Massachusetts Institute of Technology. Dr. Nilanjan Chatterjee

Transcription

1 Massachusetts Institute of Technology Dr. Nilanjan Chatterjee

2 Electron Probe Micro-Analysis (EPMA) Imaging and micrometer-scale chemical compositional analysis of solids

3 Signals produced in The Electron Microprobe Cathodoluminescence (CL) Electron beam Back-scattered electron (BSE) Characteristic X-ray Secondary electron (SE) Specimen

4 Qualitative analysis: phase identification Back-scattered electron (BSE) image: spatial variations in chemical composition X-ray spectra with energy dispersive spectrometer (EDS): element identification Semi-quantitative analysis: phase abundance Elemental X-ray X maps with EDS or wavelength dispersive spectrometer (WDS)

5 Optical Microscope versus Scanning Electron Microscope Thin section Polarized transmitted light image Function of optical properties Polished surface Back-scattered electron image Function of composition

6 Electron-specimen interactions Beam electron Specimen atom Scattered beam electron Elastic Scattering Inelastic Scattering E 1 = E 0, large φ e E 1 <E 0, small φ i - Back-scattered electron - Characteristic X-raysX - Secondary electron - Cathodoluminescence

7 Elastic scattering cross-section section Q(> (>φ e ) = 1.62x10-20 (Z 2 /E 2 )cot 2 (φ e /2) E 1 = E 0, large φ e Q: : cross section (events.cm 2 /e -.atom) φ e : elastic scattering angle Z: atomic number E: : beam energy

8 Electron Back-scattering (High angle elastic scattering) η = n BSE /n B where n B : # beam electrons, n BSE : # back-scattered electrons For compounds, η = Σ c j η j where c j : concentration of element j

9 BSE compositional contrast BSE image of rock

10 JEOL JXA-8200 Superprobe Condenser lens LN2 Electron gun Optical Microscope Objective lens Liq N2 for EDS SE EDS BSE WDS WDS Sample exchange Stage High vacuum Perpendicular geometry

11 Features: Electron gun: W or LaB 6 filament, thermionic emission Accelerating voltage: 1-40 kv (typical: kv) Electron lenses (condenser and objective): Beam diameter: 1 nm-1 μm (typical: μm) Beam current: 1 pa-1 μa (typical: na) Capabilities: Scanning electron imaging: Back-scattered electron (BSE): compositional imaging, x Secondary electron (SE): topographic imaging, ,000x X-ray spectrometry: Energy dispersive (EDS): element detection 5 Wavelength dispersive (WDS): concentration measurement

12 Back-scattered electron detector Annular, solid-state state diode, split into two semi-circles, A and B. Top view of specimen

13 Compositional and topographic imaging with BSE detector A+B Compositional mode A-B Topographic mode

14 BSE image resolution: electron interaction volume Electron Range (Kanaya-Okayama) Okayama): R = KE n 0 /ρ where, K = A/Z n = 1.67; ρ = density Typical ranges (15( kv,, perpendicular beam): C 1.8 μm Fe 1.1 μm U 0.8 μm

15 Phase identification: EDS X-ray X spectra Ilmenite (ilm): FeTiO 3 hbl: hydrous Ca-Fe-Mg-Al-silicate plg: Na-Ca-Al-silicate Ato# ilm: O Ti Fe grt: O Mg Al Si Ca Fe bt: H O Mg Al Si K Fe hbl: H O Mg Al Si Ca Fe plg: O Na Al Si Ca

16 Energy Dispersive X-ray X Spectrometer (EDS) EDS detector: a p-n layer of intrinsic Si(Li) semiconductor; Be window (Si is usually electron deficient because it contains B, a p-type impurity. It is coated with Li, an n-type dopant with excess electrons to create a p-n intrinsic semiconductor) Multichannel analyzer (MCA) processes the X-ray X signal

17 Phase abundance: BSE & X-ray maps BSE O Mg Ca Olivine (Mg,Fe)2SiO4 - light in O and Mg maps 19.9% Augite Ca(Mg,Fe)Si2O6 - light in Ca map 19.6% Crystallized fine-grained matrix 60.5% Scale bar: 4 mm

18 Other imaging techniques: secondary electron (SE) (Inelastic scattering) Electrons from specimen surface are mobilized by beam electrons (useful in studying surface features) E 1 <E 0, small φ i Emitted at low energies (typical: <10 ev)

19 Secondary electron detector Side view of specimen

20 Imaging with secondary electron detector -ve Faraday cage bias less SE less topographic contrast +ve Faraday cage bias more SE better topographic contrast

21 Other imaging techniques: Cathodoluminescence (CL) Band gap energy, E gap, is a property of the semiconductor Trace impurities change E gap by adding additional energy states in the band gap

22 Cathodoluminescence imaging Zircon Light from CL is intercepted at the optical microscope and analyzed with a wavelength discriminating spectrometer Auto focus JXA-733 Optical grating spectrometer JXA-8200

23 MIT OpenCourseWare Electron Microprobe Analysis by Wavelength Dispersive X-ray Spectrometry January (IAP) 2010 For information about citing these materials or our Terms of Use, visit: