Scanning Tunnelling Microscopy (STM)

This is a technique for probing the surface of a solid with sufficient resolution (about 0.1 nm laterally) to capture ‘images’ of individual atoms. It relies on the quantum mechanical process of tunnelling. A potential difference is applied between the sample and a very sharp tip and electrons are able to pass from the sample through a vacuum gap to the tip, producing a measurable current. This would be impossible in classical physics – the effect is explained in quantum mechanics by the wave-like nature of the electrons.

The tunnelling current decreases exponentially with an increase in distance between the tip and sample; as the tip scans across the surface, if it moves over a raised feature, then the measured current will be greater.

The nature of the STM method requires the sample to be electrically conducting or semiconducting but it also means that it is possible to perform electrical measurements on the sample. In general, interpretation of STM data is not simple, since the images produced are convolutions of both topographic and electronic information. However, this also means that there are a number of ways of manipulating the tip and interpreting the tunnelling current in order to gain useful information about the surface of the sample. (One way to distinguish topographic from electronic information is to image the sample at different biases – the topography will remain unchanged, while areas with different electronic properties will change in appearance.)

Modes of operation:

In constant current mode, a piezoelectric transducer (or piezodrive) adjusts the tip’s height to keep the current constant. Hence the tip remains at a constant height above the entire surface, and the height of the tip shows the height of each point on the surface. This results in a map of the topography of the surface.

In constant height mode, the tip’s height does not change; instead, the current changes as the distance between the tip and sample varies. This has the advantage of running faster than constant current mode, as the piezodrive does not need to make adjustments to the height of the tip, but can only be used with very flat surfaces (to avoid damage to the tip).

Various spectroscopy measurements can also be made – these determine the electrical properties of the surface. These techniques fall under the category of Scanning Tunnelling Spectroscopy (STS).

At different points on the surface, the tip height can be made to oscillate and the way in which the current varies with changes in height is then measured – this is called I(z) spectroscopy. This determines the decay constant in the exponential relationship between I and z, which itself depends on the work function of the sample directly below the tip. This process therefore gives information about the material on the surface.

Alternatively, at each point the dependence of the tunnelling current on the bias (or potential difference) can be measured – this is I(V) spectroscopy. The function of I against V (often dI/dV is measured) depends on the electronic properties of the surface, more specifically the Local Density of States (LDOS) which is a measure of the number of states available for electrons to occupy. If the array of I(V) data at each point is used to build up an image of the surface, then this is known as Current Imaging Tunnelling Spectroscopy (CITS).

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