Creating and probing quantum dot molecules with the scanning tunneling microscope

A scanning tunneling microscope (STM) operated at 5 K was used to reposition positively charged In adatoms on the InAs(111)A-(2 × 2) surface with atomic-scale precision. Vertical atom manipulation allows for a reversible and highly reliable exchange of individual In atoms between the surface and the apex of the STM tip. Chains of In adatoms assembled in this way create an attractive potential for surface-state electrons. The resulting chain-confined states have wavefunctions with n lobes and n – 1 nodes, establishing the generic properties of a quantum dot (QD) with a perfectly defined level structure. In a second step, QD assemblies are constructed whose quantum coupling has no intrinsic variation but can nonetheless be tuned over a wide range. The states of these QD molecules can be described by standard molecular-orbital theory in terms of their bonding–antibonding character and degeneracy, providing a simple and transparent means to predict the level structure of coupled QDs assembled one atom at a time.