10.1117/2.1201104.003672
Magnetically responsive
photonic nanostructures
Yadong Yin
Self-assembled nanostructures offer rapid, reversible tuning over the
entire visible spectrum using external magnetic fields.
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.
We began by developing a robust procedure for preparing
magnetite (Fe3 O4 ) 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
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. : Wavelength of light.
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 Fe3 O4 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
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10.1117/2.1201104.003672 Page 2/2
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, colorswitchable paints, and video display devices.
Author Information
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.
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c 2011 SPIE