Advanced Methods of Materials Characterization (Lecture 3)

1. Advanced Methods of Materials Characterization (Lecture 3) March 14th, 2014

2. Last week we talked about…

3. Electromagnetic Spectrum • Photons are the basic units of all the electromagnetic radiations.

4. Some Microscopic Terms • Resolution • Magnification • Brightness • Contrast • Depth-of-field Homework: What are the definitions of these terms?

5. Chapter 1 Scanning Electron Microscopy (SEM)

6. Question #1:Why do we need electron microscopy?

7. Introduction and History • Electron microscopes were developed due to the limitations of optical microscopes which are limited by the physics of visible light. • In the early 1930's this theoretical limit had been reached and there was a scientific desire to see the fine details of the interior structures of organic cells (nucleus, mitochondria, etc.). • This required 10,000x plus magnification which was not possible using current optical microscopes.

8. Microstructural Features of our Interests Grain Size From sub-micrometer to the centimeter regime Grain Shapes Precipitate Size Volume Fractions and Distributions of Various Phases Defects Cracks and voids

9. What can we see with an SEM? Topography The surface features of an object or "how it looks", its texture; direct relation between these features and materials properties Morphology The shape and size of the particles making up the object; direct relation between these structures and materials properties Composition The elements and compounds that the object is composed of and the relative amounts of them; direct relationship between composition and materials properties Crystallographic Information How the atoms are arranged in the object; direct relation between these arrangements and material properties

10. Question #2:How does Scanning Electron Microscope work?

11. Scanning Electron Microscope Electron microscopes are scientific instruments that use a beam of energetic electrons to examine objects on a very fine scale.

12. How SEM works? - An electron beam is condensed, accelerated, and focused on a specimen by lens; - The electron beam hits the specimen, producing, among others, secondary and backscattered electrons; - These electrons are collected by a detector, converted to a voltage, and amplified.

13. Electromagnetic Lens

14. Electron-Solid Interactions

15. Examples of SEM Image Sources: http://tupian.hudong.com/3960/11.html

16. Examples of SEM Image Sources: http://bbs.labscn.com/showphoto.aspx?photoid=2074 http://bbs.1718china.com/thread-27547-1-1.html

17. Examples of SEM Image Source: http://www.le.ac.uk/bs/em/sem.htm

18. Examples of SEM Image

19. Examples of SEM Image

20. Today let’s look at…

21. Question #3:What is the resolution difference between an electron microscope and an optical/light microscope?

22. Optical Microscopy

23. Examples of OM Image Sources: http://www.emeraldinsight.com/journals.htm?articleid=1524385&show=html http://www.eatechnology.com/specialistbusinesses/analytical/materialstesting /opticalmicroscopy

24. OM vs. SEM: Resolution

25. Resolution Limits of OM and SEM

26. Optical Microscopy VS SEM Combination of higher magnification, larger depth of field, greater resolution, in addition to compositional and crystallographic information makes SEM one of the most heavily used instruments in academic/national lab research areas and industry.

27. Question #5:What are the key components of an SEM?

28. Components of a SEM machine 1. Electron optical column consists of: – electron source to produce electrons – magnetic lenses to de-magnify the beam – magnetic coils to control and modify the beam – apertures to define the beam, prevent electron spray 2. Vacuum systems consists of: – chamber which “holds” vacuum, pumps to produce vacuum – valves to control vacuum, gauges to monitor vacuum 3. Signal Detection & Display consists of: – detectors which collect the signal – electronics which produce an image from the signal

29. Components of SEM Electric guns Thermonic electric guns Field emission guns Lens Deflection coils Electro-magnetic lens

30. Vacuum system The electron guns should be kept at high vacuum state! Usually better than 10-7 Torr. The sample chamber work at medium vacuum status. Better than 10-3 Torr

31. Signal detecting and processing system • SE detector// BSE detector

32. Key parameters for operations of SEM HV • Accelerating voltages: 1 KeV-30 KeV • The difference in potential between the filament and the anode. As the voltage is increased, the electrons travel with higher velocity and are more energetic. • d=0.61入/NA • Reduction in structural details of the specimen surface in SE mode.//Increased electron build up in insulating samples, causing charging artifacts //Increased heating and the possibility of specimen damage

33. Key parameters for operations of SEM • WD Working distance: Working distance is the distance from the bottom of the SEM column to the sample The shorter the working distance, the smaller the diameter of the beam is at the sample surface. So, when possible, the WD is kept at 10mm or smaller for high resolution imaging. The disadvantage is that focal depth and depth of field is drastically reduced at small WD.

