D.W. McComb

Absorption Corrections for a Four-Quadrant SuperX EDS Detector

Consider a four-quadrant detector consisting of four circular active regions of area Aq each, placed symmetrically around the sample, as shown schematically in Fig. 1(a). The sample holder is inserted from the right, and the quadrants are 90° apart, oriented symmetrically at ±45° with respect to the primary tilt axis. The specimen tilt angles are labeled  and β, with positive angles corresponding to counterclockwise rotations. Each detector quadrant is located above the plane of the specimen and is tilted from the vertical plane by an angle d; the angle between the line connecting the center of a quadrant D with the eucentric point S and the horizontal plane is labeled d (Fig. 1(b)). The active circular regions have a radius of Rd, and a distance to the eucentric point of rd; the detector opening angle is R = Rd/rd. When R is not negligible, as is the case for a SuperX detector, then the x-ray photon path length inside the sample becomes a function of the position on the detector where the pho-ton hits; the absorption correction factor must thus involve an integration over the detector surface area.

HAADF/MAADF Observations and Image Simulations of Dislocation Core Structures in a High Entropy Alloy

High entropy alloys (HEAs) are a new class of multi-component alloys in which the individual elements have similar concentrations. A single-phase solid solution HEA containing 5 elements (Co, Cr, Fe, Mn, and Ni) with equiatomic composition was first discovered by Cantor [1]. Among the surprising characteristics of this fcc HEA are: strong temperature dependence of the yield strength at temperatures around and below room temperature, relatively weak strain-rate dependence over the same temperature range [3]; very large hardening rates [2,3]; and large fracture toughness at room temperature [4]. These features are linked to deformation twinning and dislocation-mediated plasticity, yet presently there is insufficient knowledge of dislocation dissociation, stacking fault energy, or core structures in this alloy. The highly planar deformation involves dislocation arrays on active slip systems (Figure 1a and 1b). This characteristic could imply the presence of short range order, low fault energy, or supplementary displacements in the wake of glide dislocations.

Cation non-stoichiometry in pulsed laser deposited Sr2+yFe1+xMo1-xO6 epitaxial films

Sr2FeMoO6 (SFMO) films were grown on SrTiO3 (100)- and (111)-oriented substrates via pulsed laser deposition (PLD). In order to study the fundamental characteristics of deposition, films were grown in two different PLD chambers. In chamber I, the best films were grown with a relatively long substrate-to-target distance (89 mm), high substrate temperature (850 °C), and low pressure (50 mTorr) in a 95% Ar/5% H2 atmosphere. Although X-ray diffraction (XRD) measurements indicate these films are single phase, Rutherford Backscattering (RBS) measurements reveal considerable non-stoichiometry, corresponding to a Sr2Fe1−xMo1+xO6 composition with x ≅ 0.2–0.3. This level of non-stoichiometry results in inferior magnetic properties. In chamber II, the best films were grown with a much shorter substrate-to-target distance (38 mm), lower temperature (680 °C), and higher pressure (225 mTorr). XRD measurements show that the films are single phase, and RBS measurements indicate that they are nearly stoichiometric. The de...

Structural and Magnetic Characterization of B20 Skyrmion Thin Films and Heterostructures Using Aberration-Corrected Lorentz TEM and Differential Phase Contrast STEM

Magnetic materials exhibiting topological spin textures have shown great promise for magnetoelectronic applications including ultra-high density magnetic memory. [1-4] Specifically, skyrmions are vortex-like spin textures that can form hexagonal magnetic lattices at temperatures near room temperature and small applied magnetic fields. The skyrmion phase results from the competition between exchange interactions and the Dzyaloshinskii-Moriya (DM) interaction, where the exchange interaction promotes parallel alignment between neighboring spins and the DM interaction promotes 90° alignments. DM interactions only occur in structures with broken inversion or mirror symmetry like the family of materials with the B20 crystal structure (space group P213). In addition to materials lacking in bulk inversion or mirror symmetry, superlattices can host the skyrmion phase due to their broken mirror symmetry. Recently, Ahmed et al. demonstrated the ability grow epitaxial B20 superlattices of [CrGe/MnGe/FeGe]n via molecular beam epitaxy (MBE) opening the door for tunable skyrmions through varying layer thicknesses. [5]

Characterizing Epitaxial Growth of Nd 2 Ir 2 O 7 Pyrochlore Thin Films via HAADF-STEM Imaging and EDX

Much interest has been given to the 5d transition metal oxides because of their strong spin-orbit coupling, leading to the prediction of new exotic phases of matter, including the Weyl semimetal, topological Mott insulator, and spin liquid [1–4]. The pyrochlore iridates with the general formula of A2Ir2O7 have been predicted to show the Weyl semimetal and topological insulator phases under epitaxial strain [5,6]. These theoretical predictions have motivated the study and growth of epitaxial thin films of such pyrochlore iridates. In this work, we report the synthesis of Nd2Ir2O7 and growth of epitaxial thin films using off-axis magnetron sputtering and ex-situ post-growth annealing. Using aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADFSTEM) and energy dispersive X-ray spectroscopy (EDX), the growth mechanisms of Nd2Ir2O7 are characterized. A thermodynamic explanation of the observed mechanisms is presented.

