Hexagonal Layers Research Articles

Pressure-stabilized hexagonal perovskite-related isolated tetrahedral anion silicate La6Sr3Si6O24

The discovery of new perovskite compounds under high pressure mainly focuses on the ABO3 compositions and the compositions highly deviated from ABO3 are less explored. Here we demonstrate that the La6Sr3Si6O24 silicate composition can be stabilized as a hexagonal perovskite-related structure with isolated tetrahedra anions under high pressure of 6 GPa. The compound adopts 9-layer shifted hexagonal perovskite-like structure with both B-cation and oxygen deficiencies and contains pseudo-cubic (c′) (La/Sr)O2 layers and hexagonal (h) (La/Sr)O3 layers stacked according to (c′hh)3 sequence. This structure features both B-cation vacancy ordering between the two consecutive hexagonal layers and oxygen vacancy ordering in c′-(La/Sr)O2 layers, resulting in isolated tetrahedral SiO4 anions and ionic conduction behavior. This work demonstrates the practicability of accessing new perovskite-related functional materials from the compositions highly deviated from ABO3 under high pressure.

Fe 5 Ge 2 Te 2 : Iron‐rich Layered Chalcogenide for Highly Efficient Hydrogen Evolution

Recent research on van der Waals (vdW) metal chalcogenides electrocatalysts for the hydrogen evolution reaction (HER) has been devoted to finding new catalysts with active basal planes. Here, we report on experimental and theoretical investigations of the HER activity of a recently discovered iron-rich vdW spintronic material, Fe5Ge2Te2 (FG2T) in alkaline media (1 M KOH). We show that a densified electrode of FG2T requires an overpotential of only −90.5 mV to drive a current density of 10 mA/cm2. Free energy calculations of hydrogen adsorption using density functional theory (DFT) proved that the numerous sites present on the hexagonal Te layer are more active than those found in the recently proposed Fe3GeTe2 (FGT) catalyst, supporting higher activity for the new Fe-richer catalyst. Like in FGT, XPS analysis has found that a thin oxide layer covers the active FG2T layers, suggesting the real active surface to be a hybrid FG2T/oxide layer. These results strengthen the idea of continued screening of iron-based vdW materials to replace the non-abundant platinum group electrocatalysts toward HER and other electrocatalytic processes.

2D Homologous Series SrFMnBiSn+2 (M = Pb, Ag0.5Bi0.5; n = 0, 1) and Commensurately Modulated Sr2F2Bi2/3S2.

We report three new mixed-anion two-dimensional (2D) compounds: SrFPbBiS3, SrFAg0.5Bi1.5S3, and Sr2F2Bi2/3S2. Their structures as well as the parent compound SrFBiS2 were refined using single-crystal X-ray diffraction data, with the sequence of SrFBiS2, SrFPbBiS3, and SrFAg0.5Bi1.5S3 defining the new homologous series SrFMnBiSn+2 (M = Pb, Ag0.5Bi0.5; n = 0, 1). Sr2F2Bi2/3S2 has a different structure, which is modulated with a q vector of 1/3b* and was refined in superspace group X2/m(0β0)00 as well as in the 1 × 3 × 1 superstructure with space group C2/m (with similar results). Sr2F2Bi2/3S2 features hexagonal layers of alternating [Sr2F2]2+ and [Bi2/3S2]2-, and the modulated structure arises from the unique ordering pattern of Sr2+ cations. SrFPbBiS3, SrFAg0.5Bi1.5S3, and Sr2F2Bi2/3S2 are semiconductors with band gaps of 1.31, 1.21, and 1.85 eV, respectively. The latter compound exhibits room temperature red photoluminescence at ∼700 nm.

In situ measurements of electrical resistivity of metals in a cubic multi-anvil apparatus by van der Pauw method.

On the basis of the van der Pauw method, we developed a new technique for measuring the electrical resistivity of metals in a cubic multi-anvil high-pressure apparatus. Four electrode wires were introduced into the sample chamber and in contact with the pre-pressed metal disk on the periphery. The sample temperature was measured with a NiCr-NiSi (K-type) thermocouple, which was separated from the sample by a thin hexagonal boron nitride layer. The electrodes and thermocouple were electrically insulated from each other and from the heater by an alumina tube as well. Their leads were in connection with cables through the gap between the tungsten carbide anvils. We performed experiments to determine the temperature dependence of electrical resistivity of pure iron at 3 and 5 GPa. The experiments produce reproducible measurements and the results provide an independent check on electrical resistivity data produced by other methods. The new technique provides reliable electrical resistivity measurements of metallic alloys and compounds at high pressure and temperature.

