Quantitative High Resolution Electron Microscopy

 With the resolution becoming sufficient to reveal individual atoms, HREM is now entering the stage where it can compete with X-ray methods to quantitatively determine atomic structures of materials without much prior knowledge, but with the advantage of being applicable to aperiodic objects such as crystal defects. In our view the future electron microscope will be characterised by a large versatility in experimental settings under computer control such as the illumination conditions (TEM-STEM), CBED, detecting conditions (diffraction, image, ptychography) and many other tunable parameters such as focus (g), voltage, spherical aberration (C _s ), beam tilt, etc. Since modern detectors can detect single electrons, also the counting statistics is known. The only limiting factor in the experiment will be the total number of electrons that interact with the object during the experiment due to the limitations in the exposure time or in the object damage. However, instrumental potentialities will never be exploited fully if not guided by an experimental strategy. Here intuitive guidelines can be very deceptive. For instance an image made with the best electron microscope (C _s  = 0) at the best focus (g = 0) from the best object (phase object) would show no contrast at all. Hence, questions such as what is the best C _s , focus, object thickness, etc. can only be answered properly if done using a method of experiment design.     

Characterization of Nanocomposite Coatings in the System Ti–B–N by Analytical Electron Microscopy and X-Ray Photoelectron Spectroscopy

Summary.  Superhard nanocomposite coatings of different composition in the quasi-binary system TiN–TiB₂ were deposited onto stainless steel sheets by means of unbalanced DC magnetron co-sputtering using segmented TiN/TiB₂ targets. The chemistry and microstructure of a TiB_0.6N_0.7 coating was investigated using X-ray and electron diffraction, photoelectron spectroscopy, energy-filtering transmission electron microscopy, and electron energy-loss spectrometry. High resolution elemental mapping of the elements Ti, B, N, and O with energy-filtering TEM reveals a homogeneous distribution on the nanometer scale. X-Ray and electron diffraction exhibit only TiN crystallites of nanometer size, but no information on the boron-rich phase. The near-edge fine structures of the BK and NK ionization edges in the EELS spectra of the Ti–B–N coatings were used to derive information on the phases by comparing the edges with those of reference compounds. It was found that the TiN nanocrystals occur together with TiO _x particles; the grains are embedded in a strongly disordered or quasi-amorphous matrix consisting mainly of TiB₂ particles and, near the steel substrate, also boron oxide (B₂O₃). Received October 4, 2001. Accepted (revised) January 10, 2002     

Contact Potential Difference of Au and GaInAs by Electrostatic Force Microscopy

 For a metallic surface (Au) and highly doped (N+) and (P+) semiconductor surfaces (GaInAs) and for localised zones (2 × 2 μm) we have measured using an electrostatic force microscope the variation of the gradient of the electrostatic force by the signal (phase of the oscillating movement of the metallised tip) as a function of the sample-tip potential difference (− 4 V to + 4 V). In both cases the signal shows a quadratic variation with the sample-tip potential difference. The variation of the signal is of the order of magnitude of the theoretical predictions obtained by modelling the shape of the tip by a truncated cone + a portion of a sphere. Using the parabolic curve that fits the experimental results, the value of the contact potential difference, corresponding to a zero value of the electrostatic force gradient, can be determined with an accuracy of 50 mV. The contact potential difference, measured between the metallised tip and the metal (Au), taken as a reference, allows the work function of the metal tip to be determined (5.25 eV). The values of the contact potential difference for the GaInAs (N+) and (P+) surfaces can be explained by the Fermi level pinning due to surface charges.     

