| 1. |
- Busch, Christian, et al.
(författare)
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A mobile setup for simultaneous and in situ neutron reflectivity, infrared spectroscopy, and ellipsometry studies
- 2022
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Ingår i: Review of Scientific Instruments. - : American Institute of Physics. - 0034-6748 .- 1089-7623. ; 93:11
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Tidskriftsartikel (refereegranskat)abstract
- Neutron reflectivity at the solid/liquid interface offers unique opportunities for resolving the structure–function relationships of interfacial layers in soft matter science. It is a non-destructive technique for detailed analysis of layered structures on molecular length scales, providing thickness, density, roughness, and composition of individual layers or components of adsorbed films. However, there are also some well-known limitations of this method, such as the lack of chemical information, the difficulties in determining large layer thicknesses, and the limited time resolution. We have addressed these shortcomings by designing and implementing a portable sample environment for in situ characterization at neutron reflectometry beamlines, integrating infrared spectroscopy under attenuated total reflection for determination of molecular entities and their conformation, and spectroscopic ellipsometry for rapid and independent measurement of layer thicknesses and refractive indices. The utility of this combined setup is demonstrated by two projects investigating (a) pH-dependent swelling of polyelectrolyte layers and (b) the impact of nanoparticles on lipid membranes to identify potential mechanisms of nanotoxicity.
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| 2. |
- Nagy, Bela, 1985-, et al.
(författare)
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4-bounce neutron polarizer for reflectometry applications
- 2018
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Ingår i: Review of Scientific Instruments. - : American Institute of Physics (AIP). - 0034-6748 .- 1089-7623. ; 89:5
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Tidskriftsartikel (refereegranskat)abstract
- A neutron polarizer using four successive reflections on m = 2.5 supermirrors was built and installed at the GINA neutron reflectometer at the Budapest Neutron Centre. This simple setup exhibits 99.6% polarizing efficiency with 80% transmitted intensity of the selected polarization state. Due to the geometry, the higher harmonics in the incident beam are filtered out, while the optical axis of the beam remains intact for easy mounting and dismounting the device in an existing experimental setup. Published by AIP Publishing.
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| 3. |
- Nagy, Bela, 1985-
(författare)
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Neutron Reflectometry Studies of the Hydrated Structure of Polymer Thin Films
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polymer coatings are inexpensive surface modifications providing a wide variety of functions. There is an ever-present motivation to improve the films’ performance and to decrease the cost and the environmental footprint of their production. This thesis includes the study of the structural and functional parameters of polymer coatings that are aimed at preventing biological fouling, the unwanted attachment of organic molecules and organisms on surfaces. The focus was on thin films prepared by the self-initiated photografting and photopolymerization method. This is a UV-initiated polymerization reaction that does not require additional chemicals beyond the monomers and the solvent. Since biofouling is a prominent problem in wet environments, the emphasis was placed on the hydrated structure of the films. Neutron reflectometry was selected as a primary method for these studies, since it is a powerful method for investigating the structure of polymer thin films, especially in the hydrated state due to the labelling offered by isotope substitution. This allows the determination of the solvent volume fraction depth profile, which reveals the chain segment density profile in the hydrated film. To resolve fast changes in the film structure and to study the chemical composition, spectroscopic ellipsometry and infrared absorption spectroscopy was implemented in a setup for in-situ measurements in parallel with neutron reflectometry. This thesis contains an introduction and five research articles, and it can be divided into two main parts: the first focusing on the polymerization reaction and the resulting polymer films and the second on the reflectivity method and instrumentation development. Firstly, uncharged hydrophilic polymer layers were prepared by self-initiated photografting and photopolymerization and the hydrated structure of these was investigated. It was found that the films follow a stretched profile indicating negligible crosslinking, and that the growth dynamics is determined by the balance of grafting and removal through radiation damage. Studying sequential grafting of oppositely charged polyelectrolytes confirmed the results on growth dynamics and showed the effects of electrostatic interactions between the monomers. This also demonstrated that the polymerization method is ill suited for preparing block co-polymers due to the removal of material from the previous layer. However, these studies also show that the growth of the second layer tends to proceed from the substrate, forming a system where the two kinds of chains co-exist and interact in the same layer. The grafting of random co-polymers was also investigated by comparing the anti-fouling performance of layers made from a mixture of oppositely charged monomers to layers made using zwitterionic polymers, resulting in no significant difference. This was attributed to the pairwise deposition of oppositely charged monomers, further emphasizing the importance of the Coulomb force in defining the structure of the charged films. The second part of this work focuses on instrument development. Here the building and testing of an angle-dispersive reflectometer is presented, and the design and first applications of an in-situ setup for measuring spectroscopic ellipsometry and infrared spectroscopy along with neutron reflectometry is described. By investigating the structure of the polymer films prepared by self-initiated photografting and photopolymerization, this work improved the understanding of this method, facilitating the development of new applications in the future. By combining additional methods with neutron reflectometry, both fast changes in the structure and the chemical evolution of the samples can be investigated. However, the differences in the sensitivities and the structural models required by the probes present new challenges in modelling.
