Electronic properties of 2D and 1D inorganic materials for applications in nano(opto)electronics


Electronic properties of 2D and 1D inorganic materials for applications in nano(opto)electronics

Kuc, A.

The nanoelectronic industry is rapidly approaching limits of the silicon-technology, what leads to a necessity of developing new technologies, which would replace silicon in the future. Therefore, searching for materials that perform better than silicon at the atomic scale became a very important topic in the electronic and materials sciences in the past decades. Recently, two-dimensional (2D) layered materials, such as graphene, black phosphorous, silicene, or transition-metal dichalcogenides (TMCs), have attracted great attention, because of their extraordinary electronic properties and, at the same time, very good mechanical stability, which are desired features for nanoelectornic applications. The progress in the production of such 2D crystals grows rapidly every year, therefore, it is very important to estimate, understand, and explore the fundamental physics of these materials, in order to boost breakthrough technologies.
Layered transition-metal dichalcogenides have gained increasing attention ever since the seminal works published in 2010 and 2011, showing phenomenal electronic properties of monolayered systems, their easy exfoliation from bulk materials, due to the weak interlayer interactions, as well as, their applications as building blocks in the nanoelectronic logical devices. In this thesis, we present selected research based of density-functional theory, which has been carried out on the subject of electronic structure of TMC and other 2D crystals. These materials exhibit electronic properties, which are easily tuned by external modulators, such as tensile strain, doping, electric or magnetic fields, formation of different polytypes. The change in the electronic properties of semiconducting TMCs due to these external modulators vary in a wide range, e.g., semiconductor-metal transition, Rashba, Zeeman and Stark effects, induced spin-orbit coupling in centrosymmetric bilayered forms by breaking of inversion symmetry, topologically protected states in topological insulators. We also present the coherent transport properties of these 2D materials using calculations based on the density functional based tight-binding method in combination with the non-equilibrium Green’s function technique and the Landauer-Büttiker formula.
We show that the intrinsic electronic structure of MoS2 and other semiconducting TMCs change with the number of layers in the film. The indirect-band gap in the bulks changes to a direct-band gap in the monolayers and the size of the band gap is nearly 1 eV larger for the latter forms. On top of the electronic band gaps, which are mainly discussed in this thesis, TMC exhibit also very large exciton binding energies, which need to be taken into account, when discussing overall electronic properties. TMC monolayers exhibit very large spin-orbit splitting in the valence bands, which varies between 150 and almost 500 meV, depending on the stoichiometry. Stacking different monolayers of TMC materials results in reduced direct-band gaps with much smaller values than the respective pure materials, which comes from the formation of the type II heterostructures. In such heterostructures, the valence band maximum is formed from the states of different layers. This results in materials with excitons localized in such a way that the electron is located in one layer and the hole in the other.
We believe that the knowledge gained from the research presented in this thesis can provide new perspectives for the applications of TMC materials in the next generation of nano(opto)electronic devices.

  • Other
    Jacobs University Bremen, 2018
    Mentor: Prof. Thomas Heine

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Publ.-Id: 28005