Pulsed Force Mode Research Articles

Lateral organization of G M1 in phase-separated monolayers visualized by scanning force microscopy

Phase separation of glycolipids in lipid mono- and bilayers is of great interest for the understanding of membrane function. The distribution of the ganglioside GM1 in sphingomyelin (SM)/1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine (POPC), SM/1,2-dipalmitoyl- sn-glycero-3-phosphocholine (DOPC) and SM/cholesterol/POPC Langmuir-Blodgett (LB) monolayers transferred at 36 mN/m has been studied by scanning force microscopy. Besides lateral organization of the glycolipid in LB monolayers as deduced from topography, material properties have been investigated by phase imaging, pulsed force mode and force modulation microscopy. It was shown that GM1 preferentially clusters in an ordered lipid matrix, i.e. the SM phase in the case of the SM/POPC and SM/DOPC mixture or in the ordered phase of POPC/SM/cholesterol monolayers. At higher local concentrations, three-dimensional protrusions enriched in GM1 occur, which may represent a precursor for the formation of micelles budding into the aqueous subphase. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00249-002-0232-4.

Structural Evolution of Octyltriethoxysilane Films on Glass Surfaces during Annealing at Elevated Temperature

Glass samples treated by octyltriethoxysilane were annealed for times ranging from 0 to 16 h, and their topography, adhesion, and stiffness properties were examined using pulsed-force mode (PFM) atomic force microscopy. While the surfaces of samples dried at room temperature were structurally featureless, those annealed at 145°C showed formation of islands, increasing in size and number with heating time. PFM adhesion and stiffness maps suggested that the glass substrate remained at least partially covered by silanes even after island formation.

Adhesion imaging of carbon‐fibre‐reinforced materials in the pulsed force mode of the atomic force microscope

AbstractThe microstructure and microscopic mechanical and adhesive properties of carbon‐fibre bundles infiltrated with pyrolytic carbon were investigated using a combination of different scanning force techniques. Contact mode atomic force microscopy imaging was combined with lateral force imaging and adhesion force and local stiffness imaging in the pulsed force mode of the atomic force microscope. A clear material contrast was obtained between carbon fibre and pyrocarbon matrix in the adhesion and stiffness images, as well as in the friction force images, which gives valuable information about the different microscopic properties of the fibre and matrix. The significantly higher adhesive forces determined on the fibres compared with the carbon matrix may be attributed to a higher concentration of polar groups on the fibre cross‐sections due to the high degree of orientation of the graphene layers along the fibre axis. In addition, a much higher degree of spatial variation of adhesive and frictional forces is observed within the fibre compared with the matrix, indicating a higher degree of homogeneity within the pyrocarbon matrix. The data derived from the experiments give microscopic information that is essential for a more detailed understanding of the resulting macroscopic properties of these materials. Copyright © 2002 John Wiley & Sons, Ltd.

Chemical Pulsed-Force Microscopy of Single Polyethyleneimine Molecules in Aqueous Solution

Using scanning probes modified with amino- and methyl-terminated groups in situ offers the possibility of scanning force microscopy (SFM) imaging with enhanced chemical sensitivity. The sample, a positively charged high molecular weight polyethyleneimine (branched), is adsorbed on the negatively charged mica surface to achieve an arrangement of single polyethyleneimine (PEI) molecules. Chemical imaging of individual PEI molecules is revealed by differences in adhesion and contact stiffness obtained from the pulsed-force mode measurements by SFM. We conclude that electrostatic and hydrophobic forces are responsible for the adhesion contrast in the system. The measurements also show a pH dependence based on the type of interaction between the modified tip and the substrate. We demonstrate that the chemical pulsed force microscopy is a suitable tool for the investigation of strongly charged polymers on an oppositely charged surface.

