Ron Pindak

Short-Range Smectic Order in Bent-Core Nematic Liquid Crystals

Small angle X-ray diffraction from the uniaxial nematic phase of certain bent-core liquid crystals is shown to be consistent with the presence of molecular clusters possessing short-range tilted smectic (smectic-C) order. Persistence of these clusters throughout the nematic phase, and even into the isotropic state, likely accounts for the unusual macroscopic behavior previously reported in bent-core nematics, including an anomalously large flexoelectric effect (∼ 1000 times that of conventional calamitic nematics), very large orientational and flow viscosities (∼ 10–100 and ∼ 100–1000 times, respectively, typical values for calamitics), and an extraordinary flow birefringence observed in the isotropic state.

Anomalous expansion of the copper-apical-oxygen distance in superconducting cuprate bilayers

We have introduced an improved x-ray phase-retrieval method with unprecedented speed of convergence and precision, and used it to determine with sub-Ångstrom resolution the complete atomic structure of epitaxial La2-xSrxCuO4 ultrathin films. We focus on superconducting heterostructures built from constituent materials that are not superconducting in bulk samples. Single-phase metallic or superconducting films are also studied for comparison. The results show that this phase-retrieval diffraction method enables accurate measurement of structural modifications in near-surface layers, which may be critically important for elucidation of surface-sensitive experiments. Specifically we find that, while the copper-apical-oxygen distance remains approximately constant in single-phase films, it shows a dramatic increase from the metallic-insulating interface of the bilayer towards the surface by as much as 0.45 Å. The apical-oxygen displacement is known to have a profound effect on the superconducting transition temperature.

Thermal crosslinking of organic semiconducting polythiophene improves transverse hole conductivity

Thermal crosslinking using a suitable radical initiator simultaneously improves electrical conductivity in the semiconducting polymer poly(3-hexylthiophene) and makes the material insoluble. Crosslinked polythiophene shows as much as a fivefold increase in hole conductivity across the film thickness without any shift in spectral light absorption. Grazing incidence x-ray diffraction reveals more in-plane polymer lamellae stacking with only a small decrease in film crystallinity. Improved transverse conductivity increases the performance of model planar solar cells by threefold, from 0.07% to 0.2%. The ability to render polythiophene insoluble without disrupting film structural order enables fabrication pathways to more complex device architectures.

Spontaneous and field-induced mesomorphism of a silyl-terminated bent-core liquid crystal as determined from second-harmonic generation and resonant X-ray scattering

The polarity and structure of the phases of a liquid crystal constituted by thiophene-based bent-core molecules is investigated by means of optical second-harmonic generation (SHG), and resonant and conventional X-ray diffraction. The material studied is representative of a wide family of mesogens that contain silyl groups at the ends of the chains. These bulky terminal groups have been reported to give rise to smectic phases showing ferroelectric switching. However, the analysis of the SHG signal before and after application of electric fields has allowed us to establish unambiguously that the reported ferroelectricity is not intrinsic to the material but stabilized by the cell substrates once an electric field has been applied. In addition, the results obtained from resonant X-ray diffraction indicate that virgin samples have antiferroelectric undulated synclinic smectic structures.

Origin of stress and enhanced carrier transport in solution-cast organic semiconductor films

Molecular packing in laterally directed solution deposition is a strong function of variables such as printing speed, substrate temperature, and solution concentration. Knowledge of the ordering mechanisms impacts on the development of new processes and materials for improved electronic devices. Here, we present real-time synchrotron x-ray scattering results combined with optical video microscopy, revealing the stages of ordering during the deposition of organic thin films via hollow capillary writing. Limited long range ordering is observed during the initial crystallization, but it gradually develops over 3–4 s for a range of deposition conditions. Buckling of thin films is typically observed for deposition above room temperature. We infer that compressive stress originates from thermal transients related to solvent evaporation on timescales similar to the development of long range ordering. Under optimized conditions, elimination of cracks and other structural defects significantly improves the average...

Oxygen Displacement in Cuprates under Ionic Liquid Field-Effect Gating.

