hBN Monolayer Research Articles

Backside Absorbing Layer Microscopy: Monolayer Counting in 2D Crystal Flakes

The properties of two‐dimensional (2D) material stacks critically depend on the number of monolayers (m) in the stack. It is therefore important to quantify this number, which is a local quantity since 2D stacks are essentially heterogeneous. Optical interferential techniques based on contrast‐enhancing surfaces may be sensitive enough to visualize m variations but the experimental determination of m requires heavy and unstable comparisons with multiparameter numerical models. Focusing on the recent backside absorbing layer microscopy, the most sensitive to date among interferential techniques, a self‐calibrating method is demonstrated allowing instantaneous monolayer counting all over the sample surface which does not require the knowledge of the instrumental parameters, the sample or ambient refractive indices or the detailed structure of the contrast‐enhancing layer. This method is introduced step by step using examples of hexagonal boron nitride (hBN) stacks with increasing complexity. Exact monolayer counting up to 36 hBN monolayers is obtained using basic image analysis.

Dielectric Environment Sensitivity of Carbon Centers in Hexagonal Boron Nitride.

A key advantage of utilizing van-der-Waals (vdW) materials as defect-hosting platforms for quantum applications is the controllable proximity of the defect to the surface or the substrate allowing for improved light extraction, enhanced coupling with photonic elements, or more sensitive metrology. However, this aspect results in a significant challenge for defect identification and characterization, as the defect's properties depend on the the atomic environment. This study explores how the environment can influence the properties of carbon impurity centers in hexagonal boron nitride (hBN). It compares the optical and electronic properties of such defects between bulk-like and few-layer films, showing alteration of the zero-phonon line energies and their phonon sidebands, and enhancements of inhomogeneous broadenings. To disentangle the mechanisms responsible for these changes, including the atomic structure, electronic wavefunctions, and dielectric screening, it combines ab initio calculations with a quantum-embedding approach. By studying various carbon-based defects embedded in monolayer and bulk hBN, it demonstrates that the dominant effect of the change in the environment is the screening of density-density Coulomb interactions between the defect orbitals. The comparative analysis of experimental and theoretical findings paves the way for improved identification of defects in low-dimensional materials and the development of atomic scale sensors for dielectricenvironments.

Thickness Mapping and Layer Number Identification of Exfoliated van der Waals Materials by Fourier Imaging Micro-Ellipsometry

As performance of van der Waals heterostructure devices is governed by the nanoscale thicknesses and homogeneity of their constituent mono- to few-layer flakes, accurate mapping of these properties with high lateral resolution becomes imperative. Spectroscopic ellipsometry is a promising optical technique for such atomically thin-film characterization due to its simplicity, noninvasive nature and high accuracy. However, the effective use of standard ellipsometry methods on exfoliated micron-scale flakes is inhibited by their tens-of-microns lateral resolution or slow data acquisition. In this work, we demonstrate a Fourier imaging spectroscopic micro-ellipsometry method with sub-5 μm lateral resolution and three orders-of-magnitude faster data acquisition than similar-resolution ellipsometers. Simultaneous recording of spectroscopic ellipsometry information at multiple angles results in a highly sensitive system, which is used for performing angstrom-level accurate and consistent thickness mapping on exfoliated mono-, bi- and trilayers of graphene, hexagonal boron nitride (hBN) and transition metal dichalcogenide (MoS2, WS2, MoSe2, WSe2) flakes. The system can successfully identify highly transparent monolayer hBN, a challenging proposition for other characterization tools. The optical microscope integrated ellipsometer can also map minute thickness variations over a micron-scale flake, revealing its lateral inhomogeneity. The prospect of adding standard optical elements to augment generic optical imaging and spectroscopy setups with accurate in situ ellipsometric mapping capability presents potential opportunities for investigation of exfoliated 2D materials.

