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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

New triazolium based ionic liquid crystals

A set of novel 1,2,3-triazolium based ionic liquid crystals was synthesized and their mesomorphic behaviour studied by DSC (differential scanning calorimetry), POM (polarizing optical microscopy) and SAXS (small angle X-ray scattering). Beside the variation of the chain length (C10, C12 and C14) at the 1,2,3-triazolium cation also the anion has been varied (Br-, I-, I3-, BF4-, SbF6-, N(CN)2-, Tf2N-) to study the influence of ion size, symmetry and H-bonding capability on the mesophase formation. Interestingly, for the 1,3-didodecyl-1,2,3-triazolium cation two totally different conformations were found in the crystal structure of the bromide (U-shaped) and the triiodide (rod shaped).

36 MATERIALS SCIENCE↗
DOE OSTI2014

Synthetic strategy for preparing chiral double-semicrystalline polyether block copolymers

Here, we report an effective strategy for the synthesis of semi-crystalline block copolyethers with well-defined architecture and stereochemistry. As an exemplary system, triblock copolymers containing either atactic (racemic) or isotactic ( R or S ) poly(propylene oxide) end blocks with a central poly(ethylene oxide) mid-block were prepared by anionic ring-opening procedures. Stereochemical control was achieved by an initial hydrolytic kinetic resolution of racemic terminal epoxides followed by anionic ring-opening polymerization of the enantiopure monomer feedstock. The resultant triblock copolymers were highly isotactic (meso triads [ mm ]% ~ 90%) with optical microscopy, differential scanning calorimetry, wide angle x-ray scattering and small angle x-ray scattering being used to probe the impact of the isotacticity on the resultant polymer and hydrogel properties.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Triazolium based ionic liquid crystals: Effect of asymmetric substitution

A new series of ten different asymmetrical 1-dodecyl-3-alkyl-triazolium bromides, [C 12 C n Tr][Br], has been synthesized and their mesomorphic behavior studied by DSC (differential scanning calorimetry), POM (polarizing optical microscopy) and SAXS (small angle X-ray scattering). The influence of the chain length of the triazolium salts is investigated to explore the effect of asymmetric substitution on the phase behaviour of these compounds. For that reason, the length of one alkyl chain was varied from 14 to 1 carbon atoms (n = 14, 12, 10, 8–4, 2, 1) while the other alkyl chain was kept at 12 carbon. Single crystal X-ray structure analysis of compounds [C 12 C 12 Tr][Br] and [C 12 C 5 Tr][Br] reveal that the cations adopt a U-shaped conformation with head-to-head arranged triazolium cores. In contrast, for [C 12 C 1 Tr][Br], a rod like shape of the cation with interdigitated alkyl chains is found. All investigated compounds are thermotropic liquid crystals. Higher ordered smectic phases, smectic C as well as smectic A phases were found depending on the chain length of the cation. Moreover, the clearing point temperature decreases with decreasing chain length with exception for the n-dodecyl-3-alkyltrizoliumbromides with the two shortest alkyl chains, [C 12 C 2 Tr][Br] and [C 12 C 1 Tr][Br], which present higher clearing temperatures (86 and 156 °C) and are structurally distinctly different.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Snapshots of a solid-state transformation: coexistence of three phases trapped in one crystal

Crystal-to-crystal transformations have been crucial in the understanding of solid-state processes, since these may be studied in detail by means of single crystal X-ray diffraction (SCXRD) techniques. The description of the mechanisms and potential intermediates of those processes remains very challenging. In fact, solid-state transient states have rarely been observed, at least to a sufficient level of detail. We have investigated the process of guest extrusion from the non-porous molecular material [Fe(bpp)(H 2 L)](ClO 4 ) 2 ·1.5C 3 H 6 O (bpp = 2,6-bis(pyrazol-3-yl)pyridine; H 2 L = 2,6-bis(5-(2-methoxyphenyl)-pyrazol-3-yl)pyridine; C 3 H 6 O = acetone), which occurs through ordered diffusion of acetone in a crystal-to-crystal manner, leading to dramatic structural changes. The slow kinetics of the transition allows thermal trapping of the system at various intermediate stages. The transiting single crystal can be then examined at these points through synchrotron SCXRD, offering a window upon the mechanism of the transformation at the molecular scale. These experiments have unveiled the development of an ordered intermediate phase, distinct from the initial and the final states, coexisting as the process advances with either of these two phases or, at a certain moment with both of them. The new intermediate phase has been structurally characterized in full detail by SCXRD, providing insights into the mechanism of this diffusion triggered solid-state phenomenon. Lastly, the process has been also followed by calorimetry, optical microscopy, local Raman spectroscopy and powder X-ray diffraction. The discovery and description of an intermediate ordered state in a molecular solid-state transformation is of great interest and will help to understand the mechanistic details and reaction pathways underlying these transformations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Visualizing Surface Plasmons with Photons, Photoelectrons, and Electrons

