ABSTRACT
The dielectric strength is shown to increase and the relaxation frequency to decrease for a large temperature range up to a certain value of the electric field in the smectic- A phase. This behavior contrasts to that observed in a conventional smectic- A , but can be explained in terms of de Vries scenerio. On assuming the reorientation of the molecular dipoles with electric field to be of the Langevin type in the de Vries smectic- A, we find that around 1,300 molecules , corresponding to a minimum correlation length of xi_{ perpendicular} approximately 45 nm in a single layer cooperatively respond to the applied field.
ABSTRACT
Photoelectron spectroscopy was used to explore changes in Fermi level alignment, within the pi-pi* gap, arising from modifications to the coupling chemistry of conjugated phenylene ethynylene oligomers to the Au surface. Self-assembled monolayers were formed employing either thiol (4,4'-ethynylphenyl-1-benzenethiol or OPE-T) or isocyanide (4,4'-ethynylphenyl-1-benzeneisocyanide or OPE-NC) coupling. The electronic density of states in the valence region of the two systems are nearly identical with the exception of a shift to higher binding energy by about 0.5 eV for OPE-NC. Corresponding shifts appear in C(1s) spectra and in the threshold near E(F). The lack of change in the optical absorption suggests that a rigid shift of the Fermi level within the pi-pi* gap is the major effect of modifying the coupling chemistry. Qualitative consideration of bonding in each case is used to suggest the influence of chemisorption-induced charge transfer as a potential explanation. Connections to other theoretical and experimental work on the effects of varying coupling chemistries are also discussed.
ABSTRACT
Two models exist for the orientational distribution of the long molecular axes in smectic-A liquid crystals: the conventional unimodal distribution and the "cone-shaped" de Vries distribution. The de Vries hypothesis provides a plausible picture of how, at a molecular level, a first-order Sm-A to Sm-C* transition may occur, especially if there is little or no concomitant shrinkage of the layer spacing. This work investigates two materials with such transitions: C7 and TSiKN65. The azimuthal distribution of in-layer directors is probed using IR and polarized Raman spectroscopy, which allows us to obtain orientational order parameters. In C7, we observe a discontinuous change in the order parameter, the magnitude of which is small compared with the corresponding change in the in-layer director tilt angle Theta . Assuming that the smectic-A liquid crystal is of the de Vries type, we calculate the Theta required to reproduce the apparent order parameter
ABSTRACT
The second- and fourth-order apparent orientational order parameters of the core part of the molecule P2 (app) and P4 (app) , have been measured by polarized vibrational Raman spectroscopy for a homogeneously aligned ferroelectric smectic liquid crystal with three dimethyl siloxane groups in the achiral terminal chain, which shows de Vries-type phenomena, i.e., very large electroclinic effect in the smectic- A (Sm-A) phase and a negligible layer contraction at the phase transition between the Sm-A and Sm- C(*) phases. The apparent orientational order parameters of the rigid core part of the molecule are extremely small both with and without the external electric field in Sm-A . These results provide evidence for the existence of the de Vries Sm-A phase, where the local molecular director is tilted at a large angle.
ABSTRACT
For molecular electronics to become a viable technology the factors that control charge transport across a metal-molecule-metal junction need to be elucidated. We use an experimentally simple crossed-wire tunnel junction to interrogate how factors such as metal-molecule coupling, molecular structure, and the choice of metal electrode influence the current-voltage characteristics of a molecular junction.
Subject(s)
Electrochemistry/instrumentation , Electrochemistry/methods , Electron Transport , Electronics/instrumentation , Electronics/methods , Models, Chemical , Models, Molecular , Computer Simulation , Computers, Molecular , Electric Conductivity , Nanotechnology/instrumentation , Nanotechnology/methods , SemiconductorsABSTRACT
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.
ABSTRACT
Unconventional softening of order-parameter fluctuations is observed at the smectic-A to smectic-C* phase transition in a chiral liquid crystal possessing multiple siloxane substituents on its hydrocarbon chains. Together with an optical "stripe" texture detected above the transition, the atypical dynamics can be explained by the pretransitional development within the smectic layers of a modulated state of the order parameter. Conventional soft-mode behavior is restored when the degree of chain substitution is reduced.
ABSTRACT
Optical and x-ray scattering studies of a chiral, organosiloxane smectic-A liquid crystal indicate a large field induced optical tilt of up to 31 degrees accompanied by a very small contraction of the smectic layers. This result suggests that the molecules have a nonzero tilt even with no applied field, and that the primary effect of the field is to induce long range order in the direction of the molecular tilt.
ABSTRACT
An asymptotic expression of the wall shear stress (WSS) in an elastic tube is deduced for small values of the Womersley parameter. In the case of a rigid tube this asymptotic expression is shown to compare better with the exact solution than Poiseuille's or Lambossy's approximations. Its integration in a one-dimensional model of the internal carotid artery blood flow predicts more marked systolic and less marked diastolic WSS than those predicted by the commonly used Poiseuille's approximation.