Atomic Force Microscopy In Air Research Articles

Human Plasma Fibrinogen Adsorption on Ultraflat Titanium Oxide Surfaces Studied with Atomic Force Microscopy

Conformational changes of proteins adsorbing on biomaterial surfaces affect biocompatibility. Titanium is among the most successful biomaterials; however, the mechanisms leading to its biocompatibility are not yet understood. The primary objective of this study was to investigate the conformation of human plasma fibrinogen (HPF) adsorbed on titanium oxide surfaces. A method for preparing ultraflat titanium substrates was developed. This allowed high-resolution investigation of both the titanium oxide surfaces and the adsorbed state of HPF. The titanium oxide surfaces were first imaged with an atomic force microscope in air and subsequently incubated in a solution of HPF in phosphate buffer and imaged in fluid with tapping mode AFM. The titanium oxide surfaces exhibited a root-mean-squared (RMS) roughness of (0.29 ± 0.03) nm over (1.00 × 1.00) μm2 areas. Different degrees of molecular order were found on the titanium oxide surface. In crystalline surface areas square lattices with parameters a0 = b0 ≈ 0.5 ...

Observation of Human Corneal and Scleral Collagen Fibrils by Atomic Force Microscopy

Purpose: We attempted to analyze the three-dimensional ultrastructure of human corneal and scleral collagen fibrils with an atomic force microscope (AFM). Methods: A normal eye removed from a 66-year-old male was used in the study. Suspended corneal and scleral collagen fibrils were individually attached to glass slides by centrifugation. These collagen fibrils were air-dried and observed with a noncontact mode AFM in air. Results: AFM imaging provided information on the surface topography of both corneal and scleral collagen fibrils. The corneal collagen fibrils had a height of 11.9 ± 1.0 (mean ± standard deviation) nm and the scleral fibrils of 82.5 ± 35.6 nm. A periodic banding pattern of grooves and ridges was clearly found in both types of fibrils: the D-periodicity and the groove depth were 65.7 ± 0.8 nm and 1.46 ± 0.50 nm in the corneal fibrils, and 67.3 ± 1.1 nm and 6.16 ± 1.23 nm in the scleral fibrils. Conclusions: Surface topographic images of human corneal and scleral collagen fibrils were clearly obtained with the AFM. This technique provides quantitative information on the surface morphology of the collagen fibrils at high resolution.

Reentrant mound formation in GaAs(001) homoepitaxy observed by ex situ atomic force microscopy

A study of the surface morphology of homoepitaxial GaAs(001) by means of ex situ atomic force microscopy in air reveals the reentrance of mounding behavior at low growth temperatures. A transition from statistical roughening to organized mound formation is observed as the growth temperature is reduced. We show by means of growth simulations that the observed morphology is compatible with anisotropic adatom diffusion in the presence of an Ehrlich-Schwoebel barrier. The mechanism leading to this kind of adatom kinetics at low temperatures is interpreted in terms of surfactant acting arsenic condensing on the surface.

Atomic force microscopy in the surface characterization of semiconductors and superconductors

Abstract In the present paper I summarize the recent experience of my laboratory in the field of atomic force microscopy (AFM) applied to the study of the surfaces of both semiconducting and superconducting materials. I show that very useful results, both in fundamental and in applied physics, can be obtained by using simple contact-mode AFM in air, provided that particular attention is paid to the vibration isolation of the experimental set-up and to the cleavage of the samples' surface just before the measurement. Some examples are presented from the study of conventional and high-T C,. layered superconductors up to the atomic resolution, to the precise determination of the thickness of GaAs/AlAs multilayers and of the best deposition parameters for the sputtering of piezoelectric thin films.

Microprocess for fabricating carbon-nanotube probes of a scanning probe microscope

We have developed microprocesses to make carbon-nanotube probes for a scanning probe microscope. The processes contain electric-field induced transportation, welding and fixation by electron-beam carbon deposition and are performed in a scanning electron microscope equipped with two individual manipulable stages. Using the nanotube probes produced, a fine structure of helical and twinned deoxyribonucleic acid and an abrupt height transition with high fidelity in a 4.7 GB digital versatile disk are imaged with tapping-mode atomic force microscopy in air.

