Showing papers in "Faraday Discussions in 1999"

Introductory Lecture Energy landscapes of biomolecular adhesion and receptor anchoring at interfaces explored with dynamic force spectroscopy

Abstract: Beyond covalent connections within protein and lipid molecules, weak noncovalent interactions between large molecules govern properties of cellular structure and interfacial adhesion in biology. These bonds and structures have limited lifetimes and so will fail under any level of force if pulled on for the right length of time. As such, the strength of interaction is the level of force most likely to disrupt a bond on a particular time scale. For instance, strength is zero on time scales longer than the natural lifetime for spontaneous dissociation. On the other hand, if driven to unbind or change structure on time scales shorter than needed for diffusive relaxation, strength will reach an adiabatic limit set by the maximum gradient in a potential of mean force. Over the enormous span of time scales between spontaneous dissociation and adiabatic detachment, theory predicts that bond breakage under steadily rising force occurs most frequently at a force determined by the rate of loading. Moreover, the continuous plot (spectrum) of strength expressed on a scale of loge(loading rate) provides a map of the prominent barriers traversed in the energy landscape along the force-driven pathway and reveals the differences in energy between barriers. Illustrated with results from recent laboratory measurements, dynamic strength spectra provide a new view into the inner complexity of receptor–ligand interactions and receptor lipid anchoring.

The chemistry of methanol on the TiO2(110) surface: the influence of vacancies and coadsorbed species

Abstract: The chemistry of methanol was explored on the vacuum annealed TiO2(110) surface, with and without the presence of coadsorbed water and oxygen, using temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), static secondary ion mass spectrometry (SSIMS) and low energy electron diffraction (LEED). The vacuum annealed TiO2(110) surface possessed about 8% oxygen vacancy sites, as determined with H2O TPD. Although evidence is presented for CH3OH dissociation to methoxy groups on the vacuum annealed TiO2(110) surface using SSIMS and HREELS, particularly at vacancy sites, the majority of the adlayer was molecularly adsorbed, evolving in TPD at 295 K. Although no evidence of irreversible decomposition was found in the TPD, dissociative CH3OH adsorption at 135 K on the vacuum annealed TiO2(110) surface led to recombinative desorption states at 350 and 480 K corresponding to methoxys adsorbed at non-vacancy and vacancy sites, respectively. Coadsorbed water had little or no influence on the chemistry of CH3OH on the vacuum annealed TiO2(110) surface, however new channels of chemistry were observed when CH3OH was adsorbed on the surface after O2 adsorption at various temperatures. In particular, O2 exposure at 300 K resulted in O adatoms (via dissociation at vacancies) that led to increased levels of CH3O–H bond cleavage. The higher surface coverage of methoxy then resulted in a disproportionation reaction to form CH3OH and H2CO above 600 K. In contrast, low temperature exposure of the vacuum annealed TiO2(110) surface to O2 resulted in low temperature state of O2 (presumably an O2- species) that oxidized CH3OH to H2CO by C–H bond cleavage. These results provide incentive to consider alternative thermal and photochemical oxidation mechanisms that involve the interaction of organics and oxygen at surface defect sites.

Influence of wetting properties on hydrodynamic boundary conditions at a fluid/solid interface

Abstract: It is well known that, at a macroscopic level, the boundary condition for a viscous fluid at a solid wall is one of “no-slip’'. The liquid velocity field vanishes at a fixed solid boundary. In this paper, we consider the special case of a liquid that partially wets the solid, i.e., a drop of liquid in equilibrium with its vapor on the solid substrate has a finite contact angle. Using extensive non-equilibrium molecular dynamics (NEMD) simulations, we show that when the contact angle is large enough, the boundary condition can drastically differ (at a microscopic level) from a “no-slip’' condition. Slipping lengths exceeding 30 molecular diameters are obtained for a contact angle of 140°, characteristic of mercury on glass. On the basis of a Kubo expression for δ, we derive an expression for the slipping length in terms of equilibrium quantities of the system. The predicted behaviour is in very good agreement with the numerical results for the slipping length obtained in the NEMD simulations. The existence of large slipping length may have important implications for the transport properties in nanoporous media under such “nonwetting’' conditions.

