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At least 613 records · Page 34

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DOE OSTI2017

Nanosecond pulsed humid Ar plasma jet in air: shielding, discharge characteristics and atomic hydrogen production

Gas phase non-equilibrium plasmas jets containing water vapor are of growing interest for many applications. In this work, we report a detailed study of an atmospheric pressure nanosecond pulsed Ar + 0.26% H 2 O plasma jet. The plasma jet operates in an atmospheric pressure air surrounding but is shielded with a coaxial argon flow to limit the air diffusion into the jet effluent core. The jet impinges on a metal plate electrode and produces a stable plasma filament (transient spark) between the needle electrode in the jet and the metal plate. The stable plasma filament is characterized by spatially and time resolved electrical and optical diagnostics. This includes Rayleigh scattering, Stark broadening of the hydrogen Balmer lines and two-photon absorption laser induced fluorescence (TaLIF) to obtain the gas temperature, the electron density and the atomic hydrogen density respectively. Electron densities and atomic hydrogen densities up to $5 \times 10^{22}$ ${\rm m}^{-3}$ and $2 \times 10^{22}$ ${\rm m}^{-3}$ have been measured. This shows that atomic hydrogen is one of the main species in high density Ar-H 2 O plasmas. The gas temperature does not exceed 550 K in the core of the plasma. To enable in situ calibration of the H TaLIF at atmospheric pressure a previously published O density calibration scheme is extended to include a correction for the line profiles by including overlap integrals as required by H TaLIF. The line width of H TaLIF, due to collision broadening has the same trend as the neutral density obtained by Rayleigh scattering. This suggests the possibility to use this technique to in situ probe neutral gas densities.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Standoff Detection of Oil and Powder Mixtures at 12 Meters Using a Tunable Quantum Cascade Laser-Based System with a Close Focus Telescope and Uncooled Infrared Detector

We have designed and demonstrated a quantum cascade laser (QCL) based standoff system that utilizes an uncooled mercury cadmium telluride (MCT) detector with lock-in signal processing for chemical identification at a distance of 12.5 meters in indoor ambient light conditions. In the system, a tunable quad-QCL operating (1 MHz) in quasi-continuous wave mode between 8.45 and 10.03 μm (~1182 to 1000 cm –1 ) serves as the active mid-infrared source for remotely interrogating mineral, powder, and thin film oil samples including powder mixtures (6, 12.5, 25, and 50%) of crystalline quartz (SiO 2 ) in KBr. Light as reflected from a given sample is collected using a 10-inch (25.4 cm) Dall Kirkham telescope and coupled with ZnSe optics to an uncooled MCT detector. Furthermore, the mixture dependence of the highly transparent KBr and strongly absorbing quartz was found to fit a modified version of the Schatz reflectance model for compacted powder mixtures. All reflectance spectra reported are relative to an Au-coated diffuse reflector. A NIST traceable polystyrene standard reflector was also used to determine the QCL wavelength tuning range and calibration.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Raman Spectroscopy to Detect and Measure NOW Pheromones. Annual Report [2021]

Insect sex pheromones are chemical compounds that insects release to attract mates over distances of hundreds of meters or even kilometers, in complete darkness and without any audible signals. Use of synthetic forms of key compounds have in some cases become an essential component of monitoring and/or managing key pests of agricultural crops, including navel orangeworm (Amyelois transitella) (NOW) in California tree nuts There are currently multiple commercially available mating disruption products available for NOW and recent studies have demonstrated that they can be effectively used to reduce crop damage. How mating disruption works is not fully established and likely varies across products and target species. For instance, it is not well understood how synthetic pheromones compete with natural pheromones and, in the case of monitoring, how efficaciously the insect follows the diffusing plumes and how those evolve from emission points, especially across large blocks and at plot borders. We proposed to evaluate the use of enhanced Raman spectroscopy for the detection of synthetic and natural NOW pheromones and, were that to be successful, use it in subsequent years to measure pheromone diffusion in orchards.

