Very low friction state of a dodecane film confined between mica surfaces

The existence of a very low friction state of a lubrication film is demonstrated in nonequilibrium molecular dynamics simulations of a six-layer dodecane film between mica walls. We argue that this low friction state is thermodynamically stable with respect to the well documented high friction film, the latter being a metastable state. These results are in striking accord with the recent report of Zhu and Granick [Phys. Rev. Lett. 93, 096101 (2004).]. The extreme low friction is the result, not of wall slip, but of layer sliding throughout the film, a mechanism similar to solid lubrication.     

Viscosity and lubricity of aqueous NaCl solution confined between mica surfaces studied by shear resonance measurement

We report the viscoelasticity of the thin film of aqueous NaCl solution confined between mica surfaces measured by shear resonance apparatus. The observed shear resonance curves at separations less than ca. 2 nm indicated that the solution exhibits the high lubrication effects under some loads. The effective viscosity (0.1-10 Pa s) obtained for the separations less than 1 nm from a mechanical model was 2-4 orders of magnitude larger than the bulk value. Our study employing a novel shear measurement provided a comprehensive picture for the dynamics of confined water thinner than a few nanometers.     

Computer simulation of the hydrophobic hydration of concave surfaces

Hemispherical objects of radius from 6.5 to 12.2 Å were constructed from tessellated icosahedra where the vertices were hydrophobic ‘atoms’. These hemispheres were hydrated in a periodic box of water and molecular dynamics simulations in the NVE ensemble were performed on this solution at 320K. Results show that water molecules in the concave region exhibit a layered structure and have their dipoles preferentially oriented approximately orthogonal to the radial direction of the hemisphere. They possess fewer hydrogen bonds and near neighbours. Changes in the translational dynamics of these water molecules are not detected, but there is an acceleration of the re-orientational dynamics. This work thus represents the first systematic study of the hydration of concave hydrophobic surfaces.     

Influence of (phospho)lipases on properties of mica supported phospholipid layers

The effect of enzymes: lipase from Candida cylindracea (L_Cc), phospholipase A₂ from hog pancreas (PLA₂) and phospholipase C from Bacillus cereus (PLC) to modulate wetting properties of solid supported phospholipid bilayers was studied via advancing and receding contact angle measurements of water, formamide and diiodomethane, and calculation of the surface free energy and its components from van Oss et al. (LWAB) and contact angle hysteresis (CAH) approaches. Simultaneously, topography of the studied layers was determined by Atomic Force Microscopy (AFM). The investigated lipid bilayers were transferred on mica plates from subphase of pure water by means of Langmuir-Blodgett and Langmuir-Schaefer techniques. The investigated phospolipid layers were: saturated DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine), unsaturated DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine), and their mixture DPPC/DOPC. The obtained results revealed that the lipid membrane degradation by the enzymes caused increase in its surface free energy due to the amphiphilic hydrolysis products, which may accumulate in the lipid bilayer. In result activity of the enzymes may increase and then break down the bilayer structure takes place. It is likely that after dissolution of the hydrolysis reaction products in the bulk phase, patches of bare mica surface are accessible, which contribute to the apparent surface free energy changes. Comparison of AFM images and the free energy changes of the layers gives better insight into changes of their properties. The observed gradual increase in the layer surface free energy allows controlling of the hydrolysis process to obtain the surfaces of defined properties.     

Response of thin polymer layers confined between solid surfaces and submitted to normal oscillatory deformations

Summary The viscoelastic properties of liquids close to solid surfaces differ from the bulk. Nanorheology has been performed by using a surface force apparatus adapted to operate as a rheometer in a sphere-plane geometry. Axial oscillatory measurements have been carried out with high polymer solutions filling the gap. The deformations were kept sufficiently small not to perturb the film structure and were applied in a large range of frequency (10⁻³ to 10² s⁻¹). It is shown that the complex modulus characterizing the confined medium can be split into two components: a shear modulus (it accounts for the viscous dissipation due to the flow of solvent molecules through the mesh created by the long polymer chains which connect the two solid surfaces) and a compression modulus which is related to the normal stress response of the chains confined between the solid surfaces. The hydrodynamic screening lengthξ _h and the correlation length ξ deduced from the two moduli are compared and are found to scale in the same way as a function of the distance between the two surfaces. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Low frequency four-wave mixing spectroscopy of biopolymer hydration layers in aqueous solutions

Four-wave mixing (FWM) spectroscopy has been applied to detection of H₂O₂ molecules rotational resonances in both DNA and denatured DNA aqueous solutions in the range ±100 cm⁻¹. A considerable growth of rotational lines intensity of H₂O and H₂O₂ has been observed in comparison with distilled water. This fact was interpreted as an exhibition of specific property of a hydration layer formation at DNA/water and denatured DNA/water interfaces. The fitting of four-wave mixing spectra shows the increasing of the H₂O₂ rotational line’s amplitude by a factor of ∼3 in DNA solutions due to denaturizing. The shifting of FWM Brillouin resonances in opposite way in protein solution and SWNT (single wall carbon nanotube) suspension to comparison with water was observed and discussed.     

