ABSTRACT
Fracturing microscale constrictions in metallic wires, such as tungsten, platinum, or platinum-iridium, is a common fabrication method used to produce atomically sharp tips for scanning tunneling microscopy (STM), field-emission microscopy and field ion microscopy. Typically, a commercial polycrystalline drawn wire is locally thinned and then fractured by means of a dislocation slip inside the constriction. We examine a special case where a dislocation-free microscale constriction is created and fractured in a single crystal tungsten rod with a long side parallel to the [100] direction. In the absence of dislocations, vacancies become the main defects in the constriction which breaks under the tensile stress of approximately 10 GPa, which is close to the theoretical fracture strength for an ideal monocrystalline tungsten. We propose that the vacancies are removed early in the tensile test by means of deformation annealing, creating a defect-free tungsten constriction which cleaves along the W(100) plane. This approach enables fabrication of new composite STM probes which demonstrate excellent stability, atomic resolution and magnetic contrast that cannot be attained using conventional methods.
ABSTRACT
Understanding molecular switching between different charge states is crucial to further progress in molecule-based nano-electronic devices. Herein we have employed scanning tunnelling microscopy to visualize different charge states of a single C60 molecule within a molecular layer grown on the WO2/W(110) surface. The results obtained demonstrate that individual C60 molecules within the layer switch between neutral and negatively charged states in the temperature range of 220-260 K over the time scale of the experiment. The charging of the C60 causes changes in the local density of electron states and consequently a variation in tunnelling current. Using density functional theory calculations, it was found that the charged state corresponds to the negatively charged C60(-), which has accepted an electron. The switching of the molecule into the charged state is triggered continuously by tunnelling electrons when the STM tip is static above an individual C60 molecule with a bias applied. Molecular movement accompanies the molecule's switching between these states.
ABSTRACT
The as-grown shape of the cylindric tilt grain boundary (GB) in Mo bicrystals grown by the floating zone method has been studied with the electron backscattering diffraction method. The seed crystals were misoriented such that the coincidence site lattice (CSL) with lowest possible inverse density of coincidence sites, a Sigma=3 was grown. The flat (100)(Sigma3CSL) facets were observed forming smooth edges (no slope discontinuity) with rounded rough GB portions. Rough GBs curve away from the plane of the (100)(Sigma3CSL) facet as x(beta) with beta=1.69+/-0.07 on one side and beta=1.72+/-0.07 on the other side. Therefore, GB roughening belongs to the Pokrovsky-Talapov universality class. Slope discontinuities between two rounded rough GB portions were also observed. This is the first experimental observation of such first-order rough-to-rough ridges predicted by the Davidson-den-Nijs model.
