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Pure thin tin films, electroplated on copper substrates, were investigated byin situX-ray diffraction analysis during room-temperature aging, using a laboratory diffractometer equipped with a two-dimensional detector. Diffraction spots of single Sn grains in the Sn film could be observed and traced over time by using the diffraction method adopted. For the as-deposited specimens, striking changes of the Sn reflection spots, concerning their position and intensity, as well as sudden appearances and disappearances of additional diffraction spots, were observed. This could be ascribed to local microstructural changes in the films such as grain rotation, grain growth and grain dissolution. In contrast to the as-deposited specimens, so-called post-baked specimens (i.e.annealed at 423 K for 1 h after layer deposition) exhibited a stable microstructure. The results obtained are discussed with respect to their relevance for the Sn whisker-growth phenomenon.

We introduce voltage-contrast scanning electron microscopy (VC-SEM) for visual characterization of the electronic properties of single-walled carbon nanotubes. VC-SEM involves tuning the electronic band structure and imaging the potential profi le along the length of the nanotube. The resultant secondary electron contrast allows to distinguish between metallic and semiconducting carbon nanotubes and to follow the switching of semiconducting nanotube devices, as confi rmed by in situ electrical transport measurements. We demonstrate that high-density arrays of individual nanotube devices can be rapidly and simultaneously characterized. A leakage current model in combination with fi nite element simulations of the device electrostatics is presented in order to explain the observed contrast evolution of the nanotube and surface electrodes. This work serves to fill a void in electronic characterization of molecular device architectures.

We report on reversible metal to insulator transitions in metallic single-walled carbon nanotube devices induced by repeated electron irradiation of a nanotube segment. The transition from a low-resistive, metallic state to a high-resistive, insulating state by 3 orders of magnitude was monitored by electron transport measurements. Application of a large voltage bias leads to a transition back to the original metallic state. Both states are stable in time, and transitions are fully reversible and reproducible. The data is evidence for a local perturbation of the nanotube electronic system by removable trapped charges in the underneath substrate and excludes structural damage of the nanotube. The result has implications for using electron-beam lithography in nanotube device fabrication.

We report on a carbon nanotube network which is composed of aligned metallic and randomly oriented semiconducting single-walled carbon nanotubes. The material is formed by using a novel radio frequency dielectrophoresis setup, which generates very large dielectrophoretic force fields and allows dielectrophoretic assembling of nanotube films up to 100 nm thickness. Polarization dependent absorption measurements provide experimental evidence for the electronic type specific alignment behavior. We explain the experimental data with an advanced model for nanotube dielectrophoresis, which explicitly takes into account both the longitudinal and transversal polarizability. On the basis of this model, we calculate the dielectrophoretic force fields and show that semiconducting nanotubes deposit under very large fields due to their transversal polarizability even for high field frequencies.

One of the biggest limitations of conventional carbon nanotube device fabrication techniques is the inability to scale up the processes to fabricate a large number of devices on a single chip. In this report, we demonstrate the directed and precise assembly of single-nanotube devices with an integration density of several million devices per square centimeter, using a novel aspect of nanotube dielectrophoresis. We show that the dielectrophoretic force fields change incisively as nanotubes assemble into the contact areas, leading to a reproducible directed assembly which is self-limiting in forming single-tube devices. Their functionality has been tested by random sampling of device characteristics using microprobes.

The authors show that single-walled carbon nanotubes are effective probes for dielectrophoretic force fields by studying their deposition pattern on surfaces. Different force fields were generated by electrodes on insulating oxides, with or without conducting substrates underneath. The patterns were recorded by scanning electron microscopy and simulated by finite element calculations. The data show that a sign change of the normal component of the dielectrophoretic force close to the substrate surface can lead to a repulsion of metallic carbon nanotubes from the surface. The authors present geometrical constraints to avoid such a sign change in the dielectrophoretic force.