Observation of Phonon Anomaly at the Armchair Edge of Single-Layer Graphene in Air

Confocal Raman spectroscopy is used to study the phonon modes of mechanically exfoliated single-layer graphene sheets in ambient air. We observe that ambient gas induces obvious shifts in the G band frequency as well as the change in intensity ratio of 2D and G bands, I(2D)/I(G), owing to the Fermi energy change by ambient gas doping. The change in I(2D)/I(G) for the armchair edge is significantly larger than those for the graphene center or zigzag edge in our graphene samples. Also, the G band phonon anomaly, the G band frequency softens and peak width broadens at the charge neutral (Dirac) point, is clearly identified at the armchair edge but not for the zigzag edge or graphene center. We conclude that Fermi level of the armchair edge is close to the Dirac point, making the phonon anomaly visible. However, the charge carrier concentration at the graphene center was too high (Fermi level away from the Dirac point), which completely smears out the phonon softening phenomenon. This study proves that the phonon anomaly can occur at the armchair edge as predicted by Sasaki et al. (J. Phys. Soc. Jpn. 2010, 79, 044603). Our results also demonstrate that the phonon property of an edge or center site in single-layer graphene is very sensitive to its local carrier concentration

Threonine and Polythreonine Accelerate Calcium Carbonate Formation

Acidic macromolecules are considered to be critical to calcium carbonate formation in organisms and have been chosen as a model chemical for modifying biomineralization. Here, this view is challenged, and it is found that low-charge amino acid threonine (Thr) and its polymer poly-Thr can accelerate calcium carbonate formation by increasing nucleation. In contrast to aspartic acid (Asp) and poly-Asp, Thr and poly-Thr did not affect the morphology of product crystals but accelerated the formation of crystal nucleation rapidly. This effect may be partially attributed to the negative charges of these chemicals that interact with calcium. This finding provides novel insights into the mechanisms of biomineralization that involve diverse chemistry control not limited to acidic macromolecules and may inspire the synthesis of calcium carbonate in a short time

Explanation of the Source of Very Large Errors in Many Exchange–Correlation Functionals for Vanadium Dimer

Vanadium dimer is a notoriously difficult case for Kohn–Sham (KS) density functional theory with currently available approximations to the exchange–correlation (xc) functionals, and many approximate xc functionals yield an exceedingly large error in the calculated bond energy. In this paper, we first test the bond energies estimated by 43 xc functionals and the Hartree–Fock (HF) method. The results further confirm the large errors and show that, with the experimental bond energy being 64.2 kcal/mol, the KS calculations give predictions all over the map with errors ranging from −61.5 to +60.5 kcal/mol, and the HF method performs much worse with an error of −124.4 kcal/mol! The reason for these very large errors is examined in this article by analyzing the atomic and molecular orbital energies calculated by various xc functionals. The results show that the errors in estimates of the bond energy of vanadium dimer can primarily be related to the calculated energy gap between the 4s and 3d_z2 atomic orbitals of the vanadium atom and especially to the 3d_z2 orbital energy. This interesting relation between the errors in the calculated bond energy and the magnitudes of the single-particle orbital energies provides a constructive alternative to the common but more sterile explanation that it is the static correlation energy due to multicenter left–right correlation that makes the vanadium dimer and many other transition metal compounds so difficult for Kohn–Sham calculations. One of the most important factors in determining the critical atomic orbital energy is the amount of nonlocal HF exchange that is included in the xc functional, but it is still difficult to explain why different local functionals (functionals with no HF exchange) yield quite different results. We conclude that improving calculations of orbital energies of atoms may provide a route to improving the accuracy of theoretical predictions of molecular bond energies for systems containing metal atoms

Viscoelastic Properties of Water Suspensions of Polymer Nanofibers Synthesized via RAFT-Mediated Emulsion Polymerization

We report on the rheological properties of water suspensions of poly­(methacrylic acid<i>-<i>co</i>-</i>poly­(ethylene oxide) methyl ether methacrylate)<i>-<i>b</i>-</i>polystyrene and poly­(methacrylic acid<i>-<i>co</i>-</i>poly­(ethylene oxide) methyl ether methacrylate)<i>-<i>b</i>-</i>poly­(methyl methacrylate) self-assembled amphiphilic block copolymer nanofibers, synthesized via RAFT-mediated aqueous emulsion polymerization. The viscoelastic properties were studied over a range of nanofiber concentrations spanning the transition from the dilute to semidilute regimes. From the measured viscoelastic parameters, two sets of suspensions could be differentiated depending on their aspect ratio (length/diameter ≈70 and ≈54) and the average length of the nanofibers was calculated in the 2.4 μm – 3.8 μm range. The viscoelastic properties appeared to depend mainly on the aspect ratio of the fibers rather than on their nature and composition. As expected the zero shear viscosity was observed to scale with the volume fraction ϕ as η₀ ∝ ϕ¹ and η₀ ∝ ϕ³ for dilute and semidilute regime, respectively. However, the deviation of the scaling law in semidilute regime at higher concentrations and the slowdown of the rotary diffusion can be related to different mechanisms. While a Brownian motion of nanofibers is the dominant mechanism of relaxation, it was also concluded that clustering phenomenon and broad length distribution of nanofibers implies that some large nanofibers could be non Brownian

Crystallinity Enhancement of Nafion Electrolyte Membranes Assisted by a Molecular Gelator

