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Radiation Tolerant Interfaces: Influence of Local Stoichiometry at the Misfit Dislocation on Radiation Damage Resistance of Metal/Oxide Interfaces
To understand how variations in interface properties such as misfit-dislocation density and local chemistry affect radiation-induced defect absorption and recombination, we have explored a model system of CrxV1-x alloy epitaxial films deposited on MgO single crystals. By controlling film composition, the lattice mismatch with MgO was adjusted so that the misfit-dislocation density varies at the interface. These interfaces were exposed to irradiation and in situ results show that the film with a semi-coherent interface (Cr) withstands irradiation while V film, which has similar semi-coherent interface like Cr, showed the largest damage. Theoretical calculations indicate that, unlike at metal/metal interfaces, the misfit dislocation density does not dominate radiation damage tolerance at metal/oxide interfaces. Rather, the stoichiometry, and the precise location of the misfit-dislocation density relative to the interface, drives defect behavior. Together, these results demonstrate the sensitivity of defect recombination to interfacial chemistry and provide new avenues for engineering radiation-tolerant nanomaterials.
Role of the interface on radiation damage in the SrTiO{sub 3}/LaAlO{sub 3} heterostructure under Ne{sup 2+} ion irradiation
We systematically investigated the microstructural evolution of heteroepitaxial SrTiO{sub 3} (STO) thin films grown on a single crystal LaAlO{sub 3} (LAO) (001) substrate, focusing on the response of the STO/LAO interface to Ne{sup 2+} irradiation at room temperature. Cross sectional transmission electron microscope (TEM) analysis reveals that the LAO crystal amorphizes first after a relatively low dose of damage followed by the amorphization of the STO film after irradiation to a higher dose. While the critical dose to amorphize differs between each material, amorphization begins at the interface and proceeds outward in both cases. Thus, a crystalline/amorphous interface first forms at the STO/LAO interface by a dose of 1 dpa, and then an amorphous/amorphous interface forms when the dose reaches 3 dpa. Scanning TEM and x-ray energy dispersive spectroscopy indicate no significant heavy cation elemental diffusion, though electron energy loss spectroscopy reveals a redistribution of oxygen across the film/substrate interface after Ne{sup 2+} irradiation. Atomistic calculations are used to interpret the experimental findings in terms of the defect properties in each of the two component phases.
Interface-driven spin-torque ferromagnetic resonance by Rashba coupling at the interface between nonmagnetic materials
The Rashba-Edelstein effect stems from the interaction between the electron's spin and its momentum induced by spin-orbit interaction at an interface or a surface. It was shown that the inverse Rashba-Edelstein effect can be used to convert a spin current into a charge current. Here, we demonstrate the reverse process of a charge-to spin-current conversion at a Bi/Ag Rashba interface. We show that this interface-driven spin current can drive an adjacent ferromagnet to resonance. We employ a spin-torque ferromagnetic resonance excitation/detection scheme which was developed originally for a bulk spin-orbital effect, the spin Hall effect. In our experiment, the direct Rashba-Edelstein effect generates an oscillating spin current from an alternating charge current driving the magnetization precession in a neighboring permalloy (Py, Ni 80 Fe 20 ) layer. As a result, electrical detection of the magnetization dynamics is achieved by a rectificationmechanism of the time dependent multilayer resistance arising from the anisotropic magnetoresistance.
Atomically Abrupt Liquid-Oxide Interface Stabilized by Self-Regulated Interfacial Defects: The Case of Al/Al2O3 Interfaces
The atomic and electronic structures of the liquid Al/(0001) {alpha}-Al{sub 2}O{sub 3} interfaces are investigated by first-principles molecular dynamics simulations. Surprisingly, the formed liquid-solid interface is always atomically abrupt and is characterized by a transitional Al layer that contains a fixed concentration of Al vacancies ({approx}10 at.%). We find that the self-regulation of the defect density in the metal layer is due to the fact that the formation energy of the Al vacancies is readjusted in a way that opposes changes in the defect density. The negative-feedback effect stabilizes the defected transitional layer and maintains the atomic abruptness at the interface. The proposed mechanism is generally applicable to other liquid-metal/metal-oxide systems, and thus of significant importance in understanding the interface structures at high temperature.
Control of Interface Order by Inverse Quasi-Epitaxial Growth of Squaraine/Fullerene Thin Film Photovoltaics
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Graded orbital occupation near interfaces in a La[subscript 2]NiO[subscript 4]-La[subscript 2]CuO[subscript 4] superlattice
X-ray absorption spectroscopy and resonant soft x-ray reflectivity show a nonuniform distribution of oxygen holes in a La{sub 2}NiO{sub 4}-La{sub 2}CuO{sub 4} (LNO-LCO) superlattice, with excess holes concentrated in the LNO layers. Weak ferromagnetism with T{sub c} = 160 K suggests a coordinated tilting of NiO{sub 6} octahedra, similar to that of bulk LNO. Ni d{sub 3z{sup 2}-r{sup 2}} orbitals within the LNO layers have a spatially variable occupation. This variation of the Ni valence near LNO-LCO interfaces is observed with resonant soft x-ray reflectivity at the Ni and Cu L edges, at a reflection suppressed by the symmetry of the structure, and is possible through graded doping with holes, due to oxygen interstitials taken up preferentially by inner LNO layers. Since the density of oxygen atoms in the structure can be smoothly varied with standard procedures, this orbital occupation, robust up to at least 280 K, is tunable.
