Study Of Photoluminescence Properties Of CuInS₂ And Other Group ?-?-? Nanocrystals

There are many reports on the synthesis of ternary chalcopyrite semiconductor?such as CuInS₂ and CuInSe₂?nanocrystals.Group I-III-VI alloyed semiconductors have applications in solar cells,in vivo bioimaging,and display devices.CuInS₂?CuInSe₂?is a semiconductor with direct bandgap of 1.5 eV?1.04 eV?,corresponding to light emission at 827 nm?1192nm?;it does not contain highly toxic heavy-metal elements.It has large optical absorption coefficient(?>10⁵ cm^-1)and high photostability.The photoluminescence intensity of the CuInS₂ quantum dots?QDs?can be dramatically improved after surface coating with ZnS.The photoluminescence mechanisms of the group I-III-VI alloyed semiconductors are still under debate owing to complexity of their structures.We focus on study of the chalcopyrite-type CuInS₂ and CuInSe₂ QDs.We study the synthesis of these QDs and their photoluminescence properties in the visible region as well as self-assembly of the CuInSe₂QDs into superlattices.We synthesize two kinds of CuInS₂-based QDs by using different solvothermal methods.One type of the CuInS₂-based QDs are synthesized by employing different Cu/In molar ratios?3/2,1/1,and 1/2?.The bandgap of the synthesized CuInS₂ QDS depends on the Cu/In ratio,the photoluminescence of the CuInS₂ QDs shifts to bule with increasing Cu deficiency.The luminescence lies in the red spectral region?650–720 nm?,and the luminescence intensity is relatively small.The luminescence originates from the donor–acceptor pair recombination,where In_Cu?In substitutes for Cu?or V_S?S vacancy?may act as the donor state and V_Cu?Cu vacancy?may act as the acceptor state.The absorption peak of the CuInS₂/ZnS QDs shifts to blue relative to that of the original CuInS₂ QDs,and their luminescence shifts to blue too,showing tunable emission wavelength.The surface passivation of the CuInS₂ QDs with the ZnS layers results in dramaticly increased luminescence intensity.Another type of CuInS₂QDs exhibit an exciton absorption?520 nm?in the absorption spectrum at room temperature,and their luminescence lies in the red spectral region?around 640 nm?.After coating ZnS on the surfaces of the CuInS₂ QDs,the exciton absorption peak disappears,and the the luminescence with double emission bands exhibits abnormal blue shift.These results show that there are cation exchange between CuInS₂ and ZnS,and the 555 nm emission originates from the Cu?Zn?InS₂ core and the 605 nm emission originates from the ZnS shell.After the Cu?Zn?InS₂/ZnS QDs experience surface treatment with acid,the luminescence from the ZnS shell diminishes dramatically.The CuInSe₂ QDs with an average size ranging from 2.23 to 5.25 nm are synthesized by using the solvothermal method.They have size-dependent photoluminescence ranging from660 to 680 nm,and the emission is defect related.We also observe self-assembly of the CuInSe₂ QDs into superlattices,and the superlattices have 2H structure.The reflection spectrum varies with the thickness of the CuInSe₂ QD layer.These studies could help to drive the research and development of the devices based on the quantum dot superlattices.