Bulk Stoichiometry Research Articles

Insights into a clock's fidelity through vesicular encapsulation.

The single-celled cyanobacterium, Synechococcus elongatus , generates circadian rhythms with exceptional fidelity and synchrony despite their femtoliter volumes. Here, we explore the mechanistic aspects of this fidelity, by reconstituting the KaiABC post-translational oscillator (PTO) in cell-mimetic giant vesicles (GUVs) under well-defined conditions in vitro . PTO proteins were encapsulated with a coefficient of variation that closely matched protein variations observed in live cells. Using fluorescently labeled KaiB and confocal microscopy, we were able to measure circadian rhythms generated by thousands of encapsulated PTOs at the single-vesicle level for several days as a function of protein concentration and GUV size. We find that PTO fidelity decreased with decreasing levels of encapsulated PTO proteins and in smaller GUVs. We also observed that in encapsulated PTOs, a significant fraction of KaiB localized to GUV membranes like it does in cyanobacteria. A mathematical model that uses empirical bulk concentration and stoichiometry limitations suggests that cyanobacteria overcome challenges to fidelity by expressing high levels of PTO proteins along with the CikA and SasA proteins, which buffer stochastic variations in the levels of KaiA and KaiB, respectively. Further, the model suggests that the transcription-translation feedback loop (TTFL) contributes at most a small percentage to the overall fidelity of the cyanobacterial circadian clock under constant conditions but is essential for maintaining phase synchrony. Our results are the first experimental demonstration of populations of synthetic cells that can autonomously keep circadian time. Additionally, the approach of using bulk relationships to understand complex phenomena in cell-like systems could be useful for understanding other collective behavior important in biology, such as liquid-liquid phase separation.

The influence of bulk stoichiometry on near-ambient pressure reactivity of bare and Pt-loaded rutile TiO2(110).

The interaction of catalyst particles with reducible support materials can drastically change their reactivity. On rutile TiO2, processes like particle encapsulation (caused by the "strong metal-support interaction", SMSI) have long been known to depend on the initial reduction state of the oxide. Despite this knowledge, sample stoichiometry has rarely been controlled in a reproducible manner in the surface science literature. Here, we use scanning tunnelling microscopy (STM) to explore systematically how near-ambient pressures (0.1-1.0 mbar) of O2, H2, CO and CO2 affect blank and Pt-loaded rutile TiO2(110) surfaces of different bulk stoichiometry at 600 K. To this end, we present preparation recipes that result in a sample stoichiometry always converging back to the same value, which allows us to use the same samples with constant reduction state over hundreds of preparation cycles. Comparing a highly reduced and a near-stoichiometric TiO2 sample, we find that surface reactivity to all four gasses differs even without Pt loading. Most surprisingly, we find that the highly reduced TiO2(110) is oxidized by CO2, but this reaction is completely inhibited on the near-stoichiometric sample. Pt nanoparticles, in turn, become encapsulated after vacuum annealing on the reduced, but not on the near-stoichiometric sample. Encapsulation on the near-stoichiometric sample is achieved only after exposing it to 0.1 mbar H2 at 600 K. Interestingly, we also see a further modification of the already encapsulated particles on the reduced sample under the same conditions, such that they become embedded deeper in the TiO2(110) surface.

Quantitative analysis of trace elements in technological materials using online-laser ablation of solids in liquids (online-LASIL)

To overcome the challenges of the 21st century, the demand for novel materials which outperforms the limits of previous material classes is high. However, the elemental composition determines crucial material properties from the bulk stoichiometry down to ultra-trace impurities. Therefore, a precise analytical characterization is inevitable to establish a connection between composition and function.For this purpose, conventional solid sampling techniques could be applied. However, matrix-matched standards are indispensable for quantitative analysis and are often unavailable in the case of technological materials. To overcome this limitation online-Laser Ablation of Solids in Liquids (online-LASIL) has been proposed recently, which combines the advantages of solution-based analysis and solid sampling techniques. However, this technique has only been applied to determine sample stoichiometry.In this work, major improvements of the online-LASIL approach were reported by the controllable segmentation of the carrier solution, leading to an improved washout behavior of the ablated material, enabling the quantitative investigation at trace levels. The accuracy of this improved measurement setup is validated by the quantitative analysis of trace elements in the standard reference material (SRM) NIST612. To demonstrate the applicability of this method, silicon carbide (SiC), a wide bandgap semiconductor, was analyzed. With the improved online-LASIL method, an Al-doped region, which is only constant for the first 250 nm, was successfully quantified.

