An Introduction to Advanced Material Identification Techniques

Often, when developing a product, we know exactly what it’s made of. After all, you developed the product and know which material was used for each part of it. However, it is sometimes the case that you don’t know what something is. Maybe there’s an unknown chemical reaction that’s causing the buildup of some unknown substance on the surface of the device. Maybe oxidation or corrosion of metal components is contaminating the inner surface. For critical components or for regulatory compliance, you may have to precisely characterize anything unexpected. This has the potential to turn into an expensive wild goose chase, using different analytical methods to try and characterize the material. Should you use FTIR? EDS? XRD? How can you make sure you don’t get upcharged for something that only takes 30 minutes? This article details a general approach to analysis of unknown residue that can help you understand the limitations and strengths of different techniques.

Different Spectra, Different Energies

It’s important to understand that many of these techniques are spectrographic in nature, but rely on significantly different energies. For example, EDS relies on electron energy transitions between ground and excited states. This corresponds with high energy photons in the x-ray spectrum. However, FTIR relies on vibrational and rotational excitations of molecules, which are much lower energy (sometimes approaching the microwave region). Understanding the phenomenon in question and its energy can help to identify the appropriate spectrographic technique.

Understanding EDS

Energy dispersive x-ray spectroscopy, often referred to as EDS or EDX, is a powerful analytical technique for identifying elements. A beam of high energy electrons is directed towards the sample, which undergoes a complex set of interactions. Some of these interactions involve the excitation of inner shell electrons. If these excited inner shell electrons undergo a transition back down to the ground state, they will emit photons, typically in the form of X-ray radiation for most many-electron atoms. This transition has a unique characteristic energy depending on the atom in question, so this technique can be used to identify the elemental composition of a surface. Typically, the electron beam is raster scanned over an area and the EDS X-ray detector measures the resulting spectrum. This allows for a fine tuning of the detection area. It is possible to characterize a single point with submicron accuracy, as well as perform a line scan. Finally, EDS mapping allows a full elemental map to be generated and is often overlaid on top of the SEM image.

Uses and Limitations of EDS

EDS is used for a wide variety of applications. Here at Nano Lab, we’ve used EDS to characterize the metallization of nanoscale solder joints, determine the presence of contaminants on sensitive medical devices, and characterize nanoelectronic devices. However, there are some limitations to consider with this technique. 

Firstly, electrons penetrate relatively deep and wide into the sample, on the order of a micron, which limits the resolution of EDS. You may get a signal from deep in the sample, below the layer you’re trying to analyze, and there is significant spatial overlap between adjacent spots, limiting the surface resolution of the technique. Secondly, SEM chambers are relatively small and susceptible to contamination. The technique must be carried out in a vacuum, and must be free of excessive moisture or residues. If the sample must be cut if it is too large, further exposing it to contamination. Finally, EDS cannot determine anything other than the elemental composition of the sample. It typically cannot be used to determine the identity of a polymer or petroleum-based product, since determining the presence of carbon and oxygen is typically not helpful in this context. It’s also not as good for lighter elements, which have lower energy transitions, which can get lost in the noise.

Beyond Elemental Analysis

While identifying elements can be useful, there is a lot that can’t be determined from this technique. For example, many polymer materials have similar elemental composition, often consisting mostly of carbon, hydrogen, and oxygen. Knowing only the composition of a polymer will often give very little information on its identity. In this case, we need to use a different type of spectroscopy. For example, we can excite molecular vibrations and rotations. These can tell us more about the molecular structure of a material, rather than the elemental composition. Tightly bonded inner electrons in an atom have a high energy excitation corresponding to the X-ray range. However, these vibrational and rotational excitations have much less energy, and typically absorb in the infrared region. Thus, FTIR, or fourier transform infrared spectroscopy, can be used to characterize the molecular composition of a material. Incident light is absorbed at certain wavelengths by the material, and the absorption of this light is used to calculate the spectrum. Since there are extensive databases available for FTIR spectra, it is possible to fit a spectra to a known sample. [1]

Raman Spectroscopy

Another similar technique for this type of measurement is Raman spectroscopy. This technique is similar to FTIR in that it measures interactions such as molecular vibrations. However, Raman spectroscopy measures a shift rather than an absorption. This makes it a more complicated technique. For example, when an incident photon hits a molecule, it can excite the molecule, which then reemits the photon. However, if the molecule is at a different vibrational state than it was before the interaction occurred, the emitted photon will have a different wavelength than the incident one. This shift is complicated and involves rotational, vibrational, and electrical interactions. Typically, the excitation laser is monochromatic, so that subtle shifts in frequency can be measured. A combination of theoretical understanding of the virtual states that are induced, along with exhaustive experimentation, have led to this technique’s widespread use. Certain systems have characteristic Raman spectra, and it can be used to measure messy and mixed material systems. [2]

X-ray Diffraction

All of the above techniques rely on energy transitions within a molecule or atom. The next technique, however, relies on a different type of interaction between light and matter. When x-rays are reflected off the surface of a crystal lattice, there is a set of angles and wavelengths that allow the reflected light to interfere constructively. Known as the Bragg condition, the following figure shows how two rays of light travel a slightly different distance. If this difference is an integer multiple of wavelengths, the light will interfere constructively, leading to a diffraction maximum at that angle. [3]

From The Oxford Solid State Basics Page 143

X-ray Diffraction (XRD) can be used to characterize not the elemental composition or molecular signature, but the lattice spacing and structure itself. The set of angles where constructive interference occurs can give insight into the orientation and phase of a crystalline material. To understand why this is useful, consider Titanium Oxide. There are two common phases of TiO2, which have the same chemical composition and similar bond vibrational states. However, the lattice spacing and crystal structure are vastly different. Because of this, XRD is a much more useful tool to differentiate between phases of a material like this. Since these phases have different mechanical, optical, and electrical properties, it is sometimes important to differentiate between them.[4]

Finding the Right Method

An introduction to XRD, FTIR, Raman spectroscopy, and EDS is a great first step to understanding how to characterize your material samples. However, there is a whole host of related techniques such as mass spectroscopy, x-ray fluorescence, and electron energy loss spectroscopy. Each of these techniques has strengths and limitations that makes it optimized for a specific subset of material analysis tasks. However, understanding the limitations and physical principles behind the common techniques described above can help you approach material analysis with more confidence, and can guide you as you learn more about this field. Here at Nano Lab, we offer a free consultation to help you get started! We’d love to explore how we can help you with your material analysis challenges.