Monolayer Hexagonal Boron Nitride Research Articles

Silicon Cluster Arrays on the Monolayer of Hexagonal Boron Nitride on Ir(111)

Periodic structures of silicon are of interest in quantum-dot-based applications because of their unique optical and electronic properties. We report on the fabrication of stable quasi-ordered Si nanocluster arrays on the moiré of a hexagonal boron nitride (h-BN) monolayer on Ir(111). The h-BN monolayer promotes the growth of regular Si nanoclusters at 130 K and electronically decouples the clusters from the underlying metallic substrate. Using scanning tunneling microscopy and spectroscopy, we have investigated the cluster binding sites, their electronic structure, and their thermal stability. We find that the clusters display a size-dependent bandgap and that they are stable up to 577 K, after which cluster coalescence degrades the arrays.

Engineering the Optoelectronic Properties of 2D Hexagonal Boron Nitride Monolayer Films by Sulfur Substitutional Doping.

Tuning the optical and electrical properties of two-dimensional (2D) hexagonal boron nitride (hBN) is critical for its successful application in optoelectronics. Herein, we report a new methodology to significantly enhance the optoelectronic properties of hBN monolayers by substitutionally doping with sulfur (S) on a molten Au substrate using chemical vapor deposition. The S atoms are more geometrically and energetically favorable to be doped in the N sites than in the B sites of hBN, and the S 3p orbitals hybridize with the B 2p orbitals, forming a new conduction band edge that narrows its band gap. The band edge positions change with the doping concentration of S atoms. The conductivity increases up to 1.5 times and enhances the optoelectronic properties, compared to pristine hBN. A photodetector made of a 2D S-doped hBN film shows an extended wavelength response from 260 to 280 nm and a 50 times increase in its photocurrent and responsivity with light illumination at 280 nm. These enhancements are mainly due to the improved light absorption and increased electrical conductivity through doping with sulfur. This S-doped hBN monolayer film can be used in the next-generation electronics, optoelectronics, and spintronics.

Rotational epitaxy of h-BN on Cu (110)

The growth of wafer-scale, single-crystalline hexagonal boron nitride (h-BN) monolayers on catalytic metallic substrates, requires a sparse nucleation density. At high temperatures (>1000 ∘C), preparation of vicinal Cu (110) with H2 annealing is reported to provide preferential nucleation of single antiphase domains of h-BN with facets parallel to Cu steps. We have used in situ low-energy electron microscopy (LEEM) to image nucleation and growth of h-BN islands on a Cu (110) single crystal at lower temperatures (650–750)∘C. With H2 annealing, diffraction (LEED) and dark field imaging (LEEM) confirmed the formation of three sets of h-BN antiphase domains. Two sets are epitaxially aligned with four Cu 〈11〉 in-plane directions, the lowest lattice mismatch available. While alignment is excellent in one direction, the other trigonal directions are 10.4∘ rotationally mismatched. A third pair of antiphase domains nucleates aligned with 〈10〉 in-plane directions. These domains are midway, rotated 5.2∘ with respect to either type of {111} domain, and have the lowest interfacial energy. Localized defects were found to correlate with every island nucleation event.

Horizontally aligned BN nanosheet array for nanometer-thick ZrO2 coating with greatly enhanced anticorrosion and hydrogen isotope resistance property

