1. Characterization of Nanomaterials…
And the magnification game!

2. During today’s notes, there will be a picture every other slide
During today’s notes, there will be a picture every other slide. Try to guess what common household item you’re looking at (it has been magnified quite a bit!!!)

3. 1

4. Observations and Measurement: Studying physical properties related to nanometer size
Needs:
Extreme sensitivity
Extreme accuracy
Atomic-level resolution
documents/webpages/nanocrystals.html

5. 2

6. Characterization Techniques
Structural Characterization
Scanning electron microscopy (SEM)
Transmission electron microscopy (TEM)
X-ray diffraction (XRD)
Scanning probe microscopy (SPM) (includes AFM)
Chemical Characterization
Optical spectroscopy
Electron spectroscopy
Structure of butterfly wing

7. 3

8. Structural Characterization
Techniques are already used for crystal structures
X-Ray Diffraction
Many techniques are already used for studying the surfaces of bulk material (They provide topographical images)
Scanning Probe Microscopy (AFM & STM)
Electron Microscopes
(5-10)
DEMO: Lattice model & laser/skewer

9. 4

10. Electron Microscopes Are used to count individual atoms
What can electron microscopes tell us?
Morphology
Size and shape
Topography
Surface features (roughness, texture,
hardness)
Crystallography
Organization of atoms in a lattice

11. 5

12. Crystallography Crystallography affects properties
Crystals have atoms in ordered lattices
Amorphous: no ordering of atoms
Crystallography affects properties
Properties: strength; electrical; etc.

13. 6

14. Microscopes: History Light microscopes Electron Microscopes
500 X to 1500 X magnification
Resolution of 0.2 µm
Limits reached by early 1930s
Optical microscopes have a resolution limit of 200 nm, meaning they cannot be used to measure objects smaller than 200 nm. (wavelength of visible light ~400 nm).
Electron Microscopes
Use focused beam of electrons instead of light
* Transmission Electron Microscope (TEM)
* Scanning Electron Microscope (SEM)

15. 7

16. Electron Microscopy Steps to form an image:
Stream of electrons formed by an electron source and accelerated toward the specimen
Electrons confined and focused into thin beam
Electron beam focused onto sample
Electron beam affected as interacts with sample
Interactions / effects are detected
Image is formed from the detected signals

17. 8

18. Electron Microscopes Electron Beam Sample Detection
Accelerated and focused using deflection coils
Energy:
,000,000 eV
Sample
TEM: conductive, very thin!
SEM: conductive
microscope&total=6&start=0&num=10&so=0&type=search&plindex=1
Detector – CRT
For SEM, beam rastered over surface
Detection
TEM: transmitted e-
SEM: emitted e-
Source: Virtual Classroom Biology

19. 9

20. EM Resolution Wavelength dependent on: Resolution dependent on:
wavelength of electrons ()
NA of lens system
Wavelength dependent on:
Electron mass (m)
Electron charge (q)
Potential difference to accelerate electrons (V)
d = resolution (spacing so that features don’t blur together)
n = index of refraction of medium btw source and lens(1 in perfect vacuum)
Alpha: half angle of cone of light from specimen plane accepted by the object (radians) – very small for EMs (spot size??)
N sin a = NA (numerical aperture)

21. 10

22. Transmission EM Magnification: ~50X to 1,000,000X
E-beam strikes sample and is transmitted through film
Scattering occurs
Unscattered electrons pass through sample and are detected
Elastic Scattering
No energy loss
Diffraction patterns
Inelastic Scattering
Occurs at heterogeneties (defects, grain boundaries)
Source: Wikipedia

23. 11

24. Scanning EM Magnification: ~10X to 300,000X
E-beam strikes sample and electron penetrate surface
Interactions occur between electrons and sample
Electrons and photons emitted from sample
Emitted e- or photons detected
Numerical aperture
1_6
Source: Wikipedia

25. 12

26. SEM: Electron Beam Interactions
Valence electrons
Inelastic scattering
Can be emitted from sample
“secondary electron”
Atomic nuclei
Elastic scattering
Bounce back - “backscattered electrons”
Core electrons
Core electron ejected from sample; atom excited
To return to ground state,
x-ray photon or Auger electron emitted +
valence e-
core e-
nucleus
Valence e-: E transferred to atom’s electron, if enough E transferred, can be emitted from sample; these secondary electrons have E <50 eV
Atomic nuclei: e- bounce off with same energy; samples with high atomic numbers cause more backscattering

27. Electron Spectroscopy
Auger e-
Ground state
e- emitted;
excited state
Relaxes to ground state
Energy
X-ray
e- or photon strikes atom; ejects core e-
e- from outer shell fills inner shell hole
Energy is released as X-ray or Auger electron
EDS: Energy Dispersive X-ray Spectroscopy
AES: Auger Electron Spectroscopy
(2:30 for electron spectroscopy)
Emitted energy is characteristic of a specific type of atom because each atom has its own unique electronic structure and energy levels.