Crossover from small polaron tunneling to correlated barrier hopping and its evolution with thermal instability of SnO surface

In this paper, we have studied the surface and bulk stoichiometry and its effect on DC-AC conductivity measurements by x-ray photoelectron spectroscopy (XPS) as well as Raman spectroscopy depth profiling respectively. The SnO surface sheets are readily oxidized to Sn3O4phases and partially SnO2. The temperature dependence of DC resistance shows three-dimensional variable range hopping (VRH) below 200 K and Nearest Neighbor Hopping (NNH) between 200 and 300 K and undergoes a crossover to the thermally activated band conduction above 300 K till it decomposes at 600 K. This crossover temperature increases beyond 300 K during the subsequent heating cycles. In the frequency and temperature dependent conductivity, the bulk SnO throughout 300–600 K shows the frequency exponent of 0.8 which is ascribed to tunneling mechanism. Whereas, the surface progressively changes from correlated barrier hopping, CBH (band conduction) to a crossover of non-overlapping small polaron tunneling to CBH in samples in 300 to 400 K range after a thermal cycling. The oxidized phases bring about the polaronic sites which tunnel below 400 K. The energy of hopping reveals that the pristine sample has band like transport due to shallow Sn vacancies acceptors and the oxidized surface repositions fermi level to midgap due to oxygen vacancy donors.

2D reciprocal space map of etched metalorganic chemical vapor deposited CdTe(001) film surface on miscut GaAs(001)

CdTe thin films have drawn continued interest in research and applications in various fields such as optoelectronics. We report on the study of a two-dimensional (2D) reciprocal space map of Br-methanol etched CdTe(001) epitaxial film grown by metalorganic chemical vapor deposition on miscut GaAs(001) using azimuthal reflection high-energy electron diffraction (ARHEED), which reveals a two-fold symmetric surface with a rectangular unit mesh. This is in contrast to the observed four-fold symmetry with a square unit mesh on an etched miscut single crystal CdTe(001). Two sets of two-fold symmetric diffraction spots with unequal intensities indicate unequal populations of two 90° orientation domains in the etched film. Based on the 2D maps and tilted azimuthal dependent RHEED patterns of the etched film, it unveils that the short length of rectangular unit mesh in the dominant domain aligns with the step edge direction. A simple method to characterize the miscut angle of a heteroepitaxial film grown on a miscut substrate is demonstrated using ARHEED. Both etched single crystal and film surfaces are Te rich with a higher Te to Cd ratio on the elongated hillocks in the etched film disclosed by scanning Auger electron spectroscopy mapping. This methodology can be applied to examine etched epitaxial film's surfaces that might deviate from bulk structure or bulk stoichiometry before interfacing with other materials to form heterojunctions or making ohmic contacts with metals.

Hardness Enhancement in CoCrFeNi1−x(WC)x High-Entropy Alloy Thin Films Synthesised by Magnetron Co-Sputtering

We demonstrate the systematic hardness enhancement of the CoCrFeNi high-entropy alloy (HEA) by the addition of tungsten carbide (WC). Mixed thin films are fabricated by magnetron co-sputtering using a home-made spark plasma-sintered CoCrFeNi target and a commercially available WC target. The WC content in the thin films is adjusted via the ratio of deposition powers applied to the targets. X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) measurements were taken to determine the surface and bulk stoichiometry, respectively. The uniform distribution of the elements is confirmed via EDX mapping. X-ray diffraction (XRD) is carried out on the samples to determine the crystal phase formation. The Vickers hardness of the thin films is investigated using nanoindentation and shows an increase in the hardness in the thin films following an increased WC content. The data obtained are presented in comparison to pure WC and CoCrFeNi thin films fabricated by magnetron sputtering, respectively.

DFT Investigation of Ammonia Formation via a Langmuir-Hinshelwood Mechanism on Mo-Terminated δ-MoN(0001).

