A small scanning tunnelling microscope with large scan range for biological studies

We have developed a scanning tunnelling microscope specially designed for biological applications presenting some new features: the scanner tube is mounted parallel to the surface of the sample which enables a high resolution optical microscope to be brought close to the sample when working in air or liquids. The maximum scan range is 5×20 μm with a vertical range of 20 μm and the total size of the system does not exceed 10×40 mm. The piezo-sensitivity of the scanner tube versus applied voltage was analysed by interferometry measurements and by using scanning tunnelling microscopes. We found a value for the piezoelectric constant d₁₃ of ?1·71 Å/V at low voltages (under a few volts) going up to ?2 Å/V for higher voltages. Large-scale images of a carbon grid showed a surprisingly good linearity of the scanner tube.     

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.     

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.     

Investigation of intrinsic crystalline Si(111) and amorphous silicon surfaces in air with STM

Crystalline (111) and amorphous silicon surfaces have been studied by scanning tunnelling microscopy. An orientated, reproducible, corrugated structure has been observed on Si(111) surfaces. The voltage dependence of the corrugation amplitude may be attributable to surface states. The surfaces of amorphous silicon thin films show some reproducible structure in the range of a few tens of ångströms, observable only when the applied voltage between the tip and sample is between ?1·3 and +0·4V.     

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.     

An electron microscopy study of the phase Al3Fe

The phase Al₃Fe (monoclinic C2/m, a = 1·549 nm, b = 0·808 nm, c = 1·248 nm, β = 107·8°) has been studied by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). Crystals were obtained from a direct chill-cast ingot of an Al-0·25 wt% Fe-0·13 wt% Si alloy. Extracted crystals were prepared by dissolving the aluminium phase in butanol and filtering off the particles. The extracted Al₃Fe crystals were mainly (100) platelets. The monclinic lattice was confirmed by tilt experiments and the mirror plane was confirmed by convergent beam electron diffraction. Experimental HREM images from the [100] and [110] projection agreed with images calculated by the multislice method. The interpretation of images in terms of a projected crystal structure is discussed. Common defects in Al₃Fe crystals were: twins on (100) and faults on (001). The (001) faults could be described by a displacement 1/2·[100] on a fault plane at z = 0·5 in the unit cell. A model for (001) faults, based on multiple twinning, is proposed.     

Surface structures of YBa2Cu3O7-x,BiCa1·7Sr0·7Cu2Ox and TlCaBaCuO4·5±x investigated by scanning tunnelling microscopy

Surface structures of the high-T_c superconductors YBa₂Cu₃O_7-x, BiCa_1·7Sr_0·7Cu₂O_x and TlCaBaCuO_4·5±x have been investigated with scanning tunnelling microscopy. The observed features include well-organized stripe corrugations in Y-Ba-Cu-O as well as orientated flake-like structures and steps with various heights in the Bi- and Tl-compounds. These observations suggest strong local variations of the elastic and electronic properties of the investigated materials.     

In-situ STM studies of electrochemical underpotential deposition of Pb on Au(111)

A modified scanning tunnelling microscope (STM) has been used to observe in-situ deposition and stripping of an electrochemical film. With STM tip and sample immersed in an acid electrolyte, single atomic steps on Au(111) have been imaged during the deposition and stripping of a monolayer-thick, underpotential deposit (UPD) of Pb. Integration of the electrochemical current passed during the film deposition and evidence from the STM images themselves confirm monolayer coverage. Our images show enhanced film growth at step edges and defect sites. Observations of single plating and stripping cycles indicate that the Au substrate returns unaltered. Except for atomic resolution images of Au(111), which we have not yet achieved in an electrolyte, all types of Au surface features seen in air are reproduced under the electrolytic solution. The modifications made to our STM in order to perform in-situ electrochemical experiments are described.     

Topography of YBa2Cu3O7-δ single crystals,spectroscopy of thin films and sintered YBa2Cu3O7-δ: theory and STM observations

We report topographic scanning tunnelling microscopy measurements on the (001) and (100) surfaces of YBa₂Cu₃O_7-δ single crystals, together with a theoretical basis by which the surface structures can be understood. We show that it is possible to obtain reproducible topographic data on a clean (001) and (100) surface in air. The results show various structures at scales from 0·1 to 100 nm. Among the observed features on the (001) surface were growth steps which had heights that were multiples of the lattice constant in the [001] direction. This indicates that the bulk crystal structure extends to the surface, and that the surface is relatively stable, even in humid air. We also found small (013) facets on the (001) surface, in accordance with theoretical pradictions. Occasionally, we found evidence for a one-dimensional corrugation on the surface. This is possibly a direct consequence of the orthorhombic distortion in the a-b plane with ordered oxygen vacancies, which gives rise to chain-like Cu-O structures along the b-axis. In addition, we measured the energy gap of sintered pellets and thin films of this superconductor, using a low-temperature STM. We discuss some effects which may influence the measurements and present criteria to check for these effects. We found an energy gap of δ=14±2 eV, in good agreement with the BCS pradiction 2δ/kT_c=3·5. This is not contradicted by the results we obtained by far infrared spectroscopy and Andreev reflection measurements.     

