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At least 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Effect of top electrode material on radiation-induced degradation of ferroelectric thin film structures

The effects of gamma irradiation on the dielectric and piezoelectric responses of Pb[Zr{sub 0.52}Ti{sub 0.48}]O{sub 3} (PZT) thin film stacks were investigated for structures with conductive oxide (IrO{sub 2}) and metallic (Pt) top electrodes. The samples showed, generally, degradation of various key dielectric, ferroelectric, and electromechanical responses when exposed to 2.5 Mrad (Si) {sup 60}Co gamma radiation. However, the low-field, relative dielectric permittivity, ε{sub r}, remained largely unaffected by irradiation in samples with both types of electrodes. Samples with Pt top electrodes showed substantial degradation of the remanent polarization and overall piezoelectric response, as well as pinching of the polarization hysteresis curves and creation of multiple peaks in the permittivity-electric field curves post irradiation. The samples with oxide electrodes, however, were largely impervious to the same radiation dose, with less than 5% change in any of the functional characteristics. The results suggest a radiation-induced change in the defect population or defect energy in PZT with metallic top electrodes, which substantially affects motion of internal interfaces such as domain walls. Additionally, the differences observed for stacks with different electrode materials implicate the ferroelectric–electrode interface as either the predominant source of radiation-induced effects (Pt electrodes) or the site of healing for radiation-induced defects (IrO{sub 2} electrodes).

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Injection of oxygen vacancies in the bulk lattice of layered cathodes

Surfaces, interfaces, and grain boundaries are, classically, known to be sinks at which defects generated within bulk lattice will preferentially migrate to and get annihilated, therefore rejuvenating a crystalline lattice to restore the intended functionalities.1, 2, 3 Here we report an inverse case by which the defects generated at the particle surface are continuously pumped into the bulk lattice and consequentially compromising the intended functionalities. We discovered that during the operation of a rechargeable battery, oxygen vacancies generated at cathode particle surfaces migrate toward inside of a perfect lattice in lithium-rich layered cathodes, featuring electrochemical driven surface vacancy creation, injection and condensation phenomenon and revealing an unrecognized surface process induced bulk lattice degradation featured with void formation and lattice reorganization. This process is associated with a high-cutoff voltage at which the anionic redox is activated. Coupling with calculations, we substantiate that triggering of anionic redox leads to sharply lowering of both formation energy of oxygen vacancy and migration barrier of oxidized oxide ions, therefore enabling migration of oxygen vacancy via oxidized oxide ion and leading to the injection of oxygen vacancy generated at the particle surface into the bulk lattice. This work unveils unpredicted behavior of oxidized oxide ion, leading to essential insight for designing of high capacity cathode for better batteries through coupled cationic and anionic redox dynamics in layered cathodes. In a broad term, this work provides insight on the intimate coupling between interfacial process and bulk lattice behavior for governing electrode functionality.

Yan, Pengfei↗
DOE OSTI2026

Reducing Non-Radiative Recombination in Perovskite Solar Cells with a SiO2-Graphene Oxide Buried Interface Layer

Perovskite solar cells (PSCs) have emerged as a transformative photovoltaic technology. However, the device performance of inverted PSCs is dramatically limited by surface defects and the underlying film morphology of metal halide perovskite (MHP) thin film. Herein, we report a novel buried interface layer consisting of SiO2 nanoparticles (NPs) chemically bonded to graphene oxide (GO), deposited atop a hydrophobic hole extraction layer (HEL) to address this limitation. The SiO2 NPs serve as insoluble scaffolds that immobilize GO sheets on the hydrophobic HEL surface, enabling the formation of homogeneous MHP thin films. Meanwhile, the GO functional groups interact with surface-uncoordinated Pb2+, suppressing interfacial defects and guiding crystal grain growth of the resultant MHP thin films. This synergistic effect suppresses non-radiative charge recombination and enhances charge extraction efficiency. As a result, a 26.07% efficiency with significantly suppressed photocurrent hysteresis is observed from PSCs incorporating the SiO2-GO buried interface layer. Moreover, the above PSCs maintain 92% of their initial PCE after 1800 h of continuous operation at the maximum power point under AM 1.5 G (100 mW/cm2) illumination at 25 degrees C in air with 50-60% relative humidity. These results demonstrate that we have developed a facile and effective way to realize high-performance inverted PSCs.

