A force sensor based on a fiber-optic interferometric displacement transducer incorporated in an ultrahigh vacuum atomic force microscope is described. The operation of the sensor is based on balancing the tip-sample interfacial force using an electrostatic actuator. The electrodes of the actuator are formed by the grounded W cantilever and the metallized end facet of the optical fiber used by the interferometer. Chemical reduction of Ag by a wet chemical method is used for metal coating of the fiber end. A special masking procedure is used to obtain a window hole in the metal coating at the position of the fiber core to allow for optical beam output. Using a window instead of a semitransparent metal film allows us to save the low-finesse characteristics of the interferometer which facilitates the calibration of cantilever displacement. The performance of the sensor is discussed and exemplified by experimental results from force-separation measurements on the W-Au system in ultrahigh vacuum. (C) 2000 American Institute of Physics. [S0034-6748(00)01401-5].

The force on a W tip has been measured in ultrahigh vacuum during approach and indentation against Si(111). The maximum separation where we detect an attractive force is 5 Å, which is within the range where wave-function overlap should occur. Calculating the van der Waals force using the sphere-flat geometry and a radius of 150 Å that we determine by scanning electron microscopy overestimates the force, while a cone-shaped tip apex is consistent with our results. After contact we observe discontinuous steps in the force curve in agreement with molecular-dynamics simulations. The stiffness of the junction increases during the first 5 Å after contact and stabilizes at ~25 N/m, which is approximately a factor of 3 lower than estimated from classical elasticity theory. The maximum adhesive force is ~5 nN. We also present a model that relates the dynamic behavior of the force-feedback controller to the formation of a connective neck over the tip-surface gap.

Carbon and carbon nitride films, grown in argon or nitrogen discharges by reactive Jc magnetron sputtering of a graphite target, were characterized by in situ scanning tunneling microscopy. When the growth temperature increased from ambient to 800 degrees C, we observed a topographic evolution of the carbon films from an amorphous to a graphitelike structure, and further to a distorted-graphitic phase with curved and intersecting basal planes, and finally to a surface containing nanotubes and nanodomes. When nitrogen was incorporated into the films, distortion of the graphitic basal planes occurred at a lower temperature compared to the pure carbon case. At temperatures of similar to 200 degrees C and above, regions of a nongraphitic phase, containing a high degree of carbon sp(3) bonds were observed. Spatially resolved tunneling spectroscopic measurements indicated that the band gaps were 0, similar to 0-0.6 eV, and similar to 0.4-2.0 eV for graphitelike structures, the distorted-graphitic phase, and the nongraphitic phase, respectively. Together with ex sial x-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy measurements, the results suggest that the incorporation of nitrogen promotes bending of the graphitic basal planes and thereby facilitates the formation of three-dimensional covalently bonded networks with a high degree of sp(3)-coordinated carbon atoms.

During the last few years the Atomic Force Microscope (AFM) has become capable of routinely obtaining atomic resolution when operated with a vibrating cantilever (ac-mode). Local measurement of the tip-sample force (force spectroscopy) is a powerful tool for investigations of contact phenomena at the atomic scale that are important in fields like friction, tribology, atom manipulation and chemical bond formation. This paper reviews several aspects of the AFM technique such as tip-surface forces, force sensors, operation modes and contrast effects. A study of the Si(111)7 x 7 reconstruction is presented as an example of high resolution AFM imaging.

