In Situ Study of Silicon Electrode Lithiation with X‑ray Reflectivity

Surface sensitive X-ray reflectivity (XRR) measurements were performed to investigate the electrochemical lithiation of a native oxide terminated single crystalline silicon (100) electrode in real time during the first galvanostatic discharge cycle. This allows us to gain nanoscale, mechanistic insight into the lithiation of Si and the formation of the solid electrolyte interphase (SEI). We describe an electrochemistry cell specifically designed for in situ XRR studies and have determined the evolution of the electron density profile of the lithiated Si layer (Li_<i>x</i>Si) and the SEI layer with subnanometer resolution. We propose a three-stage lithiation mechanism with a reaction limited, layer-by-layer lithiation of the Si at the Li_<i>x</i>Si/Si interface

Experimental Characterization of a Theoretically Designed Candidate p‑Type Transparent Conducting Oxide: Li-Doped Cr₂MnO₄

The development of a p-type transparent conducting oxide (p-TCO) requires the deliberate design of a wide band gap and high hole conductivity. Using high-throughput theoretical screening, Cr₂MnO₄ was earlier predicted to be a p-TCO when doped with lithium. This constitutes a new class of p-TCO, one based on a tetrahedrally coordinated d⁵ cation. In this study, we examine and experimentally validate a few central properties of this system. Combined neutron diffraction and anomalous X-ray diffraction experiments give site occupancy that supports the theoretical prediction that lithium occupies the tetrahedral (Mn) site. The lattice parameter of the spinel decreases with lithium content to a solubility limit of [Li]/([Li] + [Mn]) ∼ 9.5%. Diffuse reflectance spectroscopy measurements show that at higher doping levels the transparency is diminished, which is attributed to both the presence of octahedral Mn and the increased hole content. Room-temperature electrical measurements of doped samples reveal an increase in conductivity of several orders of magnitude as compared to that of undoped samples, and high-temperature measurements show that Cr₂MnO₄ is a band conductor, as predicted by theory. The overall agreement between theory and experiment illustrates the advantages of a theory-driven approach to materials design

Synthesis, Characterization, and Calculated Electronic Structure of the Crystalline Metal–Organic Polymers [Hg(SC₆H₄S)(en)]_<i>n</i> and [Pb(SC₆H₄S)(dien)]_<i>n</i>

The reaction of Hg­(OAc)₂ with 1,4-benzenedithiol in ethylenediamine at 80 °C yields [Hg­(SC₆H₄S)­(en)]_<i>n</i>, while the reaction of Pb­(OAc)₂ with 1,4-benzenedithiol in diethylenetriamine at 130 °C yields [Pb­(SC₆H₄S)­(dien)]_<i>n</i>. Both products are crystalline materials, and structure determination by synchrotron X-ray powder diffraction revealed that both are essentially one-dimensional metal–organic polymers with -M-SC₆H₄S- repeat units. Diffuse reflectance UV–visible spectroscopy indicates band gaps of 2.89 eV for [Hg­(SC₆H₄S)­(en)]_<i>n</i> and 2.54 eV for [Pb­(SC₆H₄S)­(dien)]_<i>n</i>, while density functional theory (DFT) band structure calculations yielded band gaps of 2.24 and 2.10 eV, respectively. The two compounds are both infinite polymers of metal atoms linked by 1,4-benzenedithiolate, the prototypical molecule for single-molecule conductivity studies, yet neither compound has significant electrical conductivity as a pressed pellet. In the case of [Pb­(SC₆H₄S)­(dien)]_<i>n</i> calculations indicate fairly flat bands and therefore low carrier mobilities, while the conduction band of [Hg­(SC₆H₄S)­(en)]_<i>n</i> does have moderate dispersion and a calculated electron effective mass of 0.29·<i>m</i>_<i>e</i>. Hybridization of the empty Hg 6s orbital with SC₆H₄S orbitals in the conduction band leads to the band dispersion, and suggests that similar hybrid materials with smaller band gaps will be good semiconductors

Synthesis, Characterization, and Calculated Electronic Structure of the Crystalline Metal–Organic Polymers [Hg(SC₆H₄S)(en)]_<i>n</i> and [Pb(SC₆H₄S)(dien)]_<i>n</i>

