=============================== 2015 ======================================

Spin-orbit split surface states of Au(111) and Pt(111) (Thomas Olsen)   Student: Rasmus Jensen

Investigation of dielectric properties in novel Van der Waals heterostructures (Simone Latini, Kristian S. Thygesen)  Students: Jes Ærøe Hyllested, Samuel Dechamps, and Vincent Trolé
The idea of this project is to study how quantities like exciton binding energies or plasmon dispersions are affected by stacking different 2D material on top of each other. Understanding dielectric properties of these structures is of fundamental importance in opto-electronic devices engineering. 
In the project you will learn how to perform fast response calculations for 2D single layers at first, to proceed then with the stacking and account for the Van der Waals coupling using a recently developed classical model. The idea is to try different stacking combination in order to come up with some system with exciting properties.

The influence of free rotation on the ionization potential of atmospheric relevant molecules (Kristian Ørnsø, Kristian S. Thygesen)
Some molecules called MACR and HMML play important roles in the atmosphere and are suggested to influence the formation of aerosols. However, due to the free rotation around single bonds several conformations of these molecules should be taken into account when calculating properties like the ionization potential. In the project we will use Molecular Dynamics simulations to generate an ensemble of conformers of the molecules for which the ionization potential will be calculated. The Bolztmann averaged ionization potential can then be compared to the ground state ionization potential to explore the effect of free rotation at different temperatures.

Lattice constants within the Random Phase Approximation (Per S. Schmidt, Kristian S. Thygesen)   Student: Alexander Tygesen
The project involves calculation of equilibrium lattice constants for a range of materials using the Random Phase Approximation (RPA). This requires going beyond standard DFT and use of the linear response code in GPAW. The results can be compared with standard DFT lattice constants, lattice constants including exact-exchange but neglecting correlation effects entirely, as well as experimental results. RPA calculations are computationally heavy and might require time on Niflheim. Knowledge of response functions is preferred, if one is to understand the theory behind RPA.

Oxygen Reduction at Single-Metal Active Sites of Fluorinated Porphyrins (Juanma García Lastra)    Student: Alexander Krabbe

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)   Student: Arghya Bhowmik
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)   Students: Claudio Sanna, Akshay Bhat
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)   Student: Giuseppe De Martino

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.

Nanodiamond - diamondoid molecules (Christopher Patrick, Karsten)  Student: Mads Almind
Diamondoid molecules (nanodiamond) are molecules comprising of sp3 bonded carbon cages saturated by H.  The smallest of these molecules is adamantane C10H16. It has an interesting electronic structure, with the LUMO, LUMO+1 etc being Rydberg states.  Also the HOMO is very strongly coupled to the C-C bond length.  One could do a simple study either of electronic structure as a function of size (adding cages gives adamantane, diamantane, triamantane, tetramantane) or swap C with Si or Ge and look at vibrational/electronic properties for adamantane-like molecules.

Purcell factors of vdW-heterostructure hyperbolic metamaterials. (Morten Gjerding, Kristian Thygesen)
Recently it has been possible to manufacture stacks of 2D materials such as graphene, boron nitride (BN), molybdenum disulfide (MoS2) etc. in the lab. In general, stacks such as these are denoted van der Waals (vdW) -heterostructures. A hyperbolic metamaterial is a heterostructure consisting of alternating layers of conductor and insulator. This results in interesting photonic properties such as a higher density of states for photons within the material. A direct consequence of this is that the spontaneous decay of photons emitted by quantum dots placed on these materials is enhanced. Classically, thinner layers means greater enhancement. Due to their atomically well-defined interfaces vdW-heterostructures have to potential to approach the absolute limit of hyperbolic metamaterial, namely atomically thin layers.
In this project you will use GPAW to calculate dielectric functions for different 2D materials and use these to estimate the enhancement factor of the spontaneous decay, also known as the Purcell factor.

