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Scanning electron microscopy with EDX
In 1930s, the invention of the electron microscope revolutionized microscopy. This device uses an electron beam, instead of a light beam, to illuminate the sample and produce a magnified image, reaching a magnification of up to about 10,000,000x, while optical microscopes can only achieve useful magnifications of about 2,000x. There are two main types of electron microscopes: Transmission Electron Microscope (TEM), in which the beam runs through the sample and is analyzed afterwards, and Scanning Electron Microscope (SEM), in which the beam periodically sweeps the surface of the sample interacting with it, giving us information of its topography and morphology.
EDX (aka EDS) stands for Energy Dispersive X-ray analysis, which is an analytical technique used for the elemental analysis or chemical characterization of a sample. Although EDX systems can be used separately, they are most widely used in combination with a SEM, being the X-rays produced by the interaction of the electronic beam with the surface of the sample the object of the analysis. These two techniques combined form a powerful tool for the characterization of samples, thus, can be used in inclusions studies.
How it works
A SEM machine consists of two main units: the electron optical column and the electronic console (operation unit). The column is where the beam is generated, focused on to a small spot, and scanned across the specimen to create signals that control the intensity of the image on the viewing screen. The console provides the switches and knobs for adjusting the focus, magnification and image intensity on viewing and photography screens.
In the top of the column, an electron gun of up to 40 kV generates a beam that travels down the column. As electrons only travel short distances in the air, relatively high vacuum is required in the interior of the entire column. Commonly a hot tungsten or LaB6 filament is used as the emitter of the electrons, which are immediately accelerated by an electric field. Alternatively field electron emission can be used, and sometimes it leads to a narrower, more coherent and homogeneous beam, but this requires a much more exigent vacuum.
Before reaching the sample, the beam has to go through several lenses, first of them being one to three condenser lenses, which reduce the size of the beam to a small spot about 1µm in diameter. As the beam is formed by electrons instead of light, electromagnetic lenses are used, which are basically wire coils operating in an analogous way to glass lenses in an optical microscope. The most important lens is the objective lens, situated right above the specimen chamber. This lens moves the smallest spot formed by the beam up and down in space (working distance) to meet the specimen surface. Between the condenser and the objective lenses the scanning system is located. As mentioned before, in the SEM the beam sweeps the surface of the sample every several seconds providing us the needed information. This movement of the beam sideways is achieved by the scanning system, a series of coils that push or deflect the beam in the needed direction.
The sample is placed in the specimen chamber, an evacuated space which also contains several detectors for the different signals to be collected. An important consideration is that, for conventional imaging in the SEM, specimens must be electrically conductive, at least at the surface. This means that non-conductive materials have to be coated with an ultrathin layer of a conductive material before the analysis, being gold, platinum and gold-palladium alloys the most used ones. When the primary electron beam interacts with the sample, the electrons lose energy by repeated random scattering and absorption within a teardrop-shaped volume of the specimen known as the interaction volume, which extends from less than 100 nm to approximately 5 µm into the surface. The size of the interaction volume depends on the electron’s landing energy, the atomic number of the specimen and the specimen’s density. The interaction between the incoming primary electrons and the surface or near-surface of the material at different levels of penetration results in the reflection of electrons with different energies, as well as electromagnetic radiation (mainly X-rays). The different electronic and X-ray signals produced will be approached below.
Cross section of the interaction volume
To create an SEM image, the incident electron beam is scanned in a raster pattern across the sample’s surface. The emitted electrons are detected for each position in the scanned area by an electron detector. The intensity of the emitted electron signal is displayed as brightness on a cathode ray tube (CRT). By synchronizing the CRT scan to that of the scan of the incident electron beam, the CRT display represents the morphology of the sample surface area scanned by the beam.
The operational unit attached to the electron optical column of a SEM basically consists of a computer which receives the information from the CRT as well as from the X-ray detector and monitors it to the operator. It also allows him to control all the parameters of the microscope, such as the magnification, the movement of the specimen stage, the scanning frequency, etc., to perform EDX analyses in a specific point or take pictures of an area. Other pieces of equipment inherent to a SEM are the vacuum pumping system and the refrigerating system. Altogether, the machine takes up an entire small room.
There are two main kinds of reflected electrons that can be detected by the SEM. The first ones are low-energy electrons originated when an incoming primary electron is scattered inelastically a few nanometers from the sample surface (see interaction volume picture above). As a result, a local electron, so called a secondary electron (SE), is ejected from the shells of an atom of the specimen. The second ones are backscattered electrons (BSE), which consist of high-energy electrons reflected by elastic scattering further away from the surface, conserving their energy but changing their direction.