Monolayer graphene and hexagonal boron nitride (h-BN), as representative of two-dimensional crystals, are surprisingly impenetrable to all gases and small ions. This essential characteristic boosts them as superior materials for developing novel separation and coating technologies. However, whether graphene is an effective filler in anti-corrosion coatings is controversial for its high electrical conductivity. Compared with graphene, h-BN nanosheet is almost insulative, but bad dispersibility of BN nanosheet seriously hinders its application as coating fillers. Herein we show that few-layer h-BN nanosheets fabricated by one-step mechano-chemical process are highly water-dispersible, thanks to the hydroxyl groups contained in h-BN. These water-dispersible h-BN nanosheets were horizontally aligned in nanometer-thick ZrO2 coating by a simple sol–gel method, serving as ideal blocks for hindering ions and hydrogen isotope from transfer across the coating. With h-BN nanosheets incorporated, the 50 nm thick hybrid BN/ZrO2 coating displays an outstanding anticorrosion ability. An ultra low corrosion current density of 1.59 × 10-9A/cm2 was achieved at 0.8 mg/ml BN/ZrO2, which is 2767 times lower than 316L steel substrate and 102 times lower than pure ZrO2 coating. For the first time, hydrogen isotope resistance property was tested for nanosheet-inserted hybrid coatings to evaluate the effect of nanosheets on blocking hydrogen isotopes. The deuterium permeability is 1.97 × 10-15 mol m−1 s−1 Pa−0.5 at 400 °C for 0.8 mg/ml BN/ZrO2. The hydrogen isotope resistance of the hybrid coating is 140 times that of 316L, increasing by more than 25% when compared to pure ZrO2. The results demonstrate that h-BN nanosheets are highly effective in reducing ions and gases permeation. These advantages endow nanosheet-hybrid-coating a great potential as barrier material for various applications including anticorrosion and anti-hydrogen embrittlement for structural materials.

Intrinsic strengthening and toughening in hexagonal boron nitride by ripples

Increasing strength (maximum stress until permanent deformation) and toughness (fracture resistance) is crucial in engineering applications for all materials, but improving strength often lowers the material's capacity to deform without fracturing, and vice versa. This conflict passes on uniquely to layered van der Waals materials, which are generally brittle. Here, we report simultaneous strengthening and toughening of monolayer and multilayer hexagonal boron nitride (hBN) by ripples. We show using atomistic simulations that stable ripples of varying sizes, modelled based on our experimental observations, increase the intrinsic crack nucleation toughness of hBN by improving both fracture strain and bond strength through a distortion-mediated mechanism. This effect has been traced to the extent of structural asymmetry at the onset of failure, revealing a potential strategy to bolster the fundamental fracture and strength properties of single-phase van der Waals materials significantly.

High-Resolution Ion-Flux Imaging of Proton Transport through Graphene|Nafion Membranes.

In 2014, it was reported that protons can traverse between aqueous phases separated by nominally pristine monolayer graphene and hexagonal boron nitride (h-BN) films (membranes) under ambient conditions. This intrinsic proton conductivity of the one-atom-thick crystals, with proposed through-plane conduction, challenged the notion that graphene is impermeable to atoms, ions, and molecules. More recent evidence points to a defect-facilitated transport mechanism, analogous to transport through conventional ion-selective membranes based on graphene and h-BN. Herein, local ion-flux imaging is performed on chemical vapor deposition (CVD) graphene|Nafion membranes using an “electrochemical ion (proton) pump cell” mode of scanning electrochemical cell microscopy (SECCM). Targeting regions that are free from visible macroscopic defects (e.g., cracks, holes, etc.) and assessing hundreds to thousands of different sites across the graphene surfaces in a typical experiment, we find that most of the CVD graphene|Nafion membrane is impermeable to proton transport, with transmission typically occurring at ≈20–60 localized sites across a ≈0.003 mm2 area of the membrane (>5000 measurements total). When localized proton transport occurs, it can be a highly dynamic process, with additional transmission sites “opening” and a small number of sites “closing” under an applied electric field on the seconds time scale. Applying a simple equivalent circuit model of ion transport through a cylindrical nanopore, the local transmission sites are estimated to possess dimensions (radii) on the (sub)nanometer scale, implying that rare atomic defects are responsible for proton conductance. Overall, this work reinforces SECCM as a premier tool for the structure–property mapping of microscopically complex (electro)materials, with the local ion-flux mapping configuration introduced herein being widely applicable for functional membrane characterization and beyond, for example in diagnosing the failure mechanisms of protective surface coatings.

