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Speeding Up STM Imaging
Roland Piquepaille writes "Probably not many of you have used a scanning tunneling microscope (STM), the essential tool of nanoscience. And you might think that it's as easy to take a picture of an atom with an STM as it is to take a shot with your digital camera. In fact, the imaging of individual atoms with an STM is quite slow. Now researchers at Cornell University have shown how to accelerate this process — by adding a radio transmitter, they are able to speed up atomic-level microscopy by a factor of at least 100. A typical STM currently has a sampling rate of about one KHz. This new radio-frequency STM can operate a thousand times faster."
On atomically flat surfaces with small scan areas, you can scan in constant height mode (rather than constant current, where the tunneling current is the input to the feedback loop to adjust the probe height ). Still, a 400x400 point image of a 20 x 20 nm area still used to take a couple of minutes. Not 1/1000 second.
In an STM machine, there is a single tip that moves over the sample. The sampling is done one pixel at a time, in much the same fashion as the beam of a CRT for example. So 1kHz is rather slow; for your 3 megapixel digital camera it works out at 3000 seconds (almost 1 hour) per frame. So a 1000x increase in speed is really significant!