Lithium-ion batteries have been widely used as power supplies for portable electronic devices due to their higher gravimetric and volumetric energy densities compared to other electrochemical energy storage technologies, such as lead-acid, Ni-Cd and Ni-MH batteries. Developing a novel battery chemistry, ‘‘all silicon lithium-ion batteries’’, using lithium iron silicate as the cathode and silicon as the anode, is the primary aim of this Ph.D project. This licentiate thesis is focused on improving the performance of the silicon anode via optimization of electrolyte composition and electrode formulation. Fluoroethylene carbonate (FEC) was investigated as an electrolyte additive for silicon composite electrodes, and both the capacity retention as well as coulombic efficiency were significantly improved by introducing 10 wt% FEC into the LP40 electrolyte. This is due to the formation of a stable SEI, which mainly consisted of FEC decomposition products of LiF, -CHFOCO2-, etc. The chemical composition of the SEI was identified by synchrotron radiation based photoelectron spectroscopy. This conformal SEI prevented formation of large amounts of cracks and continues electrolyte decomposition on the silicon electrode. An alternative lithium salt, lithium 4,5-dicyano-2-trifluoromethanoimidazole (LiTDI), was studied with the silicon electrode in this thesis. The SEI formation led to a rather low 1st cycle coulombic efficiency of 44.4%, and the SEI layer was found to contain hydrocarbon, ether-type and carbonate-type species. Different to conventional composite silicon electrodes, which require heavy and expensive copper current collector, a flexible silicon electrode, consisted of only silicon nanopowder, Cladophora nanocellulose and carbon nanotube, was facilely prepared via vacuum filtration. The electrode showed good mechanical, long-term cycling as well as rate capability performance.

The need for materials that enhance life span, performance, and sustainability has propelled research in alloy design from binary alloys to more complex systems such as multicomponent alloys. The CoCrFeMnNi alloy, more commonly known as the Cantor alloy, is one of the most studied systems in bulk as well as thin film. The addition of light elements such as boron, carbon, nitrogen, and oxygen is a means to alter the properties of these materials. The challenge lies in understanding the process of phase formation and microstructure evolution on addition of these light elements. To address this challenge, I investigate multicomponent alloys based on a simplified version of the Cantor alloy. My thesis investigates the addition of nitrogen into a Cantor variant system as a step towards understanding the full Cantor alloy. Me1-yNy (Me = Cr + Fe + Co, 0.14 ≤ y ≤0.28 thin films were grown by reactive magnetron sputtering. The films showed a change in structure from fcc to mixed fcc+bcc and finally a bcc-dominant film with increasing nitrogen content. The change in phase and microstructure influenced the mechanical and electrical properties of the films. A maximum hardness of 11 ± 0.7 GPa and lowest electrical resistivity of 28 ± 5 μΩcm were recorded in the film with mixed phase (fcc+bcc) crystal structure. Copper was added as a fourth metallic alloying element into the film with the mixed fcc + bcc structure, resulting in stabilization of the bcc phase even though Cu has been reported to be a fcc stabilizer. The energy brought to the substrate increases on Cu addition which promotes surface diffusion of the ions and leads to small but randomly oriented grains. The maximum hardness recorded by nanoindentation was found to be 13.7 ± 0.2 GPa for the sample Cu0.05. While it is generally believed that large amounts of Cu can be detrimental to thin film properties due to segregation, this study shows that small amounts of Cu in the multicomponent matrix could be beneficial in stabilizing phases as well as for mechanical properties. This thesis thus provides insights into the phase formation of nitrogen-containing multicomponent alloys.

