Photoelectron emission: images and spectra

We give an account of the development of electron emission imaging and the photoelectron emission process. We then describe a particular combination of photoelectron emission microscopy and photoelectron spectroscopy which we call photoelectron spectromicroscopy. This is achieved using a very strong, divergent magnetic field to define electron trajectories without the use of electron optical focusing. It is thus possible to produce electron images of the surfaces of three-dimensional objects using low energy photoelectrons in which the image contrast is dependent upon, among other things, the yield and energy distribution functions. The various image contrast mechanisms which apply are outlined with illustrative examples. Techniques for energy analysis of the photoelectron image are described and examples of the photoelectron spectra of different regions of the surface of an object presented. Electron images produced by metastable atoms and slow ions are also discussed. The use of the magnetic field introduces the opportunity for a number of novel techniques which will be described.     

Characterizing voltage contrast in photoelectron emission microscopy

A non‐destructive technique for obtaining voltage contrast information with photoelectron emission microscopy is described. Samples consisting of electrically isolated metal lines were used to quantify voltage contrast in photoelectron emission microscopy. The voltage contrast behaviour is characterized by comparing measured voltage contrast with calculated voltage contrast from two electrostatic models. Measured voltage contrast was found to agree closely with the calculated voltage contrast, demonstrating that voltage contrast in photoelectron emission microscopy can be used to probe local voltage information in microelectronic devices in a non‐intrusive fashion.     

Evaluation of quantitative procedures for X-ray microanalysis of environmental particles

Scanning electron microscopy combined with energy-dispersive X-ray spectrometry is particularly suited to characterizing morphology and elemental composition of individual microparticles. Although not straightforward, quantitative X-ray microanalysis of low-Z-containing particles is achievable using atmospheric thin-window X-ray detectors. A critical aspect of light element analysis is the choice of substrate material. In this work, particles were deposited on specially developed boron substrates. Three case studies were investigated successively in the order of increasing difficulty. Firstly, hundreds of calcium carbonate (CaCO(3)) particles ranging in size from 0.3 to 10 microm were analyzed. Three quantitative procedures were tested: the "k-ratio" method, conventional ZAF correction, and Monte Carlo simulations. Average relative errors obtained by the reverse Monte Carlo quantitative program named CASINO were better than 2.5 wt %, carbon included. Secondly, further evaluation was carried out on a finely crushed biotite mineral, containing more than nine elements. Finally, airborne particulate matter, consisting of a complex heterogeneous mixture of particles, was investigated. By applying the Monte Carlo quantitative procedure, the observed particles were easily classified into particle types. Pure compounds (e.g., CaSO(4).2H(2)O, SiO(2), CaCO(3), etc) were directly assigned according to stoichiometry. In some cases (marine-derived particles), a partial reactivity of atmospheric particles was demonstrated by quantitative analysis.     

Some theoretical considerations on X-ray microanalysis in the environmental or variable pressure scanning electron microscope

This paper presents a new correction procedure for quantitative x-ray microanalysis in the environmental or variable pressure scanning electron microscope (SEM). This method is based on a plot of the measured intensity as a function of the fraction of nonscattered beam intensity, f_p. The theory predicts that the plot should be linear and the corrected intensity is given for f_p = 1. The advantage is that such a plot is valid for any measured pressures, which is not the case with the usual pressure correction methods. To use this method, a simple equation is derived to compute f_p. Other variations of this correction procedure are also presented. Comparison with measurements performed by Mansfield (1999) show the great consistency of this method.     

A simple method for correlative light,scanning electron microscopic and X-ray microanalytical examination of the same section

Thin paraffin sections, mounted on scanning specimen holders previously coated with polyester film tape (Minnesota Mining and MFG Co., Scotch film tape No. 850 gold), were processed for light microscopy (LM) in the conventional way, then covered with celloxin shellac and examined in the LM by using the upper illuminating source. After removal of the shellac from the surface of the sample by immersion in acetone, the sections were air-dried, coated with a copper layer in a vacuum evaporator and examined in a scanning electron microscope (SEM). The method allows: (i) high-quality LM possibilities for establishment of the diagnosis in pathological cases; (ii) SEM examination of the same area as observed in LM; and (iii) EPMA measurements of insoluble precipitates embedded in the tissue. The usefulness of the proposed method is obvious in cases where the composition of a precipitate on LM scale is to be compared with the LM appearance of the surrounding tissue.     

Beam induced mass loss in high resolution biological microanalysis

Electron beam induced loss of mass from the organic matrix and from higher Z constituents of biological samples was measured by monitoring bremsstrahlung and peak changes in EDS spectra. When any effects of contamination, extraneous X-rays, beam current drift, specimen drift, and specimen shrinkage were monitored and corrected for, the three types of samples gave consistent and similar results at 296 K. Bremsstrahlung losses averaged 45%, 46% and 50% respectively for muscle homogenate, salivary gland sections and albumin. Sulphur losses average 74%, 72% and 86% for the same three sample types. No other elements suffered significant losses. D_l/e for bremsstrahlung averaged 0·14 C/cm². Bremsstrahlung loss at 93 K began approximately one order of magnitude higher in dose, and the extent of loss varied. Sulphur losses, however, were greatly reduced at low temperatures.     

Theoretical prediction of the electron backscattering coefficient for multilayer structures

Based on existing formulae for the backscattering coefficient for coated samples, i.e. for samples consisting of one film on a massive homogeneous substrate, a theory for the backscattering coefficient for multilayer structures is derived in a clear mathematical way without using approximations. The resulting expression is simple and ready to be applied in scanning electron microscopy or other related analytical techniques.     

