ARTICLE pubs.acs.org/Langmuir
Photonic Printing through the Orientational Tuning of Photonic Structures and Its Application to Anticounterfeiting Labels Ruyang Xuan and Jianping Ge* Department of Chemistry, Tongji University, Shanghai, 200092, China
bS Supporting Information ABSTRACT: A novel photonic printing technique based on the orientational tuning of photonic structures is developed. In the printing process, the θ-contrast pattern is produced by magnetic alignment, orientational tuning, and lithographical photopolymerization. Labels printed with two mirror-symmetric or multiple axially symmetric photonic orientations show a switchable color distribution or dynamic color halo when the angle of incident light changes or samples are tilted. This printing method is capable of fabricating colorimetric or invisible codes, which can be decoded by visual observation or a spectrometer.
’ INTRODUCTION A printing technique based on the fabrication and manipulation of photonic structures has attracted increasing interest because of its applications in photonic circuits, reflective display units, outdoor signage, and photonic sensors.115 Among all of the photonic structures, the colloidal crystal composite serves as one kind of potential material for photonic printing because it can be prepared inexpensively on a large scale and flexibly tuned when active components are introduced. A possible technical extension to photonic printing is the production of anticounterfeiting labels because photonic structures usually possess unique, self-displayed optical signals that can be easily distinguished by the naked eye or commercial photoelectric devices. As a matter of fact, laser anticounterfeiting labels based on the interference of sunlight and surface patterns have been widely used nowadays. For all of these applications using photonic structures, the development of novel photonic printing techniques with fast fabrication, low cost, good reproducibility, and tunability is always highly desired. Most existing photonic printing methods using colloidal crystals can be classified into two categories. One is inkjet printing with a monodisperse colloidal dispersion as ink, which self-assembles locally on a substrate.1619 Wang and Song et al. have recently fabricated macroscale-patterned photonic crystals with multiple stop bands using polystyrene@ poly(methyl methacrylate)/poly(acrylic acid) (PSt@PMMA/PAA) latex spheres with different sizes as colored inks.18 This method integrates well with commercial inkjet printing, and the quality of self-assembly has been gradually improved in the past few years through the modification of colloidal particles and the optimization of the printing process. r 2011 American Chemical Society
An alternative method of photonic printing is changing the photonic structure in a specific region of premade colloidal crystals and related composite materials, which creates a contrast of reflection or transmission at certain resolutions.2030 Although photonic structures have to be prepared, this strategy is chosen by many researchers because it offers a great diversity of printing modes by tuning the refractive index (n), lattice spacing (d), and crystal orientation (θ). Even slight differences in the refractive index, lattice spacing, and crystal orientation between neighboring regions will lead to the formation of patterns with n contrast, d contrast, and θ contrast, respectively. For instance, Sato et al. infiltrated the voids of inverse opals with liquid crystals and azobenzene derivatives and printed n-contrast patterns with UV light, which caused the nematiciostropic phase transition of azobenzene and a change in the refractive index.20,21 Ozin et al. used a laser beam to microanneal a specific area on a silicon inverse opal film, producing patterns by the phase transition of hydrogenated silicon and the following change in the refractive index..22 Besides the adjustment of the refractive index, the tuning of the lattice spacing was investigated broadly over the past few years. Foulger et al. developed a lithographical method to print patterns on a poly(2-methoxyethyl acrylate)-encapsulated polystyrene colloidal crystal film.23 Through photopolymerization, the lattice expansion caused by the infiltrated monomers was permanently fixed in the regions exposed to UV light, producing a d-contrast pattern. Yin et al. reported a rewritable printing process on a magnetically assembled photonic crystal film using a hygroscopic salt solution as an ink, where the d-contrast Received: February 14, 2011 Revised: March 23, 2011 Published: April 06, 2011 5694
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Langmuir pattern was created by the swelling of polymer matrix as a result of the water absorption of hygroscopic salt residues.24 Kwon et al. developed a lithographical structural color printing technique that combined the assembly and tuning of lattice spacing in one step.25 Among all of these photonic printing processes, hardly any method was developed on the basis of the creation of the θ-contrast pattern, mostly because of the difficulty in changing the crystal orientation in a preassembled photonic structure and controlling multiple orientations in neighboring regions during the self-assembly. In this work, a novel photonic printing process based on the creation of a θ-contrast pattern has been developed. The printing process was accomplished by repeating the magnetic alignment, orientational tuning, and lithographical photopolymerization. It is worth noting that the printing method uses only one precursor, and no other strategies except the orientational tuning were utilized in this work. Thanks to the recent development of magnetically tunable assembly techniques, both the interparticle spacing and the orientation of 1D chain structures can be conveniently adjusted and instantly fixed inside a polymer matrix.8,25 Combined with the lithographical process, the assembly is able to create photonic structures with various orientations in selected regions and print θ-contrast patterns with interesting optical signals. The as-printed labels composed of photonic structures with two mirror-symmetric orientations present a switchable color distribution or hide the optical contrast when the angle of incident light with respect to the sample changes. The labels printed with multiple axially symmetric orientations show colorful halo and dynamic optical effects when they are tilted under sunlight, which is hard to mimic by general prints. This photonic printing is also used in colorimetric coding. As the requirements of crypticity and security increase, the codes can be printed in a highly transparent and invisible form that is readable only with a microscope or microspectrometer. This work demonstrated the use of orientational printing in the fabrication of anticounterfeiting labels, and such a fast, low-cost, tunable photonic printing technique will undoubtedly find application in many related fields.
