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PubMed2002

Current and potential characterization on InAs nanowires by contact-mode atomic force microscopy and Kelvin probe force microscopy.

We fabricated InAs nanowires on GaAs giant step structures, and studied their surfaces by two methods; one was current detection by contact-mode atomic force microscopy, and the other was surface potential measurement by Kelvin probe force microscopy (KFM). In the current detection method, the regions where the large current flowed were distributed along the step edges, and these regions agreed well with the expected distribution of InAs. This result confirms that the InAs nanowires were formed along the GaAs giant step edges. The KFM measurements showed that the potential value became more negative along each step edge, where the InAs nanowire was expected to be formed. The surface potential of the InAs nanowires is more negative than that of the surrounding GaAs, which may result from the electron accumulation in the InAs nanowires.

Journal Article↗
PubMed2004

Probe-induced native oxide decomposition and localized oxidation on 6H-SiC (0001) surface: an atomic force microscopy investigation.

We report, for the first time, the native oxide decomposition/etching and direct local oxide growth on 6H-SiC (0001) surface induced by atomic force microscopy (AFM). Surface native oxide was decomposed and assembled into protruded lines when the negatively biased AFM tip was scanned over surface areas. The mechanism of decomposition was found to be governed by the Fowler-Nordheim emission current enhanced by the negatively biased AFM tip. Direct oxide growth on the SiC surface was achieved when the AFM tip was immobilized and longer bias duration applied. In particular, the aspect ratio of oxide grown on SiC was found to be several times higher than that on the Si surface. The improved aspect ratio on SiC was attributed to the anisotropic OH(-) diffusion involved in vertical and lateral oxidation along the polar and nonpolar directions such as [0001] and [110] axis in SiC crystal. The electron transport in the above AFM grown oxide on SiC was further investigated by I-V characteristics. The dielectrical strength of AFM oxide against degradation and breakdown under electrical stressing was evaluated.

Journal Article↗
PubMed1991

Atomic force microscopy: seeing molecules of lipid and immunoglobulin.

The atomic force microscope (AFM) can image individual molecules by raster-scanning a sharp tip over a surface. In this paper we present molecular-resolution images of immunoglobulin M (IgM) and of ultraviolet light-polymerized films of the lipid dimethyl-bis(pentacosadiynoyloxyethyl) ammonium bromide ("BRONCO"). The polar head groups of individual lipid molecules can be resolved on the surface of this and other lipid films. These lipid films also provide a good substrate for AFM imaging of DNA and of other molecules such as antibodies. Because the AFM scans surfaces, it is most often successful at imaging either molecules that can form an array on a surface or molecules that are quite firmly attached to a surface. The ability of the AFM to operate under water, buffers, and other liquids makes it possible to study biological molecules under conditions in which they are physiologically active. Imaging of the actual molecular process of fibrin polymerization shows the potential of the AFM for studying biological processes. In the six years since its invention, the AFM has excited much interest and has imaged molecules in a wide range of systems.

Antibodies↗
PubMed2004

Atomic force microscopy colloid-probe measurements with explicit measurement of particle-solid separation.

We describe the use of evanescent wave scattering to measure the separation between the surface of a solid and a particle that is attached to an atomic force microscope (AFM) cantilever. Termed evanescent wave atomic force microscopy, our approach involves measuring the intensity of the light scattered from an evanescent field formed by the total internal reflection of a laser beam at a solid/fluid interface. In a conventional AFM "colloid probe" measurement, this separation must be inferred from an examination of the surface forces. Direct measurement of this separation with an evanescent wave atomic force microscope (EW-AFM) removes some ambiguity in the surface force measurement and, in addition, allows new types of measurements. For example, the force can be monitored at a constant separation. Our evanescent scattering apparatus is essentially identical to that used in total internal reflection microscopy (TIRM), except that we collect the light that scatters back into the incident medium, because the AFM partly obscures the forward scattered light (i.e., light scattered into the transmitted region). Compared to a conventional TIRM measurement, where the particle moves freely, attaching the particle to the cantilever in an EW-AFM gives much greater control of the particle position.

