Frictional effects in atomic force microscopy of Langmuir-Blodgett films

Frictional effects in atomic force microscopy (AFM) of Langmuir-Blodgett films of 1, 2-dipalmitoyl-snglycero-phosphoglycerol were examined. Height measurements of the Langmuir layers are strongly influenced by the orientation of the cantilevers used in AFM relative to the sample. A simple model is used to describe the frictional effects and to calculate the real height of the monolayers.     

Adhesion artefacts in atomic force microscopy imaging

Artefacts that affect contrast and arise from adhesion forces in atomic force microscopy images of aramid fibres (both fresh and plasma-treated) are investigated. It is demonstrated that these stem not only from variations in the chemical composition of the surface but also from certain topographical features (which may appear hidden or enhanced in the images), resulting in changes in the lateral forces that are detected by the cantilever and are comparable to the vertical forces. It is also shown that both types of contribution to the forces can be uncoupled to yield images free from these artefacts, thus allowing more accurate quantitative measurements. These artefactual effects are also generally applicable to many other materials.     

Comparative studies of perfluorinated lubricants adsorbed on hydrogenated amorphous carbon and amorphous carbon nitride

Temperature-programmed desorption and scanning force microscopy have been used to probe the interaction of a perfluorinated lubricant (Fomblin ZDOL) with hydrogenated amorphous carbon (a:C-H _x ) and amorphous carbon nitride (a:C-N _x ) substrates, two materials used as hard coatings in disk drive products. Temperature-programmed desorption measurements indicate that the nitride surfaces are more reactive toward this perfluorinated lubricant and, as a result, the thin lubricant film is more tightly bound to this substrate. Frictional force microscopy has been used to measure the coefficient of friction of the lubricated surfaces, 0.18 ± 0.02 for both substrate materials, and finds that frictional properties of these interfaces in the low load regime are influenced more by the presence of the lubricant rather than the adsorbed state of the film. Likewise, similar disjoining pressures were measured for the lubricant adsorbed on the different coating materials and suggest that the ultrathin nature of the adsorbed lubricant film dominates this property rather than adsorption states.     

Nanoscale surface electrical properties of zinc oxide films investigated by conducting atomic force microscopy

In this study, conducting atomic force microscopy was employed to investigate the nanoscale surface electrical properties of zinc oxide (ZnO) films prepared by pulsed laser deposition (PLD) at different substrate temperatures for use as anode materials in polymer light-emitting diodes. The results show that the surface conductivity distribution of ZnO is related to its surface structure. At substrate temperatures of 150-200 degrees C, the conducting regions may cover over 90% of the ZnO thin-film surface, thus providing the best local conductivity. Moreover, heating at substrate temperatures of above 250 degrees C can effectively make the conductivity on the ZnO surface uniform. In particular, at substrate temperatures of around 300 degrees C, the conducting regions where currents are between 1 and 2 muA may cover as much as 83% of the surface, and furthermore, the transmission ratio in the visible range is higher than 80%. This is a rather ideal production temperature for the PLD for ZnO films.     

Dynamic friction measurements with an atomic force microscope on polymer surfaces

The combination of scanning friction force microscopy (SFFM) and lock‐in techniques leads to dynamic SFFM (DSFFM) and provides great advantages in friction force studies with sub‐micrometre resolution. In this paper are presented measurements on thin adsorbed organic films on polymers (polymer blend of 75% poly(allylaminehydrochloride) (PAA) and 25% poly(diallyl‐dimethylammonium chloride) (PDDAC)) and on mica (as a reference). The amplitude and phase response as a function of the excitation amplitude can be explained on hard surfaces by a simple static and dynamic friction model. This model allows us further to distinguish static friction forces and kinetic friction forces in a quantitative way. Furthermore, we demonstrate the use of these spectra to determine the correct modulation amplitude of the excitation to achieve the optimal friction contrasts directly. Polymer data suggest that the viscoelastic shear flow under the atomic force microscope (AFM) tip is responsible for the shape of the phase and amplitude spectrum. Lastly, we demonstrate that DSFFM is a useful technique for surface characterisation in situations where SFFM may not be adequate.     

