The Scanning Tunnelling Microscope (STM) is a special microscope. As the name already tells you, it is based on a quantummechanical effect called "tunnelling" that will be shortly explained in one of the following paragraphs. Basically, it means, that electrons can pass (or "tunnel") thin insulating layers, although this would not be possible in classical theory of electricity. Using this effect, the microscope can obtain extremely high magnification. Under good imaging conditions, we can even see individual atoms.
In the next paragraph, the physical principle underlying the STM is described in more detail. Although in principle very powerful, this idea poses some serious technical problems. In the following paragraph, we will therefore explain in more detail the technical realisation chosen by the people at Århus University (from where we bought our STMs) to overcome these problems. Our STMs run in Ultra-High Vacuum (UHV), which necessitates special UHV chambers (including a lot of other surface science techniques besides the STM). At the end of this text, you can find images of the UHV-STM chambers we have here at DTU.
To "see" atoms on the surface we want to investigate, we place a thin and very sharp metal tip (in our case tungsten) close to that surface (but without contact). We then put a small bias voltage (of the order of some 0.1 Volt) between tip and surface (that has to be a conducting surface). According to classical physics, no current can flow, because there is no mechanical contact. However, according to Quantum Theory (which is beyond the scope of this text), electrons can pass the small gap d between tip and surface if this gap is sufficiently small (about 0.5 Nanometer). As a result of the bias voltage, there will be a net flow of electrons; in other words, we will read a current on the amp meter (some Nanoampere).
If we now scan the tip across the surface (read the next paragraph to find out how to do that), this current will increase or decrease, depending on the distance of the tip above the surface. Using a fast electronic feedback, we control the tip-surface distance such, that the current remains constant. According to Quantum Theory, the current is actually extremely dependent on that distance. As a rule of thumb one says, that the current decreases by a factor of 10, if the distance increases by 1 Ångström (or 0.1 Nanometer). As atoms typically are some Ångströms in diameter, one realises that we easily can follow the contours of the atoms, if we manage to design an instrument that can control the tip movements on that fine scale.
Although the principal idea behind STM is surprisingly simple, it took nevertheless several decades from the first formulation of the idea (in 1959) to the first operational instrument. There are 2 major reasons for that:
You have to realise that the building blocks of the STM are macroscopic in size compared to the microscopic atoms we want to see. The difference in size is actually about 8 orders of magnitude, as sketched in the animation to the right. This makes it a technological problem to build such an STM.
Point 1 could be overcome by the use of so-called Piezo elements—in our case a Piezo scanner tube (bottom part of the animation). This tube consists of a special (Piezo) material that changes length when one applies small voltages on electrodes around the tube. This change can be controlled to an extend that we can steer the tip with atomic scale resolution over the surface.
Point 2 could be overcome by minituarising the instrument and using external vibration dampers. The STM in Århus was thus reduced in size until it now only measures a few centimeters in size. Below, you can see a sketch of the Århus STM (left) and photograph of the STM lying in a hand for size comparison.
The heart of the STM is the scanner (6) that scans the tip (4) as described above across the surface (1). Another scanner motor (8) is used to bring the tip into close proximity of the surface without hitting it (another problem that had to be solved).
To the left, you can see the UHV-STM chamber in 307, to the right the one in 312 (Click on the images to learn more about the respective projects). The reason, we use such chambers is, that they allow us to precisely control the atmospheric conditions around the STM—normally either ultra clean (UHV) or controlled gas atmosphere. We use UHV to keep the surface clean over several hours—long enough to perform our experiments. Actually, our vacuum (approx. 1E-10 mbar) is much better than that around the orbiting Space Shuttle!
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