INMdok (Leibniz Institute for New Materials)
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Nanotribological properties of van der Waals heterostructures
Get PDFThe 2D materials exhibit excellent tribological properties due to their weak inter-plane interactions, such as the ultra-low friction, which can be further tuned by number of layers, application of electric bias, stacking of different materials into a van der Waals heterostructure, and change of substrate. In this work, the tribological properties of 2D materials were investigated experimentally by means of atomic force microscopy techniques in ultra-high vacuum and theoretically with atomistic simulations. Friction measurements on epitaxial graphene on SiC(0001) show that the ultra-low friction is limited by a normal load threshold, above which friction increases by one order of magnitude. Simulations suggest that, at contact pressures above 10 GPa, the high-friction regime is a result of an intermittent sp3 rehybridization of graphene and the formation of covalent bonds. Friction on the MoS2/graphene heterostructure is dominated by adhesion due to the out-of-plane deformation of the MoS2 layers. Increasing the number of MoS2 layers decreases friction as the flexural compliance decreases. Higher friction was recorded on MoSe2/hBN compared to graphene/hBN heterostructure or pristine hBN. Work on exfoliated materials was facilitated by the application of navigational microstructures.2D Materialien zeigen hervorragende tribologischen Eigenschaften, die mit der schwachen Wechselwirkung zwischen den Lagen erklärt werden können. Die extrem niedrige Reibung kann zusätzlich eingestellt werden durch die Anzahl der Lagen, Anlegen einer elektrischen Spannung, das Stapeln verschiedener 2D Materialien in eine Heterostruktur, oder die Wahl des Substrats. In dieser Arbeit wurden die tribologischen Eigenschaften von 2D Materialien experimentell mit Hilfe der Rasterkraftmikroskopie im Ultrahochvakuum untersucht. Die Ergebnisse werden mit atomistischen Simulationen verglichen. Reibungsmessungen auf epitaktischem Graphen auf SiC(0001) zeigen, dass die extrem niedrige Reibung durch einen Grenzwert in der Auflagekraft begrenzt ist, oberhalb dessen die Reibung um eine Größenordnung ansteigt. Simulationen legen nahe, dass oberhalb eines Kontaktdrucks von 10˜GPa das Auftreten höherer Reibung auf eine zwischenzeitliche sp3 Rehybridisierung und die Bildung kovalenter Bindungen zurückgeführt werden kann. Reibung auf der MoS2/Graphen-Heterostruktur wird von Adhäsion bestimmt, die durch eine Verformung der MoS2-Lagen in Richtung der AFM-Spitze verstärkt wird. Eine Erhöhung der Zahl an MoS2-Lagen verringert die Reibung da die Verbiegungssteifigkeit steigt. MoSe2/hBN zeigt höhere Reibung als Graphen/hBN Heterostrukturen oder hBN. Die Untersuchung der exfolierten 2D-Materialien im Ultrahochvakuum wird erst möglich durch die Anwendung von Mikrostrukturen zur Positionsbestimmung
Functional two-dimensional high-entropy materials
Get PDFMultiple principal element or high-entropy materials have recently been studied in the two-dimensional (2D) materials phase space. These promising classes of materials combine the unique behavior of solid-solution and entropy-stabilized systems with high aspect ratios and atomically thin characteristics of 2D materials. The current experimental space of these materials includes 2D transition metal oxides, carbides/carbonitrides/nitrides (MXenes), dichalcogenides, and hydrotalcites. However, high-entropy 2D materials have the potential to expand into other types, such as 2D metal-organic frameworks, 2D transition metal carbo-chalcogenides, and 2D transition metal borides (MBenes). Here, we discuss the entropy stabilization from bulk to 2D systems, the effects of disordered multi-valent elements on lattice distortion and local electronic structures and elucidate how these local changes influence the catalytic and electrochemical behavior of these 2D high-entropy materials. We also provide a perspective on 2D high-entropy materials research and its challenges and discuss the importance of this emerging field of nanomaterials in designing tunable compositions with unique electronic structures for energy, catalytic, electronic, and structural applications
Flocking of two unfriendly species: The two-species Vicsek model
Get PDFWe consider the two-species Vicsek model (TSVM) consisting of two kinds of self-propelled particles, A and B, that tend to align with particles from the same species and to antialign with the other. The model shows a flocking transition that is reminiscent of the original Vicsek model: it has a liquid-gas phase transition and displays micro-phase-separation in the coexistence region where multiple dense liquid bands propagate in a gaseous background. The interesting features of the TSVM are the existence of two kinds of bands, one composed of mainly A particles and one mainly of B particles, the appearance of two dynamical states in the coexistence region: the PF (parallel flocking) state in which all bands of the two species propagate in the same direction, and the APF (antiparallel flocking) state in which the bands of species A and species B move in opposite directions. When PF and APF states exist in the low-density part of the coexistence region they perform stochastic transitions from one to the other. The system size dependence of the transition frequency and dwell times show a pronounced crossover that is determined by the ratio of the band width and the longitudinal system size. Our work paves the way for studying multispecies flocking models with heterogeneous alignment interactions
