M. Ahlskog

Ring formations from catalytically synthesized carbon nanotubes

Abstract Rings of typically 0.5 μm diameter have been observed with the atomic force microscope and the scanning electron microscope in carbon nanotube deposits produced catalytically by thermal decomposition of hydrocarbon gas. The carbon nanotubes are predominantly multiwalled with an average thickness of 20 nm, which is the typical thickness of the rings as well. The observations are discussed in light of the previous discoveries of helical nanotubes in the same material and the observation of rings of similar thickness and diameter (interpreted as toroids) in single wall carbon nanotube material grown by laser vaporization. In our observations, the ring formations can be interpreted as single turn coils with a short overlap between the beginning and end of the coiled nanotube, but the toroidal interpretation cannot be ruled out.

Field induced local oxidation of Ti and Ti/Au structures by an atomic force microscope with diamond coated tips

We have investigated field induced local oxidation of thin Ti films with the tip of an atomic force microscope. Tips, which have been coated with a diamond layer to improve their wear resistance, are shown to have a much longer lifetime than conventional uncoated Si tips. We have studied the oxidation characteristics as a function of the applied tip-sample voltage and scanning speed for both diamond coated and uncoated tips. We find that the diamond coated tips result in a thinner oxide layer for the same voltage and scanning speed. The dependence of the oxidation process on the film thickness was studied for diamond coated tips. Thin films can be completely transformed into an oxide layer for a thickness up to 15 nm. Moreover, for these sufficiently thin films the measured ratio between the oxide height and the Ti film thickness is a constant. It is also possible to completely oxidize Ti films which cover Au islands, opening the way to fabricate more complicated structures.

A microdeposition technique for carbon nanotubes based on electron beam lithography

We describe a deposition technique for either individual or small amounts of multiwalled carbon nanotubes which is based on electron beam lithography. Carbon nanotubes are deposited from the solution phase onto a substrate through lithographically determined openings in an electron beam resist layer. The openings vary in size from a few μm to arbitrary size. It is possible to have one or a few nanotubes in the smaller openings. Using alignment markers, the technique has been applied to deposit individual nanotubes on top of gold electrodes.

Titanium nanostructures made by local oxidation with the atomic force microscope

Abstract Surfaces can be oxidized under ambient conditions with the tip of a scanning probe microscope when applying a voltage between surface and tip. The oxidation process is voltage- and humidity-dependent, and can be explained in terms of anodic oxidation. Sufficiently thin metal films (e.g., Ti or Al) can be completely oxidized down to the substrate, enabling the direct writing of nanostructures. We report on the fabrication and electrical measurements of Ti lines and Ti/TiOx/Ti junctions defined in titanium thin films having a thickness smaller than 10 nm.

Mesoscopic electronic devices made by local oxidation of a titanium film covering gold islands

The local oxidation produced by the tip of an atomic force microscope scanning on a thin metallic film allows to define narrow oxide lines, thus providing a method to fabricate lateral tunnel junctions. In such devices, with rather thick tunnel junction barriers, the electrical transport is governed by thermally activated hopping rather than by direct electron tunneling. In this letter we show that tunneling barriers can also be produced with Ti films covering small gold islands. The gold islands significantly shorten the effective tunneling distance, allowing to observe temperature-independent electron tunneling across the lateral barriers. The mixed Ti/Au tunnel barriers reveal Coulomb blockade effects which may be used for single-electron devices consisting of a single oxide line.

The localization-interaction model applied to the direct-current conductivity of metallic conducting polymers

The low-temperature DC-transport properties of doped conducting polymers on the metallic side of the metal-insulator transition are analysed with modern localization-interaction theories. The DC conductivity at the lowest temperatures is governed by the 3D electron-electron interaction formula , while the magnetoconductance is an interplay between electron-electron interaction and weak-localization contributions. The transition from negative to positive temperature coefficient of resistivity, below 20 K, can be explained by the sign change in the interaction coefficient m. On the basis of experimental data it is shown that the resistivity ratio (300 K)) is a controlling factor in determining the sign and magnitude of these effects. Furthermore, the normalization of relevant coefficients (e.g. m) with gives the result that the relative size of the effects is independent of the degree of chain orientation, and therefore the conductivity. However, it is found that the coefficient m crosses from negative to positive values at different values of for oriented and non-oriented conducting polymers. The positive magnetoconductance stemming from weak localization is substantial and very anisotropic in highly oriented conducting polymers. It is shown that this positive magnetoconductance exhibits a maximum as a function of in the vicinity of the metal-insulator transition. These results are compared with transport in other types of disordered metal.

Low temperature conductivity of metallic conducting polymers

In several metallic conducting polymers, both positive and negative temperature coefficient of resistivity (TCR) has been observed at low temperatures; and this can be easily tuned by disorder, pressure and magnetic field. This sign change in TCR is related to the sign change in m [a = σ 0 + mT 1/2 ] as a function of the resistivity ratio [p, - p(300 K) / ρ(1.4 K)] in both disorder-tuned and pressure-tuned samples of doped polypyrrole and poly(3-methyl thiophene). In both cases, the zero-crossing of m occurs at resistivity ratio around 2. This shows that the TCR, sign of m and the resistivity ratio are consistently related to each other in metallic conducting polymers.

Conductivity Measurements of Catalytically Synthesized Carbon Nanotubes

Electrical transport measurements have been undertaken on individual multiwalled catalytically synthesized carbon nanotubes. Two different techniques have been employed: In the first one nanotubes have been deposited on top of goldelectrodes. In the second one, atomic force microscope nanolithography has been used to locally oxidize a Ti thin film structure deposited above a single nanotube, thereby defining a two probe electrode configuration. It is shown that the latter method results in a two orders of magnitude lower resistance than the former method, due to the reduced contact resistance.

Mesoscopic electronic devices made by local oxidation of a Ti/Au bilayer film

Abstract The technique of field-induced local oxidation with the tip of an atomic force microscope can be used to pattern non-noble metal thin films by creating insulating regions. We have studied the technique (oxide line width, oxidation thickness, etc.) in order to fabricate mesoscopic electronic devices. Ti thin film structures have been patterned as well as more complicated structures where the Ti covers gold islands.