Abstract
Molecular junction is a chemically-defined nanostructure whose discrete electronic states are expected to render enhanced thermoelectric figure of merit suitable for energy-harvesting applications. Here, we report on geometrical dependence of thermoelectricity in metal-molecule-metal structures. We performed simultaneous measurements of the electrical conductance and thermovoltage of aromatic molecules having different anchoring groups at room temperature in vacuum. We elucidated the mutual contributions of vacuum tunnelling on thermoelectricity in the short molecular bridges. We also found stretching-induced thermoelectric voltage enhancement in thiol-linked single-molecule bridges along with absence of the pulling effects in diamine counterparts, thereby suggested that the electromechanical effect would be a rather universal phenomenon in Au-S anchored molecular junctions that undergo substantial metal-molecule contact elongation upon stretching. The present results provide a novel concept for molecular design to achieve high thermopower with single-molecule junctions.
Highlights
From a naïve point of view, thermoelectric transport in molecular junctions should accompany an effect of direct tunnelling through vacuum[44]
First we noticed that the positive ΔVj at G > 1 G0 is suggestive of quantum thermopower of Au nanocontacts (Fig. 1c), a care should be taken on the quantitative interpretation as it is well-established that molecules are often bridging aside the Au atom-sized contacts before the opening of electrode nanogaps[45,46]. This can be seen as multiple batteries, each contributing the single-molecule thermovoltage, with internal resistance Ri connected in parallel when considering no notable quantum interference[47,48] in the thermoelectric transport (Fig. 4a)
What we measured was the combined voltage described as ΔVj =(VAumRmol +VmolRAu) (RmolRAu)/(RmolRs +RAuRs +RmolRAu)(Rmol +RAu), where Rmol =RSMJ/m is the net resistance of m molecules bridging in parallel with the single molecule resistance RSMJ, RAu is the resistance of the Au contacts, and Rs is the serial resistance
Summary
We found exponential decay in the conductance with respect to the distance dgap between the Au electrodes in case of junctions with ΔVave ≥−0.5 mV at Vh = 3.5 V (ΔT= 59 K), i.e. VP1 state, for BDAs (Fig. 2d–e), which is naturally ascribed to electron tunnelling through vacuum gaps instead of charge transport through molecular bridges (Fig. 2f)[42].
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