Digital filters to restore information from fast scanning tunnelling microscopy images

A transfer function—similar to that used in optical cases to correct blurring effects due to the circular aperture of the system—is presented here to restore scanning tunnelling microscopy (STM) images. Due to the conical geometry of the tip-sample system, we have established an analogy between the process of image formation in STM and in optical systems. The transfer function utilized, similar to that calculated by Stokseth, allows us to differentiate between the blurring effects introduced along the x and y axes. These effects are different due, mainly, to the different velocities achieved along the x and y directions. Furthermore we have measured the β parameter that characterizes the classical 1/fβ noise present in STM data, demonstrating its independence from experimental conditions. A Wiener filter is utilized to restore the images using the previous assumptions given for the transfer function and noise effects.     

Arsenic-terminated silicon and germanium surfaces studied by scanning tunnelling microscopy

The epitaxial growth of As on the (111) and (100) faces of Si and the (111) face of Ge has been studied with vacuum tunnelling microscopy. The (111) faces of both semiconductors display a principally 1×1 termination, but differ with the presence of point defects on the Si(111): As-1×1 surface and trenches separating large (～ 100 Å) domains on the Ge(111): As-1×1 surface. I-V characteristics of the tunnel junction show a surface energy gap of approximately 1·9 eV for the Si(111): As-1×1 surface and 0·9 eV for the Ge(111): As-1×1 surface, in good agreement with recent theoretical calculations for these systems. As deposition on Si(001) results in a nominal 2×1 reconstruction of symmetric As dimers and elimination of missing dimer defects characteristic of the native Si(001) 2×1 surface. Further studies on vicinal, double-stepped substrates shows the orientation of the dimers with respect to the substrate depends critically on the substrate temperature during the growth phase, with destruction of the single principle domain surface order occurring at temperatures in excess of 700°C.     

Study of Au surfaces by scanning tunnelling microscopy

The reconstructions of Au surfaces have been studied by scanning tunnelling microscopy. Topographs of Au(110)-(1×2) as a function of annealing temperature show changes in the long range order, in good agreement with diffraction measurements and theoretical studies. Some insight into the nucleation and growth of the (1×2) reconstruction was obtained by imaging the surface after deposition of Cs or O. A new structural model for Au(100)-(5×20) is proposed with a high resolution topograph. Adsorption of Si clusters on Au(100) will be described.     

Real-time scanning tunnelling microscopy of surfaces under active electrochemical control

A scanning tunnelling microscope has been designed which allows tunnelling microscopy to be performed in the presence of an externally applied electrochemical current. Separate, isolated electrodes were used for electrochemical control, and up to 1 mA was passed during real-time, video-rate, in situ STM observation of the surfaces, without interfering with the operation of the STM. The noise level of these STM images is only slightly higher than images taken with the electrochemical circuitry disconnected. Surfaces were observed during the formation of surface films in aqueous electrolytes.     

Fractal characterization by frequency analysis: III. Effect of noise

The effect of noise in the fractal characterization by frequency analysis of surface images obtained by scanning tunnelling microscopy (STM), atomic force microscopy (AFM) or profilometry has been studied. The origin of noise and its relationship to the signal is discussed. A procedure to simulate noisy images is presented. From the study it is concluded that the method usually used to characterize noise in STM is not valid and it is shown that fractal characterization of surfaces when noise is present by traditional frequency analysis methods is not possible. A new method to perform both the noise characterization and the fractal characterization of surfaces when noise is present is proposed.     

Force sensing in scanning tunnelling microscopy: observation of adhesion forces on clean metal surfaces

Tip sample interaction forces were investigated during normal tunnelling operation of the STM using an Ir tip and a polycrystalline Ir sample. Metallic adhesion interaction was observed for tunnel conductivity ranging from 10^?6 to 19^{?9 Ω-1} implying that the actual gap width was of the order of 1–4 Å. Similar experiments performed on a polycrystalline Al sample exposed to 1 Langmuir O₂ showed that tip sample interaction changed from attractive to repulsive on well-defined areas extending over ～100 Ų which we identified with the oxidized Al surface.     

