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Project Catalogue
Page last edited by Kristian Sommer Thygesen (ksth) 20/03-2020
You are most welcome to define your own projects! Please contact one of the teachers to clear with them, mainly to ensure that the project is realistic with respect to time and computational resources. Or select a project from the list below, and send an email to Kristian stating the project you want. Calculating Raman spectra of multilayer 2D materials (Kristian Thygesen)
Raman spectroscopy is a useful non-destructive tool to perform materials characterization. In the field of 2D materials, Raman spectra can be used to identify the number of layers in thin films of 2D materials from the difference in frequency between the in-plane and out-of-plane phonon modes. The project will use DFT to compute the phonon spectrum of mono- and bilayer graphene and MoS2. From these phonon modes and associated electron-phonon coupling, the Raman spectrum will be evaluated using 3-order perturbation theory (this will be done using an existing code). The results will be compared to available experimental data and conclusions regarding the reliability of this approach to infer the number of layers in thin film 2D structure will be drawn.
Twistronics: Band structure calculations for twisted bilayer MoS2 (Kristian Thygesen) It has recently been discovered that bilayer graphene with a relative twist angle of 1.1 degree becomes superconducting at temperature around 1 K. The coupling mechanism remains unknown, but it has been suggested that the formation of the superconducting state is connected to the existence of flat bands. This project will investigate the effect of twist angle on the band structure of another van der Waals material, namely bilayer MoS2. Using vdW-DFT, the atomic structure and the electronic band structure of bilayer MoS2 will be calculated and analysed for various rotational twist angles. Phonons in van der Waals bonded layered materials (Kristian Thygesen) Layered van der Waals bonded materials, such as graphite, are characterised by strong in-plane covalent bonds and weak dispersive (van der Waals) bonds between the layers. This project will calculate the phonon band structure of the layered compounds graphite and MoS2 using a vdW density functional. Based on the calculations, the sound velocity in the in-plane and out-of-plane directions will be determined and compared to experiments, and the degree to which the inter-layer coupling changes the phonons as compared to an isolated monolayer will be explored. Self-intercalated 2D materials (Kristian Thygesen)
The layers of a van der Waals bonded layered materials can be covalently bonded together by a process known as self-intercalation. For example, self-intercalation by metal atoms (M) can transform a layered material MX_2 into different crystals M_iX_j. The precise stoichiometry, i.e. the values of i and j, depends on the concentration of intercalated metal atoms which in practice can be controlled by the partial pressure of M during growth. It has recently been shown that the properties of the intercalation compounds, e.g. magnetic/non-magnetic or metallic/semiconducting, can be designed by controlling the concentration of self-intercalated atoms. This project will investigate different self-intercalated compounds and calculate their basic properties such as stability, band structure, and magnetic state.
Long-range adsorbate-adsorbate interactions in boron doped graphene (Heine Anton Hansen)
Heteroatom doped carbons are promising materials for catalyst and sensors. Long-range interactions have been reported between dopants in graphene and between certain adsorbates on N-doped graphene. In this project you will investigate interactions between key reaction intermediates in the oxygen reduction reaction on boron doped graphene.
Interaction between graphene and hBN with DFT (Mads Brandbyge) Hexagonal Boron-Nitride (hBN) is used as an insulator in van der Waals stacks of e.g. graphene layers. We want to calculate the relaxed structures and binding energy of the stack depending on stacking (Bernal or twisted). We also want to calculate the band structures and electronic coupling between the graphene layers when separated by a single layer of hBN. Try out LDA, GGA, and vdW-DF approximations. Finally, if time allows, we will look at the effect of a vacancy (missing N) in the hBN. Electronic structure of Nano-Porous Graphene (Mads Brandbyge)
Is it possible to grow graphene-like 2D materials with regular lattices of holes (Nano-Porous Graphene) using “bottom-up” self-assembly of molecular building blocks. Due to the huge tunability of the molecular building blocks these materials are promising for future nanoelectronics, bio-sensors, photo-voltaics etc. In this project you will calculate how the electronic structure can be tuned by changing the molecular building blocks, their conformation, and doping.
