J. E. Houston

A NEW FORCE SENSOR INCORPORATING FORCE-FEEDBACK CONTROL FOR INTERFACIAL FORCE MICROSCOPY

A new interfacial‐force microscope capable of measuring the forces between two surfaces over the entire range of surface separations, up to contact, is described. The design is centered around a differential‐capacitance displacement sensor where the common capacitor plate is supported by torsion bars. A force‐feedback control system is incorporated which balances the interfacial forces at the sensor, maintaining the common capacitor plate at its rest position. This control therefore eliminates the instability or ‘‘jumping’’ which occurs with conventional cantilever‐based force sensors when the attractive force gradient between the fixed sample and sensor exceeds the mechanical stiffness of the cantilever. The operating characteristics of the sensor and its ability to measure interfacial forces using the feedback control at surface separations smaller than this instability point are demonstrated.

Chemical‐state effects in Auger electron spectroscopy

We have used Auger spectroscopy as a probe of local chemical environment both in the gas and condensed phases using a systematically chosen series of molecules [H2O, CH3OH, (CH3)2O, CH4, C2H4, and C2H2]. For the series of gas phase molecules, H2O, CH3OH, (CH3)2O, and CH4, where oxygen and carbon are, respectively, in similar bonding arrangements, characteristic fingerprint spectra (methanelike for C and waterlike for O) are shown to result. Additional fine structure, which is dependent on the specific molecular environment, appears on the spectra. In contrast, dramatic differences are observed for the series CH4, C2H2, and C2H4 in which major differences in hybridization exist at the carbon site. H2O, CH3OH, and (CH3)2O were studied both in the gas phase (electron excited) and in the condensed phase (x‐ray excited). The O(KVV) (K level–valence–valence transition) and C(KVV) spectra are shown to be similar when comparing the gas–solid results only if the multilayer spectra are properly corrected for electr...

Monolayer and multilayer growth of Cu on the Ru(0001) surface

Abstract The adsorption and growth of Cu films on the Ru(0001) surface were studied by work function measurements, low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and thermal programmed desorption (TPD). The results indicate that for submonolayer depositions at 100 K the Cu grows in a dispersed mode forming 2D islands pseudomorphic to the Ru(0001) substrate upon annealing to 300 K. This behavior is seen to continue to the 1 monolayer (ML) level. Additional Cu deposition to 2 ML shows a similar 2D island growth but with an epitaxial Cu(111) structure. Subsequent annealing in both these cases to 900 K enhances the 2D character of the films but does not affect the overall structure. AES and LEED results show that a 900 K anneal of Cu films in excess of 2 ML leads to three-dimensional Cu(111) island formation exposing areas of the surface covered by the original Cu bilayer — one pseudomorphic and one epitaxial. The effects of Cu on the chemisorptive properties of Ru(0001) toward CO were also studied by TPD. It was found that Cu attenuates the CO adsorption relative to the open Ru(0001) sites on approximately a one-to-one basis. In addition, at the 1 ML level the TPD spectrum shows features which are intermediate between those for the tightly bound CO/Ru system and the weakly bound CO/Cu case. A feature in the TPD spectra of CO on submonolayer Cu deposits is identified with mixed Cu/Ru sites, i.e. at the 2D Cu island edges, and allows an estimate of the 2D Cu island sizes to be made. The results and conclusions of this study differ markedly from previous single-crystal studies but are consistent with recent observations of Cu adsorbed onto an epitaxial Ru(0001) film grown on a Mo(110) surface.

Low-energy electron diffraction from imperfect structures

Abstract For simple kinetic scattering, the problem of diffraction from imperfect structures reduces to finding the average autocorrelation function of the structure, modulating it with an appropriate instrumental transfer function, and taking the Fourier transform. Using this approach, the effect on the diffraction beam profiles of a statistical distribution of phase shift boundaries is treated for a one-dimensional analogue of the surface. Specific calculations are made for the case of continuous beam splitting observed during the transformation from a double-spaced to a triple-spaced structure. The results demonstrate the large degree of structural anarchy that can be concealed by the deficiencies of the experimental technique.

