In chemistry and physics the electronic charge on a species or material is one important determinant of its properties. In the present Minireview, the essential requirements for a model catalyst ...system suitable to study charge control are discussed. The ideal model catalyst for this purpose consists of a material system, which comprises a single crystal metal support, covered by an epitaxially grown ultrathin oxide film, and flat, two‐dimensional nanoparticles residing on this film. Several examples from the literature are selected and presented, which illustrate various aspects of electron transport from the support to the nanoparticle and vice versa. Key experiments demonstrate charge control within such model catalysts and give direct evidence for a chemical reaction at the perimeter of Au nanoparticles. The concepts derived from these studies are then taken a step further to see how they may be applied for bulk powder oxide supported nanoparticles as they are frequently found in catalytically active materials.
Taking charge of the situation: In chemistry and physics the electronic charge on a species or material is one important determinant of its properties. In this Minireview, the essential requirements for a model catalyst system suitable to study charge control are discussed. Examples from the literature illustrate various aspects of electron transport from the support to the nanoparticle and vice versa. The concepts derived from these studies are then taken a step further to see how they may be applied for bulk powder oxide supported nanoparticles as they are frequently found in catalytically active materials.
The present review reports on the preparation and atomic‐scale characterization of the thinnest possible films of the glass‐forming materials silica and germania. To this end state‐of‐the‐art surface ...science techniques, in particular scanning probe microscopy, and density functional theory calculations have been employed. The investigated films range from monolayer to bilayer coverage where both, the crystalline and the amorphous films, contain characteristic XO4 (X=Si,Ge) building blocks. A side‐by‐side comparison of silica and germania monolayer, zigzag phase and bilayer films supported on Mo(112), Ru(0001), Pt(111), and Au(111) leads to a more general comprehension of the network structure of glass former materials. This allows us to understand the crucial role of the metal support for the pathway from crystalline to amorphous ultrathin film growth.
Ultrathin films: Comparison of silica and germania thin films to understand the crucial role of the metal support for the pathway from crystalline to amorphous structures.
A new two‐dimensional (2D) germanium dioxide film has been prepared. The film consists of interconnected germania tetrahedral units forming a bilayer structure, weakly coupled to the supporting ...Pt(111) metal‐substrate. Density functional theory calculations predict a stable structure of 558‐membered rings for germania films, while for silica films 6‐membered rings are preferred. By varying the preparation conditions the degree of order in the germania films is tuned. Crystalline, intermediate ordered and purely amorphous film structures are resolved by analysing scanning tunnelling microscopy images.
A 2D germanium dioxide film consisting of interconnected germania tetrahedral units forming a bilayer structure, weakly coupled to the supporting Pt(111) metal‐substrate has been prepared. By varying the preparation conditions the degree of order in the germania films is tuned. Crystalline, intermediate ordered and purely amorphous film structures are resolved by analyzing scanning tunneling microscopy images.
Two‐dimensional oxide films are potentially useful for future technological applications, but also important objects to study model catalyst systems on the more fundamental side. Here we study ...silica, germania, and mixed silica‐germania films supported on a metal single crystal surface Ru(0001). Those mixed films are interesting objects to systematically modify the properties of silica films, which may be used as membranes or covers for model studies in confined space, due to the modification of the rather stiff silica layers by incorporating germanium atoms replacing silicon atoms. Here we report a combined experimental and theoretical study of such layers, where we show how X‐ray photoelectron spectroscopy in combination with LEED and I/V LEEM measurements allow us to judge the formation of such mixed films.
The on-going developments in laser acceleration of protons and light ions, as well as the production of strong bursts of neutrons and multi-Formula: see text photons by secondary processes now ...provide a basis for novel high-flux nuclear physics experiments. While the maximum energy of protons resulting from Target Normal Sheath Acceleration is presently still limited to around Formula: see text, the generated proton peak flux within the short laser-accelerated bunches can already today exceed the values achievable at the most advanced conventional accelerators by orders of magnitude. This paper consists of two parts covering the scientific motivation and relevance of such experiments and a first proof-of-principle demonstration. In the presented experiment pulses of Formula: see text at Formula: see text duration from the PHELIX laser produced more than Formula: see text protons with energies above Formula: see text in a bunch of sub-nanosecond duration. They were used to induce fission in foil targets made of natural uranium. To make use of the nonpareil flux, these targets have to be very close to the laser acceleration source, since the particle density within the bunch is strongly affected by Coulomb explosion and the velocity differences between ions of different energy. The main challenge for nuclear detection with high-purity germanium detectors is given by the strong electromagnetic pulse caused by the laser-matter interaction close to the laser acceleration source. This was mitigated by utilizing fast transport of the fission products by a gas flow to a carbon filter, where the Formula: see text-rays were registered. The identified nuclides include those that have half-lives down to Formula: see text. These results demonstrate the capability to produce, extract, and detect short-lived reaction products under the demanding experimental condition imposed by the high-power laser interaction. The approach promotes research towards relevant nuclear astrophysical studies at conditions currently only accessible at nuclear high energy density laser facilities.
