Showing 1–28 of 28 results for author: Barjon, J

Oxygen in diamond: thermal stability of ST1 spin centres and creation of oxygen-pair complexes

Authors: Paul Neugebauer, Xinxi Huang, Chloe Newsom, Christophe Arnold, Hjørdis Martelock, Séverine Diziain, Edoardo Monnetti, Jocelyn Achard, Tobias Lühmann, Paolo Olivero, Jan Meijer, Julien Barjon, Alexandre Tallaire, Sébastien Pezzagna

Abstract: Little is known about oxygen-related defects in diamond. Recently, the promising room-temperature spin centre named ST1 was identified as an oxygen centre, but of still unknown atomic structure and thermal stability. In this work, we report on the optically active oxygen-related centres and the conditions for their formation, using ion implantation of oxygen in various conditions of depth and flue… ▽ More Little is known about oxygen-related defects in diamond. Recently, the promising room-temperature spin centre named ST1 was identified as an oxygen centre, but of still unknown atomic structure and thermal stability. In this work, we report on the optically active oxygen-related centres and the conditions for their formation, using ion implantation of oxygen in various conditions of depth and fluence. More specifically, we establish the temperature formation/stability range of the ST1 centre, which has a maximum at about 1100°C and is narrower than for NV centres. In these conditions, optically detected magnetic resonance (ODMR) on small ST1 ensembles was measured with a spin readout contrast of > 20% at 300K. In cathodoluminescence, the 535 nm ST1 peak is not observed. Besides, a broad peak centred at 460 nm is measured for implantation of O$_2$ molecular ions. For an annealing temperature of 1500°C, a different centre is formed (with ZPL at 584.5 nm) with an intensity increasing with a power law 1.5 < p < 1.9 dependence from the implantation fluence. This suggests that this centre contains two oxygen atoms. Besides, a new spectral feature associated to an intrinsic defect was also observed, with four prominent lines (especially at 594nm). Finally, the thermal formation and stability of oxygen centres in diamond presented here are important for the identification of the atomic structure of defects such as the ST1 and possible O$_2$V$_x$ complex by means of ab initio calculations. Indeed, the formation energies and charge states of defect centres are easier to compute than the full energy level scheme, which to date still remains unsuccessful regarding the ST1 centre. △ Less

Submitted 7 January, 2026; originally announced January 2026.

Deterministic generation of single B centers in hBN by one-to-one conversion from UV centers

Authors: Andrés Núñez Marcos, Christophe Arnold, Julien Barjon, Stéphanie Buil, Jean-Pierre Hermier, Aymeric Delteil

Abstract: Among the variety of quantum emitters in hexagonal boron nitride (hBN), blue-emitting color centers, or B centers, have gathered a particular interest owing to their excellent quantum optical properties. Moreover, the fact that they can be locally activated by an electron beam makes them suitable for top-down integration in photonic devices. However, in the absence of a real-time monitoring techni… ▽ More Among the variety of quantum emitters in hexagonal boron nitride (hBN), blue-emitting color centers, or B centers, have gathered a particular interest owing to their excellent quantum optical properties. Moreover, the fact that they can be locally activated by an electron beam makes them suitable for top-down integration in photonic devices. However, in the absence of a real-time monitoring technique sensitive to individual emitters, the activation process is stochastic in the number of emitters, and its mechanism is under debate. Here, we implement an in-situ cathodoluminescence monitoring setup capable of detecting individual quantum emitters in the blue and ultraviolet (UV) range. We demonstrate that the activation of individual B centers is spatially and temporally correlated with the deactivation of individual UV centers emitting at 4.1 eV, which are ubiquitous in hBN. We then make use of the ability to detect individual B center activation events to demonstrate the controlled creation of an array with only one emitter per irradiation site. Additionally, we demonstrate a symmetric technique for heralded selective deactivation of individual emitters. Our results provide insights into the microscopic structure and activation mechanism of B centers, as well as versatile techniques for their deterministic integration. △ Less

Submitted 26 March, 2026; v1 submitted 4 November, 2025; originally announced November 2025.

Journal ref: ACS Nano (2026)

Straightforward Method to Orient Black Phosphorus from Bulk to Thin Layers using a Standard Green Laser

Authors: Etienne Carré, Frédéric Fossard, Jean-Sébastien Mérot, Denis Boivin, Nicolas Horezan, Victor Zatko, Florian Godel, Bruno Dlubak, Marie-Blandine Martin, Pierre Seneor, Etienne Gaufres, Julien Barjon, Annick Loiseau, Ingrid Stenger

Abstract: The crystallographic orientation of anisotropic 2D materials plays a crucial role in their physical properties and device performance. However, standard orientation techniques such as transmission electron microscopy (TEM) or X-ray diffraction (XRD) can be complex and less accessible for routine characterization. In this study, we investigate the orientation of black phosphorus (BP) from bulk crys… ▽ More The crystallographic orientation of anisotropic 2D materials plays a crucial role in their physical properties and device performance. However, standard orientation techniques such as transmission electron microscopy (TEM) or X-ray diffraction (XRD) can be complex and less accessible for routine characterization. In this study, we investigate the orientation of black phosphorus (BP) from bulk crystals to thin layers using angle-resolved polarized Raman spectroscopy (ARPRS) with a single-wavelength (514 nm) Raman setup. By incorporating thickness-dependent interference effects and anisotropic optical indices, this approach provides a reliable framework for orientation determination across different BP thicknesses. The method is validated through direct orientation measurements using TEM and Electron Backscattering Diffraction (EBSD), confirming its applicability to both thick and ultrathin samples. Given its simplicity and compatibility with widely available Raman setups, this approach offers a practical solution for characterizing BP orientation without requiring advanced structural characterization techniques. △ Less

Submitted 6 August, 2025; originally announced August 2025.