Both photons and electrons may be used to excite surface plasmon polaritons, the collective charge density fluctuations at the surface of metal nanostructures. By virtue of their nanoscopic and dissipative nature, a detailed characterization of surface plasmon (SP) eigenmodes in real space-time ultimately requires joint sub-nanometer spatial and sub-femtosecond temporal resolution. The latter realization has driven significant developments in the past few years, aimed at interrogating both localized and propagating SP modes over the relevant length and time scales. In this mini-review, we briefly highlight different techniques we employ to visualize the enhanced electric fields associated with SPs. Specifically, we discuss recent hyperspectral optical microscopy, tip-enhanced Raman nano-spectroscopy, nonlinear photoemission electron microscopy, as well as correlated scanning transmission electron microscopy-electron energy loss spectroscopy measurements targeting prototypical plasmonic nanostructures and constructs. Through selected practical examples, we examine the information content in multidimensional images recorded by taking advantage of each of the aforementioned techniques. In effect, we illustrate how SPs can be visualized at the ultimate limits of space and time.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2018

Reversible hierarchical structure induced by solvation and temperature modulation in an ionic liquid-based random bottlebrush copolymer

Here a solvent and temperature responsive random copolymer having a polyacrylate backbone with emanating ionic liquid side chains of 1-decyl-3-methylimidazolium chloride and 1-(2-carboxyethyl)-3-imidazolium bromide grafted poly-N-isopropylacrylamide (PNIPAM) possessing bottlebrush architecture is synthesized. The bottlebrush copolymer is prepared using an uncommon route that exploits the co-assembly of two amphiphilic ionic liquid acryloyl-based monomers in 30% (w/w) water to form a lyotropic mesophase followed by photo-initiated free radical polymerization. The copolymer adopts a temperature-invariant 2D hexagonal structure as determined by SAXS in the dehydrated (de-swollen) state. The absence of thermoresponsivity is attributed to a combination of confinement coupled with insufficient solvation of the grafted PNIPAM chains. In a water-swollen state (hydrogel), loss in long range translational ordering is observed while short range temperature-dependent ordering dominates. Below the LCST of PNIPAM (<40 °C), disassociated nanoscale discoidal assemblies predominate, as determined by SAXS. At temperatures above the LCST, the particles condense to form thicker circular discs of similar diameter. Polarized optical microscopy shows the formation of 1-D liquid-crystalline fibrillar structures spanning the meso- to micro-length scales. The assembly and disassembly are fully reversible with temperature cycling. Finally, this work demonstrates the possibility of achieving self-organization over multiple length scales (panoscopic) to create complex structures from modular components using an external stimulus to control construction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Shear-induced polydomain structures of nematic lyotropic chromonic liquid crystal disodium cromoglycate

Lyotropic chromonic liquid crystals (LCLCs) represent aqueous dispersions of organic disk-like molecules that form cylindrical aggregates. Despite the growing interest in these materials, their flow behavior is poorly understood. Here, we explore the effect of shear on dynamic structures of the nematic LCLC, formed by 14 wt% water dispersion of disodium cromoglycate (DSCG). We employ in situ polarizing optical microscopy (POM) and small-angle and wide-angle X-ray scattering (SAXS/WAXS) to obtain independent and complementary information on the director structures over a wide range of shear rates. The DSCG nematic shows a shear-thinning behavior with two shear-thinning regions (Region I at $\dot{\gamma}$ < 1 s -1 and Region III at $\dot{\gamma}$ > 10 s -1 ) separated by a pseudo-Newtonian Region II (1 s-1 < $\dot{\gamma}$ < 10 s -1 ). The material is of a tumbling type. In Region I, $\dot{\gamma}$ < 1 s -1 , the director realigns along the vorticity axis. An increase of [small gamma, Greek, dot above] above 1 s -1 triggers nucleation of disclination loops. The disclinations introduce patches of the director that deviates from the vorticity direction and form a polydomain texture. Extension of the domains along the flow and along the vorticity direction decreases with the increase of the shear rate to 10 s -1 . Above 10 s -1 , the domains begin to elongate along the flow. At $\dot{\gamma}$ > 100 s -1 , the texture evolves into periodic stripes in which the director is predominantly along the flow with left and right tilts. The period of stripes decreases with an increase of $\dot{\gamma}$. The shear-induced transformations are explained by the balance of the elastic and viscous energies. In particular, nucleation of disclinations is associated with an increase of the elastic energy at the walls separating nonsingular domains with different director tilts. The uncovered shear-induced structural effects would be of importance in the further development of LCLC applications.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Non-covalent planarizing interactions yield highly ordered and thermotropic liquid crystalline conjugated polymers

