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V. Characterization: Probing the
Structure of Nanomaterials
Experimental observations andmeasurements of the structure andbehavior of nanoscale materials are
challenges that are being met successfullywith a host of techniques involvingScanning Probe Microscopy, for the mostpart.
[Springer Handbook of Nano-Technology,ed. Bharat Bhushan, 2004]
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Methods for the Characterization
of Nanoscale Materials
-Overview and motivation
-Introduction and terminology
-Application to a system of interest
-Microscopy (EM and SPM*)
-Data collection, analysis and
interpretation-Capabilities and Limitations
*(EM) Electron microscopy (scanning and transmission SEM and TEM)
(SPM) Scanning probe microscopy (atomic force and scanning probe)
Prepared by C. B. SCSU.
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Overview and motivation Purpose
Why is characterization necessary?
Link to materials science and nanotechnology (what are therelevant structure-property relationships for the application?)
Focus Nano-size materials and structures Examples nano-application: semiconductor industry.
Semiconductor industry must specify info. of interest
Advanced characterization techniques necessary
Present and future trends for application Ultra small scale features represent challenges
Know the limitations of characterization techniques
Combine complementary methods
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Correlation between Structure
and Properties of Nano-materials Relevant features of nano-structures:
Morphology form and structural properties
Thickness or surface roughness
Elemental/Chemical composition
Qualitative what is present?
Quantitative how much is present?
Crystallographic structure and defects
Identify crystal structure; presence of defects
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Methods for observing nanostructuresOptical Microscope SEM SPM
Sampleoperating
environment
ambientliquid
vacuum
vacuum ambientliquid
vacuum
Depth of field small large medium
Depth of focus medium small small
Resolution: x,y ~0.2 m 2 nm (TEM:
0.1nm)
0.1 - 3.0 nm
Resolution: z N/A N/A 0.01 nm
Magnification
range
1X - 2 x 103X 10X - 10
6X 5 x 10
2X -
108X
Sample
preparation
required
little freeze drying,
coating
none
Characteristics
required of
sample
sample must not be
completely
transparent to light
wavelength used
surface must not
build up charge
and sample
must be vacuum
compatible
sample must
not have
excessive
variations in
surface height
*1 nm =
0.000000001 m;
1 m =0.000001 m
ResolutionHuman Eye
~0.1mm
The challenge:
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Energy Regimes for Material
Characterization
Energy Example applications
1 eV Possessed by evaporated atoms arriving at a substrate
5 eV Possessed by sputtered atoms arriving at a substrate
10-20 eV Required to ionize neutral atoms (Ar ~ 15 eV)
20eV 1keV Possessed by emitted Auger electrons
1 20 keV Possessed by Primary beams in SEM, AES, SIMS
100-300 keV Possessed by primary beams of electrons in TEM
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Characterization methods:
Scanning Probe Microscopy(SPM)SPM methods include scanning tunneling
microscopy (STM) which exploits an
electronic tunneling current, atomic forcemicroscopy (AFM) which measuresminiscule forces between the probe andsample, and transmission electron
microscopy (TEM) whereby a beam ofelectrons is transmitted through an ultrathin specimen, and scanning electronmicroscope (SEM) which images
electrons scattered from a surface.
http://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Electron -
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Scanning Tunneling Microscope
(STM)
STM was developed by Gerd Binning and
colleagues in 1981 at the IBM Zurich
Research laboratory (Nobel Prize, 1986)
Sample
Metallic tip
"
The STM image of Si(100) surface shown below
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The STM as an Atom Manipulator
a STM image showing iron atoms adsorb
on a copper (111) surface forming a
"quantum corral
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Atomic Force Microscopy (AFM)
The AFM was developed by Binning et al in1985;
Flexible
cantilever beam
Sample
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AFM Modes of Operation
Contact mode -- repulsive modeNon Contact mode -- attractive more
There are two distinct regionsdominated by: attractive andrepulsive interactions.
Veeco Practical Guide to SPM (http://www.veeco.com/library/resources.php)
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Transmission Electron Microscopy
section of a cell ofBacillus subtilis, takenwith a Tecnai T-12TEM. The scale bar is200nm.
Concept of
transmission
microscope
http://en.wikipedia.org/wiki/Bacillus_subtilishttp://en.wikipedia.org/wiki/Image:TEM.jpghttp://en.wikipedia.org/wiki/Bacillus_subtilis -
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Scanning Electron Microscopy
(SEM)
Pollen grains taken on an SEM show the
characteristic depth of field of SEM
micrographs.
http://en.wikipedia.org/wiki/Image:Misc_pollen.jpg -
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Electron MicroscopySEM vs. TEM
SEM
TEM
sample
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Some general Features of Electron
Microscopy
Data Interpretation and limitations Beam interactions image interpretation is complex
Resolution limited by beam diameter and there are
trade-offs between resolution and signal intensity.
Sample preparation Sample must be thin enough for e-
beam transparency.
Tip effects must be considered
Resolution is limited by sample
morphology
Data interpretation is complex
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Electron Microscopy at Nano-scale
Electron micrograph of typical silicon
nanocomposite cross section showing
uniform distribution of conductive carbon
nanotube network. Photo courtesy of U.S.Air Force.
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Data CollectionAtomic Force Microscope
Transmission Electron Microscope
Scanning Electron Microscope
Imaging materials at the atomic scale -- Nanotechnology
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Conventional TEM (120kV) HRES TEM (200kV)
Semiconductor
insulatormetal
An example of Nanotechnology:
Nano-scale imaging of a transistor gate
insulator
metal
Semiconductor
50 nm scale 5 nm scale
Data Collection, Analysis and Interpretation
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Additional References
Sarid, Dror., Scanning Force Microscopy; Oxford Press
Williams and Carter, Transmission Electron Microscopy, Plenum Press
Scanning Electron Microscopy and X-Ray Microanalysis, KluwerAcademic/Plenum publishing
Kevex Corporation, Energy-Dispersive X-Ray Microanalysis
Briggs and Seah, Practical Surface Analysis, Wiley
Wolf and Tauber, Silicon Processing for the VLSI Era, Lattice Press
Park Scientific Instruments,A Practical Guide to Scanning ProbeMicroscopy