Open positions in DCAFM

Research Team 1 - Van der Waals heterostructures
C. Dellago, C. Franchini, J. Kotakoski

Machine learning for simulation of defects in van der Waals heterostructures
Mono- and few-layered nano-sheets made of transition metal dichalcogenides (TMDs) are versatile materials with many potential applications ranging from energy storage to catalysis and lubrication. The electronic, optical and mechanical properties of TMDs, however, are strongly affected by defects such as vacancies, rotated bonds and grain boundaries. In this project, we will use atomistic simulations to study the formation, migration and stability of defects in TMD/graphene heterostructures. Our investigations complement the electron microscopy studies carried out in the project of J. Kotakoski, who will investigate defects in TMD/graphene heterostructures created by ion and electron irradiation. Ab initio trained neural network for energy and force calculations.

Spin-orbit effects in defective van der Waals heterostructures
There are a category of very promising 2D materials potentially exploitable in nanoelectronics, spintronics and valleytronics. Many of the interesting properties of TMDs arise from the strong spin-orbit coupling which causes (controllable) splitting in the electronic bands and can lead to the formation of non-collinear spin textures and topological phases. TMDs can contain different types of structural defects which significantly alter their properties. The objective of this research project is the ab-initio description of relativistic effects in defective TMDs-based vdW heterostructures, aiming to clarify how structural, chemical and electronic defects perturb the spin-orbit related phases.

Atomic-scale study of defects in van der Waals heterostructures
Of the large number of currently known 2D materials, TMDs offer arguable the largest variety of properties. Beyond the individual materials, the application potential of 2D materials can be further increased through their combination in heterostructures. However, many of the TMDs have been reported to contain a large number of defects. It is widely believed that these defects are responsible for interesting optical properties of TMDCs. In this project, we create heterostructures of graphene and inorganic 2D materials and study both their intrinsic defects as well as introduce defects via ion and electron irradiation.

Research Team 2 - Carbon nanomaterials
P. Ayala, G. Kresse, T. Pichler

Heteronanotubes with tunable functionality
Heteronanotubes offer a unique platform for tailoring structural electronic and optical properties towards different applications such as biosensing and imaging, new complex nanocomposites. However, the exact control of heteroatoms and their influence on the overall electronic structure and optical properties remains elusive.

Core-valence Bethe Salpeter using plane waves
The excitation of core electrons into the conduction band is an important probe to determine the local environment of atoms. To interpret experimental results, theoretical calculations are expedient, and many plane wave codes allow such calculations in the independent particle approximation (IPA). In hierarchical materials involving defects and interfaces, however, excitonic effects mediated by electron-hole attractions can be sizeable, requiring a treatment beyond the IPA. The theoretical framework then is the Bethe Salpeter equation (BSE), first considered in [1] and recently implemented in Wien2k [2].
Electron density of conduction band states for a core hole in boron-nitride corresponding to first (a) and second (b) peak in the boron K edge spectrum.
[1] J. Vinson, J. J. Rehr, J. J. Kas and E. L. Shirley, Phys. Rev. B 83, 115106 (2011), DOI: 10.1103/PhysRevB.83.115106.
[2] R. Laskowski, N. E. Christensen, P. Blaha and B. Palanivel, Phys. Rev. B 79, 165209 (2009), DOI: 10.1103/PhysRevB.79.165209.

Spectroscopic analysis of carbyne filled carbon nanotube hybrids
The attractive properties of 1D carbon nanomaterials are determined not only by their chemical composition, but they are also critically influenced by their size, shape and interaction with the environment. In this context, carbyne is predicted to be the world strongest material and we confirmed its materialization for the first time inside double walled CNTs (DWCNTs) in 2016. We still are leading this topic improving the carbyne material, accessing new properties and unravelling the energy gap of linear carbon chains and the CNT host to carbyne interaction as well as first results on extracting carbyne from the CNT host. In this project, we aim to tailor the structure and properties of confined carbyne@CNT hybrids by using metallicity sorted SWCNTs with a narrow diameter distributionas precursors. Using advancedfillingreactions withcarbonaceous functionalelements followed by nanochemical reactions we will produce functionalized DWCNT and Carbyne@CNT hybrids. The samples will be analysed regarding their local electronic structure, energygap and optical properties using resonance Raman and electron energy loss spectroscopy (EELS).

