1. Chapter 4 Other 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 Electron 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. EDS
Energy Dispersive Spectroscopy
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

18. Raman

19. Visible and ultraviolet spectroscopy

20. UV/visible

21. Nuclear magnetic resonance (NMR) spectroscopy Magic 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)
- 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 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 hv = Ek + ef + Eb Ek = kinetic energy of escaped electrons
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 of hv = Ek + ef + Eb

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 eV, a Si2O3 species at eV and a small SiO2 peak at eV. The amount of Si2O at 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

51. An example of UPS

52. Franck-Condon Principle and Changes of Vibration Frequencies

53. UPS of O2
antibonding
bonding

54. Thermal analysis Thermogravimetry (TGA)
Differential thermal analysis (DTA)
Differential scanning calorimetry (DSC)
Thermomechanical analysis (TMA)

55. Setup of Thermogravimetric Analysis
Thermogravimetric Analysis (TGA) measures weight changes in a material as a function of temperature (or time) under a controlled atmosphere.

56. TGA curve

57. Heating rate dependence of TGA curve
CuSO4.5H2O heated in air in a Pt crucible

58. TGA of CaCO3

59. Possibility of Mixtures

60. The DTA method
Differential Thermal Analysis (DTA) measures the difference in temperature between a sample and a thermally inert reference as the temperature is raised. The plot of this differential provides information on exothermic and endothermic reactions taking place in the sample.

61. Variation of peak temperature of kolin with rate of temperature increase

62. TGA and DTA curves of kaolin minerals

63. Application of DTA 1 : melting/solidification

64. Application of DTA 2 : melting behavior of crystal

65. Application of DTA 3 : phase diagram determination

66. Application of TGA : stepwise decomposition of Ca(COO)2·H2O

67. Conversion of TGA to DTG

69. Differential Scanning Calorimetry (DSC)
Differential Scanning Calorimetry (DSC) is a thermal analysis technique which is used to measure the temperatures and heat flows associated with transitions in materials as a function of time and temperature.

70. Thermomechanical analysis (TMA)
Thermomechanical analysis (TMA) is used to determine the deformation of a sample (changes in length or thickness) as a function of temperature.
Linear Variable
Displacement Transducer

71. Mössbauer spectroscopy
Oxidation state, coordination numbers, bond character

72. Mössbauer (MB) Spectroscopy
Nuclear transition: absorption of g-rays by sample.
The condition for absorption depend on the electron density about the nucleus and the number of peaks obtained is related to the symmetry of the compound.

73. Distribution of energy of emitted and absorbed g-rays
The energy of emitted Eg = Er + D – R
Er : Eexcited state – Eground state of the source nucleus D: the Doppler shift due to the transitional motion of the nucleus
R : the recoil energy of the nucleus
The energy of the g-ray absorbed Eg = Er + D + R
Distribution of energy of emitted and absorbed g-rays

74. Emitted Eg = Er + D - R = Er + D + R = Eg absorbed
The main cause for non matching of g-rays energies is the recoil energy.
R  10-1 eV for a gaseous molecule
Doppler effect D  2  104 cm sec-1
R can be reduced by increasing mass by placing the nucleus of the sample and source in a solid.
PA = mvR = -Eg/c
R = mvR2/2 = PA2/2m = Eg2/2mc2
P = momentum

75. Three main types of interaction of the nuclei with the chemical environment:
Resonance line shifts form changes in electron environment
Quardrupole interactions
Magnetic interactions

76. isomer shifts, center shifts, chemical shifts
MB of Fe3+FeIII(CN)6
Fe3+: weak field
0.53 mm sec-1
FeIII: strong field
0.03 mm sec-1

77. The isomer shift results from the electrostatic interaction of the charge distribution in the nucleus with the electron density that has a finite probability of exiting at the nucleus.
Only s electrons have a finite probability of overlapping in the nuclear density.
p, d and other electron densities  screening effect

78. Oxidation state Isomer shift
Dr/r is positive and any factor which increases in the total s electron density increases the isomer shift (IS, CS)
Increase in the covalent bonds
Decrease the p or d-populations.
Increase in oxidation state of a transition metal.
Decrease in coordination number
Tetrahedral (sp3) more s character higher IS
Octahedral (d2sp3)
Dr/r is negative for Fe
Fe2+ (d6) has an appreciably larger center shift than Fe3+ (d5)
Isomer Shift for High Spin Iron Compounds
Oxidation state +1 +2 +3 +4 +6
Isomer shift
~+2.2
~+1.4
~+0.7
~+0.2
~-0.6

79. Quadrupole interactions
Quadrupole splitting arises from the presence of an electron filed gradient (EFG) at the nucleus.
The magnitude of the QS represents the asymmetry of the electron cloud around the nucleus.

80. QS IS IS QS
57Fe, 119Sn
197Au
The I = 3/2 state is split into two sublevels by the presence of an electron-field gradient.
The magnitude of the QS is one-half of the quadrupole coupling constant (QCC) for the I = 3/2 state.

81. Strong field case t2g6 weak field case t2g4eg2
Low spin case Fe(II) high spin case Fe(II)
NO quadrupole large quadrupole
In octahedral geometry only d3, d8, d10 , high spin d5 and low spin d6 configurations make no contribution to the QS.

82. Mössbauer of KFeS2