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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 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Auger Recombination and Amplified Luminescence in InAsSb/InAsSbP Leds at 10–60 K

The Varshni parameters of the temperature dependence of the bandgap energy in the range 10–313 K and the temperature dependence of the spin–orbit splitting energy have been calculated using experimental data obtained for an InAs{sub 0.88}Sb{sub 0.12} active layer. Amplified luminescence was observed in the range 10–35 K for LEDs based on InAs{sub 0.88}Sb{sub 0.12}/InAsSbP heterostructures. The sharp drop in radiation intensity of InAsSb/InAsSbP LEDs at temperatures >32 K was due to extensive growth of the CHCC Auger-recombination process while the CHSH process was the dominant Auger-recombination process at temperatures <35 K.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2013

Dielectronic recombination lines of C{sup +}

The present paper presents atomic data generated to investigate the recombination lines of C II in the spectra of planetary nebulae. These data include energies of bound and autoionizing states, oscillator strengths and radiative transition probabilities, autoionization probabilities, and recombination coefficients. The R-matrix method of electron scattering theory was used to describe the C{sup 2+} plus electron system.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2025

Smart culture medium optimization for recombinant protein production: Experimental, modeling, and AI/ML-driven strategies

Recombinant protein production (RPP) is central to biotechnology, where recombinant proteins are used as either end products or catalysts in the synthesis of chemicals, fuels, and materials. Among the major cost drivers, culture medium plays a pivotal role in determining protein yield and quality. This review presents a comprehensive perspective on the critical stages of “smart” culture medium optimization: planning, screening, modeling, optimization, and validation. In the planning stage, we examine the nutritional and energetic roles of medium components, including carbon, nitrogen, amino acids, salts, and trace metals, and their impacts on culture parameters such as pH, oxidative state, and osmolality. We highlight the variability in trace metal content due to water sources, culture vessels, and raw materials, which can substantially influence RPP. The screening stage covers Design of Experiments (DoE) approaches, assessing their theoretical basis, implementation, and limitations. For modeling, we describe methods that integrate experimental data to develop predictive models for smart medium formulation. Model-based optimization strategies can then be employed to select optimal media compositions for a given application. The validation stage aims to evaluate model predictions and provide feedback for model training and refinement. Finally, we survey mechanistic and artificial intelligence/machine learning (AI/ML)-driven models as integrated, transformational tools for predictive modeling of bioprocess conditions, nutrient availability, cellular metabolism, and protein quality, with the goal of optimizing culture media to enhance protein yields while reducing costs and environmental impact. We conclude by addressing the challenges of translating laboratory-scale medium optimization to industrial-scale settings and exploring future AI/ML-driven approaches that may overcome current bottlenecks and accelerate medium design for RPP. Overall, this review provides a unified framework for advancing smart medium design in RPP.

Artificial Intelligence/Machine Learning (AI/ML)↗
DOE OSTI2018

Fabrication of CdTe QD-rGO composites with different linkers for controlling charge separation and recombination

Highlights: • CdTe QD-rGO heterostructures were fabricated by controlling cross linker molecules. • 1-Pyrene carboxylic acid and polyaniline acted as cross linker molecules. • Charge separation and recombination in PL was controlled by composition and structure. • PL properties have difference due to green and red CdTe QDs different surface states. - Abstract: The composites of reduced graphene oxide (rGO) and aqueous CdTe QDs were fabricated through controlling cross linker molecules (e.g. 1-pyrene carboxylic acid and polyaniline (PANI)). The thioglycolic acid (TGA) ligands on the QDs were removed when 1-pyrene carboxylic acid was used as linkers. In contrast. The TGA was retained on the QDs in the case of PANI as linkers. Because green- and red-emitting CdTe QDs have different surface states, the PL properties of the QDs changed in the heterostructure. The charge separation and recombination in PL process was controlled by the component and structure of the composite.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Novel cis-regulatory elements as synthetic promoters to drive recombinant protein expression from the Chlamydomonas reinhardtii nuclear genome

