Electron energy loss spectroscopy

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In electron energy loss spectroscopy (EELS) a material is exposed to a beam of electrons with a known, narrow range of kinetic energies. Some of the electrons will lose energy by inelastic scattering, which is primarily an interaction of the beam electron with an electron in the sample. This inelastic scattering results in both a loss of energy and a change in momentum.

These interactions may be phonon excitations, inter and intra band transitions, plasmon excitations or inner shell ionisations. The latter are particularly useful for detecting the elemental components of a material, as the energy transferred in such an interaction is related to the ionization potential of the atom, and therefore the spectrum can be compared to that of known samples.

The technique was developed by James Hillier and RF Baker in the mid 1940s but was not widely used over the next 50 years, only becoming more widespread in research in the 1990s due to advances in microscope instrumentation and vacuum technology.

Device used: Electron spectrometer

The cited text by Egerton is an excellent general introduction to the field and is highly recommended for readers interested in more in-depth coverage. However, the reader is cautioned that a great many developments in EELS have taken place since the book was published. With the high availability of modern instrumentation, this formerly somewhat unusual tool is now in common use in laboratories around the world, and the associated technical and scientific developments have consequently been extremely rapid. It is now possible to develop detailed measurements of the atomic and electronic properties of single columns of atoms (and, in a few cases, of single atoms) with EELS. The technique is able to take advantage of modern aberration-corrected probe-forming systems to attain spatial resolutions down to ~0.1 nm, while with a monochromated electron source the energy resolution can be 100 meV or better.

EELS is often spoken of as being complementary to energy-dispersive X-ray spectrometery (variously called EDX, EDS, XEDS, etc.), which is another common spectroscopy technique available on many electron microscopes. EDX excels at identifying the atomic composition of a material, is quite easy to use, and is particularly sensitive to heavier elements. EELS has historically been a more difficult technique but is in principle capable of measuring atomic composition, chemical bonding, valence and conduction band electronic properties, surface properties, and element-specific pair distance distribution functions. EELS tends to work best at relatively low atomic numbers, where the excitation edges tend to be sharp, well-defined, and at experimentally accessible energy losses (the signal being very weak beyond about 3 keV energy loss). EELS is perhaps best developed for the elements ranging from carbon through the 3d transition metals (from scandium to zinc). For carbon, an experienced spectroscopist can tell at a glance the differences among diamond, graphite, amorphous carbon, and "mineral" carbon (such as the carbon appearing in carbonates). The spectra of 3d transition metals can be analyzed to identify the oxidation states of the atoms. Cu(I), for instance, has a different so-called "white-line" intensity ratio than does Cu(II). This ability to "fingerprint" different forms of the same element is a strong advantage of EELS over EDX. The difference is mainly due to the difference in energy resolution between the two techniques (~1 eV or better for EELS, perhaps a few times ten eV for EDX).

There are several basic flavors of EELS, primarily classified by the geometry and by the kinetic energy of the incident electrons (typically measured in kiloelectron-volts, or keV). Probably the most common today is transmission EELS, in which the kinetic energies are typically 100 to 300 keV and the incident electrons pass entirely through the material sample. Usually this occurs in a transmission electron microscope (TEM), although some dedicated systems exist which enable extreme resolution in terms of energy and momentum transfer at the expense of spatial resolution.

Other flavors include reflection EELS (including reflection high-energy electron energy-loss spectroscopy (RHEELS), typically at 10 to 30 keV) and aloof EELS (sometimes called near-field EELS, in which the electron beam does not in fact strike the sample but instead interacts with it via the long-ranged Coulomb interaction; aloof EELS is particularly sensitive to surface properties but is limited to very small energy losses such as those associated with surface plasmons or direct interband transitions).

Within transmission EELS, the technique is further subdivided into valence EELS (which measures plasmons and interband transitions) and inner-shell ionization EELS (which provides much the same information as x-ray absorption spectroscopy, but from much smaller volumes of material). The dividing line between the two, while somewhat ill-defined, is in the vicinity of 50 eV energy loss.