Magnetic Assembly Route to Colloidal Responsive Photonic Nanostructures LE HE,† MINGSHENG WANG,† JIANPING GE,†, ‡ AND YADONG YIN*, † †
Department of Chemistry, University of California, Riverside, California, 92521, and ‡Department of Chemistry, Tongji University, Shanghai, China, 200092 RECEIVED ON NOVEMBER 1, 2011
CONSPECTUS
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esponsive photonic structures can respond to external stimuli by transmitting optical signals. Because of their important technological applications such as color signage and displays, biological and chemical sensors, security devices, ink and paints, military camouflage, and various optoelectronic devices, researchers have focused on developing these functional materials. Conventionally, self-assembled colloidal crystals containing periodically arranged dielectric materials have served as the predominant starting frameworks. Stimulus-responsive materials are incorporated into the periodic structures either as the initial building blocks or as the surrounding matrix so that the photonic properties can be tuned. Although researchers have proposed various versions of responsive photonic structures, the low efficiency of fabrication through self-assembly, narrow tunability, slow responses to the external stimuli, incomplete reversibility, and the challenge of integrating them into existing photonic devices have limited their practical application. In this Account, we describe how magnetic fields can guide the assembly of superparamagnetic colloidal building blocks into periodically arranged particle arrays and how the photonic properties of the resulting structures can be reversibly tuned by manipulating the external magnetic fields. The application of the external magnetic field instantly induces a strong magnetic dipoledipole interparticle attraction within the dispersion of superparamagnetic particles, which creates one-dimensional chains that each contains a string of particles. The balance between the magnetic attraction and the interparticle repulsions, such as the electrostatic force, defines the interparticle separation. By employing uniform superparamagnetic particles of appropriate sizes and surface charges, we can create one-dimensional periodicity, which leads to strong optical diffraction. Acting remotely over a large distance, magnetic forces drove the rapid formation of colloidal photonic arrays with a wide range of interparticle spacing. They also allowed instant tuning of the photonic properties because they manipulated the interparticle force balance, which changed the orientation of the colloidal assemblies or their periodicity. This magnetically responsive photonic system provides a new platform for chromatic applications: these colloidal particles assemble instantly into ordered arrays with widely, rapidly, and reversibly tunable structural colors, which can be easily and rapidly fixed in a curable polymer matrix. Based on these unique features, we demonstrated many applications of this system, such as structural color printing, the fabrication of anticounterfeiting devices, switchable signage, and field-responsive color displays. We also extended this idea to rapidly organize uniform nonmagnetic building blocks into photonic structures. Using a stable ferrofluid of highly charged magnetic nanoparticles, we created virtual magnetic moments inside the nonmagnetic particles. This “magnetic hole” strategy greatly broadens the scope of the magnetic assembly approach to the fabrication of tunable photonic structures from various dielectric materials.
1. Introduction
between the building blocks and the surrounding medium
Colloidal assembly represents a powerful bottom-up method for the fabrication of functional materials, particularly colloidal photonic crystals in which the dielectric contrast
in the periodic arrays creates a photonic band gap that
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inhibits the propagation of light within certain wavelength ranges.1,2 Structural-colored photonic materials operating Vol. XXX, No. XX
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in the visible regime are of special interest as important chromatic materials with wide applications in color painting and printing, information storage, displays, and sensors.3 Unlike top-down approaches, colloidal assembly strategies for photonic crystal fabrication are technologically favorable due to mild processing conditions, low cost, and potential for scale-up.4 However, the precision of the structural and orientational control, the efficiency, and the scalability of the colloidal assembly routes still need to be greatly improved before they can be used for a wide range of practical applications. In many cases, it is also highly desirable to have the capability to tune the optical properties of the photonic materials by chemical or physical means.5,6 Colloidal assembly approaches are particularly suitable for this purpose because the stimuli-responsive components can be easily incorporated into colloidal crystals by modifying the building blocks or their surroundings to realize tuning of their diffraction colors in response to the application of the external stimuli.7 Although various responsive mechanisms have been developed, such as mechanical stretching,8 solvent swelling,9 and temperature-dependent phase change,10 many challenges still exist, including limited tunability of the photonic properties, a slow response to the external stimuli, incomplete reversibility, and difficulty of integration into existing photonic devices. To broaden the tuning range of diffraction color, the external stimulus must