Stoichiometric Variation Research Articles

The Nature of Ion Conduction in Methylammonium Lead Iodide: A Multimethod Approach.

By applying a multitude of experimental techniques including 1H, 14N, 207Pb NMR and 127I NMR/NQR, tracer diffusion, reaction cell and doping experiments, as well as stoichiometric variation, conductivity, and polarization experiments, iodine ions are unambiguously shown to be the mobile species in CH3NH3PbI3, with iodine vacancies shown to represent the mechanistic centers under equilibrium conditions. Pb2+ and CH3NH3 + ions do not significantly contribute to the long range transport (upper limits for their contributions are given), whereby the latter exhibit substantial local motion. The decisive electronic contribution to the mixed conductivity in the experimental window stems from electron holes. As holes can be associated with iodine orbitals, local variations of the iodine stoichiometry may be fast and enable light effects on ion transport.

Elemental stoichiometry and compositions of weevil larvae and two acorn hosts under natural phosphorus variation

To understand how different trophic organisms in a parasite food chain adapt to the differences in soil nutrient conditions, we investigated stoichiometric variation and homeostasis of multiple elements in two acorn trees, Quercus variabilis and Quercus acutissima, and their parasite weevil larvae (Curculio davidi Fairmaire) at phosphorus (P)-deficient and P-rich sites in subtropical China where P-rich ores are scattered among dominant P-deficient soils. Results showed that elemental stoichiometry and compositions of both acorns and weevil larvae differed significantly between P-deficient and P-rich sites (p < 0.05), with the largest contribution of acorn and weevil larva P in distinguishing the stoichiometric compositions between the two site types. The two acorn species were statistically separated by their acorn elemental stoichiometry and compositions (p < 0.05), but no difference was observed on weevil larvae between the two acorn species. P was one of the few elements that were non strict homeostasis in both acorns and weevil larvae. These findings highlight the importance of both environmental influence in elemental stoichiometry and composition and physiological regulations of nutritional needs in organisms and provide possible stoichiometric responses of both plants and animals to P loading, a worldwide issue from excess release of P into the environment.

Specialization to Extremely Low-Nutrient Soils Limits the Nutritional Adaptability of Plant Lineages.

Specialization to extreme selective situations promotes the acquisition of traits whose coadaptive integration may compromise evolutionary flexibility and adaptability. We test this idea in the context of the foliar stoichiometry of plants native to the South African Cape. Whereas foliar concentrations of nitrogen, phosphorus (P), potassium (K), calcium, magnesium, and sodium showed strong phylogenetic signal, as did the foliar ratios of these nutrients to P, the same was not true of the corresponding soil values. In addition, although foliar traits were often related to soil values, the coefficients of determination were consistently low. These results identify foliar stoichiometry as having a strong genetic component, with variation in foliar nutrient concentrations, especially [P] and [K], being identified as potentially adaptive. Comparison of stoichiometric variation across 11 similarly aged clades revealed consistently low foliar nutrient concentrations in lineages showing specialization to extremely low-nutrient fynbos heathlands. These lineages also display lower rates of evolution of these traits as well as a reduced tendency for foliar [P] to track soil [P]. Reduced evolutionary lability and adaptability in the nutritional traits of fynbos-specialist lineages may explain the floristic distinctness of the fynbos flora and implies a reduced scope for edaphically driven ecological speciation.

Determination of U3O8 in UO2 by infrared spectroscopy

The oxygen-uranium (O-U) system has various oxides, such as UO2, U4O9, U3O8, and UO3. Uranium dioxide is the most important one because it is used as nuclear fuel in nuclear power plants. UO2 can have a wide stoichiometric variation due to excess or deficiency of oxygen in its crystal lattice, which can cause significant modifications of its proprieties. O/U relation determination by gravimetry cannot differentiate a stoichiometric deviation from contents of other uranium oxides in UO2. The presence of other oxides in the manufacturing of UO2 powder or sintered pellets is a critical factor. Fourier Transform Infrared Spectroscopy (FTIR) was used to identify U3O8 in samples of UO2 powder. UO2 can be identified by bands at 340 cm-1 and 470 cm-1, and U3O8 and UO3 by bands at 735 cm-1, 910 cm-1, respectively. The methodology for sample preparation for FTIR spectra acquisition is presented, as well as the calibration for quantitative measurement of U3O8 in UO2. The content of U3O8 in partially calcined samples of UO2 powder was measured by FTIR with good agreement with X-rays diffractometry (XRD).

