Nanorod Heterostructures: from Colloidal Solutions to Light Emitting/Harvesting Devices
The ability to efficiently separate, recombine, and direct charge carriers is central to a wide range of applications, including electronics, photovoltaics, displays and solid-state lighting. Engineering band structure and heterointerfaces with atomic precision is an obvious route to achieving such capabilities. To do so through widely-accessible and cost-effective means is not. But such a means would allow rapid advances in these critical application areas. The evolution of colloidal semiconductor nanocrystals from single-composition, “spherical” particles to complex heterostructures of diverse shapes provides many opportunities for precision band structure engineering through scalable solution synthesis. With anisotropic shapes that can be exploited for assembly, charge carrier manipulation and optical anisotropy, incorporating heterojunctions and other functional interfaces into colloidal nanorod heterostructures represents an especially promising direction. In this talk, general challenges to the synthesis of complex-yet-well-defined colloidal nanorod heterostructures will first be discussed. Approaches such as spatially selective solution epitaxy, catalytic growth, cation exchange and combinations thereof can be exploited to achieve unique heterostructures with useful properties. A specific example of double-heterojunction nanorods (DHNRs) will be highlighted. Their engineered band structure with shape anisotropy improves charge injection, enhances light outcoupling and increases device lifetime of their light-emitting diodes (LEDs). At the same time, these features of DHNRs facilitate photo-induced charge separation, leading to useful photovoltaic response in high-performance, solution-processed LEDs. Studies that probe the underlying device operation mechanisms for improving performance will also be discussed. Emerging anisotropic colloidal heterostructures such as DHNRs can not only radically improve existing function but also impart new capabilities that could open up new directions for future generations of “self-powering” devices without adding volume/weight and complexity in manufacturing.