Coating of tips for electrochemical scanning tunneling microscopy by means of silicon, magnesium, and tungsten oxides

Different combinations of metal tips and oxide coatings have been tested for possible operation in electrochemical scanning tunneling microscopy. Silicon and magnesium oxides have been thermally evaporated onto gold and platinum-iridium tips, respectively. Two different thickness values have been explored for both materials, namely, 40 and 120 nm for silicon oxide and 20 and 60 nm for magnesium oxide. Alternatively, tungsten oxide has been grown on tungsten tips via electrochemical anodization. In the latter case, to seek optimal results we have varied the pH of the anodizing electrolyte between one and four. The oxide coated tips have been first inspected by means of scanning electron microscopy equipped with microanalysis to determine the morphological results of the coating. Second, the coated tips have been electrically characterized ex situ for stability in time by means of cyclic voltammetry in 1 M aqueous KCl supporting electrolyte, both bare and supplemented with K(3)[Fe(CN)(6)] complex at 10 mM concentration in milliQ water as an analyte. Only the tungsten oxide coated tungsten tips have shown stable electrical behavior in the electrolyte. For these tips, the uncoated metal area has been estimated from the electrical current levels, and they have been successfully tested by imaging a gold grating in situ, which provided stable results for several hours. The successful tungsten oxide coating obtained at pH=4 has been assigned to the WO(3) form.     

用有限元方法计算X光源场发射阴极电流

针对场致发射阵列建立了有效的三维有限元模型来分析单个尖锥的发射电流.考虑到场致发射阵列的周期性和尖锥的轴对称性,仅对一个尖锥单元的1/4进行分析.对模型的表面施加电压边界条件,计算得到尖锥表面电场强度分布,电场强度在尖锥顶点最强,场发射电流在此处也最强.由Fowler-Nordheim函数可得到尖锥表面的场致发射电流密度分布,对整个尖锥表面进行积分后得到了单个尖锥的场致发射电流约为7μA.计算了在100V门电压下不同顶端半径的场致发射电流.结果显示,场发射对顶端半径有很强的依赖性.计算了100个顶端半径为8nm的尖锥在不同门电压下的总场致发射电流,发射电流与开启电压与实际测量值符合得很好.     

Charging effect in electron-irradiated ice

To maintain the original distribution pattern of diffusible elements in biological samples, electron probe microanalysis is carried out with frozen hydrated bulk specimens and cryosections, analysed at temperatures below 130 K. Ice has a very low intrinsic conductivity at this working temperature and surface- and space-charging appears, when uncoated specimens are irradiated with non-penetrating electrons. Although coating with a grounded conductor abolishes the surface potential, the build-up of an internal space-charge field is possible, depending on the sample thickness and beam voltage used. Consequently, the geometry of the X-ray source volume and the spectral distribution of the emitted continuous and characteristic X-rays are affected. To simulate the situation for microanalysis of frozen hydrated specimens the charging process in electron irradiated ice is studied by recording simultaneous specimen currents from the top and bottom of ice layer preparations. The external currents yield information on the build-up of internal space-charge fields which result from the balance of charge injection, storage, and transport. Irradiation of uncoated bulk specimens with a finely focused beam results in the build-up of a space-charge field close to the surface, which causes a reduction of the depth of microprobe analysis. In coated bulk specimens the induced conductivity renders possible a current flow to the front electrode, thereby limiting the space-charge field. Sections with an effective rear electrode will not charge appreciably if the electron range is larger than about half the section thickness.     

