Ion-beam techniques for Material Analysis
Techniques based on ion beams with energies in the range of MeVs have proven to be powerful tools for material analysis. In this group of non-destructive analytical techniques we find particle induced X-ray emission (PIXE), Rutherford backscattering (RBS), Elastic Recoil Detection Analysis (ERDA), nuclear reaction analysis (NRA), and ion channeling, which are being used to analyze composition and depth profiling of solids. When sensitivity to elements placed at the surface is required, low-energy ion scattering (LEIS) can be used. Almost all elements from hydrogen onward can be probed by ion beam analysis.
Particle Induced X-ray Emission (PIXE): The incident beam (usually protons) ionizes inner shell electrons from the sample atoms which results in the emission of characteristic x-rays. With a suitable x-ray detector elements heavier than C can be detected. This technique is the analog X-ray analysis with electron microprobes. However, due to the low X-ray background, the sensitivity of PIXE is highly enhanced. Depth resolution is not possible.
Rutherford Backscattering Spectrometry (RBS): It relies on the fact that the energy of an elastically backscattered particle depends on the mass of the target atom and on the depth at which the scattering take place due to the energy loss in the entry and exit paths in the sample. Typically, a helium beam with energy of 2 MeV and intensity of some nA is used, with the backscattered particles detected at an angle close to 180°. This allows to profile the elemental composition of the samples, and layers thicknesses, in a depth of a few μm.
Elastic Recoil Detection Analysis (ERDA): Allows the detection of light elements in heavy substrates or elements lighter than the incident beam particles by measuring the recoiling target atoms at grazing angle. A typical example is the depth hydrogen profiling using a 2 MeV He-beam.
Nuclear Reaction Analysis (NRA): Light projectiles with MeV energy impinging on light to medium heavy atoms can induce nuclear reactions in the target nuclei. For those reactions where gamma rays are produced a NaI detector is used. The yield of the prompt characteristic gamma rays is proportional to the concentration of the specific elements in the sample. Reactions with narrow resonances can be used for depth profiling by sweeping the accelerator energy.
Low-Energy Ion Scattering (LEIS), also known as Scattering and Recoiling Spectrometry (SARS), allows elemental and structural analysis of the topmost surface layer. The technique uses a monoenergetic, mass-selected, collimated beam of ions in the low keV energy range to irradiate a surface and analyses the projectiles and the recoiling substrate atoms emitted through cuasi-single collisions. Since the scattering process is mass-dispersive, the energy or the time-of-flight (TOF) of the atoms provides a mass spectrum of the constituent atoms of the surface (including hydrogen).
The ion beam techniques described above allow quantitative elemental composition analysis of samples. No information on the chemical state of the elements is provided. Although the samples must be compatible with the high vacuum in the analysis chamber, there are preparation methods that allow one to analyze a wide range of materials. As an example, leaves samples are oven-dried, ground and compacted in a press. The volume of the sample affected by the ion beam is limited by the beam diameter, typically, in the range between about 20 micrometers and 2 mm, and the beam path depth, some micrometers depending on the type and energy of the beam and the sample composition.
X-ray photoelectron spectroscopy
An excellent complement to the ion-beam based techniques described above is X-Ray Photoelectron Spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA), since it provides chemical information about the surfaces of solid materials. Both composition and the chemical state of surface constituents can be determined by XPS. The photoelectrons and Auger electrons emitted from the sample are detected by an electron energy analyzer, and their energy is determined as a function of their velocity entering the detector. The energy corresponding to each peak is characteristic of an element present in the sampled volume. The area under a peak in the spectrum is a measure of the relative amount of the element represented by that peak. The peak shape and precise position indicates the chemical state for the element. XPS is a surface sensitive technique because only those electrons generated near the surface escape and are detected. The photoelectrons of interest have relatively low kinetic energy. Due to inelastic collisions within the sample’s atomic structure, photoelectrons originating more than 20 to 50 Å below the surface cannot escape with sufficient energy to be detected.
XPS (ESCA) is performed in the XPS Laboratory.
Ion-beams for material irradiation
The knowledge of the damage produced on materials with ion beam irradiation is very important in relevant areas like those related to fusion technology, space weather radiation, cancer therapy, among others. The ions that penetrate into materials transfer their energy to electrons and atoms of the sample, producing, for example, the displacement of atoms from their lattice sites in a material, creating knock-on atoms which then go on to cause further damage. These displaced atoms may either recombine with a vacant site, or, they may combine with another interstitial and grow into a larger defect. Vacancy type defects may also grow in the same manner or form directly from the collapse of the cascade itself. Collectively, these processes produce defect features such as dislocations loops, lines and cavities. These defects can cause the material to swell, become brittle or harden which may lead to structural failure as well as degradation of the materials thermophysical and thermomechanical properties, reducing the efficiency and integrity of the component. The study of such type of damages that could take years in a reactor environment could in principle be emulated in few days by using low-energy ion beams. Following irradiation, the materials can be analyzed and their robustness and adequacy for nuclear reactor environment can be evaluated.
Scanning probe microscopy
Scanning probe microscopy (SPM) is a branch of microscopy that forms images of surfaces properties of materials by using a physical probe that scans the specimen from the micron all the way down to the atomic level.
The first microscope was a Scanning Tunneling Microscope (STM), an instrument for imaging surfaces at the atomic level. Its development in 1981 earned its inventors, Gerd Binnig and Heinrich Rohrer the Nobel Prize in Physics in 1986. For an STM, a conducting tip is brought very near to the surface to be examined, a bias (voltage difference) applied between the two can allow electrons to tunnel through the vacuum between them. The resulting tunneling current is a function of tip position, applied voltage, and the local density of states (LDOS) of the sample. This technique has a lateral resolution of 0.1 nm and a depth resolution of 0.01 nm. With this resolution, individual atoms within materials are routinely imaged and manipulated. The STM can be used not only in ultra-high vacuum but also in air, water, and various other liquid or gas ambients, and at temperatures ranging from near zero kelvin to over 1000 °C.
Another type of SPM is the Atomic force microscope (AFM). The probe used in an AFM is a sharp tip, typically less than 5 μm tall and often less than 10 nm in diameter at the apex. The tip is located at the free end of a cantilever that is usually 100–500 μm long. Forces between the tip and the sample surface cause the cantilever to bend, or deflect. The measured cantilever deflections allow a computer to generate a map of surface topography with resolutions on the order of fractions of nanometers.
STM-AFM is performed in the STM Laboratory.