Supercapacitor Performance of Magnetite Nanoparticles Enhanced by a Catecholate Dispersant: Experiment and Theory.

The full potential of Fe3O4 for supercapacitor applications can be achieved by addressing challenges in colloidal fabrication of high active mass electrodes. Exceptional adsorption properties of catecholate-type 3,4-dihydroxybenzoic acid (DHBA) molecules are explored for surface modification of Fe3O4 nanoparticles to enhance their colloidal dispersion as verified by sedimentation test results and Fourier-transform infrared spectroscopy measurements. Electrodes prepared in the presence of DHBA show nearly double capacitance at slow charging rates as compared to the control samples without the dispersant or with benzoic acid as a non-catecholate dispersant. Such electrodes with active mass of 40 mg cm-2 show a capacitance of 4.59 F cm-2 from cyclic voltammetry data at a scan rate of 2 mV s-1 and 4.72 F cm-2 from galvanostatic charge-discharge data at a current density of 3 mA cm-2. Experimental results are corroborated by density functional theory (DFT) analysis of adsorption behaviour of DHBA and benzoic acid at the (001) surface of Fe3O4. The strongest adsorption energy (ca. -1.8 eV per molecule) is due to the catechol group of DHBA. DFT analysis provides understanding of the basic mechanism of DHBA adsorption on the surface of nanoparticles and opens the way for fabrication of electrodes with high capacitance.

Band alignment of monolayer CaP3, CaAs3, BaAs3 and the role of p-d orbital interactions in the formation of conduction band minima.

Recently, a number of new two-dimensional (2D) materials based on puckered phosphorene and arsenene have been predicted with moderate band gaps, good absorption properties and carrier mobilities superior to those of transition metal dichalcogenides. For heterojunction applications, it is important to know the relative band alignment of these new 2D materials. We report the band alignment of puckered CaP3, CaAs3 and BaAs3 monolayers at the quasiparticle level of theory (G0W0), calculating band offsets for isolated monolayers according to the electron affinity rule. Our calculations suggest that monolayer CaP3, CaAs3 and BaAs3 all form type-II (staggered) heterojunctions which makes them suitable for solar-energy conversion applications. Their quasiparticle gaps are 2.1 (direct), 1.8 (direct) and 1.5 eV (indirect), respectively. We also examine trends in the electronic structure in the light of chemical bonding analysis. We show that the indirect band gap in monolayer BaAs3 is caused by relatively strong As 3p-Ba 5d bonding interactions. Our results provide guidance for the design of phosphorene-like materials and their heterojunction applications.

Giant spontaneous Hall effect in a nonmagnetic Weyl-Kondo semimetal.

Nontrivial topology in condensed-matter systems enriches quantum states of matter to go beyond either the classification into metals and insulators in terms of conventional band theory or that of symmetry-broken phases by Landau's order parameter framework. So far, focus has been on weakly interacting systems, and little is known about the limit of strong electron correlations. Heavy fermion systems are a highly versatile platform to explore this regime. Here we report the discovery of a giant spontaneous Hall effect in the Kondo semimetal [Formula: see text] that is noncentrosymmetric but preserves time-reversal symmetry. We attribute this finding to Weyl nodes-singularities of the Berry curvature-that emerge in the immediate vicinity of the Fermi level due to the Kondo interaction. We stress that this phenomenon is distinct from the previously detected anomalous Hall effect in materials with broken time-reversal symmetry; instead, it manifests an extreme topological response that requires a beyond-perturbation-theory description of the previously proposed nonlinear Hall effect. The large magnitude of the effect in even tiny electric and zero magnetic fields as well as its robust bulk nature may aid the exploitation in topological quantum devices.

Electronic properties of Pb-I deficient lead halide perovskites.

The electronic structure evolution of deficient halide perovskites with a general formula (A,A')1+xM1-xX3-x was investigated using the density functional theory. The focus is placed on characterization of changes in the bandgap, band alignment, effective mass, and optical properties of deficient perovskites at various concentrations of defects. We uncover unusual electronic properties of the defect corresponding to a M-X vacancy filled with an A' cation. This defect "repels" electrons and holes producing no trap states and, in moderate quantities (x ≤ 0.1), does not hinder charge transport properties of the material. This behavior is rationalized using a confinement model and provides additional insight to the defect tolerance of halide perovskites.

Adsorption of Maleic Acid Monomer on the Surface of Hydroxyapatite and TiO2: A Pathway toward Biomaterial Composites.

Poly(styrene- alt-maleic acid) adsorption on hydroxyapatite and TiO2 (rutile) was studied using experimental techniques and complemented by ab initio simulations of adsorption of a maleic acid segment as a subunit of the copolymer. Ab initio calculations suggest that the maleic acid segment forms a strong covalent bonding to the TiO2 and hydroxyapatite surfaces. If compared to vacuum, the presence of a solvent significantly reduces the adsorption strength as the polarity of the solvent increases. The results of first-principles calculations are confirmed by the experimental measurements. We found that the adsorbed poly(styrene- alt-maleic acid) allowed efficient dispersion of rutile and formation of films by the electrophoretic deposition. Moreover, rutile can be codispersed and codeposited with hydroxyapatite to form composite films. The coatings showed an enhanced corrosion protection of metallic implants in simulated body fluid solutions, which opens new avenues for the synthesis, dispersion, and colloidal processing of advanced composite materials for biomedical applications.

Aziridinium Lead Iodide: A Stable, Low-Band-Gap Hybrid Halide Perovskite for Photovoltaics.

