1. Outline History(TEM) Background Components Specimen Preparation Imaging method Contrast formation Modifications STEM References

2. Scanning Transmission Electron Microscope

13. 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:  1 st specimen less than 1mm thick is prepared  Reducing the thickness to about 0.1mm before final thinning to 100 nm thickness Final Thinning: involve  Ion Milling  Electrolytic Thinning  Ultramicrotomy

14. Ion Milling Uses a beam of energetic ions to bombard specimen surfaces to reduce the thickness by knocking atoms out of a specimen General procedure a)Dimple grinding b)ion milling  ion beam of 1–10 keV bombarded  specimen is placed in the center at an angle of 5-30◦

15. Electrolytic Thinning Reducing specimen thickness to 100nm General procedure  A 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.

16. Ultramicrotomy Reducing specimen thickness to 100nm General procedure  A 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

21. 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

22. Cont… DETECTOR 1.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 size  have poorer resolution but better contrast

24. MATERIALS CHARACTERIZATION Introduction to Microscopic and Spectroscopic Methods by Yang Leng -ISBN 978-0-470-82298-2