Formation of intermittent covalent bonds at high contact pressure limits superlow friction on epitaxial graphene
Get PDFEpitaxial graphene on SiC(0001) exhibits superlow friction due to its weak out-of-plane interactions. Friction-force microscopy with silicon tips shows an abrupt increase of friction by one order of magnitude above a threshold normal force. Density-functional tight-binding simulations suggest that this wearless high-friction regime involves an intermittent sp3 rehybridization of graphene at contact pressure exceeding 10 GPa. The simultaneous formation of covalent bonds with the tip's silica surface and the underlying SiC interface layer establishes a third mechanism limiting the superlow friction on epitaxial graphene, in addition to dissipation in elastic instabilities and in wear processes
Functional and safe encapsulation of Escherichia coli in Pluronic hydrogels for engineered living materials
Get PDFBacterial growth and metabolic activity are sensitive to the mechanical properties of their environment. Understanding how the 3D spatial confinement regulates the cell behavior is crucial not only for understanding biofilm development but also for the design and safe application of engineered materials containing living cells. This Thesis explores the use of Pluronic-based hydrogels to encapsulate genetically modified Escherichia coli bacteria. Hydrogels with different viscoelastic properties were prepared by mixing Pluronic and Pluronic diacrylate components in different ratios, giving physical hydrogels with variable degree of covalent crosslinking and different mechanical responses. Rheological properties of the hydrogels as well as the growth rate and morphology of the embedded bacterial colonies were characterized. The results provided correlations between material parameters and bacterial cell responses. Further, a bilayer thin film model was developed for long term encapsulation of the organisms, preventing leakage of cells for up to two weeks while maintaining their activity as drug/protein eluting devices or biosensing units. The bacterial bilayer thin films did not elicit significant immune responses in primary immune cells from healthy donors. The results of this Thesis demonstrate the potential of Pluronic-based biohybrid as a suitable and safe prototype for further in vitro and in vivo testing of engineered living material designs.Wachstum und Stoffwechselaktivität von Bakterien sind sensitiv gegenüber den mechanischen Eigenschaften ihrer Umgebung. Das Verständnis, wie der räumliche 3D-Einschluss das Zellverhalten reguliert, ist sowohl für die Entwicklung von Biofilmen als auch für das Design und die sichere Anwendung von technischen Materialien, die lebende Zellen enthalten, essenziell. Diese Thesis untersucht die Verwendung von Hydrogelen auf Pluronic-Basis zur Verkapselung von genetisch veränderten Escherichia coli Bakterien. Durch die Mischung von Pluronic und Pluronic-Diacrylat in verschiedenen Verhältnissen wurden physische Hydrogele mit unterschiedlichem kovalenten Vernetzungsgrad und viskoelastischen Eigenschaften hergestellt. Die Charakterisierung der rheologischen Eigenschaften der Hydrogele sowie der Wachstumsrate und Morphologie der eingebetteten Bakterien zeigte eine Korrelation zwischen den Materialparametern und dem Zellverhalten. Darüber hinaus wurde ein Doppelschicht-Dünnfilmmodell entwickelt, in dem die Organismen bis zu zwei Wochen ohne Austreten eingeschlossen wurden, während gleichzeitig Medikamenten-, Proteinfreisetzung oder die Aktivität der Zellen als Biosensoren beibehalten wird. Das Modell löste bei primären Immunzellen von gesunden Spendern keine signifikanten Immunreaktionen aus. Diese Thesis zeigt das Potenzial von Biohybriden auf Pluronic-Basis als geeigneten und sicheren Prototyp für weitere in vitro und in vivo Tests von technischen lebenden Materialien
Surfactant stabilization of vanadium iron oxide derived from Prussian blue analog for lithium-ion battery electrodes
Get PDFDue to their high energy density, Li-ion batteries have become indispensable for energy storage in many technical devices. Prussian blue and its analogs are a versatile family of materials. Apart from their direct use as an alkali-ion battery electrode, they are a promising source for templating other compounds due to the presence of carbon, nitrogen, and metallic elements in their structure, ease of synthesis, and high tunability. In this study, homogeneous iron vanadate derivatization from iron vanadium Prussian blue was successfully carried out using an energy efficient infrared furnace utilizing CO2 gas. Iron-vanadate is an inherently unstable electrode material if cycled at low potentials vs. Li/Li+. Several parameters were optimized to achieve a stable electrochemical performance of this derivative, and the effect of surfactants, such as tannic acid, sodium dodecylbenzene sulfonate, and polyvinylpyrrolidone were shown with their role in the morphology and electrochemical performance. While stabilizing the performance, we demonstrate that the type and order of addition of these surfactants are fundamental for a successful coating formation, otherwise they can hinder the formation of PBA, which has not been reported previously. Step-by-step, we illustrate how to prepare self-standing electrodes for Li-ion battery cells without using an organic solvent or a fluorine-containing binder while stabilizing the electrochemical performance. A 400 mA h g−1 capacity at the specific current of 250 mA g−1 was achieved after 150 cycles while maintaining a Coulombic efficiency of 99.2% over an extended potential range of 0.01–3.50 V vs. Li/Li+