Characterization of the Element Distribution Within TiN Coatings with SIMS

 The distribution of the relevant elements within TiN coatings, made with two different physical deposition methods as the conventional dc vacuum arc method and the filtered high current pulsed arc method (Φ-HCA) are characterized and finally compared. Despite the rougher surface of the dc-arc produced TiN layer, which is due to accumulated droplets, there is no evidence of different stoechiometric composition of Ti and N on the surface. The interface of the dc-arc produced TiN layer (600 nm) is 10 times wider than the one made with the new filtered high current pulsed arc method (60 nm). However the TiN layer made by Φ-HCA shows an inhomogeneous distribution of aluminum and chlorine in the vertical direction, whereas the dc-arc sample is homogeneous. Furthermore, the TiN layer made by Φ-HCA shows vertically an obvious local maximum of chlorine at a depth of about 130 nm. This vertical local maximum has an homogeneous distribution in horizontal direction, which means that a thin, chlorine enriched layer has been incorporated inside the TiN layer. Nevertheless, quantification by SIMS shows that aluminum as well as chlorine concentrations of both samples are too low to influence any TiN properties. Received January 3, 2000. Revision April 4, 2000.     

Evaluation of the Anticorrosive Behaviour of Organic Coatings by Using a Variant of Electrochemical Impedance Spectroscopy

 Electrochemical impedance spectroscopy (EIS) has been acknowledged as a modern and efficient method in the evaluation of the anticorrosive behaviour of coatings. However, difficulties in the interpretation of the complex experimental data present a significant drawback in the use of the method in field studies where quick results are required. The breakpoint frequency method is an extension of the basic EIS method, which allows rapid qualitative assessment of the condition of a coating. By the breakpoint frequency method, the behaviour of epoxy coatings containing various types of reinforcement and applied on steel specimens has been examined after long-term (up to 300 days) exposure to a corrosive environment. The parameter values obtained by the breakpoint frequency method, such as the delaminated area, have been compared with those obtained from fitting the experimental EIS data to a suitable equivalent circuit. These results have also been compared with those of the water permeability of free membranes and the diffusion coefficient determined by standard test methods (e.g. the cup test).     

Characterization of Two-Component Metal Coatings (Al/Sn) with SIMS

 Vacuum deposited films of immiscible metal–metal systems can be applied as tribological coatings for plain bearings in high performance diesel engines. These industrially manufactured coatings show higher lifetimes than conventional electroplated coatings. For our investigations we used aluminum-tin coatings of 1 μm thickness on a glass substrate produced by sequential deposition from two separate targets under working gas pressure of 0.4 Pa. The tough Al matrix takes high mechanical loads and the soft inclusions of Sn act as solid lubricant. While Sn is deposited it migrates on the Al surface and grows as islands. We characterized the conformation and the distribution of the Sn islands on the surface and to the interstitial area between the islands with 3-D secondary ion mass spectroscopy (SIMS). The existence of a Sn layer between the islands (“wetting layer”) has been detected by SIMS and verified by measurements with Auger electron spectroscopy (AES).     

Electron Microscopy Study of Nickel and Nickel Composite Electrocoatings

 It has been proved that the electrolysis parameters strongly influence the structural properties of nickel electrodeposits, mainly the crystalline growth, preferred orientation and surface morphology. To determine the structural characteristics of nickel and composite nickel matrix electrocoatings, prepared under either direct current or pulse current conditions, the electron microscopy techniques have proved very important tools. In particular, investigation of the distribution and percentage of microparticles incorporated in the metallic matrix during preparation of nickel composite electrocoatings demands the use of techniques such as energy dispersion spectroscopy (EDS) or image analysis.     

Examination of the Spatial Distribution of Dyes and Polymers in Thin Films by Two-Photon Microscopy

Summary.  Two-photon absorption induced fluorescence microscopy was used as a tool for the examination of the spatial distribution of a thin dye film. The two-photon absorption induced fluorescence signal is essentially the same as that produced by excitation with a single photon of equivalent energy. When femtosecond pulses are focused into a sample there is an intrinsic spatial selectivity of the two-photon emission signal, since it is dependent upon the square of the light intensity. This has tremendous implications in fluorescence microscopy. Since two-photon absorption is confined in a small region at the focal waist of an objective lens, photodamage and photobleaching of the sample are significantly reduced. In addition, the two-photon signal has inherent z-axis spatial resolution, which facilitates the construction of 3-D images. In the present work an application of this technique to a thin film of a dye is presented. The method can generally be applied to thin films made from photonic polymers. Received June 23, 2000. Accepted (revised) July 31, 2000     

