ABC s of Electrochemistry series Materials Characterization techniques: SEM and EDS Ana María Valenzuela-Muñiz November 3, 2011 CEER, Department of Chemical and Biomolecular Engineering
Outline Introduction Physical principles Applications Summary Ohio University - Avionics Engineering Center 2
Introduction Definition 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 3
Introduction Definition 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 4
Introduction How does the SEM work? 5
Introduction How does the SEM work? Electron beam 6
Introduction Interaction of the incident electrons with the sample Incident electron beam Visible light (cathodoluminiscence) Secondary e s Auger e s Back-scattered e s Characteristic X- rays Heat SAMPLE Diffracted e s Transmitted e s Adapted from: L. Fuentes y M. Reyes. Mineralogia analitica. 2002 7
Introduction Interaction region of the incident electron beam Incident electron beam ~ 5 µm SAMPLE Secondary e s Back-scattered e s Characteristic X- rays Adapted from: L. Fuentes y M. Reyes. Mineralogia analitica. 2002 8
Introduction Detection of secondary and back-scattered electrons Detector Detector SAMPLE Secondary e s SAMPLE Back-scattered e s Adapted from: L. Fuentes y M. Reyes. Mineralogia analitica. 2002 9
Introduction Detection characteristic X-rays Si (Li) crystal Field Effect Transistor (FET) and pre-amplifier 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. 10
Physical principles Interaction of the electron beam with the sample 11
Physical principles Interaction of the electron beam with the sample 12
Physical principles Production of characteristic X-Rays http://en.wikipedia.org/wiki/file:edx-scheme.svg 13
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 14
Magnification in the SEM area scanned on the monitor / area scanned on the specimen 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. from about 10 to 500,000 times 15
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) 16
SEM and EDS techniques Strengths Rapid, high-resolution imaging Quick identification of elements present Good depth of field Versatile platform that supports many other tools Limitations 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 17
SEM and EDS techniques Main Uses Reveal topographical surface details High resolution images Detect compositional differences Elemental microanalysis and particle characterization Relevant Industries Aerospace Automotive Biomedical/biotechnology Compound Semiconductor Electronics Industrial Products Pharmaceutical Photonics Polymer Semiconductor Solar Photovoltaics Telecommunications 18
Available system General Overview Located in the Institute for Corrosion and Multiphase Technology at OU JEOL JSM-6390 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 19
Applications 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 20
Applications Images using secondary electrons Topography Morphology Carbon structures form Coal Extracts 21
Applications Images using secondary electrons Carbon nanotubes Length of the CNT Alignment Homogeneity 22
Applications Secondary Electron Image (SEI) Images Back Scattered Electrons (BSE) Pt over Carbon fibers Homogeneity of the electro-plating 23
BSE Applications SEI Images Membrane electrode assembly (MEA) Carbon paper Electrocatalyst Membrane Electrocatalyst Carbon paper 24
BES Applications Images SEI 1 2 Membrane electrode assembly (MEA) 3 4 5 25
Applications Chemical analysis using EDS 2 Membrane electrode assembly (MEA) 4 3 26
SEI Applications Images BSE 5% Pt / CNT Particle size and distribution Sample analyzed in a JSM-7401F in the Nanotechnology National Laboratory, Advanced Materials Research Center, Chihuahua, México (CIMAV S.C) 27
Applications Elemental Mapping using EDS 5% Pt - Ru / NiCNT 28
Summary SEM 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 29
Summary EDS Useful for the determination of the composition Elemental mapping is possible H, He and Li cannot be detected Some elements have overlapping peaks 30
Related Literature From OU Library 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] 31
Related Literature WebPages 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 32
Acknowledgments Institute for Corrosion and Multiphase Technology (ICMT) at Ohio University, for the use of the SEM 33
Questions! For more information visit: http://www.ohio.edu/ceer/ Contact: valenzue@ohio.edu