pH-Dependent Photophysical Properties of Metallic Phase MoSe 2 Quantum Dots.

Fluorescence properties of quantum dots (QDs) are critically affected by their redox states, which is important for practical applications. In this study, we investigated the optical properties of MoSe 2 -metallic phase quantum-dots (MoSe 2 - m QDs) depending on the pH variation, in which the MoSe 2 - m QDs were dispersed in water with two sizes (Φ~3 nm and 12 nm). The larger MoSe 2 - m QDs exhibited a large red-shift and broadening of photoluminescence (PL) peak with a constant UV absorption spectra as varying the pH, while the smaller ones showed a small red-shift and peak broadening, but discrete absorption bands in the acidic solution. The excitation wavelength-dependent photoluminescence shows that the PL properties of smaller MoSe 2 - m QDs are more sensitive to the pH change compared to those of larger ones. From the time-resolved PL spectroscopy, the excitons dominantly decaying with an energy of ~3 eV in pH 2 clearly show the shift of PL peak to the lower energy (~2.6 eV) as the pH increases to 7 and 11 in the smaller MoSe 2 - m QDs. On the other hand, in the larger MoSe 2 - m QDs, the exciton decay is less sensitive to the redox states compared to those of the smaller ones. This result shows that the pH variation is more critical to the change of photophysical properties than the size effect in MoSe 2 - m QDs.