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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 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
NASA NTRS2016

Imperfection Insensitivity Analyses of Advanced Composite Tow-Steered Shells

Two advanced composite tow-steered shells, one with tow overlaps and another without overlaps, were previously designed, fabricated and tested in end compression, both without cutouts, and with small and large cutouts. In each case, good agreement was observed between experimental buckling loads and supporting linear bifurcation buckling analyses. However, previous buckling tests and analyses have shown historically poor correlation, perhaps due to the presence of geometric imperfections that serve as failure initiators. For the tow-steered shells, their circumferential variation in axial stiffness may have suppressed this sensitivity to imperfections, leading to the agreement noted between tests and analyses. To investigate this further, a numerical investigation was performed in this study using geometric imperfections measured from both shells. Finite element models of both shells were analyzed first without, and then, with measured imperfections that were then, superposed in different orientations around the shell longitudinal axis. Small variations in both the axial prebuckling stiffness and global buckling load were observed for the range of imperfections studied here, which suggests that the tow steering, and resulting circumferentially varying axial stiffness, may result in the test-analysis correlation observed for these shells.

Wu, K. Chauncey↗
NASA NTRS2016

Buckling Design and Imperfection Sensitivity of Sandwich Composite Launch-Vehicle Shell Structures

Composite materials are increasingly being considered and used for launch-vehicle structures. For shell structures, such as interstages, skirts, and shrouds, honeycomb-core sandwich composites are often selected for their structural efficiency. Therefore, it is becoming increasingly important to understand the structural response, including buckling, of sandwich composite shell structures. Additionally, small geometric imperfections can significantly influence the buckling response, including considerably reducing the buckling load, of shell structures. Thus, both the response of the theoretically perfect structure and the buckling imperfection sensitivity must be considered during the design of such structures. To address the latter, empirically derived design factors, called buckling knockdown factors (KDFs), were developed by NASA in the 1960s to account for this buckling imperfection sensitivity during design. However, most of the test-article designs used in the development of these recommendations are not relevant to modern launch-vehicle constructions and material systems, and in particular, no composite test articles were considered. Herein, a two-part study on composite sandwich shells to (1) examine the relationship between the buckling knockdown factor and the areal mass of optimized designs, and (2) to interrogate the imperfection sensitivity of those optimized designs is presented. Four structures from recent NASA launch-vehicle development activities are considered. First, designs optimized for both strength and stability were generated for each of these structures using design optimization software and a range of buckling knockdown factors; it was found that the designed areal masses varied by between 6.1% and 19.6% over knockdown factors ranging from 0.6 to 0.9. Next, the buckling imperfection sensitivity of the optimized designs is explored using nonlinear finite-element analysis and the as-measured shape of a large-scale composite cylindrical shell. When compared with the current buckling design recommendations, the results suggest that the current recommendations are overly conservative and that the development of new recommendations could reduce the acreage areal mass of many composite sandwich shell designs by between 4% and 19%, depending on the structure.

Schultz, Marc R.↗
NASA NTRSDate not supplied

Developing Procedures to Implement Geometric Imperfections Beyond Right Circular Cylindrical Shells in Finite Element Method Models