47 OTHER INSTRUMENTATION↗

DOE OSTI2021

Raman Spectroscopy to Detect and Measure NOW Pheromones. Annual Report [Dec 2021]

47 OTHER INSTRUMENTATION↗

DOE OSTI2017

Phosphides or nitrides for better NLO properties? A detailed comparative study of alkali metal doped nano-cages

Highlights: • Alkali metal doped boron and aluminum phosphide nano-cages are evaluated for NLO properties. • The NLO response of phosphide nano-cages is compared with nitride nano-cages. • Phosphide nano-cages show better NLO properties than the corresponding nitride nano-cages. • Conductivity of doped nano-cages is much higher than pristine nano-cages. • MAl{sub 11}P{sub 12} and MB{sub 11}P{sub 12} show monotonic increase in hyperpolarizability with size of alkali metal. - Abstract: Phosphide nano-cages of MX{sub 12}P{sub 11} and MX{sub 11}P{sub 12} (where X = B or Al, M = Li, Na, K) are investigated theoretically for geometric, electronic and nonlinear optical (NLO) properties. Alkali metal is doped by replacing (a) aluminum or boron atom and (b) phosphorus atom. Alkali metal doping can remarkably alter the electronic and NLO properties of the nano-cages, regardless of doping site. HOMO-LUMO gap is decreased whereas first hyperpolarizability is enhanced significantly in doped AlP and BP nano-cages. PDOS (partial density of states) analysis suggests that diffuse excess electrons from alkali metal play important role in boosting NLO properties of phosphide nano-cages. TD-DFT calculations are performed to study the effect of alkali metal doping on absorption spectra. Red shift is observed in the UV–vis spectra of all considered nano-cages. Comparison of phosphide nano-cages with nitride nano-cages indicated that doping nitride nano-cages with alkali metal leads to greater hyperpolarizability and NLO response than doping phosphide nano-cages.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Balanced Partnership between Donor and Acceptor Components in Nonfullerene Organic Solar Cells with >12% Efficiency

Abstract Relative to electron donors for bulk heterojunction organic solar cells (OSCs), electron acceptors that absorb strongly in the visible and even near‐infrared region are less well developed, which hinders the further development of OSCs. Fullerenes as traditional electron acceptors have relatively weak visible absorption and limited electronic tunability, which constrains the optical and electronic properties required of the donor. Here, high‐performance fullerene‐free OSCs based on a combination of a medium‐bandgap polymer donor (FTAZ) and a narrow‐bandgap nonfullerene acceptor (IDIC), which exhibit complementary absorption, matched energy levels, and blend with pure phases on the exciton diffusion length scale, are reported. The single‐junction OSCs based on the FTAZ:IDIC blend exhibit power conversion efficiencies up to 12.5% with a certified value of 12.14%. Transient absorption spectroscopy reveals that exciting either the donor or the acceptor component efficiently generates mobile charges, which do not suffer from recombination to triplet states. Balancing photocurrent generation between the donor and nonfullerene acceptor removes undesirable constraints on the donor imposed by fullerene derivatives, opening a new avenue toward even higher efficiency for OSCs.

Lin, Yuze↗

DOE OSTI2022

Design considerations for gallium arsenide pulse compression photoconductive switch

In this paper, we present the physics and design-space exploration of a novel pulse compression photoconductive switch (PCPS) using semi-insulating gallium arsenide (GaAs) operating in the negative differential mobility (NDM) regime of electron transport. We systematically quantify the relationship between the PCPS performance and various design options, including contact separation, laser energy and placement, and trap dynamics. Specifically, we report the full-width at half-maximum and the peak output current generated by the PCPS as a function of applied electrical and optical bias. We discuss the optimal spacing between the electrodes and the distance of the laser spot to the anode to achieve higher electron confinement and superior radio-frequency (RF) metrics. Reducing the laser energy is important to prevent the appearance of secondary peaks due to diffusive transport, but there exists a trade-off between the bandwidth and the maximum current of the PCPS. We also compare the PCPS response with and without trap dynamics and find that the electrostatic screening from the trap-induced space charge is time-independent when the trapping time constant is set larger than the recombination lifetime. Overall, trap dynamics are detrimental to performance, unless the compensation doping scheme to achieve semi-insulating GaAs is carefully selected. Results presented in this paper can be used by experimentalists to fine-tune the PCPS design parameters to meet the specifications of various RF applications. Moreover, our results will provide a strong theoretical basis to the measurements of PCPS devices using GaAs and other NDM materials under investigation.

