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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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Continuous Encodable Reshaping of Gold Nanocrystals through Facet Modulation

Shape control of nanocrystals (NCs) is crucial for tuning their assembly behavior and functional properties, yet the precise manipulation of facet composition remains challenging. Here, we present a nanocrystal reshaping strategy to control and modulate the facets of gold (Au) NCs. Our one-pot approach, conducted at room temperature, requires only initial Au NCs, Au 3+ ions, and surfactants, distinguishing it from conventional reduction-mediated “etching-and-regrowth” methods. Detailed structural studies using electron microscopy, small-angle X-ray scattering (SAXS), and UV−vis spectroscopy reveal the surfactant-encoded pathway for NC transformation from shaped particles to spheres and then into various polyhedral shapes while preserving the individual particles' volume. The proposed reshaping mechanism involves the dissolution of surface Au atoms into Au + complexes in the presence of Au 3+ and surfactant, followed by surfactant-guided redeposition and formation of facets with different atomic planes. Using the ethanol oxidation reaction (EOR) as a probe, we observe a quasi-linear decrease in onset potential and an increase in activity with increasing {100} facet exposure. This work broadens synthetic strategies by offering precise NC reshaping and facet control.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Direct Geologic Constraints on the Timing of Late Holocene Ice Thickening in the Amundsen Sea Embayment, Antarctica

Abstract Constraining past West Antarctic Ice Sheet (WAIS) change helps validate numerical models simulating future ice sheet dynamics. Following rapid deglaciation during the mid‐Holocene, ice near Thwaites Glacier was ∼35 m thinner than present; however, the timing of ice regrowth to its present configuration remains unknown. To fill this knowledge gap, we present cosmogenic nuclide exposure ages of cobbles from the surface of a moraine situated between Thwaites and Pope glaciers. We infer that the moraine formed and stabilized in the Late Holocene (∼1.4 ka) when a small glacier thickened. We also present a novel reconstruction of WAIS volume constrained by sea‐level data, which demonstrates that moraine formation coincided with a large‐scale WAIS readvance. Our new geologic constraints will help inform models of the solid Earth response to surface mass loading, improving our understanding of ice sheet dynamics in a vulnerable part of WAIS.

Nichols, Keir A. [Imperial College London London U↗
DOE OSTI2025

Visualizing degradation mechanisms in a gas-fed CO 2 reduction cell via operando X-ray tomography

We utilize operando X-ray computed tomography, coupled with real-time electrochemical analysis, to reveal the underlying failure mechanisms of membrane electrode assemblies (MEAs) for electrochemical CO 2 reduction (eCO 2 R). Through operando imaging, we can obtain unprecedented insights into the dynamic behavior of the MEA under different operating conditions, revealing critical changes in interface interactions, phase distribution, and structural integrity over time. Our findings identify phenomena giving rise to the transition from CO 2 R to the hydrogen evolution reaction (HER), as evidenced by shifts in cathode potential and CO 2 R selectivity. The formation of inhomogeneous precipitates at the gas diffusion electrode disrupts the CO 2 supply and reduces the active sites for eCO 2 R, resulting in a shift toward H2 production during low current density operation. Additionally, under high current density conditions, rapid water crossover up to the microporous layer/gas diffusion layer promotes the transition from CO 2 R to HER, further shifting cell potential toward anodic direction. Oscillating voltage conditions reveal the dissolution and regrowth of precipitates, providing direct visualization of the competing selectivity of CO 2 R and HER. This work offers new insight into the degradation mechanisms of MEAs, with implications for the design of more durable CO 2 R systems.