Journal Article↗
PubMed2005

Electrochemistry of conductive polymers 36. pH dependence of polyaniline conductivities studied by current-sensing atomic force microscopy.

We demonstrate from our current-sensing atomic force microscopic studies that both electrical and topographical properties of electrochemically prepared polyaniline (PAn) films are affected by their preparation conditions. The electrical properties of the fully doped PAn films prepared in 0.30 M nitric acid with its pH and ionic strength adjusted to 0.50 can be described as a conductor with an average conductivity of 49 (+/-13) S/cm with primarily a compact structure resulting from a relatively small growth rate. The doped PAn films prepared at pH 5.0, for example, have compact structures with large grains and lightly doped semiconducting properties with an average conductivity of about 1.54 (+/-0.09) x 10(-4) S/cm. From these data, we conclude that the degree of protonation of the monomers and the main reactions taking place during an early stage of the polymerization reaction are important factors determining the chemical structures as well as their conductivities and morphologies of the PAn films.

Journal Article↗
PubMed2000

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.

Journal Article↗
PubMed1996

Atomic force microscopy in histology and cytology.

This review briefly introduces the principles of the atomic force microscope (AFM) and shows our own results of AFM application to biological samples. The AFM, invented in 1986, is an instrument that traces the surface topography of the sample with a sharp probe while monitoring the interaction forces working between the probe and sample surface. Thus, the AFM provides three-dimensional surface images of the sample with high resolution. The advantage of the AFM for biologists is that AFM can visualize non-conductive materials in a non-vacuous (i.e., air or liquid) environment. AFM images of the plasmid DNA are comparable to those by transmission electron microscopy using a rotary shadowing technique, and have the advantage of examining directly the molecule without staining nor coating. The surface structure of human metaphase chromosomes and mouse collagen fibrils demonstrated in air by the non-contact mode AFM is comparable to that obtained by scanning electron microscopy. Quantitative information on the heights of structures is further obtainable from the AFM images. Embedment-free thin tissue-sections are useful for observing intracellular structures by AFM. The present review also shows AFM images of living cultured cells which have been collected in a contact mode in liquid. This technique afforded us three-dimensional observation of the cellular movement with high resolution. Although there are some innate limitations for AFM imaging, the AFM has great potential for providing valuable new information in histology and cytology.

Animals↗
PubMed2002

Bias-induced forces in conducting atomic force microscopy and contact charging of organic monolayers.

Contact electrification, a surface property of bulk dielectric materials, has now been observed at the molecular scale using conducting atomic force microscopy (AFM). Conducting AFM measures the electrical properties of an organic film sandwiched between a conducting probe and a conducting substrate. This paper describes physical changes in the film caused by the application of a bias. Contact of the probe leads to direct mechanical stress and the applied electric field results in both Maxwell stresses and electrostriction. Additional forces arise from charge injection (contact charging). Electrostriction and contact charging act oppositely from the normal long-range Coulomb attraction and dominate when a charged tip touches an insulating film, causing the tip to deflect away from the film at high bias. A bias-induced repulsion observed in spin-coated PMMA films may be accounted for by either mechanism. In self-assembled monolayers, however, tunnel current signals show that the repulsion is dominated by contact charging.

Journal Article↗
PubMed2006

Nanoscale wettability of self-assembled monolayers investigated by noncontact atomic force microscopy.

We report on a novel technique to nucleate nanometer-sized droplets on a solid substrate and to image them with minimal perturbation by noncontact atomic force microscopy (NC-AFM). The drop size can be accurately controlled, thus permitting hysteresis measurements. We have studied the nanoscale wettability of several methyl-terminated substrates prepared by the self-assembly of organic molecules. These substrates are alkyltrichlorosilanes on silica, alkylthiols on gold, alkyl chains on hydrogen-terminated silicon, and crystalline hexatriacontane chains on silica. For each of these systems, we report a deviation of the wetting contact angle from the macroscopic value, and we discuss this effect in term of mesoscale surface heterogeneity and long-range solid-liquid interactions.

