Hexagonal Boron Nitride Layers Research Articles

Full phonon dispersion along the stacking direction in nanoscale van der Waals materials by picosecond acoustics

Phonon dispersion in crystals determines many important material properties, but its measurement usually requires large-scale facilities and is limited to bulk samples. Here, we demonstrate the measurement of full phonon dispersion along the stacking direction in nanoscale systems by using picosecond acoustics. A heterostructure sample was prepared consisting of layers of hexagonal boron nitride (hBN) sandwiching a thin layer of black phosphorus (BP), within which a strain pulse was generated by photoexcitation and observed with an optical probe in the BP layer. The strain pulse traverses to the few nanometer thick hBN layers, where it propagates to the edge and echoes back, like acoustic waves in Newton’s cradle. The echoes returning to the BP layer provide information on the frequency-dependent time-of-flight and group velocity dispersion of the sample system. The microscopic origin of the photoinduced strain pulse generation and its propagation is revealed from first principles. Phonon frequency combs observed in the Fourier transform spectrum confirm the strain wave round trips and demonstrate the feasibility of determining group velocity dispersion through photoacoustics.

Membrane hybrid system assembled with MXene@h-BN for efficient typical pharmaceuticals removal and self-cleaning: Synergetic effect on adsorption-photocatalytic process

Membrane filtration is regarded as one of the promising technologies for water treatment. However, insufficient micropollutants removal and membrane fouling still limited the further application. Herein, this study engineered a novel membrane hybrid system integrating typical pharmaceuticals removal and membrane self-cleaning by assembled layered MXene and hexagonal boron nitride (h-BN). By doping a small amount of MXene into h-BN (mass ratio of 1 to 10), the synergetic process of adsorption and photocatalytic degradation were achieved. Based on experimental and density functional theory (DFT) calculation results, the chemisorption-photodegradation performance of the membrane hybrid system assembled with 1/10 MXene@h-BN was significantly enhanced due to the termination group and conductive property of MXene. In addition, trimethoprim (TMP) degradation by the above membrane followed the Langmuir–Hinshelwood model, indicating pollutants removal initiated not only on the membrane surface but also in the bulk solution under simulated solar irradiation. This phenomenon was attributed to the generation of •OH and 1O2 by forming Ohmic contact with altered internal band structure. The durability and practical experiments with secondary effluent from local wastewater treatment were also tested. The exceptional self-cleaning performance of the membrane hybrid system assembled with 1/10 MXene@h-BN was demonstrated by lower membrane fouling and maintaining durability even after five cycles. Furthermore, the performance (both pollutants removal and flux) of the modified membrane for secondary effluent treatment was also considerable, validating its potential for practical application. Owing to the excellent features, the membrane assembled with MXene@h-BN is anticipated to be an ideal candidate for advanced wastewater treatment.

Single-Photon Emitters inside Bubbles Formed at Homointerfaces between Hexagonal Boron Nitride Layers

Single-Photon Emitters inside Bubbles Formed at Homointerfaces between Hexagonal Boron Nitride Layers

Optical detection of passive thermal lattice deformation of monolayer WS2 in van der Waals heterostructures of WS2/h-BN

Van der Waals heterostructures (vdWHs) of the constituent two-dimensional (2D) layered materials are quickly emerging as functional structures for diverse novel electronic and optoelectronic devices. To further explore the device applications of the vdWHs, an open question, which is whether the lattice of atomic thin active layer follows the deformation of its neighboring thick layers in a vdWH, needs to be clarified. Herein, the thermal deformation of WS2 monolayers in vdWHs with and without hexagonal boron nitride (h-BN) layers on SiO2/Si substrates is investigated with combined optical spectroscopies of photoluminescence (PL) and Raman scattering. With a developed strategy extracting the pure strain-induced effects from the temperature-dependent PL and Raman scattering spectroscopic results, the relative deformations of the WS2 monolayers and the h-BN layers with respect to the substrate have been identified. Both the Raman and PL results consistently reveal that the lattice variation of the WS2 monolayers passively match the deformation of the h-BN layers, and the WS2 monolayer on the SiO2/Si substrate exhibits the largest strain. These findings have not only demonstrated the passive behavior of the thermal strain of 2D semiconductor monolayers in vdWHs heterostructures, but also paved a way to tailor the thermal strain via designing and constructing vdWHs.

