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The spontaneous co-organization of distinct biomolecules at interfaces enables many of Nature's hierarchical organizations involving both hard and soft materials. Engineering efforts to mimic such hybrid complexes rely on our ability to rationally structure biomolecules at inorganic interfaces. Control over the nanoscale structure of patterned biomolecules remains challenging due to difficulties in controlling the multifarious interactions involved. This work discusses binary peptide assembly as a means to fabricate biomolecular nano-mosaics at graphite surfaces with predictable structures. Distinct peptide-substrate interactions lead to divergent crystallographic growth directions, molecular scale immiscibility, and a symbiotic assembly phenomenon. We present a symbiotic assembly model that accurately predicts the binary assembly structure relying solely on the constituent peptide nucleation kinetics and molar fractions. The ability to tune such biomolecular nano-mosaic structures facilitates the bottom up fabrication of high-density, multifunctional interfaces for nanotechnology.

Key to the development of functional bioinorganic soft interfaces is the predictive control over the micron-scale assembly structure and energetics of biomolecules at solid interfaces. While assembly of labile biomolecules, such as short peptides, at interfaces is a great deal affected by the shape of the molecule, biomolecular conformations are prompted by external solution conditions, involving temperature, pH, and salt concentration. In this light, one can expect that the environmental conformational selection of aqueous biomolecules could potentially allow for fine-tuning of the equilibrium assembly structure at interfaces, as well as, the binding strength and molecular mobility within these assemblies. Here, we demonstrate the energetic and structural tailoring of two-dimensional surface assemblies of graphite-binding dodecapeptides, through the thermal selection of aqueous peptide conformations. Our findings based on a scanning probe energetic analysis, supplemented by molecular dynamics modeling, show that peptide-graphite and peptide-peptide intermolecular interactions strongly depend on the thermally selected molecular conformation and that the extent of the conformational change is directly related to the observed assembled structure. Enabled by these results was the design of a peptide with predictable binding and assembled structure, thus, suggesting environmental preconditioning of peptides as a means for controlling self-assembling active bioinorganic interfaces for bioelectronic implementations such as biomolecular fuel cells and biosensors.

Dynamic and environmentally directed assembly of molecules in biological systems is essential for the fabrication of micronscale, hierarchical, functional structures. Here, we demonstrate the directed assembly of genetically selected graphite binding peptides on 2D solid surfaces upon thermal stimuli. Structural and kinetic analyses as well as molecular dynamics simulations yield the self-assembly process as thermally controllable upon tuning the solvated peptide conformational states. The ability to tailor the structure of two-dimensional soft bio/nano interfaces via external stimuli shows the potential for the bottom-up fabrication of complex materials with nanotechnological importance, such as biosensors, bioelectronics, and biomolecular fuel cells.

Dense abrasive slurries are widely used in a variety of processes, but have problems in their application due to strong shear thickening and jamming. For particles in the supra-colloidal or non-Brownian, size range ( d > 10 μm), these are due primarily to the formation of hydroclusters at high shear rates and to direct particle-particle contacts. The latter appear to be a particular problem in the development of abrasive slurry jets (ASJ) used for abrasive water jet cutting. This study explores the use of two adsorbed surfactants and a covalently bonded silane as boundary lubricants for abrasive garnet particles of irregular shape with sharp asperities and average effective diameter 50 μm to reduce shear thickening and jamming of their slurries. Particle-particle friction is determined using lateral force microscopy (LFM), and it is found that adsorbed layers of either a cationic surfactant, hexadecyltrimethylammonium bromide (HTAB) or a nonionic surfactant, Pluronic ® L62, or of a grafted octadecyltrimethoxy silane (OTS) reduced the lateral friction by a factor of as much as two. Vane tool rheometry suggested similar reductions in shear thickening of dense garnet particle slurries, and an ad hoc study of jamming slurries through a flow constriction provided by a syringe yielded similar results.

Nanocomposite polymer and ultrathin film membranes have shown great promise in enhancing gas permeation and selectivity properties by interfacially straining polymer matrices, yielding structures of higher free volume. However, undesired particle aggregation and short temporal stability remain a big challenge. In the present study, an “inverse” architecture to conventional polymer nanocomposites was investigated, in which the polymer phase poly(l-trimethylsilyl-1-propyne) (PTMSP) was interfacially and dimensionally constrained in nanoporous anodic aluminum oxide (AAO) membranes. While with this architecture the benefits of nanocomposite and ultrathin film membranes could be reproduced and improved upon, also the temporal stability could be enhanced substantially. Gas permeabilities of helium, nitrogen and carbon dioxide were increased over five-fold, and selectivities of CO2/He and CO2/N2 could be enhanced by 40% compared to the pristine bulk phase, while physical aging, caused by free-volume collapse, was reduced twenty-fold compared to ultrathin membranes.

