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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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Autoionization and Dressing of Excited Excitons by Free Carriers in Monolayer WSe 2

We experimentally demonstrate dressing of the excited exciton states by a continuously tunable Fermi sea of free charge carriers in a monolayer semiconductor. It represents an unusual scenario of two-particle excitations of charged excitons previously inaccessible in conventional material systems. We identify excited state trions, accurately determine their binding energies in the zero-density limit for both electron- and hole-doped regimes, and observe emerging many-body phenomena at elevated doping. Combining experiment and theory we gain access to the intra-exciton coupling facilitated by the interaction with free charge carriers. We provide evidence for a process of autoionization for quasiparticles, a unique scattering pathway available for excited states in atomic systems. Finally, we demonstrate a complete transfer of the optical transition strength from the excited excitons to dressed Fermi-polaron states as well as the associated light emission from their nonequilibrium populations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Barriers to carrier transport in multiple quantum well nitride-based c -plane green light emitting diodes

The presence of alloy disorder in III-nitride materials has been demonstrated to play a significant role in device performance through effects such as carrier localization and carrier transport. Relative to blue light emitting diodes (LEDs), these effects become more severe at green wavelengths. Because of the potential fluctuations that arise due to alloy disorder, full three-dimensional (3D) simulations are necessary to accurately relate materials properties to device performance. We demonstrate experimentally and through simulation that increased quantum well (QW) number in c-plane green LEDs contributes to excess driving voltage, and therefore reduced electrical efficiency. Experimentally, we grew an LED series with the number of QWs varying from one to seven and observed a systematic increase in voltage with the addition of each QW. Trends in LED electrical properties obtained from 3D simulations, which account for the effects of random alloy fluctuations, are in agreement with experimental data. Agreement is achieved without the need for adjusting polarization parameters from their known values. Finally, from these results, we propose that the polarization induced barriers at the GaN/InGaN (lower barrier/QW) interfaces and the sequential filling of QWs both contribute significantly to the excess forward voltage in multiple QW c-plane green LEDs.

42 ENGINEERING↗
DOE OSTI2022

Carrier properties of Bi(111) grown on mica and Si(111)

A comparison is presented between properties of Bi(111) films grown by Stranski-Krastanov epitaxy on Si(111) 7 × 7 and by van der Waals epitaxy on mica substrates. Thin film morphologies and electronic transport properties of Bi(111) films of variable thickness are investigated for each growth method. Atomic force micrographs for Bi(111) films on mica reveal clearly defined triangular regions consisting of layered steps with height 0.4 nm, corresponding to the Bi(111) bilayer height. Variable-temperature electronic transport measurements show the existence of a quantum confinement-induced energy gap in the film interiors, resulting in a semimetal-to-semiconductor transition. Magnetotransport analysis in a three-carrier model including metallic electrons in surface states and electrons and holes in the films' interiors provides a detailed study of densities, mobilities, and mean-free paths of the three carrier types. Further, improved electronic transport properties are found in Bi(111) films of higher thickness on mica compared to on Si(111), a likely result of the largely strain-free van der Waals epitaxial growth.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Transient electron scavengers modulate carrier density at a polar/nonpolar perovskite oxide heterojunction

Here, we show how transient electron scavengers can be utilized to control the carrier concentration at polar/non-polar perovskite interfaces. By combining quantitative synchrotron x-ray-based interface structure determination with ab initio modeling, we demonstrate that Nd vacancy formation and the resulting formation of Nd adatoms, stabilized by oxygen scavengers at the growth front, can quantitatively account for the decreased carrier concentration at the SrTiO 3 /n NdTiO 3 /SrTiO 3 (001) heterojunction for n = 1 unit cell. This study yields new insights into growth mechanisms and the effect of transient species and defects on the electronic properties of oxide heterojunctions.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Entanglement Randomness and Gapped Itinerant Carriers in a Frustrated Quantum Magnet

The quantum spin liquid is a state manifesting extraordinary many-body entanglement, and the material NaYbSe 2 is thought to be one of the most promising candidates for its realization. Through low-temperature heat capacity and thermal conductivity measurements, we identify an apparent contradiction familiar to many quantum spin liquid candidates: While entropy is stored by apparently gapless excitations, the itinerant carriers of entropy are gapped. By studying the compositional series NaYb 𝑥⁢ Lu 1−𝑥⁢ Se 2 across a percolation transition of the magnetic lattice, we suggest that this contradiction can be resolved by the presence of entanglement scales of random sizes. Moreover, as we truncate the scale of entanglement by magnetic dilution, we show that the itinerant magnetic entropy carrier in NaYbSe 2 does not arise from a uniform globally entangled spin ground state but rather materializes through the stochastic propagation of boundaries between locally entangled spin objects.

