Dual-Use Self-Assembled Monolayer Controlling Charge Carrier Extraction in Organic Solar Cells.

The development and use of interface materials are essential to the continued advancement of organic solar cells (OSCs) performance. Self-assembled monolayer (SAM) materials have drawn attention because of their simple structure and affordable price. Due to their unique properties, they may be used in inverted devices as a modification layer for modifying ZnO or as a hole transport layer (HTL) in place of typical poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) in conventional devices. In this work, zinc oxide (ZnO) is modified using five structurally similar SAM materials. This resulted in a smoother surface, a decrease in work function, a suppression of charge recombination, and an increase in device efficiency and photostability. In addition, they can introduced asfor hole extraction layer between the active layer and MoO3, enabling the use of the same material at several functional layers in the same device. Through systematic orthogonal evaluation, it is shown that some SAM/active layer/SAM combinations still offered device efficiencies comparable to ZnO/SAM, but with improved device' photostability. This study may provide recommendations for future SAM material's design and development as well as a strategy for boosting device performance by using the same material across both sides of the photoactive layer in OSCs.

Dynamics of Sliding Friction between Laser-Induced Periodic Surface Structures (LIPSS) on Stainless Steel and PMMA Microspheres.

In this work, we investigated the sliding friction measured between poly(methyl methacrylate) (PMMA) colloidal probes with two different diameters D (1.5 and 15 µm) and laser-induced periodic surface structures (LIPSS) on stainless steel with periodicities Λ of 0.42 and 0.9 µm, when the probes are elastically driven along two directions, perpendicular and parallel to the LIPSS. The time evolution of the friction shows the characteristic features of a reverse stick-slip mechanism recently reported on periodic gratings. The morphologies of colloidal probes and modified steel surfaces are geometrically convoluted in the atomic force microscopy (AFM) topographies simultaneously recorded with the friction measurements. The LIPSS periodicity is only revealed with smaller probes (D = 1.5 µm) and when Λ takes the largest value of 0.9 µm. The average value of the friction force is found to be proportional to the normal load, with a coefficient of friction µ varying between 0.23 and 0.54. The values of µ are rather independent of the direction of motion, and they reach their maximum when the small probe is scanned on the LIPSS with the larger periodicity. The friction is also found to decrease with increasing velocity in all cases, which is attributed to the corresponding decrease of the viscoelastic contact time. These results can be used to model the sliding contacts formed by a set of spherical asperities of different sizes driven on a rough solid surface.

Superparamagnetic and highly bioactive SPIONS/bioactive glass nanocomposite and its potential application in magnetic hyperthermia.

Magnetic bioactive glass-ceramics are biomaterials applied for magnetic hyperthermia in bone cancer treatment, thereby treating the bone tumor besides regenerating the damaged bone. However, combining high bioactivity and high saturation magnetization remains a challenge since the thermal treatment step employed to grow magnetic phases is also related to loss of bioactivity. Here, we propose a new nanocomposite made of superparamagnetic iron oxide nanoparticles (SPIONs) dispersed in a sol-gel-derived bioactive glass matrix, which does not need any thermal treatment for crystallization of magnetic phases. The scanning and transmission electron microscopies, X-ray diffraction, and dynamic light scattering results confirm that the SPIONs are actually embedded in a nanosized glass matrix, thus forming a nanocomposite. Magnetic and calorimetric characterizations evidence their proper behavior for hyperthermia applications, besides evidencing inter-magnetic nanoparticle interactions within the nanocomposite. Bioactivity and in vitro characterizations show that such nanocomposites exhibit apatite-forming properties similar to the highly bioactive parent glass, besides being osteoinductive. This methodology is a new alternative to produce magnetic bioactive materials to which the magnetic properties only rely on the quality of the SPIONs used in the synthesis. Thereby, these nanocomposites can be recognized as a new class of bioactive materials for applications in bone cancer treatment by hyperthermia.

Oxidic 2D Materials.

