Synthesis of CdTe quantum dots in aqueous solution and their optical and structural characterization

We describe the optical and structural characterization of highly luminescent thioglycolic acid-stabilized CdTe quantum dots (CdTe-QD) synthesized in water. Samples prepared under different reflux times were characterized using transmission electron microscopy (TEM), X-ray diffraction, Raman spectroscopy, absorbance and photoluminescence techniques. TEM measurements revealed CdTe-QD having a nearly spherical shape (2.5 nm in size), so as tellurium nanorods in the same sample. The corresponding Raman spectrum displays a characteristic peaks of CdTe (broad bands at 141 cm-1 and 162 cm-1, respectively) and tellurium nanorods (121 cm-1). Three extra peaks at 228, 252 and 277 cm-1 could be associated to the TO ₁ and LO ₂ modes of CdS nanoparticles. X-ray diffraction measurements done on dried sample indicated the face centered cubic structure for CdTe and testified the presence of CdS nanoparticles, crystallizing in the hexagonal phase. All refluxed samples exhibit high luminescence that increases with reflux time. Their absorbance spectra display a well resolved excitonic peak in the 350-510 nm range, whilst photoluminescence peaks shift red (500-600 nm) due to the Stokes shift. To explain the observed strong luminescence, it is assumed that a CdS shell develops at the CdTe surface due to the thioglycolic acid decomposition, providing the surface passivation.