Surface modification of CdS quantum dots using thiols—structural and photophysicalstudies

Abstract

This study is aimed at identifying a suitable organic thiol for CdS by studying itsstructural, thermal and photophysical characteristics. Quantum dots of the II–VIsemiconductor CdS, in the size regime of 2.0–3.3 nm, were prepared in the cubic phase by awet 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) andtransmission electron microscopy (TEM), which revealed the cubic phase of CdS. Opticalproperties were studied by FT-IR, UV–visible and fluorescence spectroscopic techniques,and a comparison was made between uncapped and capped CdS. FT-IR studies suggestedtwo different bonding mechanisms of the capping agents with the CdS. GSH and DTTcapped CdS showed significant decrease in absorption wavelengths. An increase inband 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 theDTT capped CdS. DTT was found to be the best capping agent for CdS amongthese 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.