Besides, when an electron from an inner shell is ejected as a secondary electron, another electron form a higher energy level in the same atom can fill its gap, emitting an X-ray in the process, whose energy corresponds to the energy difference between the two levels. Alternatively, instead of X-ray emission, the excess energy can be transferred to a third electron from a further outer shell, prompting its ejection. This ejected species is called an Auger electron, and the method for its analysis is known as Auger electron spectroscopy (AES).
Schematic diagram of the different interactions between the primary electrons and the sample (click on the picture to enlarge)
Secondary electrons are detected by an Everhart-Thornley detector, and give us the best possible resolution (up to about 0.5). SE imaging provides us a good picture of the surface morphology, thanks to its large depth of field, yielding a characteristic three-dimensional appearance, useful for understanding the surface structure of a specimen. Since heavy elements (high atomic number) backscatter electrons more strongly than light elements (low atomic number), and thus appear brighter in the image, BSE are most valuable for illustrating contrasts in composition in multiphase samples.
(SE and BSE images pictures)
X-rays produced when a lower shell position is filled by an electron from a higher shell are characteristic or each element, as are the energy gaps between its different shells. Along with this characteristic X-rays, the so called continuum X-rays are produced when incident beam electrons are slowed to varying degrees by the strong electromagnetic field of atomic nuclei in the sample. All degrees of electron braking are possible and, thus, the resulting X-rays have a continuous range of all energies and are not useful for the sample characterization. Besides some of the of the characteristic X-rays emissions can be strong enough to act themselves as an excitation source, ejecting electrons form other atoms (usually of a different element), and therefore producing the corresponding radiation. This phenomenon is known as secondary fluorescence.
This characteristic X-rays are the basis of EDX functioning (as well as WDX and XRF, which are discussed later), in which another detector collects the X-rays and converts them into voltage signals by ionization, which are then plotted in a graph that presents several peaks, characteristic of each element. The height of the peaks also gives us reasonably accurate information on the relative amounts of the elements present, from what we can deduce the chemical composition of a mineral. The EDX process is almost instantaneous, and can be carried out upon an area or a particular point, both selected directly in the screen of the computer that operates the SEM.
Pros and cons
The unique feature which makes SEM plus EDX method so suitable for inclusions studies is the ability to view a detailed and highly magnified image of the surface of the stone, which shows us the morphology of the inclusions, and perform EDX analysis by simply clicking the desired spot in the specimen. EDX analyses are much faster than wavelength dispersive X-ray analyses (WDS or WDX), but have poorer resolution for elements in low abundances, and can fail to detect elements with low atomic numbers, less than 11 in some of them, and only H, He and Li in more advanced detectors.
Another advantage of the SEM is the scarce sample preparation needed. The conductive coating applied to non conductive samples before the observation is easily removed in most cases, while the electron beam doesn’t significantly damage most materials, which is why scanning electron microscopy is considered non-destructive. This is an advantage of the SEM in comparison with the TEM (that can be complemented with EDX as well), which requires thinly laminated specimens for the analysis.
The main limitation of SEM in inclusions studies is that the inclusion has to be in the surface of the stone, as the penetration of the electrons is always less than 5 microns. Of course, the stone can be cut and polished to reveal the inclusion, but this would turn the analysis into a destructive one.
In the face of Raman and FTIR analyses, while the SEM plus EDX gives us a much bigger variety of possibilities, there is a huge disparity in the cost of the equipment and its maintenance; the simplest of the SEMs costs about 200,000$, and the price escalates much higher in more advanced machines. Besides, there is no really compact version of the SEM, which makes it impossible to carry out field analyses.
The SEM/EDX is used in a wide range of other applications, some of them being microscopic feature measurement, fracture characterization, surface contamination examination, and many others. The main limitations of the SEM in this uses are the size of the specimen chamber, limited to about 10 cm in the horizontal dimensions and only 40 mm in vertical in the bigger ones, and that the specimen must be stable in high vacuum, which eliminates liquids and gases. This can be overcome by using the Environmental Scanning Electron Microscope (ESEM).
