Site-selective spectroscopic study on the dynamics of exciton hopping in an array of inhomogeneously broadened quantum dots

Abstract

We investigate the dynamics of exciton hopping in a CdSe/ZnS quantum dot (QD) array composed of an inhomogeneously broadened ensemble. Time- and spectrally resolved fluorescence intensities are measured by varying the excitation photon energy at the absorption edge. This method allows us to observe fluorescence from only the subdistribution of the QD ensemble, thereby allowing the dynamics of exciton hopping, which depends on the initial (donor) exciton energy, to be elucidated. Experimental results along with numerical calculations using a model of a coupled QD array show that when high-energy QDs are selectively excited, exciton energy transfer occurs repeatedly to a site of low energy, leading to a large exciton hopping length. In contrast, when the low-energy end of the QD ensemble is excited, the exciton tends to be trapped in the initial QD. Furthermore, from the analysis of the decay time of fluorescence intensities, it is suggested that there are dark QDs associated with the defect and/or off state of blinking QDs in the ensemble and energy transfer to such a site is mainly followed by quenching.