Adhesion of small spheres

Bradley 1 Bradley, RS. 1931. Phil. Mag., 11: 846[Taylor & Francis Online] , [Google Scholar] calculated the adhesive force between rigid spheres to be 2πRΔγ, where Δγ is the surface energy of the spheres. Johnson et al. (JKR) [2] calculated the adhesive force between elastic spheres to be (3/2) πRΔγ and independent of the elastic modulus. Derjaguin et al. [3] published an alternative theory for elastic spheres (DMT theory), and concluded that Bradley's value for the pull-off force was the correct one. Tabor [4] explained the discrepancy in terms of the range of action of the surface forces, z ₀, and introduced a parameter μ≡(RΔγ²/E²z ₀³)^1/3, determining which result is applicable. Subsequently, detailed calculations by Derjaguin and his colleagues [5] and others, assuming a surface force law based on the Lennard-Jones 6–12 potential law, covered the full range of the Tabor parameter. Greenwood and Johnson [6] presented a map delineating the regions of applicability of the different theories. Yao et al. [7] repeated the numerical calculations but using an exact sphere shape instead of the usual paraboloidal approximation. They found that the pull-off force could be less than one-tenth of the JKR value, depending on the value of a ‘strength limit’ σ₀/E, and modified the Johnson and Greenwood map correspondingly. Yao et al.'s numerical calculations for contact between an exact sphere and an elastic half-space are repeated and their values confirmed: but it is shown that the drastic reductions found occur only for spheres that are smaller than atomic dimensions. The limitations imposed by large strain elasticity and by the ‘Derjaguin approximation’ are discussed.     

Micro- to nanoscale surface morphology and friction response of tribological polyimide surfaces

Sintered polyimide surfaces that were worn under macroscale conditions at different temperatures, were further characterised by contact-mode atomic force microscopy for getting insight in the tribophysical and -chemical processes at the micro- to nanoscale. Depending on the temperature, either mechanical interaction (23 °C < T < 100 °C), hydrolysis (120 °C < T < 140 °C), or imidisation (180 °C < T < 260 °C) results in different microscale surface characteristics. At low temperatures, surface brittleness and inter-grain fracture has been observed with an almost homogeneous friction pattern. At intermediate temperatures, the formation of a protecting local film leads to smoother surfaces with local lubricating properties. At high temperatures, different topographical and frictional patterns are observed depending on local imidisation or degradation. From AFM scans at the sub-micronscale, local debris depositions are observed and correspond to surface locations with locally reduced friction. From AFM scans at the nanoscale, polymer chain orientation is observed with formation of zig-zag or stretched molecular conformation: the latter is not induced by purely mechanical surface interactions or hydrolysis, but mainly results from tribochemically induced imidisation at high sliding temperatures. The present investigation describes the influences of local tribological interactions onto the macroscale wear behaviour of a polymer, and therefore aims at contributing to a better understanding of scaling between macro- to nanolevel tribological response.     

Quantitative measurement of tip-sample forces by dynamic force spectroscopy in ambient conditions

We introduce a dynamic force spectroscopy technique enabling the quantitative measurement of conservative and dissipative tip-sample forces in ambient conditions. In difference to the commonly detected force-vs-distance curves dynamic force microscopy allows to measure the full range of tip-sample forces without hysteresis effects caused by a jump-to-contact. The approach is based on the specific behavior of a self-driven cantilever (frequency-modulation technique). Experimental applications on different samples (Fischer-sample, silicon wafer) are presented.     

Immobilization of protein molecules on step-controlled sapphire surfaces

We have studied effects of surface morphology on immobilization of protein molecules using step-controlled sapphire surfaces. Preferential adsorption of avidin molecules on the step edges was observed on the single-stepped sapphire surface. A randomly-stepped sapphire surface was found to be suitable for high-density immobilization of protein molecules. These results indicate atomic scale structures of the substrate surface influence the adsorption efficiency of the proteins. By using an atomic force microscopy (AFM) equipped with a biotin-modified cantilever, we have confirmed that the immobilized avidin molecules on the substrates keep their biological activity. This means that the ligand-receptor interaction can be detected using the phase image mode of a standard AFM.     