Recent advances in scanning probe methods that provide direct access to the surface free energy of inorganic layered materials in terms of the Hamaker constant yield energetic values for monolayer graphene that differ substantially, by a factor of around 0.4, from bulk graphite. The onset of bulk deviating energy values was observed at a multilayer slab thickness of ∼3 nm, corresponding to a layer number of 10. The findings, complemented with extractions from water contact angle measurements and calculated interlayer binding energies, find short-range ordinary van der Waals interactions to be most prominently affected by dimensional constraints and many-body interactions.

A broad nanoscopic study of a wide-range of dendritic organic nonlinear optical (NLO) self-assembly molecular glasses reveals an intermediate thermal phase regime responsible for both enhanced electric field poling properties and strong phase stabilization after poling. In this paper, the focus is on dendritic NLO molecular glasses involving quadrupolar, liquid crystal, and hydrogen bonding self-assembly mechanisms that, along with chromophore dipole-dipole interactions, dictate phase stability. Specifically, dendritic face-to-face interactions involving arene-perfluoroarene are contrasted to coumarin-containing liquid crystal mesogen and cinnamic ester hydrogen interactions. Both the strength of dendritic interactions and the impact of dipole fields on the relaxation behavior have been analyzed by nanoscale energetic probing and local thermal transition analysis. The presence of dendritic groups was found to fundamentally alter transition temperatures and the molecular relaxation behavior. Thermal transition analysis revealed that molecules with dendritic groups possess an incipient transition (T(1)) preceding the glass transition temperature (T(2)) that provides increased stability and a well-defined electric field poling regime (T(1)TT(2)), in contrast to molecular groups lacking dendrons that exhibit only single transitions. On the basis of enthalpic and entropic energetic analyses, thermally active modes below T(1) were found to be intimately connected to the dendron structure. Their corresponding activation energies, which are related to thermal stability, increased moving from cinnamic ester groups to coumarin moieties to arene-perfluoroarene interacting groups. While dendritic NLO materials were found to possess only enthalpic stabilization energies at temperatures relevant for device operation (TT(1)), the apparent molecular binding energies above T(1) contain a substantial amount (up to ~80%) of cooperative entropic energy. The multiple interactions (from dipole-dipole interactions to local noncovalent dendritic interactions) are discussed and summarized in a model that describes the thermal transitions and phases.

Interfacial constraints in ultrathin poly(l-trimethylsilyl-1-propyne) (PTMSP) membranes yielded gas permeabilities and CO 2 /helium selectivities that exceed bulk PTMSP membrane transport properties by up to three-fold for membranes of submicrometer thickness. Maximum permeability coefficients of 110 × 10 3 Barrer and 27 × 10 3 Barrer for carbon dioxide and helium, respectively, were found to occur in membranes of ∼750 nm thickness. Indicative of a free volume increase, a molecular energetic mobility analysis (involving intrinsic friction analysis) revealed enhanced methyl side group mobility. This was evidenced by a minimum in the activation energies of ∼4 kcal/mol in thin PTMSP membranes with maximum permeation, compared to ∼5.5 kcal/mol in bulk films. Aging studies conducted over the timescales relevant to the conducted experiments signify that the free volume states in the thin film membranes are highly unstable in the presence of sorbing gases such as CO 2 . These results are discussed and contrasted to PTMSP bulk membrane systems, which were found to be unaffected by aging over the equivalent experimental time scale.

article i nfo To enhance the ionic conductivity in solid phase polyelectrolyte systems for lithium ion battery applications demands effective control of the phase properties. Here, we report on a strategy involving a layer-by-layer methodology of two polyelectrolytes, poly(ethylene oxide) (PEO) and poly(acrylic) acid (PAA) and carboxyl- ic acid functionalized multi-walled carbon nanotubes (MWNTs). Optimization of the assembly strategy revealed that undoped and lithium-ion doped stacking of four layers provides excellent film growth and im- provement of the ionic conductivity of up to 10 �5 Scm �1 , which exceeds conventional assemblies of lithi- um-ion doped (PEO/PAA) by up to two orders of magnitude. Although ionic conductivity was most effectively enhanced for ultrathin films (b100 nm), (PEO/PAA/PEO/(PAA+MWNT)) stacking still provides an ionic conductivity of N10 �6 Scm �1 for thick films (N2 μm). The improvement of ionic conductivity was attributed to (i) interfacial phase mixing (blending) of the two polyelectrolytes, (ii) the MWNT contribution in the interfacial region, and (iii) the preferential adsorption of lithium-ions along the carbon nanotubes. This study involved a series of scanning probe methods including lateral force microscopy, and electrostatic force microscopy.

