Material Characteristics of Silicon Nanocrystals for Optical and Electrical Applications

Silicon is abundant in nature and is also non-toxic, prerequisites for low-cost devices and biological applications. Silicon nanocrystals (Si NCs) offer a promising material engineering route for the next generation optical, electrical and biomedical applications such as solar cells and bioimaging due to their energy bandgap tunability. The optical and electrical properties of Si NCs have been intensively studied; however, there are only a few investigations of the nanostructural properties. Structure, chemical composition, distribution of atoms are basic parameters necessary to understand properties of nanomaterials. Examples of studies that would lead to better understanding and control of these nanostructures include the size, shape and distribution of Si NCs and impurity doping of Si NCs, all of which are not well understood. In this thesis, various types of Si NCs embedded in an oxide matrix have been prepared and analysed. Atom probe tomography (APT) visualises that the microstructure of the Si NCs is different depending on the excess Si content, annealing conditions, and presence of dopant atoms. Proximity histogram and cluster analyses reveal the distribution and incorporation of boron (B) and/or phosphorus (P) dopant atoms in the system of the Si NC/oxide matrix. The atomic scale analysis indicates that P atoms prefer to be located in the inside of the Si NCs, whereas B atoms are more likely to be positioned at the interface or in the oxide matrix. In addition, B and P are clustered together in Si NCs when they are co-doped. Regardless of the sample fabrication methods, the size of the Si NC increases as the excess Si content and the annealing temperature increase. The presence of dopant atoms affects the microstructure of the Si NC due to the breaking of the Si dioxide barrier layers. Segregation- and bulk solubility-behaviour, kinetic and thermodynamic effects are applied to explain the obtained B and P distribution. The measured optical and electrical properties can be explained by the microstructure of the Si NC and dopant distribution in this thesis, which helps elucidate the relationship between the actual atomic structure and the measured macroscopic properties of Si NCs films. Studying the detailed structure of the Si NCs paves the way towards improving the performance of devices that could be used for next generation optoelectronic devices

A Novel Application of Oceanic Biopolymers — Strategic Regulation of Polymer Characteristics for Membrane Technology in Separation Engineering

Membranes prepared from oceanic biopolymers have a high potential in membrane separation processes and water purification. It is anticipated to result in more biocompatible and lower-cost materials compared with artificial polymers. This chapter describes the excellent performance of oceanic biopolymer membranes in separation engineering and the regulation factors controlling membrane properties. In particular, chitosan and alginate were picked up as intelligent membrane materials to provide the promised molecular size recognition and other membrane properties. Future prospective strategies for a simple methodology for preparing stable membranes from oceanic biopolymers and the development of selective separation processing were reviewed

Boron-incorporating silicon nanocrystals embedded in SiO2: absende of free carriers vs. B-induced defects

Boron (B) doping of silicon nanocrystals requires the incorporation of a B-atom on a lattice site of the quantum dot and its ionization at room temperature. In case of successful B-doping the majority carriers (holes) should quench the photoluminescence of Si nanocrystals via non-radiative Auger recombination. In addition, the holes should allow for a non-transient electrical current. However, on the bottom end of the nanoscale, both substitutional incorporation and ionization are subject to significant increase in their respective energies due to confinement and size effects. Nevertheless, successful B-doping of Si nanocrystals was reported for certain structural conditions. Here, we investigate B-doping for small, well-dispersed Si nanocrystals with low and moderate B-concentrations. While small amounts of B-atoms are incorporated into these nanocrystals, they hardly affect their optical or electrical properties. If the B-concentration exceeds ~1 at%, the luminescence quantum yield is significantly quenched, whereas electrical measurements do not reveal free carriers. This observation suggests a photoluminescence quenching mechanism based on B-induced defect states. By means of density functional theory calculations, we prove that B creates multiple states in the bandgap of Si and SiO2. We conclude that non-percolated ultra-small Si nanocrystals cannot be efficiently B-doped

