Theoretical Study of Long-Range Electron Transport in Molecular Junctions

2017

The electrical conductance of molecular junctions may strongly depend on the temperature, and weakly on molecular length, under two distinct mechanisms: phase-coherent resonant conduction, with charges proceeding via delocalized molecular orbitals, and incoherent thermally-assisted multi-step hopping. While in the case of coherent conduction the temperature dependence arises from the broadening of the Fermi distribution in the metal electrodes, in the latter case it corresponds to electron-vibration interaction effects on the junction. With the objective to distill the thermally-activated hopping component, thus expose intrinsic electron-vibration interaction phenomena on the junction, we suggest the design of molecular junctions with "spacers", extended anchoring groups that act to filter out phase-coherent resonant electrons. Specifically, we study the electrical conductance of fixed-gap and variable-gap junctions that include a tunneling block, with spacers at the boundaries. Using numerical simulations and analytical considerations, we demonstrate that in our design, resonant conduction is suppressed. As a result, the electrical conductance is dominated by two (rather than three) mechanisms: superexchange (deep tunneling), and multi-step thermally-induced hopping. We further exemplify our analysis on DNA junctions with an A:T block serving as a tunneling barrier. Here, we show that the electrical conductance is insensitive to the number of G:C base-pairs at the boundaries. This indicates that the tunneling-to-hopping crossover revealed in such sequences truly corresponds to the properties of the A:T barrier.

Section: Introductionmentioning

confidence: 99%

Controlling charge transport mechanisms in molecular junctions: Distilling thermally induced hopping from coherent-resonant conduction

Kim

2017

“…In this technique, incoherent elastic and inelastic scattering effects are included in a phenomenological manner, by augmenting the noninteracting electronic Hamiltonian with probe terminals through which electrons lose their phase memory and (possibly) exchange energy with environmental degrees of freedom 48,49 . While the technique was originally introduced to study decoherence effects in mesoscopic devices, it was recently applied to explore electronic conduction in organic and biological molecular junctions [50][51][52][53] , as well as anharmonic effects in (purely) phononic quantum conduction [54][55][56] . Particularly, in Ref.…”

Section: Introductionmentioning

confidence: 99%

Thermopower of molecular junctions: Tunneling to hopping crossover in DNA

Korol

Kilgour

2016

We study the electrical conductance G and the thermopower S of single-molecule junctions, and reveal signatures of different transport mechanisms: off-resonant tunneling, on-resonant coherent (ballistic) motion, and multi-step hopping. These mechanisms are identified by studying the behavior of G and S while varying molecular length and temperature. Based on a simple one-dimensional model for molecular junctions, we derive approximate expressions for the thermopower in these different regimes. Analytical results are compared to numerical simulations, performed using a variant of Büttiker's probe technique, the so-called voltagetemperature probe, which allows us to phenomenologically introduce environmentally-induced elastic and inelastic electron scattering effects, while applying both voltage and temperature biases across the junction. We further simulate the thermopower of GC-rich DNA molecules with mediating A:T blocks, and manifest the tunneling-to-hopping crossover in both the electrical conductance and the thermopower, in accord with measurements by Y. Li et al., Nature Comm. 7, 11294 (2016).

“…Previous studies with the LBP technique had demonstrated its utility as a means to phenomenologically describe low-bias incoherent effects in mesoscopic conductors 25,[28][29][30][31][32][33][34][35][36] and molecular junctions 27,[37][38][39][40][41][42] These simulations indicate that the LBP technique can reproduce several key features in molecular electronic conduction: A "Kramers-like" turnover of conductance as a function of the so-called dephasing strength, an onset of a thermally activated conductance at high temperature, and a transition from tunneling to ohmic conduction when increasing the molecular size 27 . It was also shown in Ref.…”

Section: Introductionmentioning

confidence: 99%

Inelastic effects in molecular transport junctions: The probe technique at high bias

Kilgour

2016

We extend the Landauer-Büttiker probe formalism for conductances to the high bias regime, and study the effects of environmentally-induced elastic and inelastic scattering on charge current in single molecule junctions, focusing on high-bias effects. The probe technique phenomenologically incorporates incoherent elastic and inelastic effects to the fully coherent case, mimicking a rich physical environment at trivial cost. We further identify environmentally-induced mechanisms which generate an asymmetry in the current, manifested as a weak diode behavior. This rectifying behavior, found in two types of molecular junction models, is absent in the coherent-elastic limit, and is only active in the case with incoherent-inelastic scattering. Our work illustrates that in the low bias -linear response regime, the commonly used "dephasing probe" (mimicking only elastic decoherence effects) operates nearly indistinguishably from a "voltage probe" (admitting inelasticdissipative effects). However, these probes realize fundamentally distinct I-V characteristics at high biases, reflecting the central roles of dissipation and inelastic scattering processes on molecular electronic transport far-from-equilibrium.