The possibility of producing stable thin films, only a few atomic layers thick, from a variety of materials beyond graphene has led to two-dimensional (2D) materials being studied intensively in recent years. By reducing the layer thickness and approaching the crystallographic monolayer limit, a variety of unexpected and technologically relevant property phenomena were observed, which also depend on the subsequent arrangement and possible combination of individual layers to form heterostructures. These properties can be specifically used for the development of multifunctional devices, meeting the requirements of the advancing miniaturization of modern manufacturing technologies and the associated need to stabilize physical states even below critical layer thicknesses of conventional materials in the fields of electronics, magnetism and energy conversion. Differences in the structure of potential two-dimensional materials result in decisive influences on possible growth methods and possibilities for subsequent transfer of the thin films. In this review, we focus on recent advances in the rapidly growing field of two-dimensional materials, highlighting those with oxidic crystal structure like perovskites, garnets and spinels. In addition to a selection of well-established growth techniques and approaches for thin film transfer, we evaluate in detail their application potential as free-standing monolayers, bilayers and multilayers in a wide range of advanced technological applications. Finally, we provide suggestions for future developments of this promising research field in consideration of current challenges regarding scalability and structural stability of ultra-thin films.

Squared Focal Intensity Distributions for Applications in Laser Material Processing.

Tailored intensity profiles within the focal spot of the laser beam offer great potential for a well-defined control of the interaction process between laser radiation and material, and thus for improving the processing results. The present paper discusses a novel refractive beam-shaping element that provides different squared intensity distributions converted from the Gaussian output beam of the utilized femtosecond (fs) laser. Using the examples of surface structuring of stainless-steel on the micro- and nano-scale, the suitability of the beam-shaping element for fs-laser material processing with a conventional f-Theta lens is demonstrated. In this context, it was shown that the experimental structuring results are in good agreement with beam profile measurements and numerical simulations of the beam-shaping unit. In addition, the experimental results reveal the improvement of laser processing in terms of a significantly reduced processing time during surface nano-structuring and the possibility to control the ablation geometry during the fabrication of micro-channels.

The differences of the impact of a lipid and protein corona on the colloidal stability, toxicity, and degradation behavior of iron oxide nanoparticles.

AIM: In this study, the influence of a serum albumin (SA) and human plasma (HP) derived protein- and lipid molecule corona on the toxicity and biodegradability of different iron oxide nanoparticles (IONP) was investigated. METHODS: IONP were synthesized and physicochemically characterized regarding size, charge, and colloidal stability. The adsorbed proteins were quantified and separated by gel electrophoresis. Adsorbed lipids were profiled by ultraperformance liquid chromatography-ESI-tandem mass spectrometry. The biocompatibility was investigated using isolated erythrocytes and a shell-less hen's egg model. The biodegradability was assessed by iron release studies in artificial body fluids. RESULTS: The adsorption patterns of proteins and lipids varied depending on the surface characteristics of the IONP like charge and hydrophobicity. The biomolecule corona modified IONP displayed favorable colloidal stability and toxicological profile compared to IONP without biomolecule coronas, reducing erythrocyte aggregation and hemolysis in vitro as well as the corresponding effects ex ovo/in vivo. The coronas decreased the degradation speed of all tested IONP compared to bare particles, but, whereas all IONP degraded at the same rate for the SA corona, substantial differences were evident for IONP with HP-derived corona depending on the lipid adsorption profile. CONCLUSION: In this study the impact of the proteins and lipids in the biomolecule corona on the entire IONP application cycle from the injection process to the degradation was demonstrated.

Dual Laser Beam Processing of Semiconducting Thin Films by Excited State Absorption.

We present a unique dual laser beam processing approach based on excited state absorption by structuring 200 nm thin zinc oxide films sputtered on fused silica substrates. The combination of two pulsed nanosecond-laser beams with different photon energies-one below and one above the zinc oxide band gap energy-allows for a precise, efficient, and homogeneous ablation of the films without substrate damage. Based on structuring experiments in dependence on laser wavelength, pulse fluence, and pulse delay of both laser beams, a detailed concept of energy transfer and excitation processes during irradiation was developed. It provides a comprehensive understanding of the thermal and electronic processes during ablation. To quantify the efficiency improvements of the dual-beam process compared to single-beam ablation, a simple efficiency model was developed.

