Growth and Characterization of InGaAsP Alloy Nanowires with Widely Tunable Bandgaps for Optoelectronic Applications

abstract: The larger tolerance to lattice mismatch in growth of semiconductor nanowires (NWs) offers much more flexibility for achieving a wide range of compositions and bandgaps via alloying within a single substrate. The bandgap of III-V InGaAsP alloy NWs can be tuned to cover a wide range of (0.4, 2.25) eV, appealing for various optoelectronic applications such as photodetectors, solar cells, Light Emitting Diodes (LEDs), lasers, etc., given the existing rich knowledge in device fabrication based on these materials. This dissertation explores the growth of InGaAsP alloys using a low-cost method that could be potentially important especially for III-V NW-based solar cells. The NWs were grown by Vapor-Liquid-Solid (VLS) and Vapor-Solid (VS) mechanisms using a Low-Pressure Chemical Vapor Deposition (LPCVD) technique. The concept of supersaturation was employed to control the morphology of NWs through the interplay between VLS and VS growth mechanisms. Comprehensive optical and material characterizations were carried out to evaluate the quality of the grown materials. The growth of exceptionally high quality III-V phosphide NWs of InP and GaP was studied with an emphasis on the effects of vastly different sublimation rates of the associated III and V elements. The incorporation of defects exerted by deviation from stoichiometry was examined for GaP NWs, with an aim towards maximization of bandedge-to-defect emission ratio. In addition, a VLS-VS assisted growth of highly stoichiometric InP thin films and nano-networks with a wide temperature window from 560◦C to 720◦C was demonstrated. Such growth is shown to be insensitive to the type of substrates such as silicon, InP, and fused quartz. The dual gradient method was exploited to grow composition-graded ternary alloy NWs of InGaP, InGaAs, and GaAsP with different bandgaps ranging from 0.6 eV to 2.2 eV, to be used for making laterally-arrayed multiple bandgap (LAMB) solar cells. Furthermore, a template-based growth of the NWs was attempted based on the Si/SiO2 substrate. Such platform can be used to grow a wide range of alloy nanopillar materials, without being limited by typical lattice mismatch, providing a low cost universal platform for future PV solar cells.Dissertation/ThesisDoctoral Dissertation Chemistry 201

Plasma oxidation of liquid precursors for complex metal oxides.

Clean energy production and storage are two of the most significant challenges in the 21st century currently limited by the discovery and development of new and advanced materials. Complex oxides and alloys made using earth-abundant elements will play a crucial role in technology development moving forward, however, current preparation techniques are limited by their inability to produce complex oxides and alloys with precise composition control at fast timescales. A concept was proposed to produce mixed metal oxides with composition control through the oxidation of liquid precursors via plasma oxidation. It was hypothesized that the oxidation process can be completed in fast timescales owing to the rapid heating and cooling of the plasma process. Even though the rapid timescales for oxidation can be understood through fast heating processes during plasma exposure, the mechanisms responsible for composition control are not immediately obvious. So, fundamental experiments were carried out to elucidate the nucleation and growth steps responsible for metastable non-stoichiometric oxide formation. Interrupted oxidation experiments completed within twenty seconds revealed the following steps during plasma exposure of liquid droplets: the nucleation of monometallic oxide phases from an amorphous nutrient, solid-state reaction into intermediate mixed oxide phase, and formation of metastable phase. Evidence also suggests the fast kinetics of the oxidation process depends on the enormous heat released from the recombinative reactions among plasma species present in the plasma. The viability of a select set of plasma-synthesized oxides were tested in energy conversion and storage technologies. The technique was successfully used to synthesize W0.99Ir0.01O3-δ alloy which showed high oxygen evolution reaction (OER) activity and stability in acid with an overvoltage reduction in the excess of 500 mV compared to the same composition prepared via standard thermal oxidation route. The structural dilution of iridium with earth-abundant tungsten will enable the efficient use of scarce iridium resources. In alkaline media OER, charge-transfer type double perovskite (La0.9Ca0.1Co0.5Ni0.5O3-δ) prepared via the rapid plasma method shows excellent activity rivaling best performing complex oxide electrocatalysts. Most importantly, the obtained experimental data, combined with density functional theory calculations allows for relating the high OER activity to the strong hybridization of the transition metal 3d and oxygen 2p bands. Again, this technique has been used to fabricate manganese-enriched nickel-manganese-cobalt (NMC) oxides. The resulting NMC materials were tested as cathodes in lithium ion battery and show competitive results compared with NMCs prepared through other routes. This dissertation presents a concept utilizing plasma oxidation of liquid precursors for composition control of complex oxides and alloys. The presented concept could expedite the accelerated discovery and development of advanced materials for energy conversion and storage. Furthermore, the underlying nucleation and growth mechanistic aspects for forming non-stoichiometric oxide phases will add scientific knowledge to our understanding of the synthesis of materials far from equilibrium

