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 Scanning Electron Microscopy (SEM) ◦ Uses ◦ Sample Preparation ◦ Instrument ◦ Principles ◦ Micrographs  Transmission Electron Microscopy (TEM) ◦ Uses.

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Presentation on theme: " Scanning Electron Microscopy (SEM) ◦ Uses ◦ Sample Preparation ◦ Instrument ◦ Principles ◦ Micrographs  Transmission Electron Microscopy (TEM) ◦ Uses."— Presentation transcript:

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2  Scanning Electron Microscopy (SEM) ◦ Uses ◦ Sample Preparation ◦ Instrument ◦ Principles ◦ Micrographs  Transmission Electron Microscopy (TEM) ◦ Uses ◦ Sample Preparation ◦ Instrument ◦ Principles ◦ Micrographs

3  Topography ◦ Texture/surface of a sample  Morphology ◦ Size, shape, order of particles  Composition ◦ Elemental composition of sample  Crystalline Structure ◦ Arrangement present within sample

4  Samples must be small enough to fit in sample chamber ◦ Most modern microscopes can safely accommodate samples up to 15cm in height  Samples must be electrically conductive  Polymer samples typically need to be sputter coated to make sample conductive ◦ Ultra-thin metal coating ◦ Usually gold or gold/palladium alloy ◦ Coating helps to improve image resolution

5  Once sample is properly prepared, it is placed inside the sample chamber  Once chamber is under vacuum, a high voltage is placed across a tungsten filament to generate a beam of high energy electrons (electron gun) and serves as the cathode  The position of the anode allows for the generated electrons to accelerate downward towards the sample  Condensing lenses “condense” the electrons into a beam and objective lenses focus the beam to a fine point on the sample

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7  Scanning coils move the focused beam across the sample in a raster scan pattern  Same principle used in televisions  Scan speed is controllable

8  As electron beam strikes sample, secondary electrons are emitted from the sample  In addition, backscattered electrons are also emitted from the sample

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10  Secondary Electrons  Back-Scattered Electrons

11  Electrons strike the sample surface  As a result, some electrons “splash” out from the sample (secondary electrons)  A detector with a strong positive charge attracts these electrons, however depending on the surface topography, not all electrons will be attracted ◦ Electrons on high “peaks” will be attracted to the positively charged detector ◦ Electrons in low “valleys” will not be attracted to the detector

12 Crystalline Latex ParticlesPolymer Hydrogel Surface

13 SEM Images of PVEA Comb-like Terpolymers

14  Electrons from high-energy beam strike the sample  Some electrons pass close to a nucleus and are deflected by the positive charge ◦ These back-scattered electrons return to the sample surface moving at high speed  Back-scattered electrons is dependent on atomic number of sample ◦ Can provide elemental composition information about a sample

15 BSE Micrograph Showing Crystalline Lamellae

16  Morphology ◦ Shape, size, order of particles in sample  Crystalline Structure ◦ Arrangement of atoms in the sample ◦ Imperfections in crystalline structure (defects)  Composition ◦ Elemental composition of the sample

17  Samples need to be extremely thin to be electron transparent so electron beam can penetrate  Ultramicrotomy is a method used for slicing samples ◦ Slices need to be 50-100nm thick for effective TEM analysis with good resolution

18  Instrument setup is similar to SEM  Instead of employing a raster scan across the sample surface, the electron beam is “transmitted” through the sample  Material density determines darkening of micrograph ◦ Darker areas on micrograph indicate a denser packing of atoms which correlates to less electrons reaching the fluorescent screen  Electrons which penetrate the sample are collected on a screen/detector and converted into an image

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20 Isotactic Polypropylene Particle

21 Large Ceria Nanoparticles Small Ceria Nanoparticles

22 Microcrystalline Cellulose

23 Pros  Easier sample preparation  Ability to image larger samples  Ability to view a larger sample area Cons  Maximum magnification is lower than TEM (500,000x)  Maximum image resolution is lower than TEM (0.5nm)  Sputter coating process may alter sample surface

24 Pros  Higher magnifications are possible (50,000,000x)  Resolution is higher (below 0.5Å)  Possible to image individual atoms Cons  Sample preparation  Sample structure may be altered during preparation process  Field of view is very narrow and may not be representative of the entire sample as a whole

25 http://www.mrs.org/s_mrs/doc.asp?CID=1803&DID=171434


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