High Chern number van der Waals magnetic topological multilayers MnBi2Te4/hBN

Chern insulators are two-dimensional magnetic topological materials that conduct electricity along their edges via the one-dimensional chiral modes. The number of these modes is a topological invariant called the first Chern number C that defines the quantized Hall conductance as Sxy = Ce2/h. Increasing C is pivotal for the realization of low-power-consumption topological electronics, but there has been no clear-cut solution to this problem so far, with the majority of existing Chern insulators showing C = 1. Here, by using state-of-the-art theoretical methods, we propose an efficient approach for the realization of the high-C state in MnBi2Te4/hBN van der Waals multilayer heterostructures. We show that a stack of n MnBi2Te4 films with C = 1 intercalated by hBN monolayers gives rise to a high Chern number state with C = n, characterized by n chiral edge modes. This state can be achieved both under the external magnetic field and without it, both cases leading to the quantized Hall conductance Sxy = Ce2/h. Our results, therefore, pave the way to practical high-C quantized Hall systems.

Electronic properties and photocatalytic water splitting with high solar-to-hydrogen efficiency in a hBNC/Janus WSSe heterojunction: First-principles calculations

Currently, designing and exploring a photocatalyst with an interfacial electric field for hydrogen production from water splitting is crucial. Hence, based on first-principles calculations, we first calculate the electronic properties of the two-dimensional hexagonal boron nitride (hBN)/Janus WSSe heterojunction. However, the inherent energy band conformational defects of the hBN/Janus WSSe heterojunction inhibit the efficiency of photocatalytic. To solve this problem, we doped $\mathrm{C}=\mathrm{C}$ bonds on the monolayer hBN to tune the band gap. The research shows that the transition from the traditional type-I band alignment to the type-II band alignment can be achieved through different contracting methods between monolayer hexagonal boron nitride carbon (hBNC) and monolayer Janus WSSe. Notably, the hBNC/SWSe heterojunction is a direct Z-scheme photocatalyst and has high carrier mobility. Compared with the hBNC/SeWS heterojunction, the hBNC/SWSe heterojunction shows higher reduction reaction overpotential (${\ensuremath{\chi}}_{{\mathrm{H}}_{2}}=2.56\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$) and oxidation reaction overpotential (${\ensuremath{\chi}}_{{\mathrm{O}}_{2}}=0.83\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$) under acidic conditions. Furthermore, the hBNC/SWSe heterojunction has a high quantum efficiency and solar hydrogen production efficiency (33.31%). Gibbs free energy calculations confirm that both oxidation evolution reaction and hydrogen evolution reaction are exothermic reactions and can proceed spontaneously under the external potential provided by photogenerated carriers. These results suggest that the hBNC/SWSe heterojunction has potential applications in visible-light-driven water splitting.

Strong gate-tunability of flat bands in bilayer graphene due to moiré encapsulation between hBN monolayers.

When using hexagonal boron-nitride (hBN) as a substrate for graphene, the resulting moiré pattern creates secondary Dirac points. By encapsulating a multilayer graphene within aligned hBN sheets the controlled moiré stacking may offer even richer benefits. Using advanced tight-binding simulations on atomistically-relaxed heterostructures, here we show that the gap at the secondary Dirac point can be opened in selected moiré-stacking configurations, and is independent of any additional vertical gating of the heterostructure. On the other hand, gating can broadly tune the gap at the principal Dirac point, and may thereby strongly compress the first moiré mini-band in width against the moiré-induced gap at the secondary Dirac point. We reveal that in hBN-encapsulated bilayer graphene this novel mechanism can lead to isolated bands flatter than 10 meV under moderate gating, hence presenting a convenient pathway towards electronically-controlled strongly-correlated states on demand.

Growth Mechanism of High-Quality hBN Monolayers on Cu through Chemical Vapor Deposition with Inductively Coupled Plasma