’ EXPERIMENTAL SECTION Materials. Iron(III) chloride (FeCl3, 97%), diethylene glycol (DEG, 99%), poly(acrylic acid) (PAA, Mw = 1800), poly(ethylene glycol) methacrylate (PEGMA, Mn = 360), poly(ethylene glycol) diacrylate (PEGDA, Mn = 700), and 2,2-dimethoxy-2-phenylacetophenone (DMPA, 99%) were purchased from Sigma-Aldrich. Sodium hydroxide (NaOH, 99%), tetraethylorthosilicate (TEOS, 98%), and ammonium hydroxide solution (28%) were purchased from Sinopharm Chemical Reagent Co. Superparamagnetic Fe3O4 colloids were prepared by using a high-temperature hydrolysis reaction and then coated with a silica layer through a modified Stober process.7 Magnetic colloids and photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPA, 5 wt %) were dispersed in the mixture of poly(ethylene glycol) methacrylate and poly(ethylene glycol) diacrylate (4:1 PEGMA/PEGDA) to form a brown, homogeneous magnetic ink. Photonic Printing of θ-Contrast Patterns. To print the pattern composed of two mirror-symmetric orientations (Figures 1 and 3), magnetic ink was sandwiched between the substrate and a fluorinated glass slide with a designed interspacing (10470 μm) and then placed above a tilted NdFeB magnet (5 5 1 cm3) and 20 cm beneath a UV light source (Spectroline SB-100P, 365 nm, 4800 μW/cm2) for 1 min of photopolymerization. It should be noted that the angle of incident light, particle chains, and external magnetic field are all defined
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Figure 1. Printing process based on the orientational tuning of photonic structures. from 90 to 90° (Figure 4) in this work. The words section was first printed with the magnetic field tilted to 15° and UV polymerization under a photomask. The unfixed magnetic particles shielded by the photomask were still fluidic and changed orientation when the external magnetic field was tilted. Then, the background section was printed with the magnetic field tilted to þ15°, followed by a second UV exposure after the photomask was removed. All plastic photomasks were designed on a computer and prepared on a commercial laser printer. To print labels composed of multiple orientations (Figure 5), the magnetic inks were first placed above a cubic magnet whose field angle was tilted to 90°, and the logo was printed with UV polymerization under a photomask. The sample was then transferred onto a cylindrical magnet, which naturally provides axially symmetric field distributions, to print the backgrounds after removing the mask. For the printing of binary codes, the lithographical photopolymerization was similar to the previous process, but three states/orientations have to be preassigned to codes 0, code 1 and the background. For example, two colorimetric codes and the background in Figure 6a were composed of photonic structure with orientations of 15, þ15, and 90°. The disorder state can also be used as the background, as in the case of transparent binary codes in Figure 6b, where codes 0 and 1 were composed of photonic structures with orientations of 0 and 90°, respectively. Characterization. A Canon A640 digital camera was used to capture the photographs of photonic prints illuminated by parallel light at a specific incident angle. The photographs were taken without a flash to guarantee that the incident light was projected from one direction. The optical microscope images were captured using an Olympus BX51M. The reflection and transmission spectra were measured on an Ocean Optics USB-4000 UVVIS spectrometer. The reflections in Figure 2a are measured with UV and visible light sources, and all other spectra are measured with a visible light source only. All reflection spectra are measured with a six-around-one fiber probe so that the incident and reflective angles are fixed at 0°. All transmission spectra are measured with a coaxial light source and a detector placed on each side of the thin film, and the incident and transmitted angles are fixed at 0° as well. For the measurement of reflections from microscale objects (Figure 6d), the spectrometer was connected to an optical microscope to collect signals from the trinocular tube, where the incident and reflective angles are also fixed at 0°
’ RESULTS AND DISCUSSION Orientational photonic printing is accomplished by magnetic alignment, orientational tuning, and lithographical photopolymerization. Figure 1 illustrates the simplest printing process, in which only one mask and two orientations are involved. In this typical process, the original disordered magnetic particles form chain structures along the direction of the external magnetic field. 5695
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Figure 3. (a, b) Scheme and (c, d) digital photographs of photonic labels in (a, c) reflection and (b, d) transmission modes. The green and blue sections in the scheme correspond to words and the background, respectively. Photographs 1 and 2 (or 3 and 4) show the same sample as the incident light projected along opposite directions. The scale bar is 1 cm. Figure 2. (a) Reflection and (b) transmission spectra of photonic crystal films as the orientation tilted from 0 to 90°. (c) Top-view optical microscope images of chain structures fixed inside a polymer matrix (film thickness, 280 μm; particle density, 8.6 mg/cm3) with the corresponding orientations. The scale bar is 10 μm.