Research Team 3 - Macromolecular aggregates
M. Arndt, A. Bismarck, L. González, S. Kantorovich, C. Likos

Photophysics and magnetism in macromolecules
Bio/molecules in the gas phase are of interest, since they are isolated and solvent-free. Large neutral molecules have still been little explored but new methods in Vienna allow to transfer them into the gas phase, and even analyze them in quantum interference experiments. We will explore new methods to volatilize molecules to study their photophysics. Intense laser beams shall open the window to absorption and photo-isomerization on the single-molecule level in high vacuum and we will explore matter-wave-enhanced electric and magnetic deflectometry. Studies in a unique set of near-field molecule interferometers have already allowed us to utilize the quantum wave nature of thermal vitamins and massive organic molecules up to about 27000 amu. Matter-wave interference imprints a nanoscale density pattern onto the molecular beam and this molecular nanoruler can be used to measure forces down to 10-26 Newton. The absorption of even a single photon shifts the molecular fringe pattern measurably and it may induce electronic or structural changes in the molecule which lead to matter-wave fringe shifts in the presence of external electric or magnetic fields.

Simulation of light-induced dynamics in complex molecules
Mechanistic studies of light-induced processes in molecules with many degrees of freedom are still a challenge but an avoidable one in order to interpret and guide time-resolved spectroscopy. In this project we will use and develop methods to investigate the photophysics and photochemistry of complex (bio)molecules in gas phase to aid laser driven matter-wave interferometry experiments, as performed in the group of Arndt and complemented by the spectroscopic measurements.

Hydrophobically modified block-copolymer aggregates for turbulent drag reduction
Small amounts (≥ 100 wppm) of high molecular weight polymers reduce drag in turbulent pipe flows (up to 80%). Numerous models exist for polymeric drag reduction but none does fully explain all experimental observations. Turbulent shear acting on polymers causes mechanical degradation of the polymers reducing their molecular weight and thus the drag reducing capability. We will synthesise water soluble block-copolymers with hydrophobic blocks to self-assemble into aggregates in water and will tackle the following questions: Can we tailor the polymer architecture? What is the degradation behaviour of such polymers? Will these polymers recover after mechanical shear degradation? Can the association strength between the hydrophobic blocks be controlled? How does turbulent flow interact with these polymers? Will it be possible to control the morphology of polymer aggregates in solution? This will enable us to investigate the importance of polymer networks in turbulent drag reduction.

Self-assembly and dynamic response of supracolloidal magnetic polymers
The main advantage of magnetic colloidal particles and materials based on them, particularly polymer-colloid hybrid materials, is that their properties and response can be controlled by applied magnetic fields. The idea to build magnetically sensitive supracolloidal magnetic polymers (SMP) is very attractive, albeit challenging, as until today the fundamental understanding of the SMPs properties has not been achieved. Even less is known about the influence of the SMP topology or crosslinking method on their magnetic response. Thus, the goals of this project: (a) to determine equilibrium phase diagram and magnetic dynamic response of linear SMPs depending on the rigidity, varied from highly flexible, polymer-like backbones to almost rod-like structures; (b) investigate the behaviour of ring-like SMPs in order to elucidate the contribution of magnetic component via comparing the results to that obtained for ring polymers; (c) for X- and Y-shaped SMPs to investigate the possibilities of phase separation.

The role of ring-polymer topology in macromolecular self-assembly and dynamics
Ring-shaped macromolecules display a variety of characteristic properties that are diverse from those of their linear counterparts, which stem from their unique topology. Recent progress in the synthesis of ring polymers with various compositions and stiffness opens the way for experimental work of the aforementioned theoretical predictions. The project will focus on investigations of the glassy dynamics and the self-assembly in ring polymers with bending rigidity, covering contour-to persistence length ratios between 5 and 20. The project will provide a reliable coarse-graining strategy that takes faithfully into account the emergence of oblate and prolate-ring subpopulations, the latter playing an important role in the stabilization of the cluster phase. Further, we will investigate the effect of the presence of rigid colloidal additives, such as magnetic filaments or carbon nanotubes in the stability and properties of the cluster phase.