be able to induce large changes in either the refractive index of the components or the symmetries, lattice parameters, or orientations of the ordered arrays. New mechanisms need to be established to also significantly enhance the response rate of the active components to the external stimuli in order to offer dynamic optical modulation that can meet the demand of practical applications. To this end, magnetic field has been regarded as an effective stimulus to guide the assembly of magnetic particles into periodic colloidal arrays and tune the diffraction of photonic structures. Strong magnetic interactions can be initiated instantly by the application of an external magnetic field, providing enough driving force for rapid assembly of colloidal particles even within one second.11 Magnetic forces, acting at a large distance, provide convenient control over the photonic properties by changing the interparticle distance. Magnetic interactions, directional in nature, not only guide the formation of anisotropic photonic structures12,13 but also enable additional control over the orientation of the magnetic assemblies so that one can conveniently tune their optical properties through rotational B ’ ACCOUNTS OF CHEMICAL RESEARCH
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FIGURE 1. (a) Magnetic field distribution around a superparamagnetic particle with a dipole moment in the same direction as the external magnetic field. The repulsive (b) and attractive (c) dipoledipole forces in different particle configurations drive the formation of particle chains along the magnetic field (d). The color bar on the right shows the relative strength of the local magnetic field.
manipulation.14 The complexity achievable in the spatial distribution of magnetic fields also makes it possible to define patterns of photonic structures by controlling the local assembly behavior of magnetic particles. In this Account, we present our efforts toward the magnetic assembly of colloidal particles into photonic crystal structures with widely and reversibly tunable structural colors. We first briefly introduce the interactions exerted on the superparamagnetic colloidal particles, and then discuss the magnetic assembly of these particles into onedimensional dynamic ordered structures with tunable diffraction across the visible and near-IR region. The magnetic assembly strategy has been successfully extended to nonmagnetic building blocks by utilizing a stable ferrofluid of highly charged magnetic nanoparticles. By taking advantage of the unique features of the magnetically responsive photonic system, we also highlight several unique applications in structural color printing, anticounterfeiting devices, switchable signage, and field-responsive color display. Finally, we conclude with an outlook in this exciting field.
2. Mechanism of Magnetic Assembly Superparamagnetic particles dispersed in a solution experience two types of magnetic forces when they are exposed to an external magnetic field. The interparticle dipoledipole force describes the interaction of a dipole with the magnetic field induced by another dipole, while the packing force results from the gradient of the external magnetic field.15 As shown in Figure 1, the application of an external magnetic field induces a magnetic dipole moment m along the
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direction of the magnetic field in a superparamagnetic particle. For a spherical particle (particle 1) with a magnetic moment of m, its induced magnetic field H1 felt by another particle (particle 2) can be described as H1 = [3(m 3 r)r m]/d3, where r is the unit vector parallel to the line pointed from the center of particle 1 to that of particle 2 and d is the centercenter distance.16,17 The dipoledipole interaction energy of particle 2 with the same magnetic moment m can be thus written as U2 = m 3 H1 = (3cos2 R 1)m2/d3, where R, ranging from 0° to 90°, is the angle between the external magnetic field and the line connecting the center of the two particles. The dipole force exerted on particle 2 induced by particle 1 can be expressed as
3(1 3cos2 R)m2 F2 ¼ r(m 3 H1 ) ¼ r d4 The above equation clearly shows the dependence of the dipoledipole force on the configuration of the two dipoles. At the critical angle of 54.09°, the interaction approaches zero. The dipoledipole interaction is attractive when 0° e R < 54.09° and repulsive when 54.09° < R e 90°. When the interaction energy is large enough to overcome thermal fluctuations, the magnetic dipoledipole force drives the self-assembly of particles into 1D chain-like structures along the external field (Figure 1d). When the magnetic attraction is balanced by long-range interparticle repulsions, such as the electrostatic force, a force equilibrium can be established, leading to a defined interparticle separation. If the force balance is altered, the interparticle separation changes accordingly. Similarly, the packing force can be understood as the interaction of a magnetic dipole with the external magnetic
long-range 1D order at relatively large interparticle separation, resulting in widely tunable optical diffraction that can be controlled by manipulating the force balance. The stability of chain structures against stacking also greatly relies on the existence of strong interchain repulsion including both electrostatic and magnetic interactions. The strength of interparticle repulsion also affects the photonic performance of the chain assemblies, typically leading to a stronger diffraction intensity and wider wavelength tunability when the particles are covered with a higher density of surface charges.
