Metallic, magnetic and molecular nanocontacts

Scanning tunnelling microscopy and break-junction experiments realize metallic and molecular nanocontacts that act as ideal one-dimensional channels between macroscopic electrodes. Emergent nanoscale phenomena typical of these systems encompass structural, mechanical, electronic, transport, and magnetic properties. This Review focuses on the theoretical explanation of some of these properties obtained with the help of first-principles methods. By tracing parallel theoretical and experimental developments from the discovery of nanowire formation and conductance quantization in gold nanowires to recent observations of emergent magnetism and Kondo correlations, we exemplify the main concepts and ingredients needed to bring together ab initio calculations and physical observations. It can be anticipated that diode, sensor, spin-valve and spin-filter functionalities relevant for spintronics and molecular electronics applications will benefit from the physical understanding thus obtained

Novel approach to identifying supersaturated metastable ambient aerosol particles.

Atomic force microscopy (AFM) was used to determine the shape of fine and ultrafine ambient aerosol particles with sizes between 25 and 700 nm after soft landing on a solid substrate. The particles were collected in summer during daytime at a relative humidity around 50%. To avoid kinetically induced deformation, as previously observed using high-velocity sampling in impactors, the particles were collected on pore filters at very low face velocities (on the order of 10 cm/s). The shape of the collected particles was quantified in terms of their height and apparent diameter. The amount of broadening introduced by the pyramidal shape of the nonideally sharp AFM tips was calibrated using Latex reference spheres with a range of diameters. The height-to-diameter ratios, H/D, of the collected aerosol particles could be extracted from the measured data. Specified in terms of volume-equivalent (dry) diameters, Dv, the size selected frequency distributions of the H/Dv-ratios were found to be bimodal. A small mode centered at H/Dv = 1.0 +/- 0.1 is attributed to nonhygroscopic particles that retained their shape after deposition on the substrate. The large mode, with a peak at H/Dv = 0.65 +/- 0.05, reflects soft particles which were strongly deformed due to vertical collapse after deposition. The pronounced deformation suggests that these particles had previously experienced deliquescence and, when collected at a comparatively low humidity, were in a metastable, supersaturated aqueous state. After landing and indoor sample storage the water evaporated, resulting in minimum H/Dv-ratios as low as 0.45. The dried metastable fraction amounted to 81 +/- 12% in the size range 150 < Dv < 700 nm, and 79 +/- 10% for 50 < or = Dv < or = 150 nm, but only 26 +/- 10% for Dv < 50 nm. Comparison with recently reported data suggests that the observed metastable fraction is the same as the hygroscopic fraction identified by other means. The interpretation is further substantiated by a comparison of the size distributions of collected and airborne particles

Chem. Mat.

We collected test reactions which can be applied in an easy and reproducible way to the chemical composition of the abalone organic matrix. Several chemical and biochemical test reactions were applied to the interlamellar organic matrix of abalone nacre to study the content and nature of polysaccharides. The preparation of the polysaccharide matrix was examined in parallel by light microscopy. The polysaccharide core is covered by a honeycomb-like structure, which can be completely removed by the protease subtilisin under release of the hydrophobic amino acid proline. The honeycomb pattern is in its size and its shape exactly the inverse matrix of the aragonite tablets, which are well-known to build up the nacreous layer of abalone shells. With this protocol we proved and verified in a straightforward and simple way the polysaccharides in abalone to be chitin. In addition, H-1 and C-13 NMR analysis of the interlamellar organic matrix of abalone nacre confirmed that it consists to a high extent of the polysaccharide chitosan or its partially/completely N-acetylated derivative chitin (beta-(1-- >4)-2-acetamido-2-deOXY-D-glucose or N-acetyl-D-glucosamine)

Micropipet-based pico force transducer: in depth analysis and experimental verification.

Measurements of forces in the piconewton range are very important for the study of molecular adhesion and mechanics. Recently, a micropipet-based force transducer for this type of experiment was presented (E. Evans, K. Ritchie, and R. Merkel, 1995, Biophys. J., 68:2580-2587). In the present article we give a detailed mechanical analysis of this transducer, including nonlinear effects. An analytical expression for the transducer stiffness at small elongations is given. Using magnetic tweezers (F. Ziemann, J. Rädler, and E. Sackmann, 1994, Biophys. J., 66:2210-2216), we were able to determine the force displacement relation of this transducer experimentally. Forces from approximately 10 pN to 500 pN were applied. Theoretical predictions and experimental results coincide remarkably well