Towards the fabrication of the top-contact electrode in molecular junctions by photoreduction of a metal precursor.

Langmuir films of 4-{[4-({4-[(trimethylsilyl)ethynyl]phenyl}ethynyl)phenyl]ethynyl} benzenaminium chloride ([1 H]Cl) undergo anion metathesis when assembled on an aqueous auric acid (HAuCl4 ) subphase. Subsequent transfer to solid supports gives well-formed Langmuir-Blodgett (LB) monolayers of [1 H]AuCl4 in which the trimethylsilyl group serves as the surface contacting group. Photoreduction of the aurate on these monolayers leads to the formation of metallic gold nanoislands, which were distributed over the surface of the film. Electrical properties of these nascent devices were determined by recording current-voltage (I-V) curves with conductive atomic force microscopy (c-AFM) using the PeakForce tunneling AFM (PF-TUNA) mode. This gives consistent sigmoidal I-V curves that are indicative of well-behaved junctions free of metallic filaments and short circuits. The photoreduction of a metal precursor incorporated onto monomolecular films is therefore proposed as an effective method for the fabrication of molecular junctions.

Directionally oriented LB films of an OPE derivative: assembly, characterization, and electrical properties.

Langmuir films have been fabricated from 4-[4'-(4''-thioacetyl-phenyleneethynylene)-phenyleneethynylene]-aniline (NOPES) after cleavage of the thioacetyl protecting group. Characterization by surface pressure vs area per molecule isotherms and Brewster angle microscopy reveal the formation of a high quality monolayer at the air-water interface. One layer Langmuir-Blodgett (LB) films were readily fabricated by the transfer of the NOPES Langmuir film onto solid substrates. X-ray photoelectron spectroscopy (XPS), surface polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS), and quartz crystal microbalance (QCM) experiments conclusively demonstrate the formation of one layer LB films in which the functional group associated with binding to the substrate can be tailored by the film transfer conditions. Using LB methods this molecule could be transferred to gold samples with either the amine or thiol group attached to the gold surface. The amine group is directly attached to the gold substrate (Au-NH(2)-OPE-SH) when the substrate is initially immersed in the subphase and withdrawn during the transfer process; in contrast, monomolecular films in which the thiolate group is attached to the gold substrate (Au-S-OPE-NH(2)) are obtained when the substrate is initially out of the subphase and immersed during the transfer process. The morphology of these films was analyzed by atomic force microscopy (AFM), showing the formation of homogeneous layers. Film homogeneity was confirmed by cyclic voltammetry, which revealed a large passivation of gold electrodes covered by NOPES monolayers. Electrical properties for both polar orientated junctions have been investigated by scanning tunnelling microscopy (STM), with both orientations featuring a nonrectifying behavior.

Highly conductive graphene nanoribbons by longitudinal splitting of carbon nanotubes using potassium vapor.

Here we demonstrate that graphene nanoribbons (GNRs) free of oxidized surfaces can be prepared in large batches and 100% yield by splitting multiwalled carbon nanotubes (MWCNTs) with potassium vapor. If desired, exfoliation is attainable in a subsequent step using chlorosulfonic acid. The low-defect density of these GNRs is indicated by their electrical conductivity, comparable to that of graphene derived from mechanically exfoliated graphite. The possible origins of directionally selective splitting of MWCNTs have been explored using computer modeling, and plausible explanations for the unique role of potassium were found.

Metal-molecule-metal junctions in Langmuir-Blodgett films using a new linker: trimethylsilane.

Herein trimethylsilane (TMS) is demonstrated to be an efficient binding group suitable for construction of metal-molecule-metal (M-mol-M') junctions, in which one of the metal contacts is an atomically flat gold surface and the other a scanning tunnelling microscopy (STM) tip. The molecular component of the M-mol-M' devices is an oligomeric phenylene ethynylene (OPE) derivative Me(3)Si C≡C{C(6)H(4)C≡C}(2)C(6)H(4)NH(2), featuring both Me(3)SiC≡C and NH(2) metal contacting groups. This compound can be assembled into Langmuir-Blodgett (LB) films on Au--substrates by surface binding through the amine groups. Alternatively, low coverage (sub-monolayer) films are formed by adsorption from solution. In the case of condensed monolayers top electrical contacts are formed to STM tips through the TMS end group. In low coverage films, single molecular bridges can be formed between the gold surface and a gold STM tip. The similarity in the I-V response of a one-layer LB film and the single molecule conductance experiments reveals several points of critical importance to the design of molecular components for use in the construction of M-mol-M' junctions. Firstly, the presence of neighbouring π systems does not have a significant effect on the conductance of the M-mol-M' junction. Secondly, in the STM configuration, intermolecular electron hopping does not significantly enhance the junction transport characteristics. Thirdly, the symmetric behaviour of the I-V curves obtained, despite the different metal-molecule contacts, indicates that the molecule is simply an amphiphilic electron-donating wire and not a molecular diode with strong rectifying characteristics. Finally, the conductance values obtained from the amine/TMS-contacted OPE described here are of the same order of magnitude as thiol anchored OPEs, making them attractive alternatives to the more conventionally used thiol-contacting chemistry for OPE molecular wires.

