Program Highlights

Frustration is a feeling known to anyone who has had to choose one course of action from a range of imperfect options.  Interestingly, similar situations arise in nature, and scientific ideas about frustration have been explored to understand materials as varied as water, ceramics, magnets and superconductors.  Geometric frustration in condensed matter arises when the geometry of the crystal lattice prevents minimization of local interaction energies. This multiplicity of imperfect choices leads to frustrated media with bizarre properties such as many ‘lowest energy’ states wherein small perturbations cause giant property fluctuations, and entropy at zero temperature that grows with sample size. Since the experimental scenario emulates classic models of spin frustration, the research builds a novel bridge between two very different fields of materials science: soft matter and frustrated magnetism.

Graduate student Wade Luhman has demonstrated a route to enhance the short exciton diffusion length (LD) of organic semiconductors by combining fluorescent and phosphorescent materials into a single electron donating thin film. Here, a near-doubling of the diffusion length is realized in a fluorescent material upon addition of the phosphorescent sensitizer. This work highlights a novel approach to improve the absorption efficiency of organic solar cells, and provides fundamental insight into exciton migration in these materials.

Using  small-angle neutron scattering, we investigated the mechanism of molecular exchange in spherical micelles formed by diblock copolymers in an organic solvent. Not only does temperature affect the micelle exchange rate in the expected manner, but increasing the core block length by 60% produced a remarkable 10,000-fold decrease in the exchange rate. This work provides fundamental understanding of block copolymer micelle dynamics, and is important for a host of potential applications of these hybrid macromolecules, including drug delivery.

Research funded in part by the MIT MRSEC has led to a discovery of one-way photonic behavior. A team made up of MIT physicists Zheng Wang, research scientist in MIT's Research Laboratory of Electronics; recent MIT PhD recipient Yidong Chong; Professor John Joannopoulos; and Professor Marin Soljacic have developed and experimentally tested photonic crystals that restrict light to travel in only one direction without back-scattering, even in the presence of large disorders.