Equivalent Circuits of a Self-Assembled Monolayer-Based Tunnel Junction Determined by Impedance Spectroscopy

The electrical characteristics of molecular tunnel junctions are normally determined by DC methods. Using these methods it is difficult to discriminate the contribution of each component of the junctions, e.g., the molecule–electrode contacts, protective layer (if present), or the SAM, to the electrical characteristics of the junctions. Here we show that frequency-dependent AC measurements, impedance spectroscopy, make it possible to separate the contribution of each component from each other. We studied junctions that consist of self-assembled monolayers (SAMs) of <i>n</i>-alkanethiolates (S­(CH₂)_<i>n</i>−1CH₃ ≡ SC_<i>n</i> with <i>n</i> = 8, 10, 12, or 14) of the form Ag^TS-SC_<i>n</i>//GaO_<i>x</i>/EGaIn (a protective thin (∼0.7 nm) layer of GaO_<i>x</i> forms spontaneously on the surface of EGaIn). The impedance data were fitted to an equivalent circuit consisting of a series resistor (<i>R</i>_S, which includes the SAM-electrode contact resistance), the capacitance of the SAM (<i>C</i>_SAM), and the resistance of the SAM (<i>R</i>_SAM). A plot of <i>R</i>_SAM vs <i>n</i>_C yielded a tunneling decay constant β of 1.03 ± 0.04 <i>n</i>_C^–1, which is similar to values determined by DC methods. The value of <i>C</i>_SAM is similar to previously reported values, and <i>R</i>_S (2.9–3.6 × 10^–2 Ω·cm²) is dominated by the SAM–top contact resistance (and not by the conductive layer of GaO_<i>x</i>) and independent of <i>n</i>_C. Using the values of <i>R</i>_SAM, we estimated the resistance per molecule <i>r</i> as a function of <i>n</i>_C, which are similar to values obtained by single molecule experiments. Thus, impedance measurements give detailed information regarding the electrical characteristics of the individual components of SAM-based junctions

Defect Scaling with Contact Area in EGaIn-Based Junctions: Impact on Quality, Joule Heating, and Apparent Injection Current

Although the tunneling rates decrease exponentially with a decay coefficient β close to 1.0 n_C^–1 across <i>n</i>-alkanethiolate (SC_<i>n</i>) monolayer based tunneling junctions determined over a multitude of test beds, the origins of the large spread of injection current densitiesthe hypothetical current density, <i>J</i>₀ (in A/cm²), that flows across the junction when <i>n</i> = 0of up to 12 orders of magnitude are unclear. Every type of junction contains a certain distribution of defects induced by, for example, defects in the electrode materials or impurities. This paper describes that the presence of defects in the junctions is one of the key factors that cause an increase in the observed values of <i>J</i>₀. We controlled the number of defects in Ag^TS-SC_<i>n</i>//GaO_<i>x</i>/EGaIn junctions by varying the geometrical contact area (<i>A</i>_geo) of the junction. The value of <i>J</i>₀ (∼10² A/cm²) is independent of the junction size when <i>A</i>_geo is small (<9.6 × 10² μm²) but increased by 3 orders of magnitude (from 10² to 10⁵ A/cm²) when <i>A</i>_geo increased from 9.6 × 10² to 1.8 × 10⁴ μm². With increasing <i>J</i>₀ values the yields in nonshorting junctions decreased (from 78 to 44%) and β increased (from 1.0 to 1.2 n_C^–1). We show that the quality of the junctions can be qualitatively determined by examining the curvature of the d<i>J</i>/d<i>V</i> curves (defects change the sign of the curvature from positiveassociated with tunnelingto negativeassociated with Joule heating) and fitting the <i>J</i>(V) curves to the full Simmons equation to (crudely) estimate the effective separation of the top- and bottom-electrode <i>d</i>_eff. This analysis confirmed that the electrical characteristics of large junctions are dominated by thin-area defects, while small junctions are dominated by the molecular structure

Plasmon-Modulated Photoluminescence of Single Gold Nanobeams

In this work, we investigate the modulation of the photoluminescence (PL) of a single Au nanobeam (NB) by the surface plasmons of a Ag nanowire (NW) and the gap plasmons between the two nanostructures. By changing the polarization of the laser that excites the nanostructure and controlling the separation distance <i>d</i> between the two nanostructures, we found that the transverse surface plasmon resonance of the Ag NW enhanced the PL (at 520 nm) of the Au NB with a maximum effect at <i>d</i> = 7 nm. The PL enhancement (at 520 nm) was quenched and a new PL peak was observed at a longer wavelength for <i>d</i> < 7 nm. The PL quenching effect could be understood by the quadrupole-like plasmonic resonance between the Ag NW and the Au NB and be qualitatively explained by the mode dispersion as a function of <i>d</i> obtained using the transfer matrix transmittance calculation. FDTD simulations show that the new PL peak at a longer wavelength is caused by the waveguide-mode gap plasmons between the Au NB and the Ag NW