Transcript Micro-XAS and Imaging

Applications of
Microscale XAS and
XAS Imaging
Ben Kocar
Outline
• SSRL Imaging Facilities
• What is micro-XAS?
– Examples
• What is XAS-Imaging?
– Examples
• Practical Aspects…
SSRL
BL 14-3
BL 6-2
BL 10-2
BL 2-3
SSRL Microfocus Beam
Lines
Optics
Spot
Size
Energy
(keV)
Flux
(ph s-1)
mprobe
K-B
2 mm
5-23
4x108
6-2
TXM
Cap, ZP
40 nm
5-14
~1012
10-2
mprobe
Cap
10 mm
5-35
6x1010
14-3
mprobe
K-B
2 mm
2-5
~5x109
Beam
Line
Type
2-3
Microfocus Needs
• As a facility, need to address the users
of the Biological, Environmental,
Material Science communities…
–
–
–
–
–
–
Small beam size
Large format – rapid mapping
High resolution mapping
High energy (10 keV +)
Low energy (2-5 keV)
Stability for spectroscopy and diffraction
Microfocus Plan
1) High spatial resolution, good
spectroscopy/diffraction (BL 2-3)
2) Moderate resolution, high flux, rapid
scanning, spectroscopy, ideal
diffraction (BL 10-2)
3) Low energy, high spatial resolution,
good spectroscopy (BL 14-3)
BL2
Si DCM
1.3 T
Bend Magnet
BL 2-3 Focusing Optics
• The use of K-B optics, imaging
the SPEAR source, was
chosen to meet needs
– Achromatic for spectroscopy
– “Reasonable” working distance
Beam
Line
Camera:
Sample camera mounted
above sample using a
Mylar mirror to get true
view of sample – no
parallax effects.
Gasbox:
Optic hardware enclosed in He
filled box to increase mirror
coating lifetime and reduce gas
absorption effects.
2 x 2 mm
CCD:
0.3 x 0.3 mm
Area detector to measure
micro-diffraction patterns
at each sample spot. Data
collection is integrated
into scanning software.
Sample stage has X-Y-Z
capability and 50nm
repeatability. Motion of
stage is programmable to
enable correlated axis
moves and digital
programmable output to
gate detectors for each
pixel element moved.
Range of motion ± 25 mm.
Working distance from
gasbox to sample is ~30
mm
2 x 15 mm
KBs:
Fluorescence:
3-element monolith
detector with small snout
in order to get close to
sample + 1-element Si for
low Z detection
Horizontal and vertical
focusing mirrors (Pt or Rd
coated) focus beam at
optimal position to 2 x 2
mm. Micro ion chamber in
design developed to
measure true I0 after
mirrors.
Slits:
Entrance slits define
entrance aperture at
~ 0.3 x 0.3 mm
14-3 Design Parameters
Small beam (~2 mm)
Low energy (S & P edge)
XRF
Need to be able to do S/P spectroscopy
Sample mount interchangeability with
BL 2-3
• Same software/hardware as BL 2-3
•
•
•
•
•
BL14
Torroidal
M1 mirror
Si DCM
Collimating
M0 mirror
1.3 T
Bend Magnet
BL 14-3 Layout
•
•
•
The KB optics will be a permanent installation in the back of the
hutch with a temperature controlled enclosure, precision sample
stages, solid-state detector, etc.
Virtual source with KB optics. By slitting down virtual source, flux
is traded for spot size - capable of sub-micron spots
The front table will be used for bulk XAS and possibly singlecrystal polarized XAS
~3m long x-ray hutch
Slit
(S3)
Fluorescence
detector
x-rays
Unfocused
XAS
Optical bench
Sample
stage
KBoptics
Optical bench
2m
What is microXAS?
• MicroXAS is “simply” doing XAS measurements on
a micron-size spatial scale using focusing optics to
obtain a small beam size as opposed to “big” beams
that give you bulk XAS.
Why use a microprobe?
•
The environment (chemistry, biology, geology, etc.) is
inhomogenous at nearly all length scales
•
For every system, there is an optimal length scale for the
information you want:
– Too small:
– Too big :
Unrepresentative sampling