Distance Dependence of the Energy Transfer Mechanism in WS_{2}-Graphene Heterostructures.

We report on the mechanism of energy transfer in Van der Waals heterostructures of the two-dimensional semiconductor WS_{2} and graphene with varying interlayer distances, achieved through spacer layers of hexagonal boron nitride (h-BN). We record photoluminescence and reflection spectra at interlayer distances between 0.5 and 5.8nm (0-16 h-BN layers). We find that the energy transfer is dominated by states outside the light cone, indicative of a Förster transfer process, with an additional contribution from a Dexter process at 0.5nm interlayer distance. We find that the measured dependence of the luminescence intensity on interlayer distances above 1nm can be quantitatively described using recently reported values of the Förster transfer rates of thermalized charge carriers. At smaller interlayer distances, the experimentally observed transfer rates exceed the predictions and, furthermore, depend on excess energy as well as on excitation density. Since the transfer probability of the Förster mechanism depends on the momentum of electron-hole pairs, we conclude that, at these distances, the transfer is driven by nonrelaxed charge carrier distributions.

Highly Thermally Conductive Flexible Biomimetic APTES-BNNS/BC Nanocomposite Paper by Sol-Gel-Film Technology.

Owing to the evolution of 5G technology, new energy vehicles, flexible electronics, miniaturization and integration of microelectronic devices, high-frequency and high-power devices, and thermal management of materials must consider additional limitations such as electrical insulation, excellent transverse heat transfer, flexibility, and weight. Boron nitride nanosheets (BNNSs) are ideal insulating materials with high thermal conductivity. However, the problem of the 3D thermal conductivity pathway and toughness strength of nanocomposite paper loaded with inorganic thermal conductivity fillers remains a huge challenge. In this study, we propose a new method for preparing ultrathin, large, and uniformly thick BNNS for quantitative production. Bulk hexagonal boron nitride (hBN) layers were exfoliated using a simple and low-cost hydrothermal reaction, and large-scale fewer-layered BNNSs were efficiently prepared by ball milling with a high yield (up to 80%). Based on the aforementioned step, a flexible insulating composite film with high thermal conductivity and a natural "brick-mud" shell structure was constructed via the sol-gel-film conversion method. After prestretching and hot-pressing treatment, the hydrogels became denser, and the modified BNNS formed a three-dimensional (3D) network structure with an ordered orientation and interconnections in the bacterial cellulose (BC) matrix. After 100 folding cycles, the tensile strength of the nanofiber composite film reached 53 MPa, and the strength retention rate exceeded 42%. By optimizing the modified BNNS content, the thermal conductivity reached 24 W/(m·K). This simple approach has wide application potential in the next-generation electronic devices, providing options for designing thermal interface materials with excellent electrical insulation, high thermal stability, and flexibility.

Photo‐Induced Electrical Gating with the Inserting of an Interfacial Carrier Blocking Layer for Organic/Graphene Phototransistors

AbstractAlthough ultrahigh photoconductive gain is achieved in organic semiconductor‐sensitized graphene phototransistors, the response speed is limited by the photo‐induced carrier injection at the organic/graphene interface. In this work, an insulating hexagonal boron nitride layer is inserted between the organic heterostructure and graphene to block the interfacial carrier transport. Above the h‐BN layer, the organic heterostructure is adopted as the light‐absorbing layer, and the photoresponse of graphene is realized through electrical gating of the atop h‐BN dielectric layer, by the accumulated carriers in the organic heterostructure. Under this “photo‐induced electrical gating” mechanism, the recombination of nonequilibrium carriers takes place only within the organic heterostructure. Furthermore, monolayer perylene‐3,4,9,10‐tetracarboxylic dianhydride (PTCDA) is deposited on graphene before transfer of the h‐BN, by which the interfacial encapsulated bubbles are effectively reduced. As a step further, the trap states are pre‐filled with background light illumination, then the carrier recombination follows the band‐to‐band pathway. Finally, the response time of the organic/h‐BN/graphene phototransistor is reduced to 7.15 ms, and a high photoresponsivity is achieved even though the carrier multiplication is sacrificed, which is reconciled by the improvement of the device quantum efficiency.

