Scattering Techniques and Geometries: SSRL Beamlines...Grazing incidence geometry is typically used...
Transcript of Scattering Techniques and Geometries: SSRL Beamlines...Grazing incidence geometry is typically used...
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Scattering Techniques and Geometries:
SSRL Beamlines
Kevin H. Stone
2018 SSRL X-Ray Scattering School
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Motivation
• X-Rays are an excellent tool for studying the properties of
a material.
• Intrinsic material properties are not always sufficient
• Texture
• Micro- or nano-scale morphology
• Material properties/structure may change depending on
the system into which they are incorporated
Need to design the experiment to the sample
Goal: Encourage careful thought regarding
experimental design
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Outline
• The Basic Scattering Experiment
• Small Angle Scattering
• Reflectivity
• Powder Diffraction
• Thin Film Diffraction
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Basic Scattering Experiment
Scattering Object (Sample)
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Basic Scattering Experiment
Scattering Object (Sample)
Incident X-Ray
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Basic Scattering Experiment
Scattering Object (Sample)
Incident X-Ray
Scattered X-Rays
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Basic Scattering Experiment
Scattering Object (Sample)
Incident X-Ray
Scattered X-Rays
What is the scattered intensity as a function
of scattering direction from an object?
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Basic Scattering Experiment
Scattering Object (Sample)
Incident X-Ray
Scattered X-Rays
How does the scattering change as we
reorient the object?
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Basic Scattering Experiment
Scattering Object (Sample)Incident X-Ray
Scattered X-Rays
𝑘0
𝑘𝑓
Momentum Transfer:
𝑄 = 𝑘𝑓 − 𝑘0
2θ
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Basic Scattering Experiment
𝑘0
𝑘𝑓
Momentum Transfer:
𝑄 = 𝑘𝑓 − 𝑘0
2θ
𝑘 =2𝜋
𝜆
𝑄
𝑘0
𝑘𝑓
θ
𝑄
sin 𝜃 =1
2𝑄 ÷ 𝑘𝑓 sin 𝜃 =
1
2𝑄 ÷2𝜋
𝜆
sin 𝜃 =1
2𝑄 ×𝜆
2𝜋
𝑄 =4𝜋 sin 𝜃
𝜆All we care about is 𝐼 𝑄
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Choosing the Appropriate Length Scale
USAXS SAXS WAXS
Q is inversely
proportional to the
length scale being
probed.
Start by asking at
what length scale do
we want to study our
sample?
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Choosing the Appropriate Geometry
Sample Preparation:
Single Crystals Multiply Twinned or
Strongly Oriented
Crystallites
Polycrystalline
(Powder) Sample
Sample:
Diffraction:
David, W. I. F.; Shankland, K.; McCusker, L. B.; Baerlocher, C. Structure Determination from Powder Diffraction Data; IUCr, Oxford
Science Publications, (2002).
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Choosing the Appropriate Technique
• Small Angle X-Ray Scattering (SAXS)
- Probes structure at the length scale of 1-100nm
• Reflectivity
- Probes layered structure at the length scale of Å to nm’s
• Powder Diffraction (WAXS)
- Probes structure at atomic length scales (Å)
• Thin Film Diffraction
• Polycrystalline
- Probes structure at atomic length scales
• Epitaxial
- Probes structure at atomic length scales, including surface and
interfacial structures
Terminology can be somewhat fluid…
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Choosing the Appropriate Beamline
Beamline BL1-5 BL2-1 BL7-2, BL10-2 BL11-3
Methods •Thin Films
•Real Time
Experiments
•Solution Phase
•Transmission
•Powders
•Thin Films
•Reflectivity
•θ-2θ
•Anomalous
diffraction
•Single crystals
•Grazing-incidence
•Anomalous
diffraction
•Surface studies
•Thin films
•Texture
•Real time
experiments
•Polycrystalline, small
grains
Detectors Area Area, Point Area, Point Area
Advantages •Fast measurement
•Looks at large
features
•Variable Energy
•Lowest Background
•High resolution
•Accurate peak
position and
shape
•Weak peaks
•Variable energy
•High resolution
•Accurate peak
position and shape
•Weak peaks
•Variable energy
•6/4 degrees of
