E lectron E nergy L oss S pectroscopy (EELS)
张 庶
元

Atomic-scale view of electron energy loss in TEM
Incident beam electron
E
0
(100 to 1000 keV)
Excited specimen electron
E
B
+ E
Scattered beam electron
E
0
- E
3

Elastic scattering
L
K
Inelastic scattering
L
Carbon atom
K
0
1 eV
Zero loss
C K
290 Electron energy loss (eV)

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

 I
I
T o



λ
为电子非弹性散射的平均自由程
I
T
为第一个等离激发峰的强度
I o
为零损失峰的强度
Rough estimate of λ :
λ ~ 0.8E
o nm so for 100-keV electrons
λ is 80-120 nm various materials

偶极跃迁： Δ l = ± 1

11

Magnetic prism spectrometer

Optical configuration at entrance
Dispersion and focusing section
Projection section
Spectrum plane
13

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

 Spectrum is enlarged and optimally coupled to detector
Final EELS readout
EELS spectrum projected onto CCD
16

 The spectrum is shifted
 Best to do by changing prism current preserve probe focus
Final EELS readout
Spectrum offset via prism current
O K edge
Mn L edge
EELS spectrum projected onto CCD
17




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
18

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
19

20

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
D x
D y image at
D
E
1 image at
D
E
2
.
.
.
.
.
.
.
.
.
image at
D
E i spectrum at
D x i
,D y i
D
E
D x,
D y spatial dimensions
D
E energy-loss dimension
21

Spectrum imaging - STEM EELS mode
22

Spectrum imaging - STEM EELS mode
23

Elemental Mapping Using Energy Filtered Imaging
SiC/Si
3
N
4

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
25

EELS spectrum extracted from the region in the red box in the EELS SI
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
26

EELS colorized elemental map As elemental map
Ga: Green
As: Red
Ga elemental map
• The GaAs dumbbell is clearly resolved with high contrast
27

Elemental maps
EELS Pd EDS Pd
Intensity line profiles extracted from the region in the blue in the Pd maps
• 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.
28

Au EDS
Elemental maps
Au EELS
Au M EELS
Map
Au M EDS
Map
Mean signal
14468
79.9
Std. Dev.
856
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
29

EDS
Colorized Elemental Maps
• 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

3
Mn L
– LaMnO3/SrMnO
3
La M
Ti L superlattice grown on SrTiO
3
– NION UltraSTEM with Enfinium ER
• 2msec/pixel @
250pA
• 8GB of data!
2012
(1024x1024)
2008
(64x64) 10nm

Atomic-Resolution Electron Energy Loss Spectroscopy
STEM-EELS
La-doped CaTiO
3
M.S. Varela, et al., Phy. Rev. Lett. 92 (2004) 095502

谢
谢 !