Tungsten based-materials are used in many different technical fields, particularly in applications requiring good temperature and/or erosion resistance. Nanostructuring of tungsten alloys and composites has the potential to dramatically improve the materials’ properties, enhancing the performance in present applications or enabling totally new possibilities. Nanostructured WC-Co composites have been the focus of researchers and industries for over two decades. New methods for powder fabrication and powder consolidation have been developed. However, the fabrication of true nanograined WC-Co materials is still a challenge. Nanostructured tungsten composites for applications as plasma facing materials in fusion reactors have in recent years attracted a growing interest. This Thesis summarizes work on the development of chemical methods for the fabrication of two different types of nanostructured tungsten based materials; WC-Co materials mainly aimed at cutting tools applications and W-ODS composites with rare earth oxide particles, intended as plasma facing materials in future fusion reactors. The approach has been to prepare powders in two steps: a) synthesis of uniform powder precursors containing ions of tungsten and the doping elements by co-precipitation from aqueous solutions, and b) further processing of the precursors into W or WC based nano-composite powders. Highly homogenous W and Co containing powder precursors for WC-Co composites were prepared via two different routes. Keggin-based precursors ((NH4)8[H2Co2W11O40]) with agglomerates of sizes up to 50 μm, were made from sodium tungstate or ammonium metatungstate and cobalt acetate. The powder composition corresponded to 5.2 % Co in the final WC-Co composites. In a second approach, paratungstate-based precursors (Cox(NH4)10-2x[H2W12O42]) were prepared from ammonium paratungstate (APT) and cobalt hydroxide with different compositions corresponding to 3.7 to 9.7 % Co in WC-Co. These particles had a plate-like morphology with sides of 5-20 μm and a thickness of less than 1 μm. Both precursors were processed and sintered into highly uniform microstructures with fine scale (<1μm). The processing of paratungstate-based precursors was also further investigated. Nanostructured WC-Co powders with grains size of less than 50 nm by decreasing processing temperatures and by applying gas phase carburization. W-ODS materials were fabricated starting from ammonium paratungstate and rare earth elements (Y or La). Paratungstate-based precursors were prepared with different homogeneity and particle sizes. The degree of the chemical uniformity varied with the particle size from ca 1 to 30 μm. Tungsten trioxide hydrate-based precursors made from APT and yttrium nitrate under acidic conditions had dramatically higher homogeneity and smaller particle size. The crystallite size was decreased to a few nanometers. These precursors were further processed to composite nanopowder and sintered to a nanostructured W-1.2%Y2O3 composite with 88% relative density. In summary, APT can be converted to highly homogenous powder precursors of different compositions. The transformations are carried out in aqueous suspensions as a solvent mediated process, in which the starting material dissolves and the precursor precipitates. Powders with fine scale morphologies are obtained, e.g. plate-like particles with thickness less than 1 μm or spherical particles with size of a few nanometers. These precursors were processed further in to nano-sized composite powders and sintered to highly uniform tungsten composites with fine microstructures. / QC 20111013

The synthesis procedure of any materials system is often considered a challenging task if performed without any prior knowledge. Theoretical models may thus be used as an external input and guide experimental efforts toward novel exotic materials which are most likely to be synthesizable. The aim of this work is to apply theoretical models and develop frameworks for reliable predictions of thermodynamically stable materials. The material in focus herein is the family of atomic layered boride-based materials referred to as MAB phases. The ground state energy of a material system may be obtained by applying firstprincipal calculations, such as density functional theory (DFT), which has thoroughly been used throughout this thesis. However, performing modern state-of-the-art quantum mechanical calculations, in general, relies on a pre-defined crystal structure which may be constructed based on an a priori known structure or obtained through the use of crystal structure prediction models. In this work, both approaches are explored. We herein perform a thermodynamical screening study to predict novel stable ternary boron-based materials by considering M2AB2, M3AB4, M4AB6, MAB and M4AB4 compositions in orthorhombic and hexagonal symmetries with inspiration from experimentally synthesized MAB phases. The considered atomic elements are M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, A = Al, Ga, In, and B is boron. Among the considered compounds, seven experimentally synthesized phases are verified as stable, and we predict the three hypothetical phases to be stable - Hf2InB2, Zr2InB2, and Mo4AlB4. Additionally, 23 phases of varying symmetries and compositions are predicted as close to stable or to be metastable. However, the assumption of assigning initial crystal structures based on neighbouring compounds may drastically limit the outcome of a screening study. State-of-the-art techniques to generate low energy crystal structures within the considered material phase space is thus explored. More specifically, the Mo-Sc-Al-B system is studied along the ternary joints of (MoxSc1-x)2AlB2 where 0 < x < 1 by using the cluster expansion (CE) and the crystal structure prediction (CSP) codes, CLEASE and USPEX, in analogy. Previous attempts to study the Mo-Sc-Al-B system has been limited by only considering either hexagonal or orthorhombic symmetries. We challenge such approaches by covering larger portions of the phase space efficiently by combining CSP and CE frameworks. The Mo4/3Sc2/3AlB2 (R ̅3m) phase, previously referred to as i-MAB, is verified stable in addition to Mo2/3Sc4/3AlB2 (R3). The suggested approach of combining CE and CSP frameworks for investigating multi-component systems consists of initially performing CSP searches on the systems of smaller order constituting the system in focus. In the pseudo-ternary (MoxSc1-x)2AlB2 system, this refers to performing CSP searches on the ternary Mo2AlB2 and Sc2AlB2 systems. In addition, we also consider the structures of experimentally known phases with similar compositions. The complete set of structures obtained either from CSP or public databases, was later used to design CE models where mixing tendencies in addition to stability determined which model to further study. The predicted low-energy structures of the CE model were relaxed and used as seed structures within a complete CSP search covering the (MoxSc1-x)2AlB2 system for 0 < x < 1. We demonstrate that the use of seed structures, obtained from CE models, efficiently improved the search for low-energy structures within a multi-component system. The suggested approach is yet to be tested on any other system but is applicable to any alternative multi-component system.

