Transmission Electron Microscopy: Application in environmental nanoscience

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About the author: Rui Filipe Serra Maia is a PhD student in Geosciences at Virginia Tech. Check out his profile on the VTSuN student page.

Today I am going to tell you about Transmission Electron Microscopy (TEM), a fascinating imaging technique. TEM takes images of things so tiny that, in its 60+ year history, if you were to combine the volume of all particles of every image ever taken (millions of them), they would only make up to approximately 10 mm3. That is about the volume of the head of a pin! Are you not impressed? Well, appearances can be deceiving! In 1986 Ernst Ruska received the Nobel Prize in Physics for his outstanding contribution to the development of Transmission Electron Microscopes. Looks like you had better keep reading!

During the 1920’s scientists concluded that the only way to improve the imaging resolution available from optical microscopes was by using electrons instead of light. Thus began the era of transmission electron microscopy. Particles as small as atoms could now potentially be imaged!

The high resolution of TEMs owes to the fact that electrons have a much smaller wavelength than photons from the visible spectrum (the source used in optical microscopy). In simpler terms, a shorter wavelength means higher resolution. Figure 1 shows the effect of different wavelengths on the interaction with particles of approximately the same size (further details can be found in Chapter 1.2 of Williams and Carter, 2009).

Figure 1. Wavelength effect on imaging resolution of particles with the same size. Photons (A) have long wavelength and they can reach the detector as if there were not any particles in their path. Wavelength of electrons (B) is short, so they can interact with very small particles and be detected.
Figure 1. Wavelength effect on imaging resolution of particles with the same size. Photons (A) have long wavelength and they can reach the detector as if there were not any particles in their path. Wavelength of electrons (B) is short, so they can interact with very small particles and be detected.

Due to their small wavelength, electrons are now frequently used to image particles at an atomic scale. TEM imaging of nanoparticles enables a very detailed characterization of the size, shape, and crystal structure of nanoparticles. The maximum sample thickness suitable for electron transmission microscopy is generally ~100 nm, which means they are “electron-transparent”.

Figure 2 shows an image of a coal ash sample, collected from the Dan River in Virginia after an industrial spill occurred on February 2nd, 2014.

Figure 2.TEM and Selected Area Electron Diffraction (SAED) images of the colloidal fraction from a river water sample. Particle 1 and 2 are discussed in the crystal structure section of his post.
Figure 2.TEM and Selected Area Electron Diffraction (SAED) images of the colloidal fraction from a river water sample. Particle 1 and 2 are discussed in the crystal structure section of his post.

Crystal Structure Analysis

TEM imaging allowed Dr. Yi Yang to discern different particles of iron oxide in terms of size and shape in the ash spill. Furthermore, these particles were discovered to be crystalline and polyphasic (meaning that there are particles with the same chemical formula, but different crystal structures, so they may behave differently from each other). The crystal structures of the different phases present in this system were investigated using Selected Area Electron Diffraction (SAED). SAED resembles X-ray diffraction in the way that it functions. Electrons interact with the sample, and the intensity of the outgoing beam will be cancelled at some angles and enhanced at other angles. Figure 3 shows a representation of electron diffraction process (further details in Chapter 12, William and Carter, 2009).

Figure 3 Electron diffraction. A) the angles the beam makes with the planes is such that the two outgoing waves are in sync, and they interact positively creating a bright spot at the detector (see Figure 4). B) the angle of the beam with the planes is such that the two outgoing waves are out of sync, and interact destructively creating a dark spot in the detector (see figure 4). (Adapted from Marc Michel’s lecture notes of Mineralogy).
Figure 3 Electron diffraction. A) the angles the beam makes with the planes is such that the two outgoing waves are in sync, and they interact positively creating a bright spot at the detector (see Figure 4). B) the angle of the beam with the planes is such that the two outgoing waves are out of sync, and interact destructively creating a dark spot in the detector (see figure 4). (Adapted from Marc Michel’s lecture notes of Mineralogy).

Scientists thus obtain diffraction spectra of their samples, which contain information about the distances between rows of atoms within crystals, which are called “d-spacings”. These d-spacings are characteristic for each mineral and therefore SAED can be used to identify crystal structures within samples. Figure 4 shows the diffraction patterns corresponding to the crystal structures present in Figure 3. Particle 1 and 2 are both iron oxides. However, the analysis of their structure by SAED shows that their structure is different. For example the distance between the center spot and its closest spot is different in the two images. Therefore, even with similar chemistry, the two particles may interact differently with the environment due to their different crystalline structures.

