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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.

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This paper presents the joint effect of strain-and doping-induced band gap change in Sn 1Àx Mn x O (0 # x # 0.05) nanoparticles. In addition, an effort was made to understand the effect of Mn doping on the structural and optical properties of SnO 2 . X-ray diffraction analysis showed a tetragonal structure and the unit cell volume decreased slightly with Mn 4+ content. The Mn:SnO 2 are spherical shaped particles with a size ranging from 7.7 to 13.8 nm as calculated by transmission electron microscopy, Scherrer's formula and Willamson-Hall plot. X-ray photoelectron spectroscopy showed clear evidence for tetragonal coordinated high-spin Mn 4+ ions occupying the lattice sites of Sn 4+ in the SnO 2 host. Electron energy loss spectroscopy further confirmed composition and oxidation states of Sn 4+ and Mn 4+ ions. Manganese doping increased the band gap of SnO 2 from 4 eV to 4.40 eV with Mn 4+ concentration. Variation in band gap energy was attributed to the increasing lattice strain with Mn content and the charge transfer transitions between Mn 4+ ions and conduction/valence bands of SnO 2 . Three photoluminescence emission bands observed at 320, 360 and 380 nm, when excited at 250 nm, proved Mn:SnO 2 to exhibit good optical emission and to have potential application in nanoscale optoelectronic devices.

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