We have developed a method that allows a systematic study of the ability of the Scanning Tunneling Microscope (STM) for imaging DNA. A drop of a solution containing the DNA is deposited on a conductive substrate. Then a mesh screening (mask) is placed on the sample followed by evaporation of a thin Platinum-Carbon film. The mask is removed after the evaporation. This exposes coated and uncoated DNA for examination. Metal-coated DNA can be identified unambiguously by the STM and it is possible to find sections with both coated and uncoated regions. Our results show that the contrast given by uncoated DNA depends on the conductive substrate used. In the cases examined, our results suggest a strong tip-molecule interaction.

Reproducible images of pBR322 plasmid molecules have been recorded by Atomic Force Microscopy (AFM) under n-propanol. By applying a stratigraphic analysis which takes advantage of the height information contained in the AFM images, it is possible to assign the chirality of the local supercoiling of the individual molecules.

We used the atomic force microscope (AFM) to observe structure of the tooth, both rat and human. The rigidity and the surface flatness of thin sections of this mineralized tissue, allow us to attain good resolution with the AFM. As enamel contains uniquely large crystals of hydroxyapatite it can be investigated at high resolution. Tooth enamel and thin slices of undecalcified developing tooth germs from 2 - 12 day old rats were observed, embedded in acrylic resin (Lowicryl K4M). In addition, as orthophosphoric acid is widely used clinically to etch tooth enamel before restoring with composites, we studied its action at pH2 on the tooth surface during 1 hour of exposition. Hydroxyapatite crystals and collagen fibers were seen in the tooth slices observed in air, and the classical structure of the enamel was visible. The etched enamel surface under liquid, showed dramatic differences to that imaged in air. Modifications to the surface were also seen during exposure to the acid.

The Atomic Force Microscope (AFM) can easily image 'hard' sample surfaces with atomic or molecular resolution. For 'soft' samples, such as organic macromolecules or biological objects, this resolution power is very difficult to reach, because the AFM tip causes large deformation. This deformation makes the sample surface to appear thinner in the AFM image. We have observed this effect on a Langmuir-Blodgett (LB) film of pentadecavaline which has been imaged in water as well as in air. The high capillary forces when imaging in air cause the film to appear half as thick as expected. In water, where the capillary forces are eliminated, the height of the LB film in the AFM image is correct. On actin fibers even a small change in the applied force has a big effect on the height of the AFM image: Changing the force from 0.9 nN to 1.8 nN decreases the apparent height from 5.7 nm to 3.6 nm. Increasing the force to 18 nN, brings the height down to 2.1 nm.

We are attempting charge mapping and potentiometry on biological and molecular systems using a modified Atomic Force Microscope (AFM)--Electrostatic Force Microscope (EFM). With an interferometric detection system, we have obtained potentiometric sensitivity better than 0.1 mV and spatial resolution of about 200 angstroms. The images of red blood cells in air clearly show surface potential variation, independent of topography. We imaged FEP ((poly)tetrafluororethylene -co- hexafluoropropylene) films lithographically patterned with APS (poly(aminopropyl)siloxane). We have been able to positively identify the modified regions (APS) from the unmodified regions (FEP) using charge information. The charge density we estimated on the unmodified FEP regions of our samples is about 10^-9C/cm².

We present several experimental results of imaging DNA on graphite. Images have been taken simultaneously in constant-current and gap-modulated mode. This way of imaging has been very helpful in order to discriminate between structures due to biological molecules and substrate artifacts. A possible way of using the tip to confirm this conclusion is shown.

The spatial resolution afforded by near-field scanning optical microscopy (NSOM) is primarily a function of tip size and tip-sample separation. Combining scanning force microscopy with NSOM allows one to maintain a small tip-sample separation distance and, consequently, optimize NSOM resolution. This provides, simultaneously, a topographic perspective of the sample as well as an NSOM image. We present, in this paper, an instrument that provides simultaneous shear force and reflection NSOM images. We also incorporate a tip deflection detection scheme that allows the force signal to be completely decoupled from the optical signal. In order to accurately analyze the NSOM images, it is important to understand the feedback mechanism so that proper image deconvolution can be performed. Considerations concerning the forces measured are made. A discussion concerning Raman scattering capabilities in this regime is also provided, along with some preliminary Raman data.

