  Quantum dots (QD) with electric-field-controlled charge state are promising
for electronics applications, e.g., digital information storage,
single-electron transistors and quantum computing. Inorganic QDs consisting of
semiconductor nanostructures or heterostructures often offer limited control on
size and composition distribution, as well as low potential for scalability
and/or nanoscale miniaturization. Owing to their tunability and self-assembly
capability, using organic molecules as building nano-units can allow for
bottom-up synthesis of two-dimensional (2D) nanoarrays of QDs. However, 2D
molecular self-assembly protocols are often applicable on metals surfaces,
where electronic hybridization and Fermi level pinning can hinder
electric-field control of the QD charge state. Here, we demonstrate the
synthesis of a single-component self-assembled 2D array of molecules [9,
10-dicyanoanthracene (DCA)] that exhibit electric-field-controlled spatially
periodic charging on a noble metal surface, Ag(111). The charge state of DCA
can be altered (between neutral and negative), depending on its adsorption
site, by the local electric field induced by a scanning tunneling microscope
tip. Limited metal-molecule interactions result in an effective tunneling
barrier between DCA and Ag(111) that enables electric-field-induced electron
population of the lowest unoccupied molecular orbital (LUMO) and hence charging
of the molecule. Subtle site-dependent variation of the molecular adsorption
height translates into a significant spatial modulation of the molecular
polarizability, dielectric constant and LUMO energy level alignment, giving
rise to a spatially dependent effective molecule-surface tunneling barrier and
likelihood of charging. This work offers potential for high-density 2D
self-assembled nanoarrays of identical QDs whose charge states can be addressed
individually with an electric field.
