Synthesis of single-wall carbon nanotubes by excimer laser ablation

We report the KrF excimer laser ablation of carbonaceous targets in an innovative laser ablation chamber. The targets have been prepared using a new approach, without pressing or hot pressing of the composition. The Co/Ni doped target has yielded single-wall carbon nanotubes with a narrow diameter distribution. High-resolution transmission electron microscopy has been used along with the confocal Raman microscopy to characterize the products obtained. Thermogravimetric analysis confirms the presence of multiple carbonaceous species with different oxidation temperatures.     

Pulse laser ablation system for carbon nano-onions fabrication

A new laser ablation chamber design for KrF excimer laser synthesis of carbon nanomaterials, including nano-onions, is reported. The conditions for carbon nano-onions deposition, using excimer laser to ablate a commercial pure graphite target, were investigated. The transmission electron microscopy analysis of the collected deposits indicates that mainly nano-onions are obtained when pure graphite targets are ablated. Raman spectroscopy identified without doubt production of carbon nano-onions.     

Observation of proportionality between friction and contact area at the nanometer scale

The nanotribological properties of a hydrogen‐terminated diamond(111)/tungsten‐carbide interface have been studied using ultra‐high vacuum atomic force microscopy. Both friction and local contact conductance were measured as a function of applied load. The contact conductance experiments provide a direct and independent way of determining the contact area between the conductive tungsten‐carbide AFM tip and the doped diamond sample. We demonstrate that the friction force is directly proportional to the real area of contact at the nanometer‐scale. Furthermore, the relation between the contact area and load for this extremely hard heterocontact is found to be in excellent agreement with the Derjaguin–Müller–Toporov continuum mechanics model. This revised version was published online in September 2006 with corrections to the Cover Date.     

An electron microscopy study of wear in polysilicon microelectromechanical systems in ambient air

Wear is a critical factor in determining the durability of microelectromechanical systems (MEMS). While the reliability of polysilicon MEMS has received extensive attention, the mechanisms responsible for this failure mode at the microscale have yet to be conclusively determined. We have used on-chip polycrystalline silicon side-wall friction MEMS specimens to study active mechanisms during sliding wear in ambient air. Worn parts were examined by analytical scanning and transmission electron microscopy, while local temperature changes were monitored using advanced infrared microscopy. Observations show that small amorphous debris particles (∼50-100 nm) are removed by fracture through the silicon grains (∼500 nm) and are oxidized during this process. Agglomeration of such debris particles into larger clusters also occurs. Some of these debris particles/clusters create plowing tracks on the beam surface. A nano-crystalline surface layer (∼20-200 nm), with higher oxygen content, forms during wear at and below regions of the worn surface; its formation is likely aided by high local stresses. No evidence of dislocation plasticity or of extreme local temperature increases was found, ruling out the possibility of high temperature-assisted wear mechanisms.     

A decade of experiments and recent upgrading at the AMS facility in Bucharest

The Bucharest AMS facility has been in operation since 1998. We shortly present the performed experiments, the major upgrade of the AMS facility at NIPNE – Bucharest and the ongoing progress resulting since. We mounted a new ion source, of NEC 40 sample MC-SNCIS type and we reinforced the vacuum on the injector deck. Computer control on all parameters of the injector deck was implemented through a build-in-house electronic set-up. By converting the Tandem accelerator from a belt-driven charging system to a Pelletron and by introducing a modern GVM we have obtained a reduction of the fluctuations of the terminal voltage by at least two orders of magnitude.     

Effect of Tritium Loading on the Superconducting Properties of Niobium and Tantalum

Niobium and tantalum sheets were loaded with tritium and their superconducting properties were investigated. Superconductivity is rather robust in both materials though they belong to different types of superconductors. The critical temperature does not change in the case of Nb at low field but decreases slightly in the case of Ta. The tritium-loaded Nb shows a higher remanent magnetization which we attribute to the subsurface accumulation of defects, but the high field irreversible magnetization is smaller than for the bare sample. In the case of Ta, both the remanent magnetization and the irreversible magnetization are slightly reduced after tritiation.     

Patterns in journal citation data revealed by exploratory multivariate analysis

The paper discusses techniques to emphasize patterns in citation data and to study their dynamics. In this context, the STATIS and the STATIS dual methods are presented. The methods are a generalization of the principal component analysis from a dynamical point of view. STATIS and its dual are applied in order to illustrate the dynamic of "citing-cited patterns" by using citation data of sixteen major journals from the statistics field.     

Laser-Driven Transient Phase Oscillations in Individual Spin Crossover Particles

An unusual expansion dynamics of individual spin crossover nanoparticles is studied by ultrafast transmission electron microscopy. After exposure to nanosecond laser pulses, the particles exhibit considerable length oscillations during and after their expansion. The vibration period of 50–100 ns is of the same order of magnitude as the time that the particles need for a transition from the low-spin to the high-spin state. The observations are explained in Monte Carlo calculations using a model where elastic and thermal coupling between the molecules within a crystalline spin crossover particle govern the phase transition between the two spin states. The experimentally observed length oscillations are in agreement with the calculations, and it is shown that the system undergoes repeated transitions between the two spin states until relaxation in the high-spin state occurs due to energy dissipation. Spin crossover particles are therefore a unique system where a resonant transition between two phases occurs in a phase transformation of first order.