SEM-CathodoLuminescence (SEM-CL) is the emission of photons of characteristic wavelengths from a material that is under high-energy electron bombardment produced in a scanning electron microscope. The nature of CL in a material is a complex function of composition, lattice structure and superimposed strain or damage on the structure of the material. Solid-state band theory provides a way to explain the luminescence phenomenon. An insulating solid material (such as quartz or calcite) can be visualized as having a valence band and a conduction band with an intervening band gap (forbidden gap).
If a crystal is bombarded by electrons with sufficient energy, electrons from the lower-energy valence band are promoted to the higher-energy conduction band. When the energetic electrons attempt to return to the ground state valence band, they may be temporarily trapped (on the scale of microseconds) by intrinsic (structural defects) and/or extrinsic (impurities) traps. If the energy lost when the electrons vacate the traps is emitted in the appropriate energy/wavelength range, luminescence will result.
Most of the photons fall in the visible portion of the electromagnetic spectrum (wavelengths of 400-700 nm) with some falling in the ultraviolet (UV) and infrared (IR) portions of the electromagnetic spectrum. There are several possible ways in which the traps can interact to produce luminescence Once the electrons are excited to the conduction band they may not encounter a trap and fall to the valence band or they move randomly through the crystal structure until a trap is encountered. From that trap, the electron might return to the ground state or it may encounter multiple traps emitting photons with wavelengths dependent on the energy differences. The intensity of the CL is generally a function of the density of the traps.
Photon energy < EGap
Recombination with impurity
eA0 : electron in CB – hole of neutral acceptor
D0h : electron of neutral donnor – hole in VB
DAP : electron of neutral donnor – hole of neutral acceptor
Cold-cathode CL is the most commonly used optical-CL system. It is an attachment to an optical microscope that allows the sample to be examined optically with the microscope and with CL in the same area. In a cold-cathode CL system the electron beam is generated by the discharge that takes place between the cathode at negative high voltage and anode at ground potential in ionized gas at a moderate vacuum of ~10-2 Torr (vs 10-5 or more for conventional SEMs). The result is relatively low-intensity CL in most CL-active materials.
The resulting luminescence in the sample can be viewed through the objective lens of the microscope or the image can be recorded with a digital camera. Cold-CL emissions can provide general information on the trace elements contained in minerals or the production of mechanically induced defects in the crystals. Perhaps more importantly for the geologic context, the distribution of the CL in a material gives fundamental insights into such processes as crystal growth, replacement, deformation and provenance. Major limitations of acquisition of CL images with the Optical-CL relative to the SEM-CL include:
Raman Spectroscopy is a non-destructive chemical analysis technique which provides detailed information about chemical structure, phase and polymorphy, crystallinity and molecular interactions. It is based upon the interaction of light with the chemical bonds within a material. Raman is a light scattering technique, whereby a molecule scatters incident light from a high intensity laser light source.
Most of the scattered light is at the same wavelength (or colour) as the laser source and does not provide useful information – this is called Rayleigh Scatter. However a small amount of light (typically 0.0000001%) is scattered at different wavelengths (or colours), which depend on the chemical structure of the analyte – this is called Raman Scatter.
SEM-Raman consists of measuring a Raman spectrum inside the EM specimen chamber. This requires to bring the excitation laser light on the sample which is under a vacuum. A Raman spectrum features a number of peaks, showing the intensity and wavelength position of the Raman scattered light. Each peak corresponds to a specific molecular bond vibration, including individual bonds such as C-C, C=C,
N-O, C-H etc, and groups of bonds such as benzene ring breathing mode, polymer chain vibrations, lattice modes etc.
Whether it is in mineralogy, in ceramics, in semi-conductors or in novel 2D materials, Raman, PL and CL provide different pieces of information on the analyte.
Whilst CL and PL typically study luminescent materials bandgaps, different excitonic recombination pathways, growth defects, and impurities, Raman spectroscopy probes the chemical structure of a material and provides information about chemical structure and identity, phases and polymorphs, intrinsic stress/strain, and contaminants. Typically a Raman spectrum is a distinct chemical fingerprint for a particular molecule or material, and can be used to very quickly identify the material, or distinguish it from others.
Raman spectral libraries are often used for the identification of a material based on its Raman spectrum – libraries containing thousands of spectra are rapidly searched to find a match with the spectrum of the sample. The primary advantages to the electron beam excitation of SEM-CL over SEM-Raman and SEM-PL is its spatial resolution. Using the scanning electron beam, the attainable resolution is on the order of a few ten nanometers, while in a (scanning) transmission electron microscope, nanometer sized features can be resolved. With SEM-Raman and SEMPL however, the spatial resolution is limited by the optical diffraction, in the micron range.
Although direct bandgap semiconductors such as GaAs or GaN are most easily examined by these techniques, indirect semiconductors such as silicon also emit weak cathodoluminescence, and can be examined as well. In particular, the luminescence of dislocated silicon is different from intrinsic silicon, and can be used to map defects in integrated circuits. Recently, cathodoluminescence performed in electron microscopes is also being used to study surface plasmon resonances in metallic Nanoparticles.
Surface plasmon in metal nanoparticles can absorb and emit light, though the process is different from that in semiconductors. Similarly, cathodoluminescence has been exploited as a probe to map the local density of states of planar dielectric photonic crystals and nanostructured photovoltaic materials.
SEM-CL is also a much demanded technique in mineralogy and geology, applications include:
SEM-CL analysis offers the advantage of indicating variations in chemical composition on a lower level than techniques based on X-ray analysis. Therefore, it is advantageous over conventional SEM-EDX and SEM-WDX analyses for detecting trace rare-earth elements. However CL is very sensitive to a wide range of factors like temperature, chemical composition, defects, strain, crystal structure that it make the interpretation of CL complex.
High spatial resolution
The volume inside the specimen in which interactions occur depends on the several factors:
CL Panchromatic image is usually displayed as a grey scale image. It consists of only one band of data corresponding to the intensity of light emitted for each pixel and collected by the detector (integrated intensity). A PMT photomultiplier directly coupled to the CL collecting interface is commonly used to collect the light.
CL RGB image is a multispectral image consists of three visual primary color bands of data (red, green, blue). The three bands are combined together to produce “True color “image.
CL hyperspectral image is an image in which each pixel contains an entire spectrum. Electron beam scans across sample for small mapping (around hundreds of micrometers scale) and at each point acquire complete spectrum. A spectrometer equipped with CCD detector is commonly used to collect the light.