3. Magnetic Assembly of Superparamagnetic CNCs Monodisperse superparamagnetic colloidal spheres are presumably the most suitable building blocks for magnetically responsive photonic crystals since their magnetic responses are much stronger than normal paramagnetic materials and their dipoledipole interactions can be fully initiated and controlled by external fields. Bibette et al. first reported the magnetic assembly of uniform emulsion droplets containing concentrated ferrofluids into 1D chains with optical diffractions tunable in a varying magnetic field.20 However, the thermodynamic instability of the emulsion droplets, their incompatibility with nonaqueous solvents, and the complicated steps necessary for obtaining uniform droplets greatly limit the practical use of their system. Efforts toward incorporation of magnetic particles into polymer spheres have been limited mainly by the low loading of magnetic content so the magnetic-field-induced assembly and tuning process is still inefficient.19,21 Increasing the magnetic moments by
field. The interaction energy of a magnetic dipole m in an
using larger magnetic particles was limited by the super-
external magnetic field H is expressed by Um = m 3 H so the
paramagneticferromagnetic transition so that the particles
packing force can be written as Fp =r(m 3 H). The packing force thus drives the movement of superparamagnetic
would be no longer dispersible in solution. These challenges
particles toward regions with the maximum magnetic field
colloidal building blocks with strong magnetic response, as
strength and induces a concentration gradient of particles18 or crystallization.19 In some cases, the induced local concentration change along with a large magnetic field gradient can cause the aggregation of the 1D chain assemblies into large 2D or 3D structures. Besides magnetic forces, the existence of strong interparticle repulsion, in most cases electrostatic interaction, is also essential in assembling magnetic colloidal particles into ordered arrays. As discussed in detail below, the long-range electrostatic force can be used to counterbalance the magnetic dipoledipole attraction to induce the formation of dynamic particle chains with
necessitate the development of novel superparamagnetic well as controllable size, morphology, stability, and surface properties. We have recently developed magnetically tunable photonic structures by assembling superparamagnetic Fe3O4 colloidal nanocrystal clusters (CNCs) with overall diameters in the range of 100200 nm.11,2224 Utilizing clusters of ∼10 nm superparamagnetic Fe3O4 nanocrystals not only increases their magnetic responses and thereby magnetic interactions, but also avoids the superparamagneticferromagnetic transition (at a domain size of ∼30 nm for Fe3O4). As a result, we are able to instantly assemble them Vol. XXX, No. XX
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FIGURE 2. (a) Digital photos showing the diffraction color change in a typical CNC dispersion encapsulated in a capillary tube with a width of 1 cm in response to a magnetic field with increasing strengths from left to right. (b) Reflectance spectra of the same sample in different magnetic fields. (c) Illustration of interparticle force inside the chain and between different chains. (d) Scheme of Bragg diffraction from the chains of CNCs.
