hBN Monolayer Research Articles

Accurate Layer-Number Determination of Hexagonal Boron Nitride Using Optical Characterization.

Precise determination of the layer number (N) of hexagonal boron nitride (hBN) is crucial for its integration with other layered materials in applications such as ferroelectric devices and moiré potential modulation. We present a nondestructive method to accurately identify N, combining optical contrast analysis with second harmonic generation (SHG) measurements. By studying the flakes on 90 nm thick SiO2/Si substrates, we demonstrate that red-filtered optical images provide a clear contrast step in N with an uncertainty of ±1 layer, while SHG measurements further reduce the error by distinguishing even and odd layers. We also introduce a real-time detection technique to identify monolayer and few-layer hBN, improving flake identification efficiency. Given the growing interest in twisted hBN interfaces and their integration in van der Waals heterostructures, this method offers a practical approach for future studies.

Step‐Directed Epitaxy of Unidirectional Hexagonal Boron Nitride on Vicinal Ge(110)

Insulating hexagonal boron nitride (hBN) films with precisely controlled thickness are ideal dielectric components to modulate various interfaces in electronic devices. To achieve this, high‐quality hBN with controlled atomic configurations must be able to form pristine interfaces with various materials in devices. However, previously reported large‐scale hBN films with uniform thickness either are polycrystalline or are not suitable for atomically clean assembly via mechanical exfoliation, limiting their applications in device technology. Herein, the large‐scale growth of monolayer (ML) single crystalline hBN films on Ge(110) substrates by using chemical vapor deposition is reported. Vicinal Ge(110) substrates are used for the step‐directed epitaxial growth of hBN, where Ge atomic steps act as the hBN nucleation sites, guiding the unidirectional alignments of multiple hBN domains. Density functional theory calculations reveal that the optimum hydrogen passivations on both hBN edges and Ge surfaces enable the epitaxial coupling between hBN and the Ge step edges and the single crystallinity of the final hBN films. Using epitaxially grown ML hBN films, a few hBN films with controlled stacking orders and pristine interfaces through a layer‐by‐layer assembly process are fabricated. These films function as high‐quality dielectrics to enhance carrier transport in graphene and MoS2 channels.

Single-crystal hBN Monolayers from Aligned Hexagonal Islands

Hexagonal boron nitride (hBN), as one of the few two-dimensional insulators, holds strategic importance for advancing post-silicon electronic devices and circuits. Achieving wafer-scale, high-quality monolayer hBN is essential for its integration into the semiconductor industry. However, the physical mechanisms behind the chemical vapor deposition (CVD) synthesis of hBN are not yet well understood. Investigating morphology engineering is critical for developing scalable synthetic techniques for the large-scale production of high-quality hBN. In this study, we explored the underlying mechanisms of the CVD growth process of hBN and found that the involvement of a small amount of oxygen effectively modulates the shape of the single-crystal hBN islands. By tuning the oxygen content in the CVD system, we synthesized well-aligned hexagonal hBN islands and achieved a continuous, high-quality single-crystal monolayer hBN film through the merging of these hexagonal islands on conventional single-crystal metal-foil substrates. Density functional theory was used to study the edges of hBN monolayers grown in an oxygen-assisted environment, providing insights into the formation mechanism. This study opens new pathways for controlling the island shape of 2D materials and establishes a foundation for the industrial-scale production of high-quality, large-area, single-crystal hBN.

Horizontal Lithium Electrodeposition on Atomically Polarized Monolayer Hexagonal Boron Nitride.

Both uncontrolled Li dendrite growth and corrosion are major obstacles to the practical application of Li-metal batteries. Despite numerous attempts to address these challenges, effective solutions for dendrite-free reversible Li electrodeposition have remained elusive. Here, we demonstrate the horizontal Li electrodeposition on top of atomically polarized monolayer hexagonal boron nitride (hBN). Theoretical investigations revealed that the hexagonal lattice configuration and polarity of the monolayer hBN, devoid of dangling bonds, reduced the energy barrier for the surface diffusion of Li, thus facilitating reversible in-plane Li growth. Moreover, the single-atom-thick hBN deposited on a Cu current collector (monolayer hBN/Cu) facilitated the formation of an inorganic-rich, homogeneous solid electrolyte interphase layer, which enabled the uniform Li+ flux and suppressed Li corrosion. Consequently, Li-metal and anode-free full cells containing the monolayer hBN/Cu exhibited improved rate performance and cycle life. This study suggests that the monolayer hBN is a promising class of underlying seed layers to enable dendrite- and corrosion-free, horizontal Li electrodeposition for sustainable Li-metal anodes in next-generation batteries.

