Microscopy as a Means for Nano-Characterization By Thomas Williams

Microscopy as a Means for Nano-Characterization

• By Thomas Williams

• Phys 3500

What is Microscopy?

• Microscopy is any technique for producing visible images of structures or details too small to otherwise be seen by the human eye.

What is Nano Characterization?

• What does it look like?

• Dimensions, structure,

Why Microscopy?

• In order to effectively study something or build something it is important to see exactly what it is we’re doing.

• As the things we are interested in get smaller and smaller we need more better, meaning more powerful microscopy.

• Eventually this will necessitate advances in the physics.

The Origens of Microscopy

• In the first century AD Romans invented glass and began experimenting with various shapes, stumbling upon the converging lens.

• In approx. 1590 Dutch eyeglass makers Hans and Zacharias Jensonn makes a compound microscope.

• Mid 17th century Anton Van Leeuwenhoek uses an improved single lens microscope to view and describe bacteria, protozoan, etc.

Age of the Optical Microscope

• In the late 17th century Robert Hooke added a third lens, greatly improving contrast issues and comfort.

• Over the next two hundred years optical microscopy revolutionizes science, especially biology.

• During this time improvements are continually made, including corrections for chromatic spherical aberrations.

• In the late 19th century, Ernst Abbe showed that the improvement of the magnification of optical microscopes was fundamentally limited by the wavelength of light.

History of Electron Microscopy

• 1931- Ernst Ruska co-invents the electron microscope.

• 1938- 10nm resolution reached.
• 1940- 2.4 nm resolution.
• 1945- 1.0nm resolution achieved.

• 1981- Gerd Binning and Heinrich Rohrer invent the scanning tunneling electron microscope (STM).

• 1986- The Atomic Force Microscope was developed in collaboration between IBM and Stanford University.

Transmission Electron Microscope (TEM)

• Same principle as optical microscope but with electrons.

• Condenser aperture stops high angle electrons, first step in improving contrast.

• The objective aperture and selected area aperture are optional but can enhance contrast by blocking high angle diffracted electrons

• Advantages: we can look at non conducting samples, i.e. polymers, ceramics, and biological samples.

TEM Images

Scanning Electron Microscope (SEM)

• The SEM functions much like an optical microscope but uses electrons instead of visible light waves.

• The SEM uses a series a series of EM coils as lenses to focus and manipulate the electron beam.

• Samples must be dehydrated and made conductive.

• Images are back and white.

SEM Images

Scanning Tunneling Electron Microscope (STM)

• Basic principle is tunneling.

• Tunneling current flows between tip and sample when separated by less than 100nm.

• The tunneling current gives us atomic information about the surface as the tip scans.

What is tunneling?

• The probability that the electron will exist outside the barrier in the vacuum is non zero.

• If these leak-out waves overlap and a small bias voltage is applied between the tip and the sample, a tunneling current flows.

• The magnitude of this tunneling current does not give the nuclear position directly, but is directly proportional to the electron density of the sample at a point.

What does piezo-electric mean?

• In 1880 Pierre Curie discovered that by applying a pressure to certain crystals he could induce a potential across the crystal.

• The STM reverses this process. Thus, by applying a voltage across a piezoelectric crystal, it will elongate or compress.

• A typical piezoelectric material used in an STM is Lead Zirconium Titanate.

STM Images

Atomic Force Microscopy (AFM)

• AFM is performed by scanning a sharp tip on the end of a flexible cantilever across the sample while maintaining a small force.

• Typical tip radii are on the order of 1nm to 10nm.

• AFM has two modes, tapping mode and contact mode.

• In scanning mode, constant cantilever deflection is maintained.

• In tapping mode, the cantilever is oscillated at its resonance frequency.

AFM Images

AFM Video

Future / Conclusions

• We still have a long way to go before we’ve exhausted the limits of electron wavelength resolution limit.

• The wave length of a high energy electron is on the order of .001nm or 1.0pm, our current best resolution with an STM is only approximately .1nm.

• Limiting factors include, aberations, contrast,

References

• Wikipedia - http://en.wikipedia.org/wiki/Main_Page

• History of the Microscope - http://www.cas.muohio.edu/~mbi-ws/microscopes/history.html

• Molecular Expressions - http://microscopy.fsu.edu/primer/museum/hornyolddissecting1920.html

• Dictionary.com - http://dictionary.reference.com/

• Micro-bus -http://www.microscope-microscope.org/microscope-home.html

• BBC H2G2 - http://www.bbc.co.uk/dna/h2g2/

• About.com - http://about.com/

• MOS - http://www.mos.org/sln/SEM/works/slideshow/semmov.html

• UNL - http://www.unl.edu/CMRAcfem/temoptic.htm

• IBM - http://www.ibm.com/us/

• AZOM.com - http://www.azom.com/default.asp

• Nanonscience Instruments - http://www.nanoscience.com/index.html

Special Thanks

• Dr. Tapas Kar & the Fall 06 Nano-Chemistry Crew.

• Google, and their amazing database of resources.

• Utah State, for seeing the growing need to offer classes in nanotechnology.