Lyu, Yuanqi 吕源祺 [University of California, Berkele↗
DOE OSTI2021

Carrier Diffusion Lengths in Continuously Grown and Etched-and-Regrown GaN Pin Diodes

Advanced GaN power devices are promising for many applications in high power electronics but performance limitations due to material quality in etched-and-regrown junctions prevent their widespread use. Carrier diffusion length is a critical parameter that not only determines device performance but is also a diagnostic of material quality. Here we present the use of electron-beam induced current to measure carrier diffusion lengths in continuously grown and etched-and-regrown GaN pin diodes as models for interfaces in more complex devices. Variations in the quality of the etched-and-regrown junctions are observed and shown to be due to the degradation of the n-type material. Finally, we observe an etched-and-regrown junction with properties comparable to a continuously grown junction.

42 ENGINEERING↗
DOE OSTI2021

Carrier Lifetime as a Function of Se Content for CdSexTe1-x Films Grown on Al2O3 and MgZnO

Time-resolved photoluminescence (TRPL) with two excitation wavelengths – 670 (standard) and 405 nm – was used to examine the effect on carrier lifetime of two significant recent advances in CdTe: the incorporation of Se to form graded CdSe x Te 1-x and the use of Mg y Zn 1-y O buffers. The two excitation wavelengths probe depths of approximately 130 and 35 nm, respectively, and their comparison helps differentiate interface and bulk contributions to carrier lifetime. It was found that x = 0.2 Se was required to obtain lifetime improvements, primarily in the bulk. Additionally, TRPL traces for Mg y Zn 1-y O/CdSe x Te 1-x samples showed fast initial decay followed by a long-lived tail, which may be indicative of trap-dominated recombination. This behavior was not present for CdSe x Te 1-x films grown on Al 2 O 3 , which is currently state-of-the-art for surface passivation in CdTe. This indicates that further work is required to sufficiently passivate the front interface.

cadmium telluride↗
DOE OSTI2020

Arsenic Doping of Polycrystalline CdSeTe Devices for Microsecond Life-times with High Carrier Concentrations

We report seminal advances in fabrication and understanding of group V (As) doped thin-film polycrystalline CdTe-based solar cells. The devices are fabricated using a novel approach, by sublimating layers of CdSeTe and CdSeTe:As. This new method allowed us to achieve minority carrier lifetime of over 1 μs, carrier concentration of more than 5×l0 15 cc -1 and external radiative efficiency of over 2 % in a device configuration. We find an increase in open-circuit voltage when comparing As-doped, Cu-doped and undoped devices. The choice of CdSeTe instead of a CdTe-only absorber has been explained using first-principle density functional theory model. A SCAPS device model is used to analyze the potential causes for lower open-circuit voltage.

Munshi, Amit H.↗
DOE OSTI2022

Hod Carrier

SAND2023-06720O Hod Carrier is a simple proof-of-concept library for moving data from host to NVIDIA BlueField device memory using remote direct memory access. The Hod Carrier software also facilitates research activities related to the potential uses of NVIDIA BlueField data processing unit devices. Using well-established technologies (e.g., RDMA over Infiniband), it will move data from the host to the data processing unit’s memory. This software is a middleware utility for moving data without regard to its semantic meaning. Its operation is fundamentally unaffected by storage allocation. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

SciDAC↗
DOE OSTI2020

Long-term hybrid stabilization of the carrier-envelope phase

Controlling the carrier envelope phase (CEP) in mode-locked lasers over practically long timescales is crucial for real-world applications in ultrafast optics and precision metrology. We present a hybrid solution that combines a feed-forward technique to stabilize the phase offset in fast timescales and a feedback technique that addresses slowly varying sources of interference and locking bandwidth limitations associated with gain media with long upper-state lifetimes. We experimentally realize the hybrid stabilization system in an Er:Yb:glass mode-locked laser and demonstrate 75 hours of stabilization with integrated phase noise of 14 mrad (1 Hz to 3 MHz), corresponding to around 11 as of carrier to envelope jitter. Additionally, we examine the impact of environmental factors, such as humidity and pressure, on the long-term stability and performance of the system.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

