Imaging basics (X

2014/7/3
Imaging basics
(X-ray microscopy)
Der-Hsin Wei
07/03/2014
Professions work with images
Astronomer
Engineer/Scientist
Photographer
D. H. Wei
National Synchrotron Radiation Research Center
1
2014/7/3
Images
Astronomy
pixel position; (i, j)
signal intensity; (IR, IG, IB)
Microscopy
Photography
Digital images
D. H. Wei
National Synchrotron Radiation Research Center
Ways of acquiring a digital image
one pixel at a time
all pixels simultaneously
D. H. Wei
National Synchrotron Radiation Research Center
2
2014/7/3
Photon-in, electron-out
SPEM @ 09A1
XPEEM @ 05B2
EPU
e-
Detector
X-ray
Sample
eContrast
aperture
X-ray
Sample
optics
(ZP/OSA)
Electrostatic lens
Image
Multi-channel
detection
XAS-based microscope
(full-field microscope)
XPS-based microscope
(scanning microscoope)
D. H. Wei
National Synchrotron Radiation Research Center
Photon-in, photon-out
To be established at TPS, BL27
Sample
Photon detector
X-ray optics
(Focusing ZP/OSA)
XAS-based microscope (scanning transmission X-ray microscope; STXM)
TLS, BL01B1
CCD
Sample
X-ray optics
(Condenser ZP/OSA)
X-ray optics
(micro ZP)
XAS-based microscope (Full-field transmission X-ray microscope; TXM)
D. H. Wei
National Synchrotron Radiation Research Center
3
2014/7/3
A “good” X-ray micrograph
Regardless of the way images are recorded;
getting more signals for a bright image
enhance the image contrast
improve the resolving power of microscope/camera
D. H. Wei
National Synchrotron Radiation Research Center
I. Ways to increase image brightness
(A) brightness of incoming photon
(B) intensity of outgoing signals
(C) sensitivity of detector
D. H. Wei
(quantum yield)
National Synchrotron Radiation Research Center
4
2014/7/3
(A-1) Synchrotron is a bright photon
source
Fact: Synchrotron facility offers ultra-bright X-rays whose
wavelength spectrum covers a wide spectrum (frequency).
D. H. Wei
National Synchrotron Radiation Research Center
(A-2) Focus all incoming probes into one spot
source
outer most zone
zone plate (ZP)
focused spot
Simple math:
-- transmission efficiency of ZP: 10%
-- de-magnify photon in an area of 100 µm in diameter
into a spot of 100 nm in diameter (intensity :106)
Brightness enhancement: 105 X
D. H. Wei
National Synchrotron Radiation Research Center
5
2014/7/3
(B-1) Signals for X-ray microscopy
Ions and Neutral Atoms
Photoelectrons
Synchrotron
Radiation
Reflected Photons
Fluorescence
Surface Adsorbate
Bulk Crystal
Inelastic Scattering
Defect
Transmitted
Photons
Bragg Diffraction
Laue Diffraction
D. H. Wei
National Synchrotron Radiation Research Center
(B-2) Collect as many outgoing signals as possible
Objective lens (物鏡)
V
eθ θ
e-
higher V
larger numerical aperture
Better resolution
Facts:
-- higher voltage attract more photo-excited electrons
-- possible voltage is limited by the dielectric breakdown
Dielectric breakdown: ~ 3 x 10 6 V/m (air)
~ 40 x 106 V/m (vacuum)
D. H. Wei
National Synchrotron Radiation Research Center
6
2014/7/3
II. Ways to enhance the image contrast
(A) use monochromatic light to “select”
signals to be used for constructing images.
(B) look for the maximum difference among
outgoing signals, and construct images
from it (energy dispersion, etc.).
D. H. Wei
National Synchrotron Radiation Research Center
Which light source to use ?
EM waves to interact with matters, and microscopes
record the maps of signals generated upon EM wave
illumination (photon, electron) .
Depending on its wavelength, an EM wave examines the
matters from different perspectives.
e.g. microwave/far infrared: rotational spectroscopy
infrared: vibrational spectroscopy
from NASA
D. H. Wei
National Synchrotron Radiation Research Center
7
2014/7/3
Images record lateral inhomogeneity of …
Molecular vibration
Four transform infrared (FTIR)
Electronic transition
XPS: core electron
vacuum
XAS: core electron -> empty state
© K. Siegbahn
D. H. Wei
National Synchrotron Radiation Research Center
(A-1) In frequency domain
IRM
李耀昌老師
Through Michelson interferometer, frequency of incoming infrared is first
“encode” through interferometry (no sample).
Same measurement is performed again after inserting the sample.
Difference of the two spectra goes through Fourier transform to find out which
frequency is responsible to the observed difference.
D. H. Wei
National Synchrotron Radiation Research Center
8
2014/7/3
(A-2) In energy domain (XPS)
Fact: each photoelectron carries a specific kinetic energy with it, and obeys
the following relation; K.E. = hv – binding energy
Signals are recorded by a hemi-sphere
analyzer. Only electrons with selected K.E
can be detected.
陳家浩老師
SPEM
D. H. Wei
National Synchrotron Radiation Research Center
(B-1) In energy domain (XAS)
Fact: absorption cross section (and thus the number of electrons excited
by photon) is a function of photon energy
A
Larger photon absorption cross section means
more photo-excited electrons.
