Enhanced Charge Transport and Excited-State Charge-Transfer Dynamics in a Colloidal Mixture of CdTe and Graphene Quantum Dots

Abstract

Semiconductor quantum dot (QD)–graphene composites are promising materials for photovoltaics and photocatalysis because of efficient charge extraction and transport property of graphene. Analysis of their ultrafast carrier-transfer dynamics has been limited to only QD–graphene sheets, such as CdTe–graphene sheets and CdS–graphene sheets. Herein, we investigate the carrier-transfer dynamics of CdTe QDs (CdTe QDs) in the presence and absence of graphene QDs (GQDs) by using femtosecond transient absorption spectroscopy. Although the surfaces of both the QDs are in negatively charged state, as evident from the zeta potential measurements, we observed a strong excited-state interaction between two similarly charged QDs, leading to an efficient quenching of the excitonic emission of the CdTe QDs in the presence of GQDs. A detailed analysis of the rise time and 1S bleach recovery provides the evidence of electron transfer from CdTe QDs to GQDs with the hole-trapping process by surface defects. On the basis of the mechanistic study, an overall charge-transfer scheme that accounts for the faster bleach recovery of CdTe QDs in the presence of GQDs is proposed. Moreover, we have evidenced the consequences of charge transfer through the measurements of improved four-probe photoconductivity (from 1.39 (±0.12) × 10–4 S m–1 to 1.47 (±0.24) × 10–3 S m–1) and decreased charge-transfer resistance [from 6944 (±0.3) to 5761 (±0.3) Ω] exhibited by CdTe QDs in the presence of GQDs. These findings described in the present study are hoped to open up design strategies for developing light-harvesting inorganic–organic hybrid QD assemblies with better efficiency.