Analysis of aerospace structures is frequently conducted using nominal dimensions and frequently assumes ideal conditions in loading, contact, constraints, et cetera. Off-nominal dimensions and nonideal conditions, however, are present in all structures. These are the result of widely ranging causes from coefficient of thermal expansion mismatches, manufacturing tooling anomalies, to assembly procedures that inadvertently alter the structure. Specifically, geometric imperfections can have potentially significant influence on the response of a structural test article observed in an experiment versus the response given by a numerical simulation. The Python Tool for Implementing Geometric Imperfections in Reduced Structures (Py_TIGIRS) was previously presented as a set of Python scripts to calculate and implement as-manufactured geometric midsurface and thickness imperfections into finite element method (FEM) shell models of nominally right circular cylinders. By taking advantage of the simple shape of a right circular cylinder, interpolations of the measured data points were able to be performed along directions that aligned to the cylindrical coordinate system axes of the entire structure. By taking advantage of the shell representation of the real structure as opposed to modeling using a continuum representation, the thickness variation was able to be implemented by shell section definitions instead of having to modify the position of multiple nodes in the thickness direction. Py_TIGIRS is a useful tool that established a procedural example on how to implement geometric imperfections in right circular cylindrical shell structures. Three new procedures, each expanded from concepts established in Py_TIGIRS, are proposed for various test-article designs and are intended to broaden the range of structures that can be modeled with measured geometric imperfections in the structural analysis community. Each test-article design introduces new challenges to successfully implement geometric imperfections into a FEM model. The first test-article design consists of a carbon fiber reinforced polymer square plate with a hat-shaped stiffener co-cured on one side. This test-article design was for a novel seven-point bend test that was also previously presented. Manufacturing and cure-cycle imperfections are observed using digital image correlation (DIC) techniques. As thermal expansion coefficient mismatches between the plate and stiffener materials were anticipated, a thermal analysis study with continuum shell and solid elements was conducted to capture the global shape observed prior to testing. The second test-article design is of a similar hat-stiffened plate configuration, but with a side length ratio near 3:1 with elongation in the stiffener direction. The test article was used to characterize the response to uniaxial compressive loading in the direction of the stiffener. Due to differing manufacturing steps, a thermal analysis like the one developed for the seven-point bend configuration was unable to mimic the observed geometric imperfections. Instead, a strategy based on applying deformations directly to the structure during analysis was developed for continuum shell and solid element representation of a stiffened panel.

Geometric imperfections↗
NASA NTRSDate not supplied

Scaling Methodology for Buckling of Composite Conical Shells in Axial Compression

Conical shells are commonly used as structural components for launch vehicles. The axial compression experienced during launch is one of the sizing load cases, because it can lead to loss of stability. As Because experimentally testing these large full-scale structures is cumbersome and expensive, it is studied how reduced-scale shells can be designed such that their buckling behavior is representative of the full-scale scale shell behavior. An analytical, sequential scaling procedure methodology is developed based on the nondimensional governing equations for composite conical shells with a symmetric, balanced layup and negligible flexural anisotropy. The buckling behavior of the shells of different size is compared using linear and nonlinear finite element analyses, and good comparisons . Accurate results are obtained for the considered shells in terms of buckling load, displacement, and mode. The inclusion of a geometric imperfections affects reduces the prediction accuracy, but it does not to the extent that the methodology is no longer valid cause the methodology to fail.

Buckling↗
NASA NTRSDate not supplied

Optimizing the Electroforming Process to Enhance the Thickness Uniformity of Full Shell X-Ray Optics

Electroforming replication technology at Marshall Space Flight Center has a long heritage of producing highquality full-shell X-ray mirrors for various applications. Nickel alloys are electroformed onto a super-polished mandrel in the electroforming process, then separated to form the replicated full-shell optic. Various parameters in the electroplating configuration could result in the nonuniformity of the shell’s thickness. Thickness nonuniformities primarily occur due to non-uniform electric field distributions in the electroforming tank during the deposition. Using COMSOL Multiphysics simulations, we have studied the electric field distributions during the deposition process. Using these studies, we have optimized the electric fields inside the tank using customized shields and insulating gaskets on the mandrel. These efforts reduced thickness non-uniformity from over 20% to under 5% percent. Improving the thickness uniformity of the shell aids in better mounting and alignment of shells in the optics module. Optimization of the electroforming process, in some cases, improved the optical performance of the shells. COMSOL optimizing of the electroforming process and the experimental results validating these simulations are presented in this article.

X-ray optics↗
DOE OSTI2012

Bandgap Engineering of InP QDs Through Shell Thickness and Composition

Fields as diverse as biological imaging and telecommunications utilize the unique photophysical and electronic properties of nanocrystal quantum dots (NQDs). The development of new NQD compositions promises material properties optimized for specific applications, while addressing material toxicity. Indium phosphide (InP) offers a 'green' alternative to the traditional cadmium-based NQDs, but suffers from extreme susceptibility to oxidation. Coating InP cores with more stable shell materials significantly improves nanocrystal resistance to oxidation and photostability. We have investigated several new InP-based core-shell compositions, correlating our results with theoretical predictions of their optical and electronic properties. Specifically, we can tailor the InP core-shell QDs to a type-I, quasi-type-II, or type-II bandgap structure with emission wavelengths ranging from 500-1300 nm depending on the shell material used (ZnS, ZnSe, CdS, or CdSe) and the thickness of the shell. Single molecule microscopy assessments of photobleaching and blinking are used to correlate NQD properties with shell thickness.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Platinum-coated non-noble metal-noble metal core-shell electrocatalysts