Near field scanning optical microscopy (NSOM) provides a number of unique capabilities for high resolution imaging. In this regard, a fundamental aspect of the technique is its ability to retain much of the characteristics available in diffraction limited optical probing. Results are presented on the use of near field scanning optical microscopy (NSOM) in imaging a variety of samples, using different contrast mechanisms. The approaches adopted are based on the recently introduced simultaneous, non-contact, near field optical microscope with atomic force regulation. Amongst the techniques discussed are linearized polarizing microscopy, as well as amplitude, and phase, interference contrast imaging modalities.

Near-field Photodetection Optical Microscopy (NPOM) is a new approach to optical surface characterization with sub-wavelength spatial resolution. The technique is based upon the direct detection of local variations in optical intensity near a surface with a photodetector of nanometer size. The ultra-small photodetector probe is raster scanned in close proximity to an optically illuminated surface and the photoresponse of the probe is measured at each point. The local optical information is electronically stored and displayed by computer in a manner similar to all Scanning Probe Microscopies. The resultant image represents the local optical intensity at each point on the illuminated surface. The probe fabrication and a recent experimental demonstration of the probe capabilities will be described.

Biological samples, molecular solids and solid state devices have been investigated by Near- Field Scanning Optical Microscopy (NSOM), Near-Field Optical Spectroscopy, and Near- Field Chemical Sensing. We report here on our progress in applying the NSOM technology to various biological and physical systems. Results demonstrating both spatial and spectral resolution as well as image contrast unique to the near-field technique are presented.

The effective amplitude of the interference pattern produced by sending a laser beam back upon itself in a BK-7 glass prism was measured in the evanescent wave region outside the prism using a tapered fiber tip scanned and retracted piezoelectrically. This effective amplitude was related to the effective aperture size of the fiber by assuming that the finite extent of the fiber averaged the optical signal over a Gaussian distribution. In this way the resolving power of the photon scanning tunnelling microscope was determined operationally. Combining this information with electron microscopic imaging of the fiber tip leads to the conclusion that upon heating and drawing the fiber, its cladding decreases in size more rapidly than the core so that final ratio of core diameter to outer diameter in the tapered fiber is approximately twice the original ratio.

Scanning Tunneling Microscopy (STM) image of adsorbed atoms and molecules on single crystal substrates provide important information on surface structure and order. In many cases images are interpreted qualitatively based on other information on the system. To obtain quantitative information a theoretical analysis of the STM image is required. A new method of calculating STM images is presented that includes a full description of the STM tip and surface structure. This method is applied to experimental STM images of sulfur adsorbed on Re(0001). The effects of adsorption site, adsorbate geometry, tip composition and tunnel gap resistance on STM image contrast are analyzed. The chemical identity of the tip apex atom and the substrate subsurface structure are both shown to significantly affect STM image contrast.

We have investigated the surface morphology of relaxed, compositionally graded Ge_xSi_1-x structures to study the influence of defect-related strain fields on film growth. Quantitative topographic measurements via scanning force microscopy show that the roughness associated with the cross-hatch patterns, due to underlying misfit dislocations beneath the surface, increases as the final Ge concentration or the grading rate increases. We further show that strain fields arising from the termination of threading dislocations at the surface result in shallow depressions. In addition to the as grown samples, we have also studied the morphology of processed Si, Ge, and Ge_xSi_1-x surfaces. Protrusions are observed on top of the long-wavelength morphology when the Ge_0.75Si_0.25 films are annealed at 900 degree(s)C for as short as 1 minute. These protrusions are unique to the alloy films and not seen in pure Si or Ge. We offer an explanation of this process induced morphological change and discuss the effect of tip shape on images.

