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Item type:Publication, Access status: with files , Solvent controlled 2D structures of bottom-up fabricated nanoparticle superlattices(Royal Society of Chemistry (RSC), 2023-02-08); ; ; ; We demonstrate that oleyl phosphate ligand-stabilized iron oxide nanocubes as building blocks can be assembled into 2D supercrystalline mono- and multilayers on flat YSZ substrates within a few minutes using a simple spin-coating process. As a bottom-up process, the growth takes place in a layer-by-layer mode and therefore by tuning the spin-coating parameters, the exact number of deposited monolayers can be controlled. Furthermore, ex situ scanning electron and atomic force microscopy as well as X-ray reflectivity measurements give evidence that the choice of solvent allows the control of the lattice type of the final supercrystalline monolayers. This observation can be assigned to the different Hansen solubilities of the solvents used for the nanoparticle dispersion because it determines the size and morphology of the ligand shell surrounding the nanoparticle core. Here, by using toluene and chloroform as solvents, it can be controlled whether the resulting monolayers are ordered in a square or hexagonal supercrystalline lattice.Publicationtype: Journal ArticleScopus© Citations 3 41 107 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Strengthening engineered nanocrystalt three-dimensional superlattices via ligand conformation and reactivity(American Chemical Society (ACS), 2022-08-23); ; ; ; Nanocrystal assembly into ordered structures provides mesostructural functional materials with a precise control that starts at the atomic scale. However, the lack of understanding on the self-assembly itself plus the poor structural integrity of the resulting supercrystalline materials still limits their application into engineered materials and devices. Surface functionalization of the nanobuilding blocks with organic ligands can be used not only as a means to control the interparticle interactions during self-assembly but also as a reactive platform to further strengthen the final material via ligand cross-linking. Here, we explore the influence of the ligands on superlattice formation and during cross-linking via thermal annealing. We elucidate the effect of the surface functionalization on the nanostructure during self-assembly and show how the ligand-promoted superlattice changes subsequently alter the cross-linking behavior. By gaining further insights on the chemical species derived from the thermally activated cross-linking and its effect in the overall mechanical response, we identify an oxidative radical polymerization as the main mechanism responsible for the ligand cross-linking. In the cascade of reactions occurring during the surface-ligands polymerization, the nanocrystal core material plays a catalytic role, being strongly affected by the anchoring group of the surface ligands. Ultimately, we demonstrate how the found mechanistic insights can be used to adjust the mechanical and nanostructural properties of the obtained nanocomposites. These results enable engineering supercrystalline nanocomposites with improved cohesion while preserving their characteristic nanostructure, which is required to achieve the collective properties for broad functional applications.Publicationtype: Journal ArticleScopus© Citations 9 58 417 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Spin‐ and Stress‐Depending Electrical Transport in Nanoparticle Supercrystals: Sensing Elastic Properties of Organic Tunnel Barriers via Tunneling Magnetoresistance(Wiley-VCH, 2022-05-26); ; ; ; Abstract The spin‐dependent electrical transport in rigid inorganic‐inorganic layered systems is extensively applied for the detection of magnetic fields in data storage. In this work, spin‐dependent electrical transport in flexible organic‐inorganic supercrystals based on superparamagnetic iron oxide nanoparticles is investigated. These nanoparticles are stabilized by oleic acid ligands, which in turn are serving as tunneling barriers between individual magnetic nanoparticles. The resulting tunneling magnetoresistance (TMR) is tunable due to the elastic properties of these organic barriers. Applying external mechanical stress on this composite material will change the average distance between adjacent nanoparticles and will hence determine the resulting TMR‐effect amplitude. Thus, measured stress‐induced changes in the barrier thickness at sub‐nanometer scale allow for determining the mechanical properties of organic barrier molecules in the confined space between the particles. These results provide the foundation for a new type of mechanical sensor.Publicationtype: Journal ArticleScopus© Citations 3 89 164 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Constitutive and fracture behavior of ultra-strong supercrystalline nanocomposites(American Institute of Physics (AIP), 2021-09-09); ; ; ; Supercrystalline nanocomposites are a new class of hybrid and nanostructured materials that can reach exceptional mechanical strength and can be fabricated at low temperatures. Hierarchically arranged, they bridge the gap from the nano- to the macro-scale. Even though their mechanical properties are starting to be characterized, their constitutive behavior is still largely unexplored. Here, the mechanical behavior of supercrystalline nanocomposites of iron oxide nanoparticles, surface-functionalized with oleic acid and oleyl phosphate ligands, is investigated in both bending and compression, with loading-unloading tests. A new bar geometry is implemented to better detect deformation prior to unstable crack propagation, and notched bending bars are tested to evaluate fracture toughness. Micro-mechanical tests result in the values of strength