Boron Nitride Bilayer Research Articles

First principles study of BN triphenylene-graphdiyne monolayer and bilayer structures with varying C-chain lengths: insights into optical behavior

First-principles calculations engaging density functional theory (DFT) are employed to systematically study the optical characteristics of monolayer and bilayer boron nitride (BN) triphenylene-graphdiyne (Tp-BNyne) structures featuring varying lengths of C-chains. The thermal stability of Tp-BNyne structures at temperatures up to 1000 K is verified. The weak van der Waals interactions due to the small binding energies and significant interlayer distances maintain the cohesion between the layers. The investigation revealed that all Tp-BNyne structures under examination exhibit semiconductor behavior with a band gap in the range of 0.97–2.74 eV. The bilayer configurations demonstrated a narrower energy band gap in comparison to the monolayer ones. Increasing the length of C-chains leads to a reduction in the energy band gap. Delving into the optical behavior of Tp-BNyne structures under photon incidence with parallel and perpendicular polarizations, a distinct anisotropy in the optical characteristics of Tp-BNyne is revealed. The static dielectric constant increases and the optical band gap decreases with increasing C-chain length. The absorption coefficients of monolayer and bilayer Tp-BNyne structures, on the order of 107/m, demonstrate that these sheets can effectively absorb light in the visible and ultraviolet regions. These findings present Tp-BNyne sheets as promising candidates for use in photovoltaic devices to convert sunlight into electrical current, as well as for designing optical devices for ultraviolet protection. Additionally, Tp-BNyne structures are transparent materials, especially in the high-energy range.

Tunable Ultrafast Charge Transfer across Homojunction Interface.

Charge transfer at heterojunction interfaces is a fundamental process that plays a crucial role in modern electronic and photonic devices. The essence of such charge transfer lies in the band offset, making charge transfer uncommon in a homojunction. Recently, sliding ferroelectricity has been proposed and confirmed in two-dimensional van der Waals stacked materials such as bilayer boron nitride. During the sliding of these layers, the band alignment shifts, creating conditions for charge separation at the interface. We employ ab initio nonadiabatic molecular dynamics simulations to elucidate the excited state carrier dynamics in bilayer boron pnictides. We propose that, akin to ferroelectric polarization flipping, the precise modulation of the distribution of excited state carriers can also be reached by sliding. Our results demonstrate that sliding induces a reversal of the frontier orbital distribution on the upper and lower layers, facilitating a robust interlayer carrier transfer. Notably, the interlayer carrier transfer is more pronounced in boron phosphide than in boron nitride, attributed to strong electron scattering in momentum space in boron nitride. We propose this novel method to manipulate carrier distribution and dynamics in a homojunction exhibiting sliding ferroelectricity, in general, paving a new way for developing advanced electronic and photonic devices.

Comparison of lattice thermal conductivity using ab-initio DFT, machine learning interatomic potentials, and temperature dependent effective potential: a case study of hexagonal BN and BP bilayer

Discovering high thermal conductivity materials is essential for various practical applications, particularly in electronic cooling. The significance of two-dimensional (2D) materials lies in their unique properties that emerge due to their reduced dimensionality, making them highly promising for a wide range of applications. Hexagonal boron nitride (BN), both monolayer and bilayer forms, has garnered attention for its fascinating properties. In this work, we focus on bilayer boron phosphide (BP), which is isostructural to its BN analogue. The lattice thermal conductivity of both bilayer BN and BP have been calculated using ab-initio density functional theory, machine learning with the moment tensor potential method, and the temperature-dependent effective-potential method (TDEP). The TDEP approach gives more accurate results for both BN and BP materials. The lattice thermal conductivity of bilayer BP is lower than that of bilayer BN at room temperature, attributed to increased phonon anharmonicity. This study highlights the importance of understanding phonon scattering mechanisms in determining the thermal conductivity of 2D materials, contributing to the broader understanding and potential applications of these materials in future technologies.

Ultrafast high-endurance memory based on sliding ferroelectrics.

