In-Place Modulation of Rectification in Tunneling Junctions Comprising Self-Assembled Monolayers

Abstract

This paper describes tunneling junctions comprising self-assembled monolayers that can be converted between resistor and diode functionality in-place. The rectification ratio is affected by the hydration of densely packed carboxylic acid groups at the interface between the top-contact and the monolayer. We studied this process by treatment with water and a water scavenger using three different top-contacts, eutectic Ga–In (EGaIn), conducting-probe atomic force microscopy (CP-AFM), and reduced graphene oxide (rGO), demonstrating that the phenomena is molecular in nature and is not platform-speciffc. We propose a mechanism in which the tunneling junctions convert to diode behavior through the lowering of the LUMO, which is suffcient to bring it close to resonance at positive bias, potentially assisted by a Stark shift. This shift in energy is supported by calculations and a change in polarization observed by X-ray photoelectron spectroscopy and Kelvin probe measurements. We demonstrate light-driven modulation using spiropyran as a photoacid, suggesting that any chemical process that is coupled to the release of small molecules that can tightly bind carboxylic acid groups can be used as an external stimulus to modulate rectification. The ability to convert a tunneling junction reversibly between a diode and a resistor via an effect that is intrinsic to the molecules in the junction extends the possible applications of Molecular Electronics to reconfigurable circuits and other new functionalities that do not have direct analogs in conventional semiconductor devices.

In Operando Modulation of Rectification in Molecular Tunneling Junctions Comprising Reconfigurable Molecular Self‐Assemblies

“…[ 1 ] Molecular tunneling junctions, in which the charge transport properties of molecules are exploited in devices, have considerable potential in this context because subtle changes at the atomic scale can translate into unique and useful effects at the device level. [ 2,3 ] By exploiting the chemical nature of molecular electronics, these effects are exemplified in photoisomerization, [ 4–8 ] , protonation, [ 9,10 ] electrostatic gating, [ 11,12 ] reduction–oxidation, [ 13 ] etc., giving rise to molecular resistors and molecular diodes in both single‐molecule junctions and large‐area junctions comprising self‐assembled monolayers (SAMs). The majority of studies on effects that rely on external inputs, such as conductance switching, are observed ex situ, i.e., new junctions are formed and characterized instead of molecules being switched in‐place, be they single‐molecule break‐junctions in which molecules are repeatedly trapped and released [ 14 ] or large‐area junctions formed by raising and lowering a tip on top of a SAM.…”

Stochastic Computing via In Operando Modulation of Rectification in Molecular Tunneling Junctions

“…The observation that a depends on the identity of the moiety in contact with the top electrode can be understood by the tunneling-hopping mechanism shown in In operando rectification modulation While it is important to verify the mechanism of rectification in GESAMs, the ability of fullerenes and ferrocenes to effect rectification is not new; however, combined with the unique properties of GESAMs, these molecules can be used to modulate rectification while a junction is being observed under bias. This in operando control over the properties of a tunneling junction is a technological step forward from in situ and in-place modulation 8 and, unlike conductance-switching, 7 is unique to molecular-electronic devices. To allow for exchange in assembled Au TS /PTEG-1//PTEG-1//EGaIn junctions, we formed them by contacting the GESAMs of PTEG-1 with conical EGaIn tips in a microfluidic channel with precise control over back pressure (see Experimental section for details), as shown in Fig.…”

“…1 Molecular electronics, which studies the charge transport properties of molecules and their applications in devices that realize their properties, has extraordinary potential in this context because subtle changes at the atomic scale can translate into unique and useful effects at the device level. 2 By exploiting the chemical nature of molecular electronics, these effects are exemplified in photoisomerization, [3][4][5][6][7] protonation, 8,9 electrostatic gating, 10,11 reduction-oxidation, 12 etc., giving rise to molecular resistors and molecular diodes in both single-molecule junctions and large-area junctions comprising self-assembled monolayers (SAMs). However, effects that rely on external inputs, such as conductance-switching, are rarely observed in situ; i.e., new junctions are formed and characterized instead of the original ones, be they single-molecules in break-junctions or raising and lowering a tip of eutectic Ga-In (EGaIn) on top of a SAM.…”

“…The origins of asymmetric charge transport in molecular electronics are often ascribed to i) the formation of a Schottky diode, 14 ii) Stark shifts in molecular orbitals, 8,15,16 iii) the alignment between the molecular dipole moment and the external electric field 17,18 or iv) thermally-activated charge transport mediated by molecular frontier orbitals. [19][20][21] SAMs of alkanethiols bearing fullerene headgroups exhibit asymmetric charge transport that is ascribed to a thermally activated process (i.e., hopping) mediated by the lowest unoccupied -state (LUPS) of the fullerene cage, though the mechanism has not been confirmed by variable-temperature measurements.…”

Electrical molecular switch addressed by chemical stimuli

“…A similar influence of the protonation on the LUMO position was reported for SAMs, in which the protonation of the terminal carboxylic acid groups converts back and force the molecular junctions between a resistor and a rectifying diode. 33 Protonation has also been used in combination, with light irradiation to block the spiropyran molecule in its merocyanine isomer and avoiding its spontaneous back switching to the spiropyran form. 34 Finally, protonation has been used to induce destructive-quantum-interference in diketopyrrolopyrrole derivative, leading to a reversible decrease of the conductance (1 order of magnitude) upon protonation.…”