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Ciston, Jim

Publications and source records attributed to Ciston, Jim.

At least 91 records · Page 5

Non-spectroscopic composition measurements of SrTiO 3 -La 0.7 Sr 0.3 MnO 3 multilayers using scanning convergent beam electron diffraction

The local atomic structure of a crystalline sample aligned along a zone axis can be probed with a focused electron probe, which produces a convergent beam electron diffraction pattern. The introduction of high speed direct electron detectors has allowed for experiments that can record a full diffraction pattern image at thousands of probe positions on a sample. By incoherently summing these patterns over crystalline unit cells, we demonstrate in this paper that in addition to crystal structure and thickness, we can also estimate the local composition of a perovskite superlattice sample. This is achieved by matching the summed patterns to a library of simulated diffraction patterns. Finally, this technique allows for atomic-scale chemical measurements without requiring a spectrometer or hardware aberration correction.

74 ATOMIC AND MOLECULAR PHYSICS↗

Optimizing disk registration algorithms for nanobeam electron diffraction strain mapping

Scanning nanobeam electron diffraction strain mapping is a technique by which the positions of diffracted disks sampled at the nanoscale over a crystalline sample can be used to reconstruct a strain map over a large area. However, it is important that the disk positions are measured accurately, as their positions relative to a reference are directly used to calculate strain. Here in this study, we compare several correlation methods using both simulated and experimental data in order to directly probe susceptibility to measurement error due to non-uniform diffracted disk illumination structure. We found that prefiltering the diffraction patterns with a Sobel filter before performing cross correlation or performing a square-root magnitude weighted phase correlation returned the best results when inner disk structure was present. Lastly, we have tested these methods both on simulated datasets, and experimental data from unstrained silicon as well as a twin grain boundary in 304 stainless steel.

36 MATERIALS SCIENCE↗

Automatic software correction of residual aberrations in reconstructed HRTEM exit waves of crystalline samples

Abstract We develop an automatic and objective method to measure and correct residual aberrations in atomic-resolution HRTEM complex exit waves for crystalline samples aligned along a low-index zone axis. Our method uses the approximate rotational point symmetry of a column of atoms or single atom to iteratively calculate a best-fit numerical phase plate for this symmetry condition, and does not require information about the sample thickness or precise structure. We apply our method to two experimental focal series reconstructions, imaging a β-Si 3 N 4 wedge with O and N doping, and a single-layer graphene grain boundary. We use peak and lattice fitting to evaluate the precision of the corrected exit waves. We also apply our method to the exit wave of a Si wedge retrieved by off-axis electron holography. In all cases, the software correction of the residual aberration function improves the accuracy of the measured exit waves.

36 MATERIALS SCIENCE↗

Theoretical and Experimental Study on the Optoelectronic Properties of Nb 3 O 7 (OH) and Nb 2 O 5 Photoelectrodes

Nb 3 O 7 (OH) and Nb 2 O 5 nanostructures are promising alternative materials to conventionally used oxides, e.g. TiO 2 , in the field of photoelectrodes in dye-sensitized solar cells and photoelectrochemical cells. Despite this important future application, some of their central electronic properties such as the density of states, band gap, and dielectric function are not well understood. In this work, we present combined theoretical and experimental studies on Nb 3 O 7 (OH) and H-Nb 2 O 5 to elucidate their spectroscopic, electronic, and transport properties. The theoretical results were obtained within the framework of density functional theory based on the full potential linearized augmented plane wave method. In particular, we show that the position of the H atom in Nb 3 O 7 (OH) has an important effect on its electronic properties. To verify theoretical predictions, we measured electron energy-loss spectra (EELS) in the low loss region, as well as, the O-K and Nb-M 3 element-specific edges. These results are compared with corresponding theoretical EELS calculations and are discussed in detail. In addition, our calculations of thermoelectric conductivity show that Nb 3 O 7 (OH) has more suitable optoelectronic and transport properties for photochemical application than the calcined H-Nb 2 O 5 phase.

