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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