Principles of quantum dot photophysics and applications to luminescent downshifting

Abstract

Colloidal quantum dots (QDs) exhibit remarkable size-tunable optical properties including broad absorbance and bright, narrow emission. This thesis presents efforts to utilize, manipulate, and expand these properties. Despite the wide range of applications that benefit from multiband spectral information, traditional p-n junction photodetectors suffer from a fundamentally limited range of sensitivity due to a trade-off between light absorption and charge extraction. In order to expand the utility of a photodetector, its region of sensitivity can be enhanced via spectrum shifting. Here the optical properties of QDs are utilized to shift high energy photons to lower energy photons that lie within the detector's sensitive region through a process called luminescent downshifting. Chapters 2-5 will explore the application of luminescent downshifting from the near-ultraviolet to the short-wavelength infrared, the far-ultraviolet to the visible, and the ultraviolet to the mid-wavelength infrared. Unique challenges are addressed for each wavelength region to achieve increases of 10-55% absolute in the external quantum efficiency. The technology has been translated from single-element detectors to imaging arrays which is demonstrated in various environments. In pursuit of a deeper understanding of the surface-related effects on the photophysical properties of infrared quantum dots, the feasibility of shell growth and the effect of stoichiometric modification of lead chalcogenide quantum dots are presented in Appendix A. The resulting impact on emission wavelength, quantum yield, photoluminescent lifetime, and carrier mobility are explored.