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, • What is it made of? • Molecules, elemental proportions • What are it’s properties? • Physical, chemical, electromagnetic
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. http://www.cas.muohio.edu/~mbi-ws/microscopes/history.html
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. http://www.microscope-microscope.org/images/BWScope.jpg
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. http://www.unl.edu/CMRAcfem/temoptic.htm
TEM Images http://www.abdn.ac.uk/emunit/emunit/temcells/index.htm
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. http://www.mos.org/sln/SEM/works/slideshow/semmov.html
SEM Images http://www.mos.org/sln/SEM/works.html
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. http://www.iap.tuwien.ac.at/www/surface/STM_Gallery/index.htmlx
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. http://www.chembio.uoguelph.ca/educmat/chm729/STMpage/stmdet.htm
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 http://www.almaden.ibm.com/vis/stm/gallery.html
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, constantcantilever deflection is maintained. • In tapping mode, the cantilever is oscillated at its resonance frequency. http://www.nanoscience.com/education/AFM.html http://www.azom.com/details.asp?ArticleID=3278
AFM Images http://www.azom.com/details.asp?ArticleID=3278 http://www.nanoscience.com/index.html
AFM Video http://www.nanoscience.com/education/gallery.html
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.