Eric Ganz

Computational study of hydrogen binding by metal-organic framework-5

We report the results of quantum chemistry calculations on H(2) binding by the metal-organic framework-5 (MOF)-5. Density functional theory calculations were used to calculate the atomic positions, lattice constant, and effective atomic charges from the electrostatic potential for the MOF-5 crystal structure. Second-order Møller-Plesset perturbation theory was used to calculate the binding energy of H(2) to benzene and H(2)-1,4-benzenedicarboxylate-H(2). To achieve the necessary accuracy, the large Dunning basis sets aug-cc-pVTZ, and aug-cc-pVQZ were used, and the results were extrapolated to the basis set limit. The binding energy results were 4.77 kJ/mol for benzene, 5.27 kJ/mol for H(2)-1,4-benzenedicarboxylate-H(2). We also estimate binding of 5.38 kJ/mol for Li-1,4-benzenedicarboxylate-Li and 6.86 kJ/mol at the zinc oxide corners using second-order Møller-Plesset perturbation theory. In order to compare our theoretical calculations to the experimental hydrogen storage results, grand canonical Monte Carlo calculations were performed. The Monte Carlo simulations identify a high energy binding site at the corners that quickly saturated with 1.27 H(2) molecules at 78 K. At 300 K, a broad range of binding sites are observed.

Scanning tunneling microscopy of Cu, Ag, Au and Al adatoms, small clusters, and islands on graphite

Abstract We have used a scanning tunneling microscope to study the static and dynamic behaviour of Cu, Ag, Au, and Al deposited in situ on highly oriented pyrolytic graphite in an ultra-high vacuum chamber. We have imaged static monomers of Ag, Au, and Al, dimers of Ag and Au, and clusters of 3 or more atoms of Ag, Al, and Au. From the lifetime of the monomers, we estimate the energy barrier against diffusion to be greater than 0.65 eV. We have studied two-dimensional islands of Ag and Au, containing up to 100 atoms, which are atomically resolved against the supporting graphite substrate. The interiors of the islands contain ordered rectangular lattices separated by grain boundaries, while the atoms at the periphery are disordered. We show a small three-dimensional Cu crystal, the decoration of a grain boundary by Cu particles with an average diameter of 44 A, and two examples of granular films. Finally, we present examples of dynamic processes: the shrinking of a small Au island, the contraction of the lattice spacing of a rectangular two-dimensional Au lattice on a time scale of minutes, and the diffusion of a Ag cluster along a graphite step edge on a time scale of seconds.

Growth and morphology of Pb on Si(111)

Abstract Tunneling microscopy, thermal desorption, Rutherford backscattering, and low-energy electron diffraction are used to study the structures and coverages of the phases of Pb on the Si(111)7 × 7 surface. For room-temperature deposition at low coverage on the 7 × 7 surface, Pb atoms occupy sites above the rest atoms and between the Si adatoms (with a preference for the faulted half of the unit cell). At 0.6 ML, the Pb forms an ordered overlayer based on the 7 × 7 unit cell. The Pb grows epitaxially up to 3 ML at which point Pb crystals start to form. On annealed samples at low coverage, Pb atoms occupy Si(111)7 × 7 adatom sites. At 1 6 ML, a new √3 × √3 mosaic phase consisting of alternating chains of Pb and Si adatoms with a high melting point is produced. At 1 3 ML the standard √3 × √3 phase is observed. Between 1 3 and 1 ML, a 1 × 1 Pb overlayer is found on annealed samples, while above 1 ML, a rotated incommensurate phase is observed. As more Pb is added above 1 ML, the 2D Pb density is increased, and the incommensuration is reduced, until island formation begins. We link our discussion of the atomic structure of the interface to the variations in Schottky barrier heights observed for Pb/Si(111) diodes.

Binding energies of hydrogen molecules to isoreticular metal-organic framework materials

Recently, several novel isoreticular metal-organic framework (IRMOF) structures have been fabricated and tested for hydrogen storage applications. To improve our understanding of these materials, and to promote quantitative calculations and simulations, the binding energies of hydrogen molecules to the MOF have been studied. High-quality second-order Moller-Plesset (MP2) calculations using the resolution of the identity approximation and the quadruple zeta QZVPP basis set were used. These calculations use terminated molecular fragments from the MOF materials. For H2 on the zinc oxide corners, the MP2 binding energy using Zn4O(HCO2)6 molecule is 6.28 kJ/mol. For H2 on the linkers, the binding energy is calculated using lithium-terminated molecular fragments. The MP2 results with coupled-cluster singles and doubles and noniterative triples method corrections and charge-transfer corrections are 4.16 kJ/mol for IRMOF-1, 4.72 kJ/mol for IRMOF-3, 4.86 kJ/mol for IRMOF-6, 4.54 kJ/mol for IRMOF-8, 5.50 and 4.90 kJ/mol for IRMOF-12, 4.87 and 4.84 kJ/mol for IRMOF-14, 5.42 kJ/mol for IRMOF-18, and 4.97 and 4.66 kJ/mol for IRMOF-993. The larger linkers are all able to bind multiple hydrogen molecules per side. The linkers of IRMOF-12, IRMOF-993, and IRMOF-14 can bind two to three, three, and four hydrogen molecules per side, respectively. In general, the larger linkers have the largest binding energies, and, together with the enhanced surface area available for binding, will provide increased hydrogen storage. We also find that adding up NH2 or CH3 groups to each linker can provide up to a 33% increase in the binding energy.