Daily life in modern society is highly dependent on many different materials and techniques for manipulating them, and the technological forefront is constantly pushed further by new discoveries. Hence, materials science is a very important field of research. The field of 2D materials is a rather young subfield within materials science, sprung from the realisation of the first 2D material graphene. 2D materials have, due to their 2D morphology, a very high surface-to-weight ratio, which makes them clearly attractive for applications where the material surface is an important characteristic, such as for energy storage and catalysis. The family of 2D materials called MXenes contrast to other 2D materials through the methods used to synthesise them. Traditionally, 2D materials are mechanically exfoliated from a 3D bulk structure in which the 2D sheets are only kept together by weak van der Waals forces, while MXenes are instead chemically exfoliated by selectively etching the A element from a member of the MAX phase family. A MAX phase is a hexagonal nanolaminated crystal structure on the formula Mn+1AXn, with n = 1 – 4, where the M indicates one or several transition metals, A stands for an "A element", commonly a metalloid, and X stands for C or N. After etching away the A element from the MAX phase the Mn+1Xn-layers are left, making up the MXene. MXenes thus show an unusual structural and chemical diversity, and the composition spectra is even further expanded by atoms and small molecules, called surface terminations, attaching to the MXene surface upon etching. These terminations in turn also influence the properties of the MXene. Hence, the MXene family shows great potential for property tailoring towards many different applications. Besides MAX phases there are many other nanolaminated materials which can not be mechanically exfoliated like graphene, and the natural question arises: can other nanolaminated materials be etched into completely new 2D materials? This thesis is concerned with the so called MAB phases – a family of laminated materials similar to MAX phases, but with B instead of C or N – and their 2D derivatives from a computational perspective. More specifically, paper I concerns the quaternary out-of-plane-ordered MAB (o-MAB) phase Ti4MoSiB2 – which has been etched into a 2D titanium oxide – and its related ternary counterparts Mo5SiB2 and Ti5SiB2. In paper II the properties and possible termination configurations of a 2D MXene-analogue named boridene is studied. Both projects concern novel materials that have recently been experimentally realised, and the main aim of the first principles calculations presented here has been to complement and explain the experimental results. In paper I bonding characteristics of Ti4MoSiB2, Mo5SiB2 and Ti5SiB2 are studied, with the goal of better understanding why the two former are experimentally realisable while the latter has never been reported. In Ti4MoSiB2 Ti and Mo populate two symmetrically inequivalent lattice sites, and the bond between these two sites was found to display a large peak of bonding states just below the Fermi level. This peak is fully populated in Ti4MoSiB2 and Mo5SiB2, but only partially populated in Ti5SiB2, which was identified to be the key difference causing Ti5SiB2 to be unstable. Paper II instead focuses on the 2D material boridene, derived from a 3D MAB phase with in-plane ordering (i-MAB). The i-MAB phase is similar in structure to i-MAX phases, and the boridene show similar structure and properties as the corresponding i-MXene etched from i-MAX, including a high activity for the hydrogen evolution reaction (HER). The boridene surface was experimentally found to be terminated by O, F and OH species, and the first principles investigations were aimed at screening the possible termination compositions using dynamical stability analysis, and how the electronic properties of boridene are influenced by the terminations. It was found that the terminations are critical to the dynamical stability of boridene, while the specific composition is less important. For termination with only a single species, the material was predicted to be a small bandgap semiconductor with varying bandgap for different species, while for termination with mixed species, the material was found to be metallic. Hence, this thesis has slightly expanded the theoretical knowledge of MAB phases and their first 2D derivative, boridene, by detailed first principles characterisation. Hopefully, these studies can contribute in further development of the considered and related materials, and bring meaningful insight into the behaviour and properties of MAB phases and their 2D derivatives.

