Tapping Mode Scanning Force Microscopy Research Articles

Epoxy-Metal Interphases: Chemical and Mechanical Aging

Aging of epoxy (EP)-metal-joints is studied by FTIR microspectroscopy, tapping mode scanning force microscopy and nanoindentation on sample cuts prepared with low angle microtomy. The influences of temperature and water are separated by exposure to dried or moist air (90% rel. humidity) at 60°C and dried air at 120°C for up to a few hundred days. In the course of aging, diffusion-controlled chemical reactions start at the interface to the metal and extend as a gradient up to 50 μm into the EP. Water is more detrimental to the epoxy than elevated temperature. The chemical aging proceeds slower than at the contact to ambient air, and, contrary to that surface zone, oxidation of amines dominates autoxidation of CH2 at other sites in the network chains. Thus, intensity and rate of chemical aging at epoxy-metal contacts cannot be deduced simply from the aging at the air-bulk contact. Chemical aging is accompanied by the formation of gradients of mechanical properties, at least in moist conditions.

Crystallographic and morphological characterization of thin pentacene films on polycrystalline copper surfaces

The degree of crystallinity, the structure and orientation of crystallites, and the morphology of thin pentacene films grown by vapor deposition in an ultrahigh vacuum environment on polycrystalline copper substrates have been investigated by x-ray diffraction and tapping-mode scanning force microscopy (TM-SFM). Depending on the substrate temperature during deposition, very different results are obtained: While at 77 K a long-range order is missing, the films become crystalline at elevated temperatures. From a high-resolution x-ray-diffraction profile analysis, the volume-weighted size of the crystallites perpendicular to the film surface could be determined. This size of the crystallites increases strongly upon changing temperature between room temperature and 333 K, at which point the size of individual crystallites typically exceeds 100 nm. In this temperature region, three different polymorphs are identified. The vast majority of crystallites have a fiber texture with the (001) net planes parallel to the substrate. In this geometry, the molecules are oriented standing up on the substrate (end-on arrangement). This alignment is remarkably different from that on single-crystalline metal surfaces, indicating that the growth is not epitaxial. Additionally, TM-SFM images show needlelike structures which suggest the presence of at least one additional orientation of crystallites (flat-on or edge-on). These results indicate that properties of thin crystalline pentacene films prepared on technologically relevant polycrystalline metal substrates for fast electronic applications may be compromised by the simultaneous presence of different local molecular aggregation states at all temperatures.

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.

Preparation of reproducible glassy carbon electrodes by removal of polishing impurities

This paper describes a simple and rapid electrochemical oxidative procedure that removes polishing contaminants from glassy carbon (GC) surfaces. The method involves oxidation of polished GC electrodes in basic media for short periods of time (∼10 s). Tapping mode scanning force microscopy (TM SFM), X-ray photoelectron spectroscopy (XPS) and nanoindentation surface characterization techniques were utilized to track the removal of the polishing layer. Several electrochemical characterizations of the modified GC surface were carried out to assess changes in electrode reactivity following removal of the microparticle polishing layer. Electrochemical measurements obtained on the anodized GC electrodes show a more reproducible surface relative to polished GC.

From chloroplasts to photosystems: in situ scanning force microscopy on intact thylakoid membranes.

Envelope-free chloroplasts were imaged in situ by contact and tapping mode scanning force microscopy at a lateral resolution of 3-5 nm and vertical resolution of approximately 0.3 nm. The images of the intact thylakoids revealed detailed structural features of their surface, including individual protein complexes over stroma, grana margin and grana-end membrane domains. Structural and immunogold-assisted assignment of two of these complexes, photosystem I (PS I) and ATP synthase, allowed direct determination of their surface density, which, for both, was found to be highest in grana margins. Surface rearrangements and pigment- protein complex redistribution associated with salt-induced membrane unstacking were followed on native, hydrated specimens. Unstacking was accompanied by a substantial increase in grana diameter and, eventually, led to their merging with the stroma lamellae. Concomitantly, PS IIalpha effective antenna size decreased by 21% and the mean size of membrane particles increased substantially, consistent with attachment of mobile light-harvesting complex II to PS I. The ability to image intact photosynthetic membranes at molecular resolution, as demonstrated here, opens up new vistas to investigate thylakoid structure and function.

