Hexagonal Bilayer Research Articles

Formation of porous ice frameworks at room temperature

Bulk crystalline ices with ultralow densities have been demonstrated to be thermodynamically metastable at negative pressures. However, the direct formation of these bulk porous ices from liquid water at negative pressures is extremely challenging. Inspired by approaches toward porous media based on host-guest chemistry, such as metal-organic frameworks and covalent organic frameworks, we herein demonstrate via molecular dynamics simulations that a class of ultralow-density porous ices with upright channels can be formed spontaneously from liquid water at 300 K with the assistance of carbon nanotube arrays. We refer to these porous ice polymorphs as water oxygen-vertex frameworks (WOFs). Notably, our simulations revealed that the liquid-WOF phase transition is first-order and occurs at room temperature. All the WOFs exhibited the unique structural feature that they can be regarded as assemblies of nanoribbons of hexagonal bilayer ice (2D ice I) at their armchair or zigzag edges. Based on density functional theory calculations, a comprehensive phase diagram of the WOFs was constructed considering both the thermodynamic and thermal stabilities of the porous ices at negative pressures. Like other types of porous media, these WOFs may be applicable to gas storage, purification, and separation. Moreover, these biocompatible porous ice networks may be exploited as medical-related carriers.

Induced magnetic states upon electron–hole injection at B and N sites of hexagonal boron nitride bilayer: A density functional theory study

AbstractWe have reported the electronic, magnetic and optical properties of carbon doped bilayer hexagonal boron nitride (h‐BN) using thedensity functional theory. A single C‐doping at B/N sites gives the large band gap similar to dilute magnetic semiconducting behaviour with a finite net magnetic moment of 1.001 and 0.998μB, respectively. For double doping at B/N sites the net magnetic moment increases to 1.998 and 1.824μB, respectively. Upon C‐doping at N‐site, we obtained transition from non‐magnetic semiconductor (pristine) → magnetic semiconductor (1C) → half‐metal ferromagnetic (2C) → metal (3C). In case of the B site, we observed metallic behaviour for 2C‐doping. As 1,2 C‐doping at the B site reduces the energy band gap from 1.8 eV to 0.81 eV, falls in the visible range and offers an opportunity to utilized as a photocatalyst material. C‐doped systems show a magnetic semiconducting behavior crucial for spintronic applications.

Enhanced electron-phonon coupling in doubly aligned hexagonal boron nitride bilayer graphene heterostructure

The relative twist angle in heterostructures of two-dimensional (2D) materials with similar lattice constants result in a dramatic alteration of the electronic properties. Here, we investigate the electrical and magnetotransport properties in bilayer graphene (BLG) encapsulated between two hexagonal boron nitride (hBN) crystals, where the top and bottom hBN are rotationally aligned with bilayer graphene with a twist angle $\theta_t\sim 0^{\circ} \text{and}~ \theta_b < 1^{\circ}$, respectively. This results in the formation of two moir\'e superlattices, with the appearance of satellite resistivity peaks at carrier densities $n_{s1}$ and $n_{s2}$, in both hole and electron doped regions, together with the resistivity peak at zero carrier density. Furthermore, we measure the temperature(T) dependence of the resistivity ($\rho$). The resistivity shows a linear increment with temperature within the range 10K to 50K for the density regime $n_{s1} <n<n_{s2}$ with a large slope d$\rho$/dT $\sim$ 8.5~$\Omega$/K. The large slope of d$\rho$/dT is attributed to the enhanced electron-phonon coupling arising due to the suppression of Fermi velocity in the reconstructed minibands, which was theoretically predicted, recently in doubly aligned graphene with top and bottom hBN. Our result establishes the uniqueness of doubly aligned moire system to tune the strength of electron-phonon coupling and to modify the electronic properties of multilayered heterostructures.

Nonadiabatic superconductivity in a Li-intercalated hexagonal boron nitride bilayer.

