Instrumental Methods Of Analysis: Transmission Electron Microscopy
Transmission electron microscopy (TEM) is a method used to analyze a wide variety of materials and compounds from the various scientific fields, from inorganic compounds to even ceramics, making this a very useful method. However, like many methods of analysis, it’s a complex method and it requires prior preparation of the samples to be analyzed, demanding expensive equipment. Transmission electron microscopy is a method used to make all kinds of analysis, and when used properly, it is with no doubt, one of the best, if not the best, type of microscopy to make complex analysis of coumpounds. In order to perform this analysis there will be an interaction between the sample and the beam. This interaction consists in the interaction of a beam and electrons against the sample to be analyzed, a sample that has to present a very reduced thickness. “The detailed knowledge of the microestructure of the materials allows the understanding and, in many cases, the prediction of their properties and behavior. ” When we are looking for analyses with a very high resolution, we are looking for an equipment like a Transmission Electron Microscope. This kind of equipment was created precisly with the intention of being capable to conduct analysis with resolutions that would help scientists to better characterize their samples.
Transmission Electron Microscopes allow the analysis of all kinds of scientific materials like inorganic compounds, glasses, even ceramics. Other materials such as woods, concrete and textiles can also be analyzed by this method, even though that’s not a very usual thing to happen. TEMs are composed by a column that presents a vacuum system and at it’s top exists na electron source from where the electrons are going to be fired. These electrons will travel through the column until they hit the sample, positioned in the midsection, interacting with it. Then the electrons will continue to travel through the column until they hit a fluorescent screen at the bottom. After they hit this screen we will be able to see the images we are or not looking for. Here we can verify one of the disadvantages of this method. The images that are collected are only presented in black and white.
As it was refered before every Transmission Electron Microscope presents an electron source, can that be from Tungsten filaments or from Lanthanum Hexaborid. In order to create a straight beam of electrons this equipment also needs an electron gun. A TEM presents also enumerous magnetic lenses that will help to transmit the electron beam. The system of lenses must have apertures, that will help to control the intesity of the beam and, in some way, will help not to damage the sample because of too many intense beams. In order to insert the sample in the equipment we must use a sample holder. There are diferent types of sample holders depending on the type of sample we want to analyze. This sample holders exist to help us insert the sample into the equipment without damaging the equipment and/or destroying the sample.
A vaccum system with the presence of ion pumps is also required. This system is very importante because it will retain the ions that form after the beam of electrons is shot, preventing these ions from reacting with the sample. Finally it’s also needed an electron detector, that can either be a semi-conducter detector or a scintillator photomultiplier. These detectors will allow us to see the final result of the analysis of our sample. All together, these components make TEMs a very solid method to conduct analyses, but also a very expensive one. One eV can cost up to $5. But these equipments use around 100, 000 to 400, 000 eV. . Calculating the price of this range of energy we get around $500, 000 to $2000, 000. As we can see this method can become a really expensive one. This is a very importante factor to be taken into account when using the method.
However, this microscopic method has always presented restrictions at a system level. The system can either be one of two types: static or it can follow a charge-coupled device (CCD). The static system produces static images. It is called this way because the beam inciding in the sample is fixed meaning, it doesn’t follow a specific timescale as the one coupled with a CCD. The CCD is a semiconductor sensor used for imaging capture. In this type of microscopy, when the system is not static, it follows the timescales of the CCD. For that to happen the charge-coupled device has to be coupled to the equipment in order to store the information. The CCD on it’s own doesn’t have the capacity to capture the images, so it is always coupled with the equipments that have to follow its timescale. This type of system is not the most conventional. Comparing this method to regular microscopes we can see some fulcral diferences. The biggest might be the relative position of the source and the sample. In regular microscopes the samples are seen from the top and the source is at the bottom. In transmission electron microscopy happens the reverse, the source is at the top and the sample is placed in the bottom. Regarding the samples, they have to have a very reduced thickness for various reasons with the most importante being that, depending on its thickness the beam may have to be more or less intense. This happens because the beam of electrons must be capable of passing through the sample without causing impact on the samples’ structure. The electrons are accelerated, but they have to have a controlled speed and force, otherwise, damage to the sample may occur.
