Statistical analysis of single‐molecule breaking traces

Abstract

We present an experimental study of charge transport through single oligo(phenylene‐ethynylene) (OPE3) molecules and anthracene substituted OPE3 (OPE3‐AC) contacted with a mechanically controlled break junction. Both molecules are measured using two different methods. In the first approach, we measure the low‐bias conductance, while the junction is repeatedly broken and fused. In the second approach, instead of applying a fixed bias, we record current–voltage characteristics. For both approaches the data is fitted to a model describing transport through a single level, and information about the level alignment and the electronic coupling to the electrodes is extracted. We find that the electronic coupling is similar for both molecules, but that the level alignment differs. The observed trends are confirmed by quantum chemistry calculations.

Transition from Strong to Weak Electronic Coupling in a Single-Molecule Junction

“…The top panel of Fig. 4(a) shows that the average level alignment is around 0.6-0.9 eV, consistent with values previously found at room temperature [20,21]. For the CB I-V's we extract ϵ 0 directly from the position of the step or linear onset in the current (ϵ 0 ¼ eV gap =4).…”

“…4(b) shows the histogram of the total electronic coupling of junction 2 in the strong coupling regime extracted from the single-level model fit of the I-V's. The average electronic coupling is about 10 meV, again consistent with previous room temperature measurements [20,21]. In the case of the CB I-V's we extract the electronic coupling from the formula I sat ¼ ðG 0 =eÞπΓ, where I sat is the saturation current at a bias beyond the resonance (eV ≫ 2ϵ 0 þ Γ) [3]; the histogram in the bottom panel of Fig.…”

Mapping the Transmission Functions of Single-Molecule Junctions

“…The Supporting Information is available free of charge on the ACS Publications website at DOI: 10 ■ REFERENCES…”

“…3,4 For many systems investigated thus far, charge transport occurs via coherent tunneling and therefore, can be described by a transmission function detailing the energy-dependent probability that an incident electron or hole tunnels through the barrier presented by the junction. 5 Several techniques have emerged to experimentally assess energy-level alignment at the metal−molecule interface in single-molecule junctions, including thermopower measurements, 6−8 alternating current (AC) techniques, 9 mapping of current−voltage curves, 10 and gating. 11 However, these methods have been unable to provide a detailed map of the transmission landscape and have instead only provided limited insight into the transmission function.…”

“…13,14 Current−voltage (I−V) curves can, in principle, be fit in order to obtain the transmission function since the measured current at a given voltage is proportional to an integral of the transmission function. 10 However, such fitting requires knowledge of how the bias drops across the junctionand how the bias window opens about the average junction Fermi levelin addition to knowing the functional form for transmission. Furthermore, these fitting procedures are valid only if high-bias effects (such as Stark shifted molecular levels 15 ) can be excluded.…”