In the recent years, nanostructured magnetic materials and their use in biomedical and biotechnological applications have received a lot of attention. In this thesis, we developed tailored magnetic nanoparticles for advanced bio-applications, such as direct detection of antibodies in biological samples and stimuli responsive drug delivery system. For sensitive and selective detection of biomolecules, thermally blocked iron oxide nanoparticles with specific magnetic properties are synthesized by thermal hydrolysis to achieve a narrow size distribution just above the superparamagnetic limit.  The prepared nanoparticles were characterized and functionalized with biomolecules for use in a successful biosensor system. We have demonstrated the applicability of this type of nanoparticles for the detection of Brucella antibodies as model compound in serum samples and very low detection limits were achieved (0.05 mg/mL). The second part is concerning an in-depth investigation of the evolution of the thermally blocked magnetic nanoparticles. In this study, the formation of the nanoparticles at different stages during the synthesis was investigated by high resolution electron microscopy and correlated to their magnetic properties.  At early stage of the high temperature synthesis, small nuclei of 3.5 nm are formed and the particles size increases successively until they reach a size of 17-20 nm. The small particles first exhibit superparamagnetic behavior at the early stage of synthesis and then transform to thermally blocked behavior as their size increases and passes the superparamagnetic limit. The last section of the Thesis is related to the development of novel drug delivery system based on magnetically controlled release rate. The system consists of hydrogel of Pluronic FP127 incorporating superparamagnetic iron oxide nanoparticles to form a ferrogel. The sol to gel formation of the hydrogel could be tailored to be solid at body temperature and thus have the ability to be injected inside living biological tissues. In order to evaluate the drug loading and release, the hydrophobic drug indomethacin was selected as a model compound. The drug could be loaded in the ferrogel owning to the oil in water micellar structure. We have studied the release rate from the ferrogel in the absence and presence of magnetic field. We have demonstrated that the drug release rate can be significantly enhanced by use of external magnetic field decreasing the half time of the release to more than 50% (from 3200 to 1500 min) upon the application of the external magnetic field. This makes the developed ferrogel a very promising drug delivery system that does not require surgical implant procedure for medical treatment and gives the possibility of enhancing the rate of release by external magnetic field. / QC 20101110

Aviation accounts for 2-3% of the carbon dioxide emitted globally. One way to reduce emissions is to develop and introduce sustainable, functional, lightweight materials and coatings that increase the lifetime and fuel efficiency of aircraft. The main lightweight materials used in the aerospace industry today are aluminum alloys and carbon fiber reinforced plastic composites. In the work presented in this licentiate thesis, a new sustainable alternative for the replacement of toxic hexavalent chromium in a low energy and chemical consumption sealing procedure of anodized aluminum alloys is suggested (paper I and II). An alternative to the conventional metal mesh used as lightning strike protection for composite structures used today is also presented. The proposed solution adds considerably less weight and has a possibility to reduce the CO2 emission from aviation (paper III).    Aluminum alloys as well as composites both exhibit high strength-to-weight ratios but come with individual drawbacks. Fiber reinforced plastics exhibit limited electrical conductivity, which is why additional protection is needed to avoid severe damage following a lightning strike. Aluminum alloys have instead the disadvantage of being susceptible to corrosion and surface protection is required to prolong the materials lifetime and to avoid devastating failures. Anodization, formation of a porous aluminum oxide coating, is the most common choice of surface treatment. This is often followed by closure of the pores through a sealing procedure. Both processes have up until recently been performed in large, energy consuming tanks with highly toxic solutions containing hexavalent chromium which must be replaced to reduce the environmental impact.   