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A cross-section analysis is presented of defects in ZnO nanospikes, investigated by means of transmission electron microscopy and defect simulation., The examination of anisotropic nanostructures, such as wires, platelets or spikes, inside a transmission electron microscope is normally performed only in plan view. However, intrinsic defects such as growth twin interfaces could occasionally be concealed from direct observation for geometric reasons, leading to superposition. This article presents the shadow-focused ion-beam technique to prepare multiple electron-beam-transparent cross-section specimens of ZnO nanospikes, via a procedure which could be readily extended to other anisotropic structures. In contrast with plan-view data of the same nanospikes, here the viewing direction allows the examination of defects without superposition. By this method, the coexistence of two twin configurations inside the wurtzite-type structure is observed, namely and , which were not identified during the plan-view observations owing to superposition of the domains. The defect arrangement could be the result of coalescence twinning of crystalline nuclei formed on the partially molten Zn substrate during the flame-transport synthesis. Three-dimensional defect models of the twin interface structures have been derived and are correlated with the plan-view investigations by simulation.

We have explored the effect of magnetic rare-earth dopants substitutionally incorporated on the Ba sites of ${\mathrm{BaSnO}}_{3}$ in terms of electronic transport, magnetism, and optical properties. We show that for ${\mathrm{Ba}}_{0.92}{R}_{0.08}{\mathrm{SnO}}_{3}$ thin films (where $R=\text{La,}\phantom{\rule{4.pt}{0ex}}\text{Pr,}\phantom{\rule{4.pt}{0ex}}\text{Nd,}\phantom{\rule{4.pt}{0ex}}\text{Gd}$), there is a linear increase of mobility with carrier concentration across all doping schemes. La-doped films have the highest mobilities, followed by Pr- and Nd-doped films. Gd-doped samples have the largest ionic size mismatch with the Ba site and correspondingly the lowest carrier concentrations and electron mobilities. However, crystallinity does not appear to be a strong predictor of transport phenomena; our results suggest that point defects more than grain boundaries are key ingredients in tuning the conduction of ${\mathrm{BaSnO}}_{3}$ films grown by pulsed laser deposition. Pronounced, nonhysteretic x-ray magnetic dichroism signals are observed for Pr-, Nd-, and Gd-doped samples, indicating paramagnetism. Finally, we probe the optical constants for each of the ${\mathrm{BaSnO}}_{3}$ doping schemes and note that there is little change in the transmittance across all samples. Together these results shed light on conduction mechanisms in ${\mathrm{BaSnO}}_{3}$ doped with rare-earth cations.

The synthesis, structure, and properties of a new three-constituent heterostructure containing BiSe, Bi2Se3, and TiSe2 layers, each with a different structural motif, are reported. (BiSe)1+δ(Bi2Se3...

Single- and few-layer metal chalcogenide compounds are of significant interest due to structural changes and emergent electronic properties on reducing dimensionality from three to two dimensions. To explore dimensionality effects in SnSe, a series of [(SnSe)1+δ]mTiSe2 intergrowth structures with increasing SnSe layer thickness (m = 1-4) were prepared from designed thin-film precursors. In-plane diffraction patterns indicated that significant structural changes occurred in the basal plane of the SnSe constituent as m is increased. Scanning transmission electron microscopy cross sectional images of the m = 1 compound indicate long-range coherence between layers whereas the m ≧ 2 compounds show extensive rotational disorder between the constituent layers. For m ≧ 2, the images of the SnSe constituent contain a variety of stacking sequences of SnSe bilayers. Density functional theory calculations suggest that the formation energy is similar for several different SnSe stacking sequences. The compounds show un...

