Surface Science Reports最新文献

The fascinating particle size dependence to the physical, photophysical, and chemical properties of gold has motivated thousands of studies focused on exploring the ability of supported gold nanoparticles to catalyze chemical transformations. In particular, titanium dioxide-supported gold (Au/TiO₂) nanoparticles may provide the right combination of electronic structure, structural dynamics, and stability to affect catalysis in important practical applications from environmental remediation to selective hydrogenation to carbon monoxide oxidation. Harnessing the full potential of Au/TiO₂ will require a detailed atomic-scale understanding of the thermal and photolytic processes that accompany chemical conversion. This review describes some of the unique properties exhibited by particulate gold before delving into how those properties affect chemistry on titania supports. Particular attention is given first to thermally driven reactions on single crystal system. This review then addresses nanoparticulate samples in an effort begin to bridge the so-called materials gap. Building on the foundation provided by the large body of work in the field of thermal catalysis, the review describes new research into light-driven catalysis on Au/TiO₂. Importantly, the reader should bear in mind throughout this review that thermal chemistry and thermal effects typically accompany photochemistry. Distinguishing between thermally-driven stages of a reaction and photo-induced steps remains a significant challenge, but one that experimentalists and theorists are beginning to decipher with new approaches. Finally, a summary of several state-of-the-art studies describes how they are illuminating new frontiers in the quest to exploit Au/TiO₂ as an efficient catalyst and low-energy photocatalyst.

Metal–oxide interfaces play a fundamental role in determining the functional properties of artificial layered heterostructures, which are at the root of present and future technological applications. Magnetic exchange and magnetoelectric coupling, spin filtering, metal passivation, catalytic activity of oxide-supported nano-particles are just few examples of physical and chemical processes arising at metal–oxide hybrid systems, readily exploited in working devices. These phenomena are strictly correlated with the chemical and structural characteristics of the metal–oxide interfacial region, making a thorough understanding of the atomistic mechanisms responsible of its formation a prerequisite in order to tailor the device properties. The steep compositional gradient established upon formation of metal–oxide heterostructures drives strong chemical interactions at the interface, making the metal–oxide boundary region a complex system to treat, both from an experimental and a theoretical point of view. However, once properly mastered, interfacial chemical interactions offer a further degree of freedom for tuning the material properties. The goal of the present review is to provide a summary of the latest achievements in the understanding of metal/oxide and oxide/metal layered systems characterized by reactive interfaces. The influence of the interface composition on the structural, electronic and magnetic properties will be highlighted. Particular emphasis will be devoted to the discussion of ultra-thin epitaxial oxides stabilized on highly oxidizable metals, which have been rarely exploited as oxide supports as compared to the much more widespread noble and quasi noble metallic substrates. In this frame, an extensive discussion is devoted to the microscopic characterization of interfaces between epitaxial metal oxides and the Fe(001) substrate, regarded from the one hand as a prototypical ferromagnetic material and from the other hand as a highly oxidizable metal.

Molybdenum disulfide, MoS₂, has recently gained considerable attention as a layered material where neighboring layers are only weakly interacting and can easily slide against each other. Therefore, mechanical exfoliation allows the fabrication of single and multi-layers and opens the possibility to generate atomically thin crystals with outstanding properties. In contrast to graphene, it has an optical gap of ~1.9 eV. This makes it a prominent candidate for transistor and opto-electronic applications. Single-layer MoS₂ exhibits remarkably different physical properties compared to bulk MoS₂ due to the absence of interlayer hybridization. For instance, while the band gap of bulk and multi-layer MoS₂ is indirect, it becomes direct with decreasing number of layers.

In this review, we analyze from a theoretical point of view the electronic, optical, and vibrational properties of single-layer, few-layer and bulk MoS₂. In particular, we focus on the effects of spin–orbit interaction, number of layers, and applied tensile strain on the vibrational and optical properties. We examine the results obtained by different methodologies, mainly ab initio approaches. We also discuss which approximations are suitable for MoS₂ and layered materials. The effect of external strain on the band gap of single-layer MoS₂ and the crossover from indirect to direct band gap is investigated. We analyze the excitonic effects on the absorption spectra. The main features, such as the double peak at the absorption threshold and the high-energy exciton are presented. Furthermore, we report on the the phonon dispersion relations of single-layer, few-layer and bulk MoS₂. Based on the latter, we explain the behavior of the Raman-active $A_{1g}$ and $E_{2g}^1$ modes as a function of the number of layers. Finally, we compare theoretical and experimental results of Raman, photoluminescence, and optical-absorption spectroscopy.