Controlling the multi-level assembly and morphological properties of conjugated polymers through structural manipulation has contributed significantly to the advancement of organic electronics. In this work, a redox active conjugated polymer, TPT–TT, composed of alternating 1,4-(2-thienyl)-2,5-dialkoxyphenylene (TPT) and thienothiophene (TT) units is reported with non-covalent intramolecular S⋯O and S⋯H–C interactions that induce controlled main-chain planarity and solid-state order. As confirmed by density functional theory (DFT) calculations, these intramolecular interactions influence the main chain conformation, promoting backbone planarization, while still allowing dihedral rotations at higher kinetic energies (higher temperature), and give rise to temperature-dependent aggregation properties. Thermotropic liquid crystalline (LC) behavior is confirmed by cross-polarized optical microscopy (CPOM) and closely correlated with multiple thermal transitions observed by differential scanning calorimetry (DSC). This LC behavior allows us to develop and utilize a thermal annealing treatment that results in thin films with notable long-range order, as shown by grazing-incidence X-ray diffraction (GIXD). Specifically, we identified a first LC phase, ranging from 218 °C to 107 °C, as a nematic phase featuring preferential face-on π–π stacking and edge-on lamellar stacking exhibiting a large extent of disorder and broad orientation distribution. A second LC phase is observed from 107 °C to 48 °C, as a smectic A phase featuring sharp, highly ordered out-of-plane lamellar stacking features and sharp tilted backbone stacking peaks, while the structure of a third LC phase with a transition at 48 °C remains unclear, but resembles that of the solid state at ambient temperature. Furthermore, the significance of thermal annealing is evident in the ∼3-fold enhancement of the electrical conductivity of ferric tosylate-doped annealed films reaching 55 S cm −1 . More importantly, thermally annealed TPT–TT films exhibit both a narrow distribution of charge-carrier mobilities (1.4 ± 0.1) × 10 −2 cm 2 V −1 s −1 along with a remarkable device yield of 100% in an organic field-effect transistor (OFET) configuration. This molecular design approach to obtain highly ordered conjugated polymers in the solid state affords a deeper understanding of how intramolecular interactions and repeat-unit symmetry impact liquid crystallinity, solution aggregation, solution to solid-state transformation, solid-state morphology, and ultimately device applications.

Chemistry↗
DOE OSTI2025

Two-dimensional talc as a natural abundant ultra-broadband hyperbolic material

Here, we demonstrate that two-dimensional talc, a naturally abundant phyllosilicate mineral, supports hyperbolic phonon-polaritons (HPhPs) across the mid- and far-infrared wavelengths. Using scattering scanning near-field optical microscopy (s-SNOM) and synchrotron infrared nano-spectroscopy (SINS), we reveal tunable HPhP modes in talc flakes with long lifetimes, high confinement, and quality factors of up to 5. We further observe Fabry–Pérot cavity modes in tapered flakes, confirmed by simulations and analytical modeling. Compared to synthetic crystals, talc offers an ultra-broadband, low-cost, and sustainable platform for infrared nanophotonics and optoelectronics.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Self assembled monolayer based liquid crystal biosensor for free cholesterol detection

A unique cholesterol oxidase (ChOx) liquid crystal (LC) biosensor, based on the disruption of orientation in LCs, is developed for cholesterol detection. A self-assembled monolayer (SAM) of Dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DMOAP) and (3-Aminopropyl)trimethoxy-silane (APTMS) is prepared on a glass plate by adsorption. The enzyme (ChOx) is immobilized on SAM surface for 12 h before utilizing the film for biosensing purpose. LC based biosensing study is conducted on SAM/ChOx/LC (5CB) cells for cholesterol concentrations ranging from 10 mg/dl to 250 mg/dl. The sensing mechanism has been verified through polarizing optical microscopy, scanning electron microscopy, and spectrometric techniques.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Linear and nonlinear optical probe of the ferroelectric-like phase transition in a polar metal, LiOsO 3

