A new method of scanning tunneling spectroscopy for study of the energy structure of semiconductors and free electron gas in metals

In this study, anew method for the scanning tunneling spectroscopy (STS) of direct and indirect gap semiconductors and free electron gas in metals is proposed. Band structures of Si, porous Si, and Ge were studied. The tunneling current-voltage characteristics of Si and porous Si surfaces were measured over different voltage intervals from tens of mV to 20 V under incident light from an Xe lamp and those of a Ge surface in the dark. The correlation between the shapes of the I-V curves and band structure of the materials was calculated. It was found that the curves are linear if measured in the voltage range V₀ E_g/(2e) and nonlinear when V₀ α E_g/(2e) (in the measurements the applied voltage was changed from -V₀ to V₀). The method was used for the observation of a new effect of tunneling of free electron gas having thermal energies from a metal tip to a band gap state of the semiconductor. The energy spectrum of free electron gas was measured.     

Study of deposit and friction films of overbased calcium sulphonate by PM-IRRAS spectroscopy

This paper deals with the study of deposit and friction films of overbased calcium sulphonate diluted in mineral oil, on metal surfaces. The technique used for this work is Infrared Reflection Absorption Spectroscopy by Fourier Transform and Polarisation Modulation (PM-IRRAS or PM FTIR). The spectra obtained from this method contain only information on the absorption occurring in the immediate neighbourhood (⩽40 nm) of the metallic substrate and allow the average orientation for molecules adsorbed on metals to be deduced. This study shows that the adsorption of overbased calcium sulphonate on a steel surface leads to a preferential orientation of the sulphonate chains perpendicular to the surface and to a preferential orientation of the carbonate, the c axis being perpendicular to the surface. During friction, the sulphonate chains are ejected from the contact zone. We observed that the boundary film consists mainly of calcium carbonate, which crystallises into calcite.     

Brighter near-field optical probes by means of improving the optical destruction threshold

The optical destruction thresholds of conventionally etched and tube-etched near-field optical probes were measured. One of the main advantages of tube-etched tips is their smooth glass surface after taper formation. Presumably for this reason, a destruction limit of over 120 μJ was obtained, almost twice as large as that of the rougher, conventionally etched fibre probes. The use of additional adhesion layers (Ti, Cr, Co and Ni) between the glass surface and the aluminium coating produced, especially for tube-etched tips, a significant increase in the optical destruction threshold. With increasingly thin metal coatings, the use of a protection coating that prevents corrosion during aging is recommended. An additional increase in optical stability was achieved by applying mixed-metal coatings: alternating thin titanium and thick aluminium layers yielded fibre probes with superior properties that achieved average optical destruction thresholds of > 270 μJ. This is an increase in stability of > 400% compared with conventionally fabricated near-field optical tips.     

Atomic force and scanning electron microscopy of atmospheric particles

Atomic force microscopy (AFM) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) have been used for both morphological and elemental mass analysis study of atmospheric particles. As part of the geometrical particle analysis, and in addition to the traditional height profile measurement of individual particles, AFM was used to measure the volume relative to the projection area for each particle separately, providing a particle shape model. The element identification was done by the EDS analysis, and the element mass content was calculated based on laboratory calibration with particles of known composition. The SEM-EDS mass measurements from two samples collected at 150 and 500 m above the surface of the Mediterranean Sea were found to be similar to mass calculations derived from the AFM volume measurements. The AFM results show that the volume of most of the aerosols that were identified as soluble marine sulfate and nitrate aerosol particles can be better estimated using cylindrical shapes than spherical or conical geometry.     

Spectroscopic studies and electronic properties of heat treated calcium sulphonate

Overbased calcium sulphonate and its derivatives have been characterised by means of thermogravimetric analysis, X-ray and IR spectroscopy. The study was carried out after thermal treatment up to 900°C. Different phases appeared, showing a transition from an amorphous system to a crystallised medium. Electrical measurements showed electron behaviour for compounds obtained at low temperatures. ESR experiments produced classical results connected with the decomposition of organic chains.     

Micellar calcium borate as an antiwear additive

The mechanisms of action of a new generation of antiwear additives is studied here by means of energy‐filtering transmission electron microscopy (EFTEM) carried out on the wear particles generated during friction tests between two ferrous surfaces (under boundary lubrication conditions). This paper deals with the structural and physico‐chemical changes that colloidal particles, calcium carbonate (CC) and calcium borate (CB) overbased salicylates detergents, have undergone during the build‐up of the interfacial antiwear tribofilm. EFTEM allowed us to investigate the nature of wear fragments originating from the film, stemming from CC and CB micelles, and to make a comparison regarding the tribofilm formation mechanisms. It appears that the CC wear debris are mainly crystalline and contain a high concentration of iron (as abrasive iron oxide Fe₂O₃), limiting their antiwear action. Consequently, CC micelles do not lead to an effective protective tribofilm. On the other hand, CB micelles do have an antiwear action, which we explained by the formation of a glassy iron borate tribofilm during the friction tests. Many of the CB wear fragments are composed of this amorphous material containing very small crystallites of residual calcite. Boron (contained in the CB micelles) is responsible for the presence of amorphous zones of the film and acts as a glass former, in a comparable way to phosphorus in zinc dithiophosphate. This revised version was published online in July 2006 with corrections to the Cover Date.     

Scanning probe microscopy for industrial applications: Selected examples

Some examples are selected to demonstrate the variety of possible scanning probe microscopy application in industry. Magnetic and magneto-optical storage media can be investigated by magnetic force microscopy, whereas a conventional scanning force microscope is used to examine surface features of many different materials, such as technical glasses, photosensitive materials, new superconductors, and biomolecules. Some other examples include the modification as well as the observation of liquid crystal devices, and the impact that scanning probe microscopy has on other techniques such as high precision stepping motors and high quality electron beam sources.     

