Surface Science Reports最新文献

Adsorption of hydrogen and hydrocarbon molecules on semiconductor surfaces plays a key role in surface science and technology. Most studies have employed silicon (Si) as a substrate because of its paramount technological importance and scientific interest. However, other semiconductor substrates are gaining an increasing interest as well. Silicon carbide (SiC), which is a material with very special properties allowing developments of novel devices and applications, offers particularly fascinating new degrees of freedom for exceptional adsorption behaviour. For example, a very unusual hydrogen-induced metallization of a SiC(001) surface has been reported and hydrogen molecules show very different adsorption behaviour on different SiC(001) reconstructions although the latter exhibit very similar surface dimers. In marked contrast to the Si(001) surface, the adsorption of hydrocarbon molecules on SiC(001) can yield structurally well-defined adlayers in favourable cases which may have large potential for organic functionalization. We review and discuss theoretical ab initio results on conceivable adsorption scenarios of atomic and molecular hydrogen as well as acetylene, ethylene, butadiene, benzene and cyclohexadiene on various reconstructions of the SiC(001) surface. The main emphasize is on a detailed understanding of these adsorption systems and on identifying the physical origin of the particular adsorption behaviour. The results will be discussed in the light of related adsorption events on the Si(001) surface and in comparison with available experimental data.

There are a wide variety of silica nanoformulations being investigated for biomedical applications. Silica nanoparticles can be produced using a wide variety of synthetic techniques with precise control over their physical and chemical characteristics. Inorganic nanoformulations are often criticized or neglected for their poor tolerance; however, extensive studies into silica nanoparticle biodistributions and toxicology have shown that silica nanoparticles may be well tolerated, and in some case are excreted or are biodegradable. Robust synthetic techniques have allowed silica nanoparticles to be developed for applications such as biomedical imaging contrast agents, ablative therapy sensitizers, and drug delivery vehicles. This review explores the synthetic techniques used to create and modify an assortment of silica nanoformulations, as well as several of the diagnostic and therapeutic applications.

The theory of elasticity accurately describes the deformations of macroscopic bodies under the action of applied stress [1]. In this review, we examine the internal mechanisms of elasticity for strained-layer semiconductor heterostructures. In particular, we present extended x-ray-absorption fine structure (EXAFS) and x-ray diffraction (XRD) measurements to show how the bond lengths and bond angles in semiconductor thin-alloy films change with strain when they are grown coherently on substrates with different lattice constants. The structural distortions measured by experiment are compared to valence-force field (VFF) calculations and other theoretical models. Atomic switching and interfacial strain at buried interfaces are also discussed.

Alkali-metal (AM) atoms adsorbed on single-crystal surfaces are a model system for understanding the properties of adsorption. AM adsorption, besides introducing new overlayer vibrational states, induces significant modifications in the surface vibrational structure of the metal substrate. Several studies of the vibrational properties of AM on metal surfaces have been carried out in last decades. Most of these investigations have been performed for low coverages of AM in order to make the lateral interaction among co-adsorbates negligible. The adsorbed phase is characterized by a stretch (S) vibrational mode, with a polarization normal to the surface, and by other two modes polarized in the surface plane, known as frustrated translation (T) modes. The frequencies and intensities of these modes depend on the coverage, thus providing a spectroscopic signature for the characterization of the adsorbed phases.

The vibrational spectroscopy joined to an ab-initio theoretical analysis can provide useful information about surface charge re-distribution and the nature of the adatom–surface bond, establishing, e.g., its partial ionicity and polarization. Gaining this information implies a significant advancement in our knowledge on surface chemical bonds and on catalytic reactions occurring in AM co-adsorption with other chemical species. Hence, systematic studies of co-adsorption systems are essential for a more complete understanding of heterogeneous catalysis.

The two principal experimental techniques for studying the vibrations of AM adsorbed phases are high-resolution electron energy loss spectroscopy (HREELS) and inelastic helium atom scattering (HAS), the former being better suited to the analysis of the higher part of the vibrational spectrum, while the latter exploits its better resolution in the study of slower dynamics, e.g., T modes, surface acoustic phonons and diffusive phenomena. Concerning AM co-adsorption systems, reflection–absorption infrared spectroscopy (RAIRS) has been also used (as well as HREELS) for obtaining information on the influence of AM adsorption on the vibrational properties of co-adsorbates.