Automated Microscope-Absorption-Spectrophotometry Of Rock-Forming Minerals In The Range 40,000–5000 Cm−1 (250–2000 Nm)

To measure polarized absorption spectra of microcrystals of 3dⁿ-ion bearing silicate minerals, computer processed, microscope-spectrophotometric methods have been developed. Absorbance, log (I₀/I), can be measured with high relative accuracy (near u.v. and vis: ±0·002 to 0·001; n.i.r.: ±0·004 to 0·002), and relatively small spectral band widths are available. Hence, weak spin-forbidden dd-bands of 3dⁿ-ions can be recognized alongside spin-allowed dd-transitions without artificial broadening of absorption bands due to finite resolution. The smallest area from which absorption spectra can be taken is 8 μm in diameter. As one example of the many applications in mineralogy and material sciences, absorption spectra of a natural spessartine garnet, Spess_69·7Alm_30·0Gross_0·05, containing Mn²⁺, Fe²⁺, and traces of Fe³⁺ as 3dⁿ-ions, and of a pure Mn²⁺-garnet, Spess₆₇Gross₃₃, are presented. From these it is evident that bands in natural spessartines at ～ 26,900, 23,200 cm^?1 which were assigned to dd-transitions in Fe³⁺⁽⁶⁾, have to be reassigned to Mn²⁺⁽⁸⁾. Comparison of spectra obtained with the microscopic equipment described with those obtained by means of conventional macroscopic equipment prove that the methods described produce true spectra.     

The electron energy-loss near-edge structure (ELNES) on the N K-edges from the transition metal mononitrides with the rock-salt structure and its comparison with that on the C K-edges from the corresponding transition metal monocarbides

This paper presents the shapes of the electron energy-loss near-edges structure (ELNES) on the N K-edge of the group IV_A (Ti, Zr, Hf) and group V_A (V, Nb, Ta) transition metal mononitrides close to stoichiometry. With the exceptions of NbN and TaN, these compounds have the rock-salt (B1) structure when close to stoichiometry. NbN exists with both the rock-salt structure and a hexagonal structure. Two distinct ELNES shapes were observed from it, one of which corresponds closely with previously published data from the rock-salt structure. Under normal conditions, TaN is considered to exist only in the hexagonal form, the rock-salt form being a high-temperature/high-pressure phase although it has been reported as the result of plasma jet heating of the hexagonal form. Again two distinct ELNES shapes were observed, one of which appeared to fit into the pattern of the shapes from the other compounds with the rock-salt structure. The systematic changes of shape observed are very similar to those observed in the equivalent carbides and qualitatively follow the behaviour expected from theoretical band structures. The change in the chemical shift of the N K-edge on going from a group IV_A nitride to a group V_A nitride is ～-0·8 eV while that on going from a group IV_A carbide to a group V_A carbide is ～+0·8 eV. This difference in behaviour is explained as the result of differences in the densities of states at the Fermi levels of the compounds. The position of the first peak in the ELNES also shows a systematic change in its energy relative to the core state as the number of valence electrons in the compound increases and also as the transition series of the metal species changes. The energies, E_r, of the peaks in the ELNES relative to the threshold follow a relationship similar to that predicted by Natoli, i.e. (E_r - V)a = const. where V is the ‘muffin tin’ potential and a is the lattice parameter. The first peak gives a negative constant in the relationship. The value of constant increases for each subsequent peak up to the sixth becoming positive for the fourth and higher peaks but drops slightly on going from the sixth to the seventh peak. Each peak gives a different value of V in the relationship. The data sets for the carbides and the nitrides are systematically different in a similar way for each peak and there are deviations from linearity within each set. The systematic difference is minimized and the linearity significantly improved if the difference in the energies of two prominent peaks is used instead of E_r. This systematic variation of peak energy with lattice parameter can be used to predict the lattice parameter. If both the nitride and the carbide data for the energy of a prominent peak relative to the threshold are used, this results in a maximum deviation of 4% (or ～0·02 nm). However, if the differences in the energies of two prominent peaks are used and the data for the carbides and the nitrides are treated independently, the maximum deviation drops to 0·4% (or ～0·002 nm). At this level, uncertainties in the lattice parameters themselves come into play and better characterized materials are required to set true limits to the accuracy of the predictions. Finally some applications in the microanalysis of materials are outlined briefly.     

