Hexagonal Layers Research Articles

Interfacial thermal conductance across hexagonal boron nitride & paraffin based thermal energy storage materials

It is of vital importance to understand the interfacial thermal conductance between the high thermal conductivity fillers and the phase change thermal energy storage materials in order to enhance the energy storage efficiency. The interfacial thermal conductance across low-dimensional hexagonal boron nitride layers (h-BN) and n-octadecane was calculated with reverse non-equilibrium molecular dynamics methods in this paper. The influences of size effect, molecular arrangement of n-octadecane, layer number of h-BN as well as the defects modified with various functional groups on the interfacial thermal conductance within the temperature range from 273.15 to 373.15 K were investigated, respectively. The results showed that the interfacial thermal conductance of all systems increase with rising temperature and the size effect is inconspicuous. And systems with n-octadecane molecules parallel to the h-BN surface possess the highest interfacial thermal conductance with respect to the vertical and amorphous arrangements. In addition, the interfacial thermal conductance will reduce when the layer number of h-BN increases. In terms of the effect of defects, the systems with h-BN containing the defects modified with hydrogen atoms, i.e., lighter atom, possess higher interfacial thermal conductance. At last, for interpreting deeper mechanism, the vibration power spectrum of the velocity autocorrelation function and the components in different directions were also calculated.

Study of one-step pyrolysis porous boron nitride with large specific surface area

Porous boron nitride (p-BN) is gaining an enormous attention due to its application in gas absorption, photocatalysis, and drug delivery. However, complicated preparation steps and low specific surface area have been restricting its application. Detailed morphology and structure change and mechanism of pyrolysis process have never been discussed before. Therefore, the facile one-step pyrolysis method was introduced to fabricate the p-BN with high specific surface area. The results demonstrated that the as-synthesized samples with high purity was prepared at 1500 °C. Unpredictable outcome suggested that the pyrolysis temperature instead of precursor ratio has remarkable effects on samples. The structure changed from amorphous form into hexagonal layer and the morphology transformed from granular into flake when temperature varied from 1000 °C to 1500 °C. Its specific surface area and total pore volume reached to 1138.4 m2 g−1 and 0.7287 cm3 g−1, respectively. Explicit mechanism was investigated as well. And an intriguing phenomenon showed that the mass loss of as-prepared p-BN partly recovered at high temperature, which enclosed its unique thermal stability. Implications of the results suggest that this work proposed a facile route to prepare p-BN with high specific surface area.

Laser‐Assisted Multilevel Non‐Volatile Memory Device Based on 2D van‐der‐Waals Few‐Layer‐ReS2/h‐BN/Graphene Heterostructures

AbstractFew‐layer rhenium disulfide (ReS2) field‐effect transistors with a local floating gate (FG) of monolayer graphene separated by a thin hexagonal boron nitride tunnel layer for application to a non‐volatile memory (NVM) device are designed and investigated. FG‐NVM devices based on two‐dimensional van‐der‐Waals heterostructures have been recently studied as important components to realize digital electronics and multifunctional memory applications. Direct bandgap multilayer ReS2 satisfies various requirements as a channel material for electronic devices as well as being a strong light‐absorbing layer, which makes it possible to realize light‐assisted optoelectronic applications. The NVM operation with a high ON/OFF current ratio, a large memory window, good endurance (>1000 cycles), and stable retention (>104 s) are observed. The successive program and erase states using 10 ms gate pulses of +10 V and −10 V are demonstrated, respectively. Laser pulses along with electrostatic gate pulses provide multibit level memory access via opto‐electrostatic coupling. The devices exhibit the dual functionality of a conventional electronic memory and can store laser‐pulse excited signal information for future all‐optical logic and quantum information processing.

In situ nanoscale imaging of moir\xe9 superlattices in twisted van der Waals heterostructures

Direct visualization of nanometer-scale properties of moiré superlattices in van der Waals heterostructure devices is a critically needed diagnostic tool for study of the electronic and optical phenomena induced by the periodic variation of atomic structure in these complex systems. Conventional imaging methods are destructive and insensitive to the buried device geometries, preventing practical inspection. Here we report a versatile scanning probe microscopy employing infrared light for imaging moiré superlattices of twisted bilayers graphene encapsulated by hexagonal boron nitride. We map the pattern using the scattering dynamics of phonon polaritons launched in hexagonal boron nitride capping layers via its interaction with the buried moiré superlattices. We explore the origin of the double-line features imaged and show the mechanism of the underlying effective phase change of the phonon polariton reflectance at domain walls. The nano-imaging tool developed provides a non-destructive analytical approach to elucidate the complex physics of moiré engineered heterostructures.

