ABC’s of Electrochemistry · Ana María Valenzuela-Muñiz November 3, 2011 CEER, Department of...
Transcript of ABC’s of Electrochemistry · Ana María Valenzuela-Muñiz November 3, 2011 CEER, Department of...
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Ana María Valenzuela-Muñiz November 3, 2011
CEER, Department of Chemical and Biomolecular
Engineering
ABC’s of Electrochemistry
series
Materials Characterization
techniques: SEM and EDS
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2 Ohio University - Avionics Engineering Center
• Introduction
• Physical principles
• Applications
• Summary
Outline
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Scanning Electron Microscopy (SEM)
- Is a method for high-resolution imaging of surfaces -
The electrons interact with the atoms that make up the sample,
producing signals that contain information about the sample's
surface topography, composition, and other properties such as electrical
conductivity
Introduction
Center for Electrochemical Engineering Research, Ohio University
Definition
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Introduction
Center for Electrochemical Engineering Research, Ohio University
Energy Dispersive X-ray Spectroscopy (EDS)
-Is an analytical technique used for the elemental analysis or chemical
characterization of a sample -
Studies the interaction between a source of X-ray excitation, and a
sample. Is based on the fundamental principle that each element has a
unique atomic structure allowing X-rays that are characteristic of an
element's atomic structure to be identified uniquely from one another
Definition
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Introduction
How does the SEM work?
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Introduction
How does the SEM work?
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Electron beam
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Interaction of the incident electrons with the sample
Center for Electrochemical Engineering Research, Ohio University
SAMPLE
Back-scattered e’s
Secondary e’s
Auger e’s
Diffracted e’s
Incident electron beam
Transmitted e’s
Visible light
(cathodoluminiscence)
Characteristic X- rays
Heat
Introduction
Adapted from: L. Fuentes y M. Reyes. Mineralogia analitica. 2002
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Introduction
Interaction region of the incident electron beam
Center for Electrochemical Engineering Research, Ohio University
Incident electron beam
SAMPLE
Back-scattered e’s
Secondary e’s
Characteristic X- rays
Adapted from: L. Fuentes y M. Reyes. Mineralogia analitica. 2002
~ 5 µm
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Detection of secondary and back-scattered electrons
Center for Electrochemical Engineering Research, Ohio University
Introduction
Adapted from: L. Fuentes y M. Reyes. Mineralogia analitica. 2002
SAMPLE
Back-scattered e’s
Detector
SAMPLE
Secondary e’s
Detector
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Detection characteristic X-rays
Center for Electrochemical Engineering Research, Ohio University
Introduction
The crystal absorbs the energy of incoming x-rays by ionization, yielding free
electrons that become conductive and produce an electrical charge bias.
X-rays are converted into electrical voltages of proportional size; the electrical pulses
correspond to the characteristic x-rays of the element.
•Si (Li) crystal
•Field Effect
Transistor (FET)
and pre-amplifier
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Physical principles
Interaction of the electron beam with the sample
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Physical principles
Interaction of the electron beam with the sample
Center for Electrochemical Engineering Research, Ohio University
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Physical principles
Production of characteristic X-Rays
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http://en.wikipedia.org/wiki/File:EDX-scheme.svg
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Resolution of the SEM
Resolution
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Resolution in a perfect optical system can be described mathematically by Abbe’s
equation.
In this equation:
d = 0.612 λ / n sinα where
d = resolution
λ = wavelength of imaging radiation
n = index of refraction of medium between point source and lens, relative to free
Space
α = half the angle of the cone of light from specimen plane accepted by the objective
(half aperture angle in radians)
n sin α is often called numerical aperture (NA)
Resolution depends on the size of the electron spot (wavelength of the electrons and
electron-optical system) and the size of the interaction volume
less than 1 nm and 20 nm
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Magnification in the SEM
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The magnification is defined as, the ratio of the dimensions of the raster on the
specimen and the raster on the display device
Assuming that the display screen has a fixed size, higher magnification is the result
from reducing the size of the raster on the specimen, and vice versa
The magnification is therefore controlled by the current supplied to the x, y
scanning coils, or the voltage supplied to the x, y deflector plates, and not by
objective lens power.
