Combination of Photoinduced Alignment and Self-Assembly to Realize Polarized Emission from Ordered Semiconductor Nanorods.

One-dimensional semiconductor nanorods are a very promising class of materials for applications in modern optoelectronic devices, such as light-emitting diodes, solar cells, displays, and lasers. Their ability to emit linearly polarized light is considered to simplify device structures and improve the overall efficiencies. To ensure macroscopic polarization in such devices, the biggest challenge is the long-range alignment of nanorods by controllable means. We propose a technique that combines photoinduced alignment with nanorod's self-assembly. With this approach, we are able to actively control the alignment directions of highly emissive semiconductor nanorods in both microscopic and macroscopic scale with the order parameter as high as 0.87. As a result, polarized emission has been achieved with the degree of polarization of 0.62. Furthermore, patterned alignment of nanorods with spatially varying local orientations has been realized to demonstrate the great flexibility of this approach. Besides opportunities for applications, our method of alignment offers insights into host-guest interactions governing self-assembly of colloidal nanocrystals within the host molecular matrix.

Optical measurement of azimuthal anchoring strength in nematic liquid crystals.

We have observed the azimuthal switching at the interface of nematic liquid crystals (LCs) and the aligning substrate, induced by the planar electric field of a fine comb electrode. Optical transmittance as a function of applied voltage was modeled by both the elastic theory with rigid anchoring and the Landau-de Gennes theory with the interfacial energy expression 1/2Wtr[(Q two down arrows -Q two down arrows (0))(2)], where Q two down arrows is the liquid crystal order parameter and Q two down arrows (0) is the surface order parameter induced by the aligning substrate. Optical data on the in-plane switching LC cells were found to differ qualitatively with the predictions of the rigid anchoring model but to agree well with those of the Landau-de Gennes theory. We obtain not only the strength W of the azimuthal anchoring, but also find the surface order parameter S(surface) to be 20-30 % less than that of the bulk. The optically measured azimuthal anchoring strength is in good agreement with the literature values determined through other means.