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At least 235 records · Page 13

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DOE OSTI2012

First-principles characterization of potassium intercalation in the hexagonal 2H-MoS2

Periodic density functional theory calculations were performed to study the structural and electronic properties of potassium intercalated into hexagonal MoS{sub 2} (2H-MoS{sub 2}). Metallic potassium (K) atoms are incrementally loaded in the hexagonal sites of the interstitial spaces between MoS2 sheets of the 2H-MoS{sub 2} bulk structure generating 2H-KxMoS2 (0.125 {<=} x {<=} 1.0) structures. To accommodate the potassium atoms, the interstitial spacing c parameter in the 2H-MoS{sub 2} bulk expands from 12.816 {angstrom} in 2H-MoS{sub 2} to 16.086 {angstrom} in 2H-K{sub 0.125}MoS{sub 2}. The second lowest potassium loading concentration (K{sub 0.25}MoS{sub 2}) results in the largest interstitial spacing expansion (to c = 16.726 {angstrom}). Our calculations show that there is a small gradual contraction of the interstitial spacing as the potassium loading increases with c = 14.839 {angstrom} for KMoS{sub 2}. This interstitial contraction is correlated with an in-plane expansion of the MoS{sub 2} sheets, which is in good agreement with experimental X-ray diffraction (XRD) measurements. The electronic analysis shows that potassium readily donates its 4s electron to the conduction band of the 2H-K{sub x}MoS{sub 2}, and is largely ionic in character. As a result of the electron donation, the 2H-K{sub x}MoS{sub 2} system changes from a semiconductor to a more metallic system with increasing potassium intercalation. For loadings 0.25 {<=} x {<=} 0.625, triangular Mo-Mo-Mo moieties are prominent and tend to form rhombitrihexagonal motifs. Intercalation of H{sub 2}O molecules that solvate the K atoms is likely to occur in catalytic conditions. The inclusion of two H{sub 2}O molecules per K atom in the K{sub 0.25}MoS{sub 2} structure shows good agreement with XRD measurements.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2017

Interfaces between hexagonal and cubic oxides and their structure alternatives

Multi-layer structure of functional materials often involves the integration of different crystalline phases. The film growth orientation thus frequently exhibits a transformation, owing to multiple possibilities caused by incompatible in-plane structural symmetry. Nevertheless, the detailed mechanism of the transformation has not yet been fully explored. Here we thoroughly probe the heteroepitaxially grown hexagonal zinc oxide (ZnO) films on cubic (001)-magnesium oxide (MgO) substrates using advanced scanning transition electron microscopy, X-ray diffraction and first principles calculations, revealing two distinct interface models of (001) ZnO/(001) MgO and (100) ZnO/(001) MgO. Here we have found that the structure alternatives are controlled thermodynamically by the nucleation, while kinetically by the enhanced Zn adsorption and O diffusion upon the phase transformation. Finally, this work not only provides a guideline for the interface fabrication with distinct crystalline phases but also shows how polar and non-polar hexagonal ZnO films might be manipulated on the same cubic substrate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Soliton superlattices in twisted hexagonal boron nitride

Properties of atomic van der Waals heterostructures are profoundly influenced by interlayer coupling, which critically depends on stacking of the proximal layers. Rotational misalignment or lattice mismatch of the layers gives rise to a periodic modulation of the stacking, the moiré superlattice. Provided the superlattice period extends over many unit cells, the coupled layers undergo lattice relaxation, leading to the concentration of strain at line defects – solitons - separating large area commensurate domains. We visualize such long-range periodic superstructures in thin crystals of hexagonal boron nitride using atomic-force microscopy and nano-infrared spectroscopy. The solitons form sub-surface hexagonal networks with periods of a few hundred nanometers. We analyze the topography and infrared contrast of these networks to obtain spatial distribution of local strain and its effect on the infrared-active phonons of hBN.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2022