14 SOLAR ENERGY↗
DOE OSTI2022

Effect of differently oriented interlayer phases on the radiation damage of Inconel-$\mathrm{Ni}$ multimetallic layered composite

Multimetallic layered composites (MMLCs) have shown an excellent potential for application under extreme environments, e.g., accident-tolerant fuel cladding, because of their low oxidation tendency and high corrosion resistance. Interfacial phases or complexions in nanocrystalline materials accelerate the annihilation of defects and enhance the radiation resistance of materials, making MMLCs with engineered interlayer phases compelling to deploy in extreme conditions. However, implementation of MMLCs in full capacity remained a challenge due to a lack of fundamental understanding of the underlying mechanisms governing the characteristics of the interface between the metallic layers. The precise role of interlayer phases in MMLCs and their interaction with defects, specifically under extreme conditions, is still unexplored. Pursuing atomistic simulations for various Inconel-Ni MMLCs model materials, we revealed accelerated defect mobility in interlayers with larger crystalline misorientation and the inverse relationship between the interface sink strength to the misorientation angle. Furthermore, we found a linear relation between interlayer misorientation angle with the density of radiation-induced defects and radiation enhanced displacements. So, our results indicate that radiation-induced material degradation is accelerated by the higher defect formation tendency of MMLCs with a high-angle interlayer interface.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2014

Lowering the density of electronic defects on organic-functionalized Si(100) surfaces

The electrical quality of functionalized, oxide-free silicon surfaces is critical for chemical sensing, photovoltaics, and molecular electronics applications. In contrast to Si/SiO{sub 2} interfaces, the density of interface states (D{sub it}) cannot be reduced by high temperature annealing because organic layers decompose above 300 °C. While a reasonable D{sub it} is achieved on functionalized atomically flat Si(111) surfaces, it has been challenging to develop successful chemical treatments for the technologically relevant Si(100) surfaces. We demonstrate here that recent advances in the chemical preparation of quasi-atomically-flat, H-terminated Si(100) surfaces lead to a marked suppression of electronic states of functionalized surfaces. Using a non-invasive conductance-voltage method to study functionalized Si(100) surfaces with varying roughness, a D{sub it} as low as 2.5 × 10{sup 11} cm{sup −2}eV{sup −1} is obtained for the quasi-atomically-flat surfaces, in contrast to >7 × 10{sup 11} cm{sup −2}eV{sup −1} on atomically rough Si(100) surfaces. The interfacial quality of the organic/quasi-atomically-flat Si(100) interface is very close to that obtained on organic/atomically flat Si(111) surfaces, opening the door to applications previously thought to be restricted to Si(111)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

In-operando hard X-ray photoelectron spectroscopy study on the impact of current compliance and switching cycles on oxygen and carbon defects in resistive switching Ti/HfO{sub 2}/TiN cells

In this study, direct experimental materials science evidence of the important theoretical prediction for resistive random access memory (RRAM) technologies that a critical amount of oxygen vacancies is needed to establish stable resistive switching in metal-oxide-metal samples is presented. In detail, a novel in-operando hard X-ray photoelectron spectroscopy technique is applied to non-destructively investigates the influence of the current compliance and direct current voltage sweep cycles on the Ti/HfO{sub 2} interface chemistry and physics of resistive switching Ti/HfO{sub 2}/TiN cells. These studies indeed confirm that current compliance is a critical parameter to control the amount of oxygen vacancies in the conducting filaments in the oxide layer during the RRAM cell operation to achieve stable switching. Furthermore, clear carbon segregation towards the Ti/HfO{sub 2} interface under electrical stress is visible. Since carbon impurities impact the oxygen vacancy defect population under resistive switching, this dynamic carbon segregation to the Ti/HfO{sub 2} interface is suspected to negatively influence RRAM device endurance. Therefore, these results indicate that the RRAM materials engineering needs to include all impurities in the dielectric layer in order to achieve reliable device performance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Controlling magnetism and transport at perovskite cobaltite interfaces via strain-tuned oxygen vacancy ordering