Miss details, small objects
•
This scale is often between 1 and 100 microns for
environmental samples
•
Want to gather chemical and spatial information on these
scales…
X-ray Microprobe
(mXRF, mXAS)
• Raster a focused x-ray beam over sample
• Map intensity of x-ray fluorescence over
various parts of sample
• Characterize interesting spots with XAS
12500
transmission
scatter
10000
Counts
beam
7500
5000
2500
0
0
2
4
6
Energy (keV)
fluorescence
8
10
Microprobe Advantages
• Separate complex
mixtures using spatial
segregation
20 mm
1.5
m(E)
1.0
1 mm
0.5
0.0
9600
9650
9700
Energy (eV)
9750
9800
Microprobe Advantages
• Better signal to noise ratios on small particles
I/I0
I/I0
E
What’s so special about
an X-ray microprobe?
• Electron microscopy and microprobes
are “standard” and can do imaging and
elemental composition…
• Electron Microscopy
– Great spatial
resolution and
can use X-EDS
for composition
– Needs vacuum
sample
environment
– Needs coated
samples for
conductivity or
very thin slices
• X-ray Microprobe
– Resolution limited.
– 500 nm resolution
with mirrors, 20
nm with zone
plates
– Can collect data in
ambient conditions
– Can perform reactions
in situ
– Sample prep relatively
easy
– Chemical information!
Micro-XAS and Imaging
• On small, discrete points:
– XANES
– EXAFS
• On “larger” maps:
– Fluorescence composition maps
– Elemental correlations
Microscale Zn XAS
• Quartz grains incubated in Zn-Mn contaminated stream
bed (Pinal Creek)
• Biogenic Mn oxides show
significant uptake of Zn
20 mm
– Bulk EXAFS shows Zn sorbed
to birnessite
• Not all uptake is reversible
600
Zn Counts
500
Si
400
300
200
100
Zn
Mn
0
0
1000
Mn Counts
2000
Microscale Zn XANES
1.5
Zn in biogenic Mn oxides
m(E)
1.0
0.5
20 mm
0.0
9600
9650
9700
9750
9800
Energy (eV)
Zn in hetaerolite (ZnMn2O4)
1.5
Si
m(E)
1.0
0.5
Zn
Mn
0.0
9600
9650
9700
Energy (eV)
9750
9800
Araneus diadematus Fangs
and Marginal Teeth
Fang 3
Fang 7
Fang2_init_14000_001
I1
Mn
Zn
Fang 2 Teeth, Mn XANES
Fang7_teeth_only_10000_001
Zn
I1
7
6
5
3
4
C
B
2
1
A
Mn
Fang 7 Tooth A, Mn XANES
Fang 7 Tooth B, Mn XANES
Micro-XAS and Imaging
• On small, discrete points:
– XANES
– EXAFS
• On “larger” maps:
– Fluorescence composition maps
– Elemental correlations
– XANES oxidation state/species maps
What is XAS Imaging?
• XAS Imaging is taking several XRF maps at various
excitation energies across and absorption edge.
3.5
As2S2
As(III)aq
As(V)aq
3.0
mtrans
2.5
2.0
1.5
1.0
0.5
0.0
11850
11875
• Examining the fluorescence
yield at these energies can
differentiate the oxidation
state or species at every
pixel in the image.
11900
Energy (eV)
Arsenite
As(SR3)
Arsenate
Pickering, et al. (2006) ES&T, 40, 5010-5014.
Why do XAS Imaging???
• A
• picture
A picture
is worth
is worth
a approximately
a thousand words
slogan from Fred R. Barnard, 1921.
13,477 squigglyAdvertising
lines…
Not really an ancient Chinese proverb…
WhenWhy
you show
couldthis?
have this!
ZVI
GreenRust
Mackinawite
XAS Image Fitting
• N species to calculate
• Need N+1 mapped energies to have
statistical weight
1.5
FHY FHY 2-line ferrihydrite
MACKMACK Mackinawite (FeS)
ZVI_NR
Unreacted ZVI (Feo, FeO)
ZVI_NR
SID SID Siderite (FeCO3)
GRCO3
GRCO3
Carbonate Green Rust
1
0.5
0
7100
7110
7120
7130
eV
7140
7150
XAS Image Fitting
• N species to calculate
• Need N+1 mapped energies to have
statistical weight
7120
7113
7122
7124
7126
7129
• Do least squares fitting at each pixel
– n energies and m components
Find f such that
you minimize:
Mf  D
2
 m1,1
m
1, 2
M 
 