Core-shell particles encapsulated by a thin film of a catalytically active metal are described. The particles are preferably nanoparticles comprising a non-noble core with a noble metal shell which preferably do not include Pt. The non-noble metal-noble metal core-shell nanoparticles are encapsulated by a catalytically active metal which is preferably Pt. The core-shell nanoparticles are preferably formed by prolonged elevated-temperature annealing of nanoparticle alloys in an inert environment. This causes the noble metal component to surface segregate and form an atomically thin shell. The Pt overlayer is formed by a process involving the underpotential deposition of a monolayer of a non-noble metal followed by immersion in a solution comprising a Pt salt. A thin Pt layer forms via the galvanic displacement of non-noble surface atoms by more noble Pt atoms in the salt. The overall process is a robust and cost-efficient method for forming Pt-coated non-noble metal-noble metal core-shell nanoparticles.

36 MATERIALS SCIENCE↗
DOE OSTI2010

Annealing effects on the structure, photoluminescence, and magnetic properties of GaN/Mn{sub 3}O{sub 4} core-shell nanowires

Nanowires consisting of GaN/Mn{sub 3}O{sub 4} were prepared using a two-step approach that involved dipping the as-synthesized GaN nanowires into an aqueous manganese acetate solution. To examine the effects of annealing, GaN/Mn{sub 3}O{sub 4} core-shell nanowires were heated thermally to 700 {sup o}C in N{sub 2} ambient. Transmission electron microscopy showed that the continuous Mn{sub 3}O{sub 4} shell layer had agglomerated to expose a bare GaN core surface after thermal annealing. The magnetic measurements showed that the ferromagnetic behavior of the GaN nanowires had been suppressed by coating with the Mn{sub 3}O{sub 4} shell, without significant change by the subsequent thermal annealing. The GaN/Mn{sub 3}O{sub 4} core-shell nanowires exhibited blue, green, and red photoluminescence (PL) emission. The red emission was enhanced by thermal annealing. This paper discusses the associated mechanism for the variations in PL and magnetic properties of GaN/Mn{sub 3}O{sub 4} core-shell nanowires. - Abstract: Novel GaN/Mn{sub 3}O{sub 4} core-shell nanowires were synthesized and the effects of thermal annealing on the structure, photoluminescence, and magnetic properties were investigated.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Hanford Double Shell Tank Integrity Inspection - Integration of Customized Robotics - 18400

28 steel, double-shell, million gallon waste storage tanks located at the Hanford site in Washington State store both radioactive and hazardous waste. To monitor their condition and ensure continued viability, these tanks are inspected periodically through a comprehensive integrity program. In 2012, the first leak in a double-shell tank at Hanford was discovered in the 241-AY tank farm (Tank AY-102) and the failure was observed to be from the primary tank bottom. However, the exact failure location and damage mechanism have not been determined. A decision was made to pursue the capability to inspect the primary tank bottom region in order to provide additional tank integrity information. This work is particularly challenging due to the confined space in the double-shell tank annulus region (i.e. the 30 inch space between the primary and secondary tank shells), the limited dimensions of the tank risers that provide access to the annulus space (risers as small as 10 inches in diameter), and also the very small dimensions of the air channels in the concrete refractory pad that the primary waste storage tank rests on (channels are approximately two inches in diameter). Veolia Nuclear Solutions, along with Key Subcontractor Inuktun, were contracted to provide the capability to conduct these tank inspections in support of Hanford's double-shell tank integrity program. The Veolia Nuclear Solutions /Inuktun team designed, built, tested and delivered a tethered miniature robotic crawler system that utilizes the marsupial approach - a larger deployment crawler positions a smaller inspection crawler. The deployment crawler is installed via the tank riser, is magnetically coupled to the side of the tank shell, and drives to the selected deployment location at the end of one of the refractory channels where the small inspection robot is subsequently deployed. Operation of the equipment is controlled from a trailer outside the tank farm (a distance of several hundred feet). The system design includes a custom mechanism that provides for positioning and cable handling as well as the controls interface (laptop connected via control cables). The solution leveraged the Veolia Nuclear Solutions/Inuktun team's experience with delivering similar modular systems for other US and international sites. This simple but innovative solution delivered on an accelerated project schedule underlines the ability to provide effective robotic solutions to the nuclear industry. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2016

