studying-supramolecular-organization-photosynthetic-membranes-within

Studying the Supramolecular Organization of Photosynthetic Membranes within Freeze-fractured Leaf Tissues by Cryo-scanning Electron Microscopy

Cryo-scanning electron microscopy (SEM) of freeze-fractured samples allows investigation of biological structures at near native conditions. Here, we ...

Weizmann Institute of Science

The mechanical properties of materials and particularly the strength are greatly affected by the presence of defects; therefore, the theoretical strength ( approximately 10% of the Young's modulus) is not generally achievable for macroscopic objects. On the contrary, nanotubes, which are almost defect-free, should achieve the theoretical strength that would be reflected in superior mechanical properties. In this study, both tensile tests and buckling experiments of individual WS(2) nanotubes were carried out in a high-resolution scanning electron microscope. Tensile tests of MoS(2) nanotubes were simulated by means of a density-functional tight-binding-based molecular dynamics scheme as well. The combination of these studies provides a microscopic picture of the nature of the fracture process, giving insight to the strength and flexibility of the WS(2) nanotubes (tensile strength of approximately 16 GPa). Fracture analysis with recently proposed models indicates that the strength of such nanotubes is governed by a small number of defects. A fraction of the nanotubes attained the theoretical strength indicating absence of defects.

On the mechanical behavior of WS2 nanotubes under axial tension and compression.

We experimentally observed atomic-scale torsional stick-slip behavior in individual nanotubes of tungsten disulfide (WS2). When an external torque is applied to a WS2 nanotube, all its walls initially stick and twist together, until a critical torsion angle, at which the outer wall slips and twists around the inner walls, further undergoing a series of stick-slip torque oscillations. We present a theoretical model based on density-functional-based tight-binding calculations, which explains the torsional stick-slip behavior in terms of a competition between the effects of the in-plane shear stiffness of the WS2 walls and the interwall friction arising from the atomic corrugation of the interaction between adjacent WS2 walls.

Torsional stick-slip behavior in WS2 nanotubes.

We present a new approach for subdiffraction-limited far-field Raman spectroscopy of single carbon nanotubes using through-the-objective total internal reflection (TIR) excitation coupled to an atomic force microscope (AFM). By using this approach, we are able to detect spectroscopic signatures of structural changes along a single nanotube with nanometer resolution. A single multiwalled carbon nanotube is mounted on an AFM tip and imaged while tapping on the surface of a glass coverslip. As the angle of incidence of the excitation field is changed, we are able to tune the penetration depth of the evanescent field by steps as small as 2-10 nm. An increase in the ratio of the Raman D band (the disorder band) to G band (the in-plane graphitic band) of the carbon nanotube was demonstrated as the penetration depth decreased, indicating that most defects are concentrated at the end of the nanotube. We also observed frequency shifts of the G band as we changed the penetration depth. By changing the polarization of the incident beam, we are able detect the orientation and possible local curvature in the nanotubes. Coupling through-the-objective TIR with AFM is a powerful technique for studying structural and chemical properties of carbon nanotubes and can be easily extended to many other nanoscale/molecular systems.

Article	Year
On the mechanical behavior of WS2 nanotubes under axial tension and compression. Proceedings of the National Academy of Sciences of the United States of America\| PubMed ID: 16407141	2006
Torsional stick-slip behavior in WS2 nanotubes. Physical review letters\| PubMed ID: 19113281	2008
Subdiffraction-limited far-field Raman spectroscopy of single carbon nanotubes: an unenhanced approach. ACS nano\| PubMed ID: 21229967	2011

Ifat Kaplan-Ashiri

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