baudrunner's space: Seeing small, smaller, smallest
"Philosophy to Science - Quark to Cosmos. Musings on the Fundamental Nature of reality"

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Monday, January 21, 2008

Seeing small, smaller, smallest

While the ordinary microscope will to some extent remain a novelty for many students, I think that probably most children who have expressed an interest in science have at some time in their young lives been at the receiving end of one of those wondrous instruments of investigation and learning, boxed in its own wooden case and complete with dissection tools and sample slides. But there are microscopes and there are microscopes. In an effort to open the world of the invisible well beyond the range of the smallest optical wavelength, the changeable eyepiece has given way to the computer keyboard and the lenses have been replaced by finely controlled magnets and coils. The price tag too has changed, from the ten dollar instrument of our youth to the million dollars and over for the latest state of the art scanning electron microscopes, focused ion beam microscopes, atomic force microscopes and transmission electron microscopes capable of resolving individual atoms.

The Electron Microscope was co-invented by Germans Max Knoll and Ernst Ruska in 1931 for which Ernst Ruska was awarded half of the Nobel Prize for Physics in 1986, the other half being divided between Heinrich Rohrer and Gerd Binnig for the Scanning Tunneling Microscope.

The Transmission Electron Microscope (TEM) was the first electron microscope to be developed and is modelled exactly after the optical light transmission microscope except that it focuses a tight beam of monochromatic (single wavelength) electrons. Instead of lenses, magnetic confinement and focusing rings and coils image the sample. Multiple stages of metallic apertures, optionally including diffraction apertures, and magnetic enlargement rings produce the image which is formed on an anode phosphor sensing screen.

Max Knoll first described the concept of The Scanning Electron Microscope (SEM) in 1935. This is the device responsible for the many popular extreme close-up images of flies and mosquitoes and other interesting things that we have all seen in popular publications. The first SEM image, with a resolution of about 50 nanometers, was obtained in the RCA laboratories in the United States in 1942. Much credit for the development of the SEM which led to the advanced devices with today's capabilities should go to C. W. Oatley, a lecturer in the Engineering Department of Cambridge University, England, who in the late 1940's became interested in the work of V. E. Cosslett, also in Cambridge at the Physics Department who was investigating the TEM. Oatley and a student by the name of Dennis McMullan built their first SEM which by 1952 would produce the first high-quality image clearly showing three dimensional characteristics. Under Oatley and chief technician Les Peters, a constant contributor to all of the Cambridge SEM designs, various research teams continued to improve on the device and by 1960 SEM stereo-micrographs with quantifiable depth information were being produced.

The SEM differs from the TEM in that the image is produced by mapping the count of the secondary electrons emitted from a target sample when it is struck by a finely focused monochromatic electron beam which scans the sample in a raster manner, like the electron beam in a TV picture Tube scans the phosphor surface of the tube. Some interesting images created by the SEM can be viewed here.

The Scanning Tunneling Microscope (STM) was invented in 1981 and provided the first images of individual atoms on the surfaces of materials. The principle involves slowly scanning a very fine conducting probe with a tip formed by a single atom over a surface at a distance approaching an atom's diameter. The STM can resolve details as tiny as 0.04 of the diameter of an atom. It is generally used for the study of surfaces and performs optimally when sampling conducting materials but can be used to resolve organic molecules when they are fixed on a surface. To that end it has been used to study DNA molecules. Needless to say this is a very sensitive fine precision instrument, and as much technology, if not more, goes into the construction of the stable foundation upon which it rests than goes into the instrument itself.

After the introduction of the Scanning Tunneling Electron Microscope in 1981 the invention of Atomic Force Microscopy in 1986 made it is possible to detect magnetic forces, electrical forces, frictional forces, surface elasticity, and visco-elasticity, etc. The Atomic Force Microscope (AFM) resolves details to fractions of a nanometer. In this device the tip of a scanning tunneling microscope is replaced with a curved silicon or silicon nitride cantilever of nanoscopic proportions which rises and falls as it is brought close to the sample's surface due to forces that exist between the tip and the surface. An image is produced by mapping the motions of the scanning tip, which reveals the surface topography. The real advantage over STMs is that the AFM can produce a true three-dimensional surface profile.

Lateral scanning of the cantilever probe with a constant separation between the probe and the sample surface and mapping capacitive reactance as the surface topography varies is the principle behind the Kelvin Probe Force Microscope (KPFM). The KPFM can produce topographical maps as well as chart the contact potential of the surface.

As if seeing individual atoms aren't enough, the Magnetic Resonance Force Microscope (MRFM) makes possible detection of the magnetic spin of a single electron. In this instrument, the AFM is combined with Magnetic Resonance Imaging principles, replacing the constant resolution coil of the MRI with a magnet-tipped nano-sized cantilever vertically positioned over the sample. With the MRFM the resolution of the image is determined by the dimensions of the device. The sensitivity of the MRFM is 10 billion times better than the MRI. The AFM was developed at IBM's Almaden Research Center. The first MRFM image was obtained in 1993 and detection of a single electron's magnetic spin was achieved in 2004.

The Focused Ion Beam Microscope (FIBM) is a multi-functional device which can be used in a similar manner to the SEM. Instead of a monochromatic electron beam, the FIBM uses a finely focused gallium ion beam. For imaging purposes the beam is operated at low currents. At high currents the FIBM is used for precision milling purposes to the sub-micron scale. This makes the FIBM a very useful tool in the nanotechnological fabrication of sub-micron devices.

Desktop industrial laboratory electron microscopes are becoming readily available which can magnify samples up to one million times and resolve individual atoms. The Field-Emission Environmental Scanning Electron Microscope (ESEM-FEG) is becoming a common sight in a variety of labs around the world. Currently, there are over 50,000 electron microscopes in use in industrial and academic laboratories around the world.

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