Surface Science Reports最新文献

Titanates are salts of polytitanic acid that can be synthesized as nanostructures in a great variety concerning crystallinity, morphology, size, metal content and surface chemistry. Titanate nanotubes (open-ended hollow cylinders measuring up to 200 nm in length and 15 nm in outer diameter) and nanowires (solid, elongated rectangular blocks with length up to 1500 nm and 30–60 nm diameter) are the most widespread representatives of the titanate nanomaterial family. This review covers the properties and applications of these two materials from the surface science point of view. Dielectric, vibrational, electron and X-ray spectroscopic results are comprehensively discussed first, then surface modification methods including covalent functionalization, ion exchange and metal loading are covered. The versatile surface chemistry of one-dimensional titanates renders them excellent candidates for heterogeneous catalytic, photocatalytic, photovoltaic and energy storage applications, therefore, these fields are also reviewed.

In the past decade there have been many theoretical and experimental efforts to study the mechanisms of solid state dewetting, that means the spontaneous agglomeration of a thin solid film on a substrate into an assembly of 3D islands. The dewetting studies of solid films on solid substrates have not yet reached the degree of maturity achieved for liquids but there is now enough experimental data to consider the possibility of a future “dewetting engineering”. By dewetting engineering we mean all the ways to tune and/or control the kinetics of dewetting as well as the morphology of the final dewetted state. The ultimate goal is to avoid dewetting when it complicates the fabrication of thin film-based devices or to use it for the spontaneous production of an assembly of nanoscaled islands on solid substrates.

For this purpose we review the different parameters that influence the dewetting then illustrate how the dewetted state may be tuned by varying the thickness of the film, the annealing temperature, or the state of strain in the film. Moreover, adsorbed or absorbed species (by deposition or ionic impingement/ion bombardment) may modify the surface properties of the film or the mobility properties of the contact line film/substrate and thus the dewetting properties. Anisotropic properties of the film may also be used to initiate the dewetting from perfectly oriented edge fronts, leading to highly ordered 3D islands. New approaches using substrate pre-patterning or film patterning are very promising to achieve the dewetting engineering.

Ideal systems for studying solid state dewetting are single crystalline films deposited or bonded on amorphous substrates, so that, among the numerous dewetting systems reported in the literature, ultra-thin crystalline silicon-on-insulator (SOI) film (a Si film bonded on an amorphous SiO₂ substrate) is considered as a model system for studying how to control solid state dewetting. Other systems, as Ni epitaxially grown on MgO, are also used to illustrate the different approaches for a “dewetting engineering”.

The accepted image of muscovite mica is that of an inert and atomically smooth surface, easily prepared by cleavage in an ambient atmosphere. Consequently, mica is extensively used a model substrate in many fundamental studies of surface phenomena and as a substrate for AFM imaging of biomolecules. In this review we present evidence from the literature that the above picture is not quite correct. The mica used in experimental work is almost invariably cleaved in laboratory air, where a reaction between the mica surface, atmospheric CO₂ and water occurs immediately after cleavage. The evidence suggests very strongly that as a result the mica surface becomes covered by up to one formula unit of K₂CO₃ per nm², which is mobile under humid conditions, and crystallises under drier conditions. The properties of mica in air or water vapour cannot be fully understood without reference to the surface K₂CO₃, and many studies of the structure of adsorbed water on mica surfaces may need to be revisited. With this new insight, however, the air-cleaved mica should provide exciting opportunities to study phenomena such as two-dimensional ion diffusion, electrolyte effects on surface conductivity, and two-dimensional crystal nucleation.

The current efficiency of various photocatalytic processes is limited by the recombination of photogenerated electron–hole pairs in the photocatalyst as well as the back-reaction of intermediate species. This review concentrates on the use of ferroelectric polarization to mitigate electron–hole recombination and back-reactions and therefore improve photochemical reactivity. Ferroelectric materials are considered as wide band gap polarizable semiconductors. Depending on the surface polarization, different regions of the surface experience different extents of band bending and promote different carriers to move to spatially different locations. This can lead to some interesting interactions at the surface such as spatially selective adsorption and surface redox reactions. This introductory review covers the fundamental properties of ferroelectric materials, effect of an internal electric field/polarization on charge carrier separation, effect of the polarization on the surface photochemistry and reviews the work done on the use of these ferroelectric materials for photocatalytic applications such as dye degradation and water splitting. The manipulation of photogenerated charge carriers through an internal electric field/surface polarization is a promising strategy for the design of improved photocatalysts.

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.