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Research reported in the Oct. 19 edition of Science offers a new way to make enhanced LED displays using “bottoms-up engineering”.
Collaborative work between researchers from the Chemistry Department of the University of Florida and the Cornell High Energy Synchrotron Source (CHESS) at Cornell University has resulted in novel way to make colloidal superparticles from oriented nanorods of semiconducting materials. The resultant superparticles exhibit enhanced light emission and polarization, features that are important for fabrication of LED televisions and computer screens. The nucleated superparticles can further be cast into macroscopic polarized films. Using currently available manufacturing techniques, these films promise to increase efficiency in polarized LED television and computer screen by as much as 50%.
The team, led by Charles Cao at the University of Florida, synthesized CdSe/CdS core/shell nanorods. Taking advantage of the strain-tuned enhancement of emission through a CdSe-CdS lattice mismatch interface, they assembled these rods into larger periodic colloidal structures, called as superparticles. The enhanced light emission and polarization are a consequence of the anisotropic ordering of the nanorods within the superparticles.
The team, including lead author Tie Wang and other members of the Cao group and CHESS scientist Zhongwu Wang, made use of a unique facility at the B1 endstation at CHESS to collect small angle x-ray scattering data from specimens inside tiny diamond-anvil cells, in combination with high resolution transmission electron microscopy, to analyze how nanorods with attached organic components could be formed into well-ordered structures (Figure 1). The nanorods first align within a layer as hexagonally ordered arrays. Then, these highly ordered nanorod arrays behave like a series of layered units and self-assemble into structures that exhibit lamellar long-range order as they grow into large superparticles (Figure 1). The elongated superparticles can then be readily aligned in a polymer matrix into macroscopic films.
Figure 1: Synthesis of superparticles from CdSe-CdS nanorods through (i) nanorod-micelle formation and (ii) superparticle formation. Numbered images show the details of the synthetic process: (1) CdSe-CdS nanorod functionalized with the organic molecules ODPA and octylamine; (2) nanorod micelle prepared using the detergent DTAB; (3) resultant double-domed cylinders and (4) irregular-multidomain particles. The particles range in size from 50 to several hundred nanometers.
This project demonstrates how scientists are learning to recognize and exploit anisotropic interactions between nanorods, which can be adjusted during the synthesis process, to create single-domain, needle-like particles. The authors conclude their report anticipating that their findings will be generalized, leading to new processes employing self-assembly to create nano-objects having other anisotropic shapes, perhaps even joining two or more types of objects to form well-defined mesoscopic and macroscopic architectures with greater and greater complexity.
Reference: Tie Wang, Jiaqi Zhuang, Jared Lynch, Ou Chen, Zhongliang Wang, Xirui Wang, Derek LaMontagne, Huimeng Wu, Zhongwu Wang, and Y. Charles Cao; "Self-assembled Colloidal Superparticles Form Nanorods", Science 338, 358-363 (2012)
Also see article by the Cornell Chronicle online here: http://www.news.cornell.edu/stories/Oct12/wangLED.html or (pdf)
Submitted by: Zhongwu Wang, CHESS, Cornell University
10/23/2012