into ordered structures (less than 1 s) and rapidly tune the photonic properties across the whole visible region through the application of a relatively weak (typically 50500 Oe) external field (Figure 2a,b). The key to the successful assembly of CNCs and tuning of the optical diffraction is the coexistence of a tunable magnetic dipoledipole force and comparable long-range electrostatic force, both of which are separation-dependent. Along the chain, the magnetic dipoledipole attraction is balanced by the electrostatic force while the interchain magnetic repulsive force, as well as the electrostatic force, keep the chains away from each other (Figure 2c). The diffraction wavelength of the colloidal arrays can be described by Bragg's law, mλ = 2nd sin θ, where m is the diffraction order, λ the wavelength of incident light, n the effective refractive index, d the lattice spacing, and θ the glancing angle between the incident light and diffraction crystal plane (Figure 2d).20 The rapid tuning of the diffraction of dynamic photonic chains is realized by controlling the interparticle separation d in response to a varying external field. For example, enhancing the magnetic field strength induces stronger magnetic attraction along the chain, which brings the particles closer and consequently blue-shifts the diffraction. The surface of the original CNCs is grafted with a layer of polyacrylate, during the synthesis, which provides a strong interparticle electrostatic force in aqueous solution.11 Engineering the surface of Fe3O4 CNCs with a layer of silica through the solgel process increases their compatibility with nonaqueous solvents; further modifying the silica surface with a layer of hydrophobic silane enables the dispersion of the particles in nonpolar solvents.25,26 The charges carried by the silica-coated Fe3O4 produce D ’ ACCOUNTS OF CHEMICAL RESEARCH
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electrostatic interactions that can work with the magnetically induced attraction and enable successful assembly.25 However, it is difficult to establish electrostatic repulsion in nonpolar solvents due to the high energy barrier to forming surface charges. We have addressed this challenge by introducing reverse micelles to promote charge separation and create electrostatic repulsions that can counteract the magnetic attraction to allow ordering of the superparamagnetic colloids.26 The 1D chain assembly can be easily observed using an optical microscope, as shown in Figure 3a. However, the study of the detailed structure requires fixing the dynamic chains. This was first achieved by magnetic assembly of superparamagnetic particles in a UV curable oligomer, such as poly(ethylene glycol) diacrylate (PEGDA), followed by an immediate photopolymerization to solidify the dispersion medium.26,27 As shown in the scanning electron microscopy (SEM) image in Figure 3b, parallel particle chains with regular interparticle spacing can be easily observed by inspecting a section that is cut from a sample along the chain direction, providing direct support for the one-dimensional assembly scheme. In the direction perpendicular to the field, the large chainchain separation (>1 μm) makes it unlikely to diffract visible light. The necklace-like chain structure was further verified by successfully wrapping the photonic chains in a silica layer directly in solution using a solgel process (Figure 3c).13 More importantly, we have been able to observe visible diffraction from each individual nanochain assembly using dark-field optical microscopy, which further confirms that these 1D chains can act individually as the smallest 1D photonic crystal unit to diffract visible light.
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FIGURE 4. (a) Schematic illustration and (b) digital photo showing the assembly of Fe3O4@SiO2 particles in a patterned magnetic field with alternating field orientation. (c) Schematic illustration and (d) digital photo showing a suspension of CNCs displaying rainbow-like colors due to the variance in the interparticle spacing d and the orientation θ in different regions controlled by the magnetic field. FIGURE 3. (a) Dark-field optical image showing the 1D chain-like structures of CNCs assembled along a horizontal magnetic field. (b) SEM image of Fe3O4@SiO2 particle chains embedded in a PEGDA matrix showing the periodic arrangement of the particles inside each chain. (c) TEM image of nanochains of Fe3O4 CNCs fixed by wrapping in a layer of SiO2 through a solgel process.
Since the tunable structural color comes from the collective diffraction of individual particle chains, it is possible to modulate the diffraction color by controlling the assembly locally, thus providing great opportunities for applications such as high-resolution field-responsive color display. As can be seen from the Bragg equation, the diffraction color of the dynamic photonic chains can be tuned through controlling the interparticle spacing d, their orientation θ, or both. While d can be easily varied by adjusting the magnetic field strength, the orientation of the photonic chains follows the direction of the magnetic field. As an example, we have recently studied the assembly behavior of silica-modified superparamagnetic CNCs (Fe3O4@SiO2) in response to a complex magnetic field produced by a nonideal linear Halbach array and found that a horizontal magnetic field sandwiched between two vertical fields would allow one to change the orientation of the particle chains, producing high contrast color patterns (Figure 4a,b).28 When subjected to a spatial magnetic field with large variance in field strength and direction, both the interparticle spacing and orientation of the photonic chains can be modulated to display multiple colors from different areas in a single sample. A rainbow-like
color effect can be successfully created in the dispersion of magnetic particles near the edge of a cubic magnet (Figure 4c,d). The blue shift of diffraction color from left to right is due to both compression of the chains and the tilting of the chaining away from the viewing angle. An important feature of the 1D tunable system is that one can achieve considerably strong diffraction intensity at a significantly low particle concentration (∼0.1% volume fraction vs ∼74% for close-packed colloidal crystals). The strong diffraction color is believed to benefit from the large refractive index difference between magnetite (2.42) and the solvent, for example, water (1.33). The absorption of light with wavelengths outside the band gap by the magnetite also provides a dark background and therefore enhances the color contrast. The low particle density, as well as the dynamic chaining structure with only one translational order, allows instant and reversible switching between the highly ordered and completely disordered states. Our recent measurement suggested a switching rate of ∼30 Hz for the aqueous system.
