Cathodoluminescence microscope

A cathodoluminescence (CL) microscope combines methods from electron and regular (light optical) microscopes. It is designed to study the luminescence characteristics of polished thin sections of solids irradiated by an electron beam. To prevent charging of the sample, the surface must be coated with a conductive layer of gold or carbon. This is usually done by a sputter deposition device or a carbon coater.

Using a cathodoluminescence microscope, structures within crystals or fabrics can be made visible which cannot be seen in normal light conditions. Thus, for example, valuable information on the growth of minerals can be obtained. CL-microscopy is used in geology, mineralogy and materials science (rocks, minerals, volcanic ash, glass, ceramic, concrete, fly ash etc.).

CL color and intensity are dependent on the characteristics of the sample and on the working conditions of the electron gun. Here, acceleration voltage and beam current of the electron beam are of major importance. Today, two types of CL microscopes are in use. One is working with a "cold cathode" generating an electron beam by a corona discharge tube, the other one produces a beam using a "hot cathode". The advantage of a hot cathode is the precisely controllable high beam intensity allowing to stimulate the emission of light even on weakly luminescing materials (e.g. quartz - see picture).

Cold-cathode CLMs are the simplest and most economic type. They are preferred by mineralogists and petrographers and have been used for a variety of applications in material characterization. Unlike other electron bombardment techniques like electron microscopy}, cold cathodoluminecence microscopy provides positive ions along with the electrons which neutralize surface charge buildup and eliminate the need for conductive coatings to be applied to the specimens.

Hot-cathode CL systems and specially designed CL systems use a hot-gun filament as the electron source. These are commonly seen in cathodoluminescence detectors as a characterization tool in electron microscopy. While these types of CL microscopes are expensive, they are used routinely ceramic characterization because they provide more stable and adjustable current-beam parameters. Disadvantages besides cost are the need for specimen conductive coating and small examination area which both are time consuming. Also hot-catlode CLMs do not allow direct viewing of emission colors, only readout of the emission wavelengths.

Higher intensity and brightness of light emission is one of the main advantages of cathodoluminescence microscopy over fluorescence microscopy.