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Magnetically responsive photonic nanostructures

Self-assembled nanostructures offer rapid, reversible tuning over the entire visible spectrum using external magnetic fields.
5 May 2011, SPIE Newsroom. DOI: 10.1117/2.1201104.003672

Responsive photonic crystals (RPCs) are appealing because their diffraction wavelengths and color emission can be controlled by external stimuli—for example, temperature or pressure—rather than modifying the material itself. This feature is well suited to applications such as color displays, bio- and chemical sensors, security devices, printing and labeling systems, and military camouflage. However, challenges—such as narrow tunability of the band gap, slow response to external stimuli, irreversibility, and difficult integration into existing photonic systems—affect the performance of some RPCs. To address these concerns, we recently reported that highly charged, uniform superparamagnetic (SPM) colloids can self-assemble in ordered arrays. Our colloidal assemblies are reversibly color-tunable across the entire visible spectrum, under the influence of external magnetic fields.1,2 Here, we provide an overview of our recent advances in magnetically responsive photonic nanostructures.

Figure 1. (A) Schematic showing the assembly of negatively charged magnetite colloidal nanocrystal clusters (CNCs) in water in response to an external magnetic field, where repulsive electrostatic forces balance magnetic attraction. (B) Scanning electron micrograph of CNC chains embedded in a polymer matrix. Photograph (C) and reflection spectra (D) of colloidal crystals assembled in water in response to an external magnetic field (decreasing in strength from left to right). H: Magnetic field. R: Reflectance. λ: Wavelength of light.

We began by developing a robust procedure for preparing magnetite (Fe3O4) colloidal nanocrystal clusters (CNCs) with uniform size (∼30–180nm).3 Our procedure relies on control of the sodium hydroxide concentration, which initiates primary nanocrystal nucleation from an aqueous iron salt solution. The particles then aggregate uniformly into larger, secondary structures that retain their SPM property at room temperature, but show higher saturation of magnetization. We coated our magnetite CNCs with polyacrylate, which rendered the particle surfaces highly charged. The ensuing repulsive electrostatic force balanced the magnetically induced attractive force,4,5 which promoted self-assembly of the SPM particles into linear chains with periodic interparticle spacing: see Figure 1(A).1,2 We confirmed the periodicity by solidifying the structures in a polymer matrix and then imaging with scanning electron microscopy: see Figure 1(B).6 Bragg diffraction (i.e., the process that produces color in crystalline materials) occurs when the periodicity of the particles matches the wavelength of incident light. We can change the periodicity of the assemblies—and the subsequent color—by manipulating the strength of the external magnetic field. We also sought to manipulate these particles in nonpolar solvents to generalize the approach for a variety of chemical environments. However, nonpolar solvents cannot accommodate charge separation and thus impede self-assembly of charged particles. Accordingly, we coated the Fe3O4 particles with a silica layer onto which we grafted a monolayer of alkoxysilanes using a silylation reaction to render them soluble in nonpolar phases.7 We found that charge control agents re-established long-range electrostatic repulsion in the solution, and permitted particle organization.8

We identified a number of unique features of our RPCs that may address the challenges encountered in conventional systems, which use hydrodynamic flow, electrophoretic deposition, electrostatic interactions, or capillary, gravitational, and centrifugal forces to assemble colloidal particles into ordered crystals. First, self-assembly can be initiated and completed almost instantly by applying external magnetic fields. Second, the diffraction wavelength can be tuned by controlling the strength of the magnetic field: see Figure 1(C) and (D). We have shown that colors across the entire visible spectrum can be obtained. Third, the optical response to changes in the external magnetic field is instantaneous and fully reversible. Fourth, assembly does not require stringent conditions, such as the special setups, devices, controlled humidity, or gaseous environments required in many conventional assembly processes. Also, the high refractive index of magnetite (∼2.42) creates high contrast with the surrounding medium. Thus, even at low particle concentrations of several milligrams per milliliter, we can obtain high diffraction intensities. Next, the instant assembly and tuning enable large-scale production, provided an appropriately sized, uniform magnetic field is used. Finally, it is possible to produce multicolored patterns using magnetic fields of defined size to locally induce particle assembly.

With these characteristics in mind, many color-related applications are possible, such as color displays. We constructed a solid film with magnetically responsive colors by encapsulating our SPM particles in the form of microdroplets.9 The SPM particles assembled into ordered structures in the presence of a magnetic field, where each droplet behaved as a small photonic crystal capable of diffracting light. The magnetic alignment allowed light diffraction by the droplets at the same wavelength—displaying uniform color—perceivable by the human eye.

