AFM Scans Research Articles

The Effect of Ectoins on the Structural Organization of the Tear Fluid Monolayer

We investigate the influence of Ectoin on the structural organization of the natural and artificial tear fluid lipid layers (ATFLL) using surface activity analysis and topographical studies. The natural meibomian lipids exhibit a continuous pressure-area isotherm without any phase transitions. In the presence of ectoin, the isotherm is expanded towards higher area per molecule implying decreased interaction between the lipid molecules. The AFM scans show presence of fiber like structures in the natural meibomian lipid film. In the presence of ectoine, droplet-like structures are observed which are hypothesized to be triacyl glycerols excluded from the lipid film. ATFLL illustrate the fluidizing effect of ectoine on the lipid films where the pressure-area isotherms are expanded in the presence of ectoin. With the addition of a triacyl glycerol to the mixture of DPPC and Chol-Palmitate, we observed the formation of similar drop-like structures in the presence of ectoine as in the case of natural meibomian lipid films. Consequently, the hypothesis explaining the exclusion of tri/di acyl glycerol from the meibomian lipid film in the presence of ectoine in the subphase is confirmed which lead us to a model describing the fluidizing effect of ectoine on meibomian lipid films.

Pathogenic Serum Amyloid A 1.1 Shows a Long Oligomer-rich Fibrillation Lag Phase Contrary to the Highly Amyloidogenic Non-pathogenic SAA2.2

Serum amyloid A (SAA) is best known for being the main component of amyloid in the inflammation-related disease amyloid A (AA) amyloidosis. Despite the high sequence identity among different SAA isoforms, not all SAA proteins are pathogenic. In most mouse strains, the AA deposits mostly consist of SAA1.1. Conversely, the CE/J type mouse expresses a single non-pathogenic SAA2.2 protein that is 94% identical to SAA1.1. Here we show that SAA1.1 and SAA2.2 differ in their quaternary structure, fibrillation kinetics, prefibrillar oligomers, and fibril morphology. At 37 °C and inflammation-related SAA concentrations, SAA1.1 exhibits an oligomer-rich fibrillation lag phase of a few days, whereas SAA2.2 shows virtually no lag phase and forms small fibrils within a few hours. Deep UV resonance Raman, far UV-circular dichroism, atomic force microscopy, and fibrillation cross-seeding experiments suggest that SAA1.1 and SAA2.2 fibrils possess different morphology. Both the long-lived oligomers of pathogenic SAA1.1 and the fleeting prefibrillar oligomers of non-pathogenic SAA2.2, but not their respective amyloid fibrils, permeabilized synthetic bilayer membranes in vitro. This study represents the first comprehensive comparison between the biophysical properties of SAA isoforms with distinct pathogenicities, and the results suggest that structural and kinetic differences in the oligomerization-fibrillation of SAA1.1 and SAA2.2, more than their intrinsic amyloidogenicity, may contribute to their diverse pathogenicity.

High Density Cobalt Nanostructures by Ion Beam Induced Dewetting

Ion beam induced dewetting of thin metallic films has emerged as a promising way to grow metallic nanoparticles in a controlled manner. Metal films tend to dewet from non-reactive substrates upon heating, forming islands, due to the high energies of metal surfaces and interfaces. Ion beam irradiation triggers the self organized formation of nanostructures, and allows control over nanoparticle size distributions, since dewetting is initiated by highly localized (in space and time) thermal spikes along the ion track. From 25 nm cobalt films grown by e-beam evaporation, followed by irradiation with 100 keV Ar+ ions, a high density array of nanostructures is formed. AFM scans show their morphological evolution as a function of ion fluence; optimum fluences for narrow particles size distribution have been identified. Glancing angle XRD and Rutherford Backscattering Spectroscopy show evidence of increased substrate exposure as dewetting proceeds. RBS indicates that within the optimal fluence range for nanostructure formation, sputtering and ion beam mixing are not the significant effects in this system. We present ion beam induced dewetting as a well controlled method of forming nanostructured catalysts from metal films. [DOI: 10.1380/ejssnt.2013.99]

