Novel materials: Silicon clathrates, carbon nanotubes and nanoropes.

摘要

There is now much interest in two classes of novel materials: silicon clathrates and carbon nanotubes. These materials have several interesting properties. Doped silicon clathrates are superconducting and carry the promise of wide, direct band-gaps in silicon. Nanotubes may form the basis for new light-weight, high-strength composites, switches, sensors or electromechanical devices. An understanding of the vibrational, mechanical, and thermal properties of these novel materials is vital for effective application of the materials.; Here I present calculations of the vibrational and bulk elastic properties of these materials using an O(N) total-energy tight-binding method. For the clathrates I compute their Raman and infrared spectra in addition to their vibrational density of states (VDOS). I find an interesting gap in the VDOS. Bulk properties such as the modulus and elastic constants are also computed. They are found to be similar to those of silicon, including negative mode-Grüneisen parameters at low energies.; For carbon nanotubes I also compute the Raman and infrared modes. The breathing mode is found to vary linearly with inverse diameter. The high energy modes show variations with tube morphology which could be used to distinguish tube morphologies spectroscopically. By adding a van der Waals term to the energy I can make similar calculations for nanotube ropes. In the nanorope I see a shift in low energy Raman modes due to the inter-tube interaction. When pressure is considered the both the breathing and high energy graphite-like modes are seen to up-shift. At high pressure a structural transformation is found. These results are in agreement with recent experiments.