OPTIMIZING CATHODOLUMINESCENCE IMAGING ON THE SEM

Cathodoluminescence (CL) images are often optimized for the contrast within a grain and are not comparable among grains or across analytical sessions or instruments. Here we describe a new technique for collecting CL data that are consistent and reproducible. Results from experiments on a RainbowCL detector with image optimization, electron beam conditions, and sample preparation suggest the following.

First, similar to “white balancing” for photomicrographs of a thin section, SEM CL images require “black balancing.” On a panchromatic CL detector, adjust the brightness, contrast, and gamma on the histogram of the live scan so that the peak is at or near zero for the portions of the sample that do not luminesce. On a color CL detector the same approach is used; in this case, the red, green, and blue peaks are all at or near zero. Then optimize the brightness and the gamma such that zoning within the target grain is evident while the black domains remain black.

Second, comparable CL images can be achieved using a range of electron beam conditions, so the actual parameters used depends upon the scale of the CL features. For example, we obtained CL images at 10-20 kV, spot size of 300-700 nm, and scan speeds of 0.032 to 3.2 ms/pixel. Raising the voltage or the beam intensity increases the brightness of the image, yet decreases the resolution due to the larger beam diameter. Therefore, choose beam conditions based on the intrinsic luminescence of the mineral and the scale of the zoning.

Third, sample preparation affects both the resolution and color of images. Although CL can be observed on uncoated samples at low vacuum, the resolution is dramatically improved under high vacuum with a carbon coat. A carbon coat diminishes the vibrancy of the CL image so apply a coat that is just thick enough to dissipate charging. The CL detector picks up additional luminescence from the reflective back of the thin section and may change the color. Carbon tape applied to the back of the sample eliminates this additional luminescence and improves the resolution of fine scale features, but darkens the image.

Luminescence is driven in part by concentrations of activator and quencher elements and by the crystallographic orientation. It is best interpreted in concert with analytical techniques that quantify chemistry and orientation.