Thermophysical study of 1-butyl-2-methylpyridinium tetrafluoroborate ionic liquid.

A thermophysical study of 1-butyl-2-methylpyridinium tetrafluoroborate in a wide range of temperatures has been performed. Thus, density, speed of sound, refractive index, kinematic viscosity, surface tension, and thermal properties have been measured, whereas coefficients of thermal expansion, molar refractions, dynamic viscosities, and entropies and enthalpies of surface formation per unit surface area at the studied temperatures have been calculated. Experimental results have been compared with those obtained for 1-butyl-3-methylpyridinium tetrafluoroborate and 1-butyl-4-methylpyridinium tetrafluoroborate to analyze the effect of the positional isomeric cationic structure on properties. A systematic characterization leads to understand the behavior of such compounds, being a fundamental step to develop their potential as new solvents.

Influence of the Hofmeister series of anions on the molecular organization of positively ionized monolayers of a viologen derivative.

The effects of the Hofmeister series of ions are ubiquitous in chemistry and biology. In this paper specific ion effects on the surface behavior of a viologen dication, namely 1,1(')-dioctadecyl-4,4(')-bipyridilium, are shown. Surface pressure and surface potential vs area isotherms were obtained on aqueous subphases containing potassium salts with several representative counterions in the Hofmeister series (C6H5O3-7, SO2 -4, HPO2-4, Cl-, Br-, NO-3, I-, and ClO-4). The parameters obtained from the compression isotherms (area per molecule, phase transitions, Young modulus, initial surface potential, and variation of the surface potential upon compression) are dependent on the nature of the counterion, indicating ion specificity. Aqueous subphases containing C6H5O3-7, SO2-4, and HPO2-4 anions yield more expanded viologen monolayers and these anions do not effectively penetrate into the monolayer. Brewster angle microscopy was used to map the different phases of the viologen monolayers at the air-water interface. The Langmuir films were also characterized by UV-vis spectroscopy, with quantitative analysis of the reflection spectra supporting an organizational model in which the viologen chromophore undergoes a gradual transition to a more vertical position with respect to the water surface upon compression. A comparison of the tilt angles of the viologen on the different subphases indicates that anions that can more easily penetrate in the monolayer permit the viologen moieties to adopt a slightly more vertical position with respect to the water surface.

Pure and mixed films of a nitrostilbene derivative at the air-water interface, Langmuir-Blodgett multilayer fabrication, and optical characterization.

This paper reports the preparation and characterization of pure Langmuir and Langmuir-Blodgett (LB) films of a stilbene derivative containing two alkyl chains, namely 4-dioctadecylamino-4'-nitrostilbene. Mixed films incorporating docosanoic acid and the stilbene derivative are also studied. Brewster angle microscopy (BAM) analysis has revealed the existence of randomly oriented three-dimensional (3D) aggregates, spontaneously formed immediately after the spreading process of the stilbene derivative onto the water surface. These 3D aggregates coexist with a Langmuir film that shows the typical gas, liquid, and solid-like phases in the surface pressure and surface potential vs area per molecule isotherms, indicative of an average preferential orientation of the stilbene compound at the air-water interface, and a gradual molecular arrangement into a defined structure upon compression. A blue shift of 55 nm of the reflection spectrum of the Langmuir film with respect to the spectrum of a chloroform solution of the nitrostilbene indicates that two-dimensional (2D) H-aggregates are formed at the air-water interface. The monolayers are transferred undisturbed onto solid substrates with atomic force microscopy (AFM) revealing that the one layer LB films are constituted by a monolayer of the stilbene derivative together with some 3D aggregates. When the nitrostilbene compound is blended with docosanoic acid, the 3D aggregation is avoided in the Langmuir and Langmuir-Blodgett films, but does not limit the formation of 2D H-aggregates, desirable for second-order nonlinear optical response in the blue domain. The AFM images of the mixed LB films show that they are formed by a docosanoic acid monolayer and, on the top of it, a bilayer of the stilbene derivative.