Figure 4. Selected area diffraction (SAED) of the particles in Figure 2. A) SAED of particle 1 – Goethite. B) SAED of particle 2 - lepidocrocite.
Figure 4. Selected area diffraction (SAED) of the particles in Figure 2. A) SAED of particle 1 – Goethite. B) SAED of particle 2 – lepidocrocite.

Chemical Analysis

Chemical analysis can be performed in TEM through Energy Dispersive Spectroscopy (EDS). This technique makes use of the interaction between the TEM’s electron beam and the sample. Some electrons from the incident beam can transmit part of their energy to an electron in the inner shells of the atoms in the sample, which causes it to be ejected from the sample and creates a vacancy. This vacancy is then filled by an electron from an outer layer from the same atom. When electrons play this game of musical chairs, they release energy in the form of X-rays that can be detected using an x-ray detector. Figure 5 sketches the interactions of electron beam with the sample and the different types of energy that are emitted.

Figure 5. Energy dispersive spectroscopy (EDS).Step 1) An incoming electron (e-) from the electron source hits an electron from an inner shell of the atom and kicks it out. Step 2) The vacancy is filled by an electron from an outer shell. The difference in energy between the two shells requires release of an X-ray. This X-ray is picked up by the detector.
Figure 5. Energy dispersive spectroscopy (EDS).Step 1) An incoming electron (e-) from the electron source hits an electron from an inner shell of the atom and kicks it out. Step 2) The vacancy is filled by an electron from an outer shell. The difference in energy between the two shells requires release of an X-ray. This X-ray is picked up by the detector.

The energy of the emitted X-rays is characteristic for different chemical elements and allows them to be distinguished within a sample. Figure 6 shows the EDS spectrum of an environmental sample containing nanoparticles of a catalyst commonly used in the catalytic converter of our vehicles.

Figure 6. A) TEM image of a Cerium dioxide nanoparticle from a catalytic converter. B) EDS spectrum of the particle in 6A.
Figure 6. A) TEM image of a Cerium dioxide nanoparticle from a catalytic converter. B) EDS spectrum of the particle in 6A.

Ph.D. candidate James Dale used chemical analysis by EDS to identify this catalyst as Cerium dioxide (CeO2). This technique can be used in a slightly different mode that scans the beam over the sample, creating a map with the location of each element within the sample. Figure 7 shows the EDS map of the particle present in image 6, where the elements Cerium, oxygen, and carbon can be located within the sample.

Figure 7. EDS mapping of the particle presented in 6a. A) merged signals of carbon (blue), oxygen (red), and cerium (green). B) Oxygen signal only. C) Cerium signal only. D) Carbon signal only.
Figure 7. EDS mapping of the particle presented in 6a. A) merged signals of carbon (blue), oxygen (red), and cerium (green). B) Oxygen signal only. C) Cerium signal only. D) Carbon signal only.

The image shows that carbon is mostly present in the support matrix (made of a carbon mesh). On the other hand, cerium and oxygen are blended together in the particle area suggesting that cerium and oxygen are bonded together and uniformly distributed in the particle, forming Cerium dioxide.

Environmental Conditions

Nowadays, a limited number of TEMs around the world are capable of performing in situ observation of reactions at the atomic scale. Those instruments are called Environmental TEM (ETEM). In normal TEM instruments, the sample is placed under high vacuum conditions so the electrons don’t run into other nanoparticles (like those found in air), before they reach the sample. This is very important in terms of imaging resolution. Technological advances allow the use of higher atmospheric pressure (more air present) around the sample in ETEM, while still maintaining atomic resolution. Using this technique, we can observe in situ reactions between samples and gases, which is remarkable for studying nanoparticles reactions.

By now you should have a good idea of the importance of TEM in environmental nanoscience. In fact, without TEMs we would not be capable of understanding the behavior of nanoparticles the way we do. The combined volume of particles ever imaged (10 mm3) may appear to be very small, but don’t be deceived. Your computer, your cell phone, your GPS, your plasma-screen TV, and many other devices would never exist if TEM instruments had not been invented!

I want to acknowledge James Dale, Dr. Yi Yang and Dr. Marc Michel for kindly supplying images included in this post. I also want to thank  Dr. Marina Vance and Sarah Ulrich for their intellectual help turning complex concepts into more comprehensible forms. 

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