Molecular electronics has received significant attention in the last decades. To hone performance of devices, eliminating structural defects in molecular components inside devices is usually needed. We herein demonstrate this problem can be turned into a strength for modulating the performance of devices. We show the systematic dilution of a monolayer of an organic rectifier (2,2'-bipyridine-terminated n-undecanethiolate) with electronically inactive diluents (n-alkanethiolates of different lengths), gives remarkable gradients of rectification. Rectification is finely tunable in a range of approximately two orders of magnitude, retaining its polarity. Trends of rectification against the length of the diluent indicate the gradient of rectification is extremely sensitive to the molecular structure of the diluent. Further studies reveal that noncovalent intermolecular interactions within monolayers likely leads to gradients of structural defect and rectification.

Interfacial chemistry at organic-inorganic contact critically determines the function of a wide range of molecular and organic electronic devices and other systems. The chemistry is, however, difficult to understand due to the lack of easily accessible in-operando spectroscopic techniques that permit access to interfacial structure on a molecular scale. Herein, we compare two analogous junctions formed with identical organic thin film and different liquid top-contacts (water droplet vs eutectic gallium indium alloy) and elucidate the puzzling interfacial characteristics. Specifically, we fine-tune the surface topography of the organic surface using mixed self-assembled monolayers (SAMs): single component SAM composed of rectifier (2,2'-bipyridyl-terminated n-undecanethiolate; denoted as SCBIPY) is systematically diluted with nonrectifying n-alkanethiolates of different lengths (denoted as SC where n = 8, 10, 12, 14, 16, 18). Characterization of the resulting mixed SAMs in wettability and tunneling currents with the two separate liquid top-contacts allows us to investigate the role of phase segregation and gauche defect in the SAM//liquid interfaces. The results reported here show the difference in length between SCBIPY and SC is translated into nanoscopic pits and gauche-conformer defects on the surface, and the difference in contact force-hydrostatic vs user pressures-and hence conformity of contact account for the difference in wettability and rectification behaviors. Our work provides an insight into the role of molecule-electrode interfacial defects in performance of molecular-scale electronic devices.

This Letter examines the interplay of important tunneling mechanismsFermi level pinning, Marcus inverted transport, and orbital gatingin a molecular rectifier. The temperature dependence of the rectifying molecular junction containing 2,2′-bipyridyl terminated n-alkanethiolate was investigated. A bell-shaped trend of activation energy as a function of applied bias evidenced the dominant occurrence of unusual Marcus inverted transport, while retention of rectification at low temperatures implied that the rectification obeyed the resonant tunneling regime. The results allowed reconciling two separately developed transport models, Marcus–Landauer energetics and Fermi level pinning-based rectification. Our work shows that the internal orbital gating can be substituted with the pinning effect, which pushes the transport mechanism into the Marcus inverted regime.

studies have relied on homogeneous, pure SAMs, that is, SAMs composed of one type of molecules. Contamination or dilution of a homogenous SAM by different molecules has typically been considered to cause negative effects because increased heterogeneity can directly translate into (supra)molecular and electronic structural changes, which can hinder the achievement of desired device performance.Although the field of molecular and organic electronics has long utilized pure molecular systems, studies on how charges traverse across multicomponent molecular systems have only recently emerged. [8] This review article focuses on the emergence of, and recent advances in mixed molecular electronics, defined as an electronics field exploiting heterogeneous molecular systems such as mixed SAMs (Figure 1). In this article, we introduce and discuss charge transport behaviors in mixed SAMs and applications for mixed molecular electronics. We further aim to provide a rational perspective on the unique features of mixed molecular systems with an eye toward potential molecular and nanoelectronics applications. Supramolecular and Electronic Structures of Mixed SAMs Supramolecular Structure of Mixed SAMsDiluting a pure, single-component SAM with another molecule leads to a mixed SAM. Depending on the number of molecular species comprising a mixed SAM, such a molecular dilution can yield binary, ternary, quaternary mixed SAMs, and so on. Mixed SAMs can be formed by several methods. Among others, the following three methods are commonly employed.

