| Quantum dots (QDs) are becoming important materials both in academic researches and intechnical applications due to their unique optical properties, such as size-tunablephotoluminescence (PL), high PL quantum yield (PLQY), narrow emission spectra, and highphoto-stability. The control of size, shape, and surface chemistry of QDs is greatly important toprovide excellent properties for promoting further progresses of nanoscience andnanotechnology. Direct aqueous synthesis of II-VI QDs by employing different short-chainthiols as stabilizing agents provides a useful alternative to widely used synthetic routes inhighly boiling organic solvents.Zinc-blende CdTe QDs, one of the emissive II-VI semiconductor QDs, are briefly andsafely synthesized by an aqueous synthetic approach. CdTe QDs were prepared in an aqueoussolution using different mercaptocarboxylic acids such as3-mercaptopropionic acid (MPA) andthioglycolic acid (TGA) as stabilizing agents. The experimental results indicated that cappingagents play an important role in the properties of the QDs. Although both of TGA-andMPA-capped CdTe QDs exhibited tunable PL from green to red color, the TGA-capped QDsrevealed a higher PL quantum yield (QY) up to60%than that of MPA-capped QDs (up to50%)by using optimum preparation conditions, such as a pH value of11and a TGA/Cd molar ratioof1.5. Furthermore, the average lifetime of the TGA-capped QDs increased with the sizeincrease of the QDs.CdTe/CdS core/shell QDs were synthesized by depositing CdS shell on CdTe cores in anaqueous solution of sodium sulfide (Na2S·9H2O), Cd2+ions, and TGA. These core/shell QDsexhibited tunable PL spectra and a high photoluminescence quantum yield up to67%. PL decayprofiles became slower with an increase in the shell thickness, contrary to an ordinary behavior(average lifetime of79ns for CdTe/CdS QDs and31ns for CdTe cores). Both of CdTe andCdTe/CdS QDs exhibited temperature dependent photoluminescence. This is ascribed tothermally activated carrier trapping and nonradiative recombination at defects.ZnS is non-toxic, chemically stable, and is often employed as the outer layer for QDs.CdTe cores with various sizes were fabricated in aqueous solutions. Inorganic shells includingCdS, ZnS, and CdS/ZnS were subsequently deposited on the cores through a similar aqueous synthesis for investigating the effect of shells on the PL properties of the cores. The outer ZnSshell provides an efficient confinement of electron and hole wavefunctions inside the QDs,while the middle CdS shell sandwiched between the CdTe core and ZnS shell can be introducedto obviously reduce the strain on the QDs because the lattice parameters of CdS is situated at theintermediate-level between those of CdTe and ZnS. It is believed that the passivation of thewide band-gap ZnS shell in CdTe/CdS/ZnS QDs plays an important role in restricting andisolating the excitons on the CdTe/CdS interface from the solution environment. In comparisonwith CdTe/ZnS core/shell QDs, the as-prepared water-soluble CdTe/CdS/ZnS QDs in our casecan exhibit higher PL efficiency, stronger photochemical stability, and higher PL quantum yieldof80%in an aqueous solution, which implies the promising applications in the field ofbiomedical labeling.To create core/shell/shell QDs with high stability against harmful chemical environment,CdTe/CdS QDs were coated with a ZnO shell in an aqueous solution. The PL peak wavelengthof the core/shell/shell QDs can be shifted from569to615nm by adjusting the size of CdTecores and the thickness of CdS and ZnO shells, along with the highest PL quantum yield of thecore/shell/shell QDs reaching up to80%. Due to the decrease of surface defects, it is observedthat PL lifetimes have a significant increase at room temperature, which are29.634.2, and47.5ns for CdTe (537nm), CdTe/CdS (555nm), and CdTe/CdS/ZnO (581nm) QDs, respectively. |
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