Polysaccharide properties probed with atomic force microscopy

In recent years, polysaccharides have been extensively studied using atomic force microscopy (AFM). Owing to its high lateral and vertical resolutions and ability to measure interaction forces in liquids at pico‐ or nano‐Newton level, the AFM is an excellent tool for characterizing biopolymers. The first imaging studies showed the morphology of polysaccharides, but gradually more quantitative image analysis techniques were developed as the AFM grew easier to use in aqueous liquids and in non‐contact modes. Recently, AFM has been used to stretch polysaccharides and characterize their physicochemical properties by application of appropriate polymer stretching models, using a technique called single‐molecule force spectroscopy. From application of such models as the wormlike chain, freely jointed chain, extensible‐freely jointed chain, etc., properties such as the contour length, persistence length and segment elasticity or spring constant can be calculated for polysaccharides. The adhesion between polysaccharides and surfaces has been quantified with AFM, and this application is particularly useful for studying polysaccharides on microbial and other types of cells, because their adhesion is controlled by biopolymer characteristics. This review presents a synthesis of the theory and techniques currently in use to probe the physicochemical properties of polysaccharides with AFM.     

Atomic vacancy-induced friction on the graphite surface: observation by lateral force microscopy

Lateral force microscopy has been employed to investigate the frictional behaviour of atomic vacancies on the graphite surface. Such a study was only made possible by the controlled expansion of originally single‐atom vacancies into multiatom vacancies, employing oxygen plasma etching for this purpose. Enhanced friction was observed on the vacancy regions compared with pristine areas of graphite, the origin of which is examined and discussed.     

Adhesion,unfolding forces,and molecular elasticity of fibronectin coatings: An atomic force microscopy study

Fibronectin is an extracellular matrix protein that is involved in cell adhesion, growth, migration, differentiation, and wound healing. Fibronectin coatings are currently used in many laboratories for biomedical and biotechnology purposes. In this study we have investigated the adhesion and mechanical properties of fibronectin coatings. The coatings were also used to study the role of the residence time and the influence of the loading rate in nonspecific interactions. The results showed that the adhesion force between silica and fibronectin increased with loading rate delivering similar values for residence times of 1 and 2 s. Further analysis indicated that the distance to the transition state was about 0.5 nm. Moreover, the adhesion force did not vary with the loading rate for contact time of 0 s. The unfolding of fibronectin domains also depended of the Dwell time (no unfolding events were observed for zero residence time). Applied loads of 2 nN were able to stretch the fibronectin layer up to 200 nm and to unfold the three fibronectin domains, which were similar for a Dwell time of 1 and 2 s. However, the unfolding length increased with loading rate: below 2.5 µm s^?1 the obtained lengths matched the value of FN I (13.5 nm), while for higher speeds the measured values corresponded to the lengths of FN II (18 nm) and FN III (27 nm). This investigation has answered and opened new questions about the mechanical stability and function of fibronectin coatings. The results have also raised theoretical questions about the difference between specific and nonspecific interactions to be addressed in future work.     

C-banding visualized by atomic force microscopy

C-banding is a method used for studying chromosome rearrangements near centromeres and for investigating polymorphisms. In human chromosomes, the C-bands are located at the centromere of all the chromosomes and the distal long arm of the Y chromosome. In this study, we aimed to detect the structural changes in chromosomes during the stages of C-banding by atomic force microscopy. We observed crater-like structures in the chromosomes after 2xSSC (saline sodium citrate) treatment and measured the relative difference between the heights of chromatid and centromere of the chromosomes. Results showed that the relative difference was 3 nm in chromosomes 1, 9, 16, and Y, whereas in the other chromosomes this value was 11.6 nm. After Giemsa staining, the relative difference increased by a factor of 16 in chromosomes 1, 9, 16, and Y. The other chromosomes showed no such increase, which is in accordance with our suggestion that nonhiston proteins associated with DNA in constitutive heterochromatin can make the constitutive heterochromatin resistant to C-banding.     

Micromorphology analysis of TiO2 thin films by atomic force microscopy images: The influence of postannealing

This work describes an analysis of titanium dioxide (TiO₂) thin films prepared on silicon substrates by direct current (DC) planar magnetron sputtering system in O₂/Ar atmosphere in correlation with three‐dimensional (3D) surface characterization using atomic force microscopy (AFM). The samples were grown at temperatures 200, 300, and 400°C on silicon substrate using the same deposition time (30 min) and were distributed into four groups: Group I (as‐deposited samples), Group II (samples annealed at 200°C), Group III (samples annealed at 300°C), and Group IV (samples annealed at 400°C). AFM images with a size of 0.95 μm × 0.95 μm were recorded with a scanning resolution of 256 × 256 pixels. Stereometric analysis was carried out on the basis of AFM data, and the surface topography was described according to ISO 25178‐2:2012 and American Society of Mechanical Engineers (ASME) B46.1‐2009 standards. The maximum and minimum root mean square roughnesses were observed in surfaces of Group II (Sq = 7.96 ± 0.1 nm) and Group IV (Sq = 3.87 ± 0.1 nm), respectively.     