Low-symmetry vacancy-related spin qubit in hexagonal boron nitride

Point defect qubits in semiconductors have demonstrated their outstanding capabilities for high spatial resolution sensing generating broad multidisciplinary interest. Hexagonal boron nitride (hBN) hosting point defect qubits have recently opened up new horizons for quantum sensing by implementing sensing foils. The sensitivity of point defect sensors in hBN is currently limited by the linewidth of the magnetic resonance signal, which is broadened due to strong hyperfine couplings. Here, we report on a vacancy-related spin qubit with an inherently low symmetry configuration, the VB2 center, giving rise to a reduced magnetic resonance linewidth at zero magnetic fields. The VB2 center is also equipped with a classical memory that can be utilized for storing population information. Using scanning transmission electron microscopy imaging, we confirm the existence of the VB2 configuration in free-standing monolayer hBN.

Chemical vapor deposition of hexagonal boron nitride on germanium from borazine.

The growth of hexagonal boron nitride (hBN) directly onto semiconducting substrates, like Ge and Ge on Si, promises to advance the integration of hBN into microelectronics. However, a detailed understanding of the growth and characteristics of hBN islands and monolayers on these substrates is lacking. Here, we present the growth of hBN on Ge and Ge epilayers on Si via high-vacuum chemical vapor deposition from borazine and study the effects of Ge sublimation, surface orientation, and vicinality on the shape and alignment of hBN islands. We find that suppressing Ge sublimation is essential for growing high quality hBN and that the Ge surface orientation and vicinality strongly affect hBN alignment. Interestingly, 95% of hBN islands are unidirectionally aligned on Ge(111), which may be a path toward metal- and transfer-free, single-crystalline hBN. Finally, we extend the growth time and borazine partial pressure to grow monolayer hBN on Ge and Ge epilayers on Si. These findings provide new insights into the growth of high-quality hBN on semiconducting substrates.

Enhanced Performance of Self‐Powered Ge Schottky Photodetectors Enabled by 2D hBN Monolayer Passivation

Enhanced Performance of Self‐Powered Ge Schottky Photodetectors Enabled by 2D hBN Monolayer Passivation

Near-Ideal Subthreshold Swing in Scaled 2D Transistors: The Critical Role of Monolayer hBN Passivation

Near-Ideal Subthreshold Swing in Scaled 2D Transistors: The Critical Role of Monolayer hBN Passivation

Computational insights into the electronic structure of functionalized monolayer hBN supercapacitor electrodes

In this study, we examined a collection of functionalized hBN monolayers’ electronic structure and quantum capacitance. Different ad-atom adsorption techniques have been used to functionalize the hBN monolayer. Density functional theory calculations are performed to precisely determine the electrical structure of an ad-atom doped hBN monolayer with a range of doping concentrations. The quantum capacitance of every functionalized system was then calculated. Above 500 μF/cm2, a noticeable quantum capacitance has been seen. Our calculations demonstrate that ad-atom doping at hollow position of hBN honeycomb lattice greatly increases the quantum capacitance of hBN monolayer. The microscopic cause of this system’s increased quantum capacitance has been investigated. Our DFT-based calculation shows that the system has a very large quantum capacitance due to the creation of new electronic states in the vicinity of the band edge and the Fermi level shift caused by the ad-atom adsorption.

First-principles study on shape of intrinsic hBN island nucleated during CVD initial growth on Cu(111)

Using the first-principles calculation, the shape of the hexagonal boron nitride (hBN) islands was investigated, with a focus on the edge of the small monolayer hBN islands on Cu(111) and intrinsic nucleation during the CVD initial growth. Several key observations were made. First, desorption does not play a decisive role in shaping the islands. Second, for small islands, there is no chemical potential range where the armchair edge is stable. Third, the bonds between Cu atoms on the surface and N atoms at the edge are strong, whereas the bonds between Cu atoms on the surface and B atoms at the edge are comparatively weaker. Finally, triangular islands with N edge tend to grow more spontaneously than those with B edge because the critical size is smaller for those with N edge across a wide chemical potential range.

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

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

Coexistence of Quantum-Spin-Hall and Quantum-Hall-Topological-Insulating States in Graphene/hBN on SrTiO3 Substrate.

SrTiO3 (STO) substrate, a perovskite oxide material known for its high dielectric constant (ɛ), facilitates the observation of various (high-temperature) quantum phenomena. A quantum Hall topological insulating (QHTI) state, comprising two copies of QH states with antiparallel two ferromagnetic edge-spin overlap protected by the U(1) axial rotation symmetry of spin polarization, has recently been achieved in low magnetic field (B) even as high as ≈100 K in a monolayer graphene/thin hexagonal boron nitride (hBN) spacer placed on an STO substrate, thanks to the high ɛ of STO. Despite the use of the heavy STO substrate, however, proximity-induced quantum spin Hall (QSH) states in 2D TI phases, featuring a topologically protected helical edge spin phase within time-reversal-symmetry, is not confirmed. Here, with the use of a monolayer hBN spacer, it is revealed the coexistence of QSH (at B = 0T) and QHTI (at B ≠ 0) states in the same single graphene sample placed on an STO, with a crossover regime between the two at low B. It is also classified that the different symmetries of the two nontrivial helical edge spin phases in the two states lead to different interaction with electron-puddle quantum dots, caused by a local surface pocket of the STO, in the crossover regime, resulting in a spin dephasing only for the QHTI state. The results obtained using STO substrates open the doors to investigations of novel QH spin states with different symmetries and their correlations with quantum phenomena. This exploration holds value for potential applications in spintronic devices.

