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 Beam– Accelerated and focused

using deflection coils– Energy:

200 - 1,000,000 eV

• Sample– TEM: conductive, very

thin!– SEM: conductive

Electron Microscopes

Detection◦TEM: transmitted

e-◦SEM: emitted e-

Source: Virtual Classroom Biology

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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)

•Potential difference to accelerate electrons (V)

h

2mqV

NAd

612.0

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Transmission EM

• Magnification:

~50X to 1,000,000X

1. E-beam strikes sample and is transmitted through film

2. Scattering occurs3. Unscattered electrons pass

through sample and are detected

Source: Wikipedia

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

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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

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

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• 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

SEM: Electron Beam Interactions

+++

+++

valence e-

core e-

nucleus

1. e- or photon strikes atom; ejects core e-2. e- from outer shell fills inner shell hole3. Energy is released as X-ray or Auger electron

EDS: Energy Dispersive X-ray SpectroscopyAES: Auger Electron Spectroscopy

Electron SpectroscopyE

nerg

y

Ground state e- emitted; excited state

Relaxes to ground state

X-ray

Auger e-

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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

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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 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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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

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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 nAResolution:

0.01 nm (in X and Y directions)0.002 nm in Z direction Source: Univ. of Michigan

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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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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: NanosurfSource: photonics.com

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/

Scanning Probe Techniques

Other tip-surface force microscopes:• Magnetic force microscope• Scanning capacitance microscope• Scanning acoustic microscope

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

Uses:• Imaging of surfaces• Measuring chemical/physical properties of surfaces• Fabrication/Processing of nanostructures• Nanodevices

Some instruments combine STM and AFM

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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