Alloyed ZnxCd1-xSe quantum dots (QDs) have been successfully prepared at low temperatures by reacting a mixture of Cd(ClO4)2 and Zn(ClO4)2 with NaHSe using cysteine as a surface-stabilizing agent. The photoluminescence (PL) spectra of the alloyed QDs are determined on the basis of the Zn2+/Cd2+ molar ratio, reaction pH, intrinsic Zn2+and Cd2+ reactivities toward NaHSe, concentration of NaHSe, and the kind of thiols. A systematic blue shift in emission wavelength of the alloyed QDs was found with the increase in the Zn mole fraction. This result provides clear evidence of the formation of ZnxCd1-xSe QDs by the simultaneous reaction of Zn2+ and Cd2+ with NaHSe, rather than the formation of separate CdSe and ZnSe nanocrystals or core-shell structure CdSe/ZnSe nanocrystals. The size and inner structure of these QDs are also corroborated by using high-resolution transmission electron microscopy and X-ray powder diffraction. To further understand the formation mechanism, the growth kinetics of Zn0.99Cd0.01Se was studied by measuring the PL spectra at different growth intervals. The results demonstrated that, in the initial stage of growth, Zn0.99Cd0.01Se has a structure with a Cd-rich core and a Zn-rich shell. The post-preparative irradiation of these QDs improved their PL properties, resulting in stronger emission.
Alloyed Zn(x)Cd(1-x)Se quantum dots (QDs) have been successfully prepared at low temperatures by reacting a mixture of Cd(ClO(4))(2) and Zn(ClO(4))(2) with NaHSe using 3-mercaptopropionic acid as a surface-stabilizing agent. The optical properties and composition of the alloyed QDs were highly dependent on the molar ratio of Zn(2+) to Cd(2+). With an increase in the Zn content, a systematic blue shift in the first exciton absorption and band-edge emission indicated the formation of the alloyed QDs. Moreover, X-ray diffraction peaks of the alloyed QDs systematically shifted to larger angles as the Zn molar fraction of the Zn(x)Cd(1-x)Se QDs was increased. This systematic shift further confirmed the appearance of alloyed QDs. Interestingly, among these alloyed QDs, the Zn(0.93)Cd(0.07)Se QDs exhibited white-light emission with quantum yields of 12%. In addition, we discovered that we could adjust the Zn(0.93)Cd(0.07)Se QD intensity ratio of the band-edge (431 nm) to trap-state (499 nm) emissions by controlling the reaction time. Careful control of the reaction time allowed us to balance the relative strength of the band-edge and trap-state emissions, thereby attaining white-light-emitting QDs. The Zn(0.93)Cd(0.07)Se QDs offer unique advantages, including one-step synthesis, tunable white-light emission, easy manipulation, a low-temperature requirement, and low fabrication costs.
A new composite of poly(diallyldimethylammonium chloride) (PDDAC) and glutathione-capped ZnHgSe quantum dots (GSH-QDs) has been developed for sensing Cu 2+ in aqueous solution on the basis of fluorescence quenching. The formation of the composite is dominated through the electrostatic interaction between cationic PDDAC and anionic GSH-QDs. When Cu 2+ collides with PDDA/GSH-QDs composites, Cu 2+ displaces the Zn and/or Hg in the ZnHgSe QDs and forms extremely low soluble particles of CuSe onto the surface of QDs. As a result, the fluorescence intensity of QDs is quenched efficiently. Compared to GSH-QDs, PDDA/GSH-QDs composites exhibited better selectivity toward Cu 2+ as a result of minimizing the electrostatic interaction between metal ions and the ligands. The selectivity of PDDA/GSH-QDs composites toward Cu 2+ was further improved by increasing glycine concentration and optimizing the pH of the solution. Under the optimal conditions, PDDA/GSH-QDs composites provided the limits of detection for Cu 2+ at a signal-to-noise ratio of 3 of 0.2 nM (~2.0 ppt). We believe that this probe has great potential for the detection of Cu 2+ in environmental waters.
Luminescence D 6540One-Step Synthesis of White-Light-Emitting Quantum Dots at Low Temperature.-Zn x Cd 1-x Se quantum dots with x = 0.02, 0.3, 0.93, and 0.97 are prepared by reaction of an aqueous solution of Cd(ClO4)2, Zn(ClO4)2, and HS-(CH2)2-COOH as surface--stabilizing agent (pH 11 adjusted with NaOH) with NaHSe (80°C, 20 h; rapid cooling). The optical properties of the alloyed quantum dots (QDs) are highly dependent on the Zn/Cd molar ratio. With an increase in the Zn content, a systematic blue shift in the first exciton absorption and band-edge emission indicates the formation of the alloyed QDs. The Zn0.93Cd0.07Se QDs exhibit white-light emission with quantum yields of 12%. The intensity ratio of the band-edge (431 nm) to trap-state (499 nm) emissions of the Zn0.93Cd0.07Se QD can be adjusted by controlling the reaction time. The Zn0.93Cd0.07Se QDs offer unique advantages, including one-step synthesis, tunable white-light emission, easy manipulation, a low-temperature requirement, and low fabrication costs. -
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