Electron Spectroscopic Study of Indium Nitride Layers

Surface structure, chemical composition, bonding configuration, film polarity, and electronic properties of InN layers grown by high pressure chemical vapor deposition (HPCVD) have been investigated. Sputtering at an angle of 50-70 degrees followed by atomic hydrogen cleaning (AHC) was successful in removing the carbon contaminants. AHC is found to be the most effective cleaning process to remove oxygen contaminants from InN layers in an ultrahigh vacuum (UHV) system and produced a well ordered surface. Auger electron spectroscopy (AES) confirmed the cleanliness of the surface, and low energy electron diffraction (LEED) yielded a 1×1 hexagonal pattern demonstrating a well-ordered surface. High resolution electron energy loss spectra (HREELS) taken from the InN layers exhibited loss features at 550 cm-1, 870 cm-1 and 3260 cm-1 which were assigned to Fuchs-Kliewer phonon, N-H bending, and N-H stretching vibrations, respectively. Assignments were confirmed by observation of isotopic shifts following atomic deuterium dosing. No In-H species were observed indicating N-termination of the surface and N-polarity of the film. Broad conduction band plasmon excitations were observed centered at 3100 cm-1 to 4200 cm-1 in HREEL spectra acquired with 25 eV electrons, for a variety of samples grown with different conditions. Infrared reflectance data shows a consistent result with HREELS for the bulk plasma frequency. The plasmon excitations are shifted about 300 cm-1 higher in HREEL spectra acquired using 7 eV electrons due to the higher plasma frequency and carrier concentration at the surface than in the bulk, demonstrating a surface electron accumulation. Hydrogen completely desorbed from the InN surface upon annealing for 900 s at 425 ºC or upon annealing for 30 s at 500 ºC. Fitting the coverage versus temperature for anneals of either 30 or 900 s indicated that the desorption was best described by second order desorption kinetics with an activation energy and pre-exponential factor of 1.3±0.2 eV and 10-7.3±1.0 cm2/s, respectively. Vibrational spectra acquired from HREEL can be utilized to explain the surface composition, chemical bonding and surface termination, and film polarity of InN layers. The explanation of evidence of surface electron accumulation and extraction of hydrogen desorption kinetic parameters can be performed by utilizing HREEL spectra

Mechanistic Studies of Reducible Metal Oxides as Hydrodeoxygenation Catalysts

Hydrodeoxygenation of phenol to benzene using ruthenium supported titania catalysts strongly varies depending on the support crystal structure and preparation conditions. Here, we performed spectroscopic characterization of titania supports to identify the surface impurities common to commercial and synthesized titania samples using a variety of spectroscopic methods. Sulfate impurities were detected for the commercial anatase samples and a procedure for their elimination was proposed so that inactive catalysts gained reactivity. Surface hydroxyls of different TiO2 samples (anatase, rutile, and pyrogenic) were identified using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments performed on vigorously cleaned surfaces and a facet-specific assignment was proposed using DFT calculations performed by our collaborators. In addition, the electronic structure of TiO2 samples were studied using the reaction of vigorously cleaned TiO2 samples with H2/D2. Our results revealed that sulfate impurities of the commercial anatase samples change their electronic structure consistent with creation of deep electronic trap states within the band gap. Our results are used to derive structure-activity relationships for the Ru/titania catalyzed hydrodeoxygenation reactions of phenol

