Effects of confinement and surface enhancement on superconductivity

Within the Ginzburg-Landau approach a theoretical study is performed of the effects of confinement on the transition to superconductivity for type-I and type-II materials with surface enhancement. The superconducting order parameter is characterized by a negative surface extrapolation length $b$. This leads to an increase of the critical field $H_{c3}$ and to a surface critical temperature in zero field, $T_{cs}$, which exceeds the bulk $T_c$. When the sample is {\em mesoscopic} of linear size $L$ the surface induces superconductivity in the interior for $T T_{cs}$. In analogy with adsorbed fluids, superconductivity in thin films of type-I materials is akin to {\em capillary condensation} and competes with the interface delocalization or "wetting" transition. The finite-size scaling properties of capillary condensation in superconductors are scrutinized in the limit that the ratio of magnetic penetration depth to superconducting coherence length, $\kappa \equiv \lambda/\xi$, goes to zero, using analytic calculations. While standard finite-size scaling holds for the transition in non-zero magnetic field $H$, an anomalous critical-point shift is found for H=0. The increase of $T_c$ for H=0 is calculated for mesoscopic films, cylindrical wires, and spherical grains of type-I and type-II materials. Surface curvature is shown to induce a significant increase of $T_c$, characterized by a shift $T_c(R)-T_c(\infty)$ inversely proportional to the radius $R$.Comment: 37 pages, 5 figures, accepted for PR

Effect of chain architecture on the swelling and thermal response of star-shaped thermo-responsive (poly (methoxy diethylene glycol acrylate)-block-polystyrene)3 block copolymer films

The effect of chain architecture on the swelling and thermal response of thin films obtained from an amphiphilic three-arm star-shaped thermo-responsive block copolymer poly(methoxy diethylene glycol acrylate)-block-polystyrene ((PMDEGA-b-PS)3) is investigated by in situ neutron reflectivity (NR) measurements. The PMDEGA and PS blocks are micro-phase separated with randomly distributed PS nanodomains. The (PMDEGA-b-PS)3 films show a transition temperature (TT) at 33 °C in white light interferometry. The swelling capability of the (PMDEGA-b-PS)3 films in a D2O vapor atmosphere is better than that of films from linear PS-b-PMDEGA-b-PS triblock copolymers, which can be attributed to the hydrophilic end groups and limited size of the PS blocks in (PMDEGA-b-PS)3. However, the swelling kinetics of the as-prepared (PMDEGA-b-PS)3 films and the response of the swollen film to a temperature change above the TT are significantly slower than that in the PS-b-PMDEGA-b-PS films, which may be related to the conformation restriction by the star-shape. Unlike in the PS-b-PMDEGA-b-PS films, the amount of residual D2O in the collapsed (PMDEGA-b-PS)3 films depends on the final temperature. It decreases from (9.7 ± 0.3)% to (7.0 ± 0.3)% or (6.0 ± 0.3)% when the final temperatures are set to 35 °C, 45 °C and 50 °C, respectively. This temperature-dependent reduction of embedded D2O originates from the hindrance of chain conformation from the star-shaped chain architecture

Time‐Resolved Orientation and Phase Analysis of Lead Halide Perovskite Film Annealing Probed by In Situ GIWAXS

Scalable thin-film deposition methods are increasingly important for hybrid lead halide perovskite thin films. Understanding the structure evolution during non-equilibrium processes helps to find suitable materials and processing parameters to produce films with well-performing optoelectronic properties. Here, spin-cast and slot-die coated bilayers of lead iodide (PbI2) and methylammonium iodide (MAI) are investigated by in situ grazing-incidence wide-angle X-ray scattering during the thermal annealing process, which converts the bilayer into methylammonium lead iodide (MAPI). Photoluminescence (PL) and UV/Vis measurements show increasing crystallinity during the annealing process and a slight PL red-shift of the spin-cast film, attributed to crystallite coalescence that is not prominent for the slot-die coated film. The disintegration of the solvent-precursor complex (MA)$_2$(Pb$_3$I$_8$) ⋅ 2 DMSO and conversion into perovskite are followed in situ and differences in the morphology and time evolution are observed. In both, spin-cast and slot-die coated thin-films, the isotropic orientation is dominant, however, in the slot-die coated films, the perovskite crystallites have an additional face-on orientation ((110) parallel to substrate) that is not detected in spin-cast films. An Avrami model is applied for the perovskite crystal growth that indicates reduced dimensionality of the growth for the printed thin films