Frequency-domain terahertz spectroscopy using long-carrier-lifetime photoconductive antennas

We present a telecommunication-compatible frequency-domain terahertz spectroscopy system realized by novel photoconductive antennas without using short-carrier-lifetime photoconductors. Built on a high-mobility InGaAs photoactive layer, these photoconductive antennas are designed with plasmonics-enhanced contact electrodes to achieve highly confined optical generation near the metal/semiconductor surface, which offers ultrafast photocarrier transport and, hence, efficient continuous-wave terahertz operation including both generation and detection. Consequently, using two plasmonic photoconductive antennas as a terahertz source and a terahertz detector, we successfully demonstrate frequency-domain spectroscopy with a dynamic range more than 95 dB and an operation bandwidth of 2.5 THz. Moreover, this novel approach to terahertz antenna design opens up a wide range of new possibilities for many different semiconductors and optical excitation wavelengths to be utilized, therefore bypassing short-carrier-lifetime photoconductors with limited availability.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Long interior carrier lifetime in selective-area InAs nanowires on silicon

Catalyst-free, position-controlled indium arsenide (InAs) nanowires (NWs) of variable diameters were grown on Si (111) by selective-area epitaxy (SAE). Ultrafast pump-probe spectroscopy was conducted, from which carrier recombination mechanisms on the NW surface and interior were resolved and characterized. NWs grown using SAE demonstrated high optical quality, showing minority carrier lifetimes more than two-fold longer than that of the randomly-positioned (RP) NWs. The extracted SAE-InAs NW interior recombination lifetime was found to be as long as 7.2 ns , 13X longer than previous measurements on RP-NWs; and the surface recombination velocity 4154 cm · s - 1 . Transmission electron microscopy revealed a high density of stacking defects within the NWs, suggesting that interior recombination lifetime can be further increased by improving NW interior crystalline quality.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Hearing carrier-envelope offset frequency and phase in air with a microphone

Attosecond science and frequency metrology rely on the precise measurement and control of the laser pulse waveform, a feat traditionally achieved using optoelectronic techniques. In this study, we conducted a laser-induced acoustic experiment in air ionized by carrier-envelope phase (CEP)-stabilized sub-4 fs pulses. Our results reveal that the acoustic signal exhibits CEP dependence in few-cycle pulses, primarily through amplitude modulation from laser-driven ionization. This optoacoustic phenomenon is, to the best of our knowledge, a novel phenomenon that enables not only the measurement of the carrier-envelope offset frequency but also the direct characterization of the waveform of optical pulses through a microphone. Our study highlights the potential of laser-induced acoustic waves for advancing ultrafast science.

Han, Meng [Kansas State University, Manhattan, KS ↗
DOE OSTI2022

Cross sections and calculated yields of some radionuclides of yttrium, strontium and rubidium formed in proton-induced reactions on enriched strontium-86: possibility of production of 85g Sr, 83 Rb and 82m Rb in no-carrier-added form

Here, cross sections of the 86 Sr(p,3n) 84m Y, 86 Sr(p,αn) 82m Rb, and 86 Sr(p,x) 85g Sr reactions were measured from their respective thresholds up to 16.2 MeV and from 23.0 to 44.1 MeV at FZJ, and from 14.3 to 24.5 MeV at LBNL, using 96.4% enriched 86 SrCO 3 as target material. Thin targets prepared by sedimentation were irradiated with protons in a stacked-form, and the induced radioactivity was measured by high-resolution γ-ray spectrometry. Nuclear model calculations based on the code TALYS reproduced our experimental cross section data well. From the excitation functions, the integral yields of the above three radionuclides were calculated. The yield of 85g Sr via the natSr(n,γ) process was also measured using the TRIGA Mark-II reactor at AERE, Savar. A comparison of the reactor and cyclotron production of carrier-added 85g Sr is given. The production possibilities of the three investigated radionuclides in no-carrier-added forms at a 30 MeV cyclotron via new routes are discussed.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2020