In this work, we employed density functional theory to elucidate the energetics associated with elementary steps along a Langmuir–Hinshelwood mechanism for the Haber–Bosch synthesis of ammonia from N2 and H2 on a hexagonal, Mo-terminated molybdenum nitride surface. Using nudged elastic band calculations, we determined the energy barriers involved in the reaction processes. An active site consisting of four nearest-neighbor Mo atoms, previously identified as an active site on similar surfaces, was chosen to investigate the reaction processes. Using this approach, we calculate a barrier of ∼0.5 eV for the dissociation of N2. The superior activity of the dissociation of the strong N2 bonds is rationalized based on the unique geometric and electronic configurations present at these active sites. Despite the favorable energetics for nitrogen dissociation, the energy cost for hydrogenation of NHx (0 ≤ x ≤ 2) species is shown to be energetically limiting for the formation of ammonia through the Langmuir–Hinshelwood mechanism at these sites, with elementary step activation barriers calculated to be as large as ∼2 eV. A comparison to Haber–Bosch results derived from a similar γ-Mo2N model system suggests the relative independence of surface chemistry and bulk stoichiometry for rhombic Mo4 active sites present on molybdenum nitrides.

Lanthanide Oxides: From Diatomics to High-Spin, Strongly Correlated Homo- and Heterometallic Clusters.

Small lanthanide (Ln) oxide clusters present both experimental and theoretical challenges because of their partially filled, core-like 4f n orbitals, a feature that results in a plethora of close-lying and fundamentally similar electronic states. These clusters provide a bottom-up approach toward understanding the electronic structure of defective or doped bulk material but also can offer a challenge to the theorists to find a method robust enough to capture electronic structure patterns that emerge from within the 4f n (0 < n < 14) series. In this Feature Article, we explore the electronic structures of small lanthanide oxide clusters that deviate from bulk stoichiometry using anion photoelectron spectroscopy and supporting density functional theory calculations. We will describe the evolution of electronic structure with oxidation and how LnxOy- cluster reactivities can be correlated with specific Ln-local orbital occupancies. These strongly correlated systems offer additional insights into how interactions between electrons and electronically complex neutrals can lead to detachment transitions that lie outside of the sudden one-electron detachment approximation generally assumed in anion photoelectron spectroscopy. With a better understanding of how we can control nominally forbidden transitions to sample an array of spin states, we suggest that more in-depth studies on the magnetic states of these systems can be explored. Extending these studies to other Ln-based materials with hidden magnetic phases, along with sequentially ligated single molecule magnets, could advance current understanding of these systems.

Infrared furnace for in situ neutron single-crystal diffraction studies in controlled gas atmospheres at high temperatures

To understand oxygen diffusion mechanisms in non-stoichiometric oxides, the possibility to explore structural changes as a function of the oxygen partial pressure with temperature and related oxygen bulk stoichiometry is mandatory. This article reports on the realization of a high-temperature furnace, suitable for single-crystal neutron diffraction, working continuously at temperatures of up to 1000°C at different and adjustable partial gas pressures of up to 2 bar (1 bar = 100 kPa). This allows exploration of the phase diagrams of non-stoichiometric oxides under in situ conditions and controlled oxygen partial pressure. As a pilot study, the structural changes of Pr2NiO4+δ were explored at room temperature (δ ≃ 0.24) and at 900°C under 1 bar P(O2) (δ ≃ 0.13) as well as under secondary vacuum (approximately 10−5 mbar) conditions yielding a δ close to zero. The strong anharmonic displacements of the apical oxygen atoms along the [110] shallow diffusion pathway, which were previously observed at room temperature and 400°C, become more isotropic at 900°C. The study shows that the anisotropic oxygen displacements, here related to lattice instabilities, play a major role in understanding oxygen diffusion pathways and related activation energies at moderate temperatures. This also shows the importance of the availability of reaction cells for single-crystal neutron diffraction to explore the phase diagram and associated structural changes of non-stoichiometric oxygen ion conductors and respective diffusion mechanisms.

