals and oxygen plasma (step (vii)). The grating patterns on the master stamps were replicated onto a transparent polymer layer in two steps. Prior to replication, the quartz stamps were first coated with an antisticking monolayer of PFOTS for easy release. In the first step of replication, liquid UV resin was dispensed onto a 250-μm-thick transparent polyethylene terephthalate (PET) sheet, and the quartz stamp was applied on the UV resin. After removing the bubbles with pressure, the UV resin was cured with sufficient UV exposure (step (viii)). After separation, a polymer stamp with nanostructures negative to the master stamp was obtained (step (ix)). This polymer stamp was then used to produce multiple replications using the same UV nanoimprint process again, and each replica has nanostructures identical to the master stamp (steps (x), (xi)). Prior to printing, the substrate surface was treated with a PFOTS monolayer to reduce the wettability of ink in order to improve printing resolution. Preparation of TiO2 NP Ink and the Inkjet Printer. A commercially available TiO2 nanocrystalline colloidal paste (SigmaAldrich, nanoparticle diameter 18 nm, concentration 16%, waterbased) was used as the raw material for preparing the ink. The paste was diluted with deionized water and ethylene glycol at a volume ratio of 1:3:6. The diluted ink was sonicated for 1 h before use. Epson Stylus C88+ was used as the inkjet printer. The printing resolution is 5760 × 1440 dots per inch (DPI), and the volume of smallest ink droplets is 3 pL. Diluted ink was slowly injected from a syringe into a refillable inkjet cartridge for the black channel (Epson refillable inkjet cartridges, NovaCentrix). Another three refillable inkjet cartridges for three color channels were filled with a mixed solution of water and ethylene glycol (volume ratio of 1:1). After installing all four refillable cartridges inside the printer, a few cycles of head cleaning procedures were carried out to calibrate the quality of the printed patterns, and then the printer was ready for printing TiO2 NPs on the polymer substrates.
ηROI =
IS(λ) − Idark(λ) , Iref (λ) − Idark(λ)
where Idark(λ) is a dark spectrum recorded
when the light source is switched off. The normalized diffraction η efficiency is given by ηnorm = RROI , where RA is the area ratio that the A
bright pixels occupy within the selected spot. RA is calculated by digital image processing of the collected microscope color image. ηnorm is a property of a molded grating pixel, independent from the size. In all spectra presented in this article, the default light incident angle is α = 62° and viewing angle is β = −17°, unless otherwise specified. Color photos of the samples were captured using a digital camera (Canon 50D) with a 50 mm lens. The aperture of the camera was set to f/8 to match closely with the numerical aperture of the optical measurement setup. The sample was mounted onto a rotation stage. The camera and a halogen light source were fixed at a distance of about 50 cm from the sample, and the angle between the lighting angle and viewing angle was about 45°. The sample was rotated to vary the viewing angle β from −5° to −17°, and color photos were captured at specific angles. 3123
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b08580. Figures of the fabrication process of polymer grating surfaces, calculation of grating wavelength, SEM images of TiO2 NPs printed on red and green grating surfaces, transmission properties of printed layers, color shifting of grating pixels, and optical measurement setup (PDF) Video showing the function of a display box using a transparent diffractive color image printed with the MMIONS technique (AVI)
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Hao Jiang: 0000-0003-4450-1467 Author Contributions
H.J. and B.K. conceived the concepts. H.J. conducted all experiments and analysis. Both authors reviewed the manuscript. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was funded by the Natural Sciences and Engineering Research Council (NSERC) of Canada, under Collaborative Research and Development (CRD) Grant Nos. 614108 and 569260. This work made use of the 4D LABS shared facilities supported by the Canada Foundation for Innovation (CFI), British Columbia Knowledge Development Fund (BCKDF), Western Economic Diversification Canada (WD), and Simon Fraser University (SFU). The master stamps (on quartz wafers) have been fabricated in collaboration with Moxtek Inc. (Utah, USA). The replications of the grating surfaces have been done in collaboration with NanoTech Security Corp. (Vancouver, British Columbia, Canada). Special thanks goes to Mr. Y. Wang (School of Mechatronic Systems Engineering, Simon Fraser University) for drawing the 3-D schematics and constructing the motion-activated display box. REFERENCES (1) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Surface Plasmon Subwavelength Optics. Nature 2003, 424, 824−830. (2) Laux, E.; Genet, C.; Skauli, T.; Ebbesen, T. W. Plasmonic Photon Sorters for Spectral and Polarimetric Imaging. Nat. Photonics 2008, 2, 161−164. (3) Xu, T.; Wu, Y.-K.; Luo, X.; Guo, L. J. Plasmonic Nanoresonators for High-Resolution Colour Filtering and Spectral Imaging. Nat. Commun. 2010, 1, 59. (4) Kumar, K.; Duan, H.; Hegde, R. S.; Koh, S. C.; Wei, J. N.; Yang, J. K. Printing Colour at the Optical Diffraction Limit. Nat. Nanotechnol. 2012, 7, 557−561. (5) Wu, Y.-K. R.; Hollowell, A. E.; Zhang, C.; Guo, L. J. AngleInsensitive Structural Colours Based on Metallic Nanocavities and Coloured Pixels beyond the Diffraction Limit. Sci. Rep. 2013, 3, 1194. (6) Zeng, B.; Gao, Y.; Bartoli, F. J. Ultrathin Nanostructured Metals for Highly Transmissive Plasmonic Subtractive Color Filters. Sci. Rep. 2013, 3, 2840. (7) Goh, X. M.; Zheng, Y.; Tan, S. J.; Zhang, L.; Kumar, K.; Qiu, C.W.; Yang, J. K. Three-Dimensional Plasmonic Stereoscopic Prints in Full Colour. Nat. Commun. 2014, 5, 5361. 3124
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Article
ACS Nano
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DOI: 10.1021/acsnano.7b08580 ACS Nano 2018, 12, 3112−3125