Theory of PbTiO3, BaTiO3, and SrTiO3 surfaces

Abstract: First-principles total-energy calculations are carried out for (001) surfaces of the cubic perovskite ATiO3 compounds PbTiO3, BaTiO3, and SrTiO3. Both AO-terminated and TiO2-terminated surfaces are considered, and fully-relaxed atomic configurations are determined. In general, BaTiO3 and SrTiO3 are found to have a rather similar behavior, while PbTiO3 is different in many respects because of the partially covalent character of the Pb–O bonds. PbTiO3 and BaTiO3 are ferroelectrics, and the influence of the surface upon the ferroelectric distortions is studied for a tetragonal ferroelectric distortion parallel to the surface. The surface relaxation energies are found to be substantial, i.e., many times larger than the bulk ferroelectric well depth. Nevertheless, the influence of the surface upon the ferroelectric order parameter is modest, and is qualitatively as well as quantitatively different for the two materials. Surface energies and electronic properties are also computed. It is found that for BaTiO3 and SrTiO3 surfaces, both AO-terminated and TiO2-terminated surfaces can be thermodynamically stable, whereas for PbTiO3 only the PbO surface termination is stable.

Introductory Lecture: Oxide surfaces

Abstract: Oxides have gained increasing interest in surface science during recent years because of their important role in applications. In the first part of the lecture we review the current knowledge on morphology and structure of surfaces of bulk single crystals as well as oxide films. The interaction of oxide surfaces with molecules is thoroughly discussed and the role of defects on adsorption is highlighted. In a further part, structure and morphology of deposited aggregates on clean and modified substrates are discussed. Such systems may serve as models for heterogeneous catalysts. Electronic structure as a function of the size of the deposited particle is studied, as well as size dependent adsorption properties and reactivities.

Characterization of the physical properties of model biomembranes at the nanometer scale with the atomic force microscope.

Abstract: Interaction forces and topography of mixed phospholipid–glycolipid bilayers were investigated by atomic force microscopy (AFM) in aqueous conditions with probes functionalized with self-assembled monolayers terminating in hydroxy groups. Short-range repulsive forces were measured between the hydroxy-terminated probe and the surface of the two-dimensional (2-D) solid-like domains of distearoyl-phosphatidylethanolamine (DSPE) and digalactosyldiglyceride (DGDG). The form and range of the short-range repulsive force indicated that repulsive hydration/steric forces dominate the interaction at separation distances of 0.3–1.0 nm after which the probe makes mechanical contact with the bilayers. At loads <5 nN the bilayer was elastically deformed by the probe, while at higher loads plastic deformation of the bilayer was observed. Surprisingly, a short-range repulsive force was not observed at the surface of the 2-D liquid-like dioleoylphosphatidylethanolamine (DOPE) film, despite the identical head groups of DOPE and DSPE. This provides direct evidence for the influence of the structure and mechanical properties of lipid bilayers on their interaction forces, an effect which may be of major importance in the control of biological processes such as cell adhesion and membrane fusion. The step height measured between lipid domains in the AFM topographic images was larger than could be accounted for by the thickness and mechanical properties of the molecules. A direct correlation was observed between the repulsive force range over the lipid domains and the topographic contrast, which provides direct insight into the fundamental mechanisms of AFM imaging in aqueous solutions. This study demonstrates that chemically modified AFM probes can be used in combination with patterned lipid bilayers as a novel and powerful approach to characterize the nanometer scale chemical and physical properties of heterogeneous biosurfaces such as cell membranes.

Delayed sedimentation of transient gels in colloid–polymer mixtures: dark-field observation, rheology and dynamic light scattering studies

Abstract: The addition of enough non-adsorbing polymer to a hard-sphere suspension causes the particles to aggregate to form a space-filling gel. The integrity of the gel persists for a finite period of time, and then the space-filling structure collapses suddenly to form a denser sediment. This phenomenon of ‘delayed sedimentation’ is ubiquitous in many weakly-flocculated suspensions. In this work, we observe the processes occurring in the bulk of a colloid–polymer gel using dark-field imaging, and probe the arrangement and dynamics of the particles in the system using two-colour dynamic light scattering. The effect of shear is also studied. A number of physical mechanisms relevant to a comprehensive explanation of delayed sedimentation are proposed and discussed.

