Search NASA⌕ Search

SEARCH · Search NASA

Results for “hexagonal”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Landau–Zener Transition Enhanced Quantum Sensing in Spin Defects of Hexagonal Boron Nitride

Negatively charged boron vacancies (V $^{–}_{B}$ ) in hexagonal boron nitride (hBN) comprise a promising quantum sensing platform, optically addressable at room temperature and transferable onto samples. However, broad hyperfine-split spin transitions of the ensemble pose challenges for quantum sensing with conventional resonant excitation due to limited spectral coverage. While V $^{–}_{B}$ in isotopically enriched hBN using 10 B and 15 N isotopes (h 10 B 15 N) exhibits sharper spectral features, significant inhomogeneous broadening persists. We show that, implemented via frequency modulation on an FPGA, a frequency-ramped microwave pulse achieves around 4-fold greater |0⟩→|−1⟩ spin-state population transfer and thus contrast than resonant microwave excitation and thus 16-fold shorter measurement time for spin relaxation-based quantum sensing. Quantum dynamics simulations reveal that an effective two-state Landau–Zener model captures the complex relationship between population inversion and pulse length with relaxations incorporated. Our approach is robust and valuable for quantum relaxometry with spin defects in hBN, especially in noisy environments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Hexagonal boron nitride nanomechanical resonators with spatially visualized motion

Atomic layers of hexagonal boron nitride (h-BN) crystal are excellent candidates for structural materials as enabling ultrathin, two-dimensional (2D) nanoelectromechanical systems (NEMS) due to the outstanding mechanical properties and very wide bandgap (5.9 eV) of h-BN. In this work, we report the experimental demonstration of h-BN 2D nanomechanical resonators vibrating at high and very high frequencies (from ~ 5 to ~ 70 MHz), and investigations of the elastic properties of h-BN by measuring the multimode resonant behavior of these devices. First, we demonstrate a dry-transferred doubly clamped h-BN membrane with ~ 6.7 nm thickness, the thinnest h-BN resonator known to date. In addition, we fabricate circular drumhead h-BN resonators with thicknesses ranging from ~ 9 to 292 nm, from which we measure up to eight resonance modes in the range of ~ 18 to 35 MHz. Combining measurements and modeling of the rich multimode resonances, we resolve h-BN’s elastic behavior, including the transition from membrane to disk regime, with built-in tension ranging from 0.02 to 2 N m -1 . The Young’s modulus of h-BN is determined to be EY≈392 GPa from the measured resonances. The ultrasensitive measurements further reveal subtle structural characteristics and mechanical properties of the suspended h-BN diaphragms, including anisotropic built-in tension and bulging, thus suggesting guidelines on how these effects can be exploited for engineering multimode resonant functions in 2D NEMS transducers.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Characterization of two-dimensional hexagonal boron nitride using scanning electron and scanning helium ion microscopy

Characterization of the structural and physical properties of two-dimensional (2D) materials, such as layer number and inelastic mean free path measurements, is very important to optimize their synthesis and application. In this study, we characterize the layer number and morphology of hexagonal boron nitride (h-BN) nanosheets on a metallic substrate using field emission scanning electron microscopy (FE-SEM) and scanning helium ion microscopy (HIM). Using scanning beams of various energies, we could analyze the dependence of the intensities of secondary electrons on the thickness of the h-BN nanosheets. Based on the interaction between the scanning particles (electrons and helium ions) and h-BN nanosheets, we deduced an exponential relationship between the intensities of secondary electrons and number of layers of h-BN. With the attenuation factor of the exponential formula, we calculate the inelastic mean free path of electrons and helium ions in the h-BN nanosheets. Our results show that HIM is more sensitive and consistent than FE-SEM for characterizing the number of layers and morphology of 2D materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Scanning tunneling microscopic and spectroscopic studies on a crystalline silica monolayer epitaxially formed on hexagonal SiC(0001{sup ¯}) surfaces

An epitaxial silicon-oxide monolayer of chemical composition of Si{sub 2}O{sub 3} (the Si{sub 2}O{sub 3} layer) formed on hexagonal SiC(0001{sup ¯}) surfaces has been observed by scanning tunneling microscopy (STM). Filled- and empty-state STM images with atomic resolution support the previously reported model. Typical structural defects in the Si{sub 2}O{sub 3} layer are found to be missing SiO{sub n} (n = 1, 2, 3) molecules. The band gap of the Si{sub 2}O{sub 3} layer obtained by point tunneling spectroscopy is 5.5±0.5 eV, exhibiting considerable narrowing from that of bulk SiO{sub 2}, 8.9 eV. It is proposed that the Si{sub 2}O{sub 3} layer is suitable as a relevant interface material for formation of SiC-based metal-oxide-semiconductor devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Temperature dependence of the energy bandgap of two-dimensional hexagonal boron nitride probed by excitonic photoluminescence