14 SOLAR ENERGY↗

Development of a fast framing detector for electron microscopy

A high frame rate detector system is described that enables fast real-time data analysis of scanning diffraction experiments in scanning transmission electron microscopy (STEM). This is an end-to-end development that encompasses the data producing detector, data transportation, and real-time processing of data. The detector will consist of a central pixel sensor that is surrounded by annular silicon diodes. Both components of the detector system will synchronously capture data at almost 100 kHz frame rate, which produces an approximately 400 Gb/s data stream. Low-level preprocessing will be implemented in firmware before the data is streamed from the National Center for Electron Microscopy (NCEM) to the National Energy Research Scientific Computing Center (NERSC). Live data processing, before it lands on disk, will happen on the Cori supercomputer and aims to present scientists with prompt experimental feedback. This online analysis will provide rough information of the sample that can be utilized for sample alignment, sample monitoring and verification that the experiment is set up correctly. Only a compressed version of the relevant data is then selected for more in-depth processing.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

Atomic scale imaging of competing polar states in a Ruddlesden–Popper layered oxide

Layered complex oxides offer an unusually rich materials platform for emergent phenomena through many built-in design knobs such as varied topologies, chemical ordering schemes and geometric tuning of the structure. A multitude of polar phases are predicted to compete in Ruddlesden-Popper (RP), A n+1 B n O 3n+1 , thin films by tuning layer dimension (n) and strain; however, direct atomic-scale evidence for such competing states is currently absent. Using aberration-corrected scanning transmission electron microscopy with sub-Ångstrom resolution in Sr n+1 Ti n O 3n+1 thin films, we demonstrate the coexistence of antiferroelectric, ferroelectric and new ordered and low-symmetry phases. We also directly image the atomic rumpling of the rock salt layer, a critical feature in RP structures that is responsible for the competing phases; exceptional quantitative agreement between electron microscopy and density functional theory is demonstrated. The study shows that layered topologies can enable multifunctionality through highly competitive phases exhibiting diverse phenomena in a single structure.

36 MATERIALS SCIENCE↗

When does atomic resolution plan view imaging of surfaces work?

We present that surface structures that are different from the corresponding bulk, reconstructions, are exceedingly difficult to characterize with most experimental methods. Scanning tunneling microscopy, the workhorse for imaging complex surface structures of metals and semiconductors, is not as effective for oxides and other insulating materials. This paper details the use of transmission electron microscopy plan view imaging in conjunction with image processing for solving complex surface structures. We address the issue of extracting the surface structure from a weak signal with a large bulk contribution. This method requires the sample to be thin enough for kinematical assumptions to be valid. The analysis was performed on two sets of data, c(6×2) on the (100) surface and (3×3) on the (111) surface of SrTiO 3 , and was unsuccessful in the latter due to the thickness of the sample and a lack of inversion symmetry. In conclusion, the limits and the functionality of this method are discussed.

36 MATERIALS SCIENCE↗

Synthesis and characterization of (Ga 1-x Zn x )(N 1-x O x ) nanocrystals with a wide range of compositions

