Preparation of Photoactive Transition-Metal Layered Double Hydroxides (LDH) to Replace Dye-Sensitized Materials in Solar Cells

This work highlights the use of Fe-modified MgAl-layered double hydroxides (LDHs) to replace dye and semiconductor complexes in dye-sensitized solar cells (DSSCs), forming a layered double hydroxide solar cell (LDHSC). For this purpose, a MgAl-LDH and a Fe-modified MgAl LDH were prepared. X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) spectroscopy were used to analyze the structural properties, morphology, and success of the Fe-modification of the synthesized LDHs. Ultraviolet-visible (UV-Vis) absorption spectroscopy was used to analyze the photoactive behavior of these LDHs and compare it to that of TiO2 and dye-sensitized TiO2. Current-voltage (I–V) solar simulation was used to determine the fill factor (FF), open circuit voltage (VOC), short circuit current (ISC), and efficiency of the LDHSCs. It was shown that the MgFeAl-LDH can act as a simultaneous photoabsorber and charge separator, effectively replacing the dye and semiconductor complex in DSSCs and yielding an efficiency of 1.56%

Internal structure of soot particles in a diffusion flame

The evolution of the internal structure of soot particles was studied in a coflow diffusion flame. Soot particles from the flame were imaged using high resolution transmission electron microscopy. An algorithm to quantify the nano-structure of the particles was extended to study the radial distribution of fringes within the particles. The approximate size of the molecules in the particles was calculated from the fringe lengths, assuming planar pericondensed PAHs. The molecules are slightly larger (~16 rings) and more stacked at the core than at the surface (~12 rings) of the youngest particles sampled, suggesting that the particles could be formed via the stabilisation of a nuclei of larger PAHs and condensation of smaller PAHs on their surface. In the lower-temperature region of the flame the molecules grow mainly at the surface of the particles, whereas the molecules in the core of the particles become less stacked and slightly smaller, indicating some degree of nano-structural mobility. In the higher-temperature region of the flame, a graphitisation process takes place, with the development of a shell of longer (~20 rings), flatter and more compact molecules, and an immobilised amorphous core. At the tip of the flame the particles are oxidised, mainly through surface oxidation

Study of the role of dysprosium substitution in tuning structural, optical, electrical, dielectric, ferroelectric, and magnetic properties of bismuth ferrite multiferroic

Magnetoelectric multiferroics, which combine ferroelectric and magnetic characteristics, have potential use in a variety of electronic devices. In this work, Dy3+ substituted bismuth ferrites with the chemical formula Bi1−xDyxFeO3 (x = 0.0, 0.15, 0.30, 0.45, and 0.60) were synthesized using the sol-gel auto combustion process. The effect of Dysprosium substitution in BiFeO3 (BFO), on its structural, surface morphology, optical, electrical, dielectric, ferroelectric, and magnetic properties were studied. The rhombohedral perovskite structure of the space group (R3c) was confirmed via X-ray diffraction (XRD) analysis. Moreover, the crystallite size had a maximum value of 59.57 nm for x = 0.30. XRD and FTIR confirmed the substitution of Dy3+ into BFO ferrite. Further, the structural change and absorption bands confirmed the substitution of Dy3+ ions into the lattice. For x = 0.30, the energy bandgap of 2.81 eV was found. The resistivity and activation energy were minimum and drift mobility was maximum at x = 0.30 as compared to Dy3+ doped BFO samples. At low frequency, the dielectric loss was reduced, while at high frequency, the dielectric loss increased with increasing frequency. The saturated polarization (PS), electric polarization (EC), and remnant polarization (Pr) have values of 6.95 µC/cm2, 3.49 µC/cm2, and 1.53 kV/cm for x = 0.30, respectively. The maximum saturation magnetization and microwave frequencies were 10.89 emu/g and 2.41 GHz, respectively at Dy3+ concentration x = 0.30. These materials are suitable for electronic and microwave devices

Smartphone-powered, ultrasensitive, and selective, portable and stable multi-analyte chemiresistive immunosensing platform with PPY/COOH-MWCNT as bioelectrical transducer: Towards point-of-care TBI diagnosis

Traumatic Brain Injury, one of the significant causes of mortality and morbidity, affects worldwide and continues to be a diagnostic challenge. The most desirable and partially met clinical need is to simultaneously detect the disease-specific-biomarkers in a broad range of readily available body fluids on a single platform with a rapid, low-cost, ultrasensitive and selective device. Towards this, an array of interdigitated microelectrodes was fabricated on commercially existing low-cost single-side copper cladded printed-circuit-board substrate followed by the bioelectrodes preparation through covalent immobilization of brain injury specific biomarkers on carboxylic functionalized multi-walled carbon nanotubes embedded polypyrrole nanocomposite modified interdigitated microelectrodes. Subsequently, the immunological binding events were transduced as the normalized change in bioelectrode resistance with and without the target analyte via current-voltage analysis. As proof of concept, current-voltage responses were primarily recorded using a conventional probe station, and later, a portable handheld-electronic-readout was developed for the point-of-care application. The data compilation and analysis were carried out using the in-house developed android-based mobile app. Notably, the smartphone powered the readout through a PL-2303 serial connector, avoiding integrating power sources with the readout. Further, this technology can be adapted to other point-of-care biosensing applications