Preparation of SrTiO3 nanocubes by CO2 laser vaporization (LAVA) and hydrothermal maturation.

SrTiO3 is of particular interest for numerous applications such as photocatalytic water splitting, as an electrode material for thermoelectrics or as piezoceramics for sensors. Here we report on an advanced CO2 laser vaporization (LAVA) method for the production of faceted, single-phase SrTiO3 nanoparticles with an average particle size of 35 nm. Starting from a coarse SrTiO3 raw powder, spherical SrTiO3 nanoparticles were obtained by a laser-induced gas-phase condensation process. The composition of the nanoparticles corresponds to that of the starting powder, as XRD and FT-IR measurements show. Further hydrothermal treatment at 275 °C for 4 hours leads to the formation of faceted nanocubes with increasing crystallite size, as demonstrated by TEM, HR-TEM and XRD measurements. During a final washing step in 0.1 M HCl, SrCO3 impurities were dissolved and thus single-phase SrTiO3 nanocubes were successfully obtained, as shown by FT-IR, XRD and TEM analyses. The presented process facilitates the production of single-phase, highly crystalline SrTiO3 nanopowders in sufficient quantities for subsequent use in a variety of applications, in particular for hydrogen production by photocatalytic water splitting.

Large-Area Fabrication of Laser-Induced Periodic Surface Structures on Fused Silica Using Thin Gold Layers.

Despite intensive research activities in the field of laser-induced periodic surface structures (LIPSS), the large-area nanostructuring of glasses is still a challenging problem, which is mainly caused by the strongly non-linear absorption of the laser radiation by the dielectric material. Therefore, most investigations are limited to single-spot experiments on different types of glasses. Here, we report the homogeneous generation of LIPSS on large-area surfaces of fused silica using thin gold layers and a fs-laser with a wavelength λ = 1025 nm, a pulse duration τ = 300 fs, and a repetition frequency frep = 100 kHz as radiation source. For this purpose, single-spot experiments are performed to study the LIPSS formation process as a function of laser parameters and gold layer thickness. Based on these results, the generation of large-area homogenous LIPSS pattern was investigated by unidirectional scanning of the fs-laser beam across the sample surface using different line spacing. The nanostructures are characterized by a spatial period of about 360 nm and a modulation depth of around 160 nm. Chemical surface analysis by Raman spectroscopy confirms a complete ablation of the gold film by the fs-laser irradiation. The characterization of the functional properties shows an increased transmission of the nanostructured samples accompanied by a noticeable change in the wetting properties, which can be additionally modified within a wide range by silanization. The presented approach enables the reproducible LIPSS-based laser direct-writing of sub-wavelength nanostructures on glasses and thus provides a versatile and flexible tool for novel applications in the fields of optics, microfluidics, and biomaterials.

Self-healing capacity of fiber-reinforced calcium phosphate cements.

A major problem concerning the mechanical properties of calcium phosphate cements (CPC) is related to their inherent brittleness, which limits their applicability to non-load bearing bone defects. In this work the preparation of a damage tolerant CPC is presented, where the incorporation of functionalized carbon fibers facilitates steady state flat crack propagation with crack openings below 10 µm. A subsequent self-healing process in simulated body fluid, that mimics the in vivo mineralization of bioactive surfaces, closes the cracks and completely restores the mechanical properties. Hereby, two pathways of self-healing are presented: i) intrinsic healing that bases on the inherent bioactive properties of the cement matrix and chemically treated fibers, and ii) capsule based extrinsic healing, where H2PO4- is released as an initiator for the apatite formation. Such damage tolerant CPCs with self-healing capacity are of particular interest to increase the lifetime of implants as well as in the field of load-bearing bioceramics.

Wettability Analysis of Water on Metal/Semiconductor Phases Selectively Structured with Femtosecond Laser-Induced Periodic Surface Structures.

