Doctor of Philosophy (PhD)
Physics and Astronomy
Complex Materials, such as transition-metal oxides (TMOs) with exotic properties provide immense opportunities in condensed matter and materials science. The signature and challenge of these materials is the multitude of competing ground states that can be tuned or manipulated by doping, structural modification, strain induction, or the application of external stimulus. In the past few years, it is becoming increasingly clear that surfaces/interfaces, thin films, and heterostructures of TMOs, display a rich diversity of fascinating properties that are related to, but not identical to, the bulk phenomena. The fundamental issues for the understanding of these emergent phenomena include the structure and chemical composition in the proximity of surface/interface. In the thesis, I have developed a method using angle resolved X-ray photoelectron spectroscopy (ARXPS) to characterize the surface structure and chemical composition. In particular, I have considered the photoelectron diffraction effects on the relative intensities of different core electron levels which are essential to reveal the variation of chemical composition. I developed the basic methodology for the data analysis of the ARXPS spectra and used the well-known crystal surface of Sr2RuO4 (100) to verify our methodology. Using the angle-dependence of Sr 3d/ Ru 3p core level intensity ratio, our method clearly confirms that the termination layer of the surface is SrO-layer with minor amount of oxygen vacancies. The diffraction pattern matches very well with the theoretical calculation of forward scattering peaks. Then I have used the developed ARXPS method coupled with low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) to investigate the surface structure and chemical composition of a widely used crystal surface: SrTiO3(100). I found that, although maintaining in-plane unreconstructed: primary p(1×1), the surface exhibits out-of-the-plane bulking relaxation. More importantly, the systematic analysis of ARXPS spectra show that the surface is TiO2-layer terminated and has significant oxygen vacancies. These results confirm the conjecture from LEED-I(V) refinement on structure. The existence of surface oxygen-vacancies may explain the observed surface metallicity of SrTiO3. Finally, I have studied the chemical composition of the ultrathin crystalline films of La2/3Sr1/3MnO3 on SrTiO3 (100), especially focusing on the Sr surface segregation. I found that Sr concentration at the surface is appreciably higher than the corresponding bulk value. Such an off-stoichiometric behavior should link to the different physical properties such as nonmetallic/nonmagnetic “dead layer” behavior in the ultrathin films compared with the bulk crystal. To conclude, by considering the photoelectron diffraction effects, I have developed a method of ARXPES to characterize the surface chemical composition, which is essential for the understanding of emergent phenomena at surface, interface and thin film of complex materials.
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Li, Yi, "Chemical composition and structure study of surfaces and ultrathin films of complex compounds" (2013). LSU Doctoral Dissertations. 3651.