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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 1,315 records · Page 73
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

III/V nano ridge structures for optical applications on patterned 300 mm silicon substrate

We report on an integration approach of III/V nano ridges on patterned silicon (Si) wafers by metal organic vapor phase epitaxy (MOVPE). Trenches of different widths (≤500 nm) were processed in a silicon oxide (SiO{sub 2}) layer on top of a 300 mm (001) Si substrate. The MOVPE growth conditions were chosen in a way to guarantee an efficient defect trapping within narrow trenches and to form a box shaped ridge with increased III/V volume when growing out of the trench. Compressively strained InGaAs/GaAs multi-quantum wells with 19% indium were deposited on top of the fully relaxed GaAs ridges as an active material for optical applications. Transmission electron microcopy investigation shows that very flat quantum well (QW) interfaces were realized. A clear defect trapping inside the trenches is observed whereas the ridge material is free of threading dislocations with only a very low density of planar defects. Pronounced QW photoluminescence (PL) is detected from different ridge sizes at room temperature. The potential of these III/V nano ridges for laser integration on Si substrates is emphasized by the achieved ridge volume which could enable wave guidance and by the high crystal quality in line with the distinct PL.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Interface-mediated ferroelectric patterning and Mn valency in nano-structured PbTiO{sub 3}/La{sub 0.7}Sr{sub 0.3}MnO{sub 3}

We employed a multitechnique approach using piezo-force response microscopy and photoemission microscopy to investigate a self-organizing polarization domain pattern in PbTiO{sub 3}/La{sub 0.7}Sr{sub 0.3}MnO{sub 3} (PTO/LSMO) nanostructures. The polarization is correlated with the nanostructure morphology as well as with the thickness and Mn valence of the LSMO template layer. On the LSMO dots, the PTO is upwards polarized, whereas outside the nanodots, the polarization appears both strain and interface roughness dependent. The results suggest that the electronic structure and strain of the PTO/LSMO interface contribute to determining the internal bias of the ferroelectric layer.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Observation of electron states of small period artificial graphene in nano-patterned GaAs quantum wells

Engineered honeycomb lattices, called artificial graphene (AG), are tunable platforms for the study of novel electronic states related to Dirac physics. Here in this work, we report the achievement of electronic bands of the honeycomb topology with the period as low as 40 nm on the nano-patterned modulation-doped AlGaAs/GaAs quantum wells. Resonant inelastic light scattering spectra reveal peaks which are interpreted as combined electronic transitions between subbands of the quantum well confinement with a change in the AG band index. Spectra lineshapes are explained by joint density of states obtained from the calculated AG electron band structures. These results provide a basis for further advancements in AG physics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Controlling and patterning the effective magnetization in Y 3 Fe 5 O 12 thin films using ion irradiation

We report an approach to controlling the effective magnetization ( M eff ), a combination of the saturation magnetization and uniaxial anisotropy, of the ferrimagnet Y 3 Fe 5 O 12 (YIG) using different species of ions: He + and Ga + . The effective magnetization can be tuned as a function of the fluence, with He + providing the largest effect. We quantified the change in effective magnetization through an angular dependence of the ferromagnetic resonance before and after irradiation. Increases in 4π M eff were observed to be as much as 400 G with only a 15% increase in Gilbert damping, α (from 8.2e-4 to 9.4e-4). This result was combined with a method for accurate ion pattering, a focused ion beam, providing a mechanism for shaping the magnetic environment with submicron precision. We observe resonance modes localized by ion patterning of micron-sized dots, whose resonances match well with micromagnetic simulations. Lastly, this technique offers a flexible tool for precision nanoscale control and characterization of the magnetic properties of YIG.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Influence of surface nano-patterning on the placement of InAs quantum dots

We have examined the influence of spontaneous nano-patterning on the placement of InAs quantum dots (QDs) on (Al)GaAs surfaces using an experimental-computational approach. Both atomically flat and mounded surfaces, generated via a surface instability induced by the Ehrlich- Schwoebel barrier, are employed as templates for the subsequent deposition of InAs QDs. Using height profiles from atomic-force micrographs, we simulate QD deposition with a 2D phase field model, which describes the time evolution of the InAs layer driven by a chemical potential gradient. For flat surfaces, phase-field simulations result in QD densities comparable to experimental observations. For mounded surfaces, the simulations reveal QDs preferentially positioned in regions of positive curvature (substrate valleys), e.g., at the edge of surface mounds, consistent with the anisotropic QD placement observed experimentally. We discuss the role of curvaturedriven diffusion in the spontaneous ordering of QDs, demonstrating the applicability of this mechanism to AlGaAs mounds.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Reversibility, pattern formation, and edge transport in active chiral and passive disk mixtures