When considering a Li-intercalated hexagonal boron nitride bilayer (Li-hBN), the vertex corrections of electron–phonon interaction cannot be omitted. This is evidenced by the very high value of the ratio λωD/εF ≈ 0.46, where λ is the electron–phonon coupling constant, ωD is the Debye frequency, and εF represents the Fermi energy. Due to nonadiabatic effects, the phonon–induced superconducting state in Li-hBN is characterized by much lower values of the critical temperature (TLOVCC ∈ {19.1, 15.5, 11.8} K, for μ* ∈ {0.1, 0.14, 0.2}, respectively) than would result from calculations not taking this effect into account (TMEC∈ {31.9, 26.9, 21} K). From the technological point of view, the low value of TC limits the possible applications of Li-hBN. The calculations were carried out under the classic Migdal–Eliashberg formalism (ME) and the Eliashberg theory with lowest-order vertex corrections (LOVC). We show that the vertex corrections of higher order (λ3) lower the value of TLOVCC by a few percent.

Low-Energy Phases of Bi Monolayer Predicted by Structure Search in Two Dimensions.

We employ an ab-initio structure search algorithm to explore the configurational space of bismuth in quasi-two dimensions. A confinement potential is introduced to restrict the movement of atoms within a predefined thickness to find the stable and metastable forms of monolayer Bi. In addition to the two known low-energy structures (puckered monoclinic and buckled hexagonal), our calculations predict three new phases: α, β, and γ. Each phase exhibits peculiar electronic properties, ranging from metallic (α and γ) to semiconducting (puckered monoclinic, buckled hexagonal, and β)monolayers. Topologically nontrivial features are predicted for buckled hexagonal and γ phases. We also remark on the role of 5d electrons on the electronic properties of Bi monolayer. We conclude that Bi provides a rich playground to study distortion-mediated metal-insulator phase transitions in quasi-2D.

Phase Diagram of Nanoscale Water on Solid Surfaces with Various Wettabilities.

Understanding structural and dynamic properties of water in contact with solid surfaces is essential for diverse fields, including environmental sciences, nanofluidics, lubrication, and electrochemistry. Despite tremendous efforts, how interfacial water phase behaviors correlate with a surface's wettability remains elusive. Here, we investigate the structure and dynamics of nanoscale water droplets or adlayers on solid surfaces with wettabilities spanning from strongly hydrophobic to strongly hydrophilic using extensive molecular dynamics simulations. It is shown that liquid water drops on solid surfaces with contact angles greater than 42.6° transform into drops of ordinary hexagonal ice (Ih) upon cooling. In contrast, water forms a liquid disc on a completely wetted surface with a zero contact angle, which freezes into a hexagonal bilayer ice disc at low temperatures. Unexpectedly, on surfaces with a mild contact angle in the range of 21.9°-29.2°, the originally stable liquid drop at room temperature further wets the surface upon cooling and eventually transforms into a bilayer ice disc. These results establish a phase diagram of nanoscale water at the wettability versus temperature plane, which may expand our knowledge of water-surface interactions as well as enrich the complexity of water behaviors at interfaces.

Direct growth of mm-size twisted bilayer graphene by plasma-enhanced chemical vapor deposition

Plasma enhanced chemical vapor deposition (PECVD) techniques have been shown to be an efficient method to achieve single-step synthesis of high-quality monolayer graphene (MLG) without the need of active heating. Here we report PECVD-growth of single-crystalline hexagonal bilayer graphene (BLG) flakes and mm-size BLG films with the interlayer twist angle controlled by the growth parameters. The twist angle has been determined by three experimental approaches, including direct measurement of the relative orientation of crystalline edges between two stacked monolayers by scanning electron microscopy, analysis of the twist angle-dependent Raman spectral characteristics, and measurement of the Moiré period with scanning tunneling microscopy. In mm-sized twisted BLG (tBLG) films, the average twist angle can be controlled from 0° to approximately 20°, and the angular spread for a given growth condition can be limited to < 7°. Different work functions between MLG and BLG have been verified by the Kelvin probe force microscopy and ultraviolet photoelectron spectroscopy. Electrical measurements of back-gated field-effect-transistor devices based on small-angle tBLG samples revealed high-quality electric characteristics at 300 K and insulating temperature dependence down to 100 K. This controlled PECVD-growth of tBLG thus provides an efficient approach to investigate the effect of varying Moiré potentials on tBLG.

