Hexagonal Boron Nitride Layers Research Articles

Nonconductive layered hexagonal boron nitride preparation via a co-exfoliation strategy and nanohybrid coating construction for corrosion protection

Hexagonal boron nitride (h-BN), an analogue of graphene with similar layered structure, is considered as the promising material in place of graphene for application as nanofiller into polymeric matrices to develop the smart coatings for corrosion protection of metallic substrates, owing to its mechanical robustness and thermal stability together with unique electrically insulating nature. In this work, on the basis of a co-exfoliation strategy, h-BN were exfoliated by riboflavin sodium phosphate (FMN) intercalator, assisted with ball-milling to obtain few-layered nanosheets with improved water-dispersibility as well as improve their compatibility with waterborne epoxy matrix. The results from electrochemical measurement and corrosion product analysis indicate that the incorporation of only 1.0 wt% FMN-exfoliated h-BN (h-BN@FMN nanohybrids) content into waterborne epoxy matrix can realize the optimized long-term corrosion resistance of composite coatings in 3.5 wt% NaCl solution. The two-dimensional morphology and impenetrability of h-BN, together with the compatibility of waterborne epoxy system from the bridge role of FMN molecules to act as a physical barrier against corrosive species (i.e., H2O, Cl− and O2), are the main reasons behind the prominent anticorrosion performance of our composite coating.

Effect of Ion Flux Variation in DC Magnetron Sputtering on the Deposition of Cubic Boron Nitride Film

Cubic boron nitride (cBN) thin films were deposited using DC magnetron sputtering, and the effect of the ion flux on the deposition behavior and residual stress of the cBN thin films was investigated. To increase the ion flux, the magnetic force ratio of the central/peripheral permanent magnets inserted in the magnetron sputtering source was reduced. Due to the complementary relationship between ion flux and energy for cBN deposition, the critical bias voltage required for cBN nucleation decreased as the ion flux increased. The cBN content of the films was relatively higher under the deposition condition of the increased ion flux. This was interpreted to indicate the thinning of the intervening hexagonal boron nitride (hBN) layer formed prior to cBN nucleation. Comparing the compressive residual stress of the cBN films, the residual stress was relieved as the bias voltage decreased regardless of the ion flux. The increase in ion flux made it possible to deposit the cBN films at a low bias voltage, thereby depositing cBN films with lower residual stress. The results showed that reducing ion energy by increasing ion flux for cBN deposition is a promising method for depositing low-stress cBN thin film having thin intervening hBN layer.

Excellent high frequency magnetic loss and novel loss behavior of FeSiAl/hBN composites

Layered hexagonal boron nitride(hBN,1 wt.%, 2 wt.% and 3 wt.%)-added FeSiAl composites are prepared by ball milling and their soft magnetic properties are measured by B-H analyzer. The experimental results showed that FeSiAl/hBN composites have low loss at high frequency, and show the lowest loss only 175 kW/m3 at 20 mT and 1 MHz. It is also found that the relative loss decreases with the increase of frequency in the low frequency range.

Kapitza-resistance-like exciton dynamics in atomically flat MoSe2-WSe2 lateral heterojunction

Being able to control the neutral excitonic flux is a mandatory step for the development of future room-temperature two-dimensional excitonic devices. Semiconducting Monolayer Transition Metal Dichalcogenides (TMD-ML) with extremely robust and mobile excitons are highly attractive in this regard. However, generating an efficient and controlled exciton transport over long distances is a very challenging task. Here we demonstrate that an atomically sharp TMD-ML lateral heterostructure (MoSe2-WSe2) transforms the isotropic exciton diffusion into a unidirectional excitonic flow through the junction. Using tip-enhanced photoluminescence spectroscopy (TEPL) and a modified exciton transfer model, we show a discontinuity of the exciton density distribution on each side of the interface. We introduce the concept of exciton Kapitza resistance, by analogy with the interfacial thermal resistance referred to as Kapitza resistance. By comparing different heterostructures with or without top hexagonal boron nitride (hBN) layer, we deduce that the transport properties can be controlled, over distances far greater than the junction width, by the exciton density through near-field engineering and/or laser power density. This work provides a new approach for controlling the neutral exciton flow, which is key toward the conception of excitonic devices.

Two-dimensional phases of confined 5-cyano-biphenyl: Computer simulation study.