Boron Nitride Monolayers: Charge State Control of F16CoPc on h‐BN/Cu(111) (Adv. Mater. Interfaces 15/2020)

The image visualizes three parameters used to control the electronic structure of adsorbed metal-organic complexes, namely the nanostructured hexagonal boron nitride support, the tip of a scanning probe microscope, and the molecular environment. The templating functionality of the atomically thin hexagonal boron nitride layer is highlighted by the array of individual fluorinated Co-phthalocyanine molecules in the background. The tip (top right) gates and probes the molecule's electronic structure, which can be tuned by the molecular arrangement. This effect is indicated by the green and red density-functional-theory-derived density contours of the molecular trimer in the foreground, where the electronic structure of the central molecule (in red) differs from the one of the outer molecules (in green). In addition, an insight into the chemical structure of the metal-organic complexes is provided. More details can be found in article number 2000080 by Yinying Wei, Willi Auwärter, and co-workers.

Universal description of potential energy surface of interlayer interaction in two-dimensional materials by first spatial Fourier harmonics

We propose a hypothesis that the potential energy surface (PES) of interlayer interaction in diverse 2D materials can be universally described by the first spatial Fourier harmonics. This statement (checked previously for the interactions between graphene and hexagonal boron nitride layers in different combinations) is verified in the present paper for the case of hydrofluorinated graphene (HFG) bilayer with hydrogen bonding between fluorine and hydrogen at the interlayer interface. The PES for HFG bilayer is obtained through density functional theory calculations with van der Waals corrections. An analytical expression based on the first Fourier harmonics describing the PES which corresponds to the symmetry of HFG layers is derived. It is found that the calculated PES can be described by the first Fourier harmonics with the accuracy of 3\% relative to the PES corrugation. The shear mode frequency, shear modulus and barrier for relative rotation of the layers to incommensurate states of HFG bilayer are estimated. Additionally it is shown that HFG bilayer is stable relative to the formation of HF molecules as a result of chemical reactions between the layers.

Probing the physical and superconducting properties of hexagonal ZrRuAs: A first-principles calculation

In the presented work, ab initio pseudopotential calculations have been realized on structural, electronic, elastic, mechanical, phonon and electron-phonon interaction properties for ZrRuAs with the hexagonal ZrNiAl-type layer crystal structure. In the vicinity of the Fermi level, Ru 4d and Zr 4d states dominate and mainly contribute to the conduction properties of ZrRuAs. A critical assessment of elastic and mechanical properties for ZrRuAs reveals that, this superconductor is soft (ductile) due to remarkable metallic bonding. Phonon anomalies have been observed in the phonon spectrum of ZrRuAs for the lowest transverse acoustic branch and low-frequency transverse optical branches along the Γ-K and Γ-M-K symmetry directions. Our electron-phonon interaction calculations indicated that ZrRuAs is a conventional phonon-mediated superconductor with strong electron-phonon coupling parameter of 1.32. The strong electron-phonon coupling parameter of ZrRuAs produces its superconducting transition temperature to be 11.12 K which accords nicely with its experimental values of 12 K.

Crystal structure of the [(THF)Cs(μ-η5:η5-Cp')3Yb] n oligomer.

The green compound poly[(tetra-hydro-furan)-tris-[μ-η5:η5-1-(tri-methyl-sil-yl)cyclo-penta-dien-yl]caesium(I)ytterbium(II)], [CsYb(C8H13Si)3(C4H8O)] n or [(THF)Cs(μ-η5:η5-Cp')3YbII] n was synthesized by reduction of a red THF solution of (C5H4SiMe3)3YbIII with excess Cs metal and identified by X-ray diffraction. The compound crystallizes as a two-dimensional array of hexa-gons with alternating CsI and YbII ions at the vertices and cyclo-penta-dienyl groups bridging each edge. This, based off the six-electron cyclo-penta-dienyl rings occupying three coordination positions, gives a formally nine-coordinate tris-(cyclo-penta-dien-yl) coordination environment to Yb and the Cs is ten-coordinate due to the three cyclo-penta-dienyl rings and a coordinated mol-ecule of THF. The complex comprises layers of Cs3Yb3 hexa-gons with THF ligands and Me3Si groups in between the layers. The Yb-C metrical parameters are consistent with a 4f 14 YbII electron configuration.