Tuning colour centres at a twisted hexagonal boron nitride interface

The colour centre platform holds promise for quantum technologies, and hexagonal boron nitride has attracted attention due to the high brightness and stability, optically addressable spin states and wide wavelength coverage discovered in its emitters. However, its application is hindered by the typically random defect distribution and complex mesoscopic environment. In this study, employing cathodoluminescence, we demonstrate on-demand activation and control of colour centre emission at the twisted interface of two hexagonal boron nitride flakes. Further, we show that colour centre emission brightness can be enhanced by two orders of magnitude by tuning the twist angle. Additionally, by applying an external voltage, nearly 100% brightness modulation is achieved. Our ab initio GW and GW plus Bethe–Salpeter equation calculations suggest that the emission is correlated to nitrogen vacancies and that a twist-induced moiré potential facilitates electron–hole recombination. This mechanism is further exploited to draw nanoscale colour centre patterns using electron beams.

36 MATERIALS SCIENCE↗

DOE OSTI2026

Tunable symmetry breaking in a hexagonal-stacked moiré magnet

Symmetry plays a central role in defining magnetic phases, making tunable symmetry breaking across magnetic transitions highly desirable for discovering non-trivial magnetism. Magnetic moiré superlattices, formed by twisting two-dimensional magnetic crystals, have been theoretically proposed and experimentally explored as platforms for unconventional magnetic states. However, despite recent advances, the ability to tune symmetry breaking in moiré magnetism remains limited, as twisted two-dimensional magnets predominantly inherit the magnetic properties and symmetries of their constituent layers. Here, in this work, we demonstrate a clear evolution of symmetry in hexagonal-stacked twisted double-bilayer CrI 3 as the twist angle increases from 180° to 190°. Although the net magnetization remains zero across this twist-angle range, the magnetic phase breaks only the threefold rotational symmetry at 180°, but it breaks all the rotational, mirror and time-reversal symmetries at intermediate twist angles between 181° and 185°, and all broken symmetries are recovered at 190°. This pronounced symmetry breaking at intermediate twist angles is accompanied by metamagnetic behaviour, evidenced by symmetric double hysteresis loops around zero magnetic field. Together, these results reveal that hexagonal-stacked twisted double-bilayer CrI3 at intermediate twist angles hosts a distinct moiré magnetic phase, featuring periodic in-plane spin textures with broken rotational, mirror and time-reversal symmetries.

Sun, Zeliang (ORCID:0009000770748064)↗

DOE OSTI2022

Angularly quantized spin rotations in hexagonal LuMnO 3

Optical control of the spin degree of freedom is often desired in application of the spin technology. Here we report spin-rotational excitations observed through inelastic light scattering of the hexagonal LuMnO 3 in the antiferromagnetically (AFM) ordered state. We propose a model based on the spin–spin interaction Hamiltonian associated with the spin rotation of the Mn ions, and find that the spin rotations are angularly quantized by 60°, 120°, and 180°. Angular quantization is considered to be a consequence of the symmetry of the triangular lattice of the Mn-ion plane in the hexagonal LuMnO 3 . These angularly-quantized spin excitations may be pictured as isolated flat bubbles in the sea of the ground state, which may lead to high-density information storage if applied to spin devices. Optically pumped and detected spin-excitation bubbles would bring about the advanced technology of optical control of the spin degree of freedom in multiferroic materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2016