Other of the reasons because of which the sample must present such a reduced thickness is because we want as many electrons as possible passing through the sample. This has to happen because we need to originate enough intensity to be recognized from the screen or CCD and originate the image for us to be able to collect and analyze. We must also know that the thickness of a determined material will be different from the thickness of some other material. If we think a little about the subject we’ll get to the conclusion that all the materials have different propreties and so, some materials let the radiation pass through more easily than others. An example of this is the fact that inorganic compounds don’t have to presente such reduced thickness by comparison to a ceramic material. The need to use thin samples is, with no doubt, the biggest limitation in this tipe of microscopy.
Transmission Electron Microscopic analyses have to be very well prepared, not only because we may damage and even destroy our sample, but also because it can be harmful for our health. The ionization of the source emits radiation that is extremely dangerous because it can kill living tissues such as ours. However, this kind of equipment and technique have evolved to a point nowadayws where it’s much safer to work with TEM without so many adicional risks associated. Besides the usual transmission electron microscope there can be other equipments utilized. The type depends on the type of study to be conducted. It could be used a High Resolution Transmission Electron Microscope (HRTEM), a High Voltage Electron Microscope (HVEM), a Intermediate Voltage Electron Microscope (IVEM), a Scanning Transmission Electron Microscope (STEM) or even a Analytical Electron Microscope (AEM). The Transmission Electron Microscopes were developed firstly as a response to a need. As we know, the optic microscopes that are more commonly used don’t present a great resolution. We even have to focate in order for us to be able to see what we’re looking for, and in some cases, we can’t even see it. Because of this began to exist the need to create a microscope that would make possible for us to see the images we were looking for with better resolution and faster.
The Transmission Electron Microscopes were developed as a response to that need. When this type of microscopes began to be used, the scientists realized that they would have many more uses beyond the better resolution. They realized that they could analyze even more compounds and materials, something that was only possible because, as said before, with the use of a beam of electrons, the analysis of very thin compound is possible. An exemple of the wide utilization of Transmission Electron Microscopy is “Effective celulose nanocrystal imaging using transmission electron microscopy”. The main objective of this work was to characterize celulose nanocrystals, also called CNCs, by finding a method that allows them to do a thorough characterization of these nanocrystals. What makes this nanocrystals so difficult to characterize is the fact that they are composed mostly by carbon atoms which gives them low electron density. This nanocrystals are also full of hydrogen bonds. These two facts makes them really hard to characterize when not stained. So what scientists did was to try different ways of staining to make the nanocrystals gain some contrast, making it possible to analyze them. Though most of the techiniques tried didn’t work, they did find a way to analyze the celulose nanocrystals. The better way found to create a good contrast and characterize the crystals was by using low quantities of celulose nanocrystals with Bovine Serum Albumine (BSA) in solution and analyzed in a grid with silicon monoxide support surface. Other works conducted in which TEM was used are “Induction and characterization of oil palm pro-embryogenic masses” and “Source identification of individual soot agglomerates in Arctic air by Transmission Electron Microscopy”. These works are just an exemple of the many uses that Transmission Electron Microscopes have, and shows us the complexity of the method, how we have to prepare the sample, how sometimes we are not able to make the analysis without a thorough study of the method, the equipment and how our sample may interact with the beam of electrons.
Conclusions
As every analytical method, Transmission Electron Microscopy has its advantages and disadvantages. This method gives us the structure of our sample, the compounds in it, allowing us to do a thorough characterization, presents a broad area of analysis and high resolution images, that may just be its greatest advantage. On the other side it is a very expensive method that requires a very thorough preparation. It is also a very sensible method and although the images have high resolution, they are only presented in black and white, with this being its biggest disadvantage besides its very high cost.
