Catalyzing Bond-Dissociation in Graphene via Alkali-Iodide Molecules.

Atomic design of a 2D-material such as graphene can be substantially influenced by etching, deliberately induced in a transmission electron microscope. It is achieved primarily by overcoming the threshold energy for defect formation by controlling the kinetic energy and current density of the fast electrons. Recent studies have demonstrated that the presence of certain species of atoms can catalyze atomic bond dissociation processes under the electron beam by reducing their threshold energy. Most of the reported catalytic atom species are single atoms, which have strong interaction with single-layer graphene (SLG). Yet, no such behavior has been reported for molecular species. This work shows by experimentally comparing the interaction of alkali and halide species separately and conjointly with SLG, that in the presence of electron irradiation, etching of SLG is drastically enhanced by the simultaneous presence of alkali and iodine atoms. Density functional theory and first principles molecular dynamics calculations reveal that due to charge-transfer phenomena the CC bonds weaken close to the alkali-iodide species, which increases the carbon displacement cross-section. This study ascribes pronounced etching activity observed in SLG to the catalytic behavior of the alkali-iodide species in the presence of electron irradiation.

Long-Time Relaxation of Stress-Induced Birefringence of Microcrystalline Alkali Halide Crystals.

Alkali halide single crystals are most commonly used as the diluent matrix in the tablet method or disk technique for spectroscopic measurements. However, stress-induced birefringence (SIB) of alkali halides as well as intrinsic birefringence manifest during the disk formation process. Thus, the true chiroptical measurement is disturbed by optical anisotropies (OA) containing SIB and intrinsic birefringence, except in the case of optical homogeneity. SIB is generally larger than intrinsic birefringence and has a value of several thousand millidegrees in the ultraviolet-visible wavelength range, although this varies with disk type. Here, to investigate the SIB origin, alkali halide crystals were examined using polarized light, X-ray diffraction, Fourier-transform infrared, and electron backscattering diffraction spectroscopic measurements. It was found that, after stress release, the SIB exhibited nonlinear long-time relaxation, which roughly converged within several hours, with the only time-invariant intrinsic birefringence remaining being due to OA. This behavior was strongly related to an increase in the quasi-amorphous domain and the generation of an air gap between the crystallite boundaries and their pellets. Further, a straightforward correlation was found between amorphization and an increase in the disk water content caused by deliquescence. Thus, the OA of alkali halide single crystals was found to have two different origins yielding intrinsic birefringence and SIB.

Water Will Be the Coal of the Future-The Untamed Dream of Jules Verne for a Solar Fuel.

This article evokes the futuristic visions of two giants, one a writer, Jules Verne, who foresaw water as the coal of the future, and the other a scientist, Giacomo Ciamician, who foresaw the utilization of solar energy as an energy source with which to drive photochemical and photocatalytic reactions for the betterment of mankind. Specifically, we examine briefly the early work of the 1960s and 1970s on the photosplitting of free water and water adsorbed on solid supports, based mostly on metal oxides, from which both hydrogen and oxygen evolve in the expected stoichiometric ratio of 2 to 1. The two oil crises of the 1970s (1973 and 1979) spurred the interest of researchers from various disciplines (photochemistry, photo-catalysis and photoelectrochemistry) in search of a Holy Grail photocatalyst, process, or strategy to achieve efficient water splitting so as to provide an energy source alternative to fossil fuels. Some approaches to the photosplitting of water adsorbed on solid insulators (high bandgap materials; Ebg ≥ 5 eV) and semiconductor photocatalysts (metal oxides) are described from which we deduce that metal oxides with bandgap energies around 5 eV (e.g., ZrO2) are more promising materials to achieve significant water splitting on the basis of quantum yields than narrower bandgap photocatalysts (e.g., TiO2; Ebg ≈ 3.0-3.2 eV), which tend to be relatively inactive by comparison. Although proof of concept of the photosplitting of water has been demonstrated repeatedly in the last four decades, much remains to be done to find the Holy Grail photocatalyst and/or strategy to achieve significant yields of hydrogen.

An Investigation of Ion-Pairing of Alkali Metal Halides in Aqueous Solutions Using the Electrical Conductivity and the Monte Carlo Computer Simulation Methods.

The ion pairing is, in very dilute aqueous solutions, of rather small importance for solutions' properties, which renders its precise quantification quite a laborious task. Here we studied the ion pairing of alkali halides in water by using the precise electric conductivity measurements in dilute solutions, and in a wide temperature range. The low-concentration chemical model was used to analyze the results, and to estimate the association constant of different alkali halide salts. It has been shown that the association constant is related to the solubility of salts in water and produces a 'volcano relationship', when plotted against the difference between the free energy of hydration of the corresponding individual ions. The computer simulation, using the simple MB+dipole water model, were used to interprete the results, to find a microscopic basis for Collins' law of matching water affinities.

On two different objectives of the concepts of ionic radii.

Experimentally and theoretically derived interatomic distances (D) and ionic radii (R) of more than a hundred monomeric (AX), dimeric (A2X2, ABXY), and crystalline ([AX]) alkali halide species (A=Li, Na, K, Rb, Cs, Fr; X=H, F, Cl, Br, I, At) have been analyzed. Chemists use the word "atomic radius" for two antithetic concepts. Let D(CiEE'jj') be the "billion" of distances i between two adjacent atoms in the millions of known compounds C from a hundred different elements E in bonding states j. The common chemical aim is partitioning D approximately into increments R(Ej) +R(E'j'). This can be achieved with a few (say

Integer versus Fractional Charge Transfer at Metal(/Insulator)/Organic Interfaces: Cu(/NaCl)/TCNE.

Semilocal and hybrid density functional theory was used to study the charge transfer and the energy-level alignment at a representative interface between an extended metal substrate and an organic adsorbate layer. Upon suppressing electronic coupling between the adsorbate and the substrate by inserting thin, insulating layers of NaCl, the hybrid functional localizes charge. The laterally inhomogeneous charge distribution resulting from this spontaneous breaking of translational symmetry is reflected in observables such as the molecular geometry, the valence and core density of states, and the evolution of the work function with molecular coverage, which we discuss for different growth modes. We found that the amount of charge transfer is determined, to a significant extent, by the ratio of the lateral spacing of the molecules and their distance to the metal. Therefore, charge transfer does not only depend on the electronic structure of the individual components but, just as importantly, on the interface geometry.

Band-bending in organic semiconductors: the role of alkali-halide interlayers.

Band-bending in organic semiconductors, occurring at metal/alkali-halide cathodes in organic-electronic devices, is experimentally revealed and electrostatically modeled. Metal-to-organic charge transfer through the insulator, rather than doping of the organic by alkali-metal ions, is identified as the origin of the observed band-bending, which is in contrast to the localized interface dipole occurring without the insulating buffer layer.