Journal ref: ChemNanoMat 250018 (2025)

Benchmarking the integration of hexagonal boron nitride crystals and thin films into graphene-based van der Waals heterostructures

Authors: Taoufiq Ouaj, Christophe Arnold, Jon Azpeitia, Sunaja Baltic, Julien Barjon, Jose Cascales, Huanyao Cun, David Esteban, Mar Garcia-Hernandez, Vincent Garnier, Subodh K. Gautam, Thomas Greber, Said Said Hassani, Adrian Hemmi, Ignacio Jimenéz, Catherine Journet, Paul Kögerler, Annick Loiseau, Camille Maestre, Marvin Metzelaars, Philipp Schmidt, Christoph Stampfer, Ingrid Stenger, Philippe Steyer, Takashi Taniguchi , et al. (3 additional authors not shown)

Abstract: We present a benchmarking protocol that combines the characterization of boron nitride (BN) crystals and films with the evaluation of the electronic properties of graphene on these substrates. Our study includes hBN crystals grown under different conditions and scalable BN films deposited by either chemical or physical vapor deposition (CVD or PVD). We explore the complete process from boron nitri… ▽ More We present a benchmarking protocol that combines the characterization of boron nitride (BN) crystals and films with the evaluation of the electronic properties of graphene on these substrates. Our study includes hBN crystals grown under different conditions and scalable BN films deposited by either chemical or physical vapor deposition (CVD or PVD). We explore the complete process from boron nitride growth, over its optical characterization by time-resolved cathodoluminescence (TRCL), to the optical and electronic characterization of graphene by Raman spectroscopy after encapsulation and Hall bar processing. Within our benchmarking protocol we achieve a homogeneous electronic performance within each Hall bar device through a fast and reproducible processing routine. We find that a free exciton lifetime of 1 ns measured on as-grown hBN crystals by TRCL is sufficient to achieve high graphene room temperature charge carrier mobilities of 80,000 cm$^2$/(Vs) at a carrier density of |n| = 10$^{12}$ cm$^{-2}$, while respective exciton lifetimes around 100 ps yield mobilities up to 30,000 cm$^2$/(Vs). For scalable PVD-grown BN films, we measure carrier mobilities exceeding 10,000 cm$^2$/(Vs) which correlates with a graphene Raman 2D peak linewidth of 22 cm$^{-1}$. Our work highlights the importance of the Raman 2D linewidth of graphene as a critical metric that effectively assesses the interface quality (i.e. surface roughness) to the BN substrate, which directly affects the charge carrier mobility of graphene. Graphene 2D linewidth analysis is suitable for all BN substrates and is particularly advantageous when TRCL or BN Raman spectroscopy cannot be applied to specific BN materials such as amorphous or thin films. This underlines the superior role of spatially-resolved spectroscopy in the evaluation of BN crystals and films for the use of high-mobility graphene devices. △ Less

Submitted 5 September, 2024; originally announced September 2024.

Comments: 18 pages, 11 figures

Journal ref: 2D Materials 12, 015017 (2025)

Exciton self-trapping in twisted hexagonal boron nitride homostructures

Authors: Sébastien Roux, Christophe Arnold, Etienne Carré, Alexandre Plaud, Lei Ren, Frédéric Fossard, Nicolas Horezan, Eli Janzen, James H. Edgar, Camille Maestre, Bérangère Toury, Catherine Journet, Vincent Garnier, Philippe Steyer, Takashi Taniguchi, Kenji Watanabe, Cédric Robert, Xavier Marie, Annick Loiseau, Julien Barjon

Abstract: One of the main interests of 2D materials is their ability to be assembled with many degrees of freedom for tuning and manipulating excitonic properties. There is a need to understand how the structure of the interfaces between atomic layers influences exciton properties. Here we use cathodoluminescence and time-resolved cathodoluminescence experiments to study how excitons interact with the inter… ▽ More One of the main interests of 2D materials is their ability to be assembled with many degrees of freedom for tuning and manipulating excitonic properties. There is a need to understand how the structure of the interfaces between atomic layers influences exciton properties. Here we use cathodoluminescence and time-resolved cathodoluminescence experiments to study how excitons interact with the interface between two twisted hexagonal boron nitride (hBN) crystals with various angles. An efficient capture of free excitons by the interface is demonstrated, which leads to a population of long-lived and interface-localized (2D) excitons. Temperature dependent experiments indicate that for high twist angles, these excitons localized at the interface further undergo a selftrapping. It consists in a distortion of the lattice around the exciton on which the exciton traps itself. Our results suggest that this exciton-interface interaction causes the broad 4-eV optical emission of highly twisted hBN-hBN structures. Exciton self-trapping is finally discussed as a common feature of sp2 hybridized boron nitride polytypes and nanostructures due to the ionic nature of the B-N bond and the small size of their excitons. △ Less

Submitted 27 February, 2025; v1 submitted 15 May, 2024; originally announced May 2024.

Comments: 17 pages, 15 figures, accepted for publication in Physical Review X

Surface recombination and out of plane diffusivity of free excitons in hexagonal boron nitride

Authors: Sébastien Roux, Christophe Arnold, Etienne Carré, Eli Janzen, James H. Edgard, Camille Maestre, Bérangère Toury, Catherine Journet, Vincent Garnier, Philippe Steyer, Takashi Taniguchi, Kenji Watanabe, Annick Loiseau, Julien Barjon