Complex oxides such as perovskite cobaltites exhibit rich phenomena at interfaces due to the complex interplay between their structural, defect, electronic, and magnetic degrees of freedom. Here, we study this interplay here in the ferromagnetic metallic cobaltite La 1−𝑥 ⁢Sr 𝑥⁢ CoO 3−𝛿 , using specific substrates to vary both the heteroepitaxial strain (compressive vs tensile) and growth orientation ((001) vs (110)). Transmission electron microscopy, electron energy-loss spectroscopy, high-resolution X-ray diffraction, magnetometry, polarized neutron reflectometry, and electronic magnetotransport measurements are applied. Lattice mismatch and growth orientation are found to enable the precise control of interfacial oxygen vacancy ordering in La 1−𝑥⁢ Sr 𝑥 ⁢CoO 3−𝛿 , thus dictating strain relaxation and oxygen vacancy depth profiles, in turn controlling thickness-dependent magnetic and electronic properties. In particular, compressive strain and (110) orientations minimize deleterious magnetic/electronic dead layer effects, leading to the optimization of interfacial magnetism and transport. Strain and orientation tuning of oxygen vacancy ordering are thus established as powerful means to control physical properties at cobaltite-based interfaces, relevant to several fields.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Variation in Cation Adsorption Mechanism Controlled by Chemical and Structural Heterogeneities at the Quartz (101)–Water Interface

Mineral–water interfacial reactions are central to chemical processes that control the fate of nutrients and contaminants in natural environments. Mineral surfaces commonly have complex structures and compositions whose impact on interfacial reactivity is poorly understood. Here, in this work, we investigated the effects of surface heterogeneities on Rb + sorption at the quartz (101)–10 mM RbCl solution interface at pH 9.8 using in situ high-resolution X-ray reflectivity. Two surface locales (i.e., Spots A and B) having distinct interfacial structures were chosen: Spot A was characterized by its low defect density (≤20% topmost Si vacancies) and Rb + adsorption occurred predominantly as an inner-sphere complex. In comparison, Spot B had a higher defect density (~50% vacancies) and was covered with poorly crystalline SiO 2 . A substantially larger Rb + uptake (i.e., 7-times higher coverage) was observed on this defective surface where Rb + incorporated in the vacancy sites (confirmed by density functional tight binding-based molecular dynamics simulations) or adsorbed directly on the disordered film. These results provide a direct quantification of how surface heterogeneity influences the geochemical behavior of mineral–water interfaces, in particular highlighting the important role of chemical and structural defects on the sorbate speciation and coverage at silicate mineral surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Impact of extended defects on recombination in CdTe heterostructures grown by molecular beam epitaxy

Heterostructures with CdTe and CdTe{sub 1-x}Se{sub x} (x ∼ 0.01) absorbers between two wider-band-gap Cd{sub 1-x}Mg{sub x}Te barriers (x ∼ 0.25–0.3) were grown by molecular beam epitaxy to study carrier generation and recombination in bulk materials with passivated interfaces. Using a combination of confocal photoluminescence (PL), time-resolved PL, and low-temperature PL emission spectroscopy, two extended defect types were identified and the impact of these defects on charge-carrier recombination was analyzed. The dominant defects identified by confocal PL were dislocations in samples grown on (211)B CdTe substrates and crystallographic twinning-related defects in samples on (100)-oriented InSb substrates. Low-temperature PL shows that twin-related defects have a zero-phonon energy of 1.460 eV and a Huang-Rhys factor of 1.50, while dislocation-dominated samples have a 1.473-eV zero-phonon energy and a Huang-Rhys factor of 1.22. The charge carrier diffusion length near both types of defects is ∼6 μm, suggesting that recombination is limited by diffusion dynamics. For heterostructures with a low concentration of extended defects, the bulk lifetime was determined to be 2.2 μs with an interface recombination velocity of 160 cm/s and an estimated radiative lifetime of 91 μs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Properties of Pt-supported iron oxide ultra-thin films: Similarity of Hubbard-corrected and hybrid density functional theory description

