Defense PhD Thesis

On August 22, 2024, the Institute of Electrical Engineering, Slovak Academy of Sciences , will host the defenses of the following dissertation theses:

 

Mgr. Jana Hrdá

IEE SAS, August 22, 2024,  9,00 a.m.

Title: Study of the doping effect on the properties of ultra-thin layers of 2D materials
Abstract: Two-dimensional transition metal dichalcogenides belong to a widely studied group of materials mainly due to their extraordinary properties. In this dissertation thesis, we theoretically describe their atomic and electronic structure, possible fabrication methods, and applications. We focused on the doping of thin films, as it might positively influence their intrinsic properties. In the first part of this thesis, we investigated the lithium doping of molybdenum disulfide (MoS₂) and its influence on structural, optical, and electrical properties. As the doping method, we used a novel approach based on sulfurization using lithium sulfide (Li₂S) powder as the source of lithium. As the initial layers, we used Mo and already prepared MoS₂. The undoped and Li-doped layers were investigated by Raman spectroscopy, X-ray diffraction, optical measurements, synchrotron-based X-ray photoelectron spectroscopy (XPS), and resistance vs. temperature measurements. The XPS measurements showed that Li is intercalated, and it is present in all doped MoS₂ samples. Lithium promoted epitaxial growth and horizontal alignment of few-layers MoS₂. We observed a reorientation from vertical to horizontal alignment of vertically aligned MoS₂ films upon lithiation. The electrical R(T) measurements revealed that all doped and undoped samples exhibited the insulating dependence ln(R/R_ES) = (T_ES/T)^1/2, which is typical for the Efros–Shklovskii variable range hopping in a disordered semiconductor. The Li intercalation induced additional disorder enhancing this regime. In addition to lithium doping, we also focused on the second 2D material – platinum diselenide (PtSe₂). For PtSe₂ we studied the influence of selenium content on charge carrier mobilities. We found out that charge carrier mobility increases while Se:Pt ratio increases. Our findings may be helpful in further scientific research focused on optimizing the properties of TMD layers.

 

 

Ing. Martin Kucharovič

IEE SAS, August 22, 2024,  10,30 a.m.

Title: Optimizing the shielding capabilities of the magnetic cloak
Abstract: This dissertation addresses the issue of trapped geomagnetic fields in a magnetic cloak using CC HTS tapes. During the cooling of the cloak containing such a superconducting layer in the Earth’s magnetic field, this field remains trapped in the cloak. Therefore, after cooling, the cloak shields magnetic fields, but the magnetic field within its volume is not zero. The study investigates the dynamic magnetoresistance effect to reduce the residual magnetic field by applying an axial alternating magnetic field, which involves alternating periods of demagnetization and relaxation. For this purpose, an apparatus was developed to achieve such a demagnetization effect. The results of the demagnetization measurements showed that our method can reduce the trapped magnetic field. However, during these experiments, a problem was identified where the results were not consistently reproducible after a certain point. Therefore, in parallel with the demagnetization measurements, susceptibility was measured since reference data on the cloaks were available, allowing for comparison to determine if their properties had changed in any way. It was found that the cloaks’ shielding effectiveness had changed, prompting an investigation into the cause of this phenomenon. As part of the degradation investigation, transport measurements, magnetic mapping, and SEM and EDX analyses were carried out on the tapes themselves after disassembling the cloak. The study identifies potential factors that could have caused or contributed to the degradation of the HTS tapes. The most likely scenario is that damage to the silver coating layer of the CC tape initiated the decomposition process of the REBCO layer into a non-superconducting phase. External forces that damaged the silver layer could have also damaged the REBCO layer itself, as indicated by the mapping measurements. Therefore, the reduction in tape performance appears to be a combination of several factors. The research shows that it is possible to remove the magnetic field from the volume of magnetic cloaks using dynamic magnetoresistance. However, it also underscores the need for a better understanding of degradation processes to increase the long-term stability and effectiveness of HTS materials in magnetic shielding applications.

 

Ing. Andrej Novák

IEE SAS, August 22, 2024,  1,00 p.m.

Title: Enhanced radiation-hard semiconductor sensors for Timepix and Timepix3 pixel detectors
Abstract: Thesis devotes to the development and characterisation of new semiconductor sensors for hybrid pixel detectors Timepix which are extensively used in applications of radiation detection and spectral analysis as well as in imaging applications utilizing X-rays, neutrons and high-energy charged particles. The work focuses on sensors made from radiation-hard materials GaAs (gallium arsenide) and 4H-SiC (hexagonal silicon carbide). These sensors have been implemented for Timepix readout electronics. Such hybrid pixel detectors can be used to analyse different types of radiation under laboratory conditions and also in particle accelerators producing high-intensity fields of high-energy electrons, protons and neutrons. The experimental data were processed at the level of individual detected events. Results are compared with existing available Timepix detectors with Si (silicon) and CdTe (cadmium telluride) sensors.