Growth of Single-Crystalline and Layer-Controllable Hexagonal Boron Nitride

School of Energy and Chemical Engineering (Energy Engineering)Two-dimensional (2D) materials provide great potential for their applications in electronics and photonics because they can offer opportunity for extending Moore's law in beyond-CMOS (complementary metal-oxide-semiconductor) devices. Among 2D materials, hexagonal boron nitride (hBN) is a representative 2D insulting material with bandgap (~6 eV). Owing to atomically flat surface without dangling bonds yet with excellent thermal and chemical stabilities, hBN has been introduced as a promising material for an excellent dielectric layer to efficiently reduce charge scattering and a screening layer from surroundings. A key technological challenge is the scalable manufacture of single-crystal 2D hBN film to avoid a lack of durability and a poor performance influenced by inhomogeneities and grain boundaries. In addition, the controllability of the number of layers is also highly required due to the electron tunneling properties depending on the thickness of hBN. Even though several approaches to achieve large-scale single-crystal hBN and control the number of layers have been demonstrated, a growth method for few-layer single-crystalline hBN and precise control of the number of layers is still unknown. In this thesis, I demonstrate an approach to grow large-scale single-crystal hBN by chemical vapor deposition (CVD) method. First, I show the epitaxial growth of single-crystal trilayer hBN on Ni (111) foil of 2 x 5 cm at 100 oC higher temperature than normal growth temperature for Ni substrate. The trilayer hBN grains show unidirectional alignment and seamless stitching to form single-crystal film on Ni23B6/Ni (111) where a Ni23B6 layer is formed between hBN and Ni (111) during cooling. Microscopic investigations reveal epitaxial relationship between hBN, Ni23B6, and Ni (111) and enable to understand the hBN growth mechanism, the surface-mediated growth. Furthermore, single-crystal trilayer hBN on Ni23B6/Ni (111) plays a role of a catalytic-transparent protection layer for enhanced long-term stability of hydrogen evolution reaction catalyst and a dielectric layer to prevent electron doping from SiO2 substrate in MoS2 transistors. Our results suggest that few-layer single-crystal hBN allows wide applications for 2D devices and catalytic-transparent protection layer of (electro)catalysts. Next, I demonstrate a method for controlling the number of layers of 2-inch wafer-scale single-crystal hBN film on sapphire substrate by remote inductively coupled plasma CVD, which is a temperature-dependent growth method for mono-, bi-, and trilayer hBN. The x-ray photoelectron spectroscopic and transmission electron microscopic investigations show the formation of a Al-N buffer layer between sapphire substrate and the first layer and the reduction of the interlayer spacing of hBN by the Al-N bond. However, the transferred hBN onto SiO2/Si substrate shows a typical interlayer spacing of hBN. This work takes a step towards the layer-controlled growth of wafer-scale uniform hBN films.ope

Zinc Tin Oxide Thin Films by Pulsed Laser Deposition for use in Transparent Thin-Film Transistors

Zinc tin oxide (ZTO) films deposited by pulsed laser deposition (PLD) are investigated as a channel layers for transparent thin-film transistors (TTFTs). Films are deposited on glass for characterization, and transistor channel layers are deposited onto aluminum oxide-titanium oxide/tin doped indium oxide/glass substrates (ATO/ITO/glass) to produce TTFTs. UV-visible spectroscopy on films gives average T/( 1-R) values around 80% at 500 nm and optical bandgaps between 3.03 and 3.7 eV. EPMA and RBS analyses show that the films consistently contain about 15% less zinc than the targets from which they are deposited. Van der Pauw measurements yield a minimum resistivity of 2.35 Ω-cm, and a maximum Hall mobility of 10.4 cm²-V⁻¹-s⁻¹. Annealing the TTFTs in air at 600 °C for 30 minutes improves performance. XRD analysis shows this anneal changes the amorphous ZTO to a mixture of amorphous ZTO and crystalline ZnO. The highest achieved mobilities for TTFTs produced are μₐᵥg = 6.63 cm²-V⁻¹-s⁻¹ and μᵢₙc = 8.35 cm²-V⁻¹-s⁻¹ with drain current on-to-off ratios in the 10⁵ range

Smart Materials and Devices for Energy Harvesting

This book is devoted to energy harvesting from smart materials and devices. It focusses on the latest available techniques recently published by researchers all over the world. Energy Harvesting allows otherwise wasted environmental energy to be converted into electric energy, such as vibrations, wind and solar energy. It is a common experience that the limiting factor for wearable electronics, such as smartphones or wearable bands, or for wireless sensors in harsh environments, is the finite energy stored in onboard batteries. Therefore, the answer to the battery “charge or change” issue is energy harvesting because it converts the energy in the precise location where it is needed. In order to achieve this, suitable smart materials are needed, such as piezoelectrics or magnetostrictives. Moreover, energy harvesting may also be exploited for other crucial applications, such as for the powering of implantable medical/sensing devices for humans and animals. Therefore, energy harvesting from smart materials will become increasingly important in the future. This book provides a broad perspective on this topic for researchers and readers with both physics and engineering backgrounds

