arXiv subjects

Loïc Moczko

Publications and source records attributed to Loïc Moczko.

4 recordsLinked to original sources

arXiv2026

Sub-nm range momentum-dependent exciton transfer from a 2D semiconductor to graphene

Heterostructures made from atomically thin semiconductors (here MoSe$_2$) and graphene are uniquely poised to investigate photoinduced charge and energy transfer in the 2D limit. Here, using picosecond time-resolved photoluminescence spectroscopy at cryogenic temperatures on two types of MoSe$_2$/graphene heterostrutures, we unveil key features of the underlying mechanisms. First, the shortening of the MoSe$_2$ bright exciton lifetime is marginally affected by the number of graphene layers to which MoSe$_2$ is coupled. Second, exciton transfer vanishes when a sub-nm thick spacer of hexagonal boron nitride decouples MoSe$_2$ from graphene. These results indicate that charge tunneling govern bright exciton relaxation in MoSe$_2$/graphene and that longer-range, Förster-type energy transfer (FRET) does not affect bright excitons. However, sub-ps FRET to graphene accelerates the relaxation of ``hot'' excitons formed upon optical excitation, leading to photoluminescence quenching factors that exceed expectations based on the shortening of the bright exciton lifetime. Our work has direct implications for energy harvesting and funneling using van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2025

Magnetic excitons in a suspended 2D antiferromagnetic membrane

Layered magnetic and strongly correlated materials present a rich platform for condensed matter physics with intrinsic properties intertwined by magnetism and low-dimensionality. A suspended light-emitting 2D antiferromagnetic membrane forms a highly controllable hybrid system in which the interplay between spin ordering, optical and mechanical degrees of freedom can be uniquely explored. NiPS$_3$ hosts excitons responsible for a puzzling luminescence down to a few atomic layers, linked to its zigzag antiferromagnetic order. The nature of these excitons remains unclear. Here we report on the magnetic excitons of a suspended few-layer NiPS$_3$ membrane. We reveal nematic zigzag states and study the optical transitions induced by the magnon-mediated motion of the excitation in the magnetic lattice, resulting in magnon-dressed excitons. We observe a strain tuning of these emission lines, dependent on their microscopic origin, with rates that sign a strong localization of the magnetic excitons, fading with the number of magnon-mediated hops.

cond-mat.mes-hall↗

arXiv2023

Symmetry-dependent dielectric screening of optical phonons in monolayer graphene

Quantised lattice vibrations (i.e., phonons) in solids are robust and unambiguous fingerprints of crystal structures and of their symmetry properties. In metals and semimetals, strong electron-phonon coupling may lead to so-called Kohn anomalies in the phonon dispersion, providing an image of the Fermi surface in a non-electronic observable. Kohn anomalies become prominent in low-dimensional systems, in particular in graphene, where they appear as sharp kinks in the in-plane optical phonon branches. However, in spite of intense research efforts on electron-phonon coupling in graphene and related van der Waals heterostructures, little is known regarding the links between the symmetry properties of optical phonons at and near Kohn anomalies and their sensitivity towards the local environment. Here, using inelastic light scattering (Raman) spectroscopy, we investigate a set of custom-designed graphene-based van der Waals heterostructures, wherein dielectric screening is finely controlled at the atomic layer level. We demonstrate experimentally and explain theoretically that, depending exclusively on their symmetry properties, the two main Raman modes of graphene react differently to the surrounding environment. While the Raman-active near-zone-edge optical phonons in graphene undergo changes in their frequencies due to the neighboring dielectric environment, the in-plane, zone-centre optical phonons are symmetry-protected from the influence of the latter. These results shed new light on the unique electron-phonon coupling properties in graphene and related systems and provide invaluable guidelines to characterise dielectric screening in van der Waals heterostructures and moiré superlattices.

cond-mat.mes-hall↗

arXiv2021

Single- and narrow-line photoluminescence in a boron nitride-supported MoSe$_2$/graphene heterostructure

Heterostructures made from van der Waals materials provide a template to investigate proximity effects at atomically sharp heterointerfaces. In particular, near-field charge and energy transfer in heterostructures made from semiconducting transition metal dichalcogenides (TMD) have attracted interest to design model 2D "donor-acceptor" systems and new optoelectronic components. Here, using of Raman scattering and photoluminescence spectroscopies, we report a comprehensive characterization of a molybedenum diselenide (MoSe$_2$) monolayer deposited onto hexagonal boron nitride (hBN) and capped by mono- and bilayer graphene. Along with the atomically flat hBN susbstrate, a single graphene epilayer is sufficient to passivate the MoSe$_2$ layer and provides a homogenous environment without the need for an extra capping layer. As a result, we do not observe photo-induced doping in our heterostructure and the MoSe$_2$ excitonic linewidth gets as narrow as 1.6~meV, hence approaching the homogeneous limit. The semi-metallic graphene layer neutralizes the 2D semiconductor and enables picosecond non-radiative energy transfer that quenches radiative recombination from long-lived states. Hence, emission from the neutral band edge exciton largely dominates the photoluminescence spectrum of the MoSe$_2$/graphene heterostructure. Since this exciton has a picosecond radiative lifetime at low temperature, comparable with the energy transfer time, its low-temperature photoluminescence is only quenched by a factor of $3.3 \pm 1$ and $4.4 \pm 1$ in the presence of mono- and bilayer graphene, respectively. Finally, while our bare MoSe$_2$ on hBN exhibits negligible valley polarization at low temperature and under near-resonant excitation, we show that interfacing MoSe$_2$ with graphene yields a single-line emitter with degrees of valley polarization and coherence up to $\sim 15\,\%$.

cond-mat.mes-hall↗