电子能量损失谱 张 庶 元张 庶 元 Electron Energy Loss Spectroscopy (EELS)
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Transcript of 电子能量损失谱 张 庶 元张 庶 元 Electron Energy Loss Spectroscopy (EELS)
电子能量损失谱
张 庶 元
Electron Energy Loss Spectroscopy (EELS)
入射高能电子与样品的相互作用
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Atomic-scale view of electron energy loss in TEMAtomic-scale view of electron energy loss in TEM
Incident beam electronE0 (100 to 1000 keV)
Excited specimen electronEB + E
Scattered beam electronE0 - E
K
L
Zero loss
Electron energy loss (eV)
Ele
ctro
ns c
ount
0
1 eV
290
C K
Inelastic scattering
K
L
Elastic scattering
Carbon
atom
What is an EELS spectrum?What is an EELS spectrum?
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
t
ln
IT
Io
Rough estimate of λ : λ ~ 0.8Eo nmso for 100-keV electronsλ is 80-120 nm various materials
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
偶极跃迁: Δl = ±1
内壳层电子激发
1111
Correlation between EELS and specimen feature
Magnetic prism spectrometer
EELS spectrometerEELS spectrometer
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Optical configuration at entrance
Dispersion and focusing section
Projection section
Spectrum plane
In-column omega-filter
Energy-filter imaging and electron diffraction, CBED
Inserted in the imaging lens system
Post-column imaging filter
Gatan (Tridiem) imaging filter (GIF).
Attached to the TEM column below the viewing chamber
Energy-loss spectroscopy (EELS - low loss)
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Spectrum is enlarged and optimally coupled to detector
Final EELS readout
EELS spectrum projected onto CCD
Energy-loss spectroscopy (EELS - core loss)Energy-loss spectroscopy (EELS - core loss)
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Final EELS readout O K edge
Mn L edge
The spectrum is shifted
Best to do by changing prism current preserve probe focus
Spectrum offset via prism current
EELS spectrum projected onto CCD
EFTEM: Energy Filtered TEM: GIF onlyEFTEM: Energy Filtered TEM: GIF only
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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
Energy-filtered TEM imaging (EFTEM - core loss)Energy-filtered TEM imaging (EFTEM - core loss)
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Core-loss image projected onto CCD detector
image mode
Spectrum offset via high tension
The spectrum is shifted relative to the slit opening Best to do by increasing beam energy to preserve image focus
2020
EFTEM - a five-stage processEFTEM - a five-stage process
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
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x, y spatial dimensionsE energy-loss dimension
y
x
E
image at E1
image at E2
image at Ei
.
.
.
.
.
.
.
.
.
spectrum at xi yi
Spectrum imaging - STEM EELS modeSpectrum imaging - STEM EELS mode
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Spectrum imaging - STEM EELS modeSpectrum imaging - STEM EELS mode
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Elemental Mapping Using Energy Filtered Imaging
SiC/Si3N4
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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 As 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
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As elemental map
Ga elemental map
EELS colorized elemental map
Ga: GreenAs: Red
• The GaAs dumbbell is clearly resolved with high contrast
Atomic Resolved EELS of GaAs in the bulk
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• 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.
Intensity line profiles extracted from the region in the blue in the Pd maps
Elemental maps
EDS Pd EELS Pd
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Mean sign
alStd. Dev.
SNR
Au M EELS Map
14468 85617:1
Au M EDS Map
79.9 10.17.9:1
• 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
Elemental maps
Au EDS Au EELS
EDS
EELS
• 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 o
nly in the EELS elemental maps
Colorized Elemental Maps
State of the Art SrTiO3 Example– LaMnO3/SrMnO3
superlattice grown on SrTiO3
– NION UltraSTEM with Enfinium ER
• 2msec/pixel @ 250pA
• 8GB of data!
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2008(64x64)
Acknowledgements: Julia Mundy, Carolina Adamo, Darrell Schlom, David Muller, Cornell University
2012(1024x1024)
10nm
Mn LLa M Ti L
M.S. Varela, et al., Phy. Rev. Lett. 92 (2004) 095502
STEM-EELS
Atomic-Resolution Electron Energy Loss Spectroscopy
La-doped CaTiO3
谢 谢 !