Ab initio calculations on the Al2O3(0001) surface

Abstract: We calculate using a density functional pseudopotential method the atomic and electronic structure of the (0001) surface of α-alumina (Al2O3). The material is studied in the form of a slab with periodic boundary conditions, containing up to eight layers of the stoichiometric Al2O3 units. Five different terminations of the surface are calculated, representing different surface excesses of oxygen, and their free energies are estimated as a function of oxygen partial pressure. Internal relaxations of the atomic positions are obtained. The aluminium terminated surface, which is stoichiometric, has the lowest surface energy for a wide range of oxygen pressures.

Array formation in nano-colloids: Theory and experiment in 2D

Abstract: We discuss theoretical and experimental aspects of the array formation of nano-colloids in two-dimensional (2D) situations. In particular, we treat metal nanocrystals which have been passivated by surfactant monolayers and then deposited on the free surface of water. Their self-organization properties follow from the fact that the relevant interparticle attractions do not greatly exceed thermal energies, thereby allowing for equilibrium structures to form and to evolve reversibly as a function of temperature and concentration. In the case of large enough metal cores, spatially-modulated phases arise because of long-range repulsions between particles; for smaller cores, the interactions are highly directional, giving rise to linear chain structures at low concentrations and to extended networks at higher densities.

Sensing isothermal changes in the lateral pressure in model membranes using di-pyrenyl phosphatidylcholine

Abstract: In this work we present data from a homologous series of di-pyrenyl phosphatidylcholine (dipyPC) probes which can sense lateral pressure variations in the chain region of the amphiphilic membrane (lateral pressures are tangential to the interface). The dipyPC has pyrene moieties attached to the ends of equal length acyl chains on a phosphatidylcholine molecule. Ultraviolet stimulation produces both monomer and excimer fluorescence from pyrene. At low dilutions of dipyPC in model membranes the excimer signal is entirely intra-molecular and since it depends on the frequency with which the pyrene moieties are brought into close proximity, the relative intensity of the excimer to monomer signal, η, is a measure of the pressure. We synthesised or purchased dipyPC probes with the pyrene moieties attached to acyl chains having 4, 6, 8 and 10 carbon atoms and then measured η in fully hydrated bilayers composed of dioleoylphosphatidylcholine and dioleoylphosphatidylethanolamine (DOPC and DOPE respectively). Although the resolution of our measurements of lateral pressure as a function of distance into the monolayer was limited, we did observe a dip in the excimer signal in the region of the DOPC/DOPE cis double bond. As we isothermally increased the DOPE composition, and hence the desire for interfacial curvature, we observed, as expected, that the net excimer signal increased. However this net increase was apparently brought about by a transfer of pressure from the region around the glycerol backbone to the region near the chain ends, with the lateral pressure dropping above the cis double bond but increasing at a greater rate beyond the double bond.

Femtosecond time-resolved photoelectron imaging

Abstract: Femtosecond time-resolved photoelectron imaging (FS-PEI) is presented for the first time. This novel method is tested with two-color photoionization of NO via the A 3sσ state and applied to ultrafast dephasing in the intermediate case molecule pyrazine. The results illustrate the high-performance of FS-PEI in probing short-time dynamics in isolated molecules and clusters.

A scattering study of nucleation phenomena in polymer crystallisation

Abstract: The mechanism of primary nucleation in polymer crystallisation has been investigated experimentally and theoretically. Two types of experiments have been performed on polypropylene, polyethylene, and poly(ethylene terpthalate). Crystallisations with long induction times, studied by small and wide angle X-ray scattering (SAXS and WAXS), reveal the onset of large scale ordering prior to crystal growth. Rapid crystallisations studied by melt extrusion indicate the development of well resolved oriented SAXS patterns associated with large scale order before the development of crystalline peaks in the WAXS region. The results suggest pre-nucleation density fluctuations play an integral role in polymer crystallisation. A theoretical model has been developed which qualitatively describes the experimental results.