Optic phonons and anisotropic thermal conductivity in hexagonal Ge 2 Sb 2 Te 5

The lattice thermal conductivity ($κ$) of hexagonal Ge 2 Sb 2 Tesub>5 (h-GST) is studied via direct first-principles calculations. We find significant intrinsic anisotropy of ( $κ$ a /$κ$ c ~2) of $κ$ in bulk h-GST along different transport directions. The dominant contribution to$κ$ is from optic phonons, ~75%. This is extremely unusual as the acoustic phonon modes carry most of the heat in typical semiconductors and insulators with small unit cells. Very recently, Lee et. al. observed anisotropic in GST thin films and attributed this to thermal resistance of amorphous regions near grain boundaries. However, our results suggest an additional strong intrinsic anisotropy for the pure hexagonal phase. This derives from bonding anisotropy along different crystal directions, specifically from weak interlayer coupling, which gives anisotropic phonon dispersions. The phonon spectrum of h-GST has very dispersive optic branches with higher group velocities along the a-axis as compared to flat optic bands along the c-axis. The importance of optic mode contributions for the thermal conductivity in low-$κ$ h-GST is unusual, and development of fundamental physical understanding of these contributions may be critical to better understanding of thermal conduction in other complex layered materials.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Reversible oxygen intercalation in hexagonal Y 0.7 Tb 0.3 MnO 3+δ : toward oxygen production by temperature-swing absorption in air

The oxygen storage capacity, structure and thermodynamic stability were studied for hexagonal Y 0.7 Tb 0.3 MnO 3+δ in oxygen and air to assess its applicability for oxygen separation from air by a temperature-swing adsorption process. We show that large amounts of oxygen excess can be reversibly incorporated into and extracted from small particle-size samples of Y 0.7 Tb 0.3 MnO 3+δ prepared by sol–gel synthesis for fixed oxygen partial pressures. The hyperstoichiometric material, with δ ≥ 0.45 prepared in oxygen or high-pressure oxygen atmospheres, assumes a new hexagonal structure with interstitial oxygen defects near the nominally five coordinated Mn site as determined by neutron diffraction and supported by scanning and transmission electron microscopy. Thermogravimetric measurements demonstrate reversible intercalation of oxygen in a pure O 2 atmosphere at around 300 °C, but more importantly, also in air over a remarkably narrow temperature range of ~20 °C, albeit producing smaller oxygen amounts δ ≤ 0.25. Comparison of samples' properties obtained by the sol–gel and solid-state synthesis methods confirms enhanced oxygen storage capacity and oxygen exchange kinetics for the small particle-size samples which exhibit larger specific surface areas. Sequential temperature-swing absorption and long-term annealing experiments demonstrate the high practical potential of Y 0.7 Tb 0.3 MnO 3 compounds for the industrial production of oxygen enriched gases by utilization of waste heat at 250–350 °C.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

Polymerization in soft nanoconfinement of lamellar and reverse hexagonal mesophases

This work describes the kinetics of thermal polymerization in nanoconfined domains of lyotropic liquid crystal (LLC) templates by using chemorheological studies at different temperatures. Furthermore, we investigate lamellar and reverse hexagonal LLC phases with the same concentration of the monomeric phase. Results show that the mesophase structures remain intact during thermal polymerization with very slight changes in the domain size. The polymerization rate decreases in the nanoconfined structure compared to the bulk state due to the segregation effect, which increases the local monomer concentration and enhances the termination rate. Additionally, the polymerization rate is faster in the studied reverse hexagonal systems compared to the lamellar ones due to their lower degree of confinement. A higher degree of confinement also induces a lower monomer conversion. Differential scanning calorimetry confirms the obtained results from chemorheology.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Hexagonal layered group IV–VI semiconductors and derivatives: fresh blood of the 2D family

New phases of group IV–VI semiconductors in 2D hexagonal structures are predicted and their unusual physical properties are revealed. The structures of monolayer group IV–VI semiconductors are similar to those of blue phosphorene and each unit has the same ten valence electrons. The band gap of 2D hexagonal group IV–VI semiconductors depends on both the thickness and stacking order. Atomic functionalization can induce ferromagnetism, and the Curie temperature can be tuned. Gapped Dirac fermions with zero mass are developed and this makes it exceed that of graphene. Finally, the Fermi velocity can be compared to or even above that of graphene.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2025

Deterministic fabrication of highly reproducible monochromatic quantum emitters in hexagonal boron nitride