Once the orientation is determined, the photonic structure exposed under a photomask (green part) can be instantly and permanently fixed by UV polymerization. The unfixed magnetic particles reassemble into chains and change their orientation as the external field tilted to the opposite direction. Those photonic structures (blue part) are fixed in the second curing process after the mask is removed. Finally, a photonic crystal film with two crystal orientations are produced, forming θ-contrast patterns as expected. With multiple masks and orientational tuning introduced, it is believed that more complicated and unique patterns can be produced by this method. The fundamental principle of orientational printing is that the anisotropic 1D photonic structures show angle-dependent reflection and transmission. With angles of incident, reflected and transmitted light fixed at 0°, the reflection and transmittance of photonic structures with various orientations were measured by fiber spectrometry (Figure 2a,b). As the chain angle increases from 0 to 90°, the reflection peak blue shifts and gradually loses its intensity, which is similar to the optical behavior of magnetochromatic microspheres.8 The evolution of the reflection wavelength can be explained by applying Bragg’s formula to a physical model (Figure S1). It is interesting that as more particles appearing to block the transmitted light when the chains are tilted to 90°, the transmittance, on the contrary, increases within the visible range, which is probably caused by the decrease in the reflection intensity. There is an irregular decrease in transmittance as the sample is tilted to 45° because the blocking effect of particles still works even if it does not always dominate the transmission. These two angle-dependent properties suggest that the θ contrast can be realized by the variation of colors in reflection mode and the change in transparency in transmission mode. The top-view optical microscope images of the fixed particle chains well match the reflection measurement with respect to color evolution and the change in the reflection intensity (Figure 2c). Each bright green dot or rod in the optical microscopy image actually represents one aligned particle chain. The magnetic particles have an average diameter of 160 nm, and the interparticle spacing is calculated to be 187 nm according to Bragg’s equation. The density of the magnetic particles inside the
polymer matrix is estimated to be 8.6 mg/cm3. With the chains titled from vertical (0°) to horizontal (90°), bright isolated spots gradually turn into a long line, which demonstrates that photopolymerization does not strongly interfere with the magnetic alignment and orientational tuning. The separation between particle chains is about 2 to 3 μm with a typical particle density, indicating that the limiting resolution of the current printing technique is several micrometers. This resolution is close to the theoretical limit of photonic printing using colloidal crystals, where the colloidal building block is usually several hundred nanometers. In fact, the practical resolution is also determined by the alignment of boundary particles between two domains with different orientations. There might be transition orientations within the boundary regions, but their negative influence on the resolution is negligible using the reported printing process (Figure S2). On the basis of the above discussions and observations, interesting photonic prints can be prepared by this novel method. As a demonstration, an alphabetic logo composed of photonic structures with two mirror-symmetric and coplanar orientations was fabricated by the procedures illustrated in Figure 1. As shown in Figure 3a, the word and background sections are made of particle chains with tilting angles of 15 and þ15°, respectively. When the incident light has a smaller separation angle with respect to the particle chains in the word section (case 1), the words and background show diffractions with longer wavelength (green) and shorter wavelength (blue), respectively. On the contrary, as the incident light is projected with a smaller separation angle to the particle chains in the background section (case 2), the words and background instantly switch their colors. Similarly, the alphabetic logo has switchable transmitted signals when the angle of incident light changes, as shown in Figure 3b. Either the words or the background becomes less transparent as the incident light is projected close to its orientation. In the sunlight where the incident angle is fixed, the color or transparency switching can also be realized by rotating the samples horizontally 180°. The θ contrast and color switching can be well explained by the theoretical calculations. As illustrated in Figure 4, a physical model was built to investigate the interference between incident light and colloidal particles, where light is projected at a specific angle and the reflections are collected along the normal direction of the film. The diffraction wavelength of photonic structures in the word and background sections are described by eqs 1 and 2, respectively, which suggests that λ in the word section is larger 5696
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Figure 4. Calculation of the reflection of the mirror-symmetric photonic structure when incident light is projected at a specific angle and reflected light is collected along the normal direction of the film.