LiOsO 3 is one of the first materials identified in the current literature as a “polar metal,” a class of materials that are simultaneously noncentrosymmetric and metallic. In this work, the linear and nonlinear optical susceptibility of LiOsO 3 is studied by means of ellipsometry and optical second harmonic generation (SHG). Robust optical birefringence is observed using spectroscopic ellipsometry. The nonlinear optical susceptibility extracted from SHG polarimetry reveals that the tensor components are of the same magnitude as in the isostructural insulator LiNbO 3 , except the component along the polar axis d 33 is suppressed by an order of magnitude. Temperature-dependent SHG measurements in combination with Raman spectroscopy indicate a continuous order-disorder type polar phase transition at 140 K. Linear and nonlinear optical microscopy measurements reveal 109°/71° ferroelastic domain walls, like in other trigonal ferroelectrics. No 180° polar domain walls are observed to emerge across the phase transition.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

High-pressure isothermal equation of state of composite materials: A case study of LX-17 polymer bonded explosive

Experimental determination of the isothermal high-pressure equation of state (EOS) of composites is not feasible by using conventional diffraction techniques. To solve this problem in the case of polymer bonded explosives (PBXs), composites made of an energetic material and a polymeric binder, we have expanded the applicability of the optical microscopy and interferometry technique previously developed in our group. To accommodate representative samples of a PBX with large grains, we modified the diamond culets of a diamond anvil cell to include etched micrometer-scale pits. This enabled us to measure the isothermal EOS of a PBX, namely, LX-17, up to 8 GPa. The conclusions are compared with the EOSs of the constituent materials and previously published shock measurements. The technique employed in this study is not limited to PBXs and could be potentially used for the EOS determination of other materials ranging from composites to alloys and granular materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Variations of light emission and carrier dynamics around V-defects in InGaN quantum wells

Time- and spectrally-resolved scanning near-field optical microscopy was applied to study spatial variations of photoluminescence (PL) spectra and carrier dynamics in polar InGaN/GaN single quantum wells (QWs) emitting from 410 nm to 570 nm. The main attention was devoted to variations of PL properties and carrier dynamics around V-defects. The PL intensity, peak wavelength, and linewidth, as well as the radiative and nonradiative recombination times, were found to be different in V-defect-rich and defect-free regions. The radiative lifetime close to the defects was longer up to several times, which is attributed to an increased electron and hole wave function separation in the QW plane. PL decay times, measured using excitation and collection through the near-field probe, were one to two orders of magnitude shorter than PL decay times measured in the far field. This shows that the near-field PL decay and the integrated PL intensity are primarily determined by the carrier out-diffusion from under the probe. Only in the immediate vicinity of the V-defects, the near-field PL decays due to the nonradiative recombination at dislocations. The area of such enhanced nonradiative recombination is limited to just a few percent of the total QW area. This shows that recombination via dislocations and V-defects does not play a decisive role in the overall nonradiative recombination and internal quantum efficiency of polar InGaN/GaN QWs.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of sample anisotropy on scanning near-field optical microscope images

Scattering-type scanning near-field optical microscopy (s-SNOM) has been widely used to characterize strongly correlated electronic, two dimensional, and plasmonic materials, and it has enormous potential for biological applications. Many of these materials exhibit anisotropic responses that complicate the extraction of dielectric constants from s-SNOM measurements. Here, we generalize our recently developed approach for retrieving the near-field scattering signal from isotropic systems and apply it to anisotropic dielectrics. Specifically, we compare our theoretical results with experimental measurements on modestly anisotropic sapphire that exhibit strong resonances at the infrared frequency range. Good agreement with the experimental result is found. Finally, our result is important for understanding the near-field response of low damping, anisotropic polaritonic states in dielectric media.

