Diaziridyl Ether of Bisphenol A

Increased complexities in applications involving curable materials virtually need new materials that can overcome the limitations of existing ones. Resins, the structure of which is based on bisphenol A backbone terminated with three membered N-heterocyclesaziridineshave been synthesized, and their thermal-curing performance in solution and solid state was evaluated by NMR and FT-IR spectroscopies, differential scanning calorimetry, and single lap shear strength test and compared with that of analogous epoxy resin (diglycidyl ether of bisphenol A; DGEBA). Results reveal that the chemical reactivity of the aziridine-based resins is fine-tunable by controlling the <i>N</i>-substituent of aziridine. These resins can undergo ring-opening polymerization in the presence of various curing agents under unprecedentedly mild conditions and show remarkably rapid curing rate, wide substrate scope, and excellent chemoselectivity as compared to the analogous epoxy resin. Our results demonstrate superb curing ability of aziridine, making it promising for applications in materials and polymer sciences

Deconvolution of Tunneling Current in Large-Area Junctions Formed with Mixed Self-Assembled Monolayers

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 <i>n</i>-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

Maskless Arbitrary Writing of Molecular Tunnel Junctions

Since fabricating geometrically well-defined, noninvasive, and compliant electrical contacts over molecular monolayers is difficult, creating molecular-scale electronic devices that function in high yield with good reproducibility is challenging. Moreover, none of the previously reported methods to form organic–electrode contacts at the nanometer and micrometer scales have resulted in directly addressable contacts in an untethered form under ambient conditions without the use of cumbersome equipment and nanolithography. Here we show that in situ encapsulation of a liquid metal (eutectic Ga–In alloy) microelectrode, which is used for junction formation, with a convenient photocurable polymeric scaffold enables untethering of the electrode and direct writing of arbitrary arrays of high-yielding molecular junctions under ambient conditions in a maskless fashion. The formed junctions function in quantitative yields and can afford tunneling currents with high reproducibility; they also function at low temperatures and under bent. The results reported here promise a massively parallel printing technology to construct integrated circuits based on molecular junctions with soft top contacts

Seebeck Effect in Molecular Wires Facilitating Long-Range Transport

The study of molecular wires facilitating long-range charge transport is of fundamental interest for the development of various technologies in (bio)organic and molecular electronics. Defining the nature of long-range charge transport is challenging as electrical characterization does not offer the ability to distinguish a tunneling mechanism from the other. Here, we show that investigation of the Seebeck effect provides the ability. We examine the length dependence of the Seebeck coefficient in electrografted bis-terpyridine Ru(II) complex films. The Seebeck coefficient ranges from 307 to 1027 μV/K, with an increasing rate of 95.7 μV/(K nm) as the film thickness increases to 10 nm. Quantum-chemical calculations unveil that the nearly overlapped molecular-orbital energy level of the Ru complex with the Fermi level accounts for the giant thermopower. Landauer–Büttiker probe simulations indicate that the significant length dependence evinces the Seebeck effect dominated by coherent near-resonant tunneling rather than thermal hopping. This study enhances our comprehension of long-range charge transport, paving the way for efficient electronic and thermoelectric materials

Implication of Current–Voltage Curve Shape in Molecular Electronics

The transmission function, T(E), widely used as a toy model in molecular electronics, relies exclusively on a Lorentzian-shaped energy level. The shape of the energy level may be sensitive to the inhomogeneity of the active monolayer in a tunnel junction, yet it is usually ignored or underestimated in explaining the charge tunneling behavior. This article describes the interplay between the supramolecular packing feature of a self-assembled monolayer (SAM) and the shape of an energy level in T(E). Using a T(E) based on the Gaussian–Lorentzian product (GLP), line-fitting analysis was conducted over experimentally obtained current density–voltage curves of n-alkanethiolate SAMs to determine the mixing ratio between Gaussian and Lorentzian functions. It was revealed that the contribution of the Gaussian to the shape of the energy level in T(E) was dominant for solid-like SAMs whereas that of the Lorentzian was dominant for liquid-like SAMs. The energy-level shape also responded to defects induced by the surface roughness of the bottom electrode. We further demonstrated that our approach can be applied to rectifying junctions, using ferrocenyl-terminated n-alkanethiolate SAM. These findings indicate that shape analysis over the current–voltage curve, intimately related to the shape of the energy level of T(E), may provide implications for the packing features of SAMs reminiscent of spectral line fitting in spectroscopy

The Rate of Charge Tunneling Is Insensitive to Polar Terminal Groups in Self-Assembled Monolayers in Ag^TSS(CH₂)_<i>n</i>M(CH₂)_<i>m</i>T//Ga₂O₃/EGaIn Junctions

This paper describes a physical-organic study of the effect of uncharged, polar, functional groups on the rate of charge transport by tunneling across self-assembled monolayer (SAM)-based large-area junctions of the form Ag^TSS­(CH₂)_<i>n</i>M­(CH₂)_<i>m</i>T//Ga₂O₃/EGaIn. Here Ag^TS is a template-stripped silver substrate, -M- and -T are “middle” and “terminal” functional groups, and EGaIn is eutectic gallium–indium alloy. Twelve uncharged polar groups (-T = CN, CO₂CH₃, CF₃, OCH₃, N­(CH₃)₂, CON­(CH₃)₂, SCH₃, SO₂CH₃, Br, P­(O)­(OEt)₂, NHCOCH₃, OSi­(OCH₃)₃), having permanent dipole moments in the range 0.5 < μ < 4.5, were incorporated into the SAM. A comparison of the electrical characteristics of these junctions with those of junctions formed from <i>n</i>-alkanethiolates led to the conclusion that the rates of charge tunneling are insensitive to the replacement of terminal alkyl groups with the terminal polar groups in this set. The current densities measured in this work suggest that the tunneling decay parameter and injection current for SAMs terminated in nonpolar <i>n</i>-alkyl groups, and polar groups selected from common polar organic groups, are statistically indistinguishable