34. Key parameters for operations of SEM Spot size The size (cross sectional diameter) that the cone of the beam makes on the surface of the sample affects : 1) the resolution of the image. 2) the number of electrons generated (therefore the graininess of the image). At low magnifications we use a larger spot size than at higher magnifications.

35. Key parameters for operations of SEM Magnification-- Magnification is the enlargement of an image, or portion of an image By reducing the size of the area scanned by the scan coils, the SEM changes the magnification of the image!

36. SEM #2 SEM #3 SEM #1

37. Safety in an EM Lab Chemicals Common chemicals Handling Disposal, waste management Cleaning and exposure Equipment Radiation Electrical safety Servicing Physical and Mechanical Hazards

38. Primary Hazard Color Code FlammablesRedToxics/HealthBlueReactives/OxidizersYellow Contact Hazards White GeneralGray,Green,Orange Many labs color code bottles to aid in segregated chemical storage. The assignments given above are standard for most labs and are based upon chemical manufacturer’s color code designations. Liquids should also be stored away from solids.

39. Chemicals found in an EM lab Aldehydes (glutaraldehyde, paraformaldehyde) Carcinogenic, allergies, sensitivity Minimize exposure to fumes Cacodylate salts ~50% arsenic Carcinogenic, toxic Readily absorbed through skin (garlic taste) Osmium tetroxide (osmic acid) Toxic, irritant, volatile Spills reduced to metallic osmium with corn oil or Na Ascorbate powder, then cat litter to pick up.

40. Acetone and alcohols Used as solvents, dehydrants and cleaners Flammable- keep in flammables cabinet Toxic Chemicals dissolved can penetrate skin Propylene oxide Highly flammable Carcinogen Picric acid Dried salts explosive Resin Components Carcinogenic Allergic reactions when using antihistamines Heavy metal salts (lead and uranium) Toxic Carcinogenic

41. Handling Wear: Gloves Lab coat Dust mask Closed toe shoes Work deep in ventilation hood Measuring: New spatula for each chemical Minimize dust Have clean-up equipment available All spills are hazardous waste Clean after yourself !

42. Disposal Spent, expired, or surplus chemicals Minimize waste! Use less toxic alternatives if available. Use a minimal amount - avoid large amounts. Keep Waste in separate containers - avoid mixing Some can be recycled. Easier to keep track of amounts for manifests. Some chemicals are not compatible. Clean up after yourself!

43. Clean Up! Always clean up any spills, messes. Make a spill kit Mercury difficult to clean up Can’t wipe or pick up Use a vacuum with trap, not vacuum cleaner (volatilizes the mercury) Treat the cleaning materials as hazardous waste -Not into trash can or down sink

44. Pumps Oil filters to minimize inhaling Liquid N2 and compressed gasses Explosion of tank SF6 gas – changes to toxic Fluorine if heated above 200 C X-ray exposure

45. Physical damage Cuts Razor Blades Glass knives Inhalation Mercury bulbs can explode if old and overheated (usually after 200-300 hours of use). Symptoms of mercury poisoning include tremors, tunnel vision, loss of balance, slurred speech, and unpredictable emotions.

46. Fires: Extinguishers Emergency showers and eye washes

47. Chapter 2 Transmission Electron Microscopy (TEM)

48. Electron-solid interactions Secondary electrons (SEM)//Backscattered electrons (SEM）//Auger electrons (AES)//X-rays (EDS) Transmitted electrons (TEM)

49. What is TEM? Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor such as a CCD camera.

50. What is TEM used for? • Morphology • The size, shape and arrangement of the particles which make up the specimen as well as their relationship to each other on the scale of atomic diameters. • Crystallographic Information • The arrangement of atoms in the specimen and their degree of order, detection of atomic-scale defects in areas a few nanometers in diameter • Compositional Information (if so equipped) • The elements and compounds the sample is composed of and their relative ratios, in areas a few nanometers in diameter