Room temperature ionic liquids with two symmetric ions
Get PDFRoom temperature ionic liquids typically contain asymmetric organic cations. The asymmetry is thought to enhance disorder, thereby providing an entropic counter-balance to the strong, enthalpic, ionic interactions, and leading, therefore, to lower melting points. Unfortunately, the synthesis and purification of such asymmetric cations is typically more demanding. Here we introduce novel room temperature ionic liquids in which both cation and anion are formally symmetric. The chemical basis for this unprecedented behaviour is the incorporation of ether-containing side chains – which increase the configurational entropy – in the cation. Molecular dynamics simulations indicate that the ether-containing side chains transiently sample curled configurations. Our results contradict the long-standing paradigm that at least one asymmetric ion is required for ionic liquids to be molten at room temperature, and hence open up new and simpler design pathways for these remarkable materials
On the afferrante-carbone theory of ultratough tape peeling
Get PDFIn a simple and interesting theory of ultratough peeling of an elastic tape from a viscoelastic substrate, Afferrante and Carbone find that there are conditions for which the load for steady state peeling could be arbitrarily large in steady state peeling, at low angles of peeling - what they call "ultratough" peeling (Afferrante, L., Carbone, G., 2016, The ultratough peeling of elastic tapes from viscoelastic substrates, Journal of the Mechanics and Physics of Solids, 96, pp.223-234). Surprisingly, this seems to lead to toughness enhancement higher than the limit value observed in a very large crack in an infinite viscoelastic body, possibly even considering a limit on the stress transmitted. The Afferrante-Carbone theory seems to be a quite approximate, qualitative theory and many aspects and features of this "ultratough" peeling (e.g. conformity with the Rivlin result at low peel angles) are obtained also through other mechanisms (Begley, M.R., Collino, R.R., Israelachvili, J.N., McMeeking, R.M., 2013, Peeling of a tape with large deformations and frictional sliding, Journal of the Mechanics and Physics of Solids, 61(5), pp. 1265-1279) although not at “critical velocities”. Experimental and/or numerical verification would be most useful
In vitro assembly of plasmid DNA for direct cloning in Lactiplantibacillus plantarum WCSF1
Get PDFLactobacilli are gram-positive bacteria that are growing in importance for the healthcare industry and genetically engineering them as living therapeutics is highly sought after. However, progress in this field is hindered since most strains are difficult to genetically manipulate, partly due to their complex and thick cell walls limiting our capability to transform them with exogenous DNA. To overcome this, large amounts of DNA (>1 µg) are normally required to successfully transform these bacteria. An intermediate host, like E. coli, is often used to amplify recombinant DNA to such amounts although this approach poses unwanted drawbacks such as an increase in plasmid size, different methylation patterns and the limitation of introducing only genes compatible with the intermediate host. In this work, we have developed a direct cloning method based on in-vitro assembly and PCR amplification to yield recombinant DNA in significant quantities for successful transformation in L. plantarum WCFS1. The advantage of this method is demonstrated in terms of shorter experimental duration and the possibility to introduce a gene incompatible with E. coli into L. plantarum WCFS1
Degradation analysis of tribologically loaded carbon nanotubes and carbon onions
Get PDFCoating laser-patterned stainless-steel surfaces with carbon nanotubes (CNT) or carbon onions (CO) forms a tribological system that provides effective solid lubrication. Lubricant retention represents the fundamental mechanism of this system, as storing the particles inside the pattern prevents lubricant depletion in the contact area. In previous works, we used direct laser interference patterning to create line patterns with three different structural depths on AISI 304 stainless-steel platelets. Electrophoretic deposition subsequently coated the patterned surfaces with either CNTs or COs. Ball-on-disc friction tests were conducted to study the effect of structural depth on the solid lubricity of as-described surfaces. The results demonstrated that the shallower the textures, the lower the coefficient of friction, regardless of the applied particle type. This follow-up study examines the carbon nanoparticles’ structural degradation after friction testing on substrates patterned with different structural depths (0.24, 0.36, and 0.77 µm). Raman characterization shows severe degradation of both particle types and is used to classify their degradation state within Ferrari’s three-stage amorphization model. It was further shown that improving CNT lubricity translates into increasing particle defectivity. This is confirmed by electron microscopy, which shows decreasing crystalline domains. Compared to CNTs, CO-derived tribofilms show even more substantial structural degradation