Lee, Sol A [California Institute of Technology (Ca↗
DOE OSTI2020

GaSb-based heterostructure with buried vacuum pocket photonic crystal layer

The vacuum pocket retaining molecular beam epitaxial regrowth of the nano-patterned GaSb surface was demonstrated. The high contrast 2D photonic crystal layer was incorporated into the test 2 μm emitting laser heterostructure. The photonic dispersion determined from angle-resolved electroluminescence experiment showed four well-resolved bands corresponding to the model predictions for the square lattice. The single-mode lasing near 2 μm has been observed at the temperature corresponding to the alignment of the photonic crystal band-edge states and the quantum well gain peak. The reference devices without the photonic crystal layer emitted trivial spectra and did not lase at any temperature.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

External charge compensation in etched gallium nitride measured by x-ray photoelectron spectroscopy

Electronic states at GaN surfaces and at regrowth and heteroepitaxy interfaces inhibit electronic device performance. Understanding electronic state configuration at the GaN surface is, therefore, crucial for the development of GaN-based devices, which are currently of considerable interest in power electronic applications. GaN and other wurtzite III-nitrides possess large spontaneous polarization along the c-axis, producing a bound sheet charge at the surface, which affects the electronic state configuration through the formation of internal and external compensation charges. Defects induced by conventional plasma-based dry etching methods may inhibit the internal screening of this bound charge and thus increase the concentration of external charged states. The surface band bending of n-type Ga-face GaN (0001) was measured with x-ray photoelectron spectroscopy after inductively coupled plasma etching to investigate the impact of dry etching on external charge compensation. GaN samples were etched using inductively coupled plasma with varying rf power and a novel plasma-enhanced atomic layer etching method using an oxidation, fluorination, and ligand-exchange mechanism. The band bending varied from 0.0 to 0.8 ± 0.1 eV for the samples measured.

Hatch, Kevin A. (ORCID:0000000283118623)↗
DOE OSTI2023

Selective area doping of GaN toward high-power applications

Abstract Selective area doping in GaN, especially p-type, is a critical and inevitable building block for the realization of advanced device structures for high-power applications, including, but not limited to, current-aperture vertical electron transistors, junction termination extensions, junction barrier Schottky diodes, junction field-effect transistors (JFETs), vertical-channel JFETs, U-shaped metal–oxide–semiconductor field-effect transistors (U-MOSFETs), and Fin MOSFETs. This paper reviews and summarizes some of the recent advances in the fields of selective area etching and regrowth, ion implantation, and polarity-dependent doping that may lead to the practical realization of GaN-based power devices.

Physics↗
DOE OSTI2022

(110)-Oriented GaAs Devices and Spalling as a Platform for Low-Cost III-V Photovoltaics

Here, we demonstrate the growth of GaAs solar cells by hydride vapor phase epitaxy (HVPE) on epi-ready and previously spalled (110) GaAs wafers as an advance towards a potentially low-cost (110)-based device platform. Controlled spalling offers a fracture-based path to substrate cost amortization, enabling device exfoliation and substrate reuse, but the faceted surface generated during the spalling of (100)-oriented GaAs presents hurdles to direct regrowth of subsequent devices. Spalling of (110) substrates instead eliminates faceting by aligning the substrate surface with the predominant crystal cleavage plane. III-V epitaxy of solar cells is significantly less developed on (110)-oriented substrates, however. Here, we develop (110)-based GaAs solar cells grown by HVPE on epi-ready substrates, demonstrating equal performance to devices grown on the more standard (100) orientation. We also characterize the surface of spalled, (110)-oriented substrates, revealing flat terraces separated by steps with sub-micron-scale height on average. Finally, we present an initial device grown on a previously spalled surface without additional surface re-preparation with nearly 16% efficiency under a simulated AM1.5G spectrum. Together, these results provide preliminary evidence of a potentially low-cost path to enable terrestrial III-V photovoltaics via the (110) substrate orientation.