Abstract: We present a novel experimental protocol using Cathodoluminescence measurements as a function of the electron incident energy to study both exciton diffusion in a directional way and surface exciton recombination. Our approach overcomes the challenges of anisotropic diffusion and the limited applicability of existing methods to the bulk counterparts of 2D materials. The protocol is then applied at… ▽ More We present a novel experimental protocol using Cathodoluminescence measurements as a function of the electron incident energy to study both exciton diffusion in a directional way and surface exciton recombination. Our approach overcomes the challenges of anisotropic diffusion and the limited applicability of existing methods to the bulk counterparts of 2D materials. The protocol is then applied at room and at cryogenic temperatures to four bulk hexagonal boron nitride crystals grown by different synthesis routes. The exciton diffusivity depends on the sample quality but not on the temperature, indicating it is limited by defect scattering even in the best quality crystals. The lower limit for the diffusivity by phonon scattering is 0.2 cm$^{2}$.s$^{-1}$. Diffusion lengths were as much as 570 nm. Finally, the surface recombination velocity exceeds 10$^{5}$ cm$^{2}$.s$^{-1}$, at a level similar to silicon or diamond. This result reveals that surface recombination could strongly limit light-emitting devices based on 2D materials. △ Less

Submitted 11 August, 2023; v1 submitted 10 August, 2023; originally announced August 2023.

Comments: Main text : 8 pages, 5 figures. Annexes : 3 pages, 3 figures. Submitted to Physical Review B

Distinguishing different stackings in layered materials via luminescence spectroscopy

Authors: Matteo Zanfrognini, Alexandre Plaud, Ingrid Stenger, Frédéric Fossard, Lorenzo Sponza, Léonard Schué, Fulvio Paleari, Elisa Molinari, Daniele Varsano, Ludger Wirtz, François Ducastelle, Annick Loiseau, Julien Barjon

Abstract: Despite its simple crystal structure, layered boron nitride features a surprisingly complex variety of phonon-assisted luminescence peaks. We present a combined experimental and theoretical study on ultraviolet-light emission in hexagonal and rhombohedral bulk boron nitride crystals. Emission spectra of high-quality samples are measured via cathodoluminescence spectroscopy, displaying characterist… ▽ More Despite its simple crystal structure, layered boron nitride features a surprisingly complex variety of phonon-assisted luminescence peaks. We present a combined experimental and theoretical study on ultraviolet-light emission in hexagonal and rhombohedral bulk boron nitride crystals. Emission spectra of high-quality samples are measured via cathodoluminescence spectroscopy, displaying characteristic differences between the two polytypes. These differences are explained using a fully first-principles computational technique that takes into account radiative emission from ``indirect'', finite-momentum, excitons via coupling to finite-momentum phonons. We show that the differences in peak positions, number of peaks and relative intensities can be qualitatively and quantitatively explained, once a full integration over all relevant momenta of excitons and phonons is performed. △ Less

Submitted 27 May, 2023; originally announced May 2023.

Comments: Main: 6 pages and 4 figures, Supplementary: 6 pages and 7 figures

Investigating the fast spectral diffusion of a quantum emitter in hBN using resonant excitation and photon correlations

Authors: Clarisse Fournier, Kenji Watanabe, Takashi Taniguchi, Stéphanie Buil, Julien Barjon, Jean-Pierre Hermier, Aymeric Delteil

Abstract: The ability to identify and characterize homogeneous and inhomogeneous dephasing processes is crucial in solid-state quantum optics. In particular, spectral diffusion leading to line broadening is difficult to evidence when the associated timescale is shorter than the inverse of the photon detection rate. Here, we show that a combination of resonant laser excitation and second-order photon correla… ▽ More The ability to identify and characterize homogeneous and inhomogeneous dephasing processes is crucial in solid-state quantum optics. In particular, spectral diffusion leading to line broadening is difficult to evidence when the associated timescale is shorter than the inverse of the photon detection rate. Here, we show that a combination of resonant laser excitation and second-order photon correlations allows to access such fast dynamics. The resonant laser drive converts spectral diffusion into intensity fluctuations, leaving a signature in the second-order coherence function $g^{(2)}(τ)$ of the scattered light that can be characterized using two-photon coincidences -- which simultaneously provides the homogeneous dephasing time. We experimentally implement this method to investigate the fast spectral diffusion of a color center generated by an electron beam in the two-dimensional material hexagonal boron nitride. The $g^{(2)}(τ)$ function of the quantum emitter measured over more than ten orders of magnitude of delay times, at various laser powers, establishes that the color center experiences spectral diffusion at a characteristic timescale of a few tens of microseconds, while emitting Fourier-limited single photons ($T_2/2T_1 \sim 1$) between spectral jumps. △ Less

Submitted 25 May, 2023; v1 submitted 9 March, 2023; originally announced March 2023.

Journal ref: Phys. Rev. B 107, 195304 (2023)

Quantum well confinement and competitive radiative pathways in the luminescence of black phosphorus layers

Authors: Etienne Carré, Lorenzo Sponza, Alain Lusson, Ingrid Stenger, Sébastien Roux, Victor Zatko, Bruno Dlubak, Pierre Seneor, Etienne Gaufrès, Annick Loiseau, Julien Barjon

Abstract: Black phosphorus (BP) stands out from other 2D materials by the wide amplitude of the band-gap energy (Delta(Eg)) that sweeps an optical window from Visible (VIS) to Infrared (IR) wavelengths, depending on the layer thickness. This singularity made the optical and excitonic properties of BP difficult to map. Specifically, the literature lacks in presenting experimental and theoretical data on the… ▽ More Black phosphorus (BP) stands out from other 2D materials by the wide amplitude of the band-gap energy (Delta(Eg)) that sweeps an optical window from Visible (VIS) to Infrared (IR) wavelengths, depending on the layer thickness. This singularity made the optical and excitonic properties of BP difficult to map. Specifically, the literature lacks in presenting experimental and theoretical data on the optical properties of BP on an extended thickness range. Here we report the study of an ensemble of photoluminescence spectra from 79 passivated BP flakes recorded at 4 K with thicknesses ranging from 4 nm to 700 nm, obtained by mechanical exfoliation. We observe that the exfoliation steps induce additional defects states that compete the radiative recombination from bound excitons observed in the crystal. We also show that the evolution of the photoluminescence energy versus thickness follows a quantum well confinement model appreciable from a thickness predicted and probed at 25 nm. The BP slabs placed in different 2D heterostructures show that the emission energy is not significantly modulated by the dielectric environment. Introduction Confinement effects △ Less

Submitted 2 December, 2022; originally announced December 2022.