We studied structural changes in a 5 unit cell thick La1.96Sr0.04CuO4 film, epitaxially grown on a LaSrAlO4 substrate with a single unit cell buffer layer, when ultra-high electric fields were induced in the film by applying a gate voltage between the film (ground) and an ionic liquid in contact with it. Measuring the diffraction intensity along the substrate-defined Bragg rods and analyzing the results using a phase retrieval method we obtained the three-dimensional electron density in the film, buffer layer, and topmost atomic layers of the substrate under different applied gate voltages. The main structural observations were: (i) there were no structural changes when the voltage was negative, holes were injected into the film making it more metallic and screening the electric field; (ii) when the voltage was positive, the film was depleted of holes becoming more insulating, the electric field extended throughout the film, the partial surface monolayer became disordered, and equatorial oxygen atoms were displaced towards the surface; (iii) the changes in surface disorder and the oxygen displacements were both reversed when a negative voltage was applied; and (iv) the c-axis lattice constant of the film did not change in spite of the displacement of equatorial oxygen atoms.

The limits of ultrahigh-resolution x-ray mapping: estimating uncertainties in thin-film and interface structures determined by phase retrieval methods

Capturing subtle details at the sub-Angstrom level is key to understanding the structural basis of many intriguing interfacial phenomena in epitaxial thin films and nanostructures. X-ray phase retrieval methods are ideally suited to this task but the usual approaches for determination of uncertainties, based on refining a parametrized model, are not applicable in this case. Here we describe a method to estimate the uncertainties of the system electron density, obtained by phase retrieval, and of parameters of interest obtained from it. The method is based on the bootstrap approach and it can be generally applied to surface x-ray scattering data. Several examples are given which illustrate the methods utility in determining uncertainties arising from random and systematic errors. The approach also provides a quantitative measure of the validity of structural solutions obtained by phase retrieval methods.

Direct determination of epitaxial film and interface structure: Gd2O3 on GaAs (1 0 0)

Abstract We present a new method of sub-Angstrom resolution imaging of the 3D structure of epitaxial films and their interface with the substrate. The method utilizes the diffraction intensities along the substrate-defined Bragg rods and some crude knowledge of the system structure to determine the complex scattering factors (CSFs) along the Bragg rods. The system electron density and the structure is obtained by Fourier transforming the CSFs into real space. We have applied this method to study the structure of a Gd2O3 film and its interface with the GaAs substrate. The results show that the Gd2O3 abandons the bulk stacking order and adopts that of GaAs. Moreover, the atoms in the first few layers move to in-plane positions that overlap those of the underlying Ga and As. This behavior may be at the heart of its ability to passivate GaAs.

Liquid crystal mesophases beyond commensurate four-layer periodicity

For more than one decade, were the only three confirmed commensurate SmC* variant phases with periodicities less than or equal four layers. In 2006, employing ellipsometry and resonant X-ray diffraction (RXRD), our research team first discovered a new liquid crystal mesophase having a six-layer periodicity in one ternary mixture which includes one sulfur-containing compound. From our ellipsometric results, this phase showed antiferroelectric-like optical response. This novel discovery inspired renewed interest to search for liquid crystal mesophases with commensurate periodicities greater than four layers. Soon after, another mesophase having a six-layer structure and showing a ferrielectric-like dielectric response, instead, was uncovered by RXRD measurements on a different binary mixture which has one bromine-containing compound. Meanwhile mesophases having a 5-, 8-, 12- or 15-layer periodicity were reported. However, numerous questions remain to be addressed associated with these unusual reported phases. Theoretical models giving rise to mesophases with periodicities greater than four layers have been developed; but, to date, none of them have provided satisfactory explanations of all the physical phenomena related to the mesophases exhibiting a six-layer structure. Moreover, the question “what is the source of long-range interactions between liquid-like smectic layers, which are responsible for establishing mesophases with long periodicities and mean-field behavior of the smectic-A–smectic-C transition?” remains unanswered for more than three decades.

Discovery of Liquid Crystal Mesophases with a Six-Layer Periodicity

In 2006, employing ellipsometry and resonant x-ray diffraction, our research group discovered a liquid crystal mesophase having a six-layer periodicity in a ternary mixture (mixture A) as well as in a binary mixture (mixture B). This phase shows antiferroelectric-like properties. Subsequently, J. K. Vijs group used field-induced birefringence to explore the physical properties of various binary mixtures similar to mixture B. Recently, Y. Takanishi et al. obtained dielectric responses and two-dimensional microbeam resonant x-ray diffraction profiles as a function of temperature from a different binary mixture with one compound of the mixture containing a central bromine atom. They discovered another new mesophase which shows a six-layer structure and displays ferrielectric-like responses along with a different phase sequence. This article will review the sequence of events leading up to the discovery of the new phases with a six-layer periodicity and highlight differences in conclusions about the new phases and an ongoing debate about the existence of a phase with five-layer periodicity.