Empirical Parameter to Compare Molecule-Electrode Interfaces in Large-Area Molecular Junctions

Abstract

This paper describes a simple model for comparing the degree of electronic coupling between molecules and electrodes across different large-area molecular junctions. The resulting coupling parameter can be obtained directly from current-voltage data or extracted from published data without fitting. We demonstrate the generalizability of this model by comparing over 40 different junctions comprising different molecules and measured by different laboratories. The results agree with existing models, reflect differences in mechanisms of charge transport and rectification, and are predictive in cases where experimental limitations preclude more sophisticated modeling. We also synthesized a series of conjugated molecular wires, in which embedded dipoles are varied systematically and at both molecule-electrode interfaces. The resulting current-voltage characteristics vary in nonintuitive ways that are not captured by existing models, but which produce trends using our simple model, providing insights that are otherwise difficult or impossible to explain. The utility of our model is its demonstrative generalizability, which is why simple observables like tunneling decay coefficients remain so widely used in molecular electronics despite the existence of much more sophisticated models. Our model is complementary, giving insights into molecule-electrode coupling across series of molecules that can guide synthetic chemists in the design of new molecular motifs, particularly in the context of devices comprising large-area molecular junctions.

Figure 1

(a) Schematic of the single-level model used for this study showing the energy offset that is proportional to V_trans and the coupling parameters to the tip and the substrate as Γ_s and Γ_t, respectively. The SLM only considers the contribution of the most dominant molecular orbital, which can either be the highest occupied (HOMO) or the lowest unoccupied molecular orbital (LUMO). (b) The functionalized OPE3 molecular wires investigated individually in this study with thiol as the anchoring group; parent OPE3 is taken as a reference molecule, shown together in a large-area molecular junction with EGaIn as the top electrode and Au^TS as the bottom electrode. (c) Functionalized mOPE3 molecular wires with a methylene bridge connecting the conjugated core to the thiol anchoring group; parent mOPE3 was taken as a reference. The abbreviations used in this study for all these wires are included at the bottom of the figure. Structures of a few other molecules of these two series are shown in Table S2 in the Supporting Information.

Figure 2

Plots of log|J| vs V for Au^TS/SAM//Ga₂O₃/EGaIn junctions comprising the following compounds: (a) OPE3, diSAc-OPE3, and diSAc-OPE4F, (b) OPEFDown and OPEFUp compared to OPE3, (c) mOPEFDown and mOPEFUp compared to mOPE3, (d) fluorinated analogues of OPE3: TailDown, TailUp, FMidUp, and FMidDown of the OPEF series, (e) OPE-OMe and mOPE-OMe compared to OPE3, and (f) OPPy and mOPPy compared to OPE3. Error bars represent 95% confidence intervals. See Figure 1 and Table S2 for molecular structures.

Figure 3

Semilog plot of the predicted surface interaction parameter (α) using SLM for the SAM//EGaIn interface for the OPE series (green data points) with a thiol anchoring group with OPE3 as the reference (*for diSAc-OPE4F, green and black data points represent log(α) with OPE3 and TailDown—see Supporting Information for structure—as references, respectively). The orange data points represent log(α) for the mOPE series with a methylene spacer to the thiol anchoring group and, therefore, mOPE3 as the reference molecule. A horizontal line at Y = 0 is drawn to highlight the trends. The values of α are provided in Table S2 in the Supporting Information.

Figure 4

(a) Schematic of a binary SAM containing CH₃–BPT/CF₃-BPT molecules in different ratios taken from ref (78). The CF₃ group induces the dipole moment away from thiol group, in contrast to the CH₃ group whose dipole moment is pointed toward the thiol group. (b) Semilog plot of the predicted surface interaction parameter (α) with an increasing portion of CF₃–BPT in the solution from which the binary SAMs were prepared. The predicted α value shows a reducing trend with increased CF₃ groups on the SAM//EGaIn interface.

Figure 5

(a) Molecular structures with an aliphatic tail and headgroups as thiol (C16diSH), carboxylic acid (C15COOH), and hydrated carboxylic acid (C15COOH-H₂O) taken from ref (22).; naphthyl (NapC11), phenanthrenyl (PheC11), anthracenyl (AntC11), pyrenyl (PyrC11), benzo[a]pyrenyl (BPC11), and bipyridyl (BiPyC11) from refs (45) and (46); and tetrathiafulvalene (BTTF), ferrocene (FcC11), fullerene (C60C11), and ferrocene-diphenylacetylene (Fc-Cn-DPA for n = 0, 1). The R group represents the undecanethiol (C₁₁H₂₂SH). (b) Semilog plot of the predicted surface interaction parameter (α) using SLM for the SAM//EGaIn interface for (left panel) alkylcarboxylic acid in rectifying and nonrectifying states and hexadecanedithiol, using C16SH as the reference molecule; (middle panel) all the molecules in the arene series on Ag^TS substrates using the data from Yoon group, Korea University; and (right panel) miscellaneous rectifiers with C18SH reference measured on Ag^TS, except for BiPyC11, BTTFC11, and Fc-Cn-DPA, which were measured on Au^TS, and hence, a C18SH measured on Au^TS was used as a reference. Note that, for several rectifying molecules, the SLM was only applied at the nonrectifying bias polarity due to ambiguities in the extraction of V_trans at rectifying bias polarities.