SPM analysis of fibrinogen adsorption on solid surfaces

The adsorption kinetics, adhesion and orientation of human fibrinogen on solid surfaces have been studied by surface probe microscopy (SPM) and quartz crystal microbalance techniques (QCM). CF₃-, NH₂-terminated organo-silane self-assembled monolayers (SAM) and OH-terminated silicon dioxide have been used as model surfaces. Furthermore, the interaction of fibrinogen with nanocomposite Ti/hydrocarbon plasma polymer films (Ti/ppCH) deposited by dc magnetron sputtering has also been studied.     

Fracture Mechanics Studies of Adhesion in Biological Systems

A fracture mechanics based methodology for quantifying adhesive interactions between soft solids, or between a soft solid and a rigid substrate, is reviewed. An emphasis is placed on the application of these techniques to the characterization of adhesive interactions in biological systems. Results from experiments involving the adhesion of gelatin hydrogels to hydrophilic and hydrophobic substrates are described as an illustration of the application of these methods. In these experiments a hemispherical gelatin cap is brought into contact with a flat surface. Separation of the two materials is described in terms of crack propagation along the gelatin/substrate interface. Simultaneous measurements of the applied load, the resulting displacement, and the contact area between the two materials enable us to determine the elastic modulus of the cap, in addition to the crack driving force, or energy release rate. The adhesive behavior of the interface is quantified by the relationship between the energy release rate and the crack velocity. Analogies are made to information obtained from contact angle measurements, and from measurements made with the Israelachvili surface forces apparatus.     

Oxidation and adhesion on the quasicrystalline AlPdMn surface studied by nanolithography

We present the novel application of a relatively new technique (nanolithography) to the study of quasicrystalline surface oxidation. The 5-fold surface of an AlPdMn alloy was oxidized using a metallized AFM tip. The electrochemical nature of this process was confirmed by investigating the influence of humidity and polarity of the applied voltage on the quasicrystalline oxide. Oxides of different thickness and adhesive properties were created by altering the applied voltage and the humidity during the lithographic process. The technique can be used in an exhaustive study of properties of the various types of oxides that form on the AlPdMn surface and the preliminary results of one such study are reported.     

Microstructural studies of bulk and thin film GDC

Ceria rare earth solid solutions are known as solid electrolyte with potential application in oxygen sensors and solid oxide fuel cells. We report the preparation of gadolinia-doped ceria, Ce_0.90Gd_0.10O_1.95, by the conventional solid-state reaction method and the preparation of thin films from a sintered pellet of gadolinia-doped ceria by the pulsed laser deposition technique. The effect of process conditions, such as substrate temperature, oxygen partial pressure, and laser energy on microstructural properties of these films are examined using powder X-ray diffraction, scanning electron microscopy, atomic force microscopy, and Raman spectroscopy.     

A simplified but intuitive analytical model for intermittent-contact-mode force microscopy based on Hertzian mechanics

The forces acting on the substrate in intermittent-contact-mode (IC mode, tapping mode) atomic force microscopy are not accessible to a direct measurement. For an estimation of these forces, a simple analytical model is developed by considering only the shift of the cantilever resonance frequency caused by Hertzian (contact) forces. Based on the relationship between frequency shift and tip–sample force for large-amplitude frequency-modulation atomic force microscopy, amplitude and phase versus distance curves are calculated for the intermittent contact mode, and the forces on the substrate are calculated. The results show a qualitative agreement with numerical calculations, yielding typical maximal forces of 50–150 nN. When working above the unperturbed resonance, forces are found to be significantly larger than below the resonance.     