Silk fibers and silk‐producing organs ofHarpactea rubicunda(C. L. Koch 1838) (Araneae, Dysderidae)

Scanning electron microscopy and atomic force microscopy were used to study the silk spinning apparatus and silks of Harpactea rubicunda spiders. Three types of silk secretions that are produced by three kinds of silk spinning glands (ampullate, piriform, and pseudaciniform) and released through three types of spigots, were confirmed for both adult and juvenile spiders. Silk secretions for the construction of spider webs for shelter or retreat are produced by the pseudaciniform silk glands. Silk secretions that are released from spigots in the course of web construction are not processed by the legs during the subsequent process of hardening. Pairs of nanofibril bundles seemed to be part of the basic microarchitecture of the web silk fibers as revealed by AFM. These fiber bundles frequently not only overlap one another, but occasionally also interweave. This structural variability may strengthen the spider web. High-resolution AFM scans of individual nanofibrils show a distinctly segmented nanostructure. Each globular segment is ∼30-40 nm long along the longitudinal axis of the fiber, and resembles a nanosegment of artificial fibroin described by Perez-Rigueiro et al. (2007).

Importance of the dynamics of adsorption and of a transient interfacial stress on the formation of aggregates of IgG antibodies

It is common knowledge that aggregation of proteins may occur in aqueous solutions under mechanical stress (shaking or high shear), even in solutions that are stable at rest. Addition of surfactants is a practical generic means to prevent this stress-induced aggregation (e.g. in formulations of therapeutic proteins), which suggests that interfaces contribute to destabilization. We studied here the role of interfacial stress by applying brief mechanical impacts on the air–water interface, in the presence or absence of surfactants, in solutions of immunoglobulin G (IgG), a class of proteins of high importance to the developments of new therapeutics. A variety of surfactants was tested including the neutral ones Tween80, C10–C14 fos-cholines, alkylaminoxide, surfactin, and two ionic ones, TTAB and lauroylsarcosine sodium salt. We determined the presence of aggregates in solution by light scattering. Irrespective of the type of antibody, either human polyclonal or a monoclonal one, we show that the amount of aggregated IgG increases in proportion to the number of impacts on the interface. In the absence of stress, we recorded images of oblate aggregates of IgG (ca. 12 nm height and 200–1200 nm diameter) present at the air–water interface (fluorescence microscopy using anti-Fab or anti-Fc markers, and AFM scans after transfer on freshly cleaved mica). Our results evidence that aggregates are formed at the air–water interface, and are brought in solution by transient stresses applied on the water surface. Rupture of interfacial films is an important source of aggregates in solution. Finally, the role of surface dynamics in the protection brought by surfactants is discussed based on the comparison of protective efficiencies with dynamic surface tension properties (measured by the maximum bubble pressure method). Our work indicates that better protection is conferred by surfactants showing the faster interfacial dynamics, which corresponds also to conditions of faster lowering of the interfacial energy at a short time scale.

Registration of AFM and SEM Scans Using Local Features

Surface investigations combining the atomic force microscope and scanning electron microscope benefit from the complementarity of both technologies. For the analysis and visualization of a pair of corresponding scans, identifying their spatial alignment is essential. This article presents an automatic registration scheme based on finding correspondences between local image features. The result is then refined by maximizing a similarity measure between the aligned scans. Two feature extraction algorithms have been integrated and tested: Scale-invariant feature transform and speeded-up robust feature. For the experimental evaluation, five types of sample objects showing a variety of different surface structures have been used. The registration scheme succeeds in all application scenarios.

Elastic characterization of Au thin films utilizing laser induced acoustic Rayleigh waves

Wide frequency-band Rayleigh waves (~100 MHz) were utilized to characterize the elastic constants of thin Au/Cr films deposited on glass substrates. The Rayleigh waves were excited utilizing laser induced thermoelastic mechanism and detected using a knife-edge technique apparatus. The dispersion of the signals in glass substrates coated with Au/Cr was measured and fitted to theory using a non-linear regression algorithm. From the fitting, the Au films Young modulus and the film thickness were extracted. The results were analyzed with regards to AFM scans performed on the samples and independent thickness measurement done by a dektak3 profiler. Results show a good agreement between the two measurements.