Observation of the Interlayer Exciton Gases in WSe2 -p:WSe2 Heterostructures

Interlayer excitons (IXs) possess a much longer lifetime than intralayer excitons due to the spatial separation of the electrons and holes, and hence they have been pursued to create exciton condensates for decades. The recent emergence of 2D materials, such as transition-metal dichalcogenides (TMDs), and of their van der Waals heterostructures, in which two different 2D materials are layered together, has created new opportunities to study IXs. Here we present the observation of IX gases within two stacked structures consisting of hBN/WSe2/hBN/p:WSe2/hBN. The IX energies of the two different structures differed by 82 meV due to the different thicknesses of the hexagonal boron nitride spacer layer between the TMD layers. We demonstrate that the lifetime of the IXs is shortened when the temperature and the pump power increase. We attribute this nonlinear behavior to an Auger process.

Magnetic, thermal, and electronic-transport properties of EuMg2Bi2 single crystals

The trigonal compound EuMg2Bi2 has recently been discussed in terms of its topological band properties. These are intertwined with its magnetic properties. Here detailed studies of the magnetic, thermal, and electronic transport properties of EuMg2Bi2 single crystals are presented. The Eu{+2} spins-7/2 in EuMg2Bi2 exhibit an antiferromagnetic (AFM) transition at a temperature TN = 6.7 K, as previously reported. By analyzing the anisotropic magnetic susceptibility chi data below TN in terms of molecular-field theory (MFT), the AFM structure is inferred to be a c-axis helix, where the ordered moments in the hexagonal ab-plane layers are aligned ferromagnetically in the ab plane with a turn angle between the moments in adjacent moment planes along the c axis of about 120 deg. The magnetic heat capacity exhibits a lambda anomaly at TN with evidence of dynamic short-range magnetic fluctuations both above and below TN. The high-T limit of the magnetic entropy is close to the theoretical value for spins-7/2. The in-plane electrical resistivity rho(T) data indicate metallic character with a mild and disorder-sensitive upturn below Tmin = 23 K. An anomalous rapid drop in rho(T) on cooling below TN as found in zero field is replaced by a two-step decrease in magnetic fields. The rho(T) measurements also reveal an additional transition below TN in applied fields of unknown origin that is not observed in the other measurements and may be associated with an incommensurate to commensurate AFM transition. The dependence of TN on the c-axis magnetic field Hperp was derived from the field-dependent chi(T), Cp(T), and rho(T) measurements. This TN(Hperp) was found to be consistent with the prediction of MFT for a c-axis helix with S = 7/2 and was used to generate a phase diagram in the Hperp-T plane.

Ab Initio Calculations of Carbon Bilayers with Diamond-Like Structures

Ab initio calculations of monoatomic and diatomic carbon layers composed of four-coordinated (sp3 hybridized) atoms are reported. According to the calculations, monoatomic diamond-like layers L4, L3–6, and L3–4–6 are unstable even at a temperature of 0 K. The structure of diatomic layers can be formed by the polymerization of hexagonal graphene layers. The analysis of all possible ways of cross-linking graphene layer pairs shows that only one diatomic layer can be stable. The crystal lattice of this layer belongs to the p6/mmm symmetry group (four atoms in a hexagonal unit cell, a = 2.737 A). The surface density, sublimation energy, and direct band gap values of the layer are 0.123 µg/cm2, 6.64 eV/atom, and 3.37 eV, respectively. The hexagonal structure of the diamond-like layer is predicted to be stable up to 250 K at normal pressure. The powder XRD pattern of the diamond-like layers is calculated to be used for their experimental identification in carbon materials.

Fast Production of High Performance LiNi0.815Co0.15Al0.035O2 Cathode Material via Urea-Assisted Flame Spray Pyrolysis

The high throughput and rapid flame-assisted spray pyrolysis method has been adapted to synthesize cathode materials LiNi0.apCo0.15Al0.035O2 (NCA). This method is considered low cost and simple. By varying the precursor solution concentration and sintering temperature, the optimal condition was established at temperature sintering of 800 °C and precursor solution concentration of 1 M. X-ray diffraction patterns showed the as-prepared NCA particles exhibit a pure well-ordered hexagonal layer structure with high crystallinity. Polyhedral shaped micro-sized particles are confirmed by SEM images. Galvanostic charge–discharge tests were conducted using cylindrical full-cell utilizing artificial graphite as the anode. The highest specific initial discharge capacity measured between 2.7 and 4.3 V is 155 mAh g−1 with capacity retention of 92% after cycled at 0.2 C for 50 cycles. Thus, this method is considered as a satisfying approach for NCA mass production.