In this study, we numerically examine mixtures of circularly moving and passive disks as a function of density and active orbit radius. For low or intermediate densities and/or small orbit radii, the system can organize into a reversible partially phase separated labyrinth state in which there are no collisions between disks, with the degree of phase separation increasing as the orbit radius increases. As a function of orbit radius, we find a divergence in the number of cycles required to reach a collision-free steady state at a critical radius, while above this radius, the system remains in a fluctuating liquid state. For high densities, the system can organize into a fully phase separated state that is mostly reversible, but collisions at the boundaries between the phases lead to a net transport of disks along the boundary edges in a direction determined by the chirality of the active disk orbits. We map the dynamic phases as a function of density and orbit radii and discuss the results in terms of the reversible-irreversible transition found in other periodically driven non-thermal systems. Finally, we also consider mixtures of circularly driven disks and ac driven disks where the ac drive is either in or out of phase with the circular motion and find a rich variety of pattern forming and reentrant disordered phases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Lateral and vertical growth of Mg-doped GaN on trench-patterned GaN films

Growth of Mg-doped GaN on trench-patterned GaN films consists of competing lateral and vertical growth fronts that result in regions with different electronic properties. Under typical growth conditions, lateral growth from the trench sidewall occurs at a faster rate than vertical growth from the trench base. When the trench width is sufficiently narrow, the growth fronts from opposite sidewalls coalesce and lead to eventual planarization of the top surface. Secondary electron imaging and cathodoluminescence mapping are used to correlate the morphology and the optical properties of regions resulting from lateral and vertical growth. For our growth conditions, the lateral-to-vertical growth rate ratio is found to be about 2.

Su, Po-Yi (ORCID:000000018339783X)↗
DOE OSTI2021

A patterned phase-changing vanadium dioxide film stacking with VO 2 nanoparticle matrix for high performance energy-efficient smart window applications

A vanadium dioxide (VO 2 ) based solid-to-solid phased changing material has been attracting great interest in smart window applications. However, achieving high solar modulation and high transparency simultaneously in visible light is the major challenge for the practical application of this smart material. To resolve this issue, in this paper, a smart film composed of a VO 2 nanoparticle matrix and a patterned VO 2 film is presented. Furthermore, numerical modeling and electromagnetic simulation are carried out to characterize the performance in terms of solar modulation and luminous transmittance, and a parametric study is carried out to optimize the proposed smart window film. Compared with the VO 2 nanoparticle matrix, the proposed structure can obtain 23% solar modulation and 57% luminous transmittance but with a much thinner thickness, which will significantly reduce the cost and fabrication complexity and extend the environment stability.

42 ENGINEERING↗
DOE OSTI2021

Understanding curvature effects on the magnetization reversal of patterned permalloy Archimedean spirals

Geometric curvature in magnetic systems can induce several unique magnetic interactions, the most notable of which are the curvature induced magnetic anisotropy and the curvature introduced Dzyaloshinski-Moria (DM) like interaction. Of the two, the DM-like interaction is particularly interesting as it provides a unique way to control domain wall motion, which is particularly relevant to the field of spintronics. In this work, we study the effect of geometric curvature on magnetization reversal in Permalloy strips patterned into Archimedean spirals with varying widths. Specifically, we simulated the magnetization reversal of Permalloy strips for several widths ranging from microscale to nanoscale to determine a lengthscale at which the curvature effects are strong. Simulations showed that in the microscale spirals, magnetization reversal primarily occurs through the appearance of magnetization ripple in which domains with reversed magnetization nucleate. On the other hand, the nanoscale spirals showed that reversal primarily proceeded through the motion of domain walls into the arms of the spiral. Our experimental in situ Lorentz transmission electron microscopy data of the microscale spirals, however, showed that magnetization reversal occurred with both mechanisms. At the nanoscale, the effect of local variation of curvature leads to hysteresis loops with stepped behavior. This behavior is characteristic of the curvature induced DM-like interaction affecting domain wall motion in the arms of the nanoscale spirals.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Wide wavelength-tunable narrow-band thermal radiation from moiré patterns

