The interaction of water with solid surfaces: fundamental aspects revisited

Water is perhaps the most important and most pervasive chemical on our planet. The influence of water permeates virtually all areas of biochemical, chemical and physical importance, and is especially evident in phenomena occurring at the interfaces of solid surfaces. Since 1987, when Thiel and Madey (TM) published their review titled ‘The interaction of water with solid surfaces: fundamental aspects’ in Surface Science Reports, there has been considerable progress made in further understanding the fundamental interactions of water with solid surfaces. In the decade and a half, the increased capability of surface scientists to probe at the molecular-level has resulted in more detailed information of the properties of water on progressively more complicated materials and under more stringent conditions. This progress in understanding the properties of water on solid surfaces is evident both in areas for which surface science methodology has traditionally been strong (catalysis and electronic materials) and also in new areas not traditionally studied by surface scientists such as electrochemistry, photoconversion, mineralogy, adhesion, sensors, atmospheric chemistry and tribology. Researchers in all these fields grapple with very basic questions regarding the interactions of water with solid surfaces such as how is water adsorbed, what are the chemical and electrostatic forces that constitute the adsorbed layer, how is water thermally or non-thermally activated and how do coadsorbates influence these properties of water. The attention paid to these and other fundamental questions in the past decade and a half has been immense. In this review, experimental studies published since the TM review are assimilated with those covered by TM to provide a current picture of the fundamental interactions of water with solid surfaces.     

Electronic structure and magnetism of surfaces,interfaces and modulated structures (superlattices)

Recent developments in the study of surfaces and interfaces of metals and of artificial materials such as bimetallic sandwiches and modulated structures are described. Key questions include the effects on magnetism of reduced dimensionality and the possibility of magnetically dead layers. These developments have stimulated an intensified theoretical effort to investigate and describe the electronic and magnetic structure of surfaces and interfaces. One notable success has been the development of a highly accurate full-potential all-electron method (the FLAPW method) for solving the local spin density equations self-consistently for a single slab geometry. We describe here this advanced state of ab initio calculations in determining the magnetic properties of transition metal surfaces such as those of the ferromagnetic metals Ni(001) and Fe(001) and the Ni/Cu(001) interface. For both clean Fe and Ni(001) we find an enhancement of the magnetic moments in the surface layer. The magnetism of surface and interface Ni layers on Cu(001) (no dead layers are found) is described and compared to the clean Ni(001) results. Finally, the role ofSR experiments in answering some of the questions raised in these studies will be discussed.     

Electronic properties of alkali metal/silicon interfaces: A new picture

Adsorption of Na and Cs on the Si(100)2 × 1 surface in the monolayer range is investigated by core level and valence band photoemission spectroscopy using synchrotron radiation. The alkali metals are found to induce an electronic interface state near the Fermi level while hybridization between alkali adsorbate “s” and silicon substrate “3p” valence electrons occurs. These results provide evidence that the alkali metal/silicon bonding is covalent. This covalent bond is weak and polarized while plasmon at the alkali metal core level indicates adsorbate rather than substrate metallization.     

Electronic and magnetic properties of SiC nanoribbons by F termination

By using the first-principles calculations, the electronic properties are studied for the F-terminated SiC nanoribbons (SiCNRs) with either zigzag edges (ZSiCNRs) or armchair edges (ASiCNRs). The results show that the broader F-terminated ZSiCNRs are metallic and the edge states appear at the Fermi level, while the F-terminated ASiCNRs are always semiconductors independent of their width but the edge states do not appear due to the Si-C dimer bonds at the edges. The charge density contours analyses shows that the Si-F and Si-C bonds are all ionic bonds due to the much stronger electronegativities of the F and C atoms than that of the Si atom. However, the C-F bonds display a typical non-polar covalent bonding feature because of the electronegativity difference between the F and C atoms of 1.5 is a much smaller than that of between the F and Si atoms of 2.2, as well as the tighter bounded C 2s ²2p ² electrons with smaller orbital radius than the Si 3s ²3p ² electrons. For both the F- and the H-terminated ZSiCNRs, the ground state is a ferromagnetic semiconductor.     

Some new fundamental properties of GaN single-crystal films on SiC and sapphire substrates

Il Nuovo Cimento D - The reflectance spectra of GaN/6H-SiC, films and the absorption spectra of GaN/Al2O3 films were studied and several fundumental parameters of GaN-energy positions of exciton,...     

