Self-assembled Molecular Wires Research Articles

Augmenting Photoinduced Charge Transport in a Single-Component Gel System: Controlled In Situ Gel-Crystal Transformation at Room Temperature.

Smart single-component materials with versatile functions require pre-programming of a higher order molecular assembly. An electroactive supergelator (c=0.07 wt %) triphenylamine core-appended poly(aryl ether) dendron (TPAPAE) is described, where substantial dendritic effects improve the order and crystallinity by switching the local minima from self-assembled molecular wires to thermodynamically favorable global minima of ordered crystals, ripened within the fibers. Controlled in situ phase change at room temperature ultimately stabilized the mixed valence states in the single-component supramolecular assembly with photoluminescence and photoinduced charge transport amplified by two orders of magnitude.

Directed Nanoscale Self-Assembly of Molecular Wires Interconnecting Nodal Points Using Monte Carlo Simulations

The influence of directing agents in the self-assembly of molecular wires to produce two-dimensional electronic nanoarchitectures is studied here using a Monte Carlo approach to simulate the effect of arbitrarily locating nodal points on a surface, from which the growth of self-assembled molecular wires can be nucleated. This is compared to experimental results reported for the self-assembly of molecular wires when 1,4-phenylenediisocyanide (PDI) is adsorbed on Au(111). The latter results in the formation of (Au-PDI)n organometallic chains, which were shown to be conductive when linked between gold nanoparticles on an insulating substrate. The present study analyzes, by means of stochastic methods, the influence of variables that affect the growth and design of self-assembled conductive nanoarchitectures, such as the distance between nodes, coverage of the monomeric units that leads to the formation of the desired architectures, and the interaction between the monomeric units. This work proposes an approa...

Conductance of Well-Defined Porphyrin Self-Assembled Molecular Wires up to 14 nm in Length

We show that molecular wires up to 14 nm in length composed by zinc-porphyrins bridged by bipyridines stand efficient electrical transport. Self-assembled molecular wires were prepared step-by-step, alternating up to 13 units of zinc-octaethylporphyrin with axially coordinated 4,4′-bipyridine, on highly oriented pyrolytic graphite (HOPG). A combination of molecular resolution imaging and scanning tunneling spectroscopy allowed us to follow molecules self-assembly in real time during wire fabrication and to measure wires current, respectively. A statistical analysis of hundreds of current–voltage curves was carried out to determine the conductance of individual porphyrin/bipyridine wires. From the conductance dependence on the wires length an ultra low attenuation factor (β = 0.015 ± 0.006 A–1) was obtained for shorter wires, with a transition in conduction regime occurring at ca. 6.5 nm long wires.

Self-Assembled Molecular Wires and Highly Ordered Monolayer: Thiazole on Ge(100)

Adsorption of organic molecules on Ge(100) is important because of its potential applications in organic–semiconductor devices and semiconductor fabrication. On Ge(100), buckled Ge dimers composed of up-Ge and down-Ge atoms align to dimer rows separated by troughs. Adsorption of thiazole on Ge(100) surfaces was investigated using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. Two distinct features were observed when coverages were less than 0.25 monolayer (ML), including feature I on top of down-Ge atoms and feature II in dimer troughs. At 0.25 ML, feature II dominates and forms a highly ordered c(4 × 2) pattern. For coverage >0.25 ML, feature I self-assembles to one-dimensional molecular wires. DFT calculations reveal that feature I can be attributed to thiazole binding vertically to the down-Ge through N–Ge dative bonds, while feature II can be related to cross-dimer [4 + 2] cycloadducts through two C–Ge bonds. π–π interaction between T-shaped and parallel-displaced pairs of feature I facilitate the formation of the molecular wires. Our results indicate that thiazole may have application in both molecular electronics as well as Ge surface processing.

