Scanning tunnelling microscopy of charge density waves in 1T-TaS2

Using a variable temperature scanning tunnelling microscope we have observed both the lattice and the triple charge density wave (CDW) in 1T-TaS₂. We obtained images of the nearly-commensurate (NC) phase between 350 and 200 K and the triclinic (T) phase between 220 and 280 K. We analysed the images to determine the local angle of rotation of the CDW relative to the lattice to determine if commensurate domains exist in this material. In the NC phase we do not see the hexagonal discommensurate domains proposed by Nakanishi el al. (1977), but observe a structure which is uniformly incommensurate. The orientation of the CDW relative to the lattice is observed to be locally incommensurate and to vary continuously with temperature as reported by Scruby et al. (1975). However, in the T phase we observe long narrow commensurate domains very similar to those described by Nakanishi and Shiba (1984) for this phase.     

Scanning tunnelling microscopy of oxidized graphite

Scanning tunnelling microscopy (STM) and transmission electron microscopy (TEM) have been used to investigate the surface of a pyrolitic graphite oxidized in liquid phase by NaClO. Two main features of the oxidized HOPG are revealed by STM. First, a large number of steps of different heights have developed on the graphite surface. These steps can be observed by TEM on another kind of graphite, HSAG 12, but this technique cannot give any information on their heights. Another kind of defect on the previously flat surface of HOPG consists in patches where the surface is rough and perturbed. These domains are very difficult to observe by TEM due to a poor contrast. Thus for the study of surface heterogeneities intentionally created on graphite, STM, providing information along three directions, appears to be complementary of TEM which gives only images of project area.     

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.     

Scanning tunnelling and atomic force microscopy performed with the same probe in one unit

The technique demonstrated here provides features of both scanning tunnelling microscopy (STM) and atomic force microscopy (AFM). The metallic probe acts to record current variations and sense forces from the same sample area simultaneously. Thus, separate images may be recorded, in registry. The collected data allows real space correlations between some electrical properties and the geometric structure of a sample surface. The same tip is used since the geometry and condition of the tip can effect the data recordings. Platinum alloys, tungsten and graphite tips have been employed successfully. An AFM lever can respond to surface contact forces, within the elastic limits of the sample, while electric current is sensed by the tip of the lever. The usefulness of this experimental procedure is tested here by an application to semiconducting samples of Ag-doped CdTe in air and in paraffin oil media.     

Scanning tunnelling microscopy on reconstructed Si(110)

We have prepared clean surfaces of reconstructed Si(110), which showed a distinct 4×5 LEED pattern with particularly high intensity of the diffraction beams corresponding to a 2×1 periodicity of the surface. Images obtained by scanning tunnelling microscopy on the same surfaces are characterized by rows of reconstructed Si atoms along the direction. On high-resolution images, atomic subunits in a 2×5 periodicity of ideal Si(110) have been identified. By using these subunits as building blocks for both the 4×5 and the 2×1 reconstruction, a previously reported phase transition between both reconstructions may be explained by a loss of order along the direction.     

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.     

Anomalous voltage dependence of tunnelling microscopy in WSe2

We present scanning tunnelling microscopy (STM) investigations of the layered semiconductor WSe₂. The tunnelling experiments were performed in air and under silicone oil with markedly different results. In air, atomic resolution images of the hexagonally structured surface could be obtained for sample-to-tip voltages of both negative and positive polarities, from ?1·5 to ?0·3 V for negative sample and from +0·6 to +1·6 V for positive sample, respectively. Under silicone oil, however, good atomic images could be seen for negative sample biases down to at least ?14 V, while for positive sample biases no difference with respect to the tunnelling in air was found.     

Scanning tunnelling microscopy and spectroscopy on Cu(111) and Au(111)

We have studied Cu(111) and Au(111) by means of scanning tunnelling microscopy and spectroscopy. The constant current topographies showed flat parts as well as regions with a high density of monoatomic steps (in particular on Au(111)). Local I/U characteristics have been determined at a fixed sample-tip distance in the range of ?10 V≤U≤10 V. They show a linear behaviour near the Fermi level and a nearly exponential dependency for larger values of U. Neither an influence of the sp-like surface states or an onset due to d electron contributions of the sample could be observed.     

Scanning tunnelling engineering

The great potential for building molecular scale machines and other structures was first noted by Richard Feynman (1960). He also proposed the development of tools to construct nanoscale mechanisms and devices. The range of technology that he proposed is now called nanotechnology, a term coined by Taniguchi (1974). Emergence of STM and related technology should greatly facilitate the development of nanotechnology in the decades to come. Franks' (1987b) review of nanotechnology notes the tremendous potential of STM-derived tools for what he terms ‘scanning tunneling engineering’. However, STM technology can also augment the ‘bottom up’ approaches to nanotechnology, exemplified for example by Forrest Carter's (1979, 1980, 1982, 1983) proposals for building molecular electronic devices (nanocomputers) using synthetic ‘modular chemistry’ and related techniques of supramolecular chemistry (Kuhn & Mobius, 1971; Kuhn, 1983; Lehn, 1980, 1988). A multi-tip STM system integrated with an optical microscope was proposed in 1986 as a ‘nanotechnology workstation’ suitable for scanning tunnelling engineering (Schneiker & Hameroff, 1988). In this paper, we describe an overview of scanning tunnelling engineering, present concepts for nanosensors and nanoswitches, and discuss design considerations for dual tip STM. Finally, we repeat an announcement of a series of ‘Feynman’ prizes for miniature STM.     