We report a first principles study on the properties of Pt(111)-supported FeO(111) monolayer. We confront results issued from PBE+U and HSE06 approximations, and analyze the impact of the more accurate hybrid description of the electronic structure of the metal/oxide interface on a large variety of calculated characteristics of this system. In particular, we analyze the behavior of its work function and its consequences on the spontaneous charging of adsorbed Au adatoms. We also consider the FeO{sub 2} nano-oxide phase and its peculiar oxygen storage characteristics, responsible for the unusual catalytic properties of FeO{sub x}/Pt system. We show that while the hybrid approximation does indeed substantially improve the electronic characteristics of iron oxide, of individual Au adatoms, or oxygen molecules, its overall impact on the calculated properties of the composed FeO/Pt system is very small. We assign this to the relatively small effect of the hybrid approximation on the band structure alignment. This shows that the less computationally demanding DFT+U approximation remains a fully adequate tool in theoretical studies on this kind of systems. This is particularly important for calculations on realistic systems, with large-size reconstructions induced by the lattice mismatch at the interface between the two materials.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2016

Ab-initio simulations of higher Miller index Si:SiO{sub 2} interfaces for fin field effect transistor and nanowire transistors

Models of three representative higher Miller index interfaces, Si(310):SiO{sub 2}, Si(410):SiO{sub 2}, and Si(331):SiO{sub 2}, have been built by an ab-initio molecular dynamics method. We show that each interface can be made as a fully bonded network without any defects and has a reasonable electronic structure for use in fin field effect transistors or gate-all-around nanowire devices. The differences in numbers of oxygen bridges are attributed to the intermediate sub-oxide components and the atomic step structure. The interface bonding schemes to passivate different densities of dangling bonds on different facets are also analyzed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Polarized micro Raman scattering spectroscopy for curved edges of epitaxial graphene

This letter performed polarized microscopic laser Raman scattering spectroscopy on the curved edges of transferred epitaxial graphene on SiO{sub 2}/Si. The intensity ratio between the parallel and perpendicular polarized D band is evolved, providing a spectroscopy-based technique to probe the atomic-scale edge structures in graphene. A detailed analysis procedure for non-ideal disordered curved edges of graphene is developed combining the atomic-scale zigzag and armchair edge structures along with some point defects. These results could provide valuable information of the realistic edges of graphene at the atomic-scale that can strongly influence the performance of graphene-based nanodevices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

The Impacts of Cation Stoichiometry and Substrate Surface Quality on Nucleation, Structure, Defect Formation, and Intermixing in Complex Oxide Heteroepitaxy–LaCrO3 on SrTiO3(001)

Our ability to design and fabricate electronic devices with reproducible properties using complex oxides is critically dependent on our ability to controllably synthesize these materials in thin-film form. Structure-property relationships are intimately tied to film and interface composition. Here we report on the effects of cation stoichiometry in LaCrO3 heteroepitaxial films prepared using molecular beam epitaxy. We show that LaCrO3 films grow pseudomorphically on SrTiO3(001) over an wide range of La-to-Cr atom ratios. However, the growth mode and structural quality are sensitive to the La-to-Cr ratio, with La-rich films being of considerably lower structural quality than Cr-rich films. Cation mixing occurs at the interface for all La-to-Cr ratios investigated, and is not quenched by deposition at ambient temperature. Indiffused La atoms occupy Sr sites in the substrate. The presence of defects in the SrTiO3 substrate is implicated in promoting La indiffusion by comparing the properties of LaCrO3/SrTiO3 with those of LaCrO3/Si, both prepared at ambient temperature. Additionally, pulsed laser deposition is shown to result in more extensive interfacial mixing than molecular beam epitaxy for deposition at ambient temperature on Si.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2021

Anomalous interfacial dynamics of single proton charges in binary aqueous solutions

Our understanding of the dynamics of charge transfer between solid surfaces and liquid electrolytes has been hampered by the difficulties in obtaining interface, charge, and solvent-specific information at both high spatial and temporal resolution. Here, we measure at the single charge scale the dynamics of protons at the interface between an hBN crystal and binary mixtures of water and organic amphiphilic solvents (alcohols and acetone), evidencing a marked influence of solvation on interfacial dynamics. Applying single-molecule localization microscopy to emissive crystal defects, we observe correlated activation between adjacent ionizable surface defects, mediated by the transport of single excess protons along the solid/liquid interface. Solvent content has a nontrivial effect on interfacial dynamics, leading at intermediate water fraction to an increased surface diffusivity, as well as an increased affinity of the proton charges to the solid surface. Our measurements evidence the notable role of solvation on interfacial proton charge transport.