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been successfully applied to investigate the surface structure and the electron density of states of organic conductors and superconductors. The structural nature of organic conductors and superconductors makes their transport properties susceptible to one-dimensional effects. Low- dimensionality effects in the electrical conductivity of these materials are investigated using scanning tunneling microscopy at room and low temperatures. Effects such as charge density waves and Peierls instabilities are directly observed with the STM. The consequences of low- dimensionality in the electrical conductivity of these materials will be presented.

The vortex lattice in clean 2H-NbSe₂ is investigated by STM at 4.2 K. The experimental results predominantly address three major issues of current importance. (1) Sufficiently large in-plane transport currents through the superconducting sample cause the motion of vortices, which can directly be detected. (2) The apparent vortex diameter is about twice that expected from elementary theory. A pronounced shrinking of the vortex core with increasing external magnetic field is observed. (3) The STM results combined with some elementary magnetostatic model calculations shed some light on the difficulties encountered in the imaging of flux line lattices by magnetic force microscopy.

A high resolution scanning Hall probe microscope is used to spatially resolve vortices in high temperature superconducting Bi₂Sr₂CaCu₂O_8+(delta) crystals. We observe a partially ordered vortex lattice at several different applied magnetic fields and temperatures. At higher temperatures, a limited amount of vortex re-arrangement is observed, but most vortices remain fixed for periods long compared to the imaging time of several hours even at temperatures as high as 75 degree(s)K (the superconducting transition temperature for these crystals is approximately 84 degree(s)K). A measure of these local magnetic penetration depth can be obtained from a fit to the surface field of several neighboring vortices, and has been measured as a function of temperature. In particular, we have measured the zero temperature penetration depth and found it to be 275 +/- 40 nm.

A fundamental study on a magnetic shielding material known as METSHIELD has been conducted. The material is basically a new fabric made of thin ribbons of a metallic glass, amorphous alloy. The study was focused on determining the physical and magnetic properties of the alloy in as received (amorphous) and annealed (crystalline) conditions. Furthermore characterizations of the internal and surface structures of the alloy have been carried out by a transmission electron microscope (TEM) and a scanning tunneling microscope (STM), respectively. A comparison between the internal and surface structures was performed on both as-received and annealed samples. As a result, a correlation between the obtained properties and structures of the material is established.

We have developed magnetic force scanning tunneling microscopy as a powerful tool to analyze magnetic patterns on recording media with sub-micron resolution. The technique employs the interaction of surface magnetic fields with a flexible thin-film magnetic probe. We have made a thorough theoretical analysis of the interaction between the probe and the surface magnetic fields emanating from a typical recorded pattern. Quantitative data about the constituent magnetic fields and the underlying magnetization patterns can then be obtained. We have employed these techniques in studies of two of the most important issues of magnetic recording: data-density and data overwrite. In the course of these studies we have developed new techniques to analyze the magnetic fields of recorded media. These studies are both theoretical and experimental and combined with the use of our magnetic force scanning tunneling microscope should lead to further breakthroughs in the field of magnetic recording.

Work is described on the development of a scanning near-field optical microscope (SNOM) for the primary purpose of imaging magnetic systems with resolution on the order of 10 nm. Since many magnetic materials are optically opaque, it is desired to have a probe which is appropriate for reflection mode. In addition, the near-field probe must be linearly polarizable, since the magneto-optic Kerr effect (MOKE) will provide the contrast mechanism necessary for magnetic imaging. Data is presented on the characterization and use of approximately 30 nm diameter Ag particles as probes for MOKE-sensitive SNOM. Such small metal particles exhibit a localized plasmon resonance in the visible, which greatly enhances their optical scattering cross-section. Ag particles prepared by colloidal chemistry techniques have been deposited on bismuth doped YIG and silica-coated Permalloy to measure the magnitude of the near-field MOKE. The optical apparatus used to measure near-field MOKE is described. Hysteresis loop data is presented for Bi-YIG using the near-field MOKE. The effect is also seen in Permalloy, with a surprisingly large peak-to-peak rotation of 3 milliradians. A heuristic model is proposed for explaining the near-field MOKE in longitudinally magnetized materials. Finally, it is reported that we now have conventional MOKE microscopic imaging capabilities which will allow us to observe magnetic domain structures in weak magneto-optic materials, such as Permalloy.

We have developed a novel Magnetic Force Microscope (MFM) utilizing a vertically cantilevered microprobe tip. This new geometry provides maximum sensitivity while inhibiting uncontrolled vertical deflections. We demonstrate the capability of our MFM by imaging domain structure in pre-recorded magnetic tape and domain walls in single-crystal iron whiskers. Good agreement is obtained between the observed magnetic contrast and predictions of a micromagnetic model.

Charge trapping in thin films of silicon nitride has long been studied for use as a non-volatile semiconductor memory. Recently, this technology has been combined with scanned probe technologies with the sharp probe tip serving as the upper electrode in a Si₃N₄- SiO₂Si (NOS) structure. By applying a voltage pulse between the tip and silicon substrate, charge carriers can be made to tunnel through the oxide and be trapped in the nitride. This trapped charge causes a shift in the capacitance-voltage curve along the voltage axis; the voltage at which depletion occurs is increased. It has been proposed that such a system could be used as a high density data storage device. We have begun to explore some of the issues related to such an application, including data lifetime and data rates. In thermally accelerated life tests, no sign of charge spreading was seen after 100 days at 150 degree(s)C and from the rate of charge decay we would predict room temperature lifetimes in excess of 1 million years. We have also used an air-bearing spindle to conduct high speed measurements on a spinning NOS sample and obtained data rates as high as 500 kHz with carrier-to-noise ratios of approximately 60 dB in a 3 kHz bandwidth.

We have developed a stand-along atomic force microscope featuring large scan, friction measurement, atomic resolution and capability of in liquid operation. Cantilever displacements are detected with optical beam deflection. Cantilever and laser diode are both attached at the piezo tube and thus scanned simultaneously. As a direct consequence the maximum scan range, 25 X 25 micrometers ², is solely determined by the characteristics of the piezo tube and not by the dimensions of the cantilever and/or the waist of the laser beam. The stand- along atomic force microscope is suitable to be combined with any inverted optical microscope (including the confocal laser scanning microscope), as is illustrated with fluorescence and height images of K562-cells. Results on thin films consisting of a mixture of polymers show the strength of measuring friction and height simultaneously. Images of mica show that atomic resolution can be obtained both in height and friction mode.

A combination of SEM, STM, and SFM microscopy determined structural and elastic properties of (beta) -chitin, a linear polysaccharide that serves as a skeletal material in some marine species. Each microscopy technique produced artifacts that may lead to incorrect conclusions, but consistent results from multiple techniques improve the confidence in the interpretation. Our studies confirmed the existence of 40 nm X 20 - 50 nm substructures in chitin fibers from the diatom Thalassiosira fluviatilis, and found several surface structures not previously reported. We also investigated a technique to enhance subtle features in images by vibrating an SFM sample stage. The SFM directly measured the Young's Modulus of the fibers to be 1 +/- 0.5 X 10¹¹ N/m².

We have developed a new and simple technique of thermal imaging with sub-micron spatial resolution using the atomic force microscope (AFM). By using a thermocouple as an AFM tip, we can simultaneously observe the topography and the temperature field of material surfaces. The method is particularly unique for application of biased electronic devices or interconnects where there could be different materials and potential variations on a scan surface. Application to a n-GaAs MESFET has revealed hot spots under the drain-side of the gate. Thermal images of a biased polycrystalline Al-Cu via structure show the grain boundaries to be hotter than within the grain suggesting higher electron scattering rates. We have also observed the effects of current crowding in generating hot spots in the via structure. An error analysis showed that the difference between the measured temperature and the estimated actual device temperature is about 4 percent.