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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Effect of the Number and Substitution Pattern of Carbazole Donors on the Singlet and Triplet State Energies in a Series of Carbazole-Oxadiazole Derivatives Exhibiting Thermally Activated Delayed Fluorescence

Substitution of a varying number of 9H-carbazole donors onto 2-(pentafluorophenyl)-5-phenyl-1,3,4-oxadiazole yields a series of donor–acceptor compounds with similar electrochemical band gaps, but dissimilar optical properties. Each substitution of a carbazole onto the 2- and 4-positions results in a 150–200 meV stabilization of the first singlet excited state, evident in the fluorescent emission. Here, the first triplet excited state is less sensitive to these substitutions, leading to a reduced energy separation between the singlet and triplet states, ΔE ST , with increasing carbazole content. OLED devices made using the donor–acceptor compounds exhibiting the lowest ΔE ST were fabricated and exhibit sky blue emission with an external quantum efficiency of up to 24.4% at a luminance of 10 cd m –2 , and 9.2% at 1000 cd m –2 .

36 MATERIALS SCIENCE↗
DOE OSTI2015

Measurements of population densities of metastable and resonant levels of argon using laser induced fluorescence

We present a new approach to measure population densities of Ar I metastable and resonant excited states in low temperature Ar plasmas at pressures higher than 1 Torr. This approach combines the time resolved laser induced fluorescence technique with the kinetic model of Ar. The kinetic model of Ar is based on calculating the population rates of metastable and resonant levels by including contributions from the processes that affect population densities of Ar I excited states. In particular, we included collisional quenching processes between atoms in the ground state and excited states, since we are investigating plasma at higher pressures. We also determined time resolved population densities of Ar I 2 p excited states by employing optical emission spectroscopy technique. Time resolved Ar I excited state populations are presented for the case of the post-discharge of the supersonic flowing microwave discharge at pressures of 1.7 and 2.3 Torr. The experimental set-up consists of a pulsed tunable dye laser operating in the near infrared region and a cylindrical resonance cavity operating in TE{sub 111} mode at 2.45 GHz. Results show that time resolved population densities of Ar I metastable and resonant states oscillate with twice the frequency of the discharge.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

Solid state photon upconversion utilizing thermally activated delayed fluorescence molecules as triplet sensitizer

The ability to upconvert light is useful for a range of applications, from biological imaging to solar cells. But modern technologies have struggled to upconvert incoherent incident light at low intensities. Here, we report solid state photon upconversion employing triplet-triplet exciton annihilation in an organic semiconductor, sensitized by a thermally activated-delayed fluorescence (TADF) dye. Compared to conventional phosphorescent sensitizers, the TADF dye maximizes the wavelength shift in upconversion due to its small singlet-triplet splitting. The efficiency of energy transfer from the TADF dye is 9.1%, and the conversion yield of sensitizer exciton pairs to singlet excitons in the annihilator is 1.1%. Our results demonstrate upconversion in solid state geometries and with non-heavy metal-based sensitizer materials.

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

Laser-induced fluorescence measurements of argon and xenon ion velocities near the sheath boundary in 3 ion species plasmas

The Bohm sheath criterion is studied with laser-induced fluorescence in three ion species plasmas using two tunable diode lasers. Krypton is added to a low pressure unmagnetized DC hot filament discharge in a mixture of argon and xenon gas confined by surface multi-dipole magnetic fields. The argon and xenon ion velocity distribution functions are measured at the sheath-presheath boundary near a negatively biased boundary plate. The potential structures of the plasma sheath and presheath are measured by an emissive probe. Results are compared with previous experiments with Ar–Xe plasmas, where the two ion species were observed to reach the sheath edge at nearly the same speed. This speed was the ion sound speed of the system, which is consistent with the generalized Bohm criterion. In such two ion species plasmas, instability enhanced collisional friction was demonstrated [Hershkowitz et al., Phys. Plasmas 18(5), 057102 (2011).] to exist which accounted for the observed results. When three ion species are present, it is demonstrated under most circumstances the ions do not fall out of the plasma at their individual Bohm velocities. It is also shown that under most circumstances the ions do not fall out of the plasma at the system sound speed. These observations are also consistent with the presence of the instabilities.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Development of core ion temperature gradients and edge sheared flows in a helicon plasma device investigated by laser induced fluorescence measurements

