Energetics Of Adhesion Research Articles

Nucleation of membrane adhesions

Recent experimental and theoretical studies of biomimetic membrane adhesions [Bruinsma, Phys. Rev. E 61, 4253 (2000); Boulbitch, Biophys. J. 81, 2743 (2001)] suggested that adhesion mediated by receptor interactions is due to the interplay between membrane undulations and a double-well adhesion potential, and should be a first-order transition. We study the nucleation of membrane adhesion by finding the minimum-energy path on the free energy surface constructed from the bending free energy of the membrane and the double-well adhesion potential. We find a nucleation free energy barrier around 20k(B)T for adhesion of flexible membranes, which corresponds to fast nucleation kinetics with a time scale of the order of seconds. For cell membranes with a larger bending rigidity due to the actin network, the nucleation barrier is higher and may require active processes such as the reorganization of the cortex network to overcome this barrier. Our scaling analysis suggests that the geometry of the membrane shapes of the adhesion contact is controlled by the adhesion length that is determined by the membrane rigidity, the barrier height, and the length scale of the double-well potential, while the energetics of adhesion is determined by the depths of the adhesion potential. These results are verified by numerical calculations.

Atomic and electronic structure ofCo/SrTiO3/Comagnetic tunnel junctions

First-principles density-functional calculations of the atomic and electronic structure of ${\mathrm{C}\mathrm{o}/\mathrm{S}\mathrm{r}\mathrm{T}\mathrm{i}\mathrm{O}}_{3}/\mathrm{Co}$ (001) magnetic tunnel junctions (MTJ's) are performed. Different interface terminations are considered and the most stable structure with the ${\mathrm{TiO}}_{2}$ termination is identified based on energetics of adhesion. The calculated electronic structure of the ${\mathrm{TiO}}_{2}$-terminated MTJ shows an exchange coupling between the interface Co and Ti atoms mediated by oxygen. This coupling induces a magnetic moment of $0.25{\ensuremath{\mu}}_{B}$ on the interface Ti atom, which is aligned antiparallel to the magnetic moment of the Co layer. We argue that this might cause an inversion of the spin polarization of tunneling across the ${\mathrm{SrTiO}}_{3}$ barrier which was found in recent experiments.

First-principles Modeling of Co/SrTiO3/Co Magnetic Tunnel Junctions

ABSTRACTThe atomic and electronic structure of Co/SrTiO3/Co (001) magnetic tunnel junctions (MTJs) was investigated by first-principles density-functional theory. We considered different interface terminations and the most stable structure with the TiO2 termination has been identified based on energetics of adhesion. The electronic structure of the TiO2-erminated MTJ shows an exchange coupling between the interface Co and Ti atoms mediated by oxygen. This coupling induces the magnetic moment of 0.25 μB on the interface Ti atom, which is aligned antiparallel to the magnetic moment of the Co layer. We argue that this might cause the inversion of the spin polarization of tunneling across the SrTiO3 barrier that was found in recent experiments.

Microscopic structure and bonding at the Pd/SrTiO3 (001) Interface an ab-initio local-density-functional study

The microscopic structure and energetics of a SrTiO3 (001) surface covered with thin layers of Pd have been investigated by means of ab-initio electronic-structure calculations. A mixed-basis pseudopotential technique based on the local density functional theory was employed. Supercells containing SrTiO3 substrate slabs,with either SrO or TiO2 surface terminations, and Pd films of varying thicknesses were used to model the free (001) surfaces and the (001) heterophase interfaces. Based on the calculated energetics of adhesion for the different interfaces, the microscopic energetics of wetting and layer growth has been analysed. The TiO2 terminated substrate is energetically favourable for the adhesion of Pd films, with the Pdatoms bonded on top of the O atoms. The film adhesion is strongest for one (001) layer of fcc Pd and becomes weaker with increasing film thickness.

Photoelectron spectroscopy studies of growth, alloying, and segregation for transition-metal films on tungsten (211)

High resolution soft x-ray photoelectron spectroscopy using synchrotron radiation and Auger-electron spectroscopy are used to study late-transition-metal films (Pt, Pd, Ir, Rh, Au,) on W(211). It is found that the films grow in a layer mode at 300 K. As a function of the film thickness, different ${4f}_{7/2}$ photoemission peaks are observed, corresponding to a single monolayer, to an interface layer, to bulk atoms and to surface atoms. Single physical monolayers of these late-transition metals on tungsten are stable against thermal rearrangement. In contrast, when multilayer films of Pd, Pt, Ir, Rh are annealed above 700--1000 K, tungsten atoms diffuse into the overlayer to form an alloy film. Gold constitutes a different case; upon annealing it does not alloy with tungsten but the metal in excess of one monolayer forms clusters. The evolution of the bimetallic systems as a function of coverage and annealing temperature is interpreted by analysis of intensities and shapes of the ${4f}_{7/2}$ features. Born-Haber cycles and the equivalent core approximation are used to extract thermochemical data concerning energetics of adhesion, segregation, and alloying in these early-late transition-metal systems.

Metal / ceramic adhesion: a first principles study of MgO/Al and MgO/Ag

The energetics of adhesion and the electronic structure are determined for MgO/Ag(100) and MgO/Al(100) interfaces via local density-functional calculations. At the interface, both Ag and Al atoms energetically favour the site directly above the O atom, which is consistent with recent high-resolution transmission electron microscopy experiments on MgO/Ag(100). For that site, electron density distributions in the metal surface regions are reminiscent of a charge array imaging only the surface Mg and O ions. This implies a substantial ionic component to the adhesive bond. The screening charge distributions appear smoother in the A1 case than in the Ag case, suggesting a direct role of the Ag d electrons in the screening. Despite the ionic contribution to the bonding, the adhesive energy versus interfacial separation curves obey the same, universal form originally discovered for bimetallic adhesion. This could be explained by a significant metallic and/or covalent contribution to the metal/ceramic bonds. Further evidence for a metallic/covalent contribution is found in the density of states plots, including a contribution from the (filled) Ag d-band. These plots also suggest that the surface MgO layer is metallized by wave function overlap from the contiguous metal layers. The MgO/Ag(100) interfaces have larger ideal adhesive energies and ideal peak interfacial stresses than the MgO/Al(100) interfaces have. This is apparently due to a significant contribution by Ag d electrons to the adhesion, consistent with their aforementioned role in screening, the densities of states, and with the difference between MgO/Ag and MgO/Al ideal adhesive energies being comparable to the difference in Ag and Al surface energies. Finally, using the dislocation network model of Trampert et al. for MgO/Ag(100), our computed work of adhesion and contact angle are shown to be in good agreement with experimental values.

Quantitative studies of metal-metal adhesion and interface segregation energies using photoelectron spectroscopy.

Layer-dependent core-level shifts have been studied for Yb epitaxially grown on Mo(110). Clearly resolved Yb 4f positions are simultaneously distinguished for the interface, the free Yb surface, and the intermediate layers. The measured shifts provide new and detailed quantitative information on the energetics of adhesion and interface segregation; this type of data is otherwise most difficult to obtain. The determined energies are in good agreement with results from semiempirical calculations.