Use of atomic layer deposition (ALD) in microelectromechanical systems (MEMS) has increased as ALD enables conformal growth on 3-dimensional structures at relatively low temperatures. For MEMS device ...design and fabrication, the understanding of stress and mechanical properties such as elastic modulus, hardness and adhesion of thin film is crucial. In this work a comprehensive characterization of the stress, elastic modulus, hardness and adhesion of ALD aluminum oxide (Al2O3) films grown at 110–300°C from trimethylaluminum and water is presented. Film stress was analyzed by wafer curvature measurements, elastic modulus by nanoindentation and surface-acoustic wave measurements, hardness by nanoindentation and adhesion by microscratch test and scanning nanowear. The films were also analyzed by ellipsometry, optical reflectometry, X-ray reflectivity and time-of-flight elastic recoil detection for refractive index, thickness, density and impurities. The ALD Al2O3 films were under tensile stress in the scale of hundreds of MPa. The magnitude of the stress decreased strongly with increasing ALD temperature. The stress was stable during storage in air. Elastic modulus and hardness of ALD Al2O3 saturated to a fairly constant value for growth at 150 to 300°C, while ALD at 110°C gave softer films with lower modulus. ALD Al2O3 films adhered strongly on cleaned silicon with SiOx termination.
•The residual stress of Al2O3 was tensile and stable during the storage in air.•Elastic modulus of Al2O3 saturated to at 170GPa for films grown at 150 to 300°C.•At 110°C Al2O3 films were softer with high residual hydrogen and lower density.•The Al2O3 adhered strongly on the SiOx-terminated silicon.
STM conductance spectroscopy and mapping has been used to analyze the impact of molecular adsorption on the quantized electronic structure of individual metal nanoparticles. For this purpose, ...isophorone and CO2, as prototype molecules for physisorptive and chemisorptive binding, were dosed onto monolayer Au islands grown on MgO thin films. The molecules attach exclusively to the metal-oxide boundary, while the interior of the islands remains pristine. The Au quantum well states are perturbed due to the adsorption process and increase their mutual energy spacing in the CO2 case but move together in isophorone-covered islands. The shifts disclose the nature of the molecule-Au interaction, which relies on electron exchange for the CO2 ligands but on dispersive forces for the organic species. Our experiments reveal how molecular adsorption affects individual quantum systems, a topic of utmost relevance for heterogeneous catalysis.
At present, organic molecules are among the best candidate “building blocks” for the construction of self-assembling nanoscale devices based on metal substrates. Control of the formation of specific ...patterns in the submonolayer regime is usually achieved by appropriate choice and/or functionalization of the adsorbates. The effect of this intervention, though, is limited by the typically short-range character of the bonding. We present here a theoretical study on the system rubrene/gold to show that substrate-induced molecular charging can instead determine the assembly on larger scales. DFT calculations and electrostatic considerations are used to discuss the charge transfer at the metal/organic interface. This allows rationalization of previous puzzling experimental results and, in particular, of the unusual molecular gap broadening upon adsorption observed in STS spectra. The self-assembly process is further studied by means of classical molecular dynamics simulations. The charged adsorbates are modeled as mutually repulsive standing dipoles, with van der Waals interactions intervening at short distances. The striking resemblance between the experimental STM images and the results of our MD simulations shows that this simple model is able to capture the key effects driving the assembly in this system. The competition between long-range repulsive interactions and short-range attractive forces leads to characteristic and easily recognizable 1D patterns. We suggest that experimental evidence of the presence of similar patterns in other metal/organic systems can provide crucial information on the electronic level alignment at the interface, that is, on the occurrence of charge-transfer processes between metal and organic adsorbates.
The Earth has undergone at least four great glaciations, during which the ice layers and the glaciers have expanded all over the planet, corresponding to significant drops in global temperatures that ...lasted millions of years. Since the Precambrian era, ice ages have occurred at intervals of several millions of years. It is considered that the impact on the biosphere was large, because life was on the brink of disappearing completely from the planet. During these glaciation periods, carbon was reassigned, with the subsequent formation of carbonates called cap carbonates, which present stromatolite-like activity. These findings prove that life was conserved even during the glacial period. Knowledge on life conservation during the glacial period through stromatolites that have endured until the present day, is of special relevance. In recent investigations, in vitro structures have been synthesized; these crystalline aggregates have been named biomorphs because they mirror the morphologies of primitive organisms called Precambrian cherts. These biomorphs have been synthesized at different temperatures (from room temperature to lower ones). The aim of the present work was to synthesize CaCO3, BaCO3, and SrCO3 silica–carbonate biomorphs at three low temperatures (4 °C, −20 °C, and −70 °C). CaCO3 biomorphs present almost the same morphology at all temperatures with a calcite crystalline structure, whereas BaCO3 and SrCO3 biomorphs present remarkably different morphologies depending on temperature with witherite and strontianite crystalline structure, respectively.
The energy gap Delta of superconducting Pb islands grown on Si(111) was probed in situ between 5 and 60 monolayers by low-temperature scanning tunneling spectroscopy. Delta was found to decrease from ...its bulk value as a function of inverse island thickness. Corresponding T_{c} values, estimated using bulk gap-to-T_{c} ratio, are in quantitative agreement with ex situ magnetic susceptibility measurements, however, in strong contrast to previous scanning probe results. Layer-dependent ab initio density functional calculations for freestanding Pb films show that the electron-phonon coupling constant, determining T_{c}, decreases with diminishing film thickness.
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