Understanding the bio-physical–chemical interactions at nanostructured biointerfaces and the assembly mechanisms of so-called hybrid nano-composites is nowadays a key issue for nanoscience in view of the many possible applications foreseen.

The contribution of surface science in this field is noteworthy since, using a bottom-up approach, it allows the investigation of the fundamental processes at the basis of complex interfacial phenomena and thus it helps to unravel the elementary mechanisms governing them.

Nowadays it is well demonstrated that a wide variety of different molecular assemblies can form upon adsorption of small biomolecules at surfaces. The geometry of such self-organized structures can often be tuned by a careful control of the experimental conditions during the deposition process. Indeed an impressive number of studies exists (both experimental and – to a lesser extent – theoretical), which demonstrates the ability of molecular self-assembly to create different structural motifs in a more or less predictable manner, by tuning the molecular building blocks as well as the metallic substrate.

In this frame, amino acids and small peptides at surfaces are key, basic, systems to be studied. The amino acids structure is simple enough to serve as a model for the chemisorption of biofunctional molecules, but their adsorption at surfaces has applications in surface functionalization, in enantiospecific catalysis, biosensing, shape control of nanoparticles or in emerging fields such as “green” corrosion inhibition.

In this paper we review the most recent advances in this field. We shall start from the adsorption of amino acids at metal surfaces and we will evolve then in the direction of more complex systems, in the light of the latest improvements of surface science techniques and of computational methods. On one side, we will focus on amino acids adsorption at oxide surfaces, on the other on peptide adsorption both at metal and oxide substrates. Particular attention will be drawn to the added value provided by the combination of several experimental surface science techniques and to the precious contribution of advanced complementary computational methods to resolve the details of systems of increased complexity. Finally, some hints on experiments performed in presence of water and then characterized in UHV and on the related theoretical work will be presented. This is a further step towards a better approximation of real biological systems. However, since the methods employed are often not typical of surface science, this topic is not developed in detail.

This review covers the surface chemistry of porphyrins, phthalocyanines, their metal complexes, and related compounds, with particular focus on chemical reactions at solid/vacuum interfaces. Porphyrins are not only important biomolecules, they also find, together with the artificial phthalocyanines, numerous technological and scientific applications, which often involve surface and interface related aspects. After a brief summary of fundamental properties of these molecules in the context of surface science, the following topics will be discussed: (1) Aspects of geometric structure, including self-assembly, conformation, mobility and manipulation of the adsorbed molecules. (2) Surface-related changes of the electronic structure and the magnetic properties. (3) The role of the metal center in the surface chemical bond. (4) On-surface coordination reactions, such as direct metalation and coordination of axial ligands. (5) The influence of axial ligands on the surface chemical bond and the magnetic properties.

The oxidation and corrosion of metals are fundamental problems in materials science and technology that have been studied using a large variety of experimental and computational techniques. Here we review some of the recent studies that have led to significant advances in our atomic-level understanding of copper oxide, one of the most studied and best understood metal oxides. We show that a good atomistic understanding of the physical characteristics of cuprous (Cu₂O) and cupric (CuO) oxide and of some key processes of their formation has been obtained. Indeed, the growth of the oxide has been shown to be epitaxial with the surface and to proceed, in most cases, through the formation of oxide nano-islands which, with continuous oxygen exposure, grow and eventually coalesce. We also show how electronic structure calculations have become increasingly useful in helping to characterise the structures and energetics of various Cu oxide surfaces. However a number of challenges remain. For example, it is not clear under which conditions the oxidation of copper in air at room temperature (known as native oxidation) leads to the formation of a cuprous oxide film only, or also of a cupric overlayer. Moreover, the atomistic details of the nucleation of the oxide islands are still unknown. We close our review with a brief perspective on future work and discuss how recent advances in experimental techniques, bringing greater temporal and spatial resolution, along with improvements in the accuracy, realism and timescales achievable with computational approaches make it possible for these questions to be answered in the near future.