Eukaryotic green microalgae represent a sustainable, photosynthetic biotechnology platform for generating high-value products. The model green alga Chlamydomonas reinhardtii has already been used to generate high value bioproducts such as recombinant proteins and terpenoids. However, low, unstable, and variable nuclear transgene expression has limited the ease and speed of metabolic engineering and recombinant protein expression in this system. Here, novel genetic devices for transgene expression in C. reinhardtii have been developed by identifying cis-regulatory DNA elements capable of driving high transgene expression in C. reinhardtii promoters using de novo motif discovery informatics approaches. Thirteen putative motifs were synthesized as concatemers, linked to a common minimal basal promoter, and assayed for their activity to drive expression of a yellow fluorescent protein reporter gene. Following transformation of the vectors into C. reinhardtii by electroporation, in vivo measurements of yellow fluorescent protein expression by flow cytometry revealed that five of the DNA motifs analyzed displayed significantly higher reporter expression compared to the basal promoter control. Two of the concatemerized motifs, despite being much smaller minimal cis-regulatory elements, drove reporter expression at levels approaching that of the conventionally-used AR1 promoter. This analysis provides insight into C. reinhardtii promoter structure and gene regulation, and provides a new toolbox of cis-regulatory elements that can be used to drive transgene expression at a variety of expression levels.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2016

Modeling the Free Carrier Recombination Kinetics in PTB7:PCBM Organic Photovoltaics

Currently the exact recombination mechanism of free carriers in organic photovoltaic (OPV) devices is poorly understood. Often a reduced Langevin model is used to describe the decay behavior of electrons and holes. Here we propose a novel, simple kinetic model that accurately describes the decay behavior of free carriers in the PTB7:PCBM organic photovoltaic blend. In order to accurately describe the recombination behavior of free carriers as measured by time-resolved microwave conductivity (TRMC), this model needs to only take into account free and trapped holes in the polymer, and free electrons in the fullerene. The model is consistent for different PTB7:PCBM blend ratios and spans a light intensity range of over 3 orders of magnitude. Furthermore, the model demonstrates that dark carriers exist in the polymer and interact with photoinduced charge carriers, and that the trapping and detrapping rates of the holes are of high importance to the overall carrier lifetime.

14 SOLAR ENERGY↗
DOE OSTI2022

Acceleration of Biexciton Radiative Recombination at Low Temperature in CdSe Nanoplatelets

Colloidal semiconductor nanocrystals offer bandgap tunability, high photoluminescence quantum yield, and colloidal processing of benefit to optoelectronics, however rapid nonradiative Auger recombination (AR) deleteriously affects device efficiencies at elevated excitation intensities. AR is understood to transition from temperature-dependent behavior in bulk semiconductors to temperature-independent behavior in zero-dimensional quantum dots (QDs) as a result of discretized band structure that facilitates satisfaction of linear momentum conservation. For nanoplatelets (NPLs), two-dimensional morphology renders prediction of photophysical behaviors challenging. Here, in this study, we investigate and compare the temperature dependence of excited-stated lifetime and fluence-dependent emission of CdSe NPLs and QDs. For NPLs, upon temperature reduction, biexciton lifetime surprisingly decreases (even becoming shorter lived than trion emission) and emission intensity increases nearly linearly with fluence rather than saturating, consistent with dominance of radiative recombination rather than AR. CdSe NPLs thus differ fundamentally from core-only QDs and foster increased utility of photogenerated excitons and multiexcitons at low temperatures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

CdSe Magic-Size Clusters Deviate from Nanocrystal-Size Scalings for Ultrafast Intraband Relaxation and Auger Recombination

The cluster regime of colloidal semiconductor quantum dots (QDs) occupies an interesting space on the continuum from molecule to bulk material and allow the study of QD properties at extreme quantum confinement limits. For typical QDs, several size-dependent property trends are well established including intraband relaxation rates and universal scaling of Auger recombination with particle volume. Advances in synthesis now allow isolation of atomically precise semiconductor magic-size clusters (MCs), which offer experimental insight regarding how electronic structure changes with particle size in the ultrasmall limit. Here, we report intraband cooling and Auger recombination for several stable CdSe MCs as a function of particle size and observe deviation from QD response in the cluster regime. We additionally show optical signatures ascribed to transient disordering 2 that appear in the single exciton regime for MCs, which point to complexities regarding their implementation.

Auger recombination↗
DOE OSTI2023

Green Additive Limits Runaway Crystallinity in PM6:Y6 Organic Solar Cells but Causes Field-Independent Geminate Recombination

Solvent additives are well-established for improving device performance of organic solar cells by enhancing nanodomain phase separation and crystallization. However, optimum additive concentrations are typically at trace amounts with slight fluctuations causing runaway crystallinity that short the device and limit scale-up potential. Furthermore, we show that a nonhalogenated solvent additive 1-phenylnaphthalene (PN) can be used to optimize high-performance PM6:Y6 devices, but without runaway crystallinity even at concentrations 10× above the optimum. Our holistic analysis of device losses shows that PN increases photoabsorption while decreasing bimolecular recombination. However, above the optimum, charge generation is reduced due to field-independent geminate recombination of charge transfer states that overcomes the other benefits. Rotated Y6 crystallite orientation is a likely culprit, where the device electric field is ineffective at separating the charges. Despite this, device performance remains higher than that of traditional additives, indicating a route to avoiding the high sensitivity of performance to processing conditions.

