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 nanounits 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.