New Trends in Photo(Electro)catalysis

This reprint focuses on new trends in photo-electrocatalysis, specifically addressed to the remediation of wastewater and energy production. The remediation of wastewater, up to a level that is acceptable for discharge into receiving waterbodies, involves an ever-growing demand of energy, so effective and low-energy treatment processes are highly desirable. Among the other treatments, photo- and photo-electrochemical treatment processes may be considered as advanced oxidation processes (AOP), which are based on the generation of OH radicals, strong oxidizing agents able to indiscriminately degrade even the most persistent organic compounds. Photocatalysis and photo-electrocatalysis can be considered as effective methods for organic degradation, especially when the semiconductor is active in the range of visible light. Several results are presented on new morphologies and structures, which allow more photoactive, visibly responsive, and stable materials, as well as studies on combined processes in which photo- or photo-electrochemistry contribute to an increase in the sustainability of the whole process, lowering costs and achieving the most valuable final products. In view of the circular economy concept, microbial fuel cell systems are also considered as possible way to recover energy from organic pollutants contained in wastewater

Designing semiconductors and catalysts for photoelectrochemical hydrogen production

This thesis presents the results of a number of projects dealing with the challenge of hydrogen production from sunlight, promoting a medium to store and transport renewable energy. Different nanostructured organic and inorganic semiconductors as well as metallic co-catalysts were synthesized and combined to thin film devices, which can be envisioned to work like artificial leaves. These devices were extensively studied by various physical methods like X-ray diffraction, electron microscopy, X-ray photoelectron spectroscopy, spectrophotometry, nuclear magnetic resonance spectroscopy and density functional theory calculations. Finally, their abilities regarding the harvesting of sunlight and their catalytic properties for hydrogen evolution were investigated by photoelectrochemical methods. The first chapter describes the influence of tin doping on the performance of hematite photoanodes using model photoabsorber layers with different tin doping concentrations and concentration gradients prepared via atomic layer deposition. This study aims for the basic understanding of effects of dopants on fundamental rate-determining kinetic and recombination steps of metal oxide photoelectrodes. The second chapter elucidates the phenomenon of photocorrosion with the example of lithium doped copper oxide photocathodes. While this material appears to be an efficient material at first glance, it corrodes by copper reduction from its own photogenerated electrons in contact with water. This observation was studied in depth to reveal the underlying mechanism of photocorrosion. Additionally, a suitable protection approach for this material is discussed and the hydrogen evolution of those final devices is quantified. The third chapter presents the first study of covalent organic frameworks serving as photoelectrodes. By self-organization, this organic material grows conjugated two-dimensional sheets that stack in the third dimension, forming crystalline and porous polymers. The synthesized material called BDT-ETTA was grown as flat films with its one-dimensional pores oriented perpendicular to the surface of the underlying conductive substrate. Those devices were shown to exhibit a suitable band gap alignment for hydrogen evolution and were applied to reduce water by the use of sunlight. Finally, the combination with a platinum cocatalyst revealed the catalytic activity of the photoactive material itself as bottleneck for the targeted application, whereas the diversity of possible optical and electronic properties can be tuned by the selection of appropriate building blocks, offering an auspicious material system for the evolution of hydrogen from sunlight. The fourth chapter explores electrophoretic deposition as a well-working technique for the film deposition of covalent organic frameworks, also in combination with metallic platinum nanoparticles. With the example of the previously introduced BDT-ETTA, the influence of morphology and added cocatalyst on the photoelectrochemical performance is discussed. Devices exhibiting textural porosity, in addition to the intrinsic porosity of the covalent organic framework itself, showed an increased photoactivity compared to flat electrodes. Their combination with a nanosized platinum cocatalyst leads to strongly enhanced photocurrents, alleviating the catalytic bottleneck of the discussed material. Finally, the perspectives for the continuation of the above projects are discussed in the last chapter

Biomimetic Based Applications

The interaction between cells, tissues and biomaterial surfaces are the highlights of the book "Biomimetic Based Applications". In this regard the effect of nanostructures and nanotopographies and their effect on the development of a new generation of biomaterials including advanced multifunctional scaffolds for tissue engineering are discussed. The 2 volumes contain articles that cover a wide spectrum of subject matter such as different aspects of the development of scaffolds and coatings with enhanced performance and bioactivity, including investigations of material surface-cell interactions

