Colloidal Quantum Dots
Materials known as semiconductor nanocrystals (or colloidal quantum dots) allow one to understand the effect of size on the behavior of semiconductors. While such nanoscale particles can now be obtained by various methods, we utilize simple liquid-phase chemical syntheses. The resulting materials can be used to investigate many size-dependent phenomena, including how optical properties change with nanocrystal size. For example, the colors of light that are either absorbed or emitted by nanocrystals can be tuned. When light is absorbed by the particle, an electron-hole pair (or exciton) is created. Subsequent recombination of this pair can lead to fluorescence. With decreasing nanocrystal size, the energy of these optical transitions increases significantly. This arises because the energy of the exciton increases as it is “squeezed” into a small volume. This phenomenon, known as the quantum size effect, can occur in nanocrystals when the photogenerated electron and hole are confined by the particle boundary. Since the discovery of this effect, researchers have been trying to exploit the optical tunability of nanocrystals in various applications, including lasers, light-emitting diodes, bio-imaging, and solar cells. Thus, nanocrystals have been studied for both fundamental and technological reasons.
Within OMEL, we have a variety of projects related to these materials, including continuous efforts to develop new types of nanocrystals (of different materials, shapes, etc.) and to understand their properties. We also have a large project to synthesize and characterize nanometer-scale semiconductor crystallites (or nanocrystals) in which impurities (or dopants) have been intentionally incorporated. The critical role that dopants play in conventional semiconductor devices (e.g., transistors) provides a strong motivation to study doped semiconductor nanocrystals. Doping can also address key problems in applications of nanocrystal from solar cells to bio-imaging. In particular, it can lead to films of electrically conducting nanocrystals. More generally, doping should provide further control over the properties of these materials. Project objectives include: (i) the development of methods to synthesize doped nanocrystals, (ii) the characterization of these materials to understand both the chemistry and physics of nanocrystal doping, and (iii) the investigation of devices based on electronically doped nanocrystals.
For a review, see: "Doped Nanocrystals”, D. J. Norris, Al. L. Efros, and S. C. Erwin, Science 319, 1776 (2008).