The persistence of voltage-switchable collective electronic phenomena down to the atomic scale has extensive implications for area- and energy-efficient electronics, especially in emerging nonvolatile memory technology. We investigate the performance of a ferroelectric field-effect transistor (FeFET) based on sliding ferroelectricity in bilayer boron nitride at room temperature. Sliding ferroelectricity represents a different form of atomically thin two-dimensional (2D) ferroelectrics, characterized by the switching of out-of-plane polarization through interlayer sliding motion. We examined the FeFET device employing monolayer graphene as the channel layer, which demonstrated ultrafast switching speeds on the nanosecond scale and high endurance exceeding 1011 switching cycles, comparable to state-of-the-art FeFET devices. These characteristics highlight the potential of 2D sliding ferroelectrics for inspiring next-generation nonvolatile memory technology.

Manipulating the Stacking in Two-Dimensional Hexagonal Boron Nitride Bilayers: Implications for Defect-Based Single Photon Emitters

Manipulating the Stacking in Two-Dimensional Hexagonal Boron Nitride Bilayers: Implications for Defect-Based Single Photon Emitters

Functionalized hexagonal boron nitride bilayers: desirable electro-optical properties for optoelectronic applications.

Structural, electronic, and optical properties of functionalized hexagonal boron nitride (h-BN) bilayer were deeply explored by carrying out the PBE + G0W0 + BSE calculations. Hydrogenation/hydrofluorination/fluorination can cause the planar h-BN bilayer to form a novel diamane-like monolayer by interfacial sp3 atom bonding. These functionalized h-BN bilayers are estimated to be stable dynamically due to their phonon dispersions. The functionalization on h-BN bilayer can induce its electronic nature to be transformed from an indirect wide-gap insulator to direct narrow-gap semiconductor, which is desirable for its application in optoelectronics. In particular, hydrogenated and hydrofluorinated h-BN bilayers have strong absorbance coefficients for the near-infrared and visible part of the incident sunlight (larger than 105 cm-1). More interestingly, the binding energy of the observed first bright exciton can achieve a value beyond 1 eV, which can effectively reduce the recombination of photogenerated electron-hole pairs. These results are potentially important for extending the applications of the h-BN bilayer in optoelectronic devices.

Effect of temperature on tensile and vibration properties of bilayer boron nitride

AbstractHexagonal boron nitride (h‐BN) is a semiconductor material with a wide band gap, holding promising potential for applications in thermal conductivity devices and nanoresonators in the field of microelectronics. Here, molecular dynamics is simulated to investigate the tensile and vibrational behaviors of bilayer h‐BN under five different stacking modes across varying temperatures. The mechanical properties of five different stacking modes of h‐BN at various temperatures are focused on, including Young's modulus, the ultimate stress, and the ultimate strain. Results indicate that bilayer h‐BN nanosheets exhibit anisotropic characteristics, with their tensile properties decreasing as temperature increases. Additionally, we explore the influence of temperature on the natural frequency of bilayer h‐BN under five different stacking modes. These results establish a fundamental understanding of the mechanical and vibrational characteristics of bilayer h‐BN nanosheets under different stacking modes, contributing to their potential applications in advanced nanodevices operating in extremely high‐temperature environments.

Enumeration of Moiré patterns of a hexagonal twisted bilayer: Applications to intercalated transition metals

A real-space method using generating integers is used to determine possible commensurate lattice Moiré patterns for a bilayer of two equal hexagonal lattices, which can in principle be extended to lattice mismatched bilayers. These Moiré patterns can be classified by a pair of relatively prime integers (n,m), wherein a rotation θ(n,m) of the top hexagonal lattice maps its lattice vector (n,m) to (m,n) of the bottom lattice. Within this formulation, the area of the commensurate supercell is proportional to (n2+m2+nm) and the number of coincident lattice sites per supercell is given by (n−m)2. Taking bilayer boron nitride (BN) as an example, we present how to systematically generate Moiré patterns and explore the differences in local chemistry in the interstitial region by impurity intercalation. Systematic calculations of the properties of intercalated 3d transition metals were performed in an h-BN (4,3) bilayer, corresponding to a rotation of 9.43 degrees. These calculations reveal that local symmetry in the intercalated region significantly affect the energetics and magnetization of the intercalated species. These results highlight that Moiré pattern physics is not limited to optoelectronic/electronic phenomena, such as interfacial exciton formation or magic angle superconductivity, but it also produces chemical and magnetic atomic site selectivity, which may play important roles in adsorption, catalysis, or quantum information.