42 ENGINEERING↗

DOE OSTI2023

Photophysics of Intrinsic Single‐Photon Emitters in Silicon Nitride at Low Temperatures

A robust process for fabricating intrinsic single-photon emitters in silicon nitride is recently established. These emitters show promise for quantum applications due to room-temperature operation and monolithic integration with technologically mature silicon nitride photonics platforms. Here, the fundamental photophysical properties of these emitters are probed through measurements of optical transition wavelengths, linewidths, and photon antibunching as a function of temperature from 4.2 to 300 K. Important insight into the potential for lifetime-limited linewidths is provided through measurements of inhomogeneous and temperature-dependent broadening of the zero-phonon lines. At 4.2 K, spectral diffusion is found to be the main broadening mechanism, while spectroscopy time series reveal zero-phonon lines with instrument-limited linewidths.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2019

Highly Size-Selective Water-Insoluble Cross-Linked Carboxymethyl Cellulose Membranes

Carboxymethyl cellulose (CMC) membranes have strong potential for application as molecular-scale separators. For this study, stable CMC membranes were fabricated with aluminum chloride (AlCl{sub 3}) and iron(III) chloride (FeCl{sub 3}) serving as cross-linkers. The resulting CMC-Al and CMC-Fe membranes were optically transparent and water-insoluble with sufficient mechanical strength for practical applications. The water permeation flux through the membranes was directly proportional to the operating pressure. With just a 10-fold increase in the molecular weight from 60 Da (urea) to 604 Da (bordeaux S), the effective diffusion coefficient (D{sub eff}) of the CMC-Al membrane increased 56-fold, and that of the CMC-Fe membrane increased 3500-fold. This significant correlation between D{sub eff} on molecular size indicated that the sizes of the mass transfer channels through the membrane were strictly monodisperse, in the range of molecular sizes that were tested.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Porous Microcapsule-Based Regenerating Superhydrophobic Coating on 304L SS and Its Corrosion Properties

We report fabrication of a regenerating superhydrophobic coating on 304L stainless steels, with superior corrosion resistance, using porous microcapsules made of calcium hydroxide as a core and a polymeric shell consisting of ethyl cellulose and poly methyl methacrylate. The synthesized microcapsules are characterized using optical, atomic force and scanning electron microscopic techniques, x-ray diffraction, Fourier transform infrared spectroscopy and immersion tests. The diameter of porous and permeable microcapsules ranges from 2 to 20 μm. On immersion of the coated surface in sodium stearate solution, the calcium hydroxide from the core diffuses through the pores of microcapsules and forms nanocrystalline hydrophobic calcium stearate needles pointing outward that repel water molecules. The maximum water contact angle obtained was 155.7° ± 2.05°. The micro-nanoscale roughness of the surface was evident from the AFM measurement. The cross-hatch tape adhesion test confirmed the strong adherence of the superhydrophobic coating. On damage of the superhydrophobic coating, the regeneration is achieved by immersing it in sodium stearate solution for 6 h where a 10° increase in contact angle is observed. Electrochemical studies in 0.1 M NaCl showed significant decrease in the passive current density and a delayed pitting, confirming better corrosion resistance of the coating in chloride environment.