Wavelength-tunable narrow-band thermal emitters are highly desired for various applications including multigas sensing. However, current thermal emitters suffer from either too broad bandwidth or too narrow tuning range. Here, based on the moiré effect, we provide a scheme of wavelength-tunable narrow-band thermal emitters with tunability over a wide wavelength range and operation at an arbitrary temperature. Thanks to the unique sensitivity of moiré patterns, our emitter achieves a tuning range to bandwidth ratio of 313, which is 68 times larger than the previous largest value ever reported.

42 ENGINEERING↗
DOE OSTI2022

Selective area epitaxy of GaAs films using patterned graphene on Ge

We demonstrate selective area epitaxy of GaAs films using patterned graphene masks on a Ge (001) substrate. GaAs selectively grows on exposed regions of the Ge substrate for graphene stripe widths of 10 μm. The selectivity is highly dependent on the growth temperature and annealing time, which we explain in terms of temperature dependent sticking coefficients and surface diffusion. The high nucleation selectivity over several micrometers sets constraints on experimental realizations of remote epitaxy.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Photoinduced patterning of oxygen vacancies to promote the ferroelectric phase of Hf 0.5 Zr 0.5 O 2

Photoinduced reductions in the oxygen vacancy concentration were leveraged to increase the ferroelectric phase fraction of Hf 0.5 Zr 0.5 O 2 thinfilms. Modest ( ~ 2 — 77 pJ=cm 2 ) laser doses of visible light (488 nm, 2.54 eV) spatially patterned the concentration of oxygen vacancies as monitored by photoluminescence imaging. Local, tip-based, near-field, nanoFTIR measurements showed that the photoinduced oxygen vacancy concentration reduction promoted formation of the ferroelectric phase (space group Pca2 1 ), resulting in an increase in the piezoelectric response measured by piezoresponse force microscopy. Photoinduced vacancy tailoring provides, therefore, a spatially prescriptive, postsynthesis, and low-entry method to modify phase in HfO 2 -based materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Area-selective deposition of germanium on patterned graphene/monolayer molybdenum disulfide stacks via dipole engineering

Heterogeneous integration of two-dimensional materials and the conventional semiconductor has opened opportunities for next-generation semiconductor devices and their processing. Heterogeneous integration has been studied for economical manufacturing by substrate recycling and novel functionalities by a combination of incommensurate materials. However, utilizing the integration requires controlling locations of the integrated architectures. Here, we show area-selective deposition (ASD) of germanium on the graphene/MoS 2 stack. Ge nucleation precisely occurred on the surfaces of the patterned graphene/MoS 2 stack via dipole engineering. In this study, the growth temperature of ASD of Ge was significantly lower than that based on precursor desorption on SiO 2 . The first-principles calculations revealed that Ge deposited by ASD on the graphene/MoS 2 stack was not affected by charge transfer. This work provides a viable way to utilize atomically thin materials for next-generation semiconductor devices, which can be applicable for “Beyond Moore” and “More Moore” approaches.

2D materials↗
DOE OSTI2025

Automated imaging of the annihilation of a transverse domain wall in patterned magnetic thin films

Imaging the magnetic domain wall behavior in patterned thin films under external stimuli can enable understanding the underlying energy landscape and the role of local microstructure and defects. We present an automated workflow for in situ Lorentz transmission electron microscopy to image magnetic domain walls at the nanometer length scale and at a time resolution in the sub-millisecond regime—the latter of which is limited by the speed of the available camera. Our workflow is modular and can be broadly applied to various types of in situ experiments, taking us a step closer to the future of autonomous imaging of nanomagnetic films with electron microscopy. Using our workflow, we show the transformation of a transverse domain wall with sub-millisecond time resolution under the application of an in situ transverse magnetic field, a study of whose dynamics are essential in the design of future domain wall mediated spintronic device applications.