The low ionization energy of an A site molecule is a very important factor, which determines the thermodynamical stability of APbI3 hybrid halide perovskites, while the size of the molecule governs the stable phase at room temperature and, eventually, the band gap. It is challenging to achieve both a low ionization energy and the reasonable size for the PbI3 cage to circumvent the stability issue inherent to hybrid halide perovskites. Here we propose a new three-membered charged ring radical, which demonstrates a low ionization energy that renders a good stability for its corresponding perovskite and a reasonable cation size that translates into a suitable band gap for the photovoltaic application. We use ab initio calculations to evaluate a polymorphism of the crystal structure of the proposed hybrid halide perovskite, its stability, and electronic properties in comparison with the mainstream perovskites, such as the methylammonium and formamidinium lead iodide.

Molecular Dynamics of Fibrinogen Adsorption onto Graphene, but Not onto Poly(ethylene glycol) Surface, Increases Exposure of Recognition Sites That Trigger Immune Response.

Changes in the conformation of blood proteins due to their binding to nonbiological surfaces is the initial step in the chain of immunological reactions to foreign bodies. Despite the large number of experimental studies that have been performed on fibrinogen adsorption to nonbiological surfaces, a clear picture describing this complex process has eluded researchers to date. Developing a better understanding of the behavior of bioactive fibrinogen motifs upon their interaction with surfaces may facilitate the design of advanced materials with improved biocompatibility. This is especially important within the context of medical implants. Here we present results of explicit-solvent, all-atom MD simulations of the adsorption of the fibrinogen D-domain onto a graphene surface and a poly(ethylene glycol) (PEG) surface. Our results are consistent with experimental observations that interactions with PEG do not induce significant conformational changes on immune-reactive sites present in the D-domain of fibrinogen. In contrast, our results indicate that significant conformational changes induced by adsorption to graphene surfaces may occur under conditions that promote a high density of blood proteins on the surface. The structural rearrangements observed on graphene directly affect the secondary structure content of the D-domain, with consequent exposure of the recognition sites P1 (γ190-202) and P2 (γ377-395) and the subsite P2-C (γ383-395) involved in immune response. Analysis of the structural parameters of the MD conformers was shown to accurately assess the biocompatibility of the modeled surfaces.

Efficient Driving of Piezoelectric Transducers Using a Biaxial Driving Technique.

Efficient driving of piezoelectric materials is desirable when operating transducers for biomedical applications such as high intensity focused ultrasound (HIFU) or ultrasound imaging. More efficient operation reduces the electric power required to produce the desired bioeffect or contrast. Our preliminary work [Cole et al. Journal of Physics: Condensed Matter. 2014;26(13):135901.] suggested that driving transducers by applying orthogonal electric fields can significantly reduce the coercivity that opposes ferroelectric switching. We present here the experimental validation of this biaxial driving technique using piezoelectric ceramics typically used in HIFU. A set of narrow-band transducers was fabricated with two sets of electrodes placed in an orthogonal configuration (following the propagation and the lateral mode). The geometry of the ceramic was chosen to have a resonance frequency similar for the propagation and the lateral mode. The average (± s.d.) resonance frequency of the samples was 465.1 (± 1.5) kHz. Experiments were conducted in which each pair of electrodes was driven independently and measurements of effective acoustic power were obtained using the radiation force method. The efficiency (acoustic/electric power) of the biaxial driving method was compared to the results obtained when driving the ceramic using electrodes placed only in the pole direction. Our results indicate that the biaxial method increases efficiency from 50% to 125% relative to the using a single electric field.

Binding of solvated peptide (EPLQLKM) with a graphene sheet via simulated coarse-grained approach.

Binding of a solvated peptide A1 ((1)E (2)P (3)L (4)Q (5)L (6)K (7)M) with a graphene sheet is studied by a coarse-grained computer simulation involving input from three independent simulated interaction potentials in hierarchy. A number of local and global physical quantities such as energy, mobility, and binding profiles and radius of gyration of peptides are examined as a function of temperature (T). Quantitative differences (e.g., the extent of binding within a temperature range) and qualitative similarities are observed in results from three simulated potentials. Differences in variations of both local and global physical quantities suggest a need for such analysis with multiple inputs in assessing the reliability of both quantitative and qualitative observations. While all three potentials indicate binding at low T and unbinding at high T, the extent of binding of peptide with the temperature differs. Unlike un-solvated peptides (with little variation in binding among residues), solvation accentuates the differences in residue binding. As a result the binding of solvated peptide at low temperatures is found to be anchored by three residues, (1)E, (4)Q, and (6)K (different from that with the un-solvated peptide). Binding to unbinding transition can be described by the variation of the transverse (with respect to graphene sheet) component of the radius of gyration of the peptide (a potential order parameter) as a function of temperature.

Formation energies of antiphase boundaries in GaAs and GaP: an ab Initio study.

Electronic and structural properties of antiphase boundaries in group III-V semiconductor compounds have been receiving increased attention due to the potential to integration of optically-active III-V heterostructures on silicon or germanium substrates. The formation energies of {110}, {111}, {112}, and {113} antiphase boundaries in GaAs and GaP were studied theoretically using a full-potential linearized augmented plane-wave density-functional approach. Results of the study reveal that the stoichiometric {110} boundaries are the most energetically favorable in both compounds. The specific formation energy gamma of the remaining antiphase boundaries increases in the order of gamma({113}) approximately gamma({112}) < gamma({111}), which suggests {113} and {112} as possible planes for faceting and annihilation of antiphase boundaries in GaAs and GaP.