Hydrogen-deuterium exchange at Pt clusters (Mikkel Jørgensen, Jakob Schiøtz)   Student: Sebastian Jespersen

The Hydrogen-Deuterium Exchange reaction is an important benchmark reaction when studying catalysis, as it can measure how fast hydrogen adsorbs and recombines on the surface.  In that connection, the group of Ib Chorkendorff has obtained results for the HD exchange on very small Pt clusters, that are difficult to understand but may indicate that small amounts of oxygen dramatically change the rate of the HD exchange reaction.
In this project you will be calculating hydrogen adsorption energies with GGA-DFT for various oxidation states of a small Pt-cluster(8-13 atoms) and assess how the presence of oxygen changes the H-adsorption energies. In addition, it will be useful to compare with similar calculations on a Pt(111) surface for example.
Bayesian Error Estimation Functionals can be used to asses the uncertainty of these  adsorption energies.

Interfaces in MoS2 single layers (Karsten)  Student: Martin Vinther

A single layer of MoS2 can be produced in two different structures 2H and 1T, where 2H is semiconducting and 1T is metallic. It has been shown that a way to make metallic contacts to a semiconducting 2H MoS2 is therefore to change the phase of the material from 2H to 1T in areas around the 2H phase. In the project you will investigate the interface between the two phases within a single MoS2 layer.

Intercalation in MoS2 (Karsten)   Student: Sofie Colding-Jørgensen

Single layers of MoS2 in different phases can be produced by intercalating the bulk MoS2 with alkali atoms followed by exfoliation. In the project you will study bulk MoS2 with Li intercalation to see how the intercalation affects the different phases of the material. The project requires description of van der Waals interactions and it is therefore necessary to use an xc-functional like BEEF-vdW where these interaction are taken into account.

Pentagraphene (Karsten)  Students: Mikkel Maag Pedersen

Graphene consists of rings of carbon with six atoms in each ring. Now a new form of graphene has been proposed where only five atoms participate in the rings. This has therefore been named pentagraphene. The material has not yet been produced but calculations seem to indicate stability of the material. In this project you will calculate the structure and binding energy of pentagraphene in order to get an idea about how stable the material is.

=========================== 2014 =========================================

Bandstructure of MoS₂   (Karsten)

MoS₂   is a layered material where a single layer consists of an atomic Mo layer sandwiched between two atomic S layers. The bandstructure depends on the symmetry of the system: In the most stable structure where inversion symmetry is broken the materials exhibits a bandgap, while if inversion symmetry is restored the material becomes metallic. The bandstructure should be investigated with DFT calculations. (D. Voiry, M. Salehi, R. Silva, T. Fujita, M. Chen, T. Asefa, V. B. Shenoy, G. Eda, and M. Chhowalla, “Conducting MoS₂ nanosheets as catalysts for hydrogen evolution reaction.,” Nano Lett13(12), 6222–6227 (2013) [doi:10.1021/nl403661s].)

Chemisorption on semiconducting and metallic MoS₂   (Karsten)

Kaspar

Edge states of layered materials (several projects, Kirsten, Karsten)

MoS₂ is a layered material with a bandgap which exhibits metallic states at the edges. The project investigates with DFT calculations whether other layered materials  (WS₂, MoSe₂, WSe₂) also exhibit metallic edge states. 

Elastic properties of layered materials (several projects, Karsten).

Graphene is known to be a very strong material. The energy required to stretch and bend graphene can be calculated with DFT and expressed through calculated elastic constants. Other materials made of atomically thin layers can be treated the same way.

Strain dependence of chemisorption energies for layered materials (Karsten)

The chemisorption energy of atoms and molecules are known to depend rather sensitively on surface strain. Is the dependence similar if the material is atomically thin? (M. Mavrikakis, B. Hammer, and J. K. Nørskov, “Effect of Strain on the Reactivity of Metal Surfaces,” Phys. Rev. Lett. 81(1), 2819–2822 (1998) [doi:10.1103/PhysRevLett.81.2819].)

Calculation of coverage-dependent adsorption energies: Adsorption of S, SO2, HSO3 and OH on Rh, Pd, Pt and Au  (several projects, Mårten Björketun)

Molecules binding to a surface may interact with each other and therefore the adsorption energy will depend on how many molecules are present on the surface (the coverage). In the project the adsorption energies will be calculated as a function of coverage with DFT.