Revealing Donor–Acceptor Interaction on the Printed Active Layer Morphology and the Formation Kinetics for Nonfullerene Organic Solar Cells at Ambient Conditions

Slot-die coating is a powerful method for upscaling the production of organic solar cells (OSCs) with low energy consumption print processes at ambient conditions. Herein, chlorobenzene (CB) and chloroform (CF) are compared as host solvents for printing films of the neat novel fused-ring unit based wide-bandgap donor polymer (PDTBT2T-FTBDT), the small molecule nonfullerene acceptor based on a fused ring with a benzothiadiazole core (BTP-4F) as well as the respective PDTBT2T-FTBDT:BTP-4F blend films at room temperature in air. Using CF printing of the PDTBT2T-FTBDT:BTP-4F active layer, OSCs with a high power conversion efficiency of up to 13.2% are reached in ambient conditions. In comparison to CB printed blend films, the active layer printed out of CF has a superior morphology, a smoother film surface and a more pronounced face-on orientation of the crystallites, which altogether result in an enhanced exciton dissociation, a superior charge transport, and suppressed nonradiative charge carrier recombination. Based on in situ studies of the slot-die coating process of PDTBT2T-FTBDT, BTP-4F, and PDTBT2T-FTBDT:BTP-4F films, the details of the film formation kinetics are clarified, which cause the superior behavior for CF compared to CB printing due to balancing the aggregation and crystallization of donor and acceptor

Effect of Solvent Vapor Annealing on Diblock Copolymer-Templated Mesoporous Si/Ge/C Thin Films: Implications for Li-Ion Batteries

Although amphiphilic diblock copolymer templating of inorganic materials such as TiO2 is already well investigated, sol–gel synthesis routines for porous silicon and germanium are relatively rare. Therefore, especially in the field of Li-ion batteries, novel synthesis routines with the possibility to tune the silicon and germanium ratio and the morphology in the nanometer regime are of high interest. Here, we demonstrate a synthesis method that allows a change of morphology and elemental composition with minimal effort. We evidence a morphological transformation in the nanometer regime with real space (scanning electron microscopy) and complementary reciprocal space analysis methods (grazing-incidence small-angle X-ray scattering). Although energy-dispersive X-ray spectroscopy (EDS) reveals a considerable amount of oxygen in the thin film, crystalline Ge in the bulk is detected with powder X-ray diffraction (PXRD) and Raman spectroscopy. Due to the system’s simplicity, chemical mass production options such as roll-to-roll or slot-die printing can also be considered high-yield methods compared to standard synthesis routines

Sprayed Nanometer-Thick Hard Magnetic Coatings with Strong Perpendicular Anisotropy for Data-Storage Applications

The rapid growth of digital information in the world necessitates a big leap in improving the existing technologies for magnetic recording. For the best modern perpendicular recording, the highest coercivity materials with minimal volume are required. We present a study of a facile technology for establishing mono- and multilayer surfaces from various single-domain flat magnetic nanoparticles that exhibit a strong perpendicular-oriented magnetic moment on solid and flexible substrates. Surfactant-free, hard ferromagnetic, and single-domain anisotropic strontium hexaferrite SrFe$_{12}$O$_{19}$ nanoparticles with a perpendicular magnetic moment orientation and two different aspect ratios are self-ordered into magnetic thin nanofilms, exploiting the templating effect of cellulose nanofibrils and magnetic fields. Uniform magnetic coatings obtained by the scalable layer-by-layer spray deposition from a monolayer coverage up to thicknesses of a few tens of nanometers show a preferred in-plane orientation of the hard-magnetic nanoparticles. High coercivities of the films of up to 5 kOe and a high perpendicular anisotropy of M$_{r⊥}$/M$_s$ > 80% are found. The application of the magnetic field during film deposition ensures additional improvement in perpendicular magnetic anisotropy and the appearance of residual magnetization in the film of up to 0.6M$_s$. For low-aspect-ratio nanoparticles stacked in periodic planar structures, the signs of the photonic band gap are revealed. The ability to create scalable, thin magnetic structures based on nanosized particles/building blocks opens great opportunities for their application in a wide variety of optoelectronic and magnetic storage devices