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].