P2-type Na0.59Co0.20Mn0.77Mo0.03O2 cathode with excellent cycle stability for sodium-ion batteries

Layered P2-type Mn-based material is considered as one of the most promising cathode materials for sodium-ion batteries, but its unsatisfactory cyclic performance is one of the problems that need to be overcome. Here, we used molybdenum doping strategy to synthesize P2-type Na0.59Co0.20Mn0.77Mo0.03O2 material with a hexagonal layer structure as the cathode of sodium-ion batteries through a solid-state reaction route. The maximum discharge capacity of the Mo-doped cathode is 131.9 mAh g−1 with the capacity retention rate of 91.51% after 100 cycles at 0.1 C (1 C = 156 mAh g−1). Optimized Mo doping effectively decreases the charge transfer resistance and the interface resistance of the Na0.59Co0.20Mn0.80O2 cathode at open circuit and after 100 cycles, respectively, and the growth trends of the charge transfer resistance and the interface resistance are obviously slowed down, which is proved that the cycling stability of the pristine cathode is significantly improved by Mo doping. The main reasons are as follows: (1) Mo doping expands the a-axis and shortens the TM–O bond, which enhances the structural stability of the P2-type oxide; (2) Mo doping increases the crystallinity of the pristine and decreases the degree of cation disorder in the P2-type oxide; (3) the bond energy of Mo–O is stronger than that of Mn–O and Co–O, resulting in a more stable structure of the material. The results provide a meaningful reference for improving the cycle stability and kinetic performance of P2-type cathode materials for sodium-ion batteries.

Synthesis of hexagonal boron nitride 2D layers using polymer derived ceramics route and derivatives

Hexagonal boron nitride (h-BN) is nowadays an increasingly attractive material, especially for two-dimensional material applications, due to its intrisic properties. However, its properties are highly dependent on the used synthesis approach. The polymer derived ceramics (PDCs) route allows elaboration of h-BN with tailored textural and structural properties. Here, we demonstrate the interest of the PDCs pathway for the synthesis of h-BN. Growth of h-BN single crystals with crystal sizes of a few microns at relatively low temperature and atmospheric pressure is successfully achieved from borazine precursor using PDCs. The crystallization is improved by additivation of 5 wt% of Li3N to the pre-ceramic polymer. Furthermore, by coupling PDCs with gas pressure sintering, starting from the same pre-ceramic polymer and 25 wt% of Li3N, the crystal size is enlarged up to hundreds of microns. The fabricated single crystals of pure h-BN can then be exfoliated into h-BN nanosheets. Finally, by combining PDCs with atomic layer deposition, functional BN nano-/hetero-structures are successfully synthesized from highly structured sensitive templates, making this ALD process a promising alternative for fabricating functional BN nanostructures.

Atomic layer deposition of thermoelectric layered cobalt oxides

Layered cobalt oxides based on the hexagonal CoO2 layer, e.g., NaxCoO2 and [CoCa3O3]0.62CoO2 (or “Ca3Co4O9”), are promising thermoelectric materials. Here, the authors investigate the atomic layer deposition (ALD) of these materials in a thin-film form; this is not trivial, in particular, for the former compound, as both Na and Co are little challenged as components of ALD thin films. The authors employ diketonate precursors for all the metal constituents and ozone as the source of oxygen. In both cases, a postdeposition heat-treatment in O2 is applied to get crystalline coatings; the processes are found amazingly robust in terms of metal precursor pulsing ratios. A striking difference between the two processes is the resultant morphology: while the Ca3Co4O9 films grow highly homogeneous and smooth, the NaxCoO2 coatings exhibit a rather unique reproducible 10–20 μm scale channel-like island structure for all x values investigated. Finally, the authors characterized their ALD Ca3Co4O9 films for their chemical, structural, and physical property details not previously reported.

Structure and morphology of calcium-silicate-hydrates cross-linked with dipodal organosilanes

Coupling of organic and inorganic chemistry presents a new degree of freedom in nano-engineering of thermo-mechanical properties of cement-based materials. Despite these vast technological benefits, molecular scale cross-linking of calcium-silicate-hydrate (C-S-H) gel with organic molecules still presents a significant challenge. Herein, we report experimental results on sol-gel synthesis, structure and morphology of nanocrystalline C-S-H cross-linked with dipodal organosilanes. These novel organic-inorganic gels have layered turbostratic molecular structure with similarities to C-S-H precipitating in hydrating cement paste. The organic molecules' chain length controls the interlayer spacing, which shows little to no shrinkage upon dehydration up to 105 °C. However, the structure gets distorted in the basal crystallite plane, in which dimer and trimer Si-polyhedra structures condense on a 2D hexagonal Ca-polyhedra layer. Cross-linked C-S-H gels display plate-like morphology with tendency toward stacking into agglomerates at the larger scale. If successfully realized in cement environment, e.g. high concentration seed, such novel organic-inorganic C-S-H gels could potentially provide cement-based matrices with unique properties unmatched by classical inorganic systems.