CoCrFeNi High-Entropy Alloy Thin Films Synthesised by Magnetron Sputter Deposition from Spark Plasma Sintered Targets

Two magnetron sputter targets of CoCrFeNi High-Entropy Alloy (HEA), both in equal atomic ratio, were prepared by spark plasma sintering. One of the targets was fabricated from a homogeneous HEA powder produced via gas atomisation; for the second target, a mixture of pure element powders was used. Economic benefits can be achieved by mixing pure powders in the intended ratio in comparison to the gas atomisation of the specific alloy composition. In this work, thin films deposited via magnetron sputtering from both targets are analysed. The surface elemental composition is investigated by X-ray photoelectron spectroscopy, whereas the bulk stoichiometry is measured by X-ray fluorescence spectroscopy. Phase information and surface microstructure are investigated using X-ray diffraction and scanning electron microscopy, respectively. It is demonstrated that the stoichiometry, phase composition and microscopic structure of the as-deposited HEA thin films are almost identical if the same deposition parameters are used.

(Invited) Switching on Electrocatalytic Activity for Synthetic Fuel Production

The greatest challenge facing Solid oxide cells (SOC), in both fuel and electrolysis cell modes (i.e SOFCs and SOECs) is to deliver high, long-lasting electrocatalytic activity while ensuring cost and time-efficient electrode manufacture. Ultimately, this can best be achieved by growing appropriate nanoarchitectures under operationally relevant conditions, rather than through intricate ex situ procedures. In our approach, metal particles are grown directly from the oxide support though in situ redox exsolution. We demonstrated that by understanding and manipulating the surface chemistry of an oxide support with adequately designed bulk (non)stoichiometry, one can control the size, distribution and surface coverage of produced particles. We also revealed that the emergent particles are generally epitaxially socketed in the parent perovskite which appears to be the underlying origin of their remarkable stability, including unique resistance of metal particles to agglomeration and to hydrocarbon coking, whilst retaining catalytic activit. Operando redox treatments yield emergent nanomaterials at potentials in excess of 2V. Here we apply this technique to drive steam, CO2 and coelectrolysis processes at emergent metals and alloy particles.

Homeostasis and non‐linear shift in the stoichiometry of P‐limited planktonic communities

AbstractPlanktonic communities are naturally subjected to episodic nutrient enrichments that may stress or redress the imbalances in limiting nutrients. Human‐enhanced atmospheric nitrogen deposition has caused profound N:P imbalance in many remote oligotrophic lakes in which phosphorus has largely become limiting. These lakes offer an opportunity to investigate the relationship between the changes in plankton stoichiometry, productivity, and community structure occurring during nutrient fluctuations in P‐limited conditions. We performed P () and N ( or ) pulse additions to the summer epilimnetic community of an ultraoligotrophic lake using self‐filling ~100‐L enclosures and analyzed the response to varying P availability, N:P imbalance, and N source. Seston C:N:P proportions remained fairly unchanged to P additions that were within the range of values seasonally found in the lake. However, the seston N:P ratio abruptly shifted and approached Redfield’s proportions at P additions typical of mesotrophic conditions that provided non‐limiting conditions. N surplus did not affect seston C:N:P proportions. The patterns of seston N:P stability and shift were similar for both N sources. In contrast, productivity was highly sensitive to low and medium P additions and decelerated at high P additions. Phytoplankton biomass dominated particulate organic matter. The autotrophic community differentiated almost linearly across the P gradient. Chrysophytes' dominance decreased, and diatoms and cryptophytes relative abundance increased. Nonetheless, the stoichiometry stability and non‐linear shift involved large biomass proportions of the same species, which indicates that the bulk stoichiometry was related to similar physiological behavior of phylogenetically diverse organisms according to the biogeochemical context. The C:N:P seston stability in P‐limited conditions—with loose coupling with productivity, nutrient supply ratios, and species dominance—and the sudden shift to Redfield proportions in P‐repleted conditions suggest a complex regulation of P scarcity in planktonic communities that goes beyond immediate acclimation growth responses and might include alternative physiological and biogeochemical states.