Effect of nitrogen doping on nanomechanical and surface properties of silicon film

The effect of nitrogen ion implantation on the nanomechanical properties of single crystal Si was evaluated by means of a conventional Vickers indentation and nanoindentation tests. The images of Si surfaces before and after nitrogen implantation were observed and their average surface roughnesses were measured by an atomic force microscope (AFM), while the changes in the morphology and microstructure of the single crystal Si by N implantation were examined by field emission scanning electron microscope (SEM) and X-ray diffractometer (XRD). In addition, the hydrophilic/hydrophobic surface property of the N-doping Si film was determined from the measurement of water contact angle by the sessile drop technique. Furthermore, the effects of the doping energy on the surface contact angle and the surface roughness and the Vickers hardness of the film are also investigated.     

Wetting of a self-assembled 1,4-benzenedimethanethiol monolayer on gold and silver: An adhesion force spectroscopy study

Using atomic force microscopy we investigated how local capillary phenomena are affected by the deposition of a self-assembled 1,4-benzenedimethanethiol (BDMT) layer on epitaxially grown Au(1 1 1) and Ag(1 1 1) films. Force-distance curves monitored at varying relative humidity show clear differences in the adhesion forces on the different samples, which can be explained in terms of a change in the wetting behavior due to the presence of the molecules. Moreover, we found that not only the chemical structure of the molecules but also their orientation strongly influences the strength of the capillary forces. A detailed analysis of the measurements shows that condensation of water vapor on Au(1 1 1) films is drastically enhanced due to the vertically aligned BDMT molecules, while on Ag(1 1 1) water condensation is reduced due to a parallel molecule orientation.     

Kinetic effects in the self-assembly of pure and mixed tetradecyl and octadecylamine molecules on mica

The self-assembly of tetradecylamine (C14) and of mixtures of tetradecyl and octadecylamine (C18) molecules from chloroform solutions on mica has been studied using atomic force microscopy (AFM). For pure components self-assembly proceeds more slowly for C14 than for C18. In both cases after equilibrium is reached islands of tilted molecules cover a similar fraction of the surface. Images of films formed by mixtures of molecules acquired before equilibrium is reached (short ripening time at room temperature) show only islands with the height corresponding to C18 with many pores. After a long ripening time, when equilibrium is reached, islands of segregated pure components are formed.     

Determination of the nanoscale dielectric properties in ferroelectric lead zirconate titanate (PZT) thin films

We report on nanoscale experiments with <100 nm lateral resolution being able to differentiate the effective dielectric polarisation P_z, deposited charge density σ, surface dielectric constant ε_surface, its voltage dependence ε_surface(U), as well as the built-in electric bias voltage U_int in ferroelectric lead zirconate titanate (PZT) thin films. This is possible by combining piezoresponse force microscopy (PFM) and pull-off force spectroscopy (PFS), both methods based on scanning force microscopy (SFM). The differentiation becomes possible since both P_z and σ contribute additively in PFS, while they are subtractive in PFM, hence allowing the two contributions to be separated. ε_surface can be quantified by means of the experimental PFS data and the calculated effective penetration depth of PFM developed in a finite element modelling. Finally, U_int and ε_surface(U) are derived by an absolute matching of the P_z values measured by PFM and PFS.Our nanoscale results obtained on PZT thin films reveal values for the above specified quantities that have the same order of magnitude as those obtained from macroscopic measurements reflecting the integral response using large electrode areas. However, we stress that the data reported here reveal physical properties deduced on the nanometer scale. Furthermore, they are recorded during one single experimental investigation, using one single set-up only.     

Preparation and characterization of MgO stepped surfaces

We present a study of the preparation procedure for stepped MgO surfaces which can be used as templates for the deposition of metallic nanostructures. A cleaved sample of MgO(0 0 1) was mechanically polished to reach the desired miscut angle along the [1 1 0] direction. Then a thermal annealing was performed. The effect of an intermediate chemical etching has been also studied. The surface was analyzed by means of contact AFM in air, LEED and XPS in UHV. The role of the chemical etching and the dependence of the final morphology on the annealing time and temperature were investigated. The influence of the miscut angle on the final surface topography is also briefly discussed.     

