Peter Zhdan

Accurate force measurement in the atomic force microscope: a microfabricated array of reference springs for easy cantilever calibration

Calibration of atomic force microscope (AFM) cantilevers is necessary for the measurement of nanonewton and piconewton forces, which are critical to analytical applications of AFM in the analysis of polymer surfaces, biological structures and organic molecules. We have developed a compact and easy-to-use reference artefact for this calibration. This consists of an array of dual spiral-cantilever springs, each supporting a polycrystalline silicon disc of 170 µm in diameter. These were fabricated by a two-layer polysilicon surface micromachining method. Doppler interferometry is used to measure the fundamental resonant frequency of each device accurately. We call such an array a microfabricated array of reference springs (MARS).These devices have a number of advantages. Firstly, modelling the fundamental resonant frequencies of the devices is much more straightforward than for AFM cantilevers, because the mass and spring functions are isolated in different parts of the structure. Secondly, the spring constant of each spring is in linear proportion to the mass of the device, given that the resonant frequency is measured accurately. The thickness and hence the mass can be measured accurately by AFM or interferometry.The array spans the range of spring constant important in AFM, allowing almost any AFM cantilever to be calibrated easily and rapidly. The design of the MARS makes it much less sensitive to uncertainties in its dimensions, which is expected to lead to an improvement, in principle, of approximately a factor of three compared to the most accurate previous methods of spring constant calibration, because the spring constant is proportional to the a critical thickness (after resonant frequency has been measured) rather than the cube of a critical thickness, as for a reference cantilever.

Surface characterization of micrometre-sized, polypyrrole-coated polystyrene latexes: verification of a ‘core–shell’ morphology

Micrometre-sized, polypyrrole-coated polystyrene latexes with various conducting polymer loadings have been extensively characterized using X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), Raman and UV–VIS reflectance spectroscopy, scanning force microscopy (SFM) and scanning electron microscopy (SEM). Both XPS and TOF-SIMS studies are consistent with relatively uniform, chloride-doped polypyrrole overlayers. Raman studies also indicated a ‘core–shell’ morphology since only bands attributable to polypyrrole were observed; no evidence was found for the underlying polystyrene component even at the lowest polypyrrole loadings. This is most likely due to remarkably efficient attenuation of the polystyrene bands by the highly absorbing polypyrrole overlayer. UV–VIS reflectance spectroscopy studies confirmed that a coated latex had a much lower reflectance (higher absorbance) than a heterogeneous admixture of polypyrrole and polystyrene with a similar polypyrrole content. High-resolution images of the polypyrrole overlayer nanomorphology were obtained using SFM. At low polypyrrole loadings (1.0 mass%) the overlayer was relatively smooth and uniform, but higher loadings (8.9 mass%) resulted in a rougher, more globular morphology. Finally, the underlying polystyrene latex ‘core’ was quantitatively removed by solvent extraction. SEM studies of the polypyrrole residues revealed a ‘broken egg-shell’ morphology, thus providing irrefutable evidence for the ‘core–shell’ morphology of the original polystyrene/polypyrrole particles.

A solvent effect on the morphology of PMMA-coated polypyrrole surfaces

This paper reports on the morphology of poly (methyl methacrylate) (PMMA) films cast from chloroform (CHCl3) and tetrahydrofuran (THF) onto tosylate-doped polypyrrole (PPyTos) surfaces. We have examined the bare and PMMA-coated PPyTos by X-ray photoelectron spectroscopy (XPS) to determine the surface composition and by atomic force microscopy (AFM) to characterize the surface morphology. We have found that the polymer coating morphology depends significantly on the nature of the solvent. Indeed, the basic PMMA film cast from CHCl3 (an acidic good solvent) is much smoother than that cast from THF (a basic poor solvent). We interpret our results in terms of acid-base properties of the different species involved in the polymer film formation, i.e. the substrate, and the solute and solvent of the polymer solution.

Gelatine and Gelatine/Elastin Nanocomposites for Vascular Grafts: Processing and Characterization

This study involves the preparation, microstructural, physical, mechanical, and biological characterization of novel gelatine and gelatine/elastin gels for their use in the tissue engineering of vascular grafts. Gelatine and gelatine/elastin nanocomposite gels were prepared via a sol-gel process, using soluble gelatine. Gelatine was subsequently cross-linked by leaving the gels in 1% glutaraldehyde. The cross-linking time was optimized by assessing the mass loss of the cross-linked gels in water and examining their mechanical properties in dynamic mechanical tests. Atomic force microscopy (AFM) studies revealed elastin nanodomains, homogeneously distributed and embedded in a bed of gelatine nanofibrils in the 30/70 elastin/gelatine gel. It was concluded that the manufactured nanocomposite gels resembled natural arteries in terms of microstructure and stiffness. The biological characterization involved the culture of rat smooth muscle cells (SMCs) on tubular gelatine and gelatine/ elastin nanocomposite gels, and measurements of the scaffold diameter and the cell density as a function of time.

A combined atomic force microscopy (AFM)/X-ray photoelectron spectroscopy (XPS) study of organosilane molecules adsorbed on the aluminium alloy L157-T6

Organosilanes are extensively used to promote the adhesion of surface coatings and adhesives to different substrates, including metals. The surface concentration and spatial distribution of the organosilane on a particular material are important factors in determining its efficiency as an adhesion promoter. The powerful combination of atomic force microscopy (AFM) with X-ray photoelectron spectroscopy (XPS) was adopted to characterise the adsorption of aminopropyltriethoxysilane (APS) on the Al alloy L157-T6. Complementary information on the distribution of the adsorbed APS was obtained using the force–volume (FV) approach in AFM. The combined AFM/XPS study revealed that the coverage and stuctural integrity of the APS film were dependent on the concentration of the silane in solution. Greater coverage and better structural integrity were obtained by adsorbing the APS film from a 3% silane (in methanol) solution than from a 6% solution.