(Invited) Switching on Elctrocatalytic Activity for Synthetic Fuel Production

The greatest challenge facing Solid oxide cells (SOC), in both fuel and electrolysis cell modes (i.e SOFCs and SOECs) is to deliver high, long-lasting electrocatalytic activity while ensuring cost and time-efficient electrode manufacture. Ultimately, this can best be achieved by growing appropriate nanoarchitectures under operationally relevant conditions, rather than through intricate ex situ procedures.In our approach, metal particles are grown directly from the oxide support though in situ redox exsolution. We demonstrated that by understanding and manipulating the surface chemistry of an oxide support with adequately designed bulk (non)stoichiometry, one can control the size, distribution and surface coverage of produced particles. We also revealed that the emergent particles are generally epitaxially socketed in the parent perovskite which appears to be the underlying origin of their remarkable stability, including unique resistance of metal particles to agglomeration and to hydrocarbon coking, whilst retaining catalytic activit.Operando redox treatments yield emergent nanomaterials at potentials in excess of 2V. Here we apply this technique to drive steam, CO2 and coelectrolysis processes at emergent metals and alloy particles.

Carboxylic acid induced near-surface restructuring of a magnetite surface

A fundamental knowledge of the interaction of carboxylic acids, such as formic acid, with magnetite surfaces is of prime importance for heterogeneous catalysis and the synthesis of novel materials. Despite this, little is known about the atomic scale adsorption mechanisms. Here we show by in-situ surface X-ray diffraction that the oxygen rich subsurface cation vacancy reconstruction of the clean magnetite (001) surface is lifted by dissociative formic acid adsorption, reestablishing a surface with bulk stoichiometry. Using density functional theory, the bulk terminated, fully formic acid covered surface is calculated to be more stable than the corresponding clean, reconstructed surface. A comparison of calculated and experimental infrared bands supports the bidentate adsorption geometry and a specific adsorption site. Our results pave the way for a fundamental understanding of the bonding mechanism at carboxylic acid/oxide interfaces.

Air-Stable Methylammonium Lead Iodide Perovskite Thin Films Fabricated via Aerosol-Assisted Chemical Vapor Deposition from a Pseudohalide Pb(SCN)2 Precursor

Ambient-air-stable methylammonium lead iodide (MAPI) perovskite thin films have been fabricated via one-step aerosol-assisted chemical vapor deposition (AACVD) from a pseudohalide Pb(SCN)2 precursor. We compare both the bulk and surface properties of the perovskite films grown using AACVD with those made by the widely used spin-coating method. Films with larger grain sizes and much better stability in ambient air can be obtained by AACVD. By addition of excess MAI to the precursor solution, MAPI films with negligible PbI2 impurities, as determined by X-ray diffraction, are obtained. The AACVD-grown MAPI films retain high phase purity with limited PbI2 formation after aging in air for approximately one month. The films exhibit an optical bandgap energy of ca. 1.55 eV and the expected nominal bulk stoichiometry (within error). In addition to probing bulk properties, we utilize X-ray photoelectron spectroscopy (XPS) to scrutinize the surface characteristics in detail. We find that the use of excess MAI resul...

Monolayer doping of silicon-germanium alloys: A balancing act between phosphorus incorporation and strain relaxation

This paper presents the application of monolayer doping (MLD) to silicon-germanium (SiGe). This study was carried out for phosphorus dopants on wafers of epitaxially grown thin films of strained SiGe on silicon with varying concentrations of Ge (18%, 30%, and 60%). The challenge presented here is achieving dopant incorporation while minimizing strain relaxation. The impact of high temperature annealing on the formation of defects due to strain relaxation of these layers was qualitatively monitored by cross-sectional transmission electron microscopy and atomic force microscopy prior to choosing an anneal temperature for the MLD drive-in. Though the bulk SiGe wafers provided are stated to have 18%, 30%, and 60% Ge in the epitaxial SiGe layers, it does not necessarily mean that the surface stoichiometry is the same, and this may impact the reaction conditions. X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS were carried out to compare the bulk and surface stoichiometry of SiGe to allow tailoring of the reaction conditions for chemical functionalization. Finally, dopant profiling was carried out by secondary ion mass spectrometry to determine the impurity concentrations achieved by MLD. It is evident from the results that phosphorus incorporation decreases for increasing mole fraction of Ge, when the rapid thermal annealing temperature is a fixed amount below the melting temperature of each alloy.