4. Magnetic Assembly of Nonmagnetic Particles As discussed above, magnetic fields can allow instant assembly of photonic structures and wide, rapid and reversible tuning of the diffraction colors. While the choices for uniform superparamagnetic colloidal particles are very limited, the literature contains many well-developed processes for the Vol. XXX, No. XX
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FIGURE 5. (a) Magnetic field distribution around a nonmagnetic particle with a dipole moment in the opposite direction of the external magnetic field. (b) The interparticle dipoledipole force is repulsive or attractive depending on different particle configurations. The color bar on the right shows the strength of the magnetic field. (c) Time-dependent reflection spectra of the 1 mm thick film of mixed polystyrene beads and ferrofluid aqueous solution in response to a fixed magnetic field of 2530 G with gradient of 2500 G/cm.
synthesis of nonmagnetic colloidal particles with uniform sizes. It would be of great advantage to extend the fast magnetic assembly strategy to nonmagnetic building blocks. As nonmagnetic particles have a negligible response to normal external magnetic fields, the key in the magnetic assembly strategy is to establish an effective magnetic moment in nonmagnetic particles, which can be achieved by dispersing them in magnetized ferrofluid. The resulting magnetic moment of nonmagnetic particles, behaving as magnetic “holes”, can be approximately considered as equal to the total moment of the displaced ferrofluid but in the opposite direction of the magnetic field, m = VχeffH, where V is the volume of the particles, χeff is the effective volume susceptibility of the ferrofluid, and H is the local magnetic field strength (Figure 5a).29 The dipoledipole interaction between magnetic holes has the same directional nature as the real moments and likewise drives the chaining of the nonmagnetic particles (Figure 5b). However, since nonmagnetic particles have positive magnetostatic energy, the F ’ ACCOUNTS OF CHEMICAL RESEARCH
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gradient of the external magnetic field drives their movement toward regions with minimum magnetic field strengths. Previously, manipulation of magnetic holes with sizes below a micrometer has been difficult because they do not possess a magnetic moment strong enough to overcome Brownian motion during their assembly. Increasing the response of a magnetic hole requires the use of concentrated ferrofluid in strong external fields, which would be problematic in practical use due to the instability of the ferrofluid commonly used under these conditions. To address this issue, we extended the polyol synthesis method to produce highly surface-charged superparamagnetic nanocrystals to direct the assembly of nonmagnetic particles.30 Ferrofluids consisting of ∼12 nm Fe3O4 nanocrystals were found to be stable against aggregation at high concentrations (4% volume fraction) and in strong and large-gradient magnetic fields, allowing the efficient assembly of 185 nm nonmagnetic polymer beads into different photonic structures, from 1D periodic chains to 3D crystals, in response to the varying magnetic fields.31 The assembly process has been found to be driven by the interplay of the magnetic forces and electrostatic forces experienced by the nonmagnetic particles. In particular, in a large-gradient field, the strong packing force results in a substantial concentration gradient of nonmagnetic particles driven by the minimization of their magnetic potential energies. When the local concentration of nonmagnetic particles reaches a critical value, the interparticle electrostatic repulsive force becomes effective and drives the formation of 3D photonic crystals to minimize the electrostatic repulsive potentials. The assembly process of local enrichment of nonmagnetic particles into more ordered structures with shorter interparticle distance is clearly evidenced from a typical sample by the gradual blue shift of the diffraction maximum with enhanced diffraction intensity (Figure 5c). The formation of 3D photonic crystals with high reflectance within several minutes provides a promising method for the fast creation of large-area photonic crystals from nonmagnetic building blocks.