The properties of composites of mesogens and two-dimensional (2D) materials are of great interest due to their practical applications in flexible displays, optoelectronics, microelectronics, and novel nanodevices. The properties of such composites are very complex and strongly depend on the interactions between the host material and the mesogen filling. We have performed molecular dynamics simulations for 4-cyano-4^{'}-pentylbiphenyl embedded between graphene and hexagonal 2D boron nitride layers. The structural and dynamical properties of such systems were investigated in terms of the order parameters, density profiles, mean square displacement, and autocorrelation function of the single-molecule dipole moment. Our simulations have shown that the mesogenic molecules form highly stable ordered layered structures and that their dynamics are strongly related to the structural properties. We have investigated not only the effects of the polarization of the host material, but also the effects of the spatial repetition of such composites by using two models of mesogens embedded in 2D layers: the direct sheet and the structure formed by multiplying a single unit of the composite in the direction perpendicular to the substrate surface.

Designing a perfect surface plasmon resonance absorber based on graphene and hexagonal boron nitride photonic crystal nanorods

In this work, absorbers in the terahertz region based on one-dimensional photonic crystal nanorods coated with phosphorene, phosphorene-graphene, and phosphorene–hexagonal boron nitride layers were designed. On addition of a graphene layer to the structure, strong localized surface plasmon resonance was observed in two different plasmonic modes via two significant peaks at wavelengths of 10.09 μm and 12.47 μm. The absorption of the structure was affected by the presence of an antireflection layer, and the two modes showed an absorption rate of 99% at wavelengths of 9.72 μm and 9.86 μm, increasing the absorption band to 0.54 μm full width at half maximum. By our changing the dielectric environment with a higher refractive index, better localization and greater absorption occurred. Furthermore, a perfect absorber with two absorption peaks at wavelengths of 11.39 μm and 14.02 μm with 99% and 97% absorption and full width at half maximum of 0.35 μm and 1.1 μm, respectively, was created. Finally, a perfect absorber with an absorption bandwidth of 2.1 μm was obtained in the structure in which hexagonal boron nitride had been placed as a spacer between the monolayers of phosphorene.

Electrostatic moiré potential from twisted hexagonal boron nitride layers.

Moiré superlattices host a rich variety of correlated electronic phases. However, the moiré potential is fixed by interlayer coupling, and it is dependent on the nature of carriers and valleys. In contrast, it has been predicted that twisted hexagonal boron nitride (hBN) layers can impose a periodic electrostatic potential capable of engineering the properties of adjacent functional layers. Here, we show that this potential is described by a theory of electric polarization originating from the interfacial charge redistribution, validated by its dependence on supercell sizes and distance from the twisted interfaces. This enables controllability of the potential depth and profile by controlling the twist angles between the two interfaces. Employing this approach, we further demonstrate how the electrostatic potential from a twisted hBN substrate impedes exciton diffusion in semiconductor monolayers, suggesting opportunities for engineering the properties of adjacent functional layers using the surface potential of a twisted hBN substrate.

A magnetically-induced Coulomb gap in graphene due to electron-electron interactions

Insights into the fundamental properties of graphene’s Dirac-Weyl fermions have emerged from studies of electron tunnelling transistors in which an atomically thin layer of hexagonal boron nitride (hBN) is sandwiched between two layers of high purity graphene. Here, we show that when a single defect is present within the hBN tunnel barrier, it can inject electrons into the graphene layers and its sharply defined energy level acts as a high resolution spectroscopic probe of electron-electron interactions in graphene. We report a magnetic field dependent suppression of the tunnel current flowing through a single defect below temperatures of ~2 K. This is attributed to the formation of a magnetically-induced Coulomb gap in the spectral density of electrons tunnelling into graphene due to electron-electron interactions.

Low-frequency noise in hBN/MoS2/hBN transistor at cryogenic temperatures toward low-noise cryo-CMOS device applications

As an application of cryo-CMOS technology for use in future quantum computing, we have explored low-frequency noise reduction in a two-dimensional (2D) system consisting of a molybdenum disulfide (MoS2) channel sandwiched by hexagonal boron nitride (hBN) layers. Due to the passivation effect of hBN layers, low-frequency noise in hBN/MoS2/hBN channel devices exhibited substantial reduction compared to the case of MoS2 channel directly on a silicon dioxide substrate and also silicon devices, suggesting that the clean interface of substrate and gate dielectric layers, as well as the protected surface of the MoS2 channel by hBN passivation from the damage by the fabrication process, contribute to the strong reduction in low-frequency noise. The results indicated that 2D materials are suitable for cryo-CMOS technology in terms of low-frequency noise since they will bring about mitigation of one of the most serious causes of quantum phase decoherence of qubits in future large-scale integrated quantum computers.