Electronic and structural properties of hydrogen on semiconductor surfaces

In this paper we will review the scientific literature which addresses the atomic geometry and electronic structure of clean and hydrogenated semiconductor surfaces. In particular, results related to vibrational studies will be presented. First, surfaces of elemental semiconductors (Ge, Si), Ge/Si-alloys, and III–V compound semiconductors chemisorb in a first stage atomic hydrogen by saturating surface atom dangling bonds. In a second step surface bonds are broken and a change of the geometrical structure results. Finally, higher hydrogen exposures are able to etch semiconductor surfaces. Best understood to date are surfaces of Si(1 0 0), Si(1 1 1), Ge_xSi_1−x(1 0 0), and III–V's after cleavage which have been modeled by dimerized and undimerized structures. (1 0 0) surfaces of III–V semiconductors, like GaAs and InP, tend to be dimerized, too.     

Magnetism at surfaces and interfaces

The current state-of-the-art of ab-initio calculations of the magnetic structures of surfaces and interfaces is highlighted by presenting results obtained with the recently developed full-potential linearized augmented plane wave method for thin films. In particular, spin density maps, (induced) magnetic moments and hyperfine-fields are presented for the clean metal surfaces Fe(001), Ni(001) and Pt(001). The magnetic moments on an interface are discussed for the prototypical case Ni/Cu.     

Dynamical and thermodynamical properties of elastic surface waves at hexagonal surfaces and interfaces

A surface Green function matching analysis is used to study free surfaces and interfaces of hexagonal media. All the basic results required for a complete dynamical and thermodynamical study of the hexagonal case are here derived and collected, in particular the surface wave dispersion relation in explicit analytic form and the formulae for the density of modes and the surface specific heat. Examples of numerical applications are given and compared with other theoretical and experimental results, where available. This extends an analysis previously used by the authors to study isotropic systems.     

An overview of some experimental and theoretical aspects of fundamental symmetry violations in atoms

We present some of the advances in our experimental and theoretical studies of violations in fundamental symmetries in atoms. A part of this work was performed under the auspices of a NSF-DST project. During this period, a number of experimental techniques and theoretical methods were developed and employed for precision measurements and their interpretation from first principles. Future directions of these studies are briefly mentioned.     

Analysis of semiconductor surfaces and interfaces

Recent development in the experimental and theoretical analysis of semiconductor surfaces is described. Special attention is given to the Secondary Ion Mass Spectroscopy technique and to its use in the ultrasensitive elemental analysis of semiconductors. Applications to III–V compounds are described.     

On the topographic and optical properties of SiC/SiO2 surfaces

The roughness of the semiconductor surface substantially influences properties of the whole structure, especially when thin films are created. In our work 3C SiC, 4H SiC and Si/a-SiC:H/SiO₂ structures treated by various oxidation a passivation procedures are studied by atomic force microscopy (AFM) and scanning tunnelling microscopy (STM). Surface roughness properties are studied by fractal geometry methods. The complexity of the analysed surface is sensitive to the oxidation and passivation steps and the proposed fractal complexity measure values enable quantification of the fine surface changes. We also determined the optical properties of oxidized and passivated samples by using visual modelling and stochastic optimization.      

Electronic and structural properties of steps on cleaved semiconductor surfaces

The work function of clean, cleaved p-Si(111) surfaces was measured in dependence of the density of cleavage steps. The contact potential difference CPD was observed first to increase then to decrease linearly with increasing step density. This dependence is caused by an increase in band bending, which saturates at a density of approximately 1 × 10⁶ steps per cm, and a decrease of the surface dipole which is proportional to the density of steps. Since the 2 × 1 reconstruction of the cleaved Si surface is ionic and buckled it increases the surface dipole moment compared with the 1 × 1 structure. Thus, a decrease of the surface dipole may be caused by removal of the 2 × 1 reconstruction on part of the step terraces. The experimental data give a width of about 28 Å for the unreconstructed stripe along the step edge.     

Electronic properties of cleaved CdTe(110) surfaces

Photoemission yield spectroscopy measurements were performed on a set of n- and p-doped CdTe single crystals. The surfaces were obtained by cleavage in ultrahigh vacuum and characterized by low energy electron diffraction and Auger electron spectroscopy. On clean and properly cleaved surfaces, no band bending was found, neither on n- nor on p-type samples, showing the absence of intrinsic surface states in the gap. The ionization energy is found at 5.80±0.05 eV. Oxygen adsorption removes defect-induced surface states on the valence band side of the gap and develops a band bending on n-type samples which indicates the presence of acceptor surface states in the gap down to 0.70 eV below the conduction band edge. The ionization energy remains constant.     

Hyperfine fields at surfaces and interfaces

Magnetism and hyperfine fields at transition metal surfaces are discussed using state-of-the-art local spin density methods. Emphasis is placed on recent results obtained for the Fe(001), Ni(001), Cr(001) and Ag/Fe(001) ferromagnetic surfaces, and for the Knight shift in Pt(001).