לוח שנה משולב

Traditionally, electron microscopes are mainly used to characterize structures while electron beam damage to the sample should be avoided. The combination of electron microscopes and nanomanipulators provides an intuitionistic, real-time and in situ way to study nanomaterials, enabling us to relate directly the properties of nanomaterials to their structures. Although scanning electron microscope (SEM) does not provide atomic structure information like transmission electron microscope (TEM) does, its large specimen chamber can accommodate multi-nanomanipulators and enables various multi-terminal measurements, such as electrical and mechanical measurements.
Here, we demonstrate a set of novel methods for performing in situ measurements on individual thin carbon nanotubes (CNTs) using nanomanipulators inside a SEM. With the methods, the mechanical and electronic properties of individual thin CNTs were studied. In addition, the measured properties were correlated directly to the atomic structures of the CNTs.
The mechanical response of individual double-walled and triple-walled carbon nanotubes (CNTs) and CNT ropes consisting of only two double-walled CNTs (DWCNTs) under tensile load was measured using nanomanipulators in a SEM. The breaking strain and strength and Young’s modulus of individual CNTs were measured. Carbon nanotube ropes exhibited one-step or stepwise breaks depending on the relative breaking strains of the two CNTs.
The electronic properties of individual thin CNTs under a known axial tensile load were studied. A strain induced metallic-to-semiconducting transition of a DWCNT and a bandgap increase of a SWCNT were observed. The electromechanical properties of the SWCNT were also correlated to its chirality determined by electron diffraction.
The vibration properties of individual CNTs under axial tension were quantitatively determined experimentally. A gradual beam-to-string transition from multi-walled CNTs to SWCNTs was observed. The continuum beam theory was found to be applicable to even single-walled CNTs.

One of the very early discoveries in istope geochemistry was that atmospheric oxygen is enriched in 18O with respect to seawater. This enrichment was named Dole effect. By the mid 50' it became clear that the main driving force causing the Dole effect is respiration that favors 16O over 18O. It was also demonstrated that photosynthesis produces oxygen close in isotopic composition to the substrate water. Yet, the Dole effect, whose magnitue is almost 24 ‰, cannot be explained exclusively by respiratory fractionation. Over the past four decades the consensus has been that evapo-transpiration that enriches leaf water in heavy isotopes, enhaces the Dole effect through the production of 18O enriched O2 in leaves of land vegetation. In this concensus, the respiratory fractionation on land has been taken to be about 18 ‰. This assumption, however, was proven wrong by our group. We showed that slow oxygen-gas diffusion in roots and soils reduces the effective respiratory fractionation, and globally, fractionation by soil respiration, which consumes most of the land production of O2, is about 14 ‰. This relatively small fractionation compensates for the evapotranspiration enhancement, and overall, the magnitude of the terrestrial component of the Dole effects remains relatively small, thus leaving the global effect unexplained. Our recent experiments, using 17O/16O and 18O/16O ratios, show that marine photosynthesis produces O2 that is enriched by several ‰ with respect to seawater. This new discovery solves the long standing question about the magnitude of the global Dole effect.