Scanning Electron Microscopy - Cathodoluminescence (SEM-CL)
What is SEM-CL?
A cathodoluminescence detector attached to a Scanning Electron Microscope (SEM), Field Emission Microscope (FEM) or an Electron Microprobe (EPMA) is capable of producing high-resolution digital cathodoluminescent (CL) images of luminescent materials. Whether this CL detector is attached to an SEM, FEM or EMPA, this mode of acquiring a CL image or CL spectrum is commonly termed SEM-CL.
Fundamental Principles of SEM-CL
The theory behind the production of the luminescent response by SEM-CL is the same as that for (Optical-CL) instrumentation (see CL theory).
SEM-CL Instrumentation - How Does It Work?
The SEM-CL operates in the same manner as a hot-cathode CL attachment to an optical system i.e. electrons are generated with a heated filament and accelerated to an anode. However, in SEM-CL there is a column under high vacuum (<10-5 Torr) in which:
- the electrons are accelerated toward the anode under potential differences generally of 1-30 kV
- the sample current can range from 1 pa to 10 nA
- the electrons can be focused to a narrow beam (5 nm to 1 µm) that is capable of producing a CL response on a small area of the sample.
- Total CL (gray level image) for the entire spectral range (~200-800 nm) - commonly used for general textural and chemical-zoning features.
- Collection of three consecutive gray-level images using a red then green then blue series of color filters. A "true-color" image is reconstructed from the separate R-G-B images via an image processing program such as Photoshop.
- Simultaneous collection of a "live" color image with an array detector system such as the Gatan Chroma-CL system (Figure 1.)
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. These applications include:
- investigations of cementation and diagenesis processes in sedimentary rocks
- provenance of clastic material in sedimentary and metasedimentary rocks
- details of internal structures of fossils
- growth/dissolution features in igneous and metamorphic minerals
- deformation mechanisms in metamorphic rocks.
- discrimination of different generations of the same mineral as a result of differences in trace amounts of activator elements. For example, a sandstone may include a variety of quartz grains from different source areas, multiple generations of quartz cements, and a cross-cutting quartz vein--all of which have different CL signals (Figure 2). These differences in luminescence could not otherwise be detected by SEI imaging, BSE imaging (due to the grains having the same mean atomic number, Z) or EDS analysis (trace elements below detection limits, ca. 0.1 wt%) (See figure 3a and 3b).
Strengths and Limitations of SEM-CL
Strengths of acquisition of CL images with the SEM-CL relative to the Optical-CL include:
- Better spatial resolution
- Improved current control
- Generation of a color CL image of the sample with the appropriate filters or detectors
- Examination of UV or IR CL responses beyond those obtained with Optical-CL.
Limitations of acquisition of CL images with the SEM-CL relative to the Optical-CL include:
- Necessary to have an electron beam instrument i.e. SEM, FEM or EMPA
- Machine time is generally more expensive
- Conductive coating required on the sample
- Nonlinear absorption of the RGB filters and challenges in proper color reintegration
- Problems of phorphorescence of important CL-emitting minerals such as carbonate minerals and apatite.
For more detailed information regarding the theory and practice of SEM-CL, please see:
- Boggs, S.,Jr. and Krinsley, D. (2006) Application of Cathodoluminescence Imaging to the Study of Sedimentary Rocks. New York, Cambridge University Press, 165 p.
- Reed, Robert M., and Milliken, Kitty L. (2003) How to overcome imaging problems associated with carbonate minerals on SEM-based cathodoluminescence systems. Journal of Sedimentary Research, 73, 326-330.
- Barker, C. E. (1986) Notes on cathodoluminescence microscopy using the technosyn stage, and a bibliography of applied cathodoluminescence. USGS, I 19.76:86-85.
For more detailed information regarding the theory and practice of SEM-CL, please visit:
- Rob Reed's CL Web pages - this is a series of images that illustrates applications of SEM-CL to a variety of rock types.