Monolayer Hexagonal Boron Nitride Research Articles

2D hexagonal boron nitride for solar energy conversions

The optoelectronic properties of free standing monolayer (ML) hexagonal boron nitride (h-BN) is investigated for potential solar energy conversion applications using the density functional theory (DFT) full potential linearized augmented plane wave (FP-LAPW) method. In addition, the bulk optical properties have also been calculated for the sake of comparison. The dielectric functions, optical conductivities and the optical constants are evaluated using the relaxed structures from electronic total energy pseudopotential calculations. The results reinforce previous research on h-BN DUV optoelectronics and demonstrate the suitability of its use as a component in deep ultraviolet (DUV) and energy conversion devices.

Dicarbon defect in hexagonal boron nitride monolayer—a theoretical study

A comprehensive theoretical study of the lowest electronic vertical excitations of the CBCN defect in the monolayer of hexagonal boron nitride has been performed. Both the periodic boundary conditions approach and the finite-cluster simulation of the defect have been utilized at the density-functional theory (DFT) level. Clusters of increasing sizes have been used in order to estimate artefacts resulting from edge effects. The stability of the results with respect to several density functionals and various basis sets has been also examined. High-level ab initio calculations with methods like equation-of-motion coupled cluster method with single and double excitations (EOM-CCSD), algebraic-diagrammatic construction to the second order (ADC(2)), and time-dependent approximate coupled cluster theory to the second order (TD-CC2) were performed for the smallest clusters. It turns out that TD-DFT with the CAM-B3LYP functional gives similar lowest excitation energies as EOM-CCSD, ADC(2), and TD-CC2. The lowest excitation energies resulting from the periodic-boundary calculation utilizing the Bethe–Salpeter equation are in agreement with the results for finite clusters. The analysis of important configurations and transition densities shows that for all studied methods, the lowest excited state is localized on two carbon atoms and their closest neighbours and has a large dipole transition moment. The optimized geometries for the lowest two excited states indicate that in both cases, the carbon–carbon bond becomes a single bond, while for the second excited state, additionally one from boron–nitrogen bonds loses its partially double character. The calculation of the excitation energies at the respective optimal geometry reveals that these two energies become about 0.5 eV lower than vertical excitations from the ground-state geometry.

Efficiency of electron doping to monolayer hexagonal boron nitride by alkali metals

We investigated electron doping of monolayer hexagonal boron nitride (hBN) on metallic substrates by doping alkali metals (AMs). The valence band maximum (VBM) of hBN/Rh after doping with Li and Cs was directly observed using angle-resolved photoemission spectroscopy in a wide wavenumber space. The valence band shift resulting from doping confirms the vacuum level alignment model. Furthermore, when the same AM is used, the resultant binding energy of VBM was almost identical regardless of the substrate, even if it differs by ∼1 eV before doping, which we found by comparison with the literature. This independence from the substrate is explained by an extension of the vacuum level alignment model, wherein the VBM is determined by the work function of the AM when it intercalates to the interface as well as adsorbs on surfaces of hBN. It means that the doping of Cs, which has the lowest work function, gives the deepest binding energy at VBM. This is evaluated as 5.7 eV in our experiments.

Molecular Bridges Link Monolayers of Hexagonal Boron Nitride during Dielectric Breakdown