Optical Properties of Thin-Film High-Temperature Magnetic Ferrites

Strongly-correlated electron materials reveal rich physics and exotic cross-coupled electronic and magnetic properties, with important fields results e.g. superconductivity and multiferroics. This is because of the competing interaction between charge, structure, and magnetism in the materials. In this dissertation I present a spectroscopic investigation of several model complex iron oxides under external stimuli of magnetic field, electric field, and temperature. The compounds of interest include NiFe2O4 [nickel ferrite], CoFe2O4 [cobalt ferrite], hLuFeO3 [hexagonal lutethium ferrite], and LuFe2O4 [lutethium ferrite]. These materials are attractive systems in the fields of multiferroics and high-temperature magnets for investigating optical band gap tunability, lattice and charge dynamics, spin-charge coupling, and optically-enhanced magnetoresistive effect. In these works, we have combined optical spectroscopy, magnetic circular dichroism (MCD), and (magneto-)photoconductivity, with high-quality thin-film growth, and first-principles calculations to reveal the nature of the optical excitations within these strongly correlated iron oxides. NiFe2O4 we found that optical excitations offer the opportunity for producing spin-polarized current. In CoFe2O4 we showed that the band gap is robust with temperatures up to 800 K. We found that the direct-gap excitation of LuFe2O4 is highly sensitive the strain induced by epitaxial growth

Tailored perovskite-type oxynitride semiconductors and oxides with advanced physical properties

Perovskite materials (ABX3) reveal a surprisingly large variety of technologically interesting, highly advanced properties for application in solar cells, spin-optoelectronics or magnetic field sensors. In particular, perovskite-type oxynitrides AB(O,N)3 are a well-known class of materials for visible light-driven applications and as inorganic pigments. In the last few decades, their range of applications e.g. in solar water splitting (SWS) have been expanded by the discovery of a great number of before unknown materials. However, the formation of such high-applicable materials was not totally clarified and with it the targeted tuning of their physical properties. To tailor the physical properties in perovskite-type oxynitrides substitutions on the A- and B-site are common, whereas the anionic site (X-site) is less explored. In the first part of the cumulative dissertation, the formation processes of LaTaIVO2N and LaTaVON2 from the respective oxide precursors were elucidated. Additionally, the desired oxidation state of Ta and the nitrogen content in the compounds was adjusted. This opened up new perspectives for the understanding of the ammonolysis process in general, which is used for the formation of perovskite-type oxynitrides from oxide precursors. The here synthesized perovskite-type oxynitrides are promising for light-driven applications because of their measured optical bandgap and low optically active defect concentration. Additionally, the range of potentially suited candidates is expanded by degenerated semiconducting oxynitrides. In the second part, in addition to the nitrogen content and the oxidation state of Ta in La1–xYxTaIVO2N (x = 0, 0.1, 0.25, 0.3, and 1.0) the cationic ratio between Y3+ and La3+ (A-site substitution) was modified. This resulted in controlled physical properties such as an adjusted optical and effective band gap size and a significant charge carrier transport rate. These are important features for SWS and the orthorhombic strain is added to the key descriptors for the band gap size in perovskite-type oxynitrides. In the third part, instead of an A-site substitution a B-site substitution of Taz+ for Coz+ in LaTa(O,N)3 was applied. This led to the previously unknown perovskite-type oxynitrides LaTa1–xCox(O,N)3–δ (x = 0.01, 0.03, and 0.05). The material exhibited a ferromagnetic order with a Curie temperature exceeding 600 K. The synthesized material corresponds – to the best of one’s knowledge – to the first diluted ferromagnetic semiconducting perovskite-type oxynitride. Hence, by substitution of a tiny amount of magnetic B-site cations (≤ 1 at%) in the pristine diamagnetic LaTa(O,N)3 physical properties such as ferromagnetism can be tuned. In the fourth part, through a targeted B-site substitution in La0.6Ca0.4Co1–xFexO3−δ (x =0, 0.3, 0.5, 0.7, 1) physical properties such as CO2 adsorption abilities, oxygen permeability, and electrical conductivity were tuned. These are – amongst other features – important for the application in carbon capture and utilization. The variation of the Fe/Co ratio led to an improvement of the measured oxygen permeation flux. Furthermore, conducted DFT calculations opened up the possibility to determine the effect of the Fe/Co ratio on the oxygen migration behavior and formation energy of the found oxygen vacancies. The results shown in this thesis can be used to synthesize further targeted perovskite-type oxynitrides and oxides exhibiting advanced physical properties for future applications