Doctoral College TU-D Unravelling advanced 2D materials
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Theory of electronic and transport properties of 2D materials

Florian Libisch, Institute for Theoretical Physics

Research in the FL group focuses on the simulation of realistic nanodevices, including finite-size effects, edges, defects, and substrate interactions. We describe quantum transport through nanodevices using a tight-binding based modular Green’s function approach, enabling the simulation of large-scale structures (up to 1 × 1 μm2), and direct quantitative comparison to experimental results on the local density of states (from STM) [1] and transport [2]. Using Arnoldi-Lanczos propagation we simulate the electronic response to ultrashort laser pulses, including high-harmonic generation spectra [3]. Density-based embedding approaches [4] allow for the combination of different levels of theory within one simulation, enabling the high-level treatment of local excitations or charge transfer processes at defects or interfaces, while the surrounding environment is handled by a lower-level approach. The simulation of an entire nanodevice is complemented by a self-consistent treatment of the local charge accumulation, yielding critical information on the local alignment of Fermi levels in, e.g., photovoltaic devices investigated by the TM group [5].

PhD Project 1: Theory of high harmonic generation in graphene

Co-supervisor: Thomas Müller    

Simulation of the non-linear condensed matter response to ultrashort laser pulses: High harmonic generation (HHG), the generation of a high-frequency response of materials using few-cycle, ultra-short laser pulses,  is one of the most prominent and technologically most relevant signatures of the non-linear response of matter to ultrashort intense laser pulses. The topic of this Ph.D. project is the microscopic, ab-initio simulation of the response of two-dimensional materials to intense laser pulses using time-dependent density functional theory methods. Collaborations with PB on state-of-the-art exchange-correlation functionals (e.g., Tran-Blaha modified Becke-Johnson functionals) will allow for an accurate treatment of band gaps in semiconducting transition metal dichalcinogenides (MoS2, WSe2), which is critical for the faithful description of inter-band tunneling processes. By stacking different layers of 2D materials, the material response can be tailored for increased high-frequency output. Furthermore, our simulations will provide key insights on the role of competing inter-band tunneling and intra-band Bloch oscillations in these materials. Collaborations with the TM group will allow for experimental verification of results.

PhD Project 2: Describing single atom catalysis using embedding

Co-supervisor: Gareth Parkinson    

Heterogeneous catalysis at surfaces or in solution is challenging to model from a theoretical point of view: simulation cell size is large to correctly capture the catalytic conditions, rendering highly accurate quantum chemical methods unsuitable due to the large computational effort involved. By contrast, conventional density functional theory can handle the problem size, but is challenged by the charge transfer processes involved. Embedding methods allow for an elegant solution, by offering to combine different levels of theory in one calculation: we have applied these techniques successfully to surface catalysis problems like the dissociative adsorption of oxygen on aluminum. The single-atom catalysis invested by the GP group is a prime example where our techniques could offer valuable insight. The topic of this Ph.D. project is the systematic investigation of single-atom catalysis on magnetite, within close collaboration with the GP group. This project will lead to key insights in exploiting catalytic effects on heterogeneous surfaces.


Literature

  1. D. Subramaniam et al., Wave-Function Mapping of Graphene Quantum Dots with Soft Confinement, Phys. Rev. Lett. 108, 046801 (2012) DOI:10.1103/PhysRevLett.108.046801
  2. Güttinger et al., Electron-Hole Crossover in Graphene Quantum Dots. Phys. Rev. Lett. 103, 046810 (2009) DOI: 10.1103/PhysRevLett.103.046810
  3. L. Chizhova, J. Burgdörfer, and F. Libisch    , Magneto-optical response of graphene: probing substrate interactions,     Phys. Rev. B. 92, 125411 (2015) DOI: 10.1103/PhysRevB.92.125411
  4. Libisch F., Huang C., Carter E. A. Embedded Correlated Wavefunction Schemes: Theory and Applications, Accounts of Chemical Research 47, 2768 (2014) DOI: 10.1021/ar500086h
  5. Furchi M., Posposchil A., Libisch F. Burgdörfer J., Müller T. Photovoltaic effect in an electrically tunable van der Waals heterojunction. Nano Letters 14, 4785 (2014) DOI:  10.1021/nl501962c
  6. N. Freitag, L. A. Chizhova, P. Nemes-Incze, C. R. Woods, R. Gorbachev, Y. Cao, A. K. Geim, K. Novoselov, J. Burgdörfer, F. Libisch, and M. Morgenster, Electrostatically confined monolayer graphene quantum dots with orbital and valley splittings , Nano Letters, doi: 10.1021/acs.nanolett.6b02548 (2016)