Comments: 11 pages, 3 figures - Main text 12 pages, 5 figures - Supporting information

Two-photon interference from a quantum emitter in hexagonal boron nitride

Authors: Clarisse Fournier, Sébastien Roux, Kenji Watanabe, Takashi Taniguchi, Stéphanie Buil, Julien Barjon, Jean-Pierre Hermier, Aymeric Delteil

Abstract: Recently discovered quantum emitters in two-dimensional (2D) materials have opened new perspectives of integrated photonic devices for quantum information. Most of these applications require the emitted photons to be indistinguishable, which has remained elusive in 2D materials. Here, we investigate two-photon interference of a quantum emitter generated in hexagonal boron nitride (hBN) using an el… ▽ More Recently discovered quantum emitters in two-dimensional (2D) materials have opened new perspectives of integrated photonic devices for quantum information. Most of these applications require the emitted photons to be indistinguishable, which has remained elusive in 2D materials. Here, we investigate two-photon interference of a quantum emitter generated in hexagonal boron nitride (hBN) using an electron beam. We measure the correlations of zero-phonon-line photons in a Hong-Ou-Mandel (HOM) interferometer under non-resonant excitation. We find that the emitted photons exhibit a partial indistinguishability of $0.44 \pm 0.11$ in a 3 ns time window, which corresponds to a corrected value of $0.56 \pm 0.11$ after accounting for imperfect emitter purity. The dependence of the HOM visibility on the width of the post-selection time window allows us to estimate the dephasing time of the emitter to be $\sim 1.5$ ns, about half the limit set by spontaneous emission. A visibility above 90 % is under reach using Purcell effect with up-to-date 2D material photonics. △ Less

Submitted 28 April, 2023; v1 submitted 11 October, 2022; originally announced October 2022.

Cathodoluminescence monitoring of quantum emitter activation in hexagonal boron nitride

Authors: Sébastien Roux, Clarisse Fournier, Kenji Watanabe, Takashi Taniguchi, Jean-Pierre Hermier, Julien Barjon, Aymeric Delteil

Abstract: The ability to locally activate or generate quantum emitters in two-dimensional materials is of major interest for the realization of integrated quantum photonic devices. In particular, hexagonal boron nitride (hBN) has recently been shown to allow a variety of techniques for obtaining quantum emitters at desired locations. Here, we use cathodoluminescence (CL) to monitor in situ the local activat… ▽ More The ability to locally activate or generate quantum emitters in two-dimensional materials is of major interest for the realization of integrated quantum photonic devices. In particular, hexagonal boron nitride (hBN) has recently been shown to allow a variety of techniques for obtaining quantum emitters at desired locations. Here, we use cathodoluminescence (CL) to monitor in situ the local activation of color centers by an electron beam in hBN. We observe that the CL signal saturates at a given surface dose, independently of the electron current density. Based on photoluminescence and photon correlations, we show that the number of photoactive color centers is proportional to the CL signal, and we estimate the maximum density of quantum emitters that can be generated by our technique. Our results provide insights about the activation mechanism and could help to optimize the controlled generation of single photon sources in hexagonal boron nitride. △ Less

Submitted 10 October, 2022; originally announced October 2022.

From the synthesis of hBN crystals to their use as nanosheets for optoelectronic devices

Authors: Camille Maestre, Yangdi Li, Vincent Garnier, Philippe Steyer, Sébastien Roux, Alexandre Plaud, Annick Loiseau, Julien Barjon, Lei Ren, Cédric Robert, Bo Han, Xavier Marie, Catherine Journet, Bérangère Toury

Abstract: In the wide world of 2D materials, hexagonal boron nitride (hBN) holds a special place due to its excellent characteristics. In addition to its thermal, chemical and mechanical stability, hBN demonstrates high thermal conductivity, low compressibility, and wide band gap around 6 eV, making it promising candidate for many groundbreaking applications and more specifically for optoelectronic devices.… ▽ More In the wide world of 2D materials, hexagonal boron nitride (hBN) holds a special place due to its excellent characteristics. In addition to its thermal, chemical and mechanical stability, hBN demonstrates high thermal conductivity, low compressibility, and wide band gap around 6 eV, making it promising candidate for many groundbreaking applications and more specifically for optoelectronic devices. Millimeters scale hexagonal boron nitride crystals are obtained through a disruptive dual method (PDC/PCS) consisting in a complementary coupling of the Polymer Derived Ceramics route and a Pressure-Controlled Sintering process. In addition to their excellent chemical and crystalline quality, these crystals exhibit a free exciton lifetime of 0.43 ns, as determined by time-resolved cathodoluminescence measurements, confirming their interesting optical properties. To go further in applicative fields, hBN crystals are then exfoliated, and resulting Boron Nitride NanoSheets (BNNSs) are used to encapsulate transition metal dichalcogenides (TMDs). Such van der Waals heterostructures are tested by optical spectroscopy. BNNSs do not luminesce in the emission spectral range of TMDs and the photoluminescence width of the exciton at 4K is in the range 2-3 meV. All these results demonstrate that these BNNSs are relevant for future opto-electronic applications. △ Less

Submitted 19 January, 2022; originally announced January 2022.