36 MATERIALS SCIENCE↗
DOE OSTI2021

AFM-IR and s-SNOM-IR measurements of chemically addressable monolayers on Au nanoparticles

The performance of catalysts depends on their nanoscale properties, and local variations in structure and composition can have a dramatic impact on the catalytic reactivity. Therefore, probing the localized reactivity of catalytic surfaces using high spatial resolution vibrational spectroscopy, such as infrared (IR) nanospectroscopy and tip-enhanced Raman spectroscopy, is essential for mapping their reactivity pattern. Two fundamentally different scanning probe IR nanospectroscopy techniques, namely, scattering-type scanning near-field optical microscopy (sSNOM) and atomic force microscopy-infrared spectroscopy (AFM-IR), provide the capabilities for mapping the reactivity pattern of catalytic surfaces with a spatial resolution of ~20 nm. Herein, we compare these two techniques with regard to their applicability for probing the vibrational signature of reactive molecules on catalytic nanoparticles. For this purpose, we use chemically addressable self-assembled molecules on Au nanoparticles as model systems. We identified significant spectral differences depending on the measurement technique, which originate from the fundamentally different working principles of the applied methods. While AFM-IR spectra provided information from all the molecules that were positioned underneath the tip, the s-SNOM spectra were more orientation-sensitive. Due to its field-enhancement factor, the s-SNOM spectra showed higher vibrational signals for dipoles that were perpendicularly oriented to the surface. The s-SNOM sensitivity to the molecular orientation influenced the amplitude, position, and signal-to-noise ratio of the collected spectra. Ensemble-based IR measurements verified that differences in the localized IR spectra stem from the enhanced sensitivity of s-SNOM measurements to the adsorption geometry of the probed molecules.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Origins of epitaxial macro-terraces and macro-steps on GaN substrates

Localized lattice distortions in GaN substrates can serve as nucleation sites for epitaxial macro-steps and macro-terraces. These detrimental macro-scale features give rise to optically hazy homoepitaxial GaN surfaces. After nucleating, these macro-features grow laterally along the surface and coalesce, leading to significant coverage of the wafer surface. Dot-core GaN substrates consisting of a periodic array of cores were used as a defect-engineered system, where dislocations are intentionally concentrated at the cores. The high density of threading dislocations at the cores induced localized lattice distortions. Here, these distortions are associated predominantly with lattice tilt on the order of hundreds of arcsec across ~0.5 mm laterally along the wafer surface. The resulting macro-features that nucleated at these localized distorted sites were made up of macro-terraces with lengths ranging ~30–~150 μm and macro-step heights ranging ~200–~400 nm. Another source of localized distortion was threading screw dislocations or GaN nanopipes that resulted in spiral growth and hillock formation. Based on x-ray topography and optical microscopy measurements, we speculate that the coalescence of hillocks evolves into macro-terraces and macro-steps. While previous studies focused on the substrate miscut as a means to control macro-feature formation, we show that localized lattice tilt from defects is another important contributor to macro-feature formation.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Nano-infrared imaging of epitaxial graphene on SiC revealing doping and thickness inhomogeneities

Here, we report on a nano-infrared (IR) imaging and spectroscopy study of epitaxial graphene on silicon carbide (SiC) by using scattering-type scanning near-field optical microscopy (s-SNOM). With nano-IR imaging, we reveal in real space microscopic domains with distinct IR contrasts. By analyzing the nano-IR, atomic force microscopy, and scanning tunneling microscopy imaging data, we conclude that the imaged domains correspond to single-layer graphene, bilayer graphene (BLG), and higher-doped BLG. With nano-IR spectroscopy, we find that graphene can screen the SiC phonon resonance, and the screening is stronger at more conductive sample regions. Our work offers insights into the rich surface properties of epitaxial graphene and demonstrates s-SNOM as an efficient and effective tool in characterizing graphene and possibly other two-dimensional materials.

2D materials↗
DOE OSTI2024

Carrier diffusion in long wavelength InGaN quantum well LEDs after injection through V-defects

The efficiency of operation of GaN-based light emitting diodes (LEDs) to a large degree relies on realization of a uniform hole distribution between multiple quantum wells (QWs) of the active region. Since the direct thermionic transport between the QWs is inefficient, the hole injection through semipolar {$10\bar{1}1$} QWs that form on the facets of V-defects has been suggested as an alternative approach. However, for an efficient LED operation, the carrier distribution should be uniform not only vertically, between the QWs but also laterally, within individual QWs. In this work, the lateral carrier distribution in long wavelength InGaN/GaN QW LEDs is studied by the scanning near-field optical microscopy. The measurements have shown that emission is concentrated around the V-defect injectors. At high currents, the diffusion length of holes in polar QWs was found to be ∼0.6–1 μm and the hole diffusion coefficient ∼0.6 cm 2 /s. The obtained data should aid design of the V-defect injectors for a laterally uniform carrier distribution in the active region QWs.

Current crowding↗