Recently, pulsed laser deposition (PLD) technology makes viable the epitaxial growth of group III-nitrides on thermally active substrates at low temperature. The precursors generated from the pulsed laser ablating the target has enough kinetic energy when arriving at substrates, thereby effectively suppressing the interfacial reactions between the epitaxial films and the substrates, and eventually makes the film growth at low temperature possible. So far, high-quality group III-nitride epitaxial films have been successfully grown on a variety of thermally active substrates by PLD. By combining PLD with other technologies such as laser rastering technique, molecular beam epitaxy (MBE), and metal-organic chemical vapor deposition (MOCVD), III-nitride-based light-emitting diode (LED) structures have been realized on different thermally active substrates, with high-performance LED devices being demonstrated. This review focuses on the epitaxial growth of group III-nitrides on thermally active substrates by PLD and their use in the development of LED devices. The surface morphology, interfacial property between film and substrate, and crystalline quality of as-grown group III-nitride films by PLD, are systematically reviewed. The corresponding solutions for film homogeneity on large size substrates, defect control, and InGaN films growth by PLD are also discussed in depth, together with introductions to some newly developed technologies for PLD in order to realize LED structures, which provides great opportunities for commercialization of LEDs on thermally active substrates.

Among nanoalloys, Co–Pt type (CoPt or FePt) supported nanostructures are very interesting systems due to the direct link between atom arrangement and magnetic behavior. In addition, these alloys become model systems in the field of nanoalloys, due to the diversity of atom arrangements either present in the bulk state or specific to the nanoscale (chemically ordered L1₀, L1₂, or disordered fcc structures, core–shell, five-fold structures – icosahedral or decahedral, etc.). The synergy between experimental and modeling efforts has allowed the emergence of an overview of the structural, morphological and chemical behaviors of CoPt-based supported nanoparticles in terms of phase diagrams (temperature, composition, size effect), kinetic behavior (growth, annealing, ordering), and also in terms of environment effects (substrate, capping, matrix, gas) and of magnetic properties. All aspects of this complexity are reviewed: synthesis strategies (physical deposition, cluster beam deposition and wet chemical methods), magnetic behavior (atomic magnetic moment, magnetic anisotropy energy), structural transitions (non-crystalline/crystalline structures, order/disorder, surface/interface segregation), etc. In this field, the investigation techniques, such as electron microscopy and X-ray scattering or absorption techniques, are generally used at their ultimate limit due the small size of the studied objects. Finally, several aspects of the annealing process, which is a key phenomenon to achieve the chemical order, have been discussed in both thermodynamic and kinetic points of view (size effect, critical temperature, annealing time, twinning, coalescence, etc.).

In recent years, the field of catalysis has experienced an astonishing transformation, driven in part by more demanding environmental standards and critical societal and industrial needs such as the search for alternative energy sources. Thanks to the advent of nanotechnology, major steps have been made towards the rational design of novel catalysts. Striking new catalytic properties, including greatly enhanced reactivities and selectivities, have been reported for nanoparticle (NP) catalysts as compared to their bulk counterparts. However, in order to harness the power of these nanocatalysts, a detailed understanding of the origin of their enhanced performance is needed. The present review focuses on the role of the NP size and shape on chemisorption and catalytic performance. Since homogeneity in NP size and shape is a prerequisite for the understanding of structure–reactivity correlations, we first review different synthesis methods that result in narrow NP size distributions and shape controlled NPs. Next, size-dependent phenomena which influence the chemical reactivity of NPs, including quantum size-effects and the presence of under-coordinated surface atoms are examined. The effect of the NP shape on catalytic performance is discussed and explained based on the existence of different atomic structures on the NP surface with distinct chemisorption properties. The influence of additional factors, such as the oxidation state of the NPs and NP–support interactions, is also considered in the frame of the size- and shape-dependency that these phenomena present. Ultimately, our review highlights the importance of achieving a systematic understanding of the factors that control the activity and selectivity of a catalyst in order to avoid trial and error methods in the rational design of the new generation of nanocatalysts with properties tunable at the atomic level.