Quantum emitters in hexagonal boron nitride are important room temperature single-photon sources. However, conventional fabrication methods yield quantum emitters with dispersed and inconsistent spectral profiles, limiting their potential for practical quantum applications, which demand reproducible high quality single-photon sources. Here, we report the deterministic creation of highly reproducible monochromatic quantum emitters by applying carbon-ion implantation on freestanding hexagonal boron nitride flakes, while a carbon mask with suitable thickness was adapted to optimize the implantation results. Quantum emitters fabricated using this approach exhibited thermally limited monochromaticity, with an emission center wavelength of 590.7 ± 2.7 nm, a narrow full width at half maximum of 7.1 ± 1.7 nm, an emission rate of 1 MHz without optical engineering, and exceptional stability under ambient conditions. Density functional theory calculations and scanning transmission electron microscopy suggest that these emitters are comprised of boron centered carbon tetramers. This method provides a reliable single-photon source for optical quantum computing and potential future industry-scale applications.

Hua, Muchuan [Argonne National Laboratory (ANL), A↗

DOE OSTI2012

Hexagonal and pentagonal shapes of self-catalyzed one-dimensional GaAs nanostructures: Shape dependence of the phase evolutions

Hexagonal and pentagonal shapes of one-dimensional (1-D) nanostructures were observed at the growth of GaAs by molecular beam epitaxy method without catalyst aid. We report on the phase evolution dependence on the shapes of 1-D GaAs nanostructures. The hexagonal-shaped nanostructures showed a transition region composed of twin boundaries and stacking faults during the phase transition from a zinc-blende (ZB) structure to a wurtzite (WZ) structure. On the other hand, the pentagonal-shaped nanostructures appeared to have an abrupt transition from a ZB structure to a WZ structure and to be elongated along <112> directions of a ZB structure.

36 MATERIALS SCIENCE↗

DOE OSTI2012

Controlled growth and multi-photon luminescence of hexagonal arrays of Au nanoparticles on anodic aluminum oxide templates

Au nanoparticles were deposited onto anodic aluminum oxide (AAO) templates by using a rotating sputtering technique. Interestingly, hexagonal arrays of Au nanoparticles were obtained at an appropriate rotating rate and deposition time. Strong three-photon luminescence was observed from the hexagonally arrayed Au nanoparticles, which is attributed to the strong enhancements of local electromagnetic fields at both excitation and emission wavelengths. Our findings provide a new method to prepare Au nanoparticle arrays with large field enhancements and could have prospective applications in plasmonic nanodevices, such as surface-enhanced Raman scattering substrates, and biosensors.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2012

Superstructure of self-aligned hexagonal GaN nanorods formed on nitrided Si(111) surface

We present here the spontaneous formation of catalyst-free, self-aligned crystalline (wurtzite) nanorods on Si(111) surfaces modified by surface nitridation. Nanorods grown by molecular beam epitaxy on bare Si(111) and non-stoichiometric silicon nitride interface are found to be single crystalline but disoriented. Those grown on single crystalline Si{sub 3}N{sub 4} intermediate layer are highly dense c-oriented hexagonal shaped nanorods. The morphology and the self-assembly of the nanorods shows an ordered epitaxial hexagonal superstructure, suggesting that they are nucleated at screw dislocations at the interface and grow spirally in the c-direction. The aligned nanorod assembly shows high-quality structural and optical emission properties.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2013

K and Mn co-doped BaCd{sub 2}As{sub 2}: A hexagonal structured bulk diluted magnetic semiconductor with large magnetoresistance