Nanocrystallites, acting as physical cross-links in Nafion membranes, play a crucial role in building blocks for improving mechanical durability and stopping fuel crossover. However, Nafion membranes suffer from low crystallinity due to the irregular pendent side chains, which hinder self-aggregation of the poly(tetrafluoroethylene) (PTFE) backbones. For the first time, a molecular gelator was introduced in the membrane casting process to enhance the rate of self-assembly of PTFE backbones so as to increase the membrane’s crystallinity as well as proton conductivity without sacrificing the purity of Nafion. The molecular gelator used was 3,4-dimethylbenzaldehyde (DMBA). Addition of 0.5 wt % DMBA led to a 42% increase in crystallinity, a 32% increase in yield strength, a 22% increase in tensile modulus and an 18% increase in proton conductivity at 60 °C and 90% relative humidity. Additionally, the membrane electrode assembly (MEA) prepared from the membranes cast from the solution containing 0.5 wt % DMBA also showed an increase of 17% in maximum power density in comparison to the MEA prepared from pure Nafion membrane in a single cell polarization test without any external humidification. Transmission electron microscopy (TEM) and molecular dynamics simulation were used to elucidate the structural changes in Nafion membrane due to the introduction of DMBA. It was observed that the presence of DMBA gives wider crystalline regions under TEM. The molecular dynamics simulation at 500 K shows that the PTFE backbones become elongated in the presence of DMBA due to the enhanced mobility. This is consistent with the observed increase in crystallinity in the membrane as it means reduced entropic change upon crystallization

Spiro[indene-1,4′-oxa-zolidinones] Synthesis via Rh(III)-Catalyzed Coupling of 4‑Phenyl-1,3-oxazol-2(3<i>H</i>)‑ones with Alkynes: A Redox-Neutral Approach

Transition-metal-catalyzed C–H activation synthesis of heterocyclic spiro­[4,4]­nonanes has persistently witnessed the use of additional stoichiometric transition-metal oxidant when employing CC bond as the spiro ring closure site. Herein, we have addressed the issue by reporting a redox-neutral strategy for spiro­[indene-1,4′-oxa-zolidinones] synthesis via Rh­(III)-catalyzed coupling of 4-phenyl-1,3-oxazol-2­(3H)-ones with alkynes. The synthesis features a broad substrate scope and high regiospecificity

MDS ordination of microeukaryotic communities based on DGGE profiles (A) and Illumina Miseq data (B).

<p>MDS ordination of microeukaryotic communities based on DGGE profiles (A) and Illumina Miseq data (B).</p

Spiro[indene-1,4′-oxa-zolidinones] Synthesis via Rh(III)-Catalyzed Coupling of 4‑Phenyl-1,3-oxazol-2(3<i>H</i>)‑ones with Alkynes: A Redox-Neutral Approach

Transition-metal-catalyzed C–H activation synthesis of heterocyclic spiro­[4,4]­nonanes has persistently witnessed the use of additional stoichiometric transition-metal oxidant when employing CC bond as the spiro ring closure site. Herein, we have addressed the issue by reporting a redox-neutral strategy for spiro­[indene-1,4′-oxa-zolidinones] synthesis via Rh­(III)-catalyzed coupling of 4-phenyl-1,3-oxazol-2­(3H)-ones with alkynes. The synthesis features a broad substrate scope and high regiospecificity

Spiro[indene-1,4′-oxa-zolidinones] Synthesis via Rh(III)-Catalyzed Coupling of 4‑Phenyl-1,3-oxazol-2(3<i>H</i>)‑ones with Alkynes: A Redox-Neutral Approach

Transition-metal-catalyzed C–H activation synthesis of heterocyclic spiro­[4,4]­nonanes has persistently witnessed the use of additional stoichiometric transition-metal oxidant when employing CC bond as the spiro ring closure site. Herein, we have addressed the issue by reporting a redox-neutral strategy for spiro­[indene-1,4′-oxa-zolidinones] synthesis via Rh­(III)-catalyzed coupling of 4-phenyl-1,3-oxazol-2­(3H)-ones with alkynes. The synthesis features a broad substrate scope and high regiospecificity

Water-Repellent Surfaces Consisting of Nanowires on Micropyramidal Structures

Super-repellent surfaces are relevant for several practical applications, such as water collection and self-cleaning and anti-icing surfaces. However, designing surfaces that can maintain their super-repellency when the surface is subjected to a humid environment is still a challenge. Here, we present a two-tier roughness surface consisting of nanowires on micropyramidal structures. We compare the wetting properties of this surface with other single-level roughness surfaces and surfaces with nanowires on micropillars, so as to investigate the role of the two-tier roughness with micropyramidal structures. Surfaces are characterized by both the static contact angle and sliding angle of a water droplet on the surfaces. The characterization is performed also for the surfaces after these ones have been subjected to condensation conditions. Compared to the single-level roughness surfaces and surfaces with nanowires on pillars, the surface with nanowires on pyramidal structures shows no degradation of water repellency properties during condensation, and shows better performance in terms of low droplet adhesion than similar surfaces composed of the more commonly used pillar structures. This is thanks to the nanowires’ roughness that minimizes the contact area of the droplets with the base surface and the V-shaped cavities between the pyramids that provide the droplets with an upward driving force due to Laplace pressure. Furthermore, this study shows the importance of characterizing surface wetting properties not only on dry but also on wet conditions. The combination of a nanoscale roughness with micropyramidal structures appears as an attractive solution for super-repellent substrates under humid and wet conditions