| AVS 57th International Symposium & Exhibition | |
| Tribology Focus Topic | Thursday Sessions |
| Session TR-ThP |
| Session: | Tribology Focus Session Poster Session |
| Presenter: | E. Kalfon-Cohen, Weizmann Institute of Science, Israel |
| Authors: | E. Kalfon-Cohen, Weizmann Institute of Science, Israel O. Goldbart, Weizmann Institute of Science, Israel R. Schreiber, Weizmann Institute of Science, Israel D. Barlam, Ben Gurion University, Israel T. Lorenz, Technical University Dresden, Germany G. Seifert, Technical University Dresden, Germany S.R. Cohen, Weizmann Institute of Science, Israel |
| Correspondent: | Click to Email |
Understanding the mechanical properties of nanotubes is of significant practical and fundamental interest. Multiwalled nanotubes and nanoparticles of metal dichalcogenides such as WS2 express unique mechanical and tribological characteristics.[1] The structure of WS2 nanotubes consists of layers of covalently bound trigonal bipyramidal WS2. The interaction between the layers is a van der Waals interaction between adjacent sulfur sheets. One of the intriguing aspects of these structures is the response of these layers under mechanical stress. Whereas some of the elastic constants of these unique structures have been addressed by experimental and theoretical work, the radial compression mode has not yet been studied. Relatively few studies of radial modulus of multiwalled carbon nanotubes have been made, and these also do not explicitly include the multilayered aspect of the structures. Here, we report an experimental and modeling study of this mode in the WS2 nanotubes.
Three independent atomic force microscope (AFM) experiments were employed to measure the nanomechanical response, using both large (R=200 nm) and small (R=3-15 nm) probe tips. Two different analytical models were applied to analyze the results.[2] For a large AFM tip, a Hertzian model presuming an elliptical contact was applied. A shell continuum model is applied in the case of the sharper AFM tips. This model treats the nanotube wall as a thin, curved membrane. The results indicate that the derived modulus varies with nanotube diameter and compression depth
The modulus values derived from the analytical models were used as initial input for finite element analysis (FEA). The FEA model described the nanotubes as alternating high stiffness (representing the covalent shells) and low stiffness (representing the vdW gap) layers for the outer two shells, with a homogeneous inner core. This model was fit with the experimental results over the initial linear elastic region of the first few nm of deformation. Values obtained varied for different nanotube diameters, and compression depths, showing the importance of the inter-layer contact. In addition, first-principles calculations using density functional theory tight binding give qualitative agreement with a reversible collapse of the nanotubes, seen at larger deformations.
[1] L. Rapoport, et al, Nature 387, 791-93 (1997); I. Kaplan-Ashiri et al, Proc. Natl. Acad. Sci. USA. 103, 523 (2006); I. Kaplan-Ashiri et al, J. Phys. Chem C. 111 8432-8436 (2007)