• Elu/nommé au comité national CNRS

• Membre d'un pool d'experts

• Physique/Matière Condensée/Autre
  

Hexagonal boron nitride (hBN) is attracting tremendous interest as an essential component in van der Waals heterostructures due to its ability to provide weakly interacting interfaces and because of its large bandgap. Although most of theoretical calculations yield the standard AA′ stacking for few-layer hBN, the exact determination of its structural and electronic properties remains unrevealed to date. Here, we provide the direct observation of structural and electronic transitions in few layers of isotopically pure exfoliated h11BN flakes. Our nanoscopic angle-resolved photoemission spectroscopy measurements combined with density-functional theory calculations indicate that the stacking and the band structure can be strongly affected by the thickness of h11BN. Hence, we show that hBN presents an AA′ stacking in its bulk form and another more exotic stacking for three and four layers. Our findings open perspectives in understanding and controlling the stackings in hBN, which could be of great interest for optoelectronic applications.

Hexagonal boron nitride (hBN) is an important insulator that is incorporated into numerous 2D electronic, optoelectronic, and photonic devices, whereas natural hBN is a mixture of 20% 10B and 80% 11B isotopes, and monoisotopic hBN is a variant with just a single boron isotope, either 10B or 11B. Consequently, monoisotopic hBN has a higher thermal conductivity and a stronger neutron absorption (in the case of h10BN), making it superior for neutron detectors, heat management materials in nano flexible electronic devices, and phonon polariton-based nanophotonics. Here we synthesized approximately monoisotopic hBN using boron powder containing a single boron isotope and nitrogen, and grew single crystals from a Fe–Cr metal flux at atmospheric pressure. Narrow Raman peaks from the shear (r1.3 cm

The UV community for the fourth time took part in the International Workshop on Ultraviolet Materials and Devices (IWUMD4), which was organized in September 2019 by the Ioffe Institute, St. Petersburg, Russia. This event, following the successful meetings in Beijing, China (2016), Fukuoka, Japan (2017), Kunming, China (2018), was intended to show new horizons beyond the UV-blue LEDs based on wide-bandgap III-Nitrides. Material science, modeling and technology for the “UV gap” (200-300 nm) are at the frontier of applied physics. The studies are aimed at developing devices for such areas as medicine – for human-safely disinfection and rapid detection of viral threats, communication – for noise-protected and solar- blind UV-optical communication, nanotechnology – for high- resolution photolithography and for many other applications in the field of biology, pharmaceuticals and forensics.The Workshop was attended by 125 scientists from 15 coun- tries, including half from Asia and half from Europe and theDOI: 10.1002/pssa.202000378USA. The Program was very eventful and included 2 plenary talks by Prof. Chris G. Van de Walle (UCSB) and Prof. Michael Kneissl (TU Berlin&FBH), 4 keynotes, and 25 invited papers, as well as 35 oral presentations and 36 poster contributions displayed during two poster sessions. Prior to Workshop, the Tutorials on the basic properties of wide-gap semiconductor materials and nanostructures with lectures of outstanding experts in this field were held for one and a half days. The scope of the presentations at the Workshop was quite wide. Besides the fundamental properties of wide-gap semiconductor materials and nanostructures, their optical and structural characterization and applications, it highlighted new trends, such as the growth, basic properties and applications of Ga2O3 and physics of BN from bulk to 2D materials.This issue features selected reports from the Workshop participants. Recent results on the manufacture of high-quality templates of the nitrides and oxides by using differenttechnological approaches are presented. The fundamental problems of low-dimensional heterostructures and the techno- logical challenges of various epitaxial growth methods, such as molecular beam epitaxy and metalorganic chemical vapor deposition, are examined in detail to further improve the quan- tum efficiency and output power of UV light-emitting devices. A study of the optical properties of AlGaN layers and laser diode heterostructures are presented, which elucidate the nature and characteristics of spontaneous and stimulated emis- sions in AlGaN laser structures. In addition, the fabrication and radiation of GaN hexagonal microcavities are discussed. A variety of topics reflects the wide range of interests of the Workshop.The Editors would like to thank all the members of the International Advisory and Program Committees for their encouragement and great help during the Workshoporganization. We are grateful to the referees for their efforts and time devoted to the paper review process. We owe our best thanks to the official partners and sponsors of the workshop, as well as all participants at the symposium exhibition. Finally, we would like to express our sincere gratitude to Dr. Stefan Hildebrandt and the production team of physica status solidi for their friendly assistance and collaboration in the editorial process.The next 5th International Workshop on Ultraviolet Materials and Devices (IWUMD5) is planned to be held in November 2020 in Jeju, Korea. We wish the organizers a gorgeous and successful meeting.

we report the development of a scanning confocal microscope dedicated to photoluminescence in the 200 nm-wavelength range for samples at cryogenic temperatures (5-300K). We demonstrate the performances of our deep ultraviolet cryomicroscope in high- quality hexagonal boron nitride (hBN) crystals, although it can be utilized for biological studies in its range of operating wavelengths. From the mapping of the photoluminescence, we bring evidence for the suppression of extrinsic recombination channels in regions free from defects. The observation of emission spectra dominated by intrinsic recombination processes was never reported before in hBN, by means of photoluminescence spectroscopy. We show that photoluminescence tomography now competes with cathodoluminescence, and that deep ultraviolet cryomicroscopy by photoluminescence is a novel powerful tool in materials science applications, with the great advantage of an efficient non-invasive photo- excitation of carriers.

We review the recent progress regarding the physics and applications of boron nitride bulk crystals and its epitaxial layers in various fields. First, we high- light its importance from optoelectronics side, for simple devices operating in the deep ultraviolet, in view of sanitary applications. Emphasis will be directed towards the unusually strong efficiency of the exciton–phonon coupling in this indirect band gap semiconductor. Sec- ond, we shift towards nanophotonics, for the manage- ment of hyper-magnification and of medical imaging. Here, advantage is taken of the efficient coupling of the electromagnetic field with some of its phonons, those interacting with light at 12 and 6 μm in vacuum. Third, we present the different defects that are currently studied for their propensity to behave as single photon emitters, in the perspective to help them becoming challengers of the NV centres in diamond or of the double vacancy in silicon carbide in the field of modern and developing quantum technologies.