arXiv subjects

Léonard Schué

Publications and source records attributed to Léonard Schué.

4 recordsLinked to original sources

arXiv2023

Distinguishing different stackings in layered materials via luminescence spectroscopy

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.

cond-mat.mtrl-sci↗

arXiv2021

How high is a MoSe$_2$ monolayer?

Transition metal dichalcogenides (TMDCs) have attracted significant attention for optoelectronic, photovoltaic and photoelectrochemical applications. The properties of TMDCs are highly dependent on the number of stacked atomic layers, which is usually counted post-fabrication, using a combination of optical methods and atomic force microscopy (AFM) height measurements. Here, we use photoluminescence spectroscopy and three different AFM methods to demonstrate significant discrepancies in height measurements of exfoliated MoSe$_2$ flakes on SiO$_2$ depending on the method used. We highlight that overlooking effects from electrostatic forces, contaminants and surface binding can be misleading when measuring the height of a MoSe$_2$ flake. These factors must be taken into account as a part of the protocol for counting TMDC layers.

physics.app-ph↗

arXiv2019

Exciton-exciton annihilation in hBN

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.

cond-mat.mes-hall↗

arXiv2017

Angular resolved electron energy loss spectroscopy in hexagonal boron nitride

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.

cond-mat.mtrl-sci↗