Shell-Dependent Energy Transfer from 1,3,5-Tris(N-phenylbenzimidazol-2,yl) Benzene to CdSe Core/Shell Quantum Dots

We studied the energy transfer between CdSe core/shell quantum dots (QDs) and 1,3,5-tris(N-phenylbenzimidazol-2,yl) benzene (TPBI) in inorganic/organic blend films using steady-state and time-resolved photoluminescence (PL) spectroscopy. The shortening in PL lifetime of TPBI molecules and the resulting lengthening in PL lifetime of the QDs demonstrated an efficient energy transfer process from donor to acceptor. The slowest PL decays of CdSe core/shell QDs observed in the blend films with low QD concentration were considered to result from the maximum energy transfer process from the surrounding TPBI molecules of a QD to itself. The PL decay curves of the core/shell QDs with a CdS, ZnS, and CdS/ZnCdS/ZnS shells were simulated to obtain the excited state lifetimes of the surrounding TPBI molecules for understandimg the effect of the shells on the energy transfer process. It was surprisingly found that the obtained energy transfer rate to a QD with a thick CdS/ZnCdS/ZnS multishell from the surrounding TPBI molecules with the maximum contribution of the energy transfer was almost the same as that to a QD with a thin ZnS monoshell and smaller than that to a QD with a CdS monoshell. The experimental results indicated the energy level alignment and the structure of shells in CdSe core/shell QDs determined the energy transfer efficiency from TPBI molecules to the core/shell QDs.