To get data from a specimen at the nanoscale, scanning electron microscopes (SEMs) use electron beams. The primary signals identified are the backscattered (BSE) and secondary electrons (SE). These signals generate a grayscale image of the sample at extreme magnifications. There are numerous other signals however, which can result from electron-matter interaction, and these can supply further information about a sample.
Phenom World illustrate how energy-dispersive X-ray (EDX or EDS) analysis works on a SEM in this article.
A variety of signals that carry different details about the sample are produced by the electron beam-matter interaction (Figure 1).
- Cathodoluminescence can give information on the electronic structure and the chemical composition of materials.
- Backscattered electrons create images with contrast that carries information on the differences in atomic number.
- Secondary electrons give topographic information.
- Transmitted electrons can characterize the sample’s inner structure and crystallography. Another type of signal that is often used in SEMs is X-Rays.
Figure 1. Illustration of the electron-matter interaction showing its different products.
EDX Analysis in SEM: The Principle Explained
Every atom possesses a unique quantity of electrons that reside under normal conditions in particular positions, as shown in Figure 2. These positions belong to certain shells, which have different, separate energies.
The creation of the X-rays in a SEM is a two-step process:
- Firstly, the electron beam hits the sample and transports some of its energy to the atoms of the sample. Energy which can then be utilized by the electrons of the atoms to “jump” to an energy shell with more energy or be knocked-off from the atom. The electron leaves behind a hole if such a transition occurs.
- Secondly, the holes possess a positive charge and so attract the negatively-charged electrons from higher-energy shells. When an electron from such a higher-energy shell occupies the hole of the lower-energy shell, the energy difference of this transition can be released in the form of an X-Ray.
Depending on the atomic number, which is a unique property of every element, the X-ray has energy attributed to the energy difference between these two shells. Using this method, X-rays are a “fingerprint” of each element and can be employed to establish the type of elements that exist in a sample.
Figure 2. X-ray generation process: 1) The energy transferred to the atomic electron knocks it off leaving behind a hole, 2) Its position is filled by another electron from a higher energy shell and the characteristic X-ray is released.
EDX Material Analysis: How X-Ray Detection Works
Unlike SE, BSE, and TE, X-rays are electromagnetic radiation, just like light, and are made up of photons. The latest systems utilize the so-called silicon-drift detectors (SDDs) to identify them.
SDDs are superior to the standard Si(Li) detectors due to higher resolution, better count rates, and more rapid analytical ability. These detectors are positioned under an angle, extremely close to the sample, and can to calculate the energy of the incoming photons belonging to the X-rays.
The bigger the solid angle between the sample and the detector, the higher the X-rays’ detection prospect, and in turn, the probability of receiving the best results.
Figure 3. Typical EDX spectrum: y-axis depicts the number of counts and x-axis the energy of the X-rays. The position of the peaks leads to the identification of the elements and the peak height helps in the quantification of each element’s concentration in the sample.
EDX analysis generates data made up of spectra with peaks corresponding to the various components that are present in the sample. An example of this is in Figure 3. Every element has extensively documented characteristic peaks of unique energy.
EDX can be utilized for qualitative (the type of elements) and also quantitative (the percentage of the concentration of individual elements of the sample) analysis. Most SEMs have software dedicated to auto-identification of the peaks and calculation of the atomic percentage of each element identified. The EDX technique is a non-destructive characterization technique, it needs minimal or no sample preparation which is another reason that it is advantageous.
Choosing A SEM That Best Suits Your Research Processes
EDX analysis has now become standard practice. Its practicality has ensured that it is now a crucial part of a SEM. An easy experiment enables you to know what your sample contains. SEM can help you optimize your research processes, execute better analyses and save time. Selecting the correct microscope for your research can help you to work more efficiently.
This information has been sourced, reviewed and adapted from materials provided by Phenom-World BV.
For more information on this source, please visit Phenom-World BV.