Synthesis And Properties Of Pb_1-xSn_xSe Quantum Dots

Lead chalcogenide quantum dots?PbX QDs,X=S,Se,Te?have recently been intensively studied for their potential applications in various fields such as solar cells,photodetectors,biomedical labeling/imaging and telecommunication.Strong quantum confinement effects can be achieved in these materials due to their relatively large exciton Bohr radii.Especially for PbSe,the exciton Bohr radii is 46nm,which is approximately eight times larger than that of CdSe.In addition,extensive studies were inspired by the property of multiple-exciton generation?MEG?,which enhances the potential for application in QDs solar cells.The optimal size and band gap of PbSe QDs for application on the solar cell are 2.2 nm and 1.3eV,respectively.Now,it's very difficult to synthesis small PbSe QDs with high stability.At the same time,Well developed synthesis of PbSe QDs often involves the use of trioctylphosphine?TOP?,tributylphosphane?TBP?and TMS₂Se,which are unstable,expensive or toxic,and require processing in glove box.Current trend in synthesizing PbSe QDs involves the use of phosphine-free method.At present,it's very easy to tune the band gap of QDs through changing the size for binary system.However,to some extent,the size will affect the properties of materials.For example,small nanocrystal can influence the stability of materials and devices;large nanocrystal can lead to the disappearance of the quantum confined effect.The band gap of QDs can be tuned effectively through doping the third element,avoiding the dependence on the size.Here we report the synthesis of the small and surface passivated PbSe quantum dots by addition of SnCl₂ as the nucleation promoting agent and PbCl₂ as the in-situ passivation agent.Common PbCl2?dissolved in oleylamine,oleic acid and 1-octadecene?and elemental Se?dissolved in oleylamine?were used to synthesize small-sized PbSe QDs by addition of SnCl₂ to promote the fast nucleation of PbSe nanocrystals.The size of the PbSe QDs are between 2⁸ nm and tunable by controlling the ratio of PbCl₂/SnCl₂ and the reaction time.High photostability of PbSe QDs approved by the XPS analysis and photoluminescence spectra of PbSe QDs was also achieved by the in-situ formation of thin halide adlayers on surface of QDs.This new method is green with accessible raw materials and simple in-situ passivation scheme and will enable the broader utilization of PbSe QDs in ambient conditions in many optoelectronic applications.The mechanism of cation exchange between PbS QDs and SnCl₂ was explored.When the concentration of SnCl₂ was low,PbS QDs would decompose partly.Then the obtained S element reacted with Sn element,forming a shell of SnS on the surface of PbS QDs.This layer of shell led to the quenching of fluorescence.When the concentration of SnCl₂ was high,PbS QDs would decompose totally.Then the obtained S element reacted with Sn element,nucleating and growing.This mechanism of cation exchange between PbS QDs and SnCl₂ was different from the traditional mechanism with the anion lattice unchangeable.With the change of Sn content,the band gap tunable Pb_1-xSn_xSe QDs was synthesized by injecting the TOP-Se into the OA-Pb,using the OA-Sn as the Sn source.Through changing the concentration of OA-Sn,the content of Sn in the Pb_1-x-x Sn_xSe QDs could be changed.The higher content of Sn,the lower band gap of Pb1-x SnxSe QDs,then the absorption and fluorescence spectra would red shift.