Synthrsis Of CdTe Nanocrystals And Its Reversible Phase Transfer

CdTe nanocrystals have attracted much attention due to their narrow band gap, quantum size effect and special photoluminescence (PL) properties. A variety of methods, including both aqueous and metallicorganic route, have been developed to obtain CdTe nanocrystals with controlled particle size and shape. In most cases, however, the resulted CdTe can only be dispersed in the corresponding media being used in the synthesis process, and thus limited its further application in different fields. A possible way to solve this problem is to adjust the hydrophilicity of the CdTe nanocrystals via a surface modification.In this thesis, CdTe nanocrystals were prepared in aqueous solution in the presence of mercaptoacetic acid. The particle size of CdTe nanocrystals was controlled by adjusting the reflux time where CdTe nanoparticle is growing. Phase transfer of CdTe nanocrystal from water to chloroform was proceeded by using hexadecylamine (HDA). Interestingly, it is found that the CdTe nanocrystals can be transferred back to water when the pH of the aqueous solution was increased, indicating a completed reversible phase transfer process in our case. Detail investigation shows that both the concentration of HDA and solution pH are important factor to affect the transfer rate of CdTe nanocrystals from water to chloroform. Higher HDA amount and lower solution pH was found to benefit the transfer of CdTe nanocrystals from water to chloroform. On the other hand, the amount of HDA required to initiate the phase transfer was found to strongly depend on the particle size of CdTe nanocrystal, while more HDA is needed for the large size particles.XRD, UV-vis and PL investigation shows that, the crystal structure as well as its particle size and size distribution of CdTe nanocrystals are kept unchanged after the CdTe nanocrystals were subjected to the reversible phase transfer. However, the PL of CdTe nanocrystals was found slightly increased when CdTe nanocrystls were transfer to chloroform and back to water, as compared to the as prepared CdTe nanocrystals. Surface structures of the CdTe nanocrystals before and after reversible phase transfer were analyzed by FTIR, and a formation of COO^-NH₃⁺ is suggested to responsible for the reversible phase transfer of CdTe nanocrystals.