Atomistic Model of Fluorescence Intermittency of Colloidal Quantum Dots

Optoelectronic applications of colloidal quantum dots demand a high emission efficiency, stability in time, and narrow spectral bandwidth. Electronic trap states interfere with the above properties but understanding of their origin remains lacking, inhibiting the development of robust passivation techniques. Here we show that surface vacancies improve the fluorescence yield compared to vacancy-free surfaces, while dynamic vacancy aggregation can temporarily turn fluorescence off. We find that infilling with foreign cations can stabilize the vacancies, inhibiting intermittency and improving quantum yield, providing an explanation of recent experimental observations.

Figure 1

Surface states in a spherical CdSe CQD with unpassivated (111) facets. (a) Model, (b) projected DOS, (c) Se-related surface state on the valence band side, (d) Cd-related state on the conduction band side.

Figure 2

Elimination of surface traps with the aid of Cd and Se vacancies. (a) and (b) (111) facets with vacancies, (c) corelike electron and hole states in a passivated CQD.

Figure 3

Relation of vacancy surface diffusion to blinking and spectral diffusion. (a) regularly spaced Cd vacancies, (b) vacancy aggregate, (c) Ag-infilled vacancy aggregate.

Figure 4

Shallow trap dynamics of a CdSe CQD passivated by Cd and Se vacancies. Trap levels are represented by dashed lines.