Supported metal model catalyst surfaces examined by scanning tunnelling microscopy

Topographic and/or barrier-height images of ultrafine Pt and Au metal particles supported on a vacuum-deposited carbon film or titanium oxide thin films grown on titanium metal sheets were obtained. The topographic images of colloidal Au particles (5-nm diameter) adsorbed on a titanium oxide thin film showed a structure elongated in the direction normal to the x scan, indicating their weak interaction with the support surface. The topographic images of Pt vacuum-deposited on a carbon film showed c. 4-nm diameter particles, larger than those observed in electron microscopy. The problems inherent to the STM observation of such dispersed metal systems are identified. In the case of Pt particles vacuum-deposited on titanium oxide film, its barrier-height image gave better indication of different phases on the surface than its topographic image. The significance of obtaining barrier-height images along with topographic images for such sample systems is demonstrated.     

Imaging an optical disc by the combined use of scanning tunnelling microscopy and scanning electron microscopy

We present the data obtained by scanning tunnelling microscopy combined with scanning electron microscopy of the digitally encoded structure on a stamper used to fabricate optical discs. The combination allows us to focus the STM tip on a preselected spot with a precision of ?0·3 μm. The data show the superiority of STM for a more detailed characterization of shape, width, length, height and fine structure appearing on the sample. We also show the influence of tip shape on STM resolution. Simultaneous use of both microscopes is possible but high electron doses produce an insulating layer of contaminants thick enough to make STM operation impossible.     

Investigation of Si–Au vicinal surfaces using scanning tunnelling microscopy and reflection high-energy electron diffraction

Silicon vicinal surfaces can be successfully used as substrates for the preparation of one‐dimensional nanostructures. The quality of the structures prepared may be controlled using scanning tunnelling microscopy, as shown in this work. Additionally, it is possible to obtain valuable information using reflection high‐energy electron diffraction. A typical way of employing reflection high‐energy electron diffraction is to observe patterns of scattered electrons on a screen. However, it is possible to obtain more detailed information on the arrangement of atoms at the surface if azimuthal plots are collected. Azimuthal plots are measured by recording the intensity of specularly reflected electrons during the rotation of the sample around an axis perpendicular to its surface. So far, only flat surfaces have been examined in such a way. In this work, it is shown that such data, containing interesting features, can also be collected for vicinal surfaces.     

Spectroscopy with scanning near-field optical microscopy using photon tunnelling mode

We have demonstrated Raman spectroscopy using scanning near-field optical microscopy (SNOM). Photon tunnelling mode was employed, in which the sample is illuminated using an attenuated total reflection (ATR) configuration and the evanescent wave perturbed by the sample is picked up by a sharpened optical fibre probe. By this experimental arrangement Raman scattering from the optical fibre probe was greatly reduced, therefore we were able to excite the sample using more intense laser light compared to the illumination mode SNOM. Raman spectra of copper phthalocyanine (CuPc) were obtained in the off-resonance condition and without using surface-enhanced Raman scattering (SERS).     

Optical characterization of probes for photon scanning tunnelling microscopy

The photon scanning tunnelling microscope is a well-established member of the family of scanning near-field optical microscopes used for optical imaging at the sub-wavelength scale. The quality of the probes, typically pointed uncoated optical fibres, used is however, difficult to evaluate in a direct manner and has most often been inferred from the apparent quality of recorded optical images. Complicated near-field optical imaging characteristics, together with the possibility of topographically induced artefacts, however, has increased demands for a more reliable probe characterization technique. Here we present experimental results obtained for optical characterization of two different probes by imaging of a well-specified near-field intensity distribution at various spatial frequencies. In particular, we observe that a sharply pointed dielectric probe can be highly suitable for imaging when using p -polarized light for the illumination. We conclude that the proposed scheme can be used directly for probe characterization and, subsequently, for determination of an optical transfer function, which would allow one to deduce from an experimentally obtained image of a weakly scattering sample the field distribution existing near the sample surface in the absence of the probe.     

Limits of topographic measurement by the scanning tunnelling and atomic force microscopes

Parameters describing the topographic character of a surface (height, surface wavelength, slope and curvature) can be derived from equivalent sinusoidal profiles. The response of a surface-measuring instrument may be modelled in terms of instrument parameters such as stylus radius, and scanning range and resolution. The performance of the instrument may then be mapped as a zone in amplitude-wavelength (AW) space to show the sinusoidal profiles it is capable of measuring. In a first-order analysis the STM and AFM are considered as equivalent to contact-stylus instruments with a notional stylus radius equal to the tip radius plus the gap. Comparisons between different instruments and types of instrument are readily made by mapping in AW space. The error arising from convolution of the sinusoidal profile with that of the finite tip may be quantified and plotted as contours in AW space.     