We describe the synthesis and characterization of wurtzite (Ga 1-x Zn x )(N 1-x O x ) nanocrystals with a wide range of compositions and a focus on properties relevant for solar fuel generation. (Ga 1-x Zn x )(N 1-x O x ), a solid solution of GaN and ZnO, is an intriguing material because it exhibits composition-dependent visible absorption even though the parent semiconductors absorb in the UV. When functionalized with co-catalysts, (Ga 1-x Zn x )(N 1-x O x ) is also capable of water splitting under visible irradiation. Here, we examine the synthesis of (Ga 1-x Zn x )(N 1-x O x ) nanocrystals to understand how they form by nitridation of ZnO and ZnGa 2 O 4 nanocrystalline precursors. We find that the ZnO precursor is critical for the formation of crystalline (Ga 1-x Zn x )(N 1-x O x ) at 650 °C, consistent with a mechanism in which wurtzite (Ga 1-x Zn x )(N 1-x O x ) nucleates topotactically on wurtzite ZnO at an interface with ZnGa 2 O 4 . Using this information, we expand the range of compositions from previously reported 0.30 ≤ x ≤ 0.87 to include the low-x and high-x ends of the range. The resulting compositions, 0.06 ≤ x ≤ 0.98, constitute the widest range of (Ga 1-x Zn x )(N 1-x O x ) compositions obtained by one synthetic method. We then examine how the band gap depends on sample composition and find a minimum of 2.25 eV at x = 0.87, corresponding to a maximum possible solar-to-H 2 power conversion efficiency of 12%. Finally, we examine the photoelectrochemical (PEC) oxidation behavior of thick films of (Ga 1-x Zn x )(N 1-x O x ) nanocrystals with x = 0.40, 0.52, and 0.87 under visible illumination. (Ga 1-x Zn x )(N 1-x O x ) nanocrystals with x = 0.40 exhibit solar PEC oxidation activity that, while too low for practical applications, is higher than that of bulk (Ga 1-x Zn x )(N 1-x O x ) of the same composition. The highest photocurrents are observed at x = 0.52, even though x = 0.87 absorbs more visible light, illustrating that the observed photocurrents are a result of an interplay of multiple parameters which remain to be elucidated. This set of characterizations provides information useful for future studies of composition-dependent PEC properties of nanoscale (Ga 1-x Zn x )(N 1-x O x ).

36 MATERIALS SCIENCE↗

Heat-Induced Phase Transformation of Three-Dimensional Nb 3 O 7 (OH) Superstructures: Effect of Atmosphere and Electron Beam

Nanostructured niobium oxides and hydroxides are potential candidates for photochemical applications due to their excellent optical and electronic properties. In the present work the thermal stability of Nb 3 O 7 (OH) superstructures prepared by a simple hydrothermal approach is investigated at the atomic scale. Transmission electron microscopy and electron energy-loss spectroscopy provide insights into the phase transformation occurring at elevated temperatures and probe the effect of the atmospheric conditions. In the presence of oxygen, H 2 O is released from the crystal at temperatures above 500 °C, and the crystallographic structure changes to H-Nb 2 O 5 . In addition to the high thermal stability of Nb 3 O 7 (OH), the morphology was found to be stable, and first changes in the form of a merging of nanowires are not observed until 850 °C. Under reducing conditions in a transmission electron microscope and during electron beam bombardment, an oxygen-deficient phase is formed at temperatures above 750 °C. This transformation starts with the formation of defects in the crystal lattice at 450 °C and goes along with the formation of pores in the nanowires which accommodate the volume differences of the two crystal phases.

36 MATERIALS SCIENCE↗

Statistical analysis of support thickness and particle size effects in HRTEM imaging of metal nanoparticles

High-resolution transmission electron microscopy (HRTEM) examination of nanoparticles requires their placement on some manner of support – either TEM grid membranes or part of the material itself, as in many heterogeneous catalyst systems – but a systematic quantification of the practical imaging limits of this approach has been lacking. Here, in this paper, we address this issue through a statistical evaluation of how nanoparticle size and substrate thickness affects the ability to resolve structural features of interest in HRTEM images of metallic nanoparticles on common support membranes. The visibility of lattice fringes from crystalline Au nanoparticles on amorphous carbon and silicon supports of varying thickness was investigated with both conventional and aberration-corrected TEM. Over the 1–4 nm nanoparticle size range examined, the probability of successfully resolving lattice fringes differed significantly as a function both of nanoparticle size and support thickness. Statistical analysis was used to formulate guidelines for the selection of supports and to quantify the impact a given support would have on HRTEM imaging of crystalline structure. For nanoparticles ≥1 nm, aberration-correction was found to provide limited benefit for the purpose of visualizing lattice fringes; electron dose is more predictive of lattice fringe visibility than aberration correction. These results confirm that the ability to visualize lattice fringes is ultimately dependent on the signal-to-noise ratio of the HRTEM images, rather than the point-to-point resolving power of the microscope. Lastly, this study provides a benchmark for HRTEM imaging of crystalline supported metal nanoparticles and is extensible to a wide variety of supports and nanostructures.