28. 13

29. Electron Spectroscopy
Emitted energy is characteristic of a specific type of atom
Each atom has its own unique electronic structure and energy levels
AES is a surface analytical technique
<1.5 nm deep
AES can detect almost all elements
EDS only detects elements Z > 11
EDS can perform quantitative chemical analysis
To study deeper using AES, much etch, repeat, etch, repeat

30. 14

31. SEM and TEM Comparison SEM makes clearer images than TEM
SEM has easier sample preparation than TEM
TEM has greater magnification than SEM
SEM has large depth of field
SEM is often paired with detectors for elemental analysis (chemical characterization)

32. SEM and TEM Data Images
Ag thin film deposited on Si substrate (thermal or e-beam evaporation)
TCNQ (7,7,8,8-tetracyanoquinodimethane) powder and Ag thin film are enclosed in a vacuum glass tube, then heated in a furnace.

33. 15

34. Chemical Characterization
Optical Spectroscopy
Absorption and Emission
Photoluminescence (PL)
Infrared Spectroscopy (IR or FTIR)
Raman Spectroscopy
Electron Spectroscopy
Energy-Dispersive X-ray Spectroscopy (EDS)
Auger Electron Spectroscopy (AES)

35. Optical Spectroscopy: Absorbance/Transmittance
Absorbance: electron excited from ground to excited state
Emission: electron relaxed from excited state to ground state
Transmittance: “opposite” of absorbance: A = -log(T)
Radiation only penetrates ~50 nm
N&N Fig. 8.10

36. 16

37. Scanning Probe Microscopy (SPM)
AFM & STM
Measure forces
Many types of forces (dependent on tip)
Electrostatic Force Microscopy
Distribution of electric charge on surface
Magnetic Force Microscopy
Magnetic material (iron) coated tip
magnetized along tip axis
Scanning Thermal Microscopy
Scanning Capacitance Microscopy
Capacity changes between tip and sample

38. 17

39. Scanning Tunneling Microscopy (STM)
Developed by Binnig and Rohrer in 1982
Tunneling
Very dependent on distance between the two metals or semiconductors
By making the distance 1 nm smaller, tunneling will increase 10X

40. Scanning Tunneling Microscopy (STM)
Instrument: Scanning Tip
Extremely sharp
Metal or metal alloys (Tungsten); Conductive
Mounted on a stage that controls position of tip in all three dimensions
Typically kept nm from surface
Tunneling Current: ~ nA
Resolution:
0.01 nm (in X and Y directions)
0.002 nm in Z direction
Source: Univ. of Michigan

41. 18

42. Scanning Tunneling Microscopy (STM)
Constant Current Mode:
As tip moves across the surface, it constantly adjusts height to keep the tunneling current constant
Uses a feedback mechanism
Height is measured at each point
Constant Height Mode:
As tip moves across surface, it keeps height constant
Tunneling current is measured at each point
Constant current: directly related to electron charge density (not just topography)
Constant height: faster (no feedback loop)

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44. Atomic Force Microscopy (AFM)
Can be used for most samples
Measures:
Small distances:
Van der Waals interactions
Larger distances:
Electrostatic interactions (attraction, repulsion)
Magnetic interactions
Capillary forces (condensation of water between sample and tip)
Source: photonics.com
Source: Nanosurf

45. Atomic Force Microscopy (AFM)
Scan tip across surface with constant force of contact
Measure deflections of cantilever

46. Scanning Probe Techniques
Some instruments combine STM and AFM
Other tip-surface force microscopes:
Magnetic force microscope
Scanning capacitance microscope
Scanning acoustic microscope
Uses:
Imaging of surfaces
Measuring chemical/physical properties of surfaces
Fabrication/Processing of nanostructures
Nanodevices

47. Summary: Techniques used to study nanostructures
Bulk characterization techniques
Information is average for all particles
Surface characterization techniques
Information about individual nanostructures
Bulk: XRD
Surface: SPM, TEM

48. Mosquito Eye 1

49. Salt & Pepper 2

50. Deer Tick Head 3

51. Foot of House Fly 4

52. Porcupine Quill 5

53. Tip of Dentist’s Drill 6

54. Toilet Paper 7

55. Velcro 8

56. Staple in Paper 9

57. Scratch & Sniff Paper 10

58. Needle & Thread 11

59. 12
Toothbrush bristles

60. 13
Used Q-tip

61. Dust 14

62. Used Dental Floss 15

63. Human Eyelashes 16

64. Instant Coffee Granule17

65. Butterfly Wing 18

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Red Blood cells & Virus