The discovery of emergent phenomena in two-dimensional (2D) materials has sparked substantial research efforts in the materials community. A significant experimental challenge for this field is exerting atomistic control over the structure and composition of the constituent 2D layers and understanding how the interactions between layers drives both structure and properties. Segregation of Pb to the surface of three bilayer thick PbSe-SnSe alloy layers was discovered within [(PbxSn1-xSe)1+δ]n(TiSe2)1 heterostructures using electron microscopy. We demonstrate that this segregation is thermodynamically favored to occur when PbxSn1-xSe layers are interdigitated with TiSe2 monolayers. Density-functional theory (DFT) calculations indicate that the observed segregation depends on what is adjacent to the PbxSn1-xSe layers. The interplay between interface and volume free energies controls both the structure and composition of the constituent layers, which can be tuned using layer thickness.

(BiSe)1+δ(NbSe2)n heterostructures with n = 1–4 were synthesized using modulated elemental reactants. The BiSe bilayer structure changed from a rectangular basal plane with n = 1 to a square basal plane for n = 2–4. The BiSe in-plane structure was also influenced by small changes in the structure of the precursor, without significantly changing the out-of-plane diffraction pattern or value of the misfit parameter, δ. Density functional theory calculations on isolated BiSe bilayers showed that its lattice is very flexible, which may explain its readiness to adjust shape and size depending on the environment. Correlated with the changes in the BiSe basal plane structure, analysis of scanning transmission electron microscope images revealed that the occurrence of antiphase boundaries, found throughout the n = 1 compound, is dramatically reduced for the n = 2–4 compounds. X-ray photoelectron spectroscopy measurements showed that the Bi 5d3/2, 5d5/2 doublet peaks narrowed toward higher binding energies as n in...

© 2018 Author(s). Magnetic properties arising at interfaces of perovskite oxides such as La 0.7 Sr 0.3 CoO 3 (LSCO) and La 0.7 Sr 0.3 MnO 3 (LSMO) depend sensitively on the fine details of their structural properties. In this work, we use high-resolution transmission electron microscopy and spectroscopy to examine the structural and electronic phenomena at the interfaces in two LSCO/LSMO bilayers with reversed growth order. Two different strain mechanisms are at work in these films: compressive or tensile epitaxial strain, and distortion of the octahedral tilt pattern to maintain a network of corner-sharing octahedra. While the epitaxial strain is constant regardless of the growth order, the modification of the octahedral tilt pattern depends on whether the film is grown directly on the substrate or as the second sublayer. As a consequence, exchange spring behavior is observed only when the LSCO sublayer is grown first. The different mechanisms of strain accommodation within the oxygen octahedra network in each material proved to be of critical importance in determining the interfacial structure and thus magnetic and electronic properties of the bilayers.

A series of ferecrystalline compounds ([SnSe]1.15)1(V1+xSe2)n with n=1–6 and a thin film V1+xSe2 were synthesized utilizing the modulated elemental reactant technique. The effect of interstitial V-atoms ranging from 0.13≤x≤0.42 in different compounds on structure and electrical properties of these intergrowth compounds is reported. The presence of the interstitial V-atoms for n>1 was confirmed by Rietveld refinements as well as HAADF-STEM cross sections. The off-stoichiometry in the thin film V1.13Se2 causes a suppression of the charge density wave, similar to the effect of non-stoichiometry observed for the bulk compound. The charge density wave of ([SnSe]1.15)1(V1+xSe2)1, however, is not affected by the non-stoichiometry due to its incorporation as volume inclusions or due to the quasi 2-dimensionality of the isolated VSe2 layer. In the compounds ([SnSe]1.15)1(V1+xSe2)n with n=2–6, the temperature dependence of the electrical resistivity approaches bulk-like behavior.

