Question

In Scanning Tunneling Microscope what is the benefit of using an ultra high vacuum chamber? Also explain why a metal surface can be used as a good substrate

Answer 1

Ultra-high vacuum (UHV) is characterised by very low residual gas pressures of Torr or lower. At such low pressures, the gas is in free molecular flow i.e., the mean free path of the molecules is larger than the size of the chamber so that all interactions take place at the various surfaces in the chamber. Moreover, the experiment requires an extremely clean and stable sample surface with the absence of any unwanted adsorbates. So, the transmission of electrons during the tunneling process requires UHV conditions.

For understanding the importance of metals as a good substrate, consider the following example:

If, say, two metals A and B are brought within tunneling distance, a rapid charge transfer takes place between them until an equilibrium state is established in which their Fermi levels gets aligned. Once in equilibrium, no net charge is transferred on average. Now, in a slightly modified situation, let metal A is biased relative to metal B by applying a voltage . The bias offsets the Fermi energies on either side of the vacuum barrier by . Under steady-state conditions, this potential difference establishes a net tunneling current between A and B. Thus, a metal substrate is required for electron transmission across the barrier.