Since our magnetic RPCs allow instant color creation, they are also useful for direct color printing. For example, we formulated a magnetic ‘ink’ by dispersing magnetite CNCs in the liquid precursor of a UV-curable resin.10 We tuned the periodicity by varying the strength of the external magnetic field and produced multiple colors from a single ink. Once we obtained the desired color, we applied UV radiation to rapidly polymerize the resin. This not only fixed the position of SPM particles but also allowed retention of the diffraction color. This magnetic assembly followed by UV immobilization can be accomplished within seconds, which is a significant advantage over traditional methods for generating structural colors. Additionally, this printing method can be applied to uneven surfaces. We also showed high resolution patterning of multiple structural colors using a spatially modulated focused UV beam as the ‘printing’ tool, with a mixture of SPM particles and UV-curable resin as the ink.11 We micropatterned different structural colors and geometries by repeating the tuning and fixing process.

In summary, the unique features of magnetic RPCs, such as rapid switching, full reversibility, and wide tunability, lend themselves to a range of different color display systems. Potential applications include dynamic displays suitable for antifraud devices, rewritable photonic paper, and full-color, high-resolution printing systems. Our future developments will extend to areas such as bio- and chemical sensors, color-switchable paints, and video display devices.

Yadong Yin
Department of Chemistry
University of California, Riverside
Riverside, CA 

Yadong Yin received his PhD 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 started his independent career at Riverside.

1. J. Ge, Y. Hu, Y. Yin, Highly tunable superparamagnetic colloidal photonic crystals, Angew. Chem. Int'l Ed. 46, pp. 7428-7431, 2007. doi:10.1002/anie.200701992
2. J. Ge, Y. Hu, T. Zhang, T. Huynh, Y. Yin, Self-assembly and field-responsive optical diffractions of superparamagnetic colloids, Langmuir 24, pp. 3671-3680, 2008. doi:10.1021/la7039493
3. J. Ge, Y. Hu, M. Biasini, W. P. Beyermann, Y. Yin, Superparamagnetic magnetite colloidal nanocrystal clusters, Angew. Chem. Int'l Ed. 46, pp. 4342-4345, 2007. doi:10.1002/anie.200700197
4. R. W. Chabay, B. A. Sherwood, Electric and Magnetic Interactions, Wiley, 1995.
5. Y. Kraftmakher, Magnetic field of a dipole and the dipole-dipole interaction, Eur. J. Phys. 28, pp. 409, 2007. doi:10.1088/0143-0807/28/3/003
6. J. Ge, H. Lee, L. He, J. Kim, Z. Lu, H. Kim, J. Goebl, S. Kwon, Y. Yin, Magnetochromatic microspheres: rotating photonic crystals, J. Am. Chem. Soc. 131, pp. 15687-15694, 2009. doi:10.1021/ja903626h
7. N. García, E. Benito, J. Guzmán, P. Tiemblo, Use of p-toluenesulfonic acid for the controlled grafting of alkoxysilanes onto silanol containing surfaces: preparation of tunable hydrophilic, hydrophobic, and super-hydrophobic silica, J. Am. Chem. Soc. 129, pp. 5052-5069, 2007. doi:10.1021/ja067987a
8. J. Ge, L. He, J. Goebl, Y. Yin, Assembly of magnetically tunable photonic crystals in nonpolar solvents, J. Am. Chem. Soc. 131, pp. 3484-3486, 2009. doi:10.1021/ja809772v
9. J. Ge, Y. Yin, Magnetically tunable colloidal photonic structures in alkanol solutions, Adv. Mater. 20, pp. 3485-3491, 2008. doi:10.1002/adma.200800657
10. J. Ge, J. Goebl, L. He, Z. Lu, Y. Yin, Rewritable photonic paper with hygroscopic salt solution as ink, Adv. Mater. 21, pp. 4259-4264, 2009. doi:10.1002/adma.200901562
11. H. Kim, J. Ge, J. Kim, S. Choi, H. Lee, H. Lee, W. Park, Y. Yin, S. Kwon, Structural colour printing using a magnetically tunable and lithographically fixable photonic crystal, Nat. Photon. 3, pp. 534-540, 2009. doi:10.1038/nphoton.2009.141