Energy transport and relaxation of phonon polaritons in two-dimensional polar crystals

Energy transport and relaxation of phonon polaritons (PhPs) are studied, based on a macroscopic phonon model, for atomic layers of hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDs). The velocity of the energy flow (energy velocity) is derived from the energy flow and density; it equals the group velocity, similar to the results of bulk and surface PhPs. In electrostatic approximation, valid once the frequency is slightly above ω0 (e.g., ω>1.002ω0∼224cm−1 for pentalayer MoTe2; ω0 is the zone-center optical-phonon frequency), simple formulas are obtained for the energy velocity and relaxation rate (ERR). While the energy velocity increases proportionally with the number of layers N, the ERR is independent of N. The ERR equals the phonon damping rate in freestanding layers, but it is slightly decreased in SiO2-supported layers and has a non-monotonic frequency dependence (the decrease is smaller than 5.4% for hBN layers and negligible for TMD layers). The energy velocity decreases significantly with frequency in both freestanding and supported layers. Near ω0, however, the PhP properties should be calculated rigorously, and they all depend on N as well as the dielectric environment. High-frequency screening should be included to study the energy transport. The energy velocity can be engineered by varying N and the dielectric environment; it also can be tuned together with the propagation quality factor by the incident light frequency. The MoTe2 layers should be exploited for a far-infrared PhP material (wavelengths 43–45 μm); this is just an example of application of the proposed model to the considered materials.

Cavity Quantum Electrodynamics with Hyperbolic van der Waals Materials.

The ground-state properties and excitation energies of a quantum emitter can be modified in the ultrastrong coupling regime of cavity quantum electrodynamics (QED) where the light-matter interaction strength becomes comparable to the cavity resonance frequency. Recent studies have started to explore the possibility of controlling an electronic material by embedding it in a cavity that confines electromagnetic fields in deep subwavelength scales. Currently, there is a strong interest in realizing ultrastrong-coupling cavity QED in the terahertz (THz) part of the spectrum, since most of the elementary excitations of quantum materials are in this frequency range. We propose and discuss a promising platform to achieve this goal based on a two-dimensional electronic material encapsulated by a planar cavity consisting of ultrathin polar van der Waals crystals. As a concrete setup, we show that nanometer-thick hexagonal boron nitride layers should allow one to reach the ultrastrong coupling regime for single-electron cyclotron resonance in a bilayer graphene. The proposed cavity platform can be realized by a wide variety of thin dielectric materials with hyperbolic dispersions. Consequently, van der Waals heterostructures hold the promise of becoming a versatile playground for exploring the ultrastrong-coupling physics of cavity QED materials.

Highly confined low-loss light transmission in linear array-enabled hybrid plasmonic waveguides

Deep subwavelength highly confined and long-range optical propagation is vital for photonics integration. However, the performance of the guided mode could be improved by the trade-off between light confinement and loss. Here, we demonstrate a high-performance hybrid waveguide consisting of a high-index nanowire separated from a linear array by a low-index dielectric gap. The array significantly achieves the plasmonic platform optimization based on a linear combination of few-layer graphene (FLG) and hexagonal boron nitride (hBN) layers. Through the hybridization of graphene plasmon polaritons and hyperbolic phonon polaritons mode, the resulting hybrid waveguide shows at least double times larger propagation distance and smaller mode area than the multilayer waveguide. Further, modulated by altering material configuration and geometric effects, the mode properties reveal that it is more flexible to adjust the optical transmission, along with a strong deep-subwavelength mode with low loss. Because of highly confined low-loss propagation, the hybrid waveguide is expected to be an excellent building block for various mid-infrared photonic integrated circuits. The present structure also has the potential to be extended to other FLGs, like magic-angle twisted bilayer graphene and trilayer graphene/hBN moiré superlattice.

Borazine Promoted Growth of Highly Oriented Thin Films.