Localized Visualization of Chemical Cross-Talk in Microsensor Arrays by Using Scanning Electrochemical Microscopy

 An investigation of an array of four Pt microband electrodes 25 μm wide and spaced 25 μm apart was performed with the scanning electrochemical microscope (SECM). Where possible the SECM measurements were confirmed with conventional electrochemical measurements. It is shown how the sensiti- vity of the SECM recycling current to the activity of the underlying surface can be used to probe the homogeneity of enzyme-modified microelectrodes. The diffusion of H₂O₂ between these micro enzyme- electrodes and unmodified electrodes was investigated and it was demonstrated how the SECM can be a powerful tool in the elucidation of the properties of these electrodes. Received June 8, 1998. Revision November 12, 1998.     

Quantitative Analysis of Thin Aluminium-Oxynitride Films by EPMA

 Thin films of aluminium oxynitride with diverse composition were prepared by dc-magnetron sputtering of aluminium, utilising sputtering power as well as argon, oxygen and nitrogen gas flows to vary the composition. Since film properties depend mainly on the content of incorporated oxygen and nitrogen, a method for quantitative analysis of the main constituents based on electron probe micro analysis with energy dispersive detection was developed. The excellent precision of the quantitative results for aluminium as well as oxygen and nitrogen are shown. Furthermore, a film layer analysis program was applied for the quantification of several films deposited under the same deposition parameters on silicon wafers, from 520 nm down to 40 nm thickness, showing that electron probe micro analysis with energy dispersive detection is a reliable method for quantitative compositional analysis of thin aluminium oxynitride films down to approximately 20 nm thickness. Since this method of analysis provides only bulk information, expected inhomogeneities of the depth distribution of the film components were checked by secondary ion mass spectrometry depth profiles of two thin films and correlated to the EPMA results. The thickness of the films was determined by ellipsometry. Received September 1, 1998     

A Bulk Acoustic Wave Viscosity Sensor for Determination of Lysozyme Based on Lysis of Micrococcus Lysodeikeicus

 A method for determination of lysozyme with a Bulk Acoustic Wave (BAW) viscosity sensor is presented. It is based on the bacteriolytic action of lysozyme on Micrococcus lysodeikeicus (M. lysodeikeicus) and the response of the sensor to the viscosity and density change of this process. There was a good correlation between the frequency shift and the concentration of lysozyme in the range 10–100 μg/ml. The content of lysozyme in human saliva was determined by this method and the results obtained were in good agreement with those from the conventional turbidimetric method. This method has an advantage over the conventional turbidimetric method in that the amount of sample needed is smaller, the procedure is simpler and the concentration range of the bacterium suspension which can be used in the detection was extended. Received September 11, 1998. Revision March 15, 1999.     

Scanning Probe Microscopy and Spectroscopy of CVD Diamond Films

 The surface morphology and electronic properties of as-deposited CVD diamond films and the diamond films which have been subjected to boron ion implantation or hydrogen plasma etching have been systematically studied by high resolution scanning probe microscopy and spectroscopy techniques. AFM and STM image observations have shown that (a) both the as-deposited CVD diamond films and the boron ion implanted films exhibit similar hillock morphologies on (100) crystal faces and these surface features are formed during the deposition process; (b) boron ion implantation does not cause a discernible increase in surface roughness; (c) atomic flatness can be achieved on crystal faces by hydrogen plasma etching of the film surface. Scanning tunnelling spectroscopy analysis has indicated that (a) the as-deposited diamond films and the hydrogen plasma etched diamond films possess typical p-type semiconductor surface electronic properties; (b) the as-deposited diamond films subjected to boron implantation exhibit surface electronic properties which change from p-type semiconducting behaviour to metallic behaviour; (c) the damage in the boron implanted diamond films is restricted to the surface layers since the electronic properties revert to p-type on depth profiling.     

