SYNTHESIS AND SPECTROSCOPIC CHARACTERIZATION OF NANOSTRUCTURED THERMOELECTRIC MATERIALS

Bismuth in the bulk form is a semimetal with a rhombohedral structure. It has a small band overlap between the conduction and valence bands and a highly anisotropic electron effective-mass tensor. Thermoelectric materials, in which one of the three dimensions is in the nanometer regime, exhibit unique quantum confinement properties and have generated much interest in recent years. Theoretical investigations have suggested that nanowires with diameters ≤ 10 nm will possess a figure-of-merit ZT \u3e 2. Prior to this study, it has been shown that Bi nanowires with small enough diameters (~10 nm), prepared via the pulsed laser vaporization method, undergo a transition from a semimetal with a small band overlap to a semiconductor with a small indirect band gap. Infrared absorption and UV-visible measurements were used to confirm this semimetal-to-semiconductor phase transition. In this thesis, we report the synthesis and optical characteristics of a variety of various potential thermoelectric materials including bismuth, nickel sulfide and cadmium sulfide. The infrared absorption in our Bi nanorods is blue-shifted in energy when compared to the corresponding spectra in bulk Bi, and when cooled down to liquid nitrogen temperatures, group theory suggests a strong temperature dependence in the Bi band structure. We also find that the Bi nanorod suspension displays excellent optical limiting properties at both 532 and 1064 nm excitations in the nanosecond laser pulse regime. We have also synthesized nickel sulfide nanoparticles with an average size of 5 nm by a one-step solid phase reaction. The intensity-dependent nonlinear transmission study was carried out using a 7 ns Nd:YAG laser at 532nm using Z-scan, and the nonlinear scattering was found to be the dominant mechanism for the observed response. Importantly, the modified Z-scan method allowed us to measure two competing mechanisms simultaneously - the optical limiting and saturable absorption in surface-modified nickel sulfide nanoparticles suspensions