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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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46 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Electrolyte Immersion Increases Photoconductivity in a Model Polymer Photocathode

Immersing polymer solar cells in aqueous electrolyte for photoelectrochemical (PEC) hydrogen production is likely to cause photophysical changes that could present both challenges and opportunities for engineering functional and durable devices. Herein we study the bulk heterojunction blend poly(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)-alt-(2-(((2-ethylhexyl)oxy)carbonyl)-3-fluorothieno[3,4-b]thiophene-4,6-diyl):poly(N,N'-di(2-octyldodecyl)naphthalene-1,8:4,5-bis(dicarboximide)-2,6-diyl)-alt-(2,2-bithiophene-5,5'-diyl) (PTB7-Th:N2200) excited-state dynamics in electrolyte from femtosecond to millisecond time scales using pump-probe microwave conductivity and absorption spectroscopy. While the blend swells very little, electrolyte exposure increases the microwave-frequency mobility and possibly the yield of photogenerated charges while also decreasing crystallinity. These results indicate an enhancement in key performance metrics, implying that any limitations on the performance of PEC test devices do not arise from active layer-electrolyte interactions. For the PTB7-Th:N2200 blend or similar photocathode systems, our results indicate that improving the interfacial kinetics and/or the carrier lifetime should be prioritized, not protecting the active layer from the electrolyte. Since this observation may not be universal to all polymer systems, future research should focus on identifying their limiting photophysical processes. electrolyte.

14 SOLAR ENERGY↗
DOE OSTI2023

Light-induced ultrafast spin transport in multilayer metallic films originates from sp-d spin exchange coupling

Ultrafast interaction between the femtosecond laser pulse and the magnetic metal provides an efficient way to manipulate the magnetic states of matter. Numerous experimental advancements have been made on multilayer metallic films in the last two decades. However, the underlying physics remains unclear. Here, relying on an efficient ab initio spin dynamics simulation algorithm, we revealed the physics that can unify the progress in different experiments. We found that light-induced ultrafast spin transport in multilayer metallic films originates from the sp - d spin-exchange interaction, which can induce an ultrafast, large, and pure spin current from ferromagnetic metal to nonmagnetic metal without charge carrier transport. The resulting trends of spin demagnetization and spin flow are consistent with most experiments. It can explain a variety of ultrafast light-spin manipulation experiments with different systems and different pump-probe technologies, covering a wide range of work in this field.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Giant spin-selective bandgap renormalization in CsPbBr 3 colloidal nanocrystals

The spin-dependence of the strongly correlated phenomena and many-body interactions play an important role in quantum information science. In particular, it is quite interesting but unclear how the spin degree of freedom ramifies the bandgap renormalization, one of the fundamental many-body phenomena. We report the first room-temperature observation of giant spin-selective bandgap renormalization (SS-BGR) in CsPbBr 3 colloidal nanocrystals using time-resolved circularly polarized femtosecond pump-probe spectroscopy. The SS-BGR results from many-body interactions among carriers with the same spin that renormalize their joint density of states by 57+/- 1 meV, visualized here as photoinduced absorption (PIA) below band-edge transition energy when the pump and probe are co-polarized. The hallmark result of spectrally resolved SS-BGR is in stark contrast to the usually submerged signal in II-VI and III-V semiconductors and is 3 orders of magnitude larger than that observed in Ge/SiGe quantum wells, highlighting the unique and beneficial band structure of the metal-halide semiconductors. We propose that the PIA, due to SS-BGR, can be used to describe the spin-polarization and spin-relaxation dynamics. The experimental and theoretical findings open up new possibilities for optical manipulation of spin degrees of freedom and their many-body interactions in metal-halide perovskite nanocrystals for potential room-temperature applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Electro-optic characterization of synthesized infrared-visible light fields