Ion conductivities of layer-by-layer (LBL) assemblies of solid thin film polyelectrolyte systems involving poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) were found to be a strong function of the number of bilayer stacks, n , with conductivities approaching 10 − 7 S/cm for n − 9 S/cm for n ≥ 10 and 10 − 10 S/cm for bulk PEO. Increased ion conductivity for low LBL stack numbers ( n n ≥ 10) were attributed to low concentration autoblending caused by PEO-PAA hydrogen bonding, and an average layer thickness of noticeably less than 100 nm. The effect of interfacial constraints was evident in the degree of intermixing, addressed by a thin film extended Fox blend analysis, in the glass and melting transitions of PEO and PAA pure film components. While the glass transition value of PAA decreased by 55% to 46 °C for an 8 nm film, the melting transition for PEO decreased by 15% to 64 °C caused by surface tension effects.

An overview of the development and utilization of organic electro-optic materials is presented with emphasis on the role played by quantum and statistical mechanical calculations in understanding critical structure/function relationships that have guided the improvement of such materials over the past two decades. This review concentrates largely on three classes of organic electro-optic materials prepared by electric field poling of materials near their glass transition temperature: (1) chromophore/polymer composite materials, (2) dendrimers and polymers containing covalently incorporated chromophores, and (3) matrix-assisted-poling (MAP) materials where specific spatially anisotropic interactions enhance poling efficiency. In particular, the role of chromophore shape, restrictions on chromophore motion associated with covalent bonds, and lattice dimensionality effects are reviewed. The role of device design and auxiliary properties (optical loss, thermal stability, photochemical stability, processabilit...

Poly(tert butyl acrylate) (PTBA) is found to exhibit enhanced mobility when spun cast into thin films or impregnated into cylindrical anodic aluminum oxide (AAO) nanoscale pores. In a thin film configuration, the glass transition temperature of 20 nm thick PTBA is found to decrease almost 20 °C compared to the bulk. Consistent with this mobility increase, an increased volume fraction of interphase polymer leads to at least a 2.4 times viscosity reduction when PTBA is impregnated in 100 nm pores versus 200 nm pores. Such increases in mobility result in a 15-fold increase in CO2 permeability for an AAO confined geometry compared to a bulk film. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 434–441, 2010

Novel scanning force microscopy techniques (SFM) were employed to investigate poly[l-(trimethylsilyl)-1-propyne] (PTMSP), a high-free volume, highly permeable reverse-selective membrane material. This study reports, for the first time, reverse selectivity in relation to the interfacial gas adsorption capacity of the PTMSP membrane with the gas permeants CO 2 and helium. With flux-lateral force microscopy (F-LFM), mechanical property changes caused by permeant gas infiltration were recorded within the polymer interfacial downstream region. In conjunction with bulk permeation measurements and varying sequential exposure to the two permeants, CO 2 is found to saturate the membrane faster, i.e. at a lower differential pressure by about 0.3 bar, in comparison to helium. It is also identified as modifying agent for PTSMP causing a significant change in the mechanical properties of the polymer matrix, which consequentially leads to a considerable helium transport reduction, and thus, an increase in reverse selectivity from 1.2 to 4.7. Also in this study, thermally available activation modes of 6–8 kcal/mol were revealed by intrinsic friction analysis (IFA) that were attributed to backbone methyl-group rotations in accordance with conformational calculations. Bulk thermally activated modes were found to be modestly affected by interfacial constraints on the sub-100 nanometer scale, which is an important finding for interpreting interfacial constraints in PTMSP nanocomposites involving silicon-oxides.

The underpinnings of material properties in nanoscopic polymer systems are reviewed in light of the relaxation modes available for molecular motion. When motion is altered due to the presence of constraints, a rich variety of material and transport behaviors become apparent. On the one hand, external constraints imposed by interfaces and system boundaries generate structural and dynamical anisotropies that can propagate over device-relevant length scales. On the other hand, the ability to cater relaxation behavior through molecularly-engineered internal constraints offers a path to optimize material properties in nanoscopic systems. These two aspects are highlighted throughout the review, and their technological implications are discussed for MEMS/NEMS operations, including frictional and mechanical loading of polymer thin films. In this regard, material performance attributes and feedback for molecular designs are drawn from perturbation techniques that provide access to the energetic signatures and char...

