RESUMO
The understanding of interfacial effects and adhesion at oxide-metal contacts is of key importance in modern technology. Metal-silica interfaces specifically are relevant in electronics, catalysis and nanotechnology. However, adhesion at these interfaces is hindered by a formation of siloxane rings on the silica surface which saturate the dangling bonds at stoichiometric terminations. In this context, we report a thorough density functional theory study of the interaction between ß-cristobalite and selected 3d transition metals under different oxygen conditions. For any given interface stoichiometry, we find a progressive decrease of the metal/silica interaction along the series, following the increase of metal electronegativity. Crucially, in presence of early transition metals (Ti or Cr) the surface siloxane rings are spontaneously broken, allowing for strong adhesion. Late transition metals interact only weakly with reconstructed surfaces, similarly to what was found for zinc. In absence of reconstruction, stoichiometric silica/metal contacts behave similarly to alumina/metal contacts, but display larger interactions across the interface. Based on these results, we show that early transition metal or stainless steel buffers can significantly improve the weak adhesion between silica and zinc, responsible for a poor performance of anti-corrosive galvanic zinc coatings on modern advanced high strength steels.
RESUMO
Epitaxial silicon thin films grown from the deposition of plasma-born hydrogenated silicon nanoparticles using plasma-enhanced chemical vapor deposition have widely been investigated due to their potential applications in photovoltaic and nanoelectronic device technologies. However, the optimal experimental conditions and the underlying growth mechanisms leading to the high-speed epitaxial growth of thin silicon films from hydrogenated silicon nanoparticles remain far from being understood. In the present work, extensive molecular dynamics simulations were performed to study the epitaxial growth of silicon thin films resulting from the deposition of plasma-born hydrogenated silicon clusters at low substrate temperatures under realistic reactor conditions. There is strong evidence that a temporary phase transition of the substrate area around the cluster impact site to the liquid state is necessary for the epitaxial growth to take place. We predict further that a non-normal incidence angle for the cluster impact significantly facilitates the epitaxial growth of thin crystalline silicon films.
RESUMO
The weak interaction between zinc and silica is responsible for the poor performance of anti-corrosive galvanic zinc coatings on modern advanced high-strength steels, which are fundamental in the automotive industry, and important for rail transport, shipbuilding, and aerospace. With the goal of identifying possible methods for its improvement, we report an ab initio study of the effect of surface hydroxylation on the adhesion characteristics of model zinc/ß-cristobalite interfaces, representative of various surface hydroxylation/hydrogenation conditions. We show that surface silanols resulting from dissociative water adsorption at the most stable stoichiometric (001) and (111) surfaces prevent strong zinc-silica interactions. However, dehydrogenation of such interfaces produces oxygen-rich zinc/silica contacts with excellent adhesion characteristics. These are due to partial zinc oxidation and the formation of strong iono-covalent Zn-O bonds between zinc atoms and the under-coordinated excess anions, remnant of the hydroxylation layer. Interestingly, these interfaces appear as the most thermodynamically stable in a wide range of realistic oxygen-rich and hydrogen-lean environments. We also point out that the partial oxidation of zinc atoms in direct contact with the oxide substrate may somewhat weaken the cohesion in the zinc deposit itself. This fundamental analysis of the microscopic mechanisms responsible for the improved zinc wetting on pre-hydroxylated silica substrates provides useful guidelines towards practical attempts to improve adhesion.
RESUMO
The weak interaction between zinc and silica is responsible for a poor performance of anti-corrosive galvanic zinc coatings on modern advanced high strength steels. With the goal of identifying its microscopic origin, we report an extensive ab initio study on the structural, electronic, and adhesion characteristics of a variety of model zinc/ß-cristobalite interfaces, representative for different oxidation conditions. We show that the weakness of the zinc-silica interaction at non polar interfaces is driven by the presence of surface siloxane rings. These latter are drastically detrimental to interface adhesion when intact and their breaking is impeded by a large energy barrier. Conversely, the characteristics of polar interfaces are principally driven by the capacity of zinc to screen the surface compensating charges and to form O-Zn bonds. This screening is especially efficient in an oxygen-rich environment where the substrate-induced partial oxidation of the zinc deposit produces a considerable enhancement of interface adhesion. The identified microscopic mechanisms of interface interactions furnish precious guidelines towards practical attempts to improve adhesion. In particular, processes which enable breaking the surface siloxane rings are expected to noticeably reinforce the interaction at non-polar interfaces.
RESUMO
A new family of over-coordinated hydrogenated silicon nanoclusters with outstanding optical and mechanical properties has recently been proposed. For one member of this family, namely the highly symmetric Si19 H12 nanocrystal, strain calculations have been presented with the goal to question its thermal stability and the underlying mechanism of ultrastability and electron-deficiency aromaticity. Here, the invalidity of these strain energy (SE) calculations is demonstrated mainly based on a fundamentally wrong usage of homodesmotic reactions, the miscounting of atomic bonds, and arithmetic errors. Since the article in question is entirely anchored on those erroneous SE values, all of its conclusions and predictions become without meaning. We provide evidence here that the nanocrystal in question suffers from such low levels of strain that its thermodynamical stability should be largely sufficient for device fabrication in a realistic plasma reactor. Most remarkably, the two "alternative," irregular isomers explicitly proposed in the aforementioned article are also electron-deficient, nontetrahedral, ultrastable, and aromatic nicely underlining the universality of the ultrastability concept for nanometric hydrogenated silicon clusters.
RESUMO
Based on ab initio molecular dynamics simulations, we show that small nanoclusters of about 1 nm size spontaneously generated in a low-temperature silane plasma do not possess tetrahedral structures, but are ultrastable. Apparently small differences in the cluster structure result in substantial modifications in their electric, magnetic, and optical properties, without the need for any dopants. Their non-tetrahedral geometries notably lead to electron deficient bonds that introduce efficient electron delocalization that strongly resembles the one of a homogeneous electron gas leading to metallic-like bonding within a semiconductor nanocrystal. As a result, pure hydrogenated silicon clusters that form by self-assembly in a plasma reactor possess optical gaps covering most of the solar spectrum from 1.0 eV to 5.2 eV depending simply on their structure and, in turn, on their degree of electron delocalization. This feature makes them ideal candidates for future bandgap engineering not only for photovoltaics, but also for many nano-electronic devices employing nothing else but silicon and hydrogen atoms.