Lattice deformation is a powerful way to engineer the properties of 2D materials, making their precise measurement an important challenge for both fundamental science and technological applications. Here, we demonstrate that boron-vacancy () color centers in hexagonal boron nitride (hBN) enable quantitative strain sensing with sub-micrometer spatial resolution. Using this approach, we precisely quantify the strain-induced shift of the Raman mode in a multilayer hBN flake under uniaxial stress, establishing centers as a new tool for strain metrology in van der Waals heterostructures. Beyond strain sensing, our work also highlights the unique multimodal sensing functionalities offered by centers, which will be valuable for future studies of strain-engineered 2D materials.

The optical performance of nitride-based heterostructures that spontaneously emit light in the deep-ultraviolet range (wavelengths below 230 nm) is limited by various previously elucidated phenomena related to the details of the valence band structure, the need to resolve various technological issues with a view to improving electrical injection, and the need to address light extraction issues. In this article, we compare the light-matter interaction strategy in high-quality multiple quantum wells developed by both molecular beam epitaxy and organometallic vapor phase epitaxy, using temperature-dependent photoluminescence measurements performed in the range 8-300 K. The deterioration of light emission between 8 and 300 K is governed by two recombination mechanisms operating at low temperatures (below approximately 100 K) and at higher temperatures, respectively. The efficiency of the non-radiative recombination channel at low temperatures is extrinsic in origin; it is mediated by impurities and by the density of defects in the crystal. The second process is intrinsic in nature and is related to the thermal ionization of excitons at higher temperatures. We believe that the ultimate solution to partially reduce these phenomena could be homoepitaxy on high-quality AlN substrates.

We report the growth of hexagonal boron nitride (hBN) layers on multiwalled carbon nanotubes (MWCNTs) by high-temperature molecular beam epitaxy (HT-MBE). Using a variety of characterisation techniques, including transmission electron microscopy (TEM) and aberration-corrected scanning transmission electron microscopy combined with electron energy loss spectroscopy (AC-STEM-EELS), we demonstrated the epitaxy of boron nitride on the external surfaces of MWCNTs. This process formed coaxial heterostructures consisting of an inner multiwalled carbon nanotube coated with a few monolayers of hBN. The high structural quality of the carbon surface of the MWCNTs enabled smooth, uniform deposition of several monolayers of hBN. We proposed nucleation at defects on the MWCNT surface, followed by a step-flow-like "wrapping" mechanism for the hBN epitaxy around the carbon nanotubes. Additionally, boron nitride nanotubes grew from the open step edges at the tips of the multiwalled carbon nanotubes, facilitating the formation of circular carbon nanotube - boron nitride nanotube (BNNT) heterojunctions. Spatially-resolved photoluminescence (PL) spectroscopy in the UV-C spectral range confirmed the excellent structural quality of the hBN monolayers on the CNTs. The PL spectrum of BNNTs exhibited a line around 5.8 eV, which corresponds to the phonon replica in bulk hBN associated with phonon-assisted recombination involving optical phonons. This PL spectrum provides the first evidence of intrinsic radiative exciton recombination in quasi-one-dimensional hBN, with an estimated indirect bandgap of approximately 5.95 eV.

Abstract We measure the temperature profile and investigate the thermal conductivity of suspended monoisotopic hexagonal boron nitride (h10BN) heterostructures by combining suspended microbridge technique and Raman spectroscopy. The thermal conductivities exceed 1650 W.m−1.K−1 at room temperature, significantly higher than in previous reports, highlighting the crucial influence of the measurement conditions on the experimental results. By including more data points, we refine our models beyond the accuracy of conventional approaches. Our results show a striking deviation of thermal transport from the classical diffusion regime described by Fourier’s law: while the temperature profiles are linear above 300 K, they become clearly nonlinear below this temperature, indicating a strong non-diffusive heat transport regime. This behavior underscores the need for a new theoretical framework to fully account for heat transport in two-dimensional materials. Ultimately, our findings pave the way for innovative heat dissipation technologies and challenge conventional paradigms in nano-heat engineering. This study establishes a practical framework linking Raman-based temperature mapping, the number of measurement points, and thermal simulations to reliably determine the in-plane thermal conductivity of 2D materials.