Journal Article↗
PubMed1996

Observation of baker's yeast strains used in biotransformation by atomic force microscopy.

Different strains of baker's yeast (Saccharomyces cerevisiae) were imaged with an atomic force microscope (AFM). The images of uncoated and nonfixed samples were reproducible with high-constrast and nanometer-resolution. Molecules from the polysaccharide surface of the cell wall were pictured and the distance of atoms was measured. The preparation of samples was easy, suggesting that AFM is a useful tool in this type of analyses.

Biotransformation↗
PubMed1995

Imaging purple membranes in aqueous solutions at sub-nanometer resolution by atomic force microscopy.

Purple membranes adsorbed to mica were imaged in buffer solution using the atomic force microscope. The hexagonal diffraction patterns of topographs from the cytoplasmic and the extracellular surface showed a resolution of 0.7 and 1.2 nm, respectively. On the cytoplasmic surface, individual bacteriorhodopsin molecules consistently exhibited a distinct substructure. Depending on the pH value of the buffer solution, the height of the purple membranes decreased from 5.6 nm (pH 10.5) to 5.1 nm (pH 4). The results are discussed with respect to the structure determined by cryo-electron microscopy.

Adsorption↗
PubMed2000

Atomic Force Microscopy Study of Ultrafine Particles Prepared in Reverse Micelles.

The imaging of ultrafine Au, Pd, CdS, and ZnS particles prepared in reverse micelles has been studied by atomic force microscopy (AFM). Mica substrates, derivatized with a monolayer of amine or thiol-terminated silanes, were used to immobilize the particles. The substrates were exposed to reverse micellar solutions containing the particles and were then immersed in appropriate solvent media to remove surfactants. This resulted in a partial coating of the surfaces by the particles. The particle size was estimated as the height of protrusion, shown on the AFM images. The size values for the Pd and CdS particles, thus obtained, were found to be almost identical to those obtained by transmission electron microscopy (TEM), whereas those for the Au and ZnS particles were larger than those obtained by TEM. Scanning electron microscopy showed that the Au particles tended to aggregate on the surfaces, while Pd particles were isolated from one another. Copyright 2000 Academic Press.

Journal Article↗
PubMed1991

Atomic force microscopy of supported planar membrane bilayers.

Membrane bilayers of dipalmitoyl phosphatidylcholine (DPPC) and dipalmitoyl phosphatidylethanolamine (DPPE) adsorbed to a freshly cleaved mica substrate have been imaged by Atomic Force Microscopy (AFM). The membranes were mounted for imaging by two methods: (a) by dialysis of a detergent solution of the lipid in the presence of the substrate material, and (b) by adsorption of lipid vesicles onto the substrate surface from a vesicle suspension. The images were taken in air, and show lipid bilayers adhering to the surface either in isolated patches or in continuous sheets, depending on the deposition conditions. Epifluorescence light-microscopy shows that the lipid is distributed on the substrate surfaces as seen in the AFM images. In some instances, when DPPE was used, whole, unfused vesicles, which were bound to the substrate, could be imaged by the AFM. Such membranes should be capable of acting as natural anchors for imaging membrane proteins by AFM.

1,2-Dipalmitoylphosphatidylcholine↗
PubMed2006

Measuring the size dependence of Young's modulus using force modulation atomic force microscopy.

The dependence of the local Young's modulus of organic thin films on the size of the domains at the nanometer scale is systematically investigated. Using atomic force microscopy (AFM) based imaging and lithography, nanostructures with designed size, shape, and functionality are preengineered, e.g., nanostructures of octadecanethiols inlaid in decanethiol self-assembled monolayers (SAMs). These nanostructures are characterized using AFM, followed by force modulation spectroscopy and microscopy measurements. Young's modulus is then extracted from these measurements using a continuum mechanics model. The apparent Young's modulus is found to decrease nonlinearly with the decreasing size of these nanostructures. This systematic study presents conclusive evidence of the size dependence of elasticity in the nanoregime. The approach utilized may be applied to study the size-dependent behavior of various materials and other mechanical properties.