36 MATERIALS SCIENCE↗

The crystal structure, orientation relationships and interfaces of the nanoscale oxides in nanostructured ferritic alloys

The capability of nanostructured ferritic alloys (NFAs) to manage high levels of transmutation product helium will help resolve one of the grand challenges to transforming the promise of C-free fusion energy into a reality. NFAs are dispersion strengthened by an ultrahigh density of Y-Ti-O nano-oxides (NOs), which result in both high strength and temperature limits, as well as unique irradiation tolerance. We used aberration-corrected high-resolution transmission electron microscopy to characterize the NOs in four NFA conditions, including following severe deformation and extreme neutron radiation exposure. Fast Fourier Transform analysis of focal series images revealed the NO crystal structure, including the smallest at < 2 nm in diameter, to be Y 2 Ti 2 O 7 pyrochlore in all cases, consistent with both exit wave analysis and scanning transmission Z-contrast imaging of the atomic columns in a larger feature. The faceted NOs exhibit a quasi-epitaxial orientation relationship with the ferrite matrix: [110] YTO ||[100] Fe and [001] YTO ||[010] Fe , forming a 5 × 7 near coincidence site interface. Also, the NOs exhibit size-dependent strains in both the oxide and matrix ferrite phases.

36 MATERIALS SCIENCE↗

Efficient linear phase contrast in scanning transmission electron microscopy with matched illumination and detector interferometry

The ability to image light elements in soft matter at atomic resolution enables unprecedented insight into the structure and properties of molecular heterostructures and beam-sensitive nanomaterials. In this study, we introduce a scanning transmission electron microscopy technique combining a pre-specimen phase plate designed to produce a probe with structured phase with a high-speed direct electron detector to generate nearly linear contrast images with high efficiency. We demonstrate this method by using both experiment and simulation to simultaneously image the atomic-scale structure of weakly scattering amorphous carbon and strongly scattering gold nanoparticles. Our method demonstrates strong contrast for both materials, making it a promising candidate for structural determination of heterogeneous soft/hard matter samples even at low electron doses comparable to traditional phase-contrast transmission electron microscopy. Ultimately, simulated images demonstrate the extension of this technique to the challenging problem of structural determination of biological material at the surface of inorganic crystals.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

Identifying different stacking sequences in few-layer CVD-grown Mo S 2 by low-energy atomic-resolution scanning transmission electron microscopy

We present that atomically thin MoS 2 grown by chemical vapor deposition (CVD) is a promising candidate for next-generation electronics due to inherent CVD scalability and controllability. However, it is well known that the stacking sequence in few-layer MoS 2 can significantly impact electrical and optical properties. Herein we report different intrinsic stacking sequences in CVD-grown few-layer MoS 2 obtained by atomic-resolution annular-dark-field imaging in an aberration-corrected scanning transmission electron microscope operated at 50 keV. Trilayer MoS 2 displays a new stacking sequence distinct from the commonly observed 2H and 3R phases of MoS 2 . Finally, density functional theory is used to examine the stability of different stacking sequences, and the findings are consistent with our experimental observations.

36 MATERIALS SCIENCE↗

Correcting nonlinear drift distortion of scanning probe and scanning transmission electron microscopies from image pairs with orthogonal scan directions

Unwanted motion of the probe with respect to the sample is a ubiquitous problem in scanning probe and scanning transmission electron microscopies, causing both linear and nonlinear artifacts in experimental images. We have designed a procedure to correct these artifacts by using orthogonal scan pairs to align each measurement line-by-line along the slow scan direction, by fitting contrast variation along the lines. We demonstrate the accuracy of our algorithm on both synthetic and experimental data and provide an implementation of our method.

