Ultrarobust Thin‐Film Devices from Self‐Assembled Metal–Terpyridine Oligomers

Abstract

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 ]

Long-Range Activationless Photostimulated Charge Transport in Symmetric Molecular Junctions

“…The linearity of ln J with V 1/2 shown in Figure 6a for the dark current is consistent with our previous proposal 21 that tunneling barriers are lowered in an electric field, analogous to classical Schottky and Poole−Frenkel behavior, but with weak or absent temperature dependence. The apparent activation energies of 6−81 meV for the dark current are much lower than the reported Schottky barriers for injection into organometallic films, 64 although Schottky injection may still occur with a nonclassical barrier in the case of NAB. The photocurrent shares this relationship, albeit with much weaker voltage dependence and an inverse temperature dependence.…”

Control of Rectification in Molecular Junctions: Contact Effects and Molecular Signature

“…The offset between the molecular orbitals and the contact Fermi level is often used to estimate the tunneling barriers for MJs and to identify the main charge carrier flowing through the device. The Fermi level of Au is −5.1 to −5.2 eV vs vacuum while that of Ti and TiC is −4.3 eV . The BTB HOMO is close to the Au Fermi level and is separated from that of Ti or TiC by ∼1 eV, while the barrier between the BTB LUMO and the Fermi levels of either contact is much higher.…”

Structure Controlled Long-Range Sequential Tunneling in Carbon-Based Molecular Junctions

“…A recent report on transport in molecular junctions containing iron-porphyrin multilayers with d > 10 nm concluded that the linear ln JvsV 1/2 behavior observed was due to Schottky emission at the electrode interfaces. 51 Both Poole−Frankel and Schottky include a field-dependent barrier height (ϕ), given by eq 1, where ϕ 0 is the barrier height at zero field, q is the elementary charge, ε the relative dielectric constant of the molecular layer,…”