B
陳家浩老師
XPEEM/TXM
D. H. Wei
National Synchrotron Radiation Research Center
9
2014/7/3
Photo-excited electron emission
Electron Yield
Inelastic tail (XAS)
Core level (XPS)
Valence band
Auger peak
Electron Kinetic Energy
Secondary electron emission:
-- relatively strong intensity
-- no memory on how the first photo-excitation occurred
-- probing depth: 5 – 10 nm (solid samples)
D. H. Wei
National Synchrotron Radiation Research Center
III. Ways to improve the image resolution
(A) sharpen the tip
(B) reduce the lens aberration
D. H. Wei
National Synchrotron Radiation Research Center
10
2014/7/3
(A) Sharpen the tip (for ZP focus)
Focus the X-rays down to a spot as small as possible
source
outer most zone
zone plate (ZP)
focused spot
Fresnel zone plate (a diffractive element)
Dimension of the focused spot is the diffraction limit
of zone plate.
The diffraction limit of a zone plate is determined by
the width of outer most zone.
(will be covered later by 陳家浩)
http://xdb.lbl.gov/Section4/Sec_4-4.html
D. H. Wei
National Synchrotron Radiation Research Center
(B) Minimize the aberrations
Aperture helps
Signal for resolution
Spherical aberration:
due to the finite size of lens,
the off-axis rays have their
incident angles different from
that of on-axis (near-axis) rays.
Chromatic aberration:
due to the refraction index of
lens is a function of light
frequency, the incident waves will
be focused (bended) differently.
D. H. Wei
National Synchrotron Radiation Research Center
11
2014/7/3
Summary (I)
Tasks toward a high resolution image;
getting more signals for a bright image
synchrotron/undulator
enhance the image contrast
photon-matter interaction; FTIR/XPS/XAS
improve the resolving power of microscope/camera
(i) SPEM/TXM: smaller outer zone width
(ii) XPEEM: high acceleration field/ aperture
(iii) IRM: …
D. H. Wei
National Synchrotron Radiation Research Center
Summary (II)
SPEM/STXM
Scanning microscope
XPEEM/TXM
Full-field microscope
Image acquisition speed
Image resolution
Point analysis
Image analysis
Chemical sensitivity
D. H. Wei
National Synchrotron Radiation Research Center
12
2014/7/3
Wave: amplitude, frequency, phase
amplitude
So far, we focus on how microscopy can use a lens system to
detect the “intensity (amplitude)” of signals.
There is a new branch of X-ray microscopy that uses no lens
and seek into what “phase” of the signals can form images by
“retrieving” the spatial relation between individual objects
through diffraction patterns. This approach is called
“lensless imaging”.
D. H. Wei
National Synchrotron Radiation Research Center
Lensless imaging
In lensless imaging, images are “re-constructed” from
diffraction patterns.
黃玉山老師
http://www.optics.rochester.edu/workgroups/fienup/Manuel/lensless.jpg
D. H. Wei
Lensless imaging
National Synchrotron Radiation Research Center
13
2014/7/3
X-ray photoemission electron
microscopy (XPEEM)
UHV technique
Thin film deposition
MOKE
PEEM
Data analysis
…
(超高真空)
(薄膜蒸鍍)
(磁光效應)
(顯微鏡操作)
(數據分析)
D. H. Wei
National Synchrotron Radiation Research Center
What is XPEEM
XPEEM ≡ X-ray Photoemission Electron Microscopy
• Full field microscope
• Photon-in-electron-out technique using an
electron column to acquire image
High electric field (15 kV)
• Spectromicroscopy (with synchrotron only)
Spectroscopy: XAS
Microscopy: full field image
Contrast aperture
X-ray
Electrostatic Lens
eD. H. Wei
National Synchrotron Radiation Research Center
14
2014/7/3
Acquisition modes
Spectroscopy Mode
Image Mode
(scanning photon energy)
(snap shot)
1 µm
hν
FeNi ring
D. H. Wei
National Synchrotron Radiation Research Center
Contrast mechanisms in XPEEM
Element (SR)
Absorption cross section (Mb)
Topography
7
Fe
6
Co
5
Ni
4
3
2
Cu
1
0
700
750
800
850
900
950
Photon Energy (eV)
Chemical (SR)
Magnetism (SR)
Absorption intensity
Li2MnO3
Intensity
LiMn2O4
MCD intensity
LiMnO2
MnO
632
637
642
647
652
Photon Energy (eV)
30
Ni
20
10
0
5
L3
L2
0
-5
-10
870
850
Photon energy (eV)
890
J. Stöhr
D. H. Wei
National Synchrotron Radiation Research Center
15
2014/7/3
Electron Yield (a.u.)
Brief summary for principle &
instrumentation
Ni
6
Fe
3
500
600
700
800
900
Photon Energy (eV)
e
Full field electrostatic microscope :
Accelerating field
Topography
Photoelectric effect
Work
function
Chemistry
X-ray energy
X-ray polarization
Magnetism
D. H. Wei
X-Rays
25o
National Synchrotron Radiation Research Center
Take home message
Both spectroscopy and microscopy look for “fingerprints” of
something… (Is there an universal microscope?)
Spectroscopy introduced in previous days would promote you
to ask the following question when facing a challenge;
What insights I can learn when looking this particular
question in real-space, momentum-space (k-space), and
frequency-domain?
At the end of today, I hope you will ask one more question
whenever you learn about a new spectroscopy technique;
What benefit can I earn if the spatial information in
included?
D. H. Wei
National Synchrotron Radiation Research Center
16
2014/7/3
同步輻射顯微術
材料分析
魏德新
陳家浩
許瑤真
黃玉山
宋艷芳
D. H. Wei
生醫顯影
李耀昌
賴麗珍
李英玉
National Synchrotron Radiation Research Center
17

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