Abstract
Charge transfer characteristics of single-molecule junctions at the nanoscale, and consequently, their thermoelectric properties can be dramatically tuned by chemical or conformational modification of side groups or anchoring groups. In this study, we used density functional theory (DFT) combined with the non-equilibrium Green’s function (NEGF) formalism in the linear response regime to examine the thermoelectric properties of a side-group-mediated anthracene molecule coupled to gold (Au) electrodes via anchoring groups. In order to provide a comparative inspection three different side groups, i.e. amine, nitro and methyl, in two different positions were considered for the functionalization of the molecule terminated with thiol or isocyanide anchoring groups. We showed that when the anchored molecule is perturbed with side group, the peaks of the transmission spectrum were shifted relative to the Fermi energy in comparison to the unperturbed molecule (i.e. without side group) leading to modified thermoelectric properties of the system. Particularly, in the thiol-terminated molecule the amine side group showed the greatest figure of merit in both positions which was suppressed by the change of side group position. However, in the isocyanide-terminated molecule the methyl side group attained the greatest thermoelectric efficiency where its magnitude was relatively robust to the change of side group position. In this way, different combinations of side groups and anchoring groups can improve or suppress thermopower and the figure of merit of the molecular junction depending on the interplay between charge donating/accepting nature of the functionals or their position.
Highlights
Charge transfer characteristics of single-molecule junctions at the nanoscale, and their thermoelectric properties can be dramatically tuned by chemical or conformational modification of side groups or anchoring groups
The nature of anchoring group can determine the sign of the thermopower of molecular junctions by modifying energy separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), i.e. the HOMO − LUMO gap[30,34]
By employing density functional theory (DFT) calculations combined with non-equilibrium Green’s function (NEGF) formalism in linear response regime we theoretically analyzed the thermoelectric properties of an anthracene single-molecule junction that is connected to Au electrodes via thiol (− SH ) or isocyanide (− NC ) anchoring group
Summary
Charge transfer characteristics of single-molecule junctions at the nanoscale, and their thermoelectric properties can be dramatically tuned by chemical or conformational modification of side groups or anchoring groups. In the isocyanide-terminated molecule the methyl side group attained the greatest thermoelectric efficiency where its magnitude was relatively robust to the change of side group position In this way, different combinations of side groups and anchoring groups can improve or suppress thermopower and the figure of merit of the molecular junction depending on the interplay between charge donating/accepting nature of the functionals or their position. The nature of anchoring group can determine the sign of the thermopower of molecular junctions (which reveals the nature of the transport) by modifying energy separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), i.e. the HOMO − LUMO gap[30,34] These theoretical findings were consistent with experimental measurements in single-molecule junctions with Au electrodes realized by STM-BJ35 or M CBJ30 techniques, implying that anchoring groups critically determine the charge transport direction through the system by the rearrangement of HOMO or LUMO level
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