Using an in situ Diels−Alder (DA) “click chemistry” strategy for reactions between anthracene and maleimide functional groups, two prototypes of novel nonlinear optical (NLO) side-chain polymers (PMI-A7 and PMI-B7) containing highly hyperpolarizable but chemically sensitive phenyltetraene-based chromophores were synthesized and their nonlinear optical and thermal properties were characterized. Through rational material design, these NLO side-chain polymers exhibited good processibility, large electro-optic (E-O) coefficients (r33, of up to 263 and 287 pm/V at 1.31 μm wavelength respectively), and excellent temporal stability. These combined properties make them promising materials for E-O device applications. Compared to PMI-B7, PMI-A7 exhibited significantly enhanced temporal stability (87% of the initial r33 values was retained after 550 h at 85 °C) and solvent resistance, which was attributed to slight cross-linking from the side reaction between maleimide and phenyltetraenic chromophore moieties. This...

The interfacial interaction strength and transition properties in a reverse selective thin film nanocomposite system, silica-poly[(trimethylsilyl)propyne] (SiO(x)-PTMSP), are investigated locally by heated tip atomic force microscopy. SiO(x)-PTMSP has recently been introduced as a new class of reverse selective membrane materials with extraordinarily high permeability and selectivity (reverse selectivity). Here, we examine the thermal transition properties of the polymer matrix and the debonding strength between PTMSP and silica. Transitions at 330 degrees C were identified as degradation processes. Criteria for debonding were found to include polymer viscoelastic responses, particle size, embedding depth, scan speed, and frequency of impact. Probe-particle impact forces revealed a debonding energy of 2.6 J/m(2) and an impact force transition that occurs 30 degrees C below the degradation temperature in the neat polymer, confirming the presence of enhanced polymer mobility at the SiO(x)-PTMSP interface.

Atomic Force Microscopy (AFM) is a key technique for the measurement and analysis of samples when nanoscale topography is of interest. It offers a number of complementary probing modes that extend an AFM's measurement capability to a wide range of material and transport properties of surfaces, including hardness, friction, conductivity and adhesion among others. Sample temperature controlled AFM extends the study of surface morphology and properties to include changes in the material phases. Recently, silicon microfabricated AFM cantilevers that have integrated heaters, as shown in figure 1, have become commercially available. These cantilevers were initially developed for probe based data storage by researchers at IBM Zurich, Figure 1. With the availability of these cantilevers, AFM measurements can be performed where the tip is heated as opposed to the sample.

This nanorheological study addresses the water transport hindrance in perfluorinated ionomer membranes (Nafion ® H + form) in conjunction with structural relaxation properties of the polymer matrix. A low-temperature relaxation of the “dry” Nafion ® matrix at 79 °C, about 35 °C below the glass transition temperature, was found to be responsible for a steady improvement of the water transport above 70 °C. The low-temperature structural transition at 79 °C was observed for the dehydrated membrane, and thus, is specific to the polymer matrix and not a sole consequence of the kinetics of the solvent-containing aggregates within the apolar perfluorocarbon matrix. Water was found to be trapped within a hydrated perfluorinated ionomer membrane below 70 °C. The direct observation of water transport is possible with a novel scanning force microscopy (SFM) method, flux-lateral force microscopy (F-LFM). With this technique, local fluid permeation fluxes are directly obtained at the membrane downstream surface. The structural transition properties were acquired by another highly sensitive nanoscopic technique, shear modulation force microscopy (SM-FM).

Substrate constraints and interfacial boundary layers in thin polystyrene films are explored with high strain rate indentations characteristic of thermomechanical terabit data storage operations. Under these impact-like conditions, the coupling of strain-rate and inertial effects leads to large plastic deformations relative to quasi-static indentations. Strain shielding is present when the plastic deformation radius exceeds ∼65% of the film thickness. Thereafter, deformation is restricted by the rigid substrate, giving rise to elevated rim heights and interfacial shearing. The shielding effects were alleviated with use of a modulus-matched buffer layer between the polymer film and the substrate. A non-monotonic rheological gradient in the polymer films leads to the distribution of contact pressures between two asymptotic scenarios: (i) a compliant surface with a rigid sub-surface and (ii) a rigid surface with a compliant sub-surface.