Hexagonal boron nitride (hBN) is an emerging material for the exploration of new physics in two-dimensional (2D) systems that are complementary to graphene. Nanotubes with a diameter (∼60 nm) that is much larger than the exciton binding energy in hBN have been synthesized and utilized to probe the fundamental optical transitions and the temperature dependence of the energy bandgap of the corresponding 2D hBN sheets. An excitonic transition at 5.901 eV and its longitudinal optical phonon replica at 5.735 eV were observed. The excitonic emission line is blue shifted by about 130 meV with respect to that in hBN bulk crystals due to the effects of reduced dimensionality. The temperature evolution of the excitonic emission line measured from 300 to 800 K revealed that the temperature coefficient of the energy bandgap of hBN nanotubes with large diameters (or equivalently hBN sheets) is about 0.43 meV/{sup 0}K, which is a factor of about 5 times smaller than the theoretically predicted value for the transitions between the π and π* bands in hBN bulk crystals and 6 times smaller than the measured value in AlN epilayers with a comparable energy bandgap. The observed weaker temperature dependence of the bandgap than those in 3D hBN and AlN is a consequence of the effects of reduced dimensionality in layer-structured hBN.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Exchange integrals in magnetoelectric hexagonal ferrite (SrCo{sub 2}Ti{sub 2}Fe{sub 8}O{sub 19}): A density functional study

The exchange integrals in magnetoelectric hexagonal ferrite SrCo{sub 2}Ti{sub 2}Fe{sub 8}O{sub 19} have been calculated by using density functional theory. To get 10 inter-sublattice and 3 intra-sublattice exchange integrals, the electronic structures and total energies of 20 spin arrangements have been calculated with General Gradient Approximation (GGA) + U method. The dependence of exchange integrals on U has been studied. The comparison between the exchange integrals in SrFe{sub 12}O{sub 19} and those in SrCo{sub 2}Ti{sub 2}Fe{sub 8}O{sub 19} shows that substitution of Co and Ti decreases the most interactions involving the 12 k sites. The investigation based on our exchange integrals indicates that magnetic interaction between R and S blocks reduces significantly in SrCo{sub 2}Ti{sub 2}Fe{sub 8}O{sub 19}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Layer-structured hexagonal (BN)C semiconductor alloys with tunable optical and electrical properties

Hexagonal boron nitride carbon, h(BN){sub 1-x}(C{sub 2}){sub x}, semiconductor alloys have been grown on sapphire substrates by metal-organic chemical vapor deposition. Bandgap tuning through compositional variation has been demonstrated via optical absorption measurements. Furthermore, an enhancement of approximately 10 orders of magnitude in the electrical conductivity has been attained by increasing the carbon concentration (x) from 0 to 0.21. Experimental results revealed evidences that the critical carbon concentration x{sub c} to form the homogenous h(BN){sub 1-x}(C{sub 2}){sub x} alloys, or the carbon solubility in hBN is about 3.2% at a growth temperature of 1300 °C before carbon clusters form. Based on the predicted phase diagram of cubic (BN){sub 1-x}(C{sub 2}){sub x} and the excellent matches in the structural and thermal properties of hBN and graphite, it is expected that homogenous h(BN){sub 1-x}(C{sub 2}){sub x} alloys with higher x can be achieved and the alloy miscibility gap can be reduced or completely removed by increasing the growth temperature. This is a huge advantage over the InGaN alloy system in which InN decomposes at high temperatures and high growth temperature cannot be utilized to close the miscibility gap. The results indicate that the h(BN){sub 1-x}(C{sub 2}){sub x} alloy system has the potential to tackle the challenging issues facing the emerging two-dimension materials beyond graphene, which include realizing the bandgap engineering, conductivity control, and large wafers of homogeneous films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Field emission characteristics from graphene on hexagonal boron nitride

An attempt has been made to utilize uniquely high electron mobility of graphene on hexagonal boron nitride (h-BN) to electron emitter. The field emission property of graphene/h-BN/Si structure has shown enhanced threshold voltage and emission current, both of which are key to develop novel vacuum nanoelectronics devices. The field emission property was discussed along with the electronic structure of graphene investigated by Fowler-Nordheim plot and ultraviolet photoelectron spectroscopy. The result suggested that transferring graphene on h-BN modified its work function, which changed field emission mechanism. Our report opens up a possibility of graphene-based vacuum nanoelectronics devices with tuned work function.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Carbon nanotube quantum dots on hexagonal boron nitride

