(164a) Microfluidic Photodegradation Studies of Quantum Dots

Colloidal quantum dots (QDs), with their distinctive size-tunable optical properties, have demonstrated great success in a wide variety of applications, such as optoelectronic devices, photocatalysis, and bio-imaging. Photostability of QDs is an important factor for the long-term applicability of QDs in chemical and energy technologies.¹ Conventional flask-based photostability studies of QDs suffer from non-uniform and difficult-to-control experimental conditions and are labor-, time-, and material-intensive. The process-dependent nature of batch photostability studies of QDs has resulted in a knowledge gap in precise mechanistic understanding of photodegradation of QDs.² In this work, we present a material-efficient microfluidic platform for intensified photostability studies of colloidal QDs. Utilizing a single-droplet photoflow microreactor, we demonstrate 3.5X faster photodegradation while using a 100X lower amount of QDs than batch techniques.³ The developed single-droplet photoflow reactor provides in-situ access to the optical properties of QDs during the photodegradation process under a collimated and tunable high-energy photon flux. Utilizing the developed microfluidic platform, we study the mechanism and kinetics of the photodegradation of cadmium selenide (CdSe) QDs. Our in-situ intensified photodegradation studies indicate that singlet oxygen generation via triplet energy transfer from colloidal CdSe QDs can trigger photo-oxidation of CdSe QDs. Our systematic studies using the intensified photoflow microreactor unveil that the photo-oxidation phenomenon causes the etching of CdSe QDs at a constant rate linearly proportional to the incident photon flux. Moreover, a fast initial decay in the photoluminescence of colloidal CdSe QDs is observed and attributed to the emergence of surface trap states via a parallel photodegradation pathway. Next, we studied the effect of the average starting size of CdSe QDs on their photostability and found a higher photodegradation rate for CdSe QDs with smaller average starting sizes. Yet, different photodegradation pathways of CdSe QDs have rates with varying sensitivities to the average starting size. This work provides a detailed understanding of the multiplex and intertwined photodegradation phenomena of colloidal CdSe QDs and exemplifies the unique characteristics of microfluidic approaches to intensify the fundamental and applied photodegradation studies of QDs. The mechanistic photodegradation studies of colloidal QDs offered by the developed microfluidic platform are essential for the development of next-gen high-performing QDs for energy and chemical technologies.

References

1. Moon, H., et al., Stability of Quantum Dots, Quantum Dot Films, and Quantum Dot Light-Emitting Diodes for Display Applications. Adv. Mater. 31, 1–14 (2019).
2. Krivenkov, V., et al., Ligand-Mediated Photobrightening and Photodarkening of CdSe/ZnS Quantum Dot Ensembles. J. Phys. Chem. C 122, 15761–15771 (2018).
3. Morshedian, H. & Abolhasani, M. Accelerated Photostability Studies of Colloidal Quantum Dots. Sol. RRL 2201119, (2023).