Evaluation of the Dissolution Behavior of L-Bundle End Caps and HFIR Fuel Carriers

The H-Canyon facility is currently using the 6.1D dissolver for the dissolution of Material Test Reactor (MTR) fuel and the 6.4D dissolver for the dissolution of High Flux Isotope Reactor (HFIR) fuel using mercury-catalyzed HNO₃ dissolution flowsheets. The processing strategy for both dissolvers involves the dissolution of multiple charges of fuel per batch. After the designated heating cycle, the dissolvers are opened, and the charging wells are probed to determine if the MTR or HFIR fuel has dissolved. If undissolved fuel fragments are beyond a certain height, the dissolver must be closed and heated for an additional amount of time to dissolve the remaining material. In recent MTR fuel dissolutions, “high probes” (i.e., excessive undissolved material) were frequently observed, which resulted in extended dissolution times. The suspected cause of the high probes was the incomplete dissolution of the L-Bundle End Cap, rather than the fuel or fuel bundles. The End Cap is hypothesized to be binding in the insert well and not dropping into the acid as the L-Bundle and fuel dissolve. Once the End Cap is dislodged by the probe and drops into the acid, the dissolution rate of the End Cap appears to be significantly reduced compared to the dissolution rate of the fuel and other parts of the L-Bundle. A similar issue has also been observed with the lifting bail on the HFIR fuel carriers. During HFIR fuel dissolutions, the lifting bail on the outer carrier has resulted in high probes due to incomplete dissolution. In one case, a partially dissolved bail was caught in one of the insert well holes which prevented the probe from going to the bottom of the well.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2021

Toluene-Methylcyclohexane as Two-Way Carrier for Hydrogen Transmission and Storage

Recognizing the potential role of liquid hydrogen carriers in overcoming the inherent limitations in transporting and storing gaseous and liquid hydrogen, a complete production and use scenario is postulated and analyzed for a two-way carrier. MCH is produced at commercially viable scales in a central location in Texas (TX) by hydrogenation of toluene and transmitted by rail or chemical tankers to northern California (CA), see Fig. 1. MCH is dehydrogenated near city gates to generate fuel-cell quality hydrogen, and toluene is transmitted back to TX.

08 HYDROGEN↗
DOE OSTI2022

Distinguishing Bulk and Surface Recombination By Measuring and Modeling Ultrafast Carrier Dynamics

Distinguishing Bulk and Surface Recombination By Measuring and Modeling Ultrafast Carrier Dynamics The combination of modeling with TRTS and TRPL under multiple excitation conditions improves accuracy and precision of parameter estimation. Ultrafast carrier dynamics can link materials, processing, and device performance to direct the design of more efficient solar cells.

14 SOLAR ENERGY↗
DOE OSTI2023

Enrichment of H2 to CO2 ratio using formic acid as a hydrogen carrier - CRADA 581 (Abstract)

The collaboration between PNNL and OCOchem will investigate approaches to enhance the hydrogen (H2) content from a stream of H2 and carbon dioxide (CO2) gases produced in the catalytic decomposition of aqueous formic acid (FA85). OCOchem uses an electrochemical process to generate FA85 from captured CO2 using electricity from renewable resources. The FA85 is a liquid organic hydrogen carrier (LOHC) that provides the opportunity to transport and store hydrogen, in liquid form, at volumetric densities significantly greater than compressed H2 gas, i.e., 50 grams H2/liter FA. PNNL has developed and tested catalytic reactors to release H2 from LOHCs like FA and aqueous formate salts (FS). The H2 released from the LOHC can be oxidized in a proton-exchange membrane fuel cell (PEM-FC) to generate electricity with water as the only by-product. The purpose of the proposed project is to increase the purity of the hydrogen released from the LOHC to enhance the operation efficiency of the PEM FC. The PEM FC in combination with the LOHC provides an approach to demonstrate a portable generator that utilizes hydrogen as the energy carrier instead of conventional diesel generator. The ‘hydrogen generator’ can be used to supply emergency backup power and significantly reduce CO2 emissions relative to a diesel generator.

30 DIRECT ENERGY CONVERSION↗