Superfluidity of Dipolar Excitons in a Double Layer of α - T3 with a Mass Term.

We predict Bose-Einstein condensation and superfluidity of dipolar excitons, formed by electron-hole pairs in spatially separated gapped hexagonal (GHAT3) layers. In the model, the AB-honeycomb lattice structure is supplemented with C atoms located at the centers of the hexagons in the lattice. We considered the model in the presence of a mass term which opens a gap in the energy-dispersive spectrum. The gap opening mass term, caused by a weak magnetic field, plays the role of Zeeman splitting at low magnetic fields for this pseudospin-1 system. The band structure of GHAT3 monolayers leads to the formation of two distinct types of excitons in the GHAT3 double layer. We consider two types of dipolar excitons in double-layer GHAT3: (a) “A excitons”, which are bound states of electrons in the conduction band (CB) and holes in the intermediate band (IB), and (b) “B excitons”, which are bound states of electrons in the CB and holes in the valence band (VB). The binding energy of A and B dipolar excitons is calculated. For a two-component weakly interacting Bose gas of dipolar excitons in a GHAT3 double layer, we obtain the energy dispersion of collective excitations, the sound velocity, the superfluid density, and the mean-field critical temperature for superfluidity.

Domains of the adsorbed benzene monolayer on graphene

We consider the adsorbed benzene monolayer on graphene and show that the monolayer’s structure can be very complicated even for benzene, the simplest representative of the cyclic hydrocarbons. We base this on previous the first principal investigations, which found different energy states of the adsorbed benzene molecule. We find the existence of the six domains (star domains) for the stable molecular monolayer. The representatives of these domains (grains) at the graphene hexagonal layer are star vectors that describe transitions from symmetric high energy state into the stable low symmetric position. In the hexagon lattice of any benzene close pack domain, we introduce the primitive unit vectors, which are twice longer than the graphene primitive unit ones. The symmetry elements of the adsorbed benzene monolayer on graphene are found, left and right domains are introduced. Besides, half-translation of the benzene domains on the graphene primitive unit vectors leads to a further increase in the variety of the domain types. We introduce the selection procedure to decrease the number of the possible domains, find zero-block cells, and the complete number of the benzene domains — eight. We find all types of the structural domains and show their relation with the graphene unit cell construction.

Neutron irradiation induced magnetization and persistent defects at high temperatures in graphite

Structural as well as magnetization studies have been carried out on graphite samples irradiated by neutrons for over 50 years in the CIRUS research reactor at Trombay, India. Neutron diffraction studies reveal that the defects in irradiated graphite samples are not well annealed and remain significant up to high temperatures much greater than 653 K where the Wigner energy is completely released. We infer that the remnant defects may be intralayer Frenkel defects, which do not store large energy, unlike the interlayer Frenkel defects that store the Wigner energy. Magnetization studies on the irradiated graphite show ferromagnetic behavior that persists up to \ensuremath{\sim}850 K and a large additional paramagnetic contribution. Ab initio calculations based on the spin-polarized density-functional theory show that the magnetism in defected graphite is essentially confined to a single twofold-coordinated carbon atom that is located around a vacancy in the hexagonal layer.

Hexagonal Layer Manganese Metal-Organic Framework for Photocatalytic CO2 Cycloaddition Reaction.

A novel manganese metal–organic framework (Mn-MOF) termed UAEU-50 assembled from a benzenedicarboxylate linker (BDC) and trinuclear manganese clusters was synthesized and fully characterized using different spectroscopic and analytic techniques (e.g., X-ray powder diffraction, UV–vis diffuse reflectance spectroscopy, thermogravimetric analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy). UAEU-50 adopted a hexagonal layer structure and exhibited superior thermal stability and robust chemical stability. Photocatalytic activities of UAEU-50 were investigated using the cycloaddition of CO2 to different epoxides, forming cyclic carbonates. Impressively, UAEU-50 can transform up to 90% photocatalytic CO2 conversion to cyclic carbonates in the visible-light region at ambient conditions.