[Moreno et al., “Bottom-up synthesis of multifunctional nanoporous graphene”, Science, 360, 199 (2018)]
Proximity induced spin-orbit coupling in graphene (Thomas Olsen) Relativistic effects such as spin-orbit coupling become progressively more important as we move down the periodic table. In graphene the spin-orbit coupling is predicted to have extremely important consequences, but is too weak to be measurable in any experimental setup. However, if graphene is placed in proximity with heavy elements such as bulk Pt, there might be a significantly enhancement of the spin-orbit coupling. This project will investigate this effect and we will calculate the proximity induced gap of graphene resulting from spin-orbit coupling. Non-collinear spin in 2D materials (Thomas Olsen)
there is currently great interest in 2D magnetic materials. The first example of an ferromagnetic 2D material was discovered only two years ago, but there is still no reports on antiferromagnetism n 2D. The magnetic atoms in many 2D materials form a triangular lattice and in such cases there is no way to form a regular antiferromagnetic distribution of magnetic moments. Instead the ground state acquires a non-collinear structure where all spins are at a 120 degree angle. In this project we study the non-collinear magnetic order in 2D materials from self-consistent first principles and investigate whether this structure modifies the prediction of magnetic interactions form collinear calculations. Graphene on CrI3 (Thomas Olsen)
There is currently great interest in CrI3, which is the first example of a 2D ferromagnet (discovered in 2017). STM measurements on CrI3 are done with a graphene layer on top in order to supply conductive electrons for the STM. In this project we investigate the interaction between graphene and CrI3 form first pinciples. Various superstructures will be compared and the dependence on xc-functional will be critically assessed. Magnetic order in VSe2 (Thomas Olsen)
In 2018 it was shown that VSe2 exhibits magnetic order at room temperature. The origin of magnetism is still not understood since spin-orbit coupling favors spins to lie in the plane of the material, which should imply that magnetism is unstable at finite temperatures. In this project we will carefully calculate the magnetic interactions and magnetic anisotropy in VSe2. We will compare different semi-local functionals with PBE+U and hybrid functional calculations. Magnetic interactions in 2D metals (Thomas Olsen)
Magnetic insulators are usually described by Heisenberg models that describe the spins a sbeing localized on individual atoms. For metals, however, it is not clear that this assumption is justified and there is presently no reliable way of treating magnetic order in metals. It has been proposed that the Heisenberg model can also be applied to metals, but long-range magnetic interactions have to be included. In this project we investigate the long range magnetic interactions in 2D magnetic metals by calculating magnetic interactions between first, second third, fourth, and fifth nearest neighbors.
Selenium as a photoabsorber (Karsten Jacobsen)
It is of great importance to identify new and efficient materials for absorption of light from the sun. Such materials can for example be used in solar cells or to create energy to split water and produce hydrogen. Currently an experimental group at DTU physics investigates the light-absorbing properties of the elemental material Selenium. This project would involve DFT calculations to dermine the work function of different Selenium surfaces. This is of importance when Selenium is interfaced to other materials. Atomic and electronic structure of MoS nanowires (Karsten Jacobsen) MoS2 is a two-dimensional material with interesting electrical, chemical, and mechanical properties. Recently it was shown that it is possible to contact such two-dimensional layers with nanowires made also of MoS2 [1]. in this project the atomic structure and stability of different wires of MoS2 will be investigated. If time allows the stability will be compared to the one of the layered material and of small molybdenum-sulfide clusters.
[1] J. Lin, O. Cretu, W. Zhou, K. Suenaga, D. Prasai, K. I. Bolotin, N. T. Cuong, M. Otani, S. Okada, A. R. Lupini, J.-C. Idrobo, D. Caudel, A. Burger, N. J. Ghimire, J. Yan, D. G. Mandrus, S. J. Pennycook, and S. T. Pantelides, “Flexible metallic nanowires with self-adaptive contacts to semiconducting transition-metal dichalcogenide monolayers,” Nature Nanotechnology, vol. 9, no. 6, pp. 436–442, Apr. 2014.
Enhanced diffusion on metal surfaces (Jakob Schiøtz) Transmission Electron Microscopy of gold nanoparticles show that small amounts of carbon monoxide (CO) and possibly of hydrogen (H2) may significantly enhance surface diffusion. Using the Nudged Elastic Band method, the diffusion barrier of a gold adatom on a gold surface should be calculated with and without a molecule adsorbed on the ad-atom. If there is time, the calculations are extended to geometries representing the edge of a nanoparticle.
Bayesian error estimation for surface energies (Jakob Schiøtz) Using an ensemble of functionals it is possible to estimate the reliability of calculated quantities. This will be used on surface energies for a range of metals. DFT simulation of TEM images (Jakob Schiøtz) With High-Resolution Transmission Electron Microscopy (HRTEM), it is possible to image materials with atomic resolution. The electron beam is mainly scattered by the electrons in the material, for that reason one is essentially imaging the electron density of the materials. In almost all materials, the electron density is dominated by the core electrons, and is therefore only changed slightly by the chemical bonds formed in the material. For this reason, HRTEM simulation software typically ignores all chemistry, and model the electron density as a sum of atomic densities. For very light materials, such as graphene, this assumption may not hold completely. It has been shown by simulations that the chemical bonding in graphene cause a visible change in the HRTEM image. This was shown by simulating images with the electron density taken from a DFT calculation and use that as the basis for a HRTEM simulation instead of the usual sum of atomic densities.