36 MATERIALS SCIENCE↗

DOE OSTI2017

In situ investigation on ultrafast oxygen evolution reactions of water splitting in proton exchange membrane electrolyzer cells

We present that the oxygen evolution reaction (OER) is a half reaction in electrochemical devices, including low-temperature water electrolysis, which is considered as one of the most promising methods to generate hydrogen/oxygen for the storage of energy. It is affected by many factors, and its mechanism is still not completely understood. A proton exchange membrane electrolyzer cell (PEMEC) with optical access to the surface of anode catalyst layer (CL) coupled with a distinguished high-speed and micro-scale visualization system (HMVS) was developed to in situ investigate OERs. It was revealed in real time that OERs only occur on the anode CL adjacent to liquid/gas diffusion layer (LGDL). The CL electrical conductivity plays a crucial role in OERs on CLs. The large in-plane electrical resistance of CLs becomes a threshold of OERs over the entire CL, and causes a lot of catalyst waste in the middle of LGDL pores. Moreover, the oxygen bubble nucleation, growth, and detachment and the effect of current density on those processes were also characterized. Here, this study proposes a new approach for better understanding the mechanisms of OERs and optimizing the design and fabrication of membrane electrode assemblies.

25 ENERGY STORAGE↗

DOE OSTI2020

The early-stage growth and reversibility of Li electrodeposition in Br-rich electrolytes

Significance Spatial variations in chemical composition and transport properties of the material phases (interphases) formed on reactive metals in liquid electrolytes are thought to be responsible for the propensity of metal battery electrodes to electrodeposit in irregular, nonplanar morphologies. Equilibrium theoretical calculations using joint density functional analysis in vacuum and generic liquid media indicate that in-plane transport at such interphases is enhanced substantially if LiX (X = Br > Cl > F) species predominate. This study employs optical visualization experiments and nucleation theory to experimentally investigate nucleation and early-stage growth dynamics of metallic lithium in electrolytes enriched with LiBr. It is shown that the Li-Br–rich interphases formed profoundly alter the morphology of Li electrodeposits by enhancing Li-ion surface diffusion.

25 ENERGY STORAGE↗

DOE OSTI2020

Electrical control of spin relaxation anisotropy during drift transport in a two-dimensional electron gas

Spin relaxation was studied in a two-dimensional electron gas confined in a wide GaAs quantum well. Recently, the control of the spin relaxation anisotropy by diffusive motion was first shown in D. Iizasa et al., arXiv:2006.08253. Here, we demonstrate electrical control by drift transport in a system with two subbands occupied. The combined effect of in-plane and gate voltages was investigated using time-resolved Kerr rotation. Furthermore, the measured relaxation time presents strong anisotropy with respect to the transport direction. For an in-plane accelerating electric field along [110], the lifetime was strongly suppressed irrespective of the applied gate voltage. Remarkably, for transport along [$1\bara{1}0$], the data shows spin lifetime that was gate dependent and longer than in the [110] direction regardless of the in-plane voltage. In agreement, independent results of anisotropic spin precession frequencies are also presented. Nevertheless, the long spin lifetime, strong anisotropy and drift response seen in the data are beyond the existing models for spin drift and diffusion.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Pulsed Electrodeposition of Tin Electrocatalysts onto Gas Diffusion Layers for Carbon Dioxide Reduction to Formate

In this paper, we discuss a pulse electroplating method for developing tin (Sn)-decorated gas diffusion electrodes (GDEs) for the electrochemical conversion of carbon dioxide (CO 2 ) to formate. The pulse-plated Sn electrodes achieved current densities up to 388 mA/cm 2 , more than two-fold greater than conventionally prepared electrodes (150 mA/cm 2 ), both at a formate selectivity of 80%. Optical and microscopic analyses indicate improvements in deposition parameters could further enhance performance by reducing the catalyst particle size.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2017

Discovery of a Kiloparsec Extended Hard X-Ray Continuum and Fe–Kα from the Compton Thick AGN ESO 428-G014

