Materials Physics seminars

Aleksey Yekimov, Louis Brus and Moungi Bawendi trace the discovery and chemical development of quantum dots: semiconductor particles whose electronic and optical behaviour changes with their size. Yekimov examines confinement effects in small crystals embedded in glass, while Brus develops the study of semiconductor particles dispersed in solution. Bawendi explains the synthetic challenge of producing high-quality nanocrystals with controlled dimensions and how more reliable preparation made their properties experimentally and practically useful. The lectures bring together the physics of confinement, the chemistry of colloidal materials and the control of particle growth, showing why both discovery and synthesis were essential to the emergence of quantum-dot science.

Colloidal dispersions of rod-like particles are widely accepted as convenient model systems to study the phase behavior of liquid-crystal forming systems, commonly found in LCDs. This is due to the fact that colloidal rods exhibit analogous phase behavior to that of elongated molecules, while they can be directly observed by optical microscopy. Recently, there has been a surge of interest in the liquid crystalline behaviour of so-called bent-core, or banana-shaped, molecules. This is due to their ability to form exotic biaxial nematic phases such as the twist-bend and splay-bend nematic phase, which may be of particular interest inherent to their fast switching response in LCDs. Here, we develop model “banana-shaped” colloidal particles with tunable dimensions and curvature, whose structure and dynamics are accessible at the particle level. By heating initially straight rods made of SU-8 photoresist, we induce a controllable shape deformation that causes the rods to buckle into banana-shaped particles. We elucidate the phase behavior of differently curved colloidal bananas using confocal microscopy. Although highly curved bananas only form isotropic phases, less curved bananas exhibit very rich phase behavior, including biaxial nematic phases, polar and antipolar smectic-like phases, and even the long-predicted, elusive splay-bend nematic phase.