Nanoparticles in mesoporous films,a happy marriage for materials science

Mesoporous ordered materials, whose porosity is within the 2–50-nm range, are an ideal host for functional nanoparticles. Mesoporous thin films, in particular, offer a large variety of options for the fabrication of advanced materials and devices based on the host-guest combination of matrix nanoparticles. Nanocomposite mesoporous films embedding metal, oxide, and semiconductor nanoparticles have been prepared using as matrix oxides, mixed oxides, and organic-inorganic hybrids. The organization of the pores is an important peculiar property of mesoporous ordered films and allows producing nanocomposites whose nanoparticles follow a very specific array within the material. The main synthesis methods to obtain mesostructured films containing nanoparticles, together with their applications, are briefly introduced in the present review.     

Selective electroplating of copper lines on pre-patterned tantalum oxide thin films

Direct selective metal deposition on semiconductors is of interest to electronic device technology, in particular for interconnects and Schottky devices. In this study, we investigate selective copper electrodeposition on patterned tantalum oxide thin films. Cyclic voltammetry studies show that thick tantalum oxide thin films have insulating properties while oxide films thinner than a critical value are semiconductors. Copper films electrodeposited on tantalum oxide thin films are known to form Schottky contacts. We demonstrate the formation of copper patterns on pre-patterned tantalum oxide films by a simple process: an insulating tantalum oxide film was grown electrochemically, the film was then mechanically scratched followed by mild oxidation to produce a thin tantalum oxide film inside the scratch. Based on the differential behavior of thin and thick tantalum oxide films, metal lines were electrodeposited selectively under formation of Schottky junctions. The process demonstrated in this paper is compatible to standard processes for semiconductor device fabrication while permitting flexible prototyping for research at small scales.     

Thin film photocatalysis for environmental remediation: A status review

This review accounts, various metal oxide and metal sulfide thin films available for photodegradation of several organic compounds. Due to difficulties in recycling and to avoid rigorous recollection of powder catalysts, the thin film catalyst are gaining rapid attention for photocatalytic applications. The semiconducting thin films are growing as promising photocatalyst for water treatment. This review focuses mainly on the photocatalytic activity of metal oxide thin films in terms of its stability, charge transport and absorption properties. Thin film photocatalyst provides the increased efficiency and cost reduction of device. Furthermore, this review summarizes some key factors regarding the enhancement in photocatalytic performance of thin films.     

Vortex pinning in superconducting Nb thin films deposited on nanoporous alumina templates

We present a study of magnetization and transport properties of superconducting Nb thin films deposited on nanoporous aluminium oxide templates. Periodic oscillations in the critical temperature vs. field, matching effects in fields up to 700 mT and strongly enhanced critical currents were observed. These fields are considerably higher than those typical for periodic pinning arrays made by lithographic techniques, which reflects the benefits of nanostructuring superconductors by using self-organized growth. This method provides a periodic pinning potential with sub-100 nm spacing between the pinning centers, which enhances vortex pinning in broad field and temperature ranges.     

On the surface chemical reactions of metal and oxide XPS samples at 300–400° at a high vacuum produced by oil diffusion pumps

Metal and oxide surface reactions formed by heating in the spectrometer at 300–400° at a vacuum of ca. 10^?9 Torr (oil diffusion pumps) were studied. As a result of spectral observations before and after heating, the metals studied were classified into five groups. In the first group, oxide films on the metal surface are easily evaporated because of the high vapour pressure of oxide; in the second, the oxide films are easily reduced in the spectrometer; in the third, the oxide film formed on the metal is reduced but the bulk oxide is not easily reduced; in the fourth, very stable oxide films are formed and the bulk oxide is also stable; and finally in the fifth, the oxide film formed on the metal is apparently reduced, yet the bulk oxide is very stable.     

Rapid laser prototyping of plasmonic components

Renewed and growing interest in the field of surface plasmon polaritons (SPPs) comes from a rapid advance of nanostructuring technologies. In this paper, we will report on the application of two-photon polymerization (2PP) technique for the fabrication of dielectric SPP-structures, which can be used for localization, guiding, and manipulation of SPPs on a subwavelength scale. This technology is based on nonlinear absorption of near-infrared femtosecond laser pulses. Resolutions down to 100 nm (and even better) are already achievable. Characterization of these structures is performed by leakage radiation microscopy. 2PP allows the fabrication of dielectric waveguides, splitters, and couplers directly on metal surfaces. The dielectric structures on metal films are demonstrated to be very efficient for the excitation of SPPs. Using these structures, one can achieve excitation and focusing of the resulting plasmon field. PACS 42.70.Gi; 42.70.Jk; 42.82.Cr; 71.36.+c; 78.20.-e     

Optical properties change of oxide film — metal system during the film growth: computer simulation

Results of computer simulation of reflective properties of the oxide film-metal system in the process of oxidation in the air environment are presented. The complex refractive indices for oxide film and metal were used as the initial data. Thin films (the thickness is comparable with the wavelength of incident radiation) and thick films (thickness is much larger than the wavelength of incident radiation) are considered. The parameter characterizing the cyclic character of system reflectivity during the growth of film thickness was derived for the thin film. It is shown that the cyclic parameter does not depend on optical properties of a metal substrate. In the air environment, this parameter is determined by a complex refractive index of the film, its thickness, and direction of incident radiation. Relationships for the estimate of system reflectivity in the process of oxide film growth are presented for the thick film.     