The adsorption of rare earth ions using carbonized polydopamine nano shells

Herein we report the structure effects of nano carbon shells prepared by carbonized polydopamine for rare earth elements (REEs) adsorption for the first time. The solid carbon sphere, 60 nm carbon shell and 500 nm carbon shell were prepared and investigated for adsorption and desorption of REEs. The adsorption of carbon shells for REEs was found to be better than the solid carbon sphere. The effect of acidities on the adsorption and desorption properties was discussed in this study. The good adsorption performance of carbon shells can be attributed to their porous structure, large specific surface area, amine group and carbonyl group of dopamine.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Probing Nitrogen-Doping Effects in the Core-Shell Structured Catalysts for Bifunctional Electrocatalysis.

The design of core-shell structured catalysts has attracted wide interests due to their remarkable catalytic performances in many fields. Although the nitrogen doping is often integrated in such kind of catalysts, the main contribution is exclusively attributed to the metal sites or the core-shell configuration. The role of nitrogen doping is often believed to be less important, which remains largely unexplored. Here, in an effort to probe the catalytic role of heterogeneous doping in such core-shell structure, the effect of nitrogen doping on the activation of cobalt and iron carbide-based materials for the electrocatalytic reduction of oxygen and the hydrogen evolution reaction is originally revealed. The nitrogen doping in the core-shell structured biphasic interfaces is found to be of critical importance for triggering the electrocatalytic activity and selectivity. Therefore, this study shows the neglected but critical role of nitrogen doping and biphasic interactions in a core-shell configuration, paving a new pathway towards the performance enhancement.

biphasic interfaces↗
DOE OSTI2022

Soot-particle core-shell and fractal structures from small-angle X-ray scattering measurements in a flame

We have characterized soot particles measured in situ in a laminar co-flow ethylene-air diffusion flame using small-angle X-ray scattering (SAXS). The analysis includes temperature measurements made with coherent anti-Stokes Raman spectroscopy (CARS) and complements soot volume-fraction and maturity measurements made with laser-induced incandescence (LII). We compared the results of fits to the SAXS measurements using a unified model and a fractal core-shell model. Power-law parameters yielded by the unified model indicate that aggregates of primary particles are in the mass-fractal regime, whereas the primary particles are in the surface-fractal regime in the middle of the flame. Higher and lower in the flame, the primary-particle power-law parameter approaches 4, suggesting smooth primary particles. Furthermore, these trends are consistent with fits using the fractal core-shell model, which indicate that particles have an established core-shell structure in the middle of the flame and are internally homogeneous at higher and lower heights in the flame. Primary-particle size distributions derived using the fractal core-shell model demonstrate excellent agreement with distributions inferred from transmission electron microscopy (TEM) images in the middle of the flame. Higher in the flame, a second small mode appears in the size distributions, suggesting particle fragmentation during oxidation. Surface oxidation would explain (1) aggregate fragmentation and (2) loss of core-shell structure leading to smoother primary-particle surfaces by removal of carbon overlayers. SAXS measurements are much more sensitive to incipient and young soot particles than LII and demonstrate significant volume fraction from particles low in the flame where the LII signal is negligible.

42 ENGINEERING↗
DOE OSTI2019

Predicting effective thermal and elastic properties of cementitious composites containing polydispersed hollow and core-shell micro-particles

Micro- and nano-sized hollow and core-shell particles (CSPs) have attracted tremendous interests in developing functional cementitious composites and concretes. In this paper, a general method is developed to predict the thermal and elastic properties of cementitious composites containing core-shell and hollow micro-particles. The model follows a two-stage homogenization process – CSP inclusions together with their surrounding interfacial transition zone (ITZ) are first treated as equivalent solid particles, and then the homogenized properties of the composite system are obtained using numerical (i.e., finite element) or analytical (Mori-Tanaka) approaches. Compared with other homogenization methods, the proposed model is easy-to-use and versatile. The numerical model, validated by experimental results, is then used to guide the design of functional cementitious composites. The results show that CSP particle size (or relative shell thickness), volume fraction, shell property, and the size of the ITZ have significant impacts on the effective thermal and elastic properties of cementitious composites; whereas the particle size distribution pattern has relatively minor influence. In addition, the effective property of the composite is more sensitive to the particle property change when CSP inclusions are much stiffer or more thermally conductive than the matrix. Lastly, a scoping study is conducted to elucidate the relationship between the effective thermal conductivity and effective elastic moduli for cementitious composites containing CSP additives with different shell materials, particle sizes, and volume concentrations etc.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Synthesis of ZnO-TiO{sub 2} core-shell long nanowire arrays and their application on dye-sensitized solar cells

