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Master Thesis

  • Construction of an UHV thermal desorption spectroscopy setupContact: Frank Volmer

  • Time-resolved magneto-optical investigation of spin dynamics in novel 2D materialsContact: Bernd Beschoten

  • Investigating to role of edges in ultra-high mobility graphene devices Contact: Christoph Stampfer

    The absolute conductance values in transport measurements only provide limited insight into the underlying physics. One can hope to extract meaningful, quantitative results from such measurements through the use of transport models that incorporate disorder, contact resistance, and other relevant scattering mechanisms (e.g. phonons), though this is often quite difficult in practice and results are highly dependent upon the simplifying approximations that are often made in these models. However, one can directly obtain a clear, detailed picture of electronic band structure through high-resolution measurements of the capacitance. In these measurements, the so-called “quantum capacitance" is extracted, which is directly proportional to the system's density of states.

    Recently, a very sensitive capacitive read-out was proposed [1] that, in principle, can achieve an astonishing resolution of ~ 10 aF. However, this read-out circuit uses a HEMT (High Electron Mobility Transistor). Just like the graphene sample that we measure, the HEMT develops Hall plateaus at finite magnetic field. If this happens, our capacitive read-out system might not work properly anymore.

    The goal of this project is to develop and test the capacitive read-out system at high magnetic fields. We will start out with a HEMT and develop our read-out circuit based on [1] (see figure). We will use the developed system to study a capacitively coupled graphene sheet. Although the sheet is only micrometers in size, it should behave as a normal coaxial cable of macroscopic size (~ 1m).

    [1] A. Hazeghi, J.A. Sulpizio, G. Diankov, D. Goldhaber-Gordon, and H.S.P. Wong,
    Review of Scientific Instruments 82, 053904 (2011)

  • Gate defined quantum dot in two-dimensional transition metal dichalcogenide semiconductors Details, Contact: Christoph Stampfer

    The discovery of graphene in 2004 has created a whole new field of research, which led to the investigation of other two dimensional materials such as transition metal dichalcogenides (TMDCs). A monolayer of these materials consists of a layer of transition metal atoms (Mo, W, etc.) sandwiched between chalcogen atoms (S, Se, or Te.). In contrast to graphene, TMDCs have a bandgap and hence open the possibility of using standard semiconductor fabrication techniques to define an atomically thin quantum dot purely by electrostatics. The advantage compared to traditional semiconductor materials is the atomically thin geometry and dangling-bond-free interfaces which makes it easy to combine TMDCs with various substrates. The relatively strong intrinsic spin-orbit splitting in TMDC materials offer the possibility of using these quantum dots as spin-valley qubits.

    In this research project we will fabricate TMDC / hexagonal boron nitride heterostructures by exploiting a dry transfer technique to build devices with extremely high quality. After defining the quantum dot by depositing electrostatic gates and source-drain contacts, we will perform transport measurements at cryogenic temperatures and explore the quantum nature of the device. For successfully accomplishing the proposed study we will work as well in the clean room to fabricate the device as at a dilution refrigerators to perform the transport measurements.

    Suited for students from the Physics, Electrical Engineering and Material Science departments.

  • Transport properties of graphene subject to long-range strain fluctuations Contact: Christoph Stampfer

    The task is to both analytically and numerically get a detailed understanding of the electronic transport properties of graphene which is subject to long-range stain fluctuations. In the course of the project, the student will be able to understand and employ a large variety of methods in theoretical condensed matter physics. The project will be conducted in very close collaboration with the theory group of Fabian Hassler who at the moment investigates this system.

    This thesis will be supervised by both Prof. Fabian Hassler and Prof. Christoph Stampfer.

  • Suspened graphene quantum dots Details, Contact: Alexander Epping

    Since its discovery in 2004 which was awarded the Nobel Prize in 2010, graphene has gained increasing interest in the scientific community. Graphene exhibits unique electronic and mechanical properties making it a promising material for future nano-electronic applications. Because of the absence of the hyperfine field in 12C representing 99% of natural carbon, graphene quantum devices will potentially allow the realization of spin-qubits with long coherence times. Important for further progress towards this goal a better understanding of the influence phononic states have on the excited state spectrum of graphene quantum dots. Suspending the quantum dot creates an additional mechanical degree of freedom which can be exploited by local gate structures positioned underneath the device.