Charging of Multiple Interacting Particles by Contact Electrification

Many processes involve the movement of a disordered collection of small particles (e.g., powders, grain, dust, and granular foods). These particles move chaotically, interact randomly among themselves, and gain electrical charge by contact electrification. Understanding the mechanisms of contact electrification of multiple interacting particles has been challenging, in part due to the complex movement and interactions of the particles. To examine the processes contributing to contact electrification at the level of single particles, a system was constructed in which an array of millimeter-sized polymeric beads of different materials were agitated on a dish. The dish was filled almost completely with beads, such that beads did not exchange positions. At the same time, during agitation, there was sufficient space for collisions with neighboring beads. The charge of the beads was measured individually after agitation. Results of systematic variations in the organization and composition of the interacting beads showed that three mechanisms determined the steady-state charge of the beads: (i) contact electrification (charging of beads of different materials), (ii) contact de-electrification (discharging of beads of the same charge polarity to the atmosphere), and (iii) a long-range influence across beads not in contact with one another (occurring, plausibly, by diffusion of charge from a bead with a higher charge to a bead with a lower charge of the same polarity)

Thermopower in Underpotential Deposition-Based Molecular Junctions

Underpotential deposition (UPD) is an intriguing means for tailoring the interfacial electronic structure of an adsorbate at a substrate. Here we investigate the impact of UPD on thermoelectricity occurring in molecular tunnel junctions based on alkyl self-assembled monolayers (SAMs). We observed noticeable enhancements in the Seebeck coefficient of alkanoic acid and alkanethiol monolayers, by up to 2- and 4-fold, respectively, upon replacement of a conventional Au electrode with an analogous bimetallic electrode, Cu UPD on Au. Quantum transport calculations indicated that the increased Seebeck coefficients are due to the UPD-induced changes in the shape or position of transmission resonances corresponding to gateway orbitals, which depend on the choice of the anchor group. Our work unveils UPD as a potent means for altering the shape of the tunneling energy barrier at the molecule–electrode contact of alkyl SAM-based junctions and hence enhancing thermoelectric performance

Rectification in Tunneling Junctions: 2,2′-Bipyridyl-Terminated <i>n</i>‑Alkanethiolates

Molecular rectification is a particularly attractive phenomenon to examine in studying structure–property relationships in charge transport across molecular junctions, since the tunneling currents across the same molecular junction are measured, with only a change in the sign of the bias, with the same electrodes, molecule(s), and contacts. This type of experiment minimizes the complexities arising from measurements of current densities at one polarity using replicate junctions. This paper describes a new organic molecular rectifier: a junction having the structure Ag^TS/S­(CH₂)₁₁-4-methyl-2,2′-bipyridyl//Ga₂O₃/EGaIn (Ag^TS: template-stripped silver substrate; EGaIn: eutectic gallium–indium alloy) which shows reproducible rectification with a mean <i>r</i>⁺ = |<i>J</i>(+1.0 V)|/|<i>J</i>(−1.0 V)| = 85 ± 2. This system is important because rectification occurs at a polarity opposite to that of the analogous but much more extensively studied systems based on ferrocene. It establishes (again) that rectification is due to the SAM, and not to redox reactions involving the Ga₂O₃ film, and confirms that rectification is not related to the polarity in the junction. Comparisons among SAM-based junctions incorporating the Ga₂O₃/EGaIn top electrode and a variety of heterocyclic terminal groups indicate that the metal-free bipyridyl group, not other features of the junction, is responsible for the rectification. The paper also describes a structural and mechanistic hypothesis that suggests a partial rationalization of values of rectification available in the literature

Defining the Value of Injection Current and Effective Electrical Contact Area for EGaIn-Based Molecular Tunneling Junctions

Analysis of rates of tunneling across self-assembled monolayers (SAMs) of <i>n</i>-alkanethiolates SC_<i>n</i> (with <i>n</i> = number of carbon atoms) incorporated in junctions having structure Ag^TS-SAM//​Ga₂O₃/​EGaIn leads to a value for the injection tunnel current density <i>J</i>₀ (i.e., the current flowing through an ideal junction with <i>n</i> = 0) of 10^3.6±0.3 A·cm^–2 (<i>V</i> = +0.5 V). This estimation of <i>J</i>₀ does not involve an extrapolation in length, because it was possible to measure current densities across SAMs over the range of lengths <i>n</i> = 1–18. This value of <i>J</i>₀ is estimated under the assumption that values of the geometrical contact area equal the values of the effective electrical contact area. Detailed experimental analysis, however, indicates that the roughness of the Ga₂O₃ layer, and that of the Ag^TS-SAM, determine values of the effective electrical contact area that are ∼10^–4 the corresponding values of the geometrical contact area. Conversion of the values of geometrical contact area into the corresponding values of effective electrical contact area results in <i>J</i>₀(+0.5 V) = 10^7.6±0.8 A·cm^–2, which is compatible with values reported for junctions using top-electrodes of evaporated Au, and graphene, and also comparable with values of <i>J</i>₀ estimated from tunneling through single molecules. For these EGaIn-based junctions, the value of the tunneling decay factor β (β = 0.75 ± 0.02 Å^–1; β = 0.92 ± 0.02 nC^–1) falls within the consensus range across different types of junctions (β = 0.73–0.89 Å^–1; β = 0.9–1.1 nC^–1). A comparison of the characteristics of conical Ga₂O₃/​EGaIn tips with the characteristics of other top-electrodes suggests that the EGaIn-based electrodes provide a particularly attractive technology for physical-organic studies of charge transport across SAMs