motion
•Fast measurement
•Collect (nearly)
whole pattern
Disadvantages •Small q range
•Background
Sensitive
•Difficult
Interpretation
•Small q range
•Background
Sensitive
•Difficult
Interpretation
•Slow
•Can be difficult to
find textured peaks
•Complicated
•Fixed wavelength
•Low resolution
•Peak shape and
position less accurate
•Weak peaks difficult
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Beamline 1-5
Detector
BeamstopEnergy Range: 4-20keV
Low Q Limit: ~0.0009 Å-1 (d~130nm)
Detector: Fast CCD
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Beamline 11-3
DetectorBeamstop
Energy Range: 12.7keV (fixed energy)
Detector: Fast CCD
Sample-Detector Distance: 150-400mm
Beamstop is positioned close to the
sample
Gives low background scattering at
cost of lowest angles
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Diffractometer Beamlines (2-1, 7-2, 10-2)
Set of rotation stages with
a common center of
rotation
https://www.bruker.com/products/x-ray-diffraction-and-
elemental-analysis/x-ray-diffraction/d8-
advance/eco.html
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Diffractometer Beamlines (2-1, 7-2, 10-2)
Set of rotation stages with
a common center of
rotation
Sample
Detector
Typically referred to by
the total number of
rotations, e.g. 2-circle,
4-circle, 6-circle
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Diffractometer Beamlines (2-1, 7-2, 10-2)
4-circle
5-30keV energy range
6-circle
5-18keV energy range
2-circle
5-17keV energy range
Can be configured with either a point detector (PMT with analyzer crystal or Vortex)
or small area detectors (Pilatus 100K or 300K-W)
Number of in-situ chambers are compatible with all instruments, some are more
beamline specific
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Practical Considerations – Data Collection
Beamline Configuration:
Small Area Detector (Pilatus):
Reduced angular resolution (~0.01º)
Strong signal
Fast, parallel detection (many pixels)
Images need to be stitched together and integrated
No background discrimination, can have very high backgrounds
Low Resolution (Soller Slits):
Reduced angular resolution (~0.01°)
Strong signal into detector
No background discrimination (reduces signal to background)
Choice of PMT or Vortex detector
High Resolution (Analyzer Crystal):
Highest angular resolution
Reduced signal into detector
Good signal to background (can compensate for reduced total signal)
Use PMT detector only
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Practical Considerations – Data Collection
Beamline Configuration:
Small Area Detector (Pilatus):
Reduced angular resolution (~0.01º)
Strong signal
Fast, parallel detection (many pixels)
Images need to be stitched together and integrated
No background discrimination, can have very high backgrounds
Low Resolution (Soller Slits):
Reduced angular resolution (~0.01°)
Strong signal into detector
No background discrimination (reduces signal to background)
Choice of PMT or Vortex detector
High Resolution (Analyzer Crystal):
Highest angular resolution
Reduced signal into detector
Good signal to background (can compensate for reduced total signal)
Use PMT detector only
Measures Position
Measures Angle
Measures Angle
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Practical Considerations – Data Collection
Resolution:What is the time tradeoff?
Some back of the envelope calculations:
Measured 2-theta range: 5 – 65
Assuming we count for 1 second per point, how long will this take with different
step sizes?
Step Size Total Count Time
4 (Pilatus) 15sec
0.01 1hr 40min
0.005 3hr 20min
0.002 8hr 20min
0.001 (minimum feasible) 16hr 40min
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Practical Considerations – Data Collection
Resolution:What is the time tradeoff?
Some back of the envelope calculations:
Measured 2-theta range: 5 – 65
Assuming we count for 1 second per point, how long will this take with different
step sizes?
Step Size Total Count Time
4 (Pilatus) 15sec
0.01 1hr 40min
0.005 3hr 20min
0.002 8hr 20min
0.001 (minimum feasible) 16hr 40min
Why not always use the Pilatus?
