Supramolecular networks stabilise and functionalise black phosphorus

The limited stability of the surface of black phosphorus (BP) under atmospheric conditions is a significant constraint on the exploitation of this layered material and its few layer analogue, phosphorene, as an optoelectronic material. Here we show that supramolecular networks stabilised by hydrogen bonding can be formed on BP, and that these monolayer-thick films can passivate the BP surface and inhibit oxidation under ambient conditions. The supramolecular layers are formed by solution deposition and we use atomic force microscopy to obtain images of the BP surface and hexagonal supramolecular networks of trimesic acid and melamine cyanurate (CA.M) under ambient conditions. The CA.M network is aligned with rows of phosphorus atoms and forms large domains which passivate the BP surface for more than a month, and also provides a stable supramolecular platform for the sequential deposition of 1,2,4,5-tetrakis(4-carboxyphenyl)benzene to form supramolecular heterostructures

Ecology impacts the decrease of Spirochaetes and Prevotella in the fecal gut microbiota of urban humans

Compared to the huge microbial diversity in most mammals, human gut microbiomes have lost diversity while becoming specialized for animal-based diets - especially compared to chimps, their genetically closest ancestors. The lowered microbial diversity within the gut of westernized populations has also been associated with different kinds of chronic inflammatory diseases in humans. To further deepen our knowledge on phylogenetic and ecologic impacts on human health and fitness, we established the herein presented biobank as well as its comprehensive microbiota analysis. In total, 368 stool samples from 38 different animal species, including Homo sapiens, belonging to four diverse mammalian orders were collected at seven different locations and analyzed by 16S rRNA gene amplicon sequencing. Comprehensive data analysis was performed to (i) determine the overall impact of host phylogeny vs. diet, location, and ecology and to (ii) examine the general pattern of fecal bacterial diversity across captive mammals and humans.By using a controlled study design with captive mammals we could verify that host phylogeny is the most dominant driver of mammalian gut microbiota composition. However, the effect of ecology appears to be able to overcome host phylogeny and should therefore be studied in more detail in future studies. Most importantly, our study could observe a remarkable decrease of Spirochaetes and Prevotella in westernized humans and platyrrhines, which is probably not only due to diet, but also to the social behavior and structure in these communities.Our study highlights the importance of phylogenetic relationship and ecology within the evolution of mammalian fecal microbiota composition. Particularly, the observed decrease of Spirochaetes and Prevotella in westernized communities might be associated to lifestyle dependent rapid evolutionary changes, potentially involved in the establishment of dysbiotic microbiomes, which promote the etiology of chronic diseases

Substrate Templating upon Self-Assembly of Hydrogen-Bonded Molecular Networks on an Insulating Surface

Rahe P, Nimmrich M, Kühnle A. Substrate Templating upon Self-Assembly of Hydrogen-Bonded Molecular Networks on an Insulating Surface. Small. 2012;8(19):2969-2977.Molecular self-assembly on insulating surfaces, despite being highly relvant to many applications, generally suffers from the weak moleculesurface interactions present on dielectric surfaces, especially when benchmarked against metallic substrates. Therefore, to fully exploit the potential of molecular self-assembly, increasing the influence of the substrate constitutes an essential prerequisite. Upon deposition of terephthalic acid and trimesic acid onto the natural cleavage plane of calcite, extended hydrogen-bonded networks are formed, which wet the substrate. The observed structural complexity matches the variety realized on metal surfaces. A detailed analysis of the molecular structures observed on calcite reveals a significant influence of the underlying substrate, clearly indicating a substantial templating effect of the surface on the resulting molecular networks. This work demonstrates that choosing suitable molecule/substrate systems allows for tuning the balance between intermolecular and moleculesurface interactions even in the case of typically weakly interacting insulating surfaces. This study, thus, provides a strategy for deliberately exploiting substrate templating to increase the structural variety in molecular self-assembly on a bulk insulator at room temperature

Micrometre-long covalent organic fibres by photoinitiated chain-growth radical polymerization on an alkali-halide surface