Atomic force microscopy imaging of chloride ions adsorbed on etched polycrystalline copper in dilute HCl

In this paper, we study surface atomic structures of etched polycrystalline copper in dilute HCl using contact force mode atomic force microscopy (AFM). The microstructures, i.e. the average grain size, the surface roughness and the section analysis, were investigated before examination at an atomic level. For the present work, we selected for investigation the topmost grain, assumed to be fcc{111}, which is known to be the most stable face thermodynamically. The hexagonal structure of the (√3 × √3)R30°-Cl is well resolved. The imperfections of crystal grains, e.g. dislocations, steps and kinks of the adsorbed layer that are a reflection of the underlying substrate, are also revealed. Moreover, the dependence of corrugation amplitudes on the contact forces, which was anticipated from previous simulation results, is confirmed. Our experimental result has shown that the corrugation amplitude decreased as the imaging set point (equivalent to the contact force) is lowered, i.e. as the set point is decreased from −3.0 to −5.0 V the corrugation amplitude reduces from 0.46 ± 0.12 to 0.27 ± 0.07 A. Copyright

In Situ Electrochemical–AFM and Cluster-Ion-Profiled XPS Characterization of an Insulating Polymeric Membrane as a Substrate for Immobilizing Biomolecules

The electrochemical oxidation of ortho-aminophenol (oAP) by cyclic voltammetry (CV), on platinum substrates in neutral solution, produces a polymeric film (PoAP) that grows to a limiting thickness of about 10 nm. The insulating film has potential use as a bioimmobilizing substrate, with its specificity depending on the orientation of its molecular chains. Prior investigations suggest that the film consists of alternating quinoneimine and oAP units, progressively filling all the platinum sites during the electrosynthesis. This work concerns the evaluation of the growth orientation of PoAP chains, which until now was deduced only from indirect evidence. Atomic force microscopy (AFM) has been used in situ with an electrochemical cell so that PoAP deposition on a specific area can be observed, thus avoiding any surface reorganization during ex situ transport. In parallel with microscopy, XPS experiments have been performed using cluster ion beams to profile this film, which is exceptionally thin, without damage while retaining molecular information.

Observations on the full honeycomb structure of graphite as imaged by atomic force microscopy

Abstract Most atomic force microscopy (AFM) images of highly ordered pyrolytic graphite (HOPG) yield only every other atom with a lattice constant 2.46Å. However, its full honeycomb structure, revealing all six carbon atoms of each hexagon, can only be obtained under certain conditions. A particular problem is the large meniscus force generated by a thin condensed layer of water when imaging under ambient conditions. We show that this can be reduced within an enclosure box by self-heating by the piezoelectric scanner and optical system. This was confirmed by obtaining a sequence of consecutive capillary force–distance loops. We have found all six carbon atoms in simultaneous imaging of trace and retrace scan direction although the contrast of the image is inverted according to the direction of the scan. The atomic corrugations at various contact forces were also examined. We observed giant corrugation amplitudes of graphite by AFM, which were as large as 5Å. This anomalous effect, together with the inversion of contrast, was attributed to a combination of the elastic deformation of normal movement and the ‘stick–slip’ process from the harder lateral lattice of graphite. The honeycomb structures generated at high forces show clearly a warped structure with high (B site) and low (A site) carbon atoms. We have interpreted this as evidence of a surface diamond-like structure of HOPG induced by the probing tip.

Polymer coatings on conductive polypyrroles surface characterization by XPS, ToFSIMS, inverse gas chromatography and AFM

The study of PMMA adsorption on some conducting polypyrroles (PPys) using a variety of surface analytical techniques is reported. PMMA adsorption was monitored by X‐ray photoelectron spectroscopy (XPS), time of flight secondary ion mass spectroscopy (ToF‐SIMS) and inverse gas chromatography (IGC). XPS and ToF‐SIMS permit to determine the surface composition of PMMA‐coated PPy surfaces vs the solvent nature, temperature and the PPy dopant anion. Both techniques show that acid‐base interactions may govern PMMA adsorption. IGC was used to determine the coating morphology by monitoring the surface energy of the coated PPy powders. It is suggested that homogeneity of PMMA coatings increases with decreasing solvent power. Preliminary atomic force microscopy (AFM) results on PMMA films cast on flat PPy surfaces confirm the IGC observation.

Gelatine‐Hydroxyapatite Nano‐composites for Orthopaedic Applications

This study focuses on the preparation and testing of hydroxyapatite‐gelatine nano‐composite gels via a sol‐gel route and in situ formation of hydroxyapatite (HA) type salts. Four types of gels and foams were prepared with Ca/P molar ratio of 0.43 and 0.86 and hydroxyapatite/gelatine weight ratio of 0.50 and 0.70. Cross‐linking of gelatine chains was carried out in 1% glutaraldehyde and dynamic mechanical torsion tests were used to measure the viscoelastic properties of the gels and foams, optimize the cross‐linking time and assess their mechanical performance. Optical and atomic force microscopy were used to investigate the micro‐ and nano‐structure of the produced composites: in these studies, it was confirmed that the gels were nano‐composites with a nano‐structure very similar to that of bone and several similarities in the microstructural features. The best foams incorporated dual pore size distribution.