(Invited) Utilization of Photoluminescence from Semiconductor Nanoparticles to Prove Electron Transfer Reactions Influenced By Their Surface Conditions
Authors: Kuwabata Susumu , Uematsu Taro , Shimomura Eisuke , Torimoto Tsukasa 
Published: February 2020

Semiconductor nanoparticles, or quantum dots, have many advantages as photoluminescent materials such as the broad and intense optical absorption and single, narrow, and size-tunable emission. However, they have different types of fragility when compared to other organic/inorganic fluorophores. The importance of the surface on photoluminescence property is well-known by nanoparticle chemists but has hardly been investigated from both qualitative and quantitative points of view. The band edge emission can be observed when photogenerated excitons are confined in a particle having no defect levels: the whole structure including particle core, shell, and insulating ligands bound to the nanoparticle surface is essential to a high luminescence.

The present study demonstrates a simple and efficient method to investigate the condition of ligands, which typically consist of organic molecules like alkylamines, carboxylates, and thiols. The emission quenching of these nanoparticles were observed when small electron acceptors like quinones were introduced to the nanoparticle solution, because these small molecules unexpectedly penetrated the protecting ligand layers. The mathematical analyses revealed that there were many defects in the ligand layer even if they are commonly used molecules and believed to be appropriate for the synthesis and storage of nanoparticles.

Figure 1a shows the photoluminescence intensity ratio (I
0/I – 1) when the CdSe nanoparticles capped with n-alkylamine ligands were quenched by 1,4-benzoquinone introduced as an electron acceptor. The magnitude of quenching became greater with the alkyl moiety of the ligands became smaller. Although it is generally expected to be straight when the quenching mechanism follows the Stern-Volmer relationship that describes the photoinduced electron transfer between the species both dissolved and moving freely, they were all concave up. To reveal the origin of the curvatures, a model of quenching was developed. A nanoparticle has several voids of ligands on its surface, and through which the quenchers (1,4-benzoquinone) approach to the nanoparticle core to the distance that the electron transfer is possible (Figure 1b). A mathematical analysis with taking statistical distributions into account, was conducted following the proposed model, and they provided good fits with curvatures with three important parameters; the number of accessible sites existing on a nanoparticle, electron transfer rate, and the adsorption coefficients of quenchers to the nanoparticles. None of these parameters are available with conventional analysis methods, and all of them should be very import to address the intrinsic weakness of semiconductor nanoparticle fluorophores.

Time variations on the magnitude of quenching were also observed during the storage of as-prepared nanoparticles. The nanoparticle surface became gradually damaged during the storage in a pure chloroform solution, while the nearly-perfect surface was formed when only a small amount of additional ligands given in the storage solution, which were proved quantitatively for the first time by our methods.

Figure 1