Hexagonal boron nitride is a model lamellar compound where weak, non-local van der Waals interactions ensure the vertical stacking of two-dimensional honeycomb lattices made of strongly bound boron and nitrogen atoms. We study the isotope engineering of lamellar compounds by synthesizing hexagonal boron nitride crystals with nearly pure boron isotopes ( 10 B and 11 B) compared to those with the natural distribution of boron (20 at% 10 B and 80 at% 11 B). On the one hand, as with standard semiconductors, both the phonon energy and electronic bandgap varied with the boron isotope mass, the latter due to the quantum effect of zero-point renormalization. On the other hand, temperature-dependent experiments focusing on the shear and breathing motions of adjacent layers revealed the specificity of isotope engineering in a layered material, with a modification of the van der Waals interactions upon isotope purification. The electron density distribution is more diffuse between adjacent layers in 10 BN than in 11 BN crystals. Our results open perspectives in understanding and controlling van der Waals bonding in layered materials.

I will discuss our recent studies showing that the optical response in hBN displays prominent evidence for phonon-assisted optical transitions, with a very unusual phenomenology. By two-photon spectroscopy, we demonstrated that the intrinsic optical properties at the band edge are characteristic of an indirect bandgap material. Polarization-resolved experiments with a detection from the sample edge allowed us to show that the phonon symmetries can be traced back in the optical response. I will further highlight the unique properties of this material where the optical response is tailored by the phonon group velocities in the middle of the Brillouin zone. I will finally present optical characterization results of the promising high-temperature MBE growth of hBN.

I will discuss here our recent results in isotopically-purified hBN crystals. We combined Raman scattering, photoluminescence, and X-ray diffraction for an in-depth characterization. On the one hand, as with standard semiconductors, both the phonon energy and electronic bandgap varied with the boron isotope mass, the latter due to the quantum effect of zero-point renormalization. On the other hand, temperature-dependent experiments focusing on the shear and breathing motions of adjacent layers revealed the specificity of isotope engineering in a layered material, with a modification of the Van der Waals interactions upon isotope purification.

I will discuss our recent studies showing that the optical response in hBN displays prominent evidence for phonon-assisted optical transitions, with a very unusual phenomenology. By two-photon spectroscopy, we demonstrated that the intrinsic optical properties at the band edge are characteristic of an indirect bandgap material. Polarization-resolved experiments with a detection from the sample edge allowed us to show that the phonon symmetries can be traced back in the optical response. I will further highlight the unique properties of this material where the optical response is tailored by the phonon group velocities in the middle of the Brillouin zone. I will finally present optical characterization results of the promising high-temperature MBE growth of hBN, in collaboration with Nottingham University.

I will discuss our recent studies showing that the optical response in hBN displays prominent evidence for phonon-assisted optical transitions, with a very unusual phenomenology. By two-photon spectroscopy, we demonstrated that the intrinsic optical properties at the band edge are characteristic of an indirect bandgap material. Polarization-resolved experiments with a detection from the sample edge allowed us to showthat the phonon symmetries can be traced back in the optical response. I will further highlight the unique properties of this material where the optical response is tailored by the phonon group velocities in the middle of the Brillouin zone. I will finally present optical characterization results of the promising high-temperature MBE growth of hBN, in collaboration with Nottingham University.

I will discuss our recent studies showing that the optical response in hBN displays prominent evidence for phonon-assisted optical transitions. First of all, by two-photon spectroscopy, we have demonstrated that the intrinsic optical properties at the band edge are characteristic of an indirect bandgap material. I will further discuss our experimental evidence that transverse optical phonons at the K point of the Brillouin zone assist inter-K valley scattering in hexagonal boron nitride, thanks to the presence of a density of final electronic states coming from extended stacking faults. Finally, I will present our measurements of the vibronic spectrum in a point defect in hBN, displaying a remarkable mapping with the phonon density of states, and in particular a suppression of the phonon-assisted recombination signal at the phonon gap energy. I will discuss our recent studies showing that the optical response in hBN displays prominent evidence for phonon-assisted optical transitions. First of all, by two-photon spectroscopy, we have demonstrated that the intrinsic optical properties at the band edge are characteristic of an indirect bandgap material. I will further discuss our experimental evidence that transverse optical phonons at the K point of the Brillouin zone assist inter-K valley scattering in hexagonal boron nitride, thanks to the presence of a density of final electronic states coming from extended stacking faults. Finally, I will present our measurements of the vibronic spectrum in a point defect in hBN, displaying a remarkable mapping with the phonon density of states, and in particular a suppression of the phonon-assisted recombination signal at the phonon gap energy.