In paper I, the environmentally friendly tartaric sulfuric acid has been used as anodization electrolyte and cerium oxide nanoparticles have been investigated as a promising alternative for sealing. Cerium-based and hydrothermal sealing (immersion in hot water), individually and combined, were investigated. The morphological and chemical composition were studied by means of scanning electron microscopy, scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The investigation confirmed the growth of cerium oxide nanoparticles throughout the coating and closing of the pores by hydrothermal sealing. A corrosion immersion test revealed a superior corrosion resistance of surfaces treated with the two-step sealing process compared to plain anodized, cerium or hydrothermally sealed surfaces.   In paper II, the potential use of an aerosol-based wet thin film coating technique called nFOG for cerium sealing as a low chemical and energy consumption alternative to traditional bath-type sealing was investigated. Characterizations of the morphology and composition reveal cerium oxide nanoparticles evenly distributed within the porous coating by the nFOG technique. The new application of the nFOG method was also shown to provide anti-corrosion properties comparable to bath-type sealing. This wet coating technique has the potential to replace chromium and reduce the environmental impact in the treatment process.   Furthermore, the limited electrical conductivity of carbon fiber reinforced plastics can be circumvented by loading the polymer matrix of the composite structure (commonly epoxy) with highly conductive carbon nanomaterials. In paper III, graphene nanoplatelets and carbon nanotubes were loaded into the epoxy. Simulated lightning strike tests showed an improved damage tolerance for the loaded composites compared to composites prepared with plain epoxy. The results suggest that a combination of graphene nanoplatelets and carbon nanotubes increases the damage tolerance by carrying the resulting high electric current from a lightning strike.    In conclusion, the application of cerium oxide nanoparticles and carbon nanomaterials moves the aerospace industry towards a sustainable fuel efficiency using functional, lightweight materials and coatings.

The sustainability goals of the modern world and the fascinating properties of sub-micron scale materials promote development of materials in thin film form. Thin films are materials that have thicknesses ranging from sub-nanometer to several micrometers, synthesized by various deposition techniques. They are used for diverse applications, such as light emitting diodes, solar cells, semiconductor chips, etc. The primary objective of this research project is to develop a chemical vapor deposition (CVD) process for conformal boron carbide thin films. Since boron carbide is a promising neutron converter material for solid-state neutron detectors, the process was validated by depositing on prototype detector chips.   In this study, triethylboron (TEB) was used as single source CVD precursor to deposit boron carbide thin films. The initial experiments focused on low reaction rate deposition by depositing in a kinetically limited regime. The films deposited at ≤450 °C in 8:1 aspect ratio micro-trench structures were highly conformal and show a stoichiometry of about B5.2C. We attribute this observed conformality to the slow reaction kinetics of the TEB at the low deposition temperature enabling the diffusive transport of the precursor molecule down the trench. The depositions carried out on the prototype detector-chips show promising results.   We expand our studies to investigate a new strategy with the prospect of improving the step coverage at higher temperatures for better film properties. We hypothesize that adding a suitable heavier molecule, diffusion additive, with an appropriate partial pressure can enhance the step coverage by pushing the lighter precursor molecule via competitive co-diffusion. It was tested by adding Xe gas to the boron carbide CVD from TEB. The result shows that with this diffusion additive the step coverage was improved from 0.71 to 0.97. From our experimental results, we suggest a competitive diffusion model that can be adapted to other CVD processes to enhance the film step coverage.   The CVD process is further validated by depositing onto carbon nanotube membranes. The initial results show that the process was able to afford evenly deposition around the individual nanotubes in the carbon nanotube membrane. Raman spectroscopy measurements show a similar D-band to G-band intensity ratio before and after the deposition indicating that no defects were induced in the nanotubes. / <p><strong>Funding:</strong> Financial support by the Swedish Research Council (VR) and from the Swedish Government Strategic Research Area in Materials Science on Advanced Functional Materials at Linköping University have supported my studies and are gratefully acknowledged. </p>