Two-dimensional Cu2Si monolayer with planar hexacoordinate copper and silicon bonding.

Two-dimensional (2D) materials with planar hypercoordinate motifs are extremely rare due to the difficulty in stabilizing the planar hypercoordinate configurations in extended systems. Furthermore, such exotic motifs are often unstable. We predict a novel Cu2Si 2D monolayer featuring planar hexacoordinate copper and planar hexacoordinate silicon. This is a global minimum in 2D space which displays reduced dimensionality and rule-breaking chemical bonding. This system has been studied with density functional theory, including molecular dynamics simulations and electronic structure calculations. Bond order analysis and partitioning reveals 4c-2e σ bonds that stabilize the two-dimensional structure. We find that the system is quite stable during short annealing simulations up to 900 K, and predict that it is a nonmagnetic metal. This work opens up a new branch of hypercoordinate two-dimensional materials for study.

New isoreticular metal-organic framework materials for high hydrogen storage capacity

We propose new isoreticular metal-organic framework (IRMOF) materials to increase the hydrogen storage capacity at room temperature. Based on the potential-energy surface of hydrogen molecules on IRMOF linkers and the interaction energy between hydrogen molecules, we estimate the saturation value of hydrogen sorption capacity at room temperature. We discuss design criteria and propose new IRMOF materials that have high gravimetric and volumetric hydrogen storage densities. These new IRMOF materials may have gravimetric storage density up to 6.5 wt % and volumetric storage density up to 40 kg H2/m3 at room temperature.

Four Decades of the Chemistry of Planar Hypercoordinate Compounds.

The idea of planar tetracoordinate carbon (ptC) was considered implausible for a hundred years after 1874. Examples of ptC were then predicted computationally and realized experimentally. Both electronic and mechanical (e.g., small rings and cages) effects stabilize these unusual bonding arrangements. Concepts based on the bonding motifs of planar methane and the planar methane dication can be extended to give planar hypercoordinate structures of other chemical elements. Numerous planar configurations of various central atoms (main-group and transition-metal elements) with coordination numbers up to ten are discussed herein. The evolution of such planar configurations from small molecules to clusters, to nanospecies and to bulk solids is delineated. Some experimentally fabricated planar materials have been shown to possess unusual electrical and magnetic properties. A fundamental understanding of planar hypercoordinate chemistry and its potential will help guide its future development.

An ultrahigh vacuum high speed scanning tunneling microscope

We have developed an ultrahigh vacuum high speed scanning tunneling microscope (STM) which is based on a prototype STM that operates in air. The prototype operates with a tip scan speed up to 105 nm/s. At this high tip speed, we have made real time videos of nickel and gold at rates up to 15 Hz. The ultrahigh vacuum version operates at speeds up to 8000 nm/s with atomic resolution on the Si(111) surface and incorporates sample transfer and sample heating.

Scanning tunneling microscopy of silver, gold, and aluminum monomers and small clusters on graphite

We have deposited small numbers of atoms of silver, gold, and aluminum onto cleaved graphite substrates in ultrahigh vacuum. Using a scanning tunneling microscope, we have observed monomers, groups of monomers, dimers, and a trimer. The adsorption sites and atomic spacings are determined by direct observation.

SELECTIVE NANOSCALE GROWTH OF TITANIUM ON THE SI(001) SURFACE USING AN ATOMIC HYDROGEN RESIST

Nanoscale titanium structures are fabricated on a patterned Si(001)-(2×1) surface using an atomic hydrogen resist. The patterning is achieved by removing small areas of hydrogen with a scanning tunneling microscope. The large chemical reactivity of the bare Si surface compared to the hydrogen passivated surface provides selective area growth of titanium clusters grown by chemical vapor deposition using TiCl4. Titanium growth by chemical vapor deposition is normally limited by chlorine passivation of the bare Si surface. However, by removing the chlorine with the scanning tunneling microscope, the growth can be resumed.