Monolayer Hexagonal Boron Nitride Research Articles

Role of carbon substitutional and vacancy in tailoring the H2 adsorption energy over a hexagonal boron nitride monolayer: an ab initio study

AbstractThe effect of the substitutional and vacancy type defects on the H2 adsorption energy over a monolayer hexagonal boron nitride (h-BN) substrate has been studied by using the van der Waals density functional theory calculations. Carbon doping at the boron site or formation of boron vacancy can be an effective way to increase the adsorption energy value of a pristine h-BN substrate. The repulsive lateral interaction present in between the two H2 molecules plays a vital role in case of multiple H2 molecule adsorption over the substrate. Also, the carbon cluster formation during doping can have a favorable effect in the overall storage capacity of the h-BN substrate.

Guiding light with surface exciton–polaritons in atomically thin superlattices

Abstract Two-dimensional materials give access to the ultimate physical limits of photonics with appealing properties for ultracompact optical components such as waveguides and modulators. Specifically, in monolayer semiconductors, a strong excitonic resonance leads to a sharp oscillation in permittivity from positive to even negative values. This extreme optical response enables surface exciton–polaritons to guide visible light bound to an atomically thin layer. However, such ultrathin waveguides support a transverse electric (TE) mode with low confinement and a transverse magnetic (TM) mode with short propagation. Here, we propose that realistic semiconductor–insulator–semiconductor superlattices comprising monolayer WS2 and hexagonal boron nitride (hBN) can improve the properties of both TE and TM modes. Compared to a single monolayer, a heterostructure with a 1-nm hBN spacer separating two monolayers enhances the confinement of the TE mode from 1.2 to around 0.5 μm, while the out-of-plane extension of the TM mode increases from 25 to 50 nm. We propose two simple additivity rules for mode confinement valid in the ultrathin film approximation for heterostructures with increasing spacer thickness. Stacking additional WS2 monolayers into superlattices further enhances the waveguiding properties. Our results underscore the potential of monolayer-based superlattices as a platform for visible-range nanophotonics with promising optical, electrical, and magnetic tunability.

Investigation of hexagonal boron nitride monolayer with pores as an anode material for sodium-ion batteries

The hexagonal boron nitride monolayer (h-BNML) used as an anode material for metal-ion batteries was assumed pristine form. However, inspired by the imperfections in natural materials, this study investigates the effect of defects on the electrochemical properties of h-BNML. Within the current piece of research, h-BNML with pores is introduced to develop an anode material for sodium ion battery (SIBs) by undertaking density-functional theory (DFT) computations. During the diffusion and storage process of Na ions, the pores effect was thoroughly investigated. The boron atoms were removed from the structure of the pure h-BNML in order to form the h-BNML with pores, which mimicked the pore formation. Based on the DFT results, after the pore formation, the h-BNML retained its planarity. Following the introduction of vacancies, significant alterations occurred in the electronic structure characteristics of h-BNML within the pores, facilitating the effective adsorption of Na ions within the pore. The results of quantum theory of atoms in molecules indicated that there were interactions between the pores and Na ions through an electrostatic-type attraction. The h-BNML with pores was capable of retaining a large number of Na ions at its surface without any change in its planar structure. This led to an increase in its theoretical specific capacity, which increased with porosity. Hence, unlike the pure h-BNML, the pore formation improved the diffusion of Na ions. This can be promising to designing highly efficient anodes for SIBs. The methodological approach in this study can be used for tailoring porous BN materials for SIBs.

Light-wave-controlled Haldane model in monolayer hexagonal boron nitride

In recent years, the stacking and twisting of atom-thin structures with matching crystal symmetry has provided a unique way to create new superlattice structures in which new properties emerge1,2. In parallel, control over the temporal characteristics of strong light fields has allowed researchers to manipulate coherent electron transport in such atom-thin structures on sublaser-cycle timescales3,4. Here we demonstrate a tailored light-wave-driven analogue to twisted layer stacking. Tailoring the spatial symmetry of the light waveform to that of the lattice of a hexagonal boron nitride monolayer and then twisting this waveform result in optical control of time-reversal symmetry breaking5 and the realization of the topological Haldane model6 in a laser-dressed two-dimensional insulating crystal. Further, the parameters of the effective Haldane-type Hamiltonian can be controlled by rotating the light waveform, thus enabling ultrafast switching between band structure configurations and allowing unprecedented control over the magnitude, location and curvature of the bandgap. This results in an asymmetric population between complementary quantum valleys that leads to a measurable valley Hall current7, which can be detected by optical harmonic polarimetry. The universality and robustness of our scheme paves the way to valley-selective bandgap engineering on the fly and unlocks the possibility of creating few-femtosecond switches with quantum degrees of freedom.

