Thermoelectric Transport from First-Principles—Biphenyl-Based Single-Molecule Junctions
Using first-principles electronic structure methods in conjunction with nonequilibrium Green function (NEGF) techniques, we study the thermoelectric transport through biphenyl-based single-molecule junctions. We show, based on our recently published works and their present extension to include also the electron energy current, that the single-molecule conductance, junction thermopower, and electron thermal conductance strongly depend on the choice of the molecular anchor group and on the geometry of the investigated gold-biphenyl-gold contacts. We compare two different anchor groups, sulfur and cyano. The electron-donating S anchor group gives rise to a positive thermopower, while the electron-withdrawing cyano anchor results in a negative thermopower. For the S-terminated biphenyl a strong variation of the transport coefficients with respect to the binding motif is observed, for CN-terminated biphenyl such variations remain small.
Since the first realization of single-molecule devices [1, 2, 3], controlling their transport properties has become a major theme in the research in molecular electronics. Using single molecules as conducting system provides thereby a great flexibility for tailoring the transport properties by changing the geometrical and chemical structure of the molecule [4, 5, 6, 7].
Due to the strong dependence of the transport on the atomistic details, an accurate theoretical description requires a first-principles treatment of the electronic and geometrical properties of the molecular junctions. Such a first-principles approach is usually based on a combination of density functional theory (DFT) with nonequilibrium Green function (NEGF) techniques. Despite the well-known shortcomings of DFT when it comes to predict quasiparticle excitations , the DFT + NEGF scheme has demonstrated to be able to capture and explain important trends observed in experiment [4, 7, 9]. Moreover it provides the basis for higher levels of theory like, e.g., quasiparticle methods .
2 Thermoelectric Transport Coefficients from NEGF Theory
In this section we will shortly introduce the theoretical framework used to calculate the transport properties of the molecular junctions. An extensive discussion of our transport approach can be found in reference .
3.1 Contact Geometries and Electronic Structure
3.2 Thermoelectric Transport Properties
We consider two different anchor groups, connecting the biphenyl molecule to the Au electrodes, namely S and CN (Fig. 1a). Sulfur forms a covalent bond with the Au atoms of the electrode, and it is known that several metastable binding motifs exist. Therefore we select three representative geometries [10, 23] where S binds symmetrically on each side either to three Au atoms in the hollow position (S-HH), to two Au atoms in the bridge position (S-BB), or to a single one in the top position (S-TT1) (Fig. 2). CN-anchored biphenyl binds selectively via the nitrogen lone pair to a single Au atom [5, 24]. Here we consider binding to a single, highly coordinated Au atom (CN-TT1) and to a low-coordinated one (CN-TT2), respectively (Fig. 2).
Neglecting phonons, the charge and heat current through the biphenyl is carried by the π-electron system and depends strongly on the degree of π-conjugation, the broadening of the molecular orbitals due to interactions with the metal electrodes, and the alignment of the molecular frontier orbitals with respect to the electrode Fermi energy [10, 11].
Dihedral angle φ, molecular conductance G, thermopower S, and electron thermal conductance κ for the various contact geometries, evaluated at T = 300 K
The degree of π-conjugation depends on the dihedral angle φ between the two phenyl rings . For the H-terminated biphenyl, the gas-phase dihedral angle is given by φ = 35.9°. Comparing this to the angle which is adopted, if the molecule is connected to the Au electrodes (Table 1), we see that for S-HH, CN-TT1, and CN-TT2 φ remains almost at its gas-phase value, while for S-BB and S-TT1 φ decreases to 12.9° and 17.9°, respectively. The change in φ can be related to the mechanical strain, which is exerted if the molecule is connected in S-BB or S-TT1 positions. For the S-connected molecules, we find a strong dependence of the transport coefficients on the configuration of the molecule with respect to the Au surface. If connected in the HH position G, S, and κ are largely decreased, as compared to BB and TT1. The reason for this is two-fold. First, the degree of π-conjugation is larger for S-BB and S-TT1 due to the reduced φ as compared to S-HH. Second, for S-HH the overlap between the molecular orbitals and the states in the electrodes is small, which results in a relatively weak hybridization between them. On the other hand, for S-BB and S-TT1, the molecular orbitals and electrode states are rather strongly hybridized due to the tilted molecular geometry, which in turn increases the level broadening and moves the HOMO resonance closer to EF [10, 11]. For the CN-linked molecules, the thermoelectric transport coefficients are roughly comparable, which is consistent with the experimentally observed conductance histograms .
From Eq. (9) we can see that the sign of the thermopower is determined by the slope of the transmission function at the Fermi energy and can be related to the type of charge carriers present in the system . If the HOMO is closer to EF, the sign of the thermopower is positive, S > 0, and hole-like excitations are transported through the HOMO. Conversely, for S < 0, the LUMO is closer to EF and thus electron-like excitations are transported through the LUMO.
From Table 1. we can see that the sign of the thermopower is determined by the anchor group. For S-linked biphenyl we obtain a positive thermopower, and for CN-linked biphenyl it is negative. This behavior can be related to the Hammett constant of the molecules. Sulfur is electron donating and transfers negative partial charge into the molecular π-electron system , hence increasing the Coulomb repulsion. This will in turn increase the orbital energies, moving the HOMO resonance closer to EF (Fig. 3). Thus, as discussed above, we observe a positive thermopower. In contrast, for the electron withdrawing CN group  the Coulomb repulsion is decreased, which moves the orbital energies down (Fig. 3), resulting in LUMO-dominated transport and hence a negative thermopower.
Combining first-principles electronic structure, obtained within the framework of density functional theory, and the Landauer Büttiker approach, formulated with nonequilibrium Green functions, we studied the thermoelectric transport through biphenyl-based molecular junctions with Au electrodes. We investigated two different anchor groups, connecting the molecule to the Au electrodes, namely sulfur and cyano. For S-terminated molecules we observed a strong dependence of the thermoelectric transport properties on the binding motif. In hollow position the molecular conductance, the junction thermopower and the electron thermal conductance are strongly reduced as compared to geometries, where the molecule is either in bridge or top position. This behavior can be traced back to the different degree of hybridization between the molecular orbitals and states in the Au electrodes as well as to the molecular geometry itself. CN on the other hand binds selectively to single Au atoms, and we observed for the two studied top geometries just a moderate variation of the transport coefficients. Moreover, we showed that the anchor determines the type of charge carriers present in the system, which is ultimately related to the sign of the thermopower. The electron-donating S anchor gives rise to hole-like conduction through the HOMO, resulting in a positive thermopower. Oppositely, for the electron-withdrawing CN anchor we observe a negative thermopower, which is characteristic for electron-like conduction through the LUMO.
This work was partly supported by a FY2012 (P12501) Postdoctoral Fellowship for Foreign Researchers from the Japan Society for Promotion of Science (JSPS) and by a JSPS KAKENHI, i.e. ‘Grant-in-Aid for JSPS Fellows’, grant no. 24·02501.
F.P. gratefully acknowledges financial support from the Carl Zeiss Foundation as well as the collaborative research center of the German science foundation, SFB 767, through project C13.
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