Evaluation of morphology and deposits on worn polyimide/graphite composite surfaces by contact-mode AFM

Sintered polyimide/graphite composite cylinders were worn in a macroscale line-on-plate contact at different temperatures, revealing instable and relatively high coefficients of friction at 23 < T < 100 °C, decreasing coefficients of friction at 100 < T < 180 °C and constant values for coefficients of friction at 180 < T < 260 °C. The tendencies correspond with a transition from low wear rates at 23 < T < 100 °C into high and stable wear rates at 180 < T < 260 °C. These observations suggest that the lubricating effect of graphite does not prevail at all temperatures. Contact-mode atomic force microscopy of the worn surfaces was used for getting insight in the sliding processes at a micro- to nanoscale. The friction signals provide insight in the local distribution of the composite phases over the surface. Selection criterions for surface patterns representing optical interferences or shear banding patterns, were based on fast Fourier transform images. At low temperatures, severe wear grooves and shear bands within the graphite phase indicate the abrasiveness of graphite additives together with inhomogeneous graphitic transfer onto the polyimide surface areas. At intermediate temperatures, a very thin and discontinuous graphite film forms while the imide zones show a finely structured surface with homogeneous mixture of polyimide grains and graphitic particles. At high temperatures, the surfaces are relatively smooth and homogeneous with a thin graphite film covering the entire surface. The relation between coefficients of friction and surface morphology is most significantly quantified in the evolution of roughness parameters determined from AFM scans.

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.

Structural Transformation upon Nitrogen Doping of Ultrananocrystalline Diamond Films by Microwave Plasma CVD

The molecular properties and surface morphology of undoped and N‐doped ultra‐nanocrystalline diamond (UNCD) films deposited by microwave plasma CVD with addition of nitrogen are investigated with various spectroscopic techniques. The results of spatially resolved Raman scattering, ATR/FT‐IR and XPS spectra show more amorphous and sp2/sp3 ratio characteristics in N‐doped UNCD films. The surface morphology in AFM scans shows larger nanocrystalline diamond clusters in N‐doped UNCD films. Incorporation of nitrogen into UNCD films has promoted an increase of amorphous sp2‐bonded carbons in the grain boundaries and the size of nanocrystalline diamond grains that are well correlated to the reported enhancement of conductivity and structural changes of UNCD films.

Mechanical characterization of PEDOT:PSS thin films

By tensile testing the mechanical properties of thin films of the intrinsically conductive poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) (PEDOT:PSS) under different relative humidities are investigated. It can be shown that the fracture behaviour strongly depends on humidity and reaches from brittle to plastic. The fracture surfaces are first investigated by scanning electron microscopy (SEM). The surfaces change from smooth at 23% rH to rough with shear lips for samples tested at 55% rH. Atomic force microscopy then reveals the topography of fracture surfaces at the nanometer scale and thus gives insights into the morphology of PEDOT:PSS thin films. By combining the experimental findings of the tensile tests and the AFM scans a micromechanical model for the deformation behavior of PEDOT:PSS can then be derived.

Selective Epitaxial Growth of Ge-on-Si for Photodiode Applications

The goal of this work is to study and optimize the growth parameters for Ge-on-Si for photodiodes operating at 1.55 um. Approximately 1 um-thick, relaxed Ge is grown in exposed Si regions on oxide-patterned Si wafers. Germanium selectivity, faceting, morphology, and threading dislocation density are investigated as a function of growth and processing conditions. Ge faceting was reduced by increasing the growth temperature and germane partial pressure, and at the optimized growth condition of 750{degree sign}C and 10T, an RMS surface roughness of 1.3 nm was obtained for 10 x 10 um AFM scans. Threading dislocation density was reduced for structures smaller than 5 um, but the dislocation density depends on post-growth annealing conditions and Ge film thickness.