The new metastable compound Cr{sub 1+x}Sb with x up to 0.6 has been prepared via a thin film approach using modulated elemental reactants and investigated by in-situ X-ray reflectivity, X-ray diffraction, differential scanning calorimetry, energy dispersive X-ray analysis as well as transmission electron microscopy and atomic force microscopy. The new Cr-rich antimonide crystallizes in a structure related to the Ni{sub 2}In-type structure, where the crystallographic position (1/3, 2/3, 3/4) is partially occupied by excess Cr. The elemental layers of the pristine material interdiffused significantly before Cr{sub 1+x}Sb crystallized. A change in the activation energy was observed for the diffusion process when crystal growth starts. First-principles electronic structure calculations provide insight into the structural stability, magnetic properties and resistivity of Cr{sub 1+x}Sb. - Graphical abstract: 1 amorphous multilayered film 2 interdiffused amorphous film 3 metastable crystalline phase 4 thermodynamic stable phase (and by-product). - Highlights: • Interdiffusion of amorphous Cr and Sb occurs before crystallization. • Crystallization of a new metastable phase Cr{sub 1.6}Sb in Ni{sub 2}In-type structure. • The new Cr-rich phase shows half-metallic behavior.

The synthesis of multiple-component heterostructures is conventionally accomplished through mechanical or chemical exfoliation, followed by physically stacking the layers together. Here, we report the synthesis, structure, and local composition of a three-component heterostructure (SnSe)1.16(MoSe2)1.06(SnSe)1.16(NbSe2)1, which was self-assembled at relatively low temperature (450 °C) from a designed precursor. XRD and STEM studies showed that the compound consisted of alternating layers of trigonal prismatic MoSe2 and NbSe2, interleaved with SnSe layers in a distorted rock salt structure. The new three-component ferecrystal is metallic with a factor of 3 higher conductivity than the (SnSe)1+δ(NbxMo1–x)Se2 ferecrystal alloy of the same composition (x ≈ 0.5). Comparison of the in-plane lattice parameters with the (SnSe)1+δ(NbxMo1−x)Se2 alloys allowed the extent of interdiffusion of the dichalcogenide constituents to be evaluated, yielding a stoichiometry of (SnSe)1.16([Mo0.9Nb0.1]Se2)1.06(SnSe)1.16([Nb0.9Mo...

A series of (BiSe)1+δ(TiSe2)n compounds where n was varied from two to four were synthesized and electrically characterized to explore the extent of charge transfer from the BiSe layer to the TiSe2 layers. These kinetically stable heterostructures were prepared using the modulated elemental reactants (MER) method, in which thin amorphous elemental layers are deposited in an order that mimics the nanostructure of the desired product. X-ray diffraction (XRD), X-ray area diffraction, and scanning transmission electron microscopy (STEM) data show that the precursors formed the desired products. Specular diffraction scans contain only 00l reflections, indicating that the compounds are crystallographically aligned with the c-axis perpendicular to the substrate. The c-axis lattice parameter increases by 0.604(3) nm with each additional TiSe2 layer. In-plane diffraction scans contain reflections that can be indexed as the (hk0) of the BiSe and TiSe2 constituents. Area diffraction scans are also consistent with th...

A series of ferecrystalline compounds ([SnSe]1+δ)1(VSe2)1 with varying Sn/V ratios were synthesized using the modulated elemental reactant technique. Temperature-dependent specific heat data reveal a phase transition at 102 K, where the heat capacity changes abruptly. An abrupt increase in electrical resistivity occurs at the same temperature, correlated with an abrupt increase in the Hall coefficient. Combined with the magnitude and nature of the specific heat discontinuity, this suggests that the transition is similar to the charge density wave transitions in transition metal dichalcogenides. An ordered intergrowth was formed over a surprisingly wide compositional range of Sn/V ratios of 0.89 ≤ 1 + δ ≤ 1.37. X-ray diffraction and transmission electron microscopy reveal the formation of various volume defects in the compounds in response to the nonstoichiometry. The electrical resistivity and Hall coefficient data of samples with different Sn/V ratios show systematic variation in the carrier concentration with the Sn/V ratio. There is no significant change in the onset temperature of the charge density wave transition, only a variation in the carrier densities before and after the transition. Given the sensitivity of the charge density wave transitions of transition metal dichalcogenides to variations in composition, it is very surprising that the charge density wave transition observed at 102 K for ([SnSe]1.15)1(VSe2)1 is barely influenced by the nonstoichiometry and structural defects. This might be a consequence of the two-dimensional nature of the structurally independent VSe2 layers.

