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This work presents the preparation of a series of [(PbSe)1+δ]4[TiSe2]4 isomers via a low temperature synthesis approach that exploits precursor nanoarchitecture to direct formation of specific isomers. The targeted isomers formed even when the precursors did not have the correct amount of each element to make a unit cell from each repeating sequence of elemental layers deposited. This suggests that the exact composition of the precursors is less important than the nanoarchitecture in directing the formation of the compounds. The as-deposited diffraction data show that the isomers begin to form during the deposition, and Ti2Se, in addition to PbSe and TiSe2, are present in the specular diffraction patterns. HAADF-STEM images reveal impurity layers above and below an integer number of targeted isomer unit cells. The structural data suggest that Ti2Se forms as Se is deposited on the initial Ti layers and remains throughout isomer self-assembly. During growth, the isomers deplete the local supply of Ti and Pb, creating diffusion gradients that drive additional cations toward the growth front, which leaves surface impurity layers of TiSe2 and TiO2 after the supply of Pb is exhausted. The deposited stacking sequences direct formation of the targeted isomers, but fewer repeating units form than intended due to the lack of material per layer in the precursor and formation of impurity layers. All isomers have negative Hall and Seebeck coefficients, indicating that electrons are the majority carrier. The carrier concentration and conductivity of the isomers increase with the number of interfaces in the unit cell, resulting from charge donation between adjacent layers. The opposite variation of the carrier concentration and mobility with temperature result in minima in the resistivity between 50 and 100 K. The very weak temperature dependence of the carrier concentration likely results from changes in the amount of charge transfer between the layers with temperature.

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.

This work presents the preparation of a series of [(PbSe)

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.

Solid-state reaction kinetics on atomic length scales have not been heavily investigated due to the long times, high reaction temperatures, and small reaction volumes at interfaces in solid-state reactions. All of these conditions present significant analytical challenges in following reaction pathways. Herein we use in situ and ex situ X-ray diffraction, in situ X-ray reflectivity, high-angle annular dark field scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy to investigate the mechanistic pathways for the formation of a layered (Pb0.5Sn0.5Se)1+δ(TiSe2)m heterostructure, where m is the varying number of TiSe2 layers in the repeating structure. Thin film precursors were vapor deposited as elemental-modulated layers into an artificial superlattice with Pb and Sn in independent layers, creating a repeating unit with twice the size of the final structure. At low temperatures, the precursor undergoes only a crystallization event to form an intermediate (SnSe2)1+γ(TiSe2)m(PbS...

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...

Preparing homologous series of compounds allows chemists to rapidly discover new compounds with predictable structure and properties. Synthesizing compounds within such a series involves navigating a free energy landscape defined by the interactions within and between constituent atoms. Historically, synthesis approaches are typically limited to forming only the most thermodynamically stable compound under the reaction conditions. Presented here is the synthesis, via self-assembly of designed precursors, of isocompositional incommensurate layered compounds [(BiSe)1+δ] m[TiSe2] m with m = 1, 2, and 3. The structure of the BiSe bilayer in the m = 1 compound is not that of the binary compound, and this is the first example of compounds where a BiSe layer thicker than a bilayer in heterostructures has been prepared. Specular and in-plane X-ray diffraction combined with high-resolution electron microscopy data was used to follow the formation of the compounds during low-temperature annealing and the subsequent decomposition of the m = 2 and 3 compounds into [(BiSe)1+δ]1[TiSe2]1 at elevated temperatures. These results show that the structure of the precursor can be used to control reaction kinetics, enabling the synthesis of kinetically stable compounds that are not accessible via traditional techniques. The data collected as a function of temperature and time enabled us to schematically construct the topology of the free energy landscape about the local free energy minima for each of the products.

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.

Self-assembly of designed precursors has enabled the synthesis of novel heterostructures that exhibit extensive rotational disorder between constituents. In (SnSe)1.2TiSe2 nanoscale regions of long-range order were observed in scanning transmission electron microscopy (STEM) cross sectional images. Here a combination of techniques are used to determine the structure of this compound, and the information is used to infer the origin of the order. In-plane X-ray diffraction indicates that the SnSe basal plane distorts to match TiSe2. This results in a rectangular unit cell that deviates from both the bulk structure and the square in-plane unit cell previously observed in heterostructures containing SnSe bilayers separated by layers of dichalcogenides. The distortion results from lattice matching of the two constituents, which occurs along the SnSe and the TiSe2 directions as √3 × aTiSe2 equals aSnSe. Fast Fourier transform analysis of the STEM images exhibits sharp maxima in hkl families where h,...

