Cathodoluminescence

1
SEM / EPMA

• Cathodoluminescence

Modified 3/9./2018
2
Whats the point?

• Some materials, when excited by electrons,
  produce certain secondary electron excitation
  which then releases small quanta of energy in the
  range of a few eVwhich translated into
  wavelengths, is the visual light spectrum.

CL images can yield valuable information not
easily seen by other means.
Goldstein et al, 2018, SEMXRMA, p. 482
3
CL colors and eV

• The various band gap energies with their
  respective wavelengths and colors is shown here.

(image from Marshall, 1988, Fig 1.4, p. 4)
4
What its not

• Photoluminescence radiation caused by excitation
  by UV or visible light (1.8 to 4.9 eV)
• Nor
• Phosphorescence radiation that persists after
  the incident energy source is turned off
• But it is
• under the general category of Fluorescence, which
  is where the radiation ceases immediately (within
  10-8 sec) following cessation of the source.

5
Cathodoluminescence

• This is an optical (visible/nearly visible light)
  phenomenon. CL occurs in semiconductors/insulators
  , be they man-made or natural. Electrons in the
  valence band of these materials are excited into
  the conduction band for a brief time
  subsequently these electrons recombine with the
  holes left in the valence band. The energy
  difference is released as a photon of wavelength
  of light.
• Two commonly used applications are
• Locating strain (lattice mismatch) in
  semiconductors, and
• Evaluating minerals for heterogeneous growth
  (complex history, overgrowths, dissolution, crack
  infilling).

6
Seeing The Light

• There are two distinct methods to image this
  effect
• by SEM or microprobe,
• or by a small attachment to an optical microscope
  (static cold cathode electron source).
  Additionally, the light spectra can be quantified
  by a scanning monochronometer.

7
CL in living color
EXAMPLES
CL captured on color film A Casserite,
SnO2 B Crinoidal limestone
C Red dolomite, orange calcite
dark grey baddeleyite (ZrO2)
D St Peter Sandstone mature quartz with zoned
authigenic quartz overgrowths (from
Marshall,1988, CL of Geological Materials)
A
B
D
C
The CL emitted is of varying wavelengths
(colors), and can be captured with the right
equipment. Various CL microscope attachments
have been built that fit on the stage of a
regular microscope one model is the (cold
cathode) Luminoscope.
8
(Cold) CL Microscope Attachments
Cold cathode gun

• CMAs are relatively inexpensive attachments to
  microscopes. A high voltage (10-30 keV) cold
  cathode gun discharges electrons in a low vacuum
  chamber (rough pump only). A plasma results that
  provides charge neutralization (no carbon coating
  necessary). A camera (film or digital) and/or
  monochrometer are attached to acquire images
  and/or wavelength scans of the light.

(From Marshall, 1993,The present state of CL
attachments for optical microscopes, Scanning
Microscopy, Vol 7, p. 861)
9
SEM/EP CL detectors

• Above PM on SX51
• Top Right Gatan PanaCL on Hitachi mirror inside
  chamber, inserted to operating position directly
  below pole piece Below Right PM with
  filters outside chamber

10
Theories about CL

• Intrinsic CL
• Extrinsic CL

11
Intrinsic CL

• Intrinsic CL means that some minerals
  inherently emit CL, e.g. that nearly all
  silicates have CL in the blue region. However,
  this is not a totally satisfactory statement.

Goldstein et al, 2018, SEMXRMA, p. 482
12
Extrinsic CL
Goldstein et al, 2018, SEMXRMA, p. 483
13
CL jumping the band gap

• This figure demonstrates several different
  mechanisms whereby photons are emitted in the
  process of high voltage electrons promoting
  valence electrons to conduction band.

(from Marshall, 1988, CL of Geological Materials)
14
Impurities activators/quenchers

• Substitution of an element for the usual one
  (e.g. Ti 4 for Si4 in quartz, Mn 2 for Ca 2
  in calcite) is believed to be a key cause of
  distortion of the lattice (a defect) in the
  mineral. Impurities which function thusly are
  called activators.
• A few elements can perform the opposite role
  modify the energy level arrangement so that the
  CL process does not operate or is diminished.
  These are quenchers, with Fe2 being the most
  common.
• If the quencher level is low, it is possible that
  only a few ppm (or 50 ppb) of an activator is
  enough for CL emission.