Magnetoexcitons in phosphorene monolayers, bilayers, and van der Waals heterostructures

We study direct and indirect excitons in Rydberg states in phosphorene monolayers, bilayers, and van der Waals (vdW) heterostructure in an external magnetic field within the framework of the effective-mass approximation. The magnetic field is applied perpendicular to the monolayer or heterostructure and is varied between 0 and 60 T. Binding energies of magnetoexcitons are calculated by numerical integration of the Schr\"odinger equation using the Rytova-Keldysh potential for direct magnetoexcitons and both the Rytova-Keldysh and Coulomb potentials for indirect magnetoexcitons. The latter aids in the understanding of the role of screening in phosphorene. We report the magnetic field energy contribution to the binding energies and diamagnetic coefficients (DMCs) for magnetoexcitons, which depend strongly on the effective anisotropic masses of electrons and holes and can be tuned by the external magnetic field. We demonstrate that the vdW phosphorene heterostructure is a novel category of two-dimensional semiconductors with magnetoexcitonic binding energies tunable by means of the external magnetic field. The binding energies and DMCs are controlled by the number of hexagonal boron nitride layers separating two phosphorene sheets. Such tunability is potentially useful for the device design.

Interaction of Na with 2D WO3 and MoO3 Layers on Pd(100): From Doping to 2D Bronze Formation

Doping of tungsten trioxide (WO3) and molybdenum trioxide (MoO3) materials with alkali atoms, leading to the formation of the so-called sodium bronzes, is a viable approach to achieve a precise control of their electronic, optical, and magnetic properties via electron band structure engineering. Driven by the ongoing trend for thickness reduction and the resulting new functionalities at the nanoscale, using a combination of state-of-the-art experimental and computational techniques, we investigate here the interaction of two isostructural two-dimensional (2D) WO3 and MoO3 layers, grown epitaxially onto a Pd(100) surface, with Na dopants. We identify two interaction regimes as a function of the Na coverage: a low-coverage regime up to 0.3 ML, which we describe in terms of doping interactions, and a reaction regime, where at higher Na coverages, the 2D WO3/MoO3 lattices become destroyed and several ordered 2D bronze-type phases form upon thermal activation. In the doping regime, Na initially decorates the oxide domain boundaries and later adsorbs in a (2 × 2) superstructure, filling the regular adsorption sites within the oxide domains. Further Na accommodation in the 2D oxide lattice is unfavorable due to the poor lateral electrostatic screening and elastic strain increase. In the reaction regime, the most prominent and energetically stable phase is the hexagonal 2D bronze-like layer, whose atomic details are resolved in a density functional theory (DFT) analysis and compared with the structure of the bulk counterpart.

Thermal Evaporation–Deposited Hexagonal CDS Buffer Layer to Boost Performance of Sb2(S,Se)3 Solar Cells

Thermal Evaporation–Deposited Hexagonal CDS Buffer Layer to Boost Performance of Sb2(S,Se)3 Solar Cells

Synthesis of Atomically Thin h-BN Layers Using BCl3 and NH3 by Sequential-Pulsed Chemical Vapor Deposition on Cu Foil.

The chemical vapor deposition of hexagonal boron nitride layers from BCl3 and NH3 is highly beneficial for scalable synthesis with high controllability, yet multiple challenges such as corrosive reaction or by-product formation have hindered its successful demonstration. Here, we report the synthesis of polycrystalline hexagonal boron nitride (h-BN) layers on copper foil using BCl3 and NH3. The sequential pulse injection of precursors leads to the formation of atomically thin h-BN layers with a polycrystalline structure. The relationship between growth temperature and crystallinity of the h-BN film is investigated using transmission electron microscopy and Raman spectroscopy. Investigation on the initial growth mode achieved by the suppression of precursor supply revealed the formation of triangular domains and existence of preferred crystal orientations. The possible growth mechanism of h-BN in this sequential-pulsed CVD is discussed.

