As schemed in Fig. , CdTe/CdS/ZnS core/shell/shell quantum dots were fabricated in this integrated chemical aerosol flow apparatus through the epitaxial growth stage. In the nebulizer apparatus, the stock solution containing Cd(MPA) complex and NaHTe was misted to microdroplets, carried into the furnace by the 1.5 L/min N2 flow. CdTe/CdS core/shell quantum dots were directly formed in the furnace. Then, at the end of the furnace, the CdTe/CdS QDs flowed into in a three-neck flask that was kept at 80°C, capping a shell of ZnS was carried out. About 45 min later, CdTe/CdS/ZnS core/shell/shell quantum dots were harvested.
Apparatus to synthesize the CdTe/CdS/ZnS core/shell/shell quantum dots
In our experiment, the intermediate CdTe/CdS QDs synthesized show great stability due to the good crystallization at high reaction temperature, and the in situ synthesized CdS shells on CdTe cores can play as a buffer layer to further epitaxially grow ZnS shells. So with this facile method, high quality of CdTe/CdS/ZnS core/shell/shell QDs can be synthesized.
As known, in the synthesis of QDs, especially that with core/shell/shell structure, large-scale production is difficult for many methods mainly due to its difficulty to ensure the same temperature and homogeneous mixing in the large volume of solution, which have a great influence on the monodispersity of the nanocrystals. Up to now, large-scale synthesis is still a challenge [23
]. Here, in our modified integrated apparatus, the continuous synthesis method makes it possible to realize large-scale synthesis of QDs with core/shell/shell structure. In our experiment, the rate of production can reach as high as 0.1 g/h. As the currently used quartz tube in the furnace only has a diameter of 30 mm, the flow rate is 1.5 L/min. It is easy to improve the production rate using a larger diameter tube or increasing the flow rate. Owing to its continuity, as much core/shell/shell QDs can be synthesized.
CdTe/CdS/ZnS core/shell/shell QDs with different peak position of photoluminescence (PL) can be obtained by changing flow rate and temperature, consistent with our previous description for synthesizing CdTe/CdS QDs [20
]. As shown in Fig. , with a flow rate of 1.5 L/min and temperature of 200, 225, and 250°C, CdTe/CdS/ZnS core/shell/shell QDs with PL peak of 525, 554, and 567 nm were synthesized. The flow rates also had an influence on the peak position of PL. With increased flow rate, the PL would be blue-shifted (Data not shown), attributed to shortened reaction time. All the synthesized CdTe/CdS/ZnS core/shell/shell QDs showed high fluorescence (Fig. ), whose quantum yields were as high as 45%.
PL spectra of QDs synthesized at different temperature (a). Image of CdTe/CdS/ZnS QDs solution under ultraviolet light
In order to confirm the core/shell/shell structure of the synthesized QDs, several control experiments were carried out, as shown in Table and Fig. . For control experiment, the Zn and/or S precursors was absent in the part for ZnS coating. As can be seen, when the Zn precursor and S precursor were added in the three-neck flask simultaneously, the obtained QDs showed a fluorescence peak at 541 nm with high fluorescence intensity. QDs with a weaker fluorescent intensity were obtained when Zn and S precursors were both absent. The results reveal that overcoating a shell of ZnS on the CdTe/CdS greatly enhances the fluorescence intensity. Similarly, when only S precursor was added, the resultant QDs showed a red-shifted PL, because a thicker CdS shell was formed on the outer shell of the QDs. As we know, the CdTe/CdS core/shell QDs have more Cd atoms on its surface. However, when only Zn precursor was added, an aggregated agglomeration with a very weak red fluorescence was obtained. It is deduced that due to the coordination between the Zn 2+ and QDs, the Zn (MPA) complex acts as a flocculant to aggregate the synthesized QDs in water at the elevated temperature. From the results, we can clearly see that the coexisted Zn and S precursor in the second part can form a shell of ZnS on the outside of CdTe/CdS QDs.