14 SOLAR ENERGY↗
DOE OSTI2021

Impact of Photon Recycling, Grain Boundaries, and Nonlinear Recombination on Energy Transport in Semiconductors

Energy carrier transport and recombination in semiconductors can be directly monitored with optical microscopy, revealing key insights into fundamental photophysics and informs efficient optoelectronic device design. Oftentimes, energy transport is measured by monitoring a time-resolved expanding carrier profile after optical excitation under a focused laser beam. Although this approach has gained widespread adoption, the presence of competing dynamic processes, such as diffusion and photon recycling, has made it difficult to accurately extract and understand important semiconductor properties. Here, we develop and apply a new framework for modeling energy transport in both excitonic and free carrier semiconductors. Here, we demonstrate that processes such as nonlinear recombination and photon recycling can have a significant impact on the apparent energy carrier profiles, especially for excitonic materials with short radiative lifetimes. Additionally, we find that film microstructure can lead to unique transport profiles that strongly depends on material boundary behavior and the difference between feature size and diffusion length. Here, these findings provide a deeper understanding of energy transport in semiconducting materials and provide new strategies for the design and optimization of electronic and optoelectronic devices.

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

Ultrafast carrier capture and Auger recombination in single GaN/InGaN multiple quantum well nanowires

Ultrafast optical microscopy is an important tool for examining fundamental phenomena in semiconductor nanowires with high temporal and spatial resolution. In this paper, we used this technique to study carrier dynamics in single GaN/InGaN core–shell nonpolar multiple quantum well nanowires. We find that intraband carrier–carrier scattering is the main channel governing carrier capture, while subsequent carrier relaxation is dominated by three-carrier Auger recombination at higher densities and bimolecular recombination at lower densities. Finally, the Auger constants in these nanowires are approximately 2 orders of magnitude lower than in planar InGaN multiple quantum wells, highlighting their potential for future light-emitting devices.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Escape of anions from geminate recombination in THF due to charge delocalization

Geminate recombination of 24 radical anions (M˙ – ) with solvated protons (RH 2 + ) was studied in tetrahydrofuran (THF) with pulse radiolysis. The recombination has two steps: (1) diffusion of M˙ – and RH 2 + together to form intimate (contact and solvent separated) ion pairs, driven by Coulomb attraction; (2) annihilation of anions due to proton transfer (PT) from RH 2 + to M˙ – . The non-exponential time-dependence of the geminate diffusion was determined. For all molecules protonated on O or N atoms the subsequent PT step is too fast (<0.2 ns) to measure, except for the anion of TCNE which did not undergo proton transfer. PT to C atoms was as slow as 70 ns and was always slow enough to be observable. A possible effect of charge delocalization on the PT rates could not be clearly separated from other factors. For 21 of the 24 molecules studied here, a free ion yield (71.6 ± 6.2 nmol J –1 ) comprising ~29% of the total, was formed. This yield of “Type I” free ions is independent of the PT rate because it arises entirely by escape from the initial distribution of ion pair distances without forming intimate ion pairs. Furthermore, three anions of oligo(9,9-dihexyl)fluorenes, F n ˙ – (n = 2–4) were able to escape from intimate ion-pairs to form additional yields of “Type II” free ions with escape rate constants near 3 × 10 6 s –1 . These experiments find no evidence for an inverted region for proton transfer.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Photoluminescence, recombination rate, and gain spectra in optically excited n-type and tensile strained germanium layers

We theoretically investigate the optical properties of photo-excited biaxially strained intrinsic and n-type doped Ge semi-infinite layers using a multi-valley effective mass model. Spatial inhomogeneity of the excess carrier density generated near the sample surface is considered. Strain effects on the band edges, on the band dispersions, and on the orbital compositions of the near gap states involved in radiative recombinations are fully taken into account. We obtain, as a function of the distance from the sample surface, the energy resolved absorption/gain spectra resulting from the contribution of the radiative direct and phonon-assisted band-to-band transitions and from the intra-band free carrier absorption. Photoluminescence spectra are calculated from the spatially dependent spontaneous radiative recombination rate, taking into account energy-dependent self-absorption effects. For suitable combinations of doping density, strain magnitude, pump power, and emitted photon polarization, we find gain values up to 5800 cm{sup −1}.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Effects of emission layer doping on the spatial distribution of charge and host recombination rate density in organic light emitting devices: A numerical study

