The Optical Properties Of Indium Phosphide Quantum Dots

Recently,InP quantum dots（QDs）were paid tremendous attention in quantum dot optoelectronic devices and biomedicine due to heavy metal free such as cadmium and lead,which are hazardous to human health and the environment,promising to be served as the next-generation environmentally friendly material to replace conventional cadmium-based QDs.Meanwhile,among a series of non-heavy metal element QDs,InP-based QDs can cover the entire spectral range from the visible to the near-infrared region,which are attributed to quantum confinement effects induced tunable optical properties comparable to those of cadmium-containing QDs.Significant progress has been made in various optoelectronic devices based on InP QDs,such as InP/ZnSe/ZnS QDs,which can achieve an external quantum efficiency（EQE）of 21%.However,research on the temperature-dependent optical behaviors,non-linear optics,and ultrafast carrier dynamics of InP QDs still facing challenges,which will limit their deep understanding towards on applications in solid-state lighting,display panels,photodetectors and solar electric fields.Therefore,in this thesis,we investigate the photophysical behavior of InP semiconductor heterojunction QDs rely on advanced spectral techniques,such as UV-Vis absorption spectroscopy,fluorescence spectroscopy,fluorescence lifetime spectroscopy,variable temperature fluorescence spectroscopy,multiphoton fluorescence techniques,and femtosecond transient absorption,with the following main elements.We have designed and synthesized two types of InP-based QDs,InP/ZnS and InP/ZnSeS/ZnS.The results demonstrated that the multi-shell structure of InP/ZnSeS/ZnS（85%,55 nm）obtained a higher fluorescence quantum yield as well as narrower fluorescence emission linewidth relative to InP/ZnS（60%,65 nm）,as result of the small lattice mismatch between ZnSeS and InP to reduce the surface defects.At the same time,the ZnSeS intermediate shell slowed down the non-radiative coupling rate.As a result,a faster fluorescence decay rate is obtained in InP/ZnSeS/ZnS.It can be found less fluorescence emission from defect states in InP/ZnSeS/ZnS QDs via temperature dependent fluorescence spectroscopy.More importantly,more surface states present at the interface between InP and ZnS under the observation of transient absorption spectroscopy,resulted by more severe lattice mismatch.At the same time,the small conduction band difference between InP and ZnSeS forms a quasi-II electronic structure due to the presence of the ZnSeS shell layer afterwards suppressed the Auger effect.Furthermore,the InP/ZnSeS/ZnS QDs have larger two-photon（3500 vs 7000 GM）and three-photon absorption cross sections(7.8×10^-79 vs 1.06×10^-77 cm⁶ s² photon^-2)in the wavelength range of 700-1500 nm compared to the InP/ZnS QDs.This is the first report of multiphoton absorption in multishell InP/ZnSeS/ZnS QDs.Our work reveals how the ZnSeS inner shell plays an important role in the optimization of the optical properties of InP QDs.And it provides new insights into how to improve the optical properties of InP based QDs.In summary,it is shown that the non-radiative coupling rate of InP QDs is mitigated by insertion of the ZnSeS intermediate shell,as well as improves the absorption cross section of multiphoton in InP QDs.