The influence of tissue processing on quantitative histopathology in breast cancer

Objective grading of breast cancer by morphometry has been suggested for improving the precision of the prognostic prediction. However, the tissue components evaluated might be influenced by variations in the processing, reducing the clinical value. In the present study, the impact of the period of fixation, of the acidity of the fixative and of the embedding medium was investigated by allocating tissue samples from 27 surgical breast cancer specimens systematically randomly to different modes of processing. The volume-weighted mean volume of cancer cell nuclei, v?_V(nuc), was estimated using the method of point-sampled intercepts on vertical sections. In addition, estimates of the mean nuclear profile area, ā_H(nuc), the nuclear volume fraction, V_V(nuc), the nuclear profile density, ND, and the mitotic profile frequency, MF, were obtained. The quantitative histopathological estimates were stable with respect to the investigated variables of the tissue processing. No significant differences were found when comparing the estimates obtained in samples from five tumours fixed in formalin at pH 5·0, 6·0, 7·0, 7·4 and 8·0, respectively. Similarly, no significant correlations between the estimates and the period of formalin fixation (24 h, 3 days and 3 months) were found in samples from five other tumours. However, the v?_V(nuc) was 13% larger (2p = 0·004) and the mean ND 17% smaller (2p = 0·04) in hydroxyethyl-methacrylate-embedded samples from 17 tumours as compared to paraffin-embedded samples. Thus, the shrinkage observed in paraffin seems to affect nuclei less than tissue.     

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.     

Tunnelling spectroscopy of high-temperature superconductors

We measured samples of La_1·85Sr_0·15CuO_4-y and YBa₂Cu₃O_7-y with a UHV low-temperature scanning tunnelling microscope using tips made out of tungsten, aluminium and indium. The samples are either sintered pellets, single crystals, or thin films. A wide variety of I-V curves were obtained ranging from nearly ideal S-I-N and S-I-S characteristics, to strongly asymmetric and multiple-peaked characteristics. The best data are obtained with thin film samples and indium tips. Large values of 2δ/kT_c (～ 11) are observed in most cases. We discuss possible explanations for these large values. New data are presented obtained with thin films of Y-Ba-Cu-O made by e-beam evaporation in our laboratory. A system of loadlocks is used to ensure that these films are never exposed to the atmosphere.     

Low-temperature bitumen stiffness and viscous paraffinic nano- and micro-domains by cryogenic AFM and PDM

In an effort to better understand the structure and behaviour of bitumen in low temperature, we describe the first use of cryogenic atomic force microscopy and phase detection microscopy to characterize bitumen nano‐ and micro‐structures. The results were interpreted in light of glass transition temperatures (T_gs) for bitumen fractions. The domains visible by microscopy, the catana, peri and para phases, were attributed to domains rich in asphaltenes, naphthene and polar aromatics, and saturates, respectively. Between ?10°C and ?30°C, atomic force microscopy images revealed topographic features not visible in atomic force microscopy images acquired at room temperature. According to phase detection microscopy and T_gs, the features were assigned to viscous unfrozen saturates. Upon cooling to ?72°C, unfrozen domains of 20–400 nm were observed. These domains were found in the paraphase rich in saturates and in the periphase rich in naphthene aromatics and polar aromatics. The findings indicate that new viscous domains form upon cooling to low temperatures owing to phase segregation, and that some bitumens are never entirely rigid in low temperatures.     

Tribological behaviours of Cu nanoparticles recovered from electroplating effluent as lubricant additive

Nowadays, many efforts have been made to minimise the pollution risks of copper electroplating effluent, such as chemical methods, physical methods, etc. Among them, chemical reduction has been used in this paper for its simplicity and potential for industrial production, and the recovered Cu nanoparticles (CuNPs) were innovatively used as a lubricant additive to prolong the lifetime of lubrication equipment and enhance energy conservation via emission reduction. In this paper, the relationships of the remaining Cu²⁺ concentration ([Cu²⁺]) with NaBH₄/CuSO₄ mole ratio, reaction time and reaction temperature were discussed separately. Then, L₉(3³) orthogonal experiment was carried out to determine optimal reaction conditions. Finally, the tribological behaviours [e.g. friction coefficients (FCs) and wear scar diameter (WSD)] of base oil samples with and without addition of the recovered CuNPs were investigated. Results indicate that the optimal reaction conditions were as follows: NaBH₄/CuSO₄ (4∶6) react at 30°C for 25 min, under which [Cu²⁺] was minimised to 0·2 mg L^?1 with a mean particle size of 33 nm. The FC and WSD of oil with 0·3 wt-%CuNPs were decreased by 33·4 and 19% respectively compared with the base oil. This compound oil was much more suitable for moderate load and high load than for low load. This paper provides a new idea on dealing with the copper electroplating effluent.     

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.     

Measurements of polyphosphoric acid on HOPG

As a sample for initial study on biological materials by using scanning tunnelling microscope (STM), we selected phosphoric acid consisting of phosphorous atoms which play biologically an important role. We measured polyphosphoric acid coated on HOPG by STM and scanning tunnelling spectroscopy (STS) in air. In order to identify molecules on STM images, local I-V curves were taken simultaneously. The averaged I-V curves and the normalized conductance spectra of graphite coated with the acid show characteristics different from those of clean graphite surface. Around a step of the substrate, we found domains where normalized conductance spectra were different from those of clean graphite surface and ascribable to that of the adsorbed molecules.