5. Applications of Magnetically Responsive Photonic Nanostructures The instant formation of ordered arrays with widely, rapidly, and reversibly tunable structural colors makes our system a new platform for chromatic applications, such as color display, security, camouflage, and information storage. Although direct utilization of the colloidal dispersions for
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FIGURE 6. Photographs showing a flexible PDMS film displaying structural color in (a, b) a uniform magnetic field and (c) a nonuniform magnetic field. (d) Digital photos of a PDMS film that can display color patterns upon the application of an external magnetic field. The diameter of the disk in panels ac is ∼47 mm, and the length of the bar in panel d is ∼25 mm.
structural-color-based display seems to be limited, encapsulation of the suspension of superparamagnetic particles in the form of microdroplets inside a flexible polymer film provides a convenient approach for easier manipulation, more reliable performance, and local color modulation.25 In addition, the instantaneous nature of the magnetic assembly makes this method suitable for rapid production of permanent structural colors by combining assembly with a rapid photopolymerization process to fix the assembled structure.27 By taking advantage of the swelling property of the polymer matrix in response to solvents, we also demonstrated the use of photonic-chain-embedded polymer films, which can be rapidly produced in large scale for applications such as rewritable photonic paper32 and humidity sensors.33 New types of photonic structures can be fabricated by fixing the periodic chains individually or inside polymer microspheres, which allows convenient tuning of the resulting photonic properties by magnetically controlling their orientations.12,13 In the following sections, we highlight the unique applications of the magnetically tunable photonic system in structural color printing and fieldresponsive display. 5.1. Magnetically Responsive Flexible Photonic Film. Encapsulating the dispersion of magnetic particles in a solid flexible film makes it convenient to integrate the magnetically responsive photonic structures into complex devices for practical applications.34 We have demonstrated this
concept by dispersing an ethylene glycol solution of Fe3O4@SiO2 particles in a liquid prepolymer of polydimethylsiloxane (PDMS) in the form of emulsion droplets with typical sizes of several micrometers, followed by solidification of the polymer matrix.25 Applying a uniform magnetic field to the film then induces chaining of the magnetic particles inside each droplet with the same diffraction wavelength so that the film displays a uniform color that can be recognized by the naked eye. The composite film maintains the excellent flexibility of the PDMS matrix and is still able to show a rapid response to external magnetic fields even if folded into various shapes (Figure 6). Color gradient effects can be easily created by controlling the assembly of each droplet in parallel. By incorporation of different types of CNC particle solutions that have similar background colors but show different visible responses to the magnetic field, for example, by choice of CNCs with different sizes, a flexible film with a patterned display is created. Without an external field, the film shows the native brown color of iron oxide with essentially no contrast. Upon the application of the magnetic field, however, a color pattern can be clearly observed due to the diffraction contrast resulting from different sizes of CNC particles. This type of graphic display may find use in applications such as anticounterfeiting devices or switchable signage where prestored information can be hidden unless activated by external stimuli. 5.2. Structural Color Printing. Thanks to the instantaneous nature of the magnetic creation and tuning of structural colors as well as photopolymerization, structural colors can be fixed locally in a polymer film with a high spatial resolution. By using a spatially modulated focused UV beam as the printing tool, and a mixture of CNC particles and UV curable resin as the ink, we have developed a fast and highresolution color printing technique for producing various multicolored patterns with a single ink by repeating the “magnetic tuning” and “maskless lithographical fixing” processes, which can be programmed to print different color regions sequentially and eventually yield a large complex multicolored pattern (Figure 7a,b).27 Since the ultimate resolution can reach ∼2 μm, it is easy to realize grayscale modulation by varying the density of primary color dots in a pixel that is smaller than the resolution of a human eye. Spatial color mixing has also been shown to be possible by including primary dots of different colors at various ratios. With the advantages including multicolor production from a single ink, no need to move the substrate or change any physical photomask during printing, and the intrinsic characteristics of structural colors such as iridescence and Vol. XXX, No. XX
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FIGURE 7. High-resolution patterning of multiple structural colors with a single magnetic ink. (a) Schematic illustration of multicolor patterning with a single ink by the sequential steps of “tuning and fixing”. (b) High-resolution multicolor patterns produced using magnetic ink. Reproduced with permission from ref 34. Copyright 2010 Royal Society of Chemistry.