Stacks of boron nitride sheets can be arranged to emit deep ultraviolet light.

Despite intensive research on the science and technology of atomically thin (two-dimensional) semiconductors, it is still difficult to synthesize at scale such materials. Different synthetic approaches have been used, but it is not always possible to control simultaneously the layer stoichiometry and morphology. Here, we address this challenge within the wide family of indium selenide compounds, “golden challengers” to traditional semiconductor technologies. We demonstrate molecular beam epitaxy of wafer-scale indium selenide on c-plane sapphire. Precise tuning of atomic-fluxes in ultra-high vacuum and desorption-controlled growth are employed for epitaxy of single-phase In2Se3 and InSe layers with atomically smooth surfaces. Multi-dimensional growth is also demonstrated, featuring the formation of In4Se3 nanorods on InSe. The simple geometrical shape of the nanorods arises from the anisotropic crystal structure of In4Se3 and it implies direction-dependent physical properties. These developments position indium selenide as a scalable and versatile material platform with new functionalities and integration capabilities. Despite extensive research, synthesizing atomically thin semiconductors at scale remains challenging, particularly in controlling layer stoichiometry and morphology. Here, the authors employ molecular beam epitaxy to achieve wafer-scale indium selenide on c-plane sapphire, demonstrating precise atomic-flux tuning and desorption-controlled growth, positioning indium selenide as a scalable, versatile platform for advanced semiconductor applications.

The efficiency of nitride‐based optical devices, with particular focus on Al x Ga 1‐ x N/AlN quantum wells used for deep ultraviolet light emission is investigated. The study addresses the wavelength dependence of the photoluminescence and in particular the drop in efficiency as the emission wavelength decreases from 250 nm to 210 nm. A model is developed that incorporates the effects of hole state symmetry changes, built‐in strain, quantum confinement, and 2D electron‐hole localization. The latter is introduced as a Gaussian potential representing the spatial alloy disorder in the heterostructures. This model shows good agreement with experimental data and emphasizes that the localization mechanism is crucial for understanding the light emission behavior.

Defects in crystals can have a transformative effect on the properties and functionalities of solid-state systems. Dopants in semiconductors are core components in electronic and optoelectronic devices. The control of single color centers is at the basis of advanced applications for quantum technologies. Unintentional defects can also be detrimental to the crystalline structure and hinder the development of novel materials. Whatever the research perspective, the identification of defects is a key but complicated, and often long-standing issue. Here, we present a general methodology to identify point defects by combining isotope substitution and polytype control, with a systematic comparison between experiments and first-principles calculations. We apply this methodology to hexagonal boron nitride (hBN) and its ubiquitous color center emitting in the ultraviolet spectral range. From isotopic purification of the host hBN matrix, a local vibrational mode of the defect is uncovered, and isotope-selective carbon doping proves that this mode belongs to a carbon-based center. Then, by varying the stacking sequence of the host hBN matrix, we unveil different optical responses to hydrostatic pressure for the non-equivalent configurations of this ultraviolet color center. We conclude that this defect is a carbon dimer in the honeycomb lattice of hBN. Our results show that tuning the stacking sequence in different polytypes of a given crystal provides unique fingerprints contributing to the identification of defects in 2D materials.

