November 16, 2007

Prof Improves Microscopy

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With only a small coil, smaller than the average human fingernail, Prof. Keith Schwab, physics, and his associates at Boston University have improved the speed at which atoms can be photographed 100-fold. He does this by using the Scanning Tunneling Microscope.
Prior to Schwab’s discovery, scientists had to use voltage applied to a sample while moving the probe tip, in order to generate a topical image of the surface. The images were generated by the variations in the current, produced by the tunneling of electrons between the sample and the probe tip, as the probe crossed over the surface.
According to their paper “Radio-Frequency Scanning Tunneling Microscopy” published in the Nov. 1 issue of “Nature,” “The scanning tunneling microscope relies on localized electron tunneling between a sharp probe tip and a conducting sample to attain atomic-scale spatial resolution.”
This technique is so powerful that it can generate images of a surface at the atomic level, images necessary to research in semi-conduction and nano-technology. While the electrons themselves move at approximately one billion cycles per second of bandwidth, the standard technique can only collect data at one thousand cycles per second because of the limitations of the equipment.
“The electrical properties of the tip and the surface are high resistance, therefore they are very slow. It takes a long time to charge the circuit because of the high resistance which is a limitation to speed,” Schwab said.
To correct this problem, Schwab and his associate Kamil Ekinci combined Schwab’s experience with similar electronic equipment using coils and Ekinici’s research laboratory to devise a new technique for the scanning tunneling microscope with the support of the National Science Foundation.
“It was good, old-fashioned radio frequency engineering. The real trick was a well-placed coil that costs about a dollar that [one] can get free online,” Schwab said.
They managed to control, and send down the probe tip, a high frequency wave. When the wave leaves the tip and hits the sample, it reflects back. The extent of the reflected wave indicates the amount of resistance at the probe tip. Scientists had always been measuring resistance, so his discovery was in the speed at which it could be done.
“Measuring the current is a slow technique; the high frequency wave goes faster, better bandwidth; we can see changes a lot more quickly,” said Schwab.
Besides bringing response time from approximately 10 milliseconds to 100 nanoseconds, the new technique, called reflectometry, is also capable of performing thermometry, which produces an image of what is occurring thermally at the probe tip. Regarding thermometry, Schwab said, “If you put the needle at the surface and don’t even move it, the noise that the electrons make when they tunnel from tip to the surface can actually be used to measure the temperature.”
Additionally, the scanning tunneling microscope is expected to be capable of quantum-limited position measurements, which means that even things moving at the smallest of frequencies can be measured using megahertz.
“We discovered three implications that we think can lead to other things. I believe a lot of researchers will use this technique in the future; it is very simple, and it gives a huge window. The most widespread applications are things we aren’t even thinking about,” Schwab said.
“These are basic capabilities that can be used easily, but my guess is that people will use this technique for other types of experiments and do some great things. I don’t know what to expect, but it will be useful to the community,” he said.