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Pilatus Issues
• Measures position, not angle
• subject to alignment errors
• No energy discrimination
• subject to high fluorescence backgrounds
• Hard to have a constant measurement geometry
• steplike changes in both resolution and signal/background
• Limited resolution
• Increased peak overlap at high angles
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Pilatus Issues
Incident beam
sample
Ideally, each pixel on the detector will correspond to the scattered x-rays
at a given angle from the sample position.
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Pilatus Issues
Incident beam
sample
Ideally, each pixel on the detector will correspond to the scattered x-rays
at a given angle from the sample position.
But the detector will see scattering from anywhere, which may lead to
intensity at a pixel from another angle and position…
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Pilatus Issues
Incident beam
What happens we have a flat sample which is misaligned?
Scattering at the same angle hits at a different point on the detector, we
interpret as being at a different angle.
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Pilatus Issues
Incident beam
What happens we have a flat sample which is misaligned?
Scattering at the same angle hits at a different point on the detector, we
interpret as being at a different angle.
This is not a constant
offset in all angles,
the error will be a
function of scattering
angle.
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Thin Film Geometries
Sample Preparation:
Incident Beam
𝛼
𝛽
𝛼 = 𝛽
Symmetric geometry sometimes
called Bragg-Brentano geometry
Good for thick samples, gives
constant illuminated volume
Cannot satisfy this geometry for all
points on an area detector
simulaneously
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Thin Film Geometries
Sample Preparation:
Incident Beam
𝛼
𝛽
𝛼 ≠ 𝛽
Asymmetric geometry sometimes
called grazing incidence geometry
Good for thin samples, comes with a
number of corrections that need to be
considered
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Thin Film Geometries
Sample Preparation:
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Thin Film Geometries
Incident Beam
Grazing incidence geometry is typically used
Increases volume of sample illuminated → Increases scattering signal
• Large beam footprint (even if tightly focused beam is used)
• Limited penetration into (and thus scattering from) the
substrate → Reduced background
• Can isolate scattering from just the film by operating near the critical
angle or above the critical angle of the substrate
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Thin Film Geometries
Incident Beam
Grazing incidence geometry is typically used
Increases volume of sample illuminated → Increases scattering signal
• Large beam footprint (even if tightly focused beam is used)
• Footprint is projected onto the detector → gives poor scattering
resolution that changes with scattering angle
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Thin Film Geometries
Incident Beam
Grazing incidence geometry is typically used
Increases volume of sample illuminated → Increases scattering signal
• Large beam footprint (even if tightly focused beam is used)
• Footprint is projected onto the detector → gives poor scattering
resolution that changes with scattering angle
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Some Pilatus Data Examples…
2Th Degrees14.51413.51312.51211.51110.5109.598.587.57
Co
un
ts
9,000
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
LaB6 standard plate – 2 sequential images integrated and compared
Leads to steps in the background, this will
be very difficult to properly fit or account
for in a refinement.
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Some Pilatus Data Examples…
Sample for structure solution, capillary geometry (cylindrical symmetry)
Q2.42.221.81.61.41.210.80.60.40.2
Sq
rt(C
ou
nts
)
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
100.00 %
Pilatus data, no beamstop after the sample leads to a very
large air-scatter background.
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Q2.42.221.81.61.41.210.80.60.40.2
Sq
rt(C
ou
nts
)
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
100.00 %100.00 %
Some Pilatus Data Examples…
Sample for structure solution, capillary geometry
Analyzer crystal measurement (black) gives a much nicer
signal to background, although the overall signal is much
lower (data shown here is normalized)
This is actually significantly better data!
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Q2.42.221.81.61.41.210.80.60.40.2
Sq
rt(C
ou
nts
)
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
100.00 %100.00 %
Some Pilatus Data Examples…
Sample for structure solution, capillary geometry
Q1.11.091.081.071.061.051.041.031.021.011
Sq
rt(C
ou
nts
)
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
100.00 %100.00 %
Improved resolution shows
a clear splitting of the
lowest angle peaks that
would have been missed
with the Pilatus data
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Analyzer Crystal
• Measures angle
• Not subject to alignment errors
• Accepts only a single energy
• Eliminates high fluorescence backgrounds
• Highest possible resolution
• Separate closely spaced peaks
• Increases information from high angle (overlapped) peaks
• Point detector
• Collecting full data is slow
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Analyzer Crystal
Incident beam
sample
Only x-rays that are incident on the analyzer at an angle/energy that satisfies
the Bragg condition will diffract to scatter into the detector.
