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Two-dimensional electron gas has various novel properties. This kind of electronic system like two-dimensional semiconducting transition metal dichalcogenides (TMDCs) and graphene are discovered with novel physical phenomena like valley Hall effect, Ising superconductivity and circular dichroism etc. In this thesis, the negative capacitance and inverse compressibility was studied on atomically thin molybdenum disulfide (MoS2) based capacitance devices. And another technique to detect the quantum capacitance so called capacitance bridge was developed in our experiment through studying the graphene capacitance under large perpendicular magnetic field in cryogenic system. Introduction was to illustrate the basic lattice structure and electronic structure for both of our experimental materials. Electronic structure includes the energy dispersion and the density of states. Meanwhile several typical experiments were given to prove the existence of strong electron-electron interaction and negative compressibility in two-dimensional electron systems. In the end of the introduction, a brief explanation about Hartree-Fock approximation was described and showed how dimension will affect the exchange energy in electron system then induce such kind of negative capacitance. Some basic experiment and characteristic methods were given in chapter 2 and 3. To conduct the experiments, high quality of BN-materials-BN structure samples were fabricated by the try transfer method. In chapter 4, the negative capacitance effect which is induced by strong e-e interaction is focused. Our data showed how frequency and chemical potential will affect this effect. Then the penetration field data was shown and I gave my own suggest on the odd data. The last chapter shows the basic principle of capacitance bridge measurement. Then the 4-fold degeneracy of LL=0 Landau level was observed both by precise LCR meter and our homemade capacitance bridge.

Two-dimensional electron gas has various novel properties. This kind of electronic system like two-dimensional semiconducting transition metal dichalcogenides (TMDCs) and graphene are discovered with novel physical phenomena like valley Hall effect, Ising superconductivity and circular dichroism etc. In this thesis, the negative capacitance and inverse compressibility was studied on atomically thin molybdenum disulfide (MoS2) based capacitance devices. And another technique to detect the quantum capacitance so called capacitance bridge was developed in our experiment through studying the graphene capacitance under large perpendicular magnetic field in cryogenic system. Introduction was to illustrate the basic lattice structure and electronic structure for both of our experimental materials. Electronic structure includes the energy dispersion and the density of states. Meanwhile several typical experiments were given to prove the existence of strong electron-electron interaction and negative compressibility in two-dimensional electron systems. In the end of the introduction, a brief explanation about Hartree-Fock approximation was described and showed how dimension will affect the exchange energy in electron system then induce such kind of negative capacitance. Some basic experiment and characteristic methods were given in chapter 2 and 3. To conduct the experiments, high quality of BN-materials-BN structure samples were fabricated by the try transfer method. In chapter 4, the negative capacitance effect which is induced by strong e-e interaction is focused. Our data showed how frequency and chemical potential will affect this effect. Then the penetration field data was shown and I gave my own suggest on the odd data. The last chapter shows the basic principle of capacitance bridge measurement. Then the 4-fold degeneracy of LL=0 Landau level was observed both by precise LCR meter and our homemade capacitance bridge.

Two-dimensional electron gas has various novel properties. This kind of electronic system like two-dimensional semiconducting transition metal dichalcogenides (TMDCs) and graphene are discovered with novel physical phenomena like valley Hall effect, Ising superconductivity and circular dichroism etc. In this thesis, the negative capacitance and inverse compressibility was studied on atomically thin molybdenum disulfide (MoS2) based capacitance devices. And another technique to detect the quantum capacitance so called capacitance bridge was developed in our experiment through studying the graphene capacitance under large perpendicular magnetic field in cryogenic system. Introduction was to illustrate the basic lattice structure and electronic structure for both of our experimental materials. Electronic structure includes the energy dispersion and the density of states. Meanwhile several typical experiments were given to prove the existence of strong electron-electron interaction and negative compressibility in two-dimensional electron systems. In the end of the introduction, a brief explanation about Hartree-Fock approximation was described and showed how dimension will affect the exchange energy in electron system then induce such kind of negative capacitance. Some basic experiment and characteristic methods were given in chapter 2 and 3. To conduct the experiments, high quality of BN-materials-BN structure samples were fabricated by the try transfer method. In chapter 4, the negative capacitance effect which is induced by strong e-e interaction is focused. Our data showed how frequency and chemical potential will affect this effect. Then the penetration field data was shown and I gave my own suggest on the odd data. The last chapter shows the basic principle of capacitance bridge measurement. Then the 4-fold degeneracy of LL=0 Landau level was observed both by precise LCR meter and our homemade capacitance bridge.

