Microscopy

Scanning Electron Microscopy is used to determine the surface topography of a sample by scattering an electron beam and detecting the intensity of given off by different regions of the sample. Regions of the surface which are highly reflective of the sample usually show up as bright areas, and those that tend to absorb will be darken, or completely black.

Scanning Tunneling Microscopes collect electrons from points on the surface of a sample and use the number of electrons collected from each point to identify surface features. This is accomplished by moving an atomic size tip near the surface of the sample so it is close enough to counteract the attractive force exerted on the electrons by the atoms they are associated with. The tip can then pick up the electron or electrons, depending on whether the region is densely populated with electrons or not.

Transmission Electron Microscopy is most commonly used to analyze internal structural features in very thinly sliced biological samples or for advanced materials. The electron beam actually passes through the sample and is detected below on a phosphorescent screen. The different structures present within the sample (such as a cell wall) absorb more electrons than the cytoplasm, creating sufficient contrast to make out considerable detail.

One drawback to SEM and STM is that they only work well with conducting materials or materials coated with a conducting element. In order to work with a non-conducting material, Atomic Force Microscopy is used. It works in a manner similar to SEM, but the tip is moved up or down by the surface features, and measurements of how far up or down it moves are used to construct a topographic image of the surface of the material.

Atomic Force Microscopy

AFM can determine the topography of a surface by moving a sample under a small silicon nitride tip attached to acantilever which is only a few microns thick (1 micron=0.001 millimeters).

A laser beam monitors the movement of the tip as the sample is moved across the surface. The beam is reflected off the cantilever to a photodiode which monitors changes in the height of the laser, resulting from changes in the height of the cantilever. Once the entire length (y-vector) of the sample has been moved under the tip, it is shifted in the x-vector, and the length of the sample is scanned again. The “lines” resulting from the scans are placed side by side to create a surface topography image.

AFM will essentially work on any solid surface, and has even been used to image biological samples such as tissue. Its limitations are that only small areas can be imaged at a time (typically no more than 5 microns), and if the surface is too rough, the tip has difficulty imaging it.

Scanning Electron Microscopy

The scanning electron microscope generates a beam of electrons in a vacuum. Electromagnetic condensor lenses aim the beam, so it can be focused by an objective lens, before electromagnetic deflection coils expose the beam to the surface to the sample to be scanned. As the beam passes over the sample, the sample releases electrons which are detected by a scintillation material. Once the electrons come in contact with the material, flashes of light are produced. A photomultiplier tube amplifies these flashes. The intensity of the flashes is correlated to the electron beam position to produce acontrasted image of the sample surface.

Modern scanning electron microscopes often work in conjunction with x-ray detection instruments. As the sem electron beam meets the sample, x-rays characteristic of the elements comprising the sample are given off.

Scanning electron microscopy relies on the ability of a compound to interact with an electrical current, so its usefulness in limited to conducting elements or samples whose surface is coated with a conduction element.

Other imaging techniques are employed by modern scanning electron microscopes.Specimen current imaging measures the intensity of the electrical current in the specimen resulting from the electron beam. This data is used to construct a surface topography which is especially helpful in revealing surface defects. Backscatter imaging employs high energy electrons which reach detector situated 180 degrees from the electron beam source. The electron yield from backscatter imaging is directly related to the atomic number of each point scanned on the sample. This aids in determining the composition of the sample.

Scanning Tunneling Microscopy

A Scanning Tunneling Microscope is an instrument which analyzes the surface topography of a conducting material by moving a miniature probe that has a single atom at its apex over the surface of the sample. This maps the contour of the atomic surface. Regular microscopes collect light from various points on a surface and use the number of photons of light collected from each point to map the surface.

Scanning Tunneling Microscopes collect electrons in the form of a current from points on the surface of a sample, as the tip is moved over the surface any differences in height are detected as an increase or decrease in tunneling currect. A feedback circuit adjusts the height of the tip to maintain a constant current between tip and sample. Electrostatic force binds the electrons on the sample surface, thus preventing them from escaping. If another material like the STM tip is brought close enough to the sample surface, the tip electrostatic force can counteract the surface electrostatic force and pull electrons from the sample material. In order for the tip electrostatic force to overcome the surface electrostatic energy, the distance between the two conducting materials must be very small – scanning tunneling microscopes place the tip one to three atom widths from the sample surface.

