Excitons in van der Waals Heterostructures: A theoretical study

Research output: Book/ReportPh.D. thesis – Annual report year: 2016Research

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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. While it is understood that the reduced amount of electronic screening in freestanding 2D materials is the main origin of extraordinarily strongly bound excitons, 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 thesis first-principles models that overcome the limitations of standard ab-initio techniques are developed for the description of dielectric, electronic and excitonic properties in isolated 2D materials and vdWHs. The main contribution is a multi-scale method that seemingly connects the excitonic effects in the monolayer limit to the more challenging case of multi-layered structures. 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. From the vdWH dielectric properties one evaluates the screened interaction between the electron and hole forming the exciton which can then be used in a generalized hydrogenic model to compute exciton binding energies in isolated, supported, or encapsulated 2D semiconductors. The non-locality of the dielectric screening is inherently included in our method and we can successfully describe the non-hydrogenic Rydberg series of low-dimensional systems. This multi-scale method also proves successful when combined with many-body perturbation techniques for accurate prediction of electronic band structure or with complex scaling techniques for exciton dissociation rates in vdWHs. The validity of our techniques is demonstrated through numerous comparison to experimental results.
Ultimately this thesis puts forth a first-principles methodology that allows us to address scientific questions that are beyond the capability of existing state of the art techniques and enables 2D materials researcher to predict and design dielectric, electronic and excitonic properties of general vdWHs.
Original languageEnglish
PublisherDepartment of Physics, Technical University of Denmark
Number of pages196
Publication statusPublished - 2016

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