Designing Excitons in van der Waals Heterostructures
11:00 - 12:00
Technical University of Denmark (DTU)
CFEL (Bldg. 99)
Seminar Room V, O1.109
Van der Waals heterostructures (vdWHs) represent a novel and largely unexplored class of materials. Since 2013, when Geim and Grigorieva first conceived the stacking of 2D (two-dimensional) materials to create artificial layered structures with tailored properties, a number of promising (opto)electronics devices, e.g. light emitting diodes, solar cells, ultra-fast photodetectors, transistors etc. have been successfully fabricated. It is well established that for isolated 2D semiconductors and vdWHs the optical response is governed by excitonic effects. A theoretical understanding of excitonic effects and of how the electronic screening is affected for the more complex case of multi-layer structures is still lacking due to the computational limitations of standard ab-initio methods.
In this seminar, I will present the first-principles multi-scale method
(QEH) that we developed to overcome such limitations for the description
of dielectric, electronic and excitonic properties in isolated 2D
materials and vdWHs. The method is based on the analogy between vdWHs
and the popular construction toy, Lego. This analogy is much deeper than
one would first expect: it is possible to predict the dielectric
properties of a vdWH from the dielectric functions of the individual 2D
layers, which represent the dielectric genome of the heterostructure.
This multi-scale method also proves successful when combined with
many-body perturbation techniques for accurate prediction of
electronic and excitonic excitations or with complex scaling techniques
for the calculation of exciton dissociation rates in vdWHs. Our approach
can even be applied to the design of more complex many-body excitations
consisting of photo-excited electrons and holes localized in distinct
layers which result into inter-layer excitons (see figure).