Algebraic topology↗
DOE OSTI2015

Analysis of cell division patterns in the Arabidopsis shoot apical meristem

Significance Plant cells are incapable of sliding past each other, so generation of shape and structure in plant tissue is dependent on cells dividing and expanding in particular directions. Therefore, understanding how cells choose where to build new walls is critical in understanding how plant tissue is patterned. In the present study we expand on previous models of cell division to further understand what parameters of cell geometry and growth influence the position of new walls.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2020

Affinity of small-molecule solutes to hydrophobic, hydrophilic, and chemically patterned interfaces in aqueous solution

Significance Increasing global population, urbanization, and climate change pose tremendous constraints on water availability. While membrane processes offer attractive, energy-efficient options, transformative advances in membrane technology are needed to enable reuse of poor-quality waters from diverse sources. This work brings fundamental understanding of solute–surface interactions to membrane phenomena, including selectivity and fouling. Molecular simulations reveal limitations of traditional predictive measures of solute–surface partitioning, suggesting alternative thermodynamic signatures for solute binding that highlight the contextual nature of hydrophobicity. An inverse design algorithm provides a tool to pattern membrane nanoscale chemistry to optimize solute affinity and selectivity. This work demonstrates how simulations can inform practical materials design for sustainable water treatment and, more broadly, many technologies involving water-mediated solute–surface interactions.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Linked nitrogen and carbon dynamics reveal distinct pools and patterns in a deep, weathered bedrock rhizosphere

Nitrogen is one of the most limiting nutrients to forest productivity worldwide. Recently, it has been established that diverse ecosystems source a substantial fraction of their water from weathered bedrock, leading to questions about whether root-driven nitrogen cycling extends into weathered bedrock as well. In this study, we specifically examined nitrogen dynamics using specialized instrumentation distributed across a 16 m weathered bedrock vadose zone (WBVZ) underlying an old growth forest in northern California where the rhizosphere—composed of plant roots and their associated microbiome—extends meters into rock. We documented total dissolved nitrogen (TDN), dissolved organic carbon (DOC), inorganic nitrogen (ammonium and nitrate), and CO 2 and O 2 gases every 1.5 m to 16 m depth for 2 y. We found that TDN concentrations increased with depth, were an order of magnitude greater at 15 m than in the upper 30 cm, and that the majority of TDN throughout the weathered bedrock vadose zone was organic. We also found that TDN concentrations are influenced by depth, season, and interannual precipitation patterns. Carbon isotope composition of the DOC suggests that dissolved organic matter in the WBVZ is primarily derived from plant sources, and not the nitrogen-rich bedrock. We conclude that nitrogen dynamics in the WBVZ may be driven, in part, by an active rhizosphere, meters below the base of soil, and we argue that weathered bedrock horizons may play a key role in C-N cycling in ecosystems with deep-rooted plants.

Science & Technology - Other Topics↗
DOE OSTI2016

Bacillus anthracis Prolyl 4-Hydroxylase Modifies Collagen-like Substrates in Asymmetric Patterns

Proline hydroxylation is the most prevalent post-translational modification in collagen. The resulting product trans-4-hydroxyproline (Hyp) is of critical importance for the stability and thus function of collagen, with defects leading to several diseases. Prolyl 4-hydroxylases (P4Hs) are mononuclear non-heme iron α-ketoglutarate (αKG)-dependent dioxygenases that catalyze Hyp formation. Although animal and plant P4Hs target peptidyl proline, prokaryotes have been known to use free L -proline as a precursor to form Hyp. The P4H from Bacillus anthracis (BaP4H) has been postulated to act on peptidyl proline in collagen peptides, making it unusual within the bacterial clade, but its true physiological substrate remains enigmatic. Here we use mass spectrometry, fluorescence binding, x-ray crystallography, and docking experiments to confirm that BaP4H recognizes and acts on peptidyl substrates but not free l -proline, using elements characteristic of an Fe(II)/αKG-dependent dioxygenases. We further show that BaP4H can hydroxylate unique peptidyl proline sites in collagen-derived peptides with asymmetric hydroxylation patterns. We note the cofactor-bound crystal structures of BaP4H reveal active site conformational changes that define open and closed forms and mimic “ready” and “product-released” states of the enzyme in the catalytic cycle. These results help to clarify the role of BaP4H as well as provide broader insights into human collagen P4H and proteins with poly- L -proline type II helices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