We report on a phenomenon, where thin films sputter-deposited on single-crystalline Al2O3(0001) substrates exposed to borazine─a precursor commonly used for the synthesis of hexagonal boron nitride layers─are more highly oriented than those grown on bare Al2O3(0001) under the same conditions. We observed this phenomenon in face-centered cubic Pd, body-centered cubic Mo, and trigonal Ta2C thin films grown on Al2O3(0001). Interestingly, intermittent exposure to borazine during the growth of Ta2C thin films on Ta2C yields better crystallinity than direct deposition of monolithic Ta2C. We attribute these rather unusual results to a combination of both enhanced adatom mobilities on, and epitaxial registry with, surfaces exposed to borazine during the deposition. We expect that our approach can potentially help improve the crystalline quality of thin films deposited on a variety of substrates.

Effects of hexagonal boron nitride encapsulation on the electronic properties of Cu, Li, and O-doped black phosphorus monolayer

In this work, the structural and electronic properties of Cu, Li, and O-doped phosphorene/hexagonal boron nitride systems were studied based on density functional theory calculations. The intrinsic physical properties of the pristine phosphorene layer do not change with the addition of hexagonal boron nitride layers. While doping of phosphorene leads to strong binding energy at the interface, hence, improving the stability of the doped phosphorene/hexagonal boron nitride layers in systems. The electronic properties of the encapsulated systems change after doping. The band gap also decreases from 1.1200 eV in pristine encapsulated phosphorene to 0.3087, 0.0076, and 0.3100 eV in encapsulated phosphorene doped with Li, Cu, and O, respectively. The partial density of states and band structure diagrams show that the electronic properties of these systems shift from semiconductor to metallic behavior which can be exploited in electronic and optical applications.

Plasmon-Triggered Ultrafast Operation of Color Centers in Hexagonal Boron Nitride Layers.

High-quality emission centers in two-dimensional materials are promising components for future photonic and optoelectronic applications. Carbon-enriched hexagonal boron nitride (hBN:C) layers host atom-like color-center (CC) defects with strong and robust photoemission up to room temperature. Placing the hBN:C layers on top of Ag triangle nanoparticles (NPs) accelerates the decay of the CC defects down to 46 ps from their reference bulk value of 350 ps. The ultrafast decay is achieved due to the efficient excitation of the plasmon modes of the Ag NPs by the near field of the CCs. Simulations of the CC/Ag NP interaction show that higher Purcell values are expected, although the measured decay of the CCs is limited by the instrument response. The influence of the NP thickness on the Purcell factor of the CCs is analyzed. The ultrafast operation of the CCs in hBN:C layers paves the way for their use in demanding applications, such as single-photon emitters and quantum devices.

Casimir-Polder interactions of S -state Rydberg atoms with graphene

We investigate the thermal Casimir-Polder (CP) potential of $^{87}\mathrm{Rb}$ atoms in Rydberg $nS$-states near single- and double-layer graphene, and briefly look into the lifetimes near graphene-hexagonal boron nitride (hBN) multilayered structures. The dependence of the CP potential on parameters such as atom-surface distance, temperature, principal quantum number $n$, and graphene Fermi energy are explored. Through large-scale numerical simulations, we show that, in the nonretarded regime, the CP potential is dominated by the nonresonant and evanescent-wave terms which are monotonic, and that, in the retarded regime, the CP potential exhibits spatial oscillations. We identify that the most important contributions to the resonant component of the CP potential come from the $nS\text{\ensuremath{-}}nP$ and $nS\text{\ensuremath{-}}(n\ensuremath{-}1)P$ transitions. Scaling of the CP potential as a function of the principal quantum number and temperature is obtained. A heterostructure comprising hexagonal boron nitride layers sandwiched between two graphene layers is also studied. When the boron nitride layer is sufficiently thin, the CP potential can be weakened by changing the Fermi energy of the top graphene layer. Our study provides insights for understanding and controlling CP potentials experienced by Rydberg atoms near single- and multilayer graphene-based van der Waals heterostructures.

