Hello. I'm Roberto Garcia. I'm a lab manager at the Analytical Instrumentation Facility at NC State University, part of the Research Triangle Nanotechnology Network. In this video, you will learn the basics of energy-dispersive X-ray spectroscopy or EDS. EDS is a non-destructive analytical technique. It provides information on the elemental and chemical composition of a sample. EDS essentially allows us to determine what something is made out of. To perform an EDS experiment, we need an electron microscope, a special detector known as an EDS detector, and software to analyze the data. You may be wondering how a microscope, a tool that are used primarily for imaging can provide elemental or chemical information. In electron microscope, a high-energy electron beam is accelerated towards the specimen. When the electron beam hits the atoms that make up the sample, electron signals are generated such as secondary and backscattered electrons. These electrons signals are used to produce an image of the sample. However, electrons are not the only signals generated as a result of the beam interacting with the sample. The beam also causes X-rays to be emitted from the sample. To understand how this happens, let's consider a single atom in the sample. When an electron from the incident beam imparts enough energy to the sample, an electron can be ejected from one of the atom's electron shells. This leaves a vacancy like a hole in the electron shell where the electron originally was. For the atom to return to a stable state, an electron must replace the one that was emitted. An electron from a higher energy shell will drop down into this lower energy shell. When an electron from a higher energy outer shell dropped into a lower energy shell vacancy, it must lose some of its original energy. The electrons excess energy is emitted in the form of an X-ray. This X-ray can be detected by the EDS detector and analyzed by software to identify the element that it came from. How can an X-ray identify a specific element? Well, not all X-rays are alike. X-rays can have varying energies measured in kiloelectron volts or keV for short. They can also have different wavelengths. In energy-dispersive spectroscopy, we are analyzing the energy of the X-rays generated from the sample. When X-rays are emitted from a sample in an SEM, the energy of each generated X-ray will be dictated by the elements present and the electron shells that make up the atoms. Basically, this means each element will have its own unique identifying X-ray signal, just like how humans each have their own unique fingerprint. Analyzing this signal for an EDS experiment involves measuring the energy of each characteristic X-ray signal and counting how often that particular energy signal appears. The special detector must be installed on the electron microscope to detect and measure these signals, simply called an EDS detector. The beam hits the sample, characteristic energy signals are generated from the sample, and the EDS detector amplifies and measures the energy of the X-ray. Then analysis software counts the amount of X-rays at each energy and plots them on a graph called a spectrum. On this spectrum, the software plots the number of X-rays as count or intensity on the y-axis versus the energy of the photons counted in thousands of electron volts or kiloelectron volts, keV, on the x-axis. Remember, each element has its own characteristic X-rays, each with a unique energy measured in keV. EDS software helps to match the measured piece along the x-axis with their respective elements. EDS software makes it easy to match these piece with the corresponding elements that they originated from. Here are important things to note about EDS. Some elements have overlapping energy peaks with other elements. Careful analysis and experience will help avoid a misidentification. EDS requires a lot of energy to generate enough signal to get a conclusive result. Generally speaking, you want to make sure you are exciting enough X-rays by using a high voltage in the range of 15 to 30 kV to produce plenty of signal that can be used to identify elements present in the sample. Some elements are not detectable and others pose difficulties for EDS analysis. For instance, hydrogen and helium do not have characteristic X-rays. Similarly, lithium and beryllium produce X-rays that are too low-energy to be detected. Despite these challenges, EDS is nonetheless a very useful and simple narrow measurement technique to provide very fast elemental information in many different types of samples. Thank you for watching this video.
