Thermal and Optical Properties of CdS Nanoparticles in Thermotropic Liquid Crystal Monomers

Two new mesogenic monomers, namely 3,3'-dimethoxy-4,4'-di(hydroxyhexoxy)-N-benzylidene-o-Tolidine (Ia) and 4,4'-di(6-hydroxyhexoxy)-N-benzylidene-o-Tolidine (IIa), were reacted with cadmium sulfide (CdS) via an in situ chemical precipitation method in ethanol to produce CdS nanocomposites. A series of different mass compositions of CdS with Ia and IIa ranging from 0.1:1.0 to 1.0:1.0 (w/w) were prepared and characterized using X-ray Diffraction (XRD), Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FT-IR), Transmission Electron Microscopy (TEM), Polarizing Optical Microscopy (POM) and Differential Scanning Calorimetry (DSC), X-ray Photoelectron Spectroscopy (XPS) and Photoluminescence Spectroscopy (PL). XRD showed that the broad peaks are ascribed to the formation of cubic CdS nanoparticles in both Ia and IIa. The average particle size for both nanocomposites was less than 5 nm with a narrower size distribution when compared with pure CdS nanoparticles. The analyses from POM and DSC demonstrated that mass composition from 0.1:1.0 up to 0.5:1.0 of CdS: Ia nanocomposites showed their enantiotropic nematic phase. On the other hand, polarizing optical microscopy (POM) for IIa nanocomposites showed that the liquid crystal property vanished completely when the mass composition was at 0.2:1.0. PL emissions for CdS: Ia or IIa nanocomposites indicated deep trap defects occurred in these both samples. The PL results revealed that addition of CdS to Ia monomers suppressed the photoluminescence intensity of Ia. However, the introduction of CdS to IIa monomers increased the photoluminescence and was at a maximum when the mass composition was 0.3:1.0, then decreased in intensity as more CdS was added. The XPS results also showed that the stoichiometric ratios of S/Cd were close to 1.0:1.0 for both types of nanocomposites for a mass composition of 1.0:1.0 (CdS:matrix).