Acoustic impedance and interface phonon scattering in Bi 2 Te 3 and other semiconducting materials
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Reducing interface recombination for Cu(In,Ga)Se 2 by atomic layer deposited buffer layers
Partial CuInGaSe 2 (CIGS) solar cell stacks with different atomic layer deposited buffer layers and pretreatments were analyzed by photoluminescence (PL) and capacitance voltage (CV) measurements to investigate the buffer layer/CIGS interface. Atomic layer deposited ZnS, ZnO, and SnO x buffer layers were compared with chemical bath deposited CdS buffer layers. Band bending, charge density, and interface state density were extracted from the CV measurement using an analysis technique new to CIGS. The surface recombination velocity calculated from the density of interface traps for a ZnS/CIGS stack shows a remarkably low value of 810 cm/s, approaching the range of single crystalline II–VI systems. Both the PL spectra and its lifetime depend on the buffer layer; thus, these measurements are not only sensitive to the absorber but also to the absorber/buffer layer system. Pretreatment of the CIGS prior to the buffer layer deposition plays a significant role on the electrical properties for the same buffer layer/CIGS stack, further illuminating the importance of good interface formation. Lastly, ZnS is found to be the best performing buffer layer in this study, especially if the CIGS surface is pretreated with potassium cyanide.
Three-Dimensional Quantitative Chemical Roughness Of Buried ZrO2/In2O3 Interfaces Via Energy-Filtered Electron Tomography
The protocol to calculate the chemical roughness from three-dimensional (3-D) data cube acquired by energy-filtered electron tomography has been developed and applied to analyze the 3-D Zr distribution at the arbitrarily shaped interfaces in the ZrO2/In2O3 multilayer films. The calculated root-mean-square roughness quantitatively revealed the chemical roughness at the buried ZrO2/In2O3 interfaces, which is the deviation of Zr distribution from the ideal flat interface. Knowledge of the chemistry and structure of oxide interfaces in 3-D provides information useful for understanding changes in the behavior of a model ZrO2/In2O3 heterostructure that has potential to exhibit mixed conduction behavior. VC 2012 American Institute of Physics.
Potential-specific structure at the hematite-electrolyte interface
The atomic-scale structure of interfaces between metal oxides and aqueous electrolytes controls their catalytic, geochemical, and corrosion behavior. Measurements that probe these interfaces in situ provide important details of ion and solvent arrangements, but atomically precise structural models do not exist for common oxide-electrolyte interfaces far from equilibrium. Using a novel cell, we measured the structure of the hematite (a-Fe 2 O 3 ) (110$\bar{2}$)-electrolyte interface under controlled electrochemical bias using synchrotron crystal truncation rod X ray scattering. At increasingly cathodic potentials, charge-compensating protonation of surface oxygen groups increases the coverage of specifically bound water while adjacent water layers displace outwardly and became disordered. Returning to open circuit potential leaves the surface in a persistent metastable protonation state. The flux of current and ions at applied potential is thus regulated by a unique interfacial electrolyte environment, suggesting that electrical double layer models should be adapted to the dynamically changing interfacial structure far from equilibrium.
Engineering Interface Structures and Thermal Stabilities via SPD Processing in Bulk Nanostructured Metals
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Interface-Driven Plasticity: The Presence of an Interface Affected Zone in Metallic Lamellar Composites: The Presence of an Interface Affected Zone…
Large strain deformation is used to make fine nanolayered two-phase composites from stacks of conventional polycrystalline sheets. The final materials made by this technique possess a crystallographically highly oriented structure containing nearly atomically perfect interfaces prevailing ubiquitously throughout the material. How this ordered structure evolves with strain from the coarser, more disordered state is not known. Here, using microstructural analysis and computational modeling, we discover a local interface-affected zone (IAZ) possessing the same crystallographically sharp structure in coarse layered composites as the final nanolayered composites. This means that this strongly oriented interface “zone” survives the mechanical work and overtakes the structure as it refines to the nanoscale. In essence, through the formation of this interface zone, the crossover to a highly oriented nanostructure occurs. Using microstructural analysis and crystal plasticity simulations, we show that the IAZ is a consequence of slip accommodation at the interface. This insight is valuable for developing processing strategies for superior interface-dominated materials.