Dynamics of a drop at a liquid/solid interface in simple shear fields: A mesoscopic simulation study

Abstract: The dynamics of a surface-confined drop in a simple shear field has been studied, pursuing the dissipative particle dynamics (DPD) simulation approach. The shear field induces contact angle hysteresis in the drop, the degree of hysteresis increasing with the shear rate. At shear rates exceeding a critical value, the drop acquires the tendency to lift off the boundary, leading to its removal. In the equilibrium contact angle range, ϑe>120°, the drop preserves its integrity as it escapes from the boundary, whereas at lower contact angles the drop assumes a distinctly elongated shape prior to its removal, which develops “necks’' at subsequent times. The drop breaks up as the necks are ruptured upon thinning, with some fragments escaping into the bulk phase and some remaining at the surface. Under certain hydrodynamic conditions the moving drop sheds a trail of tiny droplets on the surface. The simulation results are in qualitative agreement with experimental studies on the corresponding systems published in the literature.

The influence of soft vibrational modes on our understanding of oxide surface structure

Abstract: We examine the reasons for the poor quantitative agreement between the structures predicted from the minimum energy configuration of first principles calculations and those deduced from surface X-ray diffraction experiments for the structure properties of the TiO2(110) surface. In order to confine all numerical approximations very large scale all-electron first principles calculations are used. We find a very soft, anisotropic and anharmonic surface rigid-unit vibrational mode which involves displacements of the surface ions of approximately 0.15 A for thermal vibrations corresponding to room temperature. It is concluded that in order to perform an accurate comparison between theory and experiment for this and perhaps other oxide surfaces it will be necessary to take account of such anisotropic vibrations in models used to interpret experimental data. In addition the contribution of the vibrational entropy to the surface free energy is likely to be significant and must be taken into account when computing surface energies and structures.

Complex Pattern Formation by Phase Separation of Polymer Blends in Thin Films

Abstract: During spin coating of very thin films from a solution of incompatible polymers quite interesting lateral structures are forming. From scanning force microscopy (SFM) and X-ray reflectometry it is concluded that a surface height modulation is present, which reflects at the surface the phase separated morphology of the blend in the film. Those structures depend critically on different parameters like relative concentration of the components, spinning parameters, solvent quality or compatibility of the blend. In blends of different statistical copolymers of poly-styrene-stat-para-bromo-styrene the compatibility between the blended polymers depends on the difference in the degree of bromination. This allows a variation of the interaction parameter within a wide range. Besides relatively incompatible materials, also weakly incompatible mixtures are of interest. Such a system with an almost vanishing but still positive interaction parameter is realized with blends of poly-styrene and poly-para-methyl-styrene. Ultra thin polymer films with thicknesses between 2 to 4 RG were prepared by solvent quenches from homogeneous solution. The lateral structures created during the quench are examined with SFM. The SFM pictures are statistically analysed in terms of their Fourier components. From the power spectral density function the most prominent in-plane length scale is extracted. This analysis of a mean distance is complemented by the use of Minkowski measures.

Structure and reactivity of iron oxide surfaces

Abstract: Epitaxial films of different iron oxide phases and of potassium iron oxide were grown onto Pt(111) substrates and used for studying structure–reactivity correlations. The film morphologies and their atomic surface structures were characterized by scanning tunneling microscopy and low energy electron diffraction including multiple scattering calculations. The adsorption of water, ethylbenzene, and styrene was investigated by temperature programmed desorption and photoelectron spectroscopy. A dissociative chemisorption of water and a molecular chemisorption of ethylbenzene and styrene is observed on all oxides that expose metal cations in their topmost layers, whereas purely oxygen-terminated FeO(111) monolayer films are chemically inert and only physisorption occurs. Regarding the technical styrene synthesis reaction, which is performed over iron oxide based catalysts, we find a decreasing chemisorption strength of the reaction product molecule styrene, if compared to ethylbenzene, when going from Fe3O4(111) over α-Fe2O3(0001) to KFexOy(111). Extrapolation of the adsorbate coverages to the technical styrene synthesis reaction conditions using the Langmuir isotherm for coadsorption suggests an increasing catalytic activity along the same direction. This result agrees with previous kinetic experiments performed at elevated gas pressures over the model systems studied here and over polycrystalline iron oxide catalyst samples. It indicates that the iron oxide surface chemistry does not change across the pressure gap and that the model systems simulate technical styrene synthesis catalysts in a realistic way.

