Supramolecular tunnelling junctions with robust high rectification based on assembly effects.

The performance of large-area molecular diodes can in rare cases approach the lower limit of commercial semiconductor devices but predictive structure-property design remains difficult as the rectification ratio (R) achieved by self-assembled monolayer (SAM) based diodes depends on several intertwined parameters. This paper describes a systematic approach to achieve high rectification in bisferrocenyl-based molecular diodes, HSCnFc-CC-Fc (n = 9-15) immobilised on metal surfaces (Ag, Au and Pt). Experiments supported by molecular dynamics simulations show that the molecular length and bottom electrode influence the SAM packing, which affects the breakdown voltage (VBD), the associated maximum R (Rmax), and the bias at which the Rmax is achieved (Vsat,R). From the electrical characterisation of the most stable Pt-SCnFc-CC-Fc//GaOx/EGaIn junctions, we found that VBD, Vsat,R, and Rmax all scale linearly with the spacer length of Cn, and that Rmax for all the SAMs consistently exceeds the "Landauer limit" of 103. Our data shows that the robust switching of M-SCnFc-CC-Fc//GaOx/EGaIn junctions is the result of the combined optimisation of parameters involving the molecular structure, the type of metal substrate, and the applied operating conditions (bias window), to create stable and high-performance junctions.

Molecular switching by proton-coupled electron transport drives giant negative differential resistance.

To develop new types of dynamic molecular devices with atomic-scale control over electronic function, new types of molecular switches are needed with time-dependent switching probabilities. We report such a molecular switch based on proton-coupled electron transfer (PCET) reaction with giant hysteric negative differential resistance (NDR) with peak-to-valley ratios of 120 ± 6.6 and memory on/off ratios of (2.4 ± 0.6) × 103. The switching dynamics probabilities are modulated by bias voltage sweep rate and can also be controlled by pH and relative humidity, confirmed by kinetic isotope effect measurements. The demonstrated dynamical and environment-specific modulation of giant NDR and memory effects provide new opportunities for bioelectronics and artificial neural networks.

Dynamic molecular switches with hysteretic negative differential conductance emulating synaptic behaviour.

To realize molecular-scale electrical operations beyond the von Neumann bottleneck, new types of multifunctional switches are needed that mimic self-learning or neuromorphic computing by dynamically toggling between multiple operations that depend on their past. Here, we report a molecule that switches from high to low conductance states with massive negative memristive behaviour that depends on the drive speed and number of past switching events, with all the measurements fully modelled using atomistic and analytical models. This dynamic molecular switch emulates synaptic behavior and Pavlovian learning, all within a 2.4-nm-thick layer that is three orders of magnitude thinner than a neuronal synapse. The dynamic molecular switch provides all the fundamental logic gates necessary for deep learning because of its time-domain and voltage-dependent plasticity. The synapse-mimicking multifunctional dynamic molecular switch represents an adaptable molecular-scale hardware operable in solid-state devices, and opens a pathway to simplify dynamic complex electrical operations encoded within a single ultracompact component.

Stable Universal 1- and 2-Input Single-Molecule Logic Gates.

Controllable single-molecule logic operations will enable development of reliable ultra-minimalistic circuit elements for high-density computing but require stable currents from multiple orthogonal inputs in molecular junctions. Utilizing the two unique adjacent conductive molecular orbitals (MOs) of gated Au/S-(CH2 )3 -Fc-(CH2 )9 -S/Au (Fc = ferrocene) single-electron transistors (≈2 nm), a stable single-electron logic calculator (SELC) is presented, which allows real-time modulation of output current as a function of orthogonal input bias (Vb ) and gate (Vg ) voltages. Reliable and low-voltage (ǀVb ǀ ≤ 80 mV, ǀVg ǀ ≤ 2 V) operations of the SELC depend upon the unambiguous association of current resonances with energy shifts of the MOs (which show an invariable, small energy separation of ≈100 meV) in response to the changes of voltages, which is confirmed by electron-transport calculations. Stable multi-logic operations based on the SELC modulated current conversions between the two resonances and Coulomb blockade regimes are demonstrated via the implementation of all universal 1-input (YES/NOT/PASS_1/PASS_0) and 2-input (AND/XOR/OR/NAND/NOR/INT/XNOR) logic gates.

Bias-Polarity-Dependent Direct and Inverted Marcus Charge Transport Affecting Rectification in a Redox-Active Molecular Junction.

This paper describes the transition from the normal to inverted Marcus region in solid-state tunnel junctions consisting of self-assembled monolayers of benzotetrathiafulvalene (BTTF), and how this transition determines the performance of a molecular diode. Temperature-dependent normalized differential conductance analyses indicate the participation of the HOMO (highest occupied molecular orbital) at large negative bias, which follows typical thermally activated hopping behavior associated with the normal Marcus regime. In contrast, hopping involving the HOMO dominates the mechanism of charge transport at positive bias, yet it is nearly activationless indicating the junction operates in the inverted Marcus region. Thus, within the same junction it is possible to switch between Marcus and inverted Marcus regimes by changing the bias polarity. Consequently, the current only decreases with decreasing temperature at negative bias when hopping is "frozen out," but not at positive bias resulting in a 30-fold increase in the molecular rectification efficiency. These results indicate that the charge transport in the inverted Marcus region is readily accessible in junctions with redox molecules in the weak coupling regime and control over different hopping regimes can be used to improve junction performance.

A single atom change turns insulating saturated wires into molecular conductors.

We present an efficient strategy to modulate tunnelling in molecular junctions by changing the tunnelling decay coefficient, ß, 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 ß 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 ß. 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 [Formula: see text], 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.

Electric-field-driven dual-functional molecular switches in tunnel junctions.

To avoid crosstalk and suppress leakage currents in resistive random access memories (RRAMs), a resistive switch and a current rectifier (diode) are usually combined in series in a one diode-one resistor (1D-1R) RRAM. However, this complicates the design of next-generation RRAM, increases the footprint of devices and increases the operating voltage as the potential drops over two consecutive junctions1. Here, we report a molecular tunnel junction based on molecules that provide an unprecedented dual functionality of diode and variable resistor, resulting in a molecular-scale 1D-1R RRAM with a current rectification ratio of 2.5 × 104 and resistive on/off ratio of 6.7 × 103, and a low drive voltage of 0.89 V. The switching relies on dimerization of redox units, resulting in hybridization of molecular orbitals accompanied by directional ion migration. This electric-field-driven molecular switch operating in the tunnelling regime enables a class of molecular devices where multiple electronic functions are preprogrammed inside a single molecular layer with a thickness of only 2 nm.