First-Principles Prediction of High and Low Resistance States in Ta/h-BN/Ta Atomristor.

Two-dimensional (2D) materials have received significant attention for their potential use in next-generation electronics, particularly in nonvolatile memory and neuromorphic computing. This is due to their simple metal-insulator-metal (MIM) sandwiched structure, excellent switching performance, high-density capability, and low power consumption. In this work, using comprehensive material simulations and device modeling, the thinnest monolayer hexagonal boron nitride (h-BN) atomristor is studied by using a MIM configuration with Ta electrodes. Our first-principles calculations predicted both a high resistance state (HRS) and a low resistance state (LRS) in this device. We observed that the presence of van der Waals (vdW) gaps between the Ta electrodes and monolayer h-BN with a boron vacancy (VB) contributes to the HRS. The combination of metal electrode contact and the adsorption of Ta atoms onto a single VB defect (TaB) can alter the interface barrier between the electrode and dielectric layer, as well as create band gap states within the band gap of monolayer h-BN. These band gap states can shorten the effective tunneling path for electron transport from the left electrode to the right electrode, resulting in an increase in the current transmission coefficient of the LRS. This resistive switching mechanism in monolayer h-BN atomristors can serve as a theoretical reference for device design and optimization, making them promising for the development of atomristor technology with ultra-high integration density and ultra-low power consumption.

Spatially-resolved UV-C emission in epitaxial monolayer boron nitride

We report hyperspectral imaging in the UV-C spectral domain in epitaxial monolayers of hexagonal boron nitride (hBN). Under quasi-resonant laser excitation, the UV-C emission of monolayer hBN consists in resonant Raman scattering and photoluminescence, which appear to be spatially uncorrelated. Systematic measurements as a function of the excitation energy bring evidence of a photoluminescence singlet at ∼6.045 eV. The spatial variations of the photoluminescence energy are found to be around ∼10 meV, revealing that the inhomogeneous broadening is lower than the average photoluminescence linewidth of ∼25 meV, a value close to the radiative limit in monolayer hBN. Our methodology provides an accurate framework for assessing the opto-electronic properties of hBN in the prospect of scalable hBN-based devices fabricated by epitaxy.

Hexagonal boron nitride nanosheets: Fabrication, thermal properties and application in polymers

Traditional thermally conductive materials are gradually losing their advantages as the electronics industry develops rapidly. Polymers are in urgent need of a material with high thermal conductivity, stable structure, good mechanical properties and oxidation resistance, which makes hexagonal boron nitride (h-BN) an excellent heat transfer filler for polymer composites. Boron nitride nanosheets (BNNS) are monolayers of hexagonal boron nitride, and theoretical studies have shown that BNNS have a higher thermal conductivity than h-BN (up to 400 Wm−1K−1, in-plane). This paper provides a comprehensive review of various methods for preparing BNNS, including mechanical exfoliation, liquid phase sonication exfoliation and chemical vapor deposition (CVD). In addition, various factors affecting the thermal conductivity of BNNS, including grain boundary, grain size and defects, are also discussed. Then, the influence of BNNS as filler on the thermal conductivity of the polymer was further discussed. Finally, this paper summarizes the existing BNNS applications and lists the application scenarios of BNNS in various fields. The purpose of this review is to summarize the previous work and put forward the prospect and future development direction of preparing high thermal conductivity BNNS-included polymer composites, so as to stimulate the research and improvement of new preparation methods for BNNS and promote its practical application as heat transfer materials.

Adsorption and evolution of hydrogen molecules on hexagonal boron nitride monolayer: a combined DFT and kinetic monte-carlo simulations study

Density functional theory (DFT) and kinetic Monte-Carlo (kMC) simulation code has been combinedly used to study the adsorption and evolution dynamics of hydrogen molecules over a hexagonal boron nitride (h-BN) monolayer. Maximum adsorption energy from van der Waals curve is predicted to be around 60 to 70 meV using two different DFT functionals. Repulsive lateral interaction between two hydrogen molecules plays a key role in determining the maximum number of adsorptions inside one unit cell of h-BN. Bader charge analysis, electron localization function (ELF), total and partial density of states (DOS) plots have been included to understand the weak interaction going on between the adsorbent and substrate. The input energy parameters from the DFT calculation has been used to perform the kMC simulation for describing the adsorption, desorption and the diffusion pattern of hydrogen molecules with a given time of exposure to an empty h-BN substrate along with the overall surface coverage.

