High-Resolution HAADF-STEM
This HAADF-STEM image shows WO3 segregations in a matrix of a TTB-type Nb-W oxide. The positions of the metal atom columns appear with bright contrast (c.f., HRTEM image of Nb4W13O47
X-Ray signal generation in TEM thin foil specimens
Fluorescence yield (w): w = 0 for Z ～ 5 (Boron k shell ionization) Z ～ 27 (Cobalt L shell) Z ～ 57 (lathanlum M shell)
N Error(%) = 100 N
3.2 EELS - Electron Energy Loss Spectroscopy 3.2.1 energy loss process in thin foil TEM specimens 3.2.2 Where to find the energy loss electrons? 3.2.3 Electron energy loss spectrometer 3.2.4 Comparison of the signal generating process for EDS and EELS 3.2.5 The energy loss spectrum 3.2.6 EELS Microanalysis and the limit of analysis 3.2.7 Conclusion remarks
2、Imaging in AEM
2.1.TEM 2.2.STEM - Scanning transmission electron microscopy 2.3.STEM/SEM imaging 2.4.Signal mixing - Hybrid imaging 2.5.X-Ray and EELS mapping
1. BF detector It is placed at the same site as the aperture in BF-TEM and detects the intensity in the direct beam from a point on the specimen. 2. ADF detector The annular dark field (ADF) detector is a disk with a hole in its center where the BF detector is installed. The ADF detector uses scattered electrons for image formation, similar to the DF mode in TEM. The measured contrast mainly results from electrons diffracted in crystalline areas but is superimposed by incoherent Rutherford scattering. 3. HAADF detector The high-angle annular dark field detector is also a disk with a hole, but the disk diameter and the hole are much larger than in the ADF detector. Thus, it detects electrons that are scattered to higher angles and almost only incoherent Rutherford scattering contributes to the image. Thereby, Z contrast is achieved.
STEM images of Pt particles in SiO2 nanotubes
In the BF-STEM image, the contrast is similar to that in BF-TEM images: Pt particles appear with dark contrast since they are crystalline and the heaviest scatterers in this system. In the ADF-STEM image, the Pt particles appear with bright contrast because of diffraction and incoherent scattering (cf., DF-TEM images). In the HAADF-STEM image, only the incoherently scattered electrons contribute to the image, which nonetheless appears to be very similar to the ADF-image.
HAADF-STEM image of Au/Pt particles on Silica
In the high-angle annular dark field (Z contrast) image, the metal particles, which have high atomic numbers Z compared to the SiO2 matrix, are imaged as bright dots. Thus, this method is a valuable tool to investigate catalysts and to determine the sizes of metal particles and the corresponding size distribution.
3.2.3 Optimization of the AEM for microanalysis 3.2.4 X-Ray microanalysis
Cliff and Lorimer :
CA I = k AB A CB IB
CA+CB=100%
CC IC = KCA CA IA CC IC = KCB CB IB KCA KCB = K AB
How to form a probe ?
Detectors needed for an AEM
3.Relationship between TEM, SEM and AEM
3.1 TEM Image mode Diffraction mode 3.2 SEM Image mode: SE, BSE, X-Ray Mapping Microanalysis: WDS, EDS 3.3 AEM Imaging mode: TEM, STEM, SEM, Mapping (X-Ray + EELS) Diffraction mode: Scanning probe Stationary diffraction pattern Microanalysis: EDS, EELS, micro-diffraction, convergent beam diffraction
Part III Analytical Electron Microscopy in Materials Science
1.Introduction 2.Image mode in AEM 3.Microanalysis in AEM 3.1 X-ray Energy Dispersive Spectroscopy (EDS) 3.2 Electron Energy Loss Spectroscopy (EELS) 3.3 Microdiffraction 3.4 Convergent beam diffraction
3.2 SEM Image mode: SE, BSE, X-Ray Mapping Microanalysis: WDS, EDS
Hale Waihona Puke SEM二次电子像的衬度——拓扑衬度
3.3 AEM Imaging mode: TEM, STEM, SEM, Mapping (X-Ray + EELS) Diffraction mode: Scanning probe Stationary diffraction pattern Microanalysis: EDS, EELS, Micro-diffraction, Convergent beam diffraction
Limit for microanalysis by EDS
1. Absolute accuracy 2 Minimum detectable mass: MDM 10-20g 3 Minimum mass fraction: MMF 0.1wt% 4 Spatial resolution: 10 ～20 nm 5 Low Z limit 6 Practical limitations : (1) contamination (2) Embedded particales
1、Introduction
1.Signals generated in the interaction between the incident high energy electron beam and the thin crystalline specimen 2.How to form a probe 3.Relationship between TEM, SEM and AEM 3.1 TEM Image mode Diffraction mode 3.2 SEM Image mode: SE, BSE, X-Ray Mapping Microanalysis: WDS, EDS 3.3 AEM Imaging mode: TEM, STEM, SEM, Mapping (X-Ray + EELS) Diffraction mode: Scanning probe Stationary diffraction pattern Microanalysis: EDS, EELS, micro-diffraction, convergent beam diffraction