We report experimental observation of ion heating and subsequent development of a prominent ion temperature gradient in the core of a linear magnetized plasma device, and the controlled shear de-correlation experiment. Simultaneously, we also observe the development of strong sheared flows at the edge of the device. Both the ion temperature and the azimuthal velocity profiles are quite flat at low magnetic fields. As the magnetic field is increased, the core ion temperature increases, producing centrally peaked ion temperature profiles and therefore strong radial gradients in the ion temperature. Similarly, we observe the development of large azimuthal flows at the edge, with increasing magnetic field, leading to strong radially sheared plasma flows. The ion velocities and temperatures are derived from laser induced fluorescence measurements of Doppler resolved velocity distribution functions of argon ions. These features are consistent with the previous observations of simultaneously existing radially separated multiple plasma instabilities that exhibit complex plasma dynamics in a very simple plasma system. The ion temperature gradients in the core and the radially sheared azimuthal velocities at the edge point to mechanisms that can drive the multiple plasma instabilities, that were reported earlier.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Small Changes With Big Consequences: Swapping Two Atoms In Side Chains Changes Phenylene‐Ethynylene Packing And Fluorescence

Abstract Engineering the properties of conjugated materials in the solid state is an unsolved, ongoing challenge important to fundamental understanding of how non‐covalent interactions dictate packing and key properties, as well as the development of technologies based in organic optoelectronics. The most common design paradigm of such materials divide them into a “main chain” with extended conjugation, the chemical structure of which determines optoelectronic properties, and “side chains” not conjugated to the backbone, which provide solubility when they are long alkyl chains. This paper describes comparisons between phenylene‐ethynylene molecules in which slight changes to the structure of “side chains”—swapping hydrogen and fluorine atomic position on an aromatic ring—results in unexpectedly large changes in the solid‐state optical properties. In a pair of anisyl‐terminated three‐ring phenylene‐ethynylenes, switching the side chain arenes of benzyl esters from 2,4,6‐trifluoro to 2,3,6‐trifluoro results in a shift in fluorescence emission spectra of over 100 nm, as well as the opposite direction of force‐induced shifting of emission. Through a combination X‐ray crystal structures, electronic structure calculations, and comparisons with other derivatives, we describe how the 2,4,6‐trifluorinated side chains yield cofacial fluoroarene‐arene stacking interactions that twist the PE backbone out of conjugation, while the 2,3,6‐trifluoro side chains do not stack, instead yielding more coplanar PE backbones that form intermolecular aggregates. Overall, this work demonstrates how slight modifications to parts of conjugated materials normally considered ancillary to optoelectronic properties can determine their solid‐state properties, epitomizing the challenge of rational design but at the same time offering opportunities for materials discovery and improved understanding of non‐covalent interactions.

Sharber, Seth A.↗
DOE OSTI2019

Fluorescence In Situ mRNA Hybridization for Gene Expression Detection in a Wood Decay Fungus

Wood-degrading fungi efficiently decompose wood using a series of steps that are regulated across time and space. RNA sequencing with “bulk” sampling method has indicated processes controlled at the transcription level at different decay stages and locations along hyphal networks. To further understand these regulatory mechanisms, a high-resolution technique for gene expression determination is required. In this work, Fluorescence In Situ Hybridization (FISH) of mRNA was tested and optimized in the ‘brown rot’-type (carbohydrate-selective; lignin-rich residues) model fungus Postia (Rhodonia) placenta to image gene expression at the hyphal scale, <10 micrometers. The mRNA of one housekeeping gene Tef1a and three wood-decaying genes Cel28a, Cel5b and Qrd1 were tested in hyphae grown in liquid nutrients and wood substrate. The 20 bp-long DNA probes were successfully delivered intracellularly to give sufficient hybridization signal for imaging. Ahead of hybridization, permeabilization by lyticase and proteinase K treatments proved to be the critical step to increase probe and mRNA accessibility. Tyramide signal amplification was used after hybridization to enhance signal. This gave qualitative measures of mRNA of the four target genes, which were generally in line with previous expression patterns generated from coarser-scale RNA-seq and qRT-PCR studies. These techniques were not without challenges, and future optimization using multiple probe fragments for each mRNA molecule is proposed.