The reaction of Hg­(OAc)₂ with 1,4-benzenedithiol in ethylenediamine at 80 °C yields [Hg­(SC₆H₄S)­(en)]_<i>n</i>, while the reaction of Pb­(OAc)₂ with 1,4-benzenedithiol in diethylenetriamine at 130 °C yields [Pb­(SC₆H₄S)­(dien)]_<i>n</i>. Both products are crystalline materials, and structure determination by synchrotron X-ray powder diffraction revealed that both are essentially one-dimensional metal–organic polymers with -M-SC₆H₄S- repeat units. Diffuse reflectance UV–visible spectroscopy indicates band gaps of 2.89 eV for [Hg­(SC₆H₄S)­(en)]_<i>n</i> and 2.54 eV for [Pb­(SC₆H₄S)­(dien)]_<i>n</i>, while density functional theory (DFT) band structure calculations yielded band gaps of 2.24 and 2.10 eV, respectively. The two compounds are both infinite polymers of metal atoms linked by 1,4-benzenedithiolate, the prototypical molecule for single-molecule conductivity studies, yet neither compound has significant electrical conductivity as a pressed pellet. In the case of [Pb­(SC₆H₄S)­(dien)]_<i>n</i> calculations indicate fairly flat bands and therefore low carrier mobilities, while the conduction band of [Hg­(SC₆H₄S)­(en)]_<i>n</i> does have moderate dispersion and a calculated electron effective mass of 0.29·<i>m</i>_<i>e</i>. Hybridization of the empty Hg 6s orbital with SC₆H₄S orbitals in the conduction band leads to the band dispersion, and suggests that similar hybrid materials with smaller band gaps will be good semiconductors

Understanding the Active Sites of CO Hydrogenation on Pt–Co Catalysts Prepared Using Atomic Layer Deposition

The production of liquid fuels and industrial feedstocks from renewable carbon sources is an ongoing scientific challenge. Using atomic layer deposition together with conventional techniques, we synthesize Pt–Co bimetallic catalysts that show improvement for syngas conversion to alcohols. By combining reaction testing, X-ray diffraction, electron microscopy, and <i>in situ</i> infrared spectroscopy experiments, supported by density functional theory calculations, we uncover insights into how Pt modulates the selectivity of Co catalysts. The prepared Pt–Co catalysts demonstrate increased selectivity toward methanol and low molecular weight hydrocarbons as well as a modest increase in selectivity toward higher alcohols. The <i>in situ</i> infrared spectroscopic measurements suggest that these changes in selectivity result from an interplay between linear and bridging carbon monoxide configurations on the catalyst surface

Monitoring a Silent Phase Transition in CH₃NH₃PbI₃ Solar Cells via <i>Operando</i> X‑ray Diffraction

The relatively modest temperature of the tetragonal-to-cubic phase transition in CH₃NH₃PbI₃ perovskite is likely to occur during real world operation of CH₃NH₃PbI₃ solar cells. In this work, we simultaneously monitor the structural phase transition of the active layer along with solar cell performance as a function of the device operating temperature. The tetragonal to cubic phase transition is observed in the working device to occur reversibly at temperatures between 60.5 and 65.4 °C. In these <i>operando</i> measurements, no discontinuity in the device performance is observed, indicating electronic behavior that is insensitive to the structural phase transition. This decoupling of device performance from the change in long-range order across the phase transition suggests that the optoelectronic properties are primarily determined by the local structure in CH₃NH₃PbI₃. That is, while the average crystal structure as probed by X-ray diffraction shows a transition from tetragonal to cubic, the local structure generally remains well characterized by uncorrelated, dynamic octahedral rotations that order at elevated temperatures but are unchanged locally

Scalable Synthesis and Characterization of Multilayer γ‑Graphyne, New Carbon Crystals with a Small Direct Band Gap