𝜆 = 2𝑑 sin 𝜃
Only select
wavelength
Only select
angle
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Analyzer Crystal
Incident beam
sample
Fairly insensitive to misalignment – just hits a different spot on the analyzer
and is diffracted into a different area on the detector
Can use slits to limit the accepted volume if desired (can cut down on air
scatter)
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Analyzer Crystal
What determines the resolution?
• Rocking (Darwin) width of the crystal
• Use perfectly imperfect crystals
• Can move to higher order peaks to increase resolution
• Energy resolution of the monochromator
• Typically on the order of 10-4 for double bounce Si (111)
• Minimize vertical divergence in the optics
• Can use higher order peaks
• Divergence of the incident x-ray beam
• Want a parallel beam geometry
• Minimize vertical divergence
• Can use paired slits to make the beam more parallel at the cost of
intensity
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Powder XRD Conclusions and Q&A
Think carefully about your experiment to determine the
correct setup.
Analyzer Crystal data is the gold standard, but often too
slow or unnecessary for many experiments
Sample geometry can have a large impact on data quality
when using an area detector
Discuss with your beamline scientist!
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Grazing Incidence – the “Missing Wedge”
44
Q = (4π/λ) sin θ
α ≈ 3°
This is SSRL Beamline 11-3
Grazing incidence chamber
couple with a large area
detector to collect as much of
the complete scattering solid
angle as possible.
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Grazing Incidence – the “Missing Wedge”
45
This is SSRL Beamline 11-3
Grazing incidence chamber
couple with a large area
detector to collect as much of
the complete scattering solid
angle as possible.
𝑄
When incidence and exit
angles are equivalent, 𝑄 is
purely out of plane (𝑄𝑧)
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Grazing Incidence – the “Missing Wedge”
46
This is SSRL Beamline 11-3
Grazing incidence chamber
couple with a large area
detector to collect as much of
the complete scattering solid
angle as possible.
𝑄
When incidence and exit
angles are not equivalent, 𝑄necessarily has some in-plane
component
𝑄𝑥𝑦
𝑄𝑧
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Grazing Incidence – the “Missing Wedge”
47
kin
kout
q=kout-kin
inaccessible q region
Qxy
Qz
Baker et al., Langmuir 2010, 26, 9146
Rivnay, Salleo, Toney, Chem Revs 112, 5488 (2012)
mainly edge-on mixed orientation
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Grazing Incidence – the “Missing Wedge”
48
kin
kout
q=kout-kin
inaccessible q region
Qxy
Qz
Baker et al., Langmuir 2010, 26, 9146
Rivnay, Salleo, Toney, Chem Revs 112, 5488 (2012)
Grazing incidence geometry will
always have an inaccessible
region in Q-space
Specular measurements are
needed to fill in this missing area
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Thin Film Geometries
General Considerations:
• Weakly scattering or extremely thin films
• Work at very grazing incidence to enhance film signal and reduce substrate
signal
• Typically near critical angle, ~0.1º incidence angle
• Strongly scattering or thicker films
• Work at moderate incidence angles to balance resolution and
signal/background ratio
• Typically incidence angle of a couple degrees
Decide what is best for your samples and what you are trying to learn
May need to measure at multiple incidence angles or include a specular
measurement to fill in the “missing wedge”
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X-ray Reflectivity
X-ray Reflectivity (XRR): surface-sensitive technique to
characterize surfaces, thin films and multilayers.
𝜃𝑖 = 𝜃𝑟=θ 𝑞𝑧 = 𝑘𝑟 − 𝑘𝑖 =4𝜋
𝜆sin 𝜃
𝑅 𝑞𝑧 =𝐼(𝜃)
𝐼0
ThicknessElectron Density
Roughness
o Black: Bare Substrate
o Blue: Film on Substrate
50
Total External
Reflection
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51
Reflectivity
Incident Beam
V0
Slits
∆𝑄 =2𝜋
𝑑
Q (Å-1)
Re
fle
ctivity (
a.u
.)
