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Thesis (Ph.D.)--University of Texas at Dallas, 2007. / Includes vita. Includes bibliographical references (leaves 94-96).

Thesis (Ph.D.)--Ohio University, November, 2007. / Release of full electronic text on OhioLINK has been delayed until November 30, 2010 Includes bibliographical references (leaves 142-151)

Thesis (Ph.D.)--Ohio University, November, 2007. / Title from PDF t.p. Release of full electronic text on OhioLINK has been delayed until November 30, 2010 Includes bibliographical references (leaves 142-151)

Thesis (M.S.)--University of Texas at Dallas, 2006. / Includes vita. Includes bibliographical references (leaves 50-58)

Thesis (Ph. D.)--University of Chicago, Dept. of Physics, August 2000. / Includes bibliographical references. Also available on the Internet.

The electronic structure of clean pyrite {100} and covellite {001} surfaces have been investigated in ultra-high vacuum (UHV) for the purpose of understanding the nature of sulfide surface reactivity. Using primarily scanning tunneling microscopy and spectroscopy (STM/STS), the electronic structure at atomic sites on these surfaces was directly probed, and chemical insight into the results was provided by ab-initio calculations. Pyrite is the most abundant sulfide at the earth's near surface. Its oxidation influences a wide variety of natural and industrial chemical process, but very little is known about the stepwise oxidation reactions involved. For this reason, the first two chapters are directed at understanding the surface electronic structure and fundamental reactivity of pyrite surfaces at the atomic scale. UPS spectra show a characteristic peak at ~ 1 eV forming the top of the valence band for the near surface. Ab-initio calculated densities of states for the bulk crystal suggest that this band is comprised primarily of non-bonding Fe 3d t_2g and lesser S 3p and Fe 3d e_g states. Ab-initio slab calculations predict that the broken bonding symmetry at the surface displaces a Fe 3d_Z2 dangling bond state into the bulk band gap. Evidence confirming the presence of this surface state is found in low bias STM imaging and normalized single-point tunneling spectra, which are in remarkable agreement with calculations of the LDOS at surface Fe and S sites. The results predict that due to the dangling bond surface states, Fe sites are energetically favored for redox interaction with electron donors or acceptor species. STM/STS observations of O₂/H₂O exposed surfaces are consistent with this assertion, as are ab-initio cluster calculations of adsorption reactions between O₂/H₂O derived species and the {100} surface. Furthermore, an enhancement in the "rate" of oxidation was discovered using UPS on pyrite surfaces exposed to a mixture of O₂/H₂O. Cluster calculations of adsorption energies reveal a similar result for the case where both O₂ and H₂O are dissociated on the surface and sorbed to Fe sites. Covellite, similar to pyrite, is a natural semiconducting metal sulfide. In contrast, however, precious metal bearing solutions have a curiously lower affinity for covellite surfaces than for pyrite. At the same time, its unique combination of low resistivity and perfect basal cleavage represented a unique opportunity to improve our ability to interrogate metal sulfide surfaces using STM/STS at the atomic scale. Ab-initio calculations predict that cleaving covellite exposes two slightly different surfaces, one is expected to have dangling bonds, the other is not. Atomic-scale STM images and LEED patterns indicate that the surface structure is laterally unreconstructed. The STM images are predicted to show Cu sites as high tunneling current sites on the dangling bond covered surface, and S sites on the other. Based on tunneling spectra and tip-induced effects therein, reasonable arguments are presented which allow one to uniquely differentiate between the two possible surfaces. For both pyrite and covellite, the combination of experiment and theoretical calculations afforded much more insightful conclusions than either would have alone. The calculations provided the necessary chemical framework with which to make interpretations of the experimental data and, in this sense, contribute information obtainable by no other means. This point is further developed in an investigation of Si-O interactions and the electron density distribution in the model silicate coesite, which is presented in the appendix. In addition, it breaks new ground by delving into differences and similarities between periodic vs. cluster calculations of minerals. / Ph. D.

