Cd-free Ⅰ-Ⅲ-Ⅵ group semiconductor quantum dots (QDs) like Ag-ln-S and Cu-ln-S show unstructured absorption spectra with a pronounced Urbach tail,rendering the determination of their band gap energy (Eg) and the ene...Cd-free Ⅰ-Ⅲ-Ⅵ group semiconductor quantum dots (QDs) like Ag-ln-S and Cu-ln-S show unstructured absorption spectra with a pronounced Urbach tail,rendering the determination of their band gap energy (Eg) and the energy structure of the exciton difficult.Additionally,the origin of the broad photoluminescence (PL) band with lifetimes of several hundred nanoseconds is still debated.This encouraged us to study the excitation energy dependence (EED) of the PL maxima,PL spectral band widths,quantum yields (QYs),and decay kinetics of AlS/ZnS QDs of j different size,composition,and surface capping ligands.These results were then correlated with the second derivatives of the corresponding absorption spectra.The excellent match between the onset of changes in PL band position and spectral width with the minima found for the second derivatives of the absorption spectra underlines the potential of the EED approach for deriving Eg values of these ternary QDs from PL data.The PL QY is,however,independent of excitation energy in the energy range studied.From the EED of the PL features of the AlS/ZnS QDs we could also derive a mechanism of the formation of the low-energy electronic structure.This was additionally confirmed by a comparison of the EED of PL data of as-synthesized and size-selected QD ensembles and the comparison of these PL data with PL spectra of single QDs.These results indicate a strong contribution of intrinsic inhomogeneous PL broadening to the overall emission features of AlS/ZnS QDs originating from radiative transitions from a set of energy states of defects localized at different positions within the quantum dot volume,in addition to contributions from dimensional and chemical broadening.This mechanism was confirmed by numerically modelling the absorption and PL energies with a simple mass approximation for spherical QDs and a modified donor-acceptor model,thereby utilizing the advantages of previously proposed PL mechanisms of ternary QDs.These findings will pave the road to a deeper understanding of the nature of PL in quantum confined Ⅰ-Ⅲ-Ⅵ group semiconductor nanomaterials.展开更多
Using CdSe/CdS core/shell nanocrystals with 1-10 monolayers of CdS shell as the model system, we studied effects of thiol ligands on optical properties of the nanocrystals. The core/shell nanocrystals with original li...Using CdSe/CdS core/shell nanocrystals with 1-10 monolayers of CdS shell as the model system, we studied effects of thiol ligands on optical properties of the nanocrystals. The core/shell nanocrystals with original ligands possessed near unity photoluminescence （PL） quantum yield and single-exponential PL decay dynamics. The effects of thiol ligands on optical properties were found to depend on the shell thickness, environment （with/without oxygen）, and excitation power （single- or multi-exciton）. Systematic and quantitative results reported in this work should provide necessary information for fundamental understanding and technical applications of quantum dots （QDs） coated with thiol ligands.展开更多
文摘Cd-free Ⅰ-Ⅲ-Ⅵ group semiconductor quantum dots (QDs) like Ag-ln-S and Cu-ln-S show unstructured absorption spectra with a pronounced Urbach tail,rendering the determination of their band gap energy (Eg) and the energy structure of the exciton difficult.Additionally,the origin of the broad photoluminescence (PL) band with lifetimes of several hundred nanoseconds is still debated.This encouraged us to study the excitation energy dependence (EED) of the PL maxima,PL spectral band widths,quantum yields (QYs),and decay kinetics of AlS/ZnS QDs of j different size,composition,and surface capping ligands.These results were then correlated with the second derivatives of the corresponding absorption spectra.The excellent match between the onset of changes in PL band position and spectral width with the minima found for the second derivatives of the absorption spectra underlines the potential of the EED approach for deriving Eg values of these ternary QDs from PL data.The PL QY is,however,independent of excitation energy in the energy range studied.From the EED of the PL features of the AlS/ZnS QDs we could also derive a mechanism of the formation of the low-energy electronic structure.This was additionally confirmed by a comparison of the EED of PL data of as-synthesized and size-selected QD ensembles and the comparison of these PL data with PL spectra of single QDs.These results indicate a strong contribution of intrinsic inhomogeneous PL broadening to the overall emission features of AlS/ZnS QDs originating from radiative transitions from a set of energy states of defects localized at different positions within the quantum dot volume,in addition to contributions from dimensional and chemical broadening.This mechanism was confirmed by numerically modelling the absorption and PL energies with a simple mass approximation for spherical QDs and a modified donor-acceptor model,thereby utilizing the advantages of previously proposed PL mechanisms of ternary QDs.These findings will pave the road to a deeper understanding of the nature of PL in quantum confined Ⅰ-Ⅲ-Ⅵ group semiconductor nanomaterials.
基金This work was supported in part by the National Natural Science Foundation of China （NSFC） （Nos. 21233005 and 91433204） and Fundamental Research Funds for the Central Universities （No. 2014FZA3006）.
文摘Using CdSe/CdS core/shell nanocrystals with 1-10 monolayers of CdS shell as the model system, we studied effects of thiol ligands on optical properties of the nanocrystals. The core/shell nanocrystals with original ligands possessed near unity photoluminescence （PL） quantum yield and single-exponential PL decay dynamics. The effects of thiol ligands on optical properties were found to depend on the shell thickness, environment （with/without oxygen）, and excitation power （single- or multi-exciton）. Systematic and quantitative results reported in this work should provide necessary information for fundamental understanding and technical applications of quantum dots （QDs） coated with thiol ligands.