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52
Reflectivity
Incident Beam
V0
Slits
Model Refinement Depth Resolved
Scattering
Length Density
Q (Å-1)
Re
fle
ctivity (
a.u
.)
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53
Reflectivity
Reflectivity is used to characterize out-of-plane structure in layered materials
Can give very accurate thicknesses and good estimate of interface roughness
- Cannot distinguish between conformal roughness and interdiffusion
Requires highly smooth and flat samples, typically with lengths along the
beam direction of a few mm or more
If the sample does not look mirrorlike, it is unlikely to work for XRR
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CTR – Basic Diffraction Theory
Consider the scattering of x-rays from an isolated electron:
𝐸 = 𝐸0𝑒𝑖(𝑘0∙ 𝑥 −𝜔𝑡)
Incident Plane Wave 𝐸 =𝑞2𝐸0𝑚𝑐2𝑅
𝑒𝑖(𝑘0∙𝑅 −𝜔𝑡)
“Spherically Symmetric” Scattered Wave
(assuming σ-polarization)
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CTR – Basic Diffraction Theory
Consider the scattering of x-rays from a collection
of electrons, e.g. an atom:
𝐸 = 𝐸0𝑒𝑖(𝑘0∙ 𝑥 −𝜔𝑡)
Incident Plane Wave
𝐸 =𝑞2𝐸0𝑚𝑐2𝑅
𝑒𝑖(𝑘0∙𝑅 −𝜔𝑡) 𝑒 2𝜋𝑖 𝜆 𝑘−𝑘0 ∙ 𝑟𝜌( 𝑟) 𝑑𝑉
(still assuming σ-polarization)
𝑘0
𝑘
Distribution of charge within the atom ≡ 𝑓
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CTR – Basic Diffraction Theory
Consider a 2-D plane of regularly spaced atoms
Generalize to a non-trivial unit cell
𝐸 =𝑞2𝐸0𝑚𝑐2𝑅
𝑒−𝑖𝜔𝑡𝑒𝑖𝑘0∙𝑅
𝑗
𝑓𝑗𝑒−𝑖𝑄∙𝑟𝑗
Sum over atoms in
unit cell
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CTR – Basic Diffraction Theory
Consider a 2-D plane of regularly spaced atoms
𝐸 =𝑞2𝐸0𝑚𝑐2𝑅
𝑒−𝑖𝜔𝑡𝑒𝑖𝑘0∙𝑅
𝑗
𝑓𝑗𝑒−𝑖𝑄∙𝑟𝑗
𝑛1=0
𝑁1
𝑒−𝑖𝑄∙𝑛1𝑎
𝑛2=0
𝑁2
𝑒−𝑖𝑄∙𝑛2𝑏
𝑛3=0
𝑁3
𝑒−𝑖𝑄∙𝑛3 𝑐
Sum over atoms in
unit cell
Sum over unit cells along lattice
directions
Structure Factor - F
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CTR – Basic Diffraction Theory
Consider a 2-D plane of regularly spaced atoms
𝐸 =𝑞2𝐸0𝑚𝑐2𝑅
𝑒−𝑖𝜔𝑡𝑒𝑖𝑘0∙𝑅
𝑗
𝑓𝑗𝑒−𝑖𝑄∙𝑟𝑗
𝑛1=0
𝑁1
𝑒−𝑖𝑄∙𝑛1𝑎
𝑛2=0
𝑁2
𝑒−𝑖𝑄∙𝑛2𝑏
Sum over atoms in
unit cell
Sum over unit cells
in the plane
Sums over lattice directions take the form of a geometric sum:
𝑆 =
𝑘=0
𝑁
𝑎𝑟𝑘 = 𝑎 + 𝑎𝑟 + 𝑎𝑟2 +⋯ = 𝑎1 − 𝑟𝑁+1
1 − 𝑟
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CTR – Basic Diffraction Theory
Consider a 2-D plane of regularly spaced atoms
𝑛1=0
𝑁1
𝑒−𝑖𝑄∙𝑛1𝑎
Sums over lattice directions take the form of a geometric sum:
𝑆 =
𝑘=0
𝑁
𝑎𝑟𝑘 = 𝑎 + 𝑎𝑟 + 𝑎𝑟2 +⋯ = 𝑎1 − 𝑟𝑁+1
1 − 𝑟
𝑛1=0
𝑁1
[𝑒−𝑖𝑄∙𝑎]𝑛1
lim𝑁→∞𝑎1 − 𝑟𝑁+1
1 − 𝑟=𝑎
1 − 𝑟
1
1 − 𝑒−𝑖𝑄∙𝑎
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CTR – Basic Diffraction Theory
Consider a 2-D plane of regularly spaced atoms
𝐸 =𝑞2𝐸0𝑚𝑐2𝑅
𝑒−𝑖𝜔𝑡𝑒𝑖𝑘0∙𝑅
𝑗
𝑓𝑗𝑒−𝑖𝑄∙𝑟𝑗 ×
1
1 − 𝑒−𝑖𝑄∙𝑎×
1
1 − 𝑒−𝑖𝑄∙𝑏
𝑄𝑦
𝑄𝑥
𝑄𝑧
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CTR – Basic Diffraction Theory
𝐸 =𝑞2𝐸0𝑚𝑐2𝑅
𝑒−𝑖𝜔𝑡𝑒𝑖𝑘0∙𝑅
𝑗
𝑓𝑗𝑒−𝑖𝑄∙𝑟𝑗 ×
1
1 − 𝑒−𝑖𝑄∙𝑎×
1
1 − 𝑒−𝑖𝑄∙𝑏×
1
1 − 𝑒−𝑖𝑄∙ 𝑐
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CTR – Basic Diffraction Theory
𝐸 =𝑞2𝐸0𝑚𝑐2𝑅
𝑒−𝑖𝜔𝑡𝑒𝑖𝑘0∙𝑅
𝑗
𝑓𝑗𝑒−𝑖𝑄∙𝑟𝑗 ×
1
1 − 𝑒−𝑖𝑄∙𝑎×
1
1 − 𝑒−𝑖𝑄∙𝑏×
1
1 − 𝑒−𝑖𝑄∙ 𝑐
𝐸 ∝ 𝐹𝑢.𝑐.1
1 − 𝑒−𝑖𝑄∙ 𝑐
𝑛3=0
𝑁3
𝑒−𝑖𝑄∙𝑛3 𝑐
𝑧 = 0
𝑧
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Thin Film Diffraction (Single Crystal)
Film and Substrate Bragg
peaks will not overlap
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CTR – What We Can Learn
Crystal truncation rod (CTR):
Robinson PRB 33, 3830 (1986)
(00Qz) (20Qz)(-10Qz)
Scattering that arises between bulk Bragg peaks due to the presence of a sharp termination of a crystal (a surface). The scattering is perpendicular to the surface and is a sensitive function of surface/interface structure.
-4 -2 0 2 4101
102
103
104
(20-2) (200) (202)
L or Qz
Inte
nsity
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CTR – Practical Calculations
𝐹𝐶𝑇𝑅 = 𝐹𝑠𝑢𝑟𝑓𝑎𝑐𝑒 + 𝐹𝑏𝑢𝑙𝑘
𝐹𝑏𝑢𝑙𝑘 = 𝐹𝑢.𝑐.1
1 − 𝑒𝑖2𝜋𝑙
𝐹𝑠𝑢𝑟𝑓𝑎𝑐𝑒 =
𝑗
𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑈.𝐶.
𝑓𝑗Θ𝑗𝑒𝑖2𝜋(ℎ𝑥𝑗+𝑘𝑦𝑗+𝑙𝑧𝑗)
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Ideal Truncation
Intercalated atoms
Intercalated and Displacement
Ideal Truncation
Intercalated atoms
Intercalated and Displacement
Crystal Truncation Rod Modelling
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CTR and Films – How to Measure
µ-circle
ν-circle
χ-circle (= ψ)
ϕ-circle
θ-circle (= ω)
δ-circle (= 2θ)
4-circle (or more)
Small area detectors are now common
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CTR- How to Measure
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Examples of CTR Features
Out of plane spacing changes
at the surface
In plane spacing changes at
the surface
Periodicity changes at the
surface
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Surface Reconstructions
Cross section view
Top down view
For viewing reconstructions, we will consider the top down view.