Femtosecond (fs) laser-induced periodic surface structures (LIPSS) were selectively generated on the surface of an Ag-Si alloy consisting of a metallic and a semiconducting phase. For this purpose, the alloy was irradiated with linearly polarized fs-laser pulses (τ = 300 fs, λ = 1025 nm, frep = 100 kHz) using a laser peak fluence F = 0.30 J/cm2. Due to the different light absorption behaviors of the semiconductor (Si) and the metal (Ag) phases that result in different ablation thresholds of the respective phases, pronounced LIPSS with a period of Λ ≈ 950 nm and a modulation depth of h ≈ 220 nm were generated solely on the Si phase. The alloy surface was characterized by scanning electron microscopy, optical microscopy, white-light interference microscopy, and atomic force microscopy before and after laser irradiation. The chemical analysis was carried out by energy-dispersive X-ray spectroscopy, revealing surface oxidation of the Si phase and no laser-induced chemical modification of the Ag phase. The surface wettability of the alloy was evaluated with distilled water and compared to those of the single constituents of the composites. After fs-laser irradiation, the surface is characterized by a reduced hydrophilic water contact angle. Furthermore, the alloy selectively structured with LIPSS revealed a droplet shape change due to the distinctly different contact angles on the Si (θ = 5°) and Ag (θ = 74°) phases. This phenomenon was evaluated and discussed by local contact angle analyses using a confocal laser scanning microscope and Rhodamine B dye. In addition, it was shown that the shape change due to different contact angles of the components allowed a targeted droplet movement on a macroscopic material boundary (Ag/Si) of the alloy. Selectively structured metal/semiconductor surfaces might be of particular interest for microfluidic devices with a directional droplet movement and for the fundamental research of wettability.

Simulation of the long-term fate of superparamagnetic iron oxide-based nanoparticles using simulated biological fluids.

Aim: To simulate the stability and degradation of superparamagnetic iron oxide nanoparticles (MNP) in vitro as part of their life cycle using complex simulated biological fluids. Materials & methods: A set of 13 MNP with different polymeric or inorganic shell materials was synthesized and characterized regarding stability and degradation of core and shell in simulated biological fluids. Results: All MNP formulations showed excellent stability during storage and in simulated body fluid. In endosomal/lysosomal media the degradation behavior depended on shell characteristics (e.g., charge, acid-base character) and temperature enabling the development of an accelerated stress test protocol. Conclusion: Kinetics of transformations depending on the MNP type could be established to define structure-activity relationships as prediction model for rational particle design.

Mammoth ivory was the most suitable osseous raw material for the production of Late Pleistocene big game projectile points.

Late Pleistocene societies throughout the northern hemisphere used mammoth and mastodon ivory not only for art and adornment, but also for tools, in particular projectile points. A comparative analysis of the mechanical properties of tusk dentine from woolly mammoth (Mammuthus primigenius) and African elephant (Loxodonta africana) reveals similar longitudinal stiffness values that are comparable to those of cervid antler compacta. The longitudinal bending strength and work of fracture of proboscidean ivory are very high owing to its substantial collagen content and specific microstructure. In permafrost, these properties can be fully retained for thousands of years. Owing to the unique combination of stiffness, toughness and size, ivory was obviously the most suitable osseous raw material for massive projectile points used in big game hunting.

A Graded Multifunctional Hybrid Scaffold with Superparamagnetic Ability for Periodontal Regeneration.

The regeneration of dental tissues is a still an unmet clinical need; in fact, no therapies have been completely successful in regenerating dental tissue complexes such as periodontium, which is also due to the lack of scaffolds that are able to guide and direct cell fate towards the reconstruction of different mineralized and non-mineralized dental tissues. In this respect, the present work develops a novel multifunctional hybrid scaffold recapitulating the different features of alveolar bone, periodontal ligament, and cementum by integrating the biomineralization process, and tape casting and electrospinning techniques. The scaffold is endowed with a superparamagnetic ability, thanks to the use of a biocompatible, bioactive superparamagnetic apatite phase, as a mineral component that is able to promote osteogenesis and to be activated by remote magnetic signals. The periodontal scaffold was obtained by engineering three different layers, recapitulating the relevant compositional and microstructural features of the target tissues, into a monolithic multifunctional graded device. Physico-chemical, morphological, and ultrastructural analyses, in association with preliminary in vitro investigations carried out with mesenchymal stem cells, confirm that the final scaffold exhibits a good mimicry of the periodontal tissue complex, with excellent cytocompatibility and cell viability, making it very promising for regenerative applications in dentistry.