The impact of hBN layers on guided exciton–polariton modes in WS2 multilayers

Abstract Guided exciton–polariton modes naturally exist in bare transition metal dichalcogenide (TMDC) layers due to self-hybridization between excitons and photons. However, these guided polariton modes exhibit a limited propagation distance owing to the substantial exciton absorption within the material. Here, we investigated the impact of hexagonal boron nitride (hBN) layers on guided exciton–polariton modes in WS2 multilayers. By integrating hBN layers, we demonstrate a notable enhancement in the quality of guided exciton–polariton modes. The hBN layers can reduce substrate surface roughness and provide surface protection for the WS2 layer, mitigating inhomogeneous broadening of the exciton resonance. Consequently, we experimentally observed that the propagation distance of polariton modes substantially increased with hBN layers. Additionally, the polariton spectrum broadened due to efficient exciton relaxation to the polariton states at lower energies. Comparison with simulation data emphasizes that the observed improvements are primarily attributed to enhanced exciton quality. The promising outcomes with hBN encapsulation suggest its potential to overcome strong excitonic losses of the guided exciton polariton in implementing nanophotonic devices. Furthermore, this approach provides a new avenue for exploring the novel physics of guided exciton–polariton modes and their potential applications in polariton-based all-optical integrated circuits.

Ultra-thin van der Waals magnetic tunnel junction based on monoatomic boron vacancy of hexagonal boron nitride.

We present a novel strategy to create a van der Waals-based magnetic tunnel junction (MTJ) comprising a three-atom layer of graphene (Gr) sandwiched with hexagonal boron nitride (hBN) layers by introducing a monoatomic boron vacancy in each hBN layer. The magnetic properties and electronic structure of the system were investigated using density functional theory (DFT) and the transmission probability of the MTJ was investigated using the Landauer-Büttiker formalism within the non-equilibrium Green function method. The Stoner gap was created between the spin-majority channel and the spin-minority channel on the local density of states of the hBN monoatomic boron-vacancy (VB) near the Fermi energy, creating a possible control of the spin valve by considering two magnetic alignment of hBN(VB) layers, anti-parallel configuration (APC) and parallel configuration (PC). The transmission probability calculation results showed a high electron transmission in the PC of the hBN(VB) layers and a low transmission when the APC was considered. A high tunneling magnetoresistance (TMR) ratio of approximately 400% was observed when comparing the APC and PC of the hBN(VB) layers in hBN(VB)/Gr/hBN(VB), giving the highest TMR ratio for the thinnest MTJ system.

Anisotropic thermo-mechanical response of layered hexagonal boron nitride and black phosphorus: application as a simultaneous pressure and temperature sensor.

Hexagonal boron nitride (hBN) and black phosphorus (bP) are crystalline materials that can be seen as ordered stackings of two-dimensional layers, which lead to outstanding anisotropic physical properties. Knowledge of the thermal equations of state of hBN and bP is of great interest in the field of 2D materials for a better understanding of their anisotropic thermo-mechanical properties and exfoliation mechanism towards the preparation of important single-layer materials like hexagonal boron nitride nanosheets and phosphorene. Despite several theoretical and experimental studies, important uncertainties remain in the determination of the thermoelastic parameters of hBN and bP. Here, we report accurate thermal expansion and compressibility measurements along the individual crystallographic axes, using in situ high-temperature and high-pressure high-resolution synchrotron X-ray diffraction. In particular, we have quantitatively determined the subtle variations of the in-plane and volumetric thermal expansion coefficients and compressibility parameters by subjecting these materials to hydrostatic conditions and by collecting a large number of data points in small pressure and temperature increments. In addition, based on the anisotropic behavior of bP, we propose the use of this material as a sensor for the simultaneous determination of pressure and temperature in the range of 0-5 GPa and 298-1700 K, respectively.

High Gain Graphene Based Hot Electron Transistor with Record High Saturated Output Current Density

AbstractHot electron transistors (HETs) represent an exciting new device for integration into semiconductor technology, holding the promise of high‐frequency electronics beyond the limits of SiGe bipolar hetero transistors. With the exploration of 2D materials such as graphene and new device architectures, hot electron transistors have the potential to revolutionize the landscape of modern electronics. This study highlights a novel hot electron transistor structure with a record output current density of 800 A cm−2 and a high current gain α, fabricated using a scalable fabrication approach. The hot electron transistor structure comprises 2D hexagonal boron nitride and graphene layers wet transferred to a germanium substrate. The combination of these materials results in exceptional performance, particularly in terms of the highly saturated output current density. The scalable fabrication scheme used to produce the hot electron transistor opens up opportunities for large‐scale manufacturing. This breakthrough in hot electron transistor technology holds promise for advanced electronic applications, offering high current capabilities in a practical and manufacturable device.

