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Institut für Festkörperphysik

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Charge mobility in organic semiconductors and covalent organic frameworks using the newly developed transient localization theory

Despite the fact that organic semiconductors (OSCs) have been known and extensively investigated for several decades, the mechanism of charge transport in these materials is still hotly debated. Nevertheless, it is increasingly accepted that the charge carrier dynamics in organic semiconductors is strongly determined by a coupling to low frequency inter-molecular phonons. Due to the similar energy scales of the phononic relaxation of charge carriers and the inter-molecular electronic coupling strengths, the charge carrier dynamics in OSCs is hard to model properly. Recently, a new approach – the transient localization model - has been introduced and successfully applied to various important OSCs. An advantage of this approach is that by avoiding an explicit consideration of quantum dynamics, it becomes tractable also for rather complex systems. The goal of this Master thesis is to first implement this model and to benchmark it against reported results for organic semiconductors. Subsequently, it shall for the first time be applied to covalent organic frameworks, a highly promising class or materials that is related to organic semiconductors, but consists of a fully covalently linked, porous network. From these studies unprecedented insight into the charge carrier dynamics in these advanced materials can be expected. Notably, the input parameters needed for the transient localization model (the electronic structure of the OSCs and COFs as well as the properties of phonons in these materials) are commonly calculated in our group and can be provided by other group members.

We are looking for: Highly motivated, self-propelled students with an interest in solid state physics, quantum mechanics, and computational material science. Basic programming skills are required (Matlab, Python).

Compensation: € 440,-- Forschungsbeihilfe per month for at least 6 months

Contact: Egbert Zojer (egbert.zojer@tugraz.at; Tel.: 873-8475),
Christian Winkler (christian.winkler@student.tugraz.at)

Understanding the physical origins of the electrical conductivity in Covalent Organic Frameworks

2 MASTER THESES Conductive organic materials have interesting electronic and optoelectronic properties, high mechanical flexibility and can be designed/functionalized quite easily. Thus, they are highly relevant for a huge variety of applications such as transistors, optoelectronic devices, and solar cells. Efficient charge-carrier transport is essential in all these applications.
A related materials class conductive covalent organic frameworks COFs. Their transport properties are, however, much less understood. This makes simulating their electronic structure particularly interesting from a fundamental point of view.
Notably, up to now only very few COFs with high charge carrier mobility exist and understanding of how one could systematically change the charge carrier mobility in these materials is, thus, of paramount importance. The goal of the present project is to start from existing conductive COFs, to calculate their electronic properties by using state of the art DFT models in combination with tight-binding approaches and from that to gaining a fundamental understanding of the relevant transport mechanisms and how they are impacted by the MOF structure. Based on these findings strategies for engineering the structure of the materials to exhibit even higher charge carrier mobilities will be developed.
Both theses will primarily focus on COFs consisting of covalently linked 2D layers, that are held together mostly by van der Waals forces.
The first thesis will focus on understanding, which interactions (exchange, electrostatic, van der Waals …) determine the type of inter-layer stacking, how this impacts charge transport, and how changing the “ideal stacking” can be realized by modifying the structure of the sheets.
The second thesis will systematically study the impact of the intra-layer linking units on intra-layer charge transport as well as the role played by the specific MOF topology.

We are looking for: Highly motivated, self-propelled students with an interest in solid state physics and computational material science. Basic programming skills are required (Matlab, Python).

Compensation: € 440,-- Forschungsbeihilfe per month for at least 6 months

Egbert Zojer (egbert.zojer@tugraz.at; Tel.: 873-8475)
Christian Winkler (christian.winkler@student.tugraz.at)

Search for new polymorphs by organic epitaxy

The aim of the study is to study the crystallographic property of molecular crystals at single crystalline surfaces. Organic semiconducting molecules will be crystallized at surfaces and the crystallographic properties will be investigated by grazing incidence X-ray diffraction. The position of Bragg peaks will be used for indexing to identify the crystallographic phase. The final goal of the study is to solve the crystal structure of epitaxially grown crystallites.A part of the experiments will be performed at beamline XRD1, synchrotron Elettra, Trieste.

COMPENSATION: 440€ per month / 6 months

CONTACT: Roland Resel (roland.resel@tugraz.at)

Autonomous Driving for Nanocars

Machine Learning, Exp.-Theory Collaboration, STM, Surface Science

Description: In 2016, the Nobel prize has been awarded for “molecular machines”, tiny molecules which perform various tasks on surface. One interesting application are so-called “nanocars”, for which regular races are held where these nanomachines need to go over the surface to a specific location.

The major challenges for molecular machines in general, and nanocars in particular remains their handling on the surface. Often, this is done by positioning an STM tip close the molecule and then applying a specific electric impulse. However, since the physics are not yet fully understood, the process is mostly based on trial-and-error. Moreover, since there is little human intuition about the interactions of tips, molecules, and surfaces, the handling is often quite inefficient.

The task of this master thesis is to develop and apply a machine learning algorithm (based on supervised learning) that optimizes the manipulation of a nanocar and drives it autonomously across a racetrack. The work will be done in close collaboration with the experimental group of Leonhard Grill at the KFU. Can your code outperform their human driver?

