Synthesis and characterization of CdY (Y= Te/O/Se) nanoparticles by wet chemical process
Semiconductor quantum dots are nanoparticles with unique tuneable properties. For instance, water soluble nanocrystals which have been synthesized by wet chemistry in open air environment are highly luminescent. They possess well-resolved absorption maxima, high stability and narrow emission bands. This thesis presents several aspects about the synthesis of highly luminescent water soluble, CdTe nanoparticles (NPs) and their near infrared emitting counterpart such as CdTexSe1-x and CdOxTe1-x NPs. It also investigates the synthesis of highly luminescent NPs specifically engineered to be used for biomedical applications. Here in a novel approach to synthesize CdY (Y = O/Te/Se) with tunable material properties are presented. The surface morphologies of the as-prepared NPs displayed by SEM micrographs depended strongly on their growth kinetics, probably due to the variation of reaction time, growth temperature or Te concentration. Differences in shape and size were observed depending on the growth conditions. Spherical, rod-like, oval-like and blade-like morphologies were obtained for different reaction parameters. There was observable change in size and shape at longer growth time or higher Te molar ratio. The representative HRTEM analysis showed that the as-obtained CdTe NPs appeared as spherical particles with excellent monodispersity. The images also displayed clear lattice fringes that were an indication of enhanced crystallinity. The X-ray diffraction (XRD) pattern displayed polycrystalline nature of the NPs. XRD pattern confirmed the formation of wurtzite (JCPDS no. 19-0193) and zinc blende (75-2086) phases for the CdYsamples prepared. The type of phase formed depended greatly on the composition, the molar ratios of the consequent elements and reaction conditions of the NPs. The average crystallite sizes estimated from Scherrer equation were found to increase with increase in reaction time, which was in agreement with the HRTEM measurement. The crystallite sizes of the NPs were in the range of 3 to 40 nm depending on the reaction conditions and composition of the NPs formed. Crystallinity of the samples was enhanced up to certain extent as shown by highest peak intensity of the XRD pattern. Variation in the XRD peak intensities was very much dependent on the reaction parameters. Results from XRD also showed a systematic shift in peak positions towards lower and higher 2θ degrees values for CdTe or and CdOXTe1-X NPs, respectively with an increase/decrease in reaction parameters. Results from PL showed sharp excitonic band edges of the CdTe, which loses its shoulder during the growth of the NPs. The PL spectra of all the prepared samples indicated a drastic shift in emission window of the core to longer wavelength (500 to 650 nm) which was simultaneously accompanied by variation in emission intensity with different reaction conditions. The position of the emission band was observed to shift towards the lower wavelength side for shorter durations of synthesis, lower growth temperatures and lower tellurium concentrations. Some difference in absorption edges were observed due to variation in reaction conditions of CdTe NPs. The ultraviolet and visible analysis (UV-Vis) displayed well-resolved absorption maxima which were red shifted upon increase in reaction time, growth temperature and Te concentration. There was an inverse relation between the bandgap and the reaction parameters under study (reaction time, growth temperature and tellurium concentration). The CdTe bulk band gap of 1.5 eV was tuned to even above 3.0 eV while the CdTe counterparts displayed band gap from 1.7 to 2.6 eV. A pH of 11, reaction temperature of 100 °C and Cd:Te ratio of 1:0.4 were found to be the optimum conditions so far for the preparation of CdY NPs. This method of preparation is simple, sensitive, low cost, easy to execute and efficient with profound advantages such as low reaction temperatures, broad range of pH value and wide PL emission wavelength range thus making it reliable for practical applications.