Long ZnO nanowire arrays (NAs) grown on fluorine-doped tin oxide (FTO) glasses have been synthesized via a facile hydrothermal method without refreshing the reaction solution and applied as the precursor. By adjusting growth conditions, ZnO NAs with tunable lengths can be achieved. A nanocomposite made of ZnO nanowire core and TiO{sub 2} shell was further realized by a novel 'fast-dip-coating' method conducted in a Ti(OC{sub 4}H{sub 9}){sub 4}-dissolved ethanol solution. The formed ZnO-TiO{sub 2} core-shell NAs on FTO substrates were applied as electrodes for dye sensitized solar cells (DSSCs). It is found that both the TiO{sub 2} coating and NAs length play important roles in the enhancement of photoelectric conversion efficiency (PCE) of DSSCs. When the length of ZnO-TiO{sub 2} NAs reaches up to 14 {mu}m, the electrode can exhibit a maximum PCE as high as 3.80%, which is 2.6 times higher than that of pure ZnO NAs. - Graphical abstract: ZnO nanowire arrays in the length of 14 {mu}m have been successfully synthesized on the FTO substrate and coated with a thin shell of TiO{sub 2}; the ZnO-TiO{sub 2} electrodes applied in DSSCs exhibit great photoelectric conversion efficiency as high as 3.80%. Highlights: Black-Right-Pointing-Pointer Long ZnO nanowire arrays have been synthesized by using hydrothermal method. Black-Right-Pointing-Pointer A TiO{sub 2} shell outside of ZnO nanowire is introduced by a fast dip-coating method. Black-Right-Pointing-Pointer ZnO and ZnO-TiO{sub 2} core-shell nanowires are applied as the electrodes for DSSCs. Black-Right-Pointing-Pointer The maximum conversion efficiency of ZnO-TiO{sub 2} electrode reaches to 3.80%.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Magnetically separable {gamma}-Fe{sub 2}O{sub 3}-SiO{sub 2}-Ce-doped TiO{sub 2} core-shell nanocomposites: Fabrication and visible-light-driven photocatalytic activity

Novel visible-light-induced {gamma}-Fe{sub 2}O{sub 3}-SiO{sub 2}-Ce-doped-TiO{sub 2} core-shell nanocomposite photocatalysts capable of magnetic separation have been synthesized by a facile sol-gel and after-annealing process. The as-obtained core-shell nanocomposite is composed of a central {gamma}-Fe{sub 2}O{sub 3} core with a strong response to external fields, an interlayer of SiO{sub 2}, and an outer layer of Ce-doped TiO{sub 2} nanocrystals. UV-vis spectra analysis indicates that Ce doping in the compound results in a red-shift of the absorption edge, thus offering increased visible light absorption. We show that such a {gamma}-Fe{sub 2}O{sub 3}-SiO{sub 2}-Ce-doped-TiO{sub 2} core-shell nanocomposite with appreciated Ce doping amount exhibits much higher visible-light photocatalytic activity than bare TiO{sub 2} and undoped {gamma}-Fe{sub 2}O{sub 3}-SiO{sub 2}-TiO{sub 2} core-shell nanocomposite toward the degradation of rhodamine B (RhB). Moreover, the {gamma}-Fe{sub 2}O{sub 3}-SiO{sub 2}-Ce-doped-TiO{sub 2} core-shell nanocomposite photocatalysts could be easily separated and reused from the treated water under application of an external magnetic field. - Graphical abstract: Novel {gamma}-Fe{sub 2}O{sub 3}-SiO{sub 2}-Ce-doped-TiO{sub 2} core/shell nanocomposite photocatalysts with enhanced photocatalytic activity and fast magnetic separability were prepared. Highlights: Black-Right-Pointing-Pointer Novel {gamma}-Fe{sub 2}O{sub 3}-SiO{sub 2}-Ce-doped TiO{sub 2} core/shell composite photocatalysts were prepared. Black-Right-Pointing-Pointer The resulting core/shell composite show high visible light photocatalytic activity. Black-Right-Pointing-Pointer The nanocomposite photocatalysts can be easily recycled with excellent durability.