Driven by the increasing energy crisis and environmental pollution, the renewable energy is in an urgent need to replace the traditional fossil fuels. Serving as the energy transfer station, electrochemical energy storage devices, such as batteries and supercapacitors, are capable to convert the electricity produced by solar radiation, waves, and wind into electrochemical energy, and release such energy elsewhere. Therefore, these electrochemical energy storage devices can compensate the intermittency of renewable energy sources. Especially, supercapacitors possess the appealing features such as higher specific power, faster charging/discharging rate and much longer cyclic lifetime than rechargeable batteries, thus offering possibilities in a broad range of energy-related applications such as regenerative breaks in electric vehicles, memory back-up systems, etc. For the decades, extensive research have shown that nanostructured architectures have large surface-to-volume ratio, which could be the promising candidates to serve as the scaffold for high performance supercapacitors fabrication. In particular, three-dimensional (3-D) nanostructured pseudocapacitors have been targeted as the promising energy storage devices due to their large surface/interface area, efficient ion diffusion and electron transport pathway. As a result, pseudocapacitor electrodes fabricated based on 3-D nanowires (NWs), nanotubes (NTs), nanopillars (NPLs), etc., have been broadly explored. However, they are still far from the ideal architectures mainly because they are either fabricated into small aspect ratio 3-D electrodes, which possess low specific capacitance, or they have serve performance degradation due to the unstable structural property. Furthermore, the fabrication process of these 3-D architectures always involves costly lithographic and etching processes, which limited their practical applications and capability for mass production. In this thesis, several 3-D architectures are s

Driven by the increasing energy crisis and environmental pollution, the renewable energy is in an urgent need to replace the traditional fossil fuels. Serving as the energy transfer station, electrochemical energy storage devices, such as batteries and supercapacitors, are capable to convert the electricity produced by solar radiation, waves, and wind into electrochemical energy, and release such energy elsewhere. Therefore, these electrochemical energy storage devices can compensate the intermittency of renewable energy sources. Especially, supercapacitors possess the appealing features such as higher specific power, faster charging/discharging rate and much longer cyclic lifetime than rechargeable batteries, thus offering possibilities in a broad range of energy-related applications such as regenerative breaks in electric vehicles, memory back-up systems, etc. For the decades, extensive research have shown that nanostructured architectures have large surface-to-volume ratio, which could be the promising candidates to serve as the scaffold for high performance supercapacitors fabrication. In particular, three-dimensional (3-D) nanostructured pseudocapacitors have been targeted as the promising energy storage devices due to their large surface/interface area, efficient ion diffusion and electron transport pathway. As a result, pseudocapacitor electrodes fabricated based on 3-D nanowires (NWs), nanotubes (NTs), nanopillars (NPLs), etc., have been broadly explored. However, they are still far from the ideal architectures mainly because they are either fabricated into small aspect ratio 3-D electrodes, which possess low specific capacitance, or they have serve performance degradation due to the unstable structural property. Furthermore, the fabrication process of these 3-D architectures always involves costly lithographic and etching processes, which limited their practical applications and capability for mass production. In this thesis, several 3-D architectures are s

This letter proposes a new implementation of phase-disposition pulse-width modulation (PD-PWM) for multilevel flying capacitor (FC) converters using a single triangular carrier. The proposed implementation is much simpler than conventional PD-PWM techniques based on multiple trapezoidal-shaped carriers, generates the same results as far as natural capacitor voltage balance is concerned and offers better quality line-to-line voltages when compared to phase-shifted PWM. The proposed algorithm is based on reshaping the reference signal to fit within the range of a single carrier and assigning each crossing of the reference signal with the carrier to a particular pair of switches at any time. The proposed algorithm is suitable for digital implementation taking maximum benefit from the PWM units available in the processor. Simulation and experimental results are presented from the five-level FC converter to verify the proposed PD-PWM implementation., Peer Reviewed, Postprint (author’s final draft)

The discovery of graphene in 2004 by simple micromechanical exfoliation technique has opened up a new physical world of two-dimensional (2D) materials. Graphene is a true 2D material comprised of a single layer of carbon atoms in a honeycomb structure. Extraordinary physical phenomena, such as massless Dirac Fermions, half-integer quantum Hall effect, and linear density of states (DOS), are uncovered in graphene. Inspired by the discovery of graphene, other 2D layered materials are successfully revealed, including topological insulators like Bi2Se3 and Bi2Te3 and layered transition-metal dichalcogenides (TMDs) like MoS2, MoSe2, WS2, and WSe2. Among them, MoS2 is one of the most popular 2D materials for its unusual properties, such as superconductivity at large carrier density region, controllable valley polarization and metal-insulator transition (MIT). In this thesis, I mainly study the role of disorder in modifying electronic properties of graphene and MoS2 through capacitance measurements, an indispensable complementary technique to transport measurements. We have investigated the electronic properties of graphene containing charge impurities, Anderson disorder and resonant impurities, which show distinct different characteristics. Some interesting physical phenomena, such as electron-electron (e-e) interactions, fluctuation of local DOS, resonant states, and negative quantum capacitance/compressibility are observed in graphene. Our results prove that capacitance measurements are able to detect impurity states and their related properties in 2D structures, in which strong localization effects or other scattering mechanisms of electrons may involve and thus seriously affect the conventional transport measurement data from the sample. We also accessed MoS2 properties through capacitance measurements. For capacitance measurements of MoS2b