15
Too much activator?

• It has been observed that CL intensity generally
  increases with increasing abundance of the
  activator ion, reaches a maximum and then there
  is too much of a good thing and the CL decreases
  as the activator level increases.
• The maximum CL intensity seems to occur when the
  activator level is in the 0.1 - 1 wt level e.g.
  CL intensity in doped feldspars is maximum at
  1.5 wt Fe 3 or Mn 2.

16
Pretty Colors

• A given mineral can accept different activators,
  and a give out a different CL color for each
  feldspars can accept Eu3 - blue, Mn2 - greenish
  yellow, and Fe3 - red.
• A given activator may be incorporated by
  different minerals, and produce different colored
  CL in them e.g. Mn2 can be accepted by
  feldspars - greenish yellow, apatite - yellow,
  and carbonates - reddish-orange.
• Several of the REE activators (e.g., Dy3, Sm3,
  Eu3) produce very sharp line spectra whose
  wavelength is almost independent of the host
  mineral, and can be used to qualitatively analyze
  the element content.

17
CL spectra
Goetze, 2002
18
Temperature effects

• The intensity of CL can be altered by changing
  the sample temperature there apparently are
  examples for increased intensity (in different
  materials) with both higher and lower
  temperatures.
• It has been shown that quartz CL can be enhanced
  by cooling the sample.

19
Time dependent effects

• The intensity of CL can deteriorate with time
  under electron beam bombardment, and is called
  electron beam aging in the phosphor industry.
  However, this is not believed to be a serious
  issue under normal beam conditions and viewing
  times.
• It has been reported to occur in quartz, over
  tens of seconds, and

that there is a color change from blue to red.
Goetze, 2002
20
Time dependent effects

• It is also obvious that SEMs with normal
  vacuums with oil backstreaming easily cover the
  sample with a very thin coat of carbon which
  reduces the intensity of the detected CL. Thus
  one takes a low mag image first, and high mag
  last, else the small carbon contamination
  rectangle will sit in the middle of the low mag
  image, not desirable.

21
Applications-for optical microscopy where no
SEM-BSE available

• Visualize the distribution of different phases
  that are otherwise similar, e.g. calcite and
  dolomite 2 feldspars in granite
• Pinpoint the presence and location of small or
  rare minerals in a rock e.g. apatite in granite

Applications where SEM-BSE available

• Determine that different regions of the same
  mineral have a complicated history -- growth
  zonation dissolution surfaces healed cracks,
  e.g. carbonates, quartz, zircon, feldspar

22
CL defects in GaAs
These and the following CL images are
mono-chromatic only the total light intensity at
each pixel is recorded by a photomultiplier.
This is a common (simple/cheap) attachment for an
SEM or microprobe.
GaAs on Si for optoelectronic devices can have
defects due to lattice mismatch between the film
and Si substrate. The defects are not seen in SE
image (top left). However, a CL image (bottom
left) shows the areas of reduced strain, where a
monochronometer collected 800 nm light. The right
figure shows the CL spectra of strained (top) vs
unstrained (bottom) material.
Peter Heard, 1996, Cathodoluminescence--Interestin
g phenomenon or useful technique? Microscopy and
Analysis, January, p. 25-27.
23
CL quartz, zircon
CL
BSE
CL

• Images acquired with the Cameca CL (PM) detector.
  Left quartz from Skye with complex history of
  growth or re-equilibration with hydrothermal
  system. Trace amounts of Al, Ti or Mn may be
  involved. Right CL image of zircon from
  Yellowstone tuff (false color) adjacent BSE
  image (no zonation obvious).

(from research of Valley, and Bindeman and Valley)
24
Using CL to Provide Necessary Details to Explain
SIMS Isotope Data

• Here is a sample of an olivine rich chondrule
  from the Semarkona meteorite which contains
  unusual forsterite grains (dark grey BSE) that
  show both blue and red CL, enclosed in Al,
  Ca-rich glass (light grey BSE). Blue CL olivine
  at the rim is depleted in FeO (Fogt99.5) and
  enriched and enriched in refractory elements such
  as Al and Ca, while red CL olivine at core is
  slightly FeO-richer (Folt99.5). Oxygen three
  isotope compositions of blue forsterite and glass
  are enriched in 16O relative to red forsterite.
• Note the glass emits green CL.

