Side-group-mediated thermoelectric properties of anthracene single-molecule junction with anchoring groups

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.

Voltage-dependent plasticity of spin-polarized conductance in phenyl-based single-molecule magnetic tunnel junctions

“…In this study we used the thiol anchoring unit to mediate the coupling of the PDT and BPDT molecules to the FM cobalt electrodes. However, it was previously shown that the electron donating/accepting character of anchoring units can modify the conductance of singlemolecule junctions by the energy-level line up relative to the Fermi energy of electrodes [48][49][50], ultimately resulting in an enhanced/suppressed conductance depending on the nature of the molecular structure [51,52]. Furthermore, here we used unperturbed PDT and BPDT molecules as the central molecule in the MTJ.…”

“…Furthermore, here we used unperturbed PDT and BPDT molecules as the central molecule in the MTJ. However, functionalization of the molecule with side groups can also have dramatic modulatory effects on the conductance of the system by reorganization of the frontier molecular orbitals relative to the Fermi energy level of the electrodes [52,53]. These modifications can favorably or unfavorably tune the behavior of the voltage-dependent plasticity of effective conductance in MTJ which certainly deserves further examination.…”

Anchoring Groups Determine Conductance, Thermopower and Thermoelectric Figure of Merit of an Organic Molecular Junction

“…Previously, several studies explored the effect of structural or chemical modification on the thermoelectric properties of molecular junctions by manipulating their transport properties. For example, it has been shown that the length of the molecule [14][15][16][17], functionalization of the molecule with side groups [18][19][20][21], intervention of anchoring groups [22][23][24], molecule-electrode coupling geometry [25,26] and impurity doping [7,27] can significantly tune the conductance, thermopower and figure of merit of molecular junctions, depending on the system. In this context, a key factor in the regulation of transport properties of metal-molecule-metal structures is the energy level alignment between Frontier molecular orbitals (FMOs) and the Fermi level of electrodes [28,29] that can be modulated, e.g., by the geometric position or charge transfer nature of anchoring groups [24,30,31] or side groups [20,21].…”

“…Perturbation of molecular systems with side groups in various positions can similarly modify the thermoelectric properties of the system [18][19][20][21]. The chemical character or the position of side groups can be a determinant of charge polarization between molecule and the electrodes due to the realignment of the FMOs relative to the Fermi energy of the electrodes [21,36,37].…”

“…Perturbation of molecular systems with side groups in various positions can similarly modify the thermoelectric properties of the system [18][19][20][21]. The chemical character or the position of side groups can be a determinant of charge polarization between molecule and the electrodes due to the realignment of the FMOs relative to the Fermi energy of the electrodes [21,36,37]. Side group charge transfer nature (e.g., electron-donating amine unit vs electron-accepting nitro unit) can induce different resonance effects near the Fermi energy which shapes current flow in the system [18,20].…”