Engineering the Thermopower of C₆₀ Molecular Junctions

Abstract

We report the measurement of conductance and thermopower of C60 molecular junctions using a scanning tunneling microscope (STM). In contrast to previous measurements, we use the imaging capability of the STM to determine precisely the number of molecules in the junction and measure thermopower and conductance continuously and simultaneously during formation and breaking of the molecular junction, achieving a complete characterization at the single-molecule level. We find that the thermopower of C60 dimers formed by trapping a C60 on the tip and contacting an isolated C60 almost doubles with respect to that of a single C60 and is among the highest values measured to date for organic materials. Density functional theory calculations show that the thermopower and the figure of merit continue increasing with the number of C60 molecules, demonstrating the enhancement of thermoelectric preformance by manipulation of intermolecular interactions.

Thermal conductance and thermoelectric figure of merit of C60 -based single-molecule junctions: Electrons, phonons, and photons

“…These values are close to the experimental values found in Ref. 34, where mean values of 0.1G 0 and 1.8 × 10 −3 G 0 were reported for the monomer and dimer junctions, respectively. Notice that in both cases the electronic transmission at the Fermi energy is determined by the LUMO of the molecules, as it has been reported in numerous studies, see for instance Ref.…”

“…Notice also that the thermopower value for the dimer junction almost doubles that of the monomer, similar to what was observed in Ref. 34, while the absolute values are somewhat larger than those reported experimentally.…”

“…This asymmetric situation is meant to mimic the geometries realized in the STM experiments of Ref. 34, in which one of the electrodes is an Au surface and the other one an Au STM tip. Let us stress that these geometries were obtained by minimizing the total energy of the junctions as a function of the electrode separation d, yielding the distance d 0 .…”

“…Also in the context of thermoelectricity, C 60 -based single-molecule junctions have been analyzed and several groups have reported room-temperature thermopower measurements [33][34][35]. In particular, Evangeli et al [34] employed a scanning tunneling microscope (STM) setup to report simultaneous measurements of the conductance and thermopower in "single-C 60 " or monomer molecular junctions with Au electrodes. They showed that these junctions can exhibit thermopower values ranging from −40 to 0 µV/K, depending of the contact details, with a mean value of −18 µV/K.…”

A quantum circuit rule for interference effects in single-molecule electrical junctions

“…S tudies of the electrical conductance of single molecules attached to metallic electrodes not only probe the fundamentals of quantum transport but also provide the knowledge needed to develop future molecular-scale devices and functioning circuits [1][2][3][4][5][6][7][8][9] . Owing to their small size (on the scale of Angstroms) and the large energy gaps (on the scale of eV), transport through single molecules can remain phase coherent even at room temperature, and constructive or destructive quantum interference (QI) can be utilized to manipulate their room temperature electrical 10-13 and thermoelectrical 14,15 properties. In previous studies, it was reported theoretically and experimentally that the conductance of a phenyl ring with meta (m) connectivity is lower than the isomer with para (p) connectivity by several orders of magnitude [16][17][18][19][20][21][22][23][24][25] .…”