Orbital distortion and electric field control of sliding ferroelectricity in a boron nitride bilayer

Recent experiments confirm that two-dimensional boron nitride (BN) films possess room-temperature out-of-plane ferroelectricity when each BN layer is sliding with respect to each other. This ferroelectricity is attributed to the interlayered orbital hybridization or interlayer charge transfer in previous work. In this work, we attempt to understand the sliding ferroelectricity from the perspective of orbital distortion of long-pair electrons. Using the maximally localized Wannier function method and first-principles calculations, the out-of-plane pz orbitals of BN are investigated. Our results indicate that the interlayer van der Waals interaction causes the distortion of the N pz orbitals. Based on the picture of out-of-plane orbital distortion, we propose a possible mechanism to tune the ferroelectric polarization by external fields, including electric field and stress field. It is found that both the polarization intensity and direction can be modulated under the electric field. The polarization intensity of the system can also be controlled by stress field perpendicular to the plane. This study will provide theoretical help in the device design based on sliding ferroelectrics.

Structural classification of boron nitride twisted bilayers and ab initio investigation of their stacking-dependent electronic structure

Since the discovery of superconductive twisted bilayer graphene which initiated the field of twistronics, moiré systems have not ceased to exhibit fascinating properties. We demonstrate that in boron nitride twisted bilayers, for a given moiré periodicity, there are five different stackings which preserve the monolayer hexagonal symmetry (i.e. the invariance upon rotations of 120°) and not only two as always discussed in literature. We introduce some definitions and a nomenclature that identify unambiguously the twist angle and the stacking sequence of any hexagonal bilayer with order-3 rotation symmetry. Moreover, we employ density functional theory to study the evolution of the band structure as a function of the twist angle for each of the five stacking sequences of boron nitride bilayers. We show that the gap is indirect at any angle and in any stacking, and identify features that are conserved within the same stacking sequence irrespective of the angle of twist.

Moiré patterns and inversion boundaries in graphene/hexagonal boron nitride bilayers

In this paper a systematic examination of graphene/hexagonal boron nitride (g/hBN) bilayers is presented, through a recently developed two-dimensional phase field crystal model that incorporates out-of-plane deformations. The system parameters are determined by closely matching the stacking energies and heights of g/hBN bilayers to those obtained from existing quantum-mechanical density functional theory calculations. Out-of-plane deformations are shown to reduce the energies of inversion domain boundaries in hBN, and the coupling between graphene and hBN layers leads to a bilayer defect configuration consisting of an inversion boundary in hBN and a domain wall in graphene. Simulations of twisted bilayers reveal the structure, energy, and elastic properties of the corresponding moir\'e patterns and show a crossover as the misorientation angle between the layers increases from a well-defined hexagonal network of domain boundaries and junctions to smeared-out patterns. The transition occurs when the thickness of domain walls approaches the size of the moir\'e patterns and coincides with the peaks in the average von Mises and volumetric stresses of the bilayer.

The encapsulation of atomically-thin MgB2-based superconductors in two-dimensional material bilayers

Two-dimensional (2D) superconductors are a class of materials with unique properties that can potentially exhibit novel superconductivity at a reduced dimensionality, as well as pave the way for miniaturizing superconducting devices at the atomic limit. However, such materials are highly sensitive to interaction with their substrates and the environment, and therefore it is necessary to find potential methods for chemically isolating them. We address this problem by examining the possibility of isolating the medium-temperature 2D MgB2-based superconductors. We examine two possible structures: the monolayer stoichiometric MgB2 and the B-caged Mg3B8 structure. By solving the anisotropic Eliashberg equations, we predict that the latter possesses K upon application of a small lateral (3% tensile) strain, with an electron–phonon coupling of 0.88. To investigate protecting the superconductivity of these two layers, we apply density functional theory calculations to examine their encapsulation in a graphene bilayer or a hexagonal boron nitride bilayer. We find that both have high potential as encapsulation systems to protect the superconductivity of the 2D MgB2-based superconductors, in particular B-caged (B–Mg) n systems.

