Correlation between photoluminescence intermittency of CdSe quantum dots and self-trapped states in dielectric media

We present evidence that the photoluminescence intermittency of $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}$ core/shell quantum dots is correlated with the dielectric environment surrounding the quantum dots. The statistics of dark state lifetimes in the intermittency is found to be related to the stabilization energy of charges in the local dielectric surrounding of the quantum dot. This supports the model of an ionized quantum dot in the dark state. Charges ejected from the quantum dot are suggested to be self-trapped in mid-bandgap states of the surrounding matrix due to atomic and electronic relaxation processes. These trap states are inherent to disordered materials and proposed to be a general source of power law intermittency of various types of emitters such as quantum dots and dye molecules.

Figure 1

(a) Histogram of the off time durations of $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}$ QDs embedded in PMMA. The histogram was constructed from 100 individual emission time traces, measured at $1.3\mathrm{mW}$ excitation power each with $5\bullet {10}^4$ frames and $50\mathrm{ms}$ time resolution. The inset shows the corresponding off time statistics of a single $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}$ QD in PMMA. (b) Histogram of power law exponents determined from 100 individual emission time traces as well as a simulated power law exponent distribution for emission time traces with 1000 off time events per trace (see text for simulation details).

Figure 2

Off time distributions for $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}$ QDs in PS, PMMA, PVA, and on quartz. The arrow indicates the direction of increasing reaction field $f\left(ϵ\right)$. The statistics for QDs in PVP are omitted for clarity.

Figure 3

(a) Dependence of the power law exponent of the off time statistics on the reaction field factor $f\left(ϵ\right)$ (squares), linear fit (solid line), and calculated dependence (dashed line) corresponding to the model of Verberk et al. (b) Energy level scheme of $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}$ and polymer with respect to the vacuum level. The $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{S}$ data was taken from Ref. 18. The polymer data was adopted from Refs. 13, 16, 17.