Extracting Electrons out of Colloidal Quantum Dots: Challenges and New Perspectives

December 4, 2015

Rumi Beaulac

Colloidal semiconductor nanocrystals, also known as quantum dots, are fascinating building blocks for a variety of applications that rely, one way or another, on the ability to manipulate and control the generation, the separation, and the transport of electrical charges - applications such as solid-state lighting, solar energy conversion, photoredox chemistry, to name but a few. Quantum dots are intrinsically at the intersection between large molecular species and solid-state materials, and as such exhibit hybrid properties from both limiting forms. Like molecules, quantum dots can be chemically modified by changing their chemical composition (i.e. size, surface ligand functionalization…) and, to a large extent, can be fabricated and processed using traditional chemical synthesis approaches; at the same time, the behavior of quantum dots is most easily described using the same band-structure formalism developed by solid-state physics to describe bulk semiconductors. Whereas the solid-state formalism is very well suited to describe the photophysics of quantum dots idealized as perfect particle-in-a-spherical-box models, there are challenges associated with understanding the behavior of quantum dots as electron donors and/or acceptors, a regime quite clearly outside the realm of the idealized infinite potential barrier at the surface of the quantum dot. Our research program aims at characterizing this behavior in detail by designing molecular electron-acceptors that are tuned for specific single-electron-transfer regimes. In a first time, we explored the interaction between a variety of stable nitroxide free radicals, including TEMPO and nitronyl nitroxides, and II-VI chalcogenide quantum dots. These free radicals are shown to act as efficient quenchers of the intrinsic photoluminescence of colloidal quantum dots, an effect directly associated to efficient electron transfers from the photo excited quantum dots to the free radicals. By changing the structure and functionalization of these radicals, different aspects of electronic charge transfer can be monitored, including the first indirect observation of photo-induced proton-assisted electron transfer involving colloidal quantum dots. We will also discuss the role that inhomogeneous surface disorder in colloidal quantum dots plays in interfacial electron transfer.