Voltage-controlled conductance and switching induced by single molecules or atoms are ideally studied in scanning tunneling microscope (STM) tunnel junctions. While the objects under consideration are mostly used in their original form, little is known of the possibilities of in situ adjustments of their properties. Here, we evidence properties of a tunnel junction made of a Ce atom/cluster built by atomic manipulation on Au(111) at a temperature of 4.6 K in the presence of H₂. The conductance through the object is characterized by a switching voltage corresponding to an opening or closing of an inelastic electron tunneling conductance channel at 50 mV for a Ce atom and 140 mV for a Ce cluster and by charging. We demonstrate that the electronic properties of an STM junction can be engineered in a simple way by in situ guiding of the H₂ pinning at an atomic cluster.

A study of the surface assisted self-assembly of 1, 2, 4, 5-tetracyanobenzene (TCNB) acceptor molecules and Fe atoms on an Au(111) surface is presented. While conditions to get the two-dimensional arrays of stable Fe(TCNB)₄ complexes are clearly identified, ultrahigh vacuum scanning tunneling microscopy and spectroscopy (STM/STS) coupled with first-principles calculations reveals that situations may occur where Fe and TCNB survive on the surface (as Fe–4TCNB entities) at a higher density than the original molecular monolayer without forming coordination bonds with each other. It is found that the square planar coordination of the Fe(TCNB)₄ monomer complexes cannot fully develop in the presence of lateral strain due to growth-induced confinement. A phenomenon similar to steric hindrance involving a strongly modified chirality with a Fe–N–C bond angle of 120° compared to the 180° for the stable complex may then explain why the Fe atom keeps its metallic bond with the surface. The competition between steric and electronic effects, not reported before, may arise elsewhere in surface chemistry involved in the synthesis of new and potentially useful organic nanomaterials.