A large number of electron microscope-based imaging methods exist that can provide distinctive contrasts in materials. Cathodoluminescence (CL) imaging gathers and measures the light produced by an electron beam directed at a material.
The easiest and quickest technique to obtain CL contrast is by executing intensity mapping. This technique records the CL intensity for every beam position with the help of a single-pixel light detector.
This can be achieved in two ways. Panchromatic imaging involves averaging of the CL signal over all wavelengths (colors) that fall within the detection range. Color-filtered CL intensity mapping can be carried out by introducing a color filter, for example.
The SPARC CL system employs a photomultiplier tube (PMT) light detector in combination with a filter wheel. This enables intensity mapping at seven different wavelength bands (the eighth entry is empty, enabling panchromatic imaging).
The CL emission is directed toward the PMT with the help of a motorized flip mirror (see Figure 1(a)). The flip mirror directs the light toward the PMT when it is engaged. When removed from the beam path, it enables the light to go forward to other detectors that are used for hyperspectral or angle-resolved CL.
Figure 1. (a) Schematic representation of the experimental setup used for CL intensity mapping. The ﬂip mirror is motorized and can move in and out of the beam path automatically. (b) SEM image and (c) RGB CL intensity map constructed from three color-ﬁltered PMT images (400, 500, and 600 nm bandpass ﬁlters with a bandwidth of 40 nm). The individual images were collected with 10 kV acceleration voltage, 400 pA current, and a 40 µs dwell time. Sample courtesy of Dr Chen Zhenyu (Institute of Mineral Resources, Beijing).
The PMT detector is sensitive, rapid, and has a large detector area. As a result, this approach is the method of choice for rapid scanning of large sample areas with high resolution. The pixel dwell times can be as low as 1 µs per pixel, allowing video-rate CL imaging. Moreover, an area larger than 0.5 × 0.5 mm can be covered in a single scan. This is best for scanning larger-scale structures or making CL overview images that can be employed to find the regions of interest for more detailed CL studies (for example, CL spectroscopy). A range of PMT detectors can be selected on the basis of wavelength regime of interest.
In the SPARC CL system, the output current of the PMT is monitored continuously. The amplification can thus instantly be shut down upon being overexposed to protect the detector from being damaged. This makes the system durable and powerful.
CL intensity mapping on a zircon crystal is an example that approves the effectiveness of this method. Zircon is a powerful mineral with the ability to last for longer periods of time in the earth’s crust.
Figure 1(b, c) illustrates a measurement on a zircon grain where the CL emission (partly) originates from traces of extrinsic rare-earth ion color centers. In Figure 1(b, c), an RGB CL image and an SEM image composed of three measurements at various colors of the same crystal grain are shown. The contour of the grain can be clearly seen in both the images; however, the CL image depicts much more structure within the grain.
The zircon structure and composition are affected as the growth conditions vary during formation. Such variations in conditions result in zonation in the zircon. This phenomenon can be efficiently visualized with CL since the technique is highly sensitive to such delicate changes.
Although this example has been taken from the field of geology, it would be possible to use CL intensity mapping for various other applications in materials science. These include color variations, radiation efficiency, and mapping defects in a wide range of (doped) dielectric, semiconductor, and ceramic materials.
This information has been sourced, reviewed and adapted from materials provided by Delmic B.V.
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