Optical materials
August 17, 2007
Colloidal nanocrystal clusters (CNCs) of iron oxide that change color in response to a magnetic field could lead to improved technology for making flat panel displays.
Researchers from the University of California, Riverside (UCR) claim theirs is one of the first reports of a photonic crystal that is fully tunable in the visible range of the electromagnetic spectrum, from violet to red light [Ge et al., Angew. Chem. Int. Ed. (2007), doi: 10.1002/anie.200701992].
“The key is to design the structure of the nanoparticles through chemical synthesis so they self-assemble in a magnetic field,” explains lead researcher Yadong Yin. The nanoparticles need to be superparamagnetic and have high surface charge, high magnetic moment, and optimal size.
Yin's team synthesized CNCs by hydrolyzing FeCl3 with NaOH at 220°C in a solution containing polyacrylic acid as a surfactant in diethylene glycol. This produces clusters 30–180 nm in size that contain nanocrystals 10 nm in diameter. Each nanocrystal supports a single magnetic domain. But the domain is so small that when the magnetic field is removed, the domains become thermally randomized across the sample, leaving no net magnetic moment – a property called superparamagnetism. Single nanocrystals of the same size as the cluster would become permanently magnetized and clump together.
Self-assembly into an ordered structure – a photonic crystal that strongly reflects certain wavelengths of light – occurs thanks to a trade off between electrostatic repulsion of the highly charged polyacrylate nanoparticle coating and magnetic attraction of the magnetite (Fe3O4) nanocrystals in the core. Altering the magnetic field changes the spacing between particles, so a different wavelength of light is reflected.
In similar work, Sanford Asher's group at the University of Pittsburgh has explored the properties of magnetically tunable colloidal crystals assembled from polystyrene beads that are doped with superparamagnetic iron oxide particles. “Their particles had a much smaller magnetic moment because of the low loading of magnetic material,” says Yin. “In our case the magnetic interaction is much stronger”.
Future applications could be in chemical and biological sensing, integrated optical switches, and products such as large area or flexible color displays. Since the color of the photonic crystals is based on reflection, displays based on this technology should be brighter, especially in direct sunlight where conventional flat-panel displays, which are back-lit, tend to perform poorly. “What should make the technology commercially attractive is that iron oxide is cheap, nontoxic, and available in plenty,” Yin notes.
Pauline Rigby