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Quantum interferences in cross-conjugated based planar junctions

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Quantum interference due to the wave nature of electrons is a well known quantum effect in mesoscopic physics. When electrons propagate coherently through a circuit, the topology of the system has a strong influence on the transport properties. In mesoscopic systems, whenever the phase coherence of the electron wave function is preserved during propagation, different trajectories can interfere constructively or destructively.
The ability to control and use the same kind of quantum phenomena at the molecular level could open new routes towards miniaturization, where the molecule itself becomes the active part of a device, and at the same time, it will improve knowledge of electron transport through molecular systems.
Cross-conjugated molecules are the chemical compound that can act as molecular quantum interferometers. When an electron is transferred through such a molecule, different paths through the localized orbitals of the molecule are responsible for the interference which manifests as a strong suppression of conductance at low voltage. This is a fundamentally new quantum phenomena in molecular charge transport. As pointed out in different works, this quantum interference conductance change can represent a new element of the molecular electronics toolbox. This topic is the subject of an intense effort both experimental using self-assembled monolayers (SAMs)1 and theoretical2 but no direct measurement of conductance has been achieved so far mainly due to the difficulty to obtain large area stable junctions. We have shown for the first time the possibility to achieve experimental evidence of quantum interference effects on anthraquinone based planar junctions3 by direct measurements of the differential conductance.

Left : simplified scheme of the planar junction based on an AQ layer covalently grafted on the bottom electrode. Right : structure of the diazonium derivative of the AQ molecule before grafting (top). Three- level scheme showing the electron pathways through the molecular orbitals interfering destructively (bottom)
Left : simplified scheme of the planar junction based on an AQ layer covalently grafted on the bottom electrode. Right : structure of the diazonium derivative of the AQ molecule before grafting (top). Three- level scheme showing the electron pathways through the molecular orbitals interfering destructively (bottom)

Figure 1 shows a simplified scheme of a planar junction based on an AQ layer covalently grafted on the bottom electrode (a) and the dI/dV measurements for an AQ based planar junction at 300 K with an area of 30X30 µm2 where a clear zero bias conductance suppression is evident, indicating the occurrence of quantum interference.
Besides that, we plan to investigate different expected effects related to quantum interference in cross-conjugated based junctions, as listed in the following :

- measurement of the predicted power law temperature dependence of the conductance minimum
- understanding of decoherence related to electron-phonon coupling explored by a fine temperature investigation coupled to Raman as well as inelastic electron tunneling spectroscopy
- tuning of the quantum interference effect in molecular layer by external physical parameters like gate voltage, mechanical deformation
- demonstration of a new thermopower capability (giant enhancement of the Seebeck coefficient) related to quantum interference based junctions

1. C.M. Guédon et al. Nature Nanotech., 7, 305 (2012).

2. T. Markussen, R. Stadler, K. S. Thygesen, Nano Lett. 10, 4260 (2010) ; T. Markussen, J. Schiötz, K. S. Thygesen, J. Chem. Phys. 132, 224104 (2010).

3. V. Rabache, J. Chaste, P. Petit, M. L. Della Rocca, P. Martin, J.-C. Lacroix, R. L. McCreery, and P. Lafarge, J. Am. Chem. Soc. 135, 10218 (2013).