Molecular orientation in superfluid liquid helium droplets: high resolution infrared spectroscopy as a probe of solvent–solute interactions

Abstract: Reported here are three aspects of molecular stereochemistry associated with polar molecules in an exotic solvent, namely, superfluid liquid helium. Infrared spectroscopy is used to study the nature of the solvent–solute and solute–solute interactions in this unique environment. First, we use an external dc electric field to orient the helium solvated polar molecules in the laboratory frame of reference and we characterize this orientation using pendular state spectroscopy. Second, we present results that demonstrate non-equilibrium self-assembly of polar molecules into linear chain structures, due to the mutual orientation imposed by the associated dipole–dipole interactions. Finally, the spectroscopic results provide information on the anisotropic interactions experienced by the solvated molecules, which result in significant molecular orientation in the droplet frame of reference. Studies of this type provide important new insights into the nature of the interactions between the molecules and the superfluid solvent.

Oxygen-induced morphological changes of Ag nanoclusters supported on TiO2(110)

Abstract: The effect of insitu O2 exposure on TiO2(110)-supported Ag nanoclusters was investigated using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). An oxygen-induced cluster ripening was observed by STM after Ag/TiO2(110) was exposed to 10.00 Torr O2 for 2 h in an elevated-pressure reactor. The Ag clusters exhibit a clear bimodal size distribution after O2 exposure due to Ostwald ripening: some clusters increase in size while other clusters decrease in size. The cluster density also increased 5–15% after O2 exposure, indicating redispersion simultaneously occurs with ripening. It is shown that intercluster transport is likely accomplished through the formation of Ag2O.

Scanning tunnelling microscopy studies of the reactivity of the TiO2(110) surface: Re-oxidation and the thermal treatment of metal nanoparticles

Abstract: We have employed variable temperature scanning tunnelling microscopy (STM) to probe the surface structure of the TiO2(110) surface with clean, adsorbate and metal covered terminations. The aim of the work is to understand the nature of catalysis on supported metal oxide catalysts for which a good model is an admetal on a single crystal oxide surface. For Pd overlayers, annealing in vacuum shows the formation of metal particles with nanometer sized dimensions, which are comparable to those seen in real catalysts. The clean TiO2(110) surface has two commonly observed terminations, the (1×1) bulk truncation, the (1×2) reduced and reconstructed surface. Less commonly, for very reduced crystals, the formation of ordered defects occurs leading to crystallographic shear planes. We have explored all of these surfaces by low energy electron diffraction (LEED) and STM to provide structural information, while we have employed dynamic imaging of the surface in reactive conditions at elevated temperature to assess the chemistry. We find that oxygen rich atmospheres promote a re-growth of the surface that has important consequences for the surface chemistry and morphology. The oxidation and reduction of the support in this system has been shown to modify the reactive properties of the supported metal and we relate our observations to the strong metal support interaction (SMSI).

Mg clusters on MgO surfaces: study of the nucleation mechanism with MIES and ab initio calculations