Silica Adhesion on Toner Surfaces studied by Scanning Force Microscopy

After blending with commercial available pyrogenic silicas commercial available toners based on polystyrene-acrylics or polyesters have been studied by Scanning Force Microscopy (SFM). The focus is on the simultaneous mapping of topographic features and adhesion. For this purpose, dynamic SFM techniques like Tapping Mode (TM) and Pulsed Force Mode (PFM) are used for imaging and modeling of toner surfaces and toner-silica particle interactions.Topographic images of toner particles of different toner binder compositions were recorded. In the topographic images the toner surface are mapped as a rough surface. To localize the hard silica particles phase images are used.To study systematically resin surface interactions a model system has been elected: a blend of polystyrene (PS) and polymethylmethacrylate (PMMA) may simulate polystyrene-acrylics toner particle surfaces. Due to its chemical composition and its dimension a Si-SiO2 SFM tip can be used as a model for a silica particle. PFM is a suitable tool to investigate adhesive interactions between this Si-SiO2 tip and the polymer surface. Silanization of a Si-SiO2 tip has been proofed to provide a realistic model of silica particles in a silica toner system.

Detection of Single PSS Polymers on Rough Surface by Pulsed-Force-Mode Scanning Force Microscopy

Samples of nanosize single polymers to be imaged by scanning force microscopy (SFM) are preferentially deposited on a molecularly flat substrate. However, many substrates are modified and acquire rough surfaces resulting in poor definition of single-polymer molecules. This work dealt with the imaging of single poly(sodium 4-styrenesulfonate) (PSS) polymers on a rough surface with a novel measuring mode, pulsed-force-mode (PFM). Adhesive forces can be mapped simultaneously with topographic imaging with this mode. Force−distance curves show the differences of adhesive forces of the tip/sample and the tip/surface couples, so that sample materials can be distinguished in terms of surface adhesive properties. PFM increases the roughness limit of substrates from 0.8 to 6.1 nm, allowing the observation of single molecules down to 1.0 nm height, and greatly expanding potential substrates for imaging of single polymers.

Micro-thermal analysis of polymers: current capabilities and future prospects

The current state of development of micro-thermal analysis (micro-TA) and related techniques are briefly reviewed. Results for a PET/epoxy resin composite and a bilayer polymer film are given as illustrations. Details are given of a new interface that enables the micro-TA unit to be placed inside a conventional FTIR spectrometer to carry out photothermal IR microscopy. New results are presented for a micro-pyrolysis-mass spectroscopy technique. The limitations of the current instrumentation are discussed in terms of the overriding problem being one of spatial resolution. Images obtained using pulsed force mode AFM with a high-resolution heated tip indicate the scope for future development of this technique. The possibility of even higher spatial resolution with other forms of probe are discussed along with the potential for imaging micro-pyrolysis time of flight mass spectroscopy and even tomography. It is concluded that these methods offer excellent prospects for characterising a wide range of polymer systems.

The investigation of sized cellulose surfaces with scanning probe microscopy techniques

Scanning probe microscopy techniques of lateral force microscopy, pull-off force measurement, and pulsed force mode adhesion imaging were employed to study the micro-scale surface chemistry of cellulose films hydrophobized by treatment with alkenyl succinic anhydride (ASA). Lateral force microscopy showed a monotonic decrease in surface frictional dissipation with increasing sizing level. The distributions of pull-off forces measured over a (1μm)2 area showed a wide range of local adhesion values on all samples, yet disappearance of the highest adhesion sites with increasing level of sizing agent. The pulsed force mode results showed high-resolution adhesion images of cellulose that complemented the pull-off force distribution results. These results indicate that scanning probe microscopy techniques are able to detect the wettability heterogeneity of ASA-sized cellulose with nanometer resolution.

Chemical force microscopy of self-assembled monolayers on sputtered gold films patterned by phase separation