Catalyst design using an inverse strategy: From mechanistic studies on inverted model catalysts to applications of oxide-coated metal nanoparticles

Metal-oxide interfaces are of great importance in catalytic applications since each material can provide a distinct functionality that is necessary for efficient catalysis in complex reaction pathways. Moreover, the synergy between two materials can yield properties that exceed the superposition of single sites. While interfaces between metals and metal oxides can play a key role in the reactivity of traditional supported catalysts, significant attention has recently been focused on using “inverted” oxide/metal catalysts to prepare catalytic interfaces with unique properties. In the inverted systems, metal surfaces or nanoparticles are covered by oxide layers ranging from submonolayer patches to continuous films with thickness at the nanometer scale. Inverse catalysts provide an alternative approach for catalyst design that emphasizes control over interfacial sites, including inverted model catalysts that provide an important tool for elucidation of mechanisms of interfacial catalytic reactions and oxide-coated metal nanoparticles that can yield improved stability, activity and selectivity for practical catalysts.This review begins by providing a summary of recent progress in the use of inverted model catalysts in surface science studies, where oxides are usually deposited onto the surface of metal single crystals under ultra-high vacuum conditions. Surface-level studies of inverse systems have yielded key insights into interfacial catalysis and facilitated active site identification for important reactions such as CO oxidation, the water-gas shift reaction, and CO₂ reduction using well-defined model systems, informing strategies for designing improved technical catalysts. We then expand the scope of inverted catalysts, using the “inverse” strategy for preparation of higher-surface area practical catalysts, chiefly through the deposition of metal oxide films or particles onto metal nanoparticles. The synthesis techniques include encapsulation of metal nanoparticles within porous oxide shells to generate core-shell type catalysts using wet chemical techniques, the application of oxide overcoat layers through atomic layer deposition or similar techniques, and spontaneous formation of metal oxide coatings from more conventional catalyst geometries under reaction or pretreatment conditions. Oxide-coated metal nanoparticles have been applied for improvement of catalyst stability, control over transport or binding to active sites, direct modification of the active site structure, and formation of bifunctional sites. Following a survey of recent studies in each of these areas, future directions of inverted catalytic systems are discussed.     

金属氧化物薄膜光学常数计算物理模型应用研究

采用离子束溅射(IBS)方法制备了HfO₂和Ta₂O₅两种金属氧化物薄膜,通过测量薄膜的椭偏参数,使用非线性最小二乘法反演计算获得薄膜的光学常数。 在拟合过程中,采用L₈(27)正交表设计了8组反演计算实验,在初始选定Cauchy模型后,对HfO₂薄膜拟合影响最大的为表面层模型,对Ta₂O₅薄膜拟合影响最大的为折射率梯度模型。 确定了不同物理模型对拟合函数MSE的影响权重和拟合过程中模型选择的次序,按照确定的模型选择次序拟合,最后加入弱吸收模型反演计算两种薄膜的光学常数,反演计算的MSE相对初始MSE可下降79%和39%,表明拟合过程模型选择物理意义明确,具有广泛的应用价值。 在500 nm处,Ta₂O₅薄膜的折射率梯度大于HfO₂薄膜, 而HfO₂薄膜消光系数大于Ta₂O₅薄膜。 表明Hf金属与Ta金属相比容易氧化形成稳定的氧化物,HfO₂薄膜的吸收要高于Ta₂O₅薄膜。     

Development of porous metal oxide thin films by co-evaporation

This paper focuses on the development of mixed metal oxide thin films and physical characterization of the films. The films were produced by co-evaporation of titanium oxide and tungsten oxide powders. This allowed the development of titanium oxide-tungsten oxide films as analyzed using XPS. Examination in the SEM and AFM showed that the films were nanoporous with the pore size and pore orientation varying as a function of the deposition angle. UV-vis spectra of the films show an increase of transmittance with increasing deposition angle which is attributed to the structure and porosity of the films. Raman analysis indicated that the as-deposited films have broad and weak Raman characteristics, attributed to the nanocrystal nature of the films and the presence of defects, and the peak broadening deceases after annealing the film, as expected.     

