High-resolution electron microscopy of high-temperature superconductors

We review recent results obtained at Northwestern using high-resolution electron microscopy to study high-temperature superconductors. While in general these materials form large, very perfect single crystal grains which display very few imperfections, there is also evidence of slip defects, amorphous regions, and order-disorder transformations. We also report that the gadolinium-based superconductors and in one case yttrium-based superconductors show evidence for some copper solid solubility in the form of copperrich planar defects. The structure of a metastable trigonal polytype is also reported, as are the effects of electron beam and water vapor damage to the materials.     

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.     

Quantitative electron probe microanalysis of nitrogen

Quantitative electron probe microanalysis of nitrogen has been performed on 18 nitride specimens in the voltage range of 4–30 kV. A conventional lead stearate and a new W/Si multilayer analyzer crystal (2d = 5.98 nm) were used simultaneously. The net peak count rates on the latter crystal were approximately 2.8 times higher than on the lead stearate crystal. In addition, significant improvements in the peak to background ratios were achieved, which proved critical when dealing with difficult nitrides such as ZrN, Nb₂N, and Mo₂N. A most conspicuous feature of the new crystal, however, is its effective suppression of higher-order reflections in the wavelength range of N-Kα. As a result, backgrounds are free of interfering lines and spectral artifacts, which facilitates accurate background determination. Two procedures for the analysis of nitrogen in the presence of titanium are also discussed. One procedure is based on the assumption that the area–peak factor (APF, integral k-ratio/peak k-ratio) of the N-Kα peak in Ti–N compounds relative to a nitrogen standard (Cr₂N) has a fixed value which can be used in order to separate the Ti-_ll peak from the N-Kα peak. Extensive tests on a number of compositions in the Ti–N system have shown that it is possible to do quantitative analysis of nitrogen in Ti–N compounds with a relative accuracy of better than 5%. For very low levels of nitrogen (below 15 at%) the APF method was found unsuitable. In such cases, however, the well-known multiple least-squares digital-fitting techniques appeared to do an excellent job. In order to account for the effects of peak shape alterations in the N-Kα peak, all nitrogen spectra were recorded in integral fashion and compared with those of the Cr₂N standard. The shape alterations for the N-Kα peak were much smaller than for B-Kα and C-Kα radiations: The observed values for the area–peak factor relative to Cr₂N differed less than 5% from unity. The present work has resulted in a database containing 144 integral k-ratios for N-Kα and 149 peak k-ratios for the x-ray lines of the metal partners in the nitrides. Our own Gaussian ?(pz) matrix correction procedure “PROZA,” used in conjunction with a set of improved mass absorption coefficients, yielded excellent results for the nitrogen data: An average value of k_calc/K_meas of 1.0051 and a relative root mean square deviation of 3.99%.     

The application of optical microscopy and electron-probe microanalysis to a mineral processing topic—the high-temperature reduction of ilmenite

A combination of optical microscopy and electron-probe microanalysis (EPMA) has been used to study the phases which occur during reduction of the mineral ilmenite to metallic iron and titanium oxides. Quantitative EPMA of natural and chemically-processed minerals frequently suffers from variable specimen conductivity caused by the porosity and varying texture of the mineral grains. It has been found, however, that the element concentration ratios are largely independent of porosity and texture and can be used to identify the phases with precision. The microprobe has revealed some hitherto unsuspected effects arising from small concentrations of magnesium and manganese dissolved in the ilmenite lattice. The morphology and composition of the reduced product are in fact controlled by the way in which these impurity solutes segregate within the mineral grains during reduction.     

Surface microanalysis by reflection electron energy-loss spectroscopy

Several basic physical concepts of applying eq. I_k = IσN^xt to surface microanalysis by reflection electron energy-loss spectroscopy (REELS) are clarified. Here I^k and I are the integrated intensities of the core ionization edge and the low loss part, σ is the scattering cross section of element x with atomic concentration N^x, and t is the specimen thickness. The reflected inelastic electrons are found to be distributed almost symmetrically around the Bragg sports and can be reasonably described by a Lorentzian function. EELS microanalysis can be performed by using the diffracted sports. The ω correction, arising from the angular contributions of the neighbouring spots into the spectrometer collecting aperture, is required to be considered.     

Biological applications of electron probe microanalysis in france

The contribution of French science to the field of biological X-ray microanalysis is reviewed. The main analytical microscopy centers are listed, and their methods and main results are summarized.     

Auger microanalysis in a conventional scanning electron microscope

We have coupled an Auger spectrometer to a conventional scanning electron microscope. Specimens were examined without an ultrahigh vacuum by maintaining a high partial pressure of argon in the vicinity of the specimen. It was possible to examine the specimens by means of both Auger and x-ray microanalysis. Sputtering profiles could be recorded by removing surface layers by ion bombardment. Examples of the application of the method are drawn from studies of surface layers on metals, and are compared with XRMA results.     