The electronic structure of alkali-metal layers on semiconductor surfaces

Alkali-metal layers on semiconductor surfaces are model systems for metal-semiconductor contacts, Schottky barriers, and metallization processes. The strong decrease of the work function as a function of alkali-metal coverage is also technically made use of. Recently, however, interest in these systems is growing owing to ongoing controversial discussions about questions like: Is the adsorbate system at monolayer coverage metallic or semiconducting, and does the metallization take place in the alkali overlayer or in the top layer of the semiconductor? Is the bonding ionic or covalent? What ist the absolute coverage at saturation? What are the adsorption sites? Do all alkali metals behave similar on the same semiconductor surface? We try to answer some of the questions for Li, Na, K and Cs on Si(111)(2×1), K and Cs on Si(111)(7×7) and on GaAs(110), and Na and K on Si(100)(2×1) employing the techniques of direct and inverse photoemission.     

Quantum properties of strong accumulation layers on ZnO surfaces

Extremely strong accumulation layers with surface electron densities ΔN approaching 10¹⁴ cm^?1 have been achieved on ZnO surfaces in contact with an electrolyte. Quantization effects, which are very pronounced in such narrow (?10 Å) layers, are studied by measurements of ΔN versus surface barrier height V_s. Comparison of the results with self-consistent calculations shows very good agreement up to ΔN = 2 × 10¹³ cm^?2. Deviations observed at higher ΔN are probably associated with the huge electric fields (～10⁷ V/cm) experienced by the surface electrons.     

XPS study of residual oxide layers on p-GaAs surfaces

The total thickness and composition of a residual oxide layer after chemical etching of p-GaAs:Zn + In has been studied by X-ray photoelectron spectroscopy (XPS). The variation of the Ga to As oxides ratio along the depth has been determined. A concentration correlation of doping isovalent impurity and the dislocation density with the composition of residual oxides is looked for. The total thickness of the residual oxide layer on p- and semi-insulating GaAs is about 5–6 Å. It is found that the Ga₂O₃ quantity in the oxide bulk is greater than the same value of As₂O₃ in highly In-doped samples. In-doping in concentrations over 1.5 × 10¹⁹ cm⁻³ increases the Ga₂O₃ content and the density of the residual oxide. This influence is determined by reducing the dislocation density and changing the point defect environment. The presence of As-rich precipitates on the dislocations and in the matrix decreases the sputtering time and changes the composition of the residual oxide. The correlation between the type of high temperature dislocations revealed by Abrahams-Buiocchi (AB) etching and the oxide layer composition is shown. The results obtained could be used in the first stages of epitaxial growth, metallization and other technological processes of semiconductor device and ICs fabrication.     

Computer simulation of the hydrophobic hydration of convex surfaces of large radius

Three hemispherical objects of radius 6.5, 9.3 and 12.2 Å were constructed from tessellated icosahedra where the vertices were hydrophobic ‘atoms’. These objects were hydrated and molecular dynamics simulations were performed on the solvated system. Analysis of the trajectories demonstrated that water molecules around convex solutes possess different configurations and exhibit different translational and rotational dynamics from water molecules in the bulk region. No qualitative difference was observed in the alteration of these structural and dynamic properties as the hemispheres became larger.     

Nonlinear viscoelastic dynamics of nanoconfined wetting liquids

The viscoelastic dynamics of nanoconfined wetting liquids is studied by means of atomic force microscopy. We observe a nonlinear viscoelastic behavior remarkably similar to that widely observed in metastable complex fluids. We show that the origin of the measured nonlinear viscoelasticity in nanoconfined water and silicon oil is a strain rate dependent relaxation time and slow dynamics. By measuring the viscoelastic modulus at different frequencies and strains, we find that the intrinsic relaxation time of nanoconfined water is in the range 0.1-0.0001 s, orders of magnitude longer than that of bulk water, and comparable to the dielectric relaxation time measured in supercooled water at 170-210 K.     

