Master and bachelor theses
1.Raman scattering on anatase TiO2
Description: As a versatile functional material, titanium dioxide has been a subject of extensive studies since the discovery of the photocatalytic splitting of water at a TiO2 electrode under ultraviolet light exposure. Three forms of TiO2 rutile, anatase, and brookite occur in the nature with the former being a stable one, whereas the other two are metastable. Depending on the experimental conditions, the metastable polymorphs transform to the equilibrium rutile phase at temperatures 550 to 1000 oC. Of all three phases, anatase TiO2 is the material of choise, since it reveals much better efficiency in energy-related application and optoelectronics.
Hydrogen is a common impurity in oxides with a strong impact on electrical and optical properties of these materials. In large quantities, incorporation of hydrogen in anatase TiO2 can lead to a significant band gap reduction and, hence enhancement of photocatalytic activity upon illumination with the visible light. In spite of significant progress in the recent years still very little is known about the properties of hydrogen in anatase TiO2.
Tasks: The student will investigate hydrogen-related defects in anatase TiO2 by means of Raman scattering spectroscopy.
Skills needed: Basic knowledge of physics of semiconductors, optics, and quantum mechanics, inclination to experimental work.
Gained experiences: Raman scattering measurements, vibrational mode spectroscopy of light impurities in solids. Optical characterization of solids. Contact: PD Dr. E. Lavrov
2. Oxygen in antimony triselenide (Sb2Se3)
Description: Antimony triselenide (Sb2Se3) is an emerging chalcogenide semiconductor material considered as a promising photovoltaic absorber. It possesses an unique orthorhombic quasi-1-dimensional crystal structure in which covalently bonded [Sb4Se6]n `nano-ribbons' are linked by weak van der Waals interaction.
Oxygen plays an important role as key contaminant in many solids. Its decisive impact on the performance of various semiconductor devices has attracted considerable research interest over the decades leading to identification of fundamental microscopic configurations of oxygen defects in the host lattice and revealing their electrical activity.
Research related to the role of oxygen in antimony triselenide started quite early, after it has been recognized that oxygen doping has a strong influence - a combination of both beneficial and detrimental aspects - on the performance of Sb2Se3 thin film solar cells. Yet, the nature, microscopic configuration, and electrical activity of basically all oxygen-related complexes in this material remain experimentally undiscovered.
Tasks: The student will investigate oxygen-related defects in Sb2Se3 preferentially by means of IR absorption spectroscopy and photoconductivity.
Skills needed: Basic knowledge of physics of semiconductors and quantum mechanics, inclination to experimental work.
Gained experience: IR absorption and photoconductivity measurements, vibrational mode spectroscopy of light impurities in solids. Optical characterization of solids. Contact: PD Dr. E. Lavrov
3. Raman scattering on tin (di)oxyde
Description: Tin dioxide (SnO2) is an important semiconductor widely employed in transparent conductor electronics, solar cells, gas sensors, touchscreens, catalysis, and spintronics. SnO2 reveals only n-type conductivity what seriously limits the amount of possible application. Tin oxide (SnO), on the other hand, only recently received a great deal of attention since the discovery of ambipolar doping.
Hydrogen in all metal oxides including SnO2 and SnO is ubiquitous and very difficult to remove from the crystal growth environment and post-growth treatment. Controlling the conductivity of tin (di)oxide thus requires careful control of hydrogen exposure during and after growth.
In spite of significant progress in the recent years, still very little is known about the properties of hydrogen in tin (di)oxide. In particular the nature of the so-called "hidden" hydrogen remains unveiled.
Tasks: The student will investigate hydrogen-related defects in SnO2 and SnO by means of Raman scattering spectroscopy.
Skills needed: Basic knowledge of physics of semiconductors, optics, and quantum mechanics, inclination to experimental work.
Gained experiences: Raman scattering measurements, vibrational mode spectroscopy of light impurities in solids. Optical characterization of solids. Contact: PD Dr. E. Lavrov
4. Photodissociation of H2 in Si
Description: Photodissociation is a process of fragmentation of a molecule through absorption of one or more photons. It is at heart of photochemistry and plays a fundamental role in interstellar clouds, planetary atmospheres, and plasma physics.
In spite of its 'simplicity', the hydrogen molecule has fundamental importance in quantum mechanics, molecular physics, and chemistry. Because the closest dipole allowed electronic transition lies 10 eV above the ground state of H2, all optical excitations leading to photodissociation of hydrogen require either a single UV photon or multiple photons of less energies. The latter mechanism demands significant power densities ranging from 1011 to 1015 W/cm2.
Recently, it was shown that, contrary to free molecule, H2 embedded in silicon dissociates under photoexcitation with green light of power densities below 106 W/cm2, i.e. at least five orders of magnitude below the typical values. The physical mechanisms of photodisscociation, however, remain unclear.
Tasks: The student will investigate photodissociation of H2, D2, and HD molecules embedded in silicon by means of Raman scattering spectroscopy at different temperatures and photoexcitation conditions.
Skills needed: Basic knowledge of physics of semiconductors, optics, and quantum mechanics, inclination to experimental work.
Gained experiences: Raman scattering measurements, vibrational mode spectroscopy of light impurities in solids. Optical characterization of solids. Contact: PD Dr. E. Lavrov