Abstract: We combined experimental studies using ultraviolet photoelectron spectroscopy (UPS), metastable impact electron spectroscopy (MIES) and temperature programmed desorption (TPD) with abinitio calculations of metal adsorption on the perfect MgO surface and at defect sites in order to elucidate the role of surface defects in the initial stages of nucleation and growth of metal clusters at oxide surfaces. MgO films (2 nm thick) grown on Mo and W substrates were used as a prototype system. The MIES and UPS (HeI) spectra were collected insitu, and the growth of Mg clusters was observed by monitoring the dynamics of additional MIES peaks during Mg deposition. TPD experiments were made in order to monitor the surface coverage by Mg clusters and to determine the Mg desorption energies. Interpretation of the results was made on the basis of theoretical modelling using density functional theory (DFT) calculations in both periodic and embedded cluster models. The geometric and electronic structures of the surface terrace, F-centre, positively charged anion vacancy, and step edge at the MgO(001) surface were calculated, and their role in adsorption and clustering of Mg atoms on this surface was studied. The absolute position of the top of the surface valence band of MgO with respect to the vacuum was calculated and compared with the MIES results. The MIES spectra were modelled on the basis of surface density of states (SDOS). The calculated SDOS predicted the location of additional peaks in the band gap and their shift as a function of Mg concentration on the surface in agreement with the MIES data. The desorption energies of Mg atoms from small Mg clusters formed at step edges are found to be about 1.3 eV atom-1. Comparison between the theoretical results and the experimental data suggests preferential initial adsorption of Mg atoms at steps and kinks, rather than at charged and neutral vacancies. At larger exposures these Mg atoms serve as the nucleation sites.

Analysis of membrane protein cluster densities and sizes in situ by image correlation spectroscopy

Abstract: Communication between cells invariably involves interactions of a signalling molecule with a receptor at the surface of the cell. Typically, the receptor is imbedded in the membrane and it is hypothesized that the binding of the signalling molecule causes a change in the state of aggregation of the receptor which, in turn, initiates a biochemical signal within the cell. Subsequently, many of the occupied receptors bind to membrane-associated structures, called coated pits, which invaginate and pinch off to form coated vesicles, thereby removing the receptors from the cell surface. The state of aggregation of membrane receptors is obviously in constant flux. Any useful approach to measuring the state of aggregation must, therefore, allow for dynamic measurements in living cells. It is possible to use fluorescently labelled signalling molecules or antibodies directed at the receptor of interest to visualize the receptor on the cell surface with a fluorescence microscope. By employing a laser confocal microscope, high resolution images can be produced in which the fluorescence intensity is quantitatively imaged as a function of position across the surface of the cell. Calculations of autocorrelation functions of these images provide direct and accurate measures of the density of fluorescent particles on the surface. Combined with the average intensity in the image, which reflects the total average number of molecules, it is possible to estimate the degree of aggregation of the receptor molecules. We refer to this analysis as image correlation spectroscopy (ICS). We show how ICS can be used to measure the density of several receptors on a variety of cells and how it can be used to measure the density of coated pits and the number of molecules per coated pit. We also show how the technique can be used to monitor fusion of virus particles to cell membranes. Further, we illustrate that, by calculating cross-correlation functions between pairs of images, we can extend the analysis to measurements of the distributions as a function of time, on the second timescale, as well as to measurements of the movement of the receptor aggregates on the surface. Finally, we illustrate that, by this approach, we can measure the extent of interaction between two different receptors as a function of time. This represents the most quantitative measurement of the extent of co-localization of receptors available and is independent of the spatial resolution of the confocal microscope. The theory of ICS and image cross-correlation spectroscopy (ICCS), focussing on the interpretation of the data in terms of the biological phenomenon being probed, is discussed.

Laser cooling of internal degrees of freedom of molecules by dynamically trapped states

Abstract: In the last several years we have discovered a variety of remarkable pulse strategies for manipulating molecular motion by employing a design strategy we call “local optimization.'' Here we review the concept of local optimization and contrast it with optimal control theory. By way of background, we give highlights from two recent examples of the method: (1) a strategy for eliminating population transfer to one or many excited electronic states during strong field excitation, an effect we call ‘optical paralysis’; (2) a generalization of the counterintuitive STIRAP (stimulated Raman adiabatic passage) pulse sequence from three levels to N levels, a strategy we call ‘straddling STIRAP.' We then turn to a third example, which is the main subject of this paper: laser cooling of molecular internal degrees of freedom. We study a model that includes both coherent interaction with the radiation field and spontaneous emission; the latter is necessary to carry away the entropy from the molecule. An optimal control calculation was performed first and succeeded in producing vibrational cooling, but the resulting pulse sequence was difficult to interpret. Local optimization subsequently revealed the cooling mechanism: the instantaneous phase of the laser is locked to the phase of the transition dipole moment between the excited state amplitude and v=0 of the ground state. Thus, the molecules that reach v=0 by spontaneous emission become decoupled from the field, and no longer absorb, while molecules in all other states are continually repumped. The mechanism could be called “vibrationally selective coherent population trapping,'' in analogy to the corresponding mechanism of velocity selective coherent population trapping in atoms for sub-Doppler cooling of translations.