Fabrication of rubrene nanowires on vicinal (0 0 0 1) sapphire surfaces

Morphology of high-vacuum deposited rubrene thin films on the annealed (0 0 0 1) vicinal sapphire surfaces was studied by atomic force microscopy in non-contact mode. Atomic force microscopy images of rubrene thin films indicate that a regular array of steps on the sapphire surface acts as a template for the growth of the arrays of rubrene nanosize wires. To further demonstrate that morphological features of a substrate are crucial in determining the morphology of rubrene layers we have grown rubrene on the sapphire surfaces that were characterized by the terrace-and-step morphology with islands. We have found preferential nucleation of rubrene molecules at the intersection between a terrace and a step, as well as around the islands located on terraces.     

AFM and SNOM characterization of carboxylic acid terminated silicon and silicon nitride surfaces

Silicon and silicon nitride surfaces have been successfully terminated with carboxylic acid monolayers and investigated by atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM). On clean Si surface, AFM showed topographical variations of 0.3-0.4 nm while for the clean Si₃N₄ surface the corrugation was around 3-4 nm. After material deposition, the corrugation increased in both samples with a value in topography of 1-2 nm for Si and 5-6 nm for Si₃N₄. The space distribution of specific chemical species was obtained by taking SNOM reflectivity at several infrared wavelengths corresponding to stretch absorption bands of the material. The SNOM images showed a constant contribution in the local reflectance, suggesting that the two surfaces were uniformly covered.     

Growth instability and surface phase transition of Ti thin film on Si(1 1 1): An atomic force microscopy study

Evolution of surface structure during the annealing of e-beam evaporated Ti films is studied by means of atomic force microscopy (AFM). Image variography and power spectral density analysis are used to study scaling properties of the films, ranging from 50 nm to 20 μm length scale. No particular grain size is observed up to 473 K. At 673 K, grain size of ∼250 nm are formed and coalesced to form bigger grain size upon further annealing. At 473 K, RMS roughness dropped at all length scale and became rougher at 673 K with an increasing trend up to 873 K. Clustering at 673 K indicates Kosterlitz-Thaouless [J.M. Kosterlitz, D.J. Thaouless, J. Phys. Chem. 6 (1973) 1181] type phase transition at the surface. The observed transition is also consistent with existing scaling laws.     

Calculation of the optimal imaging parameters for frequency modulation atomic force microscopy

True atomic resolution of conductors and insulators is now routinely obtained in vacuum by frequency modulation atomic force microscopy. So far, the imaging parameters (i.e., eigenfrequency, stiffness and oscillation amplitude of the cantilever, frequency shift) which result in optimal spatial resolution for a given cantilever and sample have been found empirically. Here, we calculate the optimal set of parameters from first principles as a function of the tip–sample system. The result shows that the either the acquisition rate or the signal-to-noise ratio could be increased by up to two orders of magnitude by using stiffer cantilevers and smaller amplitudes than are in use today.     

PZT铁电薄膜纳米尺度铁电畴的场致位移特性

利用扫描力显微术的压电响应模式，并基于逆压电效应原理，研究了梯度组成的PZT铁电薄膜纳米尺度铁电畴的场致位移特性.获得了源于纳米尺度铁电畴的压电效应和电致伸缩效应贡献的场致位移回滞线，以及源于线性压电效应和电畴反转效应综合贡献的纳米尺度压电位移 场强蝶形曲线，证实了Caspari Merz理论在纳米尺度上的有效性.发现了梯度铁电薄膜存在纳米尺度印刻现象，认为该现象的内因源于薄膜中的内偏场. 关键词： PZT铁电薄膜 场致位移 纳米尺度 扫描力显微术     

Soft-landing of size-selected W clusters on highly ordered pyrolytic graphite surfaces: Towards the fabrication of thin films of cluster assemblies

Mass-selected W cluster anions in the size range of 20–3500 are deposited on highly ordered pyrolytic graphite (HOPG) surfaces. The structures of resulting thin films are studied using atomic force microscopy. Clusters with more than ∼600 atoms form thin films, in which the clusters are not completely merged but survive as individual species to a certain extent. The new class of materials fabricated here (cluster-assemblies) has a potential for applications in heterogeneous catalysis and in optoelectronic devices. In addition, they are interesting from the perspective of basic research.