Stress- and electric-field-induced band gap tuning in hexagonal boron phosphide layers

Two-dimensional hexagonal boron phosphide presents great potential in applications of electronics and optoelectronics due to its high carrier mobility and moderate band gap. In this work, we investigate the effect of stress and electric field on the electronic properties of hexagonal boron phosphide layers based on first-principles calculations. We find that both the band gap and the carrier effective masses of hexagonal boron phosphide monolayer gradually increase with stress from compression to tension. As for hexagonal boron phosphide bilayer with two stacking orders (AB_B-P and AB_B-B) upon applied electric field, the band gap monotonously increases with the enhancement of electric field for AB_B-P bilayer, while it undergoes a band gap closing and reopening process for AB_B-B bilayer. We employ the tight-binding model to explain the mechanism of different band gap variations of two stacking orders with electric field. Moreover, we discuss the band gap variation of hexagonal boron phosphide bilayer with combined effect of stress and electric field. The investigation here presents an insight into the effective manipulation towards the electronic properties of hexagonal boron phosphide, which will further enable the broader applications of the hexagonal boron phosphide in modern electronic and optoelectronic fields.

Tight-binding model for electronic structure of hexagonal boron phosphide monolayer and bilayer

Graphene-like hexagonal boron phosphide with its moderate band gap and high carrier mobility is considered to be a high potential material for electronics and optoelectronics. In this work, the tight-binding Hamiltonian of hexagonal boron phosphide monolayer and bilayer with two stacking orders are derived in detail. Including up to fifth-nearest-neighbor in plane and next-nearest-neighbor interlayer hoppings, the tight-binding approximated band structure can well reproduce the first-principle calculations based on the screened Heyd–Scuseria–Ernzerhof hybrid functional level over the entire Brillouin zone. The band gap deviations for monolayer and bilayer between our tight-binding and first-principle results are only 2 meV. The low-energy effective Hamiltonian matrix and band structure are obtained by expanding the full band structure close to the K point. The results show that the iso-energetic lines of maximum valence band in the vicinity of K point undergo a pseudo-Lifshitz transition from h-BP monolayer to AB_B-P or AB_B-B bilayer. The mechanism of pseudo-Lifshitz transition can be attributed to two interlayer hoppings rather than one.

Significant magnetism enhancement and weak spin-orbit coupling effect in Mn13-nCon (n = 0–13) bimetal alloy clusters

Abstract The ground state structures, spin, orbital magnetic moments and magnetic anisotropies of bimetallic clusters Mn13-nCon (n = 0–13) are systematically investigated by using density functional theory (DFT) with and without spin-orbit coupling. The studies reveal a transition from icosahedron (ICO) to the hexagonal bilayer (HBL) structure, following the rule putting Mn in a central position and Co in the surface of icosahedrons. The magnetic moments of clusters increase to the maximum in Mn9Co4 (39µB) and then decrease as a function of Co concentration. Besides, we find that the orbital moments of all clusters are dominated by Co atoms, as the composition of Co increases, the orbital moments increase from 0.01µB (Mn13) to 1.34µB (Co13). The magnetic anisotropies and magnetic anisotropy energies (MAEs) are tiny in all clusters.

High thermoelectric performance in the hexagonal bilayer structure consisting of light boron and phosphorus elements

Two-dimensional layered materials have attracted tremendous attentions due to their extraordinary physical and chemical properties. Using first-principles calculations and Boltzmann transport theory, we give an accurate prediction of the thermoelectric properties of boron phosphide (BP) bilayer, where the carrier relaxation time is treated within the framework of electron-phonon coupling. It is found that the lattice thermal conductivity of BP bilayer is much lower than that of its monolayer structure, which can be attributed to the presence of van der Waals interactions. On the other hand, the graphene-like BP bilayer shows very high carrier mobility with a moderate band gap of 0.88 eV. As a consequence, a maximum p-type ZT value of ~1.8 can be realized along the x-direction at 1200 K, which is amazingly high for systems consisting of light elements only. Moreover, we obtain almost identical p- and n-type ZT of ~1.6 along the y-direction, which is very desirable for fabrication of thermoelectric modules with comparative efficiencies. Collectively, these findings demonstrate great advantages of the layered structures containing earth-abundant elements for environment-friendly thermoelectric applications.