Influence of volume and surface properties on phase contrast in tapping mode atomic force microscopy

Phase imaging angle of the cantilever oscillation in tapping mode scanning force microscopy can be related to the energy dissipated per oscillation. Nevertheless, in many cases, the physical properties (adhesion, viscosity…) of the sample that are sources of this energy dissipation are not clearly identified. We propose a simple method to distinguish the influence of surface and volume properties on phase imaging. This method is based on equalizing of surface properties by covering the sample surface with thin gold layers. As an example, in our specific working conditions (moderate tapping on glass–polyester composite), we show that volume properties have an important influence on the phase signal in comparison to surface properties.

Controlling the self-assembly of a deoxiguaninosine on mica

We report a tapping mode scanning force microscopy investigation on the growth from solutions of a lipophilic deoxiguanosine derivative on the basal plane of a muscovite mica surface. By varying the chemical composition of the substrate surface it is possible to drive the self-assembly towards either layered thin films, which are very likely to consist of guanosine quartets, or nanoribbons, comprising hydrogen bonded networks. Both these two architectures, which are peculiar to the guanine base itself, are of notable importance for the fabrication of molecular nanowires. Moreover, the first arrangement provides evidence that the guanosine is able to behave as ionophore, thus, suggesting its utilization in the development of solid-state ion selective sensors.

CORRELATION BETWEEN MORPHOLOGY AND MECHANICAL PROPERTIES OF DIFFERENT STYRENE/BUTADIENE TRIBLOCK COPOLYMERS: A SCANNING FORCE MICROSCOPY STUDY1*

The morphology of different styrene/butadiene (SB) block copolymers with triblock architectures was investigated using tapping mode scanning force microscopy (SFM). Comparative analysis of the morphology of the samples at the polymer/substrate interface of solution-cast films and in bulk was performed. It was found that, besides the total phase volume ratio, the interfacial structure between the incompatible chains determines the phase morphology and mechanical properties of the investigated block copolymers. The asymmetric SBS triblock copolymer (φps( 74 vol%) forms, as expected, a cylindrical morphology with hexagonally packed polybutadiene (PB) cylinders in the polystyrene (PS) matrix. Depending on the interfacial structure, block configuration, and the hard/soft phase ratio, other triblock copolymers (φps( 74 vol% and 65 vol%) show lamellae and randomly distributed PS cylinders in a random styrene/butadiene copolymer S/B matrix, respectively. *Dedicated to Prof. Francisco J. Baltá Calleja on the occasion of his 65th birthday.

Layer-by-Layer Self-Assembly of Polyelectrolyte and Low Molecular Weight Species into Capsules

Composite capsules containing low molecular weight species together with charged macromolecules have been prepared both in aqueous and organic solutions. Dyes were used as building blocks to quantify the adsorption. The Figure shows a tapping mode scanning force microscopy image of capsules consisting of poly(styrene sulfonate) and benzoquinone layers templated on melamine formaldehyde particles (see also inside front cover).

Domain structure of (100) strontium bismuth tantalate (SBT) SrBi 2Ta 2O 9 films

(100) Strontium bismuth tantalate (SBT) SrBi 2Ta 2O 9 films (∼114–234 nm thickness) on (111) Pt deposited by chemical solution deposition (CSD) and chemical vapor deposition (CVD) were investigated with tapping mode scanning force microscopy (TMSFM). The domain structure was found to be strongly influenced by film composition and defect density. Ideal samples showed an a-axis-oriented material with in-plane 180° domains of ∼22 nm width (film thickness 180–225 nm). The domains were visible by a surface corrugation of 0.5–2 nm downwards at the site of domain walls, which followed 〈001〉 planes. In samples with slightly higher defect density in-plane 180° domains, normal-to-plane 180° domains (∼10 nm difference in corrugation between differently poled domains) and 90° domains (∼5 nm difference in corrugation between differently poled domains) occurred. Finally, for (100) SBT samples, which deviated from the ideal composition by several percent, this very high defect density precluded the ordered formation of domain walls. Instead, as-grown a 1 and a 2-axis oriented SBT films showed random orientation of whole crystallites poled either in or normal to the surface. The observed surface corrugation between two differently poled crystallites was >10 nm. Single crystallites as well as single domains could be activated and their polarization state was switched mechanically with an electron beam deposited (EBD) or a plasma-sharpened silicon supertip (SSS) by a force of ∼5 nN.

Investigation of the swelling of human skin cells in liquid media by tapping mode scanning force microscopy

The swelling of individual human skin cells (so-called corneocytes) was studied in distilled water and hexadecene, respectively, by tapping mode scanning force microscopy. The area, the mean height, and the volume were recorded and compared to data of the same cell, which has been obtained under ambient conditions. In distilled water, we found an average swelling in height and volume of about (±)%5010, but no significant increase of the area. Additionally, phase-contrast images suggest a significant change of the viscoelastic properties of the corneocytes. On the other hand, the corneocytes behaved inertly when exposed to hexadecene as an example of a non-polar solvent.