The ruthenates are perhaps one of the most diverse group of materials known up to date. These compounds exhibit a wide array of behaviors ranging from the exotic pwave superconductivity in Sr2RuO4, to the itinerant ferromagnetism in SrRuO3, and the Mott-insulating behavior in Ca2RuO4. One of the most intriguing compounds belonging to this group is Ca3Ru2O7 which is known to undergo an antiferromagnetic ordering at 56K and an insulating transition at 48K. Most intriguing, however, is the behavior displayed by this compound in the presence of an external magnetic field. For fields parallel to the a-axis, the compound undergoes a metamagnetic transition into the ferromagnetic region at 6 T. If the external field direction is changed to the b-axis then the result will be different. colossal magnetoresistance occurs and a fall in reistivity of up to three orders of magnitude is recorded at fields of 15T. Most interesting, however, is the energy gap observed for this material. A number of groups have measured such gap with different methods and found conflicting results. For this reason it was of vital importance to perform measurements on this compound and try to resolve this issue. Tunneling spectroscopy is one of the most powerful techniques which can be used to probe the electronic properties of a material. The method is best suited to measure the density of states of a material and hence the nature of the strong correlations which dictate the properties of the compound. We performed a series of tunneling spectroscopy measurements by means of planar tunnel junctions. These types of junctions were chosen because of their stability over a large temperature range and their stability in the presence of an external field. The anisotropies which showed up in the resistivity and magnetization measurements manifested also in our data. For tunneling parallel to the a-axis, we observed a gap opening at 48K with a width a peak to peak width of 2Δa ~258±15meV. As the temperature was lowered, the gap size increased reaching a maximum width of 2Δa ~ 845±38meVat 4.2K. Tunneling parallel to the b-axis, the gap has a much smaller size than the a-axis gap. At 48K the gap width is about 2Δb ~ 201±13 meV and reaches a maximum width of 2Δb ~ 366±33 meV at 4.2K. For the c-axis, the situation is different since the gap opens at 56K instead of 48K. The gap width at 56K is about 2Δc ~ 102±6meV and reaches a maximum width of 2Δc ~ 179±14 meV at 4.2K. In the presence of an external field, we noticed that the overall behavior was always the same in the ab-plane but differed in c-axis direction. In our experiment, an external field was applied along the a-axis and measurements were made at 4.2K. For aaxis tunneling, the gap width decreased to a value of 2Δa ~ 587±27 meV at 4.2 K at 7T. On the other hand, the gap width in the b-axis direction decreased to a value of 2Δb ~ 308±25 meV for the same field. For the c-axis direction, the gap decreased to a value of 2Δc ~ 112±8 meV at 7T. The DOS of the c-axis differs for fields of 6T and above. A third peak emerges inside the gap on the valence side of the DOS. This third peak seems to be a direct consequence of the metamagnetic transition at 6T observed by other groups and may be attributable to a spin-filtering effect.

Recently, Bi₂Se₃ was added to a new class of materials known as topological insulators. While several studies have provided tantalizing hints towards novel physical properties, such as backscatter suppression and spin-polarized transport, several concerns remain in actual materials. In particular, high defect densities, strong surface band bending, and potential fluctuations have been observed. Here, scanning tunneling microscopy and spectroscopy are used to reveal surface effects in Bi₂Se₃ and CuxBi₂Se₃. First, a detailed examination of defects in bulk-grown samples is described. Then, I provide an analysis of molecular beam epitaxy results, done in collaboration with colleague Yuxuan Chen. Following this, I provide a detailed study of individual point defects in Cu-doped Bi₂Se₃ and examine how Cu is incorporated into the Bi₂Se₃ lattice. Finally, through spectroscopic analysis, a novel depth-sensitive measurement of the local band bending field is developed. Furthermore, for the first time, fluctuations of the Dirac point can be correlated to specific near-surface defects, namely Se vacancies. These analyses provide valuable insights into the preparation of future samples for the investigation of topological insulators. / text

The electronic structures at the hetero interface of Poly(3-hexylthiophene): methanofullerene (P3HT:PCBM) have a great improvement on the solar cell efficiency due to the formation of bicontinuous nanoscaled phase separation which will enhance charge separation and carrier transport. In the present work, cross-sectional scanning tunneling microscopy and scanning tunneling spectroscopy measurements are utilized to obtain the in-situ atomic-scale band structure across the interface between P3HT and PCBM directly. The distribution of PCBM volume concentration of organic films was also analyzed and discussed in the work.