Tailoring electronic and optical properties of hBN/InTe and hBN/GaTe heterostructures through biaxial strain engineering

In this research study, we systematically investigate the electronic and optical properties of van der Waals heterostructures (HSs) consisting of InTe (GaTe) and hBN monolayers, subjected to controlled biaxial strain. Our analysis demonstrates that the application of strain induces noteworthy alterations in the electronic band structure, enabling precise manipulation of the band gap and augmentation of the absorption properties of these structures. Employing density functional theory, we conduct a comprehensive examination of the influence of strain on the electronic and optical characteristics of these HSs. Our investigation showcases the remarkable potential of strain engineering in rendering these heterostructures into efficient and robust wide-range absorbers, particularly optimised for the visible spectrum, underscoring their relevance in various photonic and optoelectronic applications, paving the way for integration into advanced nanodevices.

Dual role of hBN as an artificial solid–electrolyte interface layer for safe zinc metal anodes

Monolayer and large area hBN as an artificial SEI layer shows dual role in preventing the undesired side reaction of HER and inducing the heteroepitaxial grown of Zn, successfully resolving the dendrite issue for safe Zn metal anode.

Nonepitaxial Wafer-Scale Single-Crystal 2D Materials on Insulators.

Next-generation nanodevices require 2D material synthesis on insulating substrates. However, growing high-quality 2D-layered materials, such as hexagonal boron nitride (hBN) and graphene, on insulators is challenging owing to the lack of suitable metal catalysts, imperfect lattice matching with substrates, and other factors. Therefore, developing a generally applicable approach for realizing high-quality 2D layers on insulators remains crucial, despite numerous strategies being explored. Herein, a universal strategy is introduced for the nonepitaxial synthesis of wafer-scale single-crystal 2D materials on arbitrary insulating substrates. The metal foil in a nonadhered metal-insulator substrate system is almost melted by a brief high-temperature treatment, thereby pressing the as-grown 2D layers to well attach onto the insulators. High-quality, large-area, single-crystal, monolayer hBN and graphene films are synthesized on various insulating substrates. This strategy provides new pathways for synthesizing various 2D materials on arbitrary insulators and offers a universal epitaxial platform for future single-crystal film production.

Stamped production of single-crystal hexagonal boron nitride monolayers on various insulating substrates

Controllable growth of two-dimensional (2D) single crystals on insulating substrates is the ultimate pursuit for realizing high-end applications in electronics and optoelectronics. However, for the most typical 2D insulator, hexagonal boron nitride (hBN), the production of a single-crystal monolayer on insulating substrates remains challenging. Here, we propose a methodology to realize the facile production of inch-sized single-crystal hBN monolayers on various insulating substrates by an atomic-scale stamp-like technique. The single-crystal Cu foils grown with hBN films can stick tightly (within 0.35 nm) to the insulating substrate at sub-melting temperature of Cu and extrude the hBN grown on the metallic surface onto the insulating substrate. Single-crystal hBN films can then be obtained by removing the Cu foil similar to the stamp process, regardless of the type or crystallinity of the insulating substrates. Our work will likely promote the manufacturing process of fully single-crystal 2D material-based devices and their applications.

Stripe-Like hBN Monolayer Template for Self-Assembly and Alignment of Pentacene Molecules.

Metallic surfaces with unidirectional anisotropy are often used to guide the self-assembly of organic molecules along a particular direction. Such supports thus offer an avenue for the fabrication of hybrid organic-metal interfaces with tailored morphology and precise elemental composition. Nonetheless, such control often comes at the expense of detrimental interfacial interactions that might quench the pristine properties of molecules. Here, hexagonal boron nitride grown on Ir(100) is introduced as a robust platform with several coexisting 1D stripe-like moiré superstructures that effectively guide unidirectional self-assemblies of pentacene molecules, concomitantly preserving their pristine electronic properties. In particular, highly-aligned longitudinal arrays of equally-oriented molecules are formed along two perpendicular directions, as demonstrated by comprehensive scanning tunneling microscopy and photoemission characterization performed at the local and non-local scale, respectively. The functionality of the template is demonstrated by photoemission tomography, a surface-averaging technique requiring a high degree of orientational order of the probed molecules. The successful identification of pentacene's pristine frontier orbitals underlines that the template induces excellent long-range molecular ordering via weak interactions, preventing charge transfer.

Dielectric breakdown and sub-wavelength patterning of monolayer hexagonal boron nitride using femtosecond pulses

Hexagonal boron nitride (hBN) has emerged as a promising two-dimensional (2D) material for many applications in electronics and photonics. Although its linear and nonlinear optical properties have been extensively studied, the interaction of hBN with high-intensity laser pulses, which is important for realizing high-harmonic generation, creating deterministic defects as quantum emitters, and resist-free patterning in this material, has not been investigated. Here we report the first systematic study of dielectric breakdown in chemical vapor deposition (CVD)-grown hBN monolayers induced by single femtosecond laser pulses. We report a breakdown fluence of 0.7 J cm−2, which is at least 7× higher than that of other monolayer 2D materials. A clean removal of hBN without leaving traces behind or causing lateral damage is demonstrated. The ablation features exhibit excellent fidelity with very small edge roughness, which we attribute to its ultrahigh fracture toughness due to its heterogeneous nature with three-fold symmetry. Moreover, even though defects are known to be abundant in CVD-grown hBN, we show experimentally and theoretically that its nonlinear optical breakdown is nearly intrinsic as defects only marginally lower the breakdown threshold. On top of this, we observe that hBN monolayers have a 4–5× lower breakdown threshold than their bulk equivalent. The last two observations can be understood if the carrier generation in monolayers is intrinsically enhanced due to its 2D nature. Finally, we demonstrate laser patterning of array of holes and lines in hBN with sub-wavelength feature sizes. Our work advances the fundamental knowledge of light-hBN interaction in the strong field regime and firmly establishes femtosecond lasers as novel and promising tools for resist-free patterning of hBN monolayers with high fidelity.

Achieving controllable multifunctionality through layer sliding

Dynamical variation of physical properties in a controllable fashion provides exciting possibilities to obtain multifunctional materials. In this work, layer-sliding is employed to modify the structural, interfacial electronic and optical properties of unintercalated and Mg-intercalated two-dimensional (2D) van der Waals heterostructure (vdW-HS) consisting of buckled silicene and hexagonal boron nitride (hBN). The most stable stacking configuration of silicene over hBN is screened out and then intercalated with Mg at the interface. Dynamical-dependent changes in the properties of vdW-HS are performed by sliding silicene over hBN monolayer in the absence and presence of the intercalant. Layer-sliding is carried out in equal length intervals, and various parametric quantities related to the physical characteristics of the vdW-HS are repeatedly calculated and compared. Apart from various parametric quantities, stability of unintercalated and Mg-intercalated vdW-HS is also checked by means of relative total energies, binding energies and vdW gaps along the sliding pathway. Comparison of binding energies shows that the un-slided, half-slided, and fully-slided Mg-intercalated vdW-HS are 1.52, 1.44 and 1.42 eV more stable than the unintercalated vdW-HS. Opening of a small band gap of 12, 31 and 28 meV for un-slided, half-slided and fully-slided unintercalated vdW-HS, respectively, is worth mentioning. To study the interfacial electronic behavior, planar average charge density difference (Δρ) and charge transfer (ΔQ) are also calculated and varied via layer-sliding. Further, we calculated diverse optical spectra such as the complex dielectric function (DF), electron energy loss function [L(ω)], diagonal components of dielectric tensor [ε(iω)], refractive index [n(ω)], extinction coefficient [k(ω)], absorption coefficient [α(ω)], and reflectivity [R(ω)] for un-slided, half-slided and fully-slided unintercalated and Mg-intercalated vdW-HS. Interestingly, the polarization and energy losses have been reduced in the case of Mg-intercalated vdW-HS. The suggested layer-sliding method can be established as a general scheme for bringing multifunctionality into a layered material.

Anomalous isotope effect on mechanical properties of single atomic layer Boron Nitride

The ideal mechanical properties and behaviors of materials without the influence of defects are of great fundamental and engineering significance but considered inaccessible. Here, we use single-atom-thin isotopically pure hexagonal boron nitride (hBN) to demonstrate that two-dimensional (2D) materials offer us close-to ideal experimental platforms to study intrinsic mechanical phenomena. The highly delicate isotope effect on the mechanical properties of monolayer hBN is directly measured by indentation: lighter 10B gives rise to higher elasticity and strength than heavier 11B. This anomalous isotope effect establishes that the intrinsic mechanical properties without the effect of defects could be measured, and the so-called ultrafine and normally neglected isotopic perturbation in nuclear charge distribution sometimes plays a more critical role than the isotopic mass effect in the mechanical and other physical properties of materials.