Excited Electron Dynamics in Size-gradient PbS Quantum Dots Elucidated by Femtosecond Transient Absorption Spectroscopy

초록

Colloidal quantum dots (CQDs) have been widely used as a photosensitizer on the inorganic photovoltaic material to absorb sunlight. The advances in synthesis methodology enable us to precisely control the size of CQDs, the diameter of which fine-tunes the HOMO and LUMO energy level relevant to the optoelectronic function. To improve the efficiency of excited electron transfer in an active medium, a size-graded CQDs superstructure has been developed. According to the previous studies, it has been expected that a size-graded structure of CQDs induces a certain directional charge transfer determined by a gradient of HOMO-LUMO separations, which is called as quantum funneling. To investigate the excited electron dynamics on the size-graded PbS CQDs through quantum funneling, we performed the femtosecond transient absorption spectroscopy study using the broadband near-infrared probe. From the comparative study between the size-graded PbS CQDs and the single-sized CQDs, it is demonstrated that the excitation energy transfer between the layers containing different sizes of CQDs is accelerated in size-graded case relative to in single-sized case.