Stoichiometric variation of halophytes in response to changes in soil salinity.

Variation in soil salt may change the stoichiometry of a halophyte by altering plant ecophysiology, and exert different influences on various plant organs, which has potentially important consequences for the nutrition of consumers as well as nutrient cycling in a saline ecosystem. Using a greenhouse pot experiment, we investigated the effect of salinity variability on the growth and stoichiometry of different organs of Suaeda glauca and Salicornia europaea - two dominant species of important ecological and economic value in the saline ecosystem. Our results showed that appropriate salt stimulated the growth of both species during the vigorous growth period, while high salt suppressed growth. Na significantly increased with increased salt in the culture, whereas concentrations of other measured elements and K:Na ratio for both species significantly decreased at low salt treatments, and became more gradual under higher salt conditions. Furthermore, with the change of salt in culture, variations in leaf (degenerated leaf for S.europaea, considered as young stem) stoichiometry, except N:P ratio, were large and less in stems (old stems for S.europaea) than in roots, reflecting physiological and biochemical reactions in the leaf in response to salt stress, supported by sharp changes in trends. These results suggest that appropriate saline conditions can enhance biological C fixation of halophytes; however, increasing salt could affect consumer health and decrease cycling of other nutrients in saline ecosystems.

Deciphering the mechanistic insight into the stoichiometric ratio dependent behavior of Cu(II) on BSA fibrillation

Various metal ions are recently implicated in protein aggregates and are associated with numerous neurodegenerative diseases. In the present work, we have scrutinized the effect of stoichiometric variation of Cu(II) on BSA fibrillation at physiological pH 7.4 through Thioflavin-T dye binding study, residual protein concentration, Fourier Transform Infrared spectroscopy (FTIR), Dynamic Light Scattering (DLS), Atomic Force Microscopy (AFM) and Isothermal Titration Calorimetry (ITC). Fibrillation kinetics was studied through ThT fluorescence, which illustrated dependency on stoichiometric ratio of Cu(II). The ThT intensity data were fitted to a single exponential expression to determine the aggregation rate, k, which revealed a slight lower k value for 1:1 ratio of BSA-Cu(II) reaction as compared to BSA alone whereas k value for 1:2 ratio of BSA-Cu(II) reaction was higher. Higher equilibrium residual concentration in case of 1:1 ratio of BSA-Cu(II) agreed to the lower aggregation rate. IR spectroscopy revealed the presence of increased β-sheet proportion to the detriment of α-helix conformation with increasing concentration of Cu(II) and illustrated maximum β-sheet proportion in 1:2 ratio of BSA-Cu(II). These results were combined with scattering results that showed the higher average hydrodynamic radius (Z-average value) of aggregates in 1:2 ratio of BSA-Cu(II) with respect to BSA and 1:1 ratio of BSA-Cu(II). AFM analysis confirmed the fibril formation. ITC analysis has shown the presence of two binding sites with many fold difference in binding affinity at prescribed in vitro conditions. An N-terminal binding site with many fold higher binding affinity was found as the first Cu(II) binding site. The free thiol group of the cysteine residue positioned at 34 (cys-34) in BSA was covalently capped and this modified BSA also showed the presence of two binding sites, which declined the cys-34 site as the second Cu(II) bind site. Therefore Zn(II) binding site was predicted as the second Cu(II) binding site. The binding of Cu(II) at N-terminal binding site marks a square planner geometry; while that at zinc binding site marks a rigid coordination geometry. Conclusively, analyzed data has concluded that the variation in Cu(II) efficacy in the different stoichiometric ratio results because of two different coordination types in different stoichiometric ratios.