Charge and Spin Current Rectification through Functionalized Boron Nitride Bilayers

Charge and Spin Current Rectification through Functionalized Boron Nitride Bilayers

Moiré and non-twisted sp3-hybridized structures based on hexagonal boron nitride bilayers: Ab initio insight into infrared and Raman spectra, bands structures and mechanical properties

We investigated hydrogenated twisted (21.8°) and non-twisted bilayer structures consisting of two hexagonal boron nitride (hBN) monolayers with interlayer covalent bonds. These structures, named bornitranes, are boron nitride analogues of well-known carbon diamanes. We found that the binding energy of twisted structures was about 0.5 eV/BN higher than that of non-twisted AA’- and AB-stacked hBN layers. According to our calculations, the fully hydrogenated Moiré bornitrane with the 21.8° twisted angle has a flattened valence band and an indirect band gap of about 2.8 eV. This value is almost twice wider than that for their non-twisted counterparts. The computed Young’s modulus of bornitranes is higher than that for the hBN monolayer. The infrared and Raman spectra of the considered systems were also defined. Spectral fingerprints of the Moiré bornitrane possess many peaks due to the presence of distorted interlayer bonds and are radically different from the spectra of non-twisted systems. Calculated peaks can facilitate experimental detection of the Moiré bornitranes. Investigated structures are novel 2D semiconductors suitable for nanoelectronics and optoelectronics applications.

Giant Manipulation of Phonon Hydrodynamics in Ferroelectric Bilayer Boron Nitride at Room Temperature and Beyond

Giant Manipulation of Phonon Hydrodynamics in Ferroelectric Bilayer Boron Nitride at Room Temperature and Beyond

Ab Initio Study of Graphene/hBN Van der Waals Heterostructures: Effect of Electric Field, Twist Angles and p-n Doping on the Electronic Properties.

In this work, we study the structural and electronic properties of boron nitride bilayers sandwiched between graphene sheets. Different stacking, twist angles, doping, as well as an applied external gate voltage, are reported to induce important changes in the electronic band structure near the Fermi level. Small electronic lateral gaps of the order of few meV can appear near the Dirac points K. We further discuss how the bandstructures change applying a perpendicular external electric field, showing how its application lifts the degeneracy of the Dirac cones and, in the twisted case, moves their crossing points away from the Fermi energy. Then, we consider the possibility of co-doping, in an asymmetric way, the two external graphene layers. This is a situation that could be realized in heterostructures deposited on a substrate. We show that the co-doping acts as an effective external electric field, breaking the Dirac cones degeneracy. Finally, our work demonstrates how, by playing with field strength and p-n co-doping, it is possible to tune the small lateral gaps, pointing towards a possible application of C/BN sandwich structures as nano-optical terahertz devices.

Moiré and Non-Twisted Sp3-Hybridized Structures Based on Hexagonal Boron Nitride Bilayers: Ab Initio Insight into Infrared and Raman Spectra, Bands Structures and Mechanical Properties

Moiré and Non-Twisted Sp3-Hybridized Structures Based on Hexagonal Boron Nitride Bilayers: Ab Initio Insight into Infrared and Raman Spectra, Bands Structures and Mechanical Properties

Quasi-Unidirectional Transport Bilayer Two-Dimensional Nanopores for Highly-Efficient Molecular Sieving

Two-dimensional nanopores are very promising for high-permeance molecular sieving, but the molecular backflow from permeate-side to feed-side is not beneficial for improving molecular permeance. We study the quasi-unidirectional molecular transport through a graphene-hexagonal boron nitride bilayer nanopore, aiming to realize a high-permeance molecular sieving. Molecular dynamics simulations of CO2/CH4 separations show that the bilayer pore presents 3.7 times higher selectivity comparing to the single-layer graphene nanopore with the same size. The quasi-unidirectional molecular transport is attributed to the distinctive adsorption abilities of gas molecules on the two sides of bilayer nanopores and the inhibited molecular backflow from permeate-side to feed-side. This work provides a promising way to realize the ultra-permeable porous membranes with molecular permeance even higher than the single-layer atomic-thickness membranes.

Stacking-engineered ferroelectricity in bilayer boron nitride.

2D ferroelectrics with robust polarization down to atomic thicknesses provide building blocks for functional heterostructures. Experimental realization remains challenging because of the requirement of a layered polar crystal. Here, we demonstrate a rational design approach to engineering 2D ferroelectrics from a non-ferroelectric parent compound via employing van der Waals assembly. Parallel-stacked bilayer boron nitride exhibits out-of-plane electric polarization that reverses depending on the stacking order. The polarization switching is probed via the resistance of an adjacently stacked graphene sheet. Twisting the boron nitride sheets by a small angle changes the dynamics of switching thanks to the formation of moiré ferroelectricity with staggered polarization. The ferroelectricity persists to room temperature while keeping the high mobility of graphene, paving the way for potential ultrathin nonvolatile memory applications.

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.