The influence of the synthesis parameters on the mean characteristics of single-wall carbon nanotubes in soot produced by the laser vaporization of graphite has been analyzed using optical absorption spectroscopy. The abundance and mean diameter of the nanotubes were found to be most influenced by the furnace temperature and the cobalt/nickel catalyst mixing ratio. Via an analysis of the fine structure in the optical spectra, the existence of preferred nanotube diameters has been established and their related fractional abundance could be determined. The results are consistent with nanotubes located mainly around the armchair axis.

No abstract

Four rate-limiting processes for the formation of single-wall carbon nanotubes (SWCNT) could be identified by varying furnace temperature, gas type, and pressure in a pulsed-laser evaporation setup. One rate-limiting process accounts essentially for all relevant gas-pressure dependencies and can be quantitatively described using a single gas-specific constant. One thermally activated process is related to fullerene formation, whereas another process, following a T 2 -law, is discussed in terms of the diffusion of carbon through molten catalyst nanoparticles. The data provide strong support for an "undercooled melt" mechanism of nanotube formation.

No abstract

No abstract

A new technique of laser direct writing of nanoscale structures is proposed. The technique is based on a local intensity enhancement of optical radiation near a tip of a scanning tunneling microscope and allows to overcome the fundamental spot limitation (about a half of the wavelength λ) of the conventional lens‐focused beams. In the first experimental results unconventional tips of silver are used in order to increase the effect. By using a laser with λ = 800 nm features of 10 to 40 nm in diameter are reversibly written on the surface of gold films. Theoretical possibilities and mechanisms of the near‐field enhancement of optical radiation by very small conductive objects are briefly summarized.

To test nanosize surface patterning for application as implant material, a suitable titanium composition has to be found first. Therefore we investigated the effect of surface chemistry on attachment and differentiation of osteoblast-like cells on pure titanium prepared by pulsed laser deposition (TiPLD) and different Ti alloys (Ti6Al4V, TiNb30 and TiNb13Zr13). Early attachment (30 min) and alkaline phosphatase (ALP) activity (day 5) was found to be fastest and highest, respectively, in cells grown on TiPLD and Ti6Al4V. Osteoblasts seeded on TiPLD produced most osteopontin (day 10), whereas expression of this extracellular matrix protein was an order of magnitude lower on the TiNb30 surface. In contrast, expression of the corresponding receptor, CD44, was not influenced by surface chemistry. Thus, TiPLD was used for further experiments to explore the influence of surface nanostructures on osteoblast adhesion, differentiation and orientation. By laser-induced oxidation, we produced patterns of parallel Ti oxide lines with different widths (0.2–10 µm) and distances (2–20 and 1,000 µm), but a common height of only 12 nm. These structures did not influence ALP activity (days 5–9), but had a positive effect on cell alignment. Two days after plating, the majority of the focal contacts were placed on the oxide lines. The portion of larger focal adhesions bridging two lines was inversely related to the line distance (2–20 µm). In contrast, the portion of aligned cells did not depend on the line distance. On average, 43% of the cells orientated parallel towards the lines, whereas 34% orientated vertically. In the control pattern (1,000 µm line distance), cell distribution was completely at random. Because a significant surplus of the cells preferred a parallel alignment, the nanosize difference in height between Ti surface and oxide lines may be sufficient to orientate the cells by contact guiding. However, gradients in electrostatic potential and surface charge density at the Ti/Ti oxide interface may additionally influence focal contact formation and cell guidance.

Two different single-walled carbon nanotube (SWNT) growth modes (cap growth mode and circumference growth mode) are shown to exist. General SWNT diameter windows are derivable from catalyst particle size considerations. In addition, an almost complete picture of nanotube diameter dependencies for the cap growth mode is drawn from experiment. The nanotube diameter always scales linear with temperature, but the degree of dependence varies with the catalyst element. The nanotube diameter scales logarithmically with the gas pressure and catalyst composition. Very few or exactly one atom of a catalyst additive is sufficient to induce SWNT diameter changes. The experimental data allow the conclusion that the observed nanotube diameter is based on materials properties of sp2-bonded carbon/graphene sheets, on individual properties of the catalyst elements, and on additional kinetic components from temperature and pressure changes. Indications are found for a specific and maybe decisive role of adsorbate atoms at the surface of a catalyst particle on the nanotube diameter and therefore on the process of nanotube nucleation and growth.