QM investigations on perovskite-structured transition metal oxides: bulk, surfaces and interfaces

Abstract: We present the results of electronic-structure, abinitio calculations on a set of perovskite-structured transition metal oxides, in which the transition metal ion has electronic configuration ). We perform an analysis of the QM solution for the bulk materials, based on a phenomenological, tight binding-like examination of the band structures in reciprocal space. This treatment allows us to understand the trends in the properties of bulk perovskites as a function of their chemical composition; a parameter is defined, easily calculated from the band structure of the cubic phase, that controls the extent of covalence in the M–O interaction. Ferroelectric-like distortions from the cubic phase are seeded by a symmetry breaking around either a M or an O ion of the structure; the electronic perturbation is then transferred to the neighbouring sites via a delocalisation of the π M–O bonding levels in the valence band. Investigations on the 〈001〉 surface termination of BaTiO3 and WO3 show that the electronic perturbation induced by the surface can couple with the ferro-/antiferro-electric (FE/AFE) distortional modes of the bulk materials in the surface and sub-surface regions. We have been able to attribute to different surface terminations an FE or AFE character. Finally, via the design of suitable perovskite/perovskite interfaces, we have combined materials with FE bulk behaviour with AFE surface terminations, and viceversa AFE bulk materials with FE surfaces. Our results show that the interface strain may significantly alter the behaviour of the support. The complexity of the systems investigated is already comparable to applications of FE materials in the field of computer memories.

Oxygen-induced restructuring of rutile TiO2(110): formation mechanism, atomic models, and influence on surface chemistry

Abstract: The rutile TiO2(110) (1×1) surface is considered the prototypical ‘well-defined’ system in the surface science of metal oxides. Its popularity results partly from two experimental advantages: (i) bulk-reduced single crystals do not exhibit charging, and (ii) stoichiometric surfaces, as judged by electron spectroscopies, can be prepared reproducibly by sputtering and annealing in oxygen. We present results that show that this commonly applied preparation procedure may result in a surface structure that is by far more complex than generally anticipated. Flat, (1×1)-terminated surfaces are obtained by sputtering and annealing in ultrahigh vacuum. When re-annealed in oxygen at moderate temperatures (470–660 K), irregular networks of partially connected, pseudohexagonal rosettes (6.5×6 Awide), one-unit cell wide strands, and small (≈tens of A) (1×1) islands appear. This new surface phase is formed through reaction of oxygen gas with interstitial Ti from the reduced bulk. Because it consists of an incomplete, kinetically limited (1×1) layer, this phenomenon has been termed ‘restructuring’. We report a combined experimental and theoretical study that systematically explores this restructuring process. The influence of several parameters (annealing time, temperature, pressure, sample history, gas) on the surface morphology is investigated using STM. The surface coverage of the added phase as well as the kinetics of the restructuring process are quantified by LEIS and SSIMS measurements in combination with annealing in 18O-enriched gas. Atomic models of the essential structural elements are presented and are shown to be stable with first-principles density functional calculations. The effect of oxygen-induced restructuring on surface chemistry and its importance for TiO2 and other bulk-reduced oxide materials is briefly discussed.

First principles simulations of titanium oxide clusters and surfaces

Abstract: Electronic and structural properties of TiO2 species of various sizes, charges and stoichiometries, ranging from TinOmx clusters (n=1–3, m-n=0, 1, x=-1, 0, +1) to bulk rutile and its (110) surface, have been obtained by total energy calculation based on the density functional theory (DFT), in the local density and local spin density approximations (LSDA), and complemented by a Bader-type analysis of the total electronic density. Attention has been focused on the electron distribution to better understand how the ionocovalent character of the Ti–O bonding and the screening properties vary as a function of the size of the system, the atomic coordination and the surface orientation.