Indium Nitride Surface Structure, Desorption Kinetics and Thermal Stability

Unique physical properties such as small effective mass, high electron drift velocities, high electron mobility and small band gap energy make InN a candidate for applications in high-speed microelectronic and optoelectronic devices. The aim of this research is to understand the surface properties, desorption kinetics and thermal stability of InN epilayers that affect the growth processes and determine film quality as well as device performance and life time. We have investigated the structural properties, the surface desorption kinetics, and the thermal stability using Auger electron spectroscopy (AES), x-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), high resolution electron energy loss spectroscopy (HREELS), and temperature programmed desorption (TPD). Investigations on high pressure chemical vapor deposition (HPCVD)-grown InN samples revealed the presence of tilted crystallites, which were attributed to high group V/III flux ratio and lattice mismatch. A study of the thermal stability of HPCVD-grown InN epilayers revealed that the activation energy for nitrogen desorption was 1.6±0.2 eV, independent of the group V/III flux ratio. Initial investigations on the ternary alloy In0.96Ga0.04N showed single-phase, N-polar epilayers using XRD and HREELS, while a thermal desorption study revealed an activation energy for nitrogen desorption of 1.14 ± 0.06 eV. HREELS investigations of atomic layer epitaxy (ALE)-grown InN revealed vibrational modes assigned to N-N vibrations. The atomic hydrogen cleaned InN surface also exhibited modes assigned to surface N-H without showing In-H species, which indicated N-polar InN. Complete desorption of hydrogen from the InN surface was best described by the first-order desorption kinetics with an activation energy of 0.88 ± 0.06 eV and pre-exponential factor of (1.5 ± 0.5) ×105 s-1. Overall, we have used a number of techniques to characterize the structure, surface bonding configuration, thermal stability and hydrogen desorption kinetics of InN and In0.96Ga0.04N epilayers grown by HPCVD and ALE. High group V/III precursors ratio and lattice mismatch have a crucial influence on the film orientation. The effects of hydrogen on the decomposition add to the wide variation in the activation energy of nitrogen desorption. Presence of surface defects lowers the activation energy for hydrogen desorption from the surface

Mechanistic Insights into Elevated Temperature Photocatalytic Processes via Combined Experimental and Theoretical Approach

Developing new catalytic technologies that allow for the production of fuels and building-block chemicals from CO2 and H2O photocatalytically is one of the greatest challenges of the 21st century. H2O and CO2 are difficult-to-activate molecules and their successful reduction is potentially kinetically limited. The studies presented herein are aimed at providing a detailed understanding of the reaction such that photocatalysts may be rationally selected and optimized. Our efforts also aim at developing photocatalysts and reactor design that allow utilization of full solar spectrum (UV-vis and IR), which could greatly enhance the overall quantum efficiency of the system. Elevated temperature is commonly avoided in photocatalysis due to the perception that high temperature leads to rapid exciton recombination and loss of quantum efficiency. Instead, enhanced catalytic activity is encountered in our study at autogenous temperatures (350°C+) produced through IR heating by employing a concentrated solar photoreaction (CSPR) approach.As we explore the photocatalytic synthesis of complex molecules, reaction mechanisms will involve more complex surface-bound intermediates of varying electronic character. This study highlights the interface between the classical electrochemical understanding of photocatalytic reactions where highly destabilized reaction intermediates are common and photocatalytic synthesis reactions where vibrational barriers may be contributing to active reaction pathways. Understanding the stability and electronic nature of these species on the catalyst surface may be crucial in dictating catalyst performance and selectivity. Our results have shed light onto several chemical and physical phenomena at the mechanistic level that drive elementary reaction steps on the surface. Some insights include the isolation of new H-transfer mechanisms and variation in surface chemistry under a range of experimental environments (variable temperature, variable chemical potential of reactants, kinetic isotope effect, etc.). Results indicate that photocatalyst with a high Debye temperature, robust bulk bonding, and elevated surface chemical reactivity could directly promote new reaction intermediates produced via thermal/vibrational routes that further enhance the selectivity towards hydrogenation. Identifying and understanding the effect of the stability of atomic H and how it is energetically driven through the reaction mechanism could dramatically enhance our ability to control selectivity in photocatalytic reaction mechanisms

Structure and Adsorption at the Bastnäsite-Water Interface: Fundamental Investigations toward Rare Earth Mineral Recovery

This dissertation investigates the interfacial structure and reactivity of a rare earth mineral in the context of froth flotation. Bastnäsite [(Ce,La,Nd)FCO3], one of the primary mineral sources of rare earth elements, has been chosen for this investigation. Flotation separation relies on selective adsorption of collector ligands to the desired mineral surface in solution; fundamental understanding of these adsorption reactions will aid in the development of more effective separation technologies. Chapter 1 presents an introduction to the significance of rare earth minerals and the process of froth flotation. Chapters 2 and 3 address the adsorption reactions of ligand molecules at the interface. Chapter 2 analyzes the adsorption mechanism of octanohydroxamic acid, a popular candidate for bastnäsite flotation. In-situ FTIR reveals mechanistic information that demystifies the quantitative results of the adsorption isotherm. Chapter 3 compares several flotation ligands to determine the effect of ligand structure on the mechanism of adsorption. In Chapter 4, the structure of the bastnäsite (001) surface is investigated under varying conditions using X-Ray reflectivity. The surface termination is significant because it determines which sites are available for ligand adsorption in the flotation system. Changes in the surface structure are evaluated by fitting a model to a set of crystal truncation rods measured at this interface. This work provides fundamental information about the aqueous geochemistry of bastnäsite flotation on both sides of the interface

