  It is discovered that the zero-dimensional character of the silicon atom
dangling bond (DB) state allows controlled formation and occupation of a new
form of quantum dot assemblies. Whereas on highly doped n-type substrates
isolated DBs are negatively charged, it is found that Coulomb repulsion causes
DBs separated by less than ~2 nm to experience reduced localized charge. The
unoccupied states so created allow a previously unobserved electron
tunnel-coupling of DBs, evidenced by a pronounced change in the time-averaged
view recorded by scanning tunneling microscopy. Direct control over net
electron occupation and tunnel-coupling of multi-DB ensembles through
separation controlled is demonstrated. Through electrostatic control, it is
shown that a pair of tunnel-coupled DBs can be switched from a symmetric
bi-stable state to one exhibiting an asymmetric electron occupation. Similarly,
the setting of an antipodal state in a square assembly of four DBs is achieved,
demonstrating at room temperature the essential building block of a quantum
cellular automata device.