On-surface polymerization is a promising technique to prepare organic functional nanomaterials that are challenging to synthesize in solution, but it is typically used on metal substrates, which play a catalytic role. Previous examples on insulating surfaces have involved intermediate self-assembled structures, which face high barriers to diffusion, or annealing to higher temperatures, which generally causes rapid dewetting and desorption of the monomers. Here we report the photoinitiated radical polymerization, initiated from a two-dimensional gas phase, of a dimaleimide monomer on an insulating KCl surface. Polymer fibres up to 1 μm long are formed through chain-like rather than step-like growth. Interactions between potassium cations and the dimaleimide’s oxygen atoms facilitate the propagation of the polymer fibres along a preferred axis of the substrate over long distances. Density functional theory calculations, non-contact atomic force microscopy imaging and manipulations at room temperature were used to explore the initiation and propagation processes, as well as the structure and stability of the resulting one-dimensional polymer fibres

Absolute step and kink formation energies of Pb derived from step roughening of two-dimensional islands and facets

The shapes of (111) oriented two-dimensional (2D) islands and facets, the latter being part of three-dimensional (3D) crystallites of Pb, were equilibrated at 104-520 K. Island sizes were in the range of 15-90 nm radius, facets typically at 100-270 nm radius. They were imaged by scanning tunneling microscopy to provide the exact outline of the bounding step. Increased step roughening with increasing temperature decreases the radius anisotropy of islands and facets in a consistent manner. Products of island/facet radius times local step curvature versus temperature were obtained experimentally, serving as the basis of absolute step and kink energies at 0 K. They are f(1A)(0)=128.3+/-0.3 meV, f(1B)(0)=115.7+/-5.8 meV, and epsilon(kA)=42.5+/-1.0 meV, epsilon(kB)=60.6+/-1.6 meV, respectively. The combination of studying small 2D islands (unstable at high temperature) and large 2D facets allows measurements over a very large range of temperatures

RTM Untersuchungen an zweidimensionalen Nanostrukturen am Beispiel der reinen Au(100)-Oberfläche und des selbstassemblierten Systems Sauerstoff auf Cu(110)

In this STM study, nanostructures on metallic single crystals are observed under UHV conditions. The aim of the work at the Au(100)-surface is to evaluate an energetic quantity, namely the step free energy, from a topographic structure information. Adatom and vacancy islands are produced on the tempered reconstructed Au(100)-surface by evaporation of submonolayer quantities of gold and sputtering with Argon ions, respectively. As noted before, these islands have a rectangular shape with the long side oriented along the parallel reconstruction lines. The initial (aspect) ratio of the long to the short side is about 10. The decay of these islands is correlated with a decrease of the aspect ratio to approximately 1. After that the aspect ratio varies merely stochastically around this mean value. We could observe adatom islands at T = 353 K, which neither grow nor decay over a long period of time and whose aspect ratio varies stochastically around the mean value of 1. In conclusion, we assume that these islands are in a thermodynamic equilibrium situation or at least close to it. By averaging over these island shapes, we obtain the equilibrium shape for this temperature. From island shape fluctuations we make a quantitative determination of the mean step free energy to 170 meV per atom. In completely separated experiments from the described ones above, the formation of the so-called (2x1) oxygen stripe phase on Cu(110) is imaged in situ, and simultaneously a quantitative determination of the induced surface stress is accomplished. The bending bar technique is utilized with the bending measured by the STM. Due to the anisotropy of the Cu(110) surface, two experiments, one with the [1(-1)0]- and a second one with the [001]-direction along the bar have to be carried out for a quantitative analysis of the induced surface stress. From the two experiments we calculate the induced stresses in both directions. We find that the difference of the surface stresses of the (2x1)O phase on Cu(110) and the clean Cu(110) surface is compressive for both directions, in accordance with the adsorption of oxygen on other surfaces. The stress depends significantly on the coverage. The data can be fitted with an exponential function. Furthermore the fit for the stress difference orthogonal to the stripes is used for a quantitative analysis of the periodicity of the oxygen stripes