Nanotip electron gun for the scanning electron microscope

Experimental nanotips have shown significant improvement in the resolution performance of a cold field emission scanning electron microscope (SEM). Nanotip electron sources are very sharp electron emitter tips used as a replacement for the conventional tungsten field emission (FE) electron sources. Nanotips offer higher brightness and smaller electron source size. An electron microscope equipped with a nanotip electron gun can provide images with higher spatial resolution and with better signal-to-noise ratio. This could present a considerable advantage over the current SEM electron gun technology if the tips are sufficiently long-lasting and stable for practical use. In this study, an older field-emission critical dimension (CD) SEM was used as an experimental test platform. Substitution of tungsten nanotips for the regular cathodes required modification of the electron gun circuitry and preparation of nanotips that properly fit the electron gun assembly. In addition, this work contains the results of the modeling and theoretical calculation of the electron gun performance for regular and nanotips, the preparation of the SEM including the design and assembly of a measuring system for essential instrument parameters, design and modification of the electron gun control electronics, development of a procedure for tip exchange, and tests of regular emitter, sharp emitter and nanotips. Nanotip fabrication and characterization procedures were also developed. Using a "sharp" tip as an intermediate to the nanotip clearly demonstrated an improvement in the performance of the test SEM. This and the results of the theoretical assessment gave support for the installation of the nanotips as the next step and pointed to potentially even better performance. Images taken with experimental nanotips showed a minimum two-fold improvement in resolution performance than the specification of the test SEM. The stability of the nanotip electron gun was excellent; the tip stayed useful for high-resolution imaging for several hours during many days of tests. The tip lifetime was found to be several months in light use. This paper summarizes the current state of the work and points to future possibilities that will open when electron guns can be designed to take full advantage of the nanotip electron emitters.     

Field emission,developments and possibilities

The overall performance of any electron microscope is to a great extent determined by the electron source. For example, the current acceptance of the STEM concept is due to the stimulation provided by field emission sources, which made it possible to image single atoms in the STEM. The field emission source remains the electron source with the highest brightness and the lowest energy spread. As a consequence, considerable research and engineering work has been and is still being conducted in a number of important areas. These areas include: (i) the mechanisms which determine the beam current stability; (ii) the electron-electron interactions which lead to beam spreading and an increase of the energy spread; (iii) various types of emitters such as bare tungsten tips, oxygen processed tips, zirconium coated tips, carbon emitters, carbon coated tungsten emitters and even solid state emitters; (iv) the lifetime limiting factors, e.g. inclusions, ion bombardment and flashovers effects; (v) different types of optical systems for field emission guns, e.g. choice of magnetic and/or electrostatic lenses at high voltage potential in UHV systems, and special magnetic lenses just below the anode; (vi) field emission guns operating at higher kV's, up to 1·6 MeV. Because analytical work in the smallest possible volumes is a growing area of scientific interest, the demand for field emission sources which are reliable, easy to operate and stable will continue.     

Propagation of femtosecond light pulses through near-field optical aperture probes

The propagation of femtosecond light pulses through near-field optical fiber tips has been modelled numerically in three spatial dimensions by means of the finite integration technique. Ideally conducting as well as real metallic coatings of the tip have been considered, and the influence of surface plasmon polaritons (SPP) on shape, spectrum, and amplitude of the light pulse in the near and far fields of the tip have been investigated in this way. Special attention has been devoted to the superluminal tunneling of light through the fiber tip. The variation of phase and group velocities along the fiber axis has been characterized for a number of real metals and for different tip angles. A maximum of both velocities in the near field of the tip is characteristic for coatings of finite conductivity. For some tip angles negative values of the phase and/or group velocities are observed, which are caused by the propagation of SPP on the outer surface of the coating and their conversion into photons. It is shown, that the excitation of SPP on the metallic coating leads to strongly altered spatial emission characteristics of the tip.     

Electron beam confinement and image contrast enhancement in near field emission scanning electron microscopy

In conventional scanning electron microscopy (SEM), the lateral resolution is limited by the electron beam diameter impinging on the specimen surface. Near field emission scanning electron microscopy (NFESEM) provides a simple means of overcoming this limit; however, the most suitable field emitter remains to be determined. NFESEM has been used in this work to investigate the W (1 1 0) surface with single-crystal tungsten tips of (3 1 0), (1 1 1), and (1 0 0)-orientations. The topographic images generated from both the electron intensity variations and the field emission current indicate higher resolution capabilities with decreasing tip work function than with polycrystalline tungsten tips. The confinement of the electron beam transcends the resolution limitations of the geometrical models, which are determined by the minimum beam width.     