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| 4. |
- Nagy, Bela, 1985-, et al.
(författare)
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On the explanation of the paramagnetic Meissner effect in superconductor/ferromagnet heterostructures
- 2016
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Ingår i: Europhysics letters. - : Institute of Physics (IOP). - 0295-5075 .- 1286-4854. ; 116:1
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Tidskriftsartikel (refereegranskat)abstract
- An increase of the magnetic moment in superconductor/ferromagnet (S/F) bilayers V(40 nm)/F (F = Fe(1, 3 nm), Co(3 nm), Ni(3 nm)) was observed using SQUID magnetometry upon cooling below the superconducting transition temperature TC in magnetic fields of 10 Oe to 50 Oe applied parallel to the sample surface. A similar increase, often called the paramagnetic Meissner effect (PME), was observed before in various superconductors and superconductor/ferromagnet systems. To explain the PME effect in the presented S/F bilayers a model based on a row of vortices located at the S/F interface is proposed. According to the model the magnetic moment induced below TC consists of the paramagnetic contribution of the vortex cores and the diamagnetic contribution of the vortex-free region of the S layer. Since the thickness of the S layer is found to be 3-4 times less than the magnetic-field penetration depth, this latter diamagnetic contribution is negligible. The model correctly accounts for the sign, the approximate magnitude and the field dependence of the paramagnetic and the Meissner contributions of the induced magnetic moment upon passing the superconducting transition of a ferromagnet/superconductor bilayer. Copyright (C) EPLA, 2016.
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| 5. |
- Nagy, Bela, 1985-, et al.
(författare)
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Structure of Self-Initiated Photopolymerized Films : A Comparison of Models
- 2022
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Ingår i: Langmuir. - : American Chemical Society. - 0743-7463 .- 1520-5827. ; 38:45, s. 14004-14015
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Tidskriftsartikel (refereegranskat)abstract
- Self-initiated photografting and photopolymerization (SI-PGP) uses UV illumination to graft polymers to surfaces without additional photoinitiators using the monomers as initiators, “inimers”. A wider use of this method is obstructed by a lack of understanding of the resulting, presumably heterogeneous, polymer structure and of the parallel degradation under continuous UV illumination. We have used neutron reflectometry to investigate the structure of hydrated SI-PGP-prepared poly(HEMA-co-PEG10MA) (poly(2-hydroxyethyl methacrylate-co-(ethylene glycol)10 methacrylate)) films and compared parabolic, sigmoidal, and Gaussian models for the polymer volume fraction distributions. Results from fitting these models to the data suggest that either model can be used to approximate the volume fraction profile to similar accuracy. In addition, a second layer of deuterated poly(methacrylic acid) (poly(dMAA)) was grafted over the existing poly(HEMA-co-PEG10MA) layer, and the resulting double-grafted films were also studied by neutron reflectometry to shed light on the UV-polymerization process and the inevitable UV-induced degradation which competes with the grafting.
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