Cooperative enhancement of nonlinear optoelectronic responses in colloidal quantum dot solids

Chemically synthesized semiconductor quantum dots (QDs) are excellent materials for optoelectronic devices, e.g., solar-cells, light-emitting diodes, and lasers. The next challenge in QD devices is to maximize the full performance of inter-QD networks. Cooperative optoelectronic dynamics of QDs, i.e., synchronous responses of QD ensembles, are expected to boost optoelectronic device performances, since QD devices are composed of a large number of QDs. Here, we study on multiexciton dynamics and novel cooperative responses in QD solids. To generate inter-QD cooperativity, we fabricated QD solid films by using a ligand exchange method, where the inter-QD distance was precisely controlled on the atomic scale of carbon chains. The hybrid system of semiconductor QDs and organic molecules is a suitable system to generate inter-QD cooperativity. To measure ultrafast nonlinear cooperative effect, we performed photocurrent quantum interference spectroscopy by using a phase-stabilized femtosecond laser pulse pair. Then, we observed that the nonlinear photocurrent signals were enhanced by forming the inter-QD networks. The enhancement ratio increases monotonically with the decrease of the inter-QD distance. This is the evidence that the enhancement is generated by the cooperative behavior of QDs via the electronic coupling. The cooperativity via electronic coupling is different from that via radiative coupling observed as superfluorescence from QD superlattices. Therefore, the inter-QD cooperativity via coherent electronic coupling is a novel feature to provide amplification of optoelectronic responses.