Effects of film thickness and contact load on nanotribological properties of sputtered amorphous carbon thin films

The nanotribological properties of amorphous carbon (a-C) films of thickness in the range of 5-85 nm sputtered on Si(1 0 0) substrates were investigated with a surface force microscope (SFM), using a Berkovich diamond tip of nominal radius of curvature approximately equal to 200 nm and contact (normal) loads between 10 and 1200 μN. The dependence of the friction and wear behaviors of the a-C films on normal load and film thickness was studied in terms of nanomechanical properties, images of scratched surfaces, and numerical results obtained from a previous analytical friction model. The increase of the contact load caused the coefficient of friction to decrease initially to a minimum value and, subsequently, to increase to a maximum value, after which, it either remained constant or decreased slightly. The dominant friction mechanism in the low-load range was adhesion, while both adhesion and plowing mechanisms contributed to the friction behavior in the intermediate- and high-load ranges. Thinner (thicker) a-C films yielded higher (lower) friction coefficients for normal loads less than 50 μN (low-load range) and lower (higher) friction coefficients for normal loads greater than 150 μN (high-load range). Elastic and plastic deformation, microcracking, and delamination of the a-C films occurred, depending on the contact load and film thickness ranges. The reduced load-carrying capacity, relatively low effective hardness (strength) obtained with thinner films, and dominant friction and wear mechanisms at each load range illustrate the film thickness and contact load dependence of the nanotribological properties of the sputtered a-C films.     

Mechanical and tribological properties of amorphous carbon films

The mechanical and tribological properties of amorphous carbon films have been studied in more detail in recent years because these films (a) can be deposited near room temperature, thus allowing film deposition on common engineering alloys (i.e., aluminum and steel) without altering their mechanical properties, and (b) are smooth and conform to surface roughness of the substrate, thus requiring no post deposition processing. In addition, amorphous carbon films exhibit low unlubricated sliding friction in contact with steel and ceramics which is comparable to that of steel against steel in a lubricated contact. The wear resistance of these films is also better than Ti‐based hard coatings. Further improvement in film tribological properties can be achieved by modifying film chemical composition. Because of these attractive features, amorphous carbon films have been evaluated in several applications including automotive, electronic and biomedical engineering. However, environmental factors such as oxygen and humidity have been found to influence tribological properties significantly. This paper reviews the current understanding of the tribological properties of both hydrogenated and non‐hydrogenated amorphous carbon films, the mechanisms responsible for low friction coefficient and identifies areas that require further research. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mechanical properties of L929 cells measured by atomic force microscopy: Effects of anticytoskeletal drugs and membrane crosslinking

To shed light on the architecture of the cytoskeleton, we used the atomic force microscope (AFM) to measure the elasticity, viscoelasticity, and plasticity of L929 cells. The initial elastic response (Young's modulus ～ 4,000 Pa) of the cells to an applied force was followed by a slow compression of the cytoskeleton (τ_1/2 ≈ 10 s). When force application was terminated, the cytoskeleton underwent a sudden partial decompression and a subsequent slow, incomplete recovery. The role of the cytoskeletal elements in cell mechanics was accessed in AFM measurements carried out on cells treated with cytochalasin D, nocodazole, or col-cemid. Cytochalasin D treatment reduced both elasticity (～45%) and cytoplasmic viscosity (～65%), whereas cells treated with nocodazole or colcemid exhibited a marked increase in elasticity (～100%) and a slight increase in viscosity (～15%). The AFM force measurements also provided evidence that the cell membrane and the cytoskeleton are mechanically coupled. Tightly adherent cells were stiffer than cells that were loosely attached. Moreover, cells crosslinked with either glutaraldehyde, 3,3 ‘-dithiobis’sul-fosuccinimidylpropionate] (DTSSP), or Concanavalin A were more rigid than untreated cells. It is of interest that cells crosslinked with Concanavalin A, but not DTSSP, displayed plastic behaviors that may reflect the induction of cytoskeletal reorganization by Concanavalin A.     

Determination of a translocation chromosome by atomic force microscopy

Atomic force microscopy (AFM) has been used to study the translocation involving chromosomes 11 and 13. An amniocentesis procedure was performed at 18 weeks of pregnancy on a familial balanced translocation carrier mother whose karyotype was 46,XX,t(11;13) (q23;q34). After harvesting the tissue cultures, light microscopy studies (LM) have indicated that the fetus had the same translocation. A 0.3 microm gap region on the derivative chromosome 13 was determined by AFM; it was equivalent to a mid-sized G-band. The enhanced resolution of AFM with respect to its line measure analysis and three-dimensional image capture capability has allowed an extension and reconsideration of conclusions about chromosomal aberrations based on the study of LM preparations. In this manner, chromosomal disorders will be studied at nanoscale to help in the planning of new therapy strategies.     

