Scanning Force Microscope Tip Research Articles

Porphyrinphosphonate fibers on mica and molecular rows on graphite.

meso-Tetra(phenyl-p-phosphonate) porphyrin forms rigid and well-separated fibers of monomolecular thickness (2.8 nm) and lengths of several micrometers on mica at pH 13 (octasodium salt). The formation of these fibers could be observed directly by tapping mode scanning force microscopy (SFM) and was induced by capillary forces. Normal height images or images with a topographical inversion were observed depending on the distance of the SFM tip. Amplitude-distance curves indicated that a stable meniscus was formed on hydrophilic surface areas below a tip-sample separation of 20 nm. The meniscus let the original nanorods appear as ditches in the mica surface and enabled rearrangements. A partly protonated form of the same porphyrin (pH 11.5) gave rows of flat-lying porphyrins on graphite, which appear with molecular resolution in SFM images as well as two-dimensional platelets of monomolecular thickness.

Metrological large range scanning probe microscope

We describe a metrological large range scanning probe microscope (LR-SPM) with an Abbe error free design and direct interferometric position measurement capability, aimed at versatile traceable topographic measurements that require nanometer accuracy. A dual-stage positioning system was designed to achieve both a large measurement range and a high measurement speed. This dual-stage system consists of a commercially available stage, referred to as nanomeasuring machine (NMM), with a motion range of 25 mm×25 mm×5 mm along x, y, and z axes, and a compact z-axis piezoelectric positioning stage (compact z stage) with an extension range of 2 μm. The metrological LR-SPM described here senses the surface using a stationary fixed scanning force microscope (SFM) head working in contact mode. During operation, lateral scanning of the sample is performed solely by the NMM. Whereas the z motion, controlled by the SFM signal, is carried out by a combination of the NMM and the compact z stage. In this case the compact z stage, with its high mechanical resonance frequency (greater than 20 kHz), is responsible for the rapid motion while the NMM simultaneously makes slower movements over a larger motion range. To reduce the Abbe offset to a minimum the SFM tip is located at the intersection of three interferometer measurement beams orientated in x, y, and z directions. To improve real time performance two high-end digital signal processing (DSP) systems are used for NMM positioning and SFM servocontrol. Comprehensive DSP firmware and Windows XP-based software are implemented, providing a flexible and user-friendly interface. The instrument is able to perform large area imaging or profile scanning directly without stitching small scanned images. Several measurements on different samples such as flatness standards, nanostep height standards, roughness standards as well as sharp nanoedge samples and 1D gratings demonstrate the outstanding metrological capabilities of the instrument.

Deformation and collapse of microtubules on the nanometer scale.

We probe the local mechanical properties of microtubules at the nanometer scale by radial indentation with a scanning force microscope tip. We find a linear elastic regime that can be described by both thin-shell theory and finite element methods, in which microtubules are modeled as hollow tubes. We also find a nonlinear regime and catastrophic collapse of the microtubules under large loads. The main physics of protein shells at the nanometer scale shows simultaneously aspects of continuum elasticity in their linear response, as well as molecular graininess in their nonlinear behavior.

Lateral force microscopy profiles for amorphous potentials

In this work, the lateral force profiles of the scanning force microscope tip on an amorphous surface were simulated with the use of an independent oscillator model. The correlation between the lateral force profiles and the surface potential were studied as a function of the tip-surface normal force and relative scanning velocity. It is shown that the microscope resolution is governed by the quotient between the average potential interaction energy and the average elastic energy stored before the jumps. We show that there is an optimal velocity with which the scanning tip better senses the surface potential and we present its scaling laws.

Stress-induced suppression of piezoelectric properties in PbTiO3:La thin films via scanning force microscopy

Sol-gel derived polycrystalline La-doped PbTiO3 films are investigated by scanning force microscopy (SFM) in a piezoelectric contact mode. The SFM signal proportional to the effective piezoelectric coefficient, deff, is measured inside individual domains as a function of the mechanical force exerted by the SFM tip on the film’s surface. It is found that the piezoelectric signal can be fully suppressed under sufficiently high force (∼20–22 μN). The suppression is qualitatively different for domains of opposite polarities where deff may change its sign depending on the orientation of polarization vector. The piezoelectric hysteresis loops acquired under increasing force gradually shrink with an asymmetric decrease of the piezoelectric coefficients. This leads to a pronounced vertical shift of the loops (deff offset). The obtained results are discussed in terms of the competing electrostatic and piezoelectric contributions to the measured signal and stress-induced polarization instabilities in the vicinity of the SFM tip.

