Evidence for Quantum Interference in SAMs of Arylethynylene Thiolates in Tunneling Junctions with Eutectic Ga–In (EGaIn) Top-Contacts

Abstract

This paper compares the current density (J) versus applied bias (V) of self-assembled monolayers (SAMs) of three different ethynylthiophenol-functionalized anthracene derivatives of approximately the same thickness with linear-conjugation (AC), cross-conjugation (AQ), and broken-conjugation (AH) using liquid eutectic Ga-In (EGaIn) supporting a native skin (~1 nm thick) of Ga(2)O(3) as a nondamaging, conformal top-contact. This skin imparts non-Newtonian rheological properties that distinguish EGaIn from other top-contacts; however, it may also have limited the maximum values of J observed for AC. The measured values of J for AH and AQ are not significantly different (J ≈ 10(-1)A/cm(2) at V = 0.4 V). For AC, however, J is 1 (using log averages) or 2 (using Gaussian fits) orders of magnitude higher than for AH and AQ. These values are in good qualitative agreement with gDFTB calculations on single AC, AQ, and AH molecules chemisorbed between Au contacts that predict currents, I, that are 2 orders of magnitude higher for AC than for AH at 0 < |V| < 0.4 V. The calculations predict a higher value of I for AQ than for AH; however, the magnitude is highly dependent on the position of the Fermi energy, which cannot be calculated precisely. In this sense, the theoretical predictions and experimental conclusions agree that linearly conjugated AC is significantly more conductive than either cross-conjugated AQ or broken conjugate AH and that AQ and AH cannot necessarily be easily differentiated from each other. These observations are ascribed to quantum interference effects. The agreement between the theoretical predictions on single molecules and the measurements on SAMs suggest that molecule-molecule interactions do not play a significant role in the transport properties of AC, AQ, and AH.

Controlling the direction of rectification in a molecular diode

“…Previously, we have reported on the mechanism of charge transfer across molecular diodes with SAMs of SC 11 Fc on ultra-flat template-stripped silver-bottom electrodes (Ag TS ; henceforth left electrode) and EGaIn top electrodes (henceforth right electrode; a non-Newtonian liquid-metal alloy of eutectic In and Ga) [17][18][19] , including temperature-dependent J(V) measurements 20 (others have studied the mechanism rectification of SC n Fc SAMs in other types of junctions [21][22][23] ). This so-called 'EGaIn' technique is well established [24][25][26][27] and has been used in a wide range of physical-organic studies 20,[28][29][30][31][32][33] . The HOMO is located at the Fc unit, which is separated from the left electrode by the alkyl chain (similar to the junction schematically shown in Fig.…”

Replacing AgTSSCH2‐R with AgTSO2C‐R in EGaIn‐Based Tunneling Junctions Does Not Significantly Change Rates of Charge Transport

“…focused predominately on the influence of backbone substituents [1][2][3][4] or terminal functional groups [5][6][7][8] on rates of charge transport. The effect of changing the group (which we call the "anchoring group") that links the SAM to the metal substrate has not been explored in detail; 9,10 only a few studies have approached this issue at the single-molecular level using scanning tunneling microscopy [11][12][13][14] or conducting atomic force microscopy.…”

Inelastic electron tunneling spectroscopy in molecular junctions showing quantum interference

“…Quantum interference takes place in molecules where the π -electron system is cross-conjugated such as anthraquinone (AQ) and results in a strong suppression of conductance with respect to more conducting linearly conjugated molecules [1][2][3][4][5][6][7]. The resulting suppression of conductance has been reported experimentally using different techniques such as conducting atomic force microscopy [8], eutectic GaIn contact [9], mechanically controllable break junctions [10], and large-area molecular junctions [11]. The AQ molecule, due to its intrinsic cross-conjugated structure, has been used in most of these experimental works under the form of a single molecule and thin films [8,9,11].…”