Targeted substitutions in extended solids have been historically challenging, limited by high temperature and the synthetic routes traditionally used. Here, we report the synthesis of new compounds in the (PbxSn1–xSe)1+δTiSe2 intergrowth family from designed amorphous precursors. By controlling local composition and using low reaction temperatures, the metastable quaternary compounds can be synthesized over the entire range of 0 ≤ x ≤ 1 as crystallographically aligned thin films. X-ray diffraction and electron microscopy confirm the formation of a solid solution in the PbxSn1–xSe layer, with the overall and constituent structure both changing as a function of composition as predicted by Vegard’s law. Charge transfer between constituents and subsequent conduction in the TiSe2 describes the observed transport properties. The mobility of the charge carriers is increased in compounds with the alloyed PbxSn1–xSe layer, providing direct evidence that charge transport occurs predominantly in the dichalcogenide l...

A series of alloyed ferecrystals ([Sn1−xBixSe]1.15)1(VSe2)1 with 0 ≤ x ≤ 0.66 was synthesized via the modulated elemental reactants technique. X-ray diffraction of the compounds reveals systematic changes of the lattice parameter and the intensities of the Bragg peaks confirming the successful alloying of the compounds corroborated by Rietveld refinements. Interestingly, both constituents of the intergrowth compounds exhibit systematic structural changes as a function of the Bi-content indicating interlayer interaction. The a-axis lattice parameter of the VSe2 layer expands with increasing Bi-content, which signifies changes in the electronic structure of this constituent. Electrical resistivities, Hall and Seebeck coefficients of compounds with a varying Bi-content present a complex scenario. At low Bi-contents an enhancement of the charge density wave transition is observed, whereas further substitution results in a suppression of the effect. At Bi-contents exceeding x = 0.55 minority carriers from the Sn1−xBixSe layer contribute to the transport properties.

The quality and temperature stability of 1 nm Bi2Te3/5 nm Sb2Te3 superlattices prepared by molecular beam epitaxy for the first time was investigated by in situ and ex situ X-ray diffraction and transmission electron microscopy. Upon heating, the superlattice structures are not stable against interdiffusion of the components, with micro- and nanostructural changes occurring at temperatures as low as 200 °C. The interdiffusion preferably starts next to superlattice defects. At 300 °C the Bi2Te3 and Sb2Te3 layers were mostly interdiffused, forming the thermodynamically stable Sb1.66Bi0.33Te3 alloy. The data suggests that structural integrity of Bi2Te3/Sb2Te3 superlattices will not be stable for extended times above 200 °C, thus inhibiting application of such superlattices over a wide temperature range.

Modification of the modulated elemental reactant method allowed the first reported synthesis of quaternary [(MX)z]m[(TxT′1–x)X2]n compounds with controlled compositions. Metastable ferecrystal compounds, [(SnSe)1.16–1.09]1[(NbxMo1–x)Se2]1 with x = 0, 0.26, 0.49, 0.83, and 1, were synthesized and characterized by X-ray diffraction, electron probe microanalysis (EPMA), and scanning transmission electron microscopy (STEM). Misfit layer dichalcogenide analogues, showing controlled alloying of the dichalcogenide layers, have not been reported. Changes in lattice parameters, as a function of x, were similar to literature observations for the (NbxMo1–x)Se2 system. The c-lattice parameters of the ferecrystal alloys decreased linearly from 12.53(2) A for [(SnSe)1.09]1(MoSe2)1 to 12.27(2) A for [(SnSe)1.16]1(NbSe2)1. The a-lattice parameters of the dichalcogenide constituent increased from 3.329(8) A for [(SnSe)1.09]1(MoSe2)1 to 3.461(4) A for [(SnSe)1.16]1(NbSe2)1. The a-lattice parameter of the rocksalt constitue...

