Microstructural examination of layered coatings by scanning electron microscopy, transmission electron microscopy, and atomic force microscopy

The structure of r.f. sputtered multilayer Ti-BN coatings was investigated by low-voltage scanning electron microscopy, energy-dispersive X-ray spectrometry, transmission electron microscopy, and atomic force microscopy. Appropriate specimen preparation methods are described for each technique; these included fracture of the substrate, masking the growing film to produce a taper section, and ion-beam milling of embedded cross sections. Correlation of scanning electron micrographs with atomic force images was facilitated by the presence of similar composition contrast in both cases. Quantitative X-ray microanalysis of the layers was performed using the φ(z) approach. The crystal structures of nanocrystalline grains nucleated as a result of interdiffusion reactions during thermal annealing were identified by selected-area electron diffraction and convergent-beam microdiffraction as -titanium and f.c.c. titanium nitride.     

Elemental mapping in natural rubber latex films by electron energy loss spectroscopy associated with transmission electron microscopy

Element distribution maps from Hevea brasiliensis natural rubber latex thin films were obtained, by electron energy-loss spectroscopic imaging in a low-energy (80 kV) transmission electron microscope. C, N, O, P, Na, Ca, Mg, Al, Si, and S maps are presented for latex fractionated by centrifugation, either followed by dialysis or not. Most elements forming non-carbon compounds are concentrated in small, electron-dense spots surrounded by a carbon-rich matrix of polymer, thus showing that the rubber is filled with small particles compatible with the polyisoprene matrix. Ca distribution is unique, since it closely parallels the C distribution, evidencing an important role for -COO(-)-Ca2+-COO- ionic bridges in the structure of natural rubber.     

Morphology study of rubber based nanocomposites by transmission electron microscopy and atomic force microscopy

Rubber based nanocomposites were prepared using octadecyl amine modified Na-montmorillonite clay (OC) and Styrene Butadiene Rubber (SBR) having styrene content of 15, 23 and 40% respectively and Acrylonitrile Butadiene Rubber (NBR) having acrylonitrile content of 19, 34 and 50% respectively. The morphology of the nanocomposites was investigated using Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) and X-ray Diffraction Technique (XRD). The TEM photographs of the unmodified clay loaded SBR nanocomposite showed agglomeration, while the modified clay loaded SBRs of all the grades revealed complete exfoliation. The NBRs, on the other hand, gave unexfoliated and intercalated clay structures both with the unmodified and the modified clays, except in the case of NBR having 19% of acrylonitrile and 4% of the unmodified clay. The AFM data were in good accord with the TEM results. The particle dimensions were within the range of 10–20 nm in the case of SBR sample having 4 parts of the modified clay. NBRs having 34 and 50% acrylonitrile contents and 4 parts of OC showed clay particles ranging from 50–70 nm and 70–100 nm respectively. On comparison of the rubbers having different nature and contents of functional groups and filler loadings, significant effect on the morphology of the composite was observed. The nature of solvent used to prepare the nanocomposites also affected the morphology. XRD data further corroborated the facts in all the above cases.     

Nanofabrication with atomic force microscopy

Atomic force microscopy (AFM) was developed in 1986. It is an important and versatile surface technique, and is used in many research fields. In this review, we have summarized the methods and applications of AFM, with emphasis on nanofabrication. AFM is capable of visualizing surface properties at high spatial resolution and determining biomolecular interaction as well as fabricating nanostructures. Recently, AFM-based nanotechnologies such as nanomanipulation, force lithography, nanografting, nanooxidation and dip-pen nanolithography were developed rapidly. AFM tip (typical radius ranged from several nanometers to tens of nanometers) is used to modify the sample surface, either physically or chemically, at nanometer scale. Nanopatterns composed of semiconductors, metal, biomolecules, polymers, etc., were constructed with various AFM-based nanotechnologies, thus making AFM a promising technique for nanofabrication. AFM-based nanotechnologies have potential applications in nanoelectronics, bioanalysis, biosensors, actuators and high-density data storage devices.     

In situ tensile testing of nanofibers by combining atomic force microscopy and scanning electron microscopy

A nanomechanical testing set-up is developed by integrating an atomic force microscope (AFM) for force measurements with a scanning electron microscope (SEM) to provide imaging capabilities. Electrospun nanofibers of polyvinyl alcohol (PVA), nylon-6 and biological mineralized collagen fibrils (MCFs) from antler bone were manipulated and tensile-tested using the AFM-SEM set-up. The complete stress-strain behavior to failure of individual nanofibers was recorded and a diversity of mechanical properties observed, highlighting how this technique is able to elucidate mechanical behavior due to structural composition at nanometer length scales.     

High-resolution and large dynamic range nanomechanical mapping in tapping-mode atomic force microscopy

High spatial resolution imaging of material properties is an important task for the continued development of nanomaterials and studies of biological systems. Time-varying interaction forces between the vibrating tip and the sample in a tapping-mode atomic force microscope contain detailed information about the elastic, adhesive, and dissipative response of the sample. We report real-time measurement and analysis of the time-varying tip-sample interaction forces with recently introduced torsional harmonic cantilevers. With these measurements, high-resolution maps of elastic modulus, adhesion force, energy dissipation, and topography are generated simultaneously in a single scan. With peak tapping forces as low as 0.6?nN, we demonstrate measurements on blended polymers and self-assembled molecular architectures with feature sizes at 1, 10, and 500?nm. We also observed an elastic modulus measurement range of four orders of magnitude (1?MPa to 10?GPa) for a single cantilever under identical feedback conditions, which can be particularly useful for analyzing heterogeneous samples with largely different material components.     

Antibacterial action of lipidic nanoemulsions using atomic force microscopy and scanning electron microscopy on Escherichia coli

The present study demonstrates the mechanism of bactericidal effect rendered by cationic and non-cationic lipidic emulsions using atomic force microscopy (AFM) and scanning electron microscopy. The AFM images of treated Escherichia coli cells indicated the conformational alteration from rod-shaped bacteria to a fluid-flattened structure with the presence of pore in the centre of bacterium, indicating the cell lysis. Root mean square roughness increased substantially due to exposure of underlying rugose peptidoglycan layer when treated with non-cationized lipidic nanoemulsions (NCLE). The cationised-lipidic-emulsion (CLE) treated E. coli cells frequently showed division septa along the length of E. coli which was not visible in non-cationised treated bacterial cells. The enhanced adhesion between CLE and negatively charged bacteria leads to lesser time required to kill the bacteria as compared to NCLE.     

High-resolution mapping of redox-immunomarked proteins using electrochemical-atomic force microscopy in molecule touching mode

We explore the possibility of using molecule touching atomic force electrochemical microcopy (Mt/AFM-SECM) for high-resolution mapping of proteins on conducting surfaces. The proposed imaging strategy relies on making surface-immobilized proteins electrochemically "visible" via redox-immunomarking by specific antibodies conjugated to poly(ethylene glycol) (PEG) chains terminated by redox ferrocene (Fc) heads. The flexibility and length of the PEG chains are such that, upon approaching a combined AFM-SECM microelectrode tip toward the surface, the Fc moieties can efficiently shuttle electrons from the surface to the tip. The so-generated SECM positive feedback tip current allows the specific localized detection of the sought protein molecules on the surface. This new electrochemical imaging scheme is validated experimentally on the basis of a model system consisting of mouse IgGs adsorbed onto electrode surfaces and recognized by Fc-PEG-labeled antimouse antibodies. In order to estimate the resolution of Mt/AFM-SECM for protein imaging, regular arrays of submicrometer-sized spots of mouse IgGs are fabricated onto gold electrode surfaces using particle lithography. The Fc-PEG-immunomarked mouse IgG spots are imaged by Mt/AFM-SECM operated in tapping mode. Both an electrochemical image, reflecting the surface distribution of the redox-labeled IgGs, and a topography image are then simultaneously and independently acquired, with a demonstrated resolution in the ~100 nm range. The strength of Mt/AFM-SECM imaging is to combine the nanometric resolution of AFM with the selectivity of the electrochemical detection, potentially allowing individual target proteins to be identified amidst similarly sized "nano objects" present on a conducting surface.     

Nanoscale mapping of contact stiffness and damping by contact resonance atomic force microscopy

In this work, a new procedure is demonstrated to retrieve the conservative and dissipative contributions to contact resonance atomic force microscopy (CR-AFM) measurements from the contact resonance frequency and resonance amplitude. By simultaneously tracking the CR-AFM frequency and amplitude during contact AFM scanning, the contact stiffness and damping were mapped with nanoscale resolution on copper (Cu) interconnects and low-k dielectric materials. A detailed surface mechanical characterization of the two materials and their interfaces was performed in terms of elastic moduli and contact damping coefficients by considering the system dynamics and included contact mechanics. Using Cu as a reference material, the CR-AFM measurements on the patterned structures showed a significant increase in the elastic modulus of the low-k dielectric material compared with that of a blanket pristine film. Such an increase in the elastic modulus suggests an enhancement in the densification of low-k dielectric films during patterning. In addition, the subsurface response of the materials was investigated in load-dependent CR-AFM point measurements and in this way a depth dimension was added to the common CR-AFM surface characterization. With the new proposed measurement procedure and analysis, the present investigation provides new insights into characterization of surface and subsurface mechanical responses of nanoscale structures and the integrity of their interfaces.     