γ-Graphyne is the most symmetric sp2/sp1 allotrope of carbon, which can be viewed as graphene uniformly expanded through the insertion of two-carbon acetylenic units between all the aromatic rings. To date, synthesis of bulk γ-graphyne has remained a challenge. We here report the synthesis of multilayer γ-graphyne through crystallization-assisted irreversible cross-coupling polymerization. A comprehensive characterization of this new carbon phase is described, including synchrotron powder X-ray diffraction, electron diffraction, lateral force microscopy, Raman spectroscopy, infrared spectroscopy, and cyclic voltammetry. Experiments indicate that γ-graphyne is a 0.48 eV band gap semiconductor, with a hexagonal a-axis spacing of 6.88 Å and an interlayer spacing of 3.48 Å, which is consistent with theoretical predictions. The observed crystal structure has an aperiodic sheet stacking. The material is thermally stable up to 240 °C but undergoes transformation at higher temperatures. While conventional 2D polymerization and reticular chemistry rely on error correction through reversibility, we demonstrate that a periodic covalent lattice can be synthesized under purely kinetic control. The reported methodology is scalable and inspires extension to other allotropes of the graphyne family

Scalable Synthesis and Characterization of Multilayer γ‑Graphyne, New Carbon Crystals with a Small Direct Band Gap

γ-Graphyne is the most symmetric sp2/sp1 allotrope of carbon, which can be viewed as graphene uniformly expanded through the insertion of two-carbon acetylenic units between all the aromatic rings. To date, synthesis of bulk γ-graphyne has remained a challenge. We here report the synthesis of multilayer γ-graphyne through crystallization-assisted irreversible cross-coupling polymerization. A comprehensive characterization of this new carbon phase is described, including synchrotron powder X-ray diffraction, electron diffraction, lateral force microscopy, Raman spectroscopy, infrared spectroscopy, and cyclic voltammetry. Experiments indicate that γ-graphyne is a 0.48 eV band gap semiconductor, with a hexagonal a-axis spacing of 6.88 Å and an interlayer spacing of 3.48 Å, which is consistent with theoretical predictions. The observed crystal structure has an aperiodic sheet stacking. The material is thermally stable up to 240 °C but undergoes transformation at higher temperatures. While conventional 2D polymerization and reticular chemistry rely on error correction through reversibility, we demonstrate that a periodic covalent lattice can be synthesized under purely kinetic control. The reported methodology is scalable and inspires extension to other allotropes of the graphyne family

Antiferromagnetism in a Family of <i>S</i> = 1 Square Lattice Coordination Polymers NiX₂(pyz)₂ (X = Cl, Br, I, NCS; pyz = Pyrazine)

The crystal structures of Ni<i>X</i>₂(pyz)₂ (X = Cl (<b>1</b>), Br (<b>2</b>), I (<b>3</b>), and NCS (<b>4</b>)) were determined by synchrotron X-ray powder diffraction. All four compounds consist of two-dimensional (2D) square arrays self-assembled from octahedral NiN₄<i>X</i>₂ units that are bridged by pyz ligands. The 2D layered motifs displayed by <b>1</b>–<b>4</b> are relevant to bifluoride-bridged [Ni­(HF₂)(pyz)₂]­<i>E</i>F₆ (<i>E</i> = P, Sb), which also possess the same 2D layers. In contrast, terminal <i>X</i> ligands occupy axial positions in <b>1</b>–<b>4</b> and cause a staggered packing of adjacent layers. Long-range antiferromagnetic (AFM) order occurs below 1.5 (Cl), 1.9 (Br and NCS), and 2.5 K (I) as determined by heat capacity and muon-spin relaxation. The single-ion anisotropy and <i>g</i> factor of <b>2</b>, <b>3</b>, and <b>4</b> were measured by electron-spin resonance with no evidence for zero–field splitting (ZFS) being observed. The magnetism of <b>1</b>–<b>4</b> spans the spectrum from quasi-two-dimensional (2D) to three-dimensional (3D) antiferromagnetism. Nearly identical results and thermodynamic features were obtained for <b>2</b> and <b>4</b> as shown by pulsed-field magnetization, magnetic susceptibility, as well as their Néel temperatures. Magnetization curves for <b>2</b> and <b>4</b> calculated by quantum Monte Carlo simulation also show excellent agreement with the pulsed-field data. Compound <b>3</b> is characterized as a 3D AFM with the interlayer interaction (<i>J</i>_⊥) being slightly stronger than the intralayer interaction along Ni–pyz–Ni segments (<i>J</i>_pyz) within the two-dimensional [Ni­(pyz)₂]²⁺ square planes. Regardless of <i>X</i>, <i>J</i>_pyz is similar for the four compounds and is roughly 1 K