merkle@parc.xerox.com (Ralph Merkle) (03/21/90)
I'm forwarding this from out internal Xerox mail group on nanotechnology. I hope the formatting is acceptable. ---------------------------------------------------------------- Sender: JimDay:PASA:Xerox Date: 13 Mar 90 07:51:08 PST (Tuesday) Subject: Children of the STM From: JimDay To: Gloger:ES AE, Wedekind:El Segundo cc: JimDay Science, February 9: The success of the scanning tunneling microscope has inspired a whole generation of imaging devices that use everything from magnetic forces to sound waves to examine samples on an atomic scale. These children of the STM include: The Atomic Force Microscope In 1985, scientists modified the STM to avoid the need for a tunneling current. Instead of bringing the tip close to the sample, they pushed it right up against the surface. Keeping the force between the tip and surface constant, the tip is scanned across a sample like a phonograph needle running along a groove in a record. Recording the tip's motion produces a topographic image of the sample. Because the AFM doesn't depend on an electric current, it can scan nonconductors as easily as conductors. The Friction Force Microscope In 1987, scientists modified an AFM to measure how much the tip drags as it is moved across a surface. The goal was to investigate friction on an atomic scale. As the probe drags across the surface of a sample, the friction increases and decreases at regular intervals -- a distance equal to the distance between atoms on the surface. The Magnetic Force Microscope By using a magnetized tip on a probe scanned a few tens of nanometers above a surface, researchers found they could map out magnetic fields along a sample with a resolution of 100 nanometers. That number has since been improved to 25 nanometers. The MFM has been especially useful in studying magnetic recording devices such as computer hard disks. The Electrostatic Force Microscope A scanning microscope with an electrically charged tip will respond to electric charges on a surface. In early 1988, scientists reported using such a microscope to study the electronic properties of a silicon sample on an atomic scale. More recently, the EFM has been used to study contact electrification, a phenomenon important to such things as xerography. The Attractive Mode Force Microscope The AMFM depends on the attractive forces between molecules. When the probe tip is brought within 2 to 20 nanometers of a sample, the two are pulled together by van der Waals forces as well as by the surface tension created by water molecules that condense out of the air between the tip and the sample. Monitoring the attractive force produces a topographic map of the sample's surface. The Scanning Thermal Microscope This is the world's smallest thermometer. It consists of a tiny thermocouple on a scanning probe. Since the voltage induced across the thermocouple is proportional to its temperature, the probe can be scanned across a surface and record temperature changes. The scanning thermal microscope has a resolution of tens of nanometers and could be used to monitor temperature variations in living cells. The Optical Absorption Microscope This is similar to the scanning thermal microscope. It can be used to determine the chemical composition of a surface by using absorption spectroscopy. The technique depends on the fact that elements vary in the efficiency with which they absorb light of different wavelengths. Shining a laser on a sample heats up some atoms more than others, and the thermal microscope can detect these temperature differences. Varying the wavelength of the laser light gives an absorption spectrum of the surface, which i n turn gives its chemical composition. This method gives a resolution of 1 nanometer, allowing it to record the spectrum of a single molecule. The Scanning Ion-Conductance Microscope This microscope uses probes made from glass micropipettes with tips having an inner diameter of 50 to 100 nanometers. When a probe tip is placed in a salt solution covering a sample, the sodium and chlorine atoms will carry a current through the pipette until it comes in contact with the sample. The current flowing through the pipette thus provides a sensitive indication of how close the tip is to the surface. The Scanning Near-Field Optical Microscope A normal optical microscope is unable to give a resolution better than about 200 nanometers, or about half a wavelength of light. But this limit can be circumvented by use of a near-field technique where either the light source or the detector is smaller than the light's wavelength and is brought very close to the subject. Using this method, researchers have pieced together visible light pictures of samples, pixel by pixel, by scanning them with probes having apertures of 50 nanometers or less. The Scanning Acoustic Microscope A project to develop a scanning acoustic microscope actually predates the invention of the STM. Stanford researchers have been imaging samples with sound, using a sonar-like technique. They have obtained resolution of 30 to 50 nanometers by using sound frequencies of 8 gigahertz and cooling samples to less than 0.5 K in liquid helium. The Molecular Dipstick Microscope A clever application of the atomic force microscope, the molecular dipstick illustrates the versatility of the new microscopes. To measure the thickness of very thin lubricant films, researchers have lowered the tip of an atomic force microscope into the lubricant like a dipstick probing an automobile's oil supply. The tip feels a strong attractive force from surface tension when it touches the lubricant and then a repulsive force when it reaches the underlying surface. The molecular dipstick can determ ine the depth of a lubricant to an accuracy of 5 angstroms.