A homologous series of quasi-2D ([PbSe]1+δ)m(TiSe2)m nanolayered heterostructures are prepared via self-assembly of designed precursors with 1 ≤m≤ 4 and their structures and properties investigated. All heterostructures have the same global composition but vary in their interface density. X-ray diffraction and electron microscopy studies show that the structures consist of rock salt structured PbSe layers alternating with TiSe2 layers, and that grain size increases with m. The compounds are all metallic with upturns in resistivity at low temperature suggesting electron localization, with room temperature resistivity of 1-3 10(-5)Ω m, negative Hall coefficients and Seebeck coefficients between -50 and -100 μV K(-1). A decrease in the mobile carrier concentration with temperature is observed for all m and the rate increases with increasing low-dimensionality. Decreasing the interface density also decreases the average carrier concentration while increasing the electron mobility. The Seebeck coefficients systematically increase in magnitude as m is increased, but the net effect to the power factor is small due to a compensating increase in resistivity. The observed transport behavior is not described by the simple rigid band models with charge transfer between constituents used previously. Charge exchange between constituents stabilizes the intergrowth, but also introduces mobile carriers and interfacial band bending that must play a role in the transport behavior of the heterostructures. As chemical potentials equilibrate in high m heterostructures there is a decrease in total coulombic stabilization as there are fewer interfaces, so m = 1 is likely to be most stable. This rationalizes why the structurally similar misfit layer compounds with m = 1 are often the only intergrowths that can be prepared. Charge transfer and band bending at interfaces should occur in other heterostructures with similar type II broken-gap band alignments and are important considerations regarding both their stability and transport properties.

Targeted heterostructures containing intergrown two dimensional (2D) layers of 3 different constituent layers, SnSe2, PbSe and TiSe2, were prepared by controlling the composition and sequence of elemental bilayers within a designed precursor. Varying the structure of the precursor enabled the number of structural units of each constituent and the sequence of crystalline 2D layers to be precisely controlled. The stacking of the 2D layers, their structures, and the segregation of the elements between them were determined using X-ray diffraction and electron microscopy techniques, with the observed sequence of the 2D layers consistent with the targeted intergrowth. This ability to prepare targeted heterostructures is critical, since the number of possible configurations in the final compound increases rapidly as the number of constituents increases, from almost 60 000 with two constituents to over 130 million with three constituents and to over 35 billion with four constituents for 20 or fewer distinct layers in the unit cell. This general route for synthesizing specific multiple component heterostructures will accelerate the feedback loop in this growing research area, permitting theorists to assume specific structures in the search for enhanced properties and providing experimentalists with crystallographically aligned samples to test these predictions.

We report the low temperature synthesis of highly textured CuCr2Se4 thin films using the modulated elemental reactant (MER) method. The structure of CuCr2Se4 is determined for the first time in its thin film form and exhibits cell parameters that are smaller than found in bulk CuCr2Se4. X-ray diffraction and precession electron diffraction show a strong degree of crystallographic alignment of the crystallites, where the axis is oriented perpendicular to the substrate surface, while being rotationally disordered within the plane. Temperature and field dependent in-plane and out-of-plane magnetization measurements show that the film is ferromagnetic with a Curie temperature of 406 K CuCr2Se4 synthesized utilizing the MER method shows stronger magnetic anisotropy (effective anisotropy: 1.82 × 106 erg cm−3; shape anisotropy: 1.07 × 106 erg cm−3), with the easy axis lying out of plane, and a larger magnetic moment (6 μB/f.u.) than bulk CuCr2Se4.

A variety of metal oxide films (InGaOx, AlOx, “HafSOx”) prepared from aqueous solutions were found to have non-uniform electron density profiles using X-ray reflectivity. The inhomogeneity in HafSOx films (Hf(OH)4–2x−2y(O2)x(SO4)y·zH2O), which are currently under investigation as inorganic resists, were studied in more detail by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and medium-energy ion scattering (MEIS). The HAADF-STEM images show a greater concentration of heavy atoms near the surface of a single-layer film. MEIS data confirm the aggregation of Hf at the film surface. The denser “crust” layer in HafSOx films may directly impact patterning resolution. More generally, the phenomenon of surface-layer inhomogeneity in solution-deposited films likely influences film properties and may have consequences in other thin-film systems under investigation as resists, dielectrics, and thin-film transistor components.