Patterned self-assembled monolayers (SAMs) were formed on gold films and observed by friction force microscopy (FFM) and adhesive force mapping with pulsed-force mode atomic force microscopy (PFM–AFM). The substrate gold films were prepared by sputtering gold on flat surfaces of osmium-coated cover glass with surface roughness, R a, of 0.3 nm. The patterned samples with the CH 3 and COOH terminated regions were prepared using the Langmuir–Blodgett (LB) method, partial removal of the LB film by ultrasonication, and SAM formation. The CH 3 and COOH terminated regions of the patterned SAMs in air and in water were observed by mapping friction and adhesive forces with FFM and PFM–AFM, respectively, using gold-coated AFM tips chemically modified with a thiol compound terminating in CH 3 or COOH. The adhesive forces measured in air increased in the order of CH 3/CH 3, CH 3/COOH (or COOH/CH 3), and COOH/COOH, while those in water increased in reverse order. The enormous high adhesive force observed in water for CH 3/CH 3 was attributed to hydrophobic interaction between the CH 3 tip and the CH 3 terminated sample surface. With CH 3 tip, the lower friction force was observed, however, in water on the CH 3 terminated region than on the COOH terminated region. This experimental finding raises a question as to what is the effective normal load in friction measurements in water.

Microthermal Characterization of Segmented Polyurethane Elastomers and a Polystyrene−Poly(methyl methacrylate) Polymer Blend Using Variable-Temperature Pulsed Force Mode Atomic Force Microscopy

Pulsed force mode atomic force microscopy (AFM) has been used to study the morphology of three segmented polyurethane elastomers. High-contrast images are presented, showing their phase-separated structure on a scale of several tens of nanometers to a few hundred nanometers. The adhesion-dependent pull-off force signal is found to be far more sensitive to local variations in mechanical properties than the stiffness-dependent indentation force signal. An insight into the size, shape, and distribution of phases can be obtained from images constructed from spatial variations in surface adhesion properties. Comparison of images obtained both above and below the Tg of the soft-segment material allows the spatial distribution of different phases to be characterized and the effects of surface topography to be decoupled from real changes in localized mechanical properties. Typical domain sizes are found to be at least an order of magnitude larger than those obtained by small-angle X-ray scattering and AFM in prev...

Discrimination of Dendrimer Aggregates on Mica Based on Adhesion Force: A Pulsed Force Mode Atomic Force Microscopy Study

Two different types of aggregated polyphenylene dendrimer molecules, G2Td(COOH)16 and G4-Td, obtained by spin coating a dilute solution onto freshly cleaved mica surfaces, and their adhesion properties were measured by pulsed force mode (PFM) AFM. The adhesion properties could be related to the chemical nature of the outer surface of the dendrimers and the mica surface, and the thin film of water adsorbed on mica when imaged under ambient conditions. Most importantly, from the adhesion image of mixed aggregates of G2Td(COOH)16 and G4-Td, as measured by PFM-AFM, the two different aggregated dendrimers can be easily discriminated.

Imaging Electrochemical Controlled Chemical Gradients Using Pulsed Force Mode Atomic Force Microscopy

Abstract The electrochemical formation of gradients in self assembled monolayers has been demonstrated recently [1]. The capacity to image these gradients provides useful information on the physical chemistry of electrochemical striping. Imaging chemical gradients requires the ability to sense the chemical moiety on the top of the self-assembled monolayer. This has been accomplished by derivatizing an atomic force microscope (AFM) tip with molecules selected to have specific interactions with the sample in a technique known as chemical force microscopy [2]. Typical tapping mode AFM is then used to image the sample; the tip is oscillated vertically above the sample and the tip-sample interaction modulates the amplitude of the tip. The sample adhesion, sample stiffness, and sample topography all influence the oscillation amplitude of the tip. Pulsed Force Mode (PFM) [3] is an extension for atomic force microscopes. The PFM electronics introduces a sinusoidal modulation to the z-piezo of the AFM with an amplitude between 10 to 500 nm at a user selectable frequency between 100 Hz and 2 kHz.

Concurrent measurement of adhesive and elastic surface properties with a new modulation technique for scanning force microscopy

The combination of two well-established dynamic scanning force microscopy (SFM) modes is incorporated for SFM in combined dynamic X mode or CODY Mode® SFM. A vertical modulation of low frequency and large amplitude is superimposed with a second vertical modulation of high frequency and low amplitude leading to a combination of pulsed force mode SFM, force modulation, and phase sensitive SFM. SFM in the new mode allows the simultaneous mapping of a number of physical surface properties including adhesive force and elasticity over one scan. The new SFM technique is nondestructive and alteration or even destruction of the sample surface is reduced to a minimum. A polymer blend (two homopolymers spin coated on silicon from a tetrahydrofuorane solution of a mixture of poly-2-vinylpyridin and polytertbutylmethacrylate) was used as a sample for comparative measurements between pulsed force mode, force modulation mode and the new SFM mode.