We use conduction atomic force microscopy (CAFM) to examine the soft breakdown of monocrystalline hexagonal boron nitride (h-BN) and relate the observations to the defect generation and dielectric degradation in the h-BN by charge transport simulations and density functional theory (DFT) calculations. A modified CAFM approach is adopted, whereby 500 × 500 nm2 to 3 × 3 μm2 sized metal/h-BN/metal capacitors are fabricated on 7 to 19 nm-thick h-BN crystal flakes and the CAFM tip is placed on top of the capacitor as an electrical probe. Current–voltage (I-V) sweeps and time-dependent dielectric breakdown measurements indicate that defects are generated gradually over time, leading to a progressive increase in current prior to dielectric breakdown. Typical leakage currents are around 0.3 A/cm2 at a 10 MV/cm applied field. DFT calculations indicate that many types of defects could be generated and contribute to the leakage current. However, three defects created from adjacent boron and nitrogen monovacancies exhibit the lowest formation energy. These three defects form molecular bridges between two adjacent h-BN layers, which in turn “electrically shorts” the two layers at the defect location. Electrical shorting between layers is manifested in charge transport simulations, which show that the I-V data can only be correctly modeled by incorporating a decrease in effective electrical thickness of the h-BN as well as the usual increase in trap density, which, alone, cannot explain the experimental data. An alternative breakdown mechanism, namely, the physical removal of h-BN layers under soft breakdown, appears unlikely given the h-BN is mechanically confined by the electrodes and no change in AFM topography is observed after breakdown. High-resolution transmission electron microscope micrographs of the breakdown location show a highly localized (<1 nm) breakdown path extending between the two electrodes, with the h-BN layers fractured and disrupted, but not removed.

Vertically aligned ZnO nanoarray directly orientated on Cu paper by h-BN monolayer for flexible and transparent piezoelectric nanogenerator

Piezoelectric nanogenerators (PENGs) harvest various types of mechanical energy and exhibit remarkable potential for powering portable devices and wearable electronic systems. The insulating layer in the device can enhance the output voltage, however, people are still curious on the extreme effect when the dielectric thickness reaches the limit of atomic monolayer. Here we demonstrate vertically aligned growth of zinc oxide (ZnO) nanorods array directly on polycrystalline copper paper through the pre-orientation with two-dimensional (2D) hexagonal boron nitride (h-BN) monolayer. The h-BN layer effectively releases the misfit strain in ZnO nanoarray and leads to highly aligned and ultralong nanorods over 75 µm through chemical vapor deposition method. A flexible and transparent PENG based on h-BN/ZnO nanoarray/h-BN sandwiched structure was designed and fabricated. Ultrathin thickness and high barrier of h-BN strongly suppresses the movement of free-carriers in PENG device and improves the output voltage. It is found that the capacitance of PENG increases exponentially inversely proportional to the dielectric layer thickness and reaches 0.01 mF with 2D h-BN monolayer. The multilayer h-BN based PENG can generate open-voltage of 5 V and short-current of ∼ 18 μA, and achieve a maximum output power density up to 169 mW/cm2. A prototype transparent PENG has been achieved for harvesting walking energy and exhibits the practical potential for self-powered wearable devices in the future.

Editorial: Strong field physics and attosecond science

emission from Rydberg atoms. Substantial neutral atoms can be excited after tunneling in a strong laser 14 field, in the process known as frustrated tunneling ionization. The generation of coherent emission from 15 the excited-state atoms produced by the frustrated tunneling ionization is demonstrated to be dependent on 16 the chirp of the laser pulse. This chirp dependence also provides a new way to investigate the dynamics 17 of Rydberg states. Kong et al. (Kong et al., 2022) explored the influence of mechanical strains on the 18 high-order harmonic generation in the monolayer hexagonal boron nitride crystal by using time-dependent 19 density functional theory. They found the band structure is sensitive to the structural deformation modulated 20 by strains. This result may be useful in probing lattice deformations in crystals and heuristic to enhance the 21 optoelectronic efficiency of solid-state nano-devices. In conclusion, the Research Topic is created to present the latest advances and trends concerning the 62 strong field physics and attosecond science. Our special thanks to the Frontiers in Physics team for the 63 technical assistance with publishing. Much more efforts are still ongoing in this fascinating area. We expect 64 that this field will attract more and more attentions and benefit potential applications.

