Negative Differential Resistance Behavior in Conjugated Molecular Wires Incorporating Spacers:  A Quantum-Chemical Description

Abstract

Recent experimental studies have demonstrated that single molecules or a small number of self-assembled molecules can perform the basic functions of traditional electronic components, such as wires and diodes. In particular, molecular wires inserted into nanopores can be used as active elements for the fabrication of resonant tunneling diodes (RTDs), whose I/V characteristics reveal a Negative Differential Resistance (NDR) behavior (i.e., a negative slope in the I/V curve). Here, quantum-chemical calculations are used to describe on a qualitative basis the mechanism leading to NDR in polyphenylene-based molecular wires incorporating saturated spacers. This description is based on the characterization of the evolution of the wire electronic structure as a function of a static electric field applied along the molecular axis, which simulates the driving voltage between the two electrodes in the RTD devices. We illustrate that the main parameters controlling the NDR behavior can be modulated through molecular engineering of the wires.

Resonant Tunneling Processes along Conjugated Molecular Wires: A Quantum‐Chemical Description

“…According to the results of our recent quantum-chemical calculations, resonance, and thus current flow, do not occur at zero bias, but can be activated in a very limited range of applied static electric field (i.e., limited range of voltage), which rationalizes the appearance of a peak profile in the I/V curves. [10,11] A very sharp NDR behavior with a peak-to-valley ratio in the current density close to 1000:1 has also been measured at 60 K for a three-ring phenylene ethynylene oligomer, substituted asymmetrically by p-active groups (see Fig. 1b) and sandwiched between two gold contacts.…”

“…[10] The first resonance yields a vanishingly small electronic splitting, mainly due to the absence of electronic density over the triple bond in the HOMO-4 level, and hence will not contribute to the appearance of strong current peak in the I/V curves. In contrast, we calculate an electronic splitting of 7 meV for the second resonance; thus, this second resonance can be responsible for the sharp NDR behavior observed experimentally if hole injection is effective in the molecular wire (i.e., if the energy separation between the occupied levels and the Fermi energy of gold is small).…”

“…In contrast to previous interpretations, this new mechanism does rely neither on the presence of atoms protruding from the metallic surface, [24] nor on an abrupt conformational change switching off the current. [1] In view of our recent work, [10,11] we are led to the general conclusion that an NDR signal can be obtained in the I/V characteristics of conjugated molecular wires by introducing saturated spacers, or by inducing significant ring twists (for instance by means of bulky substituents) along the backbone. …”

“…1) when inserted into a nanopore and sandwiched between two gold contacts; [9] this molecular wire is made of a central quantum well separated from two extended conjugated segments by saturated spacers, which induce a strong localization of the electronic levels over one of the three individual p-conjugated segments. [10] The observation of a high current across such a molecular wire requires energy resonances among electronic levels belonging to all three individual conjugated segments in order to promote a full delocalization of the electronic wavefunctions. According to the results of our recent quantum-chemical calculations, resonance, and thus current flow, do not occur at zero bias, but can be activated in a very limited range of applied static electric field (i.e., limited range of voltage), which rationalizes the appearance of a peak profile in the I/V curves.…”

Molecular Electronics: Effect of External Electric Field

“…Considerable amount of theoretical work based on semi-empirical and ab-initio methods have also been performed to understand NDR behavior in the Tour molecules. Many explanations for this phenomenon based on charging [55,56,57], reduction of the acceptor moiety [58], twisting of the ring structure leading to conformational changes [59,60,61,62], bias driven changes in molecule-electrode coupling [63] etc, have been proposed. However, most of these require to impose some external factors like the rotation of the middle ring, or introduction of extra charge in the molecule in order for the external bias to cause NDR at some bias.…”