Solvothermal synthesis and photocatalytic properties of ZnO micro/nanostructures

ZnO micro/nanostructures with different morphologies were prepared though a solvothermal process in the presence of surfactants. The structures and compositions were investigated using various techniques, including X-ray diffraction (XRD), a scanning electron microscope (SEM), a UV–vis spectrophotometer, and a fluorimeter. SEM observations revealed that the as-synthesized ZnO samples exhibited nanospheres (NSs), hexagonal disks (HDs), hexagonal bilayer disk-like structures (HBDs), and 3D flower-like hierarchitectures (HAs) due to the effect of the surfactant. XRD results showed that the prepared samples were wurtzite hexagonal phase ZnO with good crystallinity. The band gap energy (Eg) values of the ZnO samples were 3.24 eV, 3.20 eV, 3.15 eV, and 3.24 eV for the ZnO NSs, HDs, HBDs, and 3D HAs, respectively. The photocatalytic properties of the as-synthesized samples were investigated by measuring the degradation on the Rhodamine-B (Rh-B). Among them, the as-synthesized 3D ZnO HAs had the highest photocatalytic activity, which showed a degradation efficiency of 97.34% for Rh-B under UV irradiation. The excellent photocatalytic efficiency is attributed to the hierarchical structures and good crystallinity, which led to the efficient separation of photogenerated electrons and holes.

Surfactant-assisted growth and optical properties of ZnO hexagonal bilayer disk-like microstructures

ZnO hexagonal bilayer disk-like microstructures are successfully fabricated using a simple solvothermal method assisted with surfactant. The structure and morphology were investigated by XRD, SEM, and EDS. XRD result indicated that the as-obtained samples were well-crystallized wurtzite hexagonal ZnO structure. SEM images showed that the ZnO hexagonal bilayer disk-like assembles consist of two uniform and smooth disks with an average edge length of 6 μm and thickness of ∼4 μm. UV–vis spectrum reveals that ZnO sampls show an appreciable red shift and the band gap energy of the obtained ZnO samples were about 3.15 eV. A very strong UV emission at the ultraviolet (UV) region was observed in the photoluminescence (PL) spectrum of the as-prepared ZnO samples tested at room-temperature. A possible growth process of the ZnO hexagonal bilayer disk-like microstructures was schematically illustrated.

Polyvinylpyrrolidone-assisted growth and optical properties of ZnO hexagonal bilayer disk-like microstructures

ZnO hexagonal bilayer disk-like microstructures are successfully fabricated using a simple solvothermal method assisted with polyvinylpyrrolidone (PVP). The structure and morphology were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX). XRD result indicated that the as-obtained samples were well-crystallized wurtzite hexagonal ZnO structure. SEM images showed that the ZnO hexagonal bilayer disk-like assembles consist of two uniform and smooth disks with an average edge length of 6μm and thickness of ∼4μm. UV-vis spectrum reveals that ZnO sampls show an appreciable red shift and the band gap energy of the obtained ZnO samples were about 3.15eV. A very strong ultraviolet (UV) emission at the UV region was observed in the photoluminescence (PL) spectrum of the as-prepared ZnO samples tested at room temperature. A possible growth process of the ZnO hexagonal bilayer disk-like microstructures was schematically illustrated.

Spin-orbit coupling effect on structural and magnetic properties of ConRh13−n (n = 0–13) clusters

The effect of spin-orbit interaction on the structures and magnetism of ConRh13−n (n = 0–13) clusters have been systematically investigated by using the spin-orbit coupling (SOC) implementation of the density functional theory (DFT). The results calculated without SOC (NSOC) show that Rh13 prefers the double simple-cubic configuration, and icosahedron is the favorable structure for n = 1–9, while n ≥ 10, clusters favor the hexagonal bilayer structure. The inclusion of SOC in calculation does not change the geometries of clusters. Compared with that in NSOC calculation, although the binding energy per atom in clusters with same composition decreases in SOC calculation, the relative stability of clusters with different compositions does not change. An interesting result is that the spin moments of clusters for n = 1–9 are almost constant (21 μB). Spin-orbit interaction recovers orbital moment and its anisotropy by removing crystal-field effect in calculation. The destruction of bonding symmetry and relaxation of bonding account for high anisotropies of orbital moments in Co11Rh2 and CoRh12 clusters. With atomic composition (Co/Rh) around 4/9–5/8 and 9/4, the Co-Rh clusters exhibit high magnetic anisotropy energies.

