Electron Microscopes

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Electron Microscopes

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1. Electron Microscopes Stephen Douglass Amy McPeak

2. What are electron microscopes? Electron microscopes are instruments that use beams of highly energetic electrons to examine and magnify very small details on objects (use wavelengths in the 10-12 m range) Can magnify at levels up to 2 million times Have high resolving power

3. History Ernst Ruska, a German Physicist, built the first electron microscope in 1932 and was awarded the Nobel Prize for it in 1986 Ruska’s invention combined two concepts : 1) electrons have a wave-like component so they can be treated in a fashion similar to light waves 2) electrons can be manipulated by magnetic fields so they can be focused as optical lenses focus light

4. Ruska’s Original Plan

5. Why Invent Electron Microscope? Ruska realized that the electron microscope would be much more powerful than an ordinary optical microscope and therefore would allow for greater magnification Why: Magnification increases with shorter wavelengths Electron waves are shorter than ordinary light waves and even X-rays 10-6m for light waves 10-10m for X-rays 10-12 m for electrons

6. Why Not Just Use Light Microscope? The maximum resolution of an imaged object is determined by the wavelength of the photons that probe the object Visible light’s wavelengths range from 400-700nm , which is larger than many objects that one might wish to image. Ultraviolet light has problems with absorption X-rays sometimes exhibit a lack of interaction with the sample and in focusing

7. Basic Plan Light Microscope Vs. Electron Microscope

8. Light Microscope Vs. Electron Microscope

9. How do Electron Microscopes Improve Resolution? Resolution refers to the finest detail that can be distinguished in an image Resolving power and magnification are NOT the same thing Ex: enlarge a picture again and again, the magnification will increase but the resolution will decrease because image will blur Minimum separation that can be resolved: d = ?/(2n sin?) (inversely proportional to resolution) Improve resolution by using lower ? or medium with higher index of refraction

10. Disadvantages of Electron Microscopes Expensive to buy and maintain Sensitive to external magnetic fields and vibrations Samples have to be viewed in a vacuum (in almost all cases) Not as useful for imaging non-conductive specimens

11. Putting His Plan into Action Ruska and German physicist Max Knoll built the first electron microscope in 1932 The microscope was fairly impractical, but it was able to magnify objects 400 times Using Knoll’s model, Eli Franklin Burton built the first practical electron microscope in 1938

12. How do Electron Microscopes Work? The Basics: 1) An electron source forms a stream of electrons 2) Electrons are accelerated toward object using positive electrical potential 3) Metal apertures and magnetic lenses are used to focus the stream of electrons into a thin, focused, monochromatic beam 4) Magnetic lenses are used to focus the beam onto the object 5) Interactions between the sample and the beam affect the flow of electrons from the beam and are recorded by the electron detector 6) These recorded effects on the electron beam are used to create an image

14. What Information Can They Provide Topography Texture, surface features Morphology Shape and size of particles making up object, reactivity Composition Elements that make up object, material properties, melting/boiling point Crystallographic Information Arrangement of atoms, conductivity, electrical properties

15. Types of Electron Microscopes Transmission Electron Microscope (TEM) Scanning Electron Microscope (SEM) Scanning Transmission Electron Microscope (STEM) Reflection Electron Microscope (REM) Scanning Tunneling Microscope (STM)

16. Scanning Tunneling Microscope (STM) Invented in 1981 Non-optical Type of scanning probe microscopy Refers to a type of microscopy where only a very small volume of sample is being “examined” at a time with a very tiny probe. By moving the probe around very small increments at a time, the entire sample can be scanned. A plot of probe-sample interaction as a function of position is used to create a 3D image of the surface Uses quantum mechanics principles to determine the height of a surface

17. Scanning Tunneling Microscope Allows for imaging on nm scale Not restrained by wavelength of light or electrons Can resolve atoms

18. How does the Scanning Tunneling Microscope Work? An “atomically sharp probe (the tip)” is brought to the specimen that is being imaged Once the tip is right above the surface of the specimen, a voltage is applied between the probe and the surface of the specimen. An electric current (usually around 1nA) is created and the electrons tunnel from the tip of the probe to the surface or move the other direction depending on the polarity of the specimen. The size of the current depends on the distance between the probe and the surface (exponential relationship) Current is kept constant by moving the tip up and down (controlled by servo (feedback) loop

20. Tunneling current obeys Ohm’s law

21. Current as a Function of D For every 0.1nm increase in the distance between the tip and the surface, the tunneling current reduces by a factor 10 Therefore over a typical atomic diameter (0.3 nm) the tunneling current changes by a factor 1000 This is why STM is so sensitive

22. Feedback Loop The tunneling current It is converted by a pre-amplifier to a voltage Vt Voltage Vt is amplified logarithmically Logarithm of the voltage Vt is a measure of the distance between the tip and the surface. (since they are exponentially related) A Vref value is chosen that gives the desired tip to surface distance If Vlog - Vref =0 then tip is where we want it If Vlog - Vref >0 tip is too close If Vlog - Vref <0 tip is too far The high-voltage amplifier amplifies the delta between Vlog and Vref and passes it on to the piezo element Piezo element controls the height of the tip

23. Feedback Electronics

24. Use Plot of Height Vs. Position to Create 3D Image of Surface

25. Another 3D Image

26. 3D Images and Manipulations Used to obtain images of conductive surfaces at an atomic scale 2 × 10-10 m or 0.2 nm Can have enough resolution to show single atoms. The STM will get within a few nanometers of what it is observing. Also can be used to alter the specimen by manipulating individual atoms. Start chemical reactions Create ions by removing electrons

27. E. Coli under SEM

28. DNA Under Electron Microscope

29. Organelle Under Electron Microscope

30. Breast Cancer Cell Under Electron Microscope

31. Some More Images Under SEM….Just for the fun of it!

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