Generalized Mie Theory for Full‐Wave Numerical Calculations of Scattering Near‐Field Optical Microscopy with Arbitrary Geometries

Scattering‐type scanning near‐field optical microscopy (s‐SNOM) is becoming a premier method for the nanoscale optical investigation of materials well beyond the diffraction limit. A number of popular numerical methods exist to predict the near‐field contrast for axisymmetric configurations of scatterers on a surface in the quasi‐electrostatic approximation. Here, a fully electrodynamic approach is given for the calculation of near‐field contrast of several scatterers in arbitrary configuration, based on the generalized Mie scattering method. Examples for the potential of this new approach are given by showing the coupling of hyperbolic phonon polaritons in hexagonal boron nitride (hBN) layers and showing enhanced scattering in core–shell systems. In general, this method enables the numerical calculation of the near‐field contrast in a variety of strongly resonant scatterers and is able to accurately recreate spatial near‐field maps.

Phonon Thermal Transport at Interfaces of a Graphene/Vertically Aligned Carbon Nanotubes/Hexagonal Boron Nitride Sandwiched Heterostructure

Molecular dynamics simulation is used to calculate the interfacial thermal resistance of a graphene/carbon nanotubes/hexagonal boron nitride (Gr/CNTs/hBN) sandwiched heterostructure, in which vertically aligned carbon nanotube (VACNT) arrays are covalently bonded to graphene and hexagonal boron nitride layers. We find that the interfacial thermal resistance (ITR) of the Gr/VACNT/hBN sandwiched heterostructure is one to two orders of magnitude smaller than the ITR of a Gr/hBN van der Waals heterostructure with the same plane size. It is observed that covalent bonding effectively enhances the phonon coupling between Gr and hBN layers, resulting in an increase in the overlap factor of phonon density of states between Gr and hBN, thus reducing the ITR of Gr and hBN. In addition, the chirality, size (diameter and length), and packing density of sandwich-layer VACNTs have an important influence on the ITR of the heterostructure. Under the same CNT diameter and length, the ITR of the sandwiched heterostructure with armchair-shaped VACNTs is higher than that of the sandwiched heterostructure with zigzag-shaped VACNTs due to the different chemical bonding of chiral CNTs with Gr and hBN. When the armchair-shaped CNT diameter increases or the length decreases, the ITR of the sandwiched heterostructure tends to decrease. Moreover, the increase in the VACNT packing density also leads to a continuous decrease in the ITR of the sandwiched heterostructure, attributed to the extremely high intrinsic thermal conductivity of CNTs and the increase of out-of-plane heat transfer channels. This work may be helpful for understanding the mechanism for ITR in multilayer vertical heterostructures, and provides theoretical guidance for a new strategy to regulate the interlayer thermal resistance of heterostructures by optimizing the design of sandwich layer thermal interface materials.

Germanium surface cleaning and ALD of a protective boron nitride overlayer

Germanium exhibits superior hole and electron mobility compared with silicon, making it a promising candidate for replacement of silicon in certain future CMOS applications. In such applications, achieving atomically clean Ge surfaces and the subsequent deposition of ultrathin passivation barriers without interfacial reaction are critical. In this study, we present in situ x-ray photoelectron spectroscopy (XPS) investigations of hydrocarbon removal from the Ge surface utilizing atomic oxygen at room temperature, as well as removal of hydrocarbons and of germanium oxide (GeO2) through atomic hydrogen treatment at 350 °C. Subsequently, atomic layer deposition (ALD) was used to create a protective layer of hexagonal boron nitride (h-BN) with an average thickness of 3 monolayers (ML). Tris(dimethylamino)borane and ammonia precursors were utilized at 450 °C for the deposition process. Intermittent in situ XPS analysis during ALD confirmed h-BN growth, stoichiometry, and the absence of interfacial reaction with Ge. XPS analysis after subsequent exposure of the Ge film with a h-BN overlayer of ∼9 Å average thickness to 7.2 × 104 l of atomic O (O3P) at room temperature yielded no evidence of Ge oxidation, with only the surface layer of the h-BN film exhibiting oxidation. These results present a practical and scalable route toward the preparation of clean Ge surfaces and subsequent deposition of protective, nanothin h-BN barriers for subsequent processing.

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.