We report the fabrication details and low-temperature characteristics of carbon nanotube (CNT) quantum dots on flakes of hexagonal boron nitride (hBN) as substrate. We demonstrate that CNTs can be grown on hBN by standard chemical vapor deposition and that standard scanning electron microscopy imaging and lithography can be employed to fabricate nanoelectronic structures when using optimized parameters. This proof of concept paves the way to more complex devices on hBN, with more predictable and reproducible characteristics and electronic stability.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2014

Spintronics with graphene-hexagonal boron nitride van der Waals heterostructures

Hexagonal boron nitride (h-BN) is a large bandgap insulating isomorph of graphene, ideal for atomically thin tunnel barrier applications. In this letter, we demonstrate large area chemical vapor deposited (CVD) h-BN as a promising spin tunnel barrier in graphene spin transport devices. In such structures, the ferromagnetic tunnel contacts with h-BN barrier are found to show robust tunneling characteristics over a large scale with resistances in the favorable range for efficient spin injection into graphene. The non-local spin transport and precession experiments reveal spin lifetime ≈500 ps and spin diffusion length ≈1.6 μm in graphene with tunnel spin polarization ≈11% at 100 K. The electrical and spin transport measurements at different injection bias current and gate voltages confirm tunnel spin injection through h-BN barrier. These results open up possibilities for implementation of large area CVD h-BN in spintronic technologies.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Kinetic arrest induced antiferromagnetic order in hexagonal FeMnP{sub 0.75}Si{sub 0.25} alloy

The magnetic state of the FeMnP{sub 0.75}Si{sub 0.25} alloy was investigated by first principles calculations. The coexistence of ferromagnetic and antiferromagnetic phases in FeMnP{sub 0.75}Si{sub 0.25} with the same hexagonal crystal structure was revealed. It was found that kinetic arrest during the transition from the high temperature disordered paramagnetic phase to the low temperature ordered ferromagnetic phase results in the intermediate metastable and partially disordered antiferromagnetic phase. We propose that the ratio of the ferromagnetic and antiferromagnetic phases in the FeMnP{sub 0.75}Si{sub 0.25} sample can be tuned by adjusting the kinetic process of atomic diffusion. The investigations suggest that careful control of the kinetic diffusion process provides another tuning parameter to design candidate magnetocaloric materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Synthesis and ionic liquid gating of hexagonal WO{sub 3} thin films

Via thin film deposition techniques, the meta-stable in bulk crystal hexagonal phase of tungsten oxide (hex-WO{sub 3}) is stabilized as a thin film. The hex-WO{sub 3} structure is potentially promising for numerous applications and is related to the structure for superconducting compounds found in WO{sub 3}. Utilizing ionic liquid gating, carriers were electrostatically induced in the films and an insulator-to-metal transition is observed. These results show that ionic liquid gating is a viable technique to alter the electrical transport properties of WO{sub 3}.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Magnetism of hexagonal Mn{sub 1.5}X{sub 0.5}Sn (X = Cr, Mn, Fe, Co) nanomaterials

Mn{sub 1.5}X{sub 0.5}Sn (X = Cr, Mn, Fe, Co) nanomaterials in the hexagonal Ni{sub 2}In-type crystal structure have been prepared using arc-melting and melt spinning. All the rapidly quenched Mn{sub 1.5}X{sub 0.5}Sn alloys show moderate saturation magnetizations with the highest value of 458 emu/cm{sup 3} for Mn{sub 1.5}Fe{sub 0.5}Sn, but their Curie temperatures are less than 300 K. All samples except the Cr containing one show spin-glass-like behavior at low temperature. The magnetic anisotropy constants calculated from the high-field magnetization curves at 100 K are on the order of 1 Merg/cm{sup 3}. The vacuum annealing of the ribbons at 550 °C significantly improved their magnetic properties with the Curie temperature increasing from 206 K to 273 K for Mn{sub 1.5}Fe{sub 0.5}Sn.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Synthesis of atomically thin hexagonal boron nitride films on nickel foils by molecular beam epitaxy

Hexagonal boron nitride (h-BN) is a layered two-dimensional material with properties that make it promising as a dielectric in various applications. We report the growth of h-BN films on Ni foils from elemental B and N using molecular beam epitaxy. The presence of crystalline h-BN over the entire substrate is confirmed by Raman spectroscopy. Atomic force microscopy is used to examine the morphology and continuity of the synthesized films. A scanning electron microscopy study of films obtained using shorter depositions offers insight into the nucleation and growth behavior of h-BN on the Ni substrate. The morphology of h-BN was found to evolve from dendritic, star-shaped islands to larger, smooth triangular ones with increasing growth temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2015

