SCANNING TUNNELING MICROSCOPY - STM
fundamentals

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Since its invention (1983) and subsequent Nobel prize (1986), Scanning Tunneling Microscopy (STM) and related Scanning Probe Microscopy techniques have become some of the most important laboratory techniques for studying sub-nanoscale surface phenomena. This technique allows scientists to visualize regions of different electron density and hence infer the position of individual atoms and molecules on the surface of a lattice. In addition to topographic information provided by STM, Scanning Tunneling Spectroscopy (STS) gives insight into electronic properties of a sample.

STM works by scanning a sharp metallic tip over a conductive surface. A bias voltage is applied between tip and sample. When the tip is approached within a few Å from the sample, a tunneling current can be established indicating the proximity of tip and sample with very high accuracy. Due to its exponential distance dependance, most of the tunneling current flows through a single tip apex atom and atomic resolution both in- and perpendicular to the scanning plane is routinely achieved on clean surfaces when using a sharp tip.

There are two different operational modes of STM:

In constant height mode, the tip is scanned in a plane parallel to the surface. In case of a constant density of states (DOS) of the sample, the tunneling current between tip and sample reflects the sample relief. In this mode, adjustment of the tip-sample separation is not required and therefore a high scan speed can be obtained. However, constant height mode is only applicable if the sample surface is very flat as surface corrugations larger than typically 5-10 Å will cause the tip to crash. In this case, maintaining a constant tunneling current and therefore a quasi-constant tip-sample sepration is preferable. This is achieved by a PI-feedback loop wich monitors the tunneling current and keeps the tip-sample separation constant by varying the voltage on the z-piezo. This mode is referred to as constant current mode.

In addition to these two different scan modes, Scanning Tunneling Spectroscopy (STS) has proven to be an enourmosly valuable technique to investigate electronic properties of the sample surface. By varying the bias voltage at constant tip-sample separation, I/V spectra can be recorded. Furthermore, the tunneling conductance dI/dV can be obtained when applying a Lock-in technique to modulate the bias voltage and demodulate the tunneling current. It can be shown that dI/dV is a valid measure of the local sample density of states (LDOS). The LDOS is a very important piece of information when investigating phenomena such as superconductivity or charge density waves.

STM gives true atomic resolution on selected samples even at ambient conditions. This technology can be applied to study conductive surfaces or thin nonconductive films and small objects deposited on conductive substrates.

attocube systems STMs

All attocube microscope systems are compatible with cryogenic and vacuum environments as well as high magnetic fields. The STM is also suited to be used in combination with a He3 insert allowing measurement temperatures down to 300 mK.

attoSTM I:
The attoSTM I is designed particularly for the use at extreme environmental conditions such as ultra low temperature, high magnetic fields, and high vacuum. To perform low temperature microscopy, the attoSTM I is cooled by a controlled exchange gas atmosphere in a liquid Helium bath cryostat. Alternatively, the STM can be operated under vacuum conditions.