Scanning-force techniques to monitor time-dependent changes in topography and adhesion force of proteins on surfaces.

Scanning-force microscopy (SFM) investigations were conducted to probe the influences of the interactions of proteins with surfaces relevant in medicine. These interactions are an important feature in the area of biofilm formation. The adsorption of proteins leads to changes in topography, which was monitored for the build up of protein layers of hen egg-white lysozyme and bovine serum albumin (BSA) on mica in real time in phosphate-buffered aqueous solution over a time period of 10 min. Phase imaging was additionally applied to compare material contrasts and to evaluate this method for further application in this field. The adhesion forces that develop on a time scale below 20 s between a protein-modified SFM tip and titanium surfaces (TiO(2), TiAl6V4 and TiAl6Nb7) were investigated. The influences of the parameters loading force and interaction time between the protein and the surface were monitored as well as the influence of protein structure. The interaction time dependency of the adhesion force could be described with a kinetic model of two consecutive first-order reactions. For the maximal adhesion force a correlation to the ratio of the amino acids cysteine, proline and glycine has been proposed.

A universal scanning-probe-microscope-based hybrid system

A modularly equipped scanning probe microscope (SPM)-based hybrid system isdeveloped, expanding application fields and performances of former instruments.The SPM head can easily be incorporated into the analysis chamber of mostscanning electron microscopes (SEMs) and both systems can be handled by theelectronics within one software simultaneously. Contact and non-contact scanningforce microscope (SFM) measurements obtained inside an environmental SEM forthe first time demonstrate resolutions of less than 0.2 nm under workingconditions. Both probes can be used simultaneously either as actuators or sensorsto deliberately modify and analyse sample properties as well as to characterize theprobe interactions. This is the essential advantage of hybrid systems,besides the great number of well known complementary analyses whichcan be performed with each microscope at the same sample area. Theuse of an electron beam and a SFM tip as two independent electricalactuators and sensors is demonstrated to be exemplary. Interaction of theprobes with each other is a new aspect, which is presented and discussed.

Imaging and engineering the nanoscale-domain structure of a Sr0.61Ba0.39Nb2O6 crystal using a scanning force microscope

We have investigated the ferroelectric domain structure formed in a Sr0.61Ba0.39Nb2O6 single crystal by cooling the crystal through the Curie point. Imaging the etched surface structure using a scanning force microscope (SFM) in both the topographic mode and the piezoresponse mode revealed that a multidomain structure of nanoscale islandlike domains was formed. The islandlike domains could be inverted by applying an appropriate voltage using a conductive SFM tip. Furthermore, a nanoscale periodically inverted-domain structure was artificially fabricated using the crystal which underwent poling treatment.

Comparison of the self‐chemisorption of azurin on gold and on functionalized oxide surfaces

AbstractThe formation of extended uniform protein monolayers by single‐ or multiple‐step self‐chemisorption was investigated on gold and on functionalized oxide surfaces. These protein layers were characterized by spectroscopic ellipsometry, x‐ray photoelectron spectroscopy (XPS) and scanning force microscopy (SFM). Chemisorption on oxygen‐terminated surfaces (SiO2, mica) was preceded by exposure of the surface to 3‐aminopropyltriethoxysilane and glutaric dialdehyde. On both substrates (gold and insulator) the protein layers appeared flat and robust under SFM imaging conditions. The thickness of the three‐layer structure (4.5 nm on mica), measured by engraving the layer with the SFM tip, matched fairly well with the value expected for the supramolecular architecture. The thickness could be verified after each chemisorption step by ellipsometry and XPS. The elements expected for the azurin layer (Cu and S in particular) were confirmed by XPS at both substrates. The multiple self‐chemisorption approach reported herein could be applicable to similar proteins, because it relies on the general principles of protein assembling. Copyright © 2002 John Wiley & Sons, Ltd.

Electrostatic scanningforce microscopy images of long molecules: single-walled carbonnanotubes and DNA

Experimental measurements of the electrostaticproperties of long molecules are presented in this work. It isshown that, in the case of carbon nanotubes, the electrostaticforce between the scanning force microscopy tip and the sample enhances the network of nanotubes electrically connected to ametal electrode. On λ-DNA molecules the same kind ofexperiments result in essentially no measurable electrostaticforce due to the molecules. From this we conclude that λ-DNA has a negligible electronic conductivity.

