In-situ nanodiamond to carbon onion transformation in metal matrix composites

In the present study, nickel matrix composites reinforced with a fine distribution of nanodiamonds (6.5 vol%) as reinforcement phase are annealed in vacuum at different temperatures ranging from 750 °C to 1300 °C. This is carried out to evaluate the in-situ transformation of nanodiamonds to carbon onions within a previously densified composite. The resulting materials are thoroughly analyzed by complementary analytical methods, including Raman spectroscopy, transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The proposed in-situ transformation method presents two main benefits. On one hand, since the particle distribution of a nanodiamond-reinforced composite is significantly more homogenous than in case of the carbon onions, it is expected that the transformed particles will preserve the initial distribution features of nanodiamonds. On the other hand, the proposed process allows for the tuning of the sp3/sp2 carbon ratio by applying a single straightforward post-processing step

Impact of geometry on chemical analysis exemplified for photoelectron spectroscopy of black silicon

For a smooth surface, the chemical composition can be readily evaluated by a variety of spectroscopy techniques; a prominent example is X-ray photoelectron spectroscopy (XPS), where the relative proportions of the elements are mainly determined by the intensity ratio of the element-specific photoelectrons. This deduction, however, is more intricate for a nanorough surface, such as black silicon, since the steep slopes of the geometry mimic local variations of the local emission angle. Here, we explicitly quantify this effect via an integral geometric analysis, by using so-called Minkowski tensors. Thus, we match the chemical information from XPS with topographical information from atomic force microscopy (AFM). Our method provides reliable estimates of layer thicknesses for nanorough surfaces. For our black silicon samples, we found that the oxide layer thickness is on average comparable to that of a native oxide layer. Our study highlights the impact of complex geometries at the nanoscale on the analysis of chemical properties with implications for a broad class of spectroscopy techniques

Is adhesion superficial? Silicon wafers as a model system to study van der Waals interactions

Adhesion is a key issue for researchers of various fields, it is therefore of uppermost importance to understand the parameters that are involved. Commonly, only surface parameters are employed to determine the adhesive forces between materials. Yet, van der Waals forces act not only between atoms in the vicinity of the surface, but also between atoms in the bulk material. In this review, we describe the principles of van der Waals interactions and outline experimental and theoretical studies investigating the influence of the subsurface material on adhesion. In addition, we present a collection of data indicating that silicon wafers with native oxide layers are a good model substrate to study van der Waals interactions with coated materials

Graphene and graphene-like layers: preparation and growth and modification

Seit dem ersten experimentellen Nachweis von Graphen im Jahre 2004 wurde dieses Material intensiv erforscht. Dabei wurden auch mögliche Anwendungs-Szenarien ausgelotet mit einer sehr großen Bandbreite von Reibungsminimierung bis hin zu elektronischen Bauteilen. Für diese Anwendungen sind zuverlässige und kostengünstige Syntheseverfahren notwendig. Im Rahmen dieser Arbeit wurde daher eine alternative Synthesemethode – die Flüssigphasenabscheidung LPD – etabliert, mit der ein Graphenwachstum auf unterschiedlichsten Metalloberflächen nachgewiesen wurde. Für schwach wechselwirkende Systeme, hier Ag(001), ist diese Methode im Gegensatz zum Standardverfahren zur Graphensynthese, dem CVD-Prozess, ebenfalls erfolgreich. Dabei liegt ein isotroper Wachstumsprozess vor, wobei solche Oberflächen insbesondere für Transferprozesse auf Isolatoren von Interesse sind. Zusätzlich wurde ein weiteres 2D-Material, Boronitren, untersucht, das ebenfalls eine interessante Rolle für die obigen Anwendungen einnimmt, insbesondere aber im Hinblick auf Stapelfolgen von 2D-Materialien, den sogenannten Van der Waals Heterostrukturen. Hier wurde das isotrope Wachstum von Boronitren auf Ag(001) nachgewiesen. Statt nur einer Funktionalisierung der Metalloberfläche durch Graphen, um wie oben vorgeschlagen eine Reduktion der Reibung auch durch Boronitren zu erreichen, wurde im Rahmen dieser Arbeit ein Rh-Film annähernd vollständig boriert, um dessen mechanischen Abrieb zu reduzieren.The experimental proof of freestanding graphene in the year 2004 has lead to an ongoing research on this topic. Several applications of this material are under consideration, with a wide scope, leading e.g. from reduction of friction to electronic devices. However, for mass production and related applications reliable and cost-efficient preparation methods are needed. In this thesis, an alternative preparation method has been established: the liquid precursor deposition LPD. This has been accomplished by performing this method on several different metal surfaces and proofing the formation of a graphene layer. A huge advantage of this method compared to the standard preparation method for graphene, the CVD process, is that it is also successful on weak interacting systems, here Ag(001), where the CVD process more or less fails. On these weakly interacting surfaces the graphene layer grows isotropic and these surfaces are of particular interest for transferring the graphene layer in a next step onto an isolator substrate. Furthermore another 2D-material has been investigated: boronitrene. It is also of interest for the application listed above, especially for the engineering of the so-called van der Waals heterostructures. In this thesis, the isotropic growth of boronitrene on Ag(001) has been proved. Besides functionalizing a metal surface by a graphene or boronitrene layer, a complete boration of a Rh-layer could be achieved to reduce the mechanical wear

Time Dependence of Fluoride Uptake in Hydroxyapatite

Fluoridation of enamel is believed to provide an effective tool to protect teeth from caries, but there is still little information on the time scale of fluoride uptake. In this study, highly compressed pellets of hydroxyapatite are used as first-order model systems to approximate the mineral component of natural enamel for investigations on the time-dependence of fluoride uptake. We found that both the overall amount of fluoride as well as the mean thickness of the fluoridated surface layer cannot be extended to any values just by increasing the application time of a fluoride containing agent. Instead, both parameters start to become constant on a time scale of about 3 min. The present results as obtained on a synthetic model “tooth” show that the time scale to provide the maximum amount of fluoride possible is of the same order of magnitude as that in usual daily practice in dental care when applying toothpastes or mouth rinses

Single-Molecule Force Spectroscopy Study on Modular Resilin Fusion Protein

The adhesive and mechanical properties of a modular fusion protein consisting of two different types of binding units linked together via a flexible resilin-like-polypeptide domain are quantified. The adhesive domains have been constructed from fungal cellulose-binding modules (CBMs) and an amphiphilic hydrophobin HFBI. This study is carried out by single-molecule force spectroscopy, which enables stretching of single molecules. The fusion proteins are designed to self-assemble on the cellulose surface, leading into the submonolayer of proteins having the HFBI pointing away from the surface. A hydrophobic atomic force microscopy (AFM) tip can be employed for contacting and lifting the single fusion protein from the HFBI-functionalized terminus by the hydrophobic interaction between the tip surface and the hydrophobic patch of the HFBI. The work of rupture, contour length at rupture and the adhesion forces of the amphiphilic end domains are evaluated under aqueous environment at different pHs.Peer reviewe