Reminders for this week• Homework #4 Due Wednesday (5/20)• Lithography Lab Due Thursday (5/21)• Quiz #3 on Thursday (5/21) – In Classroom

– Covers Lithography, Special Nanostructures, Characterization of Nanostructures

– Format similar to Quiz #2

Characterization of Nanomaterials

NANO 101Introduction to Nanotechnology

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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• Transmission electron microscopy• Scanning probe microscopy

• Chemical Characterization• Optical spectroscopy• Electron spectroscopy

Characterization Techniques• Individual Measurements

– Electron Microscopy– Scanning Probe Microscopy– Electron Spectroscopy

• Ensemble Measurements– Optical Spectroscopy– Crystallography

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Structural Characterization• Techniques already used for crystal structures

• X-Ray Diffraction

• Many techniques already used for studying the surfaces of bulk material• Scanning Probe Microscopy (AFM & STM) • Electron Microscopes• Provide topographical images

Crystallography

• Arrangement of atoms• Crystals have atoms in ordered lattices• Amorphous: no ordering of atoms

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Source: Wikipedia Source: Encyclopedia of Alternative Energy and Sustainable Living.

Bragg’s Law/Scherrer Equation

• Constructrive interference of X-rays leads to peaks

• Less planes for diffraction -> broadened peaks

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XRD

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Crystallography Controls Properties

• BiVO4 (Monoclinic is photocatalytic,

Tetragonal is not)

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Chem. Soc. Rev., 2013, 42, 2321-2337

Electron Microscopes• 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

Microscopes: History• Light microscopes

– 500 X to 1500 X magnification– Resolution of 0.2 µm– Limits reached by early 1930s

• Electron Microscopes– Use focused beam of electrons instead of

light

• Transmission Electron Microscope (TEM)• Scanning Electron Microscope (SEM)

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Source: Wikipedia

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

• 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- 14

Source: Virtual Classroom Biology

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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NA = n sin θ

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

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Source: Wikipedia

TEM

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http://www.ammrf.org.au/myscope/tem/introduction/

TEM Samples

• Must be ultra thin • More difficult for biological samples 18

http://advanced-microscopy.utah.edu/education/electron-micro/

Nature Protocols 7, 1716–1727 (2012)

TEM Information

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http://commons.wikimedia.org/wiki/File:Mitochondria,_mammalian_lung_-_TEM_(2).jpg

http://www.intechopen.com/books/nanowires-fundamental-research/silicide-nanowires-from-coordination-compound-precursors

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

20Source: Wikipediahttp://virtual.itg.uiuc.edu/training/EM_tutorial

• 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

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Secondary versus Backscattered

• Top – Secondary electron image– Shallow escape

depth -> better resolution

• Bottom – Backscattered electron image– More sensitive to

elements of different masses

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SEM Samples• Must have conductive surface

– Electrons must go around surface not stay on surface

23http://www4.nau.edu/microanalysis/Microprobe-SEM/Imaging.html

STEM• Still requires thin sample• E-beam is scanned as in SEM and secondary or

backscattered electrons can be detected

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http://www.sci.sdsu.edu/TFrey/Bio750/ElecMi8.gif

http://www.thenanoage.com/carbon.htm

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 SpectroscopyEELS: Electron Energy Loss Spectroscopy 25

Electron Spectroscopy

Ene

rgy

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• EELS is sensitive to lighter elements (Carbon –

Transition Metals)• EELs is sensitive to chemical environment

EDS• Chemical Composition

Mapping

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Pb paint chip, SEM, http://www.clemson.edu/restoration/wlcc/equipment_services/equipment/scanning_electron_microscopy.html

Metal-Polymer Core Shellhttp://www.intechopen.com/books/nanocomposites-new-trends-and-developments/ecologically-friendly-polymer-metal-and-polymer-metal-oxide-nanocomposites-for-complex-water-treatme

AES• AES -> Surface

chemical mapping, depth profile

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Battery Electrode Surfacehttp://www.azom.com/article.aspx?ArticleID=8911

J. Mater. Chem., 2010, 20, 4392-4398

EELS

29http://stem.ornl.gov/highlight_direct_determination.shtml

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)