Massive dipoles across the metal-semiconductor cluster interface: towards chemically controlled rectification.

An interface between a metallic cluster (MgAl) and a semiconducting cluster (ReSe(PMe)) is shown to be marked by a massive dipole reminiscent of a dipolar layer leading to a Schottky barrier at metal-semiconductor interfaces. The metallic cluster MgAl with a valence electron count of 38 electrons is two electrons short of 40 electrons needed to complete its electronic shells in a superatomic model and is marked by a significant electron affinity of 2.99 eV. On the other hand, the metal-chalcogenide semiconducting cluster ReSe(PMe), consisting of a ReSe core ligated with five trimethylphosphine ligands, is highly stable in the +2 charge-state owing to its electronic shell closure, and has a low ionization energy of 3.3 eV. The composite cluster ReSe(PMe)-MgAl formed by combining the MgAl cluster through the unligated site of ReSe(PMe) exhibits a massive dipole moment of 28.38 D resulting from a charge flow from ReSe(PMe) to the MgAl cluster. The highest occupied molecular orbital (HOMO) of the composite cluster is on the MgAl side, which is 0.53 eV below the lowest unoccupied molecular orbital (LUMO) localized on the ReSe(PMe) cluster, reminiscent of a Schottky barrier at metal-semiconductor interfaces. Therefore, the combination can act as a rectifier, and an application of a voltage of approximately 4.1 V via a homogeneous external electric field is needed to overcome the barrier aligning the two states: the HOMO in MgAl with the LUMO in ReSe(PMe). Apart from the bias voltage, the barrier can also be reduced by attaching ligands to the metallic cluster, which provides chemical control over rectification. Finally, the fused cluster is shown to be capable of separating electron-hole pairs with minimal recombination, offering the potential for photovoltaic applications.