Ferromagnetic dot formation in nonmagnetic amorphous FeZr films by non-contact atomic force microscope

Ferromagnetic nanometer size dots were formed in the nonmagnetic amorphous FeZr films using a non-contact atomic force microscope in air. The shape of fabricated dots changes from a mound (about 70 to a few thousand nm diameter) to a doughnut-like protuberance, depending on both the pulse voltage V p ( V p=8–28 V) applied to vibrating Si tips and the tip–sample distance d ( d=3–22 nm). The crystallization of amorphous FeZr and successive dot formation are considered to take place triggered by an electron injection into films due to electron tunneling and breakdown of air.

Formation of nano-pyramids of layered materials with AFM

We have been able to raise squares of around 2 Å in height of the layered materials Bi 1.6Sb 0.4Se 3, Bi 1.9Sb 0.1Se 3 and Bi 2Se 3 by means of contact atomic force microscopy in air. By raising squares on/beneath previously produced squares, Mayan-like pyramids have been constructed, each step of the pyramid being around 2 Å in height. We neither remove matter from the surface nor produce a localized oxidation of the sample; we elevate a portion of it. When the surface is oxidized, the AFM cantilever pulls the oxide off producing holes. The raising of the squares is also possible in a nitrogen atmosphere.

Three dimensional (3D) analysis of the morphological changes induced by 50 Hz magnetic field exposure on human lymphoblastoid cells (Raji).

Human Raji B lymphoid cells after exposure for 64 h to a 1 mT (rms) 50 Hz sinusoidal magnetic field showed a reorganization of membrane and cytoskeletal components. Atomic force microscopy in air revealed several modifications in 80% of the exposed cells, such as loss of microvilli-like structures followed by progressive appearance of membrane introflections. This change in plasma membrane morphology was also accompanied by a different actin distribution, as detected by phalloidin fluorescence. These observations support our previous hypothesis that electric and magnetic fields may modify the plasma membrane structure.

Atomic force microscopy of intact and digested collagen molecules.

The present study was performed to analyse the structure of non-digested and digested collagen type I molecules by atomic force microscopy (AFM). Collagen type I molecules from the bovine skin were diluted with 0.05 N acetic acid, spread on a mica plate, air-dried and observed by non-contact mode AFM in air. Collagen molecules digested with Clostridium histolyticum collagenase were also examined by AFM. Intact collagen type I molecules were observed as twisted threads ranging mainly between 280 and 310 nm in length. The surface of the molecules was uneven and both ends usually slightly bulged like a globule. Depressions on the molecules were found throughout the length, and were most prominent approximately 70 nm from one end of the molecules. The collagenase-treated collagen molecules were degraded into fragments with various lengths, which corresponded to the data from sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis. The end of these fragments often appeared like a tuft, suggesting that the triple-helix unraveled at these regions.

Carbon-nanotube tips for scanning probe microscopy: Preparation by a controlled process and observation of deoxyribonucleic acid

We report a controlled process to make carbon-nanotube tips for scanning probe microscopes. The process consists of three steps: (1) purification and alignment of carbon nanotubes using electrophoresis, (2) transfer of a single aligned nanotube onto a conventional Si tip under the view of a scanning electron microscope, and (3) attachment of the nanotube on the Si tip by carbon deposition. Nanotube tips fabricated using this procedure exhibit strong adhesion and are mechanically robust. Finally, the performance of these tips is demonstrated by imaging the fine structure of twinned deoxyribonucleic acid with tapping-mode atomic force microscopy in air.

Determination of the surface area of annealed continuous gold film and pyrex glass in situ by the BET adsorption isotherm and ex situ by Atomic Force Microscopy in air

The relative surface area, Rsa, of annealed continuous thin gold film, thermally evaporated onto a Pyrex glass substrate at 78 K in an ultrahigh-vacuum (UHV) environment, and Pyrex glass has been determined in situ by the BET (Brunauer–Emmett–Teller) isotherm method, as well as ex situ by Atomic Force Microscopy (AFM) in air. It has been measured by the BET isotherm method that Rsa for clean, continuous and annealed gold film, investigated under UHV conditions, is equal to 6.3, whereas after its exposure to air, at about 100 Pa, Rsa decreased from 6.3 to 5.7. Further exposure to air, at atmospheric pressure, reduced Rsa from 5.7 to 2.2, whereas the exposure of contaminated Au film to the atomic hydrogen flux caused the increase of Rsa from 2.2 to 4.2. On the other hand, Rsa calculated directly from the AFM image of gold film in air is equal to 1.0. In contrast to thin gold film, the Pyrex glass surface area, measured under UHV conditions by the BET adsorption isotherm and in air by the AFM technique, is equal to its geometric area, i.e., Rsa=1.0.