Journal Article↗
PubMed1992

Atomic force microscopy of conventional and unconventional nucleic acid structures.

Images of conventional (Watson-Crick base paired) and unconventional (G4 RNA) nucleic acid structures have been obtained by atomic force microscopy. The images are reproducibly generated from samples deposited on freshly cleaved mica. Periodic substructural features are evident in fibres observed in both cases. In the case of G4 RNA, tip-induced formation of large fibres is observed.

DNA↗
PubMed2002

Effect of hydration of polyamide membranes on the surface electrokinetic parameters: surface characterization by x-ray photoelectronic spectroscopy and atomic force microscopy.

The surface and the solid/liquid interface of two polyamide membranes, one experimental (B0) and one commercial (NF45), have been characterized by X-ray photoelectronic spectroscopy (XPS), atomic force microscopy (AFM), and zeta potential, respectively. The surface roughness, determined by AFM data analysis, is different for the two membranes, and results show that the commercial NF45 membrane presents a much lower roughness than the experimental B0 membrane. XPS data indicate that the surface of membrane NF45 is similar to that of pure polyamide, while membrane B0 contains a considerable amount of impurities. The homogeneity in depth of both membranes was also studied by determining the composition profile at different analysis angles. Streaming potential along the membrane surface or tangential streaming potential (TSP) measurements with NaCl solutions at different concentrations were carried out with both membranes to determine the zeta potential and the electrokinetic surface charge density, and a correlation between membrane surface and interface parameters is made. Some differences in atomic concentrations of membrane surface elements and X-ray photoelectronic spectra of the samples used in TSP measurements and after a drying process at 90 degrees C for 24 h can be observed when they are compared with those for fresh membranes. Electrokinetic parameters for membrane NF45 (TSP, zeta potential, and surface electrokinetic charge density) obtained from three different series of measurements strongly decrease as a result of membrane use, but for membrane B0 they are practically independent of the number of measurements. This difference in the electrokinetic behavior of the two membranes has been related to the hydration process of the surface for each sample studied by XPS and AFM.

Journal Article↗
PubMed2000

Investigations into local ferroelectric properties by atomic force microscopy

In this article, we describe nanometer scale characterization of piezoelectric thin films of Lead-Zirconate-Titanate (PZT). Using the electric field from a biased conducting atomic-force microscopy (AFM) tip, we show that it is possible to form and subsequently image ferroelectric domains. Using a sphere-plane model for the tip-sample system we calculate the distribution of electric potential, field and polarization charge, and find good agreement with the experimental values. We also discuss the effects of surface contaminants on domain formation.

Journal Article↗
PubMed2005

Electrical discharge in a nanometer-sized air/water gap observed by atomic force microscopy.

We report a method to initiate and investigate electrical discharges of ambient air/water molecules in a nanometer-sized gap. Our methodology is based on a typical atomic force microscopy (AFM) setup, in which a cylinder discharge gap of < or =5 nm could be configured between the AFM probe and substrate. We observed highly localized stochastic nanoexplosions in which the discharge probability is dominated by the electric field, material-specific surface reactions, and humidity. AFM results, coupled with the boundary element method (BEM), finite element method (FEM), and method of characteristics (MOC) simulations, further revealed the generation of transient shock waves in the nanoscale discharge. The propagation of shock fronts significantly facilitates the radial expansion of the ionized particles, leading to the formation of microscale patterns on selected substrates. Our findings provide an initial understanding of nanoscale discharge and could be relevant to a few applications including nano/microstructuring, microelectronics, and plasma-assisted depositions.

Journal Article↗