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.     

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.     

A microcomputer based digital imaging system for ion microanalysis

A microcomputer based digital imaging system was developed for a Cameca IMS-3f ion microscope permitting real-time digital acquisition of secondary ion images. Image signal-to-noise enhancement results from random noise reduction by real-time ensemble averaging and from a reduction of pattern noise in the charge injection device (CID) array by subtraction of blank frames. Acquired images comprise 244times248 pixel arrays with 8-bit intensity resolution. Images are displayed on a colour monitor in a grey scale or pseudo-colour using one of four programmable lookup tables. Image processing software permits off-line ion image enhancement and manipulation as well as multitechnique digital image correlations. System capability is illustrated by a biological example involving digital imaging studies of Al distribution in osteomalacic bone tissue, including correlative light microscopy and ion microanalysis.     

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.     

A comparison of two models for the characteristic X-ray fluorescence correction in thin foil analysis

It is shown that Philibert & Tixier's calculation for the contribution of characteristic fluorescence in thin foil X-ray microanalysis is in error. The calculation of Nockolds et al. for this contribution is shown to be correct.     

Image-EELS: Simultaneous recording of multiple electron energy-loss spectra from series of electron spectroscopic images

A new approach for element microanalysis with energy-filtering transmission electron microscopy (EFTEM) is presented which was accomplished with the CEM 902 electron microscope (Zeiss, Germany). This method is called Image-EELS, because it is a synthesis of electron energy-loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). Series of energy-filtered images at increasing energy losses are recorded from one area with a TV camera. In a second step the intensity of selected regions in the image stack is measured with an image analysis system and plotted as a function of the energy loss. Thus many spectra from different objects can be calculated from one image series and compared with each other. The spatial resolution of EELS is considerably enhanced, the noise is decreased because many pixels from irregular objects are integrated, and the information from ESI can be analysed as a function of the energy loss.     

Real-time quantitative elemental analysis and mapping: microchemical imaging in cell physiology

Recent advances in widely available microcomputers have made the acquisition and processing of digital quantitative X-ray maps of one to several cells readily feasible. Here we describe a system which uses a graphics-based microcomputer to acquire spectrally filtered X-ray elemental image maps that are fitted to standards, to display the image in real time, and to correct the post-acquisition image map with regard to specimen drift. Both high-resolution quantitative energy-dispersive X-ray images of freeze-dried cyrosections and low-dose quantitative bright-field images of frozen-hydrated sections can be acquired to obtain element and water content from the same intracellular regions. The software programs developed, together with the associated hardware, also allow static probe acquisition of data from selected cell regions with spectral processing and quantification performed on-line in real time. In addition, the unified design of the software program provides for off-line processing and analysing by several investigators at microcomputers remote from the microscope. The overall experimental strategy employs computer-aided imaging, combined with static probes, as an essential interactive tool of investigation for biological analysis. This type of microchemical microscopy facilitates studies in cell physiology and pathophysiology which focus on mechanisms of ionic (elemental) compartmentation, i.e. structure-function correlation at cellular and subcellular levels; it allows investigation of intracellular concentration gradients, of the heterogeneity of cell responses to stimuli, of certain fast physiological events in vivo at ultrastructural resolution, and of events occurring with low incidence or involving cell-to-cell interactions.     

Evaluation of several common standards for the X-ray microanalysis of thin biological specimens

The question of the best type of standard to use for X-ray microanalysis of thin biological specimens remains unanswered. Standards embedded in an organic matrix have the advantage that they resemble biological specimens, but their composition is generally not known exactly. We compared several standards and, surprisingly, inorganic binary salts sprayed onto a supporting film were the most suitable: they corresponded closely with several other methods using organic matrices; they were easily produced; and their composition is known. Glutaraldehydeurea aminoplastic resin thin sections and thin films containing dissolved salts were problematic. The composition of the polymer appears to be variable, and the thin films did not correspond with any other standard tested. Chelex¹⁰⁰ bio-standard beads and flakes loaded with accurately determined concentrations of ions, embedded in epoxy resin and thin sectioned, tended to correspond to the results obtained with the binary salts. However, the results from some bio-standards were inexplicably aberrant. An epoxy resin standard was used for bromine, and was found to agree closely with the binary standards.     

Tribological properties of 4,5-di(cetyl thio)-1,3-dithiole-2-thione as additive in liquid paraffin

The synthesis of a new type of oil additive, 4,5-di(cetyl thio)-1,3-dithiole-2-thione, is described. The thermal stability of the compound was investigated by thermal gravity analysis (TGA). The tribological properties of the compound as additive in liquid paraffin were also evaluated using a four ball tester. The results show that the novel compound has good tribological properties in liquid paraffin. The lubricating mechanism was studied by electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS).     

Fractographic analysis of fatigue damage in 7000aluminium alloys

In this paper, an attempt is made to correlate the fatigue damage in 7000 aluminium alloys with different impurity contents to the microstructural features and to explain their interdependence through fractographic observations. The Paris constants of these alloys in the form of hot‐forged plates subjected to the overaged T73 temper are evaluated and differences in the fatigue crack growth rate described by striation spacing measurements. Scanning electron microscopy analysis of fatigue fracture surfaces revealed that the type and morphological parameters of coarse intermetallic particles play a critical role in fatigue crack growth behaviour. The elemental distribution determined by means of energy‐dispersive spectroscopy analysis showed that the fractured particles accelerating the crack advances are larger particles of Fe‐rich phases. The fatigue crack growth rate increases considerably with increasing amounts of these particles. The smaller η, S and Mg₂Si particles contribute beneficially to fatigue life.