The brownian motion of mesoscopic particles is ubiquitous and usually random. But in systems with periodic asymmetric barriers to movement, directed or 'rectified' motion can arise and may even modulate some biological processes. In man-made devices, brownian ratchets and variants based on optical or quantum effects have been exploited to induce directed motion, and the dependence of the amplitude of motion on particle size has led to the size-dependent separation of biomolecules. Here we demonstrate that the one-dimensional pores of a macroporous silicon membrane, etched to exhibit a periodic asymmetric variation in pore diameter, can act as massively parallel and multiply stacked brownian ratchets that are potentially suitable for large-scale particle separations. We show that applying a periodic pressure profile with a mean value of zero to a basin separated by such a membrane induces a periodic flow of water and suspended particles through the pores, resulting in a net motion of the particles from one side of the membrane to the other without moving the liquid itself. We find that the experimentally observed pressure dependence of the particle transport, including an inversion of the transport direction, agrees with calculations of the transport properties in the type of ratchet devices used here.

Cobalt/polymer multilayer nanotubes (see Figure) are formed by wetting of alumina and silicon membranes by polystyrene containing a metallo–organic precursor, followed by a thermal decomposition process. This article describes the fabrication and magnetic properties of high quality, thin‐walled ferromagnetic nanotubes. They might be very attractive for a broad range of potential applications ranging from biotechnology to magnetic storage devices.

We report on the fabrication and optical characterization of a three-dimensional (3D) photonic crystal on the basis of macroporous silicon. The structure consists of a 2D array of air pores in silicon whose diameter is varied (modulated) periodically with depth. The bandstructure of the resulting 3D hexagonal photonic crystal is calculated and compared with transmission measurements. The described structure allows to adjust the dispersion relation along the pore axis almost independently from the dispersion relation in the plane perpendicular to the pore axis.

Optical Characterization of the Samples: Absorption spectra were recorded in the 200±900 nm range using a Lambda 9000 Perkin-Elmer spectrophotometer. The samples were diluted by a factor of around 100 in DEG; the dilution factors slightly differed for the different samples in order to ensure analysis of solutions with the same concentrations of oxide particles (0.028 mg mL ±1 ). This allows comparison of samples with the same oxide concentration diluted in the same solvent.The UV excitation and emission spectra were measured at room temperature with a broad excitation at 325 nm. The excitation source was a 450 W xenon lamp coupled to an Hitachi Jobin±Yvon monochromator with a 300 groves mm ±1 grating. The detected emission was transferred by an optical fiber, placed at 45 to the sample, and recorded by an air-cooled charge-coupled device camera. All the samples with the same size oxide particles (either oxide colloids, gold/oxide nanocomposites, or gold/oxide mixtures) have been diluted in DEG before characterization to obtain the same concentration of Tb 3+ -doped Gd 2 O 3 in mass of oxide per mL. For the solutions containing oxide particles with a diameter of 3 nm, the concentration of oxide was 2.8 mg mL ±1 , and for those containing particles with a diameter of 8 nm, the concentration of oxide was 18.9 mg mL ±1 . Finally, the emission spectra were systematically corrected from the white luminescence due to the emission of the DEG and the quartz container.

No abstract