Scanning tunnelling microscopy (STM)

About this technique

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Scanning tunelling microscopy (STM) can only be applied conducting samples and is generally used for very-high-resolution imaging – less than a nanometre. The instrument can provide information on atomic or molecular order in materials in addition to map the location of electronic states. It creates images in a very similar manner to atomic force microscopy (AFM). However, the one major difference is that STM uses current conducted between an atomically sharp probe and the substrate to build the image.

One of the capabilities that distinguishes STM from most techniques is that atomic resolution is possible for some systems. Such resolution does require careful sample preparation and generally can only be used on systems that are quite stable over long periods. As with AFM, there are two modes of operation, generally know as constant height and constant current. Constant height experiments can be done more quickly but the sample properties have to be well known and the samples flat. Otherwise there is a risk of crashing the tip into the surface, which will generally render the probe unusable. The constant current mode is slower but follows the contours of the electronic states of the surface and hence tip crashes are less likely.

Sample size is restricted to a maximum area of 1 x 1 cm and with a maximum sample thickness of approximately 3 mm.