Stable and efficient photoelectrodes for solar fuels production

[eng] The excessive consumption of non-renewable energy sources such as fossil fuels has lead the world to a global climate change, urging for new energy consumption habits together with developing cost- effective alternative renewable technologies. Photoelectrochemical (PEC) water splitting allows for direct conversion of solar light and water into hydrogen and oxygen, storing energy into chemical bonds, solving the storage problem of photovoltaic technology. It has demonstrated to produce pure hydrogen and oxygen in significant efficiencies, although this technology is not ready for market implementation due to lack of efficient, stable and scalable photoelectrodes. In this work, we undertake a journey from improving the efficiency of stable metal-oxide-based photoanodes to stabilizing efficient photovoltaic materials by the introduction of protective, transparent, conductive and catalytic layers. Efforts have focused on using cost-effective and scalable materials and techniques. Metal oxide candidate TiO2 is reported stable in alkaline electrolytes and at anodic potentials, but they present low photon to current conversion efficiencies. This is due to excessively large band gap, absorbing small part of the visible spectra, and small electron and hole mobility. Its efficiency is increased both by microstructuring the substrate and nanostructuring the thin film into nanorods, and by modifying the electronic structure with a reductive H2 treatment, enhancing potential drop inside the nanorods. The strategy is shifted into stabilizing highly efficient short band gap semiconductor materials used by the photovoltaic industry. Silicon based photocathodes are protected from acidic electrolyte corrosion by TiO2 overlayers grown by atomic layer deposition (ALD). Temperature is found to play a key role for both efficient film conductivity and stability, being this caused by polycrystalline films formation. ALD enabled high thickness control and pinhole-free layers, together with lower crystallization temperatures than other techniques. Copper-indium-gallium-selenide (CIGS) solar cells fabricated on flexible stainless steel substrates are also protected from corrosion by TiO2 ALD protective layers. The transparent conductive oxide (TCO) already used in solar cells is found necessary for efficient p-n junction formation and charge transport to the hydrogen evolution reaction. Copper-zinc-tin- sulfide/selenide (CZTS/Se) solar cells, where scarce indium and gallium are substituted by tin and zinc, are implemented for PEC devices with TiO2 overlayers too. By modifying the S/Se ratio, band gap can be tuned, an especially interesting characteristic to design tandem PEC devices. ALD deposited protective layers are also studied in anodic polarizations and alkaline electrolytes. By varying the deposition temperature of TiO2, completely amorphous, mixed amorphous and crystalline and fully crystalline films are deposited, and a clear conductivity increase is observed correlated to crystallization. Preferential conductivity paths are observed inside crystalline grains, proposed to be related to crystalline defects and grain boundaries. Few hundred hours stability tests reveals significant photocurrent decrease, with no observed dissolution of the Si photoabsorber. This is attributed to oxidative potentials and electrolyte hydroxides diminishing the n-type semiconductor behavior of TiO2 and forming a barrier to charge injection into the oxygen evolution reaction. UV superimposed illumination partially recovered conductivity. NiO films are ALD-deposited on Si photoanodes and conductivity is found to decrease when temperature is increased from 100 to 300 ºC, simultaneous to a change in preferential crystal growth direction. Higher stoichiometric film, being formed when increasing temperature, decreases Ni2+ vacancies, responsible of the p-type semiconductor behavior. Impressive 1000 hours stability measurements are obtained. Although, this is only attained under periodic cyclic voltammetries, avoiding partial deactivation of the photoanodes. This is attributed to chemical modifications at the surface in such highly oxidative conditions

Electrochemical analysis of photoelectro-, electro-, and thermal catalysis towards more efficient hydrogen peroxide production

Hydrogen peroxide is a chemical with growing industrial relevance but is plagued with high production costs. There are several compelling alternatives to produce H2O2, and most revolve around the 2-electron oxygen reduction reaction. There is a large amount of foundational research on the mechanisms and theoretical aspects of electrochemically reducing oxygen to form H2O2, but this production method remains to be implemented on the industrial scale due to a lack of effective catalysts. Explored here are alternative H2O2 production methods involving the 2-electron reduction of O2. Specifically, photoelectrochemical, electrocatalytic, and thermal catalytic methods are investigated further to draw out necessary catalyst properties and design parameters for producing H2O2. Each catalytic system is analyzed under the lens of electrochemically detecting H2O2 that is catalytically produced. Electrochemical analysis of these catalytic systems provides the added advantage of being able to utilize high throughput screening techniques to quickly discover and test novel catalyst compositions. Optimal catalyst design parameters are identified for each H2O2 production method and these parameters can be assessed over several catalyst compositions through high throughput electrochemical screening. The research presented here acts as a basis for further improvements onto these already compelling H2O2 production methods