Synthesis Of CdTe QDs Or Nanocrystals Via Aqueous-phase Approach And Their Optical Behaviors

Since NCs were synthesized in1964, rapid development on researches of themwas in process; meanwhile, researchers paid a lot attention to synthesis of QDs.Due tothe outstanding size–dependent and tunable photoluminescence (PL) in the wholevisible region, QDs undoubtedly attract great attention in the domains of thebiological applications, luminescence devices and solar energy. QD of Cd series is akind of many categories of Nanocraystals (NCs).CdTe NCs possesses highfluorescence emission, narrow FWHM and broad tunable photoluminescence.Concerning the synthesis of QDs, a lot of methods have been developed and all ofthem could be classified into organic approach or aqueous approach. Compared toorganic synthesis, aqueous phase approach can be easy to operate, low reactiontemperature and non-toxic to environment.Thus, we choose the aqueous phase way tosynthesize CdTe QDs or NCs.Firstly, we synthesized CdTe QDs via two-step method. Through changing theratios of Cd/Te, we confirm the experimental parameter of Cd with Te. The resultsshows that when the ratio is1/8, we obtain the highest PL intensity. However, becausethe growth condition requires operating easily, in our study, we choose the ratio ofCd/Te as6/1. During the process of preparing CdTe QDs, different conditions willresult in different effects on the kinetic of growth, such as temperature, ligands. Ourmeasurements on the ultraviolet–visible (UV–Vis) absorption and fluorescenceemission reveal that both the growth kinetics and the PL efficiency of the QDs vary alot with the molecular structures of the three involved ligands. In comparison withTGA and LCys, MPA endows the QDs with a wide color tuning range from cyan todeep red and strong PL emission while its full width at half maximum (FWHM) isregretfully large.Secondly, post treatment is a modified method of passivation on the surface of CdTe QDs, especially organic amine reagents. Diamines, such as ethylenediamine(EDA) and hexamethylenediamine (HDM), have better surface modified advantagesto QDs. Also they are very common reagents in organic synthesis process. Accordingto the previous report, post treatment on the synthesized CdTe QDs with EDA canincrease the PL intensity and PLQY. We introduce HDM as another diamine toexplore the effect on thiol–contained CdTe QDs. To sum up, our research provides abetter understanding of the effects of reaction temperature, ligand nature, andprolonged refluxing on aqueous growth of CdTe nanocrystals. Besides, diamineaddition offers a method to obtain CdTe QDs with high qualities. By roomtemperature treatment with EDA or HDM solution, notable enhancement on PLemission is achieved for the TGA and MPA capped QDs while in the case of the QDscapped by LCys the enhancement is practically meaningless.Thirdly, the CdTe nanocrystals are synthesized in aqueous solution with acapping ligand of TGA，MPA and LCys. After obtaining the QDs samples, topassivate the QDs surface and improve their optical properties, inorganic metal ionwas introduced. Our results show that the addition of Zn2+can greatly enhance thefluorescence emission of the colloid nanocrystals by one order of magnitude.Meanwhile, it can make the nanocrystals settle rapidly. Thus, the CdTe powdernanocrystals with high fluorescence emission can be easily obtained via a simplecentrifugation and subsequent drying process. In addition, the successful dissolutionof the powder in NaOH solution reveals a good reuse of the nanocrystals in a colloidstate.Finally, through aqueous approach synthesis, we obtained the CdTe colloid QDsthat possessing a lot of disadvantages such as more surface defect sites, exitingnonsaturation danglings, poor stabilities, low PL intensity and narrow tunable grainsize. Shell structure on QDs can improve their optical properties. Using TGA asligands to synthesis of CdTe QDs, after forming ZnS shell on CdTe QDs surface, weobtained emission spectrum rang from green to dark red.compared to CdS shell onCdTe QDs, ZnS shell on CdTe QDs showed better fluorescence emission and highPLQY.Morevoer, CdTe/ZnS had the significance in application, due to the non-toxic property of ZnS.