  Cryogenic scanning tunneling microscopy was employed in combination with
density-functional theory calculations to explore quantum dots made of In
adatoms on the InAs(110) surface. Each adatom adsorbs at a surface site
coordinated by one cation and two anions, and transfers one electron to the
substrate, creating an attractive quantum well for electrons in surface states.
We used the scanning-probe tip to assemble the positively charged adatoms into
precisely defined quantum dots exhibiting a bound state roughly 0.1 eV below
the Fermi level at an intrinsic linewidth of only ~4 meV, as revealed by
scanning tunneling spectroscopy. For quantum-dot dimers, we observed the
emergence of a bonding and an antibonding state with even and odd wave-function
character, respectively, demonstrating the capability to engineer
quasi-molecular electronic states. InAs(110) constitutes a promising platform
in this respect because highly perfect surfaces can be readily prepared by
cleavage and charged adatoms can be generated in-situ by the scanning-probe
tip.