14 SOLAR ENERGY↗
DOE OSTI2025

Enhancement-Mode GaN Monolithic Bidirectional Switch With Breakdown Voltage Over 3.3 kV

Here, this work demonstrates a GaN enhancement-mode monolithic bidirectional switch (MBDS) with breakdown voltage (BV) higher than 3.3 kV in both polarities. This MBDS is realized on a dual p-GaN gate high electron mobility transistor (HEMT) platform on sapphire substrate. It features a novel dual junction termination extension design for electric field management, which is built on the p-GaN layer in the gate stack and does not require epitaxial regrowth. The GaN MBDS exhibits symmetric on-state characteristics in both directions with a threshold voltage (V th ) of 0.6 V and a low specific on-resistance (R on,sp ) of 5.6 m Ω · cm2. This device presents the highest BV, as well as one of the best BV and R on,sp trade-offs, in all the reported MBDS devices. The R on,sp is lower than the performance limit of conventional BDS realized by two discrete devices. This 3.3 kV GaN MBDS opens the door for developing new circuit topologies and advancing system performance in medium-voltage power electronics.

42 ENGINEERING↗
DOE OSTI2021

(110)-Oriented GaAs Devices and Spalling as a Platform for Low-Cost III-V Photovoltaics

We demonstrate the growth of (110) GaAs solar cells by hydride vapor phase epitaxy (HVPE) as an advance towards a (110)-oriented device platform with substrate reuse via spalling. Controlled spalling offers a fracture-based path to substrate amortization, allowing device removal and substrate reuse, but the faceted surface generated in spalling of (100)-GaAs presents hurdles to direct regrowth of subsequent devices. Spalling of (110)-oriented substrates takes advantage of the natural (110) cleavage plane in zinc-blende III-V materials, eliminating this faceting. The vast majority of III-V epitaxy development is on (100)-oriented substrates, but epitaxy is not limited to this orientation. Here, we demonstrate (110)-based GaAs solar cells grown by HVPE with equivalent performance to devices grown on the more standard (100) orientation. We also demonstrate spalling of (110) oriented substrates, revealing surfaces with flat terraces separated by steps. These results provide preliminary evidence of a potentially low-cost path for terrestrial III-V photovoltaics via the (110) substrate orientation.

(110) orientation↗
DOE OSTI2021

Low-Cost Handle Study for Photovoltaics on Transfer Printed Virtual Substrates

This work expands upon previous research related to transfer printed virtual substrates: a technique for generating low-cost engineered substrates that is not based on wafer reuse. Here, silicon and sapphire are evaluated as candidate low-cost handle materials, validated by metal organic chemical vapor deposition regrowth of GaAs single-junction solar cells. Resulting devices are studied via materials and electrical characterization techniques, including concentration measurements that indicate GaAs//sapphire TPVS open-circuit voltage at >10 suns comparable with identical devices grown on a prime GaAs substrate.

41 EE - Solar Energy Technologies Office (EE-4S)↗
DOE OSTI2024

Impact of Sugarcane Cultivation on C Cycling in Southeastern United States Following Conversion From Grazed Pastures

ABSTRACT The expansion of sugarcane, a tropical high‐yielding feedstock, will likely reshape the Southeastern United States (SE US) bioenergy landscape. However, the sustainability of sugarcane, particularly as it displaces grazed pastures, is highly uncertain. Here, we investigated how pasture conversion to sugarcane in subtropical Florida impacts net ecosystem CO 2 exchange (NEE) and net ecosystem carbon (C) balance (NECB). Measurements were made over three full growth cycles (> 3 years) in sugarcane—plant cane, PC; first ratoon cane, FRC; second ratoon cane, SRC—and in improved (IM) and semi‐native (SN) pastures, which make up ca. 37% of agricultural land in the region. Immediately following conversion, PC was a stronger net source of CO 2 than pastures, indicating the importance of CO 2 losses related to land disturbance. Sugarcane, however, shifted to a strong net sink of CO 2 after first regrowth, and overall sugarcane was a stronger net CO 2 sink than pastures. Both stand age and low water availability during cane emergence and tillering substantially decreased its potential gross CO 2 uptake. Accounting for all C gains and removals (i.e., NECB), greater frequency of burn events and repeated harvest increased removals and overall made sugarcane a stronger C source relative to pastures despite substantial C inputs from the previous land use and a stronger CO 2 sink strength. Time since conversion substantially reduced C losses from sugarcane, and the NECB of SRC was similar to that of IM pasture but lower than that of SN pasture, indicating a rapid shift in the NECB of cane. We conclude that the C‐balance implications following conversion will depend on the proportion of IM versus SN pastures converted to sugarcane. Furthermore, our findings suggest that no‐burn harvest management strategies will be critical to the development of a sustainable bioenergy landscape in SE US.

