Highly Tunable Superparamagnetic Colloidal Photonic Crystals

Abstract

Colloidal crystals, periodic structures typically self-assembled from monodisperse colloidal building blocks, are one class of photonic band gap materials that can be fabricated at low cost and on a large scale. [1][2][3] They have attracted much attention because of their promise in optoelectronic applications that require the manipulation of photons, for example, as photonic components in telecommunication devices, lasers, and sensors. [4][5][6][7][8][9][10][11][12][13] It is highly desirable for the envisioned applications that a photonic crystal possesses a tunable stop band that can be conveniently controlled by external stimuli. Considerable effort has been devoted to achieving this goal by changing the refractive indices of the materials and the lattice constants or spatial symmetry of the crystals through the application of chemical stimuli, mechanical forces, electrical fields, or light. [14][15][16][17][18][19][20][21][22][23] However, the wide use of these systems as optoelectronic devices is hampered by the limited tunability of the stop band (changes in peak position are typically in the range of tens of nanometers), the slow response to the external stimuli, and the difficulty of integration into existing photonic systems.Adding magnetic components to the colloidal building blocks provides an opportunity for convenient and precise control of the properties of photonic crystals through an external magnetic field. Asher and co-workers explored this approach by fabricating colloidal photonic crystals using highly charged polystyrene microspheres containing superparamagnetic nanoparticles. [24,25] In this case, the magnetic forces exerted on the colloids are weak relative to interparticle electrostatic forces because of the low loading of the magnetic materials, leading to a limited tuning range, a long response time, and consequently limited practical applications. Preparation of polymer microspheres with increased loading of magnetic materials is practically difficult within the typically used emulsion polymerization schemes. In principle, colloidal particles of pure magnetic materials such as iron oxide can be directly used as the building blocks for constructing colloidal photonic crystals. Such effort, however, is limited by the superparamagnetic-to-ferromagnetic transition that occurs as particles are grown into larger domains. [26] Recently, we developed a method for the preparation of polyacrylate-capped superparamagnetic magnetite (Fe 3 O 4 ) colloidal nanocrystal clusters (CNCs) with tunable sizes from 30 to 180 nm by a high-temperature hydrolysis process. [27] Each cluster is composed of many magnetite crystallites of approximately 10 nm, thus retaining the superparamagnetic properties at room temperature. Herein, we report that such superparamagnetic clusters can be directly employed for constructing colloidal photonic crystals with highly tunable stop bands that can be moved across the entire visible spectral region owing to the highly charged polyacrylate-capped surfaces and the strong int...

Tunable Colors in Opals and Inverse Opal Photonic Crystals

“…[57][58][59] These researchers investigated variations of superparamagnetic composites of the type Fe 3 O 4 @SiO 2 , in which a balance between electrostatic repulsive forces and magnetic attractive forces produced ordered arrays of core/shell structures. Structural colors could be tuned by changing the strength of the magnetic fi eld or the thickness of silica shells with the same core size.…”

Colloidal Assembly: The Road from Particles to Colloidal Molecules and Crystals

“…[197] Opaline structures composed of spherical colloids containing a superparamagnetic component exhibit colors that are highly responsive to magnetic fields. [198] These materials have been considered for use in reusable photonic paper whose color changes locally where the paper is infiltrated with a salt-based inking solution. [199] Optical spectra of periodic sphere arrays are most responsive to changes in lattice spacing of the arrays.…”

Self-assembled colloidal structures for photonics

“…In addition, all visible colors from red to violet can be prepared by increasing the loading of magnetic materials in the colloids. Th e switching of color in such systems is very rapid, requiring only 200 ms [55]. In an alternative approach, dynamic tuning of the structural color of superparamagnetic particles can be achieved in nonaqueous solvents, which has several advantages, including nonvolatility, for applications in display devices [56].…”