We report the discovery of kiloparsec-scale diffuse emission in both the hard continuum (3–6 keV) and in the Fe–Kα line in the Compton thick (CT) Seyfert galaxy ESO 428-G014. This extended hard component contains at least ∼24% of the observed 3–8 keV emission, and follows the direction of the extended optical line emission (ionization cone) and radio jet. The extended hard component has ∼0.5% of the intrinsic 2–10 keV luminosity within the bi-cones. A uniform scattering medium of density 1 cm{sup −3} would produce this luminosity in a 1 kpc path length in the bi-cones. Alternatively, higher column density molecular clouds in the disk of ESO 428-G014 may be responsible for these components. The continuum may also be enhanced by the acceleration of charged particles in the radio jet. The steeper spectrum (Γ ∼ 1.7 ± 0.4) of the hard continuum outside of the central 1.″5 radius nuclear region suggests a contribution of scattered/fluorescent intrinsic Seyfert emission. Ultrafast nuclear outflows cannot explain the extended Fe–Kα emission. This discovery suggests that we may need to revise the picture at the base of our interpretation of CT AGN spectra.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2025

Interactions Enhance Ramp Reversal Memory in Locally Phase Separated Materials

The ramp-reversal memory (RRM) effect in metal–insulator transition metal oxides (TMOs), a non-volatile resistance change induced by repeated temperature cycling, has attracted considerable interest in neuromorphic computing and non-volatile memory devices. Our previous defect motion model successfully explained RRM in vanadium dioxide (VO 2 ), capturing observed critical temperature shifts and memory accumulation throughout the sample. However, this approach lacked interactions between metallic and insulating domains. Here, we extend our model by combining a correlated Random Field Ising Model with defect diffusion-segregation, enabling accurate hysteresis modeling while predicting the relationship between RRM and domain interactions. Our simulations demonstrate that the maximum RRM occurs when the turnaround temperature approaches the inflection point. This peak in RRM vs. turnaround temperature is consistent with prior transport measurements, as well as our own optical measurements reported here. Significantly, we find that increasing nearest-neighbor interactions enhances the maximum memory effect, thus providing a clear mechanism for optimizing RRM performance. Since our model employs minimal assumptions, we predict that RRM should be a widespread phenomenon in materials exhibiting patterned phase coexistence of electronic domains. This work not only advances fundamental understanding of memory behavior in TMOs but also establishes a much-needed theoretical framework for optimizing device applications.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Nanoscale imaging of phonon dynamics by electron microscopy

Spatially resolved vibrational mapping of nanostructures is indispensable to the development and understanding of thermal nanodevices, modulation of thermal transport and novel nanostructured thermoelectric materials. Through the engineering of complex structures, such as alloys, nanostructures and superlattice interfaces, one can significantly alter the propagation of phonons and suppress material thermal conductivity while maintaining electrical conductivity. There have been no correlative experiments that spatially track the modulation of phonon properties in and around nanostructures due to spatial resolution limitations of conventional optical phonon detection techniques. Here we demonstrate two-dimensional spatial mapping of phonons in a single silicon–germanium (SiGe) quantum dot (QD) using monochromated electron energy loss spectroscopy in the transmission electron microscope. Tracking the variation of the Si optical mode in and around the QD, we observe the nanoscale modification of the composition-induced red shift. We observe non-equilibrium phonons that only exist near the interface and, furthermore, develop a novel technique to differentially map phonon momenta, providing direct evidence that the interplay between diffuse and specular reflection largely depends on the detailed atomistic structure: a major advancement in the field. Our work unveils the non-equilibrium phonon dynamics at nanoscale interfaces and can be used to study actual nanodevices and aid in the understanding of heat dissipation near nanoscale hotspots, which is crucial for future high-performance nanoelectronics.

36 MATERIALS SCIENCE↗

DOE OSTI2017

Holographic mode wavefront sensor with an enlarged number of measured modes

A holographic mode wavefront sensor is proposed, which makes it possible to measure up to several tens of wavefront modes. The increase in the number of measured modes is implemented using the conversion of a light wave entering the sensor into a wide diffuse light beam, which allows one to record a large number of holograms, each intended for measuring one of the modes. (holographic sensors)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