Investigation on synthesis, growth and characterization of CdIn2S2Se2 single crystal grown by vertical Bridgman method

CdIn2S2Se2 polycrystalline material has been synthesized by melt oscillation method. Vertical Bridgman method was used to grow a good quality CdIn2S2Se2 single crystal. The crystalline phase and growth orientation were confirmed by powder X-ray diffraction pattern and unit cell parameters were determined by single crystal X-ray diffraction analysis. The structural uniformity of CdIn2S2Se2 was studied using Raman scattering spectroscopy at room temperature. The stoichiometric composition variation along the CdIn2S2Se2 was measured using energy dispersive spectrometry. The transmission spectra of CdIn2S2Se2 single crystal gave 42% transmission in the NIR region. Thermal property of CdIn2S2Se2 has been studied using differential thermal analysis. Thermal diffusivity, specific heat capacity and thermal conductivity were also measured. Electrical property was measured using Hall Effect measurement and it confirms the n-type semiconducting nature. The hardness behavior has been measured using Vickers micro hardness measurement and the indentation size effect has been observed.

Ternary Ta2NiSe5 Flakes for a High‐Performance Infrared Photodetector

Multielemental systems enable the use of multiple degrees of freedom for control of physical properties by means of stoichiometric variation. This has attracted extremely high interest in the field of 2D optoelectronics in recent years. Here, for the first time, multilayer 2D ternary Ta2NiSe5 flakes are successfully fabricated using a mechanical exfoliation method from chemical vapor transport synthesized high quality bulk and the optoelectronic properties are systematically investigated. Importantly, a high responsivity of 17.21 A W−1 and high external quantum efficiency of 2645% are recorded from an as‐fabricated photodetector at room temperature in air; this is superior to most other 2D materials‐based photodetectors that have been reported. More intriguingly, a usual sublinear and an unusual superlinear light‐intensity‐dependent photocurrent are observed under air and vacuum, respectively. These excellent and special properties make multilayer ternary Ta2NiSe5 a highly competitive candidate for future infrared optoelectronic applications and an interesting platform for photophysics studies.

Synergistic, ultrafast mass storage and removal in artificial mixed conductors.

Mixed conductors-single phases that conduct electronically and ionically-enable stoichiometric variations in a material and, therefore, mass storage and redistribution, for example, in battery electrodes. We have considered how such properties may be achieved synergistically in solid two-phase systems, forming artificial mixed conductors. Previously investigated composites suffered from poor kinetics and did not allow for a clear determination of such stoichiometric variations. Here we show, using electrochemical and chemical methods, that a melt-processed composite of the 'super-ionic' conductor RbAg4I5 and the electronic conductor graphite exhibits both a remarkable silver excess and a silver deficiency, similar to those found in single-phase mixed conductors, even though such behaviour is not possible in the individual phases. Furthermore, the kinetics of silver uptake and release is very fast. Evaluating the upper limit of the relaxation time [corrected] set by interfacial ambipolar diffusion reveals chemical diffusion coefficients that are even higher than those achieved for sodium chloride in bulk liquid water. These results could potentially stimulate systematic research into powerful, even mesoscopic, artificial mixed conductors.

Thermochemical properties of selected ternary phases in the Ag–Bi–S system

Thermodynamic properties of the equilibrium phases matildite (β-AgBiS2), schapbachite (α-AgBiS2), and pavonite (AgBi3S5) with their stoichiometric variations have been studied. The ternary phase AgBiS2 was synthesized from the pure binary sulfides, and characterized by the SEM–EDS and X-ray powder diffraction techniques. Thermal analysis of the synthesized phase was done by the differential scanning calorimetry and thermogravimetry techniques. Based on the calorimetric measurements, phase transformation and melting temperatures as well as enthalpies of AgBiS2 have been determined. Thermodynamic properties of both AgBiS2 and AgBi3S5, including their stoichiometric variations at saturations of bismuth and sulfur, have been compiled and critically evaluated. Heat capacities of both phases have been estimated with two different estimation methods. The estimated results have been compared with the available experimental literature data. Despite the fact that the estimation methods are partly based on assumptions, the obtained values are generally in reasonable agreement with the available literature data. The obtained results are presented and discussed.