Kondo resonance effects in emergent flat band materials

Macroscopic degrees of freedom that are involved in the transport of carriers through mesoscopic electronic devices are susceptible to the effects of strong many-body correlations. The presence of magnetic impurities in dilute magnetic alloys typically allow for insights into Kondo effect from the scattering of free carriers by localized electron states of the magnetic impurities but this effect is not well understood when there are no d-band electron states. Herein, the signatures of Kondo resonance effect are elucidated in quantum dots derived from a carbon-nanoline embedded monolayer hexagonal boron nitride whose electron states host flat band ferromagnetism as distinct broken symmetry states. Quantum transport state of mesoscopic devices modelled as quantum dots tunnel coupled to metallic leads is computed by direct diagonalization of the Hamiltonian. The possibility of realizing quantum dots with highly tunable electron states in energy interconversion devices is discussed to show the importance of screening effects on single-electron energy levels. The quantum master equation is solved within different formalisms to determine the stationary-state particle and energy currents. Stability diagrams are calculated to show the dependence of the conductance on experimental control variables of the quantum dot device. The computed responses of the stationary-state transport signatures are used to characterize Kondo resonance effects from flat band states of embedded carbon nanoline-based quantum dots. It is found that the local network structure of the hexagonal ring carbon cluster-based quantum dot has a broken particle-hole symmetry in the transport state. This signals the formation of the quasiparticle states expected in second order scattering when the macroscopic “charge” pseudospin symmetry of the tunnelling electron state is broken dynamically due to charging. The results are discussed to show the implications of a vanishing particle-hole symmetry in the carrier transport state of quantum dots for energy conversion applications.

3 nm Channel MoS2 Transistors by Electromigration of Metal Interconnection

Further scaling down the feature size of transistors is central for the development of next-generation electronic devices. However, fabrication of transistors with channel lengths down to 5 nm has been challenging due to lithography limit and short channel effects (SCEs). Here, we demonstrate an MoS2-based device with the shortest 3 nm channel length among global back-gated transistors by feedback-controlled electromigration of metal interconnection. The Si/SiO2-back-gated model device shows on/off ratios of up to 2 × 105 and exhibits a field-effect mobility of up to 33.5 cm2 V–1 s–1, which is, to the best of our knowledge, the highest value in the as-yet-reported same-type transistors with a sub-10 nm channel length. This good immunity of the device to SCEs is also corroborated by the COMSOL Multiphysics simulation. After replacing the thicker physical gate SiO2 dielectric and Si electrode with the 2D hexagonal boron nitride (h-BN) and graphene monolayer, respectively, for better gate control, the field-effect mobility is pushed to 51.2 cm2 V–1 s–1 and displays excellent switching characteristics with near-ideal subthreshold swing of 67 mV dec–1 and drain-induced barrier lowering as low as 0.378 V V–1. This work can promote further transistor downscaling and extend Moore’s law.

Geometrical and magnetic properties of small titanium and chromium clusters on monolayer hexagonal boron nitride.

Magnetic clusters on an insulating substrate are potential candidates for spin-based quantum devices. Here we investigate the geometric, electronic, and magnetic structures of small Ti and Cr clusters, from dimers to pentamers, adsorbed on a single-layer hexagonal boron nitride (h-BN) sheet within the framework of density functional theory. The stable adsorption configurations of the Ti clusters and Cr clusters composed of the same number of atoms are found to be totally different from each other. The difference in their bonding mechanisms has been revealed by the density of states and the charge density difference of the corresponding adsorption systems. While chemical bonds are formed between the Ti atoms and the supporting sheet, the Cr clusters are found in the physisorption state on the substrate. In addition, it is shown that the h-BN sheet is energetically favorable for building three-dimensional Ti clusters. These findings support the use of h-BN as a suitable decoupling substrate for manipulation of quantum spin states in small transition metal (TM) clusters and fabrication of devices based on them.

Growth mechanisms of monolayer hexagonal boron nitride (h-BN) on metal surfaces: theoretical perspectives.