References and links
WDX (Wavelength-dispersive X-ray spectroscopy)
Wavelength-dispersive X-ray spectroscopy (WDX or WDS) is a spectroscopic technique widely used to identify the chemical composition of a sample both locally and in an area, providing compositional maps. Like EDX (energy-dispersive X-ray spectroscopy), it analyzes the X-rays produced by the interaction of an electron beam with the sample, thus, is most often used in conjunction with a SEM or any other device that uses an electron microprobe.
WDX and EDX are complementary techniques and are frequently used together, given that EDX analysis is faster, while WDX offers significantly higher spectral resolution and enhanced quantitative potential. This gives us the possibility of carrying on an EDX analysis to identify which elements are present, in order to use WDX to accurately determine their respective quantities.
How it works
The principle of WDX spectroscopy is very similar to EDX, laying the main difference in the way of acquiring information from basically the same source. This difference is responsible for the pros and cons of both methods, which will be treated further on. As explained in the page dedicated to SEM-EDX, when an electron beam reaches the surface of a sample, electrons and X-rays are emitted from the volume of material right under it (interaction volume). The electrons can be used to form a SEM image of the surface, while the so called characteristic X-rays are used in WDX (and EDX) to find out the composition of the spot.
An X-ray is generated when an electron jumps from a higher energetic level to a lower one, usually as a result of the electron that was in the lower level being kicked-out by the energy of the incoming primary electrons of the beam. The energy gaps between the different levels are different for each element (and thus are the wavelengths of the emitted X-rays). WDX measures the wavelength of the emitted x-rays to characterize the sample.
The emitted X-rays are selected according to their wavelength using an analytical crystal(s) with specific lattice spacing(s) and reflected to the detector, which measures their relative quantities. When X-rays encounter the analytical crystal at a specific angle θ, only those that satisfy Bragg’s Law
nλ = 2d sinθ
are reflected and a single wavelength is passed on to the detector. In this expression, λ is the incoming wavelength, d is the crystal’s lattice spacing, θ is the Bragg’s angle and n is any integer. The wavelength of the X-rays reflected into the detector may be varied by changing the position of the analyzing crystal relative to the sample (therefore changing θ).
Most common configuration of sample, crystal and detector in a WDX spectrometer
There is commonly more than a single analytical crystal in a WD spectrometer and, in the case of most SEM instruments, there are typically multiple spectrometers with a suite of analytical crystals with dissimilar crystal lattice spacings (d-spacings), so that the spectrometers can reach all of the elemental wavelengths of interest and optimize performance in different wavelength ranges. Nevertheless, each spectrometer needs to change the position of the analytical crystal or cycle through different crystals for each one of the wavelengths of interest, known to be corresponding to a particular element, or swipe a range of wavelengths if a continuous spectrum is needed. Therefore, this method of data acquisition is significantly slower, yet more precise than EDX, whose detector contains a crystal that simultaneously absorbs the energy of incoming x-rays of a wide range of energies by ionization.
As data is obtained or each wavelength separately, it is common to seek only for wavelengths corresponding to a specific element, which is presumed to be present in the sample, or has been detected in a previous EDX analysis, and more accurate quantitative information is demanded. When combined with a SEM, this leads to element X-ray compositional maps, as a result of determining the amount of an element present in each spot of an area of the sample.
Element map of tin and lead containing solder balls over a SEM image (from http://www.bruker.com)
Alternatively, a complete spectrum for a span of wavelengths can be obtained, similar to the one in EDX, with peaks corresponding to different elements, and a higher spectral resolution, but taking much more time in the process.
Pros and cons
In general, WDX has similar applications to EDX, and the two of them are often used together with a SEM. This offers us the possibility of viewing the surface of the sample with high magnification, and performing quick local analyses to determine its composition, with the main limitation being that only the surface and a few microns deep can be studied. In this setup, compared to EDX, WDX offers a significantly higher spectral resolution, which is the ability to distinguish two or more peaks from different elements that are very close in wavelength, and can be assumed to be one. In addition, though both methods can be used for quantitative analyses, WDX is much more accurate for this purpose.
Also, WDX is more reliable when detecting very small traces of an element (up to 10 parts per million), and elements of low atomic number; WDX can detect elements from atomic number 5 (boron) and higher, while EDX is usually limited to 11 and higher, although this has been lowered in advanced detectors. For these reasons, both techniques are complimentary and are frequently used together.
As for its limitations, WDX is restricted to the study of solid samples, is not able to distinguish among the valence states of elements (e.g. Fe2+ vs. Fe3+) or their isotopes, and is slower than EDX analysis.
References and links