Luminescence of charge transfer sensitizers anchored to metal oxide nanoparticles

The photoluminescence (PL) properties of inorganic charge transfer sensitizers anchored to nanometer sized metal oxide particles are presented. The charge transfer sensitizers are inorganic coordination compounds such as ruthenium tribipyridine, Ru(bpy)²⁺₃, which have long lived metal-to-ligand charge transfer (MLCT) excited states. The metal oxides are insulators or semiconductor materials in the form of powders, colloidal solutions, and porous nanocrystalline films. Time resolved PL decays from this and related sensitizers anchored to metal oxide surfaces are highly non-exponential. The MLCT excited states are quenched on semiconducting metal oxide particles by an apparent electron transfer mechanism. With some assumptions electron transfer rates from the MLCT excited states to the nanostructured surface are calculated. The PL properties of sensitizers bound to porous nanocrystalline TiO₂ films can be controlled electrochemically.     

Tailoring oxide properties: An impact on adsorption characteristics of molecules and metals

Both density functional theory calculations and numerous experimental studies demonstrate a variety of unique features in metal supported oxide films and transition metal doped simple oxides, which are markedly different from their unmodified counterparts. This review highlights, from the computational perspective, recent literature on the properties of the above mentioned surfaces and how they adsorb and activate different species, support metal aggregates, and even catalyse reactions. The adsorption of Au atoms and clusters on metal-supported MgO films are reviewed together with the cluster׳s theoretically predicted ability to activate and dissociate O₂ at the Au–MgO(100)/Ag(100) interface, as well as the impact of an interface vacancy to the binding of an Au atom. In contrast to a bulk MgO surface, an Au atom binds strongly on a metal-supported ultra-thin MgO film and becomes negatively charged. Similarly, Au clusters bind strongly on a supported MgO(100) film and are negatively charged favouring 2D planar structures. The adsorption of other metal atoms is briefly considered and compared to that of Au. Existing computational literature of adsorption and reactivity of simple molecules including O₂, CO, NO₂, and H₂O on mainly metal-supported MgO(100) films is discussed. Chemical reactions such as CO oxidation and O₂ dissociation are discussed on the bare thin MgO film and on selected Au clusters supported on MgO(100)/metal surfaces. The Au atoms at the perimeter of the cluster are responsible for catalytic activity and calculations predict that they facilitate dissociative adsorption of oxygen even at ambient conditions. The interaction of H₂O with a flat and stepped Ag-supported MgO film is summarized and compared to bulk MgO. The computational results highlight spontaneous dissociation on MgO steps. Furthermore, the impact of water coverage on adsorption and dissociation is addressed. The modifications, such as oxygen vacancies and dopants, at the oxide–metal interface and their effect on the adsorption characteristics of water and Au are summarized. Finally, more limited computational literature on transition metal (TM) doped CaO(100) and MgO(100) surfaces is presented. Again, Au is used as a probe species. Similar to metal-supported MgO films, Au binds more strongly than on undoped CaO(100) and becomes negatively charged. The discussion focuses on rationalization of Au adsorption with the help of Born–Haber cycle, which reveals that the so-called redox energy including the electron transfer from the dopant to the Au atom together with the simultaneous structural relaxation of lattice atoms is responsible for enhanced binding. In addition, adsorption energy dependence on the position and type of the dopant is summarized.     

Structure of ultrathin oxide layers on metal surfaces from grazing scattering of fast atoms

The structure of ultrathin oxide layers grown on metal substrates is investigated by grazing scattering of fast atoms from the film surface. We present three recent experimental techniques which allow us to study the structure of ordered oxide films on metal substrates in detail. (1) A new variant of a triangulation method with fast atoms based on the detection of emitted electrons, (2) rainbow scattering under axial surface channeling conditions, and (3) fast atom diffraction (FAD) for studies on the structure of oxide films. Our examples demonstrate the attractive features of grazing fast atom scattering as a powerful analytical tool in surface physics.     

Synthesis and conductive properties of nanoisland Sn,Al, and Cu films

Nanoisland Sn, Al, and Cu films were synthesized on dielectric substrates and their morphology and conductive properties were investigated. It is shown that the initial effective film thickness significantly affects the morphological parameters of nanoislands. Study of the surface conductivity of the films at the condensation stage revealed the conductivity drop after termination of the deposition, which is related to the nanostructuring processes. It was found that the temperature dependences of the film conductivity include three portions: the low-temperature portion of the activation growth, the decrease upon nanostructuring, and the high-temperature portion of the activation growth.     