and elastic modulus that are extremely high for supercrystals, reaching record-high numbers in the oleic acid-based nanocomposites, which also show a significant tension-compression asymmetry. The constitutive behavior of both materials is predominantly linear elastic, with some more marked nonlinearities arising in the oleyl phosphate-based nanocomposites. The fracture toughness of both types of nanocomposites, ∼0.3 MPa√m, suggests that extrinsic toughening, associated with both material composition and nanostructure, plays an important role. Fractographic observations reveal analogies with shear and cleavage in atomic crystals. The influence of material composition, nanostructure, and processing method on the mechanical behavior of the nanocomposites is analyzed.Publicationtype: Journal ArticleScopus© Citations 8 107 269 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Mapping the mechanical properties of hierarchical supercrystalline ceramic-organic nanocomposites(Multidisciplinary Digital Publishing Institute (MDPI), 2020-10-19); ; ; ; Multiscale ceramic-organic supercrystalline nanocomposites with two levels of hierarchy have been developed via self-assembly with tailored content of the organic phase. These nanocomposites consist of organically functionalized ceramic nanoparticles forming supercrystalline micron-sized grains, which are in turn embedded in an organic-rich matrix. By applying an additional heat treatment step at mild temperatures (250–350 °C), the mechanical properties of the hierarchical nanocomposites are here enhanced. The heat treatment leads to partial removal and crosslinking of the organic phase, minimizing the volume occupied by the nanocomposites’ soft phase and triggering the formation of covalent bonds through the organic ligands interfacing the ceramic nanoparticles. Elastic modulus and hardness up to 45 and 2.5 GPa are attained, while the hierarchical microstructure is preserved. The presence of an organic phase between the supercrystalline grains provides a toughening effect, by curbing indentation-induced cracks. A mapping of the nanocomposites’ mechanical properties reveals the presence of multiple microstructural features and how they evolve with heat treatment temperature. A comparison with non-hierarchical, homogeneous supercrystalline nanocomposites with lower organic content confirms how the hierarchy-inducing organic excess results in toughening, while maintaining the beneficial effects of crosslinking on the materials’ stiffness and hardness.Publicationtype: Journal ArticleScopus© Citations 10 291 331 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Shape-controlling effects of hydrohalic and carboxylic acids in TiO2 nanoparticle synthesisThe ability to synthesize nanoparticles (NPs), here TiO2, of different shapes in a controlled and reproducible way is of high significance for a wide range of fields including catalysis and materials design. Different NP shapes exhibit variations of emerging facets, and processes such as adsorption, diffusion, and catalytic activity are, in general, facet sensitive. Therefore, NP properties, e.g., the reactivity of NPs or the stability of assembled NPs, depend on their shape. We combine computational modeling based on density functional theory with experimental techniques such as transmission electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray powder diffraction to investigate the ability of various adsorbates, including hydrohalic and carboxylic acids, to influence NP shape. This approach allows us to identify mechanisms stabilizing specific surface facets and thus to predict NP shapes using computational model systems and to experimentally characterize the synthesized NPs in detail. Shape-controlled anatase TiO2 NPs are synthesized here in agreement with the calculations in platelet and bi-pyramidal shapes by employing different precursors. The importance of the physical conditions and chemical environment during synthesis, e.g., via competitive adsorption or changes in the chemical potentials, is studied via ab initio thermodynamics, which allows us to set previous and new results in a broader context and to highlight potentials for additional synthesis routes and NP shapes.Publicationtype: Journal ArticleScopus© Citations 7 301 501 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Modulating the mechanical properties of supercrystalline nanocomposite materials via solvent-ligand interactions(American Chemical Society (ACS), 2019-10-03); ; ; ; Supercrystalline nanocomposite materials with micromechanical properties approaching those of nacre or similar structural biomaterials can be produced by self-assembly of organically modified nanoparticles and further strengthened by cross-linking. The strengthening of these nanocomposites is controlled via thermal treatment, which promotes the formation of covalent bonds between interdigitated ligands on the nanoparticle surface. In this work, it is shown how the extent of the mechanical properties enhancement can be controlled by the solvent used during the self-assembly step. We find that the resulting mechanical properties correlate with the Hansen solubility parameters of the solvents and ligands used for the supercrystal assembly: the hardness and elastic modulus decrease as the Hansen solubility parameter of the solvent approaches the Hansen solubility parameter of the ligands that stabilize the nanoparticles. Moreover, it is shown that self-assembled supercrystals that are subsequently uniaxially pressed can deform up to 6 %. The extent of this deformation is also closely related to the solvent used during the self-assembly step. These results indicate that the conformation and arrangement of the organic ligands on the nanoparticle surface not only control the self-assembly itself but also