Two-dimensional hexagonal boron nitride (h-BN) has appeared as a promising material in diverse areas of applications, including as an excellent substrate for graphene devices, deep-ultraviolet emitters, and tunneling barriers, thanks to its outstanding stability, flat surface, and wide-bandgap. However, for achieving such exciting applications, controllable mass synthesis of high-quality and large-scale h-BN is a precondition. The synthesis of h-BN on metal surfaces using chemical vapor deposition (CVD) has been extensively studied, aiming to obtain large-scale and high-quality materials. The atomic-scale growth process, which is a prerequisite for rationally optimizing growth circumstances, is a key topic in these investigations. Although theoretical investigations on h-BN growth mechanisms are expected to reveal numerous new insights and understandings, different growth methods have completely dissimilar mechanisms, making theoretical research extremely challenging. In this article, we have summarized the recent cutting-edge theoretical research on the growth mechanisms of h-BN on different metal substrates. On the frequently utilized Cu substrate, h-BN development was shown to be more challenging than a simple adsorption-dehydrogenation-growth scenario. Controlling the number of surface layers is also an important challenge. Growth on the Ni surface is controlled by precipitation. An unusual reaction-limited aggregation growth behavior has been seen on interfaces having a significant lattice mismatch to h-BN. With intensive theoretical investigations employing advanced simulation approaches, further progress in understanding h-BN growth processes is predicted, paving the way for guided growth protocol design.

Entrapping metal atom on hexagonal boron nitride monolayer for high performance single-atom-catalyst: Role of vacancy defects and metal support

In recent time, the research on heterogeneous catalysis has witnessed a stunning progress because of the promising future of single atom catalysis (SAC) with higher activity and selectivity. However, the application of such catalyst in industry remained limited because of its contradicting requirement of stabilizing an atom on a support without affecting its atom-like behavior on the other. Therefore, the search is on to find suitable support and ad-atom combination. Here we report the geometry and electronic structure of metal atom embedded hexagonal boron nitride (h-BN) monolayer, with and without vacancy defects, nano-sheet supported on the copper surface. The effect of vacancy defects has been demonstrated by enhanced binding strength of ad-atoms which in turn paves the way for stabilizing the single atom based catalyst support. In particular, the metal atoms are seen to be more stable on the boron vacancy than nitrogen vacancy site. However, opposite trend has been found for Au doped systems. Further calculations of O2 molecular interaction with Ni and Au embedded h-BN have been carried out to demonstrate the catalytic efficiency of these systems.

Adsorption behavior of methylene blue on graphene and hexagonal boron nitride monolayers in aqueous solution: A first-principles treatment

Understanding the adsorption of dye molecules on nanostructured materials is important in regard to innovative potential applications, especially when the corresponding entities are combined. Here, we explore the interaction of methylene blue (MB) with graphene and hexagonal boron-nitride (h-BN) as two-dimensional nanosheets with unique electronic properties and high potential in technological applications. We study these interactions using quantum-based molecular simulations and probe these through the interaction properties and electronic structure of the adsorbed MB on the considered monolayers and especially the specific role of the surrounding environment. We discuss the importance of the findings in view of contaminant removal from wastewater and specifically sensing applications of considered monolayers. Our dispersion-corrected density functional theory (DFT) results show that MB adsorbs strongly on both the graphene and h-BN with interaction energies of −47.454 and −45.072 kcal/mol, respectively, with equilibrium distances of about 3.2 Å between the aromatic rings of adsorbate and substrates. The accuracy of the DFT calculations is validated by the coupled-cluster with single, double (triple) excitations level of theory. The charge density and atom-in-molecule theory analyses reveal the existence of non-covalent and electrostatic interactions between MB and graphene/h-BN surface. The electronic structure analysis shows that the semiconducting graphene becomes metallic after MB adsorption while the h-BN monolayer retains its semiconducting characteristic upon the adsorption. Complementary assays using DFT-based molecular dynamic simulations demonstrate the strong binding of MB to both the graphene and h-BN surfaces in the presence of water molecules as competitive entities. These findings demonstrate the potential applications of graphene and h-BN for the efficient adsorption of dye molecules and for developing promising adsorbents for wastewater treatment and functionalized sensors.