Interfacial glass transition temperature (T(g)) profiles in spin cast, ultrathin films of polystyrene and derivatives were investigated using shear-modulated scanning force microscopy. The transitions were measured as a function of film thickness (delta), molecular weight, and crosslinking density. The T(g)(delta) profiles were nonmonotonic and exhibited two regimes: (a) a sublayer extending about 10 nm from the substrate, with T(g) values lowered up to approximately 10 degrees C below the bulk value, and (b) an intermediate regime extending over 200 nm beyond the sublayer, with T(g) values exceeding the bulk value by up to 10 degrees C. Increasing the molecular weight was found to shift the T(g)(delta) profiles further from the substrate interface, on the order of 10 nm/kDa. Crosslinking the precast films elevated the absolute T(g) values, but had no effect on the spatial length scale of the T(g)(delta) profiles. These results are explained in the context of film preparation history and its influence on molecular mobility. Specifically, the observed rheological anisotropy is interpreted based on the combined effects of shear-induced structuring and thermally activated interdiffusion.

The local glass transition temperature ( T g ) of heterogeneous polymer surfaces has been successfully determined by lateral force microscopy (LFM) on the submicron–nanometer scale. This paper addresses interfacial confinement effects in thin films of blends and emulsions from polystyrene (PS), poly(butylmethacrylate) (PBMA) and poly(methylmethacrylate) (PMMA). The paper addresses the difficulty of thermal rheological experiments with a microscopic scanning technique, and introduces an in-situ calibration method.

In order to study the glass transition of thin film polymer blends, high spatial resolution and temperature sensitivity is needed. In this paper, we emphasize the importance of the calibration of scanning parameters such as load and speed when measuring the glass transition temperature of polymers using lateral force microscopy. Once calibrated, this method is ideal for investigations of heterogeneous samples such as blends and co-polymers. We present an analysis technique for lateral force imaging using a fast and stable cooling/heating stage. This approach involves mapping the friction forces over a certain area and identifying regions of different frictional properties. The difference in the average friction force can then be plotted as a function of temperature. The friction force is expected to vary around the glass transition. Therefore, the glass transition temperature can be defined as the temperature at which the difference in the average friction force undergoes a slope change. We present investigations of blends using polystyrene mixed with poly(butylmethacrylate). The transition temperatures obtained are in good agreement with the bulk values of corresponding homopolymeric films.

This article discusses capillary forces measured by scanning force microscopy (SFM), which, as recently reported, show a discontinuous behavior at a low relative humidity between 20% and 40% depending on the solid surfaces. A capillary force discontinuity is very interesting in terms of a possible phase change or restructuring transition of bulk water in the interfacial solid–liquid region. Unfortunately, we have found that SFM measurements show an inherent weakness in the determination of the origin of the forces that are obtained during pull-off measurements. This article critically discusses the origin of the adhesive interactions as a function of relative humidity with chemically modified probing surfaces. Our measurements indicate that force discontinuities in pull-off measurements are strongly affected by the inability of the liquid to form capillary necks below a critical threshold in relative humidity. In the course of this article, we will discuss roughness effects on capillary forces and provide a modified capillary force equation for asperity nanocontacts.

We report results of glass transition (T(g)) measurements for polymer thin films using atomic force microscopy (AFM). The AFM mode, shear modulation force microscopy (SMFM), involves measuring the temperature-dependent shear force on a tip modulated parallel to the sample surface. Using this method we have measured the surface T(g) of thin (17-500 nm) polymer films and found that T(g) is independent of film thickness (t>17 nm), strength of substrate interactions, or even presence of substrate.

The glass transition of thin polymeric films can be profitably studied using lateral force microscopy (LFM) if the system is calibrated regarding operational parameters, in particular the applied load and the scanning velocity. We have established that these two parameters significantly influence the occurrence of an apparent glass transition. In particular, we have found that the local pressure, applied by the LFM tip, is insufficient to generate a hydrostatic pressure effect causing an increase in the apparent transition temperature. In fact, at a constant scan velocity and for increased load, the apparent transition temperature decreases towards the actual bulk value. Further discussions in this article are based on viscoelastic theories. Critical time scales that are characteristic for sliding are compared to polymer relaxation times, and provide an estimate of the viscosity temperature dependence.

This paper reviews our recent progress in determining the surface glass transition temperature, Tg, of free and substrate confined amorphous polymer films. We will introduce novel instrumental approaches and discuss surface and bulk concepts of Tg. The Tg of surfaces will be compared to the bulk, and we will discuss the effect of interfacial interactions (confinements), surface energy, disentanglement, adhesion forces, viscosity and structural changes on the glass transition. Measurements have been conducted with scanning force microscopy in two different shear modes: dynamic friction force mode and locally static shear modulation mode. The applicability of these two nano-contact modes to Tg will be discussed.