Creation And Ordering Of Oxygen Vacancies At WO3-d And Perovskite Interfaces
Changes in structure and composition resulting from oxygen deficiency can strongly impact the physical and chemical properties of transition metal oxides. However, it is extremely difficult to accurately characterize the oxygen deficiency levels and pinpoint how these materials properties change in response. In this paper, the effects of substrate strain and oxidizing power on the creation and distribution of oxygen defects in WO3 thin films are investigated in detail. An 18O2 isotope labeled time-of-flight secondary ion mass spectrometry (ToF-SIMS) study reveals that WO3-films grown on STO display a significantly larger oxygen vacancy gradient along the growth direction. This result is corroborated by scanning transmission electron microscopy (STEM) images which reveal a large number of defects present close to the interface to accommodate interfacial strain, leading to ordering of oxygen vacancies and formation of semi-aligned Magnéli phases. The strain is gradually released and tetragonal phase with much better crystallinity is observed at the film/vacuum interface. The changes in structure resulting from oxygen defect creation are shown to have a direct impact on the electronic and optical properties of the films.
Defect interactions with stepped CeO{sub 2}/SrTiO{sub 3} interfaces: Implications for radiation damage evolution and fast ion conduction
Due to reduced dimensions and increased interfacial content, nanocomposite oxides offer improved functionalities in a wide variety of advanced technological applications, including their potential use as radiation tolerant materials. To better understand the role of interface structures in influencing the radiation damage tolerance of oxides, we have conducted atomistic calculations to elucidate the behavior of radiation-induced point defects (vacancies and interstitials) at interface steps in a model CeO{sub 2}/SrTiO{sub 3} system. We find that atomic-scale steps at the interface have substantial influence on the defect behavior, which ultimately dictate the material performance in hostile irradiation environments. Distinctive steps react dissimilarly to cation and anion defects, effectively becoming biased sinks for different types of defects. Steps also attract cation interstitials, leaving behind an excess of immobile vacancies. Further, defects introduce significant structural and chemical distortions primarily at the steps. These two factors are plausible origins for the enhanced amorphization at steps seen in our recent experiments. The present work indicates that comprehensive examination of the interaction of radiation-induced point defects with the atomic-scale topology and defect structure of heterointerfaces is essential to evaluate the radiation tolerance of nanocomposites. Finally, our results have implications for other applications, such as fast ion conduction.
Dynamic Stabilization of Metal Oxide–Water Interfaces
The interaction of water with metal oxide surfaces plays a crucial role in the catalytic and geochemical behavior of metal oxides. In a vast majority of studies, the interfacial structure is assumed to arise from a relatively static lowest energy configuration of atoms, even at room temperature. Using hematite (α-Fe 2 O 3 ) as a model oxide, we show through a direct comparison of in situ synchrotron X-ray scattering with density functional theory-based molecular dynamics simulations that the structure of the ($1\bar{10}2 $) termination is dynamically stabilized by picosecond water exchange. Simulations show frequent exchanges between terminal aquo groups and adsorbed water in locations and with partial residence times consistent with experimentally determined atomic sites and fractional occupancies. Frequent water exchange occurs even for an ultrathin adsorbed water film persisting on the surface under a dry atmosphere. The resulting time-averaged interfacial structure consists of a ridged lateral arrangement of adsorbed water molecules hydrogen bonded to terminal aquo groups. Surface pK a prediction based on bond valence analysis suggests that water exchange will influence the proton-transfer reactions underlying the acid/base reactivity at the interface. Our findings provide important new insights for understanding complex interfacial chemical processes at metal oxide–water interfaces.
Charge-carrier dynamics in polycrystalline thin-film CuIn{sub 1−x}Ga{sub x}Se{sub 2} photovoltaic devices after pulsed laser excitation: Interface and space-charge region analysis
We used time-resolved photoluminescence (TRPL) spectroscopy to analyze time-domain and spectral-domain charge-carrier dynamics in CuIn{sub 1−x}Ga{sub x}Se{sub 2} (CIGS) photovoltaic (PV) devices. This new approach allowed detailed characterization for the CIGS/CdS buffer interface and for the space-charge region. We find that dynamics at the interface is dominated by diffusion, where the diffusion rate is several times greater than the thermionic emission or interface recombination rate. In the space-charge region, the electric field of the pn junction has the largest effect on the carrier dynamics. Based on the minority-carrier (electron) drift-rate dependence on the electric field strength, we estimated drift mobility in compensated CuIn{sub 1−x}Ga{sub x}Se{sub 2} (with x ≈ 0.3) as 22 ± 2 cm{sup 2}(Vs){sup −1}. Analysis developed in this study could be applied to evaluate interface and junction properties of PV and other electronic devices. For CIGS PV devices, TRPL spectroscopy could contribute to understanding effects due to absorber compositional grading, which is one of the focus areas in developing record-efficiency CIGS solar cells.