Observation of the nonanalytic behavior of optical phonons in monolayer hexagonal boron nitride

Phonon splitting of the longitudinal and transverse optical modes (LO-TO splitting), a ubiquitous phenomenon in three-dimensional polar materials, will break down in two-dimensional (2D) polar systems. Theoretical predictions propose that the LO phonon in 2D polar monolayers becomes degenerate with the TO phonon, displaying a distinctive “V-shaped” nonanalytic behavior near the center of the Brillouin zone. However, the full experimental verification of these nonanalytic behaviors has been lacking. Here, using monolayer hexagonal boron nitride (h-BN) as a prototypical example, we report the comprehensive and direct experimental verification of the nonanalytic behavior of LO phonons by inelastic electron scattering spectroscopy. Interestingly, the slope of the LO phonon in our measurements is lower than the theoretically predicted value for a freestanding monolayer due to the screening of the Cu foil substrate. This enables the phonon polaritons in monolayer h-BN/Cu foil to exhibit ultra-slow group velocity (~5 × 10−6c, c is the speed of light) and ultra-high confinement (~ 4000 times smaller wavelength than that of light). These exotic behaviors of the optical phonons in h-BN presents promising prospects for future optoelectronic applications.

Excited-state geometry relaxation of point defects in monolayer hexagonal boron nitride

Excited-state geometry relaxation of point defects in monolayer hexagonal boron nitride

Volatile and Nonvolatile Resistive Switching Coexistence in Conductive Point Hexagonal Boron Nitride Monolayer.

Recently, we demonstrated the nonvolatile resistive switching effects of metal-insulator-metal (MIM) atomristor structures based on two-dimensional (2D) monolayers. However, there are many remaining combinations between 2D monolayers and metal electrodes; hence, there is a need to further explore 2D resistance switching devices from material selections to future perspectives. This study investigated the volatile and nonvolatile switching coexistence of monolayer hexagonal boron nitride (h-BN) atomristors using top and bottom silver (Ag) metal electrodes. Utilizing an h-BN monolayer and Ag electrodes, we found that the transition between volatile and nonvolatile switching is attributed to the thickness/stiffness of chain-like conductive bridges between h-BN and Ag surfaces based on the current compliance and atomristor area. Computations indicate a "weak" bridge is responsible for volatile switching, while a "strong" bridge is formed for nonvolatile switching. The current compliance determines the number of Ag atoms that undergo dissociation at the electrode, while the atomristor area determines the degree of electric field localization that forms more stable conductive bridges. The findings of this study suggest that the h-BN atomristor using Ag electrodes shows promise as a potential solution to integrate both volatile neurons and nonvolatile synapses in a single neuromorphic crossbar array structure through electrical and dimensional designs.

Covalent functionalization of germanene employing computational simulations.

Computational simulations through density functional theory in conjunction with M06-L and HSE functional have been carried out to investigate the chemical reactivity of the germanene monolayer. It is exceptionally reactive, with an average reaction energy of -60.4 kcal mol-1 for the nineteen functional groups considered: H, F, Cl, Br, O, S, Se, Ge, OH, SH, CH3, CF3, NH, NH2, C6H5, C6H4, CCl2, CBr2, and the azomethine ylide. The results indicate that oxygen is the most reactive reagent (-96.9 kcal mol-1), followed by fluorine (-83.1 kcal mol-1). Germanene presents a rich organic chemistry, and functionalization with azomethine ylides, benzynes, and carbenes can be easily accomplished as indicated by the reaction energies computed, which lie between -45 and -65 kcal mol-1. Furthermore, germanene is significantly more reactive than graphene and hexagonal boron nitride monolayers since the reaction energy for germanene is more than 40 kcal mol-1 lower. Although, in general, germanene is slightly more reactive than black and blue phosphorene and less prone to oxidation, but its oxidation when exposed to air occurs spontaneously. The addition of functional groups works cooperatively. The reaction energies become lower as the number of functional groups increases, thus favouring the agglomeration of functional groups attached unless the steric effect alters this pattern. Finally, we analyzed the electronic properties of functionalized germanene. It is possible to fine-tune the band gap of germanene from 0.1 to 2 eV using different functional groups and coverages. For O-50% and S-50% functionalized germanene, we found that carrier recombination is the most difficult due to the considerable differences between the effective masses of holes and electrons, which is promising for optical applications.

Electronic, optical, and adsorption properties of Li-doped hexagonal boron nitride: a GW approach.