Magnetic field emission gun with zirconiated emitter

A magnetic-field-superimposed field emission gun with low aberrations and equipped with a zirconiated tungsten emitter has been developed for applications where very stable high probe currents are required. It has been tested on a conventional electron microscope at 10 kV and on an electron beam testing system at 1 kV. Probe current i = 250 nA in a probe size d = 0.4 μm is obtained at 10 kV; at 1 kV the resolution is 0.1 μm with i = 5 nA, and 0.4 μm with i = 30 nA. For these probe currents, the spatial broadening effect due to electron–electron interactions in the beam is the preponderant factor limiting the probe size.     

Preparation of STM tips for in-situ characterization of electrode surfaces

The total current between the tip and the sample in a scanning tunnelling microscopy study of a solid/liquid interface can be dominated by Faradaic charge transfer currents. In such a situation, feedback control of the tunnelling gap, and imaging, is precluded. In this contribution we describe the preparation of glass and polymer coated STM tips that possess < 100 Ų of exposed metal. These tips effectively discriminate against Faradaic current and enable STM imaging in the presence of reversible electroactive solution species at appreciable tip/sample biases.     

Characterization and fabrication of fully metal-coated scanning near-field optical microscopy SiO2 tips

The fabrication of silicon cantilever‐based scanning near‐field optical microscope probes with fully aluminium‐coated quartz tips was optimized to increase production yield. Different cantilever designs for dynamic‐ and contact‐mode force feedback were implemented. Light transmission through the tips was investigated experimentally in terms of the metal coating and the tip cone‐angle. We found that transmittance varies with the skin depth of the metal coating and is inverse to the cone angle, meaning that slender tips showed higher transmission. Near‐field optical images of individual fluorescing molecules showed a resolution < 100 nm. Scanning electron microscopy images of tips before and after scanning near‐field optical microscope imaging, and transmission electron microscopy analysis of tips before and after illumination, together with measurements performed with a miniaturized thermocouple showed no evidence of mechanical defect or orifice formation by thermal effects.     

Modification of a TEM-goniometer specimen holder to enable beam current measurements in a (S)TEM for use in quantitative X-ray microanalysis

In order to have available a specimen holder suited to measure the beam current as is often required in quantitative electron probe X-ray microanalysis, the rod of a low background beryllium specimen holder of a transmission electron microscope was modified. The tip was electrically insulated from the mass of the microscope and connected electrically to the central contact of a BNC connector mounted on the specimen holder handle. With this modified specimen holder the current absorbed by the specimen and/or the specimen holder could be measured easily and accurately. The modified specimen holder has been used to measure the beam current stability of an analytical electron microscope under various conditions. Data were obtained for tungsten as well as lanthanum hexaboride cathodes. Small changes to other types of specimen tips made it possible to exchange these for the low background tip.     

Coherent electron emission from point sources

We study the electron emission and propagation in ultrasharp tips field-emission experiments. The intensity-voltage characteristics as well as the angular spreading of the electron beam is analysed. We show that non-coherent emission cannot explain the very narrow experimental electron beams. The properties of coherent electron sources and the beam spatial coherence width are discussed. The quantum mechanical current distribution of field-emitted electrons from tubes seems to explain the experimental beam widths.     

Performance Evaluation of a Hard Composite Solid Lubricant Coating When Dry Machining of High-carbon Steel

A novel hard composite solid lubricant coating combining TiN and MoS_x has been developed using pulsed DC closed-field unbalanced magnetron sputtering (CFUBMS). The tribological and mechanical properties together with their interdependencies with the coating microstructures have been assessed and reported elsewhere. This article evaluates the machining performance and correlates the underlying tribological aspects of different TiN-MoS_x coating architectures (deposited at titanium (Ti) cathode currents of 1, 3.5, and 5 A) when dry turning AISI 1080 high-carbon steel. A comparative performance study clearly established the supremacy of the composite coating (deposited at 3.5 A Ti cathode current with ～12 wt% of MoS_x) with a hard TiN underlayer over monolayer TiN, MoS_x, and other related coating architectures in terms of cutting force, tool wear, and workpiece surface roughness. The superlubricity behavior of the said composite coated tool resulted in a reduction of cutting force (by up to ～45% compared to the uncoated tool) and exhibited a tool life of 8 min, which was eight times and more than two times longer than that of the uncoated and conventional hard TiN coated counterparts, respectively. The workpiece surface roughness, R_a, also decreased by 13 to 21% when machined with the TiN-MoS_x coated tool in comparison to the uncoated cemented carbide.     