The variation of the electronic structure of individual molecules as a function of the applied bias matters for the performance of molecular and organic electronic devices. Understanding the structure−electric-field relationship, however, remains a challenge because of the lack of in-operando spectroscopic technique and complexity arising from the ill-defined onsurface structure of molecules and organic−electrode interfaces within devices. We report that a reliable and reproducible molecular diode can be achieved by control of the conjugation length in polycyclic-aromatic-hydrocarbon (PAH)-terminated nalkanethiolate (denoted as SC 11 PAH), incorporated into liquid-metal-based large-area tunnel junctions in the form of a selfassembled monolayer. By taking advantage of the structural simplicity and tunability of SC 11 PAH and the high-yielding feature of the junction technique, we demonstrate that the increase in the conjugation length of the PAH terminal group leads to a significant rectification ratio up to ∼1.7 × 10 2 at ±740 mV. Further study suggests that the Stark shift of the molecular energy resonance of the PAH breaks the symmetry of the energy topography across the junction and induces rectification in a temperature-independent charge-transport regime.

This paper examines the ability of structural modifications using halogen atoms (F, Cl, Br, and I) to influence tunneling rates across self-assembled monolayer (SAM)-based junctions having the structure Ag(TS)/S(CH2)n(p-C6H4X)//Ga2O3/EGaIn, where S(CH2)n(p-C6H4X) is a SAM of benzenethiol (n = 0) or benzyl mercaptan (n = 1) terminated in a hydrogen (X = H) or a halogen (X = F, Cl, Br, or I) at the para-position. The measured tunneling current densities (J(V); A cm(-2)) indicate that replacing a terminal hydrogen with a halogen atom at the X//Ga2O3 interface leads to a decrease in J(V) by ∼×13 for S(p-C6H4X) and by ∼×50 for SCH2(p-C6H4X). Values of J(V) for the series of halogenated SAMs were indistinguishable, indicating that changes in dipole moment and polarizability caused by introducing different halogen atoms at the interface between the SAM and the Ga2O3/EGaIn electrode do not significantly influence the rates of charge tunneling across the junctions.

Whereas single-component self-assembled monolayers (SAMs) have served widely as organic components in molecular and organic electronics, how the performance of the device is influenced by the heterogeneity of monolayers has been little understood. This paper describes charge transport by quantum tunneling across mixed SAMs of n-alkanethiolates of different lengths formed on ultraflat template-stripped gold substrate. Electrical characterization using liquid metal comprising eutectic gallium-indium alloy reveals that the surface topography of monolayer largely depends on the difference in length between the thiolates and is translated into distribution of tunneling current density. As the length difference is more significant, more phase segregation takes place, leading to an increase in the modality of Gaussian fitting curves. Consequently, statistical analysis permits access to deconvolution of tunneling currents, mirroring the phase-segregated surface. Our work provides an insight into the role of surface topography in the performance of molecular-scale electronic devices.

Electrical breakdown is a critical problem in electronics. In molecular electronics, it becomes more problematic because ultrathin molecular monolayers have delicate and defective structures and exhibit intrinsically low breakdown voltages, which limit device performances. Here, we show that interstitially mixed self-assembled monolayers (imSAMs) remarkably enhance electrical stability of molecular-scale electronic devices without deteriorating function and reliability. The SAM of the sterically bulky matrix (SC11BIPY rectifier) molecule is diluted with a skinny reinforcement (SC n ) molecule via the new approach, so-called repeated surface exchange of molecules (ReSEM). Combined experiments and simulations reveal that the ReSEM yields imSAMs wherein interstices between the matrix molecules are filled with the reinforcement molecules and leads to significantly enhanced breakdown voltage inaccessible by traditional pure or mixed SAMs. Thanks to this, bias-driven disappearance and inversion of rectification is unprecedentedly observed. Our work may help to overcome the shortcoming of SAM’s instability and expand the functionalities.