Solution-processed quantum dots (QDs) are finding applications in a wide-range of technologies from displays and lighting to photovoltaics and photodetectors. Advances in real-world technologies have been enabled by an increasing ability to fine-tune opto-electronic properties with strategies including quantum confinement effects, advanced heterostructuring (band-structure engineering at the nanoscale), and chemical manipulation of interfaces and surfaces. Taken together, these strategies have yielded numerous breakthroughs and insights into key fundamental excited-state processes in semiconductor nanocrystals. In our lab, we have focused on developing heterostructures that lead to suppression of non-radiative processes, including blinking, photobleaching, and Auger recombination.[1-8] Despite realizing novel properties that support a wide range of applications,[9-13] we cannot claim to have found the perfect QD. Even the most robust nanocrystal will fail in its most fundamental of property – the ability to emit light – in the face of specific stressors, such as high photon flux, temperature, and exposure to atmospheric oxygen/water.

Previously, we developed a “single-QD stress test” that was used to evaluate degradation-photophysics in two types of ultrastable, “giant” core/thick-shell QDs (gQDs).[14] Here, I will describe the latest in our efforts to elucidate QD failure mechanisms, with the aim to pinpoint structural and chemical features leading to the “killing” of a QD.[15] Specifically, we developed a method based on solid-state spectroscopy to obtain kinetic and thermodynamic parameters of photo-thermal degradation in single QDs, systematically varying ambient temperature and photon-pump fluence. We described the resulting degradation in emission with a modified form of the Arrhenius equation and showed that this reaction proceeds via pseudo zero-order reaction kinetics by a surface-assisted process with an activation energy of 60 kJ/mol. We note that the rate of degradation is ~12 orders of magnitude slower than the rate of excitonic processes, indicating that the reaction rate is not determined by ultrafast electron/hole trapping. We further determined that full-power LED-like excitation can add 90-140 K of heat to the nanocrystals due to high excitation rates and associated nonradiative relaxation. The specific reactions that are responsible for photo-oxidative degradation in gQDs are as yet unknown. However, at least one of the primary degradation reactions is now known to be a zero-order process with a reaction activation energy that is independent of photon flux or wavelength. In the search for new robust light-emitting nanocrystals, the new analysis method will enable direct comparisons between differently engineered nanomaterials or different organic/inorganic surface treatments.