Scanning Electron Microscope (SEM)
The Scanning Electron Microscope (SEM) is an instrument that uses an electron beam to generate electron, photons, and X-ray signals to produce detailed images of a sample. A focused electron beam scans across the surface of a sample, interacting with the atoms within the sample and producing various signals. These signals provide information on the topography and composition of the sample. Based on the type of signal, the SEM can collect different types of images:
- Secondary Electron (SE) Images: These are generated by secondary electrons emitted from the surface of the sample, providing detailed topographic information.
- Backscattered Electron (BSE) Images: BSE images are produced from elastic and inelastic collisions between the incident electrons and the atoms in the sample. The brightness of the BSE image is influenced by the atomic number (Z) of the sample, with higher Z elements appearing brighter.
- Cathodoluminescence (CL) Imaging: CL imaging detects visible light (photons) emitted from the sample when it is bombarded by electrons, useful for studying mineral phases and their growth, dissolution, and deformation processes. Detects visible light emitted from the sample when bombarded by electrons. Useful for studying crystal zoning that can represent growth, dissolution, deformation processes in mineral phases and other crystallographic characteristics. It can also provides insights into the distribution of impurities, defects, and structural features within the sample.
- X-ray Mapping: This involves detecting characteristic X-rays emitted from the sample to analyze its elemental composition.
Types of SEM Analysis
Energy-Dispersive X-ray Spectroscopy (EDS): Used for semi-quantitative chemical analysis. The EDS detector collects all emitted X-rays and sorts them by energy, creating a spectrum that displays peaks corresponding to the elements present in the sample.
Mineral Composition Key: Red = magnetite (Fe3O4), Pink = chalcocite (CuS), Lime Green = roquesite (CuInS), Green = sphalerite (ZnS).
Electron Backscatter Diffraction (EBSD): Analyzes the crystallographic orientation of materials. EBSD patterns are used to study the microstructural and crystallographic characteristics of the sample.
Applications of SEM
SEMs serve as essential instruments across a diverse array of disciplines, catering to the needs of chemists, engineers, geologists, material scientists, biologists, and beyond. Their widespread utility extends from quality control and analytical testing to materials development, making them indispensable in both production lines and research and development laboratories. Applications encompass:
- Research and education in materials science, semiconductors, biochemistry, and beyond.
- Product testing, evaluation, failure analysis, and quality control in industries ranging from electronics, machinery, and automobiles to construction, food, textiles, and chemicals.
With the capability to analyze an extensive range of sample types including geological specimens like rocks and minerals, various materials such as powders and electrodes, as well as biological samples, SEMs empower researchers and professionals across numerous fields to delve into detailed microanalysis and imaging.
Electron microprobe Microanalysis (EPMA)
EPMA seamlessly integrates the capabilities of an SEM with advanced functionalities such as a reflected light microscope, EDS, and wavelength-dispersive spectrometers (WDS). This multifaceted instrument is indispensable for the microanalysis of both inorganic and organic materials, providing highly accurate quantitative elemental analysis alongside intricate imaging capabilities. Sample preparation for EPMA necessitates a meticulously polished surface, ideally achieving a polish of less than 1 micron. In cases where the sample lacks conductivity, a coating of conductive material is imperative to ensure optimal performance.
Advantages of EPMA
- Spectrochemical analysis at the micron scale
- More accurate quantitative elemental analysis than EDS
- Detailed chemical spatial distribution information
- Non-destructive in situ analyses
- Capable of analyzing small spots (≥1 µm)
- Capable of analyzig all elements except H, He, and Li = Major + trace elements
- Limit of detection between (up to 10 ppm)
- Can analyze minerals, thin films, metal alloys, and semi-conductors
Principle of WDS
Interactions between electrons and samples yield derivative electrons and X-rays. Characteristic X-rays are produced through inelastic collisions of incident electrons with electrons in the inner shells of atoms. When an inner-shell electron is ejected, creating a vacancy, a higher-shell electron fills the gap and emits energy as an X-ray. These quantized X-rays are unique to each element. Wavelength Dispersive Spectrometry (WDS) employs an analyzing crystal and a detector to sort X-rays by their wavelengths. Following Bragg’s Law, the crystal diffracts X-rays, allowing only those of specific wavelengths to enter the detector sequentially. This method ensures precise elemental analysis with high spectral resolution and low detection limits. An algorithm compares captured characteristic X-rays with standard references, facilitating chemical composition analysis. The detection limit of EPMA analysis can reach parts per million (ppm) levels, contingent upon selected elements and experimental conditions.
nλ = 2d sinӨ
where, n = an integer (1, 2, 3…), λ = wavelenght, d = d-spacing of the crystal, and Ө = incident angle (measured from crystal surface)
In addition to geoscience and engineering research, SEM and EPMA can be used for product testing, crystalline characters and defects, failure analysis, and quality control in the areas of electronics, machinery, automobile, construction, food, textile, etc.