Comments: 16 pages, 9 figures

Radiative lifetime of free excitons in hexagonal boron nitride

Authors: Sébastien Roux, Christophe Arnold, Fulvio Paleari, Lorenzo Sponza, Eli Janzen, James H. Edgar, Bérangère Toury, Catherine Journet, Vincent Garnier, Philippe Steyer, Takashi Taniguchi, Kenji Watanabe, François Ducastelle, Annick Loiseau, Julien Barjon

Abstract: Using a new time-resolved cathodoluminescence system dedicated to the UV spectral range, we present a first estimate of the radiative lifetime of free excitons in hBN at room temperature. This is carried out from a single experiment giving both the absolute luminescence intensity under continuous excitation and the decay time of free excitons in the time domain. The radiative lifetime of indirect… ▽ More Using a new time-resolved cathodoluminescence system dedicated to the UV spectral range, we present a first estimate of the radiative lifetime of free excitons in hBN at room temperature. This is carried out from a single experiment giving both the absolute luminescence intensity under continuous excitation and the decay time of free excitons in the time domain. The radiative lifetime of indirect excitons in hBN is equal to 27 ns, which is much shorter than in other indirect bandgap semiconductors. This is explained by the close proximity of the electron and the hole in the exciton complex, and also by the small energy difference between indirect and direct excitons. The unusually high luminescence efficiency of hBN for an indirect bandgap is therefore semi-quantitatively understood. △ Less

Submitted 2 July, 2021; originally announced July 2021.

Identification by deuterium diffusion of a nitrogen-related deep donor preventing the p-type doping of ZnO

Authors: N. Temahuki, F. Jomard, A. Lusson, I. Stenger, S. Hassani, J. Chevallier, J. M. Chauveau, C. Morhain, J. Barjon

Abstract: Deuterium diffusion is investigated in nitrogen-doped homoepitaxial ZnO layers. The samples were grown under slightly Zn-rich growth conditions by plasma-assisted molecular beam epitaxy on m-plane ZnO substrates and have a nitrogen content [N] varied up to 5x1018 at.cm-3 as measured by secondary ion mass spectrometry (SIMS). All were exposed to a radio-frequency deuterium plasma during 1h at room… ▽ More Deuterium diffusion is investigated in nitrogen-doped homoepitaxial ZnO layers. The samples were grown under slightly Zn-rich growth conditions by plasma-assisted molecular beam epitaxy on m-plane ZnO substrates and have a nitrogen content [N] varied up to 5x1018 at.cm-3 as measured by secondary ion mass spectrometry (SIMS). All were exposed to a radio-frequency deuterium plasma during 1h at room temperature. Deuterium diffusion is observed in all epilayers while its penetration depth decreases as the nitrogen concentration increases. This is a strong evidence of a diffusion mechanism limited by the trapping of deuterium on a nitrogen-related trap. The SIMS profiles are analyzed using a two-trap model including a shallow trap, associated with a fast diffusion, and a deep trap, related to nitrogen. The capture radius of the nitrogen-related trap is determined to be 20 times smaller than the value expected for nitrogen-deuterium pairs formed by coulombic attraction between D+ and nitrogen-related acceptors. The (N2)O deep donor is proposed as the deep trapping site for deuterium and accounts well for the small capture radius and the observed photoluminescence quenching and recovery after deuteration of the ZnO:N epilayers. It is also found that this defect is by far the N-related defect with the highest concentration in the studied samples. △ Less

Submitted 9 March, 2021; originally announced March 2021.

Position-controlled quantum emitters with reproducible emission wavelength in hexagonal boron nitride

Authors: Clarisse Fournier, Alexandre Plaud, Sébastien Roux, Aurélie Pierret, Michael Rosticher, Kenji Watanabe, Takashi Taniguchi, Stéphanie Buil, Xavier Quélin, Julien Barjon, Jean-Pierre Hermier, Aymeric Delteil

Abstract: Single photon emitters (SPEs) in low-dimensional layered materials have recently gained a large interest owing to the auspicious perspectives of integration and extreme miniaturization offered by this class of materials. However, accurate control of both the spatial location and the emission wavelength of the quantum emitters is essentially lacking to date, thus hindering further technological ste… ▽ More Single photon emitters (SPEs) in low-dimensional layered materials have recently gained a large interest owing to the auspicious perspectives of integration and extreme miniaturization offered by this class of materials. However, accurate control of both the spatial location and the emission wavelength of the quantum emitters is essentially lacking to date, thus hindering further technological steps towards scalable quantum photonic devices. Here, we evidence SPEs in high purity synthetic hexagonal boron nitride (hBN) that can be activated by an electron beam at chosen locations. SPE ensembles are generated with a spatial accuracy better than the cubed emission wavelength, thus opening the way to integration in optical microstructures. Stable and bright single photon emission is subsequently observed in the visible range up to room temperature upon non-resonant laser excitation. Moreover, the low-temperature emission wavelength is reproducible, with an ensemble distribution of width 3 meV, a statistical dispersion that is more than one order of magnitude lower than the narrowest wavelength spreads obtained in epitaxial hBN samples. Our findings constitute an essential step towards the realization of top-down integrated devices based on identical quantum emitters in 2D materials. △ Less

Submitted 8 July, 2021; v1 submitted 24 November, 2020; originally announced November 2020.