Impacts of Ion Segregation on Local Optical Properties in Mixed Halide Perovskite Films.

Despite the recent astronomical success of organic-inorganic perovskite solar cells (PSCs), the impact of microscale film inhomogeneities on device performance remains poorly understood. In this work, we study CH3NH3PbI3 perovskite films using cathodoluminescence in scanning transmission electron microscopy and show that localized regions with increased cathodoluminescence intensity correspond to iodide-enriched regions. These observations constitute direct evidence that nanoscale stoichiometric variations produce corresponding inhomogeneities in film cathodoluminescence intensity. Moreover, we observe the emergence of high-energy transitions attributed to beam induced iodide segregation, which may mirror the effects of ion migration during PSC operation. Our results demonstrate that such ion segregation can fundamentally change the local optical and microstructural properties of organic-inorganic perovskite films in the course of normal device operation and therefore address the observed complex and unpredictable behavior in PSC devices.

Diel variability in the elemental composition of the marine cyanobacteriumSynechococcus

Journal of Plankton Research plankt.oxfordjournals.org J. Plankton Res. (2016) 38(4): 1052–1061. First published online January 18, 2016 doi:10.1093/plankt/fbv120 Contribution to the Themed Section: Scaling from individual p lankton to marine ecosystems Diel variability in the elemental composition of the marine cyanobacterium Synechococcus JOHANN S. LOPEZ 1 , NATHAN S. GARCIA 1 *, DAVID TALMY 2 AND ADAM C. MARTINY 1,3 DEPARTMENT OF EARTH SYSTEM SCIENCE , UNIVERSITY OF CALIFORNIA , IRVINE , CA , USA , DEPARTMENT OF EARTH , ATMOSPHERE AND PLANETARY SCIENCES , MASSACHUSETTS INSTITUTE OF TECHNOLOGY , CAMBRIDGE , MA , USA AND DEPARTMENT OF ECOLOGY AND EVOLUTIONARY BIOLOGY , UNIVERSITY OF CALIFORNIA , IRVINE , CA , USA * CORRESPONDING AUTHOR : n8garcia@gmail.com Received August 26, 2015; accepted December 10, 2015 Corresponding editor: Pia Moisander The ratio of elements such as carbon:nitrogen:phosphorus (C:N:P) in phytoplankton is known to vary substantially within single isolates and across environmental gradients. In addition, C:N:P is known to vary throughout the day due to diel patterns in nutrient acquisition and storage. It has been hypothesized that small phytoplankton such as marine cyanobacteria have relatively invariable elemental ratios during a 24 h period, whereas larger phytoplankton have a greater capacity to store elements and thus a wider diel range of C:N:P. To test this hypothesis, we examined diel vari- ability in cellular C:N:P, using a chemostat culturing system, for one of the most abundant marine cyanobacteria, Synechococcus (WH8102) during two 24 h periods. The cellular C quota nearly doubled during the 14 h light period and was subsequently reduced during the dark period. The cellular N quota also varied considerably, whereas the P quota remained relatively stable. These daily changes in elemental quotas led to highly variable C:N cell and C:P cell . Furthermore, the magnitude of variability in cellular elemental stoichiometry of Synechococcus was positively related to the growth rate. We constructed a model to test the extent to which variation in C:N cell and C:P cell is related to reserve carbon accumulation and depletion over each light –dark cycle. Results imply that, in addition to growth-related respiratory losses, Synechococcus also purges excess C during the dark period in order to maintain a nutritive balance within cells. Our data suggest that diel variation in C:N cell and C:P cell of Synechococcus is of the same order of magnitude as stoichiometric variation within plankton communities between major ocean environments. KEYWORDS: cyanobacteria; Synechococcus; stoichiometry; diel; Redfield; phytoplankton available online at www.plankt.oxfordjournals.org # The Author 2016. Published by Oxford University Press. All rights reserved. For permissions, please email: journals.permissions@oup.com