Nanoscale mapping and affinity constant measurement of signal-transducing proteins by atomic force microscopy

Atomic force microscope (AFM) was used to measure the interaction force between two signal-transducing proteins, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and Ras homologue enriched in brain (Rheb), and to analyze the binding of glyceraldehyde-3-phosphate (Gly-3-P) to GAPDH. To enhance the recognition efficiency and avoid undesirable multiple interactions, the AFM probe and the substrate were each modified with a dendron, glutathione S-transferase (GST)-fused proteins were employed, and reduced glutathione (GSH) was conjugated at the apex of each immobilized dendron. The resulting median specific force between GAPDH and Rheb was 38 ± 1 pN at a loading rate of 3.7 × 10(3) pN/s. The measurements showed that the GAPDH-Rheb interaction was inhibited by binding of Gly-3-P. An adhesion force map showed individual GADPHs on the surface and that the number density of GAPDH decreased with the concentration of Gly-3-P. Maps obtained in the presence of various Gly-3-P concentrations provided information on the binding behavior, yielding a thermodynamic association constant of 2.7 × 10(5) M(-1).     

On chlorosulphonation and the electron microscopy of polyethylene

It is shown that chlorosulphonation is a major aid to the electron microscopy of polyethylene for various samples which had mostly been crystallized at high pressures and included at least a proportion of the so-called chain-extended form. It is confirmed that sheets of excess electron density are produced at lamellar surfaces, but also including lateral surfaces. This is due primarily to the incorporation of chlorine and sulphur rather than to added uranium. The time to achieve an overall reaction varies sensitively with morphology, decreasing as the number of diffusion channels increases. Crystallinity is gradually lost, but sufficient crystals remain when a sample has become uniform, and in their initial orientations, for diffraction studies to be possible. The technique has been used to demonstrate that, during melt crystallization, the thickness of one lamella changes in response to altered growth conditions. This is direct confirmation that lamellar thickness is determined by secondary nucleation at the growth front. The tapered profile of a growing lamella previously observed in thick crystals of various polymers has been observed for chain-folded polyethylene lamellae, providing further evidence that this is a general feature of melt growth.     

Single-molecule switching with non-contact atomic force microscopy

We report upon controlled switching of a single 3,4,9,10-perylene tetracarboxylic diimide derivative molecule on a rutile TiO(2)(110) surface using a non-contact atomic force microscope at room temperature. After submonolayer deposition, the molecules adsorb tilted on the bridging oxygen row. Individual molecules can be manipulated by the atomic force microscope tip in a well-controlled manner. The molecules are switched from one side of the row to the other using a simple approach, taking benefit of the sample tilt and the topography of the titania substrate. From density functional theory investigations we obtain the adsorption energies of different positions of the molecule. These adsorption energies are in very good agreement with our experimental observations.     

Conductive-atomic force microscopy study of local electron transport in nanostructured titanium nitride thin films

Simultaneous local current and topography measurements were made on the surface of titanium nitride thin films by conductive-atomic force microscopy (C-AFM). Two compositions, stoichiometric TiN and sub-stoichiometric TiN_0.76 were investigated. Local variation of current at grain and grain boundaries was examined. The current flow is filamentary in nature, with the number of percolation paths being smaller for sub-stoichiometric titanium nitride. Current-voltage characteristics of stoichiometric TiN reveal that the grain interiors are electrically conductive, while in sub-stoichiometric TiN_0.76 thin film, grains are electrically resistive, i.e., a potential barrier to electron transport exists at the junction between the grain and the grain boundary in sub-stoichiometric TiN_0.76. Therefore, electron transport in this film is due to tunneling through the junction, which leads to increased resistivity. The total resistance of the samples measured using the four probe technique is 1 and 400 kΩ for TiN and TiN_0.76 respectively. In both type of compounds the grain and grain boundary resistances are of the order of MΩ. The grain and grain boundaries are connected in a manner that causes the total resistivity to be lower than the local resistivity.     

Tour de force microscopy

Scanning probe microscopy (SPM) is capable of imaging synthetic polymers and biomolecular systems at sub-molecular resolution, without the need for staining or coating, in a range of environments including gas and liquid, so offering major advantages over other forms of microscopy. However, there are some limitations, which could be alleviated by (i) reducing the force interaction between the probe and specimen and (ii) increasing the rate of imaging. New developments in instrumentation from the SPM group at the University of Bristol to overcome these limitations are discussed and illustrated here.The invention of scanning tunneling microscopy (STM) in 1981 began a revolution in microscopy¹, which has led to a whole new family of about a dozen microscopies known collectively as scanning probe microscopy (SPM). The importance of this development is comparable to that of the invention of electron microscopy in the 1930s and arguably as fundamental as the development of the first optical microscopes, since SPM uses an entirely different principle from optical and electron microscopy to achieve imaging at high resolution.