This study employs quasiparticle-corrected DFT calculations to explore the electronic, optical, and surface adsorption properties of Li-doped hexagonal boron nitride (h-BNLi) monolayers. The results reveal that Li doping introduces two defect states into the wide band gap of the monolayer, reducing the band gap from 5.73 eV to 3.72 eV at the K-Γ point of the Brillouin zone. Using the GW approach to incorporate quasiparticle energies demonstrates a distinct advantage over conventional DFT, leading to qualitative shifts in band alignment across the Brillouin zone. Additionally, we identify intragap transitions driven by these defect states, resulting in a significant red shift in the optical gap, decreasing it from 5.73 eV to 1.61 eV in the doped monolayer. Moreover, Li doping enhances the detection of carbon-based gas molecules, raising the surface adsorption energy by -0.42 eV and -0.45 eV compared to the pristine monolayer. These findings hold substantial promise for the application of h-BNLi in electronic, optoelectronic, optical, and sensing devices, effectively subjugating the challenge posed by its wide band gap.

Symmetry constraints on the orientation dependence of high-order elastic constants for the hexagonal boron nitride monolayer.

Group theory is a powerful tool to explore fundamental symmetry constraints for the physical properties of crystal structures, e.g. it is well-known that only a few components of the elastic constants are independent due to the symmetry constraint. This work further applies group theory to derive constraint relationships for high-order elastic constants with respect to the orientation angle, where the constraint relationships are more explicit than the traditional tensor transformation law. These analytic symmetry constraints are adopted to explain the molecular dynamics simulation results, which disclose that the high-order elastic constants are highly anisotropic with an anisotropy percentage of up to 25% for the hexagonal boron nitride monolayer. The elastic constant is a basic quantity in the mechanics field, so its high anisotropy shall cause strong anisotropy for other mechanical properties. Based on the anisotropic high-order elastic constants, we demonstrate that Poisson's ratio is highly anisotropic for the hexagonal boron nitride at large strains. These findings provide fundamental insights into the symmetry dependence of high-order elastic constants and other mechanical properties.

First-principles study of NO adsorption on defective hexagonal boron nitride monolayer

This study explored the potential of hexagonal boron nitride (h-BN) as a support for selective catalytic reduction (SCR) which means converting NOx to N2 and H2O with the aid of a catalyst. Specifically, we investigated the adsorption behavior of a NO molecule on a defective h-BN monolayer using density functional theory. Our results indicated that the initial configuration of N and O atoms over vacancy defects plays a key role in determining whether the NO molecule is adsorbed on the h-BN monolayer via covalent or ionic bonding. In the case of mono-vacancy, the ON configuration of the NO gas resulted in lower total energy (Etotal) compared with the NO configuration. Furthermore, electron localization function and Bader charge analysis, as well as projected density of states, revealed that the ON configuration over B or N vacancy resulted in chemisorption by hybridizing p-orbitals. Furthermore, the di-vacancy system led to an even lower Etotal compared with the mono-vacancy case. In particular, both N and O atoms were adsorbed chemically to the edge of vacancy defects, irrespective of the NO configurations above the di-vacancy. Our calculations revealed that the presence of vacancy defects contributed to improving the adsorption of the NO molecule to the h-BN monolayer.

Tailoring thermoelectric properties through carbon doping and magnetic field variation: A comparative study in 2D h-BN and h-SiC

The purpose of this work is to study the effects of carbon doping on the thermoelectric properties of monolayer hexagonal boron nitride (h-BN) and hexagonal silicon carbide (h-SiC) in the presence of a magnetic field. We use a tight-binding model and Kubo formalism to calculate the effects of doping concentration, dopant type and applied magnetic field on the heat capacity, electrical conductivity, magnetic susceptibility, and thermoelectric figure of merit. Heat capacity displays a Schottky anomaly that is more prominent at lower dopant concentrations. In the presence of the magnetic field, the Schottky anomaly shifts to lower temperature and its intensity is reduced. The magnetic susceptibility of the doped structures is enhanced by both doping and a magnetic field. The p-type doped structures exhibit higher magnetic susceptibility intensity compared to the n-type doped structures, especially for h-SiC structure. The electrical and thermal conductivities rise from zero at low temperatures, with higher intensity for p-doped structures. Doped h-SiC has higher conductivity at low temperatures, but doped h-BN exceeds it at higher temperatures. A comparison of p- and n-doped h-SiC and h-BN reveals enhanced thermoelectric performance for p-doped structures, further improved by lower dopant concentrations and applied magnetic fields. Overall, the work provides new insights into optimizing the thermoelectric properties of doped h-BN and h-SiC nanostructures through doping and external fields.