Reconstructions will deal with the 2-dimensional plane groups.
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Surface Reconstructions
(1 x 1) surface reconstruction
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Surface Reconstructions
(2 x 1) surface reconstruction
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Surface Reconstructions
(2 x 2) surface reconstruction
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Surface Reconstructions
c(4 x 2) surface reconstruction
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Surface Reconstructions
c(4 x 2) surface reconstruction
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Surface Reconstructions
c(4 x 2) surface reconstruction
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Surface Reconstructions
c(6 x 2) surface reconstruction
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Surface Reconstructions
(6 x 2) surface reconstruction
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Surface Reconstructions
( 5 x 5)R26.6° surface reconstruction
26.6°
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Identifying Surface Reconstructions or Film Superlattices
h
k Reciprocal SpaceReal Space
Surface reconstructions or film superlattices
give rise to scattering at half order positions,
e.g. (½, ½, ½)
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Thin Film Diffraction and CTR
General Considerations:
• Very sensitive technique for exploring structure in epitaxial films (or relaxed films)
or surface structure of crystals or films
• Requires high quality samples on crystalline substrates
• Alignment is very demanding, do not take shortcuts at this stage
• Surface structures are very sensitive, these should be measured under inert
atmosphere and care should be taken to avoid beam damage
• Should always survey for fractional order rods to confirm presence or absence of
a surface reconstruction
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Other Techniques
Other Experimental Approaches and
Techniques
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Resonant X-Ray Diffraction
X-Ray Diffraction and Crystallography
• Lattice; unit cell size & strain; crystallite orientation
• Phase identification & quantify
• Where are the atoms: atomic/crystal structure
• Grain/crystallite size; defects & disorder
Resonant Diffraction
• Element specific (Cu, Zn, Sn, Se)
• Change in atomic scattering power
• Distinguishes between kesterite and stannite phases
• Quantify anti-site disorder and stoichiometry
)()( 21 EifEfff o
Y. Shi et al. (In Preparation)
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(002)
Resonant Diffraction
Phase Identification - CZTSSe
Resonant elastic X-ray diffraction
Stannite
Qualitatively Different Resonant
Behavior for Different Phases.
Kesterite
Zn Cu
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Resonant X-Ray Diffraction
Use these three peaks for
resonant diffraction
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Resonant X-Ray Diffraction
Use these three peaks for
resonant diffraction
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Resonant X-Ray Diffraction Model
Cu
Zn
Sn
S
2a
2d
2c
2a → Cu + Zn ≤ 1
2d → Cu + Zn ≤ 1
2c → Cu + Zn ≤ 1
Interstitial → Cu + Zn + Sn ≤ 1
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Resonant X-Ray Diffraction Model
Cu
Zn
Sn
S
2a
2d
2c
PositionCu
Occupancy
Zn
OccupancyVacancy
2a (0, 0, 0) 0.90 0.10 0.00
2d (0, 1 2 , 14) 0.50 0.45 0.05
2c (0, 1 2 , 34) 0.45 0.48 0.07
interstitial
( 3 4 , 14 , 18)
0.06 0.12 0.82
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Resonant X-Ray Diffraction Absorption
- d2θ
ω
x
𝐼 = 𝐼0𝑒−𝜇𝐿
𝐿 =𝑥
sin𝜔+
𝑥
sin(2𝜃 − 𝜔)
𝐴𝑏𝑠 𝐶𝑜𝑟𝑟 =
0
𝑑
𝑒−𝜇
𝑥sin 𝜔+
𝑥sin 2𝜃−𝜔 𝑑𝑥
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Resonant Reflectivity
Exploit resonance effects to
determine an element
specific depth profile
Enriched
surface layer
Q (Å-1)
Re
fle
ctivity (
a.u
.)
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Depth Profiling
Control over the incidence angle can give control over the depth of the x-ray probe
This is especially powerful if you can work near or below the critical angle
However, this is quite tricky and should be done carefully!
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In-situ and Operando Techniques
Often we want to characterize a material under a certain set of conditions or as part
of a device while operating
Rapid Thermal Processing for PV applications
Our implementation gives all
of the functionality available
in commercial RTP systems
without compromise for use
on a beamline
Can ramp up to ~1200ºC at
>100ºC per second
Coupled to a fast area
detector (Pilatus) for
collection of XRD data
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In-situ and Operando Techniques
Often we want to characterize a material under a certain set of conditions or as part
of a device while operating
MAPbI3 undergoes phase transitions:
• cubic for >~ 335K (60 C)
• tetragonal ~335 K (60 C)
• orthorhombic below ~165 K
What happens when solar cells get hot?
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In-situ and Operando Techniques
X-rays
Heating Tape
Light source (blocked)
X-ray spot
Measured device
Thermocouple
Operando chamber:
• White LED light (~ 1 Sun)
• Kapton heating tape
• Thermocouple on top of device
• He environment
• IV curves with light
• 1D and 2D XRD at temperature
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In-situ and Operando Techniques
Often we want to characterize a material under a certain set of conditions or as part
of a device while operating
If you want to measure your sample under a specific set of conditions, talk to
the beamline scientists – There is a good chance that we can help figure out
how
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SSRL Overview
5 Main Scattering Beamlines for Material Science
BL1-5 SAXS/WAXS
BL2-1 Powder XRD (High Res. and Low Res. Setups), Reflectivity
BL7-2 6-Circle Diffractometer, Surface and Thin Film Diffraction, Powder Diffraction
BL10-2 4-Circle Diffractometer, Surface and Thin Film Diffraction, Powder Diffraction
BL11-3 Large Area Detector for Powder Diffraction and GI Measurements
SSRL proposals are not beamline
specific. A single proposal can get you
time on any of the Materials Scattering
or Spectroscopy beamlines.
Most beamlines are easy to configure to
your needs, we are happy to work with
you on the best setup for your
experiments.
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Acknowledgements
Mike Toney and his group
Laura Schelhas
Chris Tassone
and his group
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Questions
Find the measurement approach and geometry
that works best for your sample, not necessarily
what makes the measurement easier
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Ewald Construction
Incident X-ray Transmitted X-ray
𝑟 = 1 𝜆
Origin
𝑄 = 0
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Ewald Construction
Incident X-ray
𝑟 = 1 𝜆
Reciprocal Lattice
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Ewald Construction
Incident X-ray
𝑟 = 1 𝜆
Reciprocal Lattice
Observable features in
reciprocal space are those
with intersect the
Ewald Sphere
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Ewald Construction
Incident X-ray
𝑟 = 1 𝜆
Reciprocal Lattice
Observable features in
reciprocal space are those
with intersect the
Ewald Sphere
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Ewald Construction
Incident X-ray
𝑟 = 1 𝜆
Reciprocal Lattice
Observable features in
reciprocal space are those
with intersect the
Ewald Sphere
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Ewald Construction
Incident X-ray
𝑟 = 1 𝜆
Reciprocal Lattice
Observable features in
reciprocal space are those
with intersect the
Ewald Sphere
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Ewald Construction
Incident X-ray
𝑟 = 1 𝜆
Reciprocal Lattice
Observable features in
reciprocal space are those
with intersect the
Ewald Sphere
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Ewald Construction
Incident X-ray
𝑟 = 1 𝜆
Reciprocal Lattice
Effect of broad bandwidth
incident beam
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Ewald Construction
Incident X-ray
𝑟 = 1 𝜆
Reciprocal Lattice
Effect of beam divergence
(poorly defined incidence
angle)
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Summary
• Reciprocal space is a convenient way to think of scattering
• Miller Indices (h,k,l) allow us to orient ourselves and identify features in
reciprocal space from crystalline (ordered) materials
• Ewald Sphere geometric construct for identifying which reciprocal space
features are observable
𝑄 =4𝜋 sin 𝜃
𝜆
𝑄 =2𝜋
𝑑𝑑 =
2𝜋
𝑄