Tunnel channels, spectroscopy and imaging in STM

Scanning tunneling microscopy (STM) obtains information about an object via tunnel channels, which severely reduce the spectroscopic and spatial information. Hence, as the first quantification step in STM, the tunnel channels must be identified. This is performed by distance and voltage tunnel spectroscopy (DTS/VTS), showing specific I(d) and I(V) dependencies if n = 0, 1, 2,..., n intermediate states are involved. Examples of this I(d) proportional to exp(-2kd/(n + 1)) analysis, with tunnel distance d and decay length k = root 2m phi/(h) over bar approximate to 5.1/nm for a tunnel with barrier height phi approximate to 1 eV are given for surfaces covered by water, air or vacuum. Despite the n = 1, 2, 3 intermediate states and their charging, imaging down to atomic resolution is achieved. Because of the enhanced tunnel probability for intermediate states (i.e. reduced tunnel resistance allowing bridging distances between 0.5 and 25 nm), those resonant tunnel processes are of crucial importance for any "real system".