Experimental and Theoretical Analysis of Nanotransport in Oligophenylene Dithiol Junctions as a Function of Molecular Length and Contact Work Function

Abstract

We report the results of an extensive investigation of metal-molecule-metal tunnel junctions based on oligophenylene dithiols (OPDs) bound to several types of electrodes (M1-S-(C6H4)n-S-M2, with 1 ≤ n ≤ 4 and M1,2 = Ag, Au, Pt) to examine the impact of molecular length (n) and metal work function (Φ) on junction properties. Our investigation includes (1) measurements by scanning Kelvin probe microscopy of electrode work function changes (ΔΦ = ΦSAM - Φ) caused by chemisorption of OPD self-assembled monolayers (SAMs), (2) measurements of junction current-voltage (I-V) characteristics by conducting probe atomic force microscopy in the linear and nonlinear bias ranges, and (3) direct quantitative analysis of the full I-V curves. Further, we employ transition voltage spectroscopy (TVS) to estimate the energetic alignment εh = EF - EHOMO of the dominant molecular orbital (HOMO) relative to the Fermi energy EF of the junction. Where photoelectron spectroscopy data are available, the εh values agree very well with those determined by TVS. Using a single-level model, which we justify via ab initio quantum chemical calculations at post-density functional theory level and additional UV-visible absorption measurements, we are able to quantitatively reproduce the I-V measurements in the whole bias range investigated (∼1.0-1.5 V) and to understand the behavior of εh and Γ (contact coupling strength) extracted from experiment. We find that Fermi level pinning induced by the strong dipole of the metal-S bond causes a significant shift of the HOMO energy of an adsorbed molecule, resulting in εh exhibiting a weak dependence with the work function Φ. Both of these parameters play a key role in determining the tunneling attenuation factor (β) and junction resistance (R). Correlation among Φ, ΔΦ, R, transition voltage (Vt), and εh and accurate simulation provide a remarkably complete picture of tunneling transport in these prototypical molecular junctions.

Tuning electronic transport via hepta-alanine peptides junction by tryptophan doping

“…In addition Trp doping, even at the peptide sequence center (W-1, W-4) slightly enhances Γ g , the effective coupling term, compared with Γ g for 7A. The 0.1-0.2-meV values of Γ g for the peptides are rather low (even lower for N > 100), compared with ∼1 meV reported for similar length fully conjugated dithiol molecules (62).…”

“…This implies that the effective barrier « 0 differs significantly from the molecular HOMO, in contrast to previous observation (62), because « 0 corresponds to the heterogeneous energy landscape of the transport, not only to a certain HOMO. This may reflect that transport (61) via peptides differs from that via a fully conjugated wire, where a closer match between transmission function and HOMO level was reported (62).…”

“…We postulate that for W-7 contact/overlap between the Au's spillover electron density and the indole ring's π-electrons is sufficient to decrease the effective transport barrier (63). Such sensitivity of the energy levels to mere physical proximity to the metallic contact was not observed for thiol-terminated polyphenyls (62), from another hand indicating that actual chemical bonding can act to buffer coupling, rather than enhance it, possibly via electron localization in the bond (64,65). In addition, the separation into Γ g and « 0 reveals that the slightly lower current via W-4 compared with W-1 (Fig.…”

A Solid‐State Protein Junction Serves as a Bias‐Induced Current Switch

“…[11] In most cases,t he molecular junctions are so delicate that electrical break down happens before the bias voltage, required for resonance,isreached. [17] We showed previously that inserting al inker molecule between the electrode and the protein [9] can weaken the coupling strength between them, and the ETp mechanism can be switched from resonant to off-resonant tunneling by chemically modulating this strength. [13] If no peaks are observed, it is often assumed that the nonlinear part of the current-voltage response reflects the "shoulders" of ar esonance,w ithout actually reaching it.…”

First-principles molecular transport calculation for the benzenedithiolate molecule

“…A more sophisticated but numerically challenging technique for calculating transport properties is the combination of DFT with GW [15][16][17]. DFT can also be used in combination with the dynamical mean field theory [18], with ME [19] if the system is in the weak coupling regime or even with ME+CPT [20].Although the 1, 4-benzenedithiolate (BDT) molecule is usually considered in the field of molecular electronics there are still severe discrepancies between experimental [21-26] and theoretical results [6,19,[27][28][29][30]. The calculated conductances tend to be much higher than the measured ones.…”