Brown rot fungi, decomposition, FISH, gene express↗
DOE OSTI2023

Elucidating the Role of Hydrogen Bonding in the Optical Spectroscopy of the Solvated Green Fluorescent Protein Chromophore: Using Machine Learning to Establish the Importance of High-Level Electronic Structure

Hydrogen bonding interactions with chromophores in chemical and biological environments play a key role in determining their electronic absorption and relaxation processes, which are manifested in their linear and multidimensional optical spectra. For chromophores in the condensed phase, the large number of atoms needed to simulate the environment has traditionally prohibited the use of high-level excited-state electronic structure methods. By leveraging transfer learning, we show how to construct machine-learned models to accurately predict the high-level excitation energies of a chromophore in solution from only 400 high-level calculations. Here, we show that when the electronic excitations of the green fluorescent protein chromophore in water are treated using EOM-CCSD embedded in a DFT description of the solvent the optical spectrum is correctly captured and that this improvement arises from correctly treating the coupling of the electronic transition to electric fields, which leads to a larger response upon hydrogen bonding between the chromophore and water.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Fluorescence and absorption spectroscopy for warm dense matter studies and ICF plasma diagnostics

The burning core of an inertial confinement fusion (ICF) plasma produces bright x-rays at stagnation that can directly diagnose core conditions essential for comparison to simulations and understanding fusion yields. These x-rays also backlight the surrounding shell of warm, dense matter, whose properties are critical to understanding the efficacy of the inertial confinement and global morphology. In this work, we show that the absorption and fluorescence spectra of mid-Z impurities or dopants in the warm dense shell can reveal the optical depth, temperature, and density of the shell and help constrain models of warm, dense matter. This is illustrated by the example of a high-resolution spectrum collected from an ICF plasma with a beryllium shell containing native iron impurities. Lastly, analysis of the iron K-edge provides model-independent diagnostics of the shell density (2.3 × 10 24 e/cm 3 ) and temperature (10 eV), while a 12-eV red shift in Kβ and 5-eV blue shift in the K-edge discriminate among models of warm dense matter: Both shifts are well described by a self-consistent field model based on density functional theory but are not fully consistent with isolated-atom models using ad-hoc density effects.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Emanation and bulk fluorescence in liquid argon from tetraphenyl butadiene wavelength shifting coatings

Here, we study the stability of three types of popularly employed TPB coatings under immersion in liquid argon. TPB emanation from each coating is quantified by fluorescence assay of molecular sieve filter material after a prolonged soak time. Two of the coatings are shown to emanate a detectable concentration of TPB into argon over a 24 hour period, which corresponds to tens of parts per billion in argon by mass. In an independent setup, the dissolved or suspended TPB is shown to produce a wavelength shifting effect in the argon bulk. Interpretations of these results and implications for present and future liquid argon time projection chamber experiments are discussed.

Scintillators, scintillation and light emission pr↗
DOE OSTI2013

Cyanine-based probe\tag-peptide pair fluorescence protein imaging and fluorescence protein imaging methods

A molecular probe comprises two arsenic atoms and at least one cyanine based moiety. A method of producing a molecular probe includes providing a molecule having a first formula, treating the molecule with HgOAc, and subsequently transmetallizing with AsCl.sub.3. The As is liganded to ethanedithiol to produce a probe having a second formula. A method of labeling a peptide includes providing a peptide comprising a tag sequence and contacting the peptide with a biarsenical molecular probe. A complex is formed comprising the tag sequence and the molecular probe. A method of studying a peptide includes providing a mixture containing a peptide comprising a peptide tag sequence, adding a biarsenical probe to the mixture, and monitoring the fluorescence of the mixture.

60 APPLIED LIFE SCIENCES↗