Based on drift-diffusion theory, a numerical model of the doping of a single energy level trap in the emission layer of an organic light emitting device (OLED) was developed, and the effects of doping of this single energy level trap on the distribution of the charge density, the recombination rate density, and the electric field in single- and double-layer OLEDs were studied numerically. The results show that by doping the n-type (p-type) emission layer with single energy electron (hole) traps, the distribution of the recombination rate density can be tuned and shifted, which is useful for improvement of the device performance by reduced electrode quenching or for realization of desirable special functions, e.g., emission spectrum tuning in multiple dye-doped white OLEDs.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Recombination dynamics and carrier lifetimes in highly mismatched ZnTeO alloys

This study investigates the recombination dynamics in highly mismatched ZnTeO alloys using time-resolved photoluminescence (PL) spectroscopy. The large PL energy redshift with increasing O content and the disappearance of the ZnTe emission verify the O-induced conduction band anticrossing effect. The incorporation of O generates electron localization below the E{sub −} conduction subband tail, which provide additional optical transitions and cause complex recombination mechanisms. Photoexcited free electrons in both the E{sub +} and the E{sub −} conduction subbands favor rapid relaxation to low energy states. Additionally, temperature-independent long carrier lifetimes (>130.0 ns) that are induced by localized electrons increase with O concentration.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Double injection, resonant-tunneling recombination, and current-voltage characteristics in double-graphene-layer structures

We evaluate the effect of the recombination associated with interlayer transitions in ungated and gated double-graphene-layer (GL) structures on the injection of electrons and holes. Using the proposed model, we derive analytical expressions for the spatial distributions of the electron and hole Fermi energies and the energy gap between the Dirac points in GLs as well as their dependences on the bias and gate voltages. The current-voltage characteristics are calculated as well. The model is based on hydrodynamic equations for the electron and hole transports in GLs under the self-consistent electric field. It is shown that in undoped double-GL structures with weak scattering of electrons and holes on disorder, the Fermi energies and the energy gap are virtually constant across the main portions of GLs, although their values strongly depend on the voltages and recombination parameters. In contrast, the electron and hole scattering on disorder lead to substantial nonuniformities. The resonant inter-GL tunneling enables N-shaped current-voltage characteristics provided that GLs are sufficiently short. The width of the current maxima is much larger than the broadening of the tunneling resonance. In the double-GL structures with relatively long GLs, the N-shaped characteristics transform into the Z-shaped characteristics. The obtained results are in line with the experimental observations [Britnell et al., Nat. Commun. 4, 1794–1799 (2013)] and might be useful for design and optimization of different devices based on double-GL structures, including field-effect transistors and terahertz lasers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Interface defects in SiC power MOSFETs - An electrically detected magnetic resonance study based on spin dependent recombination

This study presents electrically detected magnetic resonance (EDMR) measurements on a silicon carbide (SiC) MOSFET having the structure of a double-diffused silicon MOSFET (DMOS). The resonance pattern of a SiC DMOS was measured by monitoring the change of the recombination current between the source/body and the drain. The amplitude of the response has a maximum when the device is biased in depletion due to the equal concentrations of electrons and holes at the interface resulting in the most efficient recombination. The measured anisotropic g-tensor has axial symmetry with g{sub ∥} = 2.0051(4) (B ‖ c-axis), and g{sub ⊥} = 2.0029(4) (B⊥ c-axis) and the pattern shows several hyperfine (HF) peaks. We tentatively identify the observed defect as a silicon vacancy located directly at the interface.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Temperature-dependent photocarrier recombination dynamics in Cu{sub 2}ZnSnS{sub 4} single crystals

Time-resolved photoluminescence (PL) measurements have been used to study the temperature-dependent photocarrier recombination dynamics in Cu{sub 2}ZnSnS{sub 4} (CZTS) single crystals. We found a significant change of nearly four orders of magnitude of the PL decay time, from microseconds at low temperatures to subnanoseconds at room temperature. The slow PL decay at low temperatures indicates localization of the photocarriers at the band tails. Due to the large band tail states, the PL decay time depends strongly on both the photon energy and excitation density. It is pointed out that the drastically enhanced nonradiative recombination at high temperatures is one of the main factors that determine the power conversion efficiency of CZTS-based solar cells.

14 SOLAR ENERGY↗