Photoluminescent Eu3+-Doped Calcium Phosphate Bone Cement and Its Mechanical Properties.

Calcium phosphate cements (CPC) are well-established bone replacement materials that have been used in dentistry and orthopedics for more than 25 years. The monitoring of bone cements and the associated healing processes in the human body is difficult and so far has often been achieved using cytotoxic X-ray contrast agent additives. These additives have a negative effect on the mechanical properties and setting time of the bone cement. In this paper, we present a novel approach to prepare contrastive CPC by the incorporation of luminescent Eu3+-doped hydroxyapatite (Eu:HAp) nanoparticles. Eu-doped CPC (Eu:CPC) exhibited enhanced mechanical properties compared to pure CPC. Furthermore, the red photoluminescence of Eu:CPC may allow the observation of CPC-related healing processes without the use of harmful ionizing radiation.

Femtosecond Laser-Induced Periodic Surface Structures on Fused Silica: The Impact of the Initial Substrate Temperature.

The formation and properties of laser-induced periodic surface structures (LIPSS) were investigated upon fs-laser irradiation of fused silica at different initial substrate temperatures, TS. For substrate heating between room temperature, TRT, and TS = 1200 °C, a continuous wave CO2 laser was used as the radiation source. The surface structures generated in the air environment at normal incidence with five successive fs-laser pulses (pulse duration, τ = 300 fs, laser wavelength, λ = 1025 nm, repetition frequency, frep = 1 kHz) were characterized by using optical microscopy, scanning electron microscopy, and 2D-Fourier transform analysis. The threshold fluence of fused silica was systematically investigated as a function of TS. It was shown that the threshold fluence for the formation of low-spatial frequency LIPSS (LSFL) decreases with increasing TS. The results reveal that the initial spatial period observed at TRT is notably increased by increasing TS, finally leading to the formation of supra-wavelength LIPSS. The findings are discussed in the framework of the electromagnetic interference theory, supplemented with an analysis based on thermo-convective instability occurring in the laser-induced molten layer. Our findings provide qualitative insights into the formation mechanisms of LIPSS, which allow improvements of the control of nanostructure formation to be made for corresponding applications of dielectric materials in the future.

Multifunctional Hierarchical Surface Structures by Femtosecond Laser Processing.

Hierarchical surface structures were fabricated on fused silica by using a fs-laser with a pulse duration τ = 300 fs and a wavelength λ = 512 nm. The resulting surface structures were characterized by scanning electron microscopy, atomic force microscopy and white light interference microscopy. The optical properties were analyzed by transmittance measurements using an integrating sphere and the wettability was evaluated by measuring the water contact angle θ. The silanization of structured fused silica surfaces with trichloro(1H,1H,2H,2H-perfluorooctyl)silane allows to switch the wettability from superhydrophilic (θ = 0°) to superhydrophobic behavior with θ exceeding 150°. It was shown that the structured silica surfaces are a suitable master for negative replica casting and that the hierarchical structures can be transferred to polystyrene. The transmittance of structured fused silica surfaces decreases only slightly when compared to unstructured surfaces, which results in high transparency of the structured samples. Our findings facilitate the fabrication of transparent glass samples with tailored wettability. This might be of particular interest for applications in the fields of optics, microfluidics, and biomaterials.

The Mechanical Properties of Biocompatible Apatite Bone Cement Reinforced with Chemically Activated Carbon Fibers.

