abc’s of electrochemistry · ana maría valenzuela-muñiz november 3, 2011 ceer, department of...

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

2 Ohio University - Avionics Engineering Center

• Introduction

• Physical principles

• Applications

• Summary

Outline

3

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

4

Introduction


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

5

Introduction

How does the SEM work?


6

Introduction

How does the SEM work?


Electron beam

7

Interaction of the incident electrons with the sample


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

8

Introduction

Interaction region of the incident electron beam


Incident electron beam

SAMPLE


Secondary e’s

Characteristic X- rays


~ 5 µm

9

Detection of secondary and back-scattered electrons


Introduction


SAMPLE


Detector

SAMPLE

Secondary e’s

Detector

10

Detection characteristic X-rays


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

11

Physical principles

Interaction of the electron beam with the sample


12

Physical principles

Interaction of the electron beam with the sample


13

Physical principles

Production of characteristic X-Rays


http://en.wikipedia.org/wiki/File:EDX-scheme.svg

14

Resolution of the SEM

Resolution


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

15

Magnification in the SEM


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

16 Center for Electrochemical Engineering Research, Ohio University

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)

17

SEM and EDS techniques


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

18

SEM and EDS techniques


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


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


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


Topography

Morphology

Applications

Images using secondary electrons

Carbon structures

form Coal Extracts


Images using secondary electrons

Length of the CNT

Alignment

Homogeneity

Carbon

nanotubes

Applications


Images

Homogeneity of the electro-plating

Secondary Electron Image

(SEI) Back Scattered Electrons

(BSE)

Pt over

Carbon fibers

Applications


Images

SEI BSE

Applications

Membrane electrode

assembly (MEA)

Carbon paper

Carbon paper

Electrocatalyst

Electrocatalyst

Membrane


SEI BES

1

2

3

4

5

Applications

Images

Membrane electrode

assembly (MEA)


Chemical analysis using EDS

2

3

4

Applications

Membrane electrode

assembly (MEA)


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


Elemental Mapping using EDS

5% Pt - Ru / NiCNT

Applications


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


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


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


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



33

Acknowledgments

Institute for Corrosion and Multiphase Technology (ICMT) at

Ohio University, for the use of the SEM


Questions!

For more information visit:

http://www.ohio.edu/ceer/

Contact:

[email protected]

abc’s of electrochemistry · ana maría valenzuela-muñiz november 3, 2011 ceer, department of...

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