Low-temperature scanning tunnelling microscopy and spectroscopy on Pb and Au

We have designed and built a scanning tunnelling microscope (STM) in order to work at temperatures ranging from 1·2 to 300 K. Tunnelling spectroscopy has been performed in Au and Pb with this STM. Our results on Au show different conductance-voltage behaviours, their relation with the cleaning state of the tip being discussed. For Pb, the fitting of our I-V characteristics to a BCS density of states gives A = 1·25 meV for the superconducting energy gap at about 5 K.     

Oriented immobilization of Pseudomonas putida putidaredoxin at a gold (111)–buffer interface: a real time scanning tunnelling microscopy study

A scanning tunnelling microscopy study of adsorption of wild‐type Pseudomonas putida putidaredoxin at a gold (111)–buffer interface has been made in real time. Reversible adsorption has been observed reflecting weak interaction of the wild‐type protein with a gold (111) electrode. A genetically engineered mutant, C73S‐D58C, which contains a surface thiol, has been used for ‘immobilization’ and ‘orientated adsorption’ on the gold surface. The implication of such orientated immobilization in development of a bio‐electrode surface has been predicted.     

Scanning tunnelling microscopy imaging of [3 × 3] Mn nonanuclear grids

The scanning tunnelling microscopy imaging of [3 × 3] Mn(II) nonanuclear grids on gold substrates is described. Self‐assembled behaviour is observed at both high and low coverage, with submolecular resolution of individual molecules displayed at low deposition concentrations. The importance of proper image processing techniques is demonstrated in resolving the layer structure at high coverage.     

Imaging of individual atoms on an Al(111) surface by scanning tunnelling microscopy

Due to the delocalized character of metal valence electrons the atomic corrugation of metal surfaces observed in Scanning Tunnelling Microscopy (STM) is found to be much smaller than in the case of semiconductor surfaces. In fact, there is only a single study in the literature which reports the resolution of the individual atoms on a metal surface (Hallmark et al., 1987). The present paper demonstrates the resolution of individual atoms on a close packed surface of a nearly free electron metal, Al(111), and presents systematic experiments on the physical origin of this phenomenon. A more detailed discussion will be given elsewhere (Wintterlin et al., 1988).     

Lateral resolution of the scanning tunnelling microscope

A theoretical study of the current distribution in a model which extracts the prominent characteristics of a tip-surface geometry in a scanning tunnelling microscope is presented. The sample is a Sommerfeld metal with a planar surface while the tip, also made of a Sommerfeld metal with a planar surface, presents a hemispherical protrusion. Schrödinger's equation is solved by using a form of the finite element method suitable to treat the three-dimensional tunnelling problem. We have derived the current distribution for several values of the protrusion radius and for various gaps between the electrodes. From these results, we investigate the ideal resolution of the scanning tunnelling microscope for realistic values of these geometrical parameters. Though applied to a rather simple model, at this stage, our method appears to be a suitable scheme for further, more refined computations, accounting for the detailed atomic structure of the electrodes.     

Images of 16S ribosomal RNA by scanning tunnelling microscopy

We report the use of scanning tunnelling microscopy (STM) to study surface topographies of complex nucleic acid structures. From low-resolution STM images of uncoated 16S ribosomal RNA, we demonstrate the possibility of determining several objective parameters (molecular mass and radius of gyration) in order to characterize and identify the molecules observed. These parameters were compared with values obtained by other physical methods and the radius of gyration was found to be the most reliable. At high resolution, it was possible to measure the main dimensions of selected V-form particles more precisely than with electron microscopy. Images of the more compact form have been also obtained that show different domains in the macromolecular structure.     

In situ fabrication and regeneration of microtips for scanning tunnelling microscopy

Progress in STM, in particular for the interpretation of the tunnel signals of corrugated surfaces, is related to the possibility of sharpening and regenerating in situ reproducible tips for use as the probe. We proposed two techniques for the production of tips with controlled geometry at the atomic level. These two techniques are based on the production of tips with equilibrium profiles obtained under heat treatments in vacuum and in the presence of an electric field. Based on the pseudo-stationary profile principle, the microtips can be sharpened in situ and in a reproducible manner, either from initial cylindrical (111)-W wire, or from breakdown tips; in both cases without the need for heavy control devices (like FIM for example).