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

Electromagnetic moments of 215,217 Bi: Probing shell evolution beyond N = 126

The nuclear properties of bismuth isotopes (Z = 83) , with just one valence proton above the closed spherical shell at (Z = 82) , are expected to be governed by a single unpaired proton. However, already in semimagic 209 Bi (Z = 83, N = 126) , , the magnetic moment (μ) strongly deviates from the single-particle Schmidt value. A near linear decrease in μ with the increase of N after the N = 126 magic number was observed up to N = 130 . In order to test whether this trend is kept at N > 130 and to reveal the underlying mechanisms, an investigation of 215,217 Bi (N = 132, 134) has been undertaken. The magnetic dipole and electric quadrupole moments of the I π = 9/2 - nuclear ground states in these isotopes have been measured for the first time using the in-source resonance-ionization spectroscopy technique at ISOLDE (CERN). It has been shown that the linearly decreasing trend of μ( 209,211,213 Bi g ) is broken in 215,217 Bi with a nearly constant value of μ observed. Experimental data have been compared to calculations in the framework of the configuration-interaction shell model with the monopole-based universal V MU +LS interaction. The peculiarities in the behavior of μ(Bi, 9/2 - ) with increasing neutron number are explained as being due to the shell evolution, change of the neutron orbitals occupancies and strong configuration mixing beyond N = 130 . Also, the difference in the μ trends for bismuth (Z = 83) and astatine (Z = 85) isotopes with N > 126 are reproduced by the shell-model calculations. It is shown that monopole interaction plays noticeable role in the description of the peculiarities of the μ behaviour. Additionally, the extension of the application of the V MU interaction to the μ isotopic trends for heavy nuclei is important for further study of the capabilities of this promising version of the shell-model calculations.

Dipole magnetic moments↗
DOE OSTI2025

Implications of a weakening N = 126 shell closure away from stability for r -process astrophysical conditions

The formation of the third r-process abundance peak near A ∼ 195 is highly sensitive to both nuclear structure far from stability and the astrophysical conditions that produce the heaviest elements. In particular, the N = 126 shell closure plays a crucial role in shaping this peak. Experimental data hints that the shell weakens as proton number departs from Z = 82, a trend largely missed by global mass models. To investigate its impact on r-process nucleosynthesis, we employ both standard global models with strong closures and modified Duflo-Zuker (DZ) models that reproduce the weakening, combined with three sets of β − -decay rates. Strong shell closures generate sharply peaked abundances, whereas weakened closures consistent with the experimental trend produce broader, flatter patterns. Accurately reproducing the solar third peak under weakened shell strength requires sufficiently neutron-rich conditions that significant fission occurs, and slower decay rates. These results demonstrate that a weakening N = 126 shell closure away from stability imposes significant constraints on the astrophysical environments of the r-process and underscores the need for precise mass measurements and improved characterization of β − -decay properties in this region.

N = 126 closed shells↗
DOE OSTI2017

Nonpolar InGaN/GaN core–shell single nanowire lasers

We report lasing from nonpolar p-i-n InGaN/GaN multi-quantum well core–shell single-nanowire lasers by optical pumping at room temperature. The nanowire lasers were fabricated using a hybrid approach consisting of a top-down two-step etch process followed by a bottom-up regrowth process, enabling precise geometrical control and high material gain and optical confinement. The modal gain spectra and the gain curves of the core–shell nanowire lasers were measured using micro-photoluminescence and analyzed using the Hakki-Paoli method. Significantly lower lasing thresholds due to high optical gain were measured compared to previously reported semipolar InGaN/GaN core–shell nanowires, despite significantly shorter cavity lengths and reduced active region volume. Mode simulations show that due to the core–shell architecture, annular-shaped modes have higher optical confinement than solid transverse modes. Furthermore, the results show the viability of this p-i-n nonpolar core–shell nanowire architecture, previously investigated for next-generation light-emitting diodes, as low-threshold, coherent UV–visible nanoscale light emitters, and open a route toward monolithic, integrable, electrically injected single-nanowire lasers operating at room temperature.

36 MATERIALS SCIENCE↗