Introduction

Electron Energy Loss Spectroscopy (EELS) is an analytical method that measures the energy distribution of electrons that have lost energy after interacting with the sample. Low-energy electrons are usually used for reflections on a solid surface of a large sample.  Fast electrons with a energy of 10s of keV or more are used for thin films^[1]. The low-energy electrons interact only with the first few outermost atomic layers of the crystal and due to this only carry information from near the surface ^[2]. The electrons interact with the sample  atoms through electrostatic Coulomb forces causing some of the electrons to scatter. This scattering can be divided into elastic and inelastic scattering^[3]. Elastic scattering happens between an incident electron and an atomic nucleus. Due to the large difference in mass between the nucleous and electron, the energy exchange is usually so small that it can’t usually be measuered using EELS. The energy exchange in inelastic scattering is a lot greater, because ineleastic scattering happens between an incident electron and an atomic electron which have a similar mass^[4].

Experimental Performance

Figure 1 shows a basic EELS setup. In some systems, the analyzer and monochromator are stationary, but the orientation of the sample can be changed by rotation. This allows for the sampling of scattered electrons in an angle ^[2]. The sample needs to be a solid^[1]. In EELS, electrons are used for excitation and to carry information. This means the analyzer must be capable of high-energy resolution, as well as the incident beam needs to be monochromatic within a range of a few millielectron volts. To get such a monochromatic incident beam a monochromator is required^[2].

Figure 1. Diagram of a typical electron energy loss experiment setup. Electrons from an electron source, like for example a cathode, travel through the monochromator to the sample. The scattered electrons move through the analyzer to the detector. Figure: Iida Pankka.

EELS can also be performed using a transmission electron microscope (TEM), which allows for a high spatial resolution. Electromagnetic lenses of TEM allow for the probe to be focused on a very small  area (diameter of 1 nm or even 0.1 nm)^[4].  If the sample is thin enough and the kinetic energy the  is sufficient, almost all of the electrons are transmitted instead of being reflected or absorbed. If the transmitted beam is passed into the spectrometer, the internal structure of the sample can be discovered. With an incident energy of 100 keV the sample needs to less than 1 μm thick^[3]. A typical EELS setup using a TEM is shown in Figure 2.

Figure 2. A typical TEM-based EELS setup. Figure: Iida Pankka.

Examples of EELS spectra

Inelastic scattering from outer-shell electrons is visible as plasmon peak(s) in the EELS spectrum range of 2 – 50 eV. The ionization edges induced by inner-shell excitation represents ionization threshold and reflects the inner-shell binding energy. Note that typical energy loss in EELS profiles is less than 1 kV.^[5]Because the incident, high-energy electrons are transmitted through the TEM specimen, EELS is not surface sensitive, making it a technique for bulk density of states measurements. EELS measures the distribution of energies lost by incident electrons (typically 100–1000 keV) as they pass through a thin solid specimen (typically 0.5–50 nm).^[5]

Figure 3 shows a conventional EELS spectrum. The first peak appearing at zero eV has the highest intense so it means the energy loss is zero. In this case, all of the incoming electrons pass through without any Coulomb force interaction between the incoming electrons and the atomic electrons in the solid sample.^[6]

Figure 3. An EELS spectrum of a 20 nm thin titanium carbide specimen recorded in a conventional 200KV TEM equipped with an energy-filtering spectrometer. License: CC-BY 3.0.^[6]

Advantages and Disadvantages 

In a typical electron energy loss study, where electrons are low-energy, the electrons are able to penetrate only three or four of the outermost atomic layers. This means that the backscattered electrons contain information from only near the surface of the sample. The resolution in electron energy loss spectroscopy is also less than usually  in optical spectroscopies, but it is enough to study the vibrational modes of adsorbed molecules and the substrate^[2].  One of the advantages of EELS is its ability to be combined with TEM ^[7]. With the combination on EELS and TEM the internal structure of a sample can be analyzed if the sample is thin enough and the incident energy is correct. For example if the incident energy is 100 keV, the thickness of the sample has to be less than 1 μm ^[3].  Interpretation of an EELS spectra is not simple. One way to avoid the difficulties is using the fingerprinting procedure, which requires a reference spectra catalogue ^[7].

References