Restricting Conformational Space: A New Blueprint for Electrically Switchable Self-Assembled Monolayers.

Tunnel junctions comprising self-assembled monolayers (SAMs) from liquid crystal-inspired molecules show a pronounced hysteretic current-voltage response, due to electric field-driven dipole reorientation in the SAM. This renders these junctions attractive device candidates for emerging technologies such as in-memory and neuromorphic computing. Here, the novel molecular design, device fabrication, and characterization of such resistive switching devices with a largely improved performance, compared to the previously published work are reported. Those former devices suffer from a stochastic switching behavior limiting reliability, as well as from critically small read-out currents. The present progress is based on replacing Al/AlOx with TiN as a new electrode material and as a key point, on redesigning the active molecular material making up the SAM: a previously present, flexible aliphatic moiety has been replaced by a rigid aromatic linker, thereby introducing a molecular "ratchet". This restricts the possible molecular conformations to only two major states of opposite polarity. The above measures have resulted in an increase of the current density by five orders of magnitude as well as in an ON/OFF conductance ratio which is more than ten times higher than the individual scattering ranges of the high and low resistance states.

Effect of substitution on the charge transport properties of oligophenylenethiolate self-assembled monolayers.

Electrostatic effects in charge transport across the molecular framework, including those imposed by halogen atoms, have recently attracted noticeable attention of the molecular-electronics community. In this context, in the present work, we studied the effect of tail group (R) substitution on the charge transport properties of oligophenylenethiolate self-assembled monolayers (SAMs) on Au(111), with R = H, F, CH3, and CF3. The length of the molecular backbone was varied from one to three rings and the quality, basic parameters, and electrostatic properties of the SAMs were monitored. For a given length of the molecular backbone, the current density showed a strong dependence on R, being the highest for R = CH3, and then successively lower for R = H, R = CF3, and R = F. This tendency correlated neither with the molecular length of the precursors nor with the work function of the SAMs and was, therefore, exclusively ascribed to the identity of the tail group. In contrast to the current density, the tunneling decay coefficient, describing its dependence on the molecular length, was found to be independent of the identity of the tail group. The reasons behind the observed behavior are discussed and rationalized within the available experimental data and reasonable assumptions.

Location-Selective Work Function Engineering by Self-Assembled Monolayers.

Control over specific interfaces in devices represents a key challenge for modern organic electronics and photovoltaics. Such control is frequently gained by the use of self-assembled monolayers (SAMs), which, by selection of a proper anchoring group, are generally discriminative with respect to different materials but are not selective between different areas of the same material. In particular, selective tailoring of the work function may be useful for different functional devices in a circuit. Here we demonstrate an approach for solving this problem, opening a way to function-selective electrostatic engineering of chemically identical areas, such as source and drain electrodes in a specific type of organic transistor and, more importantly, the electrodes in different types of organic devices, such as p- and n-channel transistors, located on the same circuitry board. The approach is based on the ultraviolet-light-promoted exchange reaction of SAMs on gold, a standard electrode material in organic electronics.

Sweep-Character-Dependent Switching of the Conductance State in Ferrocene-Substituted Thiofluorene Self-Assembled Monolayers.

Self-assembled monolayers (SAMs) of ferrocene-substituted thiofluorene on Au(111) exhibit two distinct conductance states (CSs) in two-terminal junctions featuring a sharp tip of eutectic GaIn as the top electrode. The occurrence of these states and the resulting effective rectification by the SAM depend on the way the bias voltage is swept; when the junction is only negatively biased, the original, high CS is preserved, whereas the junction is switched to a low CS when applying only positive biases. This results in an exceptionally high effective rectification ratio (RR) of ∼2100 already at voltages as low as 0.1 V. In contrast, when sweeping the junction alternatingly to the maximum positive and negative bias voltages (as usually performed in the literature), fully symmetric J-V curves are observed. That is, for the present SAM, rectification disappears, and the effective RR is ≈1. It is noteworthy that whether the junction in these symmetric sweeps is in the high or low CS depends on the polarity of the first sweep. We attribute the occurrence of the two CSs to a (quasi) non-reversible oxidation of the ferrocenes in combination with structural changes in the monolayer geometry. The observed sweeping dependence of the conductivity switching is an additional parameter that needs to be considered when interpreting experimental J-V curves, especially when dealing with redox-active systems.