Controlling carrier concentration is critical in many device applications, and both chemical substitution and modulation doping have been used in industry. For most inorganic materials, very low doping efficiencies are observed as site occupancies depend on both thermodynamic and kinetic factors. We demonstrate that we can make kinetically controlled site-specific substitutions in a series of (BixSn1–xSe)1+δTiSe2 compounds using the modulated elemental reactants method. These compounds were characterized using a combination of X-ray diffraction, resistivity and Hall coefficient measurements, and high angle annular dark field scanning transmission electron microscopy (HAADF STEM). For small x, the doping efficiency is 0.7, close to that observed for B in silicon. For higher x values, a structural distortion is observed in X-ray diffraction data in which the symmetry of the in-plane unit cell decreases. HAADF STEM data reveals the presence of antiphase boundaries (Bi–Bi pairs) in the BixSn1–xSe layers, whic...

(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.

Solid-state reaction kinetics on atomic length scales have not been heavily investigated due to the long times, high reaction temperatures, and small reaction volumes at interfaces in solid-state reactions. All of these conditions present significant analytical challenges in following reaction pathways. Herein we use in situ and ex situ X-ray diffraction, in situ X-ray reflectivity, high-angle annular dark field scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy to investigate the mechanistic pathways for the formation of a layered (Pb

A family of multiple-component heterostructures, ([SnSe]1+δ)m([{MoxNb1–x}Se2]1+γ)1([SnSe]1+δ)m({NbxMo1–x}Se2)1 (m = 0, 1, 2, 3, 4 and 0.8 ≤ x ≤ 1), was self-assembled from designed amorphous precursors, and their structure and physical properties were characterized. The compounds consist of MoSe2 and NbSe2 layers with metal centers in a trigonal-prismatic environment interleaved with systematically increasing numbers of SnSe bilayers with a distorted rock salt structure. The extent of alloying of the miscible dichalcogenide constituents decreased from about 20 % for m = 0 to less than 1 % for m = 3 and 4. The decreased alloying with increased SnSe thickness suggests the diffusion lengths of Mo and Nb during self-assembly are about a nanometer. Resistivity and Hall coefficient measurements show that the electrical transport properties are similar to those of ([SnSe]1+δ)m(NbSe2)1 (m = 1–8) compounds, suggesting that the NbSe2 layer dominates the conductivity and that charge transfer from SnSe reduces the carrier concentration in the NbSe2 layer.

Selecting specific 2D building blocks and specific layering sequences of van der Waals heterostructures should allow the formation of new materials with designed properties for specific applications. Unfortunately, the synthetic ability to prepare such structures at will, especially in a manner that can be manufactured, does not exist. Herein, we report the targeted synthesis of new metal-semiconductor heterostructures using the modulated elemental-reactant technique to nucleate specific 2D building blocks, control their thickness, and avoid epitaxial structures with long-range order. The building blocks, VSe2 and GeSe2 , have different crystal structures, which inhibits cation intermixing. The precise control of this approach enabled us to synthesize heterostructures containing GeSe2 monolayers alternating with VSe2 structural units with specific sequences. The transport properties systematically change with nanoarchitecture and a charge-density wave-like transition is observed.

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...

Preparing homologous series of compounds allows chemists to rapidly discover new compounds with predictable structure and properties. Synthesizing compounds within such a series involves navigating a free energy landscape defined by the interactions within and between constituent atoms. Historically, synthesis approaches are typically limited to forming only the most thermodynamically stable compound under the reaction conditions. Presented here is the synthesis, via self-assembly of designed precursors, of isocompositional incommensurate layered compounds [(BiSe)

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)

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.

Self-assembly of designed precursors has enabled the synthesis of novel heterostructures that exhibit extensive rotational disorder between constituents. In (SnSe)

(SnSe)1.2TiSe2 was found to self-assemble from a precursor containing modulated layers of Sn–Se and Ti–Se over a surprisingly large range of layer thicknesses and compositions. The constituent lattices form an alternating layer superstructure with rotational disorder present between the layers. This compound was found to have the highest Seebeck coefficient measured for analogous TiX2 containing misfit layered compounds to date, suggesting potential for low-temperature thermoelectric applications. Electrical characterization suggests that electrons transferred from SnSe to TiSe2 are responsible for the higher carrier concentration observed relative to bulk TiSe2. The transfer of charge from one constituent to the other may provide a mechanism for doping layered dichalcogenides for various applications without negatively affecting carrier mobility.

