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Scanning Tunneling Microscope
Scanning Tunneling Microscope (STM)
The scanning tunneling microscope (STM) is, hands down, the most important tool for analysis and manipulation at the atomic scale. Since its invention by IBM researchers in the 1980's, it's been used to visualize images with a resolution of about two atomic diameters. Furthermore, it has found widespread use as a manipulator capable of moving atoms and molecules across a surface. Don Eigler (IBM) was the first person to move atoms with an STM, and earned his place in history by spelling out "IBM" with xenon atoms.
The STM operates as a fixed probe with a movable sample platform. The scanning portion of this microscopy is entirely handled by ultra-fine piezoelectric nano-positioners. Unlike the electron microscope and its derivatives, the STM is not based on any form of radiation. It is a non-optical microscope that utilizes 'tunneling' from quantum mechanical theory. As the theory goes, an electron that has a barrier between two possible states can actually pass right through the barrier with a fixed probability. In normal operation, the probe is brought very close to the surface, but not into contact. Hence, the air gap acts as the barrier while the probe and sample surface act as the two possible states for the electron. Since electricity is involved, the sample must be conductive for this to work. By fixing the height of the probe and measuring the change in tunnel current, the surface morphology of the sample can be deduced. In the other mode, the current is held constant while the probe must move up and down to maintain it. Ether way, the STM is capable of 'seeing' individual atoms.
One new use that researchers found early on was the STM's ability to actually pick up atoms by applying enough voltage. It can also be used to trigger chemical reactions like electrodeposition. The STM is so powerful that every serious nanotechnology laboratory either has one in the lab or has easy access to one nearby.
These days, the resolution of the STM is rivaled only by its direct descendent: the atomic force microscope. Both were invented by the same team of researchers at IBM, both involve non-optical techniques, and both can visualize atoms. The only real difference is that the STM is extremely useful for processes requiring electricity, while the AFM is used mainly for non-conductive samples.
Unlike electron microscopes, STMs and AFMs are extremely practical in that neither require a vacuum for operation. Sample preparation times are thus significantly reduced. These are the reasons behind the STMs popularity in research labs across the world today.