Mechanism of the motion of nanovehicles on hexagonal boron-nitride: A molecular dynamics study

Nanocars have been proposed to transport nanomaterials on the surface. Study of the mechanism of the motion of nanocars has attracted a lot of interests due to the potential ability of these nano-vehicles in the construction of nanostructures with bottom-up approach. Using molecular dynamics simulations, we study the motion of two nano-vehicles named “Nanocar” and “Nanotruck” on hexagonal boron-nitride monolayer. The obtained results reveals that, boron-nitride is an appropriate option to obtain higher mobility of nanocars compared with metal substrates. Considering different temperatures reveals that nanocars start to move on the BN at 200 K, while long range motions are observed at 400 K and higher temperatures. The flexibility of Nanocar chassis wastes a portion of its energy and reduces its displacement range. The anomaly parameters show that C60, Nanocar and Nanotruck experience super-diffusive regime at 100 K and higher temperatures. Using diffusion coefficient and activation energy, rotational motion of the nanocars is evaluated. To accurately investigate the mechanism of nanocars motion, we find rotation of the wheels around their axles and the speed of Nanotruck in directions parallel and perpendicular to the chassis. The mentioned parameters indicate that wheel rolling mechanism is the minority mode of the nanocars motion and the translations commonly occur through sliding. At low temperatures, two stable configurations are found for the Nanocar, and reconfiguration is energetically possible at 200 K and higher temperatures. The results presented in this work are expected to facilitate the fabrication of nanostructures using molecular machines.

Topological multipolar corner state in a supercell metasurface and its interplay with two-dimensional materials

Second-order topological insulators (SOTIs) have recently attracted much attention due to their capability to support lower-dimensional topological states, namely, the corner states. Here, we demonstrate that properly designed supercell metasurfaces can support photonic corner states, meanwhile further serving as an ideal platform for the implementations of topological polaritons and dynamically reconfigurable corner states by assembling two-dimensional materials. Such metasurfaces consist of an array of finite-sized SOTIs mimicking the two-dimensional Su–Schrieffer–Heeger model. We reveal that the topological transition happens in unit cells without the bandgap, and nondegenerate multipolar corner states emerge in the supercell metasurface due to the inter- and intrasupercell coupling effects. Especially since these corner states are above the light line of the metasurface, we realize the collective stimulation of the two dipolar corner states and their superposition state via far-field excitation. By stacking monolayer hexagonal boron nitride film onto the metasurface, we further achieve the topological phonon polaritons through the strong coupling between the corner state and the phonon, which is confirmed by the Rabi splitting as well as anticrossing behavior emerging in the transmission spectra. Furthermore, we reveal the robustness of the corner state and strong coupling by introducing defects into the metasurface. Finally, tunable corner state and strong coupling with on-demand control are realized by assembling monolayer graphene onto the metasurface. Our theoretical study proposes a unique hybrid-material platform for topological polaritonics and reconfigurable topological photonics, which can promote large-area topological applications in practice.

Flexoelectricity in hexagonal boron nitride monolayers

Two-dimensional (2D) materials are highly bendable due to their atomic thickness and, thus, can readily attain a large strain gradient hard to achieve in bulk materials. This attribute provides a unique platform to explore the coupling between strain gradient and electric polarization, referred to as flexoelectric effect. Here, we perform an intensive first-principles study of the flexoelectric effect in hexagonal boron nitride (h-BN) monolayer for its potential applications in harsh environments. Despite being an isoelectronic isomorph of graphene, h-BN exhibits a distinctly higher flexoelectric coefficient than graphene does, along with a stronger nonlinearity. The enhanced flexoelectricity is ascribed to bending-induced structural buckling that separates B and N atoms into two coaxially curved surfaces, whose effect is well quantified by a model analysis. This behavior also helps rationalize why the three-atom-thick MoS2 has the largest flexoelectric coefficient among these 2D materials. We also find that the flexoelectricity results in a staggered band gap in all double-walled BN nanotubes that can favor charge separation in photovoltaics, in contrast to the size- and chirality-dependent band alignment in double-walled carbon nanotubes.