Scanning force microscopy three-dimensional modesapplied to the study of the dielectric response of adsorbed DNAmolecules

We have developed a set of working modes for scanning probemicroscopy (SPM), which generalizes the usual method of acquiringdata. We call these modes three-dimensional (3D) modes. Using thesemodes it is possible to measure typical SPM magnitudes, such as, forexample, the tunnel current, the normal force and the amplitude orfrequency of the cantilever oscillation, as a function of any othertwo magnitudes of the system: f(x1,x2). In this paper wepresent different examples of 3D modes. In particular, we haveapplied 3D modes to the study of the electrostatic interaction ofco-adsorbed single walled carbon nanotubes and individual DNAmolecules with a metallic scanning force microscopy tip. The dataindicate that adsorbed DNA has a dielectric constant similar to thatof the glass substrate.

The Mechanical Properties of Human Angiostatin Can Be Modulated by Means of Its Disulfide Bonds: A Single-Molecule Force-Spectroscopy Study

Human angiostatin 1 ± 5 [Ang(1 ± 5)], a 57 kDa proteolitic fragment of human plasminogen, consists of five compact globular modules called Kringle domains with very similar gross architecture and remarkable sequence homology. They are built around a hydrophobic core and exhibit a triple-loop topology defined by three internal disulfide bonds: cys1 ± cys80 (1), cys22 ± cys63 (2), and cys51 ± cys75 (3) (Figure 1a).[1] Here we report on a single-molecule force-spectroscopy study that shows how the redox environment can control the topology and mechanical properties of angiostatin by modifying the extent of pairing of the internal disulfide bonds. Ang(1 ± 5) molecules were deposited on a polystyrene surface; the tip of a scanning force microscope (SFM) was brought into contact with this surface and retracted after reaching a force of 1 nN. This cycle was repeated many hundreds of times, first under phosphate buffer saline (PBS), Figure 1. a) Schematic representation of the Ang(1 ± 5) structure with the triple-loop topology of its five Kringle domains (K1 ±K5), determined in each case by a sequence of three disulfide bonds: cys1 ± cys80 (1), cys22 ± cys63 (2), and cys51 ± cys75 (3). The two asterisks (*) indicate the position of an additional, inter-Kringle disulfide bond. b) Pathway of the sequential reduction of the most exposed (1) and then of the more interior (2) and (3) bonds. This reduction pathway leads to gradually lengthening topological loops (red) that are exposed to the force exerted by the SFM tip at the two ends of the Kringle domain.

Scanning force microscopy of domain structures in Pb(Zn1/3Nb2/3)O3-8% PbTiO3 and Pb(Mg1/3Nb2/3)O3-29% PbTiO3

Domain structures of unpoled as well as poled (along [001]- and [110]-direction) Pb(Zn1/3Nb2/3)O3 (PZN)-8% PbTiO3 (PT) and Pb(Mg1/3Nb2/3)O3 (PMN)-29% PT single crystals have been investigated by scanning force microscopy (SFM) in the piezoresponse mode, at room temperature. Antiparallel domain structures have been detected mostly in unpoled crystals of both materials, with a fingerprint pattern in (001)-oriented PZN-8% PT crystal. The ferroelastic domain wall has been identified in poled (110)-oriented PZN-8% PT crystal. “Writing” of ferroelectric domains has been performed by applying a dc voltage to the SFM tip. Local re-poling has been observed for all unpoled as well as for poled (001)-oriented crystals at the voltage ±60 V. Local electrical switching was successful in poled (110)-oriented PMN-29% PT at higher voltage (±120 V) but was not successful in poled (110)-oriented PZN-8% PT crystal. Domain-engineered crystals poled in [110]-direction seem to exhibit more stable (in the sense of local re-poling properties) domain arrangement. Hysteretic d(E) dependencies were observed by local application of an ac voltage.

Formation and characterization of protein monolayers on oxygen-exposing surfaces by multiple-step self-chemisorption

