Mercury telluride crystals encapsulated within single walled carbon nanotubes: A density functional study

The structure and binding energies of mercury telluride crystals encapsulated within single walled carbon nanotubes (SWNTs) have been studied using density functional theory. The energies of three different pseudo one-dimensional crystals of HgTe with 4:4, 3:3, and 2:2 coordination are compared. The initial structure for the 4:4 crystal was a 2 × 2 cubic motif derived from rock salt bulk structure, the 3:3 crystal corresponds to a novel structure found when HgTe was intercalated within SWNTs, and the 2:2 crystal is a chain motif derived from cinnabar (HgS) bulk structure. The isolated 3:3 crystal was found to be the most thermodynamically stable of the three structures. Calculations were performed on the 3:3 crystal inserted into three different SWNTs, (15, 0), (9, 9), and (17, 0), in order to investigate the perturbations on the molecular and electronic structure of the crystal and the SWNT, and the energy of formation of the HgTe@SWNT composites. The calculated structures are in good agreement with the experimental high resolution transmission electron microscopy images of the HgTe@SWNT composite. The calculated binding energies and density of states show that the interaction between nanotubes and the HgTe crystals is noncovalent. Since the energy difference of the "free" 4:4 and 3:3 structures is small and of the order of magnitude of the binding energies with the nanotubes, we carried out calculations on 4:4 HgTe structure inserted in to two different SWNTs, (15, 0) and (17, 0). The calculated binding energies show that, when the 4:4 structure is inserted into the smallest tube, the resultant composite has an energy comparable to the 3:3 structure, suggesting that this polymporph may also be found experimentally.