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.     

Design and operation of a variable temperature scanning tunnelling microscope

A new variable temperature STM has been developed which utilizes two concentric piezoelectric tubes; an inner scanning tube, and an outer thermal compensation tube which also provides for inertial translation of the sample into tunnelling range. With this design, continuously variable temperature operation is demonstrated for the first time in an STM. Also, by eliminating all mechanical components such as springs, levers and gears, which normally couple directly to the tunnelling gap in other designs, atomic resolution operation is demonstrated in which no vibration isolation is necessary. During operation, the inside of the scanning tube is maintained at ground potential while the feedback signal is electronically summed to the scanning voltages applied to the outer quadrants. In addition to shielding the sensitive tunnelling circuit, this mode of operation enables one to electronically balance out mechanical imperfections of the scanning tube. To date, this new STM has been operated over the 77–400 K temperature range with the observed thermal drift as low as 1 Å/h and 10 Å/K. Another useful feature of this new design is the ability to reposition a sample to within 200 Å of the same location after it has been translated macroscopic distances (several mm) out of tunnelling range.     

An alternative flat scanner and micropositioning method for scanning probe microscope

An alternative flat scanner used for combining a scanning probe microscope with an inverted optical microscope is presented. The scanner has a novel structure basically consisting of eight identical piezoelectric tubes, metal flexure beams, and one sample mount. Because of the specially designed structure, the scanner is able to carry a sample of more than 120 g during imaging. By applying voltages of ±150 V, scanning range of more than 30 μm in three dimensions can be achieved. To improve the reliability of the stick-slip motion, a new method for sample micropositioning is proposed by applying a pulsed voltage to the piezotubes to produce a motion in the z-axis. Reliable translation of the sample has been thus accomplished with the step length from ～700 nm to 9 μm over a range of several millimeters. A homemade scanning probe microscope-inverted optical microscope system based on the scanner is described. Experimental results obtained with the system are shown.     

A scanner for an ultrahigh-vacuum low-temperature scanning tunneling microscope

A scanner for an ultrahigh-vacuum low-temperature scanning tunneling microscope is described. It has a high resonance frequency (>30 kHz) and a small thermal-drift rate (≤1 nm/°C) at room temperature. The scanner feeds the tip to the sample at a distance of up to 3 mm and positions it in the sample plane on a 4 × 4-mm area. These characteristics of the scanner allow one to study atomic structures at temperature variations from 5 to 300 K with objects under study remaining in view of the microscope. The scanner has a horizontal attachment for a sample with a size of up to 6 × 6 × 3mm and ensures a scanning field of 4.8 × 4.8 × 0.6 μm at 300 K and 0.8 × 0.8 × 0.1 μm at 5 K, as well as the possibility of heating to 150°C and easily replacing the sample and tip with vacuum manipulators.     

Battery-operated STM

A scanning tunnelling microscope operating with ±12 V is described. A scanning range higher than 1 μm is achieved in the three orthogonal scanning directions for that low voltage. This translator arrangement, based on the use of piezoelectric discs mounted on metallic circular plates acting like flat springs, may provide a better performance than the tube scanners or rectangular bimorphs since it allows a greater volume to be run by the tip for a given voltage.     

STM and AFM investigations of high-Tc superconductors

We have studied the (001) surface of single crystal YBa₂Cu₃O_7-x high-T_c superconductors using scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) at room temperature at ambient pressure. Both methods show flat terraces with steps which are multiples of the c-axis lattice constant (of 1·17 nm) high. Our results show that the bulk crystal structure extends to the surface and that the crystals were formed by island growth. Only occasionally tunnelling was possible with sample bias voltages below +1·0 V. We interpret the observed voltage dependence and the difficulty to get good STM images to be due to the presence of a less-conducting surface layer. Auger spectroscopy indicates that carbon is present at the surface, which is probably related to a contamination layer.     

Single-electron effects observed with a low-temperature STM

We have used a low-temperature scanning tunnelling microscope to study the effect of the Coulomb charging energy on the tunnelling behaviour of low capacitance point-contact junctions. The tunnelling I-V characteristics between a tungsten tip and various materials, such as stainless steel, aluminium, carbon and YBa₂Cu₃O_7-δ, show a quadratic behaviour at low voltages and a displaced asymptotic behaviour at high voltages. The I-V characteristics can be quantitatively understood using the model of single-electron tunnelling induced by the Coulomb blockade. The capacitances of this type of point-contact tunnel junctions are in the 10^?18 F range, and are adjustable by varying the distance between tip and surface. These capacitances are at least two orders of magnitude lower than can presently be achieved by electron lithography. In a series configuration of two low-capacitance tunnel junctions, with an isolated small particle as a common electrode, we have observed the so-called Coulomb staircase, due to the quantization of the charge on a single small particle. The experimental results are in good agreement with semi-classical Monte-Carlo simulations. The low capacitance of an STM assembly can cause a serious complication in the interpretation of low temperature spectroscopic data. We will discuss the implications for the measurements on superconductors, and for the case of inelastic tunnelling spectroscopy.     

