Electrified Liquid Crystals Make Nanorods Stand Up

Michael J. Lander

Unlike spherical and otherwise-shaped nanostructures, semiconductor nanorods cause light to polarize along their axis. But without the ability to coordinate their orientation, the nanorods’ special capability cannot be fully utilized. Scientists at National Taiwan University in Taipei have discovered a way to manipulate rod orientation with an applied voltage, which could enable control of data flow through high-tech devices.

A device incorporating liquid crystals and nanorods responds to a perpendicular electric field (E). Before exposure, both types of structures lie flat against the device in a specific direction. After the voltage is turned on, the particles stand on end. Reprinted with permission of Nano Letters.

As Yang-Fang Chen and colleagues had determined before their study, however, an applied voltage alone will not move nanorods. Thus, they turned to nematic liquid crystals, which align themselves with electric fields and bind firmly, side by side, to cadmium sulfide nanorods. Because of this behavior, they expected crystals and rods to stand up in response to an electric field perpendicular to an alignment layer. They hypothesized that, after they switched off the field, the crystals — attracted to the layer — would cause the rods to lie flat.

To test the idea, the scientists constructed a cell from two indium tin oxide-coated glass plates, each coated with a rubbed polyimide layer. They deposited CdS nanorods onto each plate and injected liquid crystals between the layers.

To visualize nanorod behavior, they excited the rods at 374 nm with a PicoQuant laser directed through an Olympus microscope. They recorded changes in polarization of nanorod photoluminescence — indicative of the structures’ reorientation — with an analyzer as well as with a Horiba Jobin Yvon spectrometer and detector screened by a depolarizer.

Before applying an electric field, the researchers found that the ratio of polarization intensity parallel to the rubbed direction to that perpendicular to it was 5.0. This indicated that the nanorods, just like the liquid crystals, were oriented in the direction of the rubbed layer’s troughs.

Next, they exposed the system to a 1-kHz electric field oriented perpendicular to the plates’ surfaces. Upon activation of the field, the intensity ratio dropped dramatically to near unity because the rods then pointed straight at the analyzer, making its measurements directionally independent. Analysis of the data showed the overall degree of nanorod polarization to be 0.63, an order of magnitude greater than previously reported for similar samples.

To verify that the nanorods — not liquid crystals — caused the polarization change, the investigators replaced CdS nanorods with CdS nanostructures. In this setup, emission was not polarized. When they replaced the liquid crystals with water molecules in another experiment, polarization also disappeared, demonstrating the importance of the crystal-rod interaction.

The researchers’ data suggest that liquid crystals permit controlled orientation of nanorods — findings that could have numerous implications. Most obvious is the potential to use induced polarization changes to regulate image and information transmission. But nanorods also have thermal and mechanical characteristics that could make them useful in integrated photonic systems and other devices. Because liquid crystal technology is well-developed, applications may not be far off.

To make the device more practical for real-world applications, the scientists stress that an electrical pumping system would have to replace the current optical one. In future research, they want to use their concept to control the color of emission from a display or the surface plasmon resonance of metal wires.

Nano Letters, July 2007, pp. 1908-1913.