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. 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. The interaction between a variety of stable nitroxide free radicals, including TEMPO and nitronyl nitroxides, and II-VI chalcogenide quantum dots has been characterized in detail. These free radicals are shown to act as efficient quenchers of the intrinsic photoluminescence of colloidal quantum dots, an effect directly associated to efficient hole transfer from the photoexcited 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 a wide tunability of the interfacial charge-transfer rate constants, from sub-picosecond to tens of nanoseconds. We show that the most efficient hole transfer processes occur in the nanosecond timescale, with very minimal driving forces for interfacial charge separation. We also discuss the role that inhomogeneously-distributed surface traps play in the interfacial photophysics of colloidal quantum dots; specifically, we show that trapped state can effectively participate in efficient charge-separation processes, albeit with slower overall time scales.

Photoredox processes involving colloidal quantum dots: Surface states, structural inhomogeneities, and ultrafast hole extraction

253rd National Meeting (2017)

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