The semiconductor tritantalum pentanitride (Ta3N5) is a promising green-energy material for photoelectrolyzing water to produce oxygen and hydrogen owing to its proper bandgap of 2.0 ± 0.2 eV and band positions to redox potential of water. Compare with the conventional setup of water splitting, such as TiO2, Fe2O3, Cu2O, and WO3, the Ta3N5 shows a proper band gap, which leads to a theoretical efficiency as high as 15.9%. However, the complexity of the Ta-N system and the metastability of the Ta3N5 result in the limited research of the growth of high quality stoichiometric Ta3N5. Conventionally, the two-step growth of oxidation and nitridation of a metal Ta using thermal annealing in oxygen and ammonia environment is used to produce the Ta3N5. However, the amount of incorporated oxygen in the Ta3N5 samples and film’s thickness and interface are hardly to be controlled, and the use of ammonia as the nitridation gas is harmful to the environment. Hence, in this thesis work, the reactive magnetron sputtering is used to synthesis the Ta3N5, which demonstrates some advantages, such as possibility to grow on a substrate with nanostructure on the surface, a simplification of growth process, usage of environmental-friendly reactive gas, and even scaling up to the industrial application. The thesis presents a successful growth of orthorhombic Ta3N5-type Ta-O-N compound thin films on Si and sapphire substrates, specifically Ta3-xN5-yOy, using reactive magnetron sputtering with a gas mixture of Ar, N2, and O2. In the deposition process, the total working pressure was increasing from 5 to 40 mTorr, while keeping same partial pressure ratio (Ar: N2: O2 = 3: 2: 0.1). When the total pressure in the region between 5-30 mTorr, a low-degree fiber-textural Ta3-xN5-yOy films were grown. In addition, with the characterization of elastic recoil detection analysis (ERDA), the atomic fraction of O, N, and Ta of as-grown Ta3-xN5-yOy films were found varying from 0.02 to 0.15, 0.66 to 0.54, and 0.33 to 0.31, respectively, which leads to a b-lattice constant decrease around 1.3 %, shown in X-ray diffraction (XRD) results. For a total working pressure up to 40 mTorr, an amorphous O-rich Ta-O-N compound film was formed mixed with non-stoichiometric TaON and Ta2O5, which further raised the oxygen atomic fraction to ~0.48. The increasing total working pressure results in an increasing band gap from 2.22 to 2.66 eV of Ta3-xN5-yOy films, and further increasing to around 2.96 eV of O-rich Ta-O-N compound films. The mechanism of increasing oxygen atomic fraction in the film is founded correlated with the forming oxide on the Ta target surface during the deposition process due to the strong reactivity of O to Ta by the characterization of optical emission spectroscopy (OES). Moreover, the sputter yield was reduced due to the target poisoning, and which is evidenced by both plasma analysis and depth profile from ERDA. A further studies with the deposition parameters for nearly pure Ta3N5 films (oxygen atomic fraction ~2%) was performed using c-axis oriented Al2O3 substrate. In this research, it is found that a Ta2O5 seed layer and a small amount of oxygen were necessary for the growth of Ta3N5. Without the help of seed layer and oxygen, only metallic TaN phases, either mixture of ε- and δ- TaN or δ-TaN were grown, evidenced by X-ray photoelectron spectroscopy (XPS). Furthermore, the structure and phase purity of Ta3N5-phase dominated films was found highly correlated with the thickness of the Ta2O5 seed layer. With the increasing thickness of the seed layer from 5, 9, to 17 nm, the composition of grown films was changed from 111-oriented δ-TaN mixed with c-axis oriented Ta3N5, c-axis oriented Ta3N5, to polycrystalline Ta3N5. In addition, the azimuthal φ-scans in grazing incident geometry demonstrates that the c-axis oriented Ta3N5 contained