IN two previous notes1, Prof. Max Born and I have shown that one can obtain a theory of superconductivity by taking account of the fact that the interaction of the electrons with the ionic lattice is appreciable only near the boundaries of Brillouin zones, and particularly strong near the corners of these. This leads to the criterion that the metal should be superconductive if a set of corners of a Brillouin zone is lying very near the Fermi surface, considered as a sphere, which limits the region in the momentum space completely filled with electrons.

On the theory of superconductivity

We introduce density functional theory and review recent progress in its application to transition metal chemistry. Topics covered include local, meta, hybrid, hybrid meta, and range-separated functionals, band theory, software, validation tests, and applications to spin states, magnetic exchange coupling, spectra, structure, reactivity, and catalysis, including molecules, clusters, nanoparticles, surfaces, and solids.

Density functional theory for transition metals and transition metal chemistry

CO oxidation, although seemingly a simple chemical reaction, provides us with a panacea that reveals the richness and beauty of heterogeneous catalysis. The Fritz Haber Institute is a place where a multidisciplinary approach to study the course of such a heterogeneous reaction can be generated in house. Research at the institute is primarily curiosity driven, which is reflected in the five sections comprising this Review. We use an approach based on microscopic concepts to study the interaction of simple molecules with well-defined materials, such as clusters in the gas phase or solid surfaces. This approach often asks for the development of new methods, tools, and materials to prove them, and it is exactly this aspect, both, with respect to experiment and theory, that is a trade mark of our institute.

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CO oxidation as a prototypical reaction for heterogeneous processes

Reducibility is an essential characteristic of oxide catalysts in oxidation reactions following the Mars–van Krevelen mechanism. A typical descriptor of the reducibility of an oxide is the cost of formation of an oxygen vacancy, which measures the tendency of the oxide to lose oxygen or to donate it to an adsorbed species with consequent change in the surface composition, from MnOm to MnOm–x. The oxide reducibility, however, can be modified in various ways: for instance, by doping and/or nanostructuring. In this review we consider an additional aspect, related to the formation of a metal/oxide interface. This can be realized when small metal nanoparticles are deposited on the surface of an oxide support or when a nanostructured oxide, either a nanoparticle or a thin film, is grown on a metal. In the past decade, both theory and experiment indicate a particularly high reactivity of the oxygen atoms at the boundary region between a metal and an oxide. Oxygen atoms can be removed from interface sites at much...

http://pubs.acs.org/doi/pdf/10.1021/acscatal.7b01913

Increasing Oxide Reducibility: The Role of Metal/Oxide Interfaces in the Formation of Oxygen Vacancies

The preparation, characterization, and catalytic activity of supported metal phosphides are reviewed. Reduction of metal compounds together with phosphate is a convenient method to prepare metal phosphides, but requires high temperature. Reduction with phosphite, hypophosphite, or phosphine and the plasma reduction of phosphate can be carried out at lower temperatures, which leads to smaller metal phosphide particles and more active catalysts. Organometallic routes allow the separate synthesis of metal phosphide nanoparticles, which have to be added to the support in a second step. LEED, STM, XPS, and DFT studies have shown that the surfaces of Ni2P reconstruct to P-rich surfaces. The investigation of metal phosphides as catalysts for hydrotreating reactions continues to be a topic of considerable research with recent advances realized in using bimetallic and noble metal phosphides to achieve high activities and tailored selectivities. Finally, hydrodeoxygenation catalysis over metal phosphides is a growing area of research given the need to develop catalysts for upgrading biomass to transportation fuels.

Metal Phosphides: Preparation, Characterization and Catalytic Reactivity

The metal work function is a suggested key parameter for charging adsorbates with high electron affinity on thin oxide films grown on metal single crystal surfaces. Here, the first experimental data for the work function shift of Ag(001) induced by 1, 3, and 8 monolayers of MgO is measured in situ by three independent scanning probe techniques on the same surface area, making the different approaches comparable. Furthermore, the results are compared to density functional theory calculations. The measurements are performed using a dual-mode dynamic force microscope and scanning tunneling microscope working in ultrahigh vacuum at low temperature (5 K). The methods to detect the work function shift are based on Kelvin probe force microscopy measuring the contact potential difference, I(z) curves and field emission resonances.

Work function measurements of thin oxide films on metals-MgO on Ag(001)

A detailed understanding of surface defects is highly desirable, e.g. to clarify their role as active sites in catalysis. Here localized defects on the surface of MgO films deposited on Ag(001) are investigated. Since the electronic structure of color centers depends on their local position, spectroscopic signals are highly convoluted and often difficult to disentangle. In this study we aimed to obtain morphological and spectroscopic information on single color centers at a microscopic level with frequency modulated dynamic force microscopy (FM-DFM) and scanning tunneling microscopy (STM) in an ultrahigh vacuum and at low temperature. Four of the major and in literature mostly discussed defect types on MgO have been characterized by their charge state and finally identified by the complementary application of FM-DFM and STM in combination with density functional theory results.

/pdf/measuring-the-charge-state-of-point-defects-on-mgo-ag-001-3i6c3u9t9p.pdf

Measuring the charge state of point defects on MgO/Ag(001).

Gold deposition onto ultrathin MgO films on Ag(001) results in the formation of linear Au clusters, as revealed from a combined scanning tunneling microscopy and density-functional theory study. The equilibrium structure of small Au clusters containing three to seven atoms is therefore different on thin film and bulk MgO(001) but also deviates from the shape of the respective gas-phase clusters. The peculiar one-dimensional growth regime of gold is stimulated by an electron transfer from the MgO/Ag interface to the deposited Au clusters, resulting in singly and doubly charged cluster anions. Only for larger atom numbers, the formation of compact two-dimensional clusters prevails on the MgO thin films.

/pdf/charge-induced-formation-of-linear-au-clusters-on-thin-mgo-hizym1q6vr.pdf

Charge-induced formation of linear Au clusters on thin MgO films: Scanning tunneling microscopy and density-functional theory study

A combination of low temperature scanning tunneling microscopy (STM) and theoretical calculations is used to investigate Au dimers, supported on thin MgO(001) films, whose thickness was chosen such that charge transfer from the Ag substrate to the deposited Au is possible. Au dimers exist not only in an upright geometry--as theoretically predicted to be the most stable configuration--but also as flat lying dimers which populate a manifold of different azimuthal orientations. Apart from the difference in adsorption configurations, these two isomers exhibit rather different electronic structures: while upright dimers are neutral, flat ones are charged.

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Au dimers on thin MgO(001) films: flat and charged or upright and neutral?

Defect sites on oxide surfaces play a dominant role in surface chemistry. The direct atomistic study of these sites is important but very difficult. We have mimicked the adsorbate−defect interaction by a dynamic force microscope tip measuring the interaction with a color center (F0) on the MgO(001) surface. The experimental findings, complemented by density functional theory calculations, show a highly attractive adsorbate−defect interaction and a charge transfer at a critical distance.

/pdf/direct-measurement-of-the-attractive-interaction-forces-on-1agrvhil12.pdf

Thomas König

Papers

Work function measurements of thin oxide films on metals-MgO on Ag(001)

Measuring the charge state of point defects on MgO/Ag(001).

Charge-induced formation of linear Au clusters on thin MgO films: Scanning tunneling microscopy and density-functional theory study

Au dimers on thin MgO(001) films: flat and charged or upright and neutral?

Direct Measurement of the Attractive Interaction Forces on F0 Color Centers on MgO(001) by Dynamic Force Microscopy