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Abstract
We examine theoretically coherent electron transport through the single-molecule magnet Mn$_{12}$, bridged between Au(111) electrodes, using the non-equilibrium Green's function method and the density-functional theory. We analyze the effects of bonding type, molecular orientation, and geometry relaxation on the electronic properties and charge and spin transport across the single-molecule junction. We consider nine interface geometries leading to five bonding mechanisms and two molecular orientations: (i) Au-C bonding, (ii) Au-Au bonding, (iii) Au-S bonding, (iv) Au-H bonding, and (v) physisorption via van der Waals forces. The two molecular orientations of Mn$_{12}$ correspond to the magnetic easy axis of the molecule aligned perpendicular [hereafter denoted as orientation (1)] or parallel [orientation (2)] to the direction of electron transport. We find that the electron transport is carried by the lowest unoccupied molecular orbital (LUMO) level in all the cases that we have simulated. Relaxation of the junction geometries mainly shifts the relevant occupied molecular levels toward the Fermi energy as well as slightly reduces the broadening of the LUMO level. As a result, the current slightly decreases at low bias voltage. Our calculations also show that placing the molecule in the orientation (1) broadens the LUMO level much more than in the orientation (2), due to the internal structure of the Mn$_{12}$. Consequently, junctions with the former orientation yield a higher current than those with the latter. Among all of the bonding types considered, the Au-C bonding gives rise to the highest current (about one order of magnitude higher than the Au-S bonding), for a given distance between the electrodes. The current through the junction with other bonding types decreases in the order of Au-Au, Au-S, and Au-H. Importantly, the spin-filtering effect in all the nine geometries stays robust and their ratios of the majority-spin to the minority-spin transmission coefficients are in the range of 10$^3$ to 10$^8$. The general trend in transport among the different bonding types and molecular orientations obtained from this study may be applied to other single-molecular magnets.
Highlights
Advances in experimental techniques have led to a great number of experimental studies on electron transport through molecular junctions formed by single molecules bridged between electrodes or molecular monolayers adsorbed onto surfaces, using three-terminal setups or scanning tunneling microscopySTMmeasurements
We examine the nine interface geometriesFig. 2͒ leading to the five bonding types and two molecular orientations that were discussed in Sec
If the electron transport is carried by the LUMOHOMOlevel of the Mn12, geometries in the orientation1͒ provide a higherlowercurrent that those in the orientation2͒
Summary
Advances in experimental techniques have led to a great number of experimental studies on electron transport through molecular junctions formed by single molecules bridged between electrodes or molecular monolayers adsorbed onto surfaces, using three-terminal setups or scanning tunneling microscopySTMmeasurements. Recent experiments show that conductance through molecular junctions based on small single molecules can be enhanced to an order of G0 by their direct bonding to the electrodes without a thiol group or any other linker molecules.[27,28] This enhancement is attributed to a strong coupling between the molecules and the electrodes, which places the transport in the transparent regime rather than in the tunneling regime.[28,29] corresponding systematic studies have not yet been carried out for electron transport through an SMM.
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