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

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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

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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 !