A new monolayer rectifier with pyrene donor (D) and perylenebisimide acceptor (A) rectifies A → D below 2.0 Volts, but A ← D beyond that.

To shrink electrical circuits beyond the looming practical limits of Moore's “Law”, it has been a common goal to create carbon‐based components. Here is the first example of a new class of donor–σ–acceptor rectifiers: hemibiquinones (HBQs). HBQs possess donor and acceptor moieties that are electronically isolated by inter‐ring torsion. A HBQ–dinitrile self‐assembles on a template‐stripped gold surface through directed chemisorption, forming a 1.1 nm‐thick monolayer. Rectification is measured through the monolayer or single molecules by using three different top electrode arrangements. Even though the HBQs are composed of relatively weak electrophores, rectification ratios ranging from 5 to 160 were observed at 2.5 V.

Sandwiches "EGaIn|Ga2O3|LB monolayer of 2|Au" and "EGaIn|Ga2O3|LB monolayer of 3|Au" rectify. They are formed from a Langmuir-Blodgett (LB) monolayer of 2 or 3 transferred onto thermally evaporated gold. Molecules 2 and 3 are of the donor-sigma-acceptor (D-σ-A) type and have the same perylenebisimide (PBI) acceptor as previously studied molecule 1. Molecule 1 has the weak donor pyrene, 2 has the good donor ferrocene, and 3 has the very strong donor N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD). All three molecules have a long swallowtail ending in a thioacetyl group, which ensures slow chemisorption onto the Au electrode. These molecules were contacted directly by a gallium indium eutectic (EGaIn) drop, covered by a defective oxide Ga2O3 layer. As before for 1, the direction of rectification for 2 is bias-dependent. In the ±1.0 V range, the rectification is at positive V, with a rectification ratio (RR) that is initially greater than 5 and then decreases on successive scans to 2, while the currents decrease by as much as 2 orders of magnitude. In the ±2.5 V range, the rectification direction for 2 reverses, while upon repeated scanning the rectification ratio (in the negative direction) increases and the currents decrease. For molecule 3, both directions have a charge-trapped state (Coulomb blockade) leading to Voffset in both biases, but at high potentials rectification set is, with large RR (up to 2,800) at ±2.5 V.

Examples of coordination compounds that act as molecular rectifiers are rare. Recently a pentacoordinate [N2O2] Fe­(III) surfactant, namely [FeIII(LN2O2)­Cl] (1), was studied as a Langmuir–Blodgett (LB) monolayer between two Au electrodes, “Au | LB1 | Au”. Rectification was observed, but only at low currents. In order to verify the current rectification of this species, a new setup is used, where an LB monolayer of 1 is placed between Au and a soft contact of gallium indium eutectic (EGaIn), as the “sandwich” “EGaIn/Ga2O3 | LB1 | Au”. When scanned from 0 to −1.5 V, 90% of the sandwiches remained stable, while scanning from 0 to + 1.5 V only 10% remained stable. For the scan range of ± 0.7 V, 90% of the sandwiches were stable on the first scan; about half of them could withstand repeated scans; the rectification ratios (RR) at 0.7 V ranged between 3 and 12. Pushing the bias range to ±1.0 V, the RR increased to between 50 and 150, but the sandwiches lasted for at most three full scans.