Tuesday, April 4, 2023 at 2:00 PM - Salle du Conseil du LCI
Seminar presented by Stéphane BERCIAUD,
University of Strasbourg, CNRS, IPCMS (UMR 7504), Strasbourg, France

Optical spectroscopy and nanoscopy in 2D semiconductor/graphene van der Waals heterostructures »
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The broad palette of two dimensional (2D) materials and the “twist”
degree of freedom between 2D layers offer countless possibilities to
conceive and assemble van der Waals (vdW) heterostructures. The physics
of such artificial materials and their performance as emergent
(opto-)electronic, and photonic devices is governed by interfacial
coupling through, interlayer charge and/or energy transfer as well as
dielectric screening. At the same time, materials-related issues, such
as micro- and nano-scale dielectric disorder, strain gradients,
unintentional doping, atomic reconstruction, /etc/. have a major impact
on the physics of vdW heterostructures. It is therefore essential to
probe vdW heterostructures in- and out-of-plane at the nano- and even
atomic scale. This seminar will describe two complementary efforts in
this direction, with a focus on the optical properties of
heterojunctions made from 2D semiconductors (here, transition metal
dichalcogenide (TMD) monolayers) coupled to graphene.

In a first part, we will exploit van der Waals engineering and
diffraction-limited optical spectroscopy to precisely probe interfacial
coupling between TMD monlayers and graphene separated only by a
sub-nanometer vdW gap [1]. We will show how graphene alters the dynamics
and radiative recombination of excitons (bound electron-hole pairs) in
TMDs, leading to a remarkable “filtering effect” (Fig. 1 and [2]) and,
complementarily, how the optical phonons in graphene provide invaluable
insights into dielectric screening and interfacial coupling in
graphene-based vdW heterostructures [3].

In a second part, we will show that a low-temperature scanning tunneling
microscope (STM) can probe, with atomic-scale spatial resolution, the
excitonic luminescence of a vdW heterostructure, made of a TMD monolayer
stacked onto a graphene flake [4]. This work paves the way towards
understanding and control of individual quantum emitters and more
broadly of optoelectronic phenomena in moiré superlattices with
atomic-scale resolution.

This was work done within the Nano-Optics and Low-Dimensional Materials
team <https://fcbg.team/>at IPCMS, in collaboration with the STM team at
IPCMS (G. Schull /et al./) and the LPCNO at INSA Toulouse (C. Robert, D.
Lagarde, X. Marie).


[1] G. Froehlicher, E. Lorchat, S. Berciaud, Phys. Rev. X *8,* 011007 (2018)

[2] E. Lorchat, L. E. Parra López /et al./, Nature Nanotechnology. *15*,
283 (2020) -arXiv:1908.10690 <https://arxiv.org/abs/1908.10690>

[3] L. Moczko /et al./, submitted (2022)

[4] L. E. Parra López /et al./, Nature Materials (in press, 2023)
-///https://doi.org/10.1038/s41563-023-01494-4 /arXiv:2204.14022