Introduction

Energy-dispersive X-ray spectroscopy (EDS, EDX, or EDXS) is a technology used to characterize chemical elements. It can detect the chemical element as light as Z=8 (oxygen).^[1] This analytical method is also called as energy dispersive analysis of X-ray (EDAX) or energy dispersive X-ray analysis (EDXA).^[1] EDS can be carried out with scanning electron microscope (SEM) or transmission electron microscope (TEM) analysis.

EDS uses characteristic X-rays emitted from an atom of the sample which has been excited by an incident electron beam as shown in Fig. 1.^[1] The emitted characteristic X-rays have specific wavelengths. The electron probe can be focused on a microscopic area of the sample. It should be noted that the probe area is not identical with the minimum specimen area. This is because the characteristic X-rays are emitted from the pear-shaped volume not from the sample surface.^[1] Figure 1 shows that the lateral size of the volume is larger than that of the probe.

Figure 1. Type of signals emitted by incident beam-specimen interactions (Figure: Jihong Yim)

Theory behind EDS

When a sample is irradiated by incident electron beam, an electron in the inner shell of an atom can be knocked off from the atom creating a vacancy. Then, the atom is in an excited state. The atom quickly returns to the normal state after an electron in an outer shell falls into the hole and refilling it. During this transition, the excessive amount of energy can be emitted as a form of a characteristic X-ray (Figure 2a).^[1][2] Each element have characteristic X-rays with specific wavelengths. The energy of the characteristic X-ray is equal to the difference between the energies of two involved shells. K-series X-rays are generated when the electron in an outer shell fills in a hole in the K shell. In the same context, L and M series X-rays can be generated depending on the involved shell energy level (Figure 2b).^[3][4] The Greek letters, such as α and β, indicates the relative intensities of X-rays. The general intensity is higher in an order of α>β>γ.^[1] For example, the nomenclature of Kα-lines indicates that L and K shells are involved in the characteristic X-ray emission.

Figure 2. a) Schematic diagram of the characteristic X-ray generation and b) nomenclature of characteristic X-rays. Figure: Eero Kuusisto, adapted from^[2][4]

How does EDS work?

EDS system consists of an electron beam, Si drift detector, pulse processor, and analyzer as shown in Fig. 3.^[5] The measured characteristic X-ray spectrum is generally plotted as the counts (the number of the processed X-rays) against wavelength or energy level as shown in Fig. 5.^[3] The emitted X-rays are detected and process as electronic signals by the detector which is a semiconductor.^[1] A charge pulse, which is proportional to the X-ray energy, is generated and converted to a voltage pulse.^[1] The energy of X-ray photon is determined by measuring the current produced.^[3] A small current is produced when the emitted X-ray photon hits the semiconductor detector; a field gradient is generated by drift rings within the detector, and it directs the electrons toward the anode (Figure 4).^[6]

Figure 3. Schematic diagram of EDS system (Figure: Eero Kuusisto, adapted from^[5])

Figure 4. Schematic picture of silicon drift detector (Figure: Eero Kuusisto, adapted from^[6])

Application

EDS can be used to identify the chemical elements existing in a sample, it can also be used as a semi-quantitative or quantitative analysis tool. In case of the qualitative analysis, elemental mapping, point analysis, line analysis, and area analysis can be obtained^[7].

Voigt et al.^[8] prepared catalytic powder comprising mesoporous zirconia substrate coated with Ni using atomic layer deposition. They used EDS line scan to verify the presence of Ni. 100 measurement points with the scan length of 600 µm were used. The measure spot is shown in Figure 5 panel (a) and the results are shown in panel (b) and (c). It is hard to verify Ni as the characteristic X-ray intensity of Ni is much lower compared to that of Zr (panel (b)). For this reason, they adjusted the intensity scale in panel (c). The intensity is proportional to the concentration of each element. Therefore, it is likely that Ni is distributed along the analyzed location within zirconia.^[8]  

Figure 5. SEM-EDS line scan: (a) backscattered electron image showing the analyzed spot. (b) A zirconia particle is analyzed using EDS line scan. (c) EDS line scan result after rescaling the intensity (License: CC BY 4.0).^[8] 

Advantages and disadvantages of EDS

6. References