The discovery of graphene in 2004 by simple micromechanical exfoliation technique has opened up a new physical world of two-dimensional (2D) materials. Graphene is a true 2D material comprised of a single layer of carbon atoms in a honeycomb structure. Extraordinary physical phenomena, such as massless Dirac Fermions, half-integer quantum Hall effect, and linear density of states (DOS), are uncovered in graphene. Inspired by the discovery of graphene, other 2D layered materials are successfully revealed, including topological insulators like Bi2Se3 and Bi2Te3 and layered transition-metal dichalcogenides (TMDs) like MoS2, MoSe2, WS2, and WSe2. Among them, MoS2 is one of the most popular 2D materials for its unusual properties, such as superconductivity at large carrier density region, controllable valley polarization and metal-insulator transition (MIT). In this thesis, I mainly study the role of disorder in modifying electronic properties of graphene and MoS2 through capacitance measurements, an indispensable complementary technique to transport measurements. We have investigated the electronic properties of graphene containing charge impurities, Anderson disorder and resonant impurities, which show distinct different characteristics. Some interesting physical phenomena, such as electron-electron (e-e) interactions, fluctuation of local DOS, resonant states, and negative quantum capacitance/compressibility are observed in graphene. Our results prove that capacitance measurements are able to detect impurity states and their related properties in 2D structures, in which strong localization effects or other scattering mechanisms of electrons may involve and thus seriously affect the conventional transport measurement data from the sample. We also accessed MoS2 properties through capacitance measurements. For capacitance measurements of MoS2b

The discovery of graphene in 2004 by simple micromechanical exfoliation technique has opened up a new physical world of two-dimensional (2D) materials. Graphene is a true 2D material comprised of a single layer of carbon atoms in a honeycomb structure. Extraordinary physical phenomena, such as massless Dirac Fermions, half-integer quantum Hall effect, and linear density of states (DOS), are uncovered in graphene. Inspired by the discovery of graphene, other 2D layered materials are successfully revealed, including topological insulators like Bi2Se3 and Bi2Te3 and layered transition-metal dichalcogenides (TMDs) like MoS2, MoSe2, WS2, and WSe2. Among them, MoS2 is one of the most popular 2D materials for its unusual properties, such as superconductivity at large carrier density region, controllable valley polarization and metal-insulator transition (MIT). In this thesis, I mainly study the role of disorder in modifying electronic properties of graphene and MoS2 through capacitance measurements, an indispensable complementary technique to transport measurements. We have investigated the electronic properties of graphene containing charge impurities, Anderson disorder and resonant impurities, which show distinct different characteristics. Some interesting physical phenomena, such as electron-electron (e-e) interactions, fluctuation of local DOS, resonant states, and negative quantum capacitance/compressibility are observed in graphene. Our results prove that capacitance measurements are able to detect impurity states and their related properties in 2D structures, in which strong localization effects or other scattering mechanisms of electrons may involve and thus seriously affect the conventional transport measurement data from the sample. We also accessed MoS2 properties through capacitance measurements. For capacitance measurements of MoS2b

In grid-connected photovoltaic (PV) power stations, improving the life expectancy and long-term reliability of three-phase PV inverters is urgently needed to match the significantly higher lifetime of the PV modules. A key contribution toward such improvement is replacing the conventional electrolytic film capacitors by metallized polypropylene film ones. This paper presents a detailed evaluation of a conventional three-phase grid-connected PV inverter performance when replacing the electrolytic capacitor with a minimum value of metallized polypropylene film capacitor. Although the minimum dc bus capacitance leads to higher voltage ripples, such ripples were found to be within acceptable limits to operate the inverter satisfactorily. Simulation and experimental results are presented for a 5-kW grid-connected inverter prototype with a nominal dc voltage of 457 V to confirm the theoretical considerations., Peer Reviewed, Postprint (published version)

Two important technical challenges associated with the Modular Multilevel Converter (MMC) are the reduction of the voltage ripple of the Sub-Module (SM) capacitors and the reduction of the converter losses. This paper conducts a study focused on these two topics. Firstly, the effect of a circulating current with a predefined second harmonic on the SM voltage ripple is assessed. Secondly, an efficiency study for two different MMCs, one with silicon (Si) and the other with silicon carbide (SiC) devices is carried out. Results suggest that a SiC MMC converter with a circulating current including a second harmonic represent a good solution since it achieves a reduction of the SM capacitors together with a high efficiency operation., Peer Reviewed, Postprint (published version)