Chemical changes may change the electronic structure of graphene. A famous example is that if you adsorb a single hydrogen atom to a graphene sheet, you break the symmetry between the two sublattices. A second hydrogen will have a significantly different adsorbtion energy depending on whether it is adsorbed on the same sublattice or the other sublattice, even if the adsorption sites are relatively far from each other. It is unknown if adsorbing one or two hydrogen atoms to graphene induces a change in the electron density that could be observed in HRTEM.
In this project you will investigate this. The first part is generating HRTEM images from a DFT calculation with the PyQSTEM package (a package allowing HRTEM simulations with ASE and GPAW), to verify that it is indeed possible to see the effect of the chemical binding. The second part is to adsorb hydrogen to the graphene sheet, and investigate if the change in the image is visible.
Electronic structure of selected 1D materials (Karsten Jacobsen) Description: The last couple of years 2D materials have been intensely studied both experimentally and theoretically. But what about 1D materials? We have recently identified a range of experimentally known 1D materials, but many of these have never been investigated theoretically before. In this project we shall select a few 1D materials and calculate stability and band structure using DFT. High-Entropy Alloys (Karsten Jacobsen) High-entropy alloys are metals composed of 4 or more different elements like for example CuNiFeCrMo. The many different elements give many possibilities of arranging the atoms on a lattice, so the entropy is large and dominates the behavior of the material. In the project we shall do DFT calculations for a high-entropy alloy and calculate the interaction energies between the different atoms on neighboring lattice sites. This makes it possible to use Monte Carlo simulations to study the thermodynamic properties of the high-entropy alloy. What is the ground state for the high-entropy alloy, and at which temperature does it disorder? Structure optimization with machine learning and DFT (Karsten Jaccobsen).
It has been recently demonstrated that the use of Machine Learning (ML) surrogate models can be used to leverage the computational cost of finding critical points in Potential Energy Surfaces (PES) described by ab-initio methods.
The goal of this project is to test if, within a Gaussian Process Regression (GPR) framework, encoding bonding information a priori into the model results in a speed-up of local relaxations as compared with no bonding information. Target applications could include testing a pretrained model including covalent radii descriptors on organic molecules on surfaces described by Density Funtional Tight Binding (DFTB) or training and testing a model to include Van der Waals a priori bonding information for graphene (described either by DFT or DFTB).
This project requires previous knowledge of Gaussian Process Regression.
Oxygen Reduction at Single-Metal Active Sites of Fluorinated Porphyrins (Juanma García Lastra) The most common ORR catalyst, platinum, has limited availability and is expensive, so work is being done to find a viable replacement for it. Recently, it has been found that the active sites of metal porphyrins may be a good ORR/OER catalyst. In this project we will investigate if it is possible to further improve the performance of these porphyrins by fluorinating them.
Optical spectra of KNiF3: Does the magnetic order influence the optical spectra (Juanma García Lastra)
KNiF3 is an antiferromagnetic material with an absorption spectra similar to KMgF3:Ni laser material (KMgF3 doped with Ni). Would it be the case if KNiF3 was a ferromagnetic material? Band gap and magnetic order of cobalt oxide (Co3O4) (Juanma García Lastra)
Cobalt oxide is one the materials used in the anodes of state-of-the-art Lithium-ion batteries. However its electronic, magnetic and optical properties are not fully understood. Optical spectra of metallo-porphyrines (Fe, Mn, Ru) (3 projects) (Juanma García Lastra)
Porphyrin molecules are the responsible for the absorption of light in dye-sensitized solar cells and photosynthesis. Can the absorption spectra of porphyrins be tailored by adding side-groups to the molecules? CsBX3 perovskites (B=Sn,Pb, X=Cl,Br,I) for solar cell applications. (Juanma García Lastra)
Halide perovskite-based solar cells have arisen as one of the innovative photovoltaic technologies in recent times, mostly because of their direct bandgap, large light absorption coefficients due to a direct transition at the bandgap that involves Pb s-states and Pb p-states and high carrier mobility. In this project we will look at the lattice parameters, band gaps and reduced electron masses in 6 different of these (cubic) perovskites. We will use different functionals to study how the choice of the functional affects the calculated parameters.
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