IR luminescence mechanism in colloidal Ag[formula omitted]S quantum dots

Abstract

In the present paper, we discuss general features of luminescent properties for various samples of colloidal Ag2S quantum dots (QDs), passivated with thioglycolic (Ag2S/TGA) and 2-mercaptopropionic acids (Ag2S/2-MPA), L-cysteine (Ag2S/L-cys), and also Ag2S QDs stabilized in gelatin (Ag2S/Gel) and Poly(N-vinylpyrrolidone) (Ag2S/PVP). It was found that for all samples, the Stokes shift of the luminescence peak relative to ground state exciton absorption is greater than the exciton binding energy in Ag2S (0.096-0.104 eV). Moreover, the Stokes shift turned out to be caused by not only the quantum dot size but also by the type of the organic ligand. With decreasing the quantum dot size, the Stokes shift increases significantly. The estimates of the energy level shifts for electrons, holes, as well as the position of the luminescence peak, led us to a conclusion that trap state luminescence occurs. The specific luminescence mechanism is determined by the recombination of an electron trapped at the luminescence center with a free hole. It was found that decay of the trap state luminescence has non-exponential behavior. A model is proposed that explains the non-exponential luminescence decay.