Cathodoluminescence

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Cathodoluminescence

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Peter Heard, 1996, Cathodoluminescence--Interesting phenomenon or useful technique? ... Found on Web: Source, Bill Griffin, Elena Belousova; funded by Rio Tinto, BHP, ... – PowerPoint PPT presentation

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Title: 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