Fluoropolymer films prepared by radio frequency glow discharge (RF-glow discharge) are of interest as biomaterials coatings. Previous studies focused on hexafluoroethane (C2F6, HFE) and tetrafluoroethylene (C2F4, TFE) as the monomer precursor have shown such surfaces to exhibit unique protein binding capabilities. In this study, slow, surface directed deposition of hexafluoropropylene (C3F6, HFP) films is shown to confer surface functional group presentation that promotes high protein retention. The surface chemistry is controllable over a range of values, and C3F6 films prepared by RF-glow discharge are smooth, homogeneous, and defect free. Surface patterns of the fluoropolymer can be created by direct or photolithographic techniques. Scanning probe microscopic analysis indicates a surface modulus in the range of 1:2 , EHFP , 5:5 GPa. These results are discussed in terms of the applicability of C3F6 films for coating biomaterials. q 1999 Published by Elsevier Science S.A. All rights reserved.

Measurements of nanomechanical properties of unannealed homopolymers show that the spin coating process in the presence of an interactive interface can be the source for molecular constraints in the interfacial boundary regime of elastomeric films. The surface mechanical response of poly(ethylenepropylene) (PEP) films of various thicknesses spin cast on hydrogen passivated silicon surfaces was studied using scanning force microscopy (SFM). At least two distinct rheological regimes can be observed in thicker films: high-entropy viscous plug flow at low load and low-entropy shear sliding at high load. The transition point from high-entropy to low-entropy shear has been found to shift to lower loads for decreasing film thickness. The transition point was found to disappear, and only shear sliding was observed at a critical film thickness, which was comparable to the radius of gyration of the sample polymer. These results were interpreted in a model wherein a highly distorted anisotropic layer induced by high-speed spinning and solvent evaporation is formed near the silicon interface. Pinning interactions prevent the layer from annealing, and a two-fluid-type response occurs. The surface free energy, determined by SFM lateral

Atomic force microscopy (AFM), neutron reflection (NR) and secondary ion mass spectroscopy (SIMS) are used to examine phase separation in symmetrically segregating thin polymer blend films (≤1000 A). Phase separation in the film leads to undulations of the liquid−air interface, provided the film is sufficiently thin to suppress surface-directed spinodal decomposition waves. Flattened droplets are formed at a very late stage of phase separation, and the aspect ratio of these droplets can be rationalized by an interfacial free energy minimization argument.

The crystal perfection of MBE-grown CaF2Si(111) heterostructures with the type-A of epitaxial relations at the CaF2Si(111) interface was investigated. A number of research methods, including X-ray and electron diffraction, photoluminescence of rare earth ions and atomic force microscopy, were applied. It was shown that the two-step low temperature growth with an intermediate annealing at 850°C is an efficient way to obtain type-A pseudomorphic structures with a CaF2 film thickness of up to 50 nm.

Transformations of the atomic structure of the CaF 2 Si(111) interface during annealing have been studied by reflection high energy electron diffraction (RHEED) and X-ray crystal truncation rod (CTR) scattering. The surface morphology after annealing has been studied by atomic force microscopy (AFM). A conversion of the epitaxial relation of the film with respect to the substrate, from type-A (nonrotated) to type-B (with the axes of the film rotated by 180°C around the interface normal), is monitored by RHEED during annealing of the films. On the basis of RHEED, CTR and AFM data, a model of the conversion is suggested. In addition, a spontaneous transition of the type-B interface formed during the growth to the interface with another atomic arrangement has been studied with CTR. The possible role of point defect motion in the CaF 2 film during this transition is discussed.

The use of the atomic force microscope (AFM) as a local probe for elastohydrodynamic lubrication is discussed. Compliances are measured with a modified AFM that allows application of ac and dc force-displacement curves on end-grafted poly(styrene) chains in good (toluene) and poor (water) solvents. In toluene, the chains form a stretched brush whose elastic modulus is an order of magnitude larger than that of the collapsed layer in water. The dc force-displacement curves are compared to those previously obtained on similar systems with the surface forces apparatus.

