Publication Search Results

Now showing 1 - 9 of 9
  • (2021) Yao, Yuchen
    Lead-based perovskites have been one of the hottest research topics due to its excellent optoelectronic performance, such as outstanding absorption coefficients, tuneable bandgap≈1.6 eV, long carrier diffusion length, high defect tolerance, convenient solution processability, and good charge carrier mobility. It has been widely studied to be applied on solar cells, photodetector, and light emitting diode. However, there are still some challenges on widely applying lead halide perovskites: nature toxicity of lead, unreliability in moisture and light environment, poor stability to heat and oxygen, and the existed self-degradation pathway. To overcome these problems, we fabricated lead-free perovskites Cs2CuBr4 single crystals and characterized the perovskites from structure, optical properties, and electrical device performance. The solution-evaporated method was introduced to grow Cs2CuBr4 single crystals. The prepared perovskites exhibit 2D orthorhombic structure and blue-green luminescence with the PL peak at 460nm. The bandgap is ~1.74 eV, which is relatively ideal for optoelectronic devices. Moreover, the resistive switching devices based on Cs2CuBr4 single crystals thin films were also fabricated, showing the excellent reversible threshold switching behaviour and robustness of 2D Cs2CuBr4 material. We believe this thesis significantly demonstrated that lead-free Cs2CuBr4 perovskites have potential to play an important role in the next-generation optoelectronic devices.

  • (2021) Yang, Ni
    Titanium oxide (TiO2) is one of the most widely studied dioxides as its specific surface properties, which makes it an ideal candidate for pollutant reducing and water splitting. TiO2 thin film has gained an increasing concern for transparent electrodes, photovoltaic application and resistive switching memory devices. Research for the highly reduced TiO2 thin film for transparent electrodes has been conducted in some research groups; however, the cost and technology present a challenge to the widespread use of TiO2 transparent electrodes. As an n-type semiconductor, TiO2 has been recognized as an ideal switching interlayer in resistive switching memory. The new challenge derived from Moore’s crisis and von Neuman architecture present obstacles to the further improvement of computer performance. Memristor, as its in-memory computing, can be applied in the next generation computer to reduce the cost and increase operational efficiency. Furthermore, till now, fabrication freestanding TiO2 with a near 100% stable (001) anatase surface is still a challenge. In this research thesis, I firstly reported a convenient way to produce highly conductive TiO2 thin film that can be used to replace ITO and FTO for the transparent electrode application. Subsequently, a TiO2/Nb: STO memristor was fabricated to realize the high-density data storage, arithmetic logical operation and neuromorphic computing, and then a state-of-the-art method was introduced to fabricate freestanding anatase TiO2 thin film with near 100% (001) surface.

  • (2023) Hu, Guangyu
    With the evolution of ever-changing intelligent electronics and the increasingly severe electricity shortages, substantial efforts have been made to explore new technologies for powering electronics. Moist-electric generation (MEG) devices, which can extract chemical energy in moisture to generate electricity, have attracted intensive interest. However, the electric outputs of the most reported MEG devices are still low. Herein, we present a novel strategy of coupling graphene oxide (GO) based MEG device with the electrochemical cell (i.e., GO/galvanic MEG device) to boost power outputs. HCl and HNO3 acids are employed to enhance the power outputs of the hybrid MEG device through unique acidification treatments. The GO/galvanic MEG device is fabricated through a simple solvent evaporation method. Polyethylene terephthalate (PET) plastic film, multi walled carbon nanotube (MWCNT), GO, and metal sheets are all the components of the device, which reflects the low-cost advantage. The power outputs of the GO/galvanic MEG device are collected using a Keysight SourceMeter. Scanning electron microscope (SEM), x-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS) and x-ray photoelectron spectroscopy (XPS) are utilized to characterize the device. After optimizing the fabrication parameters and using the unique acidification treatments, the hybrid MEG device generated exceptional power outputs based on the synergetic mechanisms of proton diffusion and galvanic oxidation. A single hybrid GO/galvanic MEG device stably generates a maximum voltage output of 1.69 V and a highest current density of 182 μA∙cm-2 under 80% RH at room temperature. Notably, the voltage output in this study is the apex among the reported GO-based MEG devices, while the current density output is top-ranked. Impressively, in room humidity, the single GO/galvanic MEG device directly powers a CASIO calculator, or a pressure sensor, or a LED light. Additionally, the simple integration of several hybrid MEG units with a capacitor easily and efficiently drives the water electrolysis and a commercial GPS tracker. This study demonstrates the vast potential of the GO/galvanic MEG device for driving practical electronics by harvesting energy from ambient moisture.

