Hexagonal boron nitride (h-BN) is an emergent optical material being an indirect bandgap yet with a bright luminescence in the far UV. In the last years a general picture of optical recombination mechanisms has been sketched and the importance of strong phonon-exciton coupling effects has been underlined. Still, no absorption was reported above the bandgap where transmission measurements are extremely challenging in the far UV. Furthermore, phonon assisted optical phenomena can be properly understood only through a fine description of dispersion relations for electron and phonon excitations, an information that can be accessed only through non-optical techniques. The interest of h-BN for optical applications goes also beyond its sole UV luminescence since it hosts several very bright and stable single photon sources in a wide spectral range. Due to the number of interconnected phenomena occurring in defining the ultimate h-BN optical response, not a single spectroscopy should be considered but an original combination of spectroscopic methods.
The BONASPES project aims to lead to a significant breakthrough in the understanding of the peculiar optical properties of h-BN. To achieve this goal the consortium combines four teams with complementary expertise and state of the art facilities: spectro-microscopies in scanning transmission electron microscopes (the STEM-LPS group in Orsay), optical spectroscopy down to the UV spectral range (The L2C group in Montpellier), resonant inelastic X-ray scattering (Sextants beamline at the Soleil Synchrotron) and modern theoretical spectroscopy techniques (the condensed matter theory group in Rome).
Several aspects of BN optics will be tackled: the absorption spectra of h-BN from the bulk to the monolayer limit and the role of exciton-phonon coupling; the momentum dependence of exciton, phonon excitations and their intercoupling; the role of stacking faults in tuning the optical response of h-BN; the correlation between specific absorption/emission lines and well characterized native point defects and dopants; the identification of possible new single-photon sources in the visible and UV spectral domains.
In the last fifteen years h-BN has often been adopted as a model system for testing new ab initio approaches in the many body perturbation theory framework. The spectroscopic measurements which will be obtained by the BONASPES project will become milestones in the field and will provide key inputs and validators for future developments in many body perturbation theory. Besides the intrinsic interest in h-BN optics, the project aims also to pave the way to new concepts and investigating tools for materials optics. A major point will be the development of novel instrumentation and techniques: a micro-photoluminescence and a reflectometry experiment operating in the far UV at the L2C; a world unique aberration corrected and monochromated STEM microscope installed at the LPS capable of electron energy loss spectroscopy and nano-cathodoluminescence hyperspectral imaging at a high energy, space and momentum resolution; a novel light collecting system allowing to perform simultaneously X-ray excited optical luminescence (XEOL) and resonant inelastic X-ray scattering (RIXS) at Soleil.
Project coordinator
Partners
University of Rome “Tor Vergata” / Physics Department
SEXTANTS Synchrotron SOLEIL
STEM-LPS Laboratoire de Physique des Solides
L2C Laboratoire Charles Coulomb
Help of the ANR 480,237 euros
Beginning and duration of the scientific project: December 2019 – 48 Months
The author of this su