Finite element analysis of PZT tube scanner motion for scanning tunnelling microscopy

This article is a presentation of the results of a finite element method analysis of piezoelectric tube scanners of the type presently in wide use in scanning tunnelling microscopes. The tube scanner moves a tunnelling tip by bending sideways when unsymmetrical voltages are applied to longitudinal stripes of metallization on its walls. It also can be extended and contracted in length by application of symmetrical voltages. One wishes to know the characteristics and magnitudes of the resulting three-dimensional motion. We divided a model tube into radially polarized elements, and constructed a computer program to perform electromechanical stress calculations. The results show the dependence of motion upon applied voltage, tube material, and tube dimensions.     

Observation of rhombic facets on platinum wire after heating in air using scanning tunnelling microscopy

Evidence is provided for the formation of surface faceting due to heating in air at 1160 K. For the first time diamond-shaped facets with angles of 60 and 120° have been observed on the surface of a sample of ultrapure platinum wire in air using scanning tunnelling microscopy. The maximum extension of the facets is about 1000 Å.     

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.     

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.     

Safe fabrication of sharp gold tips for light emission in scanning tunnelling microscopy

Gold is the optimal tip metal for light emission in scanning tunnelling microscopy (LESTM) under ambient conditions. Sharp Au-tips of 10 nm radius were produced reliably using a safe, two-step etching method in 20% (w/w) CaCl₂ solution. Previous CaCl₂-based methods have tended to produce blunter tips, while other etching techniques that do produce sharp Au-tips, do so with the use of toxic or hazardous electrolytes. The tips are characterised using scanning electron microscopy and their efficacy in LESTM is evidenced by high-resolution, simultaneous topographic and photon mapping of Au(1 1 1)- and polycrystalline Au-surfaces. Spectra of the optical emission exhibit only one or two peaks with etched tips in contrast to the more complex spectra typical of cut tips; this feature, together with the highly symmetric geometry of the tips, facilitates a definitive analysis of the light emission process.     

Scanning tunnelling microscopy of tobacco mosaic virus on evaporated and sputter-coated palladium/gold substrates

We present scanning tunnelling microscope (STM) images of untreated tobacco mosaic virus (TMV) deposited on thermally evaporated and on sputter-coated palladium/gold 40:60 (Pd/Au) substrates, and imaged under ordinary atmospheric conditions. The TMV imaged on both evaporated and sputter-coated substrates was consistently several times wider than the known diameter of the virus. TMV on evaporated Pd/Au became overcoated with Pd/Au material during sample preparation and appeared elevated in STM images, whereas TMV on sputter-coated Pd/Au appeared as depressions. When naked TMV were intentionally overcoated with Pd/Au, the STM images were found to be similar to those for TMV on evaporated Pd/Au.     

A scanning tunnelling microscopy study of the formation and chemical activation of step defects on the basal plane of pyrolytic graphite

Scanning tunnelling microscopy is used to monitor etching of the basal plane of highly orientated pyrolytic graphite by ozone, oxygen and nitric acid. These treatments are seen to produce numerous single and multilayer step defects. Subsequent modification of the graphite sheet edges flanking these cavities by cyanuric chloride, TiCl₄ and other reagents is shown to activate the edges, thereby making them capable of covalently binding various molecules.     

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.     

扫描离子电导显微镜技术及其在纳米生物学研究中的应用

扫描探针显微镜技术的出现开辟生命科学研究的新纪元并逐步发展成为在纳米尺度研究细胞结构与功能的一类新型的显微镜技术。扫描离子电导显微镜技术就是新近发展起来的这一扫描探针显微镜技术家族中的一员,可被用来在生理条件下、高分辨率及非接触地研究活细胞的表面形貌,从而帮助人们深入研究细胞微观结构与功能的关系。本文简要介绍扫描离子电导显微镜技术的基本原理,并结合国外研究现状综述该技术在纳米生物学研究中的应用。     

Scanning probe microscopy of biological samples and other surfaces

Scanning probe microscopes derived from the scanning tunnelling microscope (STM) offer new ways to examine surfaces of biological samples and technologically important materials. The surfaces of conductive and semiconductive samples can readily be imaged with the STM. Unfortunately, most surfaces are not conductive. Three alternative approaches were used in our laboratory to image such surfaces. 1. Crystals of an amino acid were imaged with the atomic force microscope (AFM) to molecular resolution with a force of order 10^?8 N. However, it appears that for most biological systems to be imaged, the atomic force microscope should be able to operate at forces at least one and perhaps several orders of magnitude smaller. The substitution of optical detection of the cantilever bending for the measurement by electron tunnelling improved the reliability of the instrument considerably. 2. Conductive replicas of non-conductive surfaces enabled the imaging of biological surfaces with an STM with a lateral resolution comparable to that of the transmission electron microscope. Unlike the transmission electron microscope, the STM also measures the heights of the features. 3. The scanning ion conductance microscope scans a micropipette with an opening diameter of 0·04-0·1 μm at constant ionic conductance over a surface covered with a conducting solution (e.g., the surface of plant leaves in saline solution).     

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.