STM imaging of the complete bacterial cell sheath of Methanospirillum hungatei

Using a large range scanning tunnelling microscope (STM) we have obtained images of the complete outer cell wall of the archaebacterium Methanospirillum hungatei, deposited on a graphite substrate and over-coated with an Au or Pt evaporated film. The width of the collapsed cylindrical sheath structure was found to be 5000 Å ± ***10%, which agrees closely with the value from previously published electron microscope (EM) studies. The double thickness of the collapsed sheath was found to be 160 Å ± 10%, which is about 20% smaller than that from the EM results. Higher resolution STM images taken on top of the collapsed sheaths show corrugations running perpendicular to the cylinder axis and having widths which are multiples of ～ 30 Å, the minimum period expected from EM studies. The height of the corrugations have a minimum value of about 4 Å. The expected 2-D crystalline structure was not seen in the STM images.     

A spectroscopic study of some high-temperature superconductors using a low-temperature STM

We used a scanning tunnelling microscope (STM) to measure both the tunnel current, I, and the dynamic conductance, dI/dV, at 4·2 K for a number of high-transition temperature oxide superconductors. Large spatial variations in the tunnelling characteristics are observed. At low tunnel resistances, all samples show evidence of single electron tunnelling and incremental charging. Results on BiSrCaCu₂O_x show the coexistence of charging with Josephson coupling between grains within the sample. Results on both the Bi sample and a single crystal of YBa₂Cu₃O_6·5+x reveal possible energy gap (2A) values of 17 and 20 meV, respectively. A very sharp 5 meV gap, observed in ceramic samples of YBa₂Cu₃O_6·5+x and Y_0·5Al_0·05Ba₂Cu₃O_6·5+x, may indicate the presence of a lower temperature phase in these samples.     

STM study of silver intercalation into 2H-NbSe2 crystals

We have made scanning tunnelling microscope (STM) studies of the lattice expansion and topography of cleaved crystals of 2H-NbSe₂ which were intercalated in situ with silver ions entering along one edge of the samples. The measurements confirm the high surface mobility of silver seen in earlier electrochemical studies. The expansion measurements made with optimum 2 Å resolution showed no convincing evidence of silver staging domains. Topographic images revealed noisy patches and shallow depressions on cleaved surfaces of intercalated samples. It is suggested that these depressions might represent silver-poor regions in the first intercalation layer.     

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.     

A fully automated, ‘thimble-size’ scanning tunnelling microscope

A novel, fully automated high-stability, high-eigenfrequency scanning tunnelling microscope (STM) has been developed. Its key design feature is the application of two piezoelectric ceramic tubes, one for the x-y-z motion of the tip and one for a linear motor (‘nano-worm’) used for the coarse positioning of the tip relative to the specimen. By means of the nano-worm, the tip can be advanced in steps between 16 and 0·2 nm. The walking distance is >2 mm, with a maximum speed of 2000 steps/s. The nano-worm positioning implies that this STM is fully controlled by electronic means, and that no mechanical coupling is needed, which makes operation of the STM extremely convenient. The axial-symmetry construction is rigid, small and temperature-compensated, yielding reduced sensitivity to mechanical and acoustic vibrations and temperature variations. The sample is simply placed on a piece of invar which surrounds the scanner tube and the nano-worm and is held by gravity alone. This allows for easy sample mounting. The performance of the microscope has been tested in air by imaging a variety of surfaces, including graphite and biological samples.     

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 microscopy of 1-D and 2-D charge-density wave systems

We have used a new variable temperature scanning tunnelling microscope (STM) to study quasi-1D and 2-D charge-density wave (CDW) systems. The 1-D systems, typified by NbSe₃ and TaS₃, are of special interest since they exhibit unusual transport phenomena associated with moving CDW above a threshold electric field. In the case of NbSe₃, room temperature STM images show both major and subtle details of the lattice structure. At present, however, images taken below the Peierls transition temperature of T_P=144 K resolve major lattice details but are not sufficiently clear to resolve the CDW. On the other hand, for the fully gapped CDW system orthorhombic-TaS₃, the CDW modulation superimposed on the lattice structure and having the correct period of four times the S-S spacing of 3·3 Å, is observed below T_P=215 K. Above T_P, the main observable feature is the S-S spacing along the chains. STM measurements have also been performed on the 2-D CDW system 1T-TaS₂ in its incommensurate, nearly commensurate, fully commensurate and trigonal phases. For the nearly commensurate phase, STM images show uniform commensurability with a relatively low concentration of small, time-varying discommensurations in contrast to models pradicting a regular domain structure. In the trigonal phase, however, evidence is seen for the striped phase composed of long, nearly parallel discommensurations.     

