Characterization of Nanomaterials…

1. Characterization of Nanomaterials… And the magnification game!

2. 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!!!)

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

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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

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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 DEMO: Lattice model & laser/skewer

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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

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12. Crystallography • Crystals have atoms in ordered lattices • Amorphous: no ordering of atoms Crystallography affects properties

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14. Microscopes: History • Light 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)

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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

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18. Electron Microscopes • Electron Beam • Accelerated and focused using deflection coils • Energy: 200 - 1,000,000 eV • Sample • TEM: conductive, very thin! • SEM: conductive • Detection • TEM: transmitted e- • SEM: emitted e- Source: Virtual Classroom Biology

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20. EM Resolution • 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)

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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 http://www.hk-phy.org/atomic_world/tem/tem04_e.html Source: Wikipedia

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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 http://virtual.itg.uiuc.edu/training/EM_tutorial/#segment 1_6 Source: Wikipedia

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26. SEM: Electron Beam Interactions valence e- + + core e- + + • 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 + + nucleus

27. Electron Spectroscopy • 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 Auger e- X-ray Energy Relaxes to ground state Ground state e- emitted; excited state

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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

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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. http://nami.eng.uci.edu/projects/Agtcnq.htm

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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

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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

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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 0.2 - 0.6 nm from surface Tunneling Current: ~ 0.1 - 10 nA Resolution: 0.01 nm (in X and Y directions) 0.002 nm in Z direction Source: Univ. of Michigan

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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

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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 http://virtual.itg.uiuc.edu/training/AFM_tutorial/

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

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

48. Mosquito Eye 1

49. Salt & Pepper 2

50. Deer Tick Head 3