Characterization of Nanomaterials…
And the magnification game!
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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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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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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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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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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Crystallography
• Crystals have atoms in ordered lattices
• Amorphous: no ordering of atoms
Crystallography
affects properties
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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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Electron Microscopy
Steps to form an image:
1. Stream of electrons formed by an electron source
and accelerated toward the specimen
2. Electrons confined and focused into thin beam
3. Electron beam focused onto sample
4. Electron beam affected as interacts with sample
5. Interactions / effects are detected
6. Image is formed from the detected signals
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Electron Microscopes
• Electron Beam
– Accelerated and focused
using deflection coils
– Energy:
200 - 1,000,000 eV
• Sample
– TEM: conductive, very
Source: Virtual Classroom Biology
 Detection
thin!
◦ TEM: transmitted e– SEM: conductive
◦ SEM: emitted e-
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EM Resolution
• Resolution dependent on:
• wavelength of electrons ()
• NA of lens system
• Wavelength dependent on:
• Electron mass (m)
• Electron charge (q)
0.612
d
NA
h

2mqV
• Potential difference to accelerate electrons (V)

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Transmission EM
• Magnification:
~50X to 1,000,000X
1. E-beam strikes sample and is
transmitted through film
2. Scattering occurs
3. Unscattered electrons pass
through sample and are detected
http://www.hkphy.org/atomic_world/tem/tem04_e.html
Source: Wikipedia
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Scanning EM
• Magnification:
~10X to 300,000X
1. E-beam strikes sample and electron
penetrate surface
2. Interactions occur between
electrons and sample
3. Electrons and photons emitted from
sample
4. Emitted e- or photons detected
http://virtual.itg.uiuc.edu/training/EM_tutori
al/#segment 1_6
Source: Wikipedia
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SEM: Electron Beam Interactions
• Valence electrons
• Inelastic scattering
• Can be emitted from sample
“secondary electron”
valence e+ +++
++
core enucleus
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
Electron Spectroscopy
Energy
Auger e-
X-ray
Ground state
e- emitted;
excited state
Relaxes to ground state
1. e- or photon strikes atom; ejects core e2. e- from outer shell fills inner shell hole
3. Energy is released as X-ray or Auger electron
EDS: Energy Dispersive X-ray Spectroscopy
AES: Auger Electron Spectroscopy
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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
14
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)
SEM and TEM Data Images
• Ag thin film deposited on Si
substrate (thermal or ebeam evaporation)
• TCNQ (7,7,8,8tetracyanoquinodimethane)
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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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)
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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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
17
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
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
18
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
19
Atomic Force Microscopy (AFM)
• Can be used for most samples
• Measures:
– Small distances:
• Van der Waals interactions
– Larger distances:
Source: photonics.com
Source: Nanosurf
• Electrostatic interactions (attraction, repulsion)
• Magnetic interactions
• Capillary forces (condensation of water between sample
and tip)
Atomic Force Microscopy (AFM)
http://virtual.itg.uiuc.edu/training/AFM_tutorial/
• Scan tip across surface with constant force of contact
• Measure deflections of cantilever
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
http://virtual.itg.uiuc.edu/training/AFM_tutorial/
Summary:
Techniques used to study nanostructures
• Bulk characterization techniques
– Information is average for all particles
• Surface characterization techniques
– Information about individual nanostructures
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