Braking performance and noise in excessive worn brake discs coated with HVOF thermal spray process

In this paper, two excessive worn brake discs of a light commercial vehicle were coated with High velocity oxygen fuel (HVOF) thermal spray process for reuse. The coated discs were compared with uncoated Original equipment manufacturer (OEM) D1 disc in terms of braking performance and noise. The D2 disc was coated with tungsten-carbide-cobalt (88 %WC-12 %Co) powder and NiCr (80/20) binder, and 500 Gm thick coating was obtained. The D3 disc was coated with Colmonoy-88 (Ni-W-Cr-B-Si) powder and NiCr (80/20) binder and 600 Gm thick coating was obtained. Several test procedures (e.g. bedding, vibration, and structural-strength tests) was applied to three discs using inertia brake dynamometer. Results showed that coated D2 disc with tungsten carbide cobalt provided lower brake noise and higher brake performance compared with OEM disc. Considering the coefficient of friction and temperature, coated D3 disc has approximately equal braking performance with OEM D1 disc despite the high braking noise value.     

EFFECT OF COATING ROUGHNESS ON PERFORMANCE OF SMALL CVD DIAMOND COATED TOOLS

□ Small milling tools for dental application were diamond coated by means of Hot Filament CVD (HFCVD). Different thicknesses were obtained by using different diamond deposition times (3 and 12 h) and seeding conditions. The surface roughness was measured for coated and uncoated mills and milling tests were performed to measure the cutting forces. A ceramic material and a polymeric matrix composite (PMC) were used as workpiece materials. The highest cutting forces were measured for the coated tool with the highest thickness of the diamond coating. Probably, roughness increase and non-optimal edge profiles affect the tool behavior during cutting. As a consequence, tool failure was observed in the case of milling of ceramic by means of the coated tool with the highest coating thickness. The coated tool with a lower thickness of the diamond coating showed an optimal behavior under cutting, above all in the case of PMC milling.     

Analysis of fiber probes of scanning near-field optical microscope by field emission microscopy.

It is shown that field emission microscopy and related methods can be used to analyze the metal coated fiber tips, which nowadays are the most frequently used sensor for the scanning near-field optical microscopy (SNOM). Metal free and thus non field-emitting aperture for the light transmission on the tip apex can be directly seen and its parameters can be measured, which is very important for the interpretation of SNOM data.     

Structure analysis of sputter-coated and ion-beam sputter-coated films: a comparative study

Gold, platinum and tungsten films were deposited by low energy input (7 mA, 450 V), or high deposition rate (80 mA, 1500 V), diode sputter coating and by ion beam sputter coating. Film structures on Formvar coated grids and on the surface of coated erythrocytes, resin embedded, sectioned, and recorded at high magnification in a TEM were compared using computer-assisted measurements and analysis of film thickness and grain size. The average grain size of the thinnest gold and platinum films was relatively independent of the mode or rate of deposition but as the film thickness increased, significant differences in grain size and film structure were observed. Thick platinum or gold films deposited by low energy input sputter coating contained large grain size and electron transparent cracks; however, more even films with narrower cracks but larger grain size were produced at high deposition rates. Ion beam sputter coated gold had relatively large grain size in 10 nm thick films, but beyond this thickness the grains coalesced to form a continuous film. Platinum films deposited by ion beam sputter coating were even and free of electron transparent cracks and had a very small grain size (1–2 nm), which was relatively independent of the film thickness. Tungsten deposition either by low energy input or ion beam sputter coating resulted in fine grained even films which were free of electron transparent cracks. Such films remained granular in substructure and had a grain size of about 1 nm which was relatively independent of film thickness. Tungsten films produced at high deposition rates were of poorer quality. We conclude that thick diode sputter coated platinum and gold films are best deposited at high deposition rates provided the specimens are not heat sensitive, the improvement in film structure being more significant than the slight increase in grain size. Thick diode or ion beam sputter coated gold films should be suitable for low resolution SEM, and thin discontinuous gold films for medium resolution SEM. Diode sputter coated platinum should be suitable for medium resolution SEM and ion beam sputter coated platinum for medium and some high resolution SEM. 1–5 nm thick tungsten films, deposited by low energy input or ion beam sputter coating should be suitable for high resolution SEM, particularly where contrast is of less importance than resolution.     

