Free-standing anisotropic side chain liquid crystalline elastomer films have been prepared using mesogens with laterally affixed polymerizable side chains. We present data on two networks:  one containing the monomer of 4‘-acryloyloxybutyl 2,5-(4‘-butyloxybenzoyloxy)benzoate and another from a 50/50 mol % mixture of the above with 4‘-acryloyloxybutyl 2,5-di(4‘-pentylcyclohexyloyloxy)benzoate. The cross-linking was achieved using 10 mol % of 1,6-hexanediol diacrylate. The calculated cross-linking density, as determined from the Young's modulus, was in the 10 -5 mol/cm3 range. Thermoelastic responses show strain changes through the nematic−isotropic phase transition to be 30−45%. The order parameters of the oriented films were determined from the dichroic ratio of IR absorption at 3343 cm-1 to the in-plane aromatic stretching overtone of the LC mesogen core. The variation of the order parameter with temperature scales similar to the strain changes at constant stress. Isostrain studies, conducted through the nematic to isotropic phase transition, show that the two networks behave as true elastomers with significant differences in the force developed. Dynamic shear measurements near the nematic to isotropic phase transition region show that the mechanical relaxation peak appears above 100 Hz, and that viscoelastic relaxations are minimal in the nematic to isotropic phase transition region below 5−10 Hz.

Charge transport studies across molecular length scales under symmetric and asymmetric metal-molecule contact conditions using a simple crossed-wire tunnel junction technique are presented. It is demonstrated that oligo(phenylene ethynylene), a conjugated organic molecule, acts like a molecular wire under symmetric contact conditions, but exhibits characteristics of a molecular diode when the connections are asymmetric. To understand this behavior, we have calculated current-voltage (I-V) characteristics using extended Huckel theory coupled with a Green's function approach. The experimentally observed I-V characteristics are in excellent qualitative agreement with the theory.

Current-voltage (I-V) characteristics for metal-molecule-metal junctions formed from three classes of molecules measured with a simple crossed-wire molecular electronics test-bed are reported. Junction conductance as a function of molecular structure is consistent with I-V characteristics calculated from extended Hückel theory coupled with a Green's function approach, and can be understood on the basis of bond-length alternation.

No abstract

We report the synthesis and physical studies of a liquid crystalline elastomer fiber consisting of two side-chain liquid crystalline acrylates and a nonmesogennic comonomer side group that acts as a reactive site for cross-linking. The terpolymer was synthesized by radical polymerization, and the cross-linking of the network was achieved by using a diisocyanate unit. The fiber formed shows good liquid crystal alignment texture under a cross-polarizer microscope. Thermoelastic response shows strain changes through the nematic−isotropic phase transition of about 30−35%. A retractive force of nearly 300 kPa was measured in the isotropic phase. Static work loop studies show the viscoelastic losses in these materials to be very small. We also present preliminary studies on the effect of doping carbon nanotubes on the induced strain at the nematic−isotropic transition.

Nanowire metal-molecule-metal junctions containing dithoilated molecules of dodecane (C12), oligo-(phenylene ethynylene) (OPE), and oligo(phenylene vinylene) (OPV) were prepared by replicating the pores of sub-40 nm diameter polycarbonate track etched membranes. Bottom Au-S or Pd-S contacts were made by potential-assisted molecule assembly onto the tips of the first segment of the electrochemically deposited nanowires. Top S-Ag or S-Pd contacts were formed by depositing Ag or Pd nanoparticles, which also served as a thin seed layer for electrodeposition of the second nanowire segment. Room-temperature currentvoltage (I-V) measurements of individual nanowires show that the conductance of junctions formed with π-conjugated oligomers are several orders of magnitude larger than the saturated alkanes, with the OPV junctions having the highest conductance. The molecular wire junction conductance was also found to be dependent on the metal contacts with symmetric Pd-Pd junctions yielding the best metal-molecule coupling and highest conductance.

Self-assembled monolayers (SAMs) of the isocyano derivative of 4,4'-di(phenylene-ethynylene)benzene (1), a member of the "OPE" family of "molecular wires" of current interest in molecular electronics, have been prepared on smooth, {111} textured films of Au and Pd. For assembly in oxygen-free environments with freshly deposited metal surfaces, infrared reflection spectroscopy (IRS) indicates the molecules assume a tilted structure with average tilt angles of 18-24 degrees from the surface normal. The combination of IRS, X-ray photoelectron spectroscopy, and density functional theory calculations all support a single sigma-type bond of the -NC group to the Au surface and a sigma/pi-type of bond to the Pd surface. Both SAMs show significant chemical instability when exposed to typical ambient conditions. In the case of the Au SAM, even a few hours storage in air results in significant oxidation of the -NC moieties to -NCO (isocyanate) with an accompanying decrease in surface chemical bonding, as evidenced by a significant increase in instability toward dissolution in solvent. In the case of the Pd SAM, similar air exposure does not result in incorporation of oxygen or loss of solvent resistance but rather results in a chemically altered interface which is attributed to polymerization of the -NC moieties to quasi-2D poly(imine) structures. Conductance probe atomic force microscope measurements show the conductance of the degraded Pd SAMs can diminish by approximately 2 orders of magnitude, an indication that the SAM-Pd electrical contact has severely degraded. These results underscore the importance of careful control of the assembly procedures for aromatic isocyanide SAMs, particularly for applications in molecular electronics where the molecule-electrode junction is critical to the operational characteristics of the device.

Charge transport across a metal−molecule−metal junction consisting of π-conjugated molecular wires in a densely packed monolayer is studied as a function of junction area. The current−voltage characteristics, measured using a crossed-wire junction at different contact forces, scale onto a single curve. The integer scaling factors, ranging from 1 to 1119, are dictated by the number of molecules contacted in each measurement. These results demonstrate that the electronic conductance of an ensemble of molecules scales directly with the number of molecules in the junction. Our results, which also imply that molecular wires connected in parallel are not strongly coupled along their molecular backbone, should aid in the future development of molecular electronic devices.