Manipulation of the high-order harmonic generation in monolayer hexagonal boron nitride by two-color laser field.

We theoretically investigate the high-order harmonic generation (HHG) of the monolayer hexagonal boron nitride by two-color laser pulses, based on the abinitio time-dependent density-functional theory. We find that the waveform of the two-color laser field can dramatically control the harmonic spectrum. The two-color laser field can enhance the harmonic radiation more efficiently than the monochromatic pulse laser with the same incident energy. We investigate the influence of incident laser pulse parameters on the harmonic radiation, such as the relative phase of the two-color field, the amplitude ratio between component electric fields, and the laser orientation. We show that the HHG spectrum is controlled by both the electric field and the vector potential. The electronic band structure and the laser-matter energy transfer play an important role in determining the laser polarization for optimal HHG in the hBN crystal. Our work supplies a scheme to manipulate HHGs in two-dimensional materials and provides a potential methodology for the generation of intense extreme-ultraviolet pulses.

Wafer-Scale Programmed Assembly of One-Atom-Thick Crystals

Crystalline films offer various physical properties based on the modulation of their thicknesses and atomic structures. The layer-by-layer assembly of atomically thin crystals provides a powerful means to arbitrarily design films at the atomic level, which are unattainable with existing growth technologies. However, atomically clean assembly of the materials with high scalability and reproducibility remains challenging. We report programmed crystal assembly of graphene and monolayer hexagonal boron nitride, assisted by van der Waals interactions, to form wafer-scale films of pristine interfaces with near-unity yield. The atomic configurations of the films are tailored with layer-resolved compositions and in-plane crystalline orientations. We demonstrate batch-fabricated tunnel device arrays with modulation of the resistance over orders of magnitude by thickness control of the hexagonal boron nitride barrier with single-atom precision and large-scale, twisted multilayer graphene with programmable electronic band structures and crystal symmetries. Our results constitute an important development in the artificial design of large-scale films.

Enhanced Performance of WS2 Field-Effect Transistor through Mono and Bilayer h-BN Tunneling Contacts.

Transition metal dichalcogenides (TMDs) are of great interest owing to their unique properties. However, TMD materials face two major challenges that limit their practical applications: contact resistance and surface contamination. Herein, a strategy to overcome these problems by inserting a monolayer of hexagonal boron nitride (h-BN) at the chromium (Cr) and tungsten disulfide (WS2 ) interface is introduced. Electrical behaviors of direct metal-semiconductor (MS) and metal-insulator-semiconductor (MIS) contacts with mono- and bilayer h-BN in a four-layer WS2 field-effect transistor (FET) are evaluated under vacuum from 77 to 300 K. The performance of the MIS contacts differs based on the metal work function when using Cr and indium (In). The contact resistance is significantly reduced by approximately ten times with MIS contacts compared with that for MS contacts. An electron mobility up to ≈115 cm2 V-1 s-1 at 300 K is achieved with the insertion of monolayer h-BN, which is approximately ten times higher than that with MS contacts. The mobility and contact resistance enhancement are attributed to Schottky barrier reduction when h-BN is introduced between Cr and WS2 . The dependence of the tunneling mechanisms on the h-BN thickness is investigated by extracting the tunneling barrier parameters.

Intervalley Excitonic Hybridization, Optical Selection Rules, and Imperfect Circular Dichroism in Monolayer h-BN.

We perform first-principles GW plus Bethe-Salpeter equation calculations to investigate the photophysics of monolayer hexagonal boron nitride (h-BN), revealing excitons with novel k-space characteristics. The excitonic states forming the first and third peaks in its absorption spectrum are s-like, but those of the second peak are notably p-like, a first finding of strong co-occurrence of bright s-like and bright p-like states in an intrinsic 2D material. Moreover, even though the k-space wave function of these excitonic states are centered at the K and K^{'} valleys as in monolayer transition metal dichalcogenides, the k-space envelope functions of the basis excitons at one valley have significant extents to the basin of the other valley. As a consequence, the optical response of monolayer h-BN exhibits a lack of circular dichroism, as well as a coupling that induces an intervalley mixing between s- and p-like states.

