Explantion about cathod luminescence

1. Scanning Electron Microscopy -
Cathodoluminescence (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.

2. Fundamental Principles of SEM-CL
• Bombarding the surface of a material with some incident
radiation or particle may result in the emission of
electromagnetic radiation beyond that produced by
thermal black body radiation. This emission can be in the
visible range (400-700 nm), ultraviolet (UV; <400 nm) and
infrared (IR; >700 nm). This general phenomenon is known
as luminescence. The types of luminescence is general
distinguished by the type of incident radiation or particles
and by the kinetics of the emission process. In the latter
case, if the luminescent radiation occurs in <10-8 seconds
after the incoming radiation ceases, it is a luminescent
feature termed fluorescence. If the luminescent radiation
continues to emit for >10-8seconds (and sometimes much
longer) after the incoming radiation ceases, it is a
luminescent feature termed phosphorescence.

3. • Cathodoluminescence (CL) is the emission of photons
of characteristic wavelengths from a material that is
under high-energy electron bombardment. The
electron beam is typically produced in an electron
microprobe (EPMA) or scanning electron microscope
(SEM-CL) or in a cathodoluminesce microscopy
attachment to a petrographic microscope (Optical-CL).
• The nature of CL in a material is a complex function of
composition, lattice structure and superimposed strain
or damage on the structure of the material. Different
minerals exhibit fluorescent or phosphorescent kinetic
behavior which can have an effect on the quality of the
CL images, depending on the manner in which the
image is obtained.

4. Theory
• Solid-state band theory provides a way to explain the
luminescence phenomenon. An insulating solid
material (such as quartz or calcite) can be visualized as
having a valence band and a conduction band with an
intervening band gap (forbidden gap).
• If a crystal is bombarded by electrons with sufficient
energy, electrons from the lower-energy valence band
are promoted to the higher-energy conduction band.
When the energetic electrons attempt to return to the
ground state valence band, they may be temporarily
trapped (on the scale of microseconds) by intrinsic
(structural defects) and/or extrinsic (impurities) traps.

5. • If the energy lost when the electrons vacate
the traps is emitted is in the appropriate
energy/wavelength range, luminescence will
result. Most of the photons fall in the visible
portion of the electromagnetic spectrum
(wavelengths of 400-700 nm) with some
falling in the ultraviolet (UV) and infrared (IR)
portions of the electromagnetic spectrum.

7. SEM-CL Instrumentation
• 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.

8. • Generally, the electron beam is rastered across a larger
area of the sample and the CL response is recorded with
digital images from the CL detector. The CL images can be
obtained over a range of magnifications (10-10,000x), but
the lowest magnification is constrained by the specific
configuration of the CL detector system. The image
acquisition procedure varies depending on the information
that is sought. The image acquisition procedures include:
• 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.

9. With the addition of a spectrophotometer it is
possible to collect a scan of the wavelength vs.
relative intensity of the CL of a given material.
One of the considerations that affects the quality of
SEM-CL images is the existence
of phosphorescence phenomena in some
important CL-active minerals.
Consequently, as the electron beam rasters across a
sample the phosphorescent minerals continue to
emit light resulting in a streaking effect on the
image. The minerals calcite, dolomite and apatite
exhibit this phosphorescence phenomenon.

10. Applications
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. 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%).

11. Sandstone sample from unknown formation, A few
spots (such as in the light-blue grain at the bottom) are
filled with quartz, mostly macros are calcite filled.
Secondary electron image of the
same field of view

12. Strengths 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.

13. Limitations:
• 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.