Anomalous Atomic Number Contrast in Compositional Backscattered Electron Images of Organic Compounds Due to Cathodoluminescence

It is generally accepted that when multi-component materials are examined with a scanning electron microscope, the contrast differences observed using compositional backscattered electron imaging are due to variations in their average atomic numbers. With increasing atomic number, there is an approximate monotonic increase in backscatter coefficient, which is observed as an increase in the brightness of specimens. However, what seems to be less generally appreciated is that light emitted from cathodoluminescent compounds, especially organic compounds, will also contribute to the brightness in backscattered electron images, and this may result in anomalous compositional contrast. Backscattered electron detectors used with scanning electron microscopes are either a scintillator or a solid state semiconductor and, in addition to detecting high energy backscattered electrons, these detectors also happen to be very sensitive to light. Anomalous contrast had been observed with several organic pharmaceutically relevant compounds materials, where the images of some low average atomic number materials were unexpectedly brighter than those having higher average atomic numbers. This anomaly was found to be caused by cathodoluminescence. This discovery was a revelation because, if the published literature is an accurate guide, then organic compounds are rarely reported as being cathodoluminescent, whereas there is an abundance of information about the cathodoluminescence of numerous inorganic materials, especially minerals, ceramics and semiconductors. This paper describes several experiments using compositional backscattered electron imaging, elemental X-ray microanalysis and cathodoluminescence spectroscopy, which were performed to explore the cathodoluminescent behavior of several organic compounds that give rise to anomalous image contrast.