Extending ζ-factor microanalysis to boron-rich ceramics: Quantification of bulk stoichiometry and grain boundary composition

Accurate quantification of light elements which produce only soft X-ray lines via X-ray energy dispersive spectrometry (XEDS) has been traditionally difficult due to poor X-ray emission and detector efficiencies at low energies and significant X-ray absorption effects. The ζ-factor microanalysis method enables one to correct for these shortcomings; however, ζ-factor microanalysis has not yet been thoroughly applied to inorganic materials which are entirely or mostly composed of light elements such as boron carbide, boron nitride, or boron suboxide. This work successfully extended ζ-factor microanalysis to boron-rich ceramics and accurately determined stoichiometries of multiple boron carbides and measured grain boundary compositions of a boron carbide mixed with additives consisting of rare-earth ions. Various strategies were employed to experimentally determine a full range of ζ-factors and measurements were validated using materials of known composition including silicon hexaboride and silicon carbide. Overall, this work has shown that XEDS is a viable technique for light element quantification in (scanning) transmission electron microscopy, in terms of both the accuracy and precision, which is comparable or superior to the complementary electron energy loss spectrometry.

High‐Temperature Photochromism of Fe‐Doped SrTiO3 Caused by UV‐Induced Bulk Stoichiometry Changes

AbstractThe impact of UV irradiation on Fe‐doped SrTiO3 (Fe:STO) single crystals is investigated at elevated temperatures. Illumination leads to incorporation of oxygen into the single crystals and thus to a decreasing oxygen vacancy concentration and oxidation of Fe3+ to Fe4+. The Fe4+ ions cause a color change from transparent/brownish to black. This photochromic blackening due to stoichiometry changes at elevated temperatures is irreversible at room temperature, but annealing at high temperatures, for example at 700 °C, can restore the original stoichiometry and color. Absorbance changes due to UV irradiation are monitored by ex situ and in situ UV–vis spectroscopy experiments and changes in electrical properties are measured by van der Pauw measurements and in‐plane electrochemical impedance spectroscopy. After 1140 min of illumination at 440 °C, for example, electrical measurements reveal a conductivity increase by more than a factor of 5 due to the enhanced hole concentration in blackened Fe:STO. In addition, UV illumination increases the oxygen chemical potential up to a calculated p(O2) of more than 109 Pa in Fe:STO. Hence, UV light can be used to tune the color, but also electrical properties of Fe:STO by directly impacting the bulk defect concentrations.

Control of surface reactivity towards unsaturated C[sbnd]C bonds and H over Ni-based intermetallic compounds in semi-hydrogenation of acetylene

Intermetallic compounds composed of transition metals and semimetals or post-transition metals have been shown to exhibit elevated selectivity in olefins and aromatics production and other reactions that require control of CC activation and functionalization. The vast number of element combinations available for intermetallic compound production necessitates down-selecting to isolate catalytically useful compositional spaces. In this study, we have investigated the semi-hydrogenation of acetylene over isoelectronic intermetallic compounds composed of nickel and the boron group elements at a bulk stoichiometry of 1:1. We have isolated that orbital overlap between IMC constituent elements tracks well with the nature of their bulk bonding, surface reactivity, and catalytic preference for the semi-hydrogenation of acetylene to ethylene. The trend isolated appears to be universal and marks a subspace within intermetallic compounds that is promising to focus further studies upon. Results also suggest the possibility of accessing New Brønsted–Evans–Polanyi relationships over a subspace of intermetallic compounds.

MoS2 Quantum Dots: Effect of Hydrogenation on Surface Stability and H2S Release

We employ density functional theory to investigate effects of hydrogenation on the energetic stability and electronic properties of triangular MoS2 nanoclusters with S-edges. Excess edge sulfur atoms relative to the bulk stoichiometry along the edges are passivated by hydrogen atoms. We find that the hydrogen coverage for maximum stability can be calculated by (n – 2)/2(n – 1), where n is the number of S atoms along an edge. The energetics reveal a preference for the zigzag arrangement of adsorbed hydrogen atoms on the edges. Our calculations show vanishing HOMO–LUMO gaps mainly due to the presence of dangling bonds at the edges and can be considered metal-like. We find that the activation energy required to release H2S lies in between 0.47 and 0.62 eV, and this value is in good agreement with the recently reported experimental value.