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.

The presence of Mn-Fe nodules in the epipedons (surface horizons) of paleosols of presumed Upper Neogene age in the northwestern Venezuelan Andes have been interpreted as products of inorganic oxidation and reduction processes operating over the full range of glacial and interglacial cycles that affected paleosol morphogenesis. New microscopic/chemical data from combined SEM-EDS-FIB analyses of representative Mn-Fe nodules indicate microbes play an important role in Mn/Fe precipitation leading to their genesis in alpine Mollisols (Argiustolls). Although the prevailing new data are based mainly on fossil forms of filamentous bacteria and fungi and other biogenic pseudomorphs that may represent the former resident bacteria, the presence of extant microbes must await field experiments/collection, followed by a molecular microbiology approach to determine the biological drivers of metal precipitation. As in other terrestrial niche environments, microbes are seen here to play a role, perhaps a key one, in the ...

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 mix of high resolution electron microscope methods imaged the textures and chemistry of rock varnish samples from 19 field sites on five continents. The vast majority of aeolian mineral is not incorporated into manganiferous rock varnish. Of those dust particles that are enveloped, submicron sized oval-shaped quartz minerals are the most common type of detritus seen, as they rest conformably between laminated layers. The dominance of quartz as the most common detrital mineral, combined with the relative rarity of feldspars – is consistent with the hypothesis that feldspars experience in situ decay into clay minerals. After the detritus is buried in varnish, mineral boundaries often develop enhanced porosity. Some porous zones around dust particles develop submicron skins of redeposited Mn–Fe. In other cases, the porous zones aid in the transport of capillary water that mobilizes and redeposits Mn–Fe as stringers in fissures. Larger dust particles ∼10 μm in diameter are deposited in microtopographic depressions, such as tubes created by acid-producing lithobionts. Varnishes growing in particularly dusty regions form alternating dust-rich and varnish-rich layers that potentially correlate to alternating dusty and less dusty periods. The very foundation of varnish, the underlying rock, is often less stable in the surficial environment than varnish – leading to enhanced porosity and mineral decay in the substrate. Sometimes, physical collapse of varnish into the underlying void space mixes varnish and rock; more commonly, however, remobilization of varnish constituents into these pore spaces creates case hardening of the weathering rind in the underlying rock.

The cross-sectional texture of rock varnish varies considerably with the scale of analysis and technique used to image a sample. Each jump in resolution results in new insight, with the current state-of-the-art resting at the nanoscale. One key to nanoscale analysis involves focused ion beam (FIB) techniques used most frequently in material science and semiconductor failure analysis. FIB preparation remains challenging, however, for samples like rock coatings with heterogeneous density and abundant porosity. A new technique involving multiangle ion thinning and in situ plan-view lift-out facilitated a scanning transmission electron microscopy study of rock varnish from Death Valley. The results reveal variability in lateral continuity of nanometer microlaminae that can be interrupted by post-depositional diagenesis involving leaching of Mn and Fe, and this variability could explain why some of the visual varnish microlaminations (VML) used in paleoclimatic research can sometimes appear discontinuous. Beca...

Thin films with tunable and homogeneous composition are required for many applications. We report the synthesis and characterization of a new class of compositionally homogeneous thin films that are amorphous solid solutions of Al2O3 and transition metal oxides (TMOx) including VOx, CrOx, MnOx, Fe2O3, CoOx, NiO, CuOx, and ZnO. The synthesis is enabled by the rapid decomposition of molecular transition-metal nitrates TM(NO3)x at low temperature along with precondensed oligomeric Al(OH)x(NO3)3–x cluster species, both of which can be processed from aq solution. The films are dense, ultrasmooth (Rrms < 1 nm, near 0.1 nm in many cases), and atomically mixed amorphous metal-oxide alloys over a large composition range. We assess the chemical principles that favor the formation of amorphous homogeneous films over rougher phase-segregated nanocrystalline films. The synthesis is easily extended to other compositions of transition and main-group metal oxides. To demonstrate versatility, we synthesized amorphous V0.1...

(BiSe)

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.