36 MATERIALS SCIENCE↗

Environmental TEM study of electron beam induced electro-chemistry of Pr₀̣₆₄Ca₀̣₃₆MnO₃ catalysts for oxygen evolution

Environmental Transmission Electron Microscopy (ETEM) studies offer great potential for gathering atomic scale information on the electronic state of electrodes in contact with reactants but also pose big challenges due to the impact of the high energy electron beam. In this article, we present an ETEM study of a Pr₀̣₆₄Ca₀̣₃₆MnO₃ (PCMO) thin film electro-catalyst for water splitting and oxygen evolution in contact with water vapor. We show by means of off-axis electron holography and electrostatic modeling that the electron beam gives rise to a positive electric sample potential due to secondary electron emission. The value of the electric potential depends on the primary electron flux, the sample -conductivity and grounding, and gas properties. We present evidence that two observed electro-chemical reactions are driven by a beam induced electrostatic potential of the order of a volt. The first reaction is an anodic electrochemical oxidation reaction of oxygen depleted amorphous PCMO which results in recrystallization of the perovskite structure. The second reaction is oxygen evolution which can be detected by the oxidation of a silane additive and formation of SiO 2–x at catalytically active surfaces. Recently published in-situ XANES observation of subsurface oxygen vacancy formation during oxygen evolution at a positive potential [³²] is confirmed in this work. The quantification of beam induced potentials is an important step for future controlled electro-chemical experiments in an ETEM.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Elastic properties, sp³ fraction, and Raman scattering in low and high pressure synthesized diamond-like boron rich carbides

Dense BC{sub x} phases with high boron concentration are predicted to be metastable, superhard, and conductors or superconductors depending on boron concentration. However, up to this point, diamond-like boron rich carbides BC{sub x} (dl-BC{sub x}) phases have been thought obtainable only through high pressure and high temperature treatment, necessitating small specimen volume. Here, we use electron energy loss spectroscopy combined with transmission electron microscopy, Raman spectroscopy, surface Brillouin scattering, laser ultrasonics (LU) technique, and analysis of elastic properties to demonstrate that low pressure synthesis (chemical vapor deposition) of BC{sub x} phases may also lead to the creation of diamond-like boron rich carbides. The elastic properties of the dl-BC{sub x} phases depend on the carbon sp²versus sp³ content, which decreases with increasing boron concentration, while the boron bonds determine the shape of the Raman spectra of the dl-BC{sub x} after high pressure-high temperature treatment. Using the estimation of the density value based on the sp³ fraction, the shear modulus μ of dl-BC₄, containing 10% carbon atoms with sp³ bonds, and dl-B₃C₂, containing 38% carbon atoms with sp³ bonds, were found to be μ = 19.3 GPa and μ = 170 GPa, respectively. The presented experimental data also imply that boron atoms lead to a creation of sp³ bonds during the deposition processes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Crystalline Fibers of Metal–Peptide Double Ladders

Despite remarkable progress in the field of MOFs, structures based on long-flexible organic linkers are scarce and the majority of such materials rely on rigid linkers. In this work, crystals of a new metal–organic double ladder (MODL) are obtained by linking a pentapeptide (NH2-Glu-pCO2Phe-pCO2Phe-Ala-Gly-OH) with cadmium acetate to produce a Cd(2-pyrrolidone-pCO2Phe-pCO2Phe-Ala-Gly)(H2O)3 framework. SEM and TEM analyses show the fibrous nature of the crystals and show that the infinite cadmium oxide rod secondary building units (SBUs) are aligned with the longitudinal axis of the nanofibers.

36 MATERIALS SCIENCE↗