Fundamental Controls on the Reactivity of Aluminum Oxide and Hydroxide Surfaces: Contributions of Surface Site Coordination States and Interfacial Water Structure

Chemical reactions at mineral-water interfaces are of great importance in many geological and environmental processes. Essential to many of these is adsorption because it directly controls contaminant fate and nutrient availability, promotes the nucleation and growth of minerals, initiates surface redox reactions, and plays a crucial role in carbon cycling and sequestration. These reactions occur at mineral surface sites having multiple possible coordination states that interact with both adsorbates and water. While general ion adsorption mechanisms and surface charging behaviors are well established, the roles of individual surface functional group types and water in affecting the structure and reactivity of the interfacial region have not been systematically investigated. Previous studies suggest that surface functional groups in different coordination environments differ in their charge states, proton affinities, and kinetics of oxygen exchange with water. This indicates that these groups may also have different reactivities toward adsorbates and may induce distinct structural arrangements of water near mineral surfaces. This latter role for charged surface groups further suggests that adsorbates, which act as new charged surface sites, potentially may alter the structure of water near surfaces. Such restructuring of interfacial water may contribute to the energetics of many important reactions at environmental interfaces, but this remains unclear currently. Thus, a direct relationship between surface reactions (involving various surface functional groups and adsorbates) and interfacial water properties needs to be established. Aluminum (hydr)oxides, as important reactive and widespread minerals in nature and in engineered systems, play significant roles in many geological and environmental processes. Surfaces of these minerals are especially important due to their ability to control the degradation and transformation of contaminants in soils and sediments and affect the composition of natural waters and geochemical element cycling. Gibbsite, the most common form of aluminum hydroxide found in nature, and its rarer polymorph bayerite, display substantially different morphologies dominated by distinct crystallographic planes. Corundum, as the only thermodynamically stable form of aluminum oxide, has been widely investigated as a proxy for aluminum hydroxide mineral surfaces and surfaces of other phases, such as the edges of Al-rich smectites and the aluminol surface of kaolinite. This study is focused on these minerals because they can serve as model analogues for understanding the surface reactivity of other naturally abundant Al-bearing minerals in soils and sediments due to the similarity in functional groups exposed on their surfaces and for studying fundamental geochemical reactions at interfaces. The main objective of this dissertation is to determine how surface site coordination states on aluminum (hydr)oxide mineral surfaces affect ion adsorption mechanisms, interfacial water structure, and the feedback between these. Arsenate is employed as the probe adsorbate because of its environmental relevance and its uptake over a wide pH range. This research specifically seeks to (1) identify the effects of surface site coordination on macroscopic arsenate adsorption and its binding mechanisms on synthetic gibbsite and bayerite particles; (2) determine how ionic strength affects arsenate adsorption on gibbsite and bayerite; (3) characterize the response of interfacial water structure to pH variations and arsenate adsorption on corundum (001) surfaces; and (4) compare the response of interfacial water structure to pH variations and arsenate adsorption on corundum (012) and (001) surfaces. Synthetic gibbsite and bayerite have distinct particle morphologies, exposing different types of functional groups. Gibbsite platelets expose large (001) basal surfaces terminated predominantly by \u3eAl2O groups, whereas bayerite microrods display mainly edge surfaces dominated by \u3eAlO groups. Macroscopic adsorption isotherms at a single ionic strength show that gibbsite adsorbs less arsenate per unit surface area than bayerite at pH 4 and 7 and suggest that two surface complexes form on each mineral. Arsenate adsorption decreases with increasing ionic strength on both minerals, with a larger effect at pH 4 than pH 7. The observed pH-dependence corresponds with a substantial decrease in surface charge, as indicated by _-potential measurements. At a single ionic strength, similar electrokinetic behavior is observed at the same relative coverages of arsenate, suggesting that similar reactive surface groups (\u3eAlO) control surface charging on both minerals. Extended X-ray absorption fine structure (EXAFS) spectroscopy shows no variation in arsenate surface speciation on a given mineral with different surface coverage, pH, and ionic strength. While bidentate binuclear inner-sphere complexes are the dominant surface species present, EXAFS results find that the number of second shell Al neighbors around arsenate is lower than required for this adsorbate to occur solely as an inner-sphere complex, suggesting that outer-sphere species also occur on both minerals, in greater abundance on gibbsite. Together, these observations reveal that arsenate adsorption mechanisms and capacities vary with mineral morphologies because of the distribution of distinct surface functional groups. These also demonstrate that arsenate displays macroscopic and spectroscopic behavior consistent with the coexistence of inner- and outer-sphere surface complexes. This dissertation also investigated interfacial water structure near single crystal corundum surfaces. Surface X-ray scattering measurements show that corundum (001) surfaces induce weak spatial ordering of interfacial water that varies little between pH 5 and 9 but is substantially altered by the adsorption of arsenate. In the absence of arsenate, interfacial water ordering near the (012) surfaces is also largely unaffected by pH. This general invariance observed on both surfaces suggest that over the pH range of most natural waters, surface site protonation-deprotonation appears inadequate to induce extensive restructuring of interfacial water. The adsorption of arsenate weakly perturbs interfacial water structure near the (012) surface, in contrast to the substantial restructuring of interfacial water seen near the (001) surface. Arsenate is observed to form coexisting inner- and outer-sphere surface complexes on both surfaces, suggesting that adsorption mechanisms may not control the resulting restructuring of interfacial water. Instead, the different surface functional groups present on the (001) and (012) surfaces, with their distinct charging behaviors, likely drive the response of interfacial water to arsenate adsorption. This study improves our understanding of the fundamental controls of chemical reactions at environmental interfaces through systematic studies of arsenate adsorption on aluminum hydroxide particles and aluminum oxide single crystals. Arsenate adsorption on aluminum hydroxide surfaces is complicated, with different types of surface complexes forming through reactions at multiple types of functional groups of different reactivities. The complex interactions between arsenate and aluminum hydroxides can be extended to systems with other naturally abundant Al/Fe-bearing minerals and this must be considered when predicting arsenate fate at environmental interfaces. The dynamic response of interfacial water structure to adsorbates observed on different corundum surfaces here suggests a relationship among surface functional group coordination states, ion adsorption mechanisms, and interfacial water structure. Such adsorbate-induced restructuring of interfacial water indicates that water structure plays an important role in the energetics of interfacial reactions. This study provides new insight into the roles of surface functional group coordination states and interfacial water restructuring in chemical reactions at mineral-water interfaces

