or inorganic [ 13,18 ] interlayers have been introduced to protect ultrathin molecular layers from invasive, vapor deposited contacts. These approaches represent a promising pathway toward robust molecular/polymer circuits; however they require an additional organic/inorganic layer that inevitably masks the intrinsic electrical response of the molecules under investigation. A possible solution has been proposed by McCreery and co-workers, involving molecular layers that are sandwiched between carbon and copper electrodes forming stable and highly reproducible molecular junctions. [ 19 ] Remarkably, these large area junctions show high yields, endurance, and temperature stability, even though the requirement of using pyrolytic carbon as a bottom electrode might limit their applicability.Here, we demonstrate that by integration of Fe II -terpyridine redox complex oligomers [20][21][22] into large area solid-state junctions, molecular thin-fi lm devices of outstanding mechanical and electrical robustness are realized. Notwithstanding the metallic crossbar junctions are deposited in a conventional thermal evaporation process, Fe II -terpyridine oligomers are operational over a period of more than two and a half years and resist to temperatures ranging from 150-360 K. The oligomer layers show a high electron mobility ( µ e = 0.1 cm 2 V −1 s −1 ) and, most remarkably, electrical transport follows an ideal RichardsonSchottky (RS) injection behavior, as demonstrated by means of complementary experimental and theoretical investigations.Bottom electrodes are prepared by thermal evaporation of an array of eight parallel Au electrodes (each 100 µm wide) on native silicon using a shadow mask. Subsequently, metal center oligomers (MCOs) are deposited by a stepwise sequential coordination reaction of a Fe II redox center by a conjugated 1,4-di(2;2′;6′;2″-terpyridine-4′-yl)benzene (TPT) ligand ( Figure 1 a), [ 21 ] as schematically depicted in Figure 1 b. In our work, oligomers of three different lengths have been assembled by incorporation of 15, 20, and 30 Fe II metal centers (MC), yielding MCO layers with a thickness of 15, 20, and 30 nm. This allows a detailed study of their electrical characteristics as a function of molecular length. A symmetric contact of the oligomers to both Au electrodes is established by using 4′-(4-mercaptophenyl)terpyridine (MPTP) as the fi rst and last ligands of the stepwise coordination.From density functional theory (DFT) calculations, a length of 1.55 nm is derived for the repeat unit of the MCO chain (Figure 1 a). A constant increment in fi lm thickness as a function of the coordination number is determined from AFM data (Figure 1 c), following a linear regression with a slope of ≈1.08 nm per coordination step. These data and the coordination effi ciency known for the stepwise coordination process [ 23 ]

We have investigated bipolar resistive switching of Cu/CuTCNQ/Al cross-junctions in both vacuum and different gas environments. While the generally observed S-shaped I-V hysteresis was reproduced in ambient air, it was reversibly suppressed in well-degassed samples in vacuum and in dry N2. The OFF-switching currents in ambient air peaked when approximately +2.6 V was applied to the Al electrode at low voltage sweep rates. OFF-switching at constant bias was accelerated in humid and oxygen-rich atmospheres. For unbiased samples stored in air, ON-state (RON) and OFF-state (ROFF) resistances increased with time, and RON surpassed the initial ROFF after approximately one week. Retention times were enhanced for samples stored in vacuum and those with a larger cross-junction area. We suggest that resistive switching occurs in a hydrated native alumina layer at the CuTCNQ/Al interface that grows in thickness during exposure to ambient humidity: ON-switching by electrochemical metallization of free Al and/or Cu ions and OFF-switching by anodic oxidation of the Al electrode and previously grown metal filaments.