This thesis explores innovative strategies to improve Fe-based polarizing multilayer neutron optics, a cornerstone technology for neutron scattering used to investigate materials at the atomic and molecular levels. Polarization analysis plays a crucial role in uncovering insights into magnetic domain structures, molecular orientations, and protein shapes, enabling breakthroughs in materials science, physics, biology, chemistry, and cultural heritage. However, the performance of conventional multilayer optics, such as Fe/Si systems, is hindered by challenges including low reflectivity due to rough interfaces, reduced polarization caused by scattering length density (SLD) mismatches, spin-flip scattering arising from magnetic inhomogeneities, and difficulties in achieving uniform magnetic behavior, to name a few. These challenges become increasingly critical at higher scattering angles or vectors, where thinner and smoother layers are essential for high performance.

This work introduces isotope-enriched boron carbide (¹¹B₄C) as a key material to address these limitations, demonstrating its versatility through two main approaches: as interlayers and as a mixed component within Fe and/or the non-magnetic layers. Initial studies focused on Fe/¹¹B₄CTi multilayers, which outperformed traditional Fe/Si systems in both reflectivity and polarization. The incorporation of ¹¹B₄C reduced interface widths, as evidenced by sharper Bragg peaks in X-ray reflectivity measurements, and enabled polarization improvements from 61% to 78% for multilayers with 25 Å bilayer thicknesses and 20 periods. These enhancements were driven by better SLD tuning, through compositional variations, and smoother interfaces, showcasing the potential of ¹¹B₄C to optimize multilayer performance.

Further studies explored the use of ¹¹B₄C as interlayers in Fe/Si multilayers, resulting in significant performance gains. Interface widths were reduced from 9.5 Å to 5.2 Å, leading to a 125% increase in reflectivity and 15% higher polarization for multilayers with a period thickness of 15 Å and 80 periods. Acting as a barrier between Fe and Si layers, the ¹¹B₄C interlayers improved interface smoothness, suppressed intermixing, and maintained the structural and magnetic integrity of the multilayers.

The most significant advancements were achieved in multilayers where ¹¹B₄C was mixed into both Fe and Si layers. In these systems, the amorphization effect of ¹¹B₄C eliminated lateral structural correlations and magnetic coercivity, and at the same time drastically reducing spin-flip scattering. These modifications allowed the multilayers to operate at much lower external magnetic fields, making them more efficient, practical and enables possibilities of new experimental setups. Magnetic off-specular polarized neutron reflectometry (PNR) and muon spin spectroscopy measurements confirmed the uniform magnetic behavior of Fe/Si + ¹¹B₄C-containing multilayers, showing the suppression of magnetic domains even at low magnetic fields. The ability to stabilize magnetic properties and enhance polarization performance underscores the significant impact of this approach.

Additionally, the tunability of ¹¹B₄C concentrations enabled precise control over magnetic properties such as coercivity and interlayer exchange coupling. Vibrating sample magnetometry and PNR measurements revealed the emergence of low-field antiferromagnetic coupling and demonstrated how multilayer properties could be tailored for specific applications. These findings highlight the versatility of ¹¹B₄C as both a structural and magnetic modifier in multilayer systems.

This thesis establishes ¹¹B₄C as a highly effective material for Fe-based polarizing neutron optics, addressing critical challenges by improving reflectivity and polarization, reducing spin-flip scattering, and enabling better control of magnetic properties. By overcoming these limitations, ¹¹B₄C expands the capabilities of neutron scattering techniques, paving the way for advancements in materials science, magnetism, and beyond.

This study investigates the effects of incorporating 11B4C interlayers into Fe/Si multilayers, with a focus on interface quality, reflectivity, polarization, and magnetic properties for polarizing neutron optics. It is found that the introduction of 1-2 & Aring; 11B4C interlayers significantly improves the interface sharpness, reducing interface width and preventing excessive Si diffusion into the Fe layers. X-ray reflectivity and polarized neutron reflectivity measurements show enhanced reflectivity and polarization, with a notable increase in polarization for 30 & Aring; period multilayers. The inclusion of interlayers also helps prevent the formation of iron-silicides, improving both the magnetic properties and neutron optical performance. However, the impact of interlayers is less pronounced in thicker-period multilayers (100 & Aring;), primarily due to the ratio between layer and interface widths. These results suggest that 11B4C interlayers offer a promising route for optimizing Fe/Si multilayer performance in polarizing neutron mirrors.