Shear-modulated scanning force microscopy (SM-SFM) is introduced as a nanorheological characterization method for the determination of optoelectronic properties of semiconducting polymer thin films (

A correlation between friction and elasticity is drawn from measurements on organic thin films with a modified scanning force microscope (SFM). Local elastic compliance has been measured simultaneously with both topography and friction on the submicrometer scale on thin films of phase-separated mixtures of fluorocarbons and hydrocarbons. On the fluorocarbon domains, higher friction and lower Young`s modulus than on the hydrocarbon domains have been found. Variations in pH during sample preparation lead to differences in film formation, detected with the SFM as changes in topography, elasticity, and friction. On increasing pH, both the Young`s modulus and friction force are found to decrease. This unexpected result is discussed in terms of film formation and cohesive energy mechanisms.

The dependence of friction and wear on sliding parameters has been investigated with an atomic force microscope. Measurements have been performed on Langmuir-Blodgett monolayers. For the purpose of this study a special counterion polymer has been used to supply a facile insight into the friction and wear dependence on the scan velocity. With a minimum constant load in the order of 10(-8) N, wear has been observed to occur randomly at film defect sites but also on initially intact film areas. Wear is always accompanied by an increase of friction forces. A wearless friction dependence on the scan velocity could not be observed. The process of wear is discussed in terms of shear forces and adhesive contact forces. Whereas the plastic deformation at the film defect sites can be explained by shear forces, the wear process on initially defect free surfaces could be related to adhesive forces and quantified.

The impact of thermally active molecular rotational and translational relaxation modes on the friction dissipation process involving smooth nano-asperity contacts has been studied by atomic force microscopy, using the widely known Eyring analysis and a recently introduced method, dubbed intrinsic friction analysis. Two distinctly different model systems, i.e., monolayers of octadecyl-phosphonic acid (ODPA) and thin films of poly(tert-butyl acrylate) (PtBA) were investigated regarding shear-rate critical dissipation phenomena originating from diverging mode coupling behaviors between the external shear perturbation and the internal molecular modes of relaxation. Rapidly (ODPA) versus slowly (PtBA) relaxing systems, in comparison to the sliding rate, revealed monotonous logarithmic and nonmonotonous spectral shear rate dependences, respectively. Shear coupled, enthalpic activation energies of 46 kJ/mol for ODPA and of 35 and ∼65 kJ/mol for PtBA (below and above the glass transition) were found that could be attributed to intrinsic modes of relaxations. Also, entropic energies involved in the cooperative backbone mobility of PtBA could be quantified, dwarfing the activation energy by more than a factor of five. This study provides (i) a material specific understanding of the molecular scale dissipation process in shear compliant substances, (ii) analyses of material intrinsic shear-rate mode coupling, shear coordination and energetics, (iii) a verification of Eyring's model applied to tribological systems toward material intrinsic specificity, and (iv) a valuable extension of the Eyring analysis for complex macromolecular systems that are slowly relaxing, and thus, exhibit shear-rate mode coupling.

Specific spatially-anisotropic interactions are identified that enhance noncentrosymmetric order required for electro-optic activity. Enhancement of electric-field-poling-induced noncentrosymmetric order by these specific interactions is shown to result from a reduction of lattice dimensionality from three to two dimensions. New analytical techniques for measurement of centrosymmetric and noncentrosymmetric order and lattice dimensionality are introduced. Measurement of order parameters is correlated with viscoelastic data to gain further insight into the influence of specific interactions on poling efficiency and thus material electro-optic activity. The integration of organic electro-optic materials into silicon photonic, plasmonic, and metamaterial devices is also discussed. These device structures can affect the "effective" optical nonlinearity of organic materials but care must be exercised to control optical loss.

The surfaces of corona-treated isotactic polypropylene films have been investigated by atomic force microscopy. The occurrence of droplets on the film surfaces is related to the energy dose of the corona discharge. The sizes of these droplets correlate with the corona dose. The loss of adhesive strength of self-adhered polypropylene films can be explained on the basis of morphology changes during corona treatment. A comparative study of uniaxial and biaxial polypropylene films is presented.

The thermal mismatch strain introduced by the different thermal expansion of fluorides, IV-VI materials and the Si(111) substrate is relieved by glide of dislocations in the primary {100}〈110〉 glide system with glide planes inclined to the surface. The morphology of the resulting straight slip steps is revealed with atomic force microscopy (AFM) for the fluorides and scanning tunneling microscopy (STM) for the IV-VI materials. The number of observed dislocation step lines agrees with the calculated thermal strain relaxation.