Biometrology? Finding Biological, Pathological, Diagnostic Meaning From Critical Length-Scale Measurements

Abstract The atomic force microscope (AFM) is now a fairly widely used metrological tool for measuring surfaces with nanometer-scale precision, in 3-D. A number of biologists have noticed that untreated biological cells and molecules deposited onto surfaces can also be analyzed by AFM in air or fluids provided the AFM imaging forces do not greatly excede the compressiblity of the specimen. What has gone largely unnoticed in the biological microscopy community is the fact that AFM provides a wealth of opportunity with regard to 3D, nanometer-scale morphometric and biochemical measurements of biomaterials, cells and molecules. The ability to obtain such precise length-scale measurements on biological structures does open up a whole new branch of biological microscopy- something we have termed, Biometrology. A broad definition of Biometrology is, the science of quantifying biological structures such that the measured quantities give rise to biological or diagnostic information.

Structural, optical and electronic properties of oxidized AIN thin films at different temperatures

We report on the properties of a novel insulator, AlO:N for application in semiconductors produced by thermally oxidizing AlN thin films. The process steps were similar to those used for SiO2, creating the possibility of a new technology for metal-insulator-semiconductor field effect devices and integrated circuits. Thin films of AlN were deposited by radio-frequency magnetron reactive sputtering on p-type silicon or fused quartz substrates. As-deposited AlN film thickness ranged from 0.05 to 0.7 µm, with polycrystalline structure revealed by x-ray diffraction. Oxidation was performed under O2 flow at 800 to 1100°C for 1–4 h. AlN films were oxidized partially or fully into Al2O3, depending on initial thickness, oxidation temperature and time. X-ray diffraction indicates the presence of several phases of Al2O3 at 1000°C, whereas at 1100°C, only the α-Al2O3 phase was found. Considering the importance of surface field effect device applications, the surfaces of oxidized films were examined with atomic force microscopy in air, and a clear change was observed in the surface structure of the oxidized film from that of as-deposited AlN films. Capacitance-voltage measurements of metal-oxide-semiconductor structures yielded a dielectric constant of AlO:N between 8–12 and a net oxide-trapped-charge density of ∼1011 cm−2. Using Fourier transform infrared spectrometry transmittance and reflectance, some α-Al2O3 modes were observed. In this paper, we describe the general properties of the oxide thin films, bulk and interface, at different temperatures.

Surface and interface properties of quantum nanostructures grown on nonplanar substrates

The surface and interface properties of self-ordered quantum well, quantum wire and quantum dot GaAs/AlGaAs and InGaAs/AlGaAs structures grown by low pressure organometallic chemical vapor deposition (OMCVD) on V-grooved substrates are described. Transmission electron microscopy as well as cross sectional and top atomic force microscopy in air are employed to investigate the self ordering mechanisms of these nanostructures. OMCVD on the patterned substrates leads to step-flow growth with monolayer steps oriented perpendicular to the grove edges and to self-ordered nanofacets at the bottom of the grooves. These ordered, patterned surfaces form the basis for the formation of AlGaAs and InGaAs quantum wells and quantum wires whose size and shape are determined by the composition and the growth conditions. The top surfaces of the V-groove quantum wires reveal quasi-periodic facet undulations, suggesting the formation of ordered, dot-like structures providing three-dimensional quantum confinement.

Self-limiting growth of GaAs surfaces on nonplanar substrates

The growth of GaAs epitaxial layers by organometallic chemical vapor deposition on top of V-grooved substrates is found to exhibit a self-limiting behavior. As in the case of the self-limiting growth of AlGaAs on similar patterned substrates, the self-limiting GaAs profile exhibits characteristic crystallographic nanofacets. However, these facets are considerably broader than in typical, self-limiting AlGaAs profiles obtained at similar growth temperatures. Atomic force microscopy in air reveals the three-dimensional structure of the self-limiting GaAs surfaces, showing monolayer steps on the central (100) nanofacets and quasiperiodic modulation caused by step bunching on the side {311}A nanofacets. The width of the self-limiting GaAs V grooves can be reduced to less than 10 nm at sufficiently low growth temperatures, thus providing useful templates for growing, e.g., self ordered InGaAs/GaAs quantum wires.