Chromate-Free Zinc Conversion Coatings Characterised by Grazing Incidence X-Ray Diffractometry

 Alternative processes for chromate conversion coatings on zinc represent a research challenge for environmental reasons. A potential concept yielding a promising corrosion behaviour without the need for toxic chemical additives is the application of alternating currents (AC) together with the addition of inorganic nitrogen compounds in an electrochemical bath. A first grazing incidence X-ray diffractometry (GIXD) study of the composition and crystallinity of this novel conversion coating system is presented. GIXD has been employed because this technique has proved to be successful in the analysis of electrochemically formed conversion coatings down to 10 nm thickness. Various ion additives like and in NaOH solution were applied in the AC treatment. The nitrite anion, , yielded the best result. It increased the generation rate and crystallinity of the passive layers. The ratio of the crystalline ZnO to Zn(OH)₂ content could be influenced by the AC amplitude and the AC cycling time. GIXD decisively contributed to the improved understanding of this chromate-free zinc conversion process.     

Analytical Electron Microscopy of a Magnesium Alloy Containing Neodymium

 Microstructure and phase composition studies of QE-22 magnesium based alloy with 2–3% Ag, 1.8–2.5% Nd and min. 0.4% Zr, before and after full heat treatment have been carried out using various electron microscopy techniques like EDX mapping, line scans and TEM with SAED. An extrapolation technique enabled to eliminate the influence of Mg-matrix on the quantitative analysis results and allowed to estimate the composition of the second phase which was mostly (Mg, Ag)₁₂Nd type in the as cast alloy and (Mg, Ag)₁₂Nd and Mg₁₂Nd₂Ag equilibrium precipitates after the precipitation process.     

Analytical Electron Microscopy Study of a ZnO-NiO Solid Solution

 Results of an analytical electron microscopy study of a binary ZnO-NiO system are reported and discussed. Emphasis was placed on the determination of Ni concentration (solubility) in the ZnO grains using quantitative TEM-EDXS. The influence on the results of beam diameter, foil thickness and corrections used are described and discussed. During the study small precipitates, presumably NiO, were found in the ZnO grains of the ZnO-NiO samples with different ZnO/NiO ratios. In TEM, the precipitates exhibited image contrast only at certain orientations and were normally invisible during the EDXS analysis. The presence of the precipitates too small to be seen using scanning electron microscopy could explain erroneous results for the Ni concentration in a ZnO solid-solution phase obtained previously using SEM-EDXS. Quantitative EDXS analyses were performed on ZnO grains using different electron beam diameters. In each sample, the spread of the results was correlated to the beam diameter (analysed volume). It was found that when the average number of precipitates was less than one per analysed volume the measured points that included precipitates could easily be identified on the basis of their deviation from the mean value of the Ni content.     

Interface of an Al–(Al2O3)p Composite Modified with Nickel

 A composite material with aluminium matrix and alumina particles coated with nickel was investigated. The nickel coating with thickness up to 1 μm formed by a chemical method consisted of a thin film directly on the alumina surface, porous zone of grains 100 nm size and discontinuous zone of grain agglomerates. As a result of the interaction between the alumina particles coated with nickel and the aluminium matrix, dissolution of nickel in aluminium occurred and reactive bonding took place. In the interface characterized by SEM and TEM methods nickel aluminide and locally nickel-iron aluminide were identified. The presence of complex aluminide was a result of local iron impurity in the aluminium powder.     