ТЕРМОДИНАМИЧЕСКИЙ АНАЛИЗ ПРОЦЕССОВ ГИДРАТАЦИИ АЛЮМОФЕРРИТА КАЛЬЦИЯ

The processes of hydration and hydrate phase formation of calcium aluminoferrites are insufficiently covered in the technical literature. Thermodynamic analysis of the hydration processes of calcium aluminoferrite of 4CaO·Al2O3·Fe2O3 is difficult due to the unreliability of the initial data for hydration products. The question of the basicity of calcium hydroferrites requires clarification. There is insufficient data on the justification of the method of calculating the thermodynamic properties of hydroferrite -∆G0298, - ∆H0298 and c(T), verification of their results, the mechanism of formation of the complex anion Fe(OH)4- and its stability in the liquid phase of hydrating C4AF.
 The hydration of 4CaO·Al2O3·Fe2O3 with the formation of 2CaO·Al2O3·8H2O as hydrate phases is considered. The energy of hydrogen bonds between the layers of Ca(OH)2, Al2(OH)3 and Fe(OH)3, from which the hexagonal layers of these complex calcium, aluminum and iron hydroxides are composed, is compared. It is shown that the values of ∆G0298 and ∆H0298 2CaO·Fe2O3·8H2O, which are equal to -937.2 and -1082.3 kcal/mol, respectively, are not consistent with experimental data on the solubility of C2FH8 and the specific heat release during C4AF hydration (100-115 kcal/g). Values for C2FH8, ∆G0298= -933, ∆H0298 = -1072 kcal/mol are recommended. It is shown that the formation of Fe(OH)3 hydroxide in the amorphous state, rather than the Fe(OH)4 ion, is preferable as an intermediate product of C4AF hydration.

Lattice Polarity Manipulation of Quasi-vdW Epitaxial GaN Films on Graphene Through Interface Atomic Configuration.

Quasi van der Waals epitaxy, a pioneering epitaxy of sp3 -hybridized semiconductor films on sp2 -hybridized 2D materials, provides a way, in principle, to achieve single-crystal epilayers with preferred atom configurations that are free of substrate. Unfortunately, this has not been experimentally confirmed in the case of the hexagonal semiconductor III-nitride epilayer until now. Here, it is reported that the epitaxy of gallium nitride (GaN) on graphene can tune the atom arrangement (lattice polarity) through manipulation of the interface atomic configuration, where GaN films with gallium and nitrogen polarity are achieved by forming CONGa(3) or COGaN(3) configurations, respectively, on artificial CO surface dangling bonds by atomic oxygen pre-irradiation on trilayer graphene. Furthermore, an aluminum nitride buffer/interlayer leads to unique metal polarity due to the formation of an AlON thin layer in a growth environment containing trace amounts of oxygen, which explains the open question of why those reported wurtzite III-nitride films on 2D materials always exhibit metal polarity. The reported atomic modulation through interface manipulation provides an effective model for hexagonal nitride semiconductor layers grown on graphene, which definitely promotes the development of novel semiconductor devices.

Influence of slicing parameters on surface quality and mechanical properties of 3D-printed CF/PLA composites fabricated by FDM technique

ABSTRACT This present study focuses on evaluation of mechanical properties of three dimensional (3D)-printed carbon fiber/polylactic acid (CF/PLA) composites, using fused deposition modeling (FDM) technique. The composites were prepared with different slicing parameters: layer heights or thicknesses, infill densities and layer patterns. The 3D-printed composite samples were subjected to tensile, flexural and interlaminar shear strength (ILSS) tests to assess the influence of the process parameters on their mechanical characteristics. Further investigations were carried out to evaluate the effect of surface roughness of the samples. From the test results, it was evident that both rectilinear and hexagonal layer patterns exhibited better mechanical properties at infill density and layer thickness of 60% and 0.64 mm, respectively. The scanning electron microscopy (SEM) images depicted that lesser layer thickness produced poor CF/PLA interfacial bonding and major failure mode was traced to fiber pull-out. Therefore, engineering application of the composite samples depends on their slicing parameters.

An investigation of the coupling of phonon-polaritons with plasmon-polaritons in hBN/nanopatterned Au layered devices

Thin hexagonal boron nitride layers have been shown to support highly confined hyperbolic phonon-polaritons, which are of interest for light guiding applications. Localized plasmon resonances in nanopatterned metal films can exhibit subwavelength-scale confinement as well as a high local field strength that is of importance to imaging and sensor applications. In this work, the interaction between hyperbolic phonon-polaritons in a hexagonal boron nitride thin film and plasmon-polaritons in a nanopatterned gold thin film is investigated by means of finite-difference time-domain simulations of a series of coupled and uncoupled layered devices. Both far-field and near-field properties are calculated and analyzed, enabling the features due to plasmon-polaritons and phonon-polaritons, individually, to be distinguished and the coupling between these excitations to be explored and characterized.