Comments: Main text and supplementary information: 22 pages, 11 figures

Journal ref: Nature Communications 12, 3779 (2021)

Excitons in bulk black phosphorus evidenced by photoluminescence at low temperature

Authors: Etienne Carré, Lorenzo Sponza, Alain Lusson, Ingrid Stenger, Etienne Gaufrès, Annick Loiseau, Julien Barjon

Abstract: Atomic layers of Black Phosphorus (BP) present unique opto-electronic properties dominated by a direct tunable bandgap in a wide spectral range from visible to mid-infrared. In this work, we investigate the infrared photoluminescence of BP single crystals at very low temperature. Near-bandedge recombinations are observed at 2 K, including dominant excitonic transitions at 0.276 eV and a weaker one… ▽ More Atomic layers of Black Phosphorus (BP) present unique opto-electronic properties dominated by a direct tunable bandgap in a wide spectral range from visible to mid-infrared. In this work, we investigate the infrared photoluminescence of BP single crystals at very low temperature. Near-bandedge recombinations are observed at 2 K, including dominant excitonic transitions at 0.276 eV and a weaker one at 0.278 eV. The free-exciton binding energy is calculated with an anisotropic Wannier-Mott model and found equal to 9.1 meV. On the contrary, the PL intensity quenching of the 0.276 eV peak at high temperature is found with a much smaller activation energy, attributed to the localization of free excitons on a shallow impurity. This analysis leads us to attribute respectively the 0.276 eV and 0.278 eV PL lines to bound excitons and free excitons in BP. As a result, the value of bulk BP bandgap is refined to 0.287 eV at 2K. △ Less

Submitted 29 October, 2020; originally announced October 2020.

Comments: 12 pages, 3 figures - Main text 13 pages, 6 figures - Supporting information

Excellent electronic transport in heterostructures of graphene and monoisotopic boron-nitride grown at atmospheric pressure

Authors: J. Sonntag, J. Li, A. Plaud, A. Loiseau, J. Barjon, J. H. Edgar, C. Stampfer

Abstract: Hexagonal boron nitride (BN), one of the very few layered insulators, plays a crucial role in 2D materials research. In particular, BN grown with a high pressure technique has proven to be an excellent substrate material for graphene and related 2D materials, but at the same time very hard to replace. Here we report on a method of growth at atmospheric pressure as a true alternative for producing… ▽ More Hexagonal boron nitride (BN), one of the very few layered insulators, plays a crucial role in 2D materials research. In particular, BN grown with a high pressure technique has proven to be an excellent substrate material for graphene and related 2D materials, but at the same time very hard to replace. Here we report on a method of growth at atmospheric pressure as a true alternative for producing BN for high quality graphene/BN heterostructures. The process is not only more scalable, but also allows to grow isotopically purified BN crystals. We employ Raman spectroscopy, cathodoluminescence, and electronic transport measurements to show the high-quality of such monoisotopic BN and its potential for graphene-based heterostructures. The excellent electronic performance of our heterostructures is demonstrated by well developed fractional quantum Hall states, ballistic transport over distances around $10\,\mathrm{μm}$ at low temperatures and electron-phonon scattering limited transport at room temperature. △ Less

Submitted 22 December, 2019; originally announced December 2019.

Comments: 7 pages, 4 figures

Journal ref: 2D Materials 7, 031009 (2020)

Exciton-exciton annihilation in hBN

Authors: Alexandre Plaud, Léonard Schué, Kenji Watanabe, Takashi Taniguchi, Annick Loiseau, Julien Barjon

Abstract: Known as a prominent recombination path at high excitation densities, exciton-exciton annihilation (EEA) is evidenced in bulk hexagonal boron nitride (hBN) by cathodoluminescence at low temperature. Thanks to a careful tune of the the exciton density by varying either the current or the focus of the incident electron beam, we could estimate an EEA rate of 2$\times$10$^{-6}$ cm$^{3}$.s$^{-1}$ at… ▽ More Known as a prominent recombination path at high excitation densities, exciton-exciton annihilation (EEA) is evidenced in bulk hexagonal boron nitride (hBN) by cathodoluminescence at low temperature. Thanks to a careful tune of the the exciton density by varying either the current or the focus of the incident electron beam, we could estimate an EEA rate of 2$\times$10$^{-6}$ cm$^{3}$.s$^{-1}$ at $T=10$ K, the highest reported so far for a bulk semiconductor. Expected to be even stronger in nanotubes or atomic layers, EEA probablly contributes to the luminescence quenching observed in low-dimensionality BN materials. △ Less

Submitted 17 May, 2019; originally announced May 2019.

Quantitative relevance of substitutional impurities to carrier dynamics in diamond

Authors: Takaaki Shimomura, Yoshiki Kubo, Julien Barjon, Norio Tokuda, Ikuko Akimoto, Nobuko Naka

Abstract: We have quantified substitutional impurity concentrations in synthetic diamond crystals down to sub parts-per-billion levels. The capture lifetimes of electrons and excitons injected by photoexcitation were compared for several samples with different impurity concentrations. Based on the assessed impurity concentrations, we have determined the capture cross section of electrons to boron impurity,… ▽ More We have quantified substitutional impurity concentrations in synthetic diamond crystals down to sub parts-per-billion levels. The capture lifetimes of electrons and excitons injected by photoexcitation were compared for several samples with different impurity concentrations. Based on the assessed impurity concentrations, we have determined the capture cross section of electrons to boron impurity, sA=1.3x10^-14 cm2, and that of excitons to nitrogen impurity, sD^ex=3.1x10^-14 cm2. The general tendency of the mobility values for different carrier species is successfully reproduced by including carrier scattering by impurities and by excitons. △ Less

Submitted 25 May, 2018; originally announced May 2018.

