Synthesis, characterization and photovoltaic integration of type II nanorod heterostructures

Motivated by a desire to control the actions of charges within materials in new and productive ways, researchers have increasingly focused their efforts on engineering materials on the nanometer scale where the laws of quantum mechanics rule supreme. Novel properties emerge when a semiconductor crystal is prepared at sizes below the hydrogenic ground state of the material, also known as the exciton Bohr radius. In addition to effects of quantum confinement, the large fraction of surface atoms can play a significant role in determining nanocrystal properties and applications. By combining two or more nanometer scale semiconductor crystals together to form a nanocrystal heterostructure, new avenues for materials engineering are opened up as nascent properties emerge. The high fraction of surface atoms means that much larger degrees of strain are possible than in the bulk. The large fraction of interface atoms means that the heterojunction properties can dominate the properties of the entire structure. Along with engineering these novel multi component properties comes new unexplored areas of science to be investigated and understood. New techniques are needed for studying these materials that require resolution of features much smaller than the wavelength of (visible) light. Along with this research comes a responsibility to share findings with the scientific community and to pursue directions that can positively impact humanity. At the same time, we should take a long term view when judging the applications of this or any new technology as we are only beginning to understand what is possible. After an introduction to the field in chapter one where we motivate our focus on anisotropic nanocrystal heterostructures, we discuss the formation of Fe3O4/CdS structures from spherical seeds in chapter two. In chapter three we turn our focus to type II CdSe/CdTe nanorod heterostructures where the anisotropy is inherent. The type II system is of particular interest because absorbed photons rapidly produce separated electrons and holes which we suspect could make these attractive materials for photovoltaics. Also in chapter three, we observe unexpectedly high levels of strain in these structures and develop a technique using an aberration corrected scanning transmission electron microscope to argue a hypothesis as to its cause. In chapter four we develop a synthetic strategy to forming alloyed type II nanorod heterostructures and show that we can tune their heterojunction energies. Also in chapter four, we take a further step in developing the structural characterization technique from chapter three by using it to spatially quantify composition in alloyed nanorod heterostructures. In chapter five we explore the time resolved absorption spectra of the various nanorod heterostrucutres discussed in previous chapters in order to probe carrier dynamics in these materials. Finally, in chapter six we tie together the previous chapters by developing a new type of solar cell integrating type II nanorod heterostructures. In a systematic comparison between different nanorod heterostruecutres with single component nanorods, we uncover the conditions under which the attractive qualities of type II nanorod heterostructures can be capitalized on