Local spectroscopy and atomic imaging of tunneling current, forces, and dissipation on graphite.

Theory predicts that the currents in scanning tunneling microscopy (STM) and the attractive forces measured in atomic force microscopy (AFM) are directly related. Atomic images obtained in an attractive AFM mode should therefore be redundant because they should be similar to STM. Here, we show that while the distance dependence of current and force is similar for graphite, constant-height AFM and STM images differ substantially depending on the distance and bias voltage. We perform spectroscopy of the tunneling current, the frequency shift, and the damping signal at high-symmetry lattice sites of the graphite (0001) surface. The dissipation signal is about twice as sensitive to distance as the frequency shift, explained by the Prandtl-Tomlinson model of atomic friction.

Imaging crystals, polymers, and processes in water with the atomic force microscope.

The atomic force microscope (AFM) can be used to image the surface of both conductors and nonconductors even if they are covered with water or aqueous solutions. An AFM was used that combines microfabricated cantilevers with a previously described optical lever system to monitor deflection. Images of mica demonstrate that atomic resolution is possible on rigid materials, thus opening the possibility of atomic-scale corrosion experiments on nonconductors. Images of polyalanine, an amino acid polymer, show the potential of the AFM for revealing the structure of molecules important in biology and medicine. Finally, a series of ten images of the polymerization of fibrin, the basic component of blood clots, illustrate the potential of the AFM for revealing subtle details of biological processes as they occur in real time.

The Origin of the Superstructure in Bi2Sr2CaCu2O8+dgr as Revealed by Scanning Tunneling Microscopy.

Real-space images with atomic resolution of the BiO plane of Bi(2)Sr(2)CaCu(2)O(8+delta) were obtained with a scanning tunneling microscope. Single-crystal samples were cleaved and imaged under ultrahigh vacuum conditions at room temperature. The images clearly show the one-dimensional incommensurate superstructure along the b-axis that is common to this phase. High-resolution images show the position of the Bi atoms, revealing the structural nature of the superlattice. A missing row of Bi atoms occurs either every nine or ten atomic sites in both (110) directions, accounting for the measured incommensurate periodicity of the superstructure. A model is proposed that includes missing rows of atoms, as well as displacements of the atomic positions along both the a- and c-axis directions.

Atomic force microscopy of an organic monolayer.

Atomic force microscope images of polymerized monolayers of n-(2-aminoethyl)-10,12-tricosadiynamide revealed parallel rows of molecules with a side-by-side spacing of approximately equal to 0.5 nanometer. Forces used for imaging (10(-8) newton) had no observable effect on the polymer strands. These results demonstrate that atomic force microscope images can be obtained for an organic system.

High-resolution electron microscopy and scanning tunneling microscopy of native oxides on silicon.

High-resolution transmission electron microscopy and scanning tunneling microscopy have been combined to examine the structure of the thin "native" oxide that forms on silicon surfaces at room temperature. Differences in the cleaning procedures for silicon wafers may affect the morphology of this oxide and critically influence further processing on the silicon substrates. An etch that ended with a dip in hydrofluoric acid provided a thinner oxide and a lower interface step density than did a sulfuric peroxide treatment. The availability of complementary information from high-resolution transmission electron microscopy and scanning tunneling microscopy is discussed.

Images of a lipid bilayer at molecular resolution by scanning tunneling microscopy.

The molecular structure of a fatty acid bilayer has been recorded with a scanning tunneling microscope operating in air. The molecular film, a bilayer of cadmium icosanoate (arachidate), was deposited onto a graphite substrate by the Langmuir-Blodgett technique. The packing of the lipid film was found to be partially ordered. Along one axis of the triclinic unit cell the intermolecular distance varied randomly around a mean of 5.84 A with a SD of 0.24 A. Along the other axis the mean distance was 4.1 A and appeared to vary monotonically over several intermolecular distances, indicating that a superstructure of longer range may exist. The molecular density was one molecular per 19.4 A2. The surprising ability of the scanning tunneling microscope to image the individual molecular chains demonstrates that electrons from the graphite can be transferred along the molecular chains for a distance of 50 A.

Acoustic microscopy of living cells.

This paper reports preliminary results of the observation by acoustic microscopy of living cells in vitro. The scanning acoustic microscope uses high-frequency sound waves to produce images with submicrometer resolution. The contrast observed in acoustic micrographs of living cells depends on the acoustic properties (i.e., density, stiffness, and attenuation) and on the topographic contour of the cell. Variation in distance separating the acoustic lens and the viewed cell also has a profound effect on the image. When the substratum is located at the focal plane, thick regions of the cell show a darkening that can be related to cellular acoustic attenuation (a function of cytoplasmic viscosity). When the top of the cell is placed near the focal plane, concentric bright and dark rings appear in the image. The location of the rings can be related to cell topography, and the ring contrast can be correlated to the stiffness and density of the cell. In addition, the character of the images of single cells varies dramatically when the substratum upon which they are grown is changed to a different material. By careful selection of the substratum, the information content of the acoustic images can be increased. Our analysis of acoustic images of actively motile cells indicates that leading lamella are less dense or stiff than the quiescent trailing processes of the cells.

Acoustic microscopy of human metaphase chromosomes.

Acoustic micrographs of human metaphase chromosomes have been recorded with wavelengths as short as 470 nm using liquid argon near 85 K as the acoustic wave coupling medium. Chromosomes prepared by trypsin-Giemsa staining exhibit acoustic banding patterns similar to the G-bands seen in optical images. Unstained chromosomes exhibit acoustic markings that do not correspond to traditional banding patterns. The observed acoustic contrast may arise from spatial variations in chromosomal mechanical properties, or from thickness variations in the fixed chromosomes.

Acoustic microscopy: resolution of subcellular detail.

Recent advances now permit the use of scanning acoustic microscopy for the analysis of subcellular components. By sequential viewing of identified fixed cells with acoustic, light, and electron microscopy, we have established that the acoustic microscope can readily detect such features as nuclei and nucleoli, mitochondria, and actin cables. Under optimal conditions, images can even be obtained of filopodia, slender projections of the cell surface that are approximately 0.1-0.2 micron in diameter. Small objects separated by as little as 0.5-0.7 micron can successfully be resolved. Three aspects of the acoustic micrographs prepared in this preliminary survey seem especially prominent. These are, first, the extraordinary level of acoustic contrast that can differentiate the various cytoplasmic organelles, even in regions of very thin cytoplasm; second, the reversals in acoustic contrast that occur when altering the plane of focus; and third, the sensitivity of the acoustic response to overall cytoplasmic thickness. The acoustic microscope uses a novel source of contrast that is based on local mechanical properties. In addition, it can provide a degree of resolution that is comparable to that of the light microscope.