A single atom change turns insulating saturated wires into molecular conductors

Abstract

We present an efficient strategy to modulate tunnelling in molecular junctions by changing the tunnelling decay coefficient, β, by terminal-atom substitution which avoids altering the molecular backbone. By varying X = H, F, Cl, Br, I in junctions with S(CH₂)_(10-18)X, current densities (J) increase >4 orders of magnitude, creating molecular conductors via reduction of β from 0.75 to 0.25 Å^-1. Impedance measurements show tripled dielectric constants (ε_r) with X = I, reduced HOMO-LUMO gaps and tunnelling-barrier heights, and 5-times reduced contact resistance. These effects alone cannot explain the large change in β. Density-functional theory shows highly localized, X-dependent potential drops at the S(CH₂)_nX//electrode interface that modifies the tunnelling barrier shape. Commonly-used tunnelling models neglect localized potential drops and changes in ε_r. Here, we demonstrate experimentally that [Formula: see text], suggesting highly-polarizable terminal-atoms act as charge traps and highlighting the need for new charge transport models that account for dielectric effects in molecular tunnelling junctions.

Fig. 1. The junctions, equivalent circuit, and energy level diagram.

a Schematic illustration of the Ag–S(CH₂)_nX//GaO_x/EGaIn junction (shown for n = 14, EGaIn is for eutectic alloy of Gallium and Indium, “-” represents covalent bond, “//” represents non-covalent contact, “/” means the interface between GaO_x and EGaIn) together with the equivalent circuit diagram. In this work we investigated junctions with n = 10, 12, 14, 16, or 18, and X = H, F, Cl, Br, or I. b Energy level diagram of the junction showing how the coupling strength between molecules and electrodes (Γ) and tunnelling barrier height (δE_ME) change with X.

Fig. 2. Characterization of the self-assembled monolayers (SAMs).

a Representative slice-through of a large-area Ag–S(CH₂)₁₄I SAM structure calculated by molecular dynamics (MD) computer simulations. b Surface coverage (Ψ_SAM) of Ag–S(CH₂)₁₄X SAMs as a function of X determined with angle-resolved X-ray photoelectron spectroscopy (ARXPS, filled circles) and thickness of SAM (d_SAM) determined with ARXPS (filled triangles) and MD (empty triangles). c Ψ_SAM of Ag–S(CH₂)_nBr SAMs as a function of n determined with ARXPS (filled circles) and d_SAM determined with ARXPS (filled triangles) and MD (open triangles). The solid and dashed blue lines are linear fits to the experimental and MD data with R² of 0.94 and 0.99, respectively. The horizontal dashed line in panels b and c indicates the Ψ_SAM used in the MD calculations. d Computed MD packing energy per molecule E_mol,MD and per methylene –CH₂– unit E_meth,MD of Ag–S(CH₂)₁₄X SAMs as a function of X. Dashed lines are guides to the eye. The errors on the XPS data represent instrumental and fitting errors of 10% in total (see Section S4). The error bars in the MD data represent the standard deviations in the time- and molecule-averages calculated across 500 snapshots taken during the final 50 ns of 100 ns of room temperature MD of 128-molecule Ag–S(CH₂)₁₄X SAMs with the average experimental coverage of 1 nmol/cm² on Ag(111).

Fig. 3. Electrical characterization of junctions.

Gaussian log-average values of the current densities 10|J| > _G vs. applied bias V obtained from Ag–S(CH₂)_nX//GaO_x/EGaIn junctions with X = F (a) or I (b) and n = 10 (solid black line), 12 (solid red line), 14 (solid blue line), 16 (solid pink line), and 18 (solid green line). The dashed-line error bars represent the Gaussian log-standard deviation, σ_log,G. c Decay plots of <log₁₀|J| > _G at –0.5 V against d_SAM,MD with X = H (black square), F (red circle), Cl (blue triangle), Br (pink inverted triangle), or I (green diamond). The solid lines are fits to Eq. (1). The dashed lines represent the 95% confidence bands. d Plots of log₁₀|J| vs. V as a function of T (T = 250–340 K) recorded from Ag–S(CH₂)₁₄X//GaO_x/EGaIn junctions.

Fig. 4. Characterization of junctions with impedance spectroscopy.

a Contact resistance R_C vs. X for Ag–S(CH₂)₁₄X//GaO_x/EGaIn junctions at DC of 0 V and sinusoidal perturbation of 30 mV. Log-resistance of SAM, log₁₀R_SAM, (b) and R_C (c) vs. n for Ag–S(CH₂)_nBr//GaO_x/EGaIn junctions. The solid black line represents a fit to Eq. (2). d Corresponding dielectric constant ε_r vs. X for Ag–S(CH₂)₁₄X//GaO_x/EGaIn junctions. The error bars are the standard deviations of three independent measurements. Dashed lines are visual guides.

Fig. 5. Density-functional theory (DFT) calculations.

a DFT-calculated plane-averaged electrostatic potential of Ag(111)–S(CH₂)₁₄X where X = H, F, Cl, Br, or I, along the surface-normal coordinate. b Work function (Φ) of the SAMs, calculated from DFT (red squares) and measured experimentally (black dots). Density of states (DOS) projected onto the molecular backbone (c) and onto the X-site (d) of Ag(111)–S(CH₂)₁₄X. Note that X contributes very little to the band edges for X = H, F.

Fig. 6. Single-level Landauer model analysis.

a The modelled current through the Ag–S(CH₂)₁₄X//GaO_x/EGaIn junctions using Landauer theory (orange solid lines are Landauer fits, symbols represent experimental data). The values of tunnelling barrier height $\left(\delta E_{\mathrm{ME}}\right)$ (b) and the coupling strength $\left(\Gamma \right)$ (c) used for modelling the current through the junctions. d Tunnelling decay coefficient β vs. $\sqrt{\delta E_{\mathrm{ME}}}$ with a linear fit (red line), the error bars represent the standard deviations of the β values from linear fits to Eq. (1). e Double-log plot of R_C vs. 1/Γ² (R_C represents contact resistance) where the red line is a power-law fit with a slope of 0.25 and R² = 0.99, error bars of R_C represent the standard deviations of three independent measurements. f Double-log plot of β vs. ${\epsilon }_r$ where the red line is a fit with a slope of −0.82 and R² = 0.99. The error bars of β represents the same as panel b, and of ${\epsilon }_{\mathrm{r}}$ represent the standard deviations of three independent measurements.