Gomez‐Casanovas, Nuria↗
DOE OSTI2025

Demography, dynamics and data: building confidence for simulating changes in the world's forests

Vegetation demographic models (VDMs) are advanced tools for simulating forest responses to climate and land-use changes, and are essential for projecting carbon cycling and large-scale forest management strategies. Despite their increasing incorporation into Earth System Models, VDMs differ in their demographic assumptions, with no prior quantitative comparison of their performance. We benchmarked nine VDMs against observational data from boreal, temperate and tropical sites, assessing their accuracy in predicting tree growth, carbon turnover, biomass stocks and size distributions. Models were simulated under consistent climate conditions with postdisturbance recovery monitored for at least 420 yr. Postdisturbance carbon recovery trajectories showed significant variability while remaining within observational ranges. Initial regrowth rates varied substantially (0.03-0.60, 0.18-0.70 and 0.35-1.10 kgCm-2 yr-1 for boreal, temperate and tropical sites, respectively), influenced by each model's initial forest state. Models captured mature forest carbon content but showed compensating effects between overestimated growth and underestimated mortality rates. This first multi-model benchmarking identifies growth and mortality rates as critical calibration targets and highlights the need to refine postdisturbance establishment conditions for model development. We outline specific benchmarking variables needed to improve predictions of forest responses to environmental change.

demographic vegetation model benchmarking↗
DOE OSTI2021

Canopy damage and recovery following Hurricane Maria using multitemporal lidar data, Mar/2017 - Mar/2020, Puerto Rico

Hurricane Maria (Category 4) snapped and uprooted canopy trees, removed large branches, and defoliated vegetation across Puerto Rico. The magnitude of forest damages and the rates and mechanisms of forest recovery following Maria provide important benchmarks for understanding the ecology of extreme events. We used airborne lidar data acquired before (2017) and after Maria (2018, 2020) to quantify landscape-scale changes in forest structure along a 439-ha elevational gradient (100 to 800 m) in the Luquillo Experimental Forest. Damages from Maria were widespread, with 73% of the study area losing ≥1 m in canopy height (mean = -7.1 m). Taller forests at lower elevations suffered more damage than shorter forests above 600 m. Yet only 13% of the study area had canopy heights ≤2 m in 2018, a typical threshold for forest gaps, highlighting the importance of damaged trees and advanced regeneration on post-storm forest structure. Heterogeneous patterns of regrowth and recruitment yielded shorter and more open forests by 2020. Nearly 45% of forests experienced initial height loss (<-1 m, 2017-2018) followed by rapid height gain (>1 m, 2018-2020), whereas 21.6% of forests with initial height losses showed little or no height gain, and 17.8% of forests exhibited no structural changes >|1| m in either period. Canopy layers <10 m accounted for most increases in canopy height and fractional cover between 2018-2020, with gains split evenly between height growth and lateral crown expansion by surviving individuals. These findings benchmark rates of gap formation, crown expansion, and canopy closure following hurricane damage. Included in the attached zip file are four TIF and four KML files.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2020

AmeriFlux US-PFs SE5 Aspen-5 CHEESEHEAD 2019

This is the AmeriFlux version of the carbon flux data for the site US-PFs SE5 Aspen-5 CHEESEHEAD 2019. Site Description - This tower (12m Rohn) is located in the southeastern quadrant of the 10 x 10km study domain. It is located in an aspen regrowth forest (canopy height: 3.1 - 6 m).