Calcium phosphate cement (CPC) is a well-established bone replacement material in dentistry and orthopedics. CPC mimics the physicochemical properties of natural bone and therefore shows excellent in vivo behavior. However, due to their brittleness, the application of CPC implants is limited to non-load bearing areas. Generally, the fiber-reinforcement of ceramic materials enhances fracture resistance, but simultaneously reduces the strength of the composite. Combining strong C-fiber reinforcement with a hydroxyapatite to form a CPC with a chemical modification of the fiber surface allowed us to adjust the fiber-matrix interface and consequently the fracture behavior. Thus, we could demonstrate enhanced mechanical properties of CPC in terms of bending strength and work of fracture to a strain of 5% (WOF5). Hereby, the strength increased by a factor of four from 9.2 ± 1.7 to 38.4 ± 1.7 MPa. Simultaneously, the WOF5 increased from 0.02 ± 0.004 to 2.0 ± 0.6 kJ∙m-2, when utilizing an aqua regia/CaCl2 pretreatment. The cell proliferation and activity of MG63 osteoblast-like cells as biocompatibility markers were not affected by fiber addition nor by fiber treatment. CPC reinforced with chemically activated C-fibers is a promising bone replacement material for load-bearing applications.

Polarization Properties and Polarization Depth Profiles of (Cd:Zn)S/P(VDF-TrFE) Composite Films in Dependence of Optical Excitation.

The influence of optical excitation intensity on the electrical, ferroelectric and pyroelectric properties of ferroelectric-semiconductor-composites was investigated. For this purpose, composite thin films consisting of poly(vinylidene fluoride-co-trifluoroethylene) and 10 vol % (Cd:Zn)S particles with a thickness of 34 µm were fabricated. The samples were used to measure the absolute pyrocoefficient and to determine the relative pyroelectric depth profile using Laser Intensity Modulated Method. It was shown that a polarization of the samples without an optical excitation at the utilized relatively small peak-to-peak voltages could not be verified by the Sawyer⁻Tower circuit and the measurement setup of the pyroelectric coefficient, respectively. Both remanent polarization and pyroelectric coefficients increased with increasing optical excitation intensity during poling as well as increasing peak-to-peak voltage. The pyrocoefficient shows a temporal decay in the first hours after poling. The specific heat and thermal conductivity or the thermal diffusivity are required for the calibration of the pyroelectric depth profile. Rule of mixture and photo-acoustic investigations proved that the thermal properties of the utilized composites do not differ significantly from those of the pristine polymer. Based on the pyroelectric depth profile which is proportional to the polarization profile, the existing "three phase model" has been extended to generate a replacement circuit diagram, explaining the local polarization due to the optical excitation dependency for both local resistivity and local field strength.

Luminomagnetic Eu3+- and Dy3+-doped hydroxyapatite for multimodal imaging.

Multimodal imaging has recently attracted much attention due to the advantageous combination of different imaging modalities, like photoluminescence (PL) and magnetic resonance imaging (MRI). In the present study, luminescent and magnetic hydroxyapatites (HAp) were prepared via doping with europium (Eu3+) and dysprosium (Dy3+), respectively. Co-doping of Eu3+ and Dy3+ was used to combine the desired physical properties. Both lanthanide ions were successfully incorporated in the HAp crystal lattice, where they preferentially occupied calcium(I) sites. While Eu-doped HAp (Eu:HAp) exhibits dopant concentration dependent persistent PL properties, Dy-doped HAp (Dy:HAp) shows paramagnetic behavior due to the high magnetic moment of Dy3+. Co-doped HAp (Eu:Dy:HAp) nanoparticles combine both properties in one single crystal. Remarkably, multimodal co-doped HAp features enhanced PL properties due to an energy transfer from Dy3+ sensitizer to Eu3+ activator ions. Eu:Dy:HAp exhibits strong transverse relaxation effects with a maximum transverse relaxivity of 83.3L/(mmol·s). Due to their tunable PL, magnetic properties and cytocompatibility Eu:-, Dy:- and Eu:Dy:HAp represent promising biocompatible ceramic materials for luminescence imaging that simultaneously may serve as a contrast agent for MRI in permanent implants or functional coatings.