Performance evaluation of PVD TIALN coated carbide tools vis-à-vis uncoated carbide tool in turning of titanium alloy (Ti-6Al-4V) by simultaneous minimization of cutting energy,dimensional deviation and tool wear

Titanium alloys are difficult-to-machine materials because of their poor machinability characteristics. Machining and machining performance evaluation for such materials is still a challenge. Individual machining performance indices like cutting forces, cutting energy and tool wear lead to ambiguous understanding. In this work, a Cumulative Performance Index (CPI) is defined which amalgamates non-dimensional forms of specific cutting energy, back force and average principal flank wear in turning. The CPI focuses upon simultaneous minimization of specific cutting energy, dimensional deviation and average principal flank wear. The defined index is then used to evaluate performance of five commercially available physical vapor deposited (PVD) TiAlN coated tungsten carbide/cobalt inserts vis-à-vis uncoated tungsten carbide/cobalt insert in turning of Ti-6Al-4V. Cutting forces were monitored during turning and tool wear was measured after turning experiments. The results showed that the performance of coated inserts was either comparable or poor than uncoated insert; and in no case, coated inserts performed better than uncoated insert. Although commercial recommendations are in place to use PVD coated inserts for enhanced machinability of titanium alloys, the use of coated inserts is not justified keeping in view the energy spent in coating and insignificant improvement in performance.     

Effects of hot electron emission on a low-conductivity tetracyanoethylene polymer layer including studies of the corrugation of the film surface

The effect of strong field electron emission (FEE) on a tetracyanoethylene (TCNE) polymer layer was studied by Field Ion Microscopy (FIM) using TCNE and Ne as the imaging gases. The TCNE polymer was formed on each tungsten tip by radical polymerisation before FEE. The FIM images show field emission spots all over the surface of the tip. The FEM images show a random distribution of several field emission areas at the onset of FEE. After sometime at a current of about microA, there is a transition to higher currents at the same voltage, in which the electron emission pattern changes to have only one emitting area. After this transition, two different types of FIM images were observed, depending on the imaging gas that was used. Neon FIM images at low tip voltages show spots in the areas where the electron emission current was greatest, and at much higher voltages these images show emission from other areas with lower surface corrugation. However, the FIM images with TCNE as the imaging gas do not show any differences between the areas with and without electron emission. The FIM images remain as before FEE, which can be explained by the formation of a new polymer by the reaction of the surface layer with the imaging gas. It is assumed that chemically reactive fragments at the polymer/vacuum interface, which are needed for the polymerisation reaction, are formed by pyrolysis and sputtering processes during FEE.     

Study on cutting forces and experiment of MoS2/Zr-coated cemented carbide tool

MoS₂/Zr composite films were deposited on the cemented carbide YT14 (WC+14%TiC+6%Co) by medium-frequency magnetron sputtered coupled with multi-arc ion plating techniques. The thickness, micro-hardness, and coating/substrate adhesion strength of the coatings were tested. Surface morphologies of the composite coatings, as well as wear features, were investigated by scanning electron microscopy. Dry machining tests on hardened steel were carried out with the coated tool and uncoated YT15 tool. The variation of cutting forces for 45# hardened steel was tested by the Kistler force tester. The result shows that the cutting forces of coated tool were decreased by 25–30%, and flank wear resistance was improved by 30–35% in comparison with the uncoated YT15 tool. Through the analysis of cutting force distribution theory, the lower mean shear stress on the MoS₂/Zr-coated tool face leads to the decrease of cutting force and increase of tool wear resistance.