Direct observation of PFPE lubricant molecules by cryogenic AFM under ultra-high vacuum

Surface lubrication is one of the essential technologies in modern magnetic disk systems and improvement of the surface lubrication is very important in the development of next generation systems. In this study, we used AFM for the direct observation of perfluoropolyether (PFPE) lubricant molecules on atomically flat surfaces. We used a cryogenic non-contact AFM to observe the molecules in a frozen state of micro-Brownian motion of PFPE segments, because the glass transition temperature of PFPE is very low. To avoid freezing a trace amount of water vapor on the sample surface at liquid nitrogen temperatures, the AFM observation was performed under ultra-high vacuum. We observed that on a gold surface the size of the molecules increases with repeated AFM scans. This is because the mechanical stimulus causes the fusion of PFPE lubricant molecules to form reversed micelles at the non-polar surface. At a hydrophilic silicon wafer surface, however, we succeeded in observing single lubricant molecules. This is because almost all PFPE lubricant molecules are fixed to the hydrophilic solid surface by polar–polar bond formation and they cannot move around on the surface and thus they cannot fuse to each other. As formation of the reversed micelle structure is a rather general phenomenon in the PFPE lubricant thin layer at non-polar surfaces, we also will discuss briefly the expected molecular structures of PFPE lubricants at the surface of the carbon overcoat of magnetic disks.

Nanoscale stripe structures in SmBCO superconductors

AFM and STM scans on SmBa2Cu3Ox (SmBCO) melt processed samples revealed nanoscale stripe-like structures, sometimes parallel over several micrometers, sometimes wavy. These structures consist of chemical compositional fluctuations and act as effective δTc pinning centres due to their wavelength of typically 10-60 nm which is comparable to the ideal pinning center size 2ξ (∼10 nm for YBa2Cu3Ox in the ab-plane). Compared to similar structures in ternary (Sm, Eu, Gd)Ba2Cu3Ox (SEG) and (Nd, Eu, Gd)Ba2Cu3Ox (NEG) systems, where the stripes appear either as plateau-like stripes or as chains of aligned clusters, the stripes in SmBCO always appear as plateau-like stripes with a height of 1Å–8Å. These pinning structures throughout the whole sample volume may be a key to improve critical current densities especially at high external magnetic fields.

Cover Picture: Plasma Process. Polym. 2/2006

Cover: AFM scans (5µm × 5µm) of 500 nm thick films as deposited on Si from processes a) RFICP (full vertical scale: 12.6 nm), b) MIRA (full vertical scale: 44.9 nm), and c) DECRP (full vertical scale: 27.7 nm). Further details can be found in the Full Paper by P. Supiot,* C. Vivien, A. Granier, A. Bousquet, A. Mackova, D. Escaich, R. Clergereaux, P. Raynaud, Z. Stryhal, and J. Pavlik on page 100.

Atomic force microscopic characterization of films grown by inverse pulsed laser deposition

Abstract Carbon nitride films have been deposited by KrF excimer laser ablation of a rotating graphite target in 5 Pa nitrogen ambient in an inverse pulsed laser deposition configuration, where the backward motion of the ablated species is utilised for film growth on substrates lying in the target plane. Topometric AFM scans of the films, exhibiting elliptical thickness distribution, have been recorded along the axes of symmetry of the deposition area. High resolution AFM scans revealed the existence of disk-like, or somewhat elongated rice-like features of 5–10 nm average thickness and ∼100 nm largest dimension, densely packed over the whole, approximately 14 × 10 cm 2 deposition area. The RMS roughness of the film decreased from 9 nm near to the laser spot down to 2 nm in the outer regions. Even the highest RMS value obtained for IPLD films was less than half of the typical, 25 nm roughness measured on simultaneously deposited PLD films.

