Microanalysis in STEM

The use of the electron microscope over many years to obtain morphological and microstructural information is self-evident in this context, although it is arguable whether with such limited objectives the technique could be said to offer any more significant a contribution than any of the foregoing. Whilst the electron microscope per se yields, in the case of the conventional transmission variant (CTEM), images down to an atomic level, it is differentiated from the techniques noted above because it makes no use of direct chemical information about the catalyst however, in common with them it is an instrument of vacuum technology. 

It has been recognized for several years that various phenomena, consequent on the interaction of the electron beam with the solid sample in the electron microscope, may be exploited in order to obtain chemical information about the sample. The emission of fluorescent Y-rays is the most obvious of these, and energy -dispersive A -ray 

Among various signals coming from the volume excited by the focused probe in STEM, we will introduce two forms of spectroscopy that have proved to be most useful in oxide superconductor research, as well as microdiffraction. 

Equations (3.2) and (3.3) are approximate formulae for spectrometers of modem design without lenses. The actual value for the VG prism spectrometer used in our HB-501 STEM is about 1.8 pmeV. In parallel detection mode, the dispersion achieved is usually further magnified by quadmpole lenses. A single quadmpole produces a line focus. Two such lenses in series can act as a 

One of the parameters characterising the EELS detection system is its Detective Quantum Efficiency, DQE, defined as the ratio of the number of counts to the mean square fluctuation in them. A detection system is said to have unit DQE if it is shot noise limited, i.e. the mean square signal variation in a chaimel is equal to the number of counts within it. However, channel-to-channel gain variations in photodiode arrays, dark current, and detector noise 

A limitation of the magnetic prism-based EELS detection system is the spectrum drift in the energy dispersion direction. Various sources of instability in the microscope contribute to this effect High voltage fluctuation, magnetic field creep etc. This places a fundamental limit on the useful exposure time. Experimental efforts in electron spectroscopy consist largely in reducing unwanted electrical noise and specimen drift. 

What is measured in electron energy loss spectroscopy is the optical absorptive properties of the materials. This statement can be made clearer if we adopt the virtual photon field picture pioneered by C. F. Weizsacker and E. J. Williams 

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Although x-ray microanalysis in the STEM is the most developed form of analytical electron microscopy, many other types of information can be obtained when an electron beam interacts with a thin specimen. Figure 2 shows the various signals generated as electrons traverse a thin specimen. The following information about heterogeneous catalysts can be obtained from these signals ... 

The introduction in catalysis of bimetallic formulations created an important area of application of microanalysis in transmission electron microscopy. In particular, with selective hydrogenation and postcombustion catalysts, where the metallic particle sizes are several nanometres, the STEM can be used to determine the composition particle by particle and thus confirm the success of the preparation. Figure 9.16 shows the analysis of individual particles in a bimetallic preparation. It is easy to detect the existence of genuinely bimetallic particles and others containing only platinum. It should, however, be noted that this analysis, obtained on a few nanometer sized particles, concerns only a very small quantity of the catalyst (in the present case approximately 10" g of metal ). As we have noted, it is dangerous to extrapolate only one result of this type to the solid as a whole. A statistical analysis of the response of a very large number of particles, in addition to a preliminary study of the chemical composition at different scales, can be used to confirm that this case indeed concerns two groups of particles. 

Linked with its qualities, assessed above, as an imaging and structural tool, the STEM assumes prime importance when considered as a microanalytical instrument. As pointed out in the introduction, the interaction of the fine probe in STEM with, potentially, only a small volume of the sample suggests the possibility of microanalysis on a scale hitherto unattainable. Two main areas will be considered here -the emission of characteristic A -rays by the sample, and the loss of energy from the primary beam in traversing the latter. Ideally, a fully equipped analytical electron microscope will utilize both techniques, since, as a result of the relative positions of A"-ray detector and the energy loss spectrometer in the electron optical column, simultaneous measurements are possible. However, for the sake of convenience we will consider the methods separately. 

Comparison of the two x-ray techniques is shown in Table 2.2. Microanalysis in the SEM is best conducted by a combination of these two techniques to take advantage of the strengths of both. Several spectrometers can be mounted on the microscope. Computer hardware and software are available which control both types of spectrometer and combine the data on a single system. Microanalysis in the TEM and STEM is conducted by EDS analysis and EELS. 

Local elemental distributions in a specimen can be investigated in TEM and STEM using the analytical techniques electron energy-loss spectroscopy (EELS) and X-ray microanalysis. X-ray microanalysis in TEM and STEM corresponds to EDX spectroscopy in SEM. However, the thin... 

The STEM is unrivaled in its ability to obtain high-resolution imaging combined with microanalysis from specimens that can be fashioned from almost any solid. Major applications include the analysis of metals, ceramics, electronic devices... 

There are three types of electron microscopes commonly used for microanalysis. These are the scanning electron microscope (SEM) with X-ray detectors, the electron probe microanalyser (EPMA), which is essentially a purpose built analytical microscope of the SEM type, and transmission microscopes (TEM and STEM) fitted with X-ray detectors. In a TEM, compositional information may also be obtained by... 

Composition Profile Measurement. Results of Zieba et al. (1997) will be given as an example of the measurement of solute distribution in an alloy undergoing a phase transformation. They studied discontinuous precipitation in cobalt-tungsten alloys, in which a Co-32 wt% W alloy was aged in the temperature range 875 K to 1025 K, and high spatial resolution X-ray microanalysis of thin foils by STEM was used to measure the solute distribution near the reaction front. 

Williams et al. (2002) have reviewed the current state of AEM X-ray microanalysis, and they suggest ways in which the highest resolution of X-ray mapping may be achieved in the STEM with an EDS spectrometer. Because of their small collection angles and thin specimens, very small numbers of X-ray counts are generated, so the minimum detection limit is typically at best 0.1 wt%. This value is an order of magnitude worse than the 0.01 wt% figure for bulk-specimen in an SEM/EPMA. 

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