influence the mechanical properties of the resulting supercrystalline material. The Hansen solubility parameters may therefore serve as a tool to predict what solvents and ligands should be used to obtain supercrystalline materials with good mechanical properties.Publicationtype: Journal Article172 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Alumina-doped zirconia submicro-particles : synthesis, thermal stability, and microstructural characterization(Multidisciplinary Digital Publishing Institute (MDPI), 2019-09-05); ; ; ; Zirconia nanoceramics are interesting materials for numerous high-temperature applications. Because their beneficial properties are mainly governed by the crystal and microstructure, it is essential to understand and control these features. The use of co-stabilizing agents in the sol-gel synthesis of zirconia submicro-particles should provide an effective tool for adjusting the particles’ size and shape. Furthermore, alumina-doping is expected to enhance the particles’ size and shape persistence at high temperatures, similar to what is observed in corresponding bulk ceramics. Dispersed alumina should inhibit grain growth by forming diffusion barriers, additionally impeding the martensitic phase transformation in zirconia grains. Here, alumina-doped zirconia particles with sphere-like shape and average diameters of 300 nm were synthesized using a modified sol-gel route employing icosanoic acid and hydroxypropyl cellulose as stabilizing agents. The particles were annealed at temperatures between 800 and 1200 degree Celsius and characterized by electron microscopy, elemental analysis, and X-ray diffraction. Complementary elemental analyses confirmed the precise control over the alumina content (0–50 mol%) in the final product. Annealed alumina-doped particles showed more pronounced shape persistence after annealing at 1000 degree Celsius than undoped particles. Quantitative phase analyses revealed an increased stabilization of the tetragonal/cubic zirconia phase and a reduced grain growth with increasing alumina content. Elemental mapping indicated pronounced alumina segregation near the grain boundaries during annealing.Publicationtype: Journal ArticleScopus© Citations 13 733 510 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Elasticity of cross-linked titania nanocrystal assemblies probed by AFM-bulge tests(Multidisciplinary Digital Publishing Institute (MDPI), 2019-08-29); ; ; ; In order to enable advanced technological applications of nanocrystal composites, e.g., as functional coatings and layers in flexible optics and electronics, it is necessary to understand and control their mechanical properties. The objective of this study was to show how the elasticity of such composites depends on the nanocrystals’ dimensionality. To this end, thin films of titania nanodots (TNDs; diameter: ~3–7 nm), nanorods (TNRs; diameter: ~3.4 nm; length: ~29 nm), and nanoplates (TNPs; thickness: ~6 nm; edge length: ~34 nm) were assembled via layer-by-layer spin-coating. 1,12-dodecanedioic acid (12DAC) was added to cross-link the nanocrystals and to enable regular film deposition. The optical attenuation coefficients of the films were determined by ultraviolet/visible (UV/vis) absorbance measurements, revealing much lower values than those known for titania films prepared via chemical vapor deposition (CVD). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed a homogeneous coverage of the substrates on the µm-scale but a highly disordered arrangement of nanocrystals on the nm-scale. X-ray photoelectron spectroscopy (XPS) analyses confirmed the presence of the 12DAC cross-linker after film fabrication. After transferring the films onto silicon substrates featuring circular apertures (diameter: 32–111 µm), freestanding membranes (thickness: 20–42 nm) were obtained and subjected to atomic force microscopy bulge tests (AFM-bulge tests). These measurements revealed increasing elastic moduli with increasing dimensionality of the nanocrystals, i.e., 2.57 ± 0.18 GPa for the TND films, 5.22 ± 0.39 GPa for the TNR films, and 7.21 ± 1.04 GPa for the TNP films.Publicationtype: Journal ArticleScopus© Citations 6 431 476 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Synthesis and thermal stability of ZrO2@SiO2 core-shell submicron particles(Royal Society of Chemistry (RSC), 2019-08-28); ; ; ; ZrO2@SiO2 core-shell submicron particles are promising candidates for the development of advanced optical materials. Here, submicron zirconia particles were synthesized using a modified sol-gel method and pre-calcined at 400 °C. Silica shells were grown on these particles (average size: ∼270 nm) with well-defined thicknesses (26 to 61 nm) using a seeded-growth Stöber approach. To study the thermal stability of bare ZrO2 cores and ZrO2@SiO2 core-shell particles they were calcined at 450 to 1200 °C. After heat treatments, the particles were characterized by SEM, TEM, STEM, cross-sectional EDX mapping, and XRD. The non-encapsulated, bare ZrO2 particles predominantly transitioned to the tetragonal phase after pre-calcination at 400 °C. Increasing the temperature to 600 °C transformed them to monoclinic. Finally, grain coarsening destroyed the spheroidal particle shape after heating to 800 °C. In striking contrast, SiO2-encapsulation significantly inhibited grain growth and the t → m transition progressed considerably only after heating to 1000 °C, whereupon the particle shape, with a smooth silica shell, remained stable. Particle disintegration was observed after heating to 1200 °C. Thus, ZrO2@SiO2 core-shell particles are suited for high-temperature applications up to ∼1000 °C. Different mechanisms are considered to explain the markedly enhanced stability of ZrO2@SiO2 core-shell particles.Publicationtype: Journal ArticleScopus© Citations 14 280 353