Insights into nonvolatile resistive switching in monolayer hexagonal boron nitride

A recent demonstration of nonvolatile resistive switching in monolayer hexagonal boron nitride (h-BN) has paved the way for the development of the thinnest memory devices feasible. Nevertheless, the exact mechanism of such remarkable resistive switching has remained obscure, which may hinder the optimization of such attractive technology. Here, we present the first dynamic description of the resistive switching process in a Ni/monolayer h-BN/Ni device at an atomistic level by employing reactive molecular dynamics simulations. We show that with the application of a suitable bias, the h-BN layer moves vertically and peels off Ni ions from the electrode, which gets adsorbed in the N vacancy center. From density-functional-theory based electron-localization-function calculations, we confirm that N vacancy generates highly delocalized electrons around the vacancy location resulting in the adsorption of Ni ions, though such a phenomenon is not likely in case of B vacancy due to the absence of electronic states around the defect center. We demonstrate the restoration of Ni ions with the application of reverse bias in case of bipolar switching, and by rising temperature in case of unipolar switching, which agrees with the experimental observations. Finally, we conduct ab initio quantum transport calculation to find an increase in zero-bias conductivity of about 7.4 times after the Ni ion adsorption. This atomistic insight enables precise defect-engineering in 2D materials for the realization of h-BN based high-performance memristive crossbar array.

Changes in the Optical Properties of an M-Doped (M = Pt, Ti) hBN Sheet and CO2 Capturing

We performed ab initio DFT calculations to explore the optical properties of a hexagonal boron nitride (hBN) monolayer, doped with a Ti or a Pt atom. Ti doping increases the adsorption capability of the boron nitride surface for capturing CO2. Both doping types increase the optical absorption and reflectivity of the hBN surface in the infrared and visible regions. For the UV region, a B vacancy increases the absorption of the hBN sheet. Captured CO2 bears substantial changes in the optical absorption and reflectivity spectra of the system considered.

Structure and electronic properties of stable facets in the 2D material hexagonal boron nitride (hBN) on curved platinum

<h2>Abstract</h2> A hexagonal boron nitride (hBN) monolayer was grown on a curved crystal c-Pt(331) substrate including all vicinal surfaces between Pt(111) and Pt(110) faces. The surface structure has been studied by a combination of low-energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). We observed that the hBN monolayer covers the entire curved crystal, but induces a faceting subsequent to the growth. We encountered (111), (221), (441), (991), and (110) as stable facets. We assign this faceting to two factors i) a better lattice coincidence and ii) the existence of local covalent bonds and non-local Van der Waals interactions. The electronic structure was characterized by near-edge x-ray absorption fine structure (NEXAFS), X-ray photoemission spectroscopy and angle-resolved photoemission spectroscopy (ARPES). The hBN/Pt(111) has the weakest interacting overlayer system, while the hBN on vicinal surfaces is more strongly bound to the Pt substrate. Additionally, we determined using angle-resolved photoemission measurements (ARPES) that the π band shifts downward when going away from the hBN/Pt(111) surface towards hBN/Pt(110). In the latter case, the π band shift is the largest.

Cesium intercalation of graphene: A 2D protective layer on alkali antimonide photocathode

Alkali antimonide photocathodes have wide applications in free-electron lasers and electron cooling. The short lifetime of alkali antimonide photocathodes necessitates frequent replacement of the photocathodes during a beam operation. Furthermore, exposure to mediocre vacuum causes loss of photocathode quantum efficiency due to the chemical reaction with residual gas molecules. Theoretical analyses have shown that covering an alkali antimonide photocathode with a monolayer graphene or hexagonal boron nitride protects it in a coarse vacuum environment due to the inhibition of chemical reactions with residual gas molecules. Alkali antimonide photocathodes require an ultra-high vacuum environment, and depositing a monolayer 2D material on it poses a serious challenge. In the present work, we have incorporated a novel method known as intercalation, in which alkali atoms pass through the defects of a graphene thin film to create a photocathode material underneath. Initially, Sb was deposited on a Si substrate, and a monolayer graphene was transferred on top of the Sb film. Heat cleaning around 550–600 °C effectively removed the Sb oxides, leaving metallic Sb underneath the graphene layer. Depositing Cs on top of a monolayer graphene enabled the intercalation process. Atomic force microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, low energy electron microscopy, and x-ray diffraction measurements were performed to evaluate photocathode formation underneath the monolayer graphene. Our analysis shows that Cs penetrated the graphene and reacted with Sb and formed Cs3Sb.

