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 mechanismsFermi level pinning, Marcus inverted transport, and orbital gatingin 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.
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