Electrostatic properties and stability of Coulomb crystals

AbstractWe calculate the electrostatic properties of more than 20 different Coulomb crystals and study their resistance to small oscillations of the ions around their equilibrium positions (phonon oscillations). We discuss the stability of multicomponent crystals against separation into set of one‐component lattices and for some cases, the influence of energy of the zero‐point vibrations. It is confirmed that the body‐centred cubic (bcc) lattice possesses the lowest electrostatic energy among all one‐component (one type of ion in the elementary cell) crystals. For systems composed of two types of ions (their charge and density numbers are Z1, n1 and Z2, n2, respectively) and for n1 = n2, it is found that the formation of a binary bcc lattice is possible at 1/2.4229 < α < 2.4229, where α ≡ Z2/Z1. Under the same conditions, the NaCl lattice forms at α > 5.197 and α < 0.192. While for n2 = 2n1, the MgB2 lattice is found be stable at 0.1 < α < 0.32. For multicomponent lattices with hexagonal structure (binary hexagonal close‐packed, MgB2 and some others lattices), it is shown that their properties depend on the distance between hexagonal layers and this distance changes with α.

Influence of annealing temperature of ZnS-coated TiO2 films on the performance of dye-sensitized solar cells

This work deals with ZnS-coated TiO2 films prepared via liquid phase deposition and immersion techniques and then applied as photoanode of dye-sensitized solar cells. The samples underwent annealing treatment at various annealing temperatures. The morphology of TiO2 is nanograss and covered by agglomerate and hexagonal ZnS layer. The anatase crystallite size, absorption peak wavelength and optical reflectance increase with annealing temperature. However, the energy gap decreases with the temperature. The device utilizing the sample annealed at 400 °C performed the highest η of 0.24 % due to this device owns the lowest leak current and the longest carrier lifetime.

Mechanical properties of [formula omitted]-bonded carbon and boron nitride 2D membranes: A first principles study

Transverse compression and chemical passivation of few graphene or hexagonal boron nitride layers have been shown to be viable means to form ultrathin membranes rich in sp3 bonds exhibiting intriguing mechanical behaviors. Here, density functional theory calculations are used to calculate second- and third-order elastic constants, as well as ideal breaking strengths of both layered and sp3-bonded arbon and boron nitride films. The sp3-bonded membranes consist of two or three planes of both existing and new in silico-designed bulk structures of carbon and boron nitride, and have either both, one, or none of the surfaces passivated with hydrogen. This study shows that carbon and boron nitride membranes rich in sp3 bonds exhibit mechanical properties and ideal breaking strengths that compete with those of sp2-bonded layered films, and that there is a plethora of plausible stable structures of such sp3-bonded membranes, showing diverse electronic properties, and longitudinal and transverse mechanical behaviors. This study also suggests that by controlling thickness of the film and by tuning chemistry of the supporting substrate, transverse compression of layered carbon or hexagonal boron nitride films could lead to the formation of stable sp3-bonded 2D materials exposing clean surfaces, and therefore transferable, and exhibiting intriguing anisotropic mechanical and electronic properties.

Molecular Nanowire Bonding to Epitaxial Single-Layer MoS2 by an On-Surface Ullmann Coupling Reaction.

Lateral heterostructures consisting of 2D transition metal dichalcogenides (TMDCs) directly interfaced with molecular networks or nanowires can be used to construct new hybrid materials with interesting electronic and spintronic properties. However, chemical methods for selective and controllable bond formation between 2D materials and organic molecular networks need to be developed. As a demonstration of a self-assembled organic nanowire-TMDC system, a method to link and interconnect epitaxial single-layer MoS2 flakes with organic molecules is demonstrated. Whereas pristine epitaxial single-layer MoS2 has no affinity for molecular attachment, it is found that single-layer MoS2 will selectively bind the organic molecule 2,8-dibromodibenzothiophene (DBDBT) in a surface-assisted Ullmann coupling reaction when the MoS2 has been activated by pre-exposing it to hydrogen. Atom-resolved scanning tunneling microscopy (STM) imaging is used to analyze the bonding of the nanowires, and thereby it is revealed that selective bonding takes place on a specific S atom at the corner site between the two types of zig-zag edges available in a hexagonal single layer MoS2 sheet. The method reported here successfully combining synthesis of epitaxial TMDCs and Ullmann coupling reactions on surfaces may open up new synthesis routes for 2D organic-TMDC hybrid materials.