Calculation of concentration depth profiles in electron probe microanalysis

Based on a multiple reflection model, the concentration depth profiles of gold diffused into a silver substrate are determined. To conclude the structure of the sample from the measured intensity of radiation, an accurate correction procedure is necessary. A new model was developed for this purpose. Since the new electron probe microanalysis (EPMA) correction procedure is also valid for tilted samples, measurements with different geometries could be performed. The different angles of incidence and take-off angles allow for a better comparison of theory with experiment. One advantage of the applied multiple reflection model is that it can be extended comparatively easily with the analysis of layered specimens. Measurements of some specimens with one layer on a substrate are reported. The mean deviation of calculated thicknesses is about 10%. To calculate depth profiles, gold was evaporated with physical vapor deposition on silver foils. The thicknesses of evaporated gold films were 50 and 100 nm. Then the specimens were annealed at a temperature of approximately 400°C. Finally, the concentration depth profiles were quantified with EPMA. The shape of the profiles was derived from a simple diffusion model valid for the samples under consideration. To check the shape of the functions obtained, a comparison with measurements by secondary ion mass spectrometry was performed.     

Characterization of atmospheric particles by electron probe X-ray microanalysis

Microchemical glass standards were used to validate a quantitation method based on peak-to-background (P/B) ratios from electron probe x-ray microanalysis spectra. This standardless method was applied to the determination of concentrations of individual particles from Malpha or Lalpha lines, as well as from Kalpha lines. The algorithm was tested on particulate glass samples for diameters ranging from 1 to 20 microm. The determined concentrations did not depend on particle size. The certified values for elements were well matched, except for Na, which may migrate under electron bombardment. Finally, classification of qualitative results obtained for aerosol particles was completed by the P/B quantitative method.     

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.     

Secondary electron imaging as an aid to STEM microanalysis

Secondary electron (SEM mode) imaging in a scanning transmission electron microscope (STEM) has been utilized as an aid to the STEM microanalysis of second phase particles in an Alloy 800 foil by identifying particles at the foil surface facing the incident beam and energy dispersive X-ray detector. Such particles were optimally situated for microanalysis because possible deleterious effects from beam spreading and matrix absorption were eliminated. This was demonstrated by orienting the sample to analyze the particles on the surface of the foil facing away from the beam and X-ray detector, and subsequently, for comparison, reorienting the sample to reanalyze the particles with the foil surface facing beam and detector. Theoretical calculations of the magnitude of the relative change in particle X-ray signal between the two orientations were in good agreement with the experimental observations. SEM imaging also provided surface reference points for parallax-effect measurements of local foil thickness and particle depth within the foil.     

Personal-computer-based system for electron beam X-ray microanalysis of biological samples

A system based on a personal computer has been developed which provides a relatively inexpensive way to equip an electron microscopy laboratory for quantitative elemental analyses of cryosectioned biological samples. This system demonstrates the feasibility of making an X-ray analyser from a personal computer, together with commercially available hardware and software components. Hardware and software have been assembled to drive the beam in a scanning electron microscope, collect and analyse X-ray spectra, and save, retrieve, and analyse data. Our software provides a menu-controlled user interface to direct spectra acquisition and analysis. Spot analyses, video images, and quantitative elemental images may be obtained and results transferred in ASCII format to other computers. Wet weight, as well as dry weight, concentrations are calculated, if measurements were made of areas of the hydrated sample before it was freeze-dried. Grey-level copies of video and quantitative elemental images may be made on a laser printer.     

用电子探针定量测定玻璃粉末样品

研究了电子探针测定玻璃粉末样品时,粒度、压片压力与X射线计数率之间的关系。实验结果表明,当样品粒度大于180目,压片压力大于700MPa时,X射线计数率稳定,结果较理想。对电了探针和化学法的测定结果作了比较,两者基本一致。     

Backscattered electron imaging of cultured cells: application to electron probe X-ray microanalysis using a scanning electron microscope

We report a simple method to study the elemental content in cultured human adherent cells by electron probe X-ray microanalysis with scanning electron microscopy. Cells were adapted to grow on polycarbonate tissue culture cell inserts, washed with distilled water, plunge-frozen with liquid nitrogen and freeze-dried. Unstained, freeze-dried cultured cells were visualized in the secondary and backscattered electron imaging modes of scanning electron microscopy. With backscattered electron imaging it was possible to identify unequivocally major subcellular compartments, i.e. the nucleus, nucleoli and cytoplasm. X-ray microanalysis was used simultaneously to determine the elemental content in cultured cells at the cellular level. In addition, we propose some improvements to optimize backscattered electron and X-ray signal collection. Our findings demonstrate that backscattered electron imaging offers a powerful method to examine whole, freeze-dried cultured cells for scanning electron probe X-ray microanalysis.     

Experimental conditions for surface microanalysis with reflection electron energy-loss spectroscopy

Experimental conditions for obtaining the optimum signal-to-background (S/B) ratio in reflection electron energy-loss spectroscopy (REELS) are investigated. It is shown that the S/ B ratio can be improved by lowering the incident energy of the electrons. The spectra taken from the GaAs (660) specular reflection spot under the surface resonance condition is demonstrated to have the best S/B ratio and lowest surface plasma excitation, which is capable of providing structural information on the top few atomic layers.     

Frontiers in electron probe microanalysis: application to cell physiology

The application of electron probe microanalysis techniques, using X-ray and electron energy loss instruments, to problems in cell physiology is reviewed. The details of the special methodological requirements for the analysis of cryosections at high spatial resolution in an analytical electron microscope are discussed together with a comprehensive review of data obtained on major organ systems and cell types.