We seek: Highly motivated, self-propelled students with an interest in solid state physics and computational material science. Basic knowledge of Matlab and/or Python is recommended.

Compensation: € 440,-- Forschungsbeihilfe for 6 months.

Contact: Oliver Hofmann o.hofmann@tugraz.at
Or talk to the students in office PH 02 152 (2nd floor, right by the stairs)

Physical Interpretation of Machine Learning Results      >> mehr >>

The advent of Machine Learning methods has unlocked great potential in computational studies. In particular the exploration of surface structures, that was previously thought to be completely unfeasible, has surged in the recent years. At the same time, machine learning studies are often criticized for their lack of physical insight.

In this master project, we will investigate Bisphenol A aggregates on Ag(111). This system shows an interesting peculiarity: The molecules adsorb in two different ways. The rotational barriers between those two differ, such that one kind of molecules is immobile at room temperature, while the other remains mobile and cannot be sharply imaged in STM studies. The target of this thesis is to investigate the interface using an in-house developed machine learning algorithm and study the relative contribution of covalent, ionic, and van-der-Waals contributions, in order to provide an explanation for the difference between the two adsorption sites.

We seek: Highly motivated, self-propelled students with an interest in solid state physics and computational material science. Basic knowledge of Matlab and/or Python is recommended.

Compensation: € 440,-- Forschungsbeihilfe for 6 months.

Oliver Hofmann email: o.hofmann@tugraz.at
Tel: 0316 873 8964 http://www.if.tugraz.at/hofmann
Or talk to the students in office PH 02 152 (2nd floor, right by the stairs)

Modelling thermal transport in organic semiconductors      >> mehr >>

Goal: Development of atomistically motivated structure-to-property relationships for heat transport in organic semiconductors – a property, that is crucial for device operation, but is still largely unexplored such that the suggested studies can have a huge impact.

Details: The ability of a material to transport heat is of considerable importance even in cases where its main application is not thermal- related. Some examples where this property plays a central role include thermoelectricity, thermal barrier coatings, phase-change memory, heat-assisted magnetic recording, and extends to the general problem of thermal management of a wide variety of de- vices.

The main objective of the master thesis is to develop structure-to-property relation- ships for thermal transport in organic semiconductors, a cutting-edge research direction with potential applications in fields as varied as microelectronics, optoelectronics, catalysis, and porous materials.

The studies will be addressed by combining molecular dynamics and electronic structure calculations. The applicants should be interested in solid state physics, should have strong motivation for computer simulations, and should be willing to develop codes and scripts.

At the end of the master thesis, the students will have expertise in computa- tional methods for modelling thermal transport, and will have practical knowledge in electronic structure calculations and molecular dynamics. These methodologies constitute a powerful tool to study the electronic, structural and thermodynamic properties of materials.

Aside of the academic profits, we offer a very friendly work environment.

Starting date: any time
Compensation: 440 € per month for 6-8 months
Natalia Bedoya Martinez (bedoyamartinez@tugraz.at, Tel.:873-8465)
Egbert Zojer (egbert.zojer@tugraz.at; Tel.: 873-8475)

      >> mehr >>

The molecule HATCN is a strong electron acceptor that is commercially used in OLEDs to modify the property of metal substrates. Adsorbed on silver, this molecule shows unusual, fascinating physics. At low coverage, the molecule forms honeycomb patterns, which can be exploited as epitaxial growth template. When the coverage is increased, however, the first monolayer rearranges. This drastically changes the material properties, in particular the system’s work function.

At present only very little is known about the geometric and electronic structure of the rearranged, high- coverage phase. This is now at the focus of a joint efforts including the groups of Prof. Resel (TU Graz), and Prof. Fritz (University Jena), which will perform x-ray and low energy electron diffraction experiments and characterize the system via optical spectroscopy. The aim of this thesis is to provide complimentary computational insight to these experiments. Density-functional theory calculations will be performed in order to predict possible geometric structures, characterize the optical and vibrational properties, and understand the driving force that leads to the observed phase transition.

Oliver Hofmann
Email: o.hofmann@tugraz.at
Tel: 0316 873 8465

Deposition and Characterization of Dielectric Bragg Reflectors       >> mehr >>

Dielectric Bragg Reflectors (DBR) are commercially manufactured onto rigid substrates, using inorganic materials (e.g. SiO2, TiO2). Recently, organic DBR are under investigation because they allow creation of tunable optical properties. During the wet fabrication processes, the choice of polymers that can be alternated is limited by the condition that the solvents of the alternating materials need to be orthogonal. Our goal is to deposit the organic DBR completely from the vapor phase by an innovative technique called initiated Chemical Vapor Deposition (iCVD). iCVD does not require the use of solvent and allows precise control over layer thickness by coupling with laser interferometry. The polymers that will be alternated will have a large refractive index contrast, as for example Teflon (n≈1.38) and polystyrene (n≈1.59), so to achieve high reflectivity with a limited number of layers. The surface roughness, interfacial roughness, the thickness and the electron densities of each layer will be characterized by X-ray reflectivity (XRR). The results will be compared with ellipsometry and microscopy data.

Compensation: 6 month / 440€ per month

Dr. Anna Maria Coclite, Assistant Professor
Dr. Roland Resel, Associate Professor