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

Dinesh Bista,Shiv N Khanna,Turbasu Sengupta

doi:10.1039/d1cp02420e

Abstract

An interface between a metallic cluster (MgAl12) and a semiconducting cluster (Re6Se8(PMe3)5) 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 MgAl12 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 Re6Se8(PMe3)5, consisting of a Re6Se8 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 Re6Se8(PMe3)5-MgAl12 formed by combining the MgAl12 cluster through the unligated site of Re6Se8(PMe3)5 exhibits a massive dipole moment of 28.38 D resulting from a charge flow from Re6Se8(PMe3)5 to the MgAl12 cluster. The highest occupied molecular orbital (HOMO) of the composite cluster is on the MgAl12 side, which is 0.53 eV below the lowest unoccupied molecular orbital (LUMO) localized on the Re6Se8(PMe3)5 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 MgAl12 with the LUMO in Re6Se8(PMe3)5. 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.

Highlights

We investigate the possibilities of connecting a metallic cluster with a semiconducting cluster to test the hypothesis that the charge transfer across the interface might induce significant shifts in the level alignment across the junction
By projecting the projected density of states (PDOS), we examine the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the two individual clusters and check whether the junction across the two clusters presents the analog of the Schottky[12,13] barrier
We would like to add that we calculated the energy required to remove a ligand from the fully ligated Re6Se8(PMe3)[6] cluster to create the binding site for combining the MgAl12 cluster

Summary

Introduction

Metal semiconductor junctions play an important role in modern electronics.[1,2,3,4,5] When a metal is interfaced with a semiconductor, the difference in chemical potentials across the junction causes diffusion of charge carriers that generate an internal dipolar layer and a Schottky barrier.[6,7,8,9,10,11,12,13,14] an in-built potential accompanied by an internal electric field at the interface results in a directional electrical current under an applied external bias voltage. We investigate the possibilities of connecting a metallic cluster with a semiconducting cluster to test the hypothesis that the charge transfer across the interface might induce significant shifts in the level alignment across the junction. The purpose of this work is to investigate these intriguing possibilities by considering a supermolecule composed of an aluminum-based cluster and a metal chalcogenide-based cluster

Objectives

Methods

Conclusion