Theoretical calculations have demonstrated that the ratio of second and third degree order parameters can define lattice dimensionality and furthermore, that an increased ratio of second to third degree order parameters represents reduced lattice dimensionality. As a result, the third degree order parameter (i.e. acentric order parameter) is increased, causing an increase in electro-optic activity with reduced lattice dimensionality. Experimentally, specific spatially-anisotropic interactions associated with coumarin moieties and Frechet-type (arene/perfluoroarene) dendrons have been incorporated into chromophore systems and have been shown to lead to lattices of reduced dimensionality, resulting in increased values of the acentric order parameter and therefore, electro-optic activity. Reductions in lattice dimensionality can also arise from guest chromophore-host chromophore interactions in binary chromophore organic glasses and from laserinduced ordering of host lattice chromophores observed in the laser-assisted electric field poling of azo-dye-containing host lattices. These interactions in various chromophore systems including investigation of EO and order properties are discussed.

Thin films of mixtures of fluorocarbons and hydrocarbons are investigated by friction force microscopy. For a mixture of equal molar parts of arachidic acid and partially fluorinated carboxylic acid, phase separation manifests itself as circular domains. By shortening the length of the hydrocarbon chain, the disorder of the system is increased and phase separation takes on a fractal geometry. The frictional forces are found to be material specific and distinguish between domains of fluorocarbons and hydrocarbons. With higher normal forces hydrocarbon particles are easily sheared away while the fluorocarbon region remains unbroken. The results are discussed in relation to the phenomenon of boundary lubrication.

An atomic force microscopy (AFM) study of the cleavage faces of CaF2, SrF2 and BaF2 is presented. On the CaF2(111) surface the elementary cleavage structure with cleavage tips has been imaged. The step heights have been determined to be multiples of 0.31 nm, the projection length of the Burgers vector of the primary glide system. Atomic resolution has been achieved in step-free regions of cleavage faces. The hexagonal surface lattice with spacings of 0.38 nm corresponds to the unreconstructed bulk lattice. The {111} surfaces of SrF2 and BaF2 are characterized by a low density of parallel cleavage steps; cleavage tips are not observed. On SrF2 surfaces the formation of erosive holes has been documented by AFM.

Within a short time, atomic force microscopy (AFM) has developed into a useful tool in surface science. The ability to image insulating surfaces is demonstrated on epitaxially grown AgBr(100) films and on Langmuir-Blodgett films of Cg-arachidate. In addition to the imaging of surfaces the technique is extended to study dynamic processes. By increasing the force, the onset of plastic deformation is observed. Small nanometer-sized indentations are created. In the case of AgBr these structures are refilled by surface diffusion on a time scale observable by AFM. The results are discussed in relation to other surface-sensitive techniques and compared to theoretical calculations. Another dynamic process which is monitored during the AFM measurements is friction. First results with a bidirectional force sensor are presented, allowing simultaneous acquisition of normal and lateral forces.

The bulk molecular structure of free-standing organic crystals is well known from analytical methods such as X-ray diffraction and phonon emission detection. With the inception of the field of scanning force microscopy, it is now also feasible to investigate the surface molecular structure. Rearrangements of the molecules to produce a surface structure different from the bulk structure have been suspected in certain organic crystals, although detection in such a thin layer is difficult. We have now observed this rearranged surface structure in an atomic force microscopy study of pyrene, a four-ring aromatic compound. The bulk structure of pyrene is composed of dimer pairs, whereas only monomers are detected by AFM on its ab-surface. Preliminary molecular modeling has been performed to create, out of the bulk data, this monomeric arrangement. In contrast, in a parallel study on tetracene, another four-ring aromatic, AFM measurements confirm that intermolecular spacings on the surface correspond with those in the bulk.

Recent developments in the field of scanning-probe methods show increasing demand for controlling and storing many different experimental parameters in addition to topographic information. In order to be able to perform these experiments (e.g. force spectroscopy, surface modification, friction measurements, etc.) we have designed a versatile acquisition system. Up to four different channels can be measured and displayed at the same time. Scan parameters are under software control, and additional output signals can be easily generated to meet experimental requirements. This flexibility allows the straightforward incorporation of new conditions and modes of measurement. We use a PC/386 system based principally on commercially available electronics. As an illustration of its performance we present the application of this system to a bidirectional atomic-force microscope. With this particular microscope it is important to control lateral as well as normal forces independently. Topography and friction information can be acquired and viewed simultaneously. The versatility of this data-acquisition system also makes it well suited for a variety of other tribological/micromechanical experiments such as friction loops, force-distance curves and surface wear.

The topography of the hexagonally packed intermediate layer of Deinococcus radiodurans and 2D crystals reconstituted from porin-lipopolysaccharide complexes and phospholipids were imaged using a scanning tunneling microscope and a scanning force microscope at ambient pressure. The results are compared with images obtained by electron microscopy and image processing.