Aqueous Phase Synthesis And Fluorescence Properties Studies Of CdTe And Its Core-shell Structure Nanocrystals

II-VI semiconductor nanocrystals （NCs） have attracted extensive interest due totheir unique optical properties and potential application in the bioimaging and labeling.At present, the synthesis method of NCs can be classified into two groups: organicphase method and aqueous phase method. The NCs prepared by organic phase methodhave good fluorescence properties, but the preparation condition is strict, high cost,and the products must be converted into water-soluble NCs by surface modificationbefore their application. However, the fluorescence quantum yield of the NCs willdecrease dramatically in the process of surface modification. While the NCs preparedby aqueous phase method can be applied directly, there still are some defects, such asenvironmental pollution in the preparation process and poor fluorescence properties ofthe NCs. Therefore, seeking suitable raw materials and method, optimizing thepreparation process of the NCs, reducing preparation cost and improving thefluorescence properties of the NCs are of great significance.CdTe was chosen due to its good fluorescence properties. Generally, telluriumpowder was used as tellurium source and was reduced to H2Te or NaHTe to react withcadmium ions, forming CdTe NCs. Because H2Te or NaHTe is highly toxic, unstableand easily oxidized in the air, during the whole experiment, nitrogen or other inert gaswas used as shielding gas. In addition，on the surface of the NCs, there are a variety ofchemical equilibriums related to solvent water molecules, so the physical andchemical properties on the surface of the NCs are unstable. The change of thechemical environment of the NCs may alter their surface structure and fluorescenceproperties. If the NCs are coated with inert materials, their photochemical stabilitywill be improved effectively. The synthesis of large-size CdTe NCs by aqueous phasemethod usually takes long time. If two or more compound semiconductor materials are composed together at nanometer scale, forming core-shell structure NCs, theiremission peak can be easily adjusted, their fluorescence quantum yield and stabilitycan be increased, and their toxicity can be effectively reduced.In order to further optimize the preparation process of CdTe NCs and to improvetheir fluorescent properties, in this paper, the synthesis and their fluorescenceproperties of CdTe NCs, CdTe/SiO₂composite, CdTe/CdS and CdTe/CdS/ZnScore-shell NCs were systematically studied. The main contributions were as follows:（1）Water-soluble CdTe NCs were synthesized by aqueous phase method usingsodium tellurite as tellurium source, thioglycolic acid （TGA） as stabilizer and reducer.And the highest fluorescence quantum yield of the CdTe NCs is27%, in the emissionwavelength ranging from516to630nm. The method avoided the cumbersomeprocesses used H2Te or NaHTe as tellurium source which is toxic and unstable in theair, and the method was simple and productive compared to traditional aqueoussolution method. The CdTe NCs were characterized by X-ray diffraction （XRD） andfluorescence spectrophotometer.The particles crystallized predominantly in cubicphase with narrow full width at half maximum （FWHM） ranged from32-78nm. Theeffects of reaction time, reaction temperature, pH value, and precursor concentrationon the photoluminescence properties were investigated in detail. The experimentresults showed that, with reaction time increasing, the fluorescence peak of the CdTeNCs showed continually red-shift. The reaction temperature at95°C and the pH valueat9.5were deemed to be most appropriate. Low precursor concentration wasbeneficial for the preparation of CdTe NCs with shorter fluorescence emissionwavelength （less than550nm）, while high precursor concentration was favorable forthe preparation of CdTe NCs with longer fluorescence emission wavelength （morethan550nm）.（2） Fluorescent CdTe/SiO₂core-shell composites were synthesized by theSt ber method without the exchange of surface ligands. The CdTe NCs were dopedinside of the silica matrix dispersedly. The composites retained the size-dependentphotoluminescence （PL） property and PL intensity, and the emission peak of thecomposites showed red-shift. The composites had better stability than that of the CdTeNCs in the PBS buffer solution. The red-shift of the fluorescence peak of CdTe/SiO₂composite particles decreased with the increase of the emission wavelength of theinitial CdTe NCs, it was to say, for the CdTe NCs with small size, the fluorescencepeak of the composite particles had relatively larger red-shift. The silica matrix couldcreate a barrier for the toxic Cd2+ions and prevent the oxidation of the CdTe. So the composites were bio-friendly, stable and easily preserved.（3） In order to achieve the rapid preparation of the red fluorescent NCs, type-IIcore-shell CdTe/CdS NCs were further synthesized based on the route of CdTe NCs.XRD patterns showed that in the entire process of growth, CdTe/CdS NCs maintainedthe sphalerite structure. The effects of reaction time, the molar ratio of S/Te, reactiontemperature on the PL properties were investigated in detail. The results showed thatthe emission wavelength of CdTe/CdS NCs can be increased to612nm in a relativelyshort time （0.5h） by the growth of CdS shell, achieving the rapid preparation of NCswith red fluorescent peak, and the fluorescence quantum yield of the CdTe/CdS NCsincreased to20%from13%. The change of light-emitting mechanism of theCdTe/CdS NCs leads to the broadening and obvious red-shift of their fluorescencepeak. If the CdS shell was too thick, the newly generating surface defects make thefluorescence quantum yield of CdTe/CdS nanocrystals decreased. With the reactiontime increasing, the fluorescent peak of the CdTe/CdS nanocrystals showedcontinuously red-shift. Within the same reaction time, the fluorescent peak ofCdTe/CdS NCs was proportional to the molar ratio of S/Te. The results showed thatthe CdS shell grew quickly on the CdTe nuclear surface, so by this method, theCdTe/CdS NCs with different shell thickness and different fluorescence peaks can besynthesized quickly.（4） In order to further enhance the fluorescence quantum yield and stability ofCdTe/CdS NCs, CdTe/CdS/ZnS multi-shell structure NCs were prepared. The effectsof reaction time and thickness of ZnS shell on the PL properties were investigated indetail. The results showed that the fluorescence quantum yield of the CdTe/CdS/ZnSNCs can be increased to64%, and the fluorescent peak and fluorescence quantumyield can be adjusted by adjusting the size of the nucleus as well as the thickness ofthe shell including CdS shell and ZnS shell. Within the same reaction time, with theincrease of the Zn/Te molar ratio, the fluorescence intensity of the CdTe/CdS/ZnSNCs increased first and then decreased, but they were higher than that of CdTe/CdSNCs. Stability studies indicated that compared with CdTe and CdTe/CdS NCs, thefluorescent stability of the CdTe/CdS/ZnS NCs improved obviously in the PBS buffersolution, and4days later, relative fluorescence intensity of the CdTe/CdS/ZnS was83%. By ultraviolet irradiation for180minutes, the relative fluorescence intensity ofthe CdTe/CdS/ZnS was87%. Compared with CdTe NCs and CdTe/CdS core-shellNCs, anti-photobleaching properties of the CdTe/CdS/ZnS core-shell NCs improvedsignificantly.