epitaxially three-variant-orientation domains, in which the a and b planes parallel to the m and a planes of c-axis oriented Al2O3. With the simulation of density functional theory (DFT), the growth of thin seed layers of orthorhombic Ta2O5 (β-Ta2O5) was found promoting by introducing a small amount of oxygen, after calculating the interplay between the topological and energy selection criteria. By the co-action of the mentioned criteria, this already grown Ta2O5 seed layer favored the growth of the orthorhombic Ta3N5 phase. Hence, the mechanism of the domain epitaxial growth of c-axis oriented Ta3N5 on c-axis oriented Al2O3 is attributed to the similar atomic arrangement Ta3N5(001) and β-Ta2O5(201) with a small lattice mismatch around of 2.6% and 4.5%, for the interface of film/seed layer and seed layer/substrate, respectively, and a favorable energetic interaction between involved materials. / Halvledaren tritantalpentanitrid (Ta3N5) är ett lovande grönenergimaterial för fotoelektrolysering av vatten för att producera syre och väte på grund av dess rätta bandgap på 2,0 ± 0,2 eV och bandpositioner till vattens redoxpotential. Jämfört med den konventionella anordningen för vattenklyvning, såsom TiO2, Fe2O3, Cu2O och WO3, visar Ta3N5 ett korrekt bandgap, vilket leder till en teoretisk effektivitet så hög som 15,9%. Komplexiteten hos Ta-N-systemet och metastabiliteten hos Ta3N5 resulterar emellertid i begränsad forskning om tillväxten av högkvalitativa filmer av stökiometrisk Ta3N5. Konventionellt används en tvåstegsmetod genom oxidation och nitridering av Ta-metall för att producera Ta3N5, med hjälp av termisk glödgning i syre- och ammoniakmiljö. Mängden inkorporerat syre i Ta3N5-proverna, filmens tjocklek och gränsytan mellan metall och film kan sällan kontrolleras, och användningen av ammoniak som nitrideringsgas är skadlig för miljön. I detta examensarbete används därför reaktiv magnetronsputtring för att syntetisera Ta3N5, vilket har flera fördelar såsom förenklingar av tillväxtprocessen, möjlighet att växa på ett substrat med nano-strukturerad yta, användning av miljövänlig reaktiv gas, och även god skalbarhet för industriell tillämpning. Avhandlingen presenterar en framgångsrik tillväxtmetod av ortorombiska Ta3N5-typ Ta-O-N sammansatta tunna filmer, specifikt Ta3-xN5-yOy, på Si- och safirsubstrat genom reaktiv magnetronsputtring med en gasblandning av Ar, N2 och O2. I tillväxtprocessen ökade det totala arbetstrycket från 5 till 40 mTorr, samtidigt som partialtrycksförhållandet bibehölls (Ar: N2: O2 = 3: 2: 0,1). När det totala trycket låg mellan 5-30 mTorr, växtes Ta3-xN5-yOy filmer med en lågvärdig fiber-textur. Dessutom, genom karakterisering med ERDA, sågs att kvoten (per atom) av O, N och Ta i Ta3-xN5-yOy -filmerna som växtes varierande från 0,02 till 0,15, 0,66 till 0,54 respektive 0,33 till 0,31, vilket leder till en minskning av b-gitterkonstanten runt 1,3 %, som visas i XRD-resultaten. Vid ett totalt arbetstryck upp till 40 mTorr bildades en amorf O-rik Ta-O-N-sammansättning blandad med icke-stökiometrisk TaON och Ta2O5, vilket ytterligare höjde syrekvoten till ~0,48. Ett ökande totalt arbetstryck resulterar i ett ökande bandgap från 2,22 till 2,66 eV för Ta3-xN5-yOy -filmer och en ytterligare ökning till cirka 2,96 eV för O-rika Ta-O-N-sammansatta filmer. Mekanismen för ökande bråkdel syreatomer i filmen karaktäriseras med hjälp av OES och korreleras med oxiden som bildas på Ta-target under sputtringsprocessen på grund av den starka reaktiviteten av O till Ta. Dessutom reducerades sputterhastigheten på grund av target-förgiftning, vilket bevisas av både plasmaanalys och djupprofiler från ERDA. Ytterligare studier av sputtringsparametrar för nästan rena Ta3N5-filmer (syrekvot ~2%) utfördes med c-Al2O3-substrat. I denna undersökning har det visat sig att ett Ta2O5-initiallager och en liten mängd syre var nödvändiga för tillväxt av Ta3N5. Utan hjälp av initiallager och syre växtes endast metalliska TaN-faser, antingen en blandning av ε- och δ-TaN eller δ-TaN, vilket framgår av X-ray Photoelectron Spectroscopy (XPS). Dessutom visades att strukturen och fasrenheten hos Ta3N5-fasdominerade filmer är starkt korrelerade med tjockleken på Ta2O5-initiallagret. Med ökande tjocklek på initiallagret från 5, 9, till 17 nm ändrades sammansättningen av filmerna från 111-orienterad δ-TaN blandat med c-orienterad Ta3N5, c- orienterad Ta3N5, till polykristallin Ta3N5. Dessutom visar azimutala φ-svepningar vid en XRD-geometri