By Unmesha RayOct 22 2018Cathodoluminescence (CL) is the emission of light from a material which is under excitation by a cathode ray or high energy electron beam. When an electron beam interacts with a material, many processes take place which is used for different kinds of microscopy. Along with the electron signals, a broad spectrum of electromagnetic radiation (X-rays to mid-IR) is generated by coherent and incoherent processes. The radiation generated is denoted as cathodoluminescence, where cathode rays refer to fast electrons and luminescence refers to the light emission. The electron beam when it returns to the ground state causes the material to fluoresce. CL is used in semiconductor physics to determine the presence and type of crystal defects, changes in elastic strain and variations in composition.
There are two different physical processes that lead to CL emission. First is the incoherent CL emission where no phase relationship exists between the CL emission and the incoming electron field. As this CL emission involves quantum mechanical transitions in a material, it bears a strong similarity with photoluminescence. Incoherent CL are dominant in phosphors, dielectric materials like glasses, ceramics, etc, and direct bandgap semiconductors (GaAs, InP). Incoherent CL are useful for failure analysis of materials, quality assurance, and material fingerprinting.
The second is the coherent CL characterized by the coherent interactions between the electron and a material. Some of the examples of coherent CL are bulk plasmon and surface plasmon polariton excitation in plasmonic metals, transition radiation, and Cherenkov radiation. Transition radiation is generated when an electron crosses an interface between two media. Cherenkov radiation is generated when the speed of the electron exceeds the phase velocity of light in the material. Sometimes electron-induced radiation emission is used instead of coherent processes to avoid confusion with the incoherent form of CL.
Application of Cathodoluminescence
CL is used in SEM imaging microscopy. It is used to characterize the optical properties at the nanoscale. CL analyses the resulting photons which are emitted in the UV to NIR region of the electromagnetic spectrum. CL is the combination of high spatial resolution of electron microscopy with functional optical information. This makes CL technique very unique and is used in different kinds of research and application purpose such as in the field of optics, geology and material science. CL is also used to explore many fundamental properties of matter like electronic structures of materials, resonant phenomena, light transport, scattering and many more. Thus it presents important information for fundamental and applied research with a direct link to industry.
CL finds application in the field of nanophotonics. It generates hyperspectral light-emission maps that reflect the local density of electromagnetic states. The directionality and polarization can be measured to give useful information about the optical properties of nanostructures. Previously, CL was not applicable to dielectric, metallic or other semiconductor nanostructures. But now with the recent advances in CL, studies are possible on all these structures. We can analyze nanoparticles, meta-molecules, nanowires, photonic crystals, and metasurfaces, with the help of advanced CL measurements.
CL imaging is used to study the geological samples. The CL emission from rock gives an idea about the crystal growth, cementation, zonation, replacement, deformation, provenance, defect structures, and trace elements. With the help of this, we can fingerprint rocks and disclose interesting spatial textures on a submicron scale. Thus CL has proven to be an excellent and non-destructive complement in geochronology when compared to the mass spectroscopy.
References
- Coenen, T., S. V. den Hoedt, and A. Polman. "A new cathodoluminescence system for nanoscale optics, materials science, and geology" Microscopy Today 24, no. 3 (2016): 12-19.
- http://ssd.phys.strath.ac.uk/techniques/scanning-electron-microscopy/cathodoluminescence/
- Engelsen, Daniel den, George R. Fern, Paul G. Harris, Terry G. Ireland, and Jack Silver. "New Developments in Cathodoluminescence Spectroscopy for the Study of Luminescent Materials" Materials 10, no. 3 (2017): 312.
- Gustafsson, Anders, and Eli Kapon. "Cathodoluminescence in the scanning electron microscope: application to low-dimensional semiconductor structures." scanning microscopy international (1996).
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