Semiconductor nanocrystals and nanocrystal arrays: Synthesis, characterization, and time-resolved terahertz spectroscopy photoconductivity measurements

The current world energy consumption is projected to more than double by the year 2050. Climate changes associated with burning such large amounts of fossil fuel may be severe, and developing alternative energy sources are vital to future world energy security. Photovoltaics have the potential to provide much of the energy needed to support the increasing world energy demand, and the development of a new generation of solar cells possibly based on nanotechnology will be needed to harness the sun's energy in an efficient and cost effective manner. Semiconductor nanocrystals (NCs) are a promising class of materials for such opto-electronic devices, and understanding and maximizing charge transport in NC arrays is critical to the development of such devices.; The goal of the project reported in this thesis was to study electronic coupling in NC arrays. To accomplish this, a time-resolved terahertz spectrometer was built, synthetic methods to produce NCs and NC arrays were developed, and THz spectroscopy was conducted on these materials to extract carrier dynamics and photoconductivity mechanisms. Spherical PbSe and PbTe NCs were synthesized, having a size distribution as low as 6%, with first excitonic transitions tuned from about 1 mum to 2.4 mum. The synthesis of colloidal cubic-like PbSe and PbTe NCs using this PbO "one pot" approach are also reported. The photoluminescence quantum yield of PbTe spherical NCs was also measured to be as high as 52 +/- 2%. The first known observation of efficient multiple exciton generation from single photons absorbed in PbTe NCs is also reported. Longitudinal and transverse Bohr radii for PbS, PbSe, and PbTe NCs that account for electronic band anisotropy are calculated, and the electronic band structure and optical properties of these lead salts is compared. Finally, the first time-resolved terahertz spectroscopy (TRTS) photoconductivity measurements on NC arrays of InP and lead salts are reported. Several methods were developed to increase the conductivity in these assemblies by decreasing the average interdot distance in the NC arrays. Treating NC arrays with solutions containing various short-chained amines resulted in great enhancements in photoconductivity, and pump power dependent TRTS studies indicate that Auger recombination is an efficient loss mechanism in these arrays.