Large negative differential conductance in single-molecule break junctions

Abstract

Molecular electronics aims at exploiting the internal structure and electronic orbitals of molecules to construct functional building blocks. To date, however, the overwhelming majority of experimentally realized single-molecule junctions can be described as single quantum dots, where transport is mainly determined by the alignment of the molecular orbital levels with respect to the Fermi energies of the electrodes and the electronic coupling with those electrodes. Particularly appealing exceptions include molecules in which two moieties are twisted with respect to each other and molecules in which quantum interference effects are possible. Here, we report the experimental observation of pronounced negative differential conductance in the current-voltage characteristics of a single molecule in break junctions. The molecule of interest consists of two conjugated arms, connected by a non-conjugated segment, resulting in two coupled sites. A voltage applied across the molecule pulls the energy of the sites apart, suppressing resonant transport through the molecule and causing the current to decrease. A generic theoretical model based on a two-site molecular orbital structure captures the experimental findings well, as confirmed by density functional theory with non-equilibrium Green's functions calculations that include the effect of the bias. Our results point towards a conductance mechanism mediated by the intrinsic molecular orbitals alignment of the molecule.

Environment-assisted quantum transport through single-molecule junctions

“…As demonstrated experimentally, the site energies are sensitive to an applied bias voltage 17,18 . We will focus on a symmetric molecular structure in which the sites capacitively couple to the source or drain electrode as follows: ε L = ε 0 +αV b /2 and ε R = ε 0 −αV b /2 for an applied bias of V b , so that αV b is the voltage drop within the molecule.…”

“…The two-site character of the molecule can be most easily achieved by breaking the conjugation within the system (introducing regions of low π-electron density). Several structures of this type have recently been investigated in the transport setting [16][17][18][19][20][21] ; the model used here is inspired by the experimental studies of Perrin et al 17,18 . We now proceed to describe the Hamiltonian governing our system (with = 1 and e = 1 throughout).…”

“…It should be recognised that α depends on the microscopic details of the junction, and will vary between different single-molecule devices. However, as values between 0.48 and 0.74 have been reported in experimental and ab initio studies 17,18,22 , we shall henceforth adopt α = 0.6 unless stated otherwise.…”

Vibrational effects in charge transport through a molecular double quantum dot

“…Finally, let us stress that one should expect this parameter regime (where the inter-dot coupling J is stronger than the coupling to the leads) to be appropriate in the case of most single-molecule junctions 68,91 . The strength of the electron-phonon coupling in these systems is often estimated experimentally by considering the relative heights of the Franck-Condon steps 15,37 .…”

“…Several examples of such molecules have recently been investigated 67,68 . While the role of electron-phonon coupling in such systems remains largely unexplored experimentally, it has attracted some attention from a theoretical perspective [69][70][71][72][73] .…”