The team of chemists from the University of California, Riverside, have developed magnetically responsive liquid crystals with optical properties that could lead to a range of display applications that benefit from the instantaneous and contactless nature of magnetic manipulation, such as posters, billboards, signage and writing tablets.
The research, which showed it was possible to combine magnetic alignment and lithography processes to create patterns of different polarizations in a thin composite film and have control over the transmittance of light in certain areas, could also find uses in anti-counterfeit technology, and for optical communication devices as optical modulators that control the amplitude, phase, polarization, propagation direction of light.
The study, which was reported in Nano Letters [Wang et al. Nano Lett. (2014) DOI: 10.1021/nl501302s], demonstrated how ferrimagnetic inorganic nanorods could be used to construct liquid crystals with optical properties that can be instantly and reversibly controlled simply by altering the direction of an external magnetic field. This approach overcame the usual problem of the utilization of a magnetic field for this purpose as being limited due to the low magnetic susceptibility of molecular species, as extremely strong magnetic fields are required to enable effective switching of the molecular order.
The nanorods “can effectively form liquid crystals and respond strongly to even very weak magnetic fields – even a fridge magnet can operate our liquid crystals.”Yadong Yin
The team used magnetic nanorods rather than the commercial non-magnetic rod-like molecules as optically they work in a similar way but have the advantage of responding rapidly to external magnetic fields. When a magnetic field is applied, the nanorods spontaneously rotate and realign themselves parallel to the field direction, thereby influencing the transmittance of polarized light. The materials involved are made of iron oxide and silica, which are also much cheaper and more environmentally friendly than the commercial organic molecules-based liquid crystals.
As study leader Yadong Yin said, the nanorods “can effectively form liquid crystals and respond strongly to even very weak magnetic fields – even a fridge magnet can operate our liquid crystals.” As the crystals can be operated remotely by an external magnetic field, electrodes are unnecessary, and the magnetic nanorods are much bigger than those used in commercial liquid crystals so their orientation can be manipulated by solidifying the dispersing matrix.
The team will now look to reduce the optical absorption of the nanorods, either through modification or by replacing them with other transparent magnetic nanorods. They also hope to explore how using the materials to optimize the technology to fit specific application needs.