3:36–4:36 pm
3:45 PM - GCIS W301/W303 929 E. 57th Street
Colloidal semiconductor nanocrystals have seen a remarkable transition from a scientific invention to an optical material with technological relevance. In particular for display, lighting or projection applications, a key process is the conversion of short wavelength pump light into longer wavelength light, either through spontaneous or stimulated emission. This talk focuses on two examples, where recent breakthroughs in our understanding of the chemistry and physics of nanocrystals expand this application potential. A first example involves the use of InP-based quantum dots (QDs) for making monochromatic LEDs through on-chip color conversion. In this approach, the color-convertor is put directly on top of a blue LED, and should convert all primary light in a secondary color. The results shown highlight the feasibility of this approach, even at elevated power levels, provided that the QDs have a photoluminescence quantum efficiency of close to 100%; a step that was recently achieved for the case of InP/ZnSe/ZnS. A second example involves a switch from quantum dots – characterized by strong confinement of electrons and holes – to bulk nanocrystals (BNCs), defined as nanocrystals with sizes larger than the exciton Bohr radius. Using the example of CdS BNCs, we show that equal or better optical gain metrics, such as the gain coefficient, the inverted state lifetime and the optical gain threshold, are obtained for such materials than for core/shell QDs. These results are related to pronounced band-gap renormalization experienced by CdS upon photo-excitation. We conclude by a critical comparison between QDs and BNCs for applications relying on optical downconversion.