Intermolecular Effects on Tunneling through Acenes in Large-Area and Single-Molecule Junctions

Abstract

This paper describes the conductance of single-molecules and self-assembled monolayers comprising an oligophenyleneethynylene core, functionalized with acenes of increasing length that extend conjugation perpendicular to the path of tunneling electrons. In the Mechanically Controlled Break Junction (MCBJ) experiment, multiple conductance plateaus were identified. The high conductance plateau, which we attribute to the single molecule conformation, shows an increase of conductance as a function of acene length, in good agreement with theoretical predictions. The lower plateau is attributed to multiple molecules bridging the junctions with intermolecular interactions playing a role. In junctions comprising a self-assembled monolayer with eutectic Ga-In top-contacts (EGaIn), the pentacene derivative exhibits unusually low conductance, which we ascribe to the inability of these molecules to pack in a monolayer without introducing significant intermolecular contacts. This hypothesis is supported by the MCBJ data and theoretical calculations showing suppressed conductance through the PC films. These results highlight the role of intermolecular effects and junction geometries in the observed fluctuations of conductance values between single-molecule and ensemble junctions, and the importance of studying molecules in both platforms.

Figure 1

Chemical structures of OPE3-like acene series.

Figure 2

Transmission spectra calculated for OPE3, NP, 9,10-AC, TC, and PC. The x-axis is referenced to an approximate Fermi level of −4.3 eV for EGaIn (see ref (38)).

Figure 3

Plots of log |J| (the units of J are A cm^–2) versus V for SAMs using EGaIn top-contacts (Au^TS/SAM//EGaIn): black squares for OPE3, blue circles for NP, lemon-yellow up-triangle for 9,10-AC, red down-triangle for TC, and dark green left-triangle for PC. Each data point is the mean from a Gaussian fit to a histogram of log |J|.

Figure 4

C 1s XP spectra of NP, 9,10-AC, TC, and PC and reference SC18 SAMs on Au/mica substrates. The positions of the observed peak are marked by red dash lines.

Figure 5

(a) 2D conductance–displacement (left panel) and 1D conductance (right panel) histograms of OPE3. (b) 2D conductance–displacement (left panel) and 1D conductance (right panel) histograms of the corresponding high-conductance class extracted through clustering. (c) 1D conductance histograms of the high-conductance class for all molecules of the studied series. Each histogram includes all measurements from the corresponding molecule obtained after clustering. Counts were rescaled by peak height.

Figure 6

(a) Experimental MCBJ (triangles) and simulated (stars) conductances of single-molecule junctions: up-triangles are from the high-conductance plateaux, down-triangles are from the low-conductance plateaux, and stars are conductances as G = G₀T(0) from the DFT simulations shown in Figure 2. (b) Conductances of SAMs of the acence series, extracted from the J/V traces of EGaIn data in Figure 3.

Figure 7

Normalized differential conductance heatmap plots of Au^TS/SAMs//EGaIn junctions comprising (a) OPE3, (b) NP, (c) 9,10-AC, (d) TC, and (e) PC. The Y-axis is normalized differential conductance log |dJ/dV|, and the X-axis is potential. The colors correspond to the frequencies of the histogram and the lighter color represents higher frequencies.