  • (2023) Chen, Junhong
    Transparent conductive films (TCFs) and electrodes based on indium tin oxide (ITO) dominate the majority of the world electronics market in the past few decades. Although the manufacture techniques of ITO are mature and relatively low cost when comparing to other TCF materials, the inherent brittleness and the utilization of scarce element (indium) imply that ITO is not suitable for next-generation device applications, which require low cost, mechanical flexibility, reliability and stability. Metal nanowires such as silver nanowires have drawn significant interest among researchers as they could achieve excellent electrical, optical and mechanical properties when coated or printed onto a variety of substrates. Many silver nanowire synthesis methods and related applications have been explored in the recent years, but some parameters and underlying mechanisms are still unclear, which requires further study and improvement. In this thesis, polymer-free synthesis of silver nanowires has been studied and explored. Nanowires with aspect ratio around 900 were synthesized by a polymer-free method. The morphology of nanowires was investigated by tuning the concentration of exotic additives, temperature and reaction time. In addition, silver nanowire-based TCFs with high transparency (84.1%) and good electrical conductivity (44.2 ohms per square) were fabricated in this project. Zinc oxide was also uniformly coated onto the nanowires through a low temperature process (150 °C) to enhance the performance. After introduction of zinc oxide layer, the TCFs could maintain its electrical and optical properties after scratching. Moreover, zinc oxide coating enhanced the thermal stability of the device and no distinct resistance change was observed in 50 °C environment for 40 days.

  • (2023) Jia, Haowei
    Oxygen evolution reaction (OER) is thought to play an essential part in electrochemical water splitting (EWS), metal-air batteries, energy storage, etc. Therefore, exploring an advanced anode material is necessary for the efficient OER process. Although manganese dioxide (MnO2) as the anode catalyst has been widely researched in recent centuries, most of the research concentrated on the specific phases of MnO2 (α-MnO2, δ-MnO2, and γ-MnO2). Very few reports were related to the ε-MnO2. In the meantime, the lower states of manganese oxides (MnOx 1 ≤ x ≤ 2) are still worth exploring. The multiple chemical states and large oxygen vacancies of MnOx will apply outstanding electrochemical performance. Moreover, the components of MnOx with other advanced materials, such as MXene, also show great catalytic activities. In addition, heat treatment was always adopted to modify the phase transformation of MnO2 samples. This dissertation mainly involved three experimental chapters: (1) ε-MnO2 was successfully covered on the carbon cloth (CC) substrate surface by electrodeposition strategy at different depositing times and temperatures. The morphological changes and OER performance of these as-prepared samples have been investigated. The results demonstrated that when the preparation duration was 30 min and the temperature was 50 °C, the as-prepared MnO2/carbon cloth (MnO2/CC) exhibited the best OER performance. From the microscopic characterization, the MnO2 was uniformly and firmly grown on the carbon cloth without binder usage. All the as-prepared samples displayed a core-shell structure, and the morphology of MnO2/CC did not have significant changes after the long-time stability test. (2) Ni-doped MnO2/CC (NMO/CC) was successfully synthesized by adding different concentrations of Ni cations into the precursor solution. Doping Ni cations increased the OER performance, which attributed to the appearance of more oxygen vacancies and the increased active surface area. In addition, MXene (MX) was used to deposit on the surface of NMO/CC to form the compounds of Ni-doped MnO2 and MXene electrode (MX-NMO/CC), which further decreased the resistance of the samples and increased their OER performance. (3) Thermal treatment was adopted to enhance the OER performance of as-prepared MnO2/CC samples. Three different heating temperatures were applied (250, 350, and 450 °C), and the duration was 1 h. The sample treated at 350 °C (MnO2/CC 350) acquired the best OER performance. The phase transition can be detected when the temperature reached 350 °C, and the effects of phase transition on OER performance were researched. This work explored a simple binder-free electrodeposition strategy to prepare different core-shell structured MnOx-based catalysts for an efficient OER process.

