Quantum delocalization of molecular hydrogen in alkali-graphite intercalates

Dynamics & Spectroscopy with Neutrons-Recent Developments & Emerging Opportunities

This work provides an up-to-date overview of recent developments in neutron spectroscopic techniques and associated computational tools to interrogate the structural properties and dynamical behavior of complex and disordered materials, with a focus on those of a soft and polymeric nature. These have and continue to pave the way for new scientific opportunities simply thought unthinkable not so long ago, and have particularly benefited from advances in high-resolution, broadband techniques spanning energy transfers from the meV to the eV. Topical areas include the identification and robust assignment of low-energy modes underpinning functionality in soft solids and supramolecular frameworks, or the quantification in the laboratory of hitherto unexplored nuclear quantum effects dictating thermodynamic properties. In addition to novel classes of materials, we also discuss recent discoveries around water and its phase diagram, which continue to surprise us. All throughout, emphasis is placed on linking these ongoing and exciting experimental and computational developments to specific scientific questions in the context of the discovery of new materials for sustainable technologies.

Assignment of the vibrational spectra of the parent polysilsesquioxane (POSS): Octahydridosilasequioxane, H8Si8O12

Polysilsesquioxanes (POSS) are molecules with the empirical formula (RSiO_1.5)_n where R is a hydrogen atom or hydroxyl or an organic moiety such as an alkyl, alkene, acrylate or epoxide. The silicon atoms occupy the corners of a cube and oxygen atoms are located on the edges, the versatility of silsesquioxanes arises from the vacant fourth position of silicon. The choice of substituent enables a wide variety of properties to be engineered in a straightforward manner. The parent POSS is octasilsesquioxane, H₈Si₈O₁₂, with R=H and n=8. The present work employs periodic density functional theory calculations in conjunction with all the available vibrational (infrared, Raman and inelastic neutron scattering) spectra to generate a complete assignment of all the modes of the parent POSS octasilsesquioxane and some of its isotopomers for both the free, (O_h), molecule and the solid state material (C_3i site symmetry) including the forbidden and very weak modes. The latter are of interest because in less symmetrical silsesquioxanes, these modes will be activated.

Improved description of soft layered materials with van der Waals density functional theory

The accurate description of van der Waals forces within density functional theory is currently one of the most active areas of research in computational physics and chemistry. Here we report results on the structural and energetic properties of graphite and hexagonal boron nitride, two layered materials where interlayer binding is dominated by van der Waals forces. Results from several density functionals are reported, including the optimized Becke88 van der Waals (optB88-vdW) and the optimized PBE van der Waals (optPBE-vdW) (Klimeš et al 2010 J. Phys.: Condens. Matter 22 022201) functionals. Where comparison to experiment and higher-level theory is possible, the results obtained from the two new van der Waals density functionals are in good agreement. An analysis of the physical nature of the interlayer binding in both graphite and hexagonal boron nitride is also reported.

Bright beaches of nanoscale potassium islands on graphite in STM imaging

We demonstrate, via scanning tunneling microscopy (STM) measurements performed at 48 K, the existence of "bright beaches" at the edges of K islands (diameter approximately 5-500 nm) on the graphite surface. The enhanced tunneling current is only observed in monolayer-high islands on graphite, and not in islands of similar geometry on top of a K monolayer film. First-principles density functional calculations and STM simulations suggest that this is an STM field effect, which appears as the positive tip attracts donated electrons back to the metallic K islands. The restored charge accumulates preferentially at the island edges.