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Maier, Sabine

Publications and source records attributed to Maier, Sabine.

Water at surfaces and interfaces: From molecules to ice and bulk liquid

The structure and growth of water films on surfaces is reviewed, starting from single molecules to two-dimensional wetting layers, and liquid interfaces. This progression follows the increase in temperature and vapor pressure from a few degrees Kelvin in ultra-high vacuum, where Scanning Tunneling and Atomic Force Microscopies (STM and AFM) provide crystallographic information at the molecular level, to ambient conditions where surface sensitive spectroscopic techniques provide electronic structure information. We show how single molecules bind to metal and non-metal surfaces, their diffusion and aggregation. We examine how water molecules can be manipulated by the STM tip via excitation of vibrational and electronic modes, which trigger molecular diffusion and dissociation. We review also the adsorption and structure of water on non-metal substrates including mica, alkali halides, and others under ambient humid conditions. Finally, we discuss recent progress in the exploration of the molecular level structure of solid-liquid interfaces, which impact our fundamental understanding of corrosion and electrochemical processes.

74 ATOMIC AND MOLECULAR PHYSICS↗

Growth and Structure of the First Layers of Ice on Ru(0001) and Pt(111)

Scanning tunneling microscopy was used to probe the structure and growth of the first few layers of water on Ru(0001) and Pt(111) at the molecular level. The surface-bound first layer is composed of a mixture of water molecules forming hexagonal structures, both in registry and out-of-registry with the substrate atoms. The hexagons are connected by pentagonal and heptagonal units. At temperatures below 140 K, this layer structure gives rise to the growth of metastable amorphous structures in the second and higher layers. We found that in the transition from amorphous to crystalline ice the structure of the original bottom layer changes to one in perfect local registry with the hexagonal surfaces of Ru(0001) and Pt(111). We further discovered structural defects in the form of extended one-dimensional lines of five- and eight-membered rings that are domain boundaries and stacking faults in the growing ice layers, which lead to the formation of metastable cubic ice.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Water Induced Surface Reconstruction of the Oxygen (2x1) covered Ru(0001)

Low temperature scanning tunneling microscopy (STM) and density functional theory (DFT) were used to study the adsorption of water on a Ru(0001) surface covered with half monolayer of oxygen. The oxygen atoms occupy hcp sites in an ordered structure with (2x1) periodicity. DFT predicts that water is weakly bound to the unmodified surface, 86 meV compared to the ~;;200 meV water-water H-bond. Instead, we found that water adsorption causes a shift of half of the oxygen atoms from hcp sites to fcc sites, creating a honeycomb structure where water molecules bind strongly to the exposed Ru atoms. The energy cost of reconstructing the oxygen overlayer, around 230 meV per displaced oxygen atom, is more than compensated by the larger adsorption energy of water on the newly exposed Ru atoms. Water forms hydrogen bonds with the fcc O atoms in a (4x2) superstructure due to alternating orientations of the molecules. Heating to 185 K results in the complete desorption of the water layer, leaving behind the oxygen honeycomb structure, which is metastable relative to the original (2x1). This stable structure is not recovered until after heating to temperatures close to 260K.

36 MATERIALS SCIENCE↗

Sensing Current and Forces with SPM

Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) are well established techniques to image surfaces and to probe material properties at the atomic and molecular scale. In this review, we show hybrid combinations of AFM and STM that bring together the best of two worlds: the simultaneous detection of atomic scale forces and conduction properties. We illustrate with several examples how the detection of forces during STM and the detection of currents during AFM can give valuable additional information of the nanoscale material properties.

36 MATERIALS SCIENCE↗