Úkolem této bakalářské práce, je provést detailní rozbor kapacitních ovládacích prvků k ovládání elektronických zařízení a realizovat kapacitní dotykové ovládání s mikrokontrolérem MSP430. Teoretická část práce se zabývá principem funkce kapacitního snímače, výhodami a nevýhodami kapacitní technologie a metodami zpracování na mikrokontroléru. Dále jsou v teoretické části popsány nejpoužívanější prvky kapacitního snímaní. Praktická část práce obsahuje ověření doporučených postupů při návrhu kapacitních tlačítek a posuvníků, použitý software pro vývoj a také samotný návrh zařízení., The task of this bachelor work is to perform a detailed analysis of capacitive controls to control electronic devices and construct capacitive touch control device with MSP430 microprocessor. The theoretical part deals with the principles of the function, advantages and disadvantages of the technology and processing methods on the microprocessor. The theoretical part also describes the most common elements of the capacitive sensing. Practical work includes recommanded practices in the design of capacitive buttons and sliders, used software and design and construct of the capacitive touch control., Katedra elektrotechniky, Bakalářská práce byla zpracována na požadované úrovni. Odpovědi na teoretické otázky odpovídají celkovému hodnocení - velmi dobře.

The flying capacitor (FC) multilevel converter has attracted a great deal of interest in the recent years because of its easier extension to a higher number of levels (n>;3), as compared to its counterpart, the diode-clamped converter (DCC). The main focus of this paper is to develop a voltage balancing scheme of FCs for a five-level FC converter based on phase disposition pulse-width modulation (PD-PWM). Since there are multiple states that produce the same output voltage at the leg of the converter, such a redundancy is used to regulate the FC voltages at their desired levels. The selection of the optimal states is performed by minimizing a cost function. A drawback observed when using standard symmetrical triangular carriers for the PD-PWM, is the additional switching events that are produced due to transitions within the same voltage level. Nevertheless, this fact can be avoided by using sawtooth carrier waveforms instead. Simulation results verify the robustness of the proposed voltage balancing scheme against static and dynamic load conditions. Moreover, using sawtooth carriers a significant reduction of the switching frequency is achieved as compared to the use of standard triangle carriers while maintaining the FC voltage balanced., Peer Reviewed, Postprint (author’s final draft)

The flying capacitor (FC) multilevel converter requires a capacitor voltage balancing mechanism for proper operation. This paper proposes a voltage balancing scheme for a five-level FC converter based on phase disposition pulse-width modulation (PD-PWM) using transitions that produce the minimum number of switching events. This strategy will be called optimal-transition voltage balancing (OTVB) scheme. Since multiple optimum switching transitions between two consecutive voltage levels may be available, such a redundancy is used to regulate the FC voltages at their desired levels. Those transitions that produce more switching events are avoided. The rest of transitions are evaluated by means of a cost function and the one that produces the minimum value is selected. Simulation results show a significant reduction of the average switching frequency as compared to the use of the optimal-state voltage balancing (OSVB) scheme, while maintaining the balance of the FC voltages. Moreover, the proposed PD-PWM voltage balancing strategy is robust to static and dynamic load conditions., Peer Reviewed, Postprint (author’s final draft)

In flying capacitor (FC) converters, phase-shifted pulse-width modulation (PS-PWM) provides natural voltage balancing. However, for a practical application, a more robust balancing mechanism of maintaining the FC voltages at the desired values is required. This paper proposes a new closed-loop voltage balancing method for multilevel FC converters using PS-PWM. The proposed method balances the voltages of the FCs by modifying the duty cycle of each switch of the FC converter using a proportional (P) controller. The crossed effect between FC currents and duty cycles is considered and is used for FC voltage balancing. The Simulation results verify that the proposed voltage balancing method is very robust to different operating conditions, such as load transients and non-linear loads., Postprint (author’s final draft)

The modular multilevel converter (MMC) is one of the most potential converter topologies for medium/high power/voltage applications. One of the main technical challenges of an MMC is to eliminate/minimize the circulating currents within the legs. Circulating currents, if not properly controlled, increase the amplitude of capacitor voltage variations, rating values of the converter components and converter losses. This paper proposes a closed-loop circulating current control strategy for an MMC to specifically minimize the amplitude of capacitor voltage variations. The proposed strategy is based on adding an offset signal to the modulating signal of each arm. To minimize the amplitude of the capacitor voltage oscillations, an optimal circulating current component is determined and used as a reference signal for the current control of each MMC leg. Performance of the proposed control strategy is evaluated based on simulation studies in the MATLAB/Simulink environment. The reported study results demonstrate effectiveness of the proposed strategy to reduce the amplitude of the capacitor voltage oscillations., Peer Reviewed, Postprint (published version)