Quantum dot and π-conjugated molecule hybrids: nanoscale luminescence and application to photoresponsive molecular electronics

Abstract

Hybrid nanoparticles (NPs) consisting of an n-type CdSe/ZnS quantum dot (QD) as a core and p-type π-conjugate molecules as a shell were fabricated, and their nanoscale photoluminescence (PL) and photoresponsive molecular electronic characteristics were investigated. π-Conjugated macromolecular dioctyloxybenzodithiophene-based polythiophene (P3000) or a single carbazole (CB) molecule with insulating molecular blocks were attached to the QD surface. The nanoscale PL characteristics for the single QD were drastically changed through close contact with P3000 owing to energy and charge transfer effects. However, for the hybrid QD-CB NP, the PL of the QD was dominant because of weak energy transfer resulting from the relatively longer insulating molecular block between the QD and the CB molecule. From time-resolved PL spectra, the exciton lifetimes of the QD in the hybrid QD-P3000 and QD-CB NPs were clearly different because of a variation in the energy transfer rate. The photocurrents of the single hybrid QD-P3000 NP were considerably higher and actively responded to both forward and reverse biases due to the energy and charge transfer effects, while those of the single QD-CB NP exhibited diode characteristics. The QD-based hybrids show distinct nanoscale PL features and photoresponsive molecular electronic characteristics depending on the structures of π-conjugated molecules. For n–p-junctioned QD-P3000 hybrid NPs, the PL intensity from the green QD drastically decreased and that of the π-conjugated P3000 was weakly changed due to energy and charge transfer effects. The photocurrent of the QD-P3000 was much higher and more symmetric than that of the QD-CB NP, which had an insulating molecular block, suggesting more active charge transfer between the p-type P3000 and the n-type QD. The nanoscale PL and molecular optoelectronic properties of the hybrids of QDs and π-conjugated molecules could be tuned by their relative distance and the degree of spectral overlapping. An enhanced control over the optical properties of quantum dots, achieved by attaching organic molecules, has been demonstrated by Kwang-Sup Lee and Jinsoo Joo from Hannam University/Korea University and their collaborators in South Korea and the United States. The addition of nanometer-sized quantum dots dramatically alters the properties of a material, which is of use in applications such as solar cells. At the same time, it is important to exert effective control over these modified properties. This can be achieved by surrounding the quantum dots with other materials that interact with them. In their study of cadmium selenide quantum dots, the researchers use organic molecules for this purpose and demonstrate that the light emission from the quantum dots depends strongly on the type of molecule attached to them. This enables a broader flexibility when designing quantum dots for specific electronic or optical applications.