Organic photovoltaics (OPVs) offer a promising solution for indoor energy harvesting. However, fundamental investigations to understand and optimize industrial processes such as roll-to-roll lamination for upscaling remain limited. This study investigates a critical failure mode in the upscaling of OPVs. One major challenge for thick semitransparent laminated OPV devices is current-voltage (J-V) asymmetry, where performance under cathode-side illumination exceeds that under anode-side illumination. X-ray reflectivity, neutron reflectivity, and drift-diffusion simulations reveal that a vertically stratified polymer-rich region within the bulk of photoactive layers is the main cause of asymmetric J-V characteristics. Based on this fundamental understanding, a model is proposed to explain the mechanism, wherein electron extraction is hindered by the polymer-rich region during anode illumination. By exploring upscaling-compatible blends, cathode/anode-balanced, high-performing, and air-stable semitransparent laminated OPVs are developed for indoor applications using commercially available PV-X-plus material. These findings provide valuable guidance for designing OPVs with balanced performance, facilitating roll-to-roll adoption and commercialization.

Harvesting indoor light to power electronic devices for the Internet of Things has become an application scenario for emerging photovoltaics, especially utilizing organic photovoltaics (OPVs). Combined liquid- and solid-state processing, such as printing and lamination used in industry for developing indoor OPVs, also provides a new opportunity to investigate the device structure, which is otherwise hardly possible based on the conventional approach due to solvent orthogonality. This study investigates the impact of fullerene-based acceptor interlayer on the performance of conjugated polymer-fullerene-based laminated OPVs for indoor applications. We observe open-circuit voltage (V-OC) loss across the interface despite this arrangement being presumed to be ideal for optimal device performance. Incorporating insulating organic components such as polyethyleneimine (PEI) or polystyrene (PS) into fullerene interlayers decreases the work function of the cathode, leading to better energy level alignment with the active layer (AL) and reducing the V-OC loss across the interface. Neutron reflectivity studies further uncover two different mechanisms behind the V-OC increase upon the incorporation of these insulating organic components. The self-organized PEI layer could hinder the transfer of holes from the AL to the acceptor interlayer, while the gradient distribution of the PS-incorporated fullerene interlayer eliminates the thermalization losses. This work highlights the importance of structural dynamics near the extraction interfaces in OPVs and provides experimental demonstrations of interface investigation between solution-processed cathodic fullerene layer and bulk heterojunction AL.

This thesis explores novel strategies for improving Fe-based polarizing neutron optics, a critical part for improving neutron scattering methods to study materials science, physics, biology, medicine, chemistry and cultural heritage. Polarization analysis is important in uncovering data on magnetic domains, protein structures, molecular composition and orientation in biological systems, and ion-diffusion mechanisms that would otherwise be inaccessible. However, conventional methods, particularly state-of-the-art multilayer polarizing neutron optics, are hindered by low specular reflectivity, low polarization at higher scattering vectors/angles, high diffuse scattering and the need for high external magnetic fields for polarizer magnetization.  

The outcomes leading to this thesis, introduces the concept of scattering length density tuning, strategies to decrease the interface width, the diminishing of lateral correlation and magnetic coercivity. All improvements realized by introducing ¹¹B₄C in clever ways.

The multilayers were deposited using ion-assisted DC magnetron sputter deposition (DCMS). Our improvement of Fe-based multilayer mirrors all revolves around the use of ¹¹B₄C. ¹¹B₄C in various concentrations can be used together with other materials to tune the scattering length density contrasts. It can amorphize the interfaces and layers to achieve smaller interface width, diminish lateral correlations and eliminate magnetic coercivity. In other words, increase reflectivity, increase polarization, decrease diffuse scattering and saturate at lower fields.   

The multilayers were mainly characterized using X-ray reflectivity (XRR), X-ray diffraction (XRD), grazing incidence small/wide angle X-ray scattering (GISAXS/GIWAXS), elastic recoil detection analysis (ERDA), magneto-optical Kerr effect (MOKE), vibrating sample magnetometry (VSM), transmission electron microscopy (TEM) and polarized neutron reflectivity (PNR).  

All results prove the benefit of using ¹¹B₄C in Fe-based polarizing neutron optics.