Absorption microanalysis with a scanning soft X-ray microscope: mapping the distribution of calcium in bone

This article describes the scanning transmission X-ray microscope operated at the National Synchroton Light Source. The application of the instrument to elemental analysis is detailed. In particular, qualitative results on the calcium distribution in human skull tissue are presented.     

Autocorrelation spectroscopy on single ultrathin layers of CdSe/ZnSe: hints for a non-thermal distribution of excitons in quantum islands

Autocorrelation spectroscopy on the basis of thousands of individual near-field photoluminescence spectra of single ultrathin CdSe layers at low temperatures exhibits a strong positive correlation peak around 18 meV energy with a width of 5 meV. Using simulations and experiments as a function of temperature and laser intensity, we can exclude interpretations along the lines of biexcitons or phonon sidebands. We attribute this feature to the splitting of ground state and an excited state in individual quantum islands. This interpretation implies that the potential minima are rather uniform in size and that the distribution of excitons is nonthermal.     

Evaluation of fracture planes and cell morphology in complementary fractures of cultured cells in the frozen-hydrated state by field-emission secondary electron microscopy: feasibility for ion localization and fluorescence imaging studies

We have employed field-emission secondary electron microscopy (FESEM) for morphological evaluation of freeze-fractured frozen-hydrated renal epithelial LLC-PK₁ cells prepared with our simple cryogenic sandwich-fracture method that does not require any high-vacuum freeze-fracture instrumentation (Chandra et al. (1986) J. Microsc. 144 , 15–37). The cells fractured on the substrate side of the sandwich were matched one-to-one with their corresponding complementary fractured faces on the other side of the sandwich. The FESEM analysis of the frozen-hydrated cells revealed three types of fracture: (i) apical membrane fracture that produces groups of cells together on the substrate fractured at the ectoplasmic face of the plasma membrane; (ii) basal membrane fracture that produces basal plasma membrane-halves on the substrate; and (iii) cross-fracture that passes randomly through the cells. The ectoplasmic face (E-face) and protoplasmic face (P-face) of the membrane were recognized based on the density of intramembranous particles. Feasibility of fractured cells was shown for intracellular ion localization with ion microscopy, and fluorescence imaging with laser scanning confocal microscopy. Ion microscopy imaging of freeze-dried cells fractured at the apical membrane revealed well-preserved intracellular ionic composition of even the most diffusible ions (total concentrations of K⁺, Na⁺ and Ca⁺). Structurally damaged cells revealed lower K⁺ and higher Na⁺ and Ca⁺ contents than in well-preserved cells. Frozen-freeze-dried cells also allowed imaging of fluorescently labelled mitochondria with a laser scanning confocal microscope. Since these cells are prepared without washing away the nutrient medium or using any chemical pretreatment to affect their native chemical and structural makeup, the characterization of fracture faces introduces ideal sample types for chemical and morphological studies with ion and electron microscopes and other techniques such as laser scanning confocal microscopy, atomic force microscopy and near-field scanning optical microscopy.     

Scanning tunneling microscope spectroscopy of polymers

This paper presents theoretical results on the relationship between density of states (DOS) and scanning tunneling microscope current-voltage curves in polymers. We considered samples of linear hydrocarbons electrically grounded at one of their extremes. The other extreme is electrically connected to the microscope tip via electron tunneling through vacuum. When a voltage, V, is applied to the tip, electric current, I, flows in the tip-sample circuit. This current varies as the voltage varies and depends on the DOS to the extent that no current would flow if no electron states exist at a certain energy (or voltage). The detailed relationship between DOS and the current-voltage (I-V) curve is not known a priori. We solve the corresponding quantum problem in the context of tight binding and find that I-V reproduces accurately the resonant energy peaks of the DOS. We apply the results to 100 atom-long alkane and alkene chains and found that there is a significant voltage shift in the corresponding curves as to discriminate one structure from the other.     

Transformation reactions of partially gold-coated bioactive Ca-P glass granules investigated by analytical scanning electron microscopy

In the present work, we have clarified the detail of the surface transformation reactions of bioactive calcium-phosphate (Ca-P) glass granules induced by in vivo implantation in rabbit dorsal muscle sites. To this aim we have compared the behaviour, during the same implantation, between the as-prepared and gold-coated only-on-one-side glass granules. The deposited gold layer enabled us to determine very precisely the initial position of the surface of the glass before the transformation took place. In addition, since the gold layer acts as a diffusion barrier, it allowed the study of the direction and the mechanism of crystal growth which occurred at the glass surface. Lapped and polished cross-sections of the samples were examined by backscattered electron (BSE) imaging and quantitative (with standards) x-ray energy dispersive spectroscopy (EDS) in a scanning electron microscope (SEM). The observations showed the presence of an interlayered structure. Quantitative EDS microanalysis performed by profiling the electron beam across the samples indicated the presence of hydrated calcium phosphate in the external layer, an inhomogeneous silica-rich gel-type layer in the middle layer, and an unaffected original Ca-P glass in the centre. From the comparison with those granules gold-coated on one side, we deduced that the hydrated calcium phosphate layer grew towards the interior of the granules at the expense of the starting glass. A simple model, based on the balance of the concentrations of the elements which have diffused in the different layers, is proposed to explain the contribution of the elements constituting the original glass to the formation of the different layers. This result agrees with the experimental data obtained from image analysis and the microstructural behaviour of this type of glass is discussed.