 m1,n
m 2 , 1  m m , 1 

m 2 , 2




m m , n 
 c1 
c 
f  2

 
 cm 
 m1,obs 
m 
2 ,obs 
D
  


m
n
,
obs


7140
XAS Image Fitting
• Essentially doing least square fitting on a
shortened XANES spectrum
1.2
7113
1.0
7120
mu
7122
0.8
0.6
7124
0.4
7126
0.2
7129
0.0
7100
7140
7120
7140
eV
7160
XAS Image Fitting
• N species to calculate
• Need N+1 mapped energies to have
statistical weight
7120
7113
7122
7124
7126
7129
7140
• Do least squares fitting at each pixel
ZVI
Mackinawite
Ferihydrite
Siderite
Green Rust
Vivianite
As in Bluegrass Roots
3.5
As2S2
As(III)aq
As(V)aq
3.0
mtrans
2.5
2.0
1.5
1.0
0.5
0.0
11850
11875
Energy (eV)
11900
As in Bluegrass Roots
3.5
As2S2
As(III)aq
As(V)aq
3.0
mtrans
2.5
2.0
1.5
1.0
0.5
0.0
11850
11875
Energy (eV)
11900
PRB Uranium Remediation
•
1997 Permeable reactive barriers
consist of:
–
–
–
Zero Valent Iron (ZVI)
Bone Char (PO4)
Amorphous Fe Oxides (AFO)
ZVI GR MACK
MACK GR U
ZVI Sid Fhy
U(IV) U(VI)
ZVI GR SID
ZVI SID Ca
MACK GR FHY
U(VI) U(IV) Ca
A Few Practical Aspects
• All of the principles of bulk XAS
apply to microXAS
–
–
–
–
Thickness
Fluorescence self-absorption
Sample damage
Stability
• Sample prep, sample prep, sample
prep…
Sample
design considerations
Sample
Design
Cooling for rad damage
Considerations
Mechanical stability
Resins or fixatives
• Mechanical Stability
– Will sample motion or drift cause problems
Se on goethite
(D. Strawn)
• Uniformity/Topography
st
No resin, 1th scan
No resin,st 4 scan
Resin, 1 scan
th
Resin, 4 scan
9500
9600
9700
9800
9900
10000
10100
E
10200
– Do you need a constant thickness?
– Will shadows of topography create artifacts?
• Substrate/Background
Substrate– Fluorescence from the substrate?
Uniformity
Topography
Background
• Glass microscope slides have major As & Co
• Transmission?
E-12660eV
– Need to have beam go through sample?
Transmission?
• Resins or fixatives
Overabsorption
– ImportantDiffraction
to use the right type
• Cooling for radiation damage?
– Do you want to look at ice?
Fe
As
SamplePreparation
Preparation
Sample
Intact specimen
+ Simple, authentic
- Topography
Need to fix
Easily damaged
Drifts
Not durable
Thin section
+ Smooth surface
Good for quantitative
Avoid overabsorption
Avoid penetration effects
Images easy to interpret
Maintains spatial relationships
May be able to use
in transmission
- Resin effects on chemistry
Can accelerate rad damage
Requires work, time ($)
Careful about choice of
substrates
Powder/sediment
on tape
+ Easy
- Nonuniform thickness
Tends to drift
Kapton tape scatters
Not durable
Grid/filter
+ Often only way to collect
- Nonuniform thickness
Substrate scatter/fluorescence
Important Sample Preparation Methods to Conduct
µ-XRF, -XAS, and –XRD
(Courtesy Y. Arai)
Substrates (backing materials) for thin section prep:
•
Absolutely “no glass slides”!
•
Quartz slides are recommended.
•
Lexan or Delrin
-Very low trace metal (e.g., Zn and Cu) impurity, but abundant in Br.
-Transmit the beam so that the energy calibration can be done without talking out the sample.
- does not scatter as much as glass, so that it is suited for trans. micro-XRD measurements.
*each batch of lexan sheet must be checked with BL scientist for the trace metal impurity.
Ideal resin medium:
Important criteria:
-Must produce a certain hardness so that the surface can be polished down less than 30
micron. Over absorption issues when the thickness of sample is too thick (e.g.,
entrapped grains in kapton films)
•
3M electrical resin (Lowest trace metal impurity in the market)
(cures well about 70oC , but it can be cured at room temp for several days)
•
For redox sensitive sample, Epotek 301 (cures at room temp. )
Warning for the low viscous medium!
-LR white resin is not recommended due to the redox changes in chemical
Properties
-Spurr's resin (Spurr, 1969) and polymerized at 70°C for 24 h. It is not clean media
Bonding adhesive:
-Superglue is highly recommended due to its low trace metal impurity.
Requirements for XAS
Imaging
• Differentiable species!
– XAS Imaging uses N+1 variables
– Need to accurately determine N species with N+1
variables
– Need variation, ideally within the absorption edge
Ideal
Good
Non-Ideal / Impossible
1.5
1.5
3.5
FHY
As2S2
As(III)aq
As(V)aq
3.0
ZVI_NR
1.0
SID
1
GRCO3
2.0
1.5
FHY
MACK
ZVI_NR
SID
GRCO3
0.5
1.0
0.5
0.0
11850
11875
Energy (eV)
11900
0
7100
mu
mtrans
2.5
MACK
Ferihydrite
Hematite
Lepidocrocite
Goethiite
0.5
0.0
7110
7120
7130
eV
7140
7150
7100
7120
7140
eV
7160
Sample Holders
• How things mount?
• Standard, thin section, films
Sample Holders
• How things mount?
• Standard, thin section, films
Sample Holders
• Stage and sample holder have magnetic mounts
• Keeps sample repositioning accurate within
approximately 10-20 microns
• Not a lot of room in sample area!
SSRL Microfocus Beam
Lines
Optics
Spot
Size
Energy
(keV)
Flux
(ph s-1)
mprobe
K-B
2 mm
5-23
4x108
6-2
TXM
Cap, ZP
40 nm
5-14
~1012
10-2
mprobe
Cap
10 mm
5-35
6x1010
14-3
mprobe
K-B
2 mm
2-5
~5x109
Beam
Line
Type
2-3
Acknowledgements
•
•
•
•
Chris Fuller
Robert Scott
Yuji Arai
Kaye Savage
• SSRL BL 2-3 Microprobe funding from
DOE-BER