area scanned on the monitor / area scanned on the specimen
from about 10 to 500,000 times
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Sample preparation
Sample preparation is an absolute prerequisite for microscopy and analysis
For conventional imaging, the specimens must be electrically conductive
(at least the surface)
Electrically grounded to prevent the accumulation of electrostatic charge
at the surface
Nonconductive materials tend to charge
Materials for specimen coating: gold, gold/palladium alloy, platinum,
osmium, iridium, tungsten, chromium, and graphite
Handbook of Sample Preparation for SEM and XRay
Microanalysis; Patrick Echlin (2009)
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SEM and EDS techniques
Center for Electrochemical Engineering Research, Ohio University
Strengths Limitations
Rapid, high-resolution imaging
Quick identification of elements
present
Good depth of field
Versatile platform that supports
many other tools
Vacuum compatibility typically
required
May need to etch for contrast
SEM may spoil sample for
subsequent analyses
Size restrictions may require cutting
the sample
Ultimate resolution is a strong
function of the sample and preparation
Some elements can not be detected
in the EDS
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SEM and EDS techniques
Center for Electrochemical Engineering Research, Ohio University
Main Uses Relevant Industries
Reveal topographical surface details
High resolution images
Detect compositional differences
Elemental microanalysis and particle
characterization
Aerospace
Automotive
Biomedical/biotechnology
Compound Semiconductor
Electronics
Industrial Products
Pharmaceutical
Photonics
Polymer
Semiconductor
Solar Photovoltaics
Telecommunications
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Available system
General Overview
Located in the Institute for Corrosion and Multiphase Technology at OU
Resolution: 3.0 nm (30kV)
Magnification: x5 to 300,000
Filament: Pre-centered W hairpin filament
Objective lens: Super conical lens
Objective lens apertures: Three position, controllable in X/Y directions
LGS Type stage: 5" diameter sample coverage
The stage can be tilted
Low vacuum
JEOL JSM-6390
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Topography: The surface features (“how it looks”), and texture
Morphology: The shape, size and arrangement of the particles
Composition: The elements present in the sample
Applications
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Topography
Morphology
Applications
Images using secondary electrons
Carbon structures
form Coal Extracts
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Images using secondary electrons
Length of the CNT
Alignment
Homogeneity
Carbon
nanotubes
Applications
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Images
Homogeneity of the electro-plating
Secondary Electron Image
(SEI) Back Scattered Electrons
(BSE)
Pt over
Carbon fibers
Applications
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Images
SEI BSE
Applications
Membrane electrode
assembly (MEA)
Carbon paper
Carbon paper
Electrocatalyst
Electrocatalyst
Membrane
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SEI BES
1
2
3
4
5
Applications
Images
Membrane electrode
assembly (MEA)
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Chemical analysis using EDS
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3
4
Applications
Membrane electrode
assembly (MEA)
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5% Pt / CNT
Sample analyzed in a JSM-7401F in the Nanotechnology National Laboratory,
Advanced Materials Research Center, Chihuahua, México (CIMAV S.C)
Particle size
and distribution
SEI BSE
Applications
Images
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Elemental Mapping using EDS
5% Pt - Ru / NiCNT
Applications
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Summary
SEM is an analytical technique that can provide a “quick
look” of a material
Resolution between less than 1 nm and 20 nm can be
achieved
Magnification from about 10 to 500,000 times
Versatile platform that supports many other tools
Samples need to be conductive
Sample preparation is an important step
SEM
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EDS
Useful for the determination of the composition
Elemental mapping is possible
H, He and Li cannot be detected
Some elements have overlapping peaks
Summary
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Related Literature
Advanced Scanning electron microscopes X-ray microanalysis; Newbury
Dale E. (1986)
A Guide to materials characterization and chemical analysis; John P.
Sibilia
Electron probe microanalysis and scanning electron microscopy; National
Measurement Laboratory (U.S.). Office of Standard Reference Materials (1981)
Encyclopedia of materials characterization: surfaces, interfaces, thin
films; C. RichardBrundle, Charles A. Evans Jr., Shaun Wilson and Lee E.
Fitzpatrick [electronic resource]
Handbook of Sample Preparation for SEM and XRay Microanalysis; Patrick
Echlin (2009) [electronic resource]
Scanning electron microscopy and X-ray microanalysis : a text for
biologists, materials scientists, and geologists; Joseph I. Goldstein (1981)
New horizons of applied scanning electron microscopy; Kenichi and
Tomoaki (2010) [electronic resource]
Scanning microscopy for nanotechnology: techniques and applications;
Weilie Zhou and Zhong Lin Wang [electronic resource]
From OU Library
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http://www.microscopy.ethz.ch/sem.htm
http://www.purdue.edu/rem/rs/sem.htm
http://www.vcbio.science.ru.nl/en/fesem/eds/
http://www.eaglabs.com/techniques/analytical_techniques/sem.php#appnotes
Related Literature
WebPages
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Acknowledgments
Institute for Corrosion and Multiphase Technology (ICMT) at
Ohio University, for the use of the SEM
Center for Electrochemical Engineering Research, Ohio University