36 MATERIALS SCIENCE↗
DOE OSTI2014

A methodology to identify and quantify mobility-reducing defects in 4H-silicon carbide power metal-oxide-semiconductor field-effect transistors

In this paper, we present a methodology for the identification and quantification of defects responsible for low channel mobility in 4H-Silicon Carbide (SiC) power metal-oxide-semiconductor field-effect transistors (MOSFETs). To achieve this, we use an algorithm based on 2D-device simulations of a power MOSFET, density functional simulations, and measurement data. Using physical modeling of carrier mobility and interface traps, we reproduce the experimental I-V characteristics of a 4H-SiC doubly implanted MOSFET through drift-diffusion simulation. We extract the position of Fermi level and the occupied trap density as a function of applied bias and temperature. Using these inputs, our algorithm estimates the number of possible trap types, their energy levels, and concentrations at 4H-SiC/SiO{sub 2} interface. Subsequently, we use density functional theory (DFT)-based ab initio simulations to identify the atomic make-up of defects causing these trap levels. We study silicon vacancy and carbon di-interstitial defects in the SiC side of the interface. Our algorithm indicates that the D{sub it} spectrum near the conduction band edge (3.25 eV) is composed of three trap types located at 2.8–2.85 eV, 3.05 eV, and 3.1–3.2 eV, and also calculates their densities. Based on DFT simulations, this work attributes the trap levels very close to the conduction band edge to the C di-interstitial defect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Mechanism for recombination of radiation-induced point defects at interphase boundaries

MD studies of collision cascades in KS-type Cu-Nb interfaces are performed, revealing that Cu interstitials are preferentially loaded into the interface while on the bcc Nb side there is very little absorption of interstitials. The interstitial loading effect at two types of heterogeneous Cu-Nb interfaces, both KS-type and SPD {l_brace}112{r_brace}-type interfaces is studied: (a) The Cu interstitials are observed to spontaneously emit from both types of interfaces to annihilate vacancies in the nearby bulk, (b) MD and NEB studies are used to characterize low barrier emission processes. Our study also indicates that interstitials do not lose their identity when absorbed even at interfaces where they become significantly delocalized.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Epitaxial oxide ionotronics: Interfaces and oxygen vacancies

Oxide ionotronics is an interdisciplinary field in which systems and devices rely on the migration of ions/ionic defects to alter or drive functionality. In this perspective, we focus on epitaxial oxide heterostructures and the contributing roles of oxygen vacancies and interfaces in ionotronics. We begin with a description of oxygen vacancy behavior, with a focus on vacancy ordering and the effects of interfaces and electric fields on particular epitaxial oxide systems. We then emphasize the use of synchrotron x-ray techniques for investigating system structure and dynamics in situ at interfaces and surfaces. Finally, an outlook on the future of epitaxial oxide ionotronics is provided, and several key areas for research are identified, such as freestanding heterostructures, combinatorial synthesis and machine learning, and next-generation synchrotron x-ray characterization.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Formation of uranium nitride nanoparticles via mechanical alloying of uranium-molybdenum alloy fuels in gaseous nitrogen

Uranium-molybdenum (U-Mo) alloys show promise as a nuclear fuel system due to their high thermal conductivity and fuel loading capability. However, U-Mo systems are susceptible to irradiation induced swelling ultimately affecting the cladding via mechanical and chemical interactions. To address these shortcomings, this research investigated the formation of uranium mononitride (UN) nanoparticles within a 90 wt% U/10 wt% Mo (U-10Mo) matrix to act as a prospective defect sink for fission products at nanometric hetero-interfaces. To promote the formation of UN, U-10Mo powders were mechanically alloyed under a high purity nitrogen atmosphere. Variations of the milling process investigated included media size, duration of milling, and number of times the milling jar was re-aerated with nitrogen gas. Characterization of the fuel microstructure was completed using light element analysis, X-ray diffraction, scanning and transmission-electron microscopy, electron energy loss spectroscopy, and atom probe tomography. UN nanoparticles measuring 1–5 nm in radius were observed in the U-Mo matrix as early as 1 h into the mechanical alloying process. Milling time in excess of 10 h was found to lead to deleterious effects induced by the stainless-steel milling media.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