Tapping mode and phase imaging of biaxially oriented polyester films

AbstractThe surface morphology and mechanical properties of biaxially drawn poly(ethylene terephthalate) film surfaces have been investigated with tapping mode scanning force microscopy. Imaging with high tapping forces caused tip‐induced wear of the polymer surface. The damage was avoided by tapping at lower force. It is then possible to utilize phase detection imaging to provide ready access to surface properties such as stiffness and adhesion. Phase imaging has revealed a wealth of detailed surface structure information not seen in previous studies of heterogeneous polyester film materials. Not only are the different components of heterogeneous materials resolved with remarkable clarity, but detailed information is also provided on the phase morphology of individual components and on homogeneous polyester film materials. Copyright © 2001 John Wiley & Sons, Ltd.

The self-assembly of lipophilic guanosine derivatives in solution and on solid surfaces

The self-assembly of lipophilic deoxyguanosine derivatives 1 and 2 has been studied in solution by NMR spectroscopy and ESI-MS (electrospray ionization mass spectrometry). NMR data show the existence of two types of self-assembled, ribbonlike structures (A and B), which are connected at the guanine moieties through two different H-bonded networks. The first species (A), which is stable in the solid state and characterised by cyclic NH(2)-O(6) and NH(1)-N(7) hydrogen bonds, is detected soon after dissolving the polycrystalline powder in rigorously anhydrous CDCl3. In solution it slowly undergoes a structural transition towards a thermodynamically stable ribbon characterised by NH(1)-O(6) and NH(2)-N(3) cyclic hydrogen bonds (B). On the other hand, at surfaces, self-assembled ribbon nanostructures have been grown from solutions of derivative 1 both on mica and at the graphite-solution interface. They have been investigated by means of tapping mode scanning force microscopy (SFM) and scanning tunnelling microscopy (STM), respectively. SFM revealed dry, micrometer-long nanoribbons with a molecular cross-section. while STM imaging at submolecular resolution indicates a molecular packing of type A, like the one detected in the solid state. This indicates that, upon adsorption at the solid-liquid interface, the guanosine moieties undergo a structural rearrangement from a B-type to an A-type ribbon.

Surface-Directed Deposition of Platinum Nanostructures on Graphite by Chemical Vapor Deposition

Nanoscale Pt structures were prepared on highly ordered pyrolytic graphite (HOPG) by chemical vapor deposition. The organometallic compound Pt(COD)(CH3)2 (1) (COD is 1,5-cyclooctadiene) was reduced by hydrogen gas to effect the deposition of Pt. The deposited Pt was characterized by cyclic voltammetry and by tapping mode scanning force microscopy. It was found that Pt initially nucleates at defect sites on HOPG. These nucleated sites then catalyze the hydrogenation of 1 to further deposit Pt adatoms. At low coverages, we observe substrate defect decoration by “nanobead” type structures. Structures that emanate from defect sites onto the basal plane form at higher coverages.

Structure and function of surfactant protein B and C in lipid monolayers: a scanning force microscopy study

This study focuses on the impact of surfactant protein C and B on lipid monolayers at various surface pressures. The artificial system is composed of the saturated phospholipids dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylglycerol (DPPG) in a molar ratio of 4:1 with 0.2 mol% SP-B and/or 0.4 mol% SP-C. A dominating influence of SP-C on the morphology of lipid monolayers at high surface pressure was found. Even in the presence of both proteins structural peculiarities typical for SP-C were found at elevated pressure employing tapping mode scanning force microscopy of LB-films. Stacked bilayer-protrusions known to be induced by SP-C are visible in films containing SP-C together with SP-B. The findings were corroborated by fluorescence microscopy at the air/water interface and are consistent with the appearance of the corresponding isotherms. In the low pressure regime, however, disc-like protrusions characteristic of SP-B containing films are discernible. Filamentous LE domains with large boundaries arise due to the reduced line tension in the presence of surface active proteins, particularly SP-B. Remarkably, SP-B fluidizes the monolayer to a larger extent than SP-C as revealed by scanning force microscopy images. These findings show that SP-B and SP-C interact independently of each other. Therefore we conclude that SP-C may be responsible for the fast respreading process during the breathing cycle while SP-B removes material from the monolayer in more discrete portions.

