Non-topographic contrast in constant-current Scanning Field-Emission Microscopy (SFEM)

Scanning Tunneling Microscopy is performed in the conventional (tunneling) and in the field-emission regime. Images of W(110)-surfaces with and without some carbon content are taken in the constant current mode, in which the tip-target vertical distance displaces to compensate for the changes of the tunneling, respectively, field emission current. In the field emission regime, we observe tip-target displacements that are not related to the topographic contrast.