Theoretical insight into the thermoelectric behavior of tri-nuclear metal-string complexes laced with gold nanoelectrodes: A first-principles study

Abstract

Metal-string complexes in the quasi-1D framework may play an important role in molecular electronics by serving not only as nanoscale interconnects but also as active functional elements for nanoelectronic devices. However, because of the potential volumetric heat generation across such nanojunctions, the circuit stability becomes often a major concern, which necessitates to study the heat transport properties at the molecular-scale. Here we report the thermoelectric behavior of various tr-nuclear metal-string complexes, [M − M − M](dpa)4(NCS)2 for M ∈{Cr,Ru}, bridging Au(111) nanowires as nanoelectrodes. Based on our charge transport analysis from first-principles, we find that the dominant transmission peaks tend to move away from the Fermi level upon systematic rutheniation in chromium-based metal-string complexes due mainly to the coupling of π* orbitals from Ru and σnb orbitals from Cr. Such type of a metal-string junction can also exhibit strong Coulomb interaction so that its thermoelectric behavior begins to deviate from the Wiedemann-Franz law. Our results further suggest that metal-string complexes can render better thermoelectric devices especially at the molecular-scale with the thermopower as high as 172 μV/K at 300 K. Considering the contributions from both electrons and phonons, even a high figure of merit of ZT ∼ 2 may be attained for Cr-Cr-Cr based metal-string molecular junctions at room temperature. Resonant enhancement in the thermoelectric efficiency appears to occur in such systems through alteration of inter-dot electrostatic interactions, which can be controlled by incorporating Cr and Ru atoms in such tri-nuclear metal-string complexes.