than that of the background when θ is less than 0 and vice versa. The equations reveals that the blue shift of the diffraction is actually caused by the separation of incident light from the photonic chain structures. The calculation also predicts that when the incident light has a same separation angle with respect to particle chains in both the word and background regions the diffractions will be the same and the θ contrast disappears. In fact, there is a specific angle of the light source (θ = 0) in which the words are blurred or invisible to observation. These photonic prints with switchable distributions of color and transparency are different from general prints, which might be used for anticounterfeiting purposes. λlef t ¼ δ1 þ δ2 ¼ nd cosð 15°Þ þ nd cos½θ ð 15°Þ ¼ nd½cos 15° þ cosðθ þ 15°Þ
ð1Þ
λright ¼ δ1 þ δ2 ¼ nd cos 15° þ nd cosðθ 15°Þ ¼ nd½cos 15° þ cosðθ 15°Þ
ð2Þ
The reflection and transmittance of the photonic crystal film are found to be closely related to its thickness and the density of particles, which should be optimized for better performance. Samples with a tilting angle of 0° are prepared at various thicknesses, which are approximately controlled by the interspacing of glass slides sandwiching the liquid precursors during the synthesis. When the thickness decreases from 470 to 10 μm, the reflection decreases and the transmittance increases. (Figure S3a,b) Here, the effective reflection (R) can be considered as the results obtained by deducting the absorption (A) of the composite film from the reflection (RPC) of the photonic structure (R = RPC A), both of which will increase with the thickening of the film. When the thickness increased from 10 to 280 μm, the enhancement of RPC was dominant and the effective reflection increased accordingly. As the thickness increased further beyond 280 μm, the reflection contribution from the bottom layer of the film was counteracted by the absorption of the film so that the effective reflection becomes saturated. Considering the angle-dependent properties, the photonic prints are expected to be thicker than 30 μm when a visual observation of reflection is required. The detection of thinner samples can be obtained with a microscope and a microspectrometer. In transmission mode, the labels are expected to have a thickness of 30120 μm so that the
Figure 5. (a) Fabrication of the graphic logo with multiple axially symmetric orientations upon a cylindrical magnet. (b) Cross section of the logo and distribution of particle chains with various orientations. (c) Digital photographs of the logo as the incident light is projected from angle 1 to 4 in part a.
transmittance of samples with various orientations can falls into the effective measuring range. For thicker films, the strong absorption of Fe3O4 particles will minimize the transparency difference caused by the tilting of photonic structures. As for thinner films, there is little room left for the increased transparency once the photonic structures are tilted away from 0°. The density of magnetic particles is increased to enhance the reflections and create more space for the enhancement of transparency. (Figure S3c,d). The transmittance monotonically decreases as the particle density rises. There is an optimal particle density for the reflection because high density shortens the distance between particle chains and induces stronger repulsion, which will interfere with the assembly and decrease the degree of order. Using various magnetic field distributions, the orientational printing is capable of creating more sophisticated patterns with multiple photonic orientations. Figure 5 shows the scheme and photographs of a graphic logo in which the patterns are composed of photonic structures with a tilting angle of 90° and the backgrounds are made of particle chains with continuously tilted orientations that are actually determined by the axially symmetric field distribution on a cylindrical magnet during the assembly process. The results reveal a convenient method for printing patterns with multiple orientations in one step because a distinctive field distribution can be designed in the manufacture of magnets or electromagnets. Generally, the logo shows a characteristic colorful halo in the region whose orientation is close to the incident light and light brown in the rest. With the projected light surrounding the logo from angle 1 to 4 (Figure 5a), the colorful halo follows and moves around (Figure 5c), showing a dynamic optical effect similar to that of laser anticounterfeiting labels. In the sunlight, this effect can be seen as the logo itself is slightly tilted by hand when the observation angle is fixed. On the basis of the above calculations and conclusions, the green-blue halo can be explained as follows (Figure S4). The color halo can be roughly regarded as the combination of five regions: central (C), left (L), right (R), up (U), and down (D). Then, five particle chains are used to illustrate the photonic orientations of these five regions. Chain 2 (C) presents the average orientation in the central region showing a green color, and chains 1 (L), 3 (R), 4 (U), and 5 (D) describe the average 5697
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Figure 6. (a) Colorimetric binary codes array and (b) an invisible replica printed with an orientational printing process. (c) Optical microscope image of two neighboring code chips in part b. (d) Reflection spectra and (e) reflection intensity at 585 nm for 10 continuous code chips in the first line of the array. The scale bars are (a, b) 5 mm and (c) 200 μm.