Percolation transition of hydration water: from planar hydrophilic surfaces to proteins

The formation of a spanning hydrogen-bonded network of hydration water is found to occur via a 2D percolation transition in various systems: smooth hydrophilic surfaces, the surface of a single protein molecule, protein powder, and diluted peptide solution. The average number of water-water hydrogen bonds at the percolation threshold varies from 2.0 to 2.3, depending on temperature, system size, and surface properties. Calculation of nH allows an easy estimation of the percolation threshold of hydration water in various systems, including biomolecules.     

Electrofriction and dynamic stern layers at planar charged surfaces

Using dynamic simulations, the electrophoretic mobility of counterions at a substrate with fixed or mobile surface charges under the action of a lateral electric field is studied. The lateral charge inhomogeneity and corrugation of the substrate is taken into account. Because of the pronounced electrofriction between counterions and surface ions, a large fraction of counterions is practically immobilized for highly charged substrates. This explains the experimentally observed saturation of the electrophoretic mobility of charged particles in the limit of high surface charge density.     

Inter-subband optical absorption in space-charge layers on semiconductor surfaces

The resonance energy and the lineshape of absorption for inter-subband transitions are calculated in an approximation scheme based on the density-functional formulation inn-channel accumulation and inversion layers on the (100) surface of Si. The exciton-like effect almost cancels the resonance screening (depolarization) effect found recently by Allen et al., and the resonance energy is rather close to subband energy-separation. The agreement between the theory and experiments is satisfactory both for the resonance energy and for the lineshape. The theory is applied to accumulation layers on the (111) and (110) surface, and explains characteristic change of the lineshape observed experimentally.A research fellow of the Allexander von Humboldt Foundation     

Controlled assembly of DNA nanostructures on silanized silicon and mica surfaces for future molecular devices

In order to establish key technology for future molecular devices, we have explored the assembly behaviour of λ-deoxyribonucleic acid (DNA) molecules adsorbed on silanized mica and silanized oxide silicon surfaces by using atomic force microscopy (AFM). AFM experiments show that λ-DNA molecules can be hardly adsorbed on untreated mica and oxidized silicon surfaces, but can be strongly adsorbed onto aminosilanized mica and oxidized silicon surfaces. Importantly, DNA molecules can be assembled into linear DNA alignment, and can also self-assemble into various network structures on the silanized surfaces. Our experimental observations have demonstrated the feasibility of assembling DNA-based nanostructures by varying surface chemistry of substrates, and offer useful clues in constructing DNA-based nanodevices for nanoelectronics and biomolecular computation as well as quantum computation.     

Elementary excitations at doped polar semiconductor surfaces with carrier-depletion layers

We make a comparative analysis of coupled carrier plasmon–polar phonon modes at doped polar semiconductor surfaces in the absence and the presence of a carrier-depletion layer. Our analysis shows how the effect of the depletion layer appears in the spatial structure of each excitation mode visualized in a contour map of the induced charge-density, the coupling character elucidated by the phase relation and the amplitude ratio of the carrier component and the polar-phonon component in the induced charge-density distribution, and the energy-loss intensity due to the surface excitation.     

“Magnetic dead layers” on chemisorption at ferromagnetic surfaces

Chemisorption induced “magnetic dead layers” have been determined from Ferromagnetic Resonance (FMR) absorption of Ni and Fe thin films. Within experimental error “magnetic dead” or “live layers” can be excluded at free Ni surfaces. Approximately one hydrogen atom diminishes the contribution of one nickel atom to the thin film ferromagnetism in the β₂-state. Twice the effect is found for CO adsorption on nickel. Possible models to explain chemisorption effects in surface magnetism are discussed briefly.     

On the reflectivity of surfaces with thin transition or contaminated layers

The spatial profile of the dielectric pemittivity of any bulk sample is always to some extent smoothed or modified by various factors including interaction with atmospheric and environmental gases. This fact hinders optical measurements and should be taken into account in the interpretation of experimental data. The modification of the Fresnel reflectivity formula for the case where the spatial permittivity distribution deviates from an abrupt stepwise profile is considered. A correction in terms of the integral characteristics of this deviation is obtained and it is valid ifa sin δ<λ, wherea is the depth of the contaminated layer, and λ and δ are the wavelength and grazing angle of the incident beam, respectively.     

Passivation of Si (111) surfaces by ordered metal layers

The properties of √3 × √3 ordered gold and silver monolayers on a Si(111) substrate have been investigated by Auger, low energy electron diffraction and photo-emission analysis. It has been found that oxygen adsorption on these surfaces is considerably weaker than on clean Si surfaces. This new result clearly emphasizes the correlation between the oxidation properties of Si atoms and their local environment. A comparison is made with previous results concerning Au-Si amorphous metallic alloys where gold atoms act as a catalyst for the oxidation.