Journal ref: Phys. Rev. Materials 2, 094601 (2018)

Bright luminescence from indirect and strongly bound excitons in hBN

Authors: Leonard Schue, Lorenzo Sponza, Alexandre Plaud, Hakima Bensalah, Kenji Watanabe, Takashi Taniguchi, François Ducastelle, Annick Loiseau, Julien Barjon

Abstract: A quantitative analysis of the excitonic luminescence efficiency in hexagonal boron nitride (hBN) is carried out by cathodoluminescence in the ultraviolet range and compared with zinc oxide and diamond single crystals. A high quantum yield value of ~50% is found for hBN at 10 K comparable to that of direct bandgap semiconductors. This bright luminescence at 215 nm remains stable up to room tempera… ▽ More A quantitative analysis of the excitonic luminescence efficiency in hexagonal boron nitride (hBN) is carried out by cathodoluminescence in the ultraviolet range and compared with zinc oxide and diamond single crystals. A high quantum yield value of ~50% is found for hBN at 10 K comparable to that of direct bandgap semiconductors. This bright luminescence at 215 nm remains stable up to room temperature, evidencing the strongly bound character of excitons in bulk hBN. Ab initio calculations of the exciton dispersion confirm the indirect nature of the lowest-energy exciton whose binding energy is found equal to 300 meV, in agreement with the thermal stability observed in luminescence. The direct exciton is found at a higher energy but very close to the indirect one, which solves the long debated Stokes shift in bulk hBN. △ Less

Submitted 8 February, 2019; v1 submitted 10 March, 2018; originally announced March 2018.

Journal ref: Phys. Rev. Lett. 122, 067401 (2019)

Low frequency Raman spectroscopy of few-atomic-layer thick hBN crystals

Authors: I Stenger, A Schué, A Boukhicha, B Berini, A Plaçais, A Loiseau, J Barjon

Abstract: Hexagonal boron nitride (hBN) has recently gained a strong interest as a strategic component in engineering van der Waals heterostructures built with two dimensional crystals such as graphene. This work reports micro-Raman measurements on hBN flakes made of a few atomic layers, prepared by mechanical exfoliation. The temperature dependence of the Raman scattering in hBN is investigated first such… ▽ More Hexagonal boron nitride (hBN) has recently gained a strong interest as a strategic component in engineering van der Waals heterostructures built with two dimensional crystals such as graphene. This work reports micro-Raman measurements on hBN flakes made of a few atomic layers, prepared by mechanical exfoliation. The temperature dependence of the Raman scattering in hBN is investigated first such as to define appropriate measurements conditions suitable for thin layers avoiding undesirable heating induced effects. We further focus on the low frequency Raman mode corresponding to the rigid shearing oscillation between adjacent layers, found to be equal to 52.5 cm-1 in bulk hBN. For hBN sheets with thicknesses below typically 4 nm, the frequency of this mode presents discrete values, which are found to decrease down to 46.0(5) cm-1 for a three-layer hBN, in good agreement with the linear-chain model. This makes Raman spectroscopy a relevant tool to quantitatively determine the number of layers in ultra thin hBN sheets, below 8L. △ Less

Submitted 3 May, 2017; originally announced May 2017.

Angular resolved electron energy loss spectroscopy in hexagonal boron nitride

Authors: Frédéric Fossard, Lorenzo Sponza, Léonard Schué, Claudio Attaccalite, François Ducastelle, Julien Barjon, Annick Loiseau

Abstract: Electron energy loss spectra have been measured on hexagonal boron nitride single crystals employing a novel electron energy loss spectroscopic set-up composed by an electron microscope equipped with a monochromator and an in-column filter. This set-up provides high-quality energy-loss spectra and allows also for the imaging of energy-filtered diffraction patterns. These two acquisition modes prov… ▽ More Electron energy loss spectra have been measured on hexagonal boron nitride single crystals employing a novel electron energy loss spectroscopic set-up composed by an electron microscope equipped with a monochromator and an in-column filter. This set-up provides high-quality energy-loss spectra and allows also for the imaging of energy-filtered diffraction patterns. These two acquisition modes provide complementary pieces of information, offering a global view of excitations in reciprocal space. As an example of the capabilities of the method we show how easily the core loss spectra at the $K$ edges of boron and nitrogen can be measured and imaged. Low losses associated to interband and/or plasmon excitations are also measured. This energy range allows us to illustrate that our method provides results of quality comparable to those obtained from non resonant X-ray inelastic scattering, but with advantageous specificities such as an enhanced sensitivity at low q and a much higher simplicity and versatility that makes it well adapted to the study of two-dimensional materials and related heterostructures. Finally, by comparing theoretical calculations against our measures, we are able to relate the range of applicability of ab initio calculations to the anisotropy of the sample and assess the level of approximation required for a proper simulation of our acquisition method. △ Less

Submitted 6 September, 2017; v1 submitted 18 January, 2017; originally announced January 2017.

Journal ref: Phys. Rev. B 96, 115304 (2017)

Characterization methods dedicated to nanometer-thick hBN layers

Authors: Leonard Schue, Ingrid Stenger, Frederic Fossard, Annick Loiseau, Julien Barjon

Abstract: Hexagonal boron nitride (hBN) regains interest as a strategic component in graphene engineering and in van der Waals heterostructures built with two dimensional materials. It is crucial then, to handle reliable characterization techniques capable to assess the quality of structural and electronic properties of the hBN material used. We present here characterization procedures based on optical spec… ▽ More Hexagonal boron nitride (hBN) regains interest as a strategic component in graphene engineering and in van der Waals heterostructures built with two dimensional materials. It is crucial then, to handle reliable characterization techniques capable to assess the quality of structural and electronic properties of the hBN material used. We present here characterization procedures based on optical spectroscopies, namely cathodoluminescence and Raman, with the additional support of structural analysis conducted by transmission electron microscopy. We show the capability of optical spectroscopies to investigate and benchmark the optical and structural properties of various hBN thin layers sources. △ Less

Submitted 21 October, 2016; originally announced October 2016.

