Quantifying Molecular Structure-Tunneling Conductance Relationships: Oligophenylene Dimethanethiol vs Oligophenylene Dithiol Molecular Junctions

Abstract

We report quantitative analysis of tunneling conductance in molecular junctions based on self-assembled monolayers (SAMs) of oligophenylene dimethanethiols (OPDMn) in which −CH2– spacers flank either side of the phenylene (n = 1), biphenylene (n = 2), and terphenylene (n = 3) aromatic cores. The current–voltage (I–V) characteristics for the OPDMn junctions with Au and Pt contacts are analyzed quantitatively with a previously validated single level model (SLM) to extract key junction metrics, namely the HOMO-to-Fermi-level offset, εh, and the electronic coupling, Γ. Independent determination of εh by ultraviolet photoelectron spectroscopy (UPS) corroborates the estimation of εh from the I–V characteristics and provides strong evidence for the validity of the SLM analysis. Further, comparison of the results for OPDMn junctions with those for oligophenylene dithiol (OPDn) junctions, which do not have −CH2– spacers, reveals that the much larger resistance for OPDMn (>1000-fold) is primarily due to a ∼50-fold decrease in Γ and not to any significant change in εh; εh is nearly identical for OPDMn and OPDn junctions for each value of n. Overall, our results provide a clear delineation of the influence of −CH2– spacers on εh and Γ and give further evidence that the analytical SLM is a useful tool for determining structure-transport relationships in molecular tunnel junctions.

Charge Transport Characteristics of Molecular Electronic Junctions Studied by Transition Voltage Spectroscopy

“…By analyzing the relation of the inflection behavior in the F-N plots and transmission functions, the inflection could occur as the nearest molecular energy level was fairly approached to the resonant position within the bias window (Figure 1b). Even though the occasion of the inflection significantly is dissimilar to the conventional Simmons model, the F-N curves presented from respective calculations appear to be very analogous to those obtained from the latest experiments [37,39].…”

“…On the contrary, the Simmons model based on a simple rectangular barrier picture seems not to tender a reasonable description for the field of molecular electronics [30][31][32][33], mostly because it is difficult to illustrate the intrinsic chemical nature of constituent molecules. Recent works by diverse research groups have consistently shown that transition voltage spectroscopy (TVS) [34][35][36][37][38][39][40][41][42], especially on the basis for the Landauer picture, can be a more accurate analytical tool, in which quantitative fits into experimental current (I)-voltage (V) curves allow conveniently extracting charge transport parameters, for instance, the alignment of the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO), the tunneling transmission, and the coupling strength of molecule-electrode contacts.…”

Quantifying Image Charge Effects in Molecular Tunnel Junctions Based on Self-Assembled Monolayers of Substituted Oligophenylene Ethynylene Dithiols

“…Doing so, we reproduce the individual measured I – V curves for the OPX junctions extremely well, as shown by the black traces in Figure . We believe that the very good match between the experimental and simulated I – V curves indicates that the single level model applies well for OPX-based molecular junctions, as we have shown for other molecular systems. − ,, …”

“…We believe that the very good match between the experimental and simulated I−V curves indicates that the single level model applies well for OPX-based molecular junctions, as we have shown for other molecular systems. [41][42][43][44][45]53,54 The average values for the transport parameters extracted from approximately 100 I−V curves for each type of OPX SAM are compiled in Table 1. Evidently, substitution of the central ring with electron donating or withdrawing groups does influence the low bias conductance G of the OPX molecular junctions, but not to a large extent.…”

“…However, we have previously demonstrated a pronounced highest-occupied molecular orbital (HOMO) level pinning in molecular junctions based on three of these molecules, OPE, OPF, and OPN . The HOMO pinning, which appears to be a general phenomenon in molecules strongly coupled to metal electrodes, ,− is striking, as OPE, OPF, and OPN have gas-phase (isolated molecule) ionization energies E I that differ by over 0.6 eV due to the different electroactive substituents on the central ring. Density functional theory (DFT) calculations reveal that the differences in E I are nearly completely wiped out in crystalline monolayers of OPE, OPF, and OPN due to intermolecular interactions (specifically electrostatic interactions); the ionization potentials (IPs) for the three monolayers are approximately equal .…”