A single atom change turns insulating saturated wires into molecular conductors

Abstract

We present an efficient strategy to modulate tunnelling in molecular junctions by changing the tunnelling decay coefficient, β, by terminal-atom substitution which avoids altering the molecular backbone. By varying X = H, F, Cl, Br, I in junctions with S(CH2)(10-18)X, current densities (J) increase >4 orders of magnitude, creating molecular conductors via reduction of β from 0.75 to 0.25 Å−1. Impedance measurements show tripled dielectric constants (εr) with X = I, reduced HOMO-LUMO gaps and tunnelling-barrier heights, and 5-times reduced contact resistance. These effects alone cannot explain the large change in β. Density-functional theory shows highly localized, X-dependent potential drops at the S(CH2)nX//electrode interface that modifies the tunnelling barrier shape. Commonly-used tunnelling models neglect localized potential drops and changes in εr. Here, we demonstrate experimentally that $$\beta \propto 1/\sqrt{{\varepsilon }_{r}}$$ β ∝ 1 / ε r , suggesting highly-polarizable terminal-atoms act as charge traps and highlighting the need for new charge transport models that account for dielectric effects in molecular tunnelling junctions.

Thickness Fluctuations Produce Apparent Long-Range Tunneling in Large-Area Junctions: The Case of Polyelectrolyte Multilayers

“…This method employs a liquid eutectic gallium/indium alloy (EGaIn) to make a conformal electrical top contact on the film, while the bottom contact is the gold substrate on which the film was deposited. Following first report of the EGaIn method, different self-assembled systems, such as alkanethiol SAMs, ,, oligophenyleneimine SAMs, redox-active benzotetrathiafulvalene SAMs, photosystem I, and others, were studied with DC or AC potentiostatic measurements. In the DC experiment, the steady-state currents are recorded as a function of the applied potential, which is typically scanned between −1 V and +1 V. Each measurement yields a I / V trace, and typically, many of such traces are measured per junction.…”

“…However, it was also shown that this value decreases when the thiol anchoring groups are replaced by −NH 2 ( β NH 2 ∼ 0.9/CH 2 ), −COOH (β COOH ∼ 0.8/CH 2 ), , and highly polarizable atoms like iodine (β I ∼ 0.5/CH 2 ) . Moreover, Chen et al showed that by varying X = H, F, Cl, Br, I in small area EGaIn junctions (350 μm 2 ) with S­(CH 2 ) (10–18) X, molecular conductors showed a reduction of β from 0.75 to 0.25 Å –1 . These effects arise because of subtle alterations in the HOMO–LUMO gaps, localized potential drops, and dielectric constants, which have an impact on tunneling barrier heights and even in the tunneling mechanism itself.…”

“…24 Moreover, Chen et al showed that by varying X = H, F, Cl, Br, I in small area EGaIn junctions (350 μm 2 ) with S(CH 2 ) (10−18) X, molecular conductors showed a reduction of β from 0.75 to 0.25 Å −1 . 25 These effects arise because of subtle alterations in the HOMO−LUMO gaps, localized potential drops, and dielectric constants, which have an impact on tunneling barrier heights and even in the tunneling mechanism itself. On the other hand, molecules that contain a conjugated moiety (phenyl, viologen, or α-terthiophene) in the center of alkane chains of varying length gave rise to resonances close to the Fermi level through coupling to the bridge moiety.…”

Interplay between Interfacial Energy, Contact Mechanics, and Capillary Forces in EGaIn Droplets

“…Due to the interesting non-Newtonian properties of eutectic gallium–indium (EGaIn) and its nontoxic properties, this liquid-metal alloy, among others, is widely employed in areas of research where it is important to have access to flexible/stretchable electrodes. − This choice stems from the unique physicochemical properties of the EGaIn alloy: it is liquid at room temperature and spontaneously forms a thin oxide skin of gallium oxide (GaO x ) under ambient atmospheric conditions that provide mechanical stability to the liquid core, − allowing EGaIn to be readily manipulated into various shapes, such as cone-shaped tip electrodes (that are routinely used to measure charge transport phenomena across organic self-assembled monolayers (SAMs) ,− ) and three-dimensional (3D) printed ,, or stable structures in microchannels. ,, This characteristic feature makes EGaIn a suitable material for additive printing, , room-temperature microwelding/sintering, or applications in soft robotics. , …”

“…Arguably, these ambiguities are the most prevalent not only in (molecular) electronic applications but also in any other applications where it is important to understand how EGaIn interacts with surfaces: how smooth are the interfaces, what are the factors that contribute to the contact resistance, or how often can an EGaIn electrode be reused? For example, EGaIn is routinely shaped into cone-shaped tips, which, in turn, are used to form electrical contacts with the surfaces of SAMs on metal electrodes to obtain metal–SAM–GaO x /EGaIn junctions (a key element in molecular electronics). − Here, the GaO x layer provides stability and prevents the bulk Ga-In alloy from alloying with the metal surface that supports the SAM, and it also is a source of uncertainty. The GaO x /EGaIn surfaces are rough (because of rupture of the GaO x during the formation of the tips), leading to high contact resistances (due to low effective contact areas). ,, We also found that the cone-shaped EGaIn tips remained unchanged after 6–7 times of repeated indentations/contacts, but the origin of this “shape-memory” behavior is unclear.…”