Lipid packing stress and polypeptide aggregation: alamethicin channel probed by proton titration of lipid charge

Abstract: Lipid membranes are not passive, neutral scaffolds to hold membrane proteins. In order to examine the influence of lipid packing energetics on ion channel expression, we study the relative probabilities of alamethicin channel formation in dioleoylphosphatidylserine (DOPS) bilayers as a function of pH. The rationale for this strategy is our earlier finding that the higher-conductance states, corresponding to larger polypeptide aggregates, are more likely to occur in the presence of lipids prone to hexagonal HII-phase formation (specifically DOPE), than in the presence of lamellar Lα-forming lipids (DOPC). In low ionic strength NaCl solutions at neutral pH, the open channel in DOPS membranes spends most of its time in states of lower conductance and resembles alamethicin channels in DOPC; at lower pH, where the lipid polar groups are neutralized, the channel probability distribution resembles that in DOPE. X-Ray diffraction studies on DOPS show a progressive decrease in the intrinsic curvature of the constituent monolayers as well as a decreased probability of HII-phase formation when the charged lipid fraction is increased. We explore how proton titration of DOPS affects lipid packing energetics, and how these energetics couple titration to channel formation.

Atomistic simulation of oxide surfaces and their reactivity with water

Abstract: Atomistic simulation is a valuable tool for interpreting and predicting surface structures. This paper describes our current work aimed at applying this approach to model oxide surfaces in contact with water. The atomistic simulation techniques used are energy minimisation and molecular dynamics, which are coupled with interatomic potentials. Energy minimisation allows us to evaluate the most stable surface configurations and molecular dynamics provides the effect of temperature on the surface. The use of interatomic potentials, which describe the forces between the atoms, allows the surface properties to be calculated rapidly hence enabling us to increase the complexity of the systems studied. We have extended our previous work in two ways, first by modelling the interaction of water with more complex materials such as magnesium silicate and iron oxide and secondly, by considering the initial stages of dissolution by evaluating the energies of replacing the surface cations with protons. We find that there is a strong interaction between the surfaces and water. The bonding of the surface to the water molecules is dominated by the cation–water interactions but is moderated by the area occupied by each water molecule, which is approximately 10 A2. In addition, as expected, the dissolution energies are highly dependent on cation coordination and the type of cation present, with Ca being energetically more favoured than Mg, and the surface structure as illustrated by Fe2O3.

Scanning tunnelling microscopy on the growth and structure of NiO(100) and CoO(100) thin films

Abstract: We have prepared ordered thin films of NiO and CoO in (100) orientation by evaporating Ni (Co) in an O2 atmosphere onto Ag(100). The films have been analysed by scanning tunnelling microscopy and low-energy electron diffraction. In the initial stage (coverage up to a few monolayers), growth and structure of the grown films drastically depend on the preparation conditions (in particular, on the temperature of the substrate during deposition and post-annealing). In this case we also observe strong interactions with the substrate. Ag atoms are partially removed from the substrate terraces and form islands or migrate to step edges. No indications for incorporation in the oxide thin films are seen. The oxidic features grow on top of the substrate or in the vacancy islands within the first layer of the substrate left behind by the removed Ag atoms. At low substrate temperatures (near room temperature) an essential part of the oxidic features corresponds to a precursor state rather than to the fully developed (100) oxide film which only develops after post-annealing to higher temperatures (typically around 500 K). I/U characteristics and the sample bias dependency of the contrast of the islands grown have been utilised for identification of whether an oxide reaction had taken place or not. The surfaces of the oxide precursor show a typical defect structure similar to those found on cleaved NiO(100) (M. R. Castell etal., Phys. Rev. B:Condens. Matter, 1997, 55, 7859). This feature shows ‘random walk’ at room temperature.

Phase transitions, aggregation and crystallization in mixed suspensions of colloidal spheres and rods

Abstract: We have studied the coupling of aggregation and crystallization in a mixture of colloidal spheres and colloidal rods. Taking advantage of specially prepared fluorescent silica spheres we are able to investigate this process at the particle level by fluorescense confocal microscopy. We find that above a limiting concentration of rods in the mixture, the spheres rapidly assemble to form compact aggregates which subsequently crystallize.