Properties of Single Dendrimer Molecules Studied by Atomic Force Microscopy

Well-separated, individual polyphenylene dendrimer molecules have been prepared by spin coating on a mica surface, and subsequently imaged by noncontact atomic force microscopy (NCAFM). The observed height is in good agreement with the size of a single dendrimer molecule, as calculated by molecular dynamics simulation. By using pulsed force mode (PFM) AFM, stiffness and adhesion properties of individual polyphenylene dendrimers have been studied. They could be related to the molecular structure and the chemical nature of the outer surface of the dendrimers and the thin film of water adsorbed on mica when imaged under ambient conditions. Finally, by changing the concentration of the spin-coating solution, two different kinds of aggregates have been characterized.

Scanning probe microscopy of heterogeneous polymers

Adhesion, elastic and viscoelastic properties are characteristic parameters of materials. These mechanical properties of polymers show a strong dependence on the manufacturing process, the molecular weight, the temperature and the environment. It would be desirable to have tools for characterization of small samples. The scanning force microscope seems to be an ideal instrument when working towards this goal. The nanometer sized probe in contact with the sample surface can be used to measure a variety of surface properties, ranging from friction to adhesion. The measurements could be truly quantitative if one knew exactly the shape of the tip and the strength and distance dependence of the interaction forces. It is shown in this paper how the measurement modes of a scanning force microscope can be tailored to obtain quantitative data. We discuss the advantages and disadvantages of the pulsed force mode investigations using homogeneous and heterogeneous polymer samples as test objects.

Pulsed force mode: a new method for the investigation of surface properties

Scanning force microscopy is extended by the pulsed force mode from simple imaging of topography to measuring elastic, electrostatic and adhesive sample properties. Lateral forces are virtually eliminated so that mapping of delicate samples with high resolution in air and fluids is easily possible. Scanning speed is comparable to that in contact mode. The new opportunities for scanning force microscopy given by the pulsed force mode is demonstrated in selected applications. Copyright © 1999 John Wiley & Sons, Ltd.

Pulsed force mode: a new method for the investigation of surface properties

Pulsed force mode: a new method for the investigation of surface properties

Characterizing Spin Finish Layers on Polypropylene Fiber Surfaces by Means of Scanning Force Microscopy Techniques

For a better understanding of the structure, distribution, and composition of spin finish layers on fiber surfaces, scanning force microscopy techniques are used to image spin finish layers on polypropylene fiber surfaces with submicrometer resolution. The polypropylene staple fibers are made into nonwovens, and common antistatic, wetting, and hydrophobizing agents are the spin finishes. Distribution and thickness profiles of the wax-like antistatic and hydrophobizing agents are successfully measured with con tact mode atomic force microscopy and lateral force microscopy. Nanometer scale measurements of local adhesion forces and elastic moduli of the spin finish layer sur faces by force modulation techniques (pulsed force mode) provide further information on the distribution and composition of the spin finish layers. In addition, the oil-like wetting agent layer, which is too soft and sticky for contact mode measurements, can be imaged with the pulsed force mode technique.

The simultaneous measurement of elastic, electrostatic and adhesive properties by scanning force microscopy: pulsed-force mode operation

We describe the pulsed-force mode, a new measuring mode for the scanning force microscope to image elastic, electrostatic and adhesive properties simultaneously with topography. The pulsed-force mode reduces lateral shear forces between the tip and the sample. Even very delicate samples can be mapped at high lateral resolution with full control over the force applied to the sample. The achieved scanning speed is comparable to that in contact-mode operation. The pulsed-force mode electronics can easily be added to many microscopes without much alteration of the original set-up. No change of the data acquisition software or of the feedback circuit is necessary.