Comparative electrochemical and impedance studies of self-assembled rigid-rod molecular wires and alkanethiols on gold substrates

A study of the charge transfer and self-assembly characteristics of two new rigid-rod molecular wires 1 and 2 assembled on polycrystalline gold electrodes was carried out using electrochemical impedance spectroscopy and cyclic voltammetry. This class of wires have precisely controlled (ca. 1.5-2.5 nm) lengths of π-conjugation, with extended HOMO and LUMO wavefunctions. While rotations can occur around the C-C single bonds, the molecules cannot isomerise or fold due to their rigid backbone structures. The behaviour of these wires was compared with SAMs of heptanethiol (HPT) and dodecanethiol (DDT). It was found that SAMs of 1, which bears flexible hexyloxy sidechains, had randomly distributed pinholes which show microelectrode behaviour even when diluted with DDT. SAMs of 2, which do not have any sidechains, were well-organised at open-circuit potentials enabling evaluation of electron transfer kinetics assuming an average film thickness. However, impedance studies show that deviations from open circuit potentials resulted in an exponential decrease in charge transfer resistance, whereas capacitance remained constant, possibly attributable to conformational changes of the SAM. The syntheses and characterisation of the molecules is described.

Enhanced Electrical Properties of Anisotropic Conductive Adhesive With $\pi$-Conjugated Self-Assembled Molecular Wire Junctions

We have investigated the electrical properties of anisotropic conductive adhesive (ACA) joint using submicrometer-sized (~500 nm in diameter) silver (Ag) particle as conductive filler with the effect of π-conjugated self-assembled molecular wires. The ACAs with submicrometer-sized Ag particles have higher current carrying capability (~3400 mA) than those with micro-sized Au-coated polymer particles (~2000 mA) and Ag nanoparticles (~2500 mA). More importantly, by construction of π-conjugated self-assembled molecular wire junctions between conductive particles and integrated circuit (IC)/substrate, the electrical conductivity has increased by one order of magnitude and the current carrying capability of ACAs has improved by 600 mA. The crucial factors that govern the improved electrical properties are discussed based on the study of alignments and thermal stability of molecules on the submicrometer-sized Ag particle surface with surface-enhanced Raman spectroscopy (SERS), providing a fundamental understanding of conduction mechanism in ACA joints and guidelines for the formulation of high-performance ACAs in electronic packaging industry.

High Performance Nonconductive Film with π-Conjugated Self-Assembled Molecular Wires for Fine Pitch Interconnect Applications

In this paper, we introduce a novel approach of using π-conjugated molecular wires to improve the electrical properties of nonconductive films (NCFs) for the electronic interconnects. Two thiol (-SH) terminated conjugated molecular wires, 1,4-benezenedithiol (BDT) and biphenyldithiol (BPD), are incorporated into an NCF formulation and investigated. The molecular wires can be well assembled on the gold (Au) electrodes and improve the interfacial properties of the NCF joints. The BPD exhibits higher thermal stability after curing at 150°C than BDT due to the higher aromaticity (more aromatic rings) and thus more rigid structure. Formation of a self-assembled monolayer on Au electrodes can tune the effective work function (Φ) of Au electrodes and reduce the tunnel resistivity. Therefore, the joint resistance of NCF joints, which is the sum of tunnel resistance and constriction resistance, can be significantly reduced from 0.15 × 10−3 Ω to 0.1 × 10−3 Ω and 0.05 × 10−3 Ω with BDT and BPD, respectively. The improved electrical conduction and current carrying capability enables the application of the NCF in fine pitch and high performance electronic interconnects in microelectronics.

Electrical transport measurements on self-assembled organic molecular wires

The electrical properties of supermolecular assemblies of oligo(p-phenylene vinylene) were studied. These materials self-assemble into well-defined cylindrical structures in solution with lengths in the range of 100 nm-10 microm and diameters between 5 and 200 nm. Atomic force microscopy showed that by adjusting the concentration, either individual molecular wires or a dense film could be deposited. The molecular wires showed poor electrical conduction. Several tests were performed that show that it was the molecular wires themselves, not the contacts, that limit the conductivity.

Experimental Evidence for Exciton Scaling Effects in Self-Assembled Molecular Wires

Resonant Rayleigh scattering from self-assembled one-dimensional molecular J-aggregate wires reveals a distinct dependence of the exciton energy on the width of lateral extension. For the J aggregates used in this study, strong in-line dipole coupling leads to a delocalization of the exciton wave function over several molecular units. Polarization dependent measurements of resonantly scattered light from the wires show that the exciton dipole moment is oriented perpendicular to the long axis. The experimental observations can be described by applying a quantization condition to the center of mass motion of the J-band exciton in the wires.