Intrafamilial phenotypic distinction of hypophosphatasia with identical tissue nonspecific alkaline phosphatase gene mutation : a family report

Hypophosphatasia (HPP) is caused by mutations in the tissue nonspecific alkaline phosphatase (TNSALP) gene in an autosomal recessive or dominant manner and characterized by defective mineralization of bone and low serum ALP levels. In this report, we present a family with HPP mother (case 1) and HPP child (case 2) who have identical TNSALP gene mutation (c.1015G>A p.Gly339Arg heterozygous mutation) but distinct clinical phenotypes. Whereas case 1 appeared to be asymptomatic despite extremely low levels of serum ALP, case 2 had several HPP-related symptoms, such as tooth loss, fractures, short stature, with slightly decreased ALP levels. Upon the diagnosis of HPP, case 1 discontinued denosumab, which was used to treat her rheumatoid arthritis, concerning the risk of atypical femoral fractures. The clinical course of this family was suggestive in a genotype-phenotype imbalance in HPP, the underdiagnosis of HPP in adults, and the risk of atypical femoral fractures using bone resorption inhibitors

Versatility of doped nanocrystalline silicon oxide for applications in silicon thin-film and heterojunction solar cells

To optimize the optical response of a solar cell, specifically designed materials with appropriate optoelectronic properties are needed. Owing to the unique microstructure of doped nanocrystalline silicon oxide, nc-SiOx:H, this material is able to cover an extensive range of optical and electrical properties. However, applying nc-SiOx:H thin-films in photovoltaic devices necessitates an individual adaptation of the material properties according to the specific functions in the device. In this study, we investigated the detailed microstructure of doped nc-SiOx:H films via atom probe tomography at the sub-nm scale, thereby, for the first time, revealing the three-dimensional distribution of the nc-Si network. Furthermore, n- and p-type nc-SiOx:H layers with various optical and electrical properties were implemented as a window, back contact, and an intermediate reflector layer in silicon heterojunction and multi-junction thin-film solar cells with a focus on the key aspects for adapting the material properties to the specific functions. Here, nc-SiOx:H effectively reduced the parasitic absorption and opened new possibilities for the photon management in the solar cells, thereby, demonstrating the versatility of this material. Remarkably, using our adapted nc-SiOx:H layers in distinct functions enabled us to achieve a combined short circuit current density of 15.1 mA cm−2 for the two a-Si:H sub-cells in an a-Si:H/a-Si:H/µc-Si:H triple-junction thin-film solar cell and an active area efficiency of 21.4% was realized for a silicon heterojunction solar cell

Advanced structural analysis of a laser additive manufactured Zr-based bulk metallic glass along the build height

Additive manufacturing of bulk metallic glasses (BMGs) has opened this material class to an exciting new range of potential applications, as bulk-scale, net-shaped amorphous components can be fabricated in a single step. However, there exists a critical need to understand the structural details of additive manufactured BMGs and how the glassy structure is linked to the mechanical properties. Here, we present a study of structure and property variations along the build height for a laser powder bed fusion (LPBF) processed Zr-based BMG with composition Zr$_{59.3}$Cu$_{28.8}$Nb$_{1.5}$Al$_{10.4}$ commercially termed AMZ4, using hardness testing, calorimetry, positron annihilation spectroscopy, synchrotron X-ray diffraction, and transmission electron microscopy. A lower hardness, more rejuvenated glassy structure was found at the bottom of the build compared to the middle region of the build, with the structure and properties of the top region between the two. Such differences could not be attributed to variability in chemical composition or crystallisation; rather, the softer bottom region was found to have a larger medium range order cluster size, attributed to heat dissipation into the build plate during processing, which gave faster cooling rates and less reheating compared to the steady-state middle of the build. However, at the top of the build less reheating occurs compared to the middle, leading to a somewhat softer and less relaxed state