Weathering-rind thicknesses on pebble- and cobble-size sediment have been used for the past half-century, at least, as an age indicator of postdepositional time following a geologic event. In mountainous terrain, rind thickness is taken as a measurement of weathering over time frames of 0.5 m.yr.; variable thicknesses are used to discriminate relative ages of glacial deposits. The effects of chemical and physical weathering that together produce rinds are only rarely considered, and most research objectives have centered on lichen alteration of clast surfaces. Recent microscopic analyses of weathering rinds on volcanic clasts of ∼70.0-ka to ∼2.0-m.yr. age produced new data on weathering products as well as unexpected incorporated biotic materials undergoing diagenesis. The question as to how much physical/mineral/chemical/biotic paleoenvironmental data might be archived in rinds is discussed. The character and classification of organic materials undergoing diagenesis are also discussed.

Samples of manganiferous rock varnish collected from fluvial, bedrock outcrop and Erie Barge Canal settings in New York state host a variety of diatom, fungal and bacterial microbial forms that are enhanced in manganese and iron. Use of a Dual-Beam Focused Ion Beam Scanning Electron Microscope to manipulate the varnish in situ reveals microbial forms that would not have otherwise been identified. The relative abundance of Mn–Fe-enriched biotic forms in New York samples is far greater than varnishes collected from warm deserts. Moisture availability has long been noted as a possible control on varnish growth rates, a hypothesis consistent with the greater abundance of Mn-enhancing bioforms. Sub-micron images of incipient varnish formation reveal that varnishing in New York probably starts with the mortality of microorganisms that enhanced Mn on bare mineral surfaces; microbial death results in the adsorption of the Mn-rich sheath onto the rock in the form of filamentous networks. Clay minerals are then cemented by remobilization of the Mn-rich material. Thus, the previously unanswered question of what comes first – clay mineral deposition or enhancement of Mn – can be answered in New York because of the faster rate of varnish growth. In contrast, very slow rates of varnishing seen in warm deserts, of microns per thousand years, make it less likely that collected samples will reveal varnish accretionary processes than samples collected from fast-accreting moist settings.

Secondary, backscattered, high-resolution transmission, and energy-dispersive spectroscopy tools of electron microscopy reexamined Alexander von Humboldt’s field site of “brownish black crust[s]” covering rocks along cataracts of the Orinoco River. Modern tools confirm eighteenth-century analysis that the basic composition includes an abundance of manganese, iron, and carbon. Additional major constituents include clay minerals, calcium, and sometimes barium and cobalt. Backscattered and secondary electron imaging confirms the 2-century-old hypothesis of an accretionary nature of the Orinoco coating. The remarkable Orinoco sheen is produced by a smooth lamellate micromorphology and high concentrations of manganese—the same conditions required to produce lustrous varnishes formed in warm deserts. Although von Humboldt deduced, and we agree, that coating constituents must derive from the Orinoco, electron microscope observations of Mn-enriched diatom fragments, Mn-enriched cocci-bacterial forms, and ...

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.

(BiSe)(1+δ)NbSe2 ferecrystals were synthesized in order to determine whether structural modulation in BiSe layers, characterized by periodic antiphase boundaries and Bi-Bi bonding, occurs. Specular X-ray diffraction revealed the formation of the desired compound with a c-axis lattice parameter of 1.21 nm from precursors with a range of initial compositions and initial periodicities. In-plane X-ray diffraction scans could be indexed as hk0 reflections of the constituents, with a rectangular basal BiSe lattice and a trigonal basal NbSe2 lattice. Electron micrographs showed extensive turbostratic disorder in the samples and the presence of periodic antiphase boundaries (approximately 1.5 nm periodicity) in BiSe layers oriented with the [110] direction parallel to the zone axis of the microscope. This indicates that the structural modulation in the BiSe layers is not due to coherency strain resulting from commensurate in-plane lattices. Electrical transport measurements indicate that holes are the dominant charge carrying species, that there is a weak decrease in resistivity as temperature decreases, and that minimal charge transfer occurs from the BiSe to NbSe2 layers. This is consistent with the lack of charge transfer from the BiX to the TX2 layers reported in misfit layer compounds where antiphase boundaries were observed. This suggests that electronic considerations, i.e., localization of electrons in the Bi-Bi pairs at the antiphase boundaries, play a dominant role in stabilizing the structural modulation.

Thin films of the new title intergrowth compound consisting of a bilayer of SnSe and TiSe2 trilayer are prepared by self-assembly from a precursor containing modulated layers of Sn-Se and Ti-Se, which is synthesized by deposition of elemental layers (annealing at 350 °C).