The tip in a STM must travel very small distances to move from atom to atom. In order to do this, the tip is attached to a piezoelectric tube which can be moved extremely small distances by applying small voltages to it. A computer instructs a small generator how much voltage to apply to the piezoelectric tube in order to move it. As the tip is moved over the sample surface in rows and columns, an image is created based on the measured distance between the tip and surface when a certain number of electrons are collected. Because the STM tip movements are so small, the instrument can’t be exposed to vibrations in the surrounding environment.

If the STM tip is close enough to the sample surface and a strong enough attractive force is present, the tip can pick up an electron from the surface, then deposit it in some other region of the sample when a repulsive force between the tip and the atom is generated. This technique has been used by IBM Almaden research lab to create some interesting “atomic” pictures, such as the Carbon Monoxide Man.

Transmission Electron Microscopy

Transmission Electron Microscopy is used to identify imperfections in the atomic-level structure of material by analysis of microscopic surfaces. A very thin slice of the material to be tested is exposed to a beam of electrons. When the electrons interact with consistent material structure, a constant fraction of electrons is transmitted back from the sample to a detector. Once a structural imperfection is encountered, the fraction of transmitted electrons changes.

Two common methods of TEM microstructural imaging reveal important information about the material being tested. Diffraction contrast is useful in identifying large structures and crystallographic features. Phase contrast is used for high magnification imaging of atomic columns.

Scanned Transmission Electron Microscopy (STEM) is useful for identifying crystal defects and mapping diffracting domains. Electron diffraction analysis can be performed in a variety of modes and provides crystal phase identification, specimen preferred orientation information, and the determination of crystal lattice constants. Electron diffraction is an important tool employed in crystal defect identification. EELS (electron energy-loss spectroscopy) utilizes electrons that have lost energy transiting the specimen. The amount of energy lost is a function of the specimen’s composition. The technique is especially suited to analyzing light elements.

Study Questions

Scanning Electron Microscopy

  1. What would be the typical SEM electron acceleration energy in keV?
  2. Differentiaite between elastic and inelastic scattering of electrons in an SEM.
  3. How many and of what type are the lenses in a typical SEM?
  4. Describe the means of focusing electrons using these lenses.
  5. What is meant by rastering?
  6. What is the typical pressure in an SEM and why is a vacuum necessary?
  7. Can non-conducting samples be imaged in SEM, and if so how?
  8. There are two kinds of electrons detected in SEM: backscattered electrons and secondary electrons. Describe the difference between these.
  9. Certain SEM instruments {mucho expensive} produce X-rays that can be used for element mapping of the surface. How are these X-rays produced?
  10. What is the typical resolution (in microns) of an SEM

Transmission Electron Microscopy

  1. What is the essential difference between a TEM and an SEM?
  2. Would you expect the electron beam to be of higher or lower energy in TEM, than the corresponding energy required in SEM?
  3. What device is used to provide very thin slices of material for TEM analysis?
  4. How is an image formed in TEM, and how is it captured?

Scanning Tunneling Microscopy

  1. What would be the typical distance between the tip and the surface in STM?
  2. What is the purpose of the piezo-electric crystal in an STM?
  3. How is the tip maintained at a constant distance from the surface in STM?
  4. How do we know looking at the image below, that what we are “seeing” can be considered atoms of carbon in graphite? What other info is necessary?
  5. How are problems such as building vibration overcome in STM?
  6. Images are formed by raster scanning. What does that mean?
  7. The best images are obtained from a tip with a single atom at the end. What do you think the image above might look like if it had several atoms at the tip?
  8. What kind of lateral resolution does an STM have with a piezo-electric scanner? (resolution being the distance between two distinguishable points)
  9. How are STM tips made? Describe two of the more common methods using the site below: http://webphysics.davidson.edu/alumni/jocowan/FinalP/finpro.htm

Atomic Force Microscopy

  1. AFM does not rely on an electrical current to form images. On what principle does AFM obtain sub-micron resolution images of surfaces?
  2. How can the problem of “soft” samples being damaged by the silicon nitride tip be overcome?
  3. What can happen to samples if the contact force on the tip is excessive? See http://www.usra.edu/~jwcross/spm/spm-blue.htm for help.
  4. The image below is of a computer hard disk (25 micron area) taken using MFM. What is MFM?