Vibrational Spectroscopy and Analytical Electron Microscopy Studies of Fe–V–O and In–V–O Thin Films

Summary.  Orthovanadate (M ³⁺VO₄; M = Fe, In) and vanadate (Fe₂V₄O₁₃) thin films were prepared using sol-gel synthesis and dip coating deposition. Using analytical electron microscopy (AEM), the chemical composition and the degree of crystallization of the phases present in the thin Fe–V–O films were investigated. TEM samples were prepared in both orientations: parallel (plan view) and perpendicular (cross section) to the substrate. In the first stages of crystallization, when the particle sizes were in the nanometer range, the classical identification of phases using electron diffraction was not possible. Instead of measuring d values, experimentally selected area electron diffraction (SAED) patterns were compared to calculated (simulated) patterns in order to determine the phase composition. The problems of evaluating the ratio of amorphous and crystalline phases in thin films are reported. Results of TEM and XRD as well as IR and Raman spectroscopy showed that the films made at lower temperatures (300°C) consisted of nanograins embedded in the dominating amorphous phase. Characteristic vibrational spectra allowed to distinguish between the different crystalline phases, since the IR and Raman bands showed broadening due to the decreasing particle size of the films thermally treated at lower temperatures. Vibrational analysis also showed that the electrochemical cycling of crystalline films led to spectra that were in close agreement with the spectra of the nanocrystalline films prepared at lower temperatures. The formation of a nanocrystalline structure is therefore a prerequisite for obtaining a higher charging/discharging stability of Fe–V–O and In–V–O films. Received October 4, 2001. Accepted (revised) November 23, 2001     

A Novel Catalytic Procedure for the Determination of Ultratrace Zirconium

 A novel catalytic procedure for zirconium was proposed based on Zr(IV) catalyzed oxidation of gallocyanine by hydrogen peroxide in hexamethylene tetramine-hydrochloric acid buffer medium. The calibration graph is linear for 0–110 ngċml⁻¹, and the detection limit is 0.4 ngċml⁻¹ Zr(IV). Most foreign ions do not interfere with the determination, except for Cu²⁺, Fe³⁺ and Cr(VI). The interferences of Cu²⁺ and Fe³⁺ could be eliminated by masking with EDTA and mannitol, and that of Cr(VI) by reducing to Cr(III) with ascorbic acid. The typical features of this procedure are that it is sensitive for zirconium, and the determination could be carried out at room temperature. It had been used to the determination of zirconium in zirconium bronze, simulated samples and a certified reference material. The recoveries were 98.6 ∼ 102%, and relative standard deviations (R.S.D.) were 0.9 ∼ 1.5%, respectively. Received September 12, 1999. Revision April 10, 2000.     

Application of SIMS in Re-Technology Studies: Characterization of Trace-Element Distributions and Quantitative of Carbon-Determination

 The content and the three dimensional distribution of impurities play an important role in the production process of high purity rhenium powder (99.99% purity grade) and for its further use as alloying and coating agent in high temperature applications. In this paper the characterization of raw Re granulate, Re powder, cleaned by heat treatment, Re coatings, produced by most common preparation methods (PVD and VPS) and PM Re by means of SIMS is presented. The analysis of the three dimensional distribution of trace elements is performed by 3D SIMS. The quantification of carbon, which was not possible with other analytical techniques as a result of the high volatility of Re₂O₇ until now, has been carried out by SIMS depth profile analysis. It is discussed if internal standards, produced by introduction of defined amounts of carbon soot to the Re powder lead to useful results.     

Quantitative Trace Analysis by Wavelength-Dispersive EPMA

 “Trace” elements may be defined as elements whose concentrations are of a similar order to the detection limit. In WD analysis the detection limit is a function of the ‘figure of merit’ P²/B, where P is the pure-element peak intensity and B the background intensity. With normal analysis conditions detection limits of ∼100 ppm are typical, but substantial improvements can be achieved by using higher values of accelerating voltage and beam current. Long counting times are also advantageous, but should preferably be divided into relatively short alternating peak and background measurements to minimise the effect of instrumental drift. Using separate routines for trace and major element analysis is desirable owing to their different requirements. As the statistically defined detection limit is reduced, errors due to background nonlinearity and interferences (overlaps) from other elemental peaks become more probable. Spectrum simulation is useful for optimising background offsets and choice of crystal to minimise interferences, and estimating interference corrections when these are necessary. ‘Blank’ standards containing none of the trace elements of interest are also useful for quantifying background nonlinearity.