Direct evidence of photo-ionization causing luminescence "off" state in CdTe Quantum dots

TL;DR

This study provides direct experimental evidence that photo-ionization causes the luminescence 'off' state in CdTe quantum dots, with ionization probabilities similar to other quantum dots and insights into ionization mechanisms and rates.

Contribution

It offers the first direct measurement of photo-ionization probability and rates in CdTe quantum dots, revealing tunneling as the dominant ionization mechanism at band-edge excitation.

Findings

Ionization probability is approximately 10^-6, similar to CdSe quantum dots.

Ionization rate increases by two orders of magnitude with thiol addition.

Ionization and neutralization occur on microsecond timescales, limiting quantum yield improvements.

Abstract

We present a direct experimental evidence of photo-ionized CdTe quantum dot core having luminescence "off" state at band-edge photo-excitation. We use the autocorrelation of fluorescence fluctuations from dilute colloidal solution of CdTe quantum dots in a diffraction limited detection volume to determine their photo-ionization probability. The ionization probability is in the same order of magnitude (\approx10-6) as the CdSe and CdSe/ZnS core/shell quantum dots. Further, we measured the ionization/ neutralization rates (\approx104 s-1) after addition of {\beta}-mercaptanol and found that ionization rate is enhanced by two orders of magnitude after the addition. However, a comparable neutralization rate enhances the light emission quantum yield. The ionization/ neutralization at microsecond time scales puts an upper limit on achieving better quantum yield by thiol addition. The results also indicate that the dominant ionization mechanism at band-edge excitation is tunneling.