Extended protein monolayers were formed by multiple-step self-chemisorption and characterized by scanning force microscopy (SFM), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry, and cyclic voltammetry. These uniform protein monolayers were deposited on oxygen-exposing surfaces (SiO 2, mica, glass, etc.) after preexposing these surfaces to 3-aminopropyltriethoxysilane (3-APTS) and reacting the silylated surfaces with glutaric dialdehyde (GD). The samples appear flat and robust under SFM imaging conditions, both at low and molecular resolution. The thickness of the three-layer structure measured by engraving the layer with the SFM tips (4.5 nm) matches fairly well the value expected for the supramolecular architecture. XPS performed after each of the three different stages of sample preparation confirmed the presence of the elements expected (such as Cu and S for the azurin layer) and allowed film growth to be followed. Similar results have been obtained for the thicknesses of the different layers by spectroscopic ellipsometry. Optical absorption spectroscopy has provided data consistent with a 75% surface coverage by a protein (sub)monolayer. Redox proteins immobilized with linkers involving a similar chemistry but providing groups binding to gold (2-mercaptoethylamine and GD) rather than to oxygen-exposing surfaces made it possible to record cyclic voltammograms of single monolayers. These electrochemical experiments have confirmed retention of the protein's redox activity upon surface immobilization. The chemisorption approach reported in the present paper appears to be applicable to all kinds of proteins.

Friction of thin water films: a nanotribological study

Lubricant thickness is known to influence the frictional properties between two contacting surfaces in relative motion. In this paper, the dependence of friction on water films with various thicknesses is examined using a scanning force microscopy (SFM) between a hydrophilic silicon tip and silicon flat that was rendered either hydrophilic or hydrophobic. Results indicate that the frictional properties are influenced by the ordering effects of water. The friction of a hydrophilic surface initially covered with 2.6 nm of water was examined as a function of the film thickness, which was separately determined using scanning tunneling microscopy and dynamic SFM. Capillary effects dominate the tribological properties for films that are between 3 and about 1 nm thick. For thinner films the friction properties can be explained by the ordering effect of water on the sample and the tip, which together cause an increased resistance to shear (higher viscosity). Further reduction of film thickness due to water desorption leads to a decrease of the friction force; this regime is dominated by cohesive forces arising from the solid–solid contact. For all applied forces in this study, liquid confinement between tip and surface does not occur. Rather, the sharp SFM tip apparently penetrates the water film with the tip apex making direct contact with the surface. The ordering effect of water occurs on the sample surface and at the sides of the SFM tip. These results, together with previous microtribological studies, highlight the significant differences existing between two contacting surfaces moving in relative motion in the micro- and nanoregime.

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.

Extremely Long Dendronized Polymers: Synthesis, Quantification of Structure Perfection, Individualization, and SFM Manipulation This work was supported by the Deutsche Forschungsgemeinschaft (Sfb 448, TPs 1 and 5) and the Fonds der Chemischen Industrie; SFM=scanning force microscopy.

Moving individual molecules: Up to 250-nm long polystyrene chains with fourth generation dendrons at each repeat unit were synthesized, individualized on highly oriented pyrolytic graphite (HOPG), and manipulated into a stretched linear conformation by a scanning force microscope tip (see images, arrow indicates direction of movement).

Visualization of single-walled carbon nanotubes electrical networks by scanning force microspy

In this letter, a method is presented to qualitatively investigate the electrical connection between single-walled carbon nanotubes and gold electrodes. By applying a time independent bias voltage to the scanning force microscope tip, there is a clear apparent growth effect in the topography of the electrically connected carbon nanotubes. This effect is induced by the electrostatic interaction of the tip with the free charges located at the conducting areas of the surface. This is a general method that could be applied to any molecular conducting nanowire.

Asymmetric nanoscale switching in ferroelectric thin films by scanning force microscopy

Scanning force microscopy (SFM) has been used to perform nanoscale studies of the switching behavior of Pb(Zr, Ti)O3 thin films via the direct observation of their domain structures. The study revealed a significant asymmetry of a switching pattern which is a function of the voltage polarity and original domain structure of individual grains. The phenomenon of asymmetric switching is attributed (1) to the presence of an internal built-in electric field at the bottom interface and (2) to the mechanical stress exerted by the SFM tip. The former effect results in incomplete 180° switching, while the latter effect leads to a 90° rotation of the polarization vector. The resulting shear stress deformation of the grain underneath the tip combined with the applied field effect propels polarization reversal in the adjacent grains.

Nanoscale fracture studies using the scanning force microscope

We present a variety of experiments concerning friction and fracture mechanisms using two-dimensional arrays of microfabricated nanotowers as templates. The scanning force microscope tip is used as a tool to apply well-defined forces to the surface of the patterned substrate. Force statistic measurements reveal information about the forces involved in the fracture process and the probability of fracture of selected towers. These methods are discussed in the context of nanometer-scale mechanisms. Using optimized parameters, a controlled removal of individual nanotowers and the ability to “write” predefined patterns on a nanometer scale can be achieved.