A bulk diluted magnetic semiconductor was found in the K and Mn co-doped BaCd{sub 2}As{sub 2} system. Different from recently reported tetragonal ThCr{sub 2}Si{sub 2}-structured II-II-V based (Ba,K)(Zn,Mn){sub 2}As{sub 2}, the Ba{sub 1−y}K{sub y}Cd{sub 2−x}Mn{sub x}As{sub 2} system has a hexagonal CaAl{sub 2}Si{sub 2}-type structure with the Cd{sub 2}As{sub 2} layer forming a honeycomb-like network. The Mn concentration reaches up to x ∼ 0.4. Magnetization measurements show that the samples undergo ferromagnetic transitions with Curie temperature up to 16 K. With low coercive field of less than 10 Oe and large magnetoresistance of about −70%, the hexagonal structured Ba{sub 1−y}K{sub y}Cd{sub 2−x}Mn{sub x}As{sub 2} can be served as a promising candidate for spin manipulations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2013

Evidence for graphite-like hexagonal AlN nanosheets epitaxially grown on single crystal Ag(111)

Ultrathin (sub-monolayer to 12 monolayers) AlN nanosheets are grown epitaxially by plasma assisted molecular beam epitaxy on Ag(111) single crystals. Electron diffraction and scanning tunneling microscopy provide evidence that AlN on Ag adopts a graphite-like hexagonal structure with a larger lattice constant compared to bulk-like wurtzite AlN. This claim is further supported by ultraviolet photoelectron spectroscopy indicating a reduced energy bandgap as expected for hexagonal AlN.

36 MATERIALS SCIENCE↗

DOE OSTI2014

Thermal conductivity of ultra-thin chemical vapor deposited hexagonal boron nitride films

Thermal conductivity of freestanding 10 nm and 20 nm thick chemical vapor deposited hexagonal boron nitride films was measured using both steady state and transient techniques. The measured value for both thicknesses, about 100 ± 10 W m{sup −1} K{sup −1}, is lower than the bulk basal plane value (390 W m{sup −1} K{sup −1}) due to the imperfections in the specimen microstructure. Impressively, this value is still 100 times higher than conventional dielectrics. Considering scalability and ease of integration, hexagonal boron nitride grown over large area is an excellent candidate for thermal management in two dimensional materials-based nanoelectronics.

36 MATERIALS SCIENCE↗

DOE OSTI2014

Development and intrinsic properties of hexagonal ferromagnetic (Zr,Ti)Fe{sub 2}

Nanocrystalline Ti{sub 0.75}Zr{sub 0.25}Fe{sub 2+x} (x = 0-0.4) and Ti{sub 0.75−y}B{sub y}Zr{sub 0.25}Fe{sub 2.4} (y = 0-0.35) with high saturation magnetization have been fabricated by the melt-spinning technique. Nanocrystalline Ti{sub 0.75}Zr{sub 0.25}Fe{sub 2+x} consists of the hexagonal C14 Laves phase (Ti,Zr)Fe{sub 2}. Fe addition decreases the lattice parameter a and shrinks the cell volume. The antiferromagnetic Fe-Fe interactions may decrease with the increase of x, leading to a significant enhancement of saturation polarization (J{sub s}) and Curie temperature (T{sub c}). The magnetocrystalline anisotropy constant K also increases with increasing x. Excessive Fe addition (x > 0.25) may induce structural disorder which lowers the J{sub s} and T{sub c}. Nanocrystalline Ti{sub 0.75−y}B{sub y}Zr{sub 0.25}Fe{sub 2.4} is composed of hexagonal (Ti,Zr)Fe{sub 2} and Fe-rich amorphous phases with relatively high J{sub s}. The lattice parameters a, c and cell volume V are almost unchanged with the increase of y for y ≥ 0.16. Simultaneously, the T{sub c} of (Ti,Zr)Fe{sub 2} remains unchanged, indicating that B does not enter this lattice but takes part in forming the amorphous phase, in good agreement with the X-ray diffraction results. The volume fraction of the amorphous phase increases with the increase of B content and results in a large enhancement of J{sub s} up to 10.8 kG. Further B addition (y > 0.30) decreases J{sub s}, possibly due to the decrease of the J{sub s} of the amorphous phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