Effect of Interlayer Coupling and Symmetry on High-Order Harmonic Generation from Monolayer and Bilayer Hexagonal Boron Nitride

High-order harmonic generation (HHG) is a fundamental process which can be simplified as the production of high energetic photons from a material subjected to a strong driving laser field. This highly nonlinear optical process contains rich information concerning the electron structure and dynamics of matter, for instance, gases, solids and liquids. Moreover, the HHG from solids has recently attracted the attention of both attosecond science and condensed matter physicists, since the HHG spectra can carry information of electron-hole dynamics in bands and inter- and intra-band current dynamics. In this paper, we study the effect of interlayer coupling and symmetry in two-dimensional (2D) material by analyzing high-order harmonic generation from monolayer and two differently stacked bilayer hexagonal boron nitrides (hBNs). These simulations reveal that high-order harmonic emission patterns strongly depend on crystal inversion symmetry (IS), rotation symmetry and interlayer coupling.

Rational Control on Quantum Emitter Formation in Carbon-Doped Monolayer Hexagonal Boron Nitride.

Single-photon emitters (SPEs) in hexagonal boron nitride (hBN) are promising candidates for quantum light generation. Despite this, techniques to control the formation of hBN SPEs down to the monolayer limit are yet to be demonstrated. Recent experimental and theoretical investigations have suggested that the visible wavelength single-photon emitters in hBN originate from carbon-related defects. Here, we demonstrate a simple strategy for controlling SPE creation during the chemical vapor deposition growth of monolayer hBN via regulating surface carbon concentration. By increasing the surface carbon concentration during hBN growth, we observe increases in carbon doping levels by 2.4-fold for B-C bonds and 1.6-fold for N-C bonds. For the same samples, we observe an increase in the SPE density from 0.13 to 0.30 emitters/μm2. Our simple method enables the reliable creation of hBN SPEs in monolayer samples for the first time, opening the door to advanced two-dimensional (2D) quantum state engineering.

No Adsorption on Hexagonal Boron Nitride Monolayer with Vacancy Defects: A First-Principles Study

No Adsorption on Hexagonal Boron Nitride Monolayer with Vacancy Defects: A First-Principles Study

Self-aligned stitching growth of centimeter-scale quasi-single-crystalline hexagonal boron nitride monolayers on liquid copper.

Two-dimensional hexagonal boron nitride (hBN) atomic crystals are excellent charge scattering screening interlayers for advanced electronic devices. Although wafer-scale single crystalline hBN monolayer films have been demonstrated on liquid Au and solid Cu (110) and (111) vicinal surfaces, their reproducible growth still remains challenging. Here, we report the facile self-aligned stitching growth of centimeter-scale quasi-single-crystalline hBN monolayer films through synergistic chemical vapor deposition growth kinetics and liquid Cu rheological kinetics control. The sublimation temperature of the ammonia borane precursor, H2 content and melting temperature of the Cu substrate are revealed to be the dominant factors that regulate hBN nucleation, growth and alignment. The flowing liquid Cu catalytic surface promotes efficient rotation of floating triangular hBN domains and provokes uniform self-alignment upon merging at a critical high temperature of 1105 °C. Identical aligned grains are constantly observed at multiple regions, which corroborate the homogeneous in-plane orientation and uniform stitching over the whole growth area. Continuous quasi-single-crystalline hBN monolayer films are produced by seamless stitching of aligned domains with the same polarity. The quasi-single-crystalline hBN monolayers are successfully included as charge scattering and trap site screening interlayers in the hBN/SiO2 gate insulator stack to build high performance InGaZnO field-effect transistors (FETs). Full suppression of hysteresis and twofold enhancement of field-effect mobility are realized for InGaZnO FETs built with hBN as the interface dielectric. The facile growth of large quasi-single-crystalline hBN monolayers on liquid Cu paves the way for future high-performance electronics.