The LEED Instrument Response Function

The effect of the measurement system is usually ignored in the interpretation of LEED patterns. This effect can, however, be treated analytically through the use of the instrument response function, which represents the intensity function the instrument would record for a perfect surface. We have measured the response function for a particular LEED system using a double aperture Faraday collector and have identified the various experimental factors which determine its detailed functional form. The Fourier transform of the response function defines the range over which the instrument behaves as an interference detector. The effect of long range correlations on the pattern is suppressed by this finite range, with the result that fairly untidy surfaces may not be easily distinguished from those which are nearly perfect.

LEED from statistical step models

Abstract Several LEED studies have recently been reported on surfaces which were forced to exhibit steps of fairly well defined average separations. The diffraction patterns in each case were qualitatively similar, showing many beams split into multiplets with separations which depended on the incident electron energy. These patterns were interpreted in terms of kinematic scattering from a stepped, two-dimensional array of point scatterers, assuming perfectly ordered steps. Such studies expose LEED as a possible tool for the analysis of surface topography if used in conjunction with an analytical approach which includes arbitrary, statistical step distributions. Such an approach is outlined in the present paper and calculations are performed for simple distributions of step spacings to illustrate the procedure and convey physical insight. The most striking result concerns the degree of scatter in the step lengths that can be tolerated and still produce “sharp” splittings.

A Soft X‐Ray Appearance Potential Spectrometer for the Analysis of Solid Surfaces

A simple, nondispersive spectrometer has been designed which allows the binding energies of core electrons to be determined for atoms at a solid surface. The spectrometer consists of a photomultiplier which measures the total soft x‐ray fluorescence of the anode surface of a vacuum diode. The derivative of the multiplier current as a function of the anode potential rises abruptly at the appearance potentials of characteristic x rays. The derivative is taken by superimposing a small oscillation on the anode potential and synchronously detecting the variation in multiplier current. The information obtained is comparable to that usually derived from absorption spectroscopy but is much more sensitive to the surface region. Thus, in addition to determining the elemental composition of the surface, chemical shifts in the binding energies of core electrons are readily observed.

Auger spectra of alkanes

The gas‐phase Auger line shapes of the linear alkanes C1 through C6 and of neopentane are presented and analyzed. The general shape of the spectra are characteristic of carbon in a tetrahedral environment with the major feature in all cases occurring at ∼249 eV. The relatively large spectral changes found between methane and ethane results from the direct interaction of the terminal methyl groups in ethane, and the spectra of the higher alkanes are shown to be a composite of contributions from terminal methyl and interior methylene group carbon atoms. Theoretical analysis based on a one‐electron approximation is shown to be capable of making a molecular orbital assignment by comparing calculated vertical transitions to features in the Auger spectra of ethane and propane, and, in the case of ethane, of differentiating between the 2Eg and 2A1g assignment of the ground state of (C2H6)+. A one‐electron based molecular orbital treatment, however, is shown to partially break down in propane and neopentane. Anal...

IONIZATION SPECTROSCOPY OF SURFACES.

A technique is described for the analysis of surface composition on the basis of characteristic loss features associated with electron impact ionization of inner atomic levels. The method involves separating the characteristic loss spectrum from the total secondary emission spectrum by superimposing a small oscillation on the primary beam energy and detecting only those features which fluctuate at the same frequency. Ionization spectroscopy is shown to have a number of inherent advantages over conventional Auger spectroscopy.

Auger spectra of tetrahedral halides and hydridesa)

Auger electron spectra are presented for the gas‐phase molecular species CF4, SiF4, CCl4, and SiCl4 and discussed in terms of the effect of the interaction of the two final‐state holes on the spectral line shapes. The highly polar character of the bonding for this series leads to systematic behavior with respect to this hole–hole interaction, showing rare gas‐like localized halide spectra for SiF4 and a delocalized component of increasing intensity as we proceed through the series to SiCl4, CF4, and CCl4. The central atom spectra show a composite line shape resulting from the sum of two versions of the same one‐electron final states but with different values of U (the hole–hole interaction energy). The Auger process occurs as a result of the electron density available at the central atom site but, since the one‐electron orbitals are highly polarized towards the halide, the two holes principally appear around the halide sites. With both holes on the same halide site a rather large U results, but with the t...