STM imaging of noble metal electrodes in solution under potentiostatic control

We have combined a three-electrode cell arrangement with a scanning tunnelling microscope (STM) to image the surfaces of polycrystalline noble metal electrodes under potentiostatic conditions. STM imaging was made in the potential range where a tip and the electrodes are ideally polarized. We have observed on a submicron scale that the electrochemically roughened Ag electrode surface relaxed over times of 40 min at ?0·16 V versus SCE in 0·1 m KCl. We have also imaged a dynamic process (formation and dissociation) of the submicron-scale reconstruction of the Au electrode subjected to electrochemical oxidation-reduction cycles. The results can be explained by the absorption of Cl^? ions and the surface diffusion of adatoms or clusters. STM imaging under potentiostatic control leads to the discovery of phenomena occurring at solid metal/electrolyte interfaces.     

A mirror electron microscope for surface analysis

The principle of mirror microscopy has been adapted to provide a relatively low resolution surface microscope (<1000 ×), a large transfer width low energy electron diffractometer and a photoelectron analyser in k_|| space. A focused electron beam of ? 10 kV is decelerated through a Johansson lens, reflected in front of the sample and reaccelerated back through the lens to produce an electron image over a field of view of a few microns. The image can be interpreted as a micrograph of work function variations on the surface if other effects (geometry, magnetic field) are uniform. In the LEED mode, diffracted beams virtually retain their positions on the screen over the whole impact energy range used (0.160 V). Secondary electrons are preferentially focused around the lens-gun electro-optic axis, thus effectively filtering them out from the diffraction pattern. The design has an inherently large coherence length, of up to 10⁴ Å. Photoelectrons can similarly be imaged in k_|| space on the detector plane. The addition of energy filtering at the screen allows the two-dimensional Fermi surface to be imaged.     

Continuous recording of microvascular dimensions with a rotating mirror microscope

A system which permits continuous recording of the dimensions of microscopic blood vessels is described. The system uses a combination of a rotating mirror and a pinhole aperture with a microscope to scan microvessel image signals at a rate of one per second. Image signals from the gate circuit are converted to digital signals; further conversion to analogue signals is obtained by integrating the digital signals. The analogue voltages are proportional to the vessel dimensions. A linear relationship was observed over a range of 20–200 μm (standard deviation ±3.0%). The contrast between vessels and surrounding tissue is amplified by using a B-12 optical filter. By using this system, the mesenteric arteriole was observed after reaction with adrenaline and noradrenaline in rats. The contractile response of the arteriole to adrenaline was greater than with noradrenaline. A maximum response of 20% decrease of the vessel outer diameter is induced by 5.5 × 10^?7 mmol adrenaline and 5.9 × 10^?6 mmol noradrenaline.     

A confocal laser microscope scanner for digital recording of optical serial sections

A confocal laser microscope scanner developed at our institute is described. Since an ordinary microscope is used, it is easy to view the specimen prior to scanning. Confocal imaging is obtained by laser spot illumination, and by focusing the reflected or fluorescent light from the specimen onto a pinhole aperture in front of the detector (a photomultiplier tube). Two rotating mirrors are used to scan the laser beam in a raster pattern. The scanner is controlled by a microprocessor which coordinates scanning, data display, and data transfer to a host computer equipped with an array processor. Digital images with up to 1024 × 1024 pixels and 256 grey levels can be recorded. The optical sectioning property of confocal scanning is used to record thin (～ 1 μm) sections of a specimen without the need for mechanical sectioning. By using computer-control to adjust the focus of the microscope, a stack of consecutive sections can be automatically recorded. A computer is then used to display the 3-D structure of the specimen. It is also possible to obtain quantitative information, both geometric and photometric. In addition to confocal laser scanning, it is easy to perform non-confocal laser scanning, or to use conventional microscopic illumination techniques for (non-confocal) scanning. The design has proved reliable and stable, requiring very few adjustments and realignments. Results obtained with this scanner are reported, and some limitations of the technique are discussed.     

Tunnelling accelerometer

Due to the strong distance dependence of tunnelling current on electrode spacing, tunnelling can be used to measure sub-ångström-scale displacements. The sensitivity and relative simplicity of this technique make it well suited for applications in highly sensitive accelerometers. We have constructed a cantilever-type accelerometer in which a mass-loaded lever deflects freely in response to accelerations. Deflection is measured by tunnelling in the constant current mode between an electrode on the lever and a tunnelling tip positioned by a piezoelectric transducer. Using a lever/mass system with a force constant of 2·3 × 10⁴N/m and an effective mass of 2·4g, we obtained a sensitivity of 79***V/g with a bandwidth greater than 200Hz. Accelerations smaller than 10^?4g could be detected using this device.     

Characterization and local modification of atomically flat gold surfaces by STM

In order to characterize the surface of epitaxial gold on mica in air we have used a scanning tunnelling microscope (STM) to image and to modify this surface. It was possible to create controlled features by applying voltage pulses on the tip while scanning. The voltage threshold for writing (about 3 V, 100 ns pulses) was dependent of the tip condition. The lowest pulses were associated with sub-50 Å feature size. We observed that at ambient temperature the written features disappeared in a time scale of half an hour for the smallest (<30 Å) to a few hours for the bigger features (～500 Å). We have used the same surface as a substrate for organic imaging. We also present images of a polymer deposited on gold.