The interaction between hexagonal boron nitride and water from first principles

The use of hexagonal boron nitride (h-BN) in microfluidic and nanofluidic applications requires a fundamental understanding of the interaction between water and the h-BN surface. A crucial component of the interaction is the binding energy, which is sensitive to the treatment of electron correlation. In this work, we use state of the art quantum Monte Carlo and quantum chemistry techniques to compute the binding energy. Compared to high-level many-body theory, we found that the second-order Møller-Plesset perturbation theory captures the interaction accurately and can thus be used to develop force field parameters between h-BN and water for use in atomic scale simulations. On the contrary, density functional theory with standard dispersion corrections tends to overestimate the binding energy by approximately 75%.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Torsional properties of hexagonal boron nitride nanotubes, carbon nanotubes and their hybrid structures: A molecular dynamics study

The torsional mechanical properties of hexagonal single-walled boron nitride nanotubes (SWBNNTs), single-walled carbon nanotubes (SWCNTs), and their hybrid structures (SWBN-CNTs) are investigated using molecular dynamics (MD) simulation. Two approaches - force approach and energy approach, are adopted to calculate the shear moduli of SWBNNTs and SWCNTs, the discrepancy between two approaches is analyzed. The results show that the shear moduli of single-walled nanotubes (SWNTs), including SWBNNTs and SWCNTs are dependent on the diameter, especially for armchair SWNTs. The armchair SWNTs show the better ability of resistance the twisting comparable to the zigzag SWNTs. The effects of diameter and length on the critical values of torque of SWNTs are obtained by comparing the torsional behaviors of SWNTs with different diameters and different lengths. It is observed that the MD results of the effect of diameter and length on the critical values of torque agrees well with the prediction of continuum shell model. The shear modulus of SWBN-CNT has a significant dependence on the percentages of SWCNT and the hybrid style has also an influence on shear modulus. The critical values of torque of SWBN-CNTs increase with the increase of the percentages of SWCNT. This phenomenon can be interpreted by the function relationship between the torque of different bonds (B-N-X, C-C-X, C-B-X, C-N-X) and the angles of bonds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Toward achieving flexible and high sensitivity hexagonal boron nitride neutron detectors

Hexagonal boron nitride (h-BN) detectors have demonstrated the highest thermal neutron detection efficiency to date among solid-state neutron detectors at about 51%. We report here the realization of h-BN neutron detectors possessing one order of magnitude enhancement in the detection area but maintaining an equal level of detection efficiency of previous achievement.

61 RADIATION PROTECTION AND DOSIMETRY↗
DOE OSTI2018

Hexagonal boron nitride neutron detectors with high detection efficiencies

Here, neutron detectors fabricated from 10B enriched hexagonal boron nitride (h- 10 BN or h-BN) epilayers have demonstrated the highest thermal neutron detection efficiency among solid-state neutron detectors to date at about 53%. In this work, photoconductive-like vertical detectors with a detection area of 1 × 1 mm 2 were fabricated from 50 μm thick free-standing h-BN epilayers using Ni/Au and Ti/Al bilayers as ohmic contacts. Leakage currents, mobility-lifetime (μτ) products under UV photoexcitation, and neutron detection efficiencies have been measured for a total of 16 different device configurations. The results have unambiguously identified that detectors incorporating the Ni/Au bilayer on both surfaces as ohmic contacts and using the negatively biased top surface for neutron irradiation are the most desired device configurations. It was noted that high growth temperatures of h- 10 BN epilayers on sapphire substrates tend to yield a higher concentration of oxygen impurities near the bottom surface, leading to a better device performance by the chosen top surface for irradiation than by the bottom. Preferential scattering of oxygen donors tends to reduce the mobility of holes more than that of electrons, making the biasing scheme with the ability of rapidly extracting holes at the irradiated surface while leaving the electrons to travel a large average distance inside the detector at a preferred choice. When measured against a calibrated 6 LiF filled micro-structured semiconductor neutron detector, it was shown that the optimized configuration has pushed the detection efficiency of h-BN neutron detectors to 58%. These detailed studies also provided a better understanding of growth-mediated impurities in h-BN epilayers and their effects on the charge collection and neutron detection efficiencies.

61 RADIATION PROTECTION AND DOSIMETRY↗