The rational design of improved electrode–electrolyte interfaces (EEI) for energy storage is critically dependent on a molecular-level understanding of ionic interactions and nanoscale phenomena. The presence of non-redox active species at EEI has been shown to strongly influence Faradaic efficiency and long-term operational stability during energy storage processes. Herein, we achieve substantially higher performance and long-term stability of EEI prepared with highly dispersed discrete redox-active cluster anions (50 ng of pure ∼0.75 nm size molybdenum polyoxometalate (POM) anions on 25 μg (∼0.2 wt%) carbon nanotube (CNT) electrodes) by complete elimination of strongly coordinating non-redox species through ion soft landing (SL). Electron microscopy provides atomically resolved images of a uniform distribution of individual POM species soft landed directly on complex technologically relevant CNT electrodes. In this context, SL is established as a versatile approach for the controlled design of novel surfaces for both fundamental and applied research in energy storage., The design and understanding of electrode–electrolyte interfaces is important for the development of improved energy storage devices. Here, the authors study the controlled deposition of molybdenum polyoxometalate anions onto carbon nanotube electrodes, and show this can result in increased specific capacitance.

Reducing the accelerating voltage used to pattern a high-resolution inorganic thin film electron beam resist, HafSOx dramatically decreases the dose required to pattern sub-10 nm lines. Ellipsometry measurements of dose curves created for HafSOx show a dramatic increase in sensitivity without any loss in contrast. High resolution patterning at 10 keV show no decrease in obtainable feature size, producing 9 nm wide lines with half the dose required previously and no increase in line edge roughness. Monte Carlo simulations of the incident electron beam into a 22 nm thick HafSOx film predict that even lower primary beam energies, down to 5 keV, could enable patterning of sub-20 nm features with significantly enhanced throughput. This improved film sensitivity at lower beam energies enables increased throughput without loss of patterning resolution and should be extendable to other inorganic photoresists.

We report results from high angle annular dark field scanning transmission electron microscopy (HAADF STEM) image analysis of complex semi-crystalline and amorphous materials, and apply the insights gained from local structure information towards global structure determination. Variations in HAADF STEM intensities for a rotationally disordered heterostructure and an amorphous oxide film are statistically analyzed to extract information regarding the inhomogeneity of the films perpendicular to the substrate. By assuming chemical homogeneity in the film axis parallel to the substrate, the signal intensity variation parallel to the substrate is used to estimate the signal noise level, allowing evaluation of the significance of intensity differences in the substrate normal direction. The positions of HAADF STEM intensity peaks in the perpendicular direction, averaged from multiple images, provide a valuable initial model for a Rietveld refinement of the global c-axis structure of the heterostructure. For an amorphous multi-coat solution-cast oxide sample, the analysis reveals statistically significant variations in the HAADF STEM intensity profile perpendicular to the substrate. These variations indicate an inhomogeneous density profile, presumably related to the spin-casting of individual layers and have implications for understanding the chemical interactions that occur between layers when preparing multilayer amorphous oxide films from solution.

Summary While investigating rock varnish, we explored novel uses for an in-situ micromanipulator, including charge collection, sample manipulation, as well as digging and dissection at the micron level. Dual-beam focused ion beam microscopes (DB-FIB or FIBSEM) equipped with micromanipulators have proven to be valuable tools for material science, semiconductor research, and product failure analysis. Researchers in many other disciplines utilize the DB-FIB and micromanipulator for site-specific transmission electron microscope (TEM) foil preparation. We have demonstrated additional applications for in-situ micromanipulators. SCANNING 34: 279–283, 2012. © 2012 Wiley Periodicals, Inc.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.