Surface modification of CdS quantum dots using thiols-structural and photophysical studies

This study is aimed at identifying a suitable organic thiol for CdS by studying its structural, thermal and photophysical characteristics. Quantum dots of the II-VI semiconductor CdS, in the size regime of 2.0-3.3 nm, were prepared in the cubic phase by a wet chemical method. Five organic thiols were used for capping: (i) 1,4-dithiothreitol (DTT), (ii) 2-mercaptoethanol (ME), (iii) cysteine (Cys), (iv) methionine (Meth), and (v) glutathione (GSH). Structural studies were carried out by x-ray diffraction (XRD) and transmission electron microscopy (TEM), which revealed the cubic phase of CdS. Optical properties were studied by FT-IR, UV-visible and fluorescence spectroscopic techniques, and a comparison was made between uncapped and capped CdS. FT-IR studies suggested two different bonding mechanisms of the capping agents with the CdS. GSH and DTT capped CdS showed significant decrease in absorption wavelengths. An increase in band gap was observed in two cases: when (i) capped and (ii) decreased in size. The band gap was increased from 2.50 eV for the uncapped to 2.77 eV for the DTT capped CdS. DTT was found to be the best capping agent for CdS among these five organic thiols in two aspects: (i) yielding lower grain size in cubic phase, and (ii) good fluorescence properties with efficient quenching of the surface traps.