  • (2022) Xue, Yifan
    With the increasing demand for large electronic devices, such as electric vehicle (EV), hybrid vehicles (HEV), energy storage devices have become more prominent. Lithium-ion rechargeable batteries have been one of the most popular vital topics in this area. For developing the next generation lithium-ion batteries with higher energy capacity and safety, solid-state electrolytes play an important role in improving ionic conductivity and preventing leakage in lithium-ion batteries. Lithium lanthanum titanate (Li3xLa2/3–xTiO3, LLTO) is a ceramic oxide solid-state electrolyte material, which has attracted many interests due to its high chemical stability, wide voltage window and high ionic conductivity (10-3 S/cm). However poor grain boundary conductivity of LLTO and electrode/electrolyte inter-facial problem limit the overall ionic conductivity and rate capacity of LLTO based solid battery systems. Therefore, optimizing the grain boundary conductivity, minimizing the interface issues and increasing the total conductivity of LLTO solid-state electrolytes are imminent. In this thesis, three approaches for enhancing ionic conductivity of LLTO based materials were developed: spark plasma sintering technology, oxygen vacancy manipulation and SiO2 doping. Spark plasma sintering technology enhances the processing methodology of LLTO to prevent lithium-ions loss at grain boundary, thus improving the grain boundary conductivity of LLTO to 1.624×10-6 S/cm. Oxygen vacancy manipulation uses post-annealing procedures to tailor oxygen levels of LLTO, which influenced the crystal structure and changed lithium-ions conduction mechanism of LLTO, resulting an enhanced overall ionic conductivity to 3.38×10-5 S/cm. SiO2 doping process creates the amorphous layers at grain boundary of LLTO to minimize the grain boundary resistance effect, thereby further improving the grain boundary conductivity to 1.96×10-4 S/cm. The purpose of this study is to understand Lithium-ions migration mechanism and optimize the electrochemical performance of LLTO solid-state electrolyte.

  • (2023) Lee, Minwoo
    Due to the unique photovoltaic properties and ease of fabrication, organic-inorganic halide perovskites have generated considerable research interest. The perovskite solar cell can be applied to many applications, by tuning the bandgap. Inter of Things (IoT) devices and tandem solar cell applications, in particular, have been required for the wide bandgap perovskite solar cells. However, wide bandgap perovskite solar cells have band alignment mismatch problems, leading to charge recombination at the interface of perovskite, resulting in encouraging low device performance and decrease device stability. The first part of this thesis includes the study of the structure and working mechanism of perovskite solar cells. In addition, the defect of the perovskite was explained about how the majority of defects formed. This is caused by shallow defect energies within the bandgap, low density of deep traps, and low trap-charge interaction cross-sections which are occurred during the interaction between traps and charges. After that, the explanation of the reason how wide bandgap is applied for the indoor application. There is previous work on the tuning of the band alignment between perovskite and hole transfer layer which improved the efficiency of hole transfer, resulting in high device performance under the low light intensity condition. Lastly, the experiment of the thesis is focused on the address of the band alignment mismatch by adding two dimensional (2D) BA2PbBr4 perovskite layer for the tunnelling effect between the electron transport layer (ETL) and perovskite layer. The tunnelling layer of 2D perovskite improved the 3D perovskite crystal quality and charge transport from the 3D perovskite to ETL. As a result, the power conversion efficiency under the 200 lux white light emitting diodes (LED) light for the IoT devices was 43.70% with around 1 V of open circuit voltage and improved the device stability under the 1000 lux of white LED up to 1200 hrs.