The measurement and control of light field oscillations enable the study of ultrafast phenomena on sub-cycle time scales. Electro-optic sampling (EOS) is a powerful field characterization approach, in terms of both sensitivity and dynamic range, but it has not reached beyond infrared frequencies. Here, we show the synthesis of a sub-cycle infrared-visible pulse and subsequent complete electric field characterization using EOS. The sampled bandwidth spans from 700 nm to 2700 nm (428 to 110 THz). Tailored electric-field waveforms are generated with a two-channel field synthesizer in the infrared-visible range, with a full-width at half-maximum duration as short as 3.8 fs at a central wavelength of 1.7 µm (176 THz). EOS detection of the complete bandwidth of these waveforms extends it into the visible spectral range. To demonstrate the power of our approach, we use the sub-cycle transients to inject carriers in a thin quartz sample for nonlinear photoconductive field sampling with sub-femtosecond resolution.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Circularly Polarized Light-Induced Microwave Conductivity Measurement: Rapid Screening Technique of Electronic Conductivity in Chiral Molecular Materials

Here, we developed circularly polarized light-time-resolved microwave conductivity (CPL-TRMC) for investigation of the CPL-dependent photoinduced charge carrier dynamics in chiral materials with chiroptical properties. Chiral R- or S-perylenediimide (PDI) molecular thin films were paired with handedness-sorted (6,5) and (11,-5) single-walled carbon nanotube (SWCNT) films to compose a donor (D)-acceptor (A) system for the spin-dependent charge separation process, and the D-A system was examined through linear and circular polarization-dependent steady-state and time-resolved measurements. The R-PDI-(6,5) film exhibited strong enhancement in circular dichroism (CD) and revealed a reversed transient conductivity signal, relative to the polarity of CD in CPL-TRMC measurement upon excitation of the E 11 state, which is interpreted as arising from a spin-dependent initial charge separation process. Through linear polarization-dependent flash photolysis TRMC and circular polarization-resolved femtosecond transient absorption, we could deduce that sub-picosecond intertubular charge separation upon E 11 excitation in SWCNT was responsible for the spin-dependent photoconductivity transients observed in CPL-TRMC measurements.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2018

Tandem LA-ICP-MS & LIBS; A New Micro-Analytical Technique for the Measurement of Every Element in the Periodic Table

Laser Ablation ICP-MS (LA-ICP-MS) has been widely accepted as a microanalytical technique for in-situ trace (ppb) elemental analysis on the micron scale in a variety of geologic materials. LA-ICP-MS (single or multi-collector) provides both elemental and isotopic measurements critical for a wide range of geological research by generating a fine grained aerosol (nm scale) during the laser ablation event and delivering that aerosol to the ICP ion source of the mass spectrometer via an inert carrier gas. LA-ICP-MS, however, suffers from limitations in analyzing high ionization potential elements as well as elements subject to atmospheric and argon based interferences. LA-ICP-MS also has limitations in analyzing major elements due to detector saturation. An alternative laser ablation technique, Laser Induced Breakdown Spectroscopy (LIBS), employs an optical spectrometer integrated into the laser ablation system that analyzes the laser induced plasma at the sample surface across the entire optical spectrum for emission lines of every element in the periodic table. Elements that are difficult or impossible to measure with LA-ICP-MS are now possible to analyze with LIBS down to low ppm levels with CCD and/or ICCD detection. We introduce a new laser based technique, “Tandem LA-LIBS”, that combines LA for ICP-MS and LIBS into one integrated laser ablation system. This system has the effect of expanding the elemental coverage and the dynamic range of the laser ablation experiment as measurements from ppb to % level matrix elements can now be analyzed in a single ablation experiment. We present both femtosecond and nanosecond Tandem LA-LIBS quantitative and qualitative data on wide range of geological materials for those elements that are difficult or impossible by traditional LA-ICP-MS techniques such as F, H, O, N, C, S, halogens, etc. We also demonstrate that the simultaneous measurement of trace, minor and major elements are now possible in a single laser ablation experiment with Tandem LA-LIBS technology.

LA-ICP-MS↗
DOE OSTI2020

A high-power, high-repetition-rate THz source for pump–probe experiments at Linac Coherent Light Source II

Experiments using a THz pump and an X-ray probe at an X-ray free-electron laser (XFEL) facility like the Linac Coherent Light Source II (LCLS II) require frequency-tunable (3 to 20 THz), narrow bandwidth (~10%), carrier-envelope-phase-stable THz pulses that produce high fields (>1 MV cm –1 ) at the repetition rate of the X-rays and are well synchronized with them. Herein, a two-bunch scheme to generate THz radiation at LCLS II is studied: the first bunch produces THz radiation in an electromagnet wiggler immediately following the LCLS II undulator that produces X-rays from the second bunch. The initial time delay between the two bunches is optimized to compensate for the path difference in THz transport. The two-bunch beam dynamics, the THz wiggler and radiation are described, as well as the transport system bringing the THz pulses from the wiggler to the experimental hall.