Height anomalies in tapping mode atomic force microscopy in air caused by adhesion

Height anomalies in tapping mode atomic force microscopy (AFM) in air are shown to be caused by adhesion. Depending on the damping of the oscillation the height of a sticking surface is reduced compared to less sticking surfaces. It is shown that the height artefacts result from a modulation of oscillatory movement of the cantilever. Damping and excitation of the cantilever by the driver continuously compete. As a consequence a severe modulation of the cantilever oscillation occurs, depending on the phase mismatch between the driver and the cantilever. Phase images of tapping mode AFM show contrast which correlates with adhesion. Examples of a partially removed gold layer on mica, a Langmuir-Blodgett film and DNA show height artefacts ranging up to 10 nm.

Imaging excised apical plasma membrane patches of MDCK cells in physiological conditions with atomic force microscopy.

We combined the patch-clamp technique with atomic force microscopy (AFM) to visualize plasma membrane proteins protruding from the extracellular surface of cultured kidney cells (MDCK cells). To achieve molecular resolution, patches were mechanically isolated from whole MDCK cells by applying the patch-clamp technique. The excised inside-out patches were transferred on freshly cleaved mica and imaged with the AFM in air and under physiological conditions (i. e. in fluid). Thus, the resolution could be increased considerably (lateral and vertical resolutions 5 and 0.1 nm, respectively) as compared to experiments on intact cells, where plasma membrane proteins were hardly detectable. The apical plasma membrane surface of the MDCK cells showed multiple protrusions which could be identified as membrane proteins through the use of pronase. These proteins had a density of about 90 per micron(2), with heights between 1 and 9 nm, and lateral dimensions of 20-60 nm. Their frequency distribution showed a peak value of 3 nm for the protein height. A simplified assumption - modelling plasma membrane proteins as spherical structures protruding from the lipid bilayer - allowed an estimation of the possible molecular weights of these proteins. They range from 50 kDa to 710 kDa with a peak value of 125 kDa. We conclude that AFM can be used to study the molecular structures of membranes which were isolated with the patch-clamp technique. Individual membrane proteins and protein clusters, and their arrangement and distribution in a native plasma membrane can be visualized under physiological conditions, which is a first step for their identification.

Charge Dissipation on Chemically Treated Thin Silicon Oxide in Air

We studied the microscopic charge dissipation of densely contact-electrified charges on silicon oxides with and without a trimethylsilyl (TMS) organosilane monolayer using a modified atomic force microscope in air. Here, the TMS film was used to change a hydrophilic surface to a hydrophobic one. As a result, for both of the deposited negative and positive charges, it was clarified that the TMS film can suppress the rapid dissipation induced by surface diffusion. For the positive charge, however, the TMS film enhanced the charge dissipation induced by recombination through the TMS film and silicon oxide.

Nanometer-scale lithography on H-passivated Si(100) by atomic force microscope in air

We report on the mechanical friction method for a fabrication of a Si nanostructure on a H-passivated Si(100) substrate using an atomic force microscope (AFM) in a contact mode in air. The bare Si surface region exposed by the mechanical friction between a silicon nitride tip and Si surface was fully oxidized by ambient oxygens. The oxide mask patterns could withstand a selective wet etching process for pattern transfer. The width of the oxide layer formed by an AFM tip was about 200 Å. As the etching time and scan rate were decreased, the oxide line shape was improved. This study also showed that there exists a critical tip force in the removal of a H-passivating layer.

Atomic step distributions on annealed periodic Si(001) gratings

One-dimensional (1D) and two-dimensional (2D) periodic square-wave gratings fabricated on Si(001) with repeat spacings of 4 and 5 μm and amplitudes of 0.1–0.25 μm were annealed in ultrahigh vacuum at temperatures between 900 and 1200 °C. The surface profiles of the gratings were determined by atomic force microscopy in air after cooling to room temperature. The microscopic structures of the steps and terraces of the gratings were investigated in situ using low-energy electron microscopy. The morphology of the gratings changes with annealing temperature. At 950 °C, both 1D and 2D gratings have surface profiles resembling truncated sinusoids, with large flat regions at the grating extrema. Between 1050 and 1100 °C, the flats at the minima of the 2D gratings expand whereas the flats at the maxima shrink. This asymmetry does not occur for the 1D gratings in this temperature range. At 1200 °C, the profiles of both the 1D and 2D gratings become sinusoidal, with no flats at the extrema. The different morphologies and their temperature dependence can be explained on the basis of the thermodynamics of the surface and the role of surface diffusion and evaporation.