orientations in the surrounding regions showing a blue color. According to the axially symmetric field distribution on the cylindrical magnet, chains 13 are in the same vertical plane with different tilting angles and chains 4 and 5 are out of this plane but have the same tilting angle as that of chain 2. When parallel light is projected along chain 2, the central region has the largest reflection wavelength (λ) and the surrounding regions have smaller λ values because all of the latter orientations are tilted away from the incident light. Therefore, the circular halo presents a green color in the center and a blue color in the surrounding regions. The particle chains in the rest regions have an even larger separation angle to the incident light, and their reflections are thereby seriously quenched and show the intrinsic color of iron oxide. The color transition from green to blue to brown is smooth and continuous, which renders a label dynamic optical effect that is hard to mimic by general ink printing. This photonic printing technique can also be applied to the fabrication of colorimetric codes. On the basis of the definition that photonic structures with tilting angle of 15 and þ15° are coded as 0 and 1, 50 binary codes can be translated into an array of color chips (0.5 1.1 mm2) through the reported printing process (Figure 6a). The background is composed of particle chains with a tilting angle of 90°, which form a contrast for both codes. For the purpose of enhanced crypticity and security, the code chips might be miniaturized by 10-fold to exceed the resolution of the human eye or be printed in a thin, highly transparent form, both of which are invisible under common conditions but readable by a microscope or microscale photoelectric sensor. Inside the dashed square in Figure 6b, there is a replicated binary code array such as that in Figure 6a, except that photonic structures with tilting angles of 0 and 90° are defined as codes 0 and 1 whereas the background is composed of disordered particles. The code array is also composed of 50 rectangular chips (0.5 1.1 mm2), but it is invisible now because its thickness is
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merely 30 μm. The optical microscope image of two neighboring code chips confirms their vertical (left) and horizontal (right) orientations, respectively (Figure 6c). It also proves that the disordered state can be an alternative choice to create the contrast. As a demonstration, the first line of the codes containing 10 continuous code chips was decoded by scanning their reflection intensity at 585 nm, which is the characteristic reflection peak for vertical photonic structures (Figure 6d,e). With the orientation of chains changed between 0 and 90°, the reflection intensity switches between 7 and 0.5 accordingly so that the invisible codes can be precisely decoded. It should be noted that there is an error in the intensities for code 0, which is caused by the nonparallel magnetic field distribution of the magnet. The consistency should be improved when a perfectly parallel magnetic field is used in the fabrication or the code array is further miniaturized. In summary, a novel photonic printing technique based on the orientational tuning of photonic structures and the creation of a θ-contrast pattern has been developed. The whole printing process can be quickly accomplished by magnetic alignment, orientational tuning, and lithographical photopolymerization using a single precursor, and no preassembly of photonic paper or the infiltration of other chemicals is required. Labels printed with two mirror-symmetric photonic orientations show a switchable color distribution or hide the reflection contrast as the angle of incident light changes. Labels printed with multiple axially symmetric orientations show a dynamic color halo as they are tilted in the sunlight. This printing technique is also capable of fabricating colorful or invisible binary codes, which can be decoded by visual observation or a microscopic photoelectric apparatus. In addition to the reflection mode, the prints can be recognized with transmission signals as well. It is believed that this effective, new photonic printing method will find application in photonic signage such as anticounterfeiting labels, photonic circuits, and display units in the near future.
’ ASSOCIATED CONTENT
bS
Supporting Information. Schematic illustration and calculation of the reflection of the tilted photonic structure when the incident or reflected light is projected or collected along the normal direction of the film. Optical microscope images of the boundary of two neighboring regions with opposite orientations. Reflection and transmission spectra of photonic crystal films. Schematic illustration of the formation of the color halo and its color distribution. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT J. Ge is grateful for support from the National Science Foundation of China (NSFC 21001083), the Shanghai Pujiang Program (10PJ1409800), and Tongji University startup funds. We also acknowledge the Experimental Chemistry Center of Tongji University. 5698
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