Ultrastructure of Trypanosoma cruzi revisited by atomic force microscopy

Most advances in atomic force microscopy (AFM) have been accomplished in recent years. Previous attempts to use AFM to analyze the organization of pathogenic protozoa did not significantly contribute with new structural information. In this work, we introduce a new perspective to the study of the ultrastructure of the epimastigote form of Trypanosoma cruzi by AFM. Images were compared with those obtained using field emission scanning electron microscopy of critical point dried cells and transmission electron microscopy of negative stained detergent-extracted and air-dried cells. AFM images of epimastigote forms showed a flagellum furrow separating the axoneme from the paraflagellar rod (PFR) present from the emergence of the flagellar pocket to the tip of the flagellum. At high magnification, a row of periodically organized structures, which probably correspond to the link between the axoneme, the PFR and the flagellar membrane were seen along the furrow. In the origin of the flagellum, two basal bodies were identified. Beyond the basal bodies, small periodically arranged protrusions, positioned at 400 nm from the flagellar basis were seen. This structure was formed by nine substructures distributed around the flagellar circumference and may correspond to the flagellar necklace. Altogether, our results demonstrate the importance of the application of AFM in the structural characterization of the surface components and cytoskeleton on protozoan parasites.     

Adhesion and wear of colloidal silica probed by force microscopy

The adhesion and wear of colloidal silica nanoparticles (50 nm diameter) dispersed in a film have been directly studied using atomic force microscopy (AFM) under aqueous solution conditions. The adhesion between surface‐bound silica particles and the AFM tip is found to peak in strength between pH 4 and 5. Using the JKR contact mechanics model, the energy for a single Si–OH/Si–OH interaction was estimated to be 0.4 ± 0.1 kcal/mol. Tribochemical wear of the silica particles, and their displacement from the film, is enhanced at high pH due to the increased facility of silica dissolution and the concomitant increase in attendant inter‐particle repulsion. This revised version was published online in September 2006 with corrections to the Cover Date.     

Investigation of nanoparticles by atomic and lateral force microscopy

Metallic nanoparticles have been produced on vitreous carbon substrates by means of thermal evaporation. From pictures of the particles, made by a high-resolution scanning electron microscope (HRSEM), a semispherical shape is suggested due to the total mass of deposited material. Atomic force microscopy (AFM) has been applied to this sample in order to get direct topographic information. The AFM has been operated with normal and super tips, the latter having a smaller cone angle and radius, thus following more precisely the contours of an object. Simultaneously lateral-force microscopic (LFM) images have been recorded. Major differences between the contents of HRSEM- and AFM-images are considered, emphasizing the important influence of the tips' geometry. Both the AFM and LFM line scans have been compared with and have qualitatively agreed with those calculated under simplifying assumptions.     

Numerical chromosomal abnormalities detected by atomic force microscopy.

The numerical abnormalities of human metaphase chromosomes, fixed according to standard procedures for optical microscopy but not treated for banding, were detected by atomic force microscopy (AFM). High-resolution AFM imaging of chromosomes in trisomy 13, 21, and Klinefelter syndrome can be compared directly with the traditional optical image. The unbanded metaphase chromosomes, including the extra ones in trisomic patients showed a structural pattern very similar to G-banding. Comparison of AFM images with light microscopic data allows the identification of specific chromosomes, and images of chromosomes showing numerical and structural abnormalities can then be analysed.     

The mechanism of G-banding detected by atomic force microscopy

The morphologic changes occurring in human chromosomes during G-banding by trypsin treatment on the same metaphase were followed with the aid of an atomic force microscope (AFM). It was found that trypsin treatment alone caused a pattern of collapse in the chromosomes that was clearly dependent on the duration of trypsinization. The progressive pattern of collapse first indicated the loss of internal differentiation between chromatids, then bands, and finally all internal structures, except for edges running around the chromosomes' perimeter. When stained with Giemsa, the collapsed chromosomes partly regained their original form, and transverse ridges appeared that correspond to G-positive band regions. However, the treatment of fixed chromosomes with trypsin for 42 s diminished the chromosomal edges, and the z-dimensions could not be measured even with the subsequent application of Giemsa.