Structure-Properties Relations in III-Nitride Nanostructures for Optoelectronics

III-Nitride based nanowire heterostructures are useful for optoelectronics applications across the visible and ultraviolet (UV) spectral range. A remarkable range of applications for these nanomaterials have been demonstrated, including solid-state-lighting, UV LEDs, lasers, photovoltaics, and photocatalysts. Compared with their thin film counterparts, III-N nanowires exhibit several key advantages including: inherently low defect densities (zero misfit dislocations and minimal stacking faults), lattice mismatch tolerance, and greater tunability of polarization and bandgap within a single heterostructure. In particular, single crystal III-N nanowires can be grown on a variety of substrates while retaining their high optical and electronic quality. Here we discuss a few illustrative examples of correlating the atomic scale structure determined by scanning transmission electron microscopy (STEM) measurements with the functional properties of the nanowire heterostructures.

Through-Focal HAADF-STEM Analysis of Dislocation Cores in a High-Entropy Alloy

High-entropy alloys (HEAs) are a new class of multi-component alloys that exhibit surprising characteristics, [1] including very large strain hardening rates, large fracture toughness at room temperature [2], and a strong temperature dependence of yield strength at or below room temperature. These properties are closely linked to nano-twinning and dislocation-mediated plasticity, yet little experimental work has explored dislocation dissociation, stacking fault energy, or core structures in these alloys [3]. In this study, an HEA, containing 5 elements (Cr, Co, Mn, Fe, and Ni) with equiatomic composition was deformed to a 5% plastic strain at room temperature [4]. Post-mortem 3mm disks were electro-polished using a solution consisting of 21% Perchloric acid and 79% Acetic acid and analyzed using a probe-corrected Titan 80-300kV along a [110] zone axis. Highly planar deformation was first observed by Otto et al. [5] and was active for this study as well. This planar deformation, involving dislocation arrays on {111} slip systems, may imply the existence of short-range order, low stacking fault energy (SFE), and/or supplementary displacements in the wake of dislocations.

Quantifying Ordering Phenomena Through High-Resolution Electron Microscopy, Spectroscopy, and Simulation

Advances in aberration corrected scanning transmission electron microscopy (STEM) have allowed researchers to investigate structure-property relationships at the atomic scale [1–4]. In many systems, ordering phenomena at the atomic level can have a dramatic impact on the structural, electronic, and magnetic properties of the material. By combining experiment, simulation, and data processing, these ordering phenomena can be studied in a quantitative way, opening the door for new insights into the structure-property relationships of a wide variety of material systems. Experimentally, high angle annular dark field (HAADF) STEM and energy dispersive X-ray spectroscopy (EDX) are two very powerful techniques for probing compositional variations at the Ångstrom scale. In order to fully understand and quantify these techniques, image simulation and ionization calculations using the quantum excitation of phonons model can be used [5,6]. For the first time, using a double aberration corrected FEI ThemisTM with a Super-XTM XEDS detector compositional mapping of a Ni-based superalloy (commercially available HL-11) was collected at atomic resolution across a stacking fault, as seen in Figure . Individual spectra in the atomic resolution XEDS maps exhibit low signal-to-noise, usually having very few counts per channel. Based on previous characterization of the fault structure determining structural periodicity[4], the data was summed over a repeating unit cell of the fault structure along the [110] projection, resulting in almost an order of magnitude increase in the peak maxima of the XEDS spectra and even larger increase in integrated peak counts. These modified 3D data cubes were then fed into the Bruker Esprit software package and quantified using experimentally determined Cliff-Lorimer k-factors from a solutionized sample of the same material. By preprocessing these data before quantification, the error in the quantification was significantly reduced, allowing for site-specific determination of solute segregation in and around the fault structure in this Ni-based superalloy, leading to the determination of a novel high temperature strengthening mechanism.

High Resolution Electron Microscopy Characterization of (La 0.5 Sr 0.5 )2CoC4 Thin Film Cathode Materials

Thin film LSC214 was grown by pulse laser deposition (PLD) on SrTiO3 (STO) (001) or LaSrAlO4 (SLAO) (100) substrate. Due to the lattice parameter constraint of the substrate, the deposited thin film will grow in different orientations. High-resolution high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) is used to characterize both samples with a focus on the surface of the films. The S/TEM samples were lifted out by dual beam focused ion beam (FIB) method. To protect the film surface from ion beam damage, samples were coated with gold before FIB lift-out.

Viability of HAADF-STEM Imaging Contrast and Simulations as a Measure of B-site Ordering for Double Perovskites

Spintronics has emerged as a promising technology that exploits both the intrinsic spin of the electron and it associated magnetic moment in a solid-state device. Utilization of the spin degree of freedom in metals and semiconductors has potential to create significant technological advances over current, charge-based technologies[1]. Therefore, accurate, atomic level characterization of half-metallic spin injectors and magnetic semiconductors will be essential to realizing these next generation technologies.