Tuning the thermoelectric properties of a single-molecule junction by mechanical stretching

Abstract

We theoretically investigate, as a function of the stretching, the behaviour of the thermoelectric properties - the Seebeck coefficient (S), the electronic heat conductance (κel) and the figure of merit (ZT) - of a molecule-based junction composed of a benzene-1,4-dithiolate molecule (BDT) coupled to Au(111) surfaces at room temperature. We show that the thermoelectric properties of a single molecule junction can be tuned by mechanic stretching. The Seebeck coefficient is positive, indicating that it is dominated by the HOMO. Furthermore, it increases as the HOMO level, which is associated to the sulphur atom, tends towards energies close to the Fermi energy. By modelling the transmission coefficient of the system as a single Lorentzian peak, we propose a scheme to obtain the maximum ZT of any molecular junction.

From DFT to machine learning: recent approaches to materials science–a review

“…DFT has been used for almost every kind of system ranging from atomic [306], molecular [307,308], and chemical systems, extended solids, surfaces [309], defects [310], 0D [311][312][313], 1D [314][315][316][317][318][319][320], and 2D [321] systems. In terms of properties, structural [322], electronic/transport [323,324], thermal [325][326][327], electron-phonon [328], optical [329], catalytic [330], magnetic [331][332][333], topological [334][335][336], and many others have been studied.…”

Roles of vacuum tunnelling and contact mechanics in single-molecule thermopower

“…S7). The stretching effects were also present in BDTs2526 but were not evident in the thermovoltage histograms, and thus not considered here in detail due to the relatively short-lived nature of the high-Δ V j states that emerges on the verge of mechanical breakdown, in comparison to the long lifetime of the Au-thiol linkages. On the other hand, the mechanically-induced thermopower enhancement was confirmed to be completely absent in BDAs indicating important roles of the thiol linkers (Fig.…”

“…Much progress has been accomplished in understanding and controlling the single-molecule thermoelectric transport properties, such as length dependence192021, intermolecular interactions22, and gate control23, owing to the advance in experimental techniques to address the electron and heat transport in molecular bridges17. In sharp contrast, the roles of electrode-molecule contacts on the thermoelectric properties have remained almost unexplored2425 albeit the theoretically predicted impact on the electronic structures2627282930, due mostly to the technical difficulty to evaluate the geometrical dependence of thermoelectricity in molecular junctions that requires a reliable method to form and hold a molecular bridge for long-enough time to measure their thermoelectric properties and meanwhile controllably deform the configurations at an atomistic level. In the present study, therefore, we report molecular junction diagnosis by thermoelectric analysis for characterizing the anchor dependence to geometrical sensitivity of single-molecule thermopower.…”

High thermopower of mechanically stretched single-molecule junctions

“…This anomalous feature presumably stems from over-elongated Au-S bonds of mechanically-stretched BDT junctions and accompanied electrode-to-thiol charge transfer that would provide additional MOs near E F with weak electronic coupling, and hence small E gap and Γ L,R . It was in fact demonstrated theoretically that thermopower increases rapidly under stretching just before junction breakdown 36 . Furthermore, the fact that BDT junctions tended to dissociate in a relatively short period of time after the RI events infers the contact mechanics involved therein: unstable electrode-molecule link is formed at the Au-S contacts probably having an adatom-coordinated metastable motif with relatively long Au-S distance through pulling-out of the Au surface atoms during mechanical stretching 17 36 For this to happen, molecular junctions are required to endure the forces and thermal fluctuations until the contact is departed at a certain extent, which is likely to be a rare case as Au-BDT-Au structure fractures spontaneously at the Au-Au contacts in vicinity of the Au-S link as illuminated by that the RI feature was found in only about 20 % of the traces measured ( Fig.…”

“…The geometrical dependence of thermoelectricity in Au-BDT-Au junctions is inferred in the Δ V – t profiles. TR and TD tendencies clearly suggest accompanying roles of the mechanical stretching along specific directions, which results in molecular tilting toward an upright conformation and elongation of the Au-S bond lengths of BDT in a Au-hollow or -top geometry, to weaken the electrode-molecule electronic coupling that leads to steady decrease in G and meanwhile inducing gradual shift of HOMO toward or away from E F , respectively 33 34 35 36 . On the other hand, the rapid increase in both G and Δ V of RI indicates a drastic change in the Fermi alignment: E gap lowered for more than 0.5 eV and Γ L,R decreased by up to 0.05 eV.…”