Ab-initio study on the hydrogen storage in h-BN monolayers doped with Mn and Fe transition metals

Two-dimensional (2D) materials are being explored for hydrogen storage purpose attributable to their large surface area. Using first principles based DFT calculations, we investigate structural and electronic properties of transition metals especially Mn and Fe decorated hexagonal boron nitride (h-BN) monolayers. The adsorption energies were found to be −3.24 eV and −2.89 eV for Mn and Fe doped h-BN monolayers, respectively. Up to 3H2 molecules can reasonably adsorbed on the surface of Mn and Fe doped h-BN monolayers with a comparable average adsorption energy of −1.49 eV and −1.65 eV, respectively. Moreover, Mn and Fe doped h-BN monolayers show chemisorption on H2 molecules and H-H bond is dissociated. Our findings suggest significant insights for using Mn and Fe doped h-BN monolayers as hydrogen storage candidates.

Highly sensitive work function type room temperature gas sensor based on Ti doped hBN monolayer for sensing CO2, CO, H2S, HF and NO. A DFT study.

The adsorptions of toxic gas molecules (CO2, CO, H2S, HF and NO) on pristine and Ti atom doped hexagonal boron nitride (hBN) monolayer are investigated by density functional theory. Weak physisorption of gas molecules on pristine hBN results in micro seconds recovery time, limiting the gas sensing ability of pristine hBN. However Ti atom doping significantly enhances the adsorption ability. Ti atom best fits to be doped at B vacancy in hBN with lowest formation energy (-3.241 eV). Structural analysis reveals that structures of gas molecules change after being chemisorbed to Ti doped hBN monolayer. Partial density of states analysis illustrates strong hybridization among Ti-3d, gas-2p and BN-2p orbitals, moreover Bader charge transfer indicates that gas molecules act as charge acceptors. Ti doped hBN monolayer undergoes transition from semiconductor to narrow band semiconductor with adsorption of CO2, H2S and NO, while with CO and HF adsorption it transforms into metal. The change of conductance of Ti doped hBN monolayer in response to adsorption of gas molecules reveals its high sensitivity, however it is not selective to HF and NO gases. The recovery times of gas molecules desorption from monolayer are too long at ambient condition however it can significantly be shortened by annealing at elevated temperature with UV exposure. Since recovery time for NO removal from monolayer is still very long at 500 K with UV exposure, Ti doped hBN monolayer is more suitable as a scavenger of NO gas rather than as a gas sensor. It is thus predicted that Ti doped hBN monolayer can be a work-function type CO2, CO, H2S and HF sensor and NO gas scavenger.

First Principles Study of Structural, Electronic and Magnetic Properties of Defected (Monovacant) Hexagonal Boron Nitride Sheet

The structural, electronic, and magnetic properties of vacancy structures with triangular shape are related to the defect in single hexagonal boron nitride (h-BN) sheet. It is investigated by using density functional theory calculations. The first-principles calculations based GGA functionals have been implemented to study the structural, electronic and magnetic properties of pure and defected hexagonal boron nitride (h-BN) monolayer sheet using Quantum ESPRESSO (QE) package, 6.5 version. The calculated values of formation energy reveal the structural stability of the defected systems. The formation energies for B and N vacant system are found to be 16.45 eV and 12.87 eV respectively. This predicts that the N vacant system is more preferable with lower formation energy. The defect in h-BN seems to be changing its band gap and magnetic properties of h-BN system. The 6.25 % B-vacancy results h-BN to be half metallic ferromagnetic with total magnetization of 2.74µB/cell. Further, 6.25 % N-vacancy causes it to be magnetic semiconductor with total magnetization 1.00µB /cell.