In this review an extended survey is presented over:

• a)

  the existing HREELS and HAS vibrational spectroscopic studies for AM adsorbed on single-crystal metal surfaces;

• b)

  the theoretical studies based on semi-empirical and ab-initio methods of vibrational structure of AM atoms on metal surfaces;

• c)

  the vibrational (HREELS, RAIRS, TRSHG) characterization of the co-adsorption on metal surfaces of AM atoms with reactive species.

Nanoscaled or porous silicon (p-Si) with and without surface passivation exhibits unusually tunable properties that its parent bulk does never show. Such property tunability amplifies the applicability of Si in the concurrent and upcoming technologies. However, consistent understanding of the fundamental nature of nanoscaled Si remains a high challenge. This article aims to address the recent progress in this regard with focus on reconciling the tunable dielectric, electronic, phononic, and photonic properties of p-Si in terms of skin dominance. We show that the skin-depth bond contraction, local quantum entrapment, and electron localization is responsible for the size-induced property tunability. The shorter and stronger bonds between undercoordinated skin atoms result in the local densification and quantum entrapment of the binding energy and the bonding electrons, which in turn polarizes the dangling bond electrons. Such local entrapment modifies the Hamiltonian and associated properties such as the band gap, core level shift, Stokes shift (electron–phonon interaction), phonon and dielectric relaxation. Therefore, given the known trend of one property change, one is expected to be able to predict the variation of the rest based on the notations of the bond order–length–strength correlation and local bond average approach (BOLS-LBA). Furthermore, skin bond reformation due to Al, Cu, and Ti metallization and O and F passivation adds another freedom to enhance or attenuate the size effect. The developed formulations, spectral analytical methods, and importantly, the established database and knowledge could be of use in engineering p-Si and beyond for desired functions.

In this report we present a novel strategy in selective epitaxial growth on top of Si pillars, which results in a tessellated Ge film, composed by self-aligned micron-sized crystals in a maskless process. Modelling by rate equations the morphology evolution of fully facetted crystal profiles is extensively outlined, showing an excellent prediction of the peculiar role played by flux shielding among microcrystals, in the case of dense array configuration. Crack formation and substrate bending, caused by the mismatch in thermal expansion coefficients, are eliminated by the mechanical decoupling among individual microcrystals, which are also shown to be dislocation- and strain-free. The method has been also tested for Si_1−xGe_x alloys, with compositions ranging from pure silicon to pure germanium. There are ample reasons to believe that this approach could be extended to other material combinations and substrate orientations, actually providing a technology platform for several device applications.

We review basic and advanced concepts needed for the correct analysis of XPS features. We place these concepts on rigorous foundations and explore their physical and chemical meanings without stressing the derivation of the mathematical formulations, which can be found in the cited literature. The significance and value of combining theory and experiment is demonstrated by discussions of the physical and chemical origins of the main and satellite XPS features for a variety of molecular and condensed phase materials.

Advances in theoretical methods, in particular density functional theory (DFT), make it possible to describe catalytic reactions at surfaces with the detail and accuracy required for computational results to compare with experiment in a meaningful way. The theoretical studies also describe chemical reaction networks and understand variations in catalytic activity from one catalyst to another. Such understanding allows the theoretical optimization for better catalysts.

In the current report we discussed the theoretical studies in the past few years on decomposition and synthesis of methanol and ethanol on various catalyst surfaces. The knowledge of reactions including the intermediates and transition states along different reaction pathways together with kinetic modeling was demonstrated. The theoretical studies on alcohol synthesis help gain better understanding of the complex kinetics and the roles that each component of a catalyst plays. In general, moving from mono-functional catalysts to multi-functional catalysts by increasing the complexity offers new opportunities to tune the behavior of a catalyst. A good multi-functional catalyst is not necessary to compromise the binding strong enough to adsorb and dissociate reactants and weak enough to allow the formation of intermediates and removal of products; instead, it may take advantage of each component, which catalyzes different elementary steps depending on its unique activity. The synergy between the different components can enable the multi-functional catalyst a novel activity in catalysis. This is of great importance for rational design of better catalysts for alcohol renewal synthesis and efficient use.