Control experiments to confirm the core/shell/shell structure of synthesized QDs
Fluorescence spectra of different kinds of core/shell QDs
The core/shell/shell structure was further confirmed by XRD (Fig. ). In the XRD pattern, three peak including (111), (220), and (311) can be clearly seen, which corresponds to the cubic CdTe/CdS and CdTe/CdS/ZnS quantum dots. Figure shows that the scattering peaks of CdTe/CdS quantum dots were just between the bulk cubic CdTe and bulk CdS. This is because of the existence of CdS shell on the edge of CdTe. For the CdTe/CdS/ZnS QDs, the diffraction patterns shift to higher angles due to the growth of ZnS shells. The composition of CdTe/CdS/ZnS was also investigated by energy-dispersive X-ray spectroscopy (EDS) (Fig. ). In the EDS pattern, the presence of Zn and S was clearly confirmed, and the atomic ratio of S:Zn:Cd:Te was calculated to be 2.84:1.64:1.38:1. The carbon and oxygen peak showed in the EDS pattern were corresponding to the capping agent of MPA. The TEM and HRTEM of CdTe/CdS/ZnS QDs synthesized with temperature of 200°C and flow rate of 1.5 L/min were shown in Fig. . We can see that the CdTe/CdS/ZnS QDs have a narrow size distribution. The average diameter of the QDs is about 2.5 nm. The existence of lattice planes on the HRTEM confirms the good crystallinity of the CdTe/CdS/ZnS QDs.
a XRD patterns of CdTe/CdS and CdTe/CdS/ZnS and b EDS of CdTe/CdS/ZnS core/shell/shell QDs
TEM image for CdTe/CdS/ZnS core/shell/shell quantum dots prepared at 200°C. Inset is the HRTEM image
It was expected that after coating with a ZnS shell, the CdTe/CdS/ZnS core/shell/shell QDs would be more stable, less toxic and have higher quantum efficiency compared to CdTe/CdS quantum dots and CdTe quantum dots. In order to investigate the difference between the three kinds of QDs, CdTe/CdS/ZnS, CdTe/CdS, and CdTe (aq), H2
was used as an oxidizing agent to examine their anti-oxide ability via detecting the change of fluorescence spectra (Fig. ). It is known that when reacted with an oxidizer such as H2
, a blue shift would be observed due to the oxidization of surface atoms [30
]. But if the QDs were core/shell structure, the blue shift would not occur because the thick shell can prevent the oxidation of cores, which determines the PL peak position. Different volumes of 0.03% H2
were added to equal amount of three kinds of QDs (4 mL). From the results (Fig. ), we can clearly see that when H2
solution was added, the fluorescence intensities were all decreased and gave an approximate liner change. The difference is that the PL peak position was almost not changed when ZnS shells existed for CdTe/CdS/ZnS, while there is obviously a blue shift of about 6 nm for CdTe (aq) QDs. Similarly, for the CdTe/CdS QDs, a smaller blue shift of about 2 nm can be seen. This result clearly shows that the CdTe/CdS via CAF has enhanced anti-oxide ability compared with CdTe (aq). While overcoating ZnS shells on the CdTe/CdS QDs can further enhance this effect. When reacted with H2
, the surface atoms of CdTe can be oxided to CdTeO3
], which causes the decrease in CdTe QDs size and further leads to the blue shift of fluorescence peak. The existence of CdS can weaken this effect, while ZnS shells can prevent the blue shift. These results reflect that the overcoating of ZnS shells can greatly enhance its anti-oxide ability and stability.
The fluorescence intensity change of different QDs when added various volumes of 0.03% H2O2 solution. a CdTe/CdS/ZnS, b CdTe/CdS c CdTe(aq)
With ZnS shells on the CdTe/CdS QDs, its toxicity can be greatly decreased. It is more suitable to use it in biological applications such as cellular imaging. To confirm that the QDs can label the cells, we chose three kinds of CdTe/CdS/ZnS QDs (brown, yellow, and green) and added them into the Chinese hamster ovary (CHO) cells. After 30 min incubation, the intracellular distribution of CdTe/CdS/ZnS QDs was observed by confocal microscopy. It can be clearly seen that the QDs penetrate into the living cells and exhibit bright fluorescence (Fig. ). The distribution of all three kinds of QDs is in the cytoplasm and the nucleus. This observation demonstrates that QDs are gradually transported inside the cytoplasm and eventually to the nucleus. On the basis of these fluorescence images, we consider that the CHO cells are efficiently labeled with QDs and can display multicolor images.
Labeled CHO (Chinese hamster ovary) with CdTe/CdS/ZnS quantum dots, a–c are the fluorescent images of cells. d–f are the corresponding co-situated picture of cells and fluorescence