Theoretical prediction of phonon-mediated superconductivity with Tc ≈ 25 K in Li-intercalated hexagonal boron nitride bilayer

A superconducting transition temperature (Tc) up to 25 K in the Li-intercalated bilayer of hexagonal boron nitride (h-BN) is predicted according to ab-initio calculations. A Tc higher than that of metal-intercalated graphene (MIG) is ascribed to the characteristic spatial distribution of electronic states near the Fermi level, which is distinctly different from that in MIG. In the Li-intercalated bilayer h-BN, the breaking of the symmetrical restriction and the increase in the overlap between the charge density and the Li in-plane motion enhance the electron–phonon coupling. Our results provide a new design guideline for two-dimensional superconductors based on intercalated layered materials.

Directing the Structure of Two-Dimensional Silica and Silicates

Density functional theory was used to assess the viability of approaches to controlling the structure of a recently discovered two-dimensional form of SiO2. In accord with prior work, a hexagonal bilayer of mirror image planes of corner-sharing SiO4 tetrahedra in six-membered rings yielded only a slightly higher energy than α-quartz. Structures including four- through eight-membered rings were evaluated and in certain cases found to be as little as 17 meV/Si higher in energy than the hexagonal bilayer. When either biaxial or uniaxial tensile strain was applied, combinations of eight-, six-, and four-membered rings became favored due to the lower density of structures with larger rings. These findings, together with experiments that reveal expansion of silica bilayers to match the lattice of metal substrates, suggest that epitaxial strain may be used to control the bilayer structure. Replacement of Si with Ge and Al as prototypical tetravalent and trivalent dopants was also investigated. Substituting Ge fo...

A many-electron perturbation theory study of the hexagonal boron nitride bilayer system*

In this article we explore methods to reduce the computational cost in many-electron wave function expansions including explicit correlation and compact one-electron basis sets for the virtual orbitals. These methods are applied to the calculation of the interlayer binding energy of the h-BN bilayer system. We summarize the optimized interlayer distances as well as their binding energies for various stacking faults on different levels of theory including second-order Moller-Plesset perturbation theory and the random phase approximation. Furthermore, we investigate the asymptotic behavior of the binding energy at large interlayer separation and find that it decays as D -4 in agreement with theoretical predictions, where D is the interlayer distance.

Hexagonal-shaped monolayer-bilayer quantum disks in graphene: A tight-binding approach

Using the tight-binding approach, we investigate confined states in two different hybrid monolayer-bilayer systems: (i) a hexagonal monolayer area surrounded by bilayer graphene in the presence of a perpendicularly applied electric field and (ii) a hexagonal bilayer graphene dot surrounded by monolayer graphene. The dependence of the energy levels on dot size and external magnetic field is calculated. We find that the energy spectrum for quantum dots with zigzag edges consists of states inside the gap which range from dot-localized states, edge states, to mixed states coexisting together, whereas for dots with armchair edges, only dot-localized states are observed.

Cooling Growth of Millimeter-Size Single-Crystal Bilayer Graphene at Atmospheric Pressure

Single-crystal bilayer graphene is attractive for functional electronic and photonic devices because of its high carrier mobility as well as a tunable band gap induced by a perpendicular electric field. Chemical vapor deposition (CVD) has been increasingly reported for the production of single-crystal bilayer graphene, but it is limited with respect to grain size because of nonsynchronic growth between the second layer and the first layer of bilayer graphene. In this research, we established a nonisothermal atmosphere CVD (NI-APCVD) system to realize synchronization of nucleation and growth of single-crystal bilayer graphene that contains the surface adsorption for the decomposed carbons and the shallow bulk phase segregation for the dissolved carbons. Based on the above process engineering, we successfully produced large hexagonal single-crystal bilayer graphene (∼1.0 mm) on the polycrystalline Cu foils. This work paves a pathway to the synchronous growth of wafer-scale single-crystal bilayer graphene, w...