Tunneling Probability Increases with Distance in Junctions Comprising Self-Assembled Monolayers of Oligothiophenes

Abstract

Molecular tunneling junctions should enable the tailoring of charge-transport at the quantum level through synthetic chemistry but are hindered by the dominance of the electrodes. We show that the frontier orbitals of molecules can be decoupled from the electrodes, preserving their relative energies in self-assembled monolayers even when a top-contact is applied. This decoupling leads to the remarkable observation of tunneling probabilities that increase with distance in a series of oligothiophenes, which we explain using a two-barrier tunneling model. This model is generalizable to any conjugated oligomers for which the frontier orbital gap can be determined and predicts that the molecular orbitals that dominate tunneling charge-transport can be positioned via molecular design rather than by domination of Fermi-level pinning arising from strong hybridization. The ability to preserve the electronic structure of molecules in tunneling junctions facilitates the application of well-established synthetic design rules to tailor the properties of molecular-electronic devices.

Figure 1

(A) Series of alkanes (Cn) and oligothiophenes (TnC4) used in the tunneling junctions. The tunneling distance (d) increases with the number of CH₂ units in the Cn series or the number of thiophene units in TnC4. (B) An energy level diagram for molecular junctions comprising TnC4 molecules between two metal electrodes, showing the fixed barrier-width d_C4 associated with the butanethiol fragment and the increasing d_Tn with n. The shaded regions in blue represent occupied states. The barrier-height (φ) is constant for the alkane series but decreases with increasing n for the TnC4 series.

Figure 2

(A) Plots of log|j| vs V for Ag^TS/Cn//EGaIn (where n = 10, 12, 14, 16) junctions. (B) Plots of log|j| vs V for Ag^TS/TnC4//EGaIn (where n = 1, 2, 3, 4, and corresponds to the number of thiophene rings) junctions. (C) Plots of log|j| vs molecular length with linear fits to eq 1 at different bias: 1.0 V (solid line), −1.0 V (dash-dot line), 0.5 V (dashed line), −0.5 V (dotted line). (D) Plots of log|j| vs molecular length at different biases with lines drawn through the points: 1.0 V (solid line), −1.0 V (dash-dot line), 0.5 V (dashed line), −0.5 V (dotted line). Error bars in all the plots represent 95% confidence intervals from measurements of multiple junctions across multiple substrates.

Figure 3

(A) Experimental plots of I vs V for Ag^TS/SAM//Au^AFM junctions for: T1C4 (red), T2C4 (orange), T3C4 (green), and T4C4 (purple). (B) Simulated plots of I vs V derived by integrating the transmission of single-molecule junctions comprising TnC4: T1C4 (red), T2C4 (orange), T3C4 (green), and T4C4 (purple). (C) Experimental plots of log|I| vs molecular length at different biases: 1.5 V (solid line), −1.5 V (dash dot line), 1.0 V (dash line), −1.0 V (dotted line). (D) Simulated plots of log|I| vs molecular length at different bias: 1.5 V (solid line), −1.5 V (dash dot line), 1.0 V (dash line), −1.0 V (dot line). For clarity, the simulated data for log|I| at −1.5 and −1.0 V are shifted by offsets of −0.1 and −0.15 V, respectively.

Figure 4

Experimental normalized differential conductance (NDC) heat-map plots of Ag^TS/SAM//EGaIn junctions comprising C10 and TnC4. Only junctions of T4C4 were robust enough to scan to ±1.5 V. The NDC plots of the entire Cn series and other, representative, conjugated molecules are shown in the Supporting Information.

Figure 5

(A) Schematic of the two-barrier model developed to explain the extraordinary length-dependence of TnC4. The gray rectangular barrier depicts the first tunneling potential barrier due to the butanethiol fragment (V₂). The second barrier is defined by the energy offset between the HOPS and the Fermi-level of the electrode (E_f – E_HOPS). Thus, the red, orange, green, and purple spheres represent the second potential barriers (V₃) corresponding to the T1C4, T2C4, T3C4, and T4C4, respectively, illustrating the decrease in barrier-height with the increasing number of thiophene rings. Values of |E_f – E_HOPS| from UPS measurements are plotted with black squares. (B) The length-dependence of transmission derived from the two-barrier model plotted as triangles. The dashed lines drawn through the T4C4 molecule show how the tunneling distance evolves with increasing n.