Growth responses of ultrathin CN x overcoats to process parameters

Ultrathin CN x overcoats were grown using pulsed dc magnetron sputtering. Substrates were mounted on a holder that allowed 45° tilt angle and rotation. Effects of process parameters on film growth were reviewed. AFM scans over large sampling areas show that thin CN x films obtained at − 100 V substrate bias with 45° substrate tilt and 20–25 rpm rotation have r.m.s. roughness about 0.2–0.3 nm when sampled over 20 × 20 μm 2 areas, increasing to ∼ 0.45 nm when sampled over ∼ 0.05 × 3 cm 2 using X-ray reflectivity measurements. These 1–2 nm thick ultrasmooth coatings reduced corrosion damage compared with coatings of the same thickness grown without substrate tilt and rotation. This improved performance is likely a result of more efficient and uniform momentum transfer parallel to the surface during deposition in this configuration. In addition, detailed X-ray reflectivity measurements showed that the mass density of these CN x films is ∼ 2.0 g/cm 3, independent of film thickness from ∼ 1 to 10 nm, consistent with ion beam analysis.

Nucleation and growth of calcium phosphate on amine-, carboxyl- and hydroxyl-silane self-assembled monolayers

Upon implantation, calcium phosphate (Ca–P) surfaces form on materials that are bone bioactive. In this study, the evolving surface characteristics associated with calcium phosphate precipitation are modeled using self-assembled monolayers (SAMs), in a one-step nucleation process. SAMs were used to create amine (–NH 2), carboxyl (–COOH) and hydroxyl (–OH) functionalized surfaces by grafting 3-aminopropyltriethoxysilane, 3-triethoxysilylpropyl succinic anhydride and glycidoxypropyl tri-methoxysilane, respectively, onto oxidized silicon wafers. The SAM surfaces were characterized using ellipsometry to establish the presence of grafted molecules. On the surfaces incubated in simulated physiological fluids for 7 days, the thickness of Ca–P layer grew slowly over the first few hours, increasing strongly between 1 and 5 days and then slowed down again. FTIR showed the dependence of calcium phosphate morphology on the type of surface groups, with stronger P–O bands seen on the OH-terminated surface. SEM analysis showed dispersed Ca–P precipitates on the –COOH and –OH terminated surfaces after 1 day immersion. After 7 days, all SAM surfaces were covered with uniformly dispersed and denser Ca–P precipitates. The underlying Ca–P layer showed cracks on the –NH 2-terminated surface. Rutherford backscattering spectrometry (RBS) data analysis confirmed that Ca/P ratio is in excellent agreement with the theoretical value of 1.67 for hydroxyapatite. X-ray diffraction (XRD) analysis also showed evidence of apatite formation on all the surfaces, with stronger evidence on the –OH-terminated surface. Highly porous Ca–P precipitates were observed on the SAM surfaces portrayed by the AFM scans with nanoscale RMS roughness. Thus, using highly controlled surface chemistry, under physiological conditions, in vitro, this study demonstrates that a hydroxylated surface enhances Ca–P nucleation and growth relative to other surfaces, thereby supporting the concept of its beneficial effect on bone tissue formation and growth.

Modelling and Measurement Uncertainty Estimation for Integrated AFM-CMM Instrument

This paper describes modelling of an integrated AFM - CMM instrument, its calibration, and estimation of measurement uncertainty. Positioning errors were seen to limit the instrument performance. Software for offline stitching of single AFM scans was developed and verified, which allows compensation of such errors. A geometrical model of the instrument was produced, describing the interaction between AFM and CMM systematic errors. The model parameters were quantified through calibration, and the model used for establishing an optimised measurement procedure for surface mapping. A maximum uncertainty of 0.8% was achieved for the case of surface mapping of 1.2×1.2 mm2 consisting of 49 single AFM scanned areas.

Roughness models for particle adhesion

The effects of different surface roughness models on a previously developed van der Waals adhesion model were examined. The van der Waals adhesion model represented surface roughness with a distribution of hemispherical asperities. It was found that the constraints used to define the asperity distribution on the surface, which were determined from AFM scans, varied with scan size and thus were not constant for all surfaces examined. The greatest variation in these parameters occurred with materials that had large asperities or with materials where a large fraction of the surface was covered by asperities. These rough surfaces were modeled with fractals and also with a fast Fourier transform algorithm. When the model surfaces generated using the Fourier transforms are used in the adhesion model, the model accurately predicts the experimentally observed adhesion forces measured with the AFM.