Natural Attributes and Agricultural Implications of Somatic Genome Variation

This article proposes the concept of genome network, describes different variations of the somatic genome network, and reviews the agricultural implications of such variations. All genetic materials in a cell constitute the genome network of the cell and can jointly influence the cell's function and fate. The somatic genome of a plant is the genome network of cells in somatic tissues and of nonreproductive cells in pollen and ovules. Somatic genome variation (SGV, approximately equivalent to somagenetic variation) occurs at multiple levels, including stoichiometric, ploidy, and sequence variations. For a multicellular organism, the term "somatic genome variation" covers both the variation in part of the organism and the generation of new genotype individuals through somatic means from a sexually produced original genotype. For unicellular organisms, genome variation in somatic nuclei occurs at the whole organism level because there is only a single cell per individual. Growth, development and evolution of living organisms require both stability and instability of their genomes. Somatic genome variation displays many more attributes than genetic mutation and has strong implications for agriculture.

“Fertile island” effects of Tamarix chinensis Lour. on soil N and P stoichiometry in the coastal wetland of Laizhou Bay, China

As a useful comprehensive index for reflecting nutrient cycling in soils, nitrogen (N) and phosphorus (P) stoichiometry is subject to influences of many external environmental and biological factors. Studies on such influences were limited, and the influential mechanism remains unclear. The purpose of this research is to investigate soil N and P stoichiometric variations and analyze “fertile island” effects of Tamarix chinensis Lour. (T. chinensis) in the coastal wetland of Laizhou Bay in China. Soil samples beneath clusters and communities of T. chinensis were collected respectively in July 2012. Amounts of ammonium, nitrate, and available phosphorus in the soil samples were measured through the corresponding standard methods for material measuring. In general, there were significant vertical variations in soil N and P stoichiometry beneath clusters and communities of T. chinensis. A downtrend was observed for N and P contents with the increase in soil depth. On the contrary, the N/P ratio revealed a trend of going up first and then dropping off along with the increase of the soil depth. Comparatively, the horizontal variations in the soil N and P stoichiometry beneath a single cluster of T. chinensis were greater in the topsoil than those in the subsoil. The N and P contents gradually decreased from the canopy center to the outside. On the contrary, an opposite trend was found for the N/P ratio. For the horizontal variations beneath T. chinensis communities, there were no significant differences for either N and P contents or N/P ratios. Similar to the ecosystems in arid and semi-arid areas, vegetations in many semi-humid areas could also form fertile islands and exert significant influences on the soil nutrient cycle. The formation of fertile islands beneath a single cluster of T. chinensis could have significant influence on soil N and P stoichiometry. Under the influence of fertile islands beneath T. chinensis, the limiting element of the biogeochemical processes in the coastal wetland of Laizhou Bay might change from N to P. However, the influences of fertile island effects on soil N and P stoichiometry beneath T. chinensis communities were relatively small, illustrating that the influences of fertile island effects was not significant at the community level. Thus, the impacts of environmental factors on soil N and P stoichiometry might be greater than that of the fertile island effects in the wetland on a larger scale.

Stoichiometric variation of carbon, nitrogen, and phosphorus in soils and its implication for nutrient limitation in alpine ecosystem of Eastern Tibetan Plateau

The main objectives of this research are to decipher the stoichiometric characteristics of carbon (C), nitrogen (N), and phosphorus (P) in soils from the alpine ecosystem and to obtain information about nutrient limitation on plants and microbes.

Dirac states with knobs on: Interplay of external parameters and the surface electronic properties of three-dimensional topological insulators

Topological insulators are a novel materials platform with high applications potential in fields ranging from spintronics to quantum computation. In the ongoing scientific effort to demonstrate controlled manipulation of their electronic structure by external means, i.e., the provision of knobs with which to tune properties, stoichiometric variation and surface decoration are two effective approaches that have been followed. In angle-resolved photoelectron spectroscopy (ARPES) experiments, both approaches are seen to lead to electronic band-structure changes. Most importantly, such approaches result in variations of the energy position of bulk and surface-related features and the creation of two-dimensional electron gases. The data presented here demonstrate that a third manipulation handle is accessible by utilizing the amount of super-band-gap light a topological insulator surface has been exposed to under typical ARPES experimental conditions. Our results show that this third knob acts on an equal footing with stoichiometry and surface decoration as a modifier of the electronic band structure, and that it is in continuous and direct competition with the latter. The data clearly point towards surface photovoltage and photoinduced desorption as the physical phenomena behind modifications of the electronic band structure under exposure to high-flux photons. We show that the interplay of these phenomena can minimize and even eliminate the adsorbate-related surface band bending on typical binary, ternary, and quaternary Bi-based topological insulators. Including the influence of the sample temperature, these data set up a detailed framework for the external control of the electronic band structure in topological insulator compounds in an ARPES setting. Four external knobs are available: bulk stoichiometry, surface decoration, temperature, and photon exposure. These knobs can be used in conjunction to fine tune the band energies near the surface and consequently influence the topological properties of the relevant electronic states.

ChemInform Abstract: Heterogeneous Foldamers from Aliphatic—Aromatic Amino Acid Building Blocks: Current Trends and Future Prospects

AbstractReview: 105 refs.

Electrochemical and structural effects of in situ Li2O extraction from Li2MnO3 for Li-Ion batteries.

Li2MnO3 is an attractive cathode material due to its low cost, nontoxicity and potentially high capacity. However, its electrochemical inactivity, its poor electronic conductivity, and uncertainty about its underlying mechanism have limited its development. In this work, an in situ technique for extraction of Li and O during deposition of the thin film cathode is developed to investigate structural and electrochemical effects in a controlled fashion. MnO2 has been observed in samples with severe O and Li deficiency (capacity of 115 mAh g(-1)), while Li2MnO3 cathodes with slight excess of O and Li (capacity of 225 mAh g(-1)) can be synthesized by tuning growth conditions appropriately. Formation of a MnO2 phase, especially in Li and O deficient structures, could be a possible reason for irreversible capacity loss in Li2MnO3 related materials. Further investigation into stoichiometric and microstructure variations enabled by this technique allows rapid investigation of Li2MnO3 as well as other Li-rich composites.

Near Surface Stoichiometry in UO2: A Density Functional Theory Study

The mechanisms of oxygen stoichiometry variation in UO2at different temperature and oxygen partial pressure are important for understanding the dynamics of microstructure in these crystals. However, very limited experimental studies have been performed to understand the atomic structure of UO2near surface and defect effects of near surface on stoichiometry in which the system can exchange atoms with the external reservoir. In this study, the near (110) surface relaxation and stoichiometry in UO2have been studied with density functional theory (DFT) calculations. On the basis of the point-defect model (PDM), a general expression for the near surface stoichiometric variation is derived by using DFT total-energy calculations and atomistic thermodynamics, in an attempt to pin down the mechanisms of oxygen exchange between the gas environment and defected UO2. By using the derived expression, it is observed that, under poor oxygen conditions, the stoichiometry of near surface is switched from hyperstoichiometric at 300 K with a depth around 3 nm to near-stoichiometric at 1000 K and hypostoichiometric at 2000 K. Furthermore, at very poor oxygen concentrations and high temperatures, our results also suggest that the bulk of the UO2prefers to be hypostoichiometric, although the surface is near-stoichiometric.

Heterogeneous Foldamers from Aliphatic–Aromatic Amino Acid Building Blocks: Current Trends and Future Prospects

AbstractChemists' constant pursuit of understanding of the underlying principles of nature's most intricate phenomenon such as protein folding has led to the development of the field of “foldamers”. The emergence of diverse classes of unnatural amino acid building blocks has unleashed countless opportunities to design, develop and explore the structural and functional aspects of synthetic peptides. One current trend in foldamer chemistry is the heterofoldamer approach, which involves systematic stoichiometric variation of various natural/unnatural amino acid residues, leading to conformational ordering with intriguing structural architectures. In this regard, the incorporation of aromatic amino acids provides efficient structural rigidification and tunability to the molecular scaffolds, which can exhibit a range of secondary structural features. Recent times have witnessed an upsurge of foldamers featuring aliphatic‐aromatic residues with diverse structural propensities. This review is an effort to cover this rapidly developing field of foldamer science and also to envisage its future perspectives.