Ultrathin oxides: bulk-oxide-like model surfaces or unique films?

To better understand the electronic and chemical properties of wide-gap oxide surfaces at the atomic scale, experimental work has focused on epitaxial films on metal substrates. Recent findings show that these films are considerably thinner than previously thought. This raises doubts about the transferability of the results to surface properties of thicker films and bulk crystals. By means of density-functional theory and approximate GW corrections for the electronic spectra we demonstrate for three characteristic wide-gap oxides (silica, alumina, and hafnia) the influence of the substrate and highlight critical differences between the ultrathin films and surfaces of bulk materials. Our results imply that monolayer-thin oxide films have rather unique properties.Comment: 5 pages, 3 figures, accepted by PR

Enzyme-Based Nitric Oxide Releasing Thin Films and Scaffolds

Nitric oxide synthase enzyme (NOS) embedded in thin films and scaffolds, when exposed to a solution of its substrate arginine, a source of reducing equivalents, and other required ingredients of the NOS reaction, can release fluxes of nitric oxide (NO). The latter is a molecule known to counteract platelet aggregation, and thus can prevent the thrombosis cascade on the surfaces of implantable medical devices. Therefore NO antithrombogenic regimens such as active coatings and embedded scaffolds have the potential to increase the lifespan of implantable biomaterials. Layer-by-layer electrostatic adsorption allows for assembly of multi-component protein/polyelectrolytes nanostructured films. Electrospun fiber matrices may embed proteins in aqueous pockets that allow the formation of functional scaffolds. Such functional coatings and polymer scaffolds have potential applications as antithrombotic surfaces. In this project, inducible nitric oxide synthase is proposed as a functional component in active thin films and electrospun scaffolds for nitric oxide release under physiologic conditions. Atomic force microscopic (AFM) imaging confirms the presence of enzyme in adsorbed thin films. Fourier transform infrared (FTIR) spectroscopic analysis was used to characterize structure-function relationships of NOS-containing thin films. Voltammetry was used to characterize the active catalyst concentration on adsorbed surfaces and activity of NOS-containing thin films. Further, analysis of cyclic voltammetric data enabled the study of Michaelis-Menten kinetics of NOS-containing thin films. Other spectrophotometric and spectrofluorometric assays were used to monitor nitric oxide release for the NOS-based thin films and scaffolds. Three polymers were characterized for their ability to embed iNOSoxy and show enzyme activity in electrospun scaffolds. For the LbL method, during protein adsorption, an adjusted pH of 8.6 or the use of a branched matrix immobilizes more enzyme units in thin layers compared to pH 7.0 or a l

Density-functional study of Cu atoms, monolayers, and coadsorbates on polar ZnO surfaces

The structure and electronic properties of single Cu atoms, copper monolayers and thin copper films on the polar oxygen and zinc terminated surfaces of ZnO are studied using periodic density-functional calculations. We find that the binding energy of Cu atoms sensitively depends on how charge neutrality of the polar surfaces is achieved. Bonding is very strong if the surfaces are stabilized by an electronic mechanism which leads to partially filled surface bands. As soon as the surface bands are filled (either by partial Cu coverage, by coadsorbates, or by the formation of defects), the binding energy decreases significantly. In this case, values very similar to those found for nonpolar surfaces and for copper on finite ZnO clusters are obtained. Possible implications of these observations concerning the growth mode of copper on polar ZnO surfaces and their importance in catalysis are discussed.Comment: 6 pages with 2 postscript figures embedded. Uses REVTEX and epsf macro

Influence of steps on the tilting and adsorption dynamics of ordered Pn films on vicinal Ag(111) surfaces

Here we present a structural study of pentacene (Pn) thin films on vicinal Ag(111) surfaces by He atom diffraction measurements and density functional theory (DFT) calculations supplemented with van der Waals (vdW) interactions. Our He atom diffraction results suggest initial adsorption at the step edges evidenced by initial slow specular reflection intensity decay rate as a function of Pn deposition time. In parallel with the experimental findings, our DFT+vdW calculations predict the step edges as the most stable adsorption site on the surface. An isolated molecule adsorbs as tilted on the step edge with a binding energy of 1.4 eV. In addition, a complete monolayer (ML) with pentacenes flat on the terraces and tilted only at the step edges is found to be more stable than one with all lying flat or tilted molecules, which in turn influences multilayers. Hence our results suggest that step edges can trap Pn molecules and act as nucleation sites for the growth of ordered thin films with a crystal structure similar to that of bulk Pn.Comment: 4 pages, 4 figures, 1 tabl

