Structural Implications on the Electrochemical and Spectroscopic Signature of CdSe-ZnS Core−Shell Quantum Dots

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We investigated the influence of the core diameter, shell thickness, and ligand length on the spectroscopic and electrochemical signature of CdSe-ZnS core−shell quantum dots and on the ability of these nanoparticles to exchange electrons with complementary acceptors or donors upon excitation. Our studies demonstrate that the core diameter controls the absorption and emission wavelengths of the quantum dots as well as the potentials for their oxidation and reduction. Both wavelengths increase monotonically and both redox potentials shift in the negative direction with an increase in diameter. The presence of a ZnS shell enhances significantly the luminescence quantum yield and shifts both reduction potentials in the positive direction. Interestingly, the shell thickness has negligible influence of the position of the absorption and emission wavelengths, but controls the electrochemical band gap energy. Specifically, an increase in thickness translates into a decrease in the electrochemical band gap energy, but does not affect the optical band gap energy. Similarly, the length of the oligomethylene chains of the alkanethiols adsorbed on the nanoparticles surface has negligible influence on the spectroscopic signature, but regulates the electrochemical response. Indeed, the elongation of the organic ligands increases the electrochemical band gap energy. The optical band gap energy and redox potentials of the quantum dots suggest that the transfer of an electron to methyl viologen or from ferrocene upon excitation is exoergonic. However, only methyl viologen quenches the luminescence of the nanoparticles. Specifically, this electron acceptor adsorbs on the surface of the quantum dots in the ground state and quenches statically their excited state. Nonetheless, an increase in shell thickness and the elongation of the organic ligands have a depressive effect on the stability of the complex and quenching rate constants. In summary, our experimental observations provide valuable insights on the structural factors dictating the spectroscopic and electrochemical behavior of CdSe-ZnS core−shell quantum dots and can facilitate the rational design of luminescent chemosensors based on these nanoparticles and photoinduced electron transfer.