Spin transport in higher $n$-acene molecules

We investigate the spin-transport properties of molecules belonging to the acenes series by using density functional theory combined with the nonequilibrium Green's function approach to electronic transport. While short acenes are found to be nonmagnetic, molecules comprising more than nine acene rings have a spin-polarized ground state. In their gas phase, these have a singlet total spin configuration, where the two unpaired electrons occupy the doubly degenerate highest molecular orbital. Such an orbital degeneracy is however lifted once the molecule is attached asymmetrically to Au electrodes via thiol linkers, leading to a net fractional magnetic moment. In this situation, the system Au/$n$-acene/Au can act as an efficient spin filter with interesting applications in the emerging field of organic spintronics.

Figure 1

Schematic illustration of the $n$-acene series. The dashed (light blue) and dotted (red) lines denote the symmetry of the two degenerate HOMOs.

Figure 2

HOMO-LUMO gap calculated from the Kohn-Sham eigenvalues for $n$-acenes as a function of $n$.

Figure 3

Isosurfaces of spin-density magnetization for the various anchoring geometries of decacene attached to Au via thiol groups: (a) two links, meta, (b) two links, para, (c) three links, and (d) four links. Positive isosurfaces are represented in dark grey (blue) and negative ones in light grey (pink).

Figure 4

DOS projected on the two carbon atoms in the middle of decacene molecule (C31 and C32) for different anchoring configurations: (a) two links, meta, (b) two links, para, (c) three links, and (d) four links. The grey shadow is the total DOS of the molecule.

Figure 5

Transmission coefficient for decacene for the different anchoring configurations: (a) two links, meta, (b) two links, para, (c) three links, and (d) four links. “Up” and “Down” indicate the majority- and minority-spin contributions, respectively.

Figure 6

Magnetic moment of the Au/$n$-acene/Au junction as a function of the molecule length, $n$.

Figure 7

(a) DOS projected on the two carbon atoms in the middle of the pentacene molecule (C19 and C20) and (b) transmission coefficient for pentacene attached to Au in meta anchoring configuration. The grey shadow is the total DOS of the molecule.