Synthesis Of CdSe Quantum Dots In Liquid Paraffin Via One-pot Method And Their Optical Behaviors

CdSe quantum dots or semiconductor nanocrystals have attracted considerableinterest in widely application, such as biological medicine, solar cells andlight-emitting diodes, etc. due to their remarkable size-tunable optical properties andversatility toward chemical modification. In this study, we conduct a growth kineticstudy on the synthesis of CdSe quantum dots using liquid paraffin as solvent viaone-pot method, and clarify the effects of amine addition on the growth kinetics andthe quality improvements of the quantum dots. Meanwhile, the optical properties ofthe as-prepared ternary alloy CdSexS1-xquantum dots are investigated against Se to Smolar ratio and the enhancement of trap emission.At first, the CdSe quantum dots are synthesized in liquid paraffin via one-potmethod. The growth kinetic of the quantum dots is investigated by the measurementsof their ultraviolet-visible absorption and fluorescence emission spectra. The resultsshow that very fast growth occurs linearly for the quantum dots regardless of themolar ratio of Cd to Se until a reaction time of approximately60s, followed byessential growth rate decrease. At a given temperature, the quantum dots thereafterpossess a two-stage kinetic feature. The growth kinetic is consisting of a rapid growthwithin approximately30min and a slow growing up to10h. The particles size is notgoverned by the reaction temperature but by the molar ratio of Cd to Se. From theviewpoint of both strong fluorescence emission and saving in raw materials, areaction temperature of200°C and a molar ratio of Cd to Se of4/1are appropriate forthe synthesis of the CdSe quantum dots.Second, the effects of n-octylamine (OA) and octadecylamine (ODA) additionon the synthesis of CdSe quantum dots in liquid paraffin via one-pot method areinvestigated via the measurements of their ultraviolet-visible absorption andfluorescence emission spectra. The results show that the in situ added amines canactivate the formation reaction of Cd precursor and, as a result, substantially decrease the initial reaction temperature and accelerate the particle growth. By adding OA athigh temperature of200°C, remarkable improvement on particle quality is achieved,giving relatively narrow size distribution of33.1nm and high photoluminescencequantum yield (PLQY) of81.9%for the CdSe quantum dots. OA addition at lowtemperature show also good-quality improvement for the quantum dots. With regardto ODA, it may be inappropriate for quality improvement of the CdSe quantum dots.Last, the ternary alloy CdSexS1-xquantum dots are synthesized in liquid paraffinvia one-pot method. The fluorescence emission of the quantum dots can be alteredfrom violet to green by adjusting the ratio of Se to S without particle size change. Byapplying subsequent surface modification of the quantum dots with amine, thefluorescence emission from yellow to red is successfully obtained that is achievedfrom trap emission enhancement. As a result, the combination of these twoapproaches make the fluorescence emission of the ternary alloy CdSexS1-xquantumdots covering entire visible region from violet to red while their sizes are similar andkeep constant.