FIGURE 8. (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 characters and the background, respectively. Photographs 1 and 2 (or 3 and 4) show the same sample as the incident light is projected along opposite directions. The scale bars in (c, d) are 1 cm. Adapted with permission from ref 35. Copyright 2011 American Chemical Society.
metallic appearance and high color durability without photobleaching, this technique represents a new platform for highresolution printing with immediate applications ranging from forgery protection to structurally colored graphic design. In addition to tuning the interparticle separation, the orientation of the photonic chains can be programmed in each “magnetic tuning and lithographical photopolymerization” cycle to print patterns with angular dependent color contrast, which shows switchable color distribution when the angle of incident light changes or the samples are tilted.35 Again, only a single magnetic ink is needed for printing a photonic crystal film with different crystal orientation distribution. As shown in Figure 8, upon incidence of light, different structural colors appear in areas with different chain orientations. When the incident angle is close to the longitudinal direction of the particle chains, the corresponding H ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 000–000
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areas diffract longer wavelength light (green, characters in case 1), while the other areas diffract shorter wavelength light (blue, background in case 1). On the contrary, as the incident light is projected close to the chain direction in the background area (case 2), the characters and background instantly switch their colors. With multiple masks, it is expected that more complicated and unique patterns can be fabricated, which may be particularly interesting for anticounterfeiting applications. 5.3. Magnetically Rotating Photonic Structures. Because structural colors strongly rely on the glancing angle, the diffraction of photonic units can be controlled by tuning their orientations. Directional magnetic forces allow easy orientational control of magnetically anisotropic structures to minimize the magnetostatic energy, as shown in Figure 9a. As discussed above, the photonic chains assembled from
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magnetic field parallel or perpendicular to the viewing direction. Separation of the assembly and tuning into two steps has several advantages including long-term stability of optical response, improved tolerance to environmental variances such as ionic strength and solvent hydrophobicity, and greater convenience for incorporation into many liquid or solid matrices without the need for complicated surface modification. These novel photonic units, including the silica-wrapped individual nanochains, may find use in broad applications such as high-resolution color display because their diffraction color can be switched individually or collectively as needed by the external magnetic field.
6. Conclusion and Outlook
FIGURE 9. (a) Plot of magnetic dipoledipole energy of a nanochain versus the orientation with respect to the external magnetic field. (b) “ON/OFF” switching of the color of a mixture of two types of microspheres, imaged by dark-field optical microscopy, from the native light brown of iron oxide to blue and green by tuning the magnetic field orientation from horizontal to vertical. Insets are SEM images of the microspheres in horizontal (off) and vertical (on) orientations. All scale bars are 20 μm.
superparamagnetic particles are magnetically anisotropic in nature. Due to the dipoledipole interaction between neighboring particles, the chains tend to align along the magnetic field to minimize their interaction energy so that the optical properties of nanochains can be controlled by magnetically tuning their orientations relative to the incident light. In addition to the above-mentioned silica wrapped photonic nanochains, we have also demonstrated the fabrication of magnetically responsive photonic units by fixing photonic chains in polymer microspheres through the instantaneous assembly of superparamagnetic particles inside emulsion droplets of UV curable resin followed by an immediate UV curing to fix the ordered structures. The photonic properties of the resulting microspheres can be conveniently tuned by magnetically controlling their orientations.12,36 Since the magnetic assembly and tuning process are separated into individual steps, color mixing can be easily realized by mixing microspheres with different diffraction colors. Figure 9b demonstrates the switching of the diffraction of these microspheres between “on” and “off” states by rotating the external
In this Account, we demonstrate the utilization of magnetic fields to organize colloidal particles rapidly and efficiently into dynamic photonic structures with widely tunable structural colors through controlling the periodicity, orientation, or both. The key to the successful assembly and color tuning of the photonic structures is the establishment of a dynamic balance between magnetic dipoledipole attractive interaction and comparably strong long-range electrostatic repulsive interaction. The rapid magnetic assembly strategy has been successfully extended to nonmagnetic particles utilizing the magnetic hole effect, which greatly broadens the choice of building blocks for photonic bandgap materials. The instantaneous assembly of colloidal particles into ordered arrays with widely, rapidly, and reversibly tunable structural colors, which can also be easily and quickly fixed, makes our magnetically responsive photonic system a new platform for chromatic applications. Future research will be focused on the optimization of this exciting system for specific applications, including but not limited to active color display, structural color printing, anticounterfeiting, security, camouflage, and information storage.