Breakdown of Adiabatic Superconductivity in Ca-Doped h-BN Monolayer

In the present paper, we report the breakdown of the adiabatic picture of superconductivity in a calcium-doped hexagonal boron nitride (Ca-h-BN) monolayer and discuss its implications for the selected properties of this phase. In particular, it is shown that the shallow conduction band of the Ca-h-BN superconductor potentially cause a violation of the adiabatic Migdal’s theorem. As a result, the pivotal parameters that describe the superconducting state in Ca-h-BN are found to be notably influenced by the non-adiabatic effects. This finding is described here within the vertex-corrected Eliashberg formalism that predicts a strong reduction of the order parameter, superconducting transition temperature and superconducting gap in comparison to the estimates obtained in the framework of the adiabatic theory. The observed trends are in agreement with the recent results on superconductivity in hexagonal monolayers and confirm that the non-adiabatic effects have to be taken into account during the design of such future low-dimensional superconductors.

Strain effects on high-harmonic generation in monolayer hexagonal boron nitride

Based on the time-dependent density functional theory, we theoretically investigate the influence of mechanical strains on the high-order harmonic generation (HHG) in the monolayer hexagonal boron nitride (hBN) crystal. We show that mechanical strains can largely modify the band structure and facilitate the harmonic emission. Compared to uniaxial strains, we find that biaxial strains may enhance the HHG yield significantly, and the HHG spectroscopy generated by a linearly polarized laser is closely related to the symmetry of the deformed hBN. Moreover, when driven by a circularly polarized laser, we find that the appearance of the 3n-order harmonics manifests the restoration of the three-fold rotational symmetry. Our results will be useful in controlling the HHG spectroscopy and probing lattice deformations in crystals.

Hydrogen Crossover Flux through Two-Dimensional Nanomaterials

Energy Storage and conversion devices require an ion-exchange membrane to accomplish high transmission of charge-balancing ions and separation of anode and cathode electrolytes/gases from mixing. These critical functions of membranes play a pivotal role in ensuring device optimum performance and higher efficiency. Most conventional membranes suffer huge ionic and molecular species cross-permeation resulting in low energy efficiency and material degradation. In this work, hydrogen permeability and proton transmission through membrane electrode assemblies (MEAs) that contain a monolayer of hexagonal boron nitride, single-layer and bi-layer graphene were investigated in a gas-phase small-scale cell and a liquid cell. We found that the hydrogen crossover flux through MEAs with 2D materials was inhibited by at least a factor of 5 as compared to the one without. Single-layer graphene and boron nitride enabled high proton transmission, but bi-layer graphene inhibited proton conduction. Defect visualization of 2D materials by chemical treatment with a low ferric chloride concentration followed by imaging using a digital microscope revealed few atomic-scale defects in graphene. These findings suggest that a monolayer of 2D material may provide good selectivity for energy conversion and storage devices by blocking species crossover while allowing high proton transmission.

Investigation of Electronic and Optical Properties of (Cs, Br, Cs-Br) Doped Mono-Layer Hexagonal Boron Nitride Using First Principles

Research on the effect of alternative doping on the photoelectric properties of boron nitride is still at an early stage. In particular, research on hexagonal boron nitride’s diatomic co-doping is still rarely studied. In this work, first-principles calculations are selected as the main method to investigate the electronic structure and optical properties of different atoms used to dope hexagonal boron nitride (h-BN). The band gap value of intrinsic h-BN is 4.66 eV. The band gap was changed after Cs, Br, and Cs-Br doping. The results show that the band gap is 4.61 eV when the Br atom replaces the N atom, while the band gap of h-BN doped with Cs is 3.52 eV. Additionally, the band gap width can be reduced to a typical narrower band gap width of 3.19 eV when Cs-Br is used for doping. At the same time, the complex dielectric function representing the optical properties is calculated after Cs, Br, and Cs-Br doping. The optical absorption peaks of Cs-Br-doped h-BN are weaker at low-frequency conditions. The optical absorption of h-BN can be modified by Cs doping, Br doping, and Cs-Br co-doping in the near-infrared, visible, or portion of the near-ultraviolet bands, which makes the doped material more suited for photoelectric detectors in the relevant frequency bands.