Dimensionality effects on the luminescence properties of hBN

Authors: éonard Schué, Bruno Berini, Bernard Plaçais, François Ducastelle, Julien Barjon, Annick Loiseau, Andreas Betz

Abstract: Cathodoluminescence (CL) experiments at low temperature have been undertaken on various bulk and exfoliated hexagonal boron nitride (hBN) samples. Different bulk crystals grown from different synthesis methods have been studied. All of them present the same so-called S series in the 5.6--6 eV range, proving its intrinsic character. Luminescence spectra of flakes containing 100 down to 6 layers hav… ▽ More Cathodoluminescence (CL) experiments at low temperature have been undertaken on various bulk and exfoliated hexagonal boron nitride (hBN) samples. Different bulk crystals grown from different synthesis methods have been studied. All of them present the same so-called S series in the 5.6--6 eV range, proving its intrinsic character. Luminescence spectra of flakes containing 100 down to 6 layers have been recorded. Strong modifications in the same UV range are observed and discussed within the general framework of 2D exciton properties in lamellar crystals. △ Less

Submitted 8 January, 2016; originally announced January 2016.

Excitonic recombinations in hBN: from bulk to exfoliated layers

Authors: Aurélie Pierret, Jorge Loayza, Bruno Berini, Andreas Betz, Bernard Plaçais, François Ducastelle, Julien Barjon, Annick Loiseau

Abstract: Hexagonal boron nitride (h-BN) and graphite are structurally similar but with very different properties. Their combination in graphene-based devices meets now a huge research focus, and it becomes particularly important to evaluate the role played by crystalline defects in them. In this work, the cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are reported and compared… ▽ More Hexagonal boron nitride (h-BN) and graphite are structurally similar but with very different properties. Their combination in graphene-based devices meets now a huge research focus, and it becomes particularly important to evaluate the role played by crystalline defects in them. In this work, the cathodoluminescence (CL) properties of hexagonal boron nitride crystallites are reported and compared to those of nanosheets mechanically exfoliated from them. First the link between the presence of structural defects and the recombination intensity of bound-excitons, the so-called D series, is confirmed. Low defective h-BN regions are further evidenced by CL spectral mapping (hyperspectral imaging), allowing us to observe new features in the near-band-edge region, tentatively attributed to phonon replica of exciton recombinations. Second the h-BN thickness was reduced down to six atomic layers, using mechanical exfoliation, as evidenced by atomic force microscopy. Even at these low thicknesses, the luminescence remains intense and exciton recombination energies are not strongly modified with respect to the bulk, as expected from theoretical calculations indicating extremely compact excitons in h-BN. △ Less

Submitted 6 February, 2014; v1 submitted 12 June, 2013; originally announced June 2013.

Journal ref: Physical Review B (Condensed Matter) 89 (2014) 035414

Exciton and interband optical transitions in hBN single crystal

Authors: Luc Museur, Gurvan Brasse, Aurélie Pierret, Sylvain Maine, Brigitte Attal-Trétout, François Ducastelle, Annick Loiseau, Julien Barjon, Kenji Watanabe, Takashi Taniguchi, Andreï Kanaev

Abstract: Near band gap photoluminescence (PL) of hBN single crystal has been studied at cryogenic temperatures with synchrotron radiation excitation. The PL signal is dominated by the D-series previously assigned to excitons trapped on structural defects. A much weaker S-series of self-trapped excitons at 5.778 eV and 5.804 eV has been observed using time-window PL technique. The S-series excitation spectr… ▽ More Near band gap photoluminescence (PL) of hBN single crystal has been studied at cryogenic temperatures with synchrotron radiation excitation. The PL signal is dominated by the D-series previously assigned to excitons trapped on structural defects. A much weaker S-series of self-trapped excitons at 5.778 eV and 5.804 eV has been observed using time-window PL technique. The S-series excitation spectrum shows a strong peak at 6.02 eV, assigned to free exciton absorption. Complementary photoconductivity and PL measurements set the band gap transition energy to 6.4 eV and the Frenkel exciton binding energy larger than 380 meV. △ Less

Submitted 24 February, 2011; originally announced February 2011.

Origin of the excitonic recombinations in hexagonal boron nitride by spatially resolved cathodoluminescence spectroscopy

Authors: Périne Jaffrennou, Julien Barjon, Jean-Sébastien Lauret, Brigitte Attal-Trétout, François Ducastelle, Annick Loiseau

Abstract: The excitonic recombinations in hexagonal boron nitride (hBN) are investigated with spatially resolved cathodoluminescence spectroscopy in the UV range. Cathodoluminescence images of an individual hBN crystallite reveals that the 215 nm free excitonic line is quite homogeneously emitted along the crystallite whereas the 220 nm and 227 nm excitonic emissions are located in specific regions of the… ▽ More The excitonic recombinations in hexagonal boron nitride (hBN) are investigated with spatially resolved cathodoluminescence spectroscopy in the UV range. Cathodoluminescence images of an individual hBN crystallite reveals that the 215 nm free excitonic line is quite homogeneously emitted along the crystallite whereas the 220 nm and 227 nm excitonic emissions are located in specific regions of the crystallite. Transmission electron microscopy images show that these regions contain a high density of crystalline defects. This suggests that both the 220 nm and 227 nm emissions are produced by the recombination of excitons bound to structural defects. △ Less

Submitted 4 July, 2007; originally announced July 2007.

Cathodoluminescence imaging and spectroscopy on a single multiwall boron nitride nanotube

Authors: Périne Jaffrennou, Fabrice Donatini, Julien Barjon, Jean-Sébastien Lauret, Aude Maguer, Brigitte Attal-Trétout, François Ducastelle, Annick Loiseau

Abstract: Cathodoluminescence imaging and spectroscopy experiments on a single bamboo-like boron nitride nanotube are reported. Imaging experiments show that the luminescence is located all along the nanotube. Spectroscopy experiments point out the important role of dimensionality in this one dimensional object. Cathodoluminescence imaging and spectroscopy experiments on a single bamboo-like boron nitride nanotube are reported. Imaging experiments show that the luminescence is located all along the nanotube. Spectroscopy experiments point out the important role of dimensionality in this one dimensional object. △ Less

Submitted 13 March, 2007; originally announced March 2007.