Infrared spectroscopy of small-molecule endofullerenes

Hydrogen is one of the few molecules which has been incarcerated in the molecular cage of C$_{60}$ and forms endohedral supramolecular complex H$_2$@C$_{60}$. In this confinement hydrogen acquires new properties. Its translational motion becomes quantized and is correlated with its rotations. We applied infrared spectroscopy to study the dynamics of hydrogen isotopologs H$_2$, D$_2$ and HD incarcerated in C$_{60}$. The translational and rotational modes appear as side bands to the hydrogen vibrational mode in the mid infrared part of the absorption spectrum. Because of the large mass difference of hydrogen and C$_{60}$ and the high symmetry of C$_{60}$ the problem is identical to a problem of a vibrating rotor moving in a three-dimensional spherical potential. The translational motion within the C$_{60}$ cavity breaks the inversion symmetry and induces optical activity of H$_2$. We derive potential, rotational, vibrational and dipole moment parameters from the analysis of the infrared absorption spectra. Our results were used to derive the parameters of a pairwise additive five-dimensional potential energy surface for H$_2$@C$_{60}$. The same parameters were used to predict H$_2$ energies inside C$_{70}$[Xu et al., J. Chem. Phys., {\bf 130}, 224306 (2009)]. We compare the predicted energies and the low temperature infrared absorption spectra of H$_2$@C$_{70}$.Comment: Updated author lis