Van der Waals (vdW) materials, such as hexagonal boron nitride (h-BN), are highly promising for applications in optoelectronics and quantum technologies. When assembled into heterostructures, h-BN can form moiré superlattices, enabling the engineering of electronic and optical properties by varying the interlayer twist angle. However, understanding the nanoscale interplay between moiré patterns and electronic properties such as the band gap or work function, particularly in optically active h-BN structures, remains a challenge. Here, we use the atomic-scale precision of scanning tunneling microscopy (STM) to uncover the role of moiré superlattices in the electronic properties of a weakly coupled h-BN/Graphite heterostructure. Our STM study reveals large moiré patterns (14.8-18.3 nm periodicity) on the surface, implying slight local variations in the h-BN/Graphite stacking throughout the sample. Spectroscopic measurements show significant modulations of 330 meV in the local work function and 170 meV in the band gap within a moiré unit cell, which are comparable to h-BN/metallic interfaces. Additionally, we identify dual moiré superlattices in twisted homobilayers of h-BN/Graphite, offering an extra degree of freedom to tune the heterostructure's properties. These findings suggest that moiré engineering in h-BN-based systems could lead to a range of effects, including exciton broadening, twist-tunable defect luminescence, and the theoretically predicted trapping of excitons within the moiré landscape. Furthermore, this tunability may also affect adjacent layered materials, providing a versatile platform for tailoring the electronic and optical properties of h-BN and its van der Waals heterostructures.

Large, high-purity single-crystals of hexagonal BN (h-BN) are essential for exploiting its many desirable and interesting properties. Here, we demonstrate via X-ray tomography, X-ray diffraction and scanning electron microscopy that h-BN crystals can be grown by traveling-solvent floating-zone (TSFZ). The diameters of grown boules range from 3-5 mm with lengths from 2-10 mm. Tomography indicates variable grain sizes within the boules, with the largest having areas of approximate to 1 mm x 2 mm and thickness approximate to 0.5 mm. Although the boules contain macroscale flux inclusions, the h-BN lattice itself is of high quality for samples grown under optimized conditions. The currently optimized growth procedure employs an Fe flux, moderate N2 pressure (PN2 approximate to 6 bar), and a growth rate of 0.1 mm/h. Raman spectroscopy for an optimized sample gives an average linewidth of 7.7(2) cm-1 for the E2g intralayer mode at 1365.46(4) cm-1 and 1.0(1) cm-1 for the E2g interlayer shear mode at 51.78(9) cm-1. The corresponding photoluminescence spectrum shows sharp phonon-assisted free exciton peaks and minimal signal in the energy range corresponding to carbon-related defects (E = 3.9-4.1 eV). Our work demonstrates the viability of growing h-BN by the TSFZ technique, thereby opening a new route towards larger, high-quality crystals and advancing the state of h-BN related research.

Interest in hexagonal boron nitride (hBN) is booming due to its exceptional properties and potential applications: including a wide bandgap that emits deep ultraviolet light, its 2D structure, and its excellent chemical and thermal stability. In this work, intralayer carrier diffusion and exciton‐exciton annihilation in hBN are investigated by time‐resolved two‐color pump‐probe experiments. Two‐photon femtosecond excitation makes it possible to monitor the carrier relaxation dynamics in hBN under conditions of negligible surface recombination. A value of the two‐photon absorption coefficient β = 1.7 ± 0.5 cm GW −1 is measured at 350 nm and its dependence in the 315–415 nm range. The obtained in‐plane exciton diffusivity increases with excitation power due to the screening of the exciton‐phonon interaction at high charge carrier densities between 10 15 and 10 17 cm −3 . Conversely, the exciton‐exciton annihilation efficiency decreases by a factor of five from 80 to 600 K. Due to these efficient Auger processes, the in‐plane diffusion length in hBN is reduced from 0.24 to 0.1 µm as the excitation density increases. The robustness of the photoluminescence intensity is mediated in the high temperature range by excitons with a binding energy of 390 meV.