42 ENGINEERING↗
DOE OSTI2021

Dynamic Tilting of Ferroelectric Domain Walls Caused by Optically Induced Electronic Screening

Optical excitation perturbs the balance of phenomena selecting the tilt orientation of domain walls within ferroelectric thin films. The high carrier density induced in a low-strain BaTiO3 thin film by an above-bandgap ultrafast optical pulse changes the tilt angle that 90° a/c domain walls form with respect to the substrate-film interface. The dynamics of the changes are apparent in time-resolved synchrotron x-ray scattering studies of the domain diffuse scattering. Tilting occurs at 298 K, a temperature at which the a/b and a/c domain phases coexist but is absent at 343 K in the better ordered single-phase a/c regime. Furthermore, phase coexistence at 298 K leads to increased domain-wall charge density, and thus a larger screening effect than in the single-phase regime. The screening mechanism points to new directions for the manipulation of nanoscale ferroelectricity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Ultrafast Excitonic Response in Two-Dimensional Hybrid Perovskites Driven by Intense Midinfrared Pulses

Two-dimensional organic-inorganic hybrid perovskites (2DHPs) are natural quantum-well-like materials, in which strong quantum and dielectric confinement effects due to the organic spacers give rise to tightly bound excitons with large binding energy. To examine the mutual interactions between the organic spacer cations and the inorganic charge-residing octahedral framework in 2DHPs, here we perform femtosecond pump-probe spectroscopy by direct vibrational pumping of the organic spacers, followed by a visible-to-ultraviolet probe covering their excitonic resonances. Measurements on prototypical lead-bromide based 2DHP compounds, (BA) 2 ⁢PbBr 4 and (BA) 2 ⁢(FA)⁢Pb 2 ⁢Br 7 (BA+ = butylammonium; FA + = formamidinium), reveal two distinct regimes of the temporal response. The first regime is dominated by a pump-induced transient expansion of the organic spacer layers that reduces the exciton oscillator strength, whereas the second regime arises from pump-induced lattice heating effects primarily associated with a spectral shift of the exciton energy. In addition, vibrational excitation enhances the biexciton emission, which we attribute to a stronger intralayer exciton confinement as well as vibrationally induced exciton detrapping from defect states. Finally, our study provides fundamental insights regarding the impact of organic spacers on excitons in 2DHPs, as well as the excited-state dynamics and vibrational energy dissipation in these structurally diverse materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Observation of a highly conductive warm dense state of water with ultrafast pump–probe free-electron-laser measurements

The electrical conductivity of water under extreme temperatures and densities plays a central role in modeling planetary magnetic fields. Experimental data are vital to test theories of high-energy-density water and assess the possible development and presence of extraterrestrial life. These states are also important in biology and chemistry studies when specimens in water are confined and excited using ultrafast optical or free-electron lasers (FELs). Here we utilize femtosecond optical lasers to measure the transient reflection and transmission of ultrathin water sheet samples uniformly heated by a 13.6 nm FEL approaching a highly conducting state at electron temperatures exceeding 20 000 K. The experiment probes the trajectory of water through the high-energy-density phase space and provides insights into changes in the index of refraction, charge carrier densities, and AC electrical conductivity at optical frequencies. At excitation energy densities exceeding 10 MJ/kg, the index of refraction falls to n = 0.7, and the thermally excited free-carrier density reaches ne = 5 × 1027 m−3, which is over an order of magnitude higher than that of the electron carriers produced by direct photoionization. Significant specular reflection is observed owing to critical electron density shielding of electromagnetic waves. The measured optical conductivity reaches 2 × 104 S/m, a value that is one to two orders of magnitude lower than those of simple metals in a liquid state. At electron temperatures below 15 000 K, the experimental results agree well with the theoretical calculations using density-functional theory/molecular-dynamics simulations. With increasing temperature, the electron density increases and the system approaches a Fermi distribution. In this regime, the conductivities agree better with predictions from the Ziman theory of liquid metals.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