Interaction of melittin with solid supported membranes

The interaction of the membrane active peptide melittin with solid supported lipid bilayers has been scrutinized by means of impedance spectroscopy and scanning force microscopy. Highly flexible lipid bilayers anchored ia a hydrophilic spacer terminated by a thiol group onto gold were used to investigate the influence of melittin on the electrical parameters of membranes by means of ac impedance analysis. A melittin induced increase in the capacitance and conductivity of the membrane was observed suggesting that melittin induces defects within these immobilized lipid bilayers. Experiments were performed in the absence and presence of EDTA to investigate the influence of remaining phospholipase A2 in the melittin sample as well as the sole impact of phospholipase A2 from Apis mellifera. It was shown that the most pronounced influence of melittin on the electrical parameters of bilayers was found when EDTA was absent, revealing a synergistic effect of phospholipase A2 and melittin. Morphological changes of solid supported membranes induced by the combined effect of melittin and phospholipase A2 were visualized by TappingMode scanning force microscopy in aqueous solution. Dissolution of the lipid layer and the formation of non-lamellar, discoid structures was observed within minutes after addition of melittin, supporting a “ carpet-like” mechanism of peptide action leaving mixed peptide lipid micelles behind. Scanning force microscopy allowed us to resolve the gradual action of melittin within one single scan.

Use of Multi-Walled Carbon Nanotubes for Conductive Probe Scanning Force Microscopy (Cp-SFM)

AbstractMulti-Walled Carbon Nanotubes (MWNTs) mounted on commercial Scanning Force Microscopy (SFM) cantilevers have proven to be excellent probes for high resolution Tapping Mode-Scanning Force Microscopy (TM-SFM). Because of their robust nature and high electrical conductance, MWNTs are also attractive for use in Conductive Probe-Scanning Force Microscopy (CP-SFM). To be used in this application, however, the MWNT must be mounted via a high conductance contact to a conductive cantilever. A technique has been developed that produces such a contact. First, a MWNT is attached using acrylic adhesive to a commercial SFM cantilever that has been vacuum coated with gold. Then, a cap of gold is deposited over the junction between the MWNT and the SFM cantilever via spatially selective electro-deposition.

Energy dissipation in tapping-mode scanning force microscopy with low quality factors

The phase angle of the cantilever oscillation in tapping mode scanning force microscopy can be related to the energy dissipated per oscillation period through an analytical model that assumes a sinusoidal movement of the cantilever [J. Tamayo and R. Garcı́a, Appl. Phys. Lett. 73, 2926 (1998); J. P. Cleveland, B. Anczykowski, E. Schmid, and V. Elings, Appl. Phys. Lett. 72, 2613 (1998)]. In this work, numerical calculations of the oscillation of cantilevers with quality factors lower than 10 show a significant contribution of higher harmonics (∼5%–20%). This contribution can lead to a significant error in the energy dissipated deduced by using the model cited above. Thus, an extended relationship between the phase shift and the energy dissipated is presented, that takes into account the higher harmonics of the oscillation. These results determine the conditions for the measurement of energy dissipation in a liquid.

Tapping mode scanning force microscopy in water using a carbon nanotube probe

An improved technique for obtaining tapping mode scanning force microscopy (TMSFM) images of soft samples submerged in water is described. This technique makes use of a carbon nanotube several microns in length mounted on a conventional silicon cantilever as the TMSFM probe. The sample is covered by a shallow water layer and during imaging only a portion of the nanotube is submerged. This mode of operation largely eliminates the undesirable effects of hydrodynamic damping and acoustic excitation that are present during conventional tapping mode operation in liquids and leads to high-quality TMSFM images. Because of their low bending force constants, carbon nanotubes are ideal for gentle imaging of soft samples. Because of their small (5–20 nm) diameter and cylindrical shape they provide excellent lateral resolution and are ideal for scanning high aspect ratio objects.

Probing Morphological and Compositional Variations of Anodized Carbon Electrodes with Tapping-Mode Scanning Force Microscopy

This paper demonstrates the first application of tapping-mode scanning force microscopy (TM SFM) in the compositional mapping of modified glassy carbon (GC) electrodes. Using TM SFM, we have been able to track both compositional and topographical changes of polished GC induced by electrochemical pretreatment (ECP). Photoresist-based microfabrication techniques were employed to produce surfaces consisting of segregated modified and unmodified regions for direct comparison in the same image. Our results show that ECP of GC via anodization in basic solutions for short times (∼10 s) initially removes the ubiquitous layer of polishing debris via an etching process. Longer anodization in basic electrolyte results in significant etching of the GC surface. ECP in acidic solutions yields little topographic change compared to basic electrolytes. Electrochemical results obtained for three redox systems studied on both modified and unmodified GC electrodes correlate with the TM SFM images collected.