Boron nitride (BN) is today considered one of the most promising materials for many novel applications including bright single photon emission, deep UV optoelectronics, small-sized solid-state neutron detectors, high-performance two-dimensional materials, etc. Recently, AA' stacking a few layers (2-5 layers) of BN films and ABC stacking multilayer (2.2-12 nm) BN films have been successfully synthesized, respectively. However, the preparation of single crystal metal foil and the subsequent growth of BN thin films are undoubtedly incompatible with the current semiconductor process. In addition, how to achieve an accurate layer-by-layer connection between adjacent multilayer BN domains still needs further research. In this work, we demonstrate an approach to grow large-area multilayer single-crystal BN films by chemical vapor deposition on face-centered cubic Fe-Ni (111) single-crystal alloy thin films with different stoichiometric phases, which are deposited on Sapphire. We show that the BN growth is greatly tunable and improved by increasing the Fe content in single-crystal Fe-Ni (111). The formation of pyramid-shaped multilayer BN domains with aligned orientation enables a continuous connection following a layer-by-layer, "first-meet-first-connect", mosaic stitching mechanism. By means of selected area electron diffraction, microphotoluminescence spectroscopy in the deep UV, and high-resolution transmission electron microscopy, the layer-by-layer connection mechanism is unambiguously evidenced, and the stacking order has been verified to occur as unidirectional AB and ABC stackings, i.e., in the Bernal and rhombohedral BN phase.

The stacking sequence of two-dimensional hexagonal boron nitride (hBN) is a critical factor that determines its polytypes and its distinct physical properties. Although most hBN layers adopt the thermodynamically stable AA' stacking sequence, achieving alternative stacking configurations has remained a long-standing challenge. Here we demonstrate the scalable synthesis of hBN featuring unprecedented AA stacking, where atomic monolayers align along the c axis without any translation or rotation. This previously considered thermodynamically unfavourable hBN polytype is achieved through epitaxial growth on a two-inch single-crystalline gallium nitride wafer, using a metal-organic chemical vapour deposition technique. Comprehensive structural and optical characterizations, complemented by theoretical modelling, evidence the formation of AA-stacked multilayer hBN and reveal that hBN nucleation on the vicinal gallium nitride surface drives the unidirectional alignment of layers. Here electron doping plays a central role in stabilizing the AA stacking configuration. Our findings provide further insights into the scalable synthesis of engineered hBN polytypes, characterized by unique properties such as large optical nonlinearity.

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 similar to 6.045 eV. The spatial variations of the photoluminescence energy are found to be around similar to 10 meV, revealing that the inhomogeneous broadening is lower than the average photoluminescence linewidth of similar to 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.

Among a broad diversity of color centers hosted in layered van der Waals materials, the negatively charged boron vacancy (VB-) center in hexagonal boron nitride (hBN) is garnering considerable attention for the development of quantum sensing units on a two-dimensional platform. In this work, we investigate how the optical response of an ensemble of VB- centers evolves with the hBN thickness in a range of a few to hundreds of nanometers. We show that the photoluminescence intensity features a nontrivial evolution with thickness, which is quantitatively reproduced by numerical calculations taking into account thickness-dependent variations of the absorption, radiative lifetime, and radiation pattern of VB- centers. Besides providing an important resource to optimize the performances of quantum sensing units based on VB- centers in hBN, the thickness-dependent nanophotonic effects discussed in this work generally apply to any type of color center embedded in a van der Waals material.

Although large efforts have been made to improve the growth of hexagonal boron nitride (hBN) by heteroepitaxy, the non-native substrates remain a fundamental factor that limits the quality. This problem can be solved by homoepitaxy, which is the growth of hBN on hBN substrates. In this report, we demonstrate the homoepitaxial growth of triangular BN grains on exfoliated hBN flakes by Metal-Organic Vapor Phase Epitaxy and show by atomic force microscopy and photoluminescence that the stacking of these triangular islands can deviate from the AA' stacking of hBN. We show that the stacking order is enforced by the crystallographic direction of the edge of the exfoliated hBN flakes, with armchair edges allowing for centrosymmetric stacking, whereas zigzag edges lead to the growth of noncentrosymmetric BN polytypes. Our results indicate pathways to grow homoepitaxial BN with tunable layer stacking, which is required to induce piezoelectricity or ferroelectricity.