Quasi‐Van der Waals Epitaxial Growth of GaN on Hexagonal Boron Nitride via Metal–Organic Chemical Vapor Deposition

Herein, GaN grown on hexagonal boron nitride (h‐BN) layer is investigated through surface treatment to improve the quality of the GaN layer. The surface treatment method comprises the annealing and nitridation methods. The surface bonding property of the h‐BN layer is confirmed after surface treatment methods by X‐ray photoelectron spectroscopy. These conditions affect the formation of the N–H bond and limit the N–O bonding. For the X‐ray diffraction results of the GaN template on the h‐BN layer, the full width at half maximum of the GaN (002) plane significantly decreases with the surface treatment of the h‐BN layer, whereas that of the GaN (102) plane changes slightly. Moreover, the Raman spectra are measured to confirm the stress relaxation caused by the effect of quasi‐van der Waals (QvdW) epitaxy between the GaN and h‐BN layers. The GaN/h‐BN with surface treatment has smaller compressive strain value than GaN/sapphire. The QvdW epitaxy between the GaN and h‐BN layers affects the improvement of the GaN quality on h‐BN layer owing to the increased dangling bond density with surface treatment. Therefore, this indicates that the GaN grown on the h‐BN layer is QvdW epitaxy.

Incommensurability and negative thermal expansion of single layer hexagonal boron nitride

The emerging field of straintronics, i.e., the control and utilization of the strain state of 2D-materials, is of great importance for their technological development, specifically in view of their future incorporation into van der Waals heterostructures. To gain fundamental insight into structural peculiarities of two-dimensional systems, single layer hexagonal boron nitride (hBN) grown on Ir(111) by chemical vapor deposition was used as a prototypical model system: High-resolution reciprocal space mapping reveals the incommensurate nature of the material system by measuring the hBN in plane lattice parameter with high precision, facilitated by the moiré magnification effect in electron diffraction. In a growth temperature (Tg) regime of 700–1150°C an average lattice parameter of 2.496 ± 0.006 Å was found. Eventually, careful disentanglement of the hBN’s and substrate’s behavior for rising Tg allowed the determination of a negative thermal expansion coefficient of αhBN = −2.4 ± 1.2 × 10-6 K−1 for free-standing hBN.

Wafer-Scale Single Crystal Hexagonal Boron Nitride Layers Grown by Submicron-Spacing Vapor Deposition.

The direct growth of wafer-scale single crystal two-dimensional (2D) hexagonal boron nitride (h-BN) layer with a controllable thickness is highly desirable for 2D-material-based device applications. Here, for the first time, a facile submicron-spacing vapor deposition (SSVD) method is reported to achieve 2-inch single crystal h-BN layers with controllable thickness from monolayer to tens of nanometers on the dielectric sapphire substrates using a boron film as the solid source. In the SSVD growth, the boron film is fully covered by the same-sized sapphire substrate with a submicron spacing, leading to an efficient vapor diffusion transport. The epitaxial h-BN layer exhibits extremely high crystalline quality, as demonstrated by both a sharp Raman E2g vibration mode (12 cm-1 ) and a narrow X-ray rocking curve (0.10°). Furthermore, a deep ultraviolet photodetector and a ZrS2 /h-BN heterostructure fabricated from the h-BN layer demonstrate its fascinating properties and potential applications. This facile method to synthesize wafer-scale single crystal h-BN layers with controllable thickness paves the way to future 2D semiconductor-based electronics and optoelectronics.

Regulation of Li+ Diffusion via an Engineered Separator to Realize a Homogeneous Lithium Microstructure in Advanced Li-Metal Batteries

Lithium metal, the most promising anode material, is receiving increasing interest owing to its high theoretical capacity (3860 mA h g-1) and low negative potential (-3.04 V vs. standard hydrogen electrode). However, the uneven Li dissolution/deposition behavior causes a degraded cycle stability and safety issues, thus seriously restricting the application of Li-metal batteries (LMBs). Separator modification is one of the most versatile and feasible approaches to overcome this problem. In this study, polypropylene (PP) separators are prepared and coated with an inert hexagonal boron nitride (h-BN) layer, which can provide sufficient ion transport channels and physical protection. The h-BN@PP separator exhibits a remarkable effect on the regulation of the diffusion and nucleation of Li+ to realize a homogeneous Li microstructure, thereby reducing the voltage polarization and improving the cycle performance of the battery. All LMBs equipped with the modified separators exhibit excellent cycling stabilities. The Li|Li symmetric cell exhibits a stable cycling for over 2300 h with a polarization voltage of 13 mV. In conclusion, the modified h-BN@PP separator has significant potential for stabilizing various Li metal anodes, which strongly promotes the applications of advanced LMBs.