  • (2023) Su, Hao
    Of the engineering alloys, magnesium-lithium (Mg-Li) alloys are the lightest structural metallic materials (density, ρ=1.3-1.6 g/cm3) that have attracted substantial scientific research due to their relatively high specific strength and good formability. However, their low strength and poor thermal stability and corrosion resistance largely limit their industrial applications, which can be addressed by microalloying and designing proper heat treatments. The aims of this thesis are to investigate the effects of alloy composition, solid solution temperature and ageing treatments on the evolution of the microstructure and properties (hardness and corrosion) of a series of thermomechanically processed body centred cubic (BCC) Mg-Li-Al-Y alloys (designated A1 to A7). It was found that the hardness of as-quenched A2 alloy increased with the increasing solution treatment temperature, with the highest hardness ~157 HV achieved at 380-400°C, this represents a hardness increase of more than 100% over its as-cast state. This substantial strengthening is closely associated with the formation of a uniform distribution of Al-rich nanostructures in the BCC matrix. Moreover, the as-quenched A2 and A7 alloys optimized via hot forging and rolling showed excellent softening resistance during ageing, with a hardness above ~ 133 HV after over 60 days, and still above ~100 HV after over 3 years of natural ageing, or with a hardness above ~110 HV after 30 h of ageing at 70 °C and ~ 95 HV after more than 3 years of natural ageing. The mechanism behind this excellent thermal stability was explained by the formation of uniformly distributed Al-rich nanostructure on water quenching that generated a significant strengthening effect and retarded the nucleation of plate-shaped nanoparticles from the supersaturated matrix, thus reducing age softening. Also, the absence of soft AlLi precipitation back to the matrix and the impeded decomposition of the strengthening MgLi2Al θ phase into the AlLi phase also helped to slow the rate of age softening. Furthermore, the HR-WQ A7 alloy was the most corrosion resistant alloy during the various ageing procedures (1.37–5.84 µA/cm2), which was consistently lower than both high purity Mg (7.56 µA/cm2) and LA113 (6.20 µA/cm2) (Xu et al., Nature Materials, vol. 14, p. 1229, 2015). The corrosion resistance of the alloy decreased during ageing, which was argued to be a result of the precipitation of new phases and coarsening of the nanostructure.

  • (2022) Al-Farsi, Mo
    Multijunction solar cells based on silicon are predicted to achieve an efficiency of 40-45% for a top cell with a band gap of 1.6-1.9 eV. However, there are currently no known materials with suitable band gaps able to deliver high efficiencies. Two classes of materials that have been proposed for top cells are alloys of CuGaSe2 and alloyed oxide perovskites. CuGaSe2 has a suitable band gap (1.68 eV) for a top cell on silicon, but the maximum efficiency achieved is only 11%, while that of the closely-related CuInGaSe2 (band gap 1.14 eV) is 23.35%. The low efficiency of CuGaSe2 has been attributed to anti-site defects. Therefore, suppressing this defect formation is critical to achieving higher efficiencies. On the other hand, most oxide perovskites have band gaps that are too high (>2 eV) to be used as top cells on silicon, hence strategies such as alloying are required to lower their band gaps. In this work, the effects of alloying CuGaSe2 with Ag, Na, K, Al, In, La and S were investigated using Density Functional Theory (DFT) calculations. The band gaps of the alloyed compounds and formation energies of anti-site defects were calculated to find alloying elements that can increase the defect formation energy but maintain the band gap. CuGaSe2 alloyed with Al at 50at% showed the highest increase (compared to unalloyed CuGaSe2) in the defect formation energy (by ~0.20 eV) followed by Na (~0.15 eV) and S (~0.10 eV), both at 50at%. However, the band gap of the Al alloy (~2.15 eV) is too high for a top cell, while those of Na (~1.95 eV) and S (~1.91 eV) are slightly above the upper limit. Thus, alloying with these elements is not an ideal route towards significantly increasing the formation energy of anti-site defects while maintaining the band gap of CuGaSe2. However, some of the factors that influence the defect formation energy are identified, potentially leading to design rules for future work. Defect formation energies were found to be higher in structures with more positively charged Ga and negatively charged Se atoms. Analysis of bond lengths revealed a positive correlation between shorter Ga and Se bonds and higher defect formation energies. Band gaps of various alloyed oxide perovskites were calculated using DFT. BiFeO3 was alloyed with Y and Sb; LaFeO3 with Cr and Sb and YFeO3 with Bi and Sb. YFeO3 alloyed with Sb at 50at%, was found to have a band gap of 1.4-2.1 eV (depending on the basis set used) which is in the range for a top cell.