Functional Oxide Thin Films and Nanostructures: Growth, Properties, and Applications

This Special Issue of Coatings is entitled “Functional Oxide Thin Films and Nanostructures: Growth, Properties, and Applications”. Recent materials nanotechnologies have created possibilities regarding the fabrication of oxide thin films at the nanometric level and other nanocomposites’ fabrication. In parallel, recent measurement technologies can characterize their unique properties arising from the limited regions of surfaces and interfaces. This Special Issue provides an opportunity to share surface-related science and engineering topics on oxide thin films and nanocomposites in an interactive and interdisciplinary manner. The ultimate goal is to elucidate the commonalities and differences between multilayer interfaces and nanocomposite grain boundaries. This Special Issue is as an effort to bridge the gap between materials science and the applications of oxide thin films and nanostructures. The topics covered in this Special Issue range from nanoparticles to thin films, heterostructures, and homojunctions and are related to various aspects of oxide materials’ preparation, characterization, and applications

How Voltage Drops are Manifested by Lithium Ion Configurations at Interfaces and in Thin Films on Battery Electrodes

Battery electrode surfaces are generally coated with electronically insulating solid films of thickness 1-50 nm. Both electrons and Li+ can move at the electrode-surface film interface in response to the voltage, which adds complexity to the "electric double layer" (EDL). We apply Density Functional Theory (DFT) to investigate how the applied voltage is manifested as changes in the EDL at atomic lengthscales, including charge separation and interfacial dipole moments. Illustrating examples include Li(3)PO(4), Li(2)CO(3), and Li(x)Mn(2)O(4) thin-films on Au(111) surfaces under ultrahigh vacuum conditions. Adsorbed organic solvent molecules can strongly reduce voltages predicted in vacuum. We propose that manipulating surface dipoles, seldom discussed in battery studies, may be a viable strategy to improve electrode passivation. We also distinguish the computed potential governing electrons, which is the actual or instantaneous voltage, and the "lithium cohesive energy" based voltage governing Li content widely reported in DFT calculations, which is a slower-responding self-consistency criterion at interfaces. This distinction is critical for a comprehensive description of electrochemical activities on electrode surfaces, including Li+ insertion dynamics, parasitic electrolyte decomposition, and electrodeposition at overpotentials.Comment: 35 pages. 10 figure

Multilayered Thin Films from Boronic Acid-Functional Poly(amido amine)s

Purpose To investigate the properties of phenylboronic acid-functional poly(amido amine) polymers (BA-PAA) in forming multilayered thin films with poly(vinyl alcohol) (PVA) and chondroitin sulfate (ChS), and to evaluate their compatibility with COS-7 cells. Methods Copolymers of phenylboronic acid-functional poly(amido amine)s, differing in the content of primary amine (DAB-BA-PAA) or alcohol (ABOL-BA-PAA) side groups, were synthesized and applied in the formation of multilayers with PVA and ChS. Biocompatibility of the resulting films was evaluated through cell culture experiments with COS-7 cells grown on the films. Results PVA-based multilayers were thin, reaching ~100 nm at 10 bilayers, whereas ChS-based multilayers were thick, reaching ~600 nm at the same number of bilayers. All of the multilayers are stable under physiological conditions in vitro and are responsive to reducing agents, owing to the presence of disulfide bonds in the polymers. PVA-based films were demonstrated to be responsive to glucose at physiological pH at the investigated glucose concentrations (10–100 mM). The multilayered films displayed biocompatibility in cell culture experiments, promoting attachment and proliferation of COS-7 cells. Conclusions Responsive thin films based on boronic acid functional poly(amido amine)s are promising biocompatible materials for biomedical applications, such as drug releasing surfaces on stents or implants