med liten infallsvinkel att den c-orienterade Ta3N5 innehöll tre varianter av epitaxiella domäner, i vilka a- och b-planen är parallella med m- och a-planen för c-Al2O3. DFT simuleringar visade att tillväxten av tunna initiallager av ortorombisk Ta2O5 (β-Ta2O5) främjades genom att introducera en liten mängd syre, efter att ha beräknat samspelet mellan de topologiska- och energi-kriterierna. Genom samverkan av de nämnda kriterierna gynnade de Ta2O5-initiallagren tillväxt av den ortorombiska Ta3N5-fasen. Därför tillskrivs mekanismen för domänens epitaxiella tillväxt av c-Ta3N5 på c-Al2O3 det liknande atomarrangemanget för Ta3N5 (001) och β-Ta2O5(201) med en liten gittermissanpassning på runt 2,6 % och 4,5 %, för gränssnittet mellan film/initiallager respektive initiallager/substrat och en gynnsam energetisk interaktion mellan inblandade material. / <p>Funding agencies: Vetenskapsrådet (grant numbers 2018-04198 and 2021-00357), Energimyndigheten (grant number 46658-1), Stiftelsen Olle Engkvist Byggmästare (grantnumber 197-0210), The Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU 2009-00971)</p>

Research in nanoscience has gained noteworthy interest over the past three decades. As novel chemical and physical properties that are vastly different from extended solids are realized in nanosized materials, nanotechnology has become the center of attention for material in research community. Much to our amazement, investigations in the past two decades revealed that the nanocrystalline semiconductors are “THE PRIME CANDIDATES” to meet the growing energy demand, sensor development, cellular imaging and a number of other optoelectronic applications. Nonetheless, synthesis of nanostructures with control over physical parameters is not sufficient, yet assembling them into functional nanoarchitectures with unique and tunable physical properties is critical for device integration studies. Among bottom-up assembling methods, sol-gel method has received noteworthy interest to produce macroscopic nanostructures of metal and semiconductor NPs with no use of intervening ligands or supports. In 2005, condensation of pre-formed semiconductor NPs (CdSe, CdS, ZnS and PbS) into voluminous gels is reported via controlled destabilization of the surfactant ligands. The resultant chalcogenide aerogels are reported to exhibit extremely low density, high surface area and porosity, and quantum confined optical properties of the NP building blocks. More recently, this method has been extended for the assembly of metal NPs, where transparent and opaque nanostructures (aerogels) of Ag and Au/Ag NPs were produced. The aerogels produced by condensation of NPs are low dimensional (fractal) nanostructures and exhibit a physically connected network of colloidal NPs. Interactions between NPs in a gel structure are intermediate of those of the ligand stabilized NPs and core/shell hetero-nanostructures (e.g. Au@CdSe NPs) with the potential to couple chemically dissimilar systems. In this research study, NP condensation strategy has been utilized to efficiently integrate the plasmonic and excitonic properties of metal and semiconductor nanostructures to produce high-efficiency hybrids that exhibit unique tunable physical and photophysical properties. Two hybrid systems composed of spherical CdSe/Ag hollow NPs and rod shaped CdSe/Ag hollow NPs were investigated for the fabrication of metal-semiconductor hybrid aerogels. The first excitonic energy of spherical CdSe NPs is overlapped with the plasmonic energy of Ag hollow NPs at 515 - 530 nm. The second excitonic energy of rod shaped CdSe is overlapped with the plasmonic energy of Ag hollow NPs at 490 - 505 nm. The photophysical properties of both systems were thoroughly probed through UV-Visible absorption, photoluminescence (PL), and time-resolved (TR) PL spectroscopy. A novel hybrid emission emerged at 640 nm (for spherical CdSe/Ag hollow NPs) and 720 nm (for rod shaped CdSe/Ag hollow NPs) with ~0.2-1% Ag loading. TRPL studies revealed 685 ns and 689 ns PL decay times for hybrid emissions, which are vastly different from the band-edge and trap state emission of phase pure spherical and rod shaped CdSe aerogels respectively, supporting the generation of novel radiative decay pathways. Overall, synthesis of CdSe/Ag hybrid aerogels with novel/tunable photophysical properties will add to the toolbox of semiconductor aerogels with the potential application in future light harvesting technologies.