Ultrafast Photoluminescence Quenching of Initially Excited States in CdSe Quantum Dots Functionalized with a Charge Acceptor Dye

• Published in: Journal of physical chemistry. C 2019-09, Vol.123 (36), p.22519-22528
• Main Authors: López-Arteaga, Rafael, Guarin, Cesar A, Herrera Cortes, Oscar Alejando, Peon, Jorge
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.9b00949

CdSe quantum dots have interesting carrier transfer characteristics and can be used as photon collectors in certain kinds of hybrid photovoltaic devices. Some of these systems work through a charge transfer process from an excitonic state to a surface-adsorbed organic dye. In this article, we explore carrier transfer time scales through the characterization of the ultrafast photoluminescence behavior of nanocrystal excitonic states in the presence of adsorbed molecular charge acceptors. We show that upon physisorption of the cyanine dye Indocyanine Green, significant emission quenching due to carrier transfer can take place in a direct way from the initially pumped states in 5.7 nm diameter CdSe dots. We show that such transfer takes place independently of the excess energy above the band gap. Importantly, this near-instantaneous quenching is responsible for the loss of an important fraction of the excitonic population on a time scale much faster than intraband (hole and electron) excitonic relaxation. The time scales for the excitonic quenching and relaxation were addressed by femtosecond photoluminescence up-conversion experiments. These experiments showed that the time constants associated with the accumulation of the band-edge excitons remain unchanged upon dye physisorption; however, the signal amplitude is significantly reduced as a function of the addition of Indocyanine Green. The transient photoluminescence from the spectral region associated with states that act as intermediaries during excitonic relaxation (like the 1P3/21P and the 2S1/21S states) shows a significant reduction in the amplitude of the exponential components, but there was no difference in the transient’s time constants. These features indicate that the yield of accumulation into these transiently populated states is diminished by the presence of the cyanine dye due to near instantaneous exciton quenching of the initially formed states.