Origin of the Broadband Photoluminescence of Pristine and Cu+/Ag+-Doped Ultrasmall CdS and CdSe/CdS Quantum Dots

Ultrasmall (similar to 2 nm) copper(I)- and silver(I)-doped CdS and core/shell CdSe/CdS quantum dots (QDs) stabilized by Cd(II) complexes with mercaptoacetate anions and ammonia were produced in aqueous solutions. The doped QDs emit broadband visible photoluminescence (PL) with a quantum yield reaching 10-12% for Cu+-doped QDs and 5-9% for Ag+-doped QDs. The broadband PL was described by a self-trapped exciton model as a sequence of phonon replicas of a zero-phonon emission line. The shape of the PL bands of CdS, Cu+-doped CdS QDs, and Ag+-doped CdS QDs was modeled by using the energies of optical phonons of CdS, CuS, and Ag2S, respectively. The dependence of the average PL lifetime of both pristine and doped CdS and CdSe/CdS QDs on PL registration wavelength was interpreted in terms of the vibrational relaxation of the self-trapped exciton. The analysis of PL properties of different ultrasmall metal chalcogenide QDs showed that the broadband PL can be described by a general model which does not require the assumption of participation of charge-trapping lattice defects.