Synthesis, Surface Properties Studies Of Water-soluble CdTe Nanocrystals

In the past two decades, luminescent semiconductor nanocrystals, especially II-VI semiconductor nanocrystals, foreshow the great potential application due to their superior optical properties. Nowadays, in many fields, researches on the II-IV semiconductor nanocrystals are widely carried on, for example, photoelectric transform, fluorescence display, sensor, bio-labeling and so on. Although only developed within less than ten years, the utilization of nanocrystals in the bio-labeling field becomes one most active direction, receiving more attention from various research areas, such as medicine, chemistry and materials science, etc.Luminescent semiconductor nanocrystals, as a new luminescent marker, have broad application in the future when they are adopted in the field of bio-labeling. Compared with the organic dyes normally used now, luminescent semiconductor nanocrystals have many notable advantages as follows. Size-dependent band edge properties, narrower and more symmetry emission spectra, and wider excitation band determine that they can be used to detect several labeled items at the same time, and semiconductor nanocrystals have very desirable light stability and anti-photobleaching properties, it is very suitable for tracing the process of labeling in a long period; the emission color can be tuned by simply changing the size of the nanocrystals, with no alternation of the surface component, which means it is possible to develop an universal method to carry out the conjugates between different size nanocrystals and bio-molecular. With the developing of the synthesis research concerning semiconductor nanocrystals, it is quite facile to larger scale produce semiconductor nanocrystals with mono-disperse properties and higher quantum yields. On the other hand, it is feasible to obtain the desiring surface through modifying nanocrystals surface with diverse functional groups, which provides the connecting point between inorganic nanocrystals and bimolecular. Therefore, it will be no far away to practical utilizing luminescent semiconductor nanocrystals in bio-labeling field.CdTe nanocrystals were chosen as the research object because they are representative luminescent semiconductor nanocrystals, which can be prepared not only in organic solvent, but also in water, and their emission can be precisely tuned by controlling the size of CdTe nanocrystals. In addition, CdTe nanocrystals have been widely applied in emitting devices, optical-electronic cells, and bio-labeling fields. In this paper, we mainly carry out some basic research work relating to the luminescent CdTe nanocrystals under the bio-labeling background. We have got some creative results as follows.In chapter 2, we had improved one method to swiftly prepare big sized CdTe nanocrystals in water. By using one weaker reducer of hydrazine, tellurite sodium was reduced to Te2- ion. Larger sized CdTe nanocrystals with near infrared emitting properties were prepared, that is a supplement for the drawbacks of other methods which can only produce CdTe nanocrystals emitting lights in visual area. We have discussed the influence of both temperature and pH value conditions on the growing speed of CdTe nanocrystals. It is proposed that that the alternation of such reaction conditions has dramatic influences on the reducing ability of hydrazine, affecting the concentration of Te2- during the growing process, thus result into different growing speed for CdTe nanocrystals. Compared with current means to synthesize CdTe nanocrystals, this way has better maneuverability, faster growing rate, and it is more suitable for the preparation of larger sized CdTe nanocrystals.In chapter 3, the influence of the reactant molar ratio on CdTe nanocrystals PL QY had been systemically studied. Using the TGA as stabilize, through changing the molar ratio of precursor Cd to Te (Cd/Te) in system, we have obtained different luminescence quantum yields (PL QY) CdTe nanocrystals in water phase. When the high Cd/Te ratio was used, the resulting CdTe nanocrystals have high PL QY in despite of different size. Both HRTEM and XRD results indicates that the resulting nanocrystals had same interplanar spacing and the zinc blende structures, which imply that the CdTe nanocrystals with different PL QY have the same interior structure. XPS analysis implies that they had surface component with different S/Te ratio. High S/Te atom ratio means more passivating surface structure. The time-resolved luminescent was used to study the photo-induced carriers recombination dynamics. It is found that there were two different lifetime, the short lifetime component could be attributed to the populated core-states emission, and the longer lifetime was definitely due to the surface related luminescence. It also can be found that the increasing PLQY was along with the increasing of surface related emission. It was proposed that the increasing of PLQY origin from the surface reconstruction under high Cd/Te precursor ratio. More perfect surface structure can be formed under high S/Te ratio, which may effectively suppress the nonradiative channel resulting into high PL QY.In chapter 4, CdTe/CdS core shell nanocrystals were prepared under the microwave heating. By utilizing the nature of releasing S^2- ion under high temperature for TGA molecular, we have synthesized CdTe/CdS core/shell nanocrystals with the assisting of microwave heating. Due to the modification of CdS on the CdTe nanocrystals surface, both PL QY and anti-photobleaching ability were obviously enhanced. At the same time, we have optimized the reaction condition to synthesis CdTe/CdS core shell structure in details. It is found that it was necessary to heat naked CdTe nanocrystals for produce core shell structure. Because this process can increase the CdS component on the surface, which provides the action sites for the continuous growth of CdS layer. It is also found that the TGA/Cd ratio of 4 is the best condition for the modification of CdS layer on the CdTe nanocrystals surface.In chapter 5, we directly capped CdTe nanocrystals in the interior of SiO₂ spheres without the exchanging of surface ligands process happening. Through studying the influence of Cd/TGA compounds layer on the CdTe nanocrystals surface, it shows that the thicker Cd/TGA compounds layer facilitates the CdTe nanocrystals to be capped in the cores of SiO₂ spheres, on the other hand, thinner compounds layer leads to CdTe nanocrystals capping only on the surface of SiO₂ spheres. We further investigate the luminescent recovery of CdTe nanocrystals in SiO₂ sphere. And it is proposed that it is due to surface reconstruction of CdTe nanocrystals in SiO₂ media. Using Rhodamine 6G as reference to calculate the PL QY of CdTe/SiO₂ nanoparticles, it is 12.9%. Due to the SiO₂ layer, the luminescent stability of CdTe nanocrystals was enhanced significantly in PBS buffer.