First-Principles Study of Titanium and Lithium Adsorption on Perfect and Defective Hexagonal Boron Nitride Monolayer Under Effects of Charging

Single Titanium (Ti) and Lithium (Li) atoms adsorption on Pristine and defective hexagonal boron nitride (P-h-BN and BV-h-BN) monolayer were employed using Density Functional Theory (DFT) under effect of charging. Obtained data reveal that Li adsorption on P-h-BN is weak, while Ti adsorption on P-h-BN is strong. When Ti and Li atoms interact with P-h-BN surface, Ti and Li generate 4 µB/cell and 1 µB/cell magnetic moments, respectively. The extraction of an electron from the systems leads to a considerable rise in the adsorption energy, notably in the case of Li-P-h-BN. There is a notable decrease in the band gap of Ti-P-h-BN in both the charged states, especially in the electron-added state. Removing an electron from the Li-P-h-BN system results in a non-magnetic state and a significant increase of the band gap to 4.07 eV. Ti-BV-h-BN system shows significantly stronger adsorption energy due to the d-orbitals of the Ti atom. When an electron is added to the systems, the interaction energy between Ti and BV-h-BN decreases, while the interaction energy between Li and BV-h-BN increases. Moreover, removing an electron from Ti-BN-h-BN increases the band gap to 2.29 eV and the disappearance of the magnetic moment.

The outstanding electrochemical performance of porous hexagonal boron nitride as a K-ion battery anode

Within the current piece of research, we propose a porous model of hexagonal boron nitride (h-BN) monolayer (h-BNML) through DFT calculations in order develop an anode material for KIBs. We in particular focused on the impact of the pore on the diffusion of K ions and upon the storage process. We created the porous model of the h-BNML by using a pure h-BNML structure via the removal of B atoms which mimics the formation of the pore. Based on the DFT calculations, there was no change in the original planar structure of the h-BNML after the formation of the pore. There was a dramatic change in the electronic and structural attributes of the h-BNML indie the poor based on the introduced vacancies, which allowed the K-ion to adhere effectively within the pore. Based calculations demonstrated the interaction between the K ions and the pores through electrostatic attraction. The model exhibited the ability to keep a high number of K ions at its surface without any change in the initial planar structure of h-BN, which was closely associated with an increased theoretical specific capacity, which increased with porosity. Hence, the pore formation increased the diffusion of K ions in comparison with the pure h-BNML. This can be insightful to designing enhanced anode materials for KIBs. Finally, the current work can provide insights into tailoring porous BN materials for application in KIBs.

Growth of aligned and twisted hexagonal boron nitride on Ir(110)

The growth of monolayer hexagonal boron nitride (h-BN) on Ir(110) through low-pressure chemical vapor deposition is investigated using low energy electron diffraction and scanning tunneling microscopy. We find that the growth of aligned h-BN on Ir(110) requires a growth temperature of 1500 K, whereas lower growth temperatures result in coexistence of aligned h-BN with twisted h-BN. The presence of the h-BN overlayer suppresses the formation of the nano-faceted ridge pattern known from clean Ir(110). Instead, we observe the formation of a reconstruction, with n such that the missing rows are in registry with the h-BN/Ir(110) moiré pattern. Our moiré analysis showcases a precise methodology for determining both the moiré periodicity and the h-BN lattice parameter on an fcc(110) surface. Aligned h-BN on Ir(110) is found to be slightly compressed compared to bulk h-BN, with a monolayer lattice parameter of nm. The lattice mismatch with the substrate along gives rise to a moiré periodicity of nm.

Optimizing thermoelectric performance of carbon-doped h-BN monolayers through tuning carrier concentrations and magnetic field

The thermoelectric properties of carbon-doped monolayer hexagonal boron nitride (h-BN) are studied using a tight-binding model employing Green function approach and the Kubo formalism. Accurate tight-binding parameters are obtained to achieve excellent fitting with Density Functional Theory results for doped h-BN structures with impurity type and concentration. The influence of carbon doping on the electronic properties, electrical conductivity, and heat capacity of h-BN is studied, especially under an applied magnetic field. Electronic properties are significantly altered by doping type, concentration, and magnetic field due to subband splitting, merging of adjacent subbands, and band gap reduction. These modifications influence the number, location, and magnitude of DOS peaks, generating extra peaks inside the band gap region. Heat capacity displays pronounced dependence on both magnetic field and impurity concentration, exhibiting higher intensity at lower dopant levels. Electrical conductivity is increased by double carbon doping compared to single doping, but is reduced at high magnetic fields because of high carrier scattering. The electronic figure of merit ZT increases with lower impurity concentration and is higher for CB versus CN doping at a given field strength. The power factor can be improved by increasing magnetic field and decreasing doping concentration. In summary, controlling doping and magnetic field demonstrates the ability to effectively engineer the thermoelectric properties of monolayer h-BN.