Manuel 

Density functional theory is an excellent theory for predicting ground state properties of a variety of materials. However, the Kohn-Sham eigenvalues do not have a physical significance and as a consequence band structures derived from DFT are typically not correct. This project will assess this problem by comparing the DFT band structure to more accurate band structure methods for the case of an MoS2 nanoribbon. The MoS2 nanoribbon is of both technological and fundamental interest due to its insulating interior and metallic edges.

Alexander.

The delocalization error of DFT  (Kristian Thygesen)
One of the great challenges in modern electronic structure theory goes under the name "delocalization error". In essence it stems from the self-interaction error, i.e. the fact that the Hartree potential of an electron is felt by the electron itself, which tend to delocalize electronic state resulting in artificial charge transfer over long distances. In this project we shall investigate this effect for a simple charge-transfer dimer, and also investigate possible solutions based on Hartree-Fock and many-body theory.

Ab-initio calculations of light dispersion in optical metamaterials  (Kristian Thygesen)
Metamaterials are a new class of artificially structured materials with unique optical properties such as diverging photonic density of states and hyperbolic dispersion relations (rather than elliptical dispersion relation). In this project we shall investigate the optical properties of metamaterials using density functional theory calculations of the dielectric constants for atomically thin layered materials and assess the range of validity of widely used classical theories for metamaterials.  

Electron transport in molecular junctions  (Kristian Thygesen)
The vision of molecular electronics is to build electronic components "bottom-up" using single organic molecules as the basic components.  In this project we use DFT to calculate the IV curve of a single molecule connected to metallic electrodes. We shall focus on molecules showing quantum interference phenomena which has recently been experimentally observed and represents a unique possibility of controling charge transport at the level of single wave functions. 

============================== Confirmed projects 2014 ==============================

Da DFT ofte beregner båndstrukturer i materialer forkert (pga. Kohn-Sham bølgefunktionerne ikke har en fysisk betydning) vil jeg i dette projekt sammenligne DFT beregninger med many-body beregeninger (GW) af båndstrukturer i en MoS2 nanoribbon.
MoS2 nanoribbons er interessante fordi det er en isolator med kanter, som har metalliske egenskaber. Det betyder, at der på kanterne er frie elektroner og i kernen er fastlåste elektroner. De frie elektroner vil måske blive påvirket af de fastlåste elektroner (pga image charge effekten) i en tilpas stor grad, at båndstrukturerne/båndenergien i MoS2 nanoribbons ændres med tykkelsen.

Student: Simon Loftager

Project title: Electron Conductivity in Silver Chloride (AgCl).

Supervisor: Juanma

Silver chloride, AgCl, is a common reference electrode in electrochemistry. Interestingly, even though AgCl is a bandgap material, it is able to conduct electricity through a polaron hopping mechanism. Pulsed EPR/ENDOR experiments [Bennebroek et al, PRB 66, 054305 (2002)], showing information about the spatial distribution of the unpaired electron, suggest that the wave function of the self-trapped hole is essentially located in the plane perpendicular to the elongation axis of one AgCl₆ complex. In this DFT-based project we want to (i) reproduce the localized nature of the hole, (ii) reproduce the covalency for the hole (~30% on Ag and ~70 % on Cl), (iii) study Jahn–Teller distortion of the AgCl₆ complex (symmetry reduction from octahedral to tetragonal), and (iv) study the barriers for the polaron hopping between two contiguous AgCl₆ complexes. In doing this we will utilize the high quality DFT functionals HSE and PBE+U.

Student: Kaspar Haume

Project title: Kantstrukturer af ledende MoS2-lag

Student: Thomas Jauho

Project title: Bandgap under strain.

Supervisor:  Ivano  Castelli

I will be using DFT and DFT+sigma; to describe electron transport and
heat dissipation in a molecular junction between two gold surfaces. I will
be analyzing alkaline chains between the contacts (C2, C4, C6) with NH3
anchoring sites. First I will analyze the heat dissipation using a
transmission function based on a simple Lorentz model, then I will

find the "actual" transmission function using DFT and DFT+sigma.