Competition between Energy and Electron Transfer from CdSe QDs to Adsorbed Rhodamine B

Understanding the dynamics of exciton quenching in quantum dots (QDs) is essential to their potential applications, such as solar cells and biological imaging. In this work, the competition between electron and energy transfer from excited CdSe QDs to adsorbed rhodamine B (RhB) molecules was examined by time-resolved fluorescence decay, steady-state emission, and transient absorption measurements. The major pathway (84%) for exciton quenching in this system is through electron transfer to RhB, whereas similar to 16% of the excitons decay by energy transfer. In a sample with similar to 2-3 RhB per QD, exciton quenching occurs with an average time constant of 54 ps, and the charge-separated state has all average lifetime of 1 mu s. The charge separation rate depends oil the number of adsorbates attached to the QD, and the dependence can be well-described by a kinetics model that assumes a Poisson distribution of the number of adsorbates on the QDs.