Transport in self-assembled molecular wires: Effect of packing and order

We show that orientation, packing, and order play a vital role in determining the current-voltage $(I\ensuremath{-}V)$ characteristics of self-assembled molecular wires, an important issue which can affect the performance of molecular electronic devices. By topographic and tunneling spectroscopic studies on monolayers of a molecule assembled with and without molecular order, we show that competing forces due to the electric field, intermolecular interactions, tip-molecule physisorption, and substrate-molecule chemisorption influence the transport measurements and its reproducibility. We also find that asymmetry in the $I\ensuremath{-}V$ is caused both by the molecular structure and the contact geometry.

Highly Anisotropic Opto‐electronic Properties of Aligned Films of Self‐Assembled Platinum Molecular Wires

Self‐assembled columns of alternating tetrachloro‐ and tetraalkylaminoplatinum moieties form stable, highly oriented, optically anisotropic films on a friction‐deposited polytetrafluoroethylene surface (see Figure). Charge transport in the films is rapid (mobility ≥ ca. 10–2 cm2 V–1 s–1) and highly one‐dimensional along the axis of the columns (anisotropy ≥ 20), further substantiating the potential application of these materials in molecular opto‐electronic devices.

Local behavior of complex materials: scanning probes and nano structure

Understanding the behavior of complex materials such as organic self-assembled monolayers, molecular and nano wires, and transition metal oxide thin films, is facilitated by probes of local properties. Recent extensions of scanning probe microscopies that extract electrical potential, capacitance, dielectric constant, electromechanical coupling coefficients and impedance, are described. In most cases, these complex properties are accessed by stimulations and/or response function detection with multiple frequency modulations. Several illustrative examples include determination of the electronic structure of individual defects in a carbon nanotube, ferroelectric domain interactions in oxide thin films, and electric potential of an alkanethiol on metal.

A One-Pot Procedure to Prepare S-Protected 4-Iodothiophenols

By means of non-aqueous reduction of pipsyl chloride followed by treatment with K2CO3 and then reacting with acetyl chloride or benzyl chloride, S-acetyl/benzyl -4-iodothiophenols were obtained in a one-pot procedure with yield as high as 90%. These S-protected arenethiols are very important intermediates to synthesize self-assembled molecular wires.

Interfacial Electronic Structure in Thiolate Self-Assembled Monolayers: Implication for Molecular Electronics

Thiolate self-assembled monolayers (SAMs) on metal surfaces have been explored recently to address the assembly and connection issue in molecular electronics. In these systems, the molecule−metal contact is detrimental to electron transport. This is manifested not only in contact resistance, but also in the nature of the molecular device, which depends on the extent of wave function mixing between the molecule and the metal surface. We probe interfacial electronic structure, particularly unoccupied electronic states, in thiolate SAMs on Cu(111) using laser two-photon photoemission spectroscopy, in conjunction with ab initio calculations of model molecules. We find that the interfacial electronic structure is dominated by two virtual orbitals localized to the thiolate anchor and strongly coupled to the metal substrate. The shapes and energies of these interfacial σ*-like orbitals are independent of the nature of the hydrocarbon group (conjugated aromatics or saturated alkyls). As low-lying acceptor orbitals, they may play important roles in electron transport through self-assembled molecular wires.

Photo-induced electron transfer to molecular quantum structures on a metal surface

Electron transfer at organic–metal interfaces is important in molecule-based electronic and optoelectronic devices. We probe interfacial electron transfer using two-photon photoemission in two model systems: (i) hexafluorobenzene (C 6F 6) weakly adsorbed on Cu(111); and (ii) SAM of thiophenolate (C 6H 5S) on Cu(111). In the first system, the σ ∗ LUMO in C 6F 6 is found to be located around 3 eV above the Fermi level. With increasing overlayer thickness, the position of this molecular resonance decreases layer by layer, suggesting the formation of molecular quantum wells, each of which extends to the total thickness of the adsorbate layer. In the second system, which is a model interface for self-assembled molecular quantum wires, electron transfer to σ ∗ states is found to dominate the two-photon photoemission spectrum. The two σ ∗ states (LUMO and LUMO+1) are localized to the CSCu anchor and are strongly coupled to the metal substrate. These interfacial σ ∗ states may play important roles in electron transport.