Nitride Bilayer Research Articles

Phonon and electronic properties of semiconducting silicon nitride bilayers

The two-dimensional (2D) IV-V semiconductors have attracted much attention due to their fascinating electronic and optical properties. In this work, we predicted three phases of silicon nitrides, denoted α-Si2N2, β-Si2N2, and γ-Si4N4, respectively. Both α-Si2N2 and β-Si2N2 consist of two buckled SiN sheets, and γ-Si4N4 consists of two puckered SiN sheets. It is challenging to transform between α-Si2N2 and β-Si2N2 because of the high energy barrier. The three dynamically stable bilayers are semiconductors with fundamental indirect band gaps from 0.25 eV to 2.92 eV. As expected, only the s and p orbitals contribute to the electronic states, and the pz orbitals dominate near the Fermi level. Furthermore, insulator-metal transitions occur in α-Si2N2 and β-Si2N2 under the biaxial strain of 16 %. These materials perhaps have potential applications in microelectronics and spintronics.

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.

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.

Relation between Mechanical Hardening and Nitrogen Profile of PBII Nitrided Titanium Alloy.

Surface treatments of Ti-6Al-4V alloys are of utmost importance for biomedical applications since they allow for tribological gain. Here, Ti-6Al-4V disks have been PBII nitrided at either 500, 600, 700 and 800 °C. A set of techniques (XRD, SEM-EDS, EBSD and GDOES) was used to characterize the surface microstructural and chemical changes. Nanoindentation was used to assess the induced changes in terms of mechanical properties. Two types of nitrided domains are revealed. Starting from the surface, a nitride bilayer composed of δ-TiN/ϵ-Ti2N with enhanced surface resistance is supported by an α-Ti(N) solid solution formed at depth. Hardness values peak at 12-14 GPa at the surface, which is almost twice as large as the bulk value (about 7 GPa). For the moderate temperatures used here, a deep (10-15 µm) and strong hardness (14 GPa) enhancement together with a smooth gradient can be achieved.

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.

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.

Tribo-Piezoelectricity in Group III Nitride Bilayers: A Density Functional Theory Investigation.

The notable out-of-plane piezoelectric effect caused by the large electronegativity of the constituent elements makes two-dimensional (2D) group III nitrides appealing for nanoscale energy-harvesting applications. Here, we demonstrate by extensive density functional theory investigations that the vertical piezoelectricity is enhanced significantly in 2D XN (X = B, Al, Ga) bilayers due to in-plane interlayer sliding. The sliding operation generates tribological energy from the vertical resistance force between the monolayers. A maximum shear strength between the monolayers of 1-25 GPa is recorded during vertical sliding. We elucidate the tribo-piezoelectricity generation mechanism of XN bilayers using the tribological energy conversion to overcome the interfacial sliding barrier. The strongest out-of-plane piezoelectricity is found when the bilayers are in the A-A stacking arrangement. Any reduction in the interlayer distance between group III nitride bilayers enhances out-of-plane polarization due to the increase in sliding energy resistances, leading to an increased inductive voltage output. An induced voltage of ∼3.5 V is achieved during vertical compressive sliding of the upper layer. Using these phenomena, we present a compression-slide XN bilayer nanogenerator strategy capable of tuning the produced tribo-piezoelectric energy through sliding and compression.

Microhardness, Resistivity and Tribological Properties of Coatings Based on Hf/Hfn Bilayers Deposited By Sputtering

In this work, coatings based on hafnium/hafnium nitride (Hf/HfN) bilayers were synthesized by the magnetron sputtering technique, producing coatings with 4, 8 and 16 Hf/HfN bilayers on AISI 316L steel and silicon (100). The surface morphology was studied, which shows a high uniformity of grain cluster groupings. Their microhardness was evaluated where the hardness of the 316L steel was improved by more than 3 orders of magnitude, with a value of 6.71 GPa for 8 Hf/HfN bilayers, this same condition reached a high value of electrical resistivity of 6.55x1014 µΩ-cm, a reduction in the coefficient of friction by about 25% with respect to 316L and presented a total delamination of the coating at 19.77 N, which could allow these coatings to be canted for corrosion protection of 316L steel.

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

­­­­ Ultra-Broadband and Highly Efficient Silicon Nitride Bi-Layer Grating Couplers

­­­­ Ultra-Broadband and Highly Efficient Silicon Nitride Bi-Layer Grating Couplers

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.

Ultrafast Interlayer Charge Separation, Enhanced Visible-Light Absorption, and Tunable Overpotential in Twisted Graphitic Carbon Nitride Bilayers for Water Splitting.

Moiré pattern superlattice formed by 2D van der Waals layered structures have attracted great attention for diverse applications. In experiments, the enhancement of catalytic performance in twisted bilayer systems is reported while its mechanism remains unclear. From high-accuracy first-principles and time-dependent ab initio nonadiabatic molecular dynamics calculations, ultrafast interlayer charge transfer within 120 fs, excellent charge separation, improved visible-light absorption, and satisfactory overpotentials for the hydrogen evolution and oxygen evolution reactions in twisted graphitic carbon nitride (g-C3 N4 ) bilayers are found, which are beneficial to photocatalytic, photo-electrocatalytic, or electrocatalytic water splitting. This work provides insightful guidance to advanced nanocatalysis based on twisted layered materials.

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.

Effects of the tantalum intermediate layer on the nanomechanical properties and biocompatibility of nanostructured tantalum/tantalum nitride bilayer coating deposited by magnetron sputtering on the nickel titanium alloy

Nanostructured tantalum nitride coatings are widely used in biomedical engineering due to the good hardness, wear resistance, and corrosion resistance. In this work, TaN and Ta/TaN nanostructured coatings were deposited on the NiTi alloy by magnetron sputtering for 150 min at a pressure of 3.8 × 10–3 mbar using a power of 143 W. The phase, structure, morphology, and thickness were evaluated by X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM), respectively. The nanomechanical properties such as Young's modulus (E) and hardness were determined by nano-indentation and nano-scratch tests. To assess the biological properties of the coatings, the antimicrobial activity was monitored by the disk diffusion antibiogram technique using E.coli as the model gram-negative bacteria and S. aureus as the gram-positive bacteria and the biocompatibility was also studied. A homogeneous, uniform, and crack-free of TaN monolayered and Ta/TaN bilayered coating with a thickness 1050 and 1310 nm, respectively, were produced by magnetron sputtering. As a result of the Ta middle layer in the Ta/TaN coating, the hardness increased by 22% from 6.3 ± 0.5 GPa to 7.6 ± 0.5GPa under a load of 700 µN and 6% from 6.1 ± 0.2 GPa to 6.5 ± 0.3 GPa under a load of 1000 µN. The critical load also increased from 3.7 ± 0.2 to 4.1 ± 0.6 N in the presence of the Ta intermediate layer which improved the adhesion strength of the Ta/TaN bilayer compared to the TaN monolayer. In addition, the Ta intermediate layer increased the flexibility leading to a adhesive wear mechanism for the friction behavior of Ta/TaN. The bacteriostatic ability against gram-negative bacteria and bacteriocidal ability against gram-positive bacteria increase. Our results demonstrate that the nanostructured Ta/TaN bilayer coating provides a better bioactive surface for the growth and proliferation of MG-63 ossicular osteosarcoma cells growth as well.

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.