CL
Kita et al. (2007b) LPSC Abstract 1791
25
Carbonate CL with SEMusing filters
CL

• One problem with some materials is that the
  luminescence has a finite persistence, so that a
  typical SEM scan causing streaking, as shown
  above left when dolomite (CaMg-carbonate) is
  imaged by SEM-CL (using no filter). Reed and
  Milliken showed that dolomite emits CL in 2 broad
  bands, one red and the other UV-violet, and that
  using a blue (transmission) filter elimated the
  red light which was causing the streaking. The
  above right image was acquired with a UV-blue
  filter. (20 kV)

CL
Reed and Milliken, 2003
26
Trace elements quenching CL
Y
P
Th
U
CL

• Sector zoning in Yellowstone Pre-LCT zircon
• Bright CL high P (and lower Y, U and Th)
• Dark CL high Y, U, and Th (compared to Bright
  CL area)

CL
Fournelle et al AGU 2000
27
Sample Prep Epoxy and Abrasive

• Many (all?) epoxies produce CL in color, dull
  green or blue. There are some minerals where any
  CL is totally quenched, and in grain mounts, the
  crystal is black CL surrounded by brighter epoxy!
• Common lapping and polishing abrasives (diamond,
  alumina, silicon carbide) emit CL! The sample
  must be well cleaned by ultrasonic treatment
  prior to examination
• Uncovered common glass microscope slides emit a
  dull blue CL

CL
CL
In this false-colored monochrome image, the epoxy
is emitting significant CL, intermediate in
intensity to the zones in the zircon!
28
Cathodoluminescence image of zircon from Yakutian
kimberlite (Russia) shows inherited core with
thin oscillatory zoning overgrown by a
homogeneous rim.
CL
CL
Found on Web Source, Bill Griffin, Elena
Belousova funded by Rio Tinto, BHP, Macquarie
University
29

• CL in common minerals -1

• Class I Native elements
• Diamond
• Class II Sulfides
• Sphalerite ZnS (important in phosphor industry)
• Cinnabar HgS
• Realgar AsS
• Class III Oxides
• Periclase MgO
• Spinel MgAl2O4 (synthetic only)
• Corundum, Ruby, Sapphire Al2O3
• Cassiterite SnO2

CL
CL
Short list. Complete list in Marshall 1988
30

• CL in common minerals -2

• Class IV Halides
• Halite NaCl
• Fluorite CaF2
• Class V Carbonates
• Calcite and Aragonite CaCO3
• Rhodochrosite MnCO3
• Witherite BaCO3
• Strontianite SrCO3
• Cerussite PbCO3

CL
CL
Short list. Complete list in Marshall 1988
31

• CL in common minerals -3

• Class VI Sulfates, Tungstates
• Barites BaSO4
• Anhydrite CaSO4
• Gypsum CaSO4-2H2O
• Scheelite CaWO4 (a common EPMA focus mineral)
• Class VII Phosphates
• Apatite (Ca5(P)4)3(F,Cl,OH)
• Class VIII Silicates
• Quartz, Chalcedony, Tridymite, Cristobalite SiO2
• Feldspars

CL
CL
Short list. Complete list in Marshall 1988
32

• CL in common minerals - 4

• Class VIII Silicates-continued
• Scapolite
• Kaolinite
• Serpentine
• Muscovite
• Tremolite
• Spodumene
• Wollastonite
• Benitoite BaTiSi3O9 - another EPMA bright light
• Beryl

CL
CL
Short list. Complete list in Marshall 1988
33

• CL in common minerals - 5

• Class VIII Silicates-continued
• Cordierite
• Epidote
• Olivine (Fe-free)
• Andalusite, Sillimanite, Kyanite Al2SiO5
• Garnets - limited sightings
• Zircon

CL
CL
Short list. Complete list in Marshall 1988