We are grateful for the financial support from the National Science Foundation (Grant DMR-0956081). Yin also thanks the Research Corporation for Science Advancement for the Cottrell Scholar Award, 3M for the Nontenured Faculty Grant, and DuPont for the Young Professor Grant.
BIOGRAPHICAL INFORMATION Le He received his B.S. in Chemistry from Nanjing University in 2008. He is now a Ph.D. candidate under the supervision of Prof. Yadong Yin at the University of California, Riverside. His research interests include the synthesis and self-assembly of nanostructured materials. Vol. XXX, No. XX
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Mingsheng Wang received his B.S. and M.S. in Materials Science from the University of Science and Technology of China in 2007 and 2010. He is now a Ph.D. candidate under the supervision of Prof. Yadong Yin at the University of California, Riverside. His research interests include the fabrication and application of photonic nanostructures. Jianping Ge received his Ph.D. in Inorganic Chemistry from Tsinghua University in China in 2006, and then spent three years with Prof. Yadong Yin at the University of California, Riverside, to carry out his postdoctoral research. In 2009, he joined the Department of Chemistry at Tongji University as a professor. His research interests include the synthesis and functionalization of magnetic nanostructures. Yadong Yin received his Ph.D. in Materials Science and Engineering from the University of Washington in 2002. He then worked as a postdoctoral fellow at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory. In 2006, he joined the faculty at the Department of Chemistry at the University of California, Riverside. His research interests include the synthesis and application of nanostructured materials, self-assembly, surface functionalization, and colloidal chemistry. FOOTNOTES The authors declare no competing financial interest. REFERENCES 1 Vos, W. L.; Sprik, R.; van Blaaderen, A.; Imhof, A.; Lagendijk, A.; Wegdam, G. H. Strong Effects of Photonic Band Structures on the Diffraction of Colloidal Crystals. Phys. Rev. B 1996, 53, 16231–16235. 2 Asher, S. A.; Holtz, J.; Liu, L.; Wu, Z. J. Self-Assembly Motif for Creating Submicron Periodic Materials - Polymerized Crystalline Colloidal Arrays. J. Am. Chem. Soc. 1994, 116, 4997– 4998. 3 Aguirre, C. I.; Reguera, E.; Stein, A. Tunable Colors in Opals and Inverse Opal Photonic Crystals. Adv. Funct. Mater. 2010, 20, 2565–2578. 4 Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Monodispersed Colloidal Spheres: Old Materials with New Applications. Adv. Mater. 2000, 12, 693–713. 5 Arsenault, A. C.; Puzzo, D. P.; Manners, I.; Ozin, G. A. Photonic-Crystal Full-Colour Displays. Nat. Photonics 2007, 1, 468–472. 6 Holtz, J. H.; Asher, S. A. Polymerized Colloidal Crystal Hydrogel Films As Intelligent Chemical Sensing Materials. Nature 1997, 389, 829–832. 7 Ge, J.; Yin, Y. Responsive Photonic Crystals. Angew. Chem., Int. Ed. 2011, 50, 1492–1522. 8 Fudouzi, H.; Sawada, T. Photonic Rubber Sheets with Tunable Color by Elastic Deformation. Langmuir 2005, 22, 1365–1368. 9 Fudouzi, H.; Xia, Y. N. Colloidal Crystals with Tunable Colors and Their Use As Photonic Papers. Langmuir 2003, 19, 9653–9660. 10 Jeong, U.; Xia, Y. Photonic Crystals with Thermally Switchable Stop Bands Fabricated from Se@Ag2Se Spherical Colloids. Angew. Chem., Int. Ed. 2005, 44, 3099–3103. 11 Ge, J.; Hu, Y.; Yin, Y. Highly Tunable Superparamagnetic Colloidal Photonic Crystals. Angew. Chem., Int. Ed. 2007, 119, 7572–7575.
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