Desai, Ankur↗
DOE OSTI2022

AmeriFlux FLUXNET-1F CA-Cbo Ontario - Mixed Deciduous, Borden Forest Site

This is the AmeriFlux Management Project (AMP) created FLUXNET-1F version of the carbon flux data for the site CA-Cbo Ontario - Mixed Deciduous, Borden Forest Site. This is the FLUXNET version of the carbon flux data for the site CA-Cbo Ontario - Mixed Deciduous, Borden Forest Site produced by applying the standard ONEFlux (1F) software. Site Description - Mixed deciduous forest from natural regrowth.

Staebler, Ralf↗
DOE OSTI2023

AmeriFlux FLUXNET-1F US-xUN NEON University of Notre Dame Environmental Research Center (UNDE)

This is the AmeriFlux Management Project (AMP) created FLUXNET-1F version of the carbon flux data for the site US-xUN NEON University of Notre Dame Environmental Research Center (UNDE). This is the FLUXNET version of the carbon flux data for the site US-xUN NEON University of Notre Dame Environmental Research Center (UNDE) produced by applying the standard ONEFlux (1F) software. Site Description - NEON's Domain 5 core site is located at the University of Notre Dame Environmental Research Center (UNDERC) East. Straddling the border between Northern Wisconsin and Michigan’s Upper Peninsula, the UNDERC property comprises approximately 7500 acres and is maintained as an environmental education and research facility. UNDERC also has 30 lakes comprising 1350 acres, including Crampton Lake, a NEON aquatics site. Region-wide logging for pine in the late 1800s and early to mid-1900's led to clear cutting of most forested areas on the property. The main parcel was donated to the University in the 1930s. Timber harvest continued into the 1950s and later, leaving a mixture of successional forest regrowth. Since the 1970s, the site has been minimally managed to maintain access for recreational, educational and research goals.

Network), NEON (National Ecological Observatory↗
DOE OSTI2021

High-Efficiency, Low-Cost III-V Solar Cells by Dynamic Hydride Vapor Phase Epitaxy Coupled with Rapid, Polishing-Free Wafer Reuse through Orientation-Optimized (110) Spalling

We demonstrated (110) GaAs solar cells grown by D-HVPE with equivalent performance and material quality relative to (100) devices. Also, we developed a procedure for repeatable wafer-scale spalling of (110) GaAs. Spalled surfaces were free of the facets founds in (100) GaAs spalling, the large arrest lines found in (100) Ge spalling, and surface features greater than 3 µm in peak-to-valley height, which can otherwise degrade the efficiency of regrown III-V devices. A sub-µm step-terrace morphology was discovered on the post-spall wafer surface, resulting from the restricted cleavage system of GaAs, but we developed understanding of the factors that determine the resulting morphology, enabling ample opportunity for optimization of morphologies suitable for device growth and spall depth waste minimization on (110) GaAs. Lastly, we demonstrated a device grown on a previously spalled surface with only 8% relative efficiency difference to a cell grown on a new unspalled wafer. Significant optimization of the substrate regrowth conditions are required to enable higher efficiency, but these results highlight the promise of (110)-oriented devices coupled with substrate reuse by spalling as a pathway for low-cost III-V photovoltaics.

14 SOLAR ENERGY↗
DOE OSTI2021

Technoeconomic and Life Cycle Analysis for Bio-Energy with Carbon Capture and Storage (BECCS) Baseline

Bio-Energy with Carbon Capture and Storage (BECCS) is an attractive option from an environmental standpoint, as biomass regrowth removes CO 2 from the atmosphere, which offsets the emissions produced by burning the biomass. When combined with carbon capture, this produces a system that is capable of zero or even negative greenhouse gas (GHG) emissions. This study examines the performance, environmental impact, and economics of co-firing biomass in pulverized coal (PC) power plants. The analysis is based on various plant configurations (with and without carbon dioxide [CO 2 ] capture) using hybrid poplar biomass at three levels of co-fire (20, 35, and 49 weight percent) with Illinois No. 6 coal. This study is an analysis of the overall performance and economics of the plant, which was used to determine the levelized cost of electricity (LCOE) and to perform a full environmental life cycle analysis (LCA) of greenfield PC plants co-firing biomass.

09 BIOMASS FUELS↗