Transmission electron microscopy (TEM) – spectroscopy

About this technique

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High energy electrons impacting on matter will cause the emission of X-rays, the energy of which is characteristic of the emitting atom. These can be measured using a technique called energy dispersive X-ray spectroscopy (EDS). Emitted X-ray signals provide compositional information on the specimen and originate from the region illuminated by the beam only. The beam can be broad, or it can be focused onto a region just a few nanometres in diameter. It is therefore possible to obtain compositional information with very high spatial resolution, e.g. from individual nanoparticles. By analysing the EDS spectrum, it is possible to obtain qualitative information (which elements are present) and semi-quantitative information (how much of each element is present) with an accuracy of perhaps a few percent.

The performance of EDS is somewhat specimen dependent, but lower detection limits of about 0.5 wt% are typical. EDS techniques have quite poor sensitivity for very light elements (B, C, N, O). It is possible to determine that a material is an oxide or a carbide for example, but little more. Fortunately, the complimentary technique of electron energy loss spectroscopy (EELS) shows excellent sensitivity for light elements and between the two techniques a complete analysis is usually possible.

EELS measures the energy lost by the electrons when they excite the atoms in the specimen to emit X-rays (and other signals). The EELS spectrum theredfore contains compositional information which is complimentary to that of the EDS signal. EELS can be used for qualitative and semi-quantitative analysis, and has the advantage over EDS that it is very sensitive to light elements down to lithium. The EELS spectrum contains a wealth of other information. For some elements the shape of the spectrum is strongly dependent on the type of bonding in the material. It is therefore possible to determine not only which elements are present, but what type of bonding configuration they are in. Some examples include differentiating between the various forms of carbon or determining the oxidation state of transition series metal oxides.

EDS/EELS spectra can be acquired quickly – requiring just a few seconds to make a qualitative measurement of a single point. Semi-quantitative measurements may take a few minutes. For accurate analysis the specimen must be thin (much less than 100 nm), but this is a general requirement for TEM work.

The EELS signal can also be used to map the chemical distribution of elements over a defined area in a sample. This typically only a takes a few minutes compared with perhaps hours for the EDS mapping. EELS-based methods are difficult and considerable expertise is required to apply them effectively.

EDS/EELS  are very complimentary, as is scanning TEM (STEM). In STEM, the TEM is operated with a finely focused probe rastered across the specimen. The resulting scanning imaging provides information analogous to that from TEM imaging. However, integration of the STEM system into the spectroscopy software permits the beam to be stopped at any point on the image and an analysis taken. A line profile may be obtained by scanning the beam across of region of interest, such as an interface. A chemical map may be acquired at the same time as the image. This enables distribution of elements within the image to be correlated with microstructural features.