Scanning transmission electron microscope (2)
Post on 07-May-2015
339 Views
Preview:
Transcript
Outline
• History(TEM)• Background• Components• Specimen Preparation• Imaging method• Contrast formation• Modifications• STEM• References
Scanning Transmission Electron Microscope
Specimen Preparation
• Prepare a specimen with at least part of its thickness at about 100 nm, depending on the atomic weight of specimen materials.
• Pre-Thinning:1st specimen less than 1mm thick is prepared Reducing the thickness to about 0.1mm before
final thinning to 100 nm thickness• Final Thinning: involveIon MillingElectrolytic ThinningUltramicrotomy
Ion Milling
• Uses a beam of energetic ions to bombard specimen surfaces to reduce the thickness by knocking atoms out of a specimen
• General procedurea) Dimple grindingb) ion milling ion beam of 1–10 keV
bombarded specimen is placed in the
center at an angle of 5-30◦
Electrolytic Thinning
• Reducing specimen thickness to 100nm
• General procedureA specimen placed in
an electrochemical cell as anode
A suitable reduce specimen thickness
Common technique is jet polishing
Electrolytic thinning completed in 3–15 minutes.
Ultramicrotomy
• Reducing specimen thickness to 100nm
• General procedureA specimen is mounted
in a holder against the cutting tool
The specimen should be trimmed to have a tip held against the knife
The holder gradually moves toward the knife while it repeatedly moves up and down
STEM
• The basic principle of image formation fundamentally different from static beam TEM
• small spot size is formed on the sample surface with the condenser lenses
• This probe is scanned on the sample surface
• the signal is detected by an electron detector, amplified and synchronously displayed on CRT with the scan coils
Cont…
• DETECTOR1. Small disk on the column axis which
detects the transmitted beam (BF STEM image) or diffracted beam (DF STEM image)
2. Annular detector (a plate with a hole) which detects all the diffracted beams except the transmitted one (ADF STEM)
• Resolution limited by the spot sizehave poorer resolution but better contrast
TEM =
STEM
MATERIALS CHARACTERIZATION Introduction to Microscopic and Spectroscopic Methods by Yang Leng -ISBN 978-0-470-82298-2
top related