Spatial Electron-hole Separation in a One Dimensional Hybrid Organic-Inorganic Lead Iodide.

The increasing efficiency of the inorganic-organic hybrid halides has revolutionised photovoltaic research. Despite this rapid progress, the significant issues of poor stability and toxicity have yet to be suitably overcome. In this article, we use Density Functional Theory to examine (Pb2I6) · (H2DPNDI) · (H2O) · (NMP), an alternative lead-based hybrid inorganic-organic solar absorber based on a photoactive organic cation. Our results demonstrate that optical properties suitable for photovoltaic applications, in addition to spatial electron-hole separation, are possible but efficient charge transport may be a limiting factor

Anion Distribution, Structural Distortion, and Symmetry-Driven Optical Band Gap Bowing in Mixed Halide Cs2SnX6 Vacancy Ordered Double Perovskites.

Mixed anion compounds in the Fm3̅m vacancy ordered perovskite structure were synthesized and characterized experimentally and computationally with a focus on compounds where A = Cs+. Pure anion Cs2SnX6 compounds were formed with X = Cl, Br, and I using a room temperature solution phase method. Mixed anion compounds were formed as solid solutions of Cs2SnCl6 and Cs2SnBr6 and a second series from Cs2SnBr6 and Cs2SnI6. Single phase structures formed across the entirety of both composition series with no evidence of long-range anion ordering observed by diffraction. A distortion of the cubic A2BX6 structure was identified in which the spacing of the BX6 octahedra changes to accommodate the A site cation without reduction of overall symmetry. Optical band gap values varied with anion composition between 4.89 eV in Cs2SnCl6 to 1.35 eV in Cs2SnI6 but proved highly nonlinear with changes in composition. In mixed halide compounds, it was found that lower energy optical transitions appeared that were not present in the pure halide compounds, and this was attributed to lowering of the local symmetry within the tin halide octahedra. The electronic structure was characterized by photoemission spectroscopy, and Raman spectroscopy revealed vibrational modes in the mixed halide compounds that could be assigned to particular mixed halide octahedra. This analysis was used to determine the distribution of octahedra types in mixed anion compounds, which was found to be consistent with a near-random distribution of halide anions throughout the structure, although some deviations from random halide distribution were noted in mixed iodide-bromide compounds, where the larger iodide anions preferentially adopted trans configurations

Phase Quantification of Heterogeneous Surfaces Using DFT-Simulated Valence Band Photoemission Spectra

Quantifying the crystallographic phases present at a surface is an important challenge in fields such as functional materials and surface science. X-ray photoelectron spectroscopy (XPS) is routinely employed in surface characterization to identify and quantify chemical species through core line analysis. Valence band (VB) spectra contain characteristic but complex features that provide information on the electronic density of states (DoS) and thus can be understood theoretically using density functional theory (DFT). Here, we present a method of fitting experimental photoemission spectra with DFT models for quantitative analysis of heterogeneous systems, specifically mapping the anatase to rutile ratio across the surface of mixed-phase TiO2 thin films. The results were correlated with mapped photocatalytic activity measured using a resazurin-based smart ink. This method allows large-scale functional and surface composition mapping in heterogeneous systems and demonstrates the unique insights gained from DFT-simulated spectra on the electronic structure origins of complex VB spectral features

Hybrid Organic-Inorganic Coordination Complexes as Tunable Optical Response Materials.

Novel lead and bismuth dipyrido complexes have been synthesized and characterized by single-crystal X-ray diffraction, which shows their structures to be directed by highly oriented π-stacking of planar fully conjugated organic ligands. Optical band gaps are influenced by the identity of both the organic and inorganic component. Density functional theory calculations show optical excitation leads to exciton separation between inorganic and organic components. Using UV-vis, photoluminescence, and X-ray photoemission spectroscopies, we have determined the materials' frontier energy levels and show their suitability for photovoltaic device fabrication by use of electron- and hole-transport materials such as TiO2 and spiro-OMeTAD respectively. Such organic/inorganic hybrid materials promise greater electronic tunability than the inflexible methylammonium lead iodide structure through variation of both the metal and organic components

Enhanced visible light absorption in layered Cs3Bi2Br9 through mixed-valence Sn(ii)/Sn(iv) doping

Funder: Cambridge TrustMixed valence Sn doping of Cs3Bi2Br9 leads to broad visible light absorption.</jats:p

Normative Reference Magnets

The concept of moral wrongness, many think, has a distinctive kind of referential stability, brought out by moral twin earth cases. This article offers a new account of the source of this stability, deriving it from a metaphysics of content: “substantive” radical interpretation, and first-order normative assumptions. This story is distinguished from extant “reference magnetic” explanations of the phenomenon, and objections and replies are considered

A 915 MHz wristwatch-integrated antenna for wireless health monitoring

A compact 915 MHz antenna integrated within a wristwatch wireless sensor device is presented. The antenna is a variant of a planar inverted-F antenna (PIFA) and uses a dual-resonator configuration. The results of simulation and measurement are shown to be in good agreement with the antenna exhibiting desirable impedance and radiation characteristics together with low Specific Absorption Rate (SAR) performance. The antenna is fabricated using a low cost flexible printed circuit and is fully integrated into the watch device. Measurements on the prototype antenna show a -10 dB impedance bandwidth of 30 MHz, a peak realized gain of -4.9 dBi and a peak radiation efficiency of 15.9% at 915 MHz. The antenna also has a low SAR value of 0.003 W/kg making it suitable for a wide range of wrist-worn wireless applications

Peak grain forecasts for the US High Plains amid withering waters

Irrigated agriculture contributes 40% of total global food production. In the US High Plains, which produces more than 50 million tons per year of grain, as much as 90% of irrigation originates from groundwater resources, including the Ogallala aquifer. In parts of the High Plains, groundwater resources are being depleted so rapidly that they are considered nonrenewable, compromising food security. When groundwater becomes scarce, groundwater withdrawals peak, causing a subsequent peak in crop production. Previous descriptions of finite natural resource depletion have utilized the Hubbert curve. By coupling the dynamics of groundwater pumping, recharge, and crop production, Hubbert-like curves emerge, responding to the linked variations in groundwater pumping and grain production. On a state level, this approach predicted when groundwater withdrawal and grain production peaked and the lag between them. The lags increased with the adoption of efficient irrigation practices and higher recharge rates. Results indicate that, in Texas, withdrawals peaked in 1966, followed by a peak in grain production 9 y later. After better irrigation technologies were adopted, the lag increased to 15 y from 1997 to 2012. In Kansas, where these technologies were employed concurrently with the rise of irrigated grain production, this lag was predicted to be 24 y starting in 1994. In Nebraska, grain production is projected to continue rising through 2050 because of high recharge rates. While Texas and Nebraska had equal irrigated output in 1975, by 2050, it is projected that Nebraska will have almost 10 times the groundwater-based production of Texas