Effects of the Gas Molecules (H2, CO, NO) and Transition Metal Atoms (Cr, Au) Adsorbed on Hexagonal Boron Nitride Layers

The adsorption characteristics of various gas molecules (H2, CO, NO) and transition metal atoms (Au, Cr) in respect of hexagonal boron nitride monolayers (hBN-MLs) were studied with computation and experimental methods. The obtained results show that the defective hBN-MLs strongly adsorbed H2 molecules and Cr atoms. This hints that those gas molecules and metal atoms play a leading role in the growth mechanism of hBN thin film deposition. This new finding provides a better understanding for the growth mechanism of hBN film using a physical vapor deposition technique with different gas mixtures and substrates. Moreover, the hBN films grew on Si(100) and Au/Cr bilayer substrates, this aims to exam the calculated data. The adsorption properties of hBN-MLs in respect of those gas molecules and transition metal atoms were exploited to convince that the defective hBN-MLs are also able to rectify some gases that are highly dependent on the density of B and N vacancies.

Role of metal contacts on the electric and thermoelectric response of hBN/WSe2 based transistors

Transition metal dichalcogenides represent an emergent platform for energy conversion solutions at the nanoscale. The thermoelectric performances of devices based on two-dimensional materials rely not only on the electric and thermal properties of the used materials but also on device engineering. In actual devices, hybridization effects at the semiconductor/metal interface strongly affect the local band structure with important consequences on charge injection and thermoelectric response. Here, we investigate the role of different metal contacts (Ag, Pd, Co, Ti) on the electric and thermoelectric properties of hexagonal boron nitride-supported few layers WSe2 transistors. In our devices, we reveal a metal contact-dependent Seebeck response with high values of the Seebeck coefficient (S), up to ∼180μV/K, and power factors (PF=S2σ) as high as 2.4μW/cmK2 (Co), in agreement with the state-of-the-art. Metal electrodes for which weak interface hybridization is theoretically expected (Ag) show the lowest electrical conductivity and the highest Seebeck coefficient. On the opposite, for expected strong interface hybridization (Pd, Co, Ti), electrical conductivity increases and slightly reduced S values are measured. Our work unveils the importance of metal contacts engineering to optimize the thermoelectric performances of actual few layers transition metal dichalcogenides based transistors.

Memory effect of vertically stacked hBN/QDs/hBN structures based on quantum-dot monolayers sandwiched between hexagonal boron nitride layer

The characteristics of a flexible write-once-read-many-times (WORM) memory fabricated with monolayered 0-dimensional (0D) CdSe–ZnS quantum dots (QDs) layers sandwiched between two insulating 2-dimensional (2D) hexagonal boron nitride (hBN) multilayers were investigated by electrical measurement method. The hBN/QDs monolayer/hBN structure was fabricated in a vertical stacked structure using a technique which control the formation of the QDs monolayer. QDs monolayer was formed by electrostatic interaction between the negative charge group on the CdSe–ZnS QDs surface and the positive charge group on the hBN surface. The device has a WORM characteristic due to the presence of QDs in the current-voltage (I–V) measurement. When a bias is applied, carriers were initially trapped by tunneling due to the QDs and then a conductive filament was formed in the hBN, which were not detrapped and exhibit characteristics of write-once-read-many-times memory. The maximum ON/OFF ratio of the current for the devices was as large as 4 × 10, and the endurance was 5 × 10 4 cycles, and a retention time was larger than 1 × 10 5 s. In order to explain the carrier transport mechanism and conductive filament of the WORM memory device caused by QDs, it through various methods such as I–V fitting data, simulation, and conductive AFM. Unlike the conventional conductive filament mechanism, through random diffusion such as Ag filament, the Au/hBN/QD/hBN/ITO/PET structures implemented a consistent conductive filament using Au metal and QDs active layer.

Electrical Properties of LaS‐TaS2 Misfit Layered Compound Nanotubes

AbstractSeveral nanotubular structures from chalcogenide‐based misfit layer compounds (MLC) were reported in recent years. MLCs consist of a stacking of two alternating and dissimilar (2D) atomic layers, e. g. one with rocksalt structure (MX) and the other‐ TX2 – with hexagonal layer structure. The layers are held together by weak van der Waals forces, i. e. they can be exfoliated with scotch‐tape. Furthermore, in analogy to intercalation compounds, partial charge transfer between the layers with dissimilar work function results also in polar forces between the MX and TX2 layers. The mismatch between the alternating (asymmetric) layers and the seaming of the dangling bonds at the edges drives them to form tubular (and also scroll‐like) structures. New structural characterization whereby the nanotubes were bisected into lamella via focused ion beam and examined by TEM, are reported.