Influence of annealing on the structure of nanoporous oxide films on the surface of titanium‒aluminum powder alloy

Oxide films obtained during anodization of Ti?40% Al sintered powder samples in fluorine-containing electrolytes are investigated. With scanning electron microscopy and X-ray phase analysis, it is demonstrated that an X-ray amorphous nanoporous anodic oxide film is formed on the surface of the powder microparticles under optimal anodization conditions. After annealing at T = 1093 K in air and vacuum (10^?2 Pa), the oxide films are revealed to crystallize with its regular porous structure retained. The composition of the polycrystalline anodic-oxide films annealed in air is a mixture involving TiO₂ (anatase and rutile) and α- and γ-Al₂O₃ phases and Ti₂O₃ and Al₂TiO₅ traces. The vacuum annealing process makes it possible to identify TiO₂, in which anatase is the main phase, α- and γ-Al₂O₃, and Ti₂O₃ and TiO traces. However, rutile is not revealed. The presented results indicate that the application of the anodic nanostructuring of Ti?40% Al powders is promising for the obtainment of new photocatalytic active nanomaterials.     

Metal-supported ultrathin oxide film systems as designable catalysts and catalyst supports

Thin oxide films lend themselves as model supports for studies in heterogeneous catalysis, for example, to study the growth and reaction of metal deposits (atoms, clusters and nanoparticles). If the thickness of the film is chosen appropriately these thin films are reasonable models to mimic the situation of bulk materials. If thin films below a critical thickness are studied these materials exhibit properties in their own right. Their structural properties may be tuned to control their functional characteristics. Possible implications for heterogeneous catalysis are discussed.     

Anodic iridium oxide films: An UPS study of emersed electrodes

Formation of anodic iridium oxide films has been monitored using Ultraviolet Photoemission Spectroscopy (UPS) of the emersed electrodes. The potential dependent valence band spectra clearly show the onset of oxide formation at about 0.6 V versus SCE. The density of states at the Fermi level and the positron of the Fermi level with respect to the maximum of the t_2g band of the oxide indicates a transition from metallic to semiconducting behaviour of the oxide. Protonation of the oxide is associated with increased emission from OH species. A linear correlation between electrode potential and workfunction change is observed for the metal as well as for the oxide. Our results confirm known band theory models and provide a fundamental understanding of the electrochromism of anodic iridium oxide films.     

Development of Oxidation Sources in Preparation of High-Tc Oxide Superconductor Thin Films Using the Molecular Beam Epitaxy Method

Film preparation of oxide superconductors, mainly of the 1-2-3 (RBa₂Cu₃O_x) and Bi-containing (Bi-Sr-Ca-Cu-O) systems, by evaporation of either metals or metal compounds by low pressure is summarized, with a particular focus on the development of oxidation sources essential to the technique. Oxidizing reagents that enable the oxidation of metal evaporates to take place in high (0·1 to 10_?3 Pa) or even ultra-high (<10_?5 Pa) vacuum are summarized using the experiments of those who tried to apply the molecular beam epitaxy method to atomically controlled fabrication of thin films of the material, especially for device processing. The evaporation in various kinds of oxidizing atmosphere, including the simple method of in situ annealing of the metal layers in oxygen to the more advanced in situ preparation of the films with strong oxidizing reagents such as atomic oxygen, ozone, nitric oxide, etc. along with the thermochemistry of the oxidation of metals by low pressure with these reagents is reviewed.     

Infrared spectroscopic characterization of the properties of oxide films grown on Zr surfaces doped under ion-beam irradiation in a vapor-water medium

Infrared spectroscopy is shown to be applicable to the nondestructive check of the state of oxide films on metal substrates doped under irradiation by an ion beam. The reflection spectra of oxide films on zirconium surfaces doped under ion-beam irradiation are obtained. Oxide films were grown by means of oxidation in a vapor-water medium. The analysis of the fine structure of IR spectra has revealed that, at wavelengths of 1–10 μm, the spectra contain resonance lines corresponding to ZrH-and ZrOH-type molecular compounds, OH groups, and other components of oxide.     

Charge transport and trapping model for scaled nitride-oxide stacked films

Experimental measurements, including capacitance-voltage and temperature-dependent current-voltage measurements of two nitride/oxide dual-layer films, have been used to characterize the charge transport and trapping mechanisms for scaled nitride-oxide stacked films. For charge transport from the cathode electrode to the adjacent oxide or nitride, electron Fowler-Nordheim tunneling is the dominant mechanism and the tunneling barriers are 3.2 eV for oxide and 2 eV for nitride, respectively. For charge transport from the nitride to the oxide, electron tunneling with limited electron supply from the nitride/oxide injecting interfaces was observed. A new charge transport and trapping model for scaled nitride-oxide stacked films is evolved from the experimental observations. According to the model, nitride-oxide stacked films can be thought of as an oxide film with electron trapping at the nitride/oxide interface. The electron trapping reduces the leakage current and lowers the incidence of early failures for nitride-oxide stacked films.