    In this research project we will fabricate heterostructures of graphene and pre-structured hexagonal boron nitride exploiting a dry transfer technique to precisely align the structure with a pre-defined local gate. After fabricating the quantum dot by electron beam lithography and depositing contacts, we will perform transport measurements at cryogenic temperatures and explore the quantum nature of the device. For successfully accomplishing the proposed study we will work as well in the clean room to fabricate the device as at a dilution refrigerators to perform the transport measurements.

  • Monolayer-WSe2 based heterostructures for light emitting diodesDetails, Contact: Beata Kardynal

  • Fabrication of graphene/TMDC- heterostructures for optospintronics Assigned to: M. Heithoff

    In this project, heterostructures of graphene and monolayers of transition metal dichalcogenides (TMDC) shall be used to optically create a spin polarization in the TMDC
    which is subsequently injected into the subjacent graphene by a DC current. The resulting spin current shall be measured by magneto-resistive electrical read-out using ferromagnetic electrodes which are placed on graphene in nonlocal geometry. The heterostructures will be fabricated by a dry transfer method (see Science Advances 1, e1500222 (2015)). The measurements will be performed in an optical cryostat which is equipped with an electromagnet. The student will be trained in the fabrication process of van der Waals heterostructures in our clean room and will perform opto-electronic device characterization together with Master and PhD students.

  • 2D mechanical resonators at the quantum limitDetails, Assigned to: Xin Ge

  • Transport in graphene nano devicesAssigned to: Wakana Okita

  • CVD growth of single and bilayer grapheneAssigned to: Christian Nick

  • Integration of Capacitance Bridge in MEMS experiments for accurate displacement and force determinationDetails, Assigned to: Tianyu Han

  • Charge trasnport in CVD grown bilayer grapheneAssigned to: Michael Schmitz

  • Optimizing large-scale CVD growth and dry dilaminatonAssigned to: Andrea Ceruti

  • Ballistic transport in graphene nanodevicesAssigned to: Zachary Winter

Bachelor Thesis

  • Fabrication of nanomechanical resonators with graphene based membranes Assigned to: Philipp Schmidt

    Nanomechanical resonators based on 2d materials show high mechanical quality factors and extraordinary force sensitivities which can be attributed to the high crystallinity and low weight of the membranes. These properties in combination with tunable resonance frequencies make them promising candidates for high-precision sensors of mass and force and provide an opportunity to study physical phenomena in the quantum regime.
    The goal of the project is to fabricate and transfer two-dimensional membranes suitable for high quality nanomechanical resonators. Tools like AFM and Raman spectroscopy are used to evaluate and where necessary improve the processing steps. Strain in the membranes can be studied and used to estimate and enhance the devices perfomance at cryogenic temperatures.

  • Time-resolved magneto-optical investigation of spin dynamics in novel 2D materialsAssigned to: Jakob Schibbert

  • Spin transport in heterostructures of 2d materialsAssigned to: A. Willmes

  • Investigating the electromechanical coupling in 2D materials Assigned to: I. Hirscher

    After its discovery in 2004, graphene and other related 2D materials sparked an enormous interest in multidisciplinary research all around the world due to their unique electronical and mechanical properties. One of these is, for example, the very high charge carrier mobility in graphene at room temperature. Nevertheless, due to its atomic thickness this 2D material is extremely sensitive to the external perturbation which influences the transport properties. Recently, it has been shown that the mobility and thus the electronic performance of graphene are limited by externally induced strain variations. This highlights the importance in understanding the electromechanical coupling of 2D-materials under strain.

    The goal of this project is to combine Raman spectroscopy and electrical transport experiments to characterize the electromechanical coupling in 2D materials, which are coupled to silicon-based electrostatic micro-actuators (comb-drives). These actuators allow to apply significant tensile strain in these materials and thus change the electromechanical coupling. The student is expected to perform the transport and Raman measurements as well as part of the device fabrication.

PhD Thesis / Postdoc

  • We are always interested in excellent candidates who want to pursue a PhD or postdoc in our group. The earlier we know about an applicant the better we can prepare the project. For details please contact Prof. Christoph Stampfer. Contact: Christoph Stampfer

Student assistant ("Hiwi")

Currently we do not have any Student assistant ("Hiwi") openings.