Chapter 4 Other Techniques: Microscopy, Spectroscopy, Thermal Analysis

1. Chapter 4Other Techniques:Microscopy, Spectroscopy, Thermal Analysis

2. Microscopic techniques • Optical microscopy - polarizing microscope - reflected light microscope • Electron microscopy - scanning electron microscopy (SEM) - transmission electron microscopy (TEM) - high resolution electron microscopy (HREM) EDS: Energy Dispersive Spectroscopy

3. Applications • Optical microscopy - phase identification, purity, and homogeneity - crystal defects : grain boundaries and dislocation - refractive index determination • Electron microscopy - particle size and shape, texture, surface detail - crystal defects - precipitation and phase transitions - chemical analysis - structure determination

4. SEM scanning electron microscopy

5. Photos of SEM

6. Energy Dispersive Spectroscopy EDS An attachment of EM

7. Transmission Electron Microscopy TEM

10. Wavelength of electrons • = h(2meV)-1/2 At 90 kV accelerating voltage, l ~ 0.04 Å Consequently, the Bragg angles for diffraction are small and the diffracted beams are concentrated into a narrow cone centered on the undiffracted beam.

11. Basic components of a TEM

12. HREM of an intergrowth tungsten bronze, Rb0.1WO3

13. Scanning tunneling microscope (STM) The STM can obtain images of conductive surfaces at an atomic scale of 0.2 nm, and also can be used to manipulate individual atoms, trigger chemical reactions, or reversibly produce ions by removing or adding individual electron from atoms or molecules.

14. Atomic force microscope (AFM)

15. Field Emission SEM (FESEM) Traditional SEM:Thermionic Emitters use electrical current to heat up a filament FESEM:A Field Emission Gun (FEG); also called a cold cathode field emitter, does not heat the filament. The emission is reached by placing the filament in a huge electrical potential gradient. FESEM uses Field Emission Gun producing a cleaner image, less electrostatic distortions and spatial resolution < 2nm.

16. Spectroscopic techniques

17. Vibrational spectroscopy : IR and Raman • IR

18. Raman

19. Visible and ultraviolet spectroscopy

20. UV/visible

21. Nuclear magnetic resonance (NMR) spectroscopyMagic Angle Spinning NMR (MAS-NMR) If a solid-state sample is allowed to spin at an angle of θ=54.7° to a strong external magnetic field, dipolar coupling (D) will be zero.

22. Example 1

23. Example 2

24. Electron spin resonance (ESR) spectroscopy: detect unpaired electrons

25. X-ray spectroscopy : XRF, AEFS, EXAFS

26. X-ray fluorescence (XRF)-coordination number -bond distance -oxidation state

27. X-ray absorption techniques • Absorption edge fine structure (AEFS) or X-ray absorption near edge structure (XANES) Information can be obtained - oxidation state, site symmetry, surrounding ligands, the nature of the bonding • Extended X-ray absorption fine structure (EXAFS) Information can be obtained - bonding distance, coordination number

28. Extended X-Ray Absorption Fine Structure This introduction to the theory of EXAFS is divided into basic, relatively simple and complicated parts. EXAFS spectra are a plot of the value of the absorption coefficient of a material against energy over a 500 - 1000 eV range (including an absorption edge near the start of the spectrum). Through careful analysis of the oscillating part of the spectrum after the edge, information relating to the coordination environment of a central excited atom can be obtained. The theory as to what information is contained in the oscillations is described here.

29. EXAFS

30. EXAFS

31. XANES

32. AEFS (or XANES)

33. Electron spectroscopies • ESCA • XPS • UPS • AES • EELS

34. Origins of ESCA and Auger spectra Electron Spectroscopy for Chemical Analysis: XPS, UPS Auger electrons are secondary electrons

35. Core, valence and virtual levels

36. X-ray photoelectron spectroscopy XPS is a surface chemical analysis technique

37. XPS is used to measure: • 1) elemental composition of the surface (1–10 nm usually) • 2) empirical formula of pure materials • 3) elements that contaminate a surface • 4) chemical or electronic state of each element in the surface • 5) uniformity of elemental composition across the top of the surface (line profiling or mapping) • 6) uniformity of elemental composition as a function of ion beam etching (depth profiling)

38. XPS and UPS • XPS: core-level photoelectron spectroscopy • UPS: valence-level photoelectron spectroscopy hv = Ek + ef + Eb Ek = kinetic energy of escaped electrons ef = work function (energy from Fermi level to continuous states) Eb = binding energy Ek is measured experimentally Eb contains information of electronic structure

39. Schematic representation ofhv = Ek + ef + Eb Ek ef Eb hv

40. XPS

41. Resolution of XPS and UPS • XPS conventionally has lower resolution (0.2 ~1.2 eV). Cannot see vibration (< 0.5 eV or 4000 cm-1) • UPS has better resolution ( < 0.01 eV). Can see vibration frequency. • For UPS, hv = Ek + ef + Eb +DEvib Synchrotron-based light source can enhance resolution

42. Different oxidation states determined by XPS Example of "High Energy Resolution XPS Spectrum" also called High Res spectrum. This is used to decide what chemical states exist for the element being analyzed. In this example the Si (2p) signal reveals pure Silicon at 99.69 eV, a Si2O3 species at 102.72 eV and a small SiO2 peak at 103.67 eV. The amount of Si2O at 100.64 eV is very small.

43. XPS and AES

44. XPS spectra of Na2S2O3 and Na2SO4

45. XPS spectrum of KCr3O8

46. XPS spectrum of NaWO3 Band Structures can be seen by XPS The relative amount of electrons filled in a band can be seen.

47. XPS of Co and its oxide

49. Binding Energy Table

50. Band Structures can be seen by XPS