We report the observation, by scanning tunneling microscopy (STM), scanning electron microscopy (SEM), Auger electron spectroscopy (AES), and atomic force microscopy (AFM), of island formation on SIC during high temperature deposition and annealing of thin Ni films. Ni films with a nominal thickness of 2.5 monolayers were sputter deposited onto H-2-etched single crystal 6H-SiC (0001) substrates heated to 600 degreesC in an ultrahigh vacuum STM system. After the substrates were annealed to 800-1000 degreesC, island formation was observed by STM. The islands were 0.1-0.5 mum in diameter, similar to 30 nm high, and separated by similar to2 mum from each other, with an exceptionally flat top with a peculiar 'stitched' surface structure. A second type of island, similar to1.5 mum in diameter, similar to 10 nm high, and separated by similar to 10 mum from each other, was observed by ex situ AFM and SEM. Microspot AES showed that the first islands are composed of Ni and C, while the second islands are composed of Ni, C, and Si. AES lineshape studies showed that the carbon in both types of islands is graphitically bound as opposed to the carbon in the substrate which is carbidically bound. From comparisons to literature, we believe that the first islands are a new type of graphite intercalation compound. An indexing of Ni on the top graphite sheets is presented for each anneal temperature.

Studies of epitaxial growth of Mo thin films on MgO(001) substrates by ultrahigh Vacuum (UHV) d.c. magnetron sputter deposition have shown independently by in situ low energy electron diffraction (LEED), scanning tunnelling microscopy (STM) and time-resolved in situ reflection high-energy electron diffraction (RHEED) measurements that oxygen is present on the Mo surface during the initial stages of growth. Oxygen induced reconstructions are only found for films thinner than similar to 50 nm, and it is shown that the oxygen originates from the MgO substrate. The oxygen is causing the growing film surface to continuously reconstruct with the (root 5 x root 5)R26 degrees 33', p(2 x 2) and c(2 x 2) structures indicating O coverages ranging from similar to 0.8-1.0 ML. STM measurements confirm earlier X-ray diffraction (XRD) and STM measurements of the (root 5 x root 5)R26 degrees 33' reconstruction and show for the first time STM images of the surface structures where p(2 x 2) and c(2 x 2) reconstructions are present. Based on our STM data we suggest surface models for the p(2 x 2) and c(2 x 2) reconstructions. The STM measurements also revealed surfaces randomly interspersed with apparent 2x2 'holes' not visible by techniques such as LEED due to their non-periodic nature. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

Under certain conditions morphology changes occur when thin Pd films, grown on SiO₂ at room temperature, are subject to elevated temperatures. First holes in the metal are observed, followed by network formation and finally isolation of metal islands. This process is known as agglomeration. The influence of gas exposures on this restructuring process has been studied by following variations in the capacitance of the structure and by atomic force microscopy, transmission electron microscopy and ultraviolet photoelectron spectroscopy. The capacitance measurements show that carbonaceous species have an impeding influence on the rate of agglomeration and may lock the film structure in a thermodynamic non-equilibrium state. By removing these species with oxygen exposure, i.e. by forming volatile CO and CO₂, a clean surface is obtained and the agglomeration process can proceed. High oxygen or hydrogen coverages also lower the rate of restructuring, compared to the case of a clean surface. For the clean Pd surface, an apparent activation energy of 0.64 eV is found for the restructuring process.

Some recent results from the scanning force microscopy activity at our laboratory are presented. A brief description of attractive mode force microscopy is followed by a discussion of the following examples: O₂/H₂-induced morphology changes in thin palladium films, structure of spin cast polysulfone films, fibrinogen adsorption on hydrophobic SiO₂, and force measurements on hydrophobic/hydrophilic substrates.

THERE is much interest in the use of chemical sensor arrays, in conjunction with pattern-recognition routines, for developing artificial olfactory devices-electronic noses-which can characterize the chemical composition of gas mixtures 1-5. Here we describe a technique that uses a continuous sensing surface and a detection method involving a scanning pulsed light source, to generate images that represent a fingerprint of the gases detected. The detector is a large-area field-effect device with a number of different catalytic metals constituting the detecting surface (the devices active gate) 6,7. A pulsed light beam scanned across this surface generates a photocapacitive current that varies with the value of the surface potential 8,9. A continuous sensing surface of this type provides information that would require an array of hundreds of discrete sensors. The technique also provides a new means of studying the coupling between the electronic properties of catalytic metals and chemical reactions taking place on their surfaces.