The growth mechanism associated with the nucleation and growth kinetics of hexagonal boron nitride (hBN) on Cu through chemical vapor deposition (CVD) has been studied. A fully covered hBN film with a large grain size has been obtained by lowering the density of the grain boundary during the thermal CVD. The Cu substrate is first annealed under the H2 ambiance in order to efficiently reduce the copper oxide on the surface and promote the Cu surface with the (111) plane. Next, the nucleation and growth kinetics of CVD hBN are investigated by the analysis of scanning electron microscopy (SEM) images and the Johnson–Mehl–Avrami–Kolmogorov model. The hBN growth is overall dominated by nucleation under a low flow rate of H2, while it is initially dominated by grain growth under a high flow rate of H2 and the grain size of hBN can be larger than 25 μm. The competition between nucleation and grain growth is reversed at higher temperatures, resulting from the wavier Cu surface. Furthermore, the hBN film is grown by the inductively coupled plasma-enhanced CVD (ICP-CVD) and its growth mechanism is compared with that of CVD hBN. The growth rate of ICP-CVD hBN is 50 times faster than that of CVD hBN due to more energetic B and N species caused by plasma. The growth mechanism of ICP-CVD hBN without a long incubation time is dominated by nucleation.

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.

Moiré and non-twisted sp3-hybridized structures based on hexagonal boron nitride bilayers: Ab initio insight into infrared and Raman spectra, bands structures and mechanical properties

We investigated hydrogenated twisted (21.8°) and non-twisted bilayer structures consisting of two hexagonal boron nitride (hBN) monolayers with interlayer covalent bonds. These structures, named bornitranes, are boron nitride analogues of well-known carbon diamanes. We found that the binding energy of twisted structures was about 0.5 eV/BN higher than that of non-twisted AA’- and AB-stacked hBN layers. According to our calculations, the fully hydrogenated Moiré bornitrane with the 21.8° twisted angle has a flattened valence band and an indirect band gap of about 2.8 eV. This value is almost twice wider than that for their non-twisted counterparts. The computed Young’s modulus of bornitranes is higher than that for the hBN monolayer. The infrared and Raman spectra of the considered systems were also defined. Spectral fingerprints of the Moiré bornitrane possess many peaks due to the presence of distorted interlayer bonds and are radically different from the spectra of non-twisted systems. Calculated peaks can facilitate experimental detection of the Moiré bornitranes. Investigated structures are novel 2D semiconductors suitable for nanoelectronics and optoelectronics applications.

All-optical valley switch and clock of electronic dephasing.

2D materials with broken inversion symmetry posses an extra degree of freedom, the valley pseudospin, that labels in which of the two energy-degenerate crystal momenta, K or K', the conducting carriers are located. It has been shown that shining circularly-polarized light allows to achieve close to 100% of valley polarization, opening the way to valley-based transistors. Yet, switching of the valley polarization is still a key challenge for the practical implementation of such devices due to the short valley lifetimes. Recent progress in ultrashort laser technology now allows to produce trains of attosecond pulses with controlled phase and polarization between the pulses. Taking advantage of such technology, we introduce a coherent control protocol to turn on, off and switch the valley polarization at faster timescales than electron-hole decoherence and valley depolarization, that is, an ultrafast optical valley switch. We theoretically demonstrate the protocol for hBN and MoS2 monolayers calculated from first principles. Additionally, using two time-delayed linearly-polarized pulses with perpendicular polarization, we show that we can extract the electronic dephasing time T2 from the valley Hall conductivity.

Enhancement of the contrast for a hexagonal boron nitride monolayer placed on a silicon nitride/silicon substrate

We propose a visualization technique for identifying an exfoliated monolayer hexagonal boron nitride (hBN) flake placed on a SiN x /Si substrate. The use of a Si substrate with a 63 nm thick SiN x film enhanced the contrast of monolayer hBN at wavelengths of 480 and 530 nm by up to 12% and −12%, respectively. The maximum contrast for the Si substrate with SiN x is more than four times as large as that for a Si substrate with a ∼90 or ∼300 nm SiO2 film. Based on the results of the reflectance spectrum measurement and numerical calculations, the enhancement is discussed.

Anisotropic magnetoexcitons in two-dimensional transition metal trichalcogenide semiconductors

Direct and indirect excitons in Rydberg states in transition metal trichalcogenide (TMTC) monolayers, bilayers, and van der Waals (vdW) heterostructures in an external magnetic field are studied within the framework of the effective mass approximation. Binding energies of magnetoexcitons are calculated using the Rytova-Keldysh potential for direct magnetoexcitons and both the Rytova-Keldysh and Coulomb potentials for indirect magnetoexcitons. We report the magnetic field energy contribution to the binding energies and diamagnetic coefficients for magnetoexcitons that depend strongly on the effective mass anisotropy of electrons and holes. The comparative study of TMTCs and phosphorene is given. In ${\mathrm{TiS}}_{3}$, ${\mathrm{TiSe}}_{3}$, and ${\mathrm{ZrSe}}_{3}$ the excitonic binding energies and diamagnetic coefficients demonstrate the same kind of anisotropy as in phosphorene. In contrast, ${\mathrm{ZrS}}_{3}$ has the opposite anisotropy to phosphorene. The tunability of the binding energy of direct and indirect magnetoexcitons by the external magnetic field and the possibility to control the binding energy of magnetoexcitons in vdW heterostructures by manipulation of numbers of hBN monolayers are shown.

Synthesis of AAB-Stacked Single-Crystal Graphene/hBN/Graphene Trilayer van der Waals Heterostructures by In Situ CVD.

van der Waals heterostructures based on graphene and hBN layers with different stacking modes are receiving considerable attention because of their potential application in fundamental physics. However, conventional exfoliation fabrication methods and layer‐by‐layer transfer techniques have various limitations. The CVD synthesis of high‐quality large‐area graphene and hBN multilayer heterostructures is essential for the advancement of new physics. Herein, the authors propose an in situ CVD growth strategy for synthesizing wafer‐scale AAB‐stacked single‐crystal graphene/hBN/graphene trilayer van der Waals heterostructures. Single‐crystal CuNi(111) alloys are prepared on sapphire, followed by the pre‐dissolution of carbon atoms. Single‐crystal monolayer hBN is synthesized on a plasma‐cleaned CuNi(111) surface. Then, a single‐crystal monolayer graphene is epitaxially grown onto the hBN surface to form graphene/hBN bilayer heterostructures. A controlled decrease in the growth temperature allows the carbon atoms to precipitate out of the CuNi(111) alloy to form single‐crystal graphene at the interface between hBN and CuNi(111), thereby producing graphene/hBN/graphene trilayer van der Waals heterostructures. The stacking modes between as‐grown 2D layers are investigated through Raman spectroscopy and transmission electron microscopy. This study provides an in situ CVD approach to directly synthesize large‐scale single‐crystal low‐dimensional van der Waals heterostructures and facilitates their application in future 2D‐material‐based integrated circuits.

Scalable Synthesis of Monolayer Hexagonal Boron Nitride on Graphene with Giant Bandgap Renormalization.

Monolayer hexagonal boron nitride (hBN) has been widely considered a fundamental building block for 2D heterostructures and devices. However, the controlled and scalable synthesis of hBN and its 2D heterostructures has remained a daunting challenge. Here, an hBN/graphene (hBN/G) interface-mediated growth process for the controlled synthesis of high-quality monolayer hBN is proposed and further demonstrated. It is discovered that the in-plane hBN/G interface can be precisely controlled, enabling the scalable epitaxy of unidirectional monolayer hBN on graphene, which exhibits a uniform moiré superlattice consistent with single-domain hBN, aligned to the underlying graphene lattice. Furthermore, it is identified that the deep-ultraviolet emission at 6.12eV stems from the 1s-exciton state of monolayer hBN with a giant renormalized direct bandgap on graphene. This work provides a viable path for the controlled synthesis of ultraclean, wafer-scale, atomically ordered 2D quantum materials, as well as the fabrication of 2D quantum electronic and optoelectronic devices.

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

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

Hexagonal boron nitride: optical properties in the deep ultraviolet

We review the recent advance in the understanding of the optoelectronic properties of hexagonal boron nitride (hBN) in the deep ultraviolet. The comparison between bulk hBN and monolayer hBN highlights some of their major differences such the bandgap nature, the excitonic binding energy and the phonon-assisted broadening of the excitonic lines. Perspectives point out the relevance of addressing the regime of hBN samples made of a very few number of monolayers, including twisted hBN monolayers in the context of twistronics.

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

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

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

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

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

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