Transcript EELS

电子能量损失谱
Electron Energy Loss Spectroscopy (EELS)
张 庶
元
入射高能电子与样品的相互作用
Atomic-scale view of electron energy loss in TEM
Incident beam electron
E0 (100 to 1000 keV)
Excited specimen electron
EB + E
Scattered beam electron
E0 - E
3
What is an EELS spectrum?
Elastic scattering
Inelastic scattering
L
L
K
Carbon
K
atom
Electrons count
Zero
loss
CK
1 eV
0
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Electron energy loss (eV)
电子能量损失谱信息
非弹性散射过程:
声子激发
(<0.1eV)
等离子激发
(<30eV)
内壳层电子激发
(＞13eV)
自由电子激发
(二次电子)
(<50eV)
(背底)
韧致辐射 (背底)
∙∙∙ ∙∙∙
根据等离子激发能量的大小，即谱峰的位置，可以确定物质的
种类和他的组成。
Na：
5.70ev(一次激发)
11.4ev(二次激发)
随试样厚度的增加，电子在试样中可能产生二次，甚至多次等离子激
发，其峰位出现在第一次激发峰的两倍或多倍能量的位置。
Al:
14.95ev
29.9ev
44.35ev
59.8ev
表中列出了几种物质的等离子激发峰的理论值和实测值
Specimen thickness measurement
IT 
 ln 

Io 
t
λ 为电子非弹性散射的平均自由程

IT
为第一个等离激发峰的强度
Io
为零损失峰的强度
Rough estimate of λ :
λ ~ 0.8Eo nm
so for 100-keV electrons
λ is 80-120 nm various
materials
内壳层电子激发
偶极跃迁：Δl = ±1
Correlation between EELS and
specimen feature
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Magnetic prism spectrometer
EELS spectrometer
Optical configuration at entrance
Dispersion and focusing section
Projection section
Spectrum plane
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In-column omega-filter
Inserted in the imaging lens system
Energy-filter imaging and electron
diffraction, CBED
Post-column imaging filter
Gatan (Tridiem) imaging filter (GIF).
Attached to the TEM column below the viewing chamber
Energy-loss spectroscopy (EELS - low loss)
 Spectrum is enlarged and optimally coupled to detector
Final EELS
readout
EELS spectrum projected onto CCD
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Energy-loss spectroscopy (EELS - core loss)

The spectrum is shifted

Best to do by changing prism current preserve probe focus
Final EELS
readout
Mn L edge
O K edge
Spectrum offset
via prism current
EELS spectrum projected onto CCD
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EFTEM: Energy Filtered TEM: GIF only



Projection section operates in imaging mode
Spectrum is projected back to an image
Just like forming an image from a diffraction pattern in TEM
Unfiltered image projected
onto CCD detector
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Energy-filtered TEM imaging (EFTEM - core loss)


The spectrum is shifted relative to the slit opening
Best to do by increasing beam energy to preserve image focus
Core-loss image projected
onto CCD detector
Spectrum offset
via high tension
image mode
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EFTEM - a five-stage process
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Spectrum Imaging – EFTEM mode
• Collects detailed spatial and spectroscopy information
–
–
–
Allows processing decisions after acquisition
Spectrum imaging can create quantitative images / profiles
Can confidently locate artifacts & understand image contrast
Dx
Dy
image at DE1
image at DE2
.
.
.
.
.
.
.
.
.
image at DEi
spectrum at Dxi ,Dyi
DE
Dx, Dy spatial dimensions
DE
energy-loss dimension
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Spectrum imaging - STEM EELS mode
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Spectrum imaging - STEM EELS mode
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Elemental Mapping Using Energy Filtered Imaging
SiC/Si3N4
Atomic Resolved EELS of GaAs in
the bulk
HAADF survey image
• Analysis was carried out using the facilities at Florida
State University
• System: ARM200 with cold FEG equipped with GIF
Quantum heavily upgraded
• Sample was provided by Glasgow University and
Sample was observed along the [110] direction
• Sample is 4 years old and shows some oxidation
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Atomic Resolved EELS of GaAs in
the bulk
EELS SI
EELS spectrum extracted from the region
in the red box in the EELS SI
Ga L2,3-edges
• Convergence angle: 25mrad
• Collection angle120mrad
• EELS data was acquired in single range mode
• Exposure time per pixel: 50ms
• Dataset size: 26x25x2048
• Total number of pixels: 650
• Total acquisition time: 51seconds
As L2,3-edges
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Atomic Resolved EELS of GaAs in
the bulk
As elemental map
EELS colorized elemental map
Ga: Green
As: Red
Ga elemental map
• The GaAs dumbbell is clearly
resolved with high contrast
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Elemental maps
EDS Pd
Intensity line profiles extracted from the
region in the blue in the Pd maps
EELS Pd
• The EELS elemental map for the Pd looks much
sharper and shows higher contrast than the
same map obtained using EDS. This can be
directly attributed to the strong forward scattering
of the EELS signal and the nearly 100%
collection efficiency of detector.
• The high signal to noise ratio in the data is
evident from intensity line profiles extracted from
the region indicated in the box in the EDS and
EELS Pd elemental maps.
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Elemental maps
Au EDS
Au M EELS
Map
Au M EDS
Map
Au EELS
Mean
signal
Std. Dev.
14468
856
79.9
10.1
SN
R
17:
1
7.9:
1
• The signal intensity was analyzed from a uniform
region of a Au particle. This 16x16 pixel region is
show by the red box in the Au elemental maps
• The SNR for the EELS data is ~17 while that for the
EDS data is ~8 giving about a 2x improvement for the
EELS data.
• the EELS signal is more than twice as sensitive than
the EDS data
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Colorized Elemental Maps
EDS
• Red: Pd
• Green: Au
• Despite the presence of heavy elements
involved in the analysis, EELS maps show
better contrast
• Some details in the maps can be observed
only in the EELS elemental maps
EELS
State of the Art SrTiO3 Example
2012
(1024x1024)
– LaMnO3/SrMnO3
superlattice grown
on SrTiO3
– NION UltraSTEM
with Enfinium ER
Mn L
La M
Ti L
• 2msec/pixel @
250pA
• 8GB of data!
2008
(64x64)
10nm
Acknowledgements: Julia Mundy, Carolina Adamo, Darrell Schlom, David Muller, Cornell University
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Atomic-Resolution Electron Energy Loss Spectroscopy
STEM-EELS
La-doped CaTiO3
M.S. Varela, et al., Phy. Rev. Lett. 92 (2004) 095502
谢
谢 !