Electrons move freely along one-dimensional nanostructures such as nanorods, quantum wires or nanotubes, but their motion in the other two spatial directions is governed by quantum effects. One important implication of this behavior is that the optical response of these structures is strongly dependent on their orientation. A device that takes advantage of this phenomenon by using liquid crystals to rotate cadmium selenide (CdSe) nanorods has now been constructed by Yang-Fang Chen and co-workers at the National Taiwan University1.

Fig. 1: A color-tunable light-emitting device. The active region is a mixture of liquid crystals, CdSe nanorods and quantum dots (QDs) between indium tin oxide (ITO) electrodes. An applied electric field (E) rotates the liquid-crystal molecules and attached nanorods to change the color of emission.© 2010 Y.-F. Chen

Chen and his co-workers mixed CdSe nanorods, just 25 nm long and 7 nm in diameter, with a liquid crystal known as E7 and CdSe quantum dots (QDs) — 5 nm-wide zero-dimensional structures in which electron motion is quantized in all three directions. To apply an electric field across the mixture while allowing any emission to be observed, the team injected the mixture between two glass slides coated with the transparent conductor indium tin oxide (ITO). The nanorods cling to the liquid-crystal molecules because the large surface areas involved generate a strong anchoring force. The liquid crystals — and therefore nanorods — can then be aligned in any direction by applying an electric field (Fig. 1). “We discovered that the physical properties of nanomaterials could be manipulated by an external bias due to the coupling between nanorods and liquid crystal molecules,” explains Chen.

The nanorods and QDs emit light when excited by a laser beam. The peak emission from the rods, which is yellow with a wavelength of 580 nm, is highly polarized because of the rod’s one-dimensional structure. The 650 nm red light from the dots, on the other hand, remains randomly polarized. This means that the contribution of the two light-emitting components — and thus the color of the emission — is dependent on which polarization, relative to the orientation of the nanorods, is measured.

Chen and his colleagues measured the optical output over a range of polarization angles and demonstrated that this approach enabled them to tune the color of light from the device. Chen hopes that further refinement of this approach, such as introducing photonic-crystal structures, will see an increase in device efficiency.