Structure, chemistry, and energetics of organic and inorganic adsorbates on Ga-rich GaAs and GaP(001) surfaces

The work described in this dissertation includes fundamental investigations into three surface processes, namely inorganic film growth, water-induced oxidation, and organic functionalization/passivation, on the GaP and GaAs(001) surfaces. The techniques used to carry out this work include scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. Atomic structure, electronic structure, reaction mechanisms, and energetics related to these surface processes are discussed at atomic or molecular levels.

First, we investigate epitaxial Zn₃P₂ films grown on the Ga-rich GaAs(001)(6×6) surface. The film growth mechanism, electronic properties, and atomic structure of the Zn₃P₂/GaAs(001) system are discussed based on experimental and theoretical observations. We discover that a P-rich amorphous layer covers the crystalline Zn3P2 film during and after growth. We also propose more accurate picture of the GaP interfacial layer between Zn₃P₂ and GaAs, based on the atomic structure, chemical bonding, band diagram, and P-replacement energetics, than was previously anticipated.

Second, DFT calculations are carried out in order to understand water-induced oxidation mechanisms on the Ga-rich GaP(001)(2×4) surface. Structural and energetic information of every step in the gaseous water-induced GaP oxidation reactions are elucidated at the atomic level in great detail. We explore all reasonable ground states involved in most of the possible adsorption and decomposition pathways. We also investigate structures and energies of the transition states in the first hydrogen dissociation of a water molecule on the (2×4) surface.

Finally, adsorption structures and thermal decomposition reactions of 1-propanethiol on the Ga-rich GaP(001)(2×4) surface are investigated using high resolution STM, XPS, and DFT simulations. We elucidate adsorption locations and their associated atomic structures of a single 1-propanethiol molecule on the (2×4) surface as a function of annealing temperature. DFT calculations are carried out to optimize ground state structures and search transition states. XPS is used to investigate variations of the chemical bonding nature and coverage of the adsorbate species.

Site-Specific Isotopes in Small Organic Molecules

Stable isotope geochemistry is a valuable toolkit for addressing a broad range of problems in the geosciences. Recent technical advances provide information that was previously unattainable or provide unprecedented precision and accuracy. Two such techniques are site-specific stable isotope mass spectrometry and clumped isotope thermometry. In this thesis, I use site-specific isotope and clumped isotope data to explore natural gas development and carbonate reaction kinetics. In the first chapter, I develop an equilibrium thermodynamics model to calculate equilibrium constants for isotope exchange reactions in small organic molecules. This equilibrium data provides a framework for interpreting the more complex data in the later chapters. In the second chapter, I demonstrate a method for measuring site-specific carbon isotopes in propane using high-resolution gas source mass spectrometry. This method relies on the characteristic fragments created during electron ionization, in which I measure the relative isotopic enrichment of separate parts of the molecule. My technique will be applied to a range of organic compounds in the future. For the third chapter, I use this technique to explore diffusion, mixing, and other natural processes in natural gas basins. As time progresses and the mixture matures, different components like kerogen and oil contribute to the propane in a natural gas sample. Each component imparts a distinct fingerprint on the site-specific isotope distribution within propane that I can observe to understand the source composition and maturation of the basin. Finally, in Chapter Four, I study the reaction kinetics of clumped isotopes in aragonite. Despite its frequent use as a clumped isotope thermometer, the aragonite blocking temperature is not known. Using laboratory heating experiments, I determine that the aragonite clumped isotope